Challenges and opportunities for a European HVDC grid

Renewable and Sustainable Energy Reviews 70 (2017) 427–456

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Challenges and opportunities for a European HVDC grid a

a

Erika Pierri , Ole Binder , Nasser G.A. Hemdan

a,b,⁎

a

, Michael Kurrat

crossmark

a

Institute of High Voltage Technology and Electrical Power Systems, Technische Universität Braunschweig, Schleinitzstraβe 23, 38106, Braunschweig, Germany b Electrical Engineering Department, Faculty of Engineering, Minia University, 61519, Minia, Egypt

A R T I C L E I N F O

A BS T RAC T

Keywords: HVDC grid Renewable energies Offshore wind power European overlay network HVDC Grid Codes

In 2009 the European governments committed their countries to a reduction in green-house-gas emissions up to 80% by 2050. Consequently, the employment of renewable energies has been strongly encouraged, making the upgrade of the existing AC grid necessary. Several associations are considering whether it would be more beneficial to build an HVDC grid instead. The main objective of the current study is to investigate possibilities and challenges in relation to the installation of a European HVDC grid. Most suitable technologies and possible configurations have been identified. Applications and existing projects of HVDC have been briefly mentioned. After examining various Supergrids models, financial aspects resulted challenging, since big investments are required. From a technical point of view, further research for DC voltage control and protection strategies is required. The publication of the HVDC Grid Codes represents a first step towards a European energy union; requirements are however not fully mandatory in all nations yet. The most probable development of the future HVDC grid has been defined: the grid is not expected to be built at once, but rather to develop in an organic way. In this sense, the installation of an offshore grid would be highly beneficial. Studies of possible supergrid architectures are still controversial and in part superficial. A solution to solve those lacks and achieve an agreement would be a strict collaboration between existing associations and involved European countries.

1. Introduction Currently, developed and emerging countries are facing a transition phase as a result of the electrification process. Innovation of the transmission grids has already began in countries like the USA [1]. One of the current challenges of the transmission networks worldwide is the integration of a large amount of renewable energy sources (RES), for instance offshore wind farms and photovoltaic (PV), into the power system [2–4]. Installed wind capacity is growing fast and need to be accommodated into the grid. Locations where wind has the maximum potential are often isolated. For instance, in Brazil the most favorable region for the installation of wind power plants is the Northeast, whereas the biggest load centers are in the South [5]. Concentrated

Solar Power (CSP) and big-scale photovoltaic plants are also more likely built or planned in remote areas, such as unpopulated regions or deserts (with higher solar radiation). In order to optimize the employment of those sustainable resources, the construction of new power grids is hence required. The choice of the transmission technology depends upon different issues, including social influence, environmental impact as well as technical and financial criteria [6]. For more than one hundred years the HVAC (High Voltage Alternating Current) technology has been the backbone of power transmission. However with an increasing size of the systems and the liberalization in the power industry, HVAC is losing its importance against HVDC (High Voltage Direct Current) [7]. Due to reactive power limitations, AC cable transmission is in fact not feasible over long

Abbreviations: AC, Alternating Current; ACER, Agency for Cooperation of Energy Regulators; CENELEC, European Committee for Electrotechnical Standardization; CTL, Cascaded Two Level; COMECON, Council for Mutual Economic Assistance; CSP, Concentrated Solar Power; DC, Direct Current; EEG, Renewable Energies Act; ENTSO-E, European Network of Transmission System Operators for Electricity; E[R], Energy Revolution; EWEA, European Wind Energy Association; FACTS, Flexible Alternating Current Transmission System; FC, Flying Capacitor; FOSG, Friends of the SuperGrid; GDP, Grid Development Plan; GTO, Gate Turn-off Thyristors; HVDC, High Voltage Direct Current; IGBT, Insulated Gate Bipolar Transistors; LCC, Line Commutaded Converter; MMC, Modular Multilevel Converter; MI, Mass Impregnated; NC, Network Codes; NREAP, National Renewable Energy Action Plan; NSCOGI, North Seas Countries Offshore Grid Initiative; NTC, Net Transfer Capacities; OEEC, Organization for European Economic Cooperation; O-GDP, Offshore-Grid Development Plan; PCI, Projects of Common Interest; PPM, Power Park Modules; PV, Photovoltaic; PWM, Pulse Width Modulation; RES, Renewable Energies Sources; TSO, Transmission System Operator; TYNDP, Ten-Year Network Development Plan; UCTE, Union for the Coordination of Transmission of Electricity; VSC, Voltage Source Converter; XLPE, Crossed-Linked PolyEthylene ⁎ Corresponding author at: Institute of High Voltage Technology and Electrical Power Systems, Technische Universität Braunschweig, Schleinitzstraβe 23, 38106, Braunschweig, Germany. E-mail address: [email protected] (N.G.A. Hemdan). http://dx.doi.org/10.1016/j.rser.2016.11.233 Received 26 October 2015; Received in revised form 20 October 2016; Accepted 19 November 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

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distances, since transmission losses are too high. HVDC is an efficient technology designed to deliver large amounts of electricity over long transmission routes with lower losses than a conventional AC system. In general applications, HVDC can carry more power per conductor of a given size than AC. Therefore the profile of pylons and wiring can be reduced, saving both money and land. HVDC also allows power transmission between grid systems running at different frequencies, improving the overall stability and reliability of the electrical power system [8]. These and further advantages have encouraged a wider use of the HVDC technology. Indeed in the second half of the last century HVDC links have been installed worldwide for the interconnection between countries, integration of green energies into the AC power system, bulk power transmission from remote energy sources and supply of electrical power from shore to oil and gas offshore platforms. Submarine cables have been developed in order to transmit power over long distances. Various commercial trademarks for different HVDC technologies are available today, using either Voltage Sourced Converters (VSC) or Line Commutated Converters (LCC). LCC is a mature technology, which has been used in many HVDC projects. However VSC is in continuous development and is today the preferred technology for the interconnection of isolated grids to the power system, such as offshore wind farms. Its main characteristic is the continuous and independent control of active and reactive power. The fast development of HVDC technology led to a new concept of electrical power grid: the HVDC Supergrid. This paper has the aim of analyzing challenges and opportunities concerning the installation of such a grid in Europe. Applications and HVDC grid arrangements are going to be also examined. An overview of existing and planned projects and their economic aspects will be provided. Different studies on the installation of future HVDC grids in Europe are going to be analyzed, including details regarding the German Grid Development Plan (GDP). HVDC Grid Codes build the basis for the technical standardization across Europe. Requirements for the implementation of a European Supergrid as well as the consequent benefits and challenges will be discussed. Finally prospects of HVDC offshore and onshore Supergrids will be presented.

Fig. 1. Components of a HVDC transmission system [13].

interest due to the lower power electronics cost and improved control electronics [12]. 2.1. System description As shown in Fig. 1, the three main components of an HVDC system are the converter station, the transmission medium and the electrodes. The transmission medium is the system employed to transport power, i.e. overhead lines or underground/submarine cables. Main components of a converter station are transformers, AC and DC filters, converter valves, the control system and, if necessary, reactive power compensation equipment [13]. The basic configurations of LCC- and VSC-HVDC systems are presented below, including details concerning the single components. 2.1.1. LCC-HVDC configuration Fig. 2 shows the configuration of a LCC-HVDC system and the function of its components are listed in Table 1.

2. High Voltage Direct Current (HVDC) HVAC remains the primary medium for general transmission and distribution of electrical energy, however HVDC has proven to be an economic media of power transmission in different applications, including [9]

2.1.2. VSC-HVDC configuration The configuration of a typical VSC-HVDC system is displayed in Fig. 3 and the components functions are described in Table 2.

1. Long distance overland overhead line transmission (above 800 km). 2. Long submarine cable crossings (up to 80 km). 3. Interconnections between asynchronous networks.

2.2. Technical facts As mentioned before, HVDC consists in the combination of a DC circuit with two power electronic converters, each one at a link terminal, for AC/DC and DC/AC conversion. The main characteristics

The invention of the high-voltage mercury arc valve was the first milestone in the development of HVDC systems. This made it possible to integrate HVDC links in AC networks. With the construction of new power generation plants at remote locations from the load centers, new transmission lines are required. There is a growing opposition to their acceptance, though. An alternative is represented by the possibility of increasing the power carrying capability of the AC transmission lines [10]. With an increasing size of interconnected systems, the technical and economical advantages diminish, due to problems regarding load flow, power oscillation and voltage quality. For instance, in the initial UCTE system the 400 kV voltage level is too low for large cross-border and inter-area power exchange. A further increase of the power transfer would lead to problems; therefore advanced solutions need to be implemented [11]. In order to increase the grid flexibility and improve its performance two complementary technologies have been developed, HVDC and FACTS (Flexible AC Transmission System). HVDC systems have been used for over 50 years and have lately gained renewed

Fig. 2. Components of a LCC-HVDC system [14].

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Table 1 Function of LCC-HVDC components. Component

Function

LCC Converters

– Heart of the system – Main component: Thyristors valves – Connecting the AC network to the valve bridges and adapting the AC voltage level. Each converter needs three transformers. – Avoiding that the current harmonics produced on the AC side of HVDC converters enter into the AC network, or at least reducing them to a suitable level. – Reducing ripple on the DC voltage – Responsible of interferences close to the DC line – They are necessary when the transmission medium is an overhead line, whereas in case of underground cables or back-to-back transmission they are not needed – Consists of two converters, one controlling the DC voltage and the other the DC current – Yielding the desired combination of voltage and current – Enhancing system stability and optimizing power availability

Transformers AC Filters DC Filters

Control System Reactive Power Compensation

control of the power flow is feasible [2]. Any power transfer can be set independently of impedance, phase angle, frequency and voltage [10]. This avoids the overloading and improves the stability of the surrounding AC power system. In addition no increase in the short-circuit capacity is imposed on the AC system switchgear: in case of shortcircuit at one of the terminals, the converters can be quickly switched off [2]. By comparing AC and DC links it is important to consider whether the synchronization of the separate systems is technical and economical possible. By choosing the DC technology this issue can be avoided, since the two connected systems do not need to be synchronized. The interconnection can be used as a generation reserve able to provide power immediately. HVDC links also have the capability to recover from power failures using adjacent grids, the so called black-start [10].

Fig. 3. Components of a VSC-HVDC system [14]. Table 2 Function of VSC-HVDC components. Component

Function

VSC Converters

– Heart of the system – Main component: IGBT valves. – Adapting the AC voltage to the DC voltage level – Currents regulation between active and reactive power – Reduce high frequency harmonics of the AC currents. – Same function as in LCC-HVDC systems, – No reactive power compensation is needed though. – Besides the current harmonics on the AC side are related to the PWM (Pulse Width Modulation) frequency. – The amount of filters is smaller compared to LCC [13]. – Reducing voltage ripple on the DC side (as DC filters in LCCHVDC transmission) – Providing energy storage, able to control power flow. Their size depends on the required DC voltage [15] – Control of active and reactive power independently. It consists of a faster vector controller.

