- Email: [email protected]
Methodology for mapping operational zones of VSC-HVDC transmission system on offshore ports
Electrical Power and Energy Systems 93 (2017) 266–275
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Methodology for mapping operational zones of VSC-HVDC transmission system on offshore ports Rodney Itiki, Silvio Giuseppe Di Santo ⇑, Eduardo C. Marques Costa, Renato Machado Monaro Department of Energy and Automation Engineering – PEA, Polytechnic School of the University of São Paulo – EPUSP, São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 9 February 2017 Received in revised form 20 April 2017 Accepted 27 May 2017
Keywords: Floating nuclear power plant Offshore ports Power ships VSC-HVDC Electrification Transshipment and subsea mining
a b s t r a c t This research proposes a methodology for mapping operational zones of transmission systems based on voltage source converter high voltage direct current technology (VSC-HVDC) linking offshore ports to onshore power grid. Various operational conditions are investigated using the simulation software PSCAD in order to evaluate the feasibility of application of VSC-HVDC technology on offshore ports whereas supporting three economic activities: transshipment, subsea mining and power generation. The electrical power source considered in this research is based on nuclear energy or driven by fossil fuel. Varying the settings of the HVDC control strategy as well as the power balance within the offshore port and running several simulations on the power system, the power and voltage profiles in the generation, distribution and transmission of the system can be evaluated in details. Based on the methodology, stability boundaries in the HVDC system can be defined and mapped for reliable operation of the system during unusual and critical scenarios. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Market forces that drive global trade among countries impose pressure on port business for efficiency in operations. Many ports face a difficult decision to expand its infrastructure in order to keep up with the competitors by capturing such opportunities of scale economy. Current onshore ports are also being challenged by many other new concerns that cannot be solved by just increasing its size and water depth. On the other hand, offshore ports may provide many advantages in face of new challenges in power generation and delivery during critical and very specific cases, for example [1,2]: Exploration of rich reservoirs of rare minerals on deep-subsea. Unexpected increase of power demand caused by fast-paced growth of population notably in underdeveloped countries. Safety concerns over vulnerability of onshore ports and nuclear and gas power plants located on onshore coastal areas to tsunamis, destructive waves and flooding caused by sea level rise. Security concerns over vulnerability in war zone of onshore power plants running on flammable gas or fuel. ⇑ Corresponding author. E-mail addresses: [email protected] (R. Itiki), [email protected] (S.G. Di Santo), [email protected] (Eduardo C. Marques Costa), [email protected] (R.M. Monaro). http://dx.doi.org/10.1016/j.ijepes.2017.05.034 0142-0615/Ó 2017 Elsevier Ltd. All rights reserved.
Public pressure for deactivation and decommissioning of onshore old nuclear power plants close to densely populated urban areas. Very large human and monetary costs for remediation work, loss of civilian life, and property damage caused by onshore nuclear accidents. Offshore port is a business opportunity and a solution for such safety and security concerns. A VSC-HVDC providing power link between onshore power grid to the demands of offshore ports was simulated under several conditions: with floating nuclear or fossil fuel power plants, in distinct VSC converter control strategies, and in different power balance of load and generation within the offshore port. The developed methodology delimits voltage stability zones providing an easy, visual, and fast situation awareness for offshore port power system operation. Section 2 of this paper introduces a conceptual design of an offshore port in support of three possible economic activities in offshore and subsea environment. Section 3 details how a VSC converter can be electrically integrated on an offshore port power system to support electrification of economic activities. Section 4 describes the developed methodology for mapping operation zones of a power system with VSC-HVDC. Section 5 illustrates that the application of the developed methodology led to identification of five operation zones. The voltage and power stability on each of these zones were investigated through five case studies. Section 6
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
highlights prospects of recent advances of VSC-HVDC technology on offshore ports. Section 7 is the final conclusions on the benefits of the developed methodology on operation and on future studies about offshore port power systems. 2. Economic activities and power supply using offshore ports Transshipment is the transfer of goods or containers between ships or barges, which involves many power demanding equipment. Some onshore shallow water ports are not safe or suitable to receive large cargo ships. Small ships or barges, in this case, are useful to transport and transfer goods or containers to a larger cargo ship moored on an offshore area, so as it does not have to incur in costs of docking on an onshore port. Several minerals are extracted from subsea mining, such as: as gold, zinc, silver, manganese, nickel, copper and cobalt [3]. However, this extraction process requires a great demand of specific equipment for subsea mining and electric power. In this context, power generation on offshore environment can be provided by power ships, floating power generation plants driven by fossil fuel, which represent a versatile solution for rapid response during shortage of power generation capacity. Fig. 1 shows an example of an offshore port with distributed generation composed of floating nuclear power station and power-ship using fossil fuel. The offshore port supplies AC electrical power for cargo ships, subsea mining machine and for a HVDC/ AC converter that is connected to an onshore converter through a submarine cable. Even though diesel combustion is not a very economic, clean, and efficient process for power generation, diesel driven power ships can be a convenient alternative in certain circumstances,
267
for example: after catastrophic events caused by natural disasters and during unexpected power demand. In addition, there are also power ships driven by natural gas, which represents a cleaner and more efficient solution if compared to power ships using diesel or other fossil fuels. Offshore power generation can also be supported by nuclear floating power plants and most of them are based on small and medium sized nuclear reactors (SMRs), defined by the International Atomic Energy Agency (IAEA) as advanced reactors that produce electric power up to 700 MW [4]. Electrification, in the context of this paper, is the distributed power generation using various sources, such as: diesel, gas, nuclear energy and even renewable sources. Electrification on offshore ports has many advantages as following: elimination of onsite fossil fuel consumption of combustion machines, such as ship loaders, unloaders and towers, to meet air and noise pollution requirements of local health and environment legislation; increase of power efficiency by making use of electrical motors and drives rather than combustion driven motors; reduction of logistic cost and operational risks caused by onsite combustible diesel transportation from tanks to local combustion machines; cost savings of locally available energy source from the utility power grid in comparison to normally expensive onsite diesel power generation; new income opportunities by providing power from the grid through sockets and feeder cables to the main switchgear of a docked ship or vessel, allowing onboard diesel generator to be switched off [5];
Fig. 1. Economic activities and electrification on a conceptual design of an offshore port.
268
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
new income opportunities by providing an access point of power coupling on offshore port for power ships and floating nuclear power stations to the utility grid. In this paper, operational and stability conditions are investigated based on a typical offshore port configuration and related economic activities, port electrification, existing technologies for power generation and VSC-HVDC power transmission to an onshore converter using submarine cables. The goal of the proposed analysis is to establish operational and stability references for projects or further researches on offshore ports taken into account distributed power generation, offshore activities and HVDC power transmission using VSC technology. 3. VSC-HVDC technology in offshore ports HVDC technology based on voltage source converters (VSC) has been widely applied for interconnection of offshore wind farms to onshore terminals and integration of power grid blocks among countries [6,7]. The VSC-HVDC technology presents some advantages over existing alternating current technology [8]. HVDC transmission is a preferred choice for cable length exceeding 50 km [9]. The same technology represents a great advantage for power transmission from offshore ports to onshore facilities or even for offshore activities. Another important advantage of the VSC is that the power flow can be instantaneously reversed without reversion of the DC voltage polarity of the HVDC link, i.e., only DC current direction reverses [8]. Thus, VSC-HVDC system can easily withstand power
flow from onshore to offshore converters (power importation) as well as in reverse direction (power exportation). This versatile dynamic in the power flow through HVDC link between VSCs represents a great advantage compared to the line commuted converters (LCC), also denominated as current source converters [8,10]. Depending on the economic activity, the offshore port power system can perform either power generation or distribution functions or even both. For example, if the offshore port is receiving power from a floating power plant, it is generating electrical power. On the other hand, if it has electrical loads such as electric cranes, conveyor belts, auxiliary loads, its power system is distributing electrical power, as illustrated in Fig. 2. Fig. 2 shows the equipment of the offshore port illustrated in Fig. 1. An onshore high voltage alternating current grid is connected to an onshore converter terminal which is linked to an offshore port converter terminal by a HVDC submarine cable. The offshore port has an VSC HVDC terminal and a high voltage AC switchyard with two buses (main and generator buses) and a tie circuit breaker. When tie circuit breaker is closed, the main bus voltage has the same AC voltage on the generator high voltage bus. When it is open, generator high voltage AC bus can be 145 kV or 0 kV, following the status ON/OFF of the AC generators. Voltage transformers on both buses of the AC switchyard allows voltage measurement. There are also medium and low voltage power distribution system, which carries out the offshore port electrification. The port is also a power coupling access point for power generation coming from power ships and floating nuclear power plants.
