The restoration of an electric power system: International survey and discussion of possible innovative enhancements for the Italian system

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Electric Power Systems Research 78 (2008) 239–247

The restoration of an electric power system: International survey and discussion of possible innovative enhancements for the Italian system Stefano Barsali a , Romano Giglioli a , Davide Poli a,∗ , Marino Sforna b , Roberto Salvati b , Roberto Zaottini b a

Department of Electric Systems and Automation, University of Pisa, Via Diotisalvi 2, 56122 Pisa, Italy b TERNA S.p.A (Italian TSO), Via Arno 64, 00198 Roma, Italy Received 5 October 2005; received in revised form 29 September 2006; accepted 8 February 2007 Available online 23 March 2007

Abstract The present paper reports the main results of a comprehensive international survey that, on behalf of the Italian independent system operator, the authors have carried out into the main strategies, critical issues and practical experiences related to the restoration plans of a wide set of deregulated systems worldwide. For many ISOs, people directly involved in designing, testing and updating restoration plans have been contacted and interviewed, in order to focus general issues and to outline common improvement trends. Taking the international survey as a starting point, the paper also proposes and discusses possible innovative enhancements being studied for the Italian system, aimed at increasing the effectiveness of the restoration service. © 2007 Elsevier B.V. All rights reserved. Keywords: Power systems; Restoration; Blackout

1. Introduction Electric power systems have a very high degree of reliability, as a consequence of the high level of interconnection and the accurate management of components and plants, in terms of inspection, operation, maintenance and planning. Nevertheless, there is still some probability that, as a consequence of some unlikely event, a given number of power units may be separated from the grid, and that large areas remain disconnected. The issue of the emergency management of the electric systems plays a primary role in the scenario of big national energy facilities; particularly delicate is the definition of the objectives and priorities for defence and restoration plans, because of the social and business aspects associated with the continuity of power supply. User safety and system security, quality of supply, social needs and the requirements of the national productive apparatus are even more intertwined with each other in a deregulated scenario, where constraints on the use of emergency resources, role sharing and priorities, as well as the adjustment of

∗

Corresponding author. Tel.: +39 050 2217351; fax: +39 050 2217333. E-mail address: [email protected] (D. Poli).

0378-7796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2007.02.005

economic transactions, heavily rely on the juridical-contractual framework which regulates the relations among the stakeholders. The main functions required to the components of an electric system, which play a key role in the restarting of the service, are briefly discussed in the following: • Supply of black-start power (black-start plants, or units tripped to house loads); • Restoration of the connections between ‘first restart’ resources and the load to be supplied; • Balance keeping between generation and load in each electric island. Nowadays, power plants, in particular thermoelectric and nuclear units, are so huge that each production plant can exceed 1000 MW, as a compromise between scale economies (in both costs and efficiency) and the reliability of systems having generation concentrated in “few” large-scale facilities. As a consequence, in poorly interconnected grids, the loss and restart of a high-capacity unit can turn out to be critical. Power plants cannot actually self-restart; they always need an energy source to supply their auxiliary services, which are essential to safely start the plant. For larger size plants, a few tens of MW can be

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necessary; furthermore, the cold starting of a thermoelectric unit can take over 24 h. The connection between the power units and the loads requires the activation of high-voltage lines, which results in the supply of plenty of reactive power, problems of grid over-voltage and under-excitation of the synchronous generators; the number of circuit breakers to be operated and the territorial extension of the electric system requires good coordination among the operators and a careful assessment of the actual functionality of components after blackout. Finally, the low interconnected generation capacity in the first restarting phases, the constraints on the loading profiles of the units, the low regulation capacity of some units after the heavy transients sustained immediately before the blackout, require a special attention to be paid for keeping a constant balance between generation and load, otherwise some generation units could trip again and the grid would very likely collapse again. 2. International survey about restoration in a deregulated environment A broad survey of the restoration practices in deregulated markets around the world has been carried out by means of questionnaire and of direct interview to the most representative system operators. Spain, PJM and CalISO have been visited in 2003, while in the same period questionnaires were sent to several other (some tens) system operators. The research dealt mainly with the strategies adopted, the criteria for service compensation and on the tests for checking the performances [5,7]. 2.1. Restoration strategies The strategy of a restoration plan includes the definition of basic assumptions, general rules, resources and priorities for the black-start process. The restoration strategies applied worldwide can be associated to two main general philosophies: restoration by path and by zone [1,2]. The restarting by path technique gives priority to the repowering of the backbone of the network using the available power sources and a strictly necessary initial load; all the rest of the load is gradually reconnected to the system after the main backbone of the grid has been restored. The adopted procedure is to first open some restarting paths that connect the black-start units with the main thermoelectric power plants, by connecting just the parts of the load which are strictly necessary for stabilizing these initial islands. The main goal of the restoration plan is the re-synchronization of as many islands as possible, by ‘mending’ the network backbone system before reactivate most of the user-load. This strategy is usually applied in those systems having reduced black-start capacity and a bulk generation based on conventional thermoelectric plants with critical ramping constraints. The survey indicates that the majority of the ISOs uses instead a restoration philosophy based on zoning, starting from balanced areas set up around black-start units or generating plants tripped