Transformers Phase Reactors AC Filters

DC Capacitors

Control System

2.3. Transmission media The overhead line technology used in HVDC connections is available at service voltages up to ± 600 kV for transfers of about 3000 MW and distances up to 1800 km. HVDC overhead lines can be designed to have a similar performance to HVAC overhead lines. This technology is economically feasible only in high power transmissions over distances above 200 km. So far HVDC transmission was mainly used in submarine applications, either connecting offshore wind farms to land or transmitting electricity over long distances through the sea, where overhead lines cannot be used. HVDC cables are beginning to be used also for land transmission projects [16]. Fig. 4 depicts the possible layouts of overhead lines and underground cables required to fulfill 5 GW power transmission using HVDC. The first option shows the configuration of two lines of 300 kV each. The distance between the towers and from vegetation is equal to 25 m, hence in total 100 m are required. An alternative is to use one single line at 800 kV, with a space requirement of 50 m. By employing underground cables the two options are using either three bipoles at 500 kV, with MI cables and line-commutated converter (LCC), or 5 bipoles at 320 kV, with extruded cables and voltage-source converter (VSC). In both cases the choice of underground cables is environmentally more advantageous, since the ecological footprint can be severely reduced. The only restriction on the use of land over an underground section is that no deeply rooted trees may be planted within the corridor plus a margin of 2 m. However the ground over the cables can be re-naturalized by planting shallow rooted trees or even through agricultural farming [16]. Underground cables can hence transmit power across densely populated areas, or regions where land is either costly or environmentally sensitive. As shown in Fig. 5, three cable technologies are available for HVDC links, namely Mass Impregnated (MI), Self-Contained Fluid Filled (SCFF) and Crossed Linked Polyethylene (XLPE). Their main characteristics are given below. A. Mass impregnated (MI): Consisting of Kraft paper layers,

of a HVDC technology are its speed and flexibility, providing several benefits, including transfer capacity enhancement, power flow control, transient stability improvement, power oscillation damping, voltage stability and control, rejection of cascading disturbance and absence of reactive power during the transmission. One of the main advantages of HVDC is the transmission line length limitation only by ohm resistance: long submarine or underground cables can transmit power with low loss levels and without the need of reactive power compensation. Thus for a given conductor cross section, HVDC can transfer more power through a conductor than a conventional HVAC transmission [8]. Despite the higher levels of initial power losses characteristic of an HVDC system, overall losses are lower compared to HVAC, for applications requiring long transmission routes. In contrast, losses levels increase with the distance in an AC system, making HVDC more feasible for long transmissions. Furthermore active power can be transmitted in both directions and reversed if necessary, hence a full

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Fig. 4. A comparison of HVDC overhead lines and underground cables: possible layouts to fulfill HVDC 5 GW power transmission requirements (based on [16]).

2.4. Converter technology Nowadays approximately 100 HVDC installations have been realized or are planned worldwide, employing two different converter technologies, e.g. Line-Commutated Converters (LCC) and VoltageSource Converters (VSC) [20]. The main features are listed in Table 3. 2.4.1. Line-Commutated Converters (LCC) Conventional HVDC transmission employs LCC. The term linecommutated indicates that the conversion process relies on the line voltage of the connected AC system. LCCs use switching devices, which can be only turned on. Early LCC systems employed mercury-arc valves, however many adaptations were necessary to make them suitable for HVDC. Nowadays mercury-arc valves are no longer used and they have been replaced by thyristors. A thyristor is a controllable semiconductor able to carry very high currents (4000 A) and to block very high voltages (up to 10 kV). Many thyristors connected in series build a thyristor valve, suitable to operate at hundreds of kV at the network frequency (50 Hz in Europe). Nevertheless a LCC requires a synchronous voltage source in order to operate. The difference in reactive power must be kept within defined values to maintain the AC voltage in the desired tolerance. The weaker the system or the further away from generation, the tighter the reactive power exchange must be to stay within the desired voltage tolerance [22]. This limitation is overcome in VSC stations.

Fig. 5. HVDC cables. (a): Mass Impregnated, (b): Self-Contained Fluid Filled, (c): XLPE [17] .

impregnated with high viscosity oil. They are not used in combination to HVAC systems, due to problems with partial discharge [18]. Nowadays MI can be provided by European manufacturers at voltages up to ± 600 kV, to a maximum pole rating of 800 MW and bipole rating of 1600 MW. Conductor sizes are typically up to 2700 mm2. Short-term achievable ratings are up to 1500 MW for a single cable and 3000 MW for bipole. They have a long operational experience, as they have been in use in submarine application for at least 40 years. On land, they are a proven and reliable transmission component [16]. B. Self-Contained Fluid Filled (SCFF): Insulated with special paper and impregnated with low viscosity oil. Conductor sizes reach 3000 mm2 and voltage levels are up to 600 kV [17]. C. Crossed Linked Polyethylene (XLPE): This technology has been in use for many years in HVDC applications but is relatively new in the HVDC market, previously dominated by MI cables. XLPE cables provide several advantages. For land applications they are lighter than MI cables, allowing longer transportation lengths and longer distances between joints. In addition pre-molded joints are available, reducing the time required for cable jointing. XLPE cables are generally more mechanically robust and they can operate at higher temperatures than MI cables. Hence they can carry more current for a given conductor cross section. However, XLPE cables suffer from a space charge phenomenon; therefore they can be employed only in combination with Voltage Source Converters (VSC), which allows power flow reversion without reversing the voltage polarity. ABB has recently developed a new XLPE HVDC cable system, which has a voltage capacity of 525 kV. Up to now, cables with up to 320 kV have been commercially available; hence ABB accomplished an increase in voltage capacity equal to 64%. By these means, the power transmission capacity, limited to 1000 MW, can be doubled and the transmission can be feasible even by links of 1500 km length [19].

2.4.2. Voltage-Source Converters (VSC) VSC was employed for the first time in 1997 in the Hellsjön project in Sweden [23]. Since then a considerable development of this technology has been achieved. As mentioned above, thyristors can only be turned on, therefore synchronous machines must be available in the AC system to which the LCC-HVDC is connected, in order to provide the commutating voltage. VSC uses other types of semiconTable 3 Main features of LCC and VSC technologies (based on current projects and [21]).

Power ratings [MW] Voltage levels [kV] Losses [%] Cables Technology

430

LCC

VSC

7200 800 0.75 MI Thyristors

2000 500 1 XLPE Transistors (IGBT, GTO)

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ductor devices, such as the Insulated Gate Bipolar Transistors (IGBT), which instead can be both turned on and off. This makes VSC advantageous over LCC, as the VSC valves are independent on the operation of the surrounding AC grid. Hence a VSC-HVDC system can supply AC networks consisting only of passive loads. Furthermore, VSC can control the power flow and provide dynamic voltage regulation to the AC system [22,24,25]. In addition no reactive power compensation is needed. Therefore less harmonic filtering is needed and the converter station is more compact, making its use advantageous for instance on offshore platforms, where space saving is an important planning issue. In LCC stations the direction of power flow can be changed only by reversing the polarity of DC voltage at both stations. In VSC it can be achieved by reversing the current direction, keeping the polarity of DC voltage constant. By this means, VSC can be easily connected to Multiterminal HVDC systems [26]. In the first VSC applications, losses in the converter station were equal to 3%. Afterwards an improvement has been accomplished, reducing the losses up to 1.7%. In the last years a percentage of 1% has been achieved [28]. Despite the fact of being an emerging technology, VSC is facing a fast development. It is thus expected to become equivalent to LCC in terms of power capabilities and voltage levels (see Fig. 6). It is hence clear that VSC-HVDC represents a promising technology for the development of future HVDC grids. Fig. 7. Location of renewable energy sources [33].

2.5. Applications

HVDC); 5. Supply of electrical power from shore to oil and gas offshore platforms.

In many applications HVDC is economically more advantageous than HVAC, such as in transmission over long distances. HVDC is sometimes the only technical feasible solution, for instance for interconnecting two asynchronous grids, reducing fault currents, long underground cable circuits, bypassing network congestion and mitigating environmental concerns [29]. Besides, in relation to photovoltaic, HVDC has the potential of minimizing the system price and making thus PV even more attractive for investors than nowadays. Indeed, in PV grid-connected systems, DC/AC converters are needed in order to integrate produced power in the conventional AC grid, increasing the overall costs [30,31]. HVDC technology has been already successfully applied for the following purposes [32]:

Long-distance transmission of solar power from desert areas situated far from consumers, as the Sahara region, is under considerations and will be probably achieved in the near future [32]. In the following paragraphs, single applications of HVDC are examined. A. Long distance bulk power transmission: Most RES sites are located far away from population and industrial centers (see Fig. 7). Therefore power has to be transmitted from the station to the load centers over distances up to 2500 km. For links of this size, more lines are needed for the HVAC transmission and reaction power compensation is necessary. With this extreme high-capacity transmission, LCC technology is the most feasible alternative, by means of economical and environmental benefits. Results of several studies showed that HVDC is the cheapest solution for transmission distances of about 700 km and above (the so-called break-even distance) [34]. B. Integration in the AC network: Most of HVDC links in operation today are installed between two different synchronous AC networks. Due to growing needs for grid development and environmental issues related to overhead lines, new underground HVDC links are planned to be embedded in meshed AC networks. Contrary to an AC line, the power set-point of an HVDC link can be chosen to optimize the distribution of flows on the network. When large amount of RES are connected to the grid overloading of weak links or bottlenecks in the existing grid can occur [34]. VSC HVDC can be used for segmenting very large grids into smaller, more stable subgrids. This segmentation can be an effective means for controlling and avoiding widespread disturbances and overcoming grid bottlenecks [32]. C. Asynchronous ties: AC transmission is only possible if the two interconnected AC systems have the same nominal frequency and operate synchronously. DC transmission does not have such requirements. Several back-to-back HVDC links have been built for such purposes, for instance between regions and countries. Asynchronous HVDC links act as an effective firewall against propagation of cascading outages between networks [29].

1. Bulk power transmission from large, remote energy sources, i.e. large-scale hydro-power plants (LCC-HVDC); 2. Offshore wind farms and remote land-based wind farms, inaccessible to present HVAC grids (VSC-HVDC); 3. Embedded HVDC links for improving HVAC grid performance and facilitating the introduction of renewable energy into the grid (VSCHVDC); 4. Interconnections of national or regional grids (LCC and VSC-

Fig. 6. Development of ABB’s HVDC technology (HVDC Classic – LCC; HVDC Light – VSC) [27].

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D. Power delivery to large urban areas: The development of new transmission systems into large cities is problematic, due to rightof-way limitations and land-use constraints. VSC underground cables can be placed on dual-use locations to supply power and to provide voltage support. Stations are compact and housed mainly indoors, making constructions permissions in urban areas easier. Furthermore, the dynamic voltage support offered by the VSC can often increase the capability of the adjacent AC transmission. E. Underground and submarine cable transmission: AC cables have physical restrictions, limiting the distance and the power level. HVDC underground/submarine cables do not present these limitations. For a given cable conductor area, HVDC losses are about the half compared to those of AC cables. Using HVAC technology in a cable system would require series-connected reactors or phaseshifting transformers, in order to balance unequal loadings or the risk of overloads. These requirements are not necessary in a controlled HVDC cable system [8]. F. Offshore transmission, production platforms and wind farms: Self-commutation, dynamic voltage control and black-start capability allow compact VSC-HVDC transmission to supply isolated loads on islands or offshore production platforms (oil and gas) with electrical power from shore through submarine cables. Offshore wind farms have several advantages over onshore wind farms, including the availability of higher wind speed, the ease of transporting large structures and hence the possibility of building larger wind turbines. In Europe the availability of inland locations for wind farms is limited, encouraging the offshore alternative [35]. In [36] a technical/economical evaluation concerning grid connection of wind farms has been performed, defining VSC-HVDC as the cheapest option for connecting a 100 MW and larger wind power plant at distances over 90 km from shore. Fig. 8. Different potential topologies for DC grids [42].

