Session_5

SESSION

5Emerging Trends in HVDC and

Facts Technology

340

New Technologies in T & D, Renewable Energy Integration,Smart Grid, Energy Efficiency and Communication

5th International Exhibition & Conference, 2015

April 8-102015

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Real Time Simulation of Stability Functions of HVDC Systems -A Case Study of 500 kV, 2500 MW Ballia-Bhiwadi HVDC Link

using Controller ReplicaMahesh Vardikar Vishwajeet Singh M.S. Rao M.M. Goswami Oommen Chandy

Powergrid Corporation of India Ltd., India

1 [email protected]

SUMMARY

Indian Power System is rapidly expanding andbecoming more complex with the installation ofsophisticated power electronic systems anddemands a higher reliability and availability. Theoccurrence of transient stability issues due tocomplexity of power system is becoming morechallenging. These issues can be resolved withinteraction of HVDC system with the inter-connected AC systems. The stability functions areoften cited as an important advantage of HVDCsystems. In order to improve the system stability,the stability functions of HVDC will be useful. Toachieve the promised advantages, these functionsmust perform appropriately for variousdisturbances and system conditions. A Real TimeSimulator allows to accurately and efficientlysimulate all the system conditions, taking thehardware controllers / protective devices intoclosed loop and presents response of controllersunder testing.

This paper presents the effectiveness of two ofthe key stability functions, viz. Power OscillationDamping (POD) and Sub Synchronous TorsionalInteraction Damping (SSTI), of 500 kV, 2500MW Ballia Bhiwadi Bipole HVDC link ofPOWERGRID, using Real Time Simulator (RTS) HVDC Controller replica setup operating atPOWERGRID, Gurgaon.

Keywords: Real Time Simulation, Hardware In Loop(HIL), Power Oscillation Damping (POD), SubSynchronous Torsional Interaction (SSTI), HighVoltage DC transmission (HVDC).

1. INTRODUCTION

High Voltage Direct Current (HVDC) transmissionwhen integrated with conventional ac transmissiontechnology, offers many advantages and benefits tothe overall power system. Many of the benefits,which are realized can be attributed to the fast,flexible and effective means by which HVDCsystems can be controlled. One of the mostimportant considerations when designing an HVDCinstallation is the application of appropriate controlsand protection strategies.

The oscillations of sub synchronous frequency rangemanifest in power system due to various reasons likeloss of generation or transmission lines, sudden loadchanges or faults etc. under different systemscenarios. Hence, auxiliary controls are required toutilize the controllability of HVDC links forimproving the AC system stability and dynamicperformance. Purpose of such controls depend onthe characteristics of the connected AC systems.The key auxiliary controls in DC links are:

(a) Power Oscillation Damping (POD): Dampspower oscillations by modulating the powerflow through the HVDC line and hence stabilizesthe system. It improves transient stabilitydamping in AC system.

(b) Frequency Control: Controls the small frequencychanges in the AC networks to which the HVDCis connected. HVDC link can control frequencyof part of power system by modulating powertransferred to or from that system. Frequencylimits can be set for such function which arethreshold for power transfer till the frequencyof the area is within limit.

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(c) Power Run back / Run up functions: Thesefunctions are used to support the systemfrequency/stability in the case of contingenciesinvolving the loss of generation, load or majortransmission lines.

(d) Sub Synchronous Torsional InteractionDamping Control (SSTI): This control is usedto damp out torsional oscillations in subsynchronous frequency range caused due tointeraction between HVDC and nearbygenerators.

2. STABILITY FUNCTIONS

In this paper following the HVDC control functionsto stabilize the network are discussed anddemonstrated through real time simulations :

zzzzz Power Oscillation Damping

zzzzz Sub Synchronous Torsional Interaction Control

2.1 Power Oscillation Damping

Large interconnected power systems are prone tointer area oscillations, where groups of generatorsswing against each other across relatively weak inter-ties. The stability of AC systems are determined bythe damping of these electromechanical oscillationsor swings. The oscillations may persist till they aredamped out by the system (Fig. 1). In real powersystems, the damping energy is obtained by themodulation of load or generation for a period of time,typically in the range of 5 to 10 seconds. Thedamping energy must have the correct phase shiftrelative to the accelerated/decelerated systems. Thewrong phase angles can even excite poweroscillations.

The frequency of power oscillations is typically inthe range of 0.2 to 3 Hz. This means that for effectivepower oscillations damping, fast acting devices arerequired. As shown in the Figure 1, various methodsare available to damp out these oscillations. ButHVDC solution is the direct modulation of activepower and is very effective. The eigen value analysisis valid for small signal perturbations only, but givesan analytical insight into the behavior of a largepower system. A factor which is frequently used forthe evaluation of the damping behavior instead ofthe absolute damping coefficient Sigma is therelative damping coefficient Zeta

where and are real and imaginary parts ofoscillation mode. Normally an oscillation is dampedsufficiently when < -5%. The choice of the inputsignal used for the modulation is very muchimportant.

zzzzz The frequency difference between the twosubsystems. In principle this signal is veryeffective, but it is not locally available and fasttelecommunication is required.

zzzzz The derivative of the angle difference betweenthe two subsystems. Fast telecommunication isrequired.

In the case study discussed in the paper, the input toPOD controller is the frequency difference betweenthe two buses (f) and damping of the networkoscillation is carried out.

2.2 Typical Power Oscillation DampingController

Fig. 1: Power Oscillation Damping Fig. 2: Block diagram of a typical POD controller

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AC system frequencies at both rectifier and inverterend are measured and compared with referencefrequency input to get the frequency deviation atboth terminals.

The filtered frequency deviations at both rectifierand inverter end are then compared to get thefrequency deviation for change of power transferred.The high frequency components from the frequencydeviation are filtered out of AC systems of rectifierand inverter ends. POD acts on this frequencydeviation in the rectifier and inverter ends and triesto minimise the deviation.

