Power Flow Controlling Devices as a Smart and Independent Grid Investment for Flexible Grid...

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IEEE TRANSACTIONS ON SMART GRID 1

Power Flow Controlling Devices as a Smart andIndependent Grid Investment for Flexible Grid

Operations: Belgian Case StudyDirk Van Hertem, Senior Member, IEEE, Johan Rimez, Member, IEEE, and Ronnie Belmans, Fellow, IEEE

Abstract—The electric power system in Europe is fundamen-tally changing, due to the liberalization of the energy market, theinternationalization of electric power system operations, the shifttowards renewable energy sources and the increased difficulty toinstall new transmission lines. The transmission system operatorsface new problems and they require new means to solve them.One option is to resort to the use of power flow controlling devicesto manage the energy flows in the transmission system and toprovide a secure operation under varying circumstances. Throughthe use of power flow controlling devices, the TSO gains a doubleadvantage: investments in new (overhead) transmission linescan be avoided and it allows more flexible grid operations. Thispaper describes the Belgian case for different stages of the gridmanagement: investment, planning, scheduling and operationsusing power flow controlling devices, including practical aspects.The Belgian case study comprises two technologies: the traditionalphase shifting transformer and voltage source converter HVDC,each with their advantages and disadvantages.

Index Terms—Coordination, HVDC, phase shifting transformer,transmission system operation, transmission system planning.

I. INTRODUCTION

T HE transmission system operators (TSOs) in Europe areexperiencing new challenges in both transmission system

planning and operations. Different factors force TSOs to use acompletely different approach compared to the situation beforeunbundling. While generation and transmission were planned ina coordinated manner before, this is no longer case. Generationinvestments are now rather independent from grid investmentsand any type of power plant can be proposed to connect to any-where in the grid. The transmission system owner is expected toaccommodate these investments. Generation investments oftenhave a shorter lead time than transmission investments and thereis considerable uncertainty with respect to these investments.Furthermore, a strong increase in renewable energy, espe-

cially wind and solar, is seen throughout the European powersystem. The highly volatile energy output of wind farms chal-lenges the TSOs in a double way: they are forced to invest in

Manuscript received June 02, 2012; revised January 07, 2013; accepted Feb-ruary 09, 2013. Paper no. TSG-00335-2012.D. Van Hertem and R. Belmans are with the Electrical Engineering

Department, Division Electa, of the KU Leuven, Kasteelpark Arenberg 10(PB2445), 3001 Heverlee, Belgium (e-mail: [email protected];[email protected])J. Rimez is with Elia, Belgian TSO, Boulevard de l’Empereur 20, 1000 Brus-

sels, Belgium, and with the KU Leuven, Kasteelpark Arenberg 10 (PB2445),3001 Heverlee, Belgium (e-mail: [email protected])Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TSG.2013.2249597

expanding their network to accommodate this new generation(as with any new generation) and they need to manage varyingpower flows from sources that are no longer centrally dispatch-able. The tendency to concentrate electricity generation, anda general move of the generation towards the borders of thesystem, causes additional transmission of energy in and betweenzones. Also the annual energy consumption is still increasing,further increasing the need for transmission capacity.At the same time, the TSO has the task to maintain and op-

erate the power system in a secure way, while facilitating themarket. As such, it has to provide sufficient capacity to themarket. Furthermore, themarket expects this capacity to be firm.Guaranteeing the capacity one or two days ahead is increasinglycomplicated. Renewables generate a variable amount of energy,which needs to be matched by other sources, generally placedat a different location. Using generation redispatch to ensure afirm capacity is possible, but considered too expensive by theTSO.Indirectly, also the liberalization of the electricity market has

caused an increase of cross-border flows, which are also lesspredictable than before. The unbundling has resulted in a strongincrease in the number of market players, each with a differenttask and a specific target to serve its purposes, including valuecreation for their shareholders.In the meantime, major investments in the transmission

system have been lacking, mostly due to heavy siting opposi-tion. This is especially true for cross-border connections due tothe regulatory gap between concerned countries or zones.As such, the system operators are struggling to operate a

system with more variable flows than before, with a system thatwas originally not intended for this. The security margins needto be adjusted to take the variability into account, even furtherreducing the transfer capacity. As a result, the TSOs are lookingto reinforce their system, both tackling the issue of controllingthe flows and increasing transmission capacity.This paper presents the solutions implemented by Elia, the