3. HVDC grid

fore the most advantageous for the implementation of an HVDC grid. A multi-terminal system consists of a number of AC/DC and DC/AC converter stations. The terminals can be connected either in radial or in meshed topologies. Employing radial arrangements, converters connected to load or generation can have only one power flow direction, whereas converters connected to AC systems have both power flow directions. Various converter stations can also be combined together to form a meshed DC system, also referred as HVDC grid. This system is more reliable, since parallel paths for power flows are available in case of equipment outages [41]. Fig. 8 displays possible topologies for a DC grid:

A HVDC Grid is defined in Cigre as an HVDC system consisting of at least three converter stations and at least one mesh formed by transmission lines [37]. There are two kinds of HVDC grids, e.g. regional and interregional. A regional DC grid is a system with one protection zone for DC earth faults, without the need of HVDC breakers, while interregional HVDC grids require several protection zones for DC earth faults. Future HVDC Grids will be built by integrating smaller systems into larger grids. This requires standardization of HVDC grid design and principles [38]. The conversion of regional HVAC grids into continental HVDC Grids offers several advantages, such as loss reduction, increased power capacity and availability combined with the AC system, less visual impact and easier permitting. There are still several technical challenges, namely HVDC breakers, grid power flow control and master control (start/stop, re-dispatching). In a long-term view also high voltage DC/DC converters for connecting different regional systems have to be developed [39]. Challenges in the conversion process are DC fault clearing strategies and regulatory frameworks, based on HVDC Grid Codes [40].

(a) A simple multi-terminal system, i.e. a DC bus with many tappings. Since no meshes and thus no redundancy are available this configuration cannot be defined as a grid. This is a typical proposal for the connection of offshore wind farms. The second arrangement, (b) A grid of independent DC lines, where all the buses are AC buses. Here all DC lines are fully controllable however several converters would be needed, resulting in high expenses. The third option, (c) A grid which represent the most feasible and can be considered as a real grid as it has a meshed approach, e.g. various connections between the AC and DC system [42].

3.1. HVDC grid configurations There are mainly two possibilities for grid connections, i.e. multiterminal or point-to-point. Today almost all HVDC systems are built point-to-point (overhead lines, cables or back-to-back). In case an additional HVDC link is needed, by employing a point-to-point set up, two new converters would be required, resulting in a double point-topoint connection with four converters. By multi-terminal configurations, breakers are used to protect the system, without the need of a converter in each node. Thus the extension of a multi-terminal link would require only one additional breaker and one converter. Hence multi-terminal arrangements are flexible to extensions and are there-

In the following typical schemes of offshore connections are presented. Using a point-to-point scheme (see Fig. 9a), each offshore unit is connected separated to the AC grid onshore; for each connection two converters are needed (AC/DC off- shore and DC/AC onshore). Employing a radial topology (see Fig. 9b), only one DC/AC converter onshore is needed, resulting to be cheaper as the previous solution. In a meshed system, depicted in Fig. 9c, two converters for each offshore 432

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unit are required; however the stations are connected, improving the reliability of the system. 4. Towards HVDC grids In this section, first steps towards HVDC grids are discussed, describing the chronological development of the current European Grid. Details about HVDC projects are also provided. 4.1. Historical background Historically energy markets followed a national level approach, e.g. European countries were managing their electricity independently. In the 1920s Italy and France built the first cross-border electricity connection in Europe. In 1948 the Electricity Committee of the Organization for European Economic Cooperation (OEEC) was founded, with the aim of refurbishing the electrical grids after the Second World War. The UCPTE (Union for the Coordination of Production and Transmission of Electricity) was created in 1951 from eight Western European countries (see Fig. 10). Eastern countries were instead coordinated by the COMECON (Council for Mutual Economic Assistance). Eastern and Western Europe were thus not interconnected at that time. In 1999 the UCTPE changed its name to UCTE, as the main focus was moved to the transmission grid, and in 2001 it became a forum of different TSOs. Fig. 11 provides an overview of the TSOs organizations across Europe

Fig. 10. UCTPE in 1951 [44].

Fig. 9. Connection of offshore wind farms: configurations [43].

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Fig. 11. TSO organizations in Europe in the 1990s [44].

and improvement in electricity efficiency were established in 2007. These challenging objectives lead to the need of a stronger collaboration between European countries, resulting in the foundation of the ENTSO-E (European Network of Transmission System Operators) in 2008. Fig. 12 depicts the current members of the ENTSO-E.

in the 1990s. The UCTE together with the ATSOI (Association of the Transmission System Operators of Ireland), the NORDEL (Northern Europe) and the UKTSOA (United Kingdom Transmission System Operators Associations) were members of the ETSO (European Transmission System Operators), which developed mostly economic and legal procedures. The 2020 objectives of CO2 emission reduction

4.2. Current situation In 2009 the European Union (EU) and the G8 Heads of Government committed their countries to a reduction in greenhouse gas emissions up to 80% by 2050. According to the Renewable Energy Directive 2009/28/EC, each European Member State was demanded to establish by 2010 a National Renewable Energy Action Plan (NREAP), setting the technology mix in order to reach a 20% renewable energy participation by 2020 [46]. This led to an increased deployment of RES, mostly wind and solar energy, and consequently higher penetration rates into the grid. Integrating those variable resources, could in some cases become problematic and yield operational constraints, making curtailment necessary [47]. One of the main issues related to renewable energy curtailment is transmission congestion [48]. In Table 4 data of wind curtailment in different European countries are provided. In order to solve curtailment issues using the available AC grid, back up power and energy storage would be needed [50]. HVDC interconnections between EU countries would play a key role in this context: they would help mitigate transmission constraints, by supplying energy surplus to neighboring countries and thus reducing curtailment. In order to achieve a “nearly zero-carbon power supply” in the EU, an open market in electricity, driven by both upgraded and new transnational transmission networks is required. Hence action is needed today in order to fulfill the 2050 challenge [51]. The fast development of HVDC technology makes it suitable for the construction of future HVDC Grids, considered at this time as

Fig. 12. ENTSO-E member countries today [45].

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Table 4 Wind generation and curtailment data in European countries (2013) [47,49]. Country

Denmark

Germany

Ireland

Italy

Portugal

Spain

Sweden

Electricity generation (TW h) Wind generation (GW h) Wind Curtailment (GW h)

35 11100 0

635 53400 127

26 5872 196

290 14811 152

52 11900 0

284 54338 1166

153 9900 0

Fig. 13. European commission: projects of common interest (PCI) [52].

connecting Norway and Denmark. Pioneers of HVDC in Europe are ABB and Siemens, which have developed comparable technologies. The NORD.LINK between Germany and Norway will be Europe’s longest HVDC interconnection (623 km) [54], representing a unique example of efficient use of renewables: it allows to transfer solar and wind energy surplus to Norway and to stabilize the German grid by importing clean and flexible hydroelectric power. The link will hence increase energy security in both countries. Norway would indeed need solar and wind power during dry seasons, whereas Germany could import hydro power in cases of demand peaks combined with low wind and solar power rates. This project is the demonstration that a sustainable green energy policy is likely to be achieved in Europe in the near future [54]. Table 5 provides an overview of the HVDC based projects. The IDnumber serves as a legend for the single projects depicted in Fig. 14.

technically possible, economically advantageous and benefiting from a standard approach across the industry. Besides HVDC has the highest expected public acceptance. The European Commission is making big efforts for improving the liberalization of the electrical markets and to ensure the EU energy union. Indeed, on 29 October 2014 the Member States have voted for the allocation of 647 million Euros to key energy infrastructure projects, defined as Projects of Common Interest (PCI). This agreement will play an important role in the revolution of the electric power system. The approved projects are displayed in Fig. 13. The design of an overlay grid is a new technological approach: it will change the electrical power system in the way it is operated and planned today. Therefore the network topologies must be well conceived and analyzed in depth [53]. 4.3. Projects in Europe: an overview The first HVDC projects were based on LCC technology. VSC was first employed in 1997 in the Hellsjön link in Sweden. VSC-HVDC projects had initially low power ratings, due to technology limitations. With its further development, VSC could be employed also for bigger projects and is today the best alternative for the connection of offshore wind farms. Power ratings of 2000 MW have been achieved so far in Europe, for instance in the INELFE link between France and Spain. Concerning DC voltages, levels of 500 kV are currently possible, as in the Skagerrak 4,

4.4. Offshore HVDC links in the North Sea Germany is one of the main drivers of innovation in the energy sector, thanks to the so-called “Energiewende”, an energy transition policy framework, which started in 1990s and became relevant after the Fukushima nuclear accident. The program consists in phasing out nuclear and coal plants, reducing CO2 emissions and increasing renewables share and energy efficiency by 2050 [56]. Germany is shutting down its nuclear plants, which have been already successfully 435

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Table 5 European HVDC projects (t.b.d. stands for “to be defined”) [18,55]. ID

Project

Location

Year commissioned

Supplier

Power rating [MW]

DC Voltage [kV]

Converter type

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Gotland Cross Channel BP Fenno-Skan Skagerrak 3 Sacoi Baltic Cable Kontek Hellsjön Gotland HVDC Light Swepol Link Tjaereborg Grita Moyle Interconnector Troll A Konti-Skan 1 Estlink NorNed Storebaelt Valhall Sapei BritNed Romulo East-West-Interconnector INELFE Skagerrak 4 Estlink 2 NordBalt NorGer Nemo Alegro NORD.LINK

Sweden France-UK Finland-Sweden Norway-Denmark Italy-Corsica-Sardinia Sweden-Germany Denmark-Germany Sweden Sweden Sweden-Poland Denmark Greece-Italy Ireland-Scotland Norway Denmark-Sweden Estonia-Finland Norway-Netherlands Denmark Norway Italy-Sardinia UK-Netherlands Spain-Mallorca Ireland-UK France-Spain Norway-Denmark Estonia-Finland Sweden-Lithuania Norway-Germany UK-Belgium Belgium-Germany Norway-Germany

1954 1985 1989 1993 1993 1994 1995 1997 1999 2000 2000 2001 2001 2004 2005 2006 2008 2010 2011 2011 2011 2011 2012 2015 2014 2014 2015 2015 20171 20191 20201

ABB CGEE Alsthom/GEC ABB/Alcatel ABB ANSADO ABB ABB/NKT ABB ABB ABB ABB Pirelli/ABB Siemens ABB AREVA ABB ABB Siemens ABB ABB Siemens Siemens ABB Siemens ABB Siemens ABB t.b.d. t.b.b. t.b.d. ABB