The deviation is then acted up on by a gain limiterto change the response to small signals. This is thengiven to the high pass filter to filter higher frequencydeviations and finally converted to resulting Pmodulating power which superimposes over the Prefgiven by the operator to get the resultant referencepower.

2.3 Case Study

For the analysis of POD, a simulation casedemonstrating the effect of this control on dampingof system generated oscillations under a faultscenario was carried out using the Real-TimeSimulator connected with replica control &protection system of +/- 500 kV, 2500 MW Ballia Bhiwadi HVDC Bipole link of POWERGRID. ThePower Oscillation Damping Control function is a partof the Pole control.

2.4 System Representation

+/- 500 kV, 2500 MW Ballia Bhiwadi HVDC Bipolelink is used for the evacuation of power from Balliain Eastern UP to Bhiwadi in Rajasthan over a distanceof approximately 780 km. The power is pooled atBallia from Barh and Kahalgaon generating stations.The transmitted power at Bhiwadi is distributed inthe Northern Region. The AC system on both sidesis represented as ac equivalent network with thirty(30) buses. The HVDC system has been representedas bipolar 12 pulse bridges with ac filters, dc filtersand HVDC line along with electrode line. The figurebelow represents the HVDC buses and AC doublecircuit line connected between Ballia and NewLucknow.

zzzzz Power flow in Ballia - Bhiwadi HVDC link wasset to 2400 MW (pre-fault power).

zzzzz 400 kV Ballia - New Lucknow double circuitline parallel to the Ballia-Bhiwadi HVDC systemwas used for observing the action of POD.

zzzzz A 3 phase to ground fault of 100 ms durationand subsequent tripping was initiated in one ofthe double circuit lines between Ballia- NewLucknow-I.

zzzzz The power oscillations in the other line wasobserved with and without the action of POD.

Fig. 3: Equivalent system representation for POD evaluation.

2.5 Study Results

As result of a fault, the ac system becomesdynamically unstable as evident from the oscillationsoccurring due to deviation in active and reactivepower balance of the system.

Without POD Control: The POD function wasdisabled from HMI. The pre-fault power in Ballia-New Lucknow Line-II was 685 MW which shot upto 1903 MW dynamically after the clearance of faultfurther settling to 1050 MW.

(i) The power oscillations of magnitude P = 1372MW was observed within 3 seconds after thefault is cleared and oscillations of around200 MW was observed even after 9 seconds.

(ii) These inter-area oscillations with the frequencyof 0.37 Hz were observed in the power flow ofthe healthy 400 kV Ballia-New LucknowLine -II. These slow oscillations persist for morethan 12 seconds eventually dying out ReferFig. 5.

(iii) The difference in power angle between Balliaand New Lucknow buses also oscillates and 3seconds after fault is cleared the variation inangle is 30 degrees. However there is no majorrotor angle instability in nearby generators. ReferFig. 8. The oscillations are lightly damped and

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system remains stable as per the stabilitycriteria.

(iv) The HVDC controls recover to pre-fault HVDCpower of 2400 MW with some slow oscillatorybehaviour. It is to be noted that to demonstratethe sole effect of the POD during disturbancethe frequency control was disabled from theHMI.

With POD Control : The POD function was enabledfrom HMI. The pre-fault power in Ballia-NewLucknow Line-II was 685 MW which shot up to1751 MW dynamically after the clearance of AC faulteventually power settling to approx. 1050 MW.

(i) The power oscillations of magnitudeP = 975 MW was observed within 3 secondsafter the fault is cleared and oscillations ofaround 60 MW was observed after 9 seconds.

(ii) These inter-area oscillations with frequency of0.37 Hz were observed from the power flow ofthe healthy 400 kV Ballia-New LucknowLine -II. These slow oscillations die-out in lessthan 9 seconds. Refer Fig. 6.

(iii) The difference in power angle between Balliaand New Lucknow buses also oscillates initiallyand 3 seconds after fault is cleared the variationin angle is 14 degrees. Refer Fig. 9.

(iv) The HVDC controls recover to pre-fault HVDCpower of 2400 MW. Considerable dynamicchanges in DC power with HVDC PowerOscillation damping control coming into actioncan be observed from Fig. 7. Due to theoscillations seen at rectifier buses, the PODcontrol picks up the change in frequency (f)and generates modulating signal (P) which gets

Fig. 4: Oscillation of Power in Ballia - New Lucknow-II linewithout POD control

Fig. 5: Damping Effect on Power Oscillation in Ballia -New Lucknow -II line with POD control

Fig. 6: DC Power Modulation in Ballia - Bhiwadi HVDC with POD

Fig. 7.: Power Angle Oscillation between Ballia andNew Lucknow bus without POD

added to the scheduled reference power. Thevariation in dc power from reference power of2400 MW is between 2057 MW to 2540 MW.

Comparing results of the cases with and withoutPOD, it is observed that POD controller improvesthe overall stability of the interconnected systemwith oscillations getting damped out and achievesfaster system recovery.

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3. SUB SYNCHRONOUS TORSIONALINTERACTION DAMPING CONTROL

Sub synchronous torsional interaction (SSTI) refersto energy exchange between HVDC rectifier terminaland closely connected turbine-generators atfrequencies below 50 Hz. Generally turbine-generators consist of several masses connected byshaft acting as torsional spring with several naturalfrequencies. HVDC rectifier operating in islandedmode with generators are most prone to subsynchronous oscillations. This is due to HVDCrectifier appears as negatively damped load fromgenerator terminals in range of 10 - 40 Hz. This maylead to excitation of torsional oscillations which maycause damage to generator shaft.

Typically thermal generator shafts are moresusceptible to torsional interaction than hydrogenerator shafts. This is due to the fact that thermalgenerators have several torsional frequencies(typically 4 to 5) in sub synchronous range whilehydro generators have typically one frequency andalso possess higher mechanical damping.