Belgian Transmission System Operator, in the form of powerflow controlling devices (PFC). Through manipulation of thecontrollable devices, the system operator gets control and ad-justs his system to the circumstances (windy day, sunny day,maintenance in part of the power system, ). This flexibilitycan be in different directions. For instance, during a sunny daythe transmission capacity to one zone can be maximized, andduring nights, the capacity to another zone is maximized. Eliachose to install three phase shifting transformers to increasethe border capacity and manage the system flows. They are

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2 IEEE TRANSACTIONS ON SMART GRID

TABLE ICOMPARISON OF CLASSICAL AC TRANSMISSION (OHL, UPRATING ANDCABLE), POWER TRANSMISSION USING FACTS OR PST FOR POWER FLOW

CONTROL (PFC) AND HVDC [1]

TABLE IIEVALUATION OF PROJECTS WITH RESPECT TO NON-GRID ISSUES [2]

also planning one VSC HVDC project embedded within thedensely meshed European power system as a non-standard ca-pacity investment having stronger control options. These op-tions are rather unconventional and the Belgian case as such isa unique experiment to invest in a more controllable grid usingits assets more closely to their limits. The paper will detail thereason for investing in these technologies, the impact on theplanning and the operation of the power system and the changesrequired at the transmission system operator side. The interna-tional, cross-border interactions are given special attention.

II. TRANSMISSION INVESTMENT SOLUTIONS

A. Available Upgrade Technologies

Different technologies are available to TSOs which providea full range of solutions to invest in grid capacity:• Overhead lines (OHL).• Uprating existing circuits.• Underground cables.• Direct current connections (HVDC).• Power flow controlling devices such as flexible ac trans-mission systems (FACTS) and phase shifting transformers(PST).

These technologies and the comparison between them hasbeen extensively discussed in literature (e.g., [1]–[3]). InTable I, a comparison of the technical properties is given and inTable II the non-technical properties are shown.AC overhead lines (OHL) is the standard solution when in-

vestments are considered. AC overhead lines have the benefitof being a well known and “cheap” technology, reliable, easy tomaintain and applicable for distances that are not too long.Uprating existing circuits can be done by using new con-

ductor types that can carry higher currents, through operation ofthe line at higher voltages or through adding another circuit toexisting right-of-way. Alternatively, existing ac overhead linesmight be converted to be fit for HVDC systems.AC cables were previously not an option at the highest

voltage levels, but are now increasingly available, also forhigher voltages. XLPE (Cross-Linked Poly Ethylene) cable

is now quite standard, which makes the application easier toinstall and cheaper. XLPE also has fewer environmental issuescompared to the traditional mass-impregnated (MI) cable types.At transmission voltages, cables are considerably more ex-pensive than OHL, but their public acceptance is considerablyhigher.HVDC allows the straightforward and relatively cheap use

of cables (without length limitation), lower cable losses and ac-tive power control. Voltage source converter (VSC) also allowsindependent reactive power control at both ends. The conver-sion between ac and dc and the other way around is done usingpower electronic converters, which are rather expensive and ex-hibit high conversion losses ( % per converter).Power flow controlling devices are a subdivision of the

FACTS (flexible ac transmission systems) family. They allowto pull power from, or push towards, parallel circuits andas such allow to redistribute the power flows in the system.Through adequate control actions, they can be used to in-troduce flexibility in the grid operations and influence gridlosses, grid security, transmission capacities between zones,Several types exist: phase shifting transformers (PST), thyristorcontrolled series capacitor (TCSC), static synchronous seriescompensator (SSSC) or unified power flow controller (UPFC)[4]. Although fundamentally different components, theirsteady-state operation is comparable. An important advantageof power flow controlling devices is that they are installedon a single spot, while influencing a wide area. As such, theinstallation is a local investment which can be done by a singleTSO (while possibly influencing other grids). Also the footprintis limited to the substation where it is installed.Through uprating of existing lines, using ac cables, installing

power flow controlling devices and using underground HVDClinks can be used to tackle some of the obstacles encounteredwith ac overhead lines. Although some technologies, for in-stance, power flow controlling devices and HVDC, are clearlyable to outperform ac overhead lines for the non-grid related cri-teria, this does not imply that it is always possible or even desir-able to use these technologies. Power flow controlling devicesare installed on existing lines and the system needs to accom-modate this solution. The same is true for uprating technologies.The extra transmission capacity created by these investments isonly incremental compared to new ac lines. In some cases thiswill not be sufficient. HVDC also has clear benefits, especiallywhen VSC technology is used. But today VSC HVDC is onlyavailable for smaller capacities (up to GW). AC cablesat extra high voltages (400 kV) are not common, especially notfor long distances. Additionally, investment costs are not explic-itly discussed in this paper, but should be carefully considered.A more expensive technology can sometimes help in reducingthe lead time. A careful cost-benefit analysis remains necessaryeven though transmission system investments are a regulatedasset of which the investments are recovered through transmis-sion tariffs.The authors want to mention gas insulated lines (GIL) and

high temperature superconducting (HTS) cables as options forthe future. Although that GIL is currently available as a trans-mission technology, it has not yet been used for long distancetransmission lines. HTS is still under development and the first