20 2000 500 500 300 600 600 3 50 600 7 500 2x250 2x40 250 350 700 600 78 1000 1000 2x200 500 2000 700 670 700 1400 1000 500–1000 1400

± 100 ± 270 400 ± 350 ± 200 450 400 ± 10 ± 60 ± 450 9 400 2x250 ± 60 ± 250 ± 150 ± 450 400 150 ± 500 ± 450 ± 250 ± 200 ± 320 500 ± 450 ± 300 ± 450–500 320–500 t.b.d. ± 525 kV

LCC LCC LCC LCC LCC LCC LCC VSC VSC LCC VSC LCC LCC VSC LCC VSC LCC LCC VSC LCC LCC LCC VSC VSC VSC LCC VSC LCC VSC VSC VSC

a

Scheduled.

topology. HVDC Light technology has evolved through three generations. It has now reached its fourth evolutionary stage, based on CTL (Cascaded Two Level) topology. ABB Offshore projects DolWin1 and DolWin2 are based on this converter configuration [58]. Fig. 15 shows a single-line diagram of DolWin 1 offshore station: both the converter platform and the collector platform have gas-insulated switchgear [59]. Wind power generated offshore is collected in an AC station and transmitted to an AC/DC converter; DC power can then be fed in the DC cable system and transferred onshore. In the following an overview of offshore HVDC links in the North Sea is provided. Table 6 summarizes the main characteristics of offshore HVDC links in the North Sea, depicted in Fig. 16. 4.4.1. Economic aspects of HVDC Projects Data of an economic analyses show that VSC converters are more expensive than LCC for comparable power ratings and voltages. As regards cables, extruded cables are in general cheaper than mass impregnated ones. Cable installation costs are extremely variable, thus a general economic analysis can be only indicative. In [18] costs related to HVDC projects are examined. Based on these data, in the following an overview of costing information is given, including converters (LCC and VSC), cables (XLPE and MI) and submarine cable installation.

Fig. 14. European HVDC projects: an overview.

replaced by wind and solar energy, thanks to the Renewable Energy Act (EEG) [57]. However, due to the variable nature of those resources, in order to ensure supply security, coal plants still need to be employed. As a result of this strategy, different offshore projects have been planned in the North-Sea. In order to collect power generated offshore, several offshore multi-terminal HVDC links are employed. The two main providers for offshore connections are Siemens and ABB, which have founded two comparable technologies, namely HVDCPLUS and HVDC Light, respectively. The projects Borwin2, Helwin1, Helwin2 and SylWin1, built by Siemens, employ the multilevel converter

A. HVDC Converters Tables 7 and 8 provide a summary of LCC and VSC costs. By VSC, AC switchyard costs are considered, whereas platform costs are not taken into account. Big ratings for VSC systems have been achieved recently. Therefore they are here just indicative. Costs are given in millions of Euro. B. Cable Systems The costs of XLPE and MI cables are given in Tables 9 and 10, respectively. All costs are calculated in Euros per meter of cable

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Fig. 15. DolWin1 offshore platform in the North Sea [59] . Table 6 German VSC-HVDC projects [60].

Table 7 HVDC projects: LCC costs.

Project

Year

Supplier

Power rating [MW]

Converter Topology

NordE.ON BorWin 1 BorWin 2 HelWin 1 DolWin 1 SylWin 1 HelWin 2 DolWin 2

2009 2012 2013 2013 2013 2014 2015 2015

ABB ABB Siemens Siemens ABB Siemens Siemens ABB

400 400 800 576 800 864 800 900

Two-level Two-level Multilevel Multilevel Cascaded-two-level Multilevel Multilevel Cascaded-two-level

Specifications

Unit Cost [Mio. €]

1000 MW, 400 kV 2000 MW, 500 kV 3000 MW, 600 kV

81–104 150–184 196–230

Table 8 HVDC projects: VSC costs.

supplied. The cost per route-km depends on the number of poles and cables per pole. Prices can change significantly, depending on market supply and demand [18]. C. ubsea Cable Installation

Specifications

Unit Cost [Mio. €]

500 MW, 300 kV 850 MW, 320 kV 1250 MW, 500 kV 2000 MW, 500 kV

75–92 98–105 121–150 144–196

Table 9 HVDC projects: HVDC XLPE subsea cable costs.

Cable installation prices range from 230 to 977.5 € per meter. Different parameters influence the cost variances, i.e. the combination of vessels, the location of cable installation, the cables configuration, the rating of cables and their size. The ground condition, chosen depth and cable type will govern the installation method. Cables can be bundled and laid in single trench or placed in adjacent trenches. Due to the high variability of cable installation costs, a detailed analysis has to be carried out in the single project [18]. Table 11 provides an overview of the installation costs, depending on the installation.

Cross sectional area [mm2]

Price [€/m] 150 kV

320 kV

1200 1500 1800 2000

230–460 288–460 345–518 345–575

345–518 345–518 345–575 403–660

Fig. 16. Offshore multi-terminal HVDC links in the North Sea area [61] .

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2. Competition and market: reducing price spikes; 3. Integration of renewable energies: reducing CO2 emissions.

Table 10 HVDC projects: HVDC MI subsea cable costs. Cross sectional area [mm2]

1200 1500 1800 2000

Price [€/m] 150 kV

320 kV

403–660 460–660 460–690 575–805

460–660 460–690 460–748 575–863

The first step of the study consists in the analysis of the offshore wind farms connection to shore. The most beneficial solution is thereby represented by hub connections, i.e. by connecting up wind farms that are close to one another, forming only one transmission line to shore. The second step is then the development of two different scenarios for the offshore grid, e.g. Direct Design and Split Design (see Fig. 19). In the Direct Design interconnections are built to promote unconstrained trade between electricity markets, since prices are very different. In the Split Design the offshore grid is developed around the planned or existing offshore wind farms. Here interconnections are built by splitting the connection of large offshore wind farms between countries. Both designs are highly beneficial from a socio-economic perspective [62].

Table 11 HVDC projects: subsea cable installation costs. Installation type

Total cost [1000 €/km]

Single cable, single trench Twin cable, single trench 2 single cables, 2 trenches 10 m apart

345–805 575–1035 690–1380

5.1.2. EWEA: twenty-years offshore network development master plan According to the European Wind Energy Association (EWEA), Europe’s offshore wind potential is able to power Europe seven times more than necessary. EWEA has a target of 40 GW offshore wind in Europe by 2020. The construction of an offshore grid will establish Europe as world leader in offshore wind power technology. Furthermore reduced import dependence, lower CO2 emissions and affordable electricity will be achieved [63]. The offshore wind energy market in 2030 will be characterized by the following goals [63]

5. Models for a European supergrid Several HVDC grids have been proposed in Europe, in order to increase the amount of renewable energy that can be accommodated in the overall energy system. Fig. 17 provides an overview of the models for offshore grids, integration of CSP modules and PV plants and finally for a European Supergrid, which are described in details in the following. 5.1. Offshore supergrid

1. 2. 3. 4. 5.

Offshore Supergrids have not been planned yet, however results of different investigations offer an optimistic view for their implementation in the near future. Below the research models of OffshoreGrid and EWEA are described.

Total installed capacity of 150000 MW; Electricity production of 563 TW h; Meeting 12.8–16.7% of total EU electricity demand; Avoiding 292 Mt of CO2 annually; Annual investments in wind turbines equal to 16.5 billion Euros.

The proposed offshore grid is displayed in Fig. 20. It is based on consideration made by Transmission System Operators (TSO) in the North and Baltic Sea and on the existing or planned offshore cables. The implementation of the EWEA offshore grid takes place around the UK and Ireland, the English Channel, the North Sea and the Baltic Sea. EWEA suggests using the existing HVDC links to improve the connections between Northern European countries. For instance the links NorGer and Nord. Link between Norway and Germany could be also connected to Denmark and the UK, respectively. The construction of a link between the UK and Norway is under consideration by TSOs. EWEA proposes to build a further link connecting Sweden and Germany. Other suggestions are a link between Ireland, Northern Ireland and Wales, one between Belgium, UK and the Netherlands as well as an upgrade between Denmark and Sweden [63].

5.1.1. Offshore grid Offshore wind capacity in Europe is expected to reach 150 GW by 2030. Most of the wind farms are located close to the European coast. Fig. 18 depicts the offshore scenario in Northern Europe in mid (2011– 2020) and long terms (2021–2030). In the mid term development, the main actors will be the UK and Germany, followed by the Netherlands, Sweden, France, Denmark and Belgium. In the long-term scenario UK and Germany will have a further increase in the offshore installation, as well as the Netherlands, Sweden and Norway. Offshore installation will also take place in the Baltic Sea after 2020. An offshore grid connecting different countries would enable offshore wind power to be transported onshore and at the same time facilitate competition and electricity trade between the Member States [62]. NSCOGI summarizes the advantages of an offshore grid as follows [62]

5.2. Integration of CSP and PV solar 1. Security of supply: improving exchange between big load centers and transmit offshore power to shore;

In this chapter projects associated to integration of CSP (MEDGRID and DESERTEC), and to large scale PV plants are described. The academic debate related to EU energy security connected to those scenarios is also presented. 5.2.1. Integration of CSP In the TRANS-CSP study performed by the German Aerospace Center (DLR) [64] the interconnection between the Middle East-North Africa (MENA) and the Mediterranean Region for the integration of CSP power was analyzed for the first time. Results of the analysis remark how CSP imports could improve power system stability and sustainability of the European grid. CSP would indeed balance fluctuations of PV and wind power in Europe, reducing the need for fossil fuels fired plants. Although the interconnection is technically

Fig. 17. Available models for HVDC grids.

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Fig. 18. Installed capacities of offshore wind farms in Northern Europe by 2030 [62] .

Fig. 19. Offshore grid scenario: split design [62].

scale solar power plants and transmission lines of high capacity will enable solar energy to be competitive and will lead to a socio-economic improvement in the entire Mediterranean region. Article 9 of the EU directive Energy-Climate (2009/28/CE) allows the import of green energy from outside the EU to reach the objective of renewable share, making solar energy imports from the Sahara possible [66]. In 2008 the Union for Mediterranean was funded in Paris, with the aim of developing RES in the Mediterranean region, installing 20 GW of additional capacity through the Mediterranean Solar Plan [67].

feasible, there are no political frameworks available yet, representing a big challenge in the implementation of such a large scale project [65]. Today the main associations dealing with CSP integration are MEDGRID and DESERTEC. A. MEDGRID In the Mediterranean South and East countries, electricity infrastructure must respond to economic and consumption growth, expected to be equal to 6% per year until 2025. The development of large439

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Fig. 20. EWEA: offshore grid [63].

Member countries (see Fig. 21) have big renewable energy potentials, mostly wind, PV and hydro power, that have not been fully exploited yet [68]. One of the main projects of the Mediterranean Solar Plan is the MEDGRID (see Fig. 22), launched by the French government [70]. This industrial initiative analyses the technical and economic feasibility of the connection of Italy, Spain and the Balkans with North Africa. The main concerns of the project are [71]: 1. 2. 3. 4.