to houseloads; the main target is forming steady load islands, to be resynchronized at a later time.1 Such an approach is considered more profitable to reduce load restoration times, thus minimizing user inconvenience. These ISOs have a large number of black-start units in their own systems. Part of the interviewed ISOs (approximately 25%) think that the tie lines to the neighbour systems, if available, are essential to start the restoration process, while just a minority of the interviewed organizations (8%) answered that the restoring of the backbone system, no matter if disconnected from the rest of the system, is a priority goal compared to the re-powering of the bulk of the load. More in dept, one of the most significant contributions highlighted by the survey concerns the interaction between market rules and emergency management logics. This farranging problem involves, just mentioning the main aspects, the incentives and the ex-ante economic remuneration of the restoration resources, the criteria and priorities for their utilization in emergency conditions, the adjustment of ex-post economic transactions of ordinary market stakeholders as well as the operators who have played an active part in the restoration of the system. The above mentioned issues are analyzed and discussed in the following. 2.2. Possible approaches to provide incentives for the service If a vertically integrated monopolistic utility is directly interested in having the service supplied by its own power units, this would not happen in a liberalised regime, as the producer could have no interest in bearing the costs for supplying special services. Therefore, the coordinated use of the generation, distribution and transmission plants, which is essential to restore the power system, becomes itself a system requirement, subject of a specific agreement between the parties. One of the first possibilities (10% of the analyzed countries) is that some “basic” services are viewed, during an emergency, as “obligations of a public service” and included, as such, in the network access rules. In the remaining part of considered countries, the restoration services are optional and remunerated according to a cost-based or market-based approach; in the former case (70% of answers), the problem is to estimate the costs that have been actually incurred by the operators and suitably push the subjects involved to find their own benefit in the supply of the service; in the latter case, the black-start service becomes the focus of a special market, where the rules must boost the development of competition among operators and oppose to any local monopoly of the restoration resources. Although a large part of the analyzed countries uses at present a cost-based approach to remunerate the black-start service, direct contacts with many system operators confirm the recent trend of setting priorities, obligations

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Infrequent and definitely more complicated is the voluntary and preventative setup of several load islands, even within the defense plan, as it happens in the French system and somewhere in the Italian grid.

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and remuneration of the restoration process, as far as possible, in a market perspective. The third possibility, being studied by some countries but not implemented yet, is that the system operator directly owns the equipments to restart the system; such resources, as well as in an emergency, could also be used during the normal condition of the system for balancing purposes and for reactive power control. This assumption belongs to a more general trend towards the total transparency of the network in the exchanges between producers and loads, as the endpoint of that “Third Party Access” approach that is behind the operation of the deregulated energy markets. As the international survey showed, during the first years of deregulation, the ISOs relied for restoring just on the same plants as those included in the restoration plan of the previous vertically integrated system. In order to ensure that such plants keep supplying the black-start service even in a liberalized regime, in most cases a cost-based remuneration was enough. Concerning power plants, it essentially considers the costs incurred to install the facilities needed to enable the plant self-start, the maintenance costs and the periodical testing costs, as well as the staff training costs. Typically an incentive factor to be applied to acknowledged costs, suitably calibrated and diversified by zone, is provided to increase the producers’ interest in installing and maintaining the restoration resources. These considerations are pushing several system operators to promote competition as for other ancillary services, thus studying the possible resort to market-oriented remuneration mechanisms for black-start service. In some cases, the idea of bilateral agreements (private negotiations) between the system operator and the plant owners is gaining ground; in some other cases, real auction mechanisms are being considered. 2.3. Performance tests and service remuneration It is essential to check at regular intervals that the plants and equipments involved in the restoration are fit for the operations scheduled in the restoration plan, as well as for ensuring a good maintenance of those components that, although critical for black-start, are rarely used during normal operation. The deregulation of the electric industry has also introduced new legal-economic problems associated with the restarting tests, because the timelines, operation, remuneration of the tests and the penalties for failing to perform the tests must be set within a market perspective. In systems where the black-start service is remunerated, a distinction can be made between ex-ante and ex-post payment criteria. In the former case, the availability to perform a given task is remunerated; in the latter case, the actual delivery of the stipulated service during a real black/brownout is rewarded. As to generation units, usually an ex-ante payment is subject to the performance of periodical physical tests on the plants. These tests involve checking some typical plant performance, for instance tripping to houseload capability, self-restarting, loading ramp time, the ability to control voltage and frequency on a cold load pick-up, proper operation of all communication systems and all devices connecting the plant to the grid. Physical