VAN HERTEM et al.: POWER FLOW CONTROLLING DEVICES AS A SMART AND INDEPENDENT GRID INVESTMENT 3

Fig. 1. Paradox in investing to relieve loop flows and the use of power flowcontrolling devices to solve it [3]. (a) Parallel flow through zone A causes con-gestion (grey area). (b) Investing in A relieves the problem, but loop flows in-crease. (c) Investing in B would be better. (d) Power flow control can be anoption.

test sites have been implemented, yet a full scale deployment isnot imminent.

B. Paradox of Investing in Additional Transmission Capacityin Multi-Zonal Environments

In case a certain transmission corridor experiences conges-tion on a regular basis, the natural long-term solution is to in-vest in new transmission lines to increase the capacity. How-ever, in a multi-zonal grid this may not be straightforward whenthe congestion is largely caused by loop flows. This is illustratedby Fig. 1. In the initial case, zone A is congested, in part by ageneration-load pattern in zone B which causes north to southflows [Fig. 1(a)]. This congestion can be relieved by investing innew transmission lines in zone A. However, this is not a likelysolution, as the “problem” does not originate in zone A, andthe investment will even increase the flows through that zone[Fig. 1(b)]. A better solution is to invest in new transmissioncapacity in zone B, but there is no incentive for that grid oper-ator or the regulator in that zone to actually invest as there areno local problems [Fig. 1(c)]. An alternative solution is depictedin Fig. 1(d) where a power flow controlling device is installedin the connection between zone B and A, limiting the transferflow through A and diverting the flow to B. This is a local andlimited investment in zone A. This solution is possibly not thebest solution on a global level.

III. STEADY-STATE COORDINATION

In transmission systems with multiple (well interconnected)zones, the coordination of power system operations is of keyimportance. The effect of PFCs controlled by different entitiesin different zones complicates the operation and calls for addi-tional coordination [3], [5].The effect of PFC on the steady-state operation of the power

system can easily be shown using a simple example. As test

Fig. 2. Test system for steady state analysis, consisting of 4 equal and mirroredsubsystems (cloverfield grid).

Fig. 3. Transfer capacities between zones 1 and 2 and between 1 and 3 (relativeto phase angles equal to 0 ) [6].

system, the cloverfield system1 is used (Fig. 2 [3]). This systemexists of 4 symmetrical interconnected subgrids. In this system,two PSTs are added, between nodes and and betweenand . Because of its symmetry, all studied effects are directlycaused by the PFC.For the system as indicated in Fig. 2, the transfer capacity2

between zones 1 and 2 and between zones 1 and 3 is cal-culated for different settings of the PFC (in this case phaseangles of a PST). Fig. 3 shows contour plots for the relative

between these zones. ThePFCs have a profound influence on the TTC, as could beexpected.Using the same test system, a simple check is per-

formed to search for the feasible region of operation for thecloverfield system. It is assumed that each system operatorchecks the internal overloads. As calculations aretypically performed per zone, the results differ for each zone(Fig. 4).

1The full data of the system is given in [3].2In this example, the TTC (total transfer capacity) is calculated without taking

into consideration, and by linearly scaling the generation in the exportingzone and the load in the importing zone.



Fig. 4. Feasibility zones with changing of PST settings [6].