3.1 Method of Investigation

To assess possibility of sub synchronous oscillation(torsional interaction) of generators with HVDCcontrols, calculation of Unit Interaction Factor (UIF)is widely accepted method. UIF for a generator unitis defined as reference [3],

... (1)

Fig. 8: Damped Power Angle Oscillation between Ballia andNew Lucknow bus with POD.

where

UIFi is Unit Interaction Factor for the ith unit.

MWHVDC

is rating of HVDC converter terminal.

MVAi is rating of ith turbine-generator unit.

SCi is short circuit level at HVDC converter busexcluding ith unit.

SCtotal is short circuit level at HVDC converter busincluding ith unit.

Generating unit with UIF less than 0.1 implies levelof interaction to be small as recommended by EPRIreport. Such a unit need not be considered for furtherdetailed study of interaction.

UIF higher than 0.1 indicates level of interaction maybe sufficient to destabilize rotor torsional modes ofoscillation and detailed analysis of torsional dampingshould be performed to accurately evaluate extentof torsional interaction.

As turbine generator system consist of several massesconnected in tandem. Torsional frequency of eachpart of the shaft is either available from generatormanufacturers or can be calculated from mechanicalparameters of turbine stages, generator and exciter.For generators near Ballia rectifier station, thetorsional modes in the sub synchronous frequencyrange between 10 Hz and 35 Hz are as follows:

13.95 Hz (Barh), 19.86 Hz (Kahalgaon 588 MVA),22.14 Hz (Kahalgaon 247 MVA), 24.21 Hz (Barh),24.75 Hz (Tenughat), 32.25 Hz (Barh), 33.73 Hz(Kahalgaon 247 MVA).

Fig. 9: Generators in the vicinity of Ballia rectifier station

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3.2 Typical Sub Synchronous TorsionalOscillation Damping Controller

The SSDC controller damps out sub synchronousoscillations by modulation of firing angle or currentorder. The modulation of current order, is preferredover firing angle, for damping control to avoiddisturbances caused by noise. This is due to filteringof current order in P-I controller, which will eliminatedisturbances due to noise.

The modulation of current order and firing angle hasbeen limited to 5% of rated current and 5 degreesrespectively.

3.3 Study Results

Sub synchronous resonance is small signal stabilityrelated problem. If this problem exist, disturbancessuch as an AC system fault near generator will initiatethe sub synchronous oscillations. Then the oscillationwill built up in shaft connected masses in turbinegenerators system.

The multi-mass turbine-generator system for Barhgenerators was modelled in RSCAD. The SSTIdamping control is part of control and protectioncubicles, which are Hardware-In-Loop connectedwith RTDS setup.

As a worst case scenario for occurrence of subsynchronous oscillations, on Ballia side, islandedoperation of rectifier is considered with Barhgenerators (3 x 660 MW). The power transmissionthrough HVDC is set as 1800 MW from Ballia toBhiwadi. Then a phase to earth fault was created toinitiate sub synchronous oscillations. The fault wasapplied on Ballia - Barh transmission line to observeaction of SSTI damping control.

Fig. 10 : Structure of typical SSTI damping controller

(i) Without SSTI damping control

Torsional oscillations in Barh generator shaft getsamplified as shown in Fig 11. The 24.2 Hz torsionalmode of Barh generators is amplified and over thetime system goes unstable. Such a case in actualgenerators will result in fatigue of shafts (due totwisting forces acting on shafts).

At increased firing angle, problem of subsynchronous torsional interaction is morepronounced as damping offered by HVDC goes morenegative. Also if size of generator is large, possibilityof SSTI increases.

Fig 11: Torque deviations without SSTI damping control

(ii) With SSTI damping control

After enabling SSTI damping control in HVDCstability functions, DC current order is modified todamp out the sub synchronous oscillations. TheSSDC control modifies current order in ramp manner,so as to bring smooth changes in the current order.After detecting SSTI oscillations, the SSTI dampingcontrol becomes fully operative after about 1-1.5seconds.

Fig. 12: Torque deviations with SSTI damping control

The DC current order modulation for the HVDC linkis shown in Figure 13.

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4. CONCLUSION

This paper has presented the stability functions ofHVDC system and demonstration of theeffectiveness of stability functions by closed looptesting of HVDC controllers replica of 500 kV, 2500MW BalliaBhiwadi Bipole HVDC link ofPOWERGRID, using Real Time Simulator installedat POWERGRID, Gurgaon.

Two number of simulation cases, Power OscillationDamping (POD) and Sub Synchronous TorsionalInteraction (SSTI) damping were presented alongwith analysis of performance and effectiveness ofHVDC controller. The power system oscillations inboth cases were effectively damped out by thecontroller and the connected AC system is stable.The post fault frequency and voltages are stablewithout any change in final value.

It is seen that the key stability functions of+/- 500 kV, 2500 MW Ballia Bhiwadi Bipole HVDClink have the capacity to effectively damp theoscillations generated due to AC system disturbanceswith fast controller action and optimum response.

BIBLIOGRAPHY

1. Resources at Power System simulator lab atPOWERGRID, Gurgaon.

2. He, J.; Lu, C.; Wu, X.; Li, P.; Wu, J., Design andexperiment of wide area HVDC supplementary

Fig 13: DC Current order modulation by SSTI damping control

damping controller considering time delay inChina southern power grid, Generation,Transmission & Distribution, IET , Vol. 3, no.1,pp. 17,25, January 2009.

3. EPRI Report EL-2708, HVDC System Controlfor Damping Sub synchronous Oscillations,Final Report of Project RP1425-1, Prepared byGeneral Electric Company , October 1982.

4. Smed, T.; Andersson, G., Utilizing HVDC todamp power oscillations, Power Delivery, IEEETransactions on, Vol. 3, no. 2, pp.620,627,Apr 1993.