Climate protection Cost reduction Reliability Security of supply

Nevertheless there are still huge financial, political and social differences between European and North African countries, making the development of a project of this kind quite complex. In addition big injections of RES from North Africa could lead to bottlenecks and compromise energy security in the European grid. Further investigation is required in order to solve these issues [72]. A. DESERTEC It was founded in 2009 on the basis of the above mentioned TRANS-CSP concept, aiming to provide North Africa, the Middle East and Europe with a sustainable supply of renewable energies by 2050. The project consists in building 100 GW of CSP in North Africa to export it to Europe and satisfy 15% of EU demand [74]. The approach is based on the use of solar energy from deserts, considering that more than 90% of the world’s population is leaving in areas located less than 3000 km away from those. Besides solar energy is abundant and constantly available. This energy source can be converted into electricity and transmitted to centers of demand. Fig. 23 shows the development of a Supergrid connecting Europe with North Africa and the Middle East. The implementation of the DESERTEC Concept and thus the substitution of fossil fuels with clean energy from deserts and arid regions could reduce 80% of global CO2 emissions. In deserts not only solar energy is abundant, but also wind blows strongly and can be included in the concept, as well as other RES.

Fig. 21. Union for Mediterranean: member countries [69].

Fig. 22. MEDGRID: potential transmission projects in the Mediterranean basin [73].

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Fig. 23. The DESERTEC concept [76].

larly by large-scale projects. The main issues are related to grid operation and stability, due to PV intermittency, lack of inertia and reactive power compensation need [79,81]. However integrating PV in the future HVDC grid (instead of using the current AC grid) would imply that no DC/AC converters would be needed in the feed-in process, being financially and technically more advantageous than integrating CSP. As PV is growing fast in the past year, new grid scenarios including large scale PV plants are expected to be contemplated in the next years.

Although CSP isn’t the cheapest technology yet, it has technical advantages over PV, as the ability of supplying energy also during the night, by storing the heated fluid [75]. The project has no technical barrier in relation to CSP and HVDC technologies. Nevertheless, as mentioned above, a project of those dimensions could be challenging from a political point of view [77]. The main concerns in this context are related to energy supply security and dependency on solar energy imports. It might be relevant to consider whether renewable electricity trade could lead to conflicts between European and North-African countries [74]. It has to be considered that North African states are nor politically stable neither united: indeed countries like Algeria, Libya and Egypt are willing to increase oil and gas exports and maximize profits, while on the other hand Morocco and Tunisia want to reduce fossil fuel dependence [78]. Key points of the academic debate are related to exploitation of lands outside European borders to produce electricity and creating a parallel with the oil dependency from the Middle East [75]. Authors claim that DESERTEC will be more insecure than today’s system, as nowadays it does not import significant amount of electricity. In addition, it is hard to evaluate the risk in a time frame of 40 years from now. In order to reduce the political risks, Europe should keep good relations with the MENA countries and defend from possible extortions by maintaining capacity reserves [74].

5.3. European supergrid According to the proposed approach, the development of a European supergrid would be the last of the five steps. This is predicted to happen before 2050. In the following the main studies focused on meshed supergrids in Europe are described. 5.3.1. ENTSO-E: ten-years network development plan (TYNDP) Key points of TYNDP are transparency, TSO cooperation platform, and stakeholder involvement; inform EU policy and investment decision. The TYNDP 2012–2020 includes the construction of 2100 km DC overhead lines, 1490 km inland cables and 9000 km submarine cables. In addition an upgrade of 28400 km overhead AC lines will be performed [82]. Through the Regulation (EU) 347/2013, in force since April 2013, ENTSO-E TYNDP is defined as the sole instrument for the selection of Projects of Common Interest (PCI). The TYNDP 2014 combines power transmission issues with environmental and resilience concerns, by analyzing four possible visions for 2030, i.e. "Slow Progress", "Money Rules", "Green Transition" and "Green Revolution" (see Fig. 24). All of them assume a significant development of RES generation, highest by Vision 4 and a RES supply between 40% and 60%. Furthermore a strong reduction of CO2 is expected, with values in the range 36–78% compared to 1999, depending on the vision. Vision 3 and 4 are the most favorable for the achievement of the energy goals, stated for 2050. The TYNDP 2014 also pointed about 100 spots of the European grid where reinforcements are needed, in order to avoid bottlenecks. The

5.2.2. Integration of large scale PV PV technology is today technically more advanced and financially more advantageous than CSP [79]. In 2015 50 GW of additional PV solar capacity have been installed worldwide, as a result of an increased competitiveness of PV. Top markets for PV expansion are China, Japan and the USA. In Europe leaders are the United Kingdom, Germany, France and Italy [80]. Developed countries are still leading the PV market, however emerging economies are starting to be part of it: in Jordan and the United Arab Emirates new projects were launched, as a result of low-bids in PV solar tenders in 2015. As regards North Africa, new projects are planned to be developed in Algeria, Egypt and Morocco [80]. PV integration at transmission levels is still challenging, particu441

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Fig. 24. Four visions of the TYNDP [44].

5.3.2. Financial issues The investment costs for the projects of pan-European significance reach 150 billion €, representing a financial challenge [83]. Most of the projects cost more than 300 million Euros, while 22% of them cost more than 1 billion Euros, e.g. the subsea interconnections, 10 times more expensive than overhead lines. Fig. 27 offers a breakdown of the investment costs in each country. Costs of tie lines are split between the two concerned countries. In order to realize the TYNDP by 2030, a significant financial engagement for TSOs is needed. However, the implementation of the project would lead to mitigation in the electricity prices on average 2–5 €/MW h, depending on the vision. In the worst-case scenario, the payback can be achieved after 20 years [83].

most critical areas are the four electrical peninsulas, e.g. the Iberian Peninsula, Italy, Great Britain/Ireland and the Baltic States. In order to increase the development of wind and solar generation, the interconnection capacity needs to be incremented. Another important issue is the integration of wind power generated in the offshore farms in the North Sea into the respective coastal nations [83]. The main features of the generation mix in 2030 are [83]: 1. New generation capacities are mostly RES, especially wind and solar; 2. RES generation mostly in Germany, countries with favorable wind conditions (Iberian and Italian peninsulas) and countries neighboring the North Sea; 3. New hydro-power capacities, expected in the Alps, the Iberian peninsula and Norway; 4. Depending on the vision, 30–45 GW of nuclear capacity is expected to be shut down; 5. Average distance between generation and load centers is tending to increase.

5.3.3. Greenpeace: PowE[R] 2030 Europe is currently debating new targets for renewable energies for 2030. Greenpeace demands a target of at least 45% RES by 2030, in order to fulfill the climate purpose of staying below 2 ºC temperature rise. The PowE[R] 2030 vision comes from a collaboration between Energynautics and Greenpeace. Three cases have been calculated, i.e. the Energy [R]evolution Case, the Reference Case and the Conflict Case. The first case supposes a percentage of renewable electricity equal to 70% in 2030 and over 95% in 2050. The Reference Case assumes a continuing progress in electricity and gas market reforms, the liberalization of cross border energy trade and environmental policies. The Conflict Case focuses on the bottleneck of the French inflexible electricity system and its conflict with Germany. The growing system conflict between Germany and its eastern neighbors Poland and Czech Republic, due to their coal and nuclear policy, is also taken into account. The Energy [R]evolution concept could be achieved through an optimization process of grid extension, grid management, storage of energy, demand side management and allocation of power generation technologies in specific regions. The other two scenarios show the impact of unchanged grid policies [84]. Greenpeace’s scenarios are used as examples for CSP technology assessments, as for instance in [85,86]. The initial network topology proposed from Energynautics uses grid nodes representing all major load and generation sites in the European power grid area covered by ENTSO-E. Greenpeace, following the three mentioned scenarios, has optimized the basis model. The reference case is a "business as usual" vision, where coal and nuclear have priority. The result is a very little line expansion (23 GVA by 2030),

Fig. 25 displays the project of pan-European relevance in a long term (2019–2030) vision. By 2018 DC interconnections (violet) between Ireland/Great Britain, Great Britain/Belgium, Spain/France/ Great Britain, Italy/Montenegro, Italy/Switzerland, Sweden/Lithuania, Denmark/Germany will be in operation. Besides the offshore wind farms in the North Sea will be connected to Germany and Norway. Many substations (blue) are planned to be built or upgraded, as well as AC connections (red). In a long-term view Iceland is planned to be included in the European Grid employing a DC link, connecting it to Great Britain. The issue of electrical isolation of Great Britain is going to be solved, by connecting it to Norway, Denmark and France. The Greek islands and Cyprus will be linked to the mainland Greece through DC. A link between Italy and France is also planned to be built and exchange between North and South Germany will be increased. In total 48000 km lines will be built or upgraded. Finally Fig. 26 illustrates the TYNDP 2014 portfolio, divided by used technology. Most new overhead lines will be built using AC technology, still the easiest to implement inland applications. However about 20000 km new HVDC lines are planned, thereby more than 75% will be built using cables. Submarine HVDC cables in the North Sea build an offshore grid, using point-to-point topologies. Offshore meshing is still not included in the current vision by 2030. 442

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Fig. 25. Projects of pan-European relevance: long term (2019–2030) [83] .

necessary between Southern Italy/Greece, Central/Southern Italy, Scotland/London, Great Britain/Ireland, Ireland/France and Copenhagen/Southern Denmark. In the conflict scenario flexible wind and solar capacities would be added across Europe except in France, Czech Republic and Poland,

poor load factors of gas generators (17%), a low coverage by renewables (37% of load) and a high curtailment across Europe (6.2%), due to inflexibility of coal and nuclear. Thus the system is inflexible and does not allow any structural change. Implementing the reference scenario, new HVDC links would be 443

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the three cases of Greenpeace. The TYNDP requires a network expansion of 50100 km, achieving a RES share equal to 37%. The Energy [R]evolution (E[R]) scenario can double the value of RES sharing, by only half the transmission line expansion compared to TYNDP. In the reference case (REF) the line expansion is equal to 3663 km, however the RES share would be 37%. This scenario is still more advantageous than the TYNDP, since the RES level is the same and the line expansion is minimal. Finally the conflict scenario would lead to a network expansion of 18781 km and a RES share of 59%. Hence the E[R] vision turns out to be the most financially sound option to facilitate the RES integration into the grid. 5.3.4. ABB: continental overlay HVDC grid In [87] an overlay HVDC grid model has been developed, by studying the power flows. The DC grid shown in Fig. 30 has been employed to simulate DC power flows. It consists of a 40-terminal meshed DC system with a 30 GW in-feed of renewable power. The main generation sources are assumed to be solar energy from the Sahara desert in the south (19200 MW), hydropower in Northern Europe (2200 MW) and wind power from Western Europe (7800 MW). The terminals are VSC, all in bipolar configuration with metallic return cables. Munich is used as the DC voltage-controlling terminal, while all the others are in power control mode. The results of the study have shown that a very large DC grid is feasible considering the load flow. However using only one DC station for controlling the DC voltage in a large DC grid has a major impact to the AC system connected to that station. Thus a new power flow control strategy is required [87].

Fig. 26. Ten-years network development plan (TYNDP): investment portfolio [83] .