IV. POWER FLOW CONTROLLING DEVICES IN THE BELGIANGRID

The Benelux (Belgium–The Netherlands–Luxembourg)region has a central role in the transmission grid of continentalEurope, the former UCTE grid. The Belgian power systemis operated by the transmission system operator Elia, and isinterconnected with France through three 380 kV and two 220kV lines, and through four 380 kV lines with the Netherlands.At the end of 2008, the latter interconnections were equippedwith phase shifting transformers. One PST is placed in Zand-vliet and two in Kinrooi at the new substation of “Van Eyck.”The PST in Zandvliet is installed in an extended coupling bay,separating the “Belgian” and “Dutch” busbar. The two PSTsat Van Eyck are placed in the lines Meerhout–Maasbracht andGramme–Maasbracht. These three PSTs have identical elec-trical parameters. A fourth PST is installed in Monceau on theline Chooz (Fr)–Jamoille–Monceau which is an interconnec-tion with France. Besides connections with Belgium, the Dutchpower system, operated by Tennet, has four interconnectionswith Germany out of which the northern one is equipped with aphase shifting transformer installation (two, placed in parallelin Meeden), a HVDC connection with Norway (Norned) andone with the UK (Britned). Another phase shifting transformeris placed in the German grid, operated by Amprion, at the sub-station of Gronau in the connection with Hengelo. Furthermore,a new set of phase shifters is installed in the grid of Tennetin the line Diele–Conneforde in vicinity of the Dutch borderand the Meeden PSTs. The placement of the PSTs is shown inFig. 5. The PSTs are placed on the interconnections and thusdirectly influence the cross border flows between them. TheBelgian TSO Elia and its neighboring German TSO Amprionare planning to build a new interconnection using HVDC toenhance the power transfer between the countries.In the federal development plan for 2010–2020 [7], an addi-

tional PST is planned in the substation of Zandvliet in the period2016–2020 and a potential coupling between the Belgian andGerman grid through the 220 kV system in Luxembourg willrequire a PST as well.

Fig. 5. Schematic overview of the placement of existing and projected PSTsin the Benelux transmission system and the planned embedded VSC HVDCsystem of Alegro.

The remainder of the paper will focus on the two cases: thecombination of 3 PSTs between Belgium and the Netherlands(1, 2, and 3 in Fig. 5) and the planned Alegro HVDC link be-tween Belgium andGermany (A). The choice for the technologyis addressed in both cases, as well as the consequences in op-eration. Both the current implementation as future coordinatedactions are discussed.

V. CASE 1: 3 PSTS FOR FULL BORDER CONTROL:INSTALLATION

Given the difficulties as stated in the introduction, the Bel-gian power system is subject to large power flows through itsgrid, often in the form of unidentified loop flows. The systemis operated closer to its limits, yet reinforcing the grid locallythrough the construction of new transmission lines is not a real-istic option. Instead, the Belgian TSO Elia chose to install threePSTs in the north of Belgium considering the following aspects:the available time frame, building space, budgetary concernsand available technological experience. These PSTs allow to re-distribute the flows within Belgium, and limit the loop flowsthrough the zone.The choice was made to install three PSTs nearly simulta-

neously on all existing interconnections with the Netherlands.The objective was to gain complete border control between Bel-gium and the Netherlands, and as such also between Belgiumand France. The Dutch border was chosen as this required fewerPSTs to be installed. In case no full border control is achieved,there is still a possibility to optimally use the transmission paththrough the Benelux, but not to shift the power transfer to usethe direct interconnections between Germany and France [8].Full border control requires, depending on the location, three orfour independent flow control units to be installed. Each device



TABLE IIIMAIN CHARACTERISTICS OF THE 1400 MVA PHASE SHIFTING TRANSFORMERS

[9]

is installed in the 400 kV grid and able to carry the full rating of1400 MVA.Based on extensive system simulations, the choice was made

to place three PSTs. One in the substation of Zandvliet and twoin the newly built station of Van Eyck (see also Section IV).These extensive simulations include standard load flow simula-tions in N and N-1 state for a number of scenarios, emergencysituations and reclosing of the transmission lines with the PSTinstalled under high phase angle.The PSTs are manufactured using proven transformer tech-

nology, providing compactness, relatively low cost, high reli-ability, limited need for maintenance, training and peripheralequipment and the use of standard spare parts. Recent projects inthe western European countries (the Netherlands, France, Italy,Spain) were also received positively, which gave a strong incen-tive to choose this option.Clearly, a PST cannot shift beyond its design values once

manufactured. Extensive planification forecast scenarios needto be calculated to verify if the angle ranges suffices. Anotherdisadvantage is that switching the PST causes aging of the tapchanger. As a result, unnecessary switching operations shouldbe avoided.

A. Practical Implementation of the PST Installations

The choice of the electrical properties of the PSTs was ratherstraightforward. The basic requirement for the design of thetransformer rating is to match the transmission line it is con-nected to (400 kV and 1400 MVA). The maximum angle dis-placement and number of tap positions was dictated by the tapchangers available on the market.Indeed, the combination of the maximum switching capa-

bility of available tap-changers together with the chosen rating,led to a maximum angle displacement of divided over 17equal steps. The total number of tap positions of each PST is 35,position 18 being the neutral one. Table III gives an overviewof the main characteristics of one of the PST units.The three PSTs are identical to reduce manufacturing and de-

sign costs, as well as to reduce maintenance and spare equip-ment. They are designed using the symmetrical indirect scheme.This scheme is the most adopted for using with devices of suchrating and rated voltages. The most critical part, the tap changer,

is decoupled from the line voltage and by current transforma-tion also from the full short circuit fault current. For this project,the most optimized method to split the PST into different trans-former units, was by designing it into two three-phase trans-formers: a series transformers making the connection with the400 kV installations and superposing the voltage in quadratureand the regulation transformer, containing the six OLTCs. Theunits are linked together using oil-ducts.Only regulation of the active power was desired for this appli-

cation, no measures were taken to influence the reactive powerflow.