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tests involving the energization of network portions are rarely performed (e.g. in Ontario); for such types of tests, simulation devices are typically adopted [6]. In most cases, the test is confirmed by a standard declaration by the unit owner, certifying the test results to the ISO. Many countries, however, are thinking of directly involving the system operator in the tests, so as to directly check the operation and its result. The remuneration, usually fixed by bilateral agreements between the producer and the ISO, is intended to cover the costs for installation of black-start equipment, staff training and regular testing. If the agreed test is not carried out or it fails, economic sanctions are generally given to the plant owner; they include: • Suspension of monthly payments from the date of the failed test to the date when the test is performed again and passed. • Refund of the remunerations received since the last successful test. • Both (for instance, in Ontario). In some cases if a test is failed, the system operator grants the plant a short extension time, during which the test can be tried again with no penalties. The ex-ante payment for the availability to supply the blackstart service, which also remunerates the performance of the tests, is based on the concept that the service must be available at all times. Heavy economic sanctions are usually given even when a black-start resource, which had been called to work during a restoration, cannot perform the tasks under the restoration plan for which it received ex-ante remuneration. Such penalties, however, are regulated by bilateral agreements, whose economic values are not disclosed. Sometimes the failure to declare temporary unavailability of the black-start service (due to a failure or maintenance) is also punished, since it prevents the system operator allocating the service to someone else during such period. At present, the main liberalised systems do not generally provide for any special remuneration for the owners of the network systems for the black-start service; the participation of the load in both the defence plan and the system restoration plan could be better set in a market perspective. At the moment, not even the self-producers seem to be particularly motivated to take part in defence or restoration activities, since they generally tend to part from the system at the first network disturbances and to get back to it in parallel when the blackout has cleared up. Usually, no ex-post remunerations are given to those black-start resources that have proactively participated in the restoration process. Usually, in the emergency condition, the economic transactions related to production and consumption are actually settled at the usual market price of that time. An exception is the system of England and Wales, where the energy produced during the system restoration by the black-start units is given special value. The costs incurred by the system operator to guarantee the restoration service are usually reversed on to the wheeling tariff that are charged to the end users. In some systems, though,

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the new trend is to view a quick restoration as an economic benefit, not only for the customers, but also for the producers, who quickly come back selling energy on the market; from this perspective, discussions are under way about the prospect of charging the costs of the black-start service to all the subjects connected to the network and not just to the loads.

also be helpful to improve the ordinary management of the system. Some of the above mentioned solutions/ideas will be proposed and discussed in the following.

3. The Italian restoration plan: possible innovative enhancements

3.2.1. Classification of generating units and network plants During emergency conditions the system operator has to locate and coordinate the available resources, according to preestablished strategies, so as to guarantee the restoration of the service. To pursue this goal, it is essential to predefine a production and grid plants classification, based on clear parameters that give prominence to the actual capacities offered by each plant for contributing to the system restoration. To collect information on these parameters, a comprehensive questionnaire has been proposed to the owners of all Italian generating units above 10 MW. The investigation focused the geographical location of each plant, the connection to the grid, the level of coordination with the ISO control centre, the selfrestarting capacity, the energetic problems related to the systems related to the prime mover, the ability to perform load rejection operations, to adjust and deliver reactive power to the grid and to regulate frequency and voltage during the loading ramp. Once information on all power plants has been collected, such information has been entered in a carefully developed database. Pending an organized collection of information from the different plants located all over the country, a production plant classification by type has been considered. Power plants are the most critical and complex key factors of service restoration, as they provide the power and energy required to re-supply the loads, but the role played by the network systems is equally important, since they not only convey energy from generation to loads, but their availability determines whether a given strategy can be implemented or not. For such plants, especially electric stations, the following data has been collected and analyzed:

