Reliable Controllable Reactive Power for the Extra High Voltage System By High Voltage Distributed Energy Resources Economic Attractiveness and Practical Implications Study committee A1 E. Kaempf1*, M. Braun (University of Kassel & Fraunhofer IWES, Germany) T. Stetz (Fraunhofer IWES, Germany) H. Abele (TransnetBW GmbH, Germany) S. Stepanescu (Netze BW GmbH, Germany)

reactive power consumption from underlying networks and for the reactive power requirements of electrical equipment in his own system.

Abstract The topic of providing controllable reactive power (CQ) to the next-higher voltage level based on controlling distributed energy resources (DER) has recently received increasing attention. From a theoretical perspective, the use case of deferring or avoiding Extra High Voltage (EHV) investment in compensating equipment by the ancillary service ‘provision of controllable reactive power from High Voltage (HV) to EHV’ would seem within future reach in certain cases. Despite the comparatively higher losses related to HV DER controllable reactive power provision, the use case is shown to be potentially economically attractive under certain circumstances. However, a considerable number of practical barriers exist that are presented in a systematic way based on a decision flowchart. Simulation results are based on a close-to-real German EHV/HV system and measured data. They highlight the benefit resulting from optimized dispatch of both EHV/HV tap changer and HV DER providing CQ.

To fulfill the task, the TSO utilizes mechanically switched compensating devices, synchronous generators connected to the transmission system and sometimes Flexible AC Transmission Systems (FACTS) devices or reactive power capability of certain types of High Voltage Direct Current (HVDC) substations. In certain regions of several countries – e.g. UK, Ireland, Germany, Spain, Poland – a significant locational penetration of wind and/or PV farms connected to the high voltage (HV) system is observed already to date or expected for the short to medium term future [1][7]. Frequently, grid codes oblige Distributed Energy Resources (DER) to be able to provide CQ free of charge to the HV system operator [8]-[12] . – The term ‚HV system operator‘ is preferred here over a choice of either ‚TSO‘ or ‚DSO‘ (Distribution System Operator), since in a number of countries, e.g. Switzerland, Germany, Poland, HV systems are operated by the DSO rather than by the TSO [13]. – In many cases the CQ will be utilized for HV system purposes, e.g. to stabilize the voltage at a weak point of common coupling (PCC). In these cases DER CQ will only sporadically be available for other applications. These cases do not qualify for the investigations carried out here. There is an increasing number of regions however, in which the abundance of CQ is to date – or is expected to become - such that providing reliable CQ as ancillary service from HV to Extra High Voltage (EHV) may be considered. This is the point of departure for the investigations carried out here.

1. Introduction 1.1. Motivation The paper at hand focuses on reactive power exchange at the interface of high voltage and extra high voltage systems. Typically, the Transmission System Operator (TSO) has – among others - the task to ensure adequate availability and dispatch of controllable reactive power (CQ). He will compensate both for the residual of vertical * [email protected]

Keywords Ancillary Services, Compensating Equipment, Distributed Energy Resources, Reactive Power Provision

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Beyond the deferral of investment in EHV compensators further use cases may be realized by HV CQ provision. For instance ‘volt/var motivated redispatch’ of conventional generation may be an issue from TSO perspective: In this case, due to locational shortage of CQ, conventional generators currently not in service might need to be taken into service, and active power generation in some other part of the system reduced. Or an existing fossilfuelled generator would need to change active power output in order to be able to supply more CQ. In both cases, the generators may need to be compensated by the TSO for the change made to their planned schedule.

The largest reactive compensating devices with the lowest losses will usually be connected to EHV. It is therefore of particular interest to compare the economic effectiveness of reactive power provision from HV DER to that of EHV reactive power resources. At hand, the EHV connected capacitor – a low-cost low-loss EHV resource of CQ - is chosen as benchmark for comparison with provision of CQ from HV DER units. One might expect that under these conditions HV DER CQ cannot compete: Based on the simulation of a close-to real German EHV/HV system it is shown that indeed even loss-minimal dispatch of HV DER units will frequently increase losses in the HV system. The increase in network and DER losses – expressed per Mvarh delivered – is in many cases significantly larger than losses related to providing overexcited reactive power from an EHV capacitor. In addition, depending on the grid code, HV DER units may need to be paid for parts of their reactive power provision.

The paper at hand channels the complexity of the discussed scenario into a decision flowchart that makes transparent how the potential area of applicability is narrowed down by a large number of practical considerations. Differences between the use cases are highlighted. The discussion is accompanied and enhanced by simulation results from a real German EHV/HV system: The influence of EHV/ HV transformer control strategy on the resulting feasible reactive power bandwidth under constraining operating conditions is demonstrated.

Yet, this fact is only the beginning of the analyses carried out here: If an EHV capacitor with expected short operating times is planned to be installed, this is equivalent to a very high specific cost of reactive power, i.e. a high cost per utilized Mvarh. The high cost is due to the investment and capital costs related to the investment in an EHV capacitor. HV DER CQ will be the more competitive, the more it is already paid off, i.e. reactive power can be provided at marginal cost. When it is not requested, it does not cause any costs. Should investment in DER – e.g. retrofit – be required exclusively to produce the desired HV CQ quantity and reliability, this competitive advantage is reduced.