Fig. 2. Simplified diagram of an offshore port operation.
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
4. Methodology for mapping operation zones of VSC-HVDC on offshore ports A very few papers on technical literature investigated VSCHVDC transmission system linking onshore power grids to offshore ports. Those who have studied HVDC on this particular application do not developed a methodology for comprehensive simulation and mapping of operation zones [11]. Most of them presents only a few power and voltage profiles on a very specific balance of load and generation [12] that would not be useful in practice for power system operators neither as a reference for comparison with the methodology developed on this paper. The methodology developed on this paper organizes on a systematic way how to simulate an offshore port power system with VSC-HVDC control in a comprehensive range of operation points and leads to a map showing all operation zones with stability in power and voltage.
269
Simulations were carried out in order to determine possible operation zones as a function of the power balance between consumption and generation on an offshore port. The calculated power flow on each equipment also must be within its minimum and maximum limits of capacity. For HVDC, the limit is 400 MW in both directions. For a power ship, it is 12 generators of 19.6 MW each operating between 20% and 100%. For floating nuclear power plant, the limit is a generator of 70 MW operating between 20% and 100%. Fig. 3 shows the developed methodology through a flow diagram with three loops. The first one asks for manual selection of switching position (on/off) of the tie circuit breaker and the selection of one of many VSC control strategies normally made available by HVDC manufacturers, power system simulation software library, technical books or scientific journals. PSCAD library in its examples allows selection of control strategies such as reactive power control, AC voltage control, DC voltage control, droop on active power and AC voltage, or just active power control. The second loop asks for manual iterative tests with incremental steps in power generation, and the last one, asks for simulation with steps in load demand. The outcome after running the methodology on a simulated power system is the definition of operational zones which satisfies all conditions for operation. In order to limit the extension of this research paper, the authors chose to study steady state power and voltage profile against capacity limits of HVDC and generators.
5. Case studies
Fig. 3. Flow diagram for operational zones mapping.
The stability boundaries of the system were obtained applying the methodology on the system. It was observed during simulation that stability on operation zones appears in five cases. Each case led to the identification of operation zones by varying the settings of control strategy [13] of the HVDC converters, status of the tie circuit breaker of the AC switchyard, simulation steps in the active power of loads and power generation units according to the input data described in Table 1. In Table 1, five operation zones are determined based on system configurations in terms of generation, load profile and HVDC control strategy. From several simulations using the software PSCAD, varying the operational conditions in Table 1, the stability boundary of the five zones are delimited in Fig. 4 as follows. Following the input data and step changes on the power system simulations as well as additional power systems data indicated on Appendix, it was observed within the boundaries of the operational zones a steady state profile of power flow in the HVDC converter, generators, loads and also at AC voltage profile on the high voltage switchyard bus within the acceptable limits (±5%) based on the nominal voltage value of 145 kV. Zone A illustrates a stable area in which it is possible to operate the VSC-HVDC system importing power from onshore grid to feed passive AC loads at the offshore port. Some technical literatures propose advanced control strategies to expand the importing power limits through HVDC links with VSCs [14–16]. In fact, as a passive AC load does not have inertia and, as a consequence, is not able to provide a reference angle and frequency, the VSC controller should internally allow the operator to set the required frequency and an internal voltage-controlled oscillator should generate a reference angle to a phase lock loop system, eventually allowing it to operate similarly to an adjusted speed drive controlling motor speed [8]. Zone B is defined by four limits: an ascendant left limit in form of a staircase with rise height of 19.6 MW in load power; a load top limit capped in 288 MW to keep a good visual proportion of all zones on the graphics; a right limit defined by twelve maximum
270
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
Table 1 Input data for simulation. Data
Zone A
Zone B
Zone C
Zone D
Zone E
Control strategy on onshore converter
Q: 0 MVAr Vdc: 400 kV Island
Q: 0 MVAr Vdc: 400 kV Island
0
1–12 53.55 MW
Open Off Off
Q: 0 MVAr Vdc: 400 kV PV droop P:400 MW Vac: 1 pu. 20 to 24 Up to 69.7 MW total on both buses Up to 69.7 MW total on both buses Close On (two) Off
Q: 0 MVAr Vdc: 400 kV PV droop P:400 MW Vac: 1 pu. 21 0 MW
Up to 235 MW Open On (one) Off
Q: 0 MVAr Vdc: 400 kV PV droop P: -63 MW Vac: 1 pu. 5–12 Up to 288 MW total on both buses Up to 288 MW total on both buses Close On (one) Off
Close Off On (Up to six)
Live Dead No
Live Live No
Live Live Yes
Live Live Yes
Live Live Yes
Control strategy on offshore converter
Quantity of 19.6 MW generators Power of loads on Main HV AC Bus Power of loads on Gen HV AC Bus Tie circuit breaker command Power ship command (quantity) Nuclear power plant command (quantity) Main AC bus status Generator AC bus status Main AC bus in parallel with generator AC bus
Up to 53.55 MW 0 MW
0 MW
Zone E is determined by the minimum quantity of generators required to reach VSC controller maximum power settings of 400 MW, which corresponds to approximately 6 nuclear generators of 70 MW, which is equivalent to 21 generators of 19.6 MW. It should be noted that Zone C and D are defined by a specific VSC setting of 63 MW in power importation with one power ship and 400 MW of power exportation with two power ships. It is expected a shift upward or downward in the operation zones depending on the VSC power setting. An increase in HVDC power importation can supply more power consumption from local loads at the offshore port and vice versa. The performance of the offshore system is evaluated for nine distinct operation points, from P1 to P9, which are distributed into the five zones in Fig. 4.
5.1. Case 1 – HVDC importing power from onshore resources to supply offshore loads
Fig. 4. Power operation zones of the offshore port.
quantity of generators in one power ship and an ascendant bottom limit in form of a staircase with a rise height in load power of 3.92 MW, equal to the minimum operation power of each generator (20% of its 19.6 MW capacity). Zone C is defined by 4 limits: a left limit in form of a descendant staircase determined by power instability caused by a combination of low levels of power consumption and generation (weak system) in parallel with an adjusted HVDC power importation [17]; a load top limit capped in 288 MW to keep a good visual proportion of all zones on the graphics; a right limit defined by twelve generators in one power ship and, finally, an ascendant bottom limit in form of a staircase with a rise height in load power of 3.92 MW, equal to the minimum operation power of each generator (20% of 19.6 MW). Zone D has an ascendant boundary on its left limit in form of a staircase with rise height of 19.6 MW, equal to the maximum operation power of each generator (100% of 19.6 MW) and a right maximum limit corresponding to all twenty-four generators of its two power ships.
This first case represents the power supply from VSC-HVDC to loads connected at the offshore port. Power flows from the onshore converter to the offshore converter and switchgear provides electrification of industrial activities (transshipment and subsea mining). Operational point P1, into the Zone A, has a load demand of 53 MW and the power supply is provided exclusively from the VSC-HVDC, without offshore generation. Fig. 5 shows that AC voltage stabilizes at the nominal voltage of 145 kV rms and HVDC succeeds in transmitting 53 MW of active power to the 53 MW load. Even though VSC power importation setting is 63 MW, HVDC is sending 53 MW because the load requires only 53 MW. Operational point P2 outside Zone A has loads demanding 89 MW and the power is intended to be supplied exclusively by the HVDC, without contribution from offshore generation. VSC power importation permission setting was increased to 105 MW in this point. Fig. 6 shows that AC voltage in point P2 does not stabilize at the nominal voltage of 145 kV rms and HVDC does not succeed in transmitting stable active power to the load. This voltage collapse and corresponding instability of load active power are due to limitations of VSC-HVDC technology to provide stable power to passive load, as reported on technical literature [14]. Instability problem observed in the simulation point P2 can be solved by:
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
271
5.2. Case 2 – HVDC importing power from onshore resources added to offshore generation
Fig. 5. Load profile for the Case 1: importing power from onshore resources to support the offshore demand.