3.1. Present situation and perspectives The current restoration plan of the Italian electric system is conceived for restarting by backbones, which mainly start from the hydroelectric power units and reach the main large thermoelectric power plants. Such Plan was designed in the previous vertically integrated system, where one operator used to manage virtually all of the generation, distribution and transmission plants and where the main first-start resource was the hydroelectric units. Considering the recent legal-contractual and technological evolution, alternative or additional solutions can be found to improve the effectiveness of the restoration service. The number of first-restart units, including non-hydroelectric ones, can be increased by means of technically feasible and economically not much expensive adjustments, in order to obtain a more even distribution of the resources on the national territory. As well as the power units, resources also exist outside the system, including interconnections with neighbouring systems, realized either by alternated current lines or by high-voltage direct current (HVDC) systems. The latter, if provided with suitable balancing and control systems, can re-power parts of the network. Another possibility could be the use of electrochemical storage systems, currently available in sizes up to 40 MW, for network stabilization and load levelling. Great problems in the implementation of the restoration plans concern the energization of very high-voltage lines (380 kV). The implementation of a restoration plan based on islands at lower voltage levels (132 kV) could reduce the need to energize long connections. This also would help to quickly exploit any unit that has tripped to houseload and is ready to energize the network without waiting for complex paths to be built. Furthermore, new defensive strategies could help the restoration process, such as the automatic formation of load islands in an emergency condition. Related to the need of keeping a balance between the generated power and the load, during the restoration stages the regulating capacity of the units, faced to the significant power steps at which the load is available, can constitute a very critical factor. The performances of new remote metering/control systems of the load can be very helpful in keeping this balance. Finally, it could be useful, for the system operator, to get personally equipped with part of the physical resources required for the defence and restoration. Such solution can be put in practice by installing synchronous compensator in direct-current conversion stations, storage systems, equipment for the planned formation of specific load islands during the restoration process and also black-start units. In addition, these plants can

3.2. Analysis of available resources

• Function. It depends on the location of the station in the network and on the presence or absence of some equipments.2 • Topology. Essentially referred to the station scheme3 and its connections with the network. • Energy. It includes all the factors4 needed to ensure the feeding of the station components in charge of protection and operation, in particular when there is no power in the network. • Information. It includes details on the information, control and communication system of the station in both local and remote mode.5 2 Switching systems (in particular to split the load fed by the station), generating units, interconnection with foreign networks, phase shifting systems, balancing equipments and automatic parallel devices. 3 Voltage levels available at the station, number of bars and sections per voltage level, transformers with their main features, parameters of the lines connected with the station. 4 Feeding scheme of the auxiliary systems in ac and dc, presence and size of the electrochemical storage system, presence and size of diesel gensets.

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3.2.2. Qualification of generating units and network plants for the restoration Italian power plants have been qualified on the basis of the functions they can provide during the restoration. In particular, as far as concerns the ability to self-start without network contribution (black-start), five different categories have been defined: (1) Plants already “suitable” for the first restoration of the system; (2) “Adjustable” plants, that could be transformed into Type 1; (3) Plants that are “usable” as early as the first stages of the restoration plan, which cannot however be transformed (for technical and/or economic reasons) into Type 1; (4) “Isolable” plants, that cannot be transformed into plants suitable for the first restoration of the system, but can be fitted for supplying load islands; (5) “Unfit” plants, i.e. plants that cannot however be transformed into plants suitable for the first restoration and that cannot quickly resume operation after a blackout. A general survey of the current situation of the Italian generation mix has been carried out. At present, the “suitable plants” category includes a large part of the hydroelectric units and some simple-cycle turbogas plants. The “adjustable plants” category includes hydro and simple-cycle turbogas plants which at present do not have a self-starting system, the turbogas units in combined-cycle or used in re-powering plants, all the geothermoelectric units. The “usable plants” category includes, instead, the conventional thermoelectric units (steam, combined cycle or re-powering) that trip to houseload after a successful load-rejection operation. The “isolable plants” category includes some plants which, due to their geographical location, the features of the adjacent network area and the plant itself, can be used for the planned formation of load islands during the defence phase, through the use of suitably set minimumfrequency relays. “Unfit plants” largely consist of plants which need extremely long times for resuming operation or plants which have low regulating capacity or which do not have it at all. Some kinds of renewable sources are in this category; a wind farm, for instance, with its fluctuating and un-controllable output would destabilise the critical balance of a just restarting power system. These plants can be reconnected as soon as a large enough regulating capacity is available. Looking at small plants mainly connected to the distribution networks (say distributed generation: DG) the ability to supply a portion of the grid during the restoration process is presently strongly affected by the rules and standards for the connection of DG. In almost all the systems, DG is not allowed to stay connected to the system in the event of an upstream system failure. Distribution systems was born and developed as passive and are usually radially operated. The possibility of DG to supply

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It is essential to assess how each network plant can be controlled during the restoration process and to estimate the time it will take the operators to reach unmanned stations for the local control.