5. To, K.W.V.; David, A.K.; Hammad, A.E., Arobust co-ordinated control scheme for HVDCtransmission with parallel AC systems, PowerDelivery, IEEE Transactions on , Vol. 3, no. 3,pp.1710,1716, Jul 1994.

6. Piwko, R.J.; Larsen, E.V., HVDC SystemControl for Damping of SubsynchronousOscillations, Power Apparatus and Systems,IEEE Transactions on, Vol. PAS-101, no. 7,pp. 2203, 2211, July 1982.

7. Changchun Zhou; Zheng Xu, Damping analysisof subsynchronous oscillation caused byHVDC, Transmission and DistributionConference and Exposition, 2003 IEEE PES,Vol. 1, no., pp. 30,34 Vol. 1, 7-12 Sept. 2003.

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STATCOM and Hybrid STATCOM Solutions Based onChan-Link Multilevel Converter Technology for

the Electrical Transmission Network

S. Hutchinson1 M. Halonen2ABB, Sweden

1 [email protected] [email protected]

SUMMARY

Todays electric power transmission grid is ever-growing, and the demand for flexible actransmission systems (FACTS) such as the StaticVar Compensator (SVC) and STATCOM areincreasing with it. Rising costs and increasedenvironmental concerns make it more difficult tobuild new power generation and transmissionfacilities, and the need for renewable penetrationin the electric power grid puts additionalconstraints on these efforts. These challengesamong many others create a demand forinnovative solutions to increase efficiency inpower transmission and to integrate renewableenergy sources, and these solutions must betailored for the specific needs of each power grid.SVC Light is ABBs brand name for its STATCOMtechnology, and the newest development for SVCLight utilizes the chain-link multilevel converterconcept to provide a unique solution to thesegrowing demands. SVC and STATCOM systemscan provide reliable reactive power support tothe grid during steady state conditions to increasetransmission capacity and prevent voltagecollapse as well as act quickly during transientdisturbances to help support the network andsupply additional transient stability.

This paper intends to: (1) address the workingprinciples of the multilevel chain-link STATCOMtechnology along with some of its inherentadvantages, (2) introduce the newest member ofthe FACTS family, the Hybrid STATCOM,

(3) provide a brief comparative analysis of theSVC, STATCOM, and Hybrid STATCOM systems,and 4) give an overview of some of the key issuesin electrical transmission networks related to thedesign of such systems.

Keywords: STATCOM, Hybrid, SVC, SVC Light,chain-link, multilevel, FACTS, Voltage SourceConverter (VSC), Thyristor Switched Capacitor(TSC), Thyristor Switched Reactor (TSR)

1. SVC LIGHT INTRODUCTION

SVC Light is ABBs brand name for its STATCOMtechnology. The newest development of SVC Lightbuilds on the legacy of clean, reliable reactive powersupply and utilizes the chain-link modular multilevel(MMC) converter concept to provide a uniquesolution to growing demands in the power system.

FACTS technologies have been installed around theworld since the early 1950s. ABB helped to pioneerthe development in ac/dc converter applications forHVDC and STATCOMs in the late 1990s. Over theyears, these versatile concepts have been well provenfor many applications in transmission grids to helpstabilize and support the network during steady-stateand transient conditions. SVC Light was firstintroduced in 1997 and has been continuallyimproved and developed since then to deliver robust,reliable reactive power to the grid through voltagesource converter (VSC) technology. Furtherdevelopments in recent years have combined theVSC technology with that of SVC classic thyristorswitched capacitor (TSC) and reactor (TSR) branchesto create a Hybrid STATCOM solution [1]. This enablessuperior performance over the full dynamic range ascompared to the classical SVC.

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The first STATCOM installations made use of 2- and3-level converter schemes [2], which will be discussedin section 2.1. A low-order harmonic filter wasnecessary in these early installations to prevent theharmonics generated by the STATCOM fromnegatively impacting power quality on the grid.Recent advances in the technology (i.e. the use ofMMC) has helped to eliminate the need for low-orderharmonic filters in most applications.

2. MULTILEVEL TECHNOLOGY

The main difference in the new generation of SVCLight is the multilevel chain-link configuration ofthe VSC branch. This chapter will further describeits working principles and effectiveness.

2.1 Working Principles

2-Level and 3-Level converters

Level in this context refers to the number of dcvoltage positions available when synthesizing asinusoidal wave form. See the figure below.

In Figure 1(a), there are two possible positions whenoperating the switches, either Ud+ or Ud-. In Figure1(b), there are three possible positions since the dccapacitor has been split into two parts with fourswitches to select between positions. These positionscorrespond to Ud+, Ud-, and also a zero-voltageposition.

Once the low order harmonics are filtered out, theproduced choppy waveforms will look likefundamental frequency sinusoids and can besynthesized based on the voltage at the Point ofCommon Coupling (PCC) to be either capacitive or

inductive, that is to produce either a leading or laggingcurrent.

Multilevel chain-link converters

The multilevel chain-link solution is built up bylinking H-bridge modules in series with one anotherto form one phase leg of the VSC branch. Figure 2(a)shows a single H-bridge with 4 IGBTs, andFigure 2(b) shows a configuration in which four H-bridge modules make up each of the three phase legs.

In Figure 2(a), there are three possible voltage levelsdepending on the switching arrangement of theIGBTs: +Udc, -Udc, and a zero-voltage, similar to the3-level converter shown in Figure 1(b). For the casewith 4 modules connected serially as in Figure 2(b),there are 9 possible voltage levels: 4 in the positivedirection, 4 in the negative direction, and a zero-voltage, depending on how the IGBTs are switched.