5.3.5. Friends of the Supergrid (FOSG) Friends of the Supergrid (FOSG) is a group of international companies promoting the concept of open markets in electricity transmission and trans-nation interconnection, known as Supergrid. This is defined by FOSG as a transmission system based on DC, with the function of facilitating the transmission of renewable power generated in remote areas to consumption centers. The Supergrid is an important tool to fulfill the target for 2050 of zero-carbon power supply. The first step is represented by the development of an offshore Supergrid in the North Sea, which is meant to be expanded afterwards to cover the entire European Union. The North Seas Countries Offshore Grid Initiative (NSCOGI) supports this project. FOSG has developed its Phase 1 proposal, which consists in the connection of offshore wind generators to existing grids. The grid is supposed to collect energy from wind generation clusters off the east coast of the UK, at Super Nodes, which are connected together and interconnected with the German and Belgian North Sea clusters and Norwegian hydro-power. The grid then delivers this power to the existing grids. By 2050 the Supergrid should cover the entire EU and integrate solar power generated in North Africa and the Middle East (see Fig. 31) [51].

Fig. 27. TYNDP: investment cost breakdown in billion Euros [83].

which would continue a "business as usual", by keeping and extending less flexible coal and nuclear power plant. The network expansion would be necessary to reduce curtailment of renewables. Conflicts might take an international dimension if different countries will pursue different policies. With the conflict scenario, Germany could still proceed with future expansion towards a renewable energy supply, however more investments in grid expansion and storage capacity would be required. Contrary to the reference and conflict cases, the Energy [R] evolution scenario has a high level of RES capacity in all European countries. Curtailment of wind and solar plants are kept to a minimum. In the E[R] scenario (see Fig. 28) important corridors were identified for the HVDC overlay network, e.g. Scotland/Southern-England, Spain/France, Southern-Italy/Northern-Italy, French-coast/Paris, Northern-Germany/Southern-Germany, France/Germany and Italy/ Germany. By these means, even countries like France and Poland will achieve shares of over 50% renewables by 2030 [84]. The power 2030 concept of Greenpeace is optimized for the highest share of renewables and a phase-out of coal and nuclear across Europe. Contrary, the TYNDP is inadequate to integrate high levels of renewable energies, since it is based on conservative RES targets. This could lead to an overcapacity of power generation. Fig. 29 displays a comparison of the TYNDP from ENTSO-E with

6. Prospects of HVDC grids In the previous section different grid drafts have been examined. Since models are quite controversial in many points, here an evaluation is carried out. In this way probable grid configurations of the future grid can be predicted. In the following different grid architectures are compared, by overlaying similar concepts in a European map. By these means similarity and differences between the various models are highlighted. Based on those comparisons, suggestions for the installation of HVDC grids in Germany, in the North Sea and across Europe are given. 6.1. Possible development of the future HVDC grid Several architectures have been proposed as results from simula444

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Fig. 28. Greenpeace [R]evolution case: extension map 2030 [84] .

links cannot be considered as HVDC grids, since they do not offer alternative paths to power flows in case a fault occurs on a cable. Nevertheless they will represent preliminary steps for a Supergrid. According to the European Wind Energy Association (EWEA) scenarios, 40 GW offshore wind farms will be installed by 2020, and 150 GW by 2030. The main markets are thereby the UK, Denmark, the Netherlands, Sweden, Norway, Belgium and Germany. Hence offshore wind farms will be mostly concentrated in the North and Baltic Seas. At the present, each wind cluster is connected to shore by a single link. However it would be more beneficial to collect offshore wind power in an offshore grid [88]. Furthermore it has been proved that the deserts have enough solar energy potential to cover the needs of the whole world. DESERTEC and MEDGRID suggest the installation of largescale Concentrated Solar Power (CSP) in the Sahara region and the transmission of generated power directly to European load centers [88]. An intermediate scenario between point-to-point and meshed multi-terminal grids could be the connection of wind farms or CSP units to different countries, the first being defined as offshore supergrid [88]. Recently, multi-terminal configurations are developed for the connection of offshore clusters in the North Sea, representing a promising advance towards offshore grids. On the basis of these considerations, we propose a possible implementation of the future European HVDC grid in five steps, displayed in Fig. 32.

Fig. 29. Network expansion in km: comparison of different scenarios [84].

tions, mathematical optimizations, technical and economical feasibility studies as well as environmental assessments. The most realistic option is that the Supergrid will be built in an organic way, taking into consideration the already existing HVDC paths, like interconnectors, offshore links to wind farms and oil and gas platforms. Point-to-point

6.2. Evaluation of HVDC grid models In this chapter the models presented above are compared in details, 445

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Fig. 30. ABB: 40-nodes overlay DC grid [87].

Fig. 31. Friends of the supergrid: scenario 2050 [51].

Fig. 33. Evaluation of different proposals for an Offshore Grid in the North Sea. The map of offshore wind farms sites from ECOFYS [89] has been used as background in order to localize the offshore clusters. The corresponding legend is on the top-left side. The legend on the bottom-right side refers instead to the grid models included in the map.

Fig. 32. Proposed implementation of the future HVDC grid.

the main challenges of HVDC grids are discussed and overall suggestion for the successful implementation of the future Supergrid are given.

integrate offshore wind energy and increase the exchange rates, the Spilt Design is here the preferred option. This model is compared with other offshore grid architectures, building the following scenario (see Fig. 33):

6.2.1. HVDC offshore grid In the previous section two different models proposed from OffshoreGrid have been described, e.g. the Direct Design, where interconnections between countries are made directly, and the Spilt Design, where the same interconnections are investigated considering the existing wind farms. As the aims of an offshore grid are mainly to

1. OffshoreGrid: Spilt Design 2. EWEA: 2030 timeframe 446

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3. Offshore German Grid Development Plan (O-GDP): North Sea 4. ENTSO-E Ten-Years Network Development Plan (TYNDP): long term 5. Friends of the Supergrid (FOSG): Phase 1 In order to have an overview of the existing and planned offshore clusters, the map of Offshore Wind Farm Sites from ECOFYS has been used as basis [89]. The focus of attention is on the North Sea. Let us analyze each model in details. The existing offshore links are represented in black. The OffshoreGrid Split Design proposes the development of the grid in three steps: first direct connections are built, like the ones between Ireland/France and England/France; then hub-to-hub links, as for instance the one connecting the UK to the Netherlands are constructed; finally, in the third step, meshed interconnections are implemented, e.g. the one having the central node in the planned wind clusters in the North Sea, collecting wind power from other wind farms and supplying it to Norway, the UK, Germany and Belgium. The EWEA model is based on a similar approach, however having several collection nodes across the North Sea. Wind power is supplied to the UK, Belgium and the Netherlands. Further interconnections are still under study. The offshore German Grid development plan (O-GDP) supposes the connection of new wind farms in the Germans portion of the North Sea to shore. Those links, using multi-terminal configurations, represent important advances in the implementation of offshore grids. In the TYNDP from ENTSO-E single interconnectors are proposed in order to improve the exchange levels of countries in the North Seas region. Besides the German’s offshore wind farms will be connected to shore using HVDC links. Finally, in the approach proposed by FOSG, the development of the HVDC Grid is expected to take place first in the North Sea (first phase), then to be extended to neighboring countries (second phase) and finally to evolve into an overlay grid across Europe (Scenario 2050). In this scenario only the first phase is displayed. The central node is placed on the North Seas portion belonging to the UK, delivering offshore wind power to Scotland, England, Belgium, Norway and Germany. In the second phase also Denmark and the Netherlands will be involved. Observing the map, shown in Fig. 33, it is evident that the TYNDP is the least beneficial option when considering an offshore grid, since more cables would be required and the concept of meshed grids is not applied. Furthermore, in order to install an offshore grid, more links would be needed, by enlarging for instance the planned connections. This would be in conclusion the most expensive option and is here not the preferred one. The EWEA vision offers more advantages, however several offshore nodes are required and the grid results less compact. The OffshoreGrid alternative, being based on a three-steps development would require a big time frame for the installation of meshed grids and is therefore, in terms of timing, not optimal. Nevertheless the choice of offshore nodes seems to be more favorable than the one proposed by EWEA. FOSG’s Vision is considered to offer the best perspective. It is in fact not only beneficial for the implementation of an offshore grid, but rather for the overall European Supergrid. During the first phase, which sees the construction of an offshore grid, power can be optimally delivered to nations adjacent to the North Sea. The offshore supergrid can be then employed as a basis for the further development of the HVDC grid trough Middle Europe and in a longer view also in South Europe, Africa and the Middle East, rendering the European energy union a reality. After examining all the scenarios, it is evident that action is needed in the North Sea, in order to improve the integration of offshore wind farms. Fig. 34 shows the links, which are considered necessary in various offshore models. TYNDP, FOSG and OffshoreGrid suggested the connection of Norway to offshore farms, not only in the Norwegian

Fig. 34. Needed action in the North Sea as a result of various scenarios. The legend on the top- left is referred to the background map from ECOFYS [89], which shows the location of offshore wind clusters .

portion of the North Sea, but also in the British one. All wind farms belonging to the UK should be connected to land, as proposed by OffshoreGrid, EWEA and FOSG. In all models it is clear that the planned wind clusters in the German’s part of the North Sea should be connected to shore. Same is valid also for the Netherlands. In conclusions new offshore links need to be built in the North Sea region in short terms, as the development of offshore wind is growing faster and faster. Some of the recently planned wind clusters haven’t been taken considered in past studies, like the ones between Ireland and the UK or in the English Channel. HVDC links in those regions need to be evaluated as well. 6.2.2. HVDC supergrid in Europe Two evaluations are carried out, building the following scenarios 1. Scenario 1 (Fig. 35a): considering single DC upgrades 1. ENTSO-E TYNDP: long term 2. Greenpeace: [R]evolution Case 3. MEDGRID: projects in the Mediterranean basin 1. Scenario 2 (Fig. 35b): based on the following meshed grid proposals 1. Friends of the Supergrid (FOSG): Scenario 2050 2. The DESERTEC Concept 3. ABB: 40 nodes overlay grid In order to provide an overview about existing HVDC links, these have been inserted in the scenarios as well. The first scenario concerns single upgrades of the European HVAC grid using HVDC links. Each model has been previously discussed. The TYNDP suggested building several links across Europe. It has been demonstrated that this concept is less beneficial in financial terms than the three visions proposed by Greenpeace (Reference, Conflict and Revolution Scenarios). It would in fact require a total network 447

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Fig. 35. Evaluation of different models for the development of the European HVDC Supergrid. (a): Scenario 1: single links, (b): Scenario 2: meshed grids.

considered as a good start for building the future HVDC grid, but it needs to be further extended. MEDGRID offers in the same way a good model for the construction of the Mediterranean Grid in order to integrate CSP from Africa. The DESERTEC Concept, part of the second scenario, provides an overall view, by suggesting nearly the same connections to Africa as MEDRGRID. It is also the only meshed approach considering the integration of Iceland power system in the European energy union. The design of the future Supergrid could be based on this model. The 40 nodes overlay grid of ABB presents a lack in the power flow control and needs to be re-designed. However, applying the meshed grid approach

expansion of 50100 km, with an investment equal to 150 billion Euros. The Greenpeace Reference Scenario is the cheapest option; however it is based on a RES share of 37%. The conflict scenario would see some countries still employing a "business as usual" energy policy (France and Check Republic based on nuclear power, Poland on coal). It is hence also not favorable. The Revolution Scenario would require half the line expansion compared to the TYNDP, with an optimal RES share, equal to 77%. Therefore this vision is taken here for the comparison. However Greenpeace focuses its attention mostly on Central Europe, without considering the integration neither of offshore wind power in the North nor of solar power in the South. It can be 448

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Fig. 36. German Grid Development Plan (GDP) [91].

is a favorable option. FOSG’s approach, based on the integration of power supplied from renewable energy sources, is comparable to DESERTEC. Indeed power generated in hydro power plants in Scandinavia and in the Alps, potential offshore wind power in the North Sea, the Baltic Sea and the Atlantic Ocean and finally CSP in Southern Italy, Africa and the Middle East could be optimally integrated in the onshore grid. Energy trade between all European countries would be also ensured.