1.2. Definitions and Scope Generally, favorable and non-favorable connecting points of DER to the power system may be distinguished: A favorable connecting point allows to integrate the full reactive power capability from connected resources based on the existing EHV/HV transformer control strategy without causing unwanted voltage or loading conditions. Existing utility-owned fossil-fuelled or hydro HV generators are frequently connected to such favorable points.

The specific examples simulated focus on wind power plants capable of providing reactive power even at zero active power output. The results are generally applicable to so far not utilized potentially controllable HV reactive resources whose reactive power control capability is financed by other use cases. Further examples are e.g. suitable PV plants or industrial synchronous generators so far frequently operated to maintain a fixed power factor.

The contribution at hand focuses on making available CQ from resources previously not actively utilized for managing the reactive power exchange, e.g. DER or industry generators. While in the following, the term DER is used for simplicity, the results may be transferred to industry generators c. p.. Focus here is on provision of controllable reactive power, i.e. on a situation where reactive power output of HV DER is influenced online according to the varying requirements of the EHV system operator with the aim of producing a defined exchange of reactive power between EHV and HV.

For the vast majority of worldwide HV systems the discussed scenario will seem rather hypothetic: Yet, with the observed fast expansion of installed DER - e.g. wind – capacity in many countries it may be of interest to analyze the ICT, control infrastructure and grid code requirements required for the discussed application.

In literature – e.g. [14], [15], this topic of CQ provision to the next-higher voltage level has so far usually been

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The research topic of CQ provision from HV to EHV addresses a situation in which EHV network planning / operation is not (yet) taking into account the possibilities available from HV resources. This may occur e.g. for the following reasons:

treated in the context of integrating CQ from DER in situations where the interface of two voltage levels coincided with the interface between TSO and DSO, Figure 1.

- HV is operated by the DSO, and therefore there is an organizational boundary - HV did previously not contain a significant amount of controllable resources that were technically able to provide CQ - EHV has so far had such an abundance of CQ available that there was no point in making the effort of looking into HV resources.

Figure 1: Sketch of HV DER Q Provision for DSO-operated HV

The article at hand will thus only be of interest for systems facing change in at least one of the above categories: At heart, a situation is addressed, where it shall be investigated whether enlarging the horizon of analyses – e.g. in the above-listed ways - may produce economic benefit. Hereafter, the effect of potential organizational boundaries between HV and EHV system operator is not further elaborated on, focus is on the principally achievable potential that can be reached by coordinated operation of the EHV/HV system.

Figure 2: Sketch of HV DER Q Provision for TSO-Operated HV

If HV is operated by the TSO – situation represented in Figure 2 - the reactive power provision process discussed here will not be noticed as a separate process any more. If the TSO possesses the ICT infrastructure described e.g. in [16], reactive power provision will be integrated into the TSO closed-loop contingency constrained optimal volt-var control. HV network areas capable of providing CQ could in this case be considered as individual voltage control zones. Alternatively, they could be integrated as sub-entities into an existing voltage control zone. Contingency constrained volt-var dispatch – the tertiary voltage control – will automatically ensure that the most cost-effective controllable resource is activated while maintaining sufficient reserves. For a closer discussion of the concept of primary, secondary and tertiary voltage control refer to [16]. Research interest in the article at hand is in this case focusing on the question whether it makes sense to integrate distributed controllable HV reactive power resources into EHV tertiary voltage control. This again is determined by how much dependable potential can be obtained from them, and at what cost. While costs of network losses are discussed here, costs related to obtaining CQ from HV resources will depend on the grid codes and possibly on additional contracts negotiated bilaterally.

1.3. Literature Review A comprehensive literature review on the topic may be found in [17]. An overview of principally relevant research questions is given in Figure 3. Kaempf et al. [18] was the first source to provide a systematic overview of use cases related to the topic. For any use case analyzed it must be ensured that an economically competitive, sufficient amount of CQ is provided at sufficient reliability. The requirements related to the term ‘sufficient’ – e.g. sufficient reliability, sufficient amount will depend on each particular use case investigated. First research results discussing feasible amounts and economic competitiveness are available [17]-[19]. In [20] it is pointed out that optimal power flow based dispatch of both DER and transformer tap changers yields relevant reactive power bandwidths even under constrained operating conditions, thus providing a contribution to the discussion of required ICT infrastructure. The question of long-term availability of HV reactive power has not been discussed so far. Neither has the topic of

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The paper is organized as follows: Section 2 describes the simulated EHV/HV system. Section 3 ‘Methodology’ derives – among others - the choice of simulated DER penetration and network infrastructure. Section 4 ‘Simulation Results’ presents achieved bandwidths of CQ for different scenarios and control strategies. The specific cost of CQ provision based on control of HV DER is presented.

sufficient reliability been investigated systematically for HV DER CQ provision. Both are most relevant when use cases related to TSO network planning are investigated: Deferring EHV capacitor investment is the comparatively most challenging use case in terms of the requirements for reliability, amount and long-term availability.

Section 5 starts by presenting the specific reactive power costs related to the investment in an EHV capacitor. Subsequently, the practical requirements related to the use case of deferring or replaying EHV capacitor investment by HV DER based Q provision are discussed.