It is noted that the more modules included in thedesign, the smoother the waveform will be. Thenumber of series-connected modules is primarilydetermined based on the power rating of theSTATCOM. Additional modules may be necessarydepending on requirements for overvoltage ride-through (see section 4) and harmonic distortion. Thewaveform can be further synthesized by use of pulse-width modulation (PWM) to reduce the lower levelharmonics present. Signals with PWM processing tomatch a sinusoidal reference can be seen in Figure 4in the next section.

Practically, the implementation of an MMCconverter-based SVC Light can be seen in Figure 3.Note that this implementation shows 8 modules inseries as opposed to the 4 shown in the exampleabove. One of the main differences with this designas opposed to 2- or 3-level is that the dc link isdistributed into several separate capacitors as canbe seen in the single line in Figure 2(b) and in thephotograph in Figure 3. Each IGBT is inside of amodular housing which is made up of a number ofsub-modules of enhanced Press-Pack typesemiconductor chips which have passed rigorousfailure mode and safety tests.

2.2 Why Multilevel?

The shift from 2- and 3-level converter technologyto multilevel for SVC Light has been based on aFig. 1: 2- and 3-level converter topologies (single phase)

(a) 2-level (b) 3-level

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number advantages, the most important of whichare outlined in this section.

Fig. 3: Modular H-bridge units, 2 stacks of 4 modules each

Harmonics and Power Quality

First, as discussed previously in section 2.1 themultilevel converter can deliver a smoother waveshape (i.e. less harmonics) with a lower switchingfrequency per each module. This concept isillustrated in Figure 4.

Notice that for the 2 and 3 level converter configu-rations, there are significant amounts of lower orderharmonics. These harmonics drive the need for filtersin 2 and 3 level converter based STATCOMinstallaions. When the multilevel chain-link conceptis applied, these harmonics decrease in magnitudeand are pushed to higher frequencies as can be seenin the bottom two plots in Figure 4(b).

Absence of low-order filters

Because of the reduced harmonics at lower orders,it is possible for the majority of cases to excludelow-order filters from the design of a multilevelconverter based on VSC technology. This is a greatadvantage as harmonic filter design can becumbersome and is heavily dependent on theimpedance of the network. This typically requirescomplex studies to be performed to evaluateharmonic impedances in the network and are onlyvalid for the cases which are studied, which means

Fig. 4: Harmonics in various converter topologies

Fig. 2: Multilevel chain-link converter setup

(a) H-bridge with IGBTs(single phase)

(b) 3-phase chain-linkof H-bridges

(a) PWM and voltage reference sinusoid (b) Voltage harmonics in per unit by integer number

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that if there are major changes to the network theimpedance will change and thereby effect theperformance of the filters. STATCOM systems basedon MMC technology are therefore more network-independent than 2- or 3-level STATCOMs orclassical SVCs, which all require low order filterdesign [3].

Reduced footprint

In addition to the reduced complexity of design dueto the absence of low-order filters, the reduction inrequired area (or footprint) for a STATCOMinstallation using MMC technology is an obviousadvantage. Low-order filters require a large spaceon site, and this space is not necessary for mostmultilevel chain-link converter STATCOMs. InFigure 5, a typical SVC Light layout can be seen fora 100 MVAR VSC.

This layout would need to be expanded if usingThyristor Switched Capacitors (TSCs) and ThyristorSwitched Reactors (TSRs) to create a HybridSTATCOM solution as there would be space neededfor the thyristor valves and the additional maincomponents and buswork. It is noted that a similarsized SVC installation might require twice the areaof the VSC-only example in Figure 5.

Modularity

Another advantage of the lack of low-order filters isthe ability to standardize components in theSTATCOM system due to less need to rate forharmonic currents and voltages. The valves,buildings, and topologies can all be modularized forspecific market needs. Overall, this would reduce

cost in the long run for markets which have continueddemands for STATCOMs.

Losses

The multilevel concept can use relatively lowswitching frequencies to produce a similar switchingpattern as a comparable 2- or 3-level converter dueto the fact that the series modules can stagger theirswitching patterns to come up with a highereffective switching frequency. This means that 10modules switching in a staggered arrangement wouldhave an effective switching frequency which is 10times higher than that of the individual IGBTswitching frequencies. The main impact of this is onlowering the losses of the STATCOM, as less frequentswitching means less switching losses.

3. STATCOM AND HYBRID STATCOMTOPOLOGIES

The V/I curve is a useful tool to see how a devicewill operate in the network, and is particularly usefulto evaluate under- and over-voltage abilities of thevarious FACTS devices. This section will examinethe V/I characteristic of a typical STATCOM andcompare it to that of a classical SVC as well as aHybrid STATCOM.

The classical SVC V/I characteristic is plotted inFigure 6(a). V/I diagrams for STATCOM and HybridSTATCOM solutions can be seen in Figure 6(b) and(c), respectively. For undervoltages, the STATCOMhas superior reactive power compensation ascompared to an SVC for similar sizing at 1 pu voltage.This can be seen by the fact that the current remainsconstant at low voltages. For a classical SVC on the

Fig. 5: SVC Light typical layout (100 Mvar)

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Fig 6: V/I diagrams for SVC, STATCOM and Hybrid STATCOM

(b) VSC-only STATCOM (c) Hybrid STATCOM

other hand, the current decreases linearly as thevoltage decreases meaning that the reactive powerwill be less as compared to the STATCOM by a factorof V (per-unit voltage). On the other side, a classicalSVC will outperform a STATCOM during overvoltagedisturbances for similar sizing at 1 pu voltage (seethe right-hand side of the V/I diagram). For each extraamount of voltage at the SVC point of commoncoupling (PCC), it will return more current andtherefore more reactive power by a factor of V (per-unit voltage) as compared to a STATCOM. More onovervoltage ride-through in Section 4.

It is noted that while a VSC solution may be neededfor a particular transmission application, in manycases (especially for very large MVAR operatingranges) the power from the VSC must also besupplemented by added thyristor-switched (TSC/TSR) branches to achieve the desired Mvar output.