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portions of the distribution systems requires a new concept for the distribution system is developed. Recently, several projects are studying the concept of active distribution networks (Microgrids, SmartGrids), and some pilot installations are currently being developed. Properly designed distribution systems will be able of profiting by the presence of embedded generation for both separating self-sustained island during the defence phase of a system emergency situation and restarting small areas in the event of a system blackout. Concerning the network asset, the existing archive of components and systems is the basis for qualifying them for restoration. Compared to the ordinary running of the system, a relevant role is played by information and control systems, essential to remotely manage the plants even when unsupplied from the network; functional schemes of the plants should be then supplemented by energetic details, to analyze whether and how long the feeding of the auxiliary services of the station is guaranteed. Depending on the functionality of the station, one can set special thresholds or a number of operations which can be performed by the plant before being re-powered, or the time during which the emergency system guarantees the proper operation of the station. As to the topological and functional aspects of the network systems, the role they can play in the restoration process can be qualified on the basis of key factors, like the availability of local load and the possibility of managing the parallel operations between two portions of the network which are separately powered. 3.3. Improvement of systems for self-starting of units and for network energization 3.3.1. Adaptation of generating units to participate to the black-start Basically, the plants that are economically and technically fit to be transformed into first restart units are those plants needing just moderate starting capacities and times for which the main improvement consists in the installation of a self-powering system for the auxiliary services. Two main categories of interventions are being investigated by the Italian ISO. 3.3.1.1. Installation of new power sources. The typical power sources to be installed to make a power plant a black-start-up unit can be: • 2–3 MW diesel gensets, which can be fed with gas, so as to ensure a higher availability, no longer restricted by the fuel tank storage capacity; • static Watt var compensator (SWVC) devices, comprising a bidirectional forced-commutated inverter and an electrochemical storage system. 3.3.1.2. Interventions on procedures, security devices and automation logics. The analysis of the Italian generating park has highlighted the following issues to be dealt with: • Self-restarting of a suitable thermoelectric unit equipped with turbogas (re-powering plants, combined-cycle plants and simple-cycle GT plants). In the usual starting procedure

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of a GT plant the majority of the houseload is supplied by the grid either through a transformer dedicated to start-up or through the same auxiliary transformer used during the normal operation. For starting from a diesel genset, all the logics for switching the houseload need to be reviewed as well as the list of the loads, which are actually essential for the start-up6 to avoid over sizing the genset. • Self-restarting of a suitable hydroelectric unit or a suitable geothermoelectric unit. The logics to be set are similar to the previous ones, except that a static starter is not provided. • Load-rejection and operation on an insulated network for a conventional thermoelectric unit. The starting sequences of the burners and the flame control logics are to be cleared of any unavailability of field signals which require the operators’ local action. Under these operating conditions this would affect the start-up time and often the success of the attempted restarting of the system. The control of the turbine speed must be calibrated and made ready and steady, and its behaviour must be documented by a test of the extensive load reduction and stepped reconnection on an insulated network (these features are not usually requested in normal operation on an interconnected network). Sudden load variations in wider and faster steps than those of normal low-load running cause the boiler to be subject to fast changes in the operating regime. The regulation of the boiler pressure must be therefore calibrated and tested. • Load-rejection and operation on an insulated grid of a combined-cycle thermoelectric unit. The turbogas speed control system must be calibrated and made ready and steady by applying the same criteria as those used for the conventional units. In the usual operation, the pressure control system in the HRSG (heat recovery steam generator) and the turbine bypass logics are only subject to smooth transients or not used at all. The level and temperature control systems operates at constant high load for long periods as well. All these systems need to be carefully checked for being sure of their correct operation in the unusual island operating condition. 3.3.2. Use of HVDC systems The use of a self-commutated HVDC system for restarting the electric grid on the inverter side shows interesting prospects and opportunities. In principle, the HVDC connections could be reactivated after a blackout to feed the auxiliary services of a large thermoelectric power plant, sustaining at the same time the load variations required during the loading ramp process. In Italy, interesting prospects have to do with the presence of two HVDC connections located in strategic positions. The first one, which connects Sardinia to the Italian continent via Corsica, could re-power the ancillary services of one power plant of the northern Tyrrhenian coast. The second one, connecting Greece to Italy and recently built, could be used to restart a large thermoelectric power plant in southern Adriatic coast, since the

6

For example, the static starter if provided, the turbine barring engine and the strictly necessary auxiliary units, such as the bearing greasing system and the hydrogen sealing system.