2. Model Description and Assumptions

Figure 3: Research Topics Related To CQ Provision

In the context of a comprehensive overview of all aspects of grid integration of wind generation the topic of providing CQ from subtransmission to transmission level is raised in [21]. It is judged that due to the fact that wind generators are frequently connected at weak points of the subtransmission system, wind generators are not able to provide ‘any substantial contribution to the reactive power balance at the transmission level’. The above statements were – as may be seen in [21] – motivated by the investigation into the impact of wind generators on system stability in South Australia.

The simulated system shall be systematically characterized which allows to more easily estimate applicability of the considerations in the readers’ own target environment. 2.1. EHV / HV System A real HV system and its related EHV subset are modelled. The EHV subset consists of a detailed part, and the adjacent external network represented by network equivalents. The subset of EHV modelled in detail consists of a total of 35 substations on 380 and 220 kV levels. It is part of an interconnected transmission system in which a minimum amount of transmission connected synchronous generation is always online [22]. The ‘connections to further modelled EHV nodes’ in Figure 4 merely summarize the modelled complex EHV system on a very schematic aggregation level, without distinguishing between internal and external network.

1.4. Contribution and Outline Based on the analysis of a German EHV/HV system the contribution at hand takes up the concerns raised in [21] in the following ways: - First, it is shown that the in southern Germany currently widely practiced local voltage control of EHV/HV transformers will – under certain conditions – indeed lead to unsatisfactory minimal ratios of installed CQ from DER to CQ that can be made available to EHV. - Second it is shown that this minimal ratio may be substantially improved by introducing optimalpower flow based computation of setpoints for both EHV/HV transformer tap positions and DER reactive power output. - Third, an economic analysis compares specific costs of EHV capacitors to the cost of reactive power provision obtained for the simulated network.

The modelled EHV subset supplies two HV network areas. One of these is the network area discussed here. This HV study network area is connected to EHV via six transformers, two of which operated in parallel. The HV system supplies 73 medium voltage (MV) systems. The most direct EHV access for most of the ten HV wind farms is via EHV/HV transformers T2 and T3a, T3b. As compared to the HV system status investigated in [17] already, some lines considered to be shortly before realization of an expansion in [17] are assumed to be successfully upgraded here. The distance between

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two EHV/HV substations, e.g. between T2 and T3 is usually less than 55 km. Wind farms are sized between Pn = 10 MW and Pn = 58 MW. The distance between HV wind farms and the closest EHV coupling point is usually less than 45 km. HV-side measurements of 15 minute averaged active and reactive power exchange with MV systems were used. For EHV, the peak load scenario was available and implemented. In this way the typical planning condition of a German HV system operator is largely replicated, who usually has one or two characteristic network loading cases to represent EHV in his simulations.

0.9 at nominal power output. The broad green curve in Figure 5 indicates the assumption made here, while the blue lines delineate the grid code requirements relevant for the simulated system [25]. The reliable generation of reactive power assumed here may be obtained in the following ways: (i) By contractually agreed retrofit of existing DER It may be worthwhile investigating integration of the frequently existing park capacitors connected to the park collector bus into the reactive power management during zero active power output: This is possible, when implemented in the context of centralized coordinated control of EHV/HV tap changers and park capacitors, as shown in [17]. The advantages of combining capacitors and DER reactive power control have already been pointed out in [26]. Beyond that, numerous possibilities for retrofit of existing plants with FACTS devices or with wind generators containing STATCOM functionality exist [27]. (ii) By deciding to change requirements for network connection in the future, or by foreseeing such capability when connecting new generators based on bilateral agreements.

Figure 4: Schematic Representation of the EHV/HV System

2.2. Choice of simulated time period

(iii) By combining different types of resources, e.g. fossil fuelled industry generators and DER in a costminimal and reliable way.

Purpose here is to assess the potential of a partially highly wind-penetrated HV system to provide overexcited CQ under challenging conditions. Provision of overexcited reactive power will usually be limited either by high voltages or by equipment loading. Therefore, in the investigated system, the high-feed-in low-load scenario constitutes a particularly challenging situation. Results are presented for a day featuring annual peak feed-back from MV systems combined with varying degrees of HV feed-in from wind power plants.

Especially to introduce utilization of existing DER collector bus capacitors may significantly increase the possibility to provide low-cost overexcited reactive power, as discussed in [17].

2.3. CQ from HV DER A significant variety of specifications regarding static reactive power provision exists internationally [12], [23], [24]. In this case study capabilities offered by manufacturers since several years – but usually not yet installed in the field – are assumed. The ability to provide overexcited reactive power Q/Pn=0.484 from zero active power generation to nominal power generation is assumed, Figure 5. This is equivalent to a power factor of

Figure 5: DER PQ Capability Assumptions

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3.2. Choice of DER penetration

It is assumed here that in reality, when utilizing DER in the context of a volt-var Optimal Power Flow (OPF), DER would receive voltage setpoints, see e.g. [28].

The following investigation shall assess whether the given system is able to provide a significant amount of CQ even under constrained operating conditions. Ideally, the obtained results should be applicable for a longer time span. One possible approach to solve this problem consists of varying DER penetration, carrying out network expansion measures if necessary, and assessing the resulting CQ bandwidth for different DER penetrations.