In the Case 2, a hybrid scenario is considered with the VSCHVDC providing the same 53 MW from onshore resources added to offshore power generation, increasing the total power availability on offshore port. A tie circuit breaker, indicated in Fig. 2, is open in order to isolate the HVDC system from the local offshore generation. The offshore generation considered in this case is equivalent to a power ship with a total capacity of 235 MW, similar to an Aysegül Sultan power ship [18], which is able to operate independently twelve generators of 19.6 MW. Since the tie circuit breaker is open, operational point P3 has three running generators of 19.6 MW each feeding through the generator HV AC bus section a 40 MW load. It should be noted that point P3 (40 MW) representation in Fig. 4 is shifted 53 MW, reaching a total load of 93 MW (point P3 s inside Zone B), accounting the original Case 1 power supply from the HVDC power importation. Fig. 7 shows steady state AC voltage close to the nominal voltage of 145 kV rms and the generators succeeded in providing 44 MW of active power to the 44-MW load. Operational point P4, outside of the Zone B, has the same three running generators of 19.6 MW each, with a power demand from the offshore loads of 74 MW. Fig. 8 shows steady state AC voltage at the nominal voltage of 145 kV rms and the offshore generators are delivering 74 MW of active power to the 74 MW load. However, since there are only three generators running, the maximum output power is 58.8 MW and the active power demand of the offshore total load is 74 MW. Thus, operation characteristics close to P4, outside Zone B, represent overload conditions. It should be noted that point P4 (74 MW load) representation in Fig. 4 is shifted 53 MW, reaching a total load of 127 MW (point P4s), accounting the original Case 1 power supply from the HVDC power importation. P4s is outside Zone B because generator in overload condition is not permitted as operational zone. Case 2 shows the operation of isolated generation, which in fact can be a preparatory step before subsequent phase synchronization with the AC side of HVDC in order to close the tie circuit breaker of AC switchyard, leading to the parallel operation of the local generation and offshore converter.
Fig. 6. Load profile for the Case 1: excessive offshore load demand through the HVDC link from onshore resources.
Installing an additional pole and submarine cable on the VSCHVDC system which would duplicate the overall power transmission capacity resulting in the upward expansion of stable Zone A as shown in Fig. 4. Keep investigating advanced control strategies on VSC-HVDC control systems based on recent researches [14–16]. Adding more power generation on the offshore port, rather based on gas, oil, nuclear or even renewable resources, which would move operational point P2 to Zone B as indicated in Fig. 4. Stability on Zone B is detailed on Case 2. High power demand in this case are supported by additional isolated power generation. It is very clear at this point that the methodology created and detailed on this paper to map all the stable operation zones represents a very useful tool for decision making on capacity expansion planning and for improvement of situational awareness of offshore port power system operators.
Fig. 7. Load profile for the Case 2: importing power from onshore resources and offshore generation from a power ship.
272
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
Fig. 8. Load profile for the Case 2: overload situation considering power supplying from offshore resources (power ship).
Fig. 9. Load profile for the Case 3: local generation in parallel with HVDC importing from onshore resources to feed offshore loads.
5.3. Case 3 – HVDC converter in parallel to power ship to feed offshore loads In the Case 3, the tie circuit breaker is closed and the generator is in parallel with the VSC-HVDC converter. The HVDC system is set to import 63 MW from the onshore grid. The operational point P5, inside of the Zone C, has six generators of 19.6 MW each and 133 MW total load. Fig. 9 shows that AC voltage on HV switchyard keeps close to the nominal value of 145 kV and generators are supplying 70 MW of active power while HVDC are providing 63 MW to the 133 MW load. Six running generators, having a maximum power capacity of 117.6 MW, in parallel with HVDC power importation (63 MW), can safety supply a required load demand of 133 MW. Operational point P6, outside of the Zone C, has five running generators of 19.6 MW each and 105 MVA load (89.25 MW at 0.85 power factor). Fig. 10 shows that AC voltage in P6 does not stabilize properly at the nominal voltage of 145 kV rms and the significant disturbances are observed in the generator and load power as consequence of disturbance in the AC voltage. Operation of five generators in parallel with the VSC-HVDC converter seems to be unstable if the offshore power generation is not sufficient to complement the 63 MW imported from the HVDC system in order to supply the total offshore demand of 89 MW. In this context, sufficient offshore power generation and HVDC power importation are necessary but not sufficient for the stability:
PGEN þ PHVDC > cos u SLOAD where PGEN is the power generation capacity (98 MW in P6); PHVDC is the module of power importation setting of converter control (63 MW in P6); SLOAD is the apparent power of the offshore load (105 MVA); cos u is the power factor of 0.85 for all loads (as considered in the Case 3). The power capacity margin, above of 71.75 MW in P6, seems to be necessary for stable operation. However, operation with a large number of generators can imply in individual under loading of these same generators, considering the underload level as 20% of the individual power capacity of each generator (19.6 MW), since the total power generation is restrained by the power balance. Case 3 and the previous one shows that the methodology for mapping operational zones is important because it allows visual
Fig. 10. Load profile for the Case 3: offshore generators in parallel with HVDC converter under overload demand.
identification of intersection between two zones (Zone B and C) which indicates potential opportunity for operators, by shifting control strategy and operational conditions (for instance, tie circuit breaker command), to carry out continuous transfer of zones for the same operational point of demand and supply. This eventually allows for greater flexibility of operation.
5.4. Case 4 – Generators exporting power to onshore grid and supplying the offshore load demand In the Case 4, the tie circuit breaker is closed and the generator is connected to the HVDC system, which is set to export 400 MW from the offshore port to the onshore grid. In the operation point P7, there are 24 running generators of 19.6 MW each that mean a total power of 470 MW, considering a total load demand of 51 MW. Fig. 11 shows that HV AC voltage keeps close to its nominal value (145 kV) and a great amount of 453 MW, from the offshore
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
Fig. 11. Load profile for the Case 4: offshore generation in parallel with the MMCHVDC system for supplying offshore load demand and exporting 400 MW to onshore grid.
generators, flows through the HVDC link to the onshore grid and the remaining power supplies the 51 MW load demand. Two principal observations are obtained from Case 4. First is that the generated power of 453 MW is below the maximum capacity of 470 MW (2 235 MW). The second is that in the operational point P8, there are 20 generators of 19.6 MW each, 68 MW load demand and the HVDC system is set to export 400 MW to onshore grid. Fig. 12 shows that part of 467 MW flows to the 68 MW offshore load and the AC voltage keeps in steady state at its nominal value of 145 kV rms. The total 467 MW is above the maximum active power capacity provided by the 20 generators of 19.6 MW. Thus, the operation in P8 is not possible considering only the power supplied from the offshore generators in the power ship. Case 4 and the previous one indicates that the methodology for mapping operational zones is important in showing, on the operation zones map, the effects of addition of power ships on the capacity of the offshore port to perform power exportation to the onshore grid.
5.5. Case 5 – HVDC exporting power from generators to onshore grid In the Case 5, the power generation is given by floating nuclear power plants. The generated power is used for floating the nuclear power plants and AC/DC conversion to the HVDC system. In this case, six floating nuclear power plants of 70 MW each, similar to the existing Akademik Lomonosov [19], are synchronized and connected to the same offshore MMC-HVDC terminal converter. The Tie circuit breaker is close and generators are exporting all power through the HVDC link. Offshore converter control is set to export 400 MW to onshore grid. Operational point P9, into the Zone E, has 6 floating nuclear power plants of 70 MW each, which are equivalent to approximately 21 generators of 19.6 MW each (as previously described in the power ship Aysegül Sultan). Fig. 13 shows that although six floating nuclear power plants have a total capacity of 420 MW, as the HVDC converter control is set to export 400 MW, the power generation is also limited to the same exporting power.