VI. CASE 1: 3 PSTS FOR FULL BORDER CONTROL: OPERATION

A. Grid Operation

In general, grid operation starts from the grid data and loadand generation forecasts, combining together results in thebranch flows. This data is getting more and more variable anduncertain. Wind power intermittency, cross border exchanges,load evolution and market clearing have led to an increasedamount of uncertainty. Also control actions of neighboringTSOs contribute to uncertainty. As a result, the day aheadoperational planning and intra-day operations of the powersystem are significantly affected. A deterministic approach thatconsists of a single best guess of the system becomes inappro-priate at this point. A set of scenarios, taking into account thevariations and uncertainties, determine the strategic decisionsmade by the TSOs, ensuring that the system security is stillguaranteed under all plausible scenarios.The control centers of the TSOs carry out security assess-

ments for both non-contingency and contingencies in their gridon a 24/7 basis, both for day-ahead and intra-day operations.Depending on the scheduled power injections for the next dayor next hours a set-point is determined which ensures safe op-eration. The set-points can be defined as the settings of thecontrollable devices required for the particular power injectionforecasts which will keep the system running in a secure state.Changes to the grid settings that deviate from the scheduledvalues are made if necessary, for instance when the forecastsare adjusted significantly, or in case of contingencies.

B. Loop and Border Flow Control

In order to fully control loop flows over a certain border, eachinterconnection requires a controllable device.As three phase shifting transformers were installed, the grid

operator received three additional freedom degrees to influencethe trans-border power flow, allowing different modes of oper-ation [10].1) Single PST Operation: Single PST operation is the

“standard” manipulation commonly applied. Field tests withchanging the tap position of the phase shifting transformershave shown that under normal grid conditions of only one ofthe three devices, the power flow through that transformer willchange with about 75 MW. As a result an active circulationcurrent is created which flows through the rest of the network.The active power through the other two PSTs is influencedby roughly 25 MW (in opposite direction). The remaining



25 MW flows through the parallel network mesh through theNetherlands, Germany and France.2) Full Border Control: When not only one, but all three

devices are moved one tap position in the same direction, a totaltrans-border active power flow variation of roughly 75 MW canbe observed. This power flow variation will adjust the flow inthe parallel paths of the foreign grids with the same value. At fullrange (17 taps), about 1250MW active power can be influenced,both in positive and negative direction.3) Load Flow Repartition: The PSTs can also be used to

alter the power flows within the Belgian grid. Internal rebal-ancing power flows can be obtained by shifting two of the threemachines in opposite direction. Doing so, a power flow adjust-ment of about 90 MW per OLTC tap can be observed. The flowthrough the third PST and the Belgian loop flow will remain un-changed. As a net effect, only the flow repartition between theeastern and western OHL corridor is altered.

C. Operational Strategy

The overall control strategy is to resolve three distinct gridissues, by operating the PSTs in a combination of the modesdescribed above:• Limiting the Belgian north to south loop flows within therange [ MW; MW]

• Relieving congestion in the Belgian grid• Minimizing congestion in the CWE area

Each of these items will be explained more in detail in the fol-lowing subsections.1) Limiting the Loop Flow: The normal operation strategy

for the PSTs is to secure border capacity between Belgium andits neighbors by limiting the loop flow. The loop flow is definedas the power flow that would go through the Belgian networkwhen all nominated (or commercial) power flows are reducedto zero. The remaining (theoretical) active power will take aportion of the NTC (net transfer capacity) not available to themarket.The loop flows are calculated in real time by combining

power measurements and calculated power transfer distributionfactors (PTDF). The PSTs provide a means to split loop flowsfrom market flows, and if necessary reduce the former toacceptable levels.Fig. 6 shows the evolution of the North-South loop flows for

the years 2001 to 2011. Positive values are north-south orientedloop flows (from the Netherlands in the direction of France),negative value indicate south-north loop flows. The extremevalues are the 15 minute peak values [MW] (both positive andnegative) and the grayed values are the values of the L80 per-centile of all positive and negative 15 minutes averages overthat year. The values of the year 2008 are indicated by a darkercolor as in the middle of that year, the three PSTs became oper-ational. On this figure, the desired control range for loop flowsis depicted as well.The spread of the values after the commissioning in 2008, is

not substantially less than before that year. Two reasons exist:• The loop flows are increasingly present in the system, andfrom Fig. 6 a slowly increase in loop flows is observed until2008. This increase has been stopped after the PSTs wentinto service.