3. Methodology 3.1. Definition of Reactive Power Bandwidth HV network areas are galvanically coupled regions that mostly have two or more connection points with EHV. Reactive power exchange is here studied on a network area basis: The residual of reactive power exchange at all network area EHV/HV connection points – Figure 6 is computed once for the base case – depicted in cyan in Figure 7 - and once for the optimized case, depicted in brown in Figure 7. The difference between the two is the achievable bandwidth of controllable overexcited reactive power, indicated by green arrows. In this contribution, only overexcited CQ is analyzed, therefore the term ‘overexcited’ is usually omitted when referring to the bandwidth in the following. The term ‘reactive flexibility’ is used as synonym to the term ‘bandwidth’ here. The consumer oriented counting system is applied throughout this contribution.

Here, a different approach is taken. Figure 8 illustrates the influence factors on the resulting CQ bandwidth of any given HV system.

Figure 8: Factors Determining Reactive Power Bandwidth

CQ provision is about making use of the remaining loading capacity, before a loading limit is hit, and making use of the remaining voltage bandwidth, before the CQprovision-relevant voltage limit is reached. The more frequently the system is close to some limit already in its base case, the harder it is to transport additional CQ. Therefore, the moment when a network expansion measure is being planned, but has not yet been carried out, is the most critical moment in the circle of increased DER penetration and increased network expansion. Therefore in this study, a DER penetration is simulated that results in base case loading and voltage conditions being close to permissible limits.

Figure 6: Reactive Power Provision on Network Area Level

3.3. Assess Reactive Power Bandwidth Based on Operational Constraints: Contingency-Constrained Optimal Power Flow If network expansion is oriented towards hosting DER

Figure 7: Bandwidth of Controllable Reactive Power

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MV systems. Measured values in 15 minute resolution at the HV terminals of HV/MV transformers were available, and one feed-in measurement from a wind farm. The selected day features the peak annual feedback from MV.

at unity power factor, it is of particular importance to verify the feasible reactive power bandwidth based on a contingency constrained dispatch, Figure 9. In this way it is ensured that

- A second scenario (‘Wind Strong’) explores CQ provision under severely limited grid capacity: The measured values from MV systems are combined with wind feed-in such that the permissible n-1 relevant loading limit of the transformer is reached between 11:00 and 18:00. This scenario could occur for a few hours per year, considering regionally possible maximum coincidence of wind and PV [29]. DER are continually being added to the system. Network expansion measures to accommodate increasing DER capacities tend to take several years from planning to implementation [30]. Thus, in the context of simulating dependable CQ provision from HV to EHV, it is an interesting and relevant scenario to investigate CQ provision in the event of capacity bottlenecks. 3.4. Formulation of the optimization problem Cost minimal delivery of reactive flexibility is simulated. The minimum overexcited flexibility to be delivered (QFlexox,min) is the constraint, objective being cost minimization:

Figure 9: Expected Advantage from Optimizing EHV/HV Taps

the dispatch solution fulfills the requirement relevant for operation of HV systems: N-1 security for loads must be observed at each time step. This approach allows to verify in how far the high reliability ancillary service provision aimed at here may be practically feasible, and at what costs.

Minimize: Cost (t) = CostHVLineloss (t) + Cost EHVHVTrfLoss (t) (1)

subject to

If the base case investigated consists of a power system already operated close to the permissible limits, as is the case here, integrating additional control variables that allow to redistribute power flows – here: EHV/ HV transformer tap changers – is expected to increase the feasible reactive power bandwidth. This will be demonstrated in section 4.

QEHVHVetwArea (t)

QFlexox,min

A loss-cost of 40 Euro/MWh was assumed, which corresponds to the average of the average annual EPEX spot market price ‘Phelix Day Base’ of the years 2012 and 2013 [31]. Cost of transformer tap-changing is not considered. For an analysis of its impact refer to [17].

While network planning may frequently be based on analyzing selected loading conditions only, it is preferred here to carry out time series simulations for the assessment of highly reliable reactive power bandwidths. Two scenarios are investigated:

Optimal power flow was implemented using the heuristic optimization algorithm Mean Variance Mapping Optimization (MVMO) [32], whose superior performance for solving mixed-integer reactive power problems was shown e.g. in [33]. For further details regarding the chosen implementation of single swarm

- The first scenario is based on the measured coincidence of HV wind feed-in and feed-back from

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MVMO see [17]. Loading limits are assessed based on the outcome of contingency screening: For lines, usually either 70 % or 50 % loading limit under normal operating conditions result. Throughout the scenarios analyzed here, transformer T2 experiences highest loading: The limit for T2 is determined to be 61.4 % based on contingency analyses. Max. permissible voltage for 110 kV

1.100 p.u.

Min. permissible voltage for 110 kV

0.982 p.u.

Max. permissible voltage for EHV side of EHV/HV transformer

1.1030 p.u. (380 kV); 1.1305 p.u. (220 kV)

Min. permissible voltage for EHV side of EHV/HV transformer

1.0650 p.u. (380 kV); 1.0363 p.u. (220 kV)

protection in case of an outage of T2. Such curtailment would be a rare event in reality. Yet, to achieve highreliability CQ provision, the feasible CQ bandwidth under this kind of condition should be verified as well, at least in systems facing considerable DER expansion. The residual of network area MV load (green) is assumed to be the same for both the ‘Wind Strong’ and the ‘Wind Measured’ scenario.

Table I: Voltage limits observed by OPF

Contingency analysis here focuses on the moment immediately after a contingency, i.e. before DER voltage control can adjust reactive power output and before EHV/ HV taps change position. For this short moment here maximum voltages of 1.105 p.u. have been permitted, otherwise values of Table I apply.