This creates what is known as a Hybrid STATCOMsolution, combining the technologies of SVC classicand STATCOM. In some cases, there could also besome required capacitive offset due to telephoneinterference and network harmonic requirements.

This would shift the entire V/I characteristic in thecapacitive direction. For applications with less strictdynamic requirements (i.e. reaction time andconsecutive number of switching operations),mechanically switched capacitors and/or reactors(MSC/MSR) may be used instead of faster thyristor-switched branches.

Figure 7 shows one line diagrams of new STATCOMsystems, the multilevel SVC Light. Combining thebest of STATCOM technology with conventionalthyristor based SVC technology will optimize thepower system performance for under-voltageperformance, over-voltage performance, TOV atfault clearing, losses, speed of response, reliability,etc.

STATCOM or Hybrid STATCOM systems fortransmission applications normally make use of apower transformer between the power grid and themedium voltage (MV) busbar. On this bus a VSC isconnected in series with a coupling reactor. Inaddition, thyristor controlled reactors and capacitorsor mechanically switched capacitors and reactors canbe used. The voltage on the MV bus is typically in

(a) SVC

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the range of 15-30 kV irrespective of the voltagelevel on the mains. A normal transformer turn ratiois 400/25 kV. This large ratio results in very highshort circuit currents on the MV bus, frequently inthe range of 50-90 kA (rms symmetrical).

MSCs and MSRs in STATCOM hybrid solutionsprovide reactive power support during steady-stateconditions and replace one or several TSCs/TCRs/TSRs. It is preferable to locate the MSC or MSR onthe primary bus to reduce the size of the transformerand minimize the stress on the breaker elements.Mechanically switched banks have relatively lowlosses and the capital cost is lower compared to atopology based on thyristor switched elements.Although such external devices are relatively simpleand fairly economical, they come with somedisadvantages which have to be taken intoconsideration. Among these are (a) the obviousdischarge time of MSC banks of several minutesbefore re-energization, (b) the overall limited numberof switching cycles of a circuit breaker reducing theoverall flexibility of the installation, (c) difficulty tofind a breaker which fulfills the requirements of amedium voltage and high current, and (d) slowerreaction time compared to thyristor-based solutions.

Regarding point c above, it is essential that thebreaker switched elements (i.e. MSCs and MSRs) areonly to be used for steady state base load. It mustbe controlled in a way so that switching is minimized.The number of allowed circuit breaker operations istypically in the range of 2000-3000.

4. STATCOMS FOR ELECTRICALTRANSMISSION NETWORKS

There are many criteria that need to be consideredwhen designing a STATCOM; many more than can

be discussed in a paper of this scope. However, it isimportant to point out a few of these which arespecific to a STATCOM system that is intended forapplication in an electrical transmission network (asopposed to a lower-kV distribution network forexample). A few such criterion are: overvoltage ridethrough, undervoltage ride through, and reliability/availability.

Overvoltage ride through

For overvoltages in the network, an SVC has superiorvoltage support as compared to a STATCOM. This isdue to the fact that the Mvar output of an SVC varieswith the square of the voltage (i.e. as voltageincreases, the Mvar output increases quadratically).For a STATCOM, as voltage increases the Mvaroutput increases linearly. However, as shown insection 3, if a TSR branch is added to the VSC tocreate a Hybrid STATCOM solution, the solution cantake advantage of the inherent ability of the reactorto consume Mvar during overvoltages.

Most power utility applications for FACTS devicesinclude an overvoltage profile for which the devicemust remain in service at full inductive conductionwithout blocking/tripping. This can be seen forexample in IEEE 1031-2011[4], where the followingoverload curve is given.

Although this curve is intended for use in specifyingSVCs, the same should be valid for STATCOMapplications since they are frequently specifiedtogether as alternatives.

The important thing for a STATCOM during operationin overvoltage conditions such as this is that thedevice should not block or trip but rather providefull inductive output in order to ensure that the gridvoltage is stabilized. This requires that the

Fig 7: STATCOM and Hybrid STATCOM topologies

(a) STATCOM (b) Hybrid STATCOM-VSC/TSC/TSR (c) Hybrid STATCOM - VSC/MSC/MSR (d) Hybrid STATCOM - VSC/MSC/MSR

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STATCOM be sized appropriately (i.e. the numberof series-connected valve modules should beincreased) in order to accommodate an overloadrequirement. This has been the design philosophyfor SVC classic installations for decades and so shouldalso apply for STATCOM and Hybrid STATCOMsystems.

Undervoltage ride through

During undervoltages, it is known that a STATCOMhas superior voltage support capability as comparedto an SVC of similar size. This can be clearly seenwhen comparing the V/I characteristic curves forSVCs and STATCOMs in Section 3. The Mvar outputof a STATCOM is linearly dependent on voltage,whereas the Mvar from an SVC varies based on thevoltage squared due to the physical constraints ofthe capacitors.

Reliability and availability

Dynamic shunt compensation such as SVC andSTATCOMs are installed in electric powertransmission grids as a kind-of power systeminsurance to support the system during severecontingency conditions, and so they should act whenthey are needed. It is therefore essential to considerreliability and availability of the installed devicewhen specifying and designing. SVCs traditionallyhave a high degree of reliability with 2-3 forced stopsper year (due to component malfunction, lightningstrikes, or the like) and availability figures greaterthan or equal to 99%. STATCOMs therefore shouldbe required to have at a minimum these requirementsfor reliability and availability.

5. SUMMARY

In conclusion, ABBs latest version of the SVC LightSTATCOM technology has built upon the legacy of

Fig 8: IEEE 1031-2011 Overload profile for SVC

reliable reactive power supply and includedmultilevel modular chain-link converter technologyfor added benefits of reduced harmonics, smallerfootprint, and less losses. The multilevel VSC branchmay be combined with TSC and TSR branches (orMSC/MSR branches for applications with less strictdynamic requirements) to form a Hybrid STATCOMsolution.