273
Fig. 12. Load profile for the Case 4: Offshore generation in parallel with the MMCHVDC system submitted to excessive load demand or exportation to onshore grid.
Fig. 13. Load profile for the Case 5: Load profile of six nuclear power generators of 70 MW each.
The AC voltage is stabilized at 145 kV, as set in the converter control. From the same settings, the exporting power profile of the HVDC link is maintained with 400 MW. Case 5 indicates that the methodology for mapping operational zones is important in showing, on the operation zones map, that generation-only operation is also possible even with different technologies of floating power generation.
6. Feasibility and future prospects on VSC-HVDC technology on offshore ports The VSC-HVDC technology on offshore ports shows to be a viable solution for various offshore activities, such as: mineral prospecting, offshore power generation by fossil and renewable resources. The power transmission between offshore and onshore systems is carried out by subsea HVDC cables considering power flow from both systems, depending of the offshore and onshore power demand.
274
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
There are many evidences proving that VSC-HVDC maximum capacity is sufficient for most of floating power plants connected to offshore ports: A VSC-HVDC link between France and Spain has power transmission capacity of 2 GW [7]. New 300-MW floating nuclear plants have been object of recent researches [20]. Osman Khan power ship produces approximately 470 MW [18]. In this context, Zone B, C, D, and E in Fig. 4, can be expanded from new technologies for offshore power generation and extending limits of power conversion and transmission using VSC-HVDC technologies. Recent researches are also aimed at improving HVDC transmission capacity by increased DC voltage [21] as well as to improve stability of weak systems through HVDC converter control emulating synchronous machine inertia [22,23]. Although such recent researches have potential to expand the boundaries of stable zones beyond the limits shown of Fig. 4, the novel methodology presented on this paper to map the stable operation zones keeps valid for application in any typical configuration and novel control strategy of HVDC in power system of offshore port. The similar boundary expansion is expected to Zone A in Fig. 4, which involves passive load applications, in face of the following recent developments: A 78-MW voltage source converter VSC-HVDC was developed in 2011 [8]. VSC-HVDC up to 1000 MW for passive loads was announced as feasible in 2016 [24]. This way, passive loads operating into the Zone A are expected to be expanded from less than a hundred megawatts to 1000 MW in the future. 7. Conclusion This research developed a methodology for mapping operational zones of offshore port power system linked to onshore power grid through VSC-HVDC transmission system. The methodology was applied to a case study involving an offshore port designed to support electrification of three different economic activities (transshipment, subsea mining and power generation). The authors submitted the offshore port power system to simulation on different configurations of power generation, distribution, exportation and importation. The simulations following the procedures of the methodology led to a graphic presenting the boundaries of five operational zones with power and voltage stability. The graphic of power operational zones has great potential to be usefully represented on man-machine interface of the power port operation workstation. This graphic reference, while confronted with the online measurement of power demand and supply, may eventually improve the situational awareness of the power system operator as for the risks of operation in proximity to instability zones foreseen on the graphic. It was observed on simulations some of the advantages of VSC HVDC technology such as bidirectional power flow and availability of different control strategies for very distinct power demand and supply levels. These characteristics make VSC-HVDC a very promising and flexible technology for electrification of offshore port and related economic activities.
As for the generation side, developments on small and medium sized nuclear reactor technology for floating nuclear power plants as well on fossil fuel driven power generation carried out on offshore environment may benefit from the feasibility of power coupling access point provided by offshore ports. The developed methodology for mapping operational zones can be considered as a normative testing procedure for future offshore ports with VSC HVDC transmission systems and other offshore economic activities such as subsea prospection and renewable power generation. Acknowledgments The authors thank the ‘‘Coordination for the Improvement of Higher Education Personnel” (CAPES) for supporting this work. Appendix A
VSC-HVDC and offshore power system main data: Power grid AC short circuit level: very strong (infinite bus) Onshore power grid nominal AC voltage: 380 kV/60 Hz VSC-HVDC power transmission capacity: 400 MW VSC-HVDC poles: monopolar VSC-HVDC cells quantity: 200 units (modular multilevel converter) Onshore converter rated voltage on AC side: 220 kV Onshore converter rated voltage on DC side: +400 kV Submarine XLPE DC cable length: 74 km Offshore converter rated voltage on DC side: +400 kV Offshore converter rated voltage on AC side: 220 kV Offshore converter AC frequency: 60 Hz Offshore transformer ratio: 220 kV:145 kV Offshore transformer power: 400 MVA Offshore switchyard high voltage AC: 145 kV Offshore medium AC voltage: 11 kV Offshore power distribution transformer ratio: 11 kV:0.48 kV Offshore low AC voltage: 0.48 kV Offshore loads power factor: 0.85 PF Power ship capacity: 12 (20–100%) 19.6 MW Power ship voltage: 145 kV Nuclear power plant capacity: (20–100%) 70 MW Floating nuclear power plant voltage: 145 kV
References [1] Flory D. IAEA report on protection against extreme earthquakes and tsunamis in the light of the accident at the Fukushima Daiichi nuclear power plant. Presented at international experts meeting; 2012.. [2] IRSN. Methodology used in IRSN nuclear accident cost estimates in France. Institut de Radioprotection et de Sûreté Nucléaire, Fontenay-aux-Roses, France; 2016. . [3] Fang H, Duan M. Offshore operation facilities, 1 ed. Elsevier Science Press; 2014. . [4] Rowinskia MK, White TJ, Zhao J. Small and Medium sized Reactors (SMR): a review of technology. Renew Sust Energy Rev 2015;44:643–56. http://dx.doi. org/10.1016/j.rser.2015.01.006. . [5] Marquart K, Haasdijk T, Ferrari GB, Schmidhalter R. Shore to ship power. ABB AB. Sweden; 2016. . [6] Pierri E, Binder O, Hemdan NGA, Kurrat M. Challenges and opportunities for a European HVDC grid. Renew Sust Energy Rev 2017;70:427–56. http://dx.doi. org/10.1016/j.rser.2016.11.233. .