5. Simulation Results Table II provides an overview of the scenarios and control strategies whose results are discussed in the following. Strategy Scenario

Figure 10: Overview of Simulated Active Power Components

Figure 11 and Figure 12 highlight the resulting network area active and reactive power exchanges with EHV. The base case for assessing the reactive power bandwidth consists of the reactive power exchange of the particular scenario: the blue line in Figure 11 represents the base case reactive power exchange for scenario ‘Wind Measured’.

Base Case: EHV/HV Taps EHV/HV EHV/HV Taps Control Variable in Taps Voltage Voltage Con- OPF(Optimized), Controlled & trolled (Local) together with HV DER Q = 0 DER Q

HV Wind Measured

x

x

x

HV Wind Strong

x

x

x

Table II: Simulated Scenarios and Strategies

5.1. Base Scenarios: Wind Measured and Wind Strong. Scenario ‘Wind Measured’ allows to assess realistic frequency distributions of specific loss costs of providing CQ. Scenario ‘Wind Strong’ allows to analyze the network area reactive power bandwidth under challenging normal operating conditions. Figure 10 features both the measured (cyan plain) and the strong (cyan dotted) HV wind feed-in. The strong feed-in is slightly reduced by curtailment between 11:00 and 18:00. Curtailment is required to make sure that lines do not get tripped by

Figure 11: Base Case HV Wind Measured: Network Area P & Q

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normal operating conditions. This clearly highlights the necessity to study reactive power ancillary service provision to EHV based on network operation considerations, rather than purely on network planning assumptions.

Figure 12: Base Case HV Wind Strong: Network Area P and Q

5.2. Analysis of Achieved CQ Bandwidths The assumed installed DER Q bandwidth amounts to 146 Mvar overexcited. It is here aimed at providing 100 Mvar of overexcited CQ to EHV, which corresponds to the size of typical smaller EHV capacitors, and is equivalent to 68 % of installed CQ. Figure 13 displays the achieved reactive power bandwidth for the different scenarios and control strategies implemented. The severe limitation of bandwidth for local voltage control of EHV/HV transformer taps during daytimes is clearly visible.

Figure 13: Network Area Q Bandwidth for Different Strategies

As expected from the analysis in Figure 9, the integration of EHV/HV transformer tap positions as variables into the optimization permits to achieve substantially improved bandwidths throughout difficult operating conditions. From Figure 14 follows that in scenario ‘Wind Strong.’ between approximately 10:30 and 18:00 the permissible loading limit is reached. In fact, curtailment is required to observe loading limits. Curtailment is assumed to be carried out in steps of 10 % of installed DER capacity. This stepwise active power reduction – see Figure 10 - results in slightly added freedom available for reactive power at certain times – compare to the red dash-dotted line in Figure 13. If curtailment was carried out in steps of 5 %, less reactive power could be made available, especially in the ‘EHV/HV Tap Local’ strategy. It shall be pointed out here that the limitation in bandwidth observed for the locally voltage controlled EHV/HV taps in Figure 13 would not be observed if the n-1 contingency constraint had not been imposed. By simulations it was determined that if the post-contingency voltage limit was chosen to be higher, e.g. 1.2 p.u., or no n-1 constraint was imposed, bandwidths would amount to 120 Mvar and more, while still observing the voltage bands of Table I under

Figure 14: Loading of Transformer T2 for Different Strategies

5.3. Analysis of Optimized EHV/HV Tap Control Effects The ability of integrated ‘Tap + DER’ optimization to

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shift power flows shall now be analyzed in more detail. Figure 15 and Figure 16 highlight the difference in network voltage level resulting from the two control strategies. For locally voltage controlled EHV/HV taps voltages remain within a band between 1.03 and 1.08 p.u. The optimization of transformer taps allowed to make use of the full available specified voltage band until the lower boundary of 0.982. The upper voltage boundary of 1.1 p.u. - Table I - is not reached, since corresponding solutions do not observe the voltage boundaries in the n-1 contingency calculations.

DER frequency distribution of reactive power during the simulated 25 h is represented for each DER by a twosided bean plot in Figure 17. Each bean plot consists of two sides (red and blue) which allows to compare DER reactive power for the two control strategies. The blue frequency distributions of reactive power values are clearly displaced towards negative, i.e. overexcited values, as compared to the red distributions that stand for locally voltage controlled EHV/HV taps. DER are sorted by the grid strength of their PCC, defined as the ratio of short circuit power Sk’’ over nominal active DER power Pn, see [23]. Summarizing, the optimized control of EHV/HV taps allowed to operate even those DER at least partially in overexcited mode that have fairly low grid strength. Bean plots have been created using package ‘beanplot’ [34] from software ‘R’.