surrounding area has no significant hydroelectric production and is very far from any foreign ac interconnection. The use of HVDC devices during restoration immediately raises a few problems to be solved. One concerns the supply of reactive power that both converters need to work; the second one concerns the sinusoidal alternated voltage system which is needed on the ac side of the self-commutated inverter to work; in addition, the current should be controlled (as the rectifier usually does) by the inverter according to the required load variations [8]. The basic idea for the operation of these systems during the restoration process consists in equipping the receiving power plant with a synchronous compensator started by an external source. The compensator will have to supply the necessary voltage reference to the inverter, the reactive power drained by the converter and a suitable level of short circuit power. Once the HVDC connection has been reactivated, the frequency measured at the synchronous compensator should be used to supply the error signal (against a desired value of 50 Hz) to the current control of the link. As a matter of fact, the compensator prime mover has the sole function to compensate the losses during the starting phase, while, after the link has been reactivated, the link itself will supply the losses and the prime mover can be disconnected. As a consequence, the compensator has no regulating capability and any unbalance between the power supplied by the link and the power of the load at the receiving end (losses included) is immediately reflected on the rotating speed of the compensator and finally on the receiving grid frequency. The frequency regulating loop on the link will therefore assign the power transfer and actually the current set-point on the dc side of the link. The compensator must be sized according to the following requirements: • supply the reactive power to the inverter, • provide enough short circuit capability to ensure the correct commutation of the valves, • ensure the stability of the control loops. The most restrictive requirement is the last one, especially when the inverter operates at a constant extinction angle (or the so called constant-γ operation).7 When the inverter operates with a control of the firing angle (usually called α), instead of the extinction angle, the stability margins are wider. Preliminary assessments have shown that for a 500MW link, the minimum short-circuit power to be supplied at the inverter terminals is roughly 2500 MVA for a constant-γ operated inverter and 1200 MVA for a constant-␣ operated one. The relevant sizes for the synchronous compensators to be used should be of 750 and 300 MVA. 7 The extinction angle γ is the angle which corresponds to the time interval between the extinction of the current in one valve, after the commutation transient, and the time at which the voltage across the valve becomes positive (direct polarisation). A minimum value for this angle must be ensured in any operating conditions to prevent the valve from remaining in the on state when the applied voltage becomes positive.

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A critical issue concerns the minimum value of current setpoint on the link. Such a limit, usually in the range of 10% of the rated current, actually means that as the link is activated it will immediately deliver at least 10% of its rated power into the receiving grid. Since the compensator has no regulating capability, a load must be promptly connected to avoid the frequency to rapidly reach unacceptable limits.8 A help to the operator is the high and prompt regulating capability of the link control above the minimum current margin. The operator should therefore be able to capture enough load in the neighbourhoods of the receiving terminal, no matter if it is sensitively larger than the minimum required. Presently, a detailed analysis of these aspects is being carried out by the authors, with numerical simulations, for the Italy–Greece link, accounting for the peculiar features of the HVDC system, such as the control logics and the protective equipment. 3.3.3. Use of storage systems The use of energy storage systems, such as BESS (battery energy storage systems), in extensive electric systems, is a recent application, although some extremely interesting examples can be found worldwide today. A high efficiency energy storage device can actually be installed in an electric system to serve several functions. The main ones can be summarized as follows: Generation

Ancillary services

Transmission and distribution

Users

Energy management Load leveling Peak generation Ramping/load following

Frequency response Spinning reserve Standby reserve Black-start

Voltage control

Energy management Load leveling Power quality System reliability

Power quality System reliability Black-start

The use of a storage system during the restoration process can differ with the type of system. Storage systems designed to have a high capacity, but a comparatively low energy, can be used to power the ancillary service of a power plant, in order to restart it. If the system has been designed for load-levelling, and has therefore more energy, it can be used for black-starting the power plant while performing the ramping and load following of the power plant and adjusting the frequency (this could be extremely useful, for instance, to restart the electric system by islands). Detailed studies have been carried out few years ago for providing 60 MW geothermal units of black-start capability [9]. Such plants are highly reliable and are used for roughly 8000 h per year. The likelihood to have it running just before a blackout is close to unity. Therefore, if provided with an auxiliary supply, they can be quickly restarted. The above mentioned studies have proved the technical feasibility of installing a 1 MW SWVC for supplying the essential loads (the condenser extraction pump, the exciter and some minor loads) thus enabling the prompt

8 50 MW on a 500 MVA compensator with inertia constant H = 1 s means a frequency derivative of 2.5 Hz/s on a 50 Hz system.