Fig. 6. Yearly North to South loop flow levels [MW] (peak and L80 percentilevalues). The PSTs became operational in 2008.

• The loop flow control scheme has a local objective for theBelgian power system. However, the operation of the PSTsinfluence also neighboring system security, which takespreference over local transfer capacity.

2) Relieving Congestion in the Belgian Grid: To avoid localoverloading, in case of planned outages or network incidents,the tap changing strategy, described in VI.B3, is applied tocreate an internal loop flow which will counteract local satu-ration in the 400 kV grid. Scheduled contractual flow is notinfluenced, and the power flows only close to the border.3) Minimizing Congestion in the CWEArea: As the influence

of the coordinated PST control is seen over a very large area inthe Central and Western Europe control zone (CWE area), it isin some cases possible to help neighboring countries to relievetheir HV grid partly from congestion. This is internationally co-ordinated and used with precaution as it may be at the expenseof losing transmission capacity, increase losses or reduce the se-curity margin locally.4) Future Operation Through International Coordination:

As different zones are connected, there is a considerable influ-ence across borders. In order to manage the system includingthe power flow controllers in an optimal manner, coordinationis needed. Different levels of coordination exist. A first level iswhere information is exchanged regarding the projected settingstogether with the day ahead forecasts and exchanges. This cor-responds to the level at which the system currently is operated.Each operator manages his own grid and informs his neighbors.In case of potential problems, ad hoc coordination is performed.For an improved operation, coordination is needed within all

stages of power system operation. Within the current system op-erations as in place in the European power system, this meansthat during the exchange of the planned power injections, doneinter-day (during two day ahead and day ahead information ex-changes) and intra-day, also the planned power flows on eachof the controllable links needs to be exchanged and an optimalcontrol setting can be obtained from a combined data set. Thisrequires an international coordination or an iterative approach.In the coordinated case, all underlying systems must be broughttogether and the system settings must be optimized towards apredefined objective (which can be zonal, international or oth-erwise). Such international coordination efforts are under study



in the Twenties project.3 [11] In this project, a security con-straint OPF algorithm is developed which allows to test andcompare multiple objectives. At a later stage, this algorithm canbe used to coordinate the operation of power flow controllers inthe CWE region as it will be implemented in the security coor-dination center of Coreso.4

One step further is to integrate the controllable devices intothe market, where the control of PFC has a monetary value. Thecontrollability has a direct influence on the transfer capacitiesand can influence the market price in different zones (e.g., onemight be able to increase the capacity between two zones A andB), while reducing the capacity between B and C. To correctlybring this into account, the set-point of the power flow control-ling device needs to be put into the market calculations (at theminimum) or better, the controller itself needs to be integratedin the market model, where the set-point of the controllable linkwill be the outcome of the market mechanism. The flow basedmarket coupling methodology needs to be extended. At this mo-ment there is not yet a regulatory framework to achieve this, nora consensus among TSOs. One of the key points is the possibleremuneration scheme for such a setup. In theory it is relativelystraightforward to implement a mechanism that optimizes theentire system, including PFC settings, towards maximal socialwelfare. Three aspects require specific attention: determiningthe correct value of controllability (and the correct division ofthe remuneration among the different parties), the control ob-jectives of the different devices and the objectives within thedifferent operating time frames (D-2, D-1, and real time). Forinstance, what is the correct level of reliability/transmission ca-pacity allocated two days ahead given that PFCs influence bothreliability and transmission capacity? Will the same objectivesbe used D-1 and real-time? Most likely we will see a transgres-sion of more market oriented criteria to more reliability orientedcriteria whenmoving towards real-time. Both the formulation ofthe operation [12]–[16] and the integration in themarket [17] arebeing researched. Nevertheless, the integration into the marketis bound to happen, especially when more HVDC lines will beconnected into the meshed ac system.Note that although the system objective aims at a maximum

welfare, the current installations are generally regulated invest-ments with a focus on a local return on investment.