Figure 15: Voltage Level: DER Optimized, EHV/HV Taps Local

Figure 16: Voltage Level: DER + EHV/HV Taps Optimized

Figure 17: HV DER PCC Grid Strength and CQ Contribution

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Figure 18: Specific Cost of HV Controllable Reactive Power

5.4. Cost of DER CQ-Provision for Simulated Period

5.5. Summary

The resulting PQ-capability curve for both the ‘Wind Measured’ and the ‘Wind Strong’ scenario is displayed in Figure 18. The ‘Base Case’ was chosen as reference for reactive power provision, therefore, no cost is associated (black dots). The specific cost of CQ provision is computed to:

Based on optimized dispatch of both EHV/HV transformers and HV DER reactive power output it was possible in simulations to reliably provide at least 57 % (83 Mvar) of assumed installed DER reactive power capability on the day featuring 2013 annual peak feed-back towards HV for this network area. Local voltage control of EHV/HV transformer taps allowed to obtain bandwidths between -10 and -55 Mvar, depending on the network loading condition. Thus, in the investigated network area, the widely practiced local voltage control of EHV/HV taps does not qualify for providing large quantities of CQ in a reliable way to EHV, if n-1 contingency constraints are to be observed.

SpCQCScen i =

Term SpCQC

CCtr,Scen i _CBase,Scen i

(2)

(QNA,Ctr,Scen i _QNA,Base,Scen i)t

Unit

Explanation

-

Scenario i: i=1: HV wind as measured i=2: HV wind strong

Ctr j

-

Controlled Case j: j=1: DER Optimized., EHV/HV Tap Voltage Controlled (Local) j=2: DER + EHV/HV Tap Optimized

Base

-

Base Case: DER Q = 0, EHV/HV Tap Voltage Controlled (Local)

C

€

Cost = Loss Cost * Losses, see (1)

QNA

Mvar

t

h

Scen i

Cost of CQ provision varies substantially depending on network operating conditions. Values between a saving of 0.23 €/Mvarh and a cost of 0.53 €/Mvarh were observed. Due to the target of achieving a bandwidth of 100 Mvar overexcited, frequently the loss-minimization mode of optimal power flow was not reached, and rather a maximization of reactive power bandwidth was carried out. The cost figures do not include the additional cost caused by CQ delivery inside the DER parks.

€/Mvarh Specific Cost of Controlled Reactive Power

To implement DER based HV CQ provision as discussed here, it would need to be ensured that DER are not disconnected from the system for other reasons, e.g. due to being part of a virtual power plant that contributes to the reduction of overfrequency by occasionally completely disconnecting DER from the grid.

Residual of Network Area Reactive Power Exchange with EHV, as measured at the EHV terminals of EHV/HV transformers Time

6. Economic attractiveness of highly dependable controllable reactive power from HV

Table III: Abbreviations used in (2)

Providing larger amounts of CQ under the simulated difficult operating conditions comes at the cost of higher losses, as may be observed by comparing the colors of filled (EHV/HV Tap local) and empty (EHV/HV Tap Optimized) symbols of same shape (e.g. circles).

6.1. Specific reactive power cost of EHV capacitors At hand it is assumed that the TSO alternative to sourcing

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Figure 19: Cost Components Considered in Cost Comparison

Figure 20: Specific Cost of Reactive Power from EHV Capacitors

overexcited reactive power from HV DER consists of installing an EHV-connected capacitor. Principally the TSO may have further investment alternatives at hand, e.g. FACTS devices. These usually have the capability to provide further ancillary services and therefore assessing the specific cost per Mvarh provided while considering investment and capital costs tends to be more complex. EHV capacitors are being installed in many countries. They are a typical source of voltage support for slow phenomena [21]. Here the aim is to compare reactive power cost of EHV capacitors to that caused by CQ provision from HV DER as illustrated above.

capacitor incl. its EHV switching field, and operational costs of 2 % of investment costs per year [35]. Loss costs were assumed to 40 €/MWh, see section 3.4. Not considered are possible long term price increases for operational costs and loss costs. From Figure 20 follows that if the capacitor is switched on only during rare contingency events, the specific costs per Mvarh delivered may become very high. Once an EHV capacitor has been purchased, it is characterized by extraordinarily low losses in the order of 0.35 kW/Mvar1, i.e. 0.014 Eur/Mvarh under the assumed loss cost. Specific reactive power cost of EHV capacitors is thus dominated by investment and capital cost.

Generally, the cost per Mvarh of reactive power delivered is influenced by the cost components shown in Figure 19. Here, costs from the perspective of the network operator(s) are computed, without distinguishing whether they occur on EHV or on HV side. Generally, HV DER CQ requires ICT & OPF integration of substantially more components than CQ from EHV capacitors. Therefore in Figure 19, ICT & OPF integration costs of the EHV capacitor are not indicated separately. ICT and OPF integration costs of HV DER CQ may constitute a relevant cost component. They require more detailed analysis to take into account potential synergies with other use cases and further ancillary services and are therefore not considered here.

With respect to HV DER CQ provision from the analyses of the previous sections the following may be concluded: The higher the amount of CQ available free of charge to the DSO due to grid codes, and the higher the requirements of grid codes with respect to providing CQ even at zero active power output, the lower the relevant fixed costs for making available reliable CQ at times of zero DER active power output. Especially if capacitors connected at the collector bus of many existing DER parks exist and are utilized, this results in a low-loss, highly available source of CQ. Moreover, this resource is already paid off, i.e. beyond the potentially required upgrade of Information and Communication Technology (ICT) and Control Infrastructure, no additional investment and capital costs ensue. If modern PV inverters are used, able to provide reactive power down to 3 % of active power output without installing additional compensating equipment, and EHV demand for reactive power is expected to occur only during daytimes, then utilityscale PV plant based reactive power provision may also be a very attractive option [36].