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restart of the unit which becomes able to supply the houseload of a neighbouring thermal power plant and to sustain its ramping by sharing the cold load pick-ups. Through storage systems, the asynchronous motors can be started at variable frequency, thus reducing the starting currents with respect to the use of diesel gensets driving a synchronous generator and keeping the size of the device at a reasonable value. 3.4. Definition of new restoration strategies 3.4.1. Problems related to the restoration by path The current restoration plan of the Italian electric system is equipped for restarting by paths, which mainly start from the hydroelectric power units and reach the main high-capacity thermoelectric power plants. Most paths are located in the north area, where the largest hydroelectric power plants are based, while just a few are located along the peninsula, because of the low number of hydroelectric power plants powerful enough to be used for the first restarting process. The building of these paths is considered a matter of priority and only after they have been developed can the service be restored through the building of secondary paths aimed at powering those load areas that are far from the primary paths. Such strategy, however, can result to be not flexible enough for exploiting possible resources not established in the Plan (plants that have tripped to houseload, network portions remained islanded) and hard to be updated to include new resources, which could supply black-start capacity (turbogas sections of simple-cycle turbogas power plants, of combined-cycle power plants and re-powering units). 3.4.2. Use of local networks to build up islands The present restoration strategy, which provides for the reconstruction of the 380–220 kV system backbone before the actual supply of the load, can result in fairly long times for picking up the loads that are not strictly necessary for the restoration process. In addition, those plants which, after a system failure, have successfully performed a load rejection operation risk being not used in the first restoration stages because they cannot be reconnected to the 380–220 kV grid and remaining isolated for long times, which would be time- and money-consuming. The number of thermoelectric power plants that can be directly connected to the 132–150 kV grid or connected to short 380 and 220 kV lines at power plants where local 60–132–150 kV networks can be powered is quite high. A large part of these plants is equipped to perform, with a good chance of success, the load rejection operation, so that they can be classed as “usable” plants as early as the first restarting stages. “Suitable” units too can be used to power load islands rather than for building restoration paths. In this case, as these plants have no particular problems with load ramps, they do not need any special care. Finally, “adjustable” units too, once suitable upgraded, are available to supply load islands. “Usable” plants, essentially consisting of thermoelectric power plants that have successfully completed a load rejection operation, can have quite stringent load ramp requirements that require some feasibility tests before they can be used in an isolated network. If they

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are started by a first restart unit, the control system of the two power plants would make the load ramp smooth by gradually transferring the load from the first start-up unit to the other one. Nevertheless, this procedure would require the building of a restoration path even when the thermoelectric power plant has tripped to houseload and needs therefore no external power source for the first restart. In the past, tests have been conducted for energizing network portions starting from tripped to houseload thermoelectric power plants, both drum boilers and UP boilers. The tests proved that, taking due caution in terms of maximum acceptable cold load pick-up, the plants can overcome the most critical load-following step and re-power the load islands. The same tests also proved that one single unit that has tripped to houseload can also re-power the auxiliary services of another unit in the same power plant and restart it. Lastly, note that many suitable and adjustable units are comparatively small, and often they are directly connected to the 132–150 kV grid rather than to the 380 kV grid. Using these units to start forming load islands rather than for restoring the whole system structure can make the procedure simpler by removing or reducing problems associated with: • need to energize long lines with no load and over-voltage problems at industrial frequencies; • need to energize large autotransformers to step up to higher voltage levels; • limited under-excitation capacity of the generators to absorb the reactive power produced by an unloaded network (65 Mvar/100 km for 380 kV lines, 15 Mvar/100 km for 220 kV lines) [3]; • problems in coordinating plants located very far from each other [4]. 3.4.3. Contribution of load to restoration The role played by the load in the restoration plans developed worldwide is a double one; it is both a means and the aim of the restoration of the electric system after a blackout. Especially in the first restoration stages, the load plays a key role as an instrument that enables the selected procedures to come to a successful end: in the first restarting stages, when the interconnected capacity is low, it is not always easy to maintain frequency within a suitable range and the regulating capability of the power units is very low, especially during the load ramp process. Furthermore, the value of the load that appears after a power outage can be significantly different from the value before the blackout and can also depend on the length of the outage. In addition, depending on the type of load, the dynamic response of the load can lead to temporary increases in absorption, up to three or four times the service value. An interesting opportunity to implement measures to make the load controllable during emergency and restoration phases is given by the campaign that was started in 2001 by the major distribution company in Italy for replacing approximately 30 millions electromechanical meters with electronic devices. Even if originally designed for market and managerial aims, the remote reading and remote control system associated with the

new meters could be very useful for the energy management of the electric system, sending load control signals from a remote centre to single utility devices. The very promising results of a reliability test campaign being carried out by the mentioned distribution company confirm in perspective the possible use of this communication system for system purposes. One of the first possible levels is sending signals that open the switch and cut off the user in case of critical conditions on the grid and in case of interruptible contracts. A second level is sending signals for changing the available capacity, in connection with some contractual term (times, prices, etc.) or in connection with system emergency. A third level is controlling each single load through the direct control of one switch or through dialogue with a load management system available in larger users, if provided. At a defensive stage, these types of services can be useful for performing more specific adjustments of the loads and often, with very quick intervals, for overcoming the critical stages and re-powering the whole load in just a few minutes. Likewise, during the restoration of the system, actions could be taken to speed up the restoration time by giving priority to the number of re-powered users rather than to the capacity provided to each of them. In many cases, the criticality of the restoration stages is due to the lengthening of the disconnection resulting from the low capacity, which is gradually provided by the power plants during the restoration process. This is because once supplied the users have no signals that could limit the consumption to just the essential services, leaving more capacity available for supplying other areas. Sending a signal to the meter for reducing the available capacity can help restore the service more quickly, in terms of number of re-powered users, even with a lower power availability. For household utilities, for instance, we can think of reducing 3 to 1 kW, which would still operate the essential appliances (refrigerators, basic lighting, heating fans, etc.). If there is no management system inside the user, the application of a load in excess of the threshold would make the switch trip. When rearmed, the user could read a message on the display, saying that availability is limited and that ask to take adequate measures. If a load management system exists, when changing the ‘available capacity’ signal, the loads to be left on and those to be cut off are to be redefined. The system is preset to be directly interfaced with the user power board; in this way, the loads can be internally selected, so as to give a list of priorities for the powering. So, when re-powering a utility, a signal can be sent for powering only those utilities that are connected downstream of the switch of the “essential services” and only when the capacity available in the grid is enough to power the other ones as well. With such a system, there is no need for the user’s manual action, since there is no reduction in the calibration of the general device, but only an exclusion of part of the loads. 3.5. Criteria for performance checking Failure to supply the service guaranteed by a plant during the restoration process causes remarkable damage to the system