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275 [7] Dennetière S, Saad H, Clerc B, Mahseredjian J. Setup and performances of the real-time simulation platform connected to the INELFE control system. Electric Power Syst Res 2016;138:180–7. http://dx.doi.org/10.1016/j.epsr.2016.03.008.. [8] Jovcic D, Ahmed K. VSC HVDC applications and topologies, performance and cost comparison with LCC HVDC. In: High-voltage direct-current transmission – converters, systems and DC grids, 1st ed. Chichester, UK: John Wiley & Sons; 2015. pp. 123, 124 and 294 [ch.12, sec.12.1]. doi:http://dx.doi.org/10.1002/ 9781118846704.fmatter.pdf. [9] Kolluri S, Thummala P, Sapkota R, Kumar Panda S, Rendusara D. Subsea power transmission cable modelling: reactive power compensation and transient response studies. In: 2016 IEEE 17th workshop on control and modeling for power electronics (COMPEL), Trondheim; 2016. p. 1–6. doi:http://dx.doi.org/ 10.1109/COMPEL.2016.7556718 . [10] Blond SL, Bertho Jr R, Coury DV, Vieira JCM. Design of protection schemes for multi-terminal HVDC systems. Renew Sust Energy Rev 2015;56:965–74. http://dx.doi.org/10.1016/j.rser.2015.12.025. [11] Solhjoukhah Z, Egea-Alvarez A, Gomis-Bellmunt O. Voltage control of offshore load connected to a multi-terminal HVDC transmission system. In: 11th IET international conference on AC and DC power transmission, Birmingham; 2015. p. 1–9. doi: http://dx.doi.org/10.1049/cp.2015.0063 . [12] Xu J, Zhao C, Zhang B. Control design and operational characteristics comparation for VSC-HVDC supplying active/passive networks. In: 2011 6th IEEE conference on industrial electronics and applications, Beijing; 2011. p. 1381–6. doi:http://dx.doi.org/10.1109/ICIEA.2011.5975803 . [13] Arrillaga J, Liu YH, Watson NR. Flexible power transmission: the HVDC options. John Wiley & Sons Ltd 2007. http://dx.doi.org/10.1002/9780470511862. . [14] Chai RZ, Zhang BH, Hao ZG, Zheng T, Ma SY, Chen DZ. Control strategies of MMC-HVDC connected to passive networks. Presented at TENCON 2013 - 2013 IEEE region 10 conference (31194); 2013. . [15] Chen Y, Damm G, Benchaib A, Lamnabhi-Lagarrigue F. A new control strategy based on static and dynamic feedback linearization for a VSC-HVDC terminal. In: ECC 2014, Jun 2014, Strasbourg, France. Proceeding of the 13th European
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
[24]
275
control conference; 2014 [] . Tanga X, Lu DDC. Enhancement of voltage quality in a passive network supplied by a VSC-HVDC transmission under disturbances. Int J Electr Power Energy Syst 2014;54:45–54. http://dx.doi.org/10.1016/j.ijepes.2013.06.030. . Geetha RS, Deekshit R, Lal G. Analysis of VSC-HVDC system connected to a weak AC system, Presented at 2013 annual IEEE India conference (INDICON), Mumbai; 2013. p.1–6 . Karadeniz Energy Group, Istanbul, Turkey. Powering the world – delivering fast track reliable electricity; 2017 . JSC Africantov OKBM, Russia. KLT-40S reactor plants for small nuclear plants; 2016 . Buongiorno J, Golay M, Todreas N, Briccetti A, Jurewicz J, Kindfuller V, et al. Offshore Small Modular Reactor (OSMR): an innovative plant design for societally acceptable and economically attractive nuclear energy in a postfukushima, Post-9/11 World. Presented at ASME 2014 small modular reactors symposium; 2014. doi: http://dx.doi.org/10.1115/SMR2014-3306 . Li R, Flextcher JE. A novel MMC control scheme to increase the DC voltage in HVDC transmission systems. Electric Power Syst Res 2017;143:544–53. http:// dx.doi.org/10.1016/j.epsr.2016.11.004. . Aouinia R, Marinescu B, Kilani KB, Elleuch M. Stability improvement of the interconnection of weak AC zones by synchronverter-based HVDC link. Electric Power Syst Res 2017;142:112–24. http://dx.doi.org/10.1016/j. epsr.2016.09.005. . D’Arco S, Suul JA, Fosso OB. A Virtual Synchronous Machine implementation for distributed control of power converters in SmartGrids. Electric Power Syst Res 2015;122:180–97. http://dx.doi.org/10.1016/j.epsr.2015.01.001. . Westman B, Eriksson K, Persson G, Maeland A. Power from shore, ABB AB. Sweden; 2016. .
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Methodology for mapping operational zones of VSC-HVDC transmission system on offshore ports Rodney Itiki, Silvio Giuseppe Di Santo ⇑, Eduardo C. Marques Costa, Renato Machado Monaro Department of Energy and Automation Engineering – PEA, Polytechnic School of the University of São Paulo – EPUSP, São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 9 February 2017 Received in revised form 20 April 2017 Accepted 27 May 2017
Keywords: Floating nuclear power plant Offshore ports Power ships VSC-HVDC Electrification Transshipment and subsea mining
a b s t r a c t This research proposes a methodology for mapping operational zones of transmission systems based on voltage source converter high voltage direct current technology (VSC-HVDC) linking offshore ports to onshore power grid. Various operational conditions are investigated using the simulation software PSCAD in order to evaluate the feasibility of application of VSC-HVDC technology on offshore ports whereas supporting three economic activities: transshipment, subsea mining and power generation. The electrical power source considered in this research is based on nuclear energy or driven by fossil fuel. Varying the settings of the HVDC control strategy as well as the power balance within the offshore port and running several simulations on the power system, the power and voltage profiles in the generation, distribution and transmission of the system can be evaluated in details. Based on the methodology, stability boundaries in the HVDC system can be defined and mapped for reliable operation of the system during unusual and critical scenarios. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Market forces that drive global trade among countries impose pressure on port business for efficiency in operations. Many ports face a difficult decision to expand its infrastructure in order to keep up with the competitors by capturing such opportunities of scale economy. Current onshore ports are also being challenged by many other new concerns that cannot be solved by just increasing its size and water depth. On the other hand, offshore ports may provide many advantages in face of new challenges in power generation and delivery during critical and very specific cases, for example [1,2]: Exploration of rich reservoirs of rare minerals on deep-subsea. Unexpected increase of power demand caused by fast-paced growth of population notably in underdeveloped countries. Safety concerns over vulnerability of onshore ports and nuclear and gas power plants located on onshore coastal areas to tsunamis, destructive waves and flooding caused by sea level rise. Security concerns over vulnerability in war zone of onshore power plants running on flammable gas or fuel. ⇑ Corresponding author. E-mail addresses: [email protected] (R. Itiki), [email protected] (S.G. Di Santo), [email protected] (Eduardo C. Marques Costa), [email protected] (R.M. Monaro). http://dx.doi.org/10.1016/j.ijepes.2017.05.034 0142-0615/Ó 2017 Elsevier Ltd. All rights reserved.
Public pressure for deactivation and decommissioning of onshore old nuclear power plants close to densely populated urban areas. Very large human and monetary costs for remediation work, loss of civilian life, and property damage caused by onshore nuclear accidents. Offshore port is a business opportunity and a solution for such safety and security concerns. A VSC-HVDC providing power link between onshore power grid to the demands of offshore ports was simulated under several conditions: with floating nuclear or fossil fuel power plants, in distinct VSC converter control strategies, and in different power balance of load and generation within the offshore port. The developed methodology delimits voltage stability zones providing an easy, visual, and fast situation awareness for offshore port power system operation. Section 2 of this paper introduces a conceptual design of an offshore port in support of three possible economic activities in offshore and subsea environment. Section 3 details how a VSC converter can be electrically integrated on an offshore port power system to support electrification of economic activities. Section 4 describes the developed methodology for mapping operation zones of a power system with VSC-HVDC. Section 5 illustrates that the application of the developed methodology led to identification of five operation zones. The voltage and power stability on each of these zones were investigated through five case studies. Section 6
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
highlights prospects of recent advances of VSC-HVDC technology on offshore ports. Section 7 is the final conclusions on the benefits of the developed methodology on operation and on future studies about offshore port power systems. 2. Economic activities and power supply using offshore ports Transshipment is the transfer of goods or containers between ships or barges, which involves many power demanding equipment. Some onshore shallow water ports are not safe or suitable to receive large cargo ships. Small ships or barges, in this case, are useful to transport and transfer goods or containers to a larger cargo ship moored on an offshore area, so as it does not have to incur in costs of docking on an onshore port. Several minerals are extracted from subsea mining, such as: as gold, zinc, silver, manganese, nickel, copper and cobalt [3]. However, this extraction process requires a great demand of specific equipment for subsea mining and electric power. In this context, power generation on offshore environment can be provided by power ships, floating power generation plants driven by fossil fuel, which represent a versatile solution for rapid response during shortage of power generation capacity. Fig. 1 shows an example of an offshore port with distributed generation composed of floating nuclear power station and power-ship using fossil fuel. The offshore port supplies AC electrical power for cargo ships, subsea mining machine and for a HVDC/ AC converter that is connected to an onshore converter through a submarine cable. Even though diesel combustion is not a very economic, clean, and efficient process for power generation, diesel driven power ships can be a convenient alternative in certain circumstances,
267
for example: after catastrophic events caused by natural disasters and during unexpected power demand. In addition, there are also power ships driven by natural gas, which represents a cleaner and more efficient solution if compared to power ships using diesel or other fossil fuels. Offshore power generation can also be supported by nuclear floating power plants and most of them are based on small and medium sized nuclear reactors (SMRs), defined by the International Atomic Energy Agency (IAEA) as advanced reactors that produce electric power up to 700 MW [4]. Electrification, in the context of this paper, is the distributed power generation using various sources, such as: diesel, gas, nuclear energy and even renewable sources. Electrification on offshore ports has many advantages as following: elimination of onsite fossil fuel consumption of combustion machines, such as ship loaders, unloaders and towers, to meet air and noise pollution requirements of local health and environment legislation; increase of power efficiency by making use of electrical motors and drives rather than combustion driven motors; reduction of logistic cost and operational risks caused by onsite combustible diesel transportation from tanks to local combustion machines; cost savings of locally available energy source from the utility power grid in comparison to normally expensive onsite diesel power generation; new income opportunities by providing power from the grid through sockets and feeder cables to the main switchgear of a docked ship or vessel, allowing onboard diesel generator to be switched off [5];
Fig. 1. Economic activities and electrification on a conceptual design of an offshore port.