D. Innovative Aspect

The innovation in this investment decision is not in the gridexpansion using the (well known) PST technology itself, butrather the system wide approach in adding control to all lineswith the Netherlands and enabling it to drastically influence thepower flows in the loop Belgium–France–Germany–the Nether-lands–Belgium.The revenues, and the correlated pay back time, are of a vir-

tual nature. Indeed, the PSTs do not generate a direct monetarybenefit for the grid operator who is the one that needs to dothe investment, and the control actions are not yet directly inte-grated into the market operations. They are rather securing thesystem and the market activities under varying circumstances.

3http://www.twenties-project.eu4http://www.coreso.eu

Comparing grid events and the remaining transfer capacity withand without the devices, one can still assess their market value.In case loop flows are expected to be within the security range

without intervention of the PSTs, they can be operated to meetsecondary goals: reduction of system losses, increasing bordertransfer capacity, assistance during planned network outages bydeviating active power through other electrical paths.Lastly, on several occasions, the transformers have been cru-

cial in resolving extreme network events and securing overallgrid security. Although tap changing by itself is merely actingon steady state conditions (there is no contribution to dynamicnetwork security) and no automatic control actions are imple-mented, these compact devices play a crucial role to fulfill thetasks of the modern TSO.• Internally: changing taps, with the condition that overallphase shift between Belgium and the Netherlands remainsequal, provides a balancing action for the power flow on theindividual borders. Market capacity remains unaffected.Different strategies can be applied to cope with local over-loading, planned and unplanned outages, severe local net-work conditions. This feature improves overall flexibilityto network operation, and is free to use without major ex-ternal impact.

• Externally: different coordination bodies and platformswere set up (e.g., Coreso and CWE), to better integrate thePST and other control devices into the Western Europeangrid. Day ahead forecasting, international security calcu-lation, and set point fixing are their main purposes: theinstallation of the control devices rendered internationalcooperation indispensable.

The effectiveness of the use of PSTs to manage the flowsinternally and internationally has been proven through interna-tionally organized tests.

VII. CASE 2: CONNECTION TO GERMANY USING VSC HVDC

Although the grids of Germany and Belgium are geographi-cally close, no direct connection exists. Such a link was neverbuilt because of the following reasons. Firstly, the generationload pattern in Western Europe results in largely north-southflows or south-north flows. With a connection from the westto the east, the natural flow through the line would be limitedunless a PFC would be installed. Secondly, it is difficult to finda suitable route for overhead lines through sensitive areas (cityareas and nature reserves).Recently, the plans to build such a line have been revitalized

in order to meet the rising demand for interconnection capacityand also to anticipate changes in the flow pattern, for instancedue to offshore wind development. Studies have resulted in thechoice for VSC HVDC.This new link of roughly 100 km with a (still undecided)

rating of 1000 to 1600 MW, will be one of the first applicationswhere a dc link embedded in a meshed ac network. For sucha short connection, the VSC HVDC solution is certainly not acheap option when compared to traditional ac OHL. However,the trajectory crosses protected natural reserves, which the useof underground cable is obligatory.Several reason have led to the choice of the VSCHVDC tech-

nology for this application:



• The dc link will be installed between two network nodeswhere in a normal situation, the voltage angle difference istoo low to fully use a normal ac link. In an early projectstage, the installation of a phase shifting transformer inseries with an high voltage cable was considered. Reactivepower compensation had to be foreseen as well.

• In this case, the independent active and reactive power reg-ulation of the VSC technology is certainly an added value.Regardless of the angle difference between the connectingnodes, the active power will be transferred in any grid con-dition as desired by the operator.

• The flexibility of the voltage support, by either generationor consumption of reactive power, is inherent to this solu-tion.

• The VSC HVDC solution has improved considerably withrespect to energy losses

• Less need for additional filtering (which is also importantfor the footprint of the substation).

• The VSC technology is adopted for later use in a multi-terminal dc link or perhaps in a fully meshed dc overlaygrid.

When ready, the VSC HVDC link control will be integratedin the scheduling and control for the PSTs.

VIII. CHANGES IN THE OPERATING ROOM

Adding controllable devices or VSC HVDC in a powersystem adds an (or multiple) additional degree of freedom. Thisadditional control also comes at a cost, namely increased com-plexity and new dynamics. The system will react differently tochanges than in normal systems. In case the link is automati-cally controlled, seemingly “unnatural” system behavior mightoccur. The operator must be able to recognize the effects andassess the actual situation, both during normal operation and incontingencies. In case the system is operated fully manually,the operator needs to fully aware of his options and be educatedto make the correct decision.The operators need to be trained in operating the new devices,

and this in all potential circumstances to be aware of the newchallenges. Existing practices need to be reviewed in light of thenew equipment (e.g., adjustments to special protection schemes,reactive power management, ).VSC HVDC can change its active and reactive output much

faster than common ac elements. The operators have to be 100%confident in the implemented automatisms and regulations andhave a good know-how of the HVDC system behavior. Alter-natively, the HVDC dynamics could be limited to match the in-tuitive behavior of the traditional ac system.In an international context, operators need to be trained to ex-

change information, especially with respect to emergency sit-uations. The control room itself, as well as the training toolsthat are used for operators, need to be adjusted to the new sit-uation. The SCADA system, power flow, OPF, state estimationand other tools all need to be able to manage the new device(s)[18].