Table 12 German Grid Development Plan (GDP) in four scenarios [90].

Installed wind power [GW] Wind power generation [TW h] New DC-corridors [km] DC power capacity [GW] New AC lines [km] DC/AC network amplification [km] Investments [Mrd. €]

6.2.3. HVDC supergrid in Germany Germany is one of the main actors of the energy revolution; hence an investigation of the German grid could provide an example for the development of future European grids in the single countries. As this research work is performed in Germany, in the following details regarding the German development plan both onshore and offshore are given.

A 2024

B 2024

C 2024

B 2034

61 140 2000 10 1300 5300

68 157 2100 12 1300 5200

103 237 3500 18 1300 5200

97 247 3600 20 1400 7000

21

22

26

unknown

6.2.5. Offshore GDP The O-GDP contains important guidelines for the coordination and construction of the offshore grid. The plan of the offshore grid is carried out similarly to the onshore GDP, however different parameters are considered. The connection of offshore wind farms in the North Sea is performed by TenneT, while in the Baltic Sea by 50Hertz. As in the onshore GDP, three scenarios are analyzed, e.g. scenario A, with a moderate development of RES, scenario B, with a middle development, close to reality, and finally scenario C, with an ambitious increase of renewable energies. The expected development for each scenario is provided in Table 13. These data are fundamental for the estimation of the needed transmission capacities. The Federal Maritime and Hydro-graphic Agency (Bundesamt für Seeschifffahrt und Hydrographie) identifies different clusters in the North and Baltic Sea for the construction of offshore wind farms. Scenario A supposes the smallest growth of offshore winds capacities. The O-GDP would consists in this vision in the construction of 735 km DC-links in the North Sea and 400 km AC-links in the Baltic Sea. The total transmission capacity is equal to 3.7 GW (2.7 GW in the North Sea and 1 GW in the Baltic Sea). The investments would amount 17

6.2.4. Onshore GDP The German onshore GDP 2014 consists of 4 scenarios (A 2024, B 2024, C 2024, B 2034). In each scenario, nuclear power stations are supposed to be shut down before the end of 2022 [90]. Fig. 36 displays the future development of the German transmission network. Three new HVDC corridors are planned, aiming to improve the transfer capacities between the North and the South. At first, four DC corridors have been studied (A–D), however corridor B resulted to be not necessary. The four scenarios employed for the evaluation of the German GDP are given in Table 12. Four parameters were used to evaluate the different 4 scenarios, e.g. reduction of CO2 emissions, and primary energy consumption, energy exchange and increase of RES generation. In all scenarios Germany is a net-exporter, due to its high power installation of RES, which influences the replacement of conventional energy sources also outside Germany (see Fig. 37). 449

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Fig. 37. GDP - energy exchange: scenario B 2034 [90].

milliard €, thereby 12 milliard € are required for the start-offshore-grid [92]. The estimated capacities and length of AC and DC links for the 4 scenarios are summarized in Table 14. Here two different evaluations are carried on. They are built as follows:

Table 13 Installed offshore wind capacities [GW] [92].

North Sea Baltic Sea Total

A 2024

B 2024

C 2024

B 2034

10.2 1.3 11.5

11 1.7 12.7

20.1 5.2 25.3

13.2 2.9 16.1

1. Scenario 1 (Fig. 38a): taking into account only the construction of single HVDC links across Germany, namely

Table 14 Offshore German Grid Development Plan (O-GDP) [92].

DC-Links North Sea [km] Ac-Links Baltic Sea [km] Transmission Capacities North Sea [GW] Transmission Capacities Baltic Sea [GW] Investments [Mrd. €]

B 2034

1. German Grid Development Plan (GDP) 2. ENTSO-E Ten-Years Network Development Plan (TYNDP): long term Greenpeace: [R]evolution Case

1525 1015 5.4

4265 2060 12.6

1. Scenario 2 (Fig. 38b): considering the proposed meshed grids in Europe, centering the focus on Germany, based on

1.5

2.5

5

19

23

unknown

A 2024

B 2024

735 400 2.7

1005 600 3.6

1 17

C 2024

1. German Grid Development Plan (GDP) 2. ABB: 40 nodes overlay grid 3. DESERTEC Concept 450

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Fig. 38. Evaluation of different models for the development of the Supergrid, focused on Germany. (a): Scenario 1: single links, (b): Scenario 2: meshed grids.

Germany an improvement of the transfer rates between the South and the North is needed, because most of the power is consumed in the South and produced in the North. Therefore this grid design is not optimal for Germany. In the DESERTEC Concept the situation would be similar as in the previous case, e.g. with two corridors crossing Germany from the West to the East and one HVDC cable supplying offshore wind power to shore. This alternative is advantageous when considering the installation of the overall European Supergrid, however its development would require long timing. A solution could be to redesign the concept by taking into consideration the German GDP corridors. In this way the German power system could be improved in short-terms and the three corridors could be used as backbones for the further construction of the grid. The FOSG model is also in this case the best option. Indeed three lines cross Germany southwards, collecting offshore power in the North and exchanging energy with the Netherlands, France, Austria and Check Republic. In this way good transfers are ensured not only in Germany, but also with adjacent nations.

4. Friends of the Supergrid (FOSG): Scenario 2050 The German government approved the construction of three new HVDC overhead lines linking North to South Germany, reinforcing the existing HVAC network. This are referred as Corridor A, Corridor C and Corridor D. As a result of the studies on the German Grid development plan (GDP), corridor B, being at first included in the plan, is considered unnecessary. Through the first two corridors, offshore wind power will be integrated in the onshore grid. Nevertheless the social acceptance of the new corridors is not high enough and it is still not certain if they will be actually built. The TYNDP proposes the construction of four corridors and two connections to the North Seas offshore wind farms. In the [R]evolution Case proposed by Greenpeace Corridor A and D are suggested to be placed where they are planned to be, according to the GDP. Instead of building Corridor C an HVDC link crossing Germany in the North- and South-West and sharing power with Belgium, Denmark and Switzerland is suggested to be built. This approach seems to be more advantageous in a global view, as not only the transmission in Germany can be improved, but also the power trade-off is promoted. Nevertheless the integration of offshore wind power would not be possible, which makes this alternative not suitable for the development of the European Supergrid as it is conceived here. The options studying meshed grids (Fig. 38b) are in this sense more feasible. The overlay grid analyzed by ABB has three of the forty nodes in Germany, e.g. in Rostock, Bremen and Munich, connected to each other. Trough the connection of Rostock and Munich the third corridor of the GDP could be used, whereas the other two are not taken into consideration. Power could be exchanged with neighboring countries using the meshed grid. For instance Munich would be connected to Vienna, Zurich and Lille (France); Bremen to Amsterdam and offshore wind farms in the North Sea, and Rostock to Malmö and Gdansk. In

6.3. Main challenges. Different challenges need to be faced in the installation of a European HVDC grid. Fig. 39 summarizes the main issues. 6.3.1. Technical challenges. A. Protection System

Protection systems represent one of the main challenges for HVDC grids, as proven technologies used in combination with AC grids are not feasible for DC grids. Fig. 40 summarizes the main issues. 451

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Fig. 39. Challenges of HVDC grids.

The only solution for such grids is the employment of fast DC breakers and isolation of the fault [93]. In this context it is hence relevant to locate the fault. In [98–100] fault detection methods in HVDC systems are presented. In general, fault detection for LCC-HVDC lines is quite advanced, whereas protection methods for VSC-HVDC system need further investigation [95]. DC breakers in multi-terminal systems ensure rebalancing control and fast isolation, in order to avoid voltage collapse. Several solutions are already available [101]. Typical schemes are depicted in Fig. 41. Passive DC circuit breakers (see Fig. 41a) are used in combination to LCC-HVDC systems, but are not suitable to VSC-HVDC. Hybrid circuit breakers (Fig. 41b) employ a mechanical switch in the main path to conduct current. Mechanical breakers (Fig. 41c) work similar as hybrid ones, but the divert part serves to limit the rate of rise across the main path. Solid-state circuit breakers (Fig. 41d) use semiconductor switches in combination to voltage limiting devices. This alternative is quite expensive, as several devices would be necessary. ABB introduced a new type of DC breaker (see Fig. 41e), which consists of a solid-state circuit breaker in parallel to the normal conduction path. In the main path a mechanical switch is in series with a semiconductor switch. This combination of devices turns out to have low on state losses [101]. Detailed configurations of semiconductor and hybrid DC breakers are shown in Figs. 42 and 43, respectively. Losses in semiconductor DC breakers are approximately 0.2% of the transmitted power. IGBT DC breakers represent a new technology, developed in 2012. Their main features can be summarized as follows [22].

Fig. 40. Challenges of DC system protection [42].

Compared with AC systems, the main issues related to HVDC protection are [93]: 1. A DC fault can be supplied by each AC system connected to the HVDC grid, 2. By DC lines the line impedance is very low, hence the DC protection scheme has to act very quickly (few ms), 3. A short circuit could result in a significant voltage drop in the whole network, 4. Breaking a DC current is more complicated, due to lack of zero crossings, 5. HVDC converters require protection for lower fault current than AC, because power electronic switches cannot withstand high fault currents.