268
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
new income opportunities by providing an access point of power coupling on offshore port for power ships and floating nuclear power stations to the utility grid. In this paper, operational and stability conditions are investigated based on a typical offshore port configuration and related economic activities, port electrification, existing technologies for power generation and VSC-HVDC power transmission to an onshore converter using submarine cables. The goal of the proposed analysis is to establish operational and stability references for projects or further researches on offshore ports taken into account distributed power generation, offshore activities and HVDC power transmission using VSC technology. 3. VSC-HVDC technology in offshore ports HVDC technology based on voltage source converters (VSC) has been widely applied for interconnection of offshore wind farms to onshore terminals and integration of power grid blocks among countries [6,7]. The VSC-HVDC technology presents some advantages over existing alternating current technology [8]. HVDC transmission is a preferred choice for cable length exceeding 50 km [9]. The same technology represents a great advantage for power transmission from offshore ports to onshore facilities or even for offshore activities. Another important advantage of the VSC is that the power flow can be instantaneously reversed without reversion of the DC voltage polarity of the HVDC link, i.e., only DC current direction reverses [8]. Thus, VSC-HVDC system can easily withstand power
flow from onshore to offshore converters (power importation) as well as in reverse direction (power exportation). This versatile dynamic in the power flow through HVDC link between VSCs represents a great advantage compared to the line commuted converters (LCC), also denominated as current source converters [8,10]. Depending on the economic activity, the offshore port power system can perform either power generation or distribution functions or even both. For example, if the offshore port is receiving power from a floating power plant, it is generating electrical power. On the other hand, if it has electrical loads such as electric cranes, conveyor belts, auxiliary loads, its power system is distributing electrical power, as illustrated in Fig. 2. Fig. 2 shows the equipment of the offshore port illustrated in Fig. 1. An onshore high voltage alternating current grid is connected to an onshore converter terminal which is linked to an offshore port converter terminal by a HVDC submarine cable. The offshore port has an VSC HVDC terminal and a high voltage AC switchyard with two buses (main and generator buses) and a tie circuit breaker. When tie circuit breaker is closed, the main bus voltage has the same AC voltage on the generator high voltage bus. When it is open, generator high voltage AC bus can be 145 kV or 0 kV, following the status ON/OFF of the AC generators. Voltage transformers on both buses of the AC switchyard allows voltage measurement. There are also medium and low voltage power distribution system, which carries out the offshore port electrification. The port is also a power coupling access point for power generation coming from power ships and floating nuclear power plants.
Fig. 2. Simplified diagram of an offshore port operation.
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
4. Methodology for mapping operation zones of VSC-HVDC on offshore ports A very few papers on technical literature investigated VSCHVDC transmission system linking onshore power grids to offshore ports. Those who have studied HVDC on this particular application do not developed a methodology for comprehensive simulation and mapping of operation zones [11]. Most of them presents only a few power and voltage profiles on a very specific balance of load and generation [12] that would not be useful in practice for power system operators neither as a reference for comparison with the methodology developed on this paper. The methodology developed on this paper organizes on a systematic way how to simulate an offshore port power system with VSC-HVDC control in a comprehensive range of operation points and leads to a map showing all operation zones with stability in power and voltage.
269
Simulations were carried out in order to determine possible operation zones as a function of the power balance between consumption and generation on an offshore port. The calculated power flow on each equipment also must be within its minimum and maximum limits of capacity. For HVDC, the limit is 400 MW in both directions. For a power ship, it is 12 generators of 19.6 MW each operating between 20% and 100%. For floating nuclear power plant, the limit is a generator of 70 MW operating between 20% and 100%. Fig. 3 shows the developed methodology through a flow diagram with three loops. The first one asks for manual selection of switching position (on/off) of the tie circuit breaker and the selection of one of many VSC control strategies normally made available by HVDC manufacturers, power system simulation software library, technical books or scientific journals. PSCAD library in its examples allows selection of control strategies such as reactive power control, AC voltage control, DC voltage control, droop on active power and AC voltage, or just active power control. The second loop asks for manual iterative tests with incremental steps in power generation, and the last one, asks for simulation with steps in load demand. The outcome after running the methodology on a simulated power system is the definition of operational zones which satisfies all conditions for operation. In order to limit the extension of this research paper, the authors chose to study steady state power and voltage profile against capacity limits of HVDC and generators.
5. Case studies
Fig. 3. Flow diagram for operational zones mapping.
The stability boundaries of the system were obtained applying the methodology on the system. It was observed during simulation that stability on operation zones appears in five cases. Each case led to the identification of operation zones by varying the settings of control strategy [13] of the HVDC converters, status of the tie circuit breaker of the AC switchyard, simulation steps in the active power of loads and power generation units according to the input data described in Table 1. In Table 1, five operation zones are determined based on system configurations in terms of generation, load profile and HVDC control strategy. From several simulations using the software PSCAD, varying the operational conditions in Table 1, the stability boundary of the five zones are delimited in Fig. 4 as follows. Following the input data and step changes on the power system simulations as well as additional power systems data indicated on Appendix, it was observed within the boundaries of the operational zones a steady state profile of power flow in the HVDC converter, generators, loads and also at AC voltage profile on the high voltage switchyard bus within the acceptable limits (±5%) based on the nominal voltage value of 145 kV. Zone A illustrates a stable area in which it is possible to operate the VSC-HVDC system importing power from onshore grid to feed passive AC loads at the offshore port. Some technical literatures propose advanced control strategies to expand the importing power limits through HVDC links with VSCs [14–16]. In fact, as a passive AC load does not have inertia and, as a consequence, is not able to provide a reference angle and frequency, the VSC controller should internally allow the operator to set the required frequency and an internal voltage-controlled oscillator should generate a reference angle to a phase lock loop system, eventually allowing it to operate similarly to an adjusted speed drive controlling motor speed [8]. Zone B is defined by four limits: an ascendant left limit in form of a staircase with rise height of 19.6 MW in load power; a load top limit capped in 288 MW to keep a good visual proportion of all zones on the graphics; a right limit defined by twelve maximum
270
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
Table 1 Input data for simulation. Data
Zone A
Zone B
Zone C
Zone D
Zone E
Control strategy on onshore converter
Q: 0 MVAr Vdc: 400 kV Island
Q: 0 MVAr Vdc: 400 kV Island
0
1–12 53.55 MW
Open Off Off
Q: 0 MVAr Vdc: 400 kV PV droop P:400 MW Vac: 1 pu. 20 to 24 Up to 69.7 MW total on both buses Up to 69.7 MW total on both buses Close On (two) Off
Q: 0 MVAr Vdc: 400 kV PV droop P:400 MW Vac: 1 pu. 21 0 MW
Up to 235 MW Open On (one) Off
Q: 0 MVAr Vdc: 400 kV PV droop P: -63 MW Vac: 1 pu. 5–12 Up to 288 MW total on both buses Up to 288 MW total on both buses Close On (one) Off
Close Off On (Up to six)
Live Dead No
Live Live No
Live Live Yes
Live Live Yes
Live Live Yes
Control strategy on offshore converter
Quantity of 19.6 MW generators Power of loads on Main HV AC Bus Power of loads on Gen HV AC Bus Tie circuit breaker command Power ship command (quantity) Nuclear power plant command (quantity) Main AC bus status Generator AC bus status Main AC bus in parallel with generator AC bus
Up to 53.55 MW 0 MW
0 MW
Zone E is determined by the minimum quantity of generators required to reach VSC controller maximum power settings of 400 MW, which corresponds to approximately 6 nuclear generators of 70 MW, which is equivalent to 21 generators of 19.6 MW. It should be noted that Zone C and D are defined by a specific VSC setting of 63 MW in power importation with one power ship and 400 MW of power exportation with two power ships. It is expected a shift upward or downward in the operation zones depending on the VSC power setting. An increase in HVDC power importation can supply more power consumption from local loads at the offshore port and vice versa. The performance of the offshore system is evaluated for nine distinct operation points, from P1 to P9, which are distributed into the five zones in Fig. 4.