Maintenance cost of HV DER would be in the responsibility of the HV DER owner. However, the higher number and different nature of components to be managed in the context of HV DER CQ will cause increased administration costs for the network operator. DER CQ is assumed to be available free of charge and potentially added losses within DER parks are not considered. Based on these assumptions Figure 20 shows the specific cost of reactive power from EHV capacitors as a function of capacitor utilization rate. The figure was compiled assuming a cost of 3.3 Mio. Euro for the 100 Mvar

Courtesy TransnetBW GmbH: Value applies to EHV mechanically switched capacitor with damping network of 250 Mvar operated at 420 kV. It includes main and ancillary capacitors as well as the inductance and damping resistor. Losses related to harmonic currents are not considered. 1

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power in the event of a contingency of a heavily loaded transmission line.

Summarizing, in many cases the dominating cost component of providing CQ based on HV DER will be the operational costs (losses, administration). Unlike an EHV capacitor – characterized by long equipment lifetime – the DER based reactive power of HV network areas may be flexibly adjusted to the EHV requirements. Therefore, the risk of investing into an EHV capacitor that may not be needed any more a decade later due to changed system operating conditions, could be avoided by recurring to CQ from HV. Thus, from the perspective prior to the investment into an EHV capacitor, it seems attractive to contemplate the HV alternative. The strong points of this alternative in case of expected low EHV capacitor utilization rates having been listed, the next section is devoted to discussing the practical impediments related to such an undertaking. 6.2. Requirements related to the use case of deferring or replacing EHV capacitor investment The decision flowchart of Figure 21 summarizes the considerations: Point of departure for the investigation is the intention to install an EHV capacitor characterized by low expected utilization rates. The first question to be clarified is whether the HV system has potentially CQ resources that have not yet been considered in the dimensioning of the EHV capacitor. If this is the case, the conditions motivating the EHV capacitor installation should be clearly defined, in order to derive the conditions under which HV CQ would be utilized.

Figure 21: Decision Flowchart

A possible scenario might be to compensate for the increase of PV-feed-in related reactive power consumption in LV and MV systems under conditions of high solar irradiation and low load (‘target condition’), see [37], [38]. This would limit relevant CQ delivery to daylight times. Thus, modern PV plants capable of providing reactive power support down to very low active power feed-in would be the ideal candidate for compensating this reactive power consumption from MV and LV systems on HV level. This compensation may in fact be complemented by similar action from MV PV plants.

Step 3 is devoted to assessing the available CQ from HV under the defined target conditions. Step 4 is of particular importance: In many cases it may be required to study HV CQ bandwidth under non-normal and contingency operating conditions, in order to satisfy the reliability requirements from EHV. The combined results of steps 3 and 4 will determine whether the CQ that can be provided at the required reliability level is sufficient. This check will rule out a large number of potential applications. During the check the CQ potential of neighboring network areas might also be considered, if tolerable from EHV perspective.

The most challenging outcome of this step consists of an EHV reactive power requirement that is uncorrelated with HV reactive power capability. Such situation may arise when the capacitor is installed to provide reactive

Check 7 constitutes a further stepping stone for the considered use case: If retrofit of ICT and control infrastructure of DSO and DER is required, and possibly

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implemented based on the heuristic optimization Mean Variance Mapping Optimization. This allowed to analyze HV CQ capability by taking into account the operational constraint of guaranteeing HV n-1 security at every time step.

also negotiation of contracts with DER owners or introduction of a corresponding market, implementation time may in some cases substantially exceed the time required for ordering and installing an EHV capacitor. Finally, the different dynamics of HV system evolvement as compared to that of a static capacitor installation must be considered: From TSO perspective, HV CQ would need to be dependably available for a longer time span, e.g. five years or more, to impact (e.g. delay) a capacitor investment decision.

As base case todays’ situation in the studied area was chosen, in which HV DER operate at unity power factor. CQ was defined as the deviation of reactive power exchange with EHV in the controlled case from the base case. It was shown that it would be possible to provide a sustained amount of 83 Mvar of overexcited CQ to EHV even under challenging normal operating conditions under the given assumptions. The closer the system is operated to its limits in the base case already, the more beneficial it is to have the added degree of freedom inherent in centrally optimized, coordinated dispatch of EHV/HV tap changers and DER reactive power.

Summarizing, the stepping stones ‘sufficient quantity’, ‘sufficient reliability’, ‘short implementation time’ and ‘sustained availability over several years’ may rule out the discussed use case in many cases. Yet, deferral of EHV capacitor investment is certainly not the only possible use case for HV CQ: In some EHV systems, volt/var motivated redispatch of conventional generation may be an issue, see section 1.1. Requirements related to this use case of ‘avoiding volt/var related redispatch’ are far lower than those discussed so far: Rather than being available in a sustained way for years, it is mostly sufficient if HV CQ can be forecasted with high precision and is then reliably available in a time span of several hours: This requirement is likely to be met by far more systems already today, given the high feasible forecast accuracies for active power today [39]. The required ability to provide substantial amounts of HV DER CQ under challenging operating conditions to EHV has been demonstrated here for a sample close to real system.