BIBLIOGRAPHY

1. M. Ibrahim and M. Halonen, Dynamic ShuntCompensation for Power Utility Application,in Cigre-GCC Power, Bahrain, 2014.

2. R. Grunbaum, T. Larsson and B. Ratering-Schnitzler, SVC Light: A utilitys aid torestructuring its grid, in IEEE PES wintermeeting, 2000.

3. A. H. Al-Mubarak, B. Thorvaldsson, M. Halonenand M. Z. Al-Kadhem, Hybrid and Classic SVCtechnology for improved efficiency andreliability in Saudi transmission grid, in IEEEPES Transmission and Distribution Conferenceand Exposition, Chicago, 2014.

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SUMMARY

This paper will provide an update on the currentstate of the art with regards to the use of MediumVoltage DC links to provide transmission networkstyle flexibility to constrained distributionnetworks.

MVDC is starting to be considered as an optionfor enhancing transfer capacity and providingincreased power quality at distribution networks.There is a term starting to be used of soft open-point which can provide controlled powertransfer between two 11kV or 33kV distributiongroups, without affecting shortcircuit levels,voltage differences, loop flows or limitations dueto phase-angle differences. The 4-quadrantconverters can also provide reactive powersupport and voltage control at each end of the linkand multi-terminal is also feasible. There arefuture technology opportunities includingenhancement of existing corridors through theconversion of existing AC lines to DC.

This paper will provide a technology update aswell as information on recently deployed projectsranging from linking of oil and gas platforms,through to an urban infeed. It will summarise thebenefits of MVDC and the applications where itmay provide a competitive or preferentialalternative solution to conventional technology.

RXPE is part of a national project in China todevelop a number of new key technologiesincluding MV level DC/DC transformers and a costeffective and practical MVDC circuit breaker. Formany years we have been leading thedevelopment of multi-level converters for utilityapplications, starting with Statcoms and now forVSC-HVDC.

Keywords: MVDC, Distribution, Soft open-point,VSC

1. INTRODUCTION

In DC circles, the story of the battle between thecurrents is well known and is almost slipping intopopular folklore via recent media conversations suchas War of the Currents Tesla Vs Edison [1].

For those unaware of this past battle, in contrast totodays power systems, the first commercialdistribution of electrical energy was realized usingdirect current (DC). Edisons Pearl Street installationwas commissioned in 1882, featuring a 24-km longtwo-wire cable system of copper conductors thatdistributed electricity at a voltage of 110 V forincandescent lighting in Lower Manhattan, NewYork. But, due to the high losses caused by the lowdistribution voltage, the dc technology at that timewas outperformed in terms of efficiency by itsalternating current (AC) counterpart, which wasdeveloped in the middle of the 1880s byWestinghouse (Tesla). The ac system could usetransformers (a voltage converter invented andpatented first in Europe) to step up the generatorvoltage to high levels, that are suitable for long-distance transmission of electric power.Consequently, the medium-voltage alternatingcurrent distribution and transmissionsystem operated with lower currents and hadconsiderably lower losses than low-voltage DCsystems.

Consequently, with the state-of-the-art technologyin those early days, ac systems could provide a higherefficiency. Soon after, thanks to the invention of thetransformer, three-phase AC transmission anddistribution systems became and have been eversince the cornerstone of efficient and reliableelectricity supplies system.

MVDC The New Technology for Distribution NetworksAshish Bangar Lalit Tejwani1

RongXin Power Engineering, UK RongXin Power Electronic India Pvt Ltd, India

1 [email protected]

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Today, owing to considerable progress in the fieldsof power semiconductor devices and cabletechnology, DC is becoming a more significantcomponent of the modern power system, and someare talking of a second battle in years to come of ACversus DC.

At the Distribution Network level (

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Fig. 1: 12 MVA / 11 kV Soft Open Point formed with back-to-back converters

The 4-quadrant converters can also providereactive power support and voltage control ateach end of the link and multi terminal is alsofeasible.

MVDC does have some complicating factors thatneed to be considered such as:

Short lifetime equipment (~15-25yrs comparedto 30-40yrs) incurs greater replacement costs;this needs to be factored into any business case.

Substation space still required for DC converterseven if provided as containerised solutions.

MVDC losses are higher than transformers orcircuit-breakers, so losses need to be consideredat an overall network level.

The following sections consider the use of MVDC atan individual substation level as well as at a networklevel.

2.2 Substation Reinforcement Opportunities

MVDC can be used at a substation level to provideincreased power transfer capacity in situations wherethe up-rating cables and transformers is disruptive,expensive and hard to achieve. Some of these optionsinclude:

Prevent voltage limits impinging before thermallimits by increasing the control options in thenetwork.

Dynamically rebalancing voltage and flows toavoid limit on individual phase.

Even up loading between transformers or createnew routes power between substations or partsof substations.

Alleviate power quality problems by splitting

These can be achieved by using MVDC in aconfiguration by the so-called Soft Open Point(SOP) application (Figure 1). This is essentially two

power converters connected in a back to backarrangement across a conventional Open-Point, orbusbar section.

MVDC in a back-to-back configuration can providesoft-open point capability to improve powerbalancing at heavily loaded substations and improvereliability, while maintaining short-circuit levelswithin circuit-breaker ratings. These back-to-backconfigurations also allow the system to overcomepotential phase-angle differences and circulatingpower-flows when the MV substations are fed fromdifferent transmission infeeds.

By using the controllable power transfer capability,and individual voltage control capabilities of eachconverter, it is possible to dynamically force activepower flows to balance feeders (and phase loadings),and thereby better utilise the existing networkcapacity.