5.1. Case 1 – HVDC importing power from onshore resources to supply offshore loads
Fig. 4. Power operation zones of the offshore port.
quantity of generators in one power ship and an ascendant bottom limit in form of a staircase with a rise height in load power of 3.92 MW, equal to the minimum operation power of each generator (20% of its 19.6 MW capacity). Zone C is defined by 4 limits: a left limit in form of a descendant staircase determined by power instability caused by a combination of low levels of power consumption and generation (weak system) in parallel with an adjusted HVDC power importation [17]; a load top limit capped in 288 MW to keep a good visual proportion of all zones on the graphics; a right limit defined by twelve generators in one power ship and, finally, an ascendant bottom limit in form of a staircase with a rise height in load power of 3.92 MW, equal to the minimum operation power of each generator (20% of 19.6 MW). Zone D has an ascendant boundary on its left limit in form of a staircase with rise height of 19.6 MW, equal to the maximum operation power of each generator (100% of 19.6 MW) and a right maximum limit corresponding to all twenty-four generators of its two power ships.
This first case represents the power supply from VSC-HVDC to loads connected at the offshore port. Power flows from the onshore converter to the offshore converter and switchgear provides electrification of industrial activities (transshipment and subsea mining). Operational point P1, into the Zone A, has a load demand of 53 MW and the power supply is provided exclusively from the VSC-HVDC, without offshore generation. Fig. 5 shows that AC voltage stabilizes at the nominal voltage of 145 kV rms and HVDC succeeds in transmitting 53 MW of active power to the 53 MW load. Even though VSC power importation setting is 63 MW, HVDC is sending 53 MW because the load requires only 53 MW. Operational point P2 outside Zone A has loads demanding 89 MW and the power is intended to be supplied exclusively by the HVDC, without contribution from offshore generation. VSC power importation permission setting was increased to 105 MW in this point. Fig. 6 shows that AC voltage in point P2 does not stabilize at the nominal voltage of 145 kV rms and HVDC does not succeed in transmitting stable active power to the load. This voltage collapse and corresponding instability of load active power are due to limitations of VSC-HVDC technology to provide stable power to passive load, as reported on technical literature [14]. Instability problem observed in the simulation point P2 can be solved by:
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
271
5.2. Case 2 – HVDC importing power from onshore resources added to offshore generation
Fig. 5. Load profile for the Case 1: importing power from onshore resources to support the offshore demand.
In the Case 2, a hybrid scenario is considered with the VSCHVDC providing the same 53 MW from onshore resources added to offshore power generation, increasing the total power availability on offshore port. A tie circuit breaker, indicated in Fig. 2, is open in order to isolate the HVDC system from the local offshore generation. The offshore generation considered in this case is equivalent to a power ship with a total capacity of 235 MW, similar to an Aysegül Sultan power ship [18], which is able to operate independently twelve generators of 19.6 MW. Since the tie circuit breaker is open, operational point P3 has three running generators of 19.6 MW each feeding through the generator HV AC bus section a 40 MW load. It should be noted that point P3 (40 MW) representation in Fig. 4 is shifted 53 MW, reaching a total load of 93 MW (point P3 s inside Zone B), accounting the original Case 1 power supply from the HVDC power importation. Fig. 7 shows steady state AC voltage close to the nominal voltage of 145 kV rms and the generators succeeded in providing 44 MW of active power to the 44-MW load. Operational point P4, outside of the Zone B, has the same three running generators of 19.6 MW each, with a power demand from the offshore loads of 74 MW. Fig. 8 shows steady state AC voltage at the nominal voltage of 145 kV rms and the offshore generators are delivering 74 MW of active power to the 74 MW load. However, since there are only three generators running, the maximum output power is 58.8 MW and the active power demand of the offshore total load is 74 MW. Thus, operation characteristics close to P4, outside Zone B, represent overload conditions. It should be noted that point P4 (74 MW load) representation in Fig. 4 is shifted 53 MW, reaching a total load of 127 MW (point P4s), accounting the original Case 1 power supply from the HVDC power importation. P4s is outside Zone B because generator in overload condition is not permitted as operational zone. Case 2 shows the operation of isolated generation, which in fact can be a preparatory step before subsequent phase synchronization with the AC side of HVDC in order to close the tie circuit breaker of AC switchyard, leading to the parallel operation of the local generation and offshore converter.
Fig. 6. Load profile for the Case 1: excessive offshore load demand through the HVDC link from onshore resources.
Installing an additional pole and submarine cable on the VSCHVDC system which would duplicate the overall power transmission capacity resulting in the upward expansion of stable Zone A as shown in Fig. 4. Keep investigating advanced control strategies on VSC-HVDC control systems based on recent researches [14–16]. Adding more power generation on the offshore port, rather based on gas, oil, nuclear or even renewable resources, which would move operational point P2 to Zone B as indicated in Fig. 4. Stability on Zone B is detailed on Case 2. High power demand in this case are supported by additional isolated power generation. It is very clear at this point that the methodology created and detailed on this paper to map all the stable operation zones represents a very useful tool for decision making on capacity expansion planning and for improvement of situational awareness of offshore port power system operators.
Fig. 7. Load profile for the Case 2: importing power from onshore resources and offshore generation from a power ship.
272
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
Fig. 8. Load profile for the Case 2: overload situation considering power supplying from offshore resources (power ship).
Fig. 9. Load profile for the Case 3: local generation in parallel with HVDC importing from onshore resources to feed offshore loads.
5.3. Case 3 – HVDC converter in parallel to power ship to feed offshore loads In the Case 3, the tie circuit breaker is closed and the generator is in parallel with the VSC-HVDC converter. The HVDC system is set to import 63 MW from the onshore grid. The operational point P5, inside of the Zone C, has six generators of 19.6 MW each and 133 MW total load. Fig. 9 shows that AC voltage on HV switchyard keeps close to the nominal value of 145 kV and generators are supplying 70 MW of active power while HVDC are providing 63 MW to the 133 MW load. Six running generators, having a maximum power capacity of 117.6 MW, in parallel with HVDC power importation (63 MW), can safety supply a required load demand of 133 MW. Operational point P6, outside of the Zone C, has five running generators of 19.6 MW each and 105 MVA load (89.25 MW at 0.85 power factor). Fig. 10 shows that AC voltage in P6 does not stabilize properly at the nominal voltage of 145 kV rms and the significant disturbances are observed in the generator and load power as consequence of disturbance in the AC voltage. Operation of five generators in parallel with the VSC-HVDC converter seems to be unstable if the offshore power generation is not sufficient to complement the 63 MW imported from the HVDC system in order to supply the total offshore demand of 89 MW. In this context, sufficient offshore power generation and HVDC power importation are necessary but not sufficient for the stability:
PGEN þ PHVDC > cos u SLOAD where PGEN is the power generation capacity (98 MW in P6); PHVDC is the module of power importation setting of converter control (63 MW in P6); SLOAD is the apparent power of the offshore load (105 MVA); cos u is the power factor of 0.85 for all loads (as considered in the Case 3). The power capacity margin, above of 71.75 MW in P6, seems to be necessary for stable operation. However, operation with a large number of generators can imply in individual under loading of these same generators, considering the underload level as 20% of the individual power capacity of each generator (19.6 MW), since the total power generation is restrained by the power balance. Case 3 and the previous one shows that the methodology for mapping operational zones is important because it allows visual
Fig. 10. Load profile for the Case 3: offshore generators in parallel with HVDC converter under overload demand.
identification of intersection between two zones (Zone B and C) which indicates potential opportunity for operators, by shifting control strategy and operational conditions (for instance, tie circuit breaker command), to carry out continuous transfer of zones for the same operational point of demand and supply. This eventually allows for greater flexibility of operation.