IX. CONCLUSION

With recent changes in the power system, the grid operatorsare experiencing considerable issues inmaintaining their system

secure, both on the level of system operation and planning. Thelack of investments and the high level of variable energy flowsmake ask for innovative solutions.This paper discusses two cases by the Belgian TSO Elia

where “smart” devices were installed to deal with the issuesat hand. The Belgian TSO Elia has chosen for power flowcontrolling devices to upgrade its grid.In the first case, three phase shifting transformers are installed

which provide full border control. As a result, the system flowscan now be managed in a flexible manner, without installingadditional transmission lines. Although not the most technolog-ically advanced devices, PST have been chosen because theyfulfill all technical requirements for the flexible control. Theyare also a well known technology with high ratings and ratherlow losses. Through coordinated use of these three devices, thepower flow through the Belgian power system can be influencedby approximately 2500 MW. In a first approach, the devices areused to manage loop flows, internal congestion and if requiredcongestion within the CWE area. In a later stage, internationalcoordination can allow optimal use of the flexibility. The de-vices can be directly integrated in the market.In a second case, VSC HVDC has been chosen for a short

( km) land line in a meshed transmission system becauseof the possibility to underground the circuit and because ad-vanced control options (mainly reactive power) at both ends.Both investment decisions are rather uncommon in a grid suchas the Belgian one, and they make the Belgian power systema unique experiment where the system operator has gained astronger hold on the power flows traversing its grid. Adding con-trollability to the system provides a level of flexibility to operatethe system closer to its limits.

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[22] D. Bekaert, L. Meeus, D. Van Hertem, E. Delarue, B. Delvaux, G.Küpper, R. Belmans, W. D’Haeseleer, K. Deketelaere, and S. Proost,“How to increase cross border transmission capacity? A case study:Belgium,” in Proc. Eur. Energy Markets, Leuven, Belgium, May27–29, 2009.

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Dirk Van Hertem (S’02–SM’09) was born in 1979,in Neerpelt, Belgium. He received the M.Eng. in2001 from the KHK, Geel, Belgium and the M.Sc.in electrical engineering from the KU Leuven,Belgium in 2003. In 2009, he received the Ph.D.,also from the KU Leuven. In 2010, he was a memberof EPS group at the Royal Institute of Technology,in Stockholm, Sweden, where he was the programmanager for controllable power systems for the

competence center at KTH. Since spring2011 he is back at the University of Leuven where

he is an Assistant Professor in the ELECTA group. His special fields of interestare power system operation and control in systems with FACTS and HVDCand building the transmission system of the future, including offshore grids andthe supergrid concept. He is an active member of IEEE PES and IAS and Cigré.

Johan Rimez (M’11) was born in 1977 and receivedthe M.Sc. degree in electro-mechanical engineeringat the Vrije Universiteit Brussel (VUB), Brussels,Belgium, in 2000. He is currently fully employedwith the Belgian Transmission System OperatorElia, Brussels, as Senior Expert. At the same time,since 2008, he is Freelance Ph.D. Researcher atthe Katholieke Universiteit Leuven (KU Leuven),Belgium. His main interests are grid perturbationsand dynamics, HVDC technology and its interactionwith the ac network, wind power plants, and network

optimization.

Ronnie Belmans (S’77–M’84–SM’89–F’05) re-ceived the M.S. degree in electrical engineeringin 1979 and the Ph.D. degree in 1984, both fromthe KU Leuven, Heverlee, Belgium, the SpecialDoctorate in 1989 and the Habilitierung in 1993,both from the RWTH, Aachen, Germany. Currently,he is a Full Professor with the KU Leuven, teachingelectric power and energy systems. His researchinterests include techno-economic aspects of powersystems, power quality, and distributed generation.He is also Guest Professor at Imperial College of

Science, Medicine and Technology, London, U.K. Dr. Belmans is a Fellow ofthe Institute of Electrical Engineers (IET, U.K.). He is the Honorary Chairmanof the board of Elia, which is the Belgian transmission grid operator. He is alsothe President of the International Union for Electricity Applications (UIE).

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