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structure, not only in terms of service quality, as it happened in the past, but, especially in a deregulated scenario, in economic terms. In order to prevent such problems, compulsory tests on the reliability of the restarting services are regarded as essential for those plants which will be called to serve such function. The tests, monitored by additional measuring systems for recording the quantities relevant to the restoration functionality, should be carried out at first to certify that the plant is currently able to supply a given service and later on to guarantee that such service is maintained over time. Proposed tests have been divided into two groups, according to their complexity, in relation to the cost, time, staff and grid disturbance that they involve: • Tests on individual components of the network and equipment used for the arrangement of the restoration paths.9 Such tests have the advantage that they are easy to perform and can be repeated at reasonable costs in terms of human and/or plant resources (for instance, during a scheduled shutdown of the power unit). • Actual restoration tests for the paths or partial tests.10 Such tests are more demanding in terms of human resources, coordinated use of the plants and involvement of utilities. 4. Conclusions If deregulated power systems require tasks and responsibilities of different market players during emergencies to be clearly set, nowadays new technologies both in terms of power plants and communication systems can be suitably exploited to reduce the extension, the length and the social impact of a large blackout. The solutions proposed in this paper for the Italian system, mainly originated by an international survey into restoration

9 For example, tests of load-rejection, tests of self-starting capacity, verifications of the time it takes to perform the main actions required by the restoration plan, measures of the dummy-loads actually available for each restoration path. 10 For example, tests of load-rejection when disconnected for low frequency and voltage, tests of line energization from a “suitable” unit to a restarting path, tests of islanding for “isolable” plants.

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techniques and future improvement trends, are intended to increase the power available for the system black-start and to reduce load restoration times, managing the emergency resources with a higher degree of flexibility. In particular, the use of HVDC links for restoration, the contribution of local networks to build up islands and specific demand side management techniques seem to be worldwide acknowledged as interesting field of research to improve present restoration plans in a deregulated environment. References [1] M. Adibi, P. Clelland, L. Fink, H. Happ, R. Kafka, J. Raine, D. Scheurer, F. Trefny, Power system restoration—a task force report, IEEE Trans. Power Syst. 2 (2) (1997). [2] C.-C Liu, K.-L. Liou, R.F. Chu, A.T. Holen, Generation capability dispatch for bulk power system restoration: a knowledge-based approach, IEEE Trans. Power Syst. 8 (1) (1993). [3] M.M. Adibi, R.W. Alexander, D.P. Milanicz, Energizing high and extra-high voltage lines during restoration, IEEE Trans. Power Syst. 14 (3) (1999). [4] M.M. Adibi, D.P. Milanicz, Estimating restoration duration, IEEE Trans. Power Syst. 14 (4) (1999). [5] CIGRE´ Working Group 34.08, Isolation and restoration policies against system collapse, CIGRE´ Brochure, April 2002. [6] J.D.Willson, Operator Training Working Group, System restoration guidelines: how to set up, conduct and evaluate a drill, IEEE Trans. Power Syst. 11 (3) (1996). [7] PJM Dispatching Operation Manual, December 2002, http://www.pjm.com. [8] IEEE Std 1204-1997, IEEE guide for planning dc links terminating at ac locations having low short-circuit capacities, IEEE Standards Board, June 1997. [9] S. Barsalia , A. Borghettib , B. Delfinoc , G.B. Denegric , R. Gigliolia , M. Invernizzic , C.A. Nuccib , M. Paoloneb (a Universit`a di Pisa, b Universit`a di Bologna, c Universit`a di Genova), Guidelines for the ISO operator aid and training for power system restoration in open electricity markets, IREP 2001 Bulk Power Systems Dynamics and Control—V, Security and Reliability in a Changing Environment, Onomichi City, Japan, August 26–31, 2001.