5.4. Case 4 – Generators exporting power to onshore grid and supplying the offshore load demand In the Case 4, the tie circuit breaker is closed and the generator is connected to the HVDC system, which is set to export 400 MW from the offshore port to the onshore grid. In the operation point P7, there are 24 running generators of 19.6 MW each that mean a total power of 470 MW, considering a total load demand of 51 MW. Fig. 11 shows that HV AC voltage keeps close to its nominal value (145 kV) and a great amount of 453 MW, from the offshore
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
Fig. 11. Load profile for the Case 4: offshore generation in parallel with the MMCHVDC system for supplying offshore load demand and exporting 400 MW to onshore grid.
generators, flows through the HVDC link to the onshore grid and the remaining power supplies the 51 MW load demand. Two principal observations are obtained from Case 4. First is that the generated power of 453 MW is below the maximum capacity of 470 MW (2 235 MW). The second is that in the operational point P8, there are 20 generators of 19.6 MW each, 68 MW load demand and the HVDC system is set to export 400 MW to onshore grid. Fig. 12 shows that part of 467 MW flows to the 68 MW offshore load and the AC voltage keeps in steady state at its nominal value of 145 kV rms. The total 467 MW is above the maximum active power capacity provided by the 20 generators of 19.6 MW. Thus, the operation in P8 is not possible considering only the power supplied from the offshore generators in the power ship. Case 4 and the previous one indicates that the methodology for mapping operational zones is important in showing, on the operation zones map, the effects of addition of power ships on the capacity of the offshore port to perform power exportation to the onshore grid.
5.5. Case 5 – HVDC exporting power from generators to onshore grid In the Case 5, the power generation is given by floating nuclear power plants. The generated power is used for floating the nuclear power plants and AC/DC conversion to the HVDC system. In this case, six floating nuclear power plants of 70 MW each, similar to the existing Akademik Lomonosov [19], are synchronized and connected to the same offshore MMC-HVDC terminal converter. The Tie circuit breaker is close and generators are exporting all power through the HVDC link. Offshore converter control is set to export 400 MW to onshore grid. Operational point P9, into the Zone E, has 6 floating nuclear power plants of 70 MW each, which are equivalent to approximately 21 generators of 19.6 MW each (as previously described in the power ship Aysegül Sultan). Fig. 13 shows that although six floating nuclear power plants have a total capacity of 420 MW, as the HVDC converter control is set to export 400 MW, the power generation is also limited to the same exporting power.
273
Fig. 12. Load profile for the Case 4: Offshore generation in parallel with the MMCHVDC system submitted to excessive load demand or exportation to onshore grid.
Fig. 13. Load profile for the Case 5: Load profile of six nuclear power generators of 70 MW each.
The AC voltage is stabilized at 145 kV, as set in the converter control. From the same settings, the exporting power profile of the HVDC link is maintained with 400 MW. Case 5 indicates that the methodology for mapping operational zones is important in showing, on the operation zones map, that generation-only operation is also possible even with different technologies of floating power generation.
6. Feasibility and future prospects on VSC-HVDC technology on offshore ports The VSC-HVDC technology on offshore ports shows to be a viable solution for various offshore activities, such as: mineral prospecting, offshore power generation by fossil and renewable resources. The power transmission between offshore and onshore systems is carried out by subsea HVDC cables considering power flow from both systems, depending of the offshore and onshore power demand.
274
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275
There are many evidences proving that VSC-HVDC maximum capacity is sufficient for most of floating power plants connected to offshore ports: A VSC-HVDC link between France and Spain has power transmission capacity of 2 GW [7]. New 300-MW floating nuclear plants have been object of recent researches [20]. Osman Khan power ship produces approximately 470 MW [18]. In this context, Zone B, C, D, and E in Fig. 4, can be expanded from new technologies for offshore power generation and extending limits of power conversion and transmission using VSC-HVDC technologies. Recent researches are also aimed at improving HVDC transmission capacity by increased DC voltage [21] as well as to improve stability of weak systems through HVDC converter control emulating synchronous machine inertia [22,23]. Although such recent researches have potential to expand the boundaries of stable zones beyond the limits shown of Fig. 4, the novel methodology presented on this paper to map the stable operation zones keeps valid for application in any typical configuration and novel control strategy of HVDC in power system of offshore port. The similar boundary expansion is expected to Zone A in Fig. 4, which involves passive load applications, in face of the following recent developments: A 78-MW voltage source converter VSC-HVDC was developed in 2011 [8]. VSC-HVDC up to 1000 MW for passive loads was announced as feasible in 2016 [24]. This way, passive loads operating into the Zone A are expected to be expanded from less than a hundred megawatts to 1000 MW in the future. 7. Conclusion This research developed a methodology for mapping operational zones of offshore port power system linked to onshore power grid through VSC-HVDC transmission system. The methodology was applied to a case study involving an offshore port designed to support electrification of three different economic activities (transshipment, subsea mining and power generation). The authors submitted the offshore port power system to simulation on different configurations of power generation, distribution, exportation and importation. The simulations following the procedures of the methodology led to a graphic presenting the boundaries of five operational zones with power and voltage stability. The graphic of power operational zones has great potential to be usefully represented on man-machine interface of the power port operation workstation. This graphic reference, while confronted with the online measurement of power demand and supply, may eventually improve the situational awareness of the power system operator as for the risks of operation in proximity to instability zones foreseen on the graphic. It was observed on simulations some of the advantages of VSC HVDC technology such as bidirectional power flow and availability of different control strategies for very distinct power demand and supply levels. These characteristics make VSC-HVDC a very promising and flexible technology for electrification of offshore port and related economic activities.
As for the generation side, developments on small and medium sized nuclear reactor technology for floating nuclear power plants as well on fossil fuel driven power generation carried out on offshore environment may benefit from the feasibility of power coupling access point provided by offshore ports. The developed methodology for mapping operational zones can be considered as a normative testing procedure for future offshore ports with VSC HVDC transmission systems and other offshore economic activities such as subsea prospection and renewable power generation. Acknowledgments The authors thank the ‘‘Coordination for the Improvement of Higher Education Personnel” (CAPES) for supporting this work. Appendix A
VSC-HVDC and offshore power system main data: Power grid AC short circuit level: very strong (infinite bus) Onshore power grid nominal AC voltage: 380 kV/60 Hz VSC-HVDC power transmission capacity: 400 MW VSC-HVDC poles: monopolar VSC-HVDC cells quantity: 200 units (modular multilevel converter) Onshore converter rated voltage on AC side: 220 kV Onshore converter rated voltage on DC side: +400 kV Submarine XLPE DC cable length: 74 km Offshore converter rated voltage on DC side: +400 kV Offshore converter rated voltage on AC side: 220 kV Offshore converter AC frequency: 60 Hz Offshore transformer ratio: 220 kV:145 kV Offshore transformer power: 400 MVA Offshore switchyard high voltage AC: 145 kV Offshore medium AC voltage: 11 kV Offshore power distribution transformer ratio: 11 kV:0.48 kV Offshore low AC voltage: 0.48 kV Offshore loads power factor: 0.85 PF Power ship capacity: 12 (20–100%) 19.6 MW Power ship voltage: 145 kV Nuclear power plant capacity: (20–100%) 70 MW Floating nuclear power plant voltage: 145 kV
References [1] Flory D. IAEA report on protection against extreme earthquakes and tsunamis in the light of the accident at the Fukushima Daiichi nuclear power plant. Presented at international experts meeting; 2012.
R. Itiki et al. / Electrical Power and Energy Systems 93 (2017) 266–275 [7] Dennetière S, Saad H, Clerc B, Mahseredjian J. Setup and performances of the real-time simulation platform connected to the INELFE control system. Electric Power Syst Res 2016;138:180–7. http://dx.doi.org/10.1016/j.epsr.2016.03.008.
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
[24]
275
control conference; 2014 [