1. Very low transfer losses in bypass (less than 0.01% of transmitted power); 2. Fast protection without time delay (less than 2 ms); 3. Immediate backup protection in DC switchyard; 4. Self-protection due to internal current limitation. A. HVDC Grid Codes As learned from the history, the installation of overlay grids is costly in terms of time. In fact the implementation of the current 400 kV AC grid lasted around seventy years [102]. In order to accelerate the process of approval and construction of HVDC grids, a strict collaboration between the involved nations is needed. One of the main issues is the availability of HVDC Grid Codes. Historically requirements have been usually written by TSOs at national levels. Grid codes from different countries were not all homogeneous or were not available in English, making them inaccessible to manufacturers and other operators. Currently most European

Various strategies have been implemented to overcome those problems in HVDC systems (see [94–97]). For instance in point-topoint VSC-HVDC systems when a fault occurs, AC side breakers can be used to interrupt current. In this way the HVDC link is disconnected from the AC terminal. This strategy can be used only in combination to overhead lines, where faults are normally temporary. In underground cables most of the faults are instead permanent, thus the HVDC system cannot restart automatically after the fault. Protection becomes even more challenging in the case of a meshed HVDC grid, as using AC breakers would lead to interruption of power flow in the whole grid. 452

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Fig. 41. Circuit breakers [101].

support the development of a single European market. European standards should encourage technological development, guarantee safety and environmental protection. Besides the standardization is a key instrument for the recovery of the European economy [103]. However Europe is still far from standardization in the electricity sector. This process is indeed challenging by many aspects, as political and legislative frameworks, energy security, technology availability, financial and social issues. In fact the supergid will incorporate facilities from different suppliers and will deal with technologies from different manufacturers. In order to ensure the compatibility of this wide variety of components, interoperability is a must [88]. Network Codes (NC) were introduced in 2009 in order to govern the European energy market with common rules [44]. The drafts are developed by ENTSO-E in close cooperation with stakeholders, based on Framework Guidelines written by the Agency for Cooperation of Energy Regulators (ACER). Europe’s energy policy goals include security of supply, sustainability and competition. The network codes seek to set out a balanced set of rules in order to promote the three objectives equally [104]. Furthermore the future power system must facilitate the integration of RES, partially located far away from load centers (offshore wind parks) and manage huge cross-border power flows caused by the pan-European electrical energy trade. Besides it has to achieve both targets with minimal environmental impact and at least costs [105]. As displayed in Fig. 44 network codes are required mainly in three areas. 6.3.2. Economic and social challenges. A big challenge by the implementation of HVDC grids is the financing process. A precise economic assessment of a European Supergrid is not available, as it strictly depends on how the grid will be developed. Europe has been recently facing a strong recession, thus politicians are considering

Fig. 42. Semiconductor DC breakers. Blue components represent semiconductor switches, violet ones varistors [22]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 43. Hybrid IGBT DC breakers. Blue components represent semiconductor switches, violet ones varistors and the green element symbolizes a mechanical switch [40]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

countries and the respective transmission system operators (TSOs) are part of the ENTSO-E [83]. The European Committee for Electrotechnical Standardization, known as CENELEC, has been founded in 1973 and is designated by the European Commission as a European Standards Organization. The main objectives are to facilitate trade between countries, create new markets, cut compliance costs and 453

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terms, as available models are very promising and first advances have been already achieved. Nevertheless improvements and more detailed studies are needed in order to solve lacks of the proposed grid architectures. Suggestions are provided in the following: 1. Adopting a global view in the scenarios, rather than focusing only on small regions; 2. Considering benefits also for single countries, rather than only for the European Union; 3. Including existing or planned links in the models, in order to avoid the construction of unnecessary lines; 4. Using HVDC paths as starting points for the construction of the meshed grids; promoting collaboration between existing associations; 5. Trying to achieve an agreement of a possible European HVDC grid model.

Fig. 44. HVDC grid codes: main areas [44].

whether the construction of an overlay grid is beneficial at this moment. According to the European Commission, investments equal to 600 billion € will be required for TSO (Transmission System Operators) and DSO (Distributions Systems Operators) in the decade 2010–2020, in order to build an interregional supergrid. Main investors will most likely be manufacturers, utilities as well as DSOs and TSOs. Public funding is also needed to speed up the development process. National and regulatory funding is employed mostly for deployment projects [106]. Another point is that many countries are not ready to accept a new electrical system or to change their energy policies. For instance France invested a big amount of money in the construction of nuclear power plants. Therefore it might not accept a green revolution, which would mean shutting down nuclear power plants, delivering today 75% of the total electric power in France. 6.3.3. Environmental challenges. As regards environmental concerns, the main drivers of the development of the Supergrid, several factors have to be considered. The construction of new HVDC cable routes should not damage neither environmental sensitive areas, as lakes and natural parks, nor agricultural lands or archaeological sites. In the same way, offshore HVDC links should not interfere with fishing or navigating areas during the construction process. Besides natural ecosystems have to be protected. To mention an example, the installation of subsea cable could affect submarine ecosystems through acoustic noises. Nevertheless there are still lacks of knowledge in regard to the characteristics of sound emission and perception by the marine fauna [107].

7. Conclusions and outlook. The present study was carried out to examine the context related to the installation of HVDC grids in Europe. HVDC applications and performed projects have been presented. HVDC grid drafts have been first analyzed and then compared. In this way, opportunities and challenges of HVDC grids have been highlighted. Previously published studies are still not coherent or not precise enough on important issues, such as economic and environmental assessments. An agreement of the possible design of the future power system hasn’t been achieved yet, as the standardization process is still in its first phase. The following research issues need to be answered: 1. Which technologies and configurations are more suitable for an HVDC grid? 2. How is the current situation in Europe, by means of existing HVDC projects and political frameworks? 3. Which of the available grid models is more advantageous and will most likely be implemented? 4. What are the main challenges in the installation of an HVDC grid in Europe? As an answer to the first question, the preferred converter technology to ensure security and reliability of the future HVDC grid is without doubt VSC. Power capabilities and voltage levels of VSC based HVDC systems are lower than those employing LCC. However, higher ratings are expected to be reached in the near future. In addition VSC can be applied in combination to multi-terminal topologies, considered as the most suitable technology for the implementation of a meshed HVDC grid. Multi-terminal schemes are in fact cheaper than point-to-point links, as less converter stations are required, which yields also lower converter losses. Besides multi-terminal grids offer the advantage of providing alternative paths to power flows in case a fault occurs on a cable. In Europe several projects have been implemented over the years. Further projects are under construction or are already planned. The main intends are to increase the transfer capability of the AC network, in order to facilitate the integration of renewable energies, improve the power exchange rates between European nations and thus achieve a better liberalization of the energy market. Advances toward the completion of an energy union have been reached through the publication of HVDC Grid Codes. Nevertheless improvements in different topics and more strict requirements are needed. Nowadays various grid models are available for consultation. The HVDC grid is expected to be built in an organic way, thus existing HVDC links and interconnectors will be included in the interregional European grid. In order to avoid unnecessary paths, an agreement on

The process of cable lying yields seabed disturbance and associated impact on flora and fauna. Sediments could be altered both by cable laying and heat dissipations, the least being however attenuated through thermal insulation. The thermal increase of the seabed depends on the burial depth of the cable and could lead to increased bacterial activities [107]. Another issue is represented by the fact that cables could contaminate grounds through leakage of filling materials. This, in long terms, could result in pollution of the underlying aquifer. Furthermore electromagnetic fields generated by power cables may affect the orientation of fish and marine mammals and the migratory behavior [107]. Precise environmental assessments for the development of HVDC grids have not been published yet, as lots of information are not known at the moment and require further research. The best approach, from an environmental point of view, is to monitor areas where HVDC equipment are likely to be installed, reduce ecological impact and increase the environmental awareness [107]. 6.4. Suggestions. On the basis of those considerations, it can be deduced that HVDC meshed grids can be implemented in the future, however in long terms. Offshore HVDC grids can be built in shorter

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the most probable design of the future grid should be achieved in short terms. The construction of an offshore grid is beneficial since it could be the first milestone in the installation of the continental grid. In this context, models focused on offshore grids, either in the North Sea or in the Mediterranean, are valuable only for short-term analysis. The same is valid for the ones taking into considerations the need for upgrades and single links or only onshore grids, as in the case of the TYNDP and Greenpeace’s scenarios. Optimal models should be based on the analysis of both offshore and onshore grids. In this sense, the draft developed by Friends of the Supergrid turned out to be the best alternative. Indeed the installation of the grid is expected to happen in three phases, the first including the installation of an offshore grid in North Europe, then extending it to Middle Europe and finally enabling the integration of solar power from external sources in Africa and the Middle East. Similar to this vision, is the one proposed by DESERTEC, also highly beneficial. However this is focused mostly on the extension of the European systems to neighboring continents, rather than on the needs within its borders. It goes hence beyond the aims of this study. The main challenges to be faced are the technical requirements of HVDC grids, as for instance DC breakers. Traditional AC protection strategy cannot be applied to HVDC, therefore new ones must be developed. The future HVDC grid should be able to transmit power at higher levels than the current AC system. This remains today, together with the further development of multi-terminal VSC systems, necessary for meshed grids, a demanding task. To be considered are also economic and social issues. Europe is in fact still recovering from the long financial crisis. The installation of the HVDC grid, would require big investments, that several European countries are not ready to face. On the other side new jobs could be created, offering the chance to restore good economic conditions. The fact that different countries will be involved makes the implementation of the grid harder, as power systems are not homogeneous, and political and cultural frames are very different. In this sense, the collaboration between engaged nations as well as between energy companies or associations should be promoted. References [1] Simões MG, Roche R, Kyriakides E, Miraoui A, Blunier B, McBee K, et al. Smartgrid technologies and progress in Europe and the USA. IEEE Energy Convers. Congr. Expo. Energy Convers. Innov. a Clean Energy Futur. ECCE 2011, Proc.. 2011. p. 383–90. [2] L’Abbate A, Migliavacca G, Häger U, Rehtanz C, Rüberg S, Ferreira H et al. The role of facts and HVDC in the future PAN-European transmission system development. 9th IET Int. Conf. AC DC Power Transm. (ACDC 2010). 2010. [p. O16–O16]. [3] Bianchi FD, Domínguez-García JL, Gomis-Bellmunt O. Control of multi-terminal HVDC networks towards wind power integration: a review. Renew Sustain Energy Rev 2016;55:1055–68. [4] Sánchez SA, Torres EA, Kalid RA. Renewable energy generation for the rural electrification of isolated communities in the Amazon Region. Renew Sustain Energy Rev 2015;49:278–90. [5] de Jong P, Kiperstok A, Sanchez AS, Dargaville R, Torres EA. Integrating large scale wind power into the electricity grid in the Northeast of Brazil. Energy 2016;100. [6] Xu L, Yao L, Sasse C. Power electronics options for large wind farm integration: VSC-based HVDC transmission. 2006 IEEE PES Power Syst. Conference Expo. PSCE 2006 - Proceedings. 2006. p. 760–7. [7] DR, Breuer W, Hartmann V, Povh D, et al. Application of HVDC for Large Power System Interconnections. 2004. [8] ABB. Introducing HVDC. 〈www.abb.com〉, 2014. [9] Brinckerhoff P. Comparison of High Voltage Transmission Options: Alternating Current Overhead and Underground, and Direct Current Underground. 2009. [10] Arrillaga J, Watson N, Liu Y. Flexible power transmission. 2007. [11] Miguel J, Andrés P De, Mühlenkamp M, Retzmann D, Walz R. Prospects for HVDC – getting more power out of the grid. Power 2006;4:29–30. [12] Buijs P, Cole S, Belmans R, TEN-E Revisited: Opportunities for HVDC Technology. [13] Rudervall R, Chapentier J, Sharma Raghuveer. High voltage direct current (HVDC) transmission systems technology review paper. Energy Week 2000;Ccc:1–19. [14] Barnes M, Beddard A. Voltage source converter HVDC links – the state of the art and issues going forward. Energy Procedia 2012;24:108–22. [15] Du C. The Control of VSC-HVDC and its Use for Large Industrial Power Systems.

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