The economic attractiveness of HV DER based CQ was compared to that of a planned EHV capacitor installation with low expected utilization rates: HV CQ may in many cases come at comparatively low investment and capital cost, since the capability to provide CQ is to a certain degree available free of charge due to grid codes. It has been shown that investment in HV DER CQ can be economically competitive with EHV capacitor investment, if it is thus possible to completely substitute an otherwise required EHV capacitor investment. A decision diagram has been developed highlighting the stepping stones of utilizing HV DER CQ to replace or delay EHV capacitor investment. The hurdles to implementing this use case were found to be comparatively high in many cases. One critical point is the requirement to provide highly reliable reactive power: While change of HV operating conditions can be largely dealt with by integrating EHV/HV tap changers into the OPF as demonstrated, the capability to provide CQ during times of DER zero active power output is mostly not available with DER installed in todays’ power systems.

7. Conclusion and Outlook HV systems in certain cases possess controllable reactive resources that are to date not fully utilized. This may be because they have been added to the system only fairly recently, or because in the past, EHV CQ was so abundant that there was no need to take care of HV resources that are comparably more cumbersome to deal with. With increasing DER penetration and increasing DER ancillary service capability, this trend may be expected to increase.

Therefore those applications of the studied use case in which DER capability to provide CQ is correlated with the demand for CQ are most promising for short to medium term applications: A possible application in this context would be to compensate for the reactive power consumption from MV or LV DER with the same primary energy source as that of the controlled HV DER: e.g. HV wind farms compensating for reactive power consumption

Here, the capability of a close-to-real German HV system to supply reliable CQ to EHV has been analyzed. A contingency constrained optimal volt-var control was

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from MV DER, or HV PV parks compensating for reactive power consumption from MV and LV DER. Generally, the implementation of CQ provision from HV DER to EHV for various use cases is easier to realize if EHV and HV are both operated by the TSO.

[4] Irish Wind Energy Association, “Wind Farms in Ireland,” [Online]. Available: http://www.iwea.com/windfarmsinireland, last accessed: 29.04.2015. [5] Iberdrola, “Facilities Map,” [Online]. Available: www.iberdrola.es, last accessed: 29.04.2015. [6] TPA Horvath, “Wind Energy in Poland 2013,” [Online]. Available: http://www.psew.pl/en/biblioteka/raporty/, last accessed: 29.04.2015.

The optimal combination of reactive power provision at HV vs. reactive power provision / balancing at MV level is to date a field of research. One advantage of CQ provision by HV DER as compared to that by MV DER is that their integration into the DSO ICT infrastructure is frequently more advanced than that of MV DER, and the HV system status is fully known to the HV system operator, thus making OPF easier to implement.

[7] TheWindPowerNet, “Countries,” [Online]. Available: http://www. thewindpower.net/, last accessed: 29.04.2015. [8] ENTSO-E, “ENTSO-E Network Code for Requirements for Grid Connection Applicable to all Generators,” version 8th March 2013, [Online]. Available: http://networkcodes.entsoe.eu/connectioncodes/requirements-for-generators/, last accessed: 29.04.2015. [9] National Grid Electricity Transmission plc., “THE GRID CODE,” issue 5, revision 13, [Online]. Available: http://www2. nationalgrid.com/UK/Industry-information/Electricity-codes/ Grid-code/The-Grid-code/, last accessed: 29.04.2015.

Further use cases beyond that of delaying or replacing investment in EHV capacitors will be studied in more detail in future works, as well as the capability of HV systems to provide CQ under non-normal switching conditions or n-1 conditions, combined with voltage control of HV DER or industry generators and dynamic simulations. Sensitivities with respect to different permissible short term voltage bands in the event of a contingency could be studied as well. The feasible CQ bandwidth in the event of reaching HV line loading limits related to delayed network expansion may be of interest in some networks. For systems with high DER penetration on 230 or 275 kV level it may be of interest to study in how far the considerations carried out here would also apply to the provision of CQ from 230 kV or 275 kV systems to higher voltage levels.

[10] Eirgrid, “Grid Code,” version 5, [Online]. Available: http://www. eirgrid.com/operations/gridcode/, last accessed: 29.04.2015. [11] VDE FNN, “Technische Bedingungen für den Anschluss und Betrieb von Kundenanlagen an das Hochspannungsnetz (TAB Hochspannung),” VDE-AR-N 4120, version 1.0, in force since 01.01.2015. [12] C. Sourkounis, and P. Tourou, “Grid Code Requirements for Wind Power Integration in Europe,” Conf. Papers in Energy, Volume 2013 (2013), Article ID 437674, Hindawi Publishing Corporation, pp. 1-9. [13] PSE, “Company Information,” [Online]. Available: www.pse.pl., last accessed: 29.04.2015.

Acknowledgment

[14] H. Barth, D. Hidalgo, A. Pohlemann, M. Braun, L. H. Hansen, and H. Knudsen, “Technical and Economical Assessment of Reactive Power Provision from Distributed Generators: Case Study Area of East Denmark,” in Proc. 2013 IEEE Power Tech Conf., June 2013, pp. 1-6.

The authors gratefully acknowledge the information provide by DSO Netze BW GmbH and TSO TransnetBW GmbH.

[15] N. Etherden, M.H.J. Bollen, and J. Lundkvist, “Quantification of Ancillary Services from a Virtual Power Plant in an Existing Subtransmission Network,” in Proc. 4th IEEE PES ISGT Europe, Copenhagen, Oct. 2013, pp. 1-5.

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