This has further advantages where it is not otherwisepossible to close up this open-point due to loop-flows, short-circuit levels, voltage limits, or poorpower quality on one of the groups.

2.3 Corridor Reinforcement Opportunities

At HV transmission level, there has previously beendiscussion and feasibility studies to convert anexisting AC right of way to DC, as there can beconsiderable increases in power transfer. Thisbecomes particularly important where the corridoris highly constrained for non-electrical reasons suchas environmental restrictions or restricted corridorwidth.

MVDC can provide capacity increases on existingMV circuits through a combination of increasedcurrent and voltage operation and avoidance ofvoltage drop, phase-angle and power-factorlimitations. MV here is defined as from 10 kV up to70kV - but is not a limiting term.

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MV networks typically only have a maximum of fourterminals within a specific protection group, thereforelimiting the likely maximum number of converterstations within a specific multi-terminal group.Within each protection group the circuit breakersare only at each end, therefore for a MVDC equivalentthere is no requirement for DC circuit breakers.

In the UK, 33 kV circuits are typically in the orderof 20-25MVA and 300-400A continuous winterrating. They are generally operated with a singleearthing point at the bulk supply substation feedingfrom 132kV, remote ends are delta connected withoutearthing transformers. The 33 kV networks are oftenoperated as part of a ring or mesh system, or as radialcircuits in parallel with separate supply points andthen interconnected on the 11 kV busbar of thesecondary substations to provide security.

By converting a specific 33kV circuit or group toMVDC, it is in theory possible to increase the specifictransfer capacity of that circuit compared to thenominal AC rating. Clearly as this is now fullycontrollable, it may have additional network capacityincreases (i.e. removal of power-flow or voltagelimitations). The MVDC capacity increase is basedon the following fundamentals:

DC able to use full peak voltage capability ofAC circuits compared to the RMS rating (1.4x)

DC does not suffer from skin effect so potentialfor increased current capability withoutaffecting sag (1.1x)

DC will need metallic return so can only utilise2 of 3 conductors on single circuit (0.67x)

Existing 33kV AC circuits run with single groundpoint at BSP, AC insulation rated for 1.7xnominal voltage for single-phase voltagedisplacement at remote ends. DC does not havevoltage displacement if grounded at both ends,therefore can utilise full insulation capacity(1.7x)

By combining these factors together, the totaltheoretical increase from simple conversion of singlecircuit 33kV AC to MVDC can be shown up to 185%.This is based on the following comparison for atypical point to point application:

AC: 400A at 33kV gives 21.7MW at 0.95pf

DC: 440A at 45kV gives 40MW with 0.9pf(assumes 45MVA converter)

DC creepage and contamination issues may requireincreased safety factors so reducing away fromtheoretical voltage capabilities.

If a double-circuit is available, then the third wirecan be also utilised for current transfer and thetheoretical transfer capacity can be increasedsignificantly.

It is possible in the MVDC configuration to includehybrid operation of the existing cable or overheadline to provide fail-safe use for consumer security inevent of converter maintenance/failure by revertingto AC operation. This could also provide a lossreduction option of operating at AC at low loads andwould only require a bypass switch at eachsubstation terminal.

Clearly before any conversion of an existing ACcircuit, it is important to test and verify theperformance of all affected equipment such asinsulators, line isolators, cables and cable joints underDC conditions.

3. MVDC CASE STUDY - WENCHANGPROJECT

Recently a revolutionary MVDC system has beenprovided for CNOOC as part of the Wenchang 19-1A to 14-3A platform submarine cable repair project.This was provided by the specialist power electronicequipment manufacturing RXPE on a turn-key basis.

The Wengchang project was started in 2010 as a resultof customer negotiations and participation with theRXPE business, product development and

Fig. 2 : MVDC corridor capacity enhancement

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engineering departments to help address the urgentloss of supply security problem.

3.1 Problem: Failed Transmission Cable Phase

Initially, the power transmission between Wengchangoff-shore drilling platforms 19 1A and 14 3Aused three 29.2 km long single phase submarinecables. Due to the switching surge impact damagingthe insulation of one of the cables, the powertransmission between the two platforms wasinterrupted, and so only Diesel Generator could beused on the remote platform.

However, the reliance on the Diesel generator wasundesirable to the client due to the following reasons:

High cost of operation and maintenance.

Serious air and noise pollution.

Limited generator capacity.

Serious energy waste.

The alternative to MVDC for CNOOC was to installadditional submarine cables and surge mitigationequipment. This was viewed as time-consuming andexpensive.

3.2 Solution: MVDC Converter

Extensive joint design evaluation was undertakenbefore settling on their existing high power MV driveplatform to form the technology platform for theMVDC application. This is a multiple branchconverter topology that has good efficiency,flexibility, modular configuration, and can directly

connect at MV AC voltage levels to remove therequirement for external transformers.

Each end of the link was installed into marinisedcontainers, fully tested onshore before beingtransferred to the offshore platforms and installed.

Using the MVDC system, the remaining two phasesof AC submarine cables with good insulation serveas the positive and negative DC cables, and thedamaged phase served as the neutral line. Power wasthen transported to the 14-3A platform via the MVDClink from 19-1A. The 14-3A platform inverter devicethen transforms the DC into AC, and supplies powerfor the 14-3A, 8-3A and 8-3B platforms via theexisting AC distribution network.

The MVDC was configured as a symmetrical bipole,such that the positive and negative poles are identicaland each pole can work independently of the otherto provide security of supply to the remoteplatforms. Powerflow was predominantly uni-directional from 19-1A to 14-3A.

The MVDC system main parameters are:

Rectifier Substation (19 1A)

Converters: 2 x 4MVA, AC 10.5 kV / DC 15 kV

Transformer: 10.5 KV / 1320 V x 8, 400 V x 1(auxiliary), 4 MVA

Power module: AC Input 1320 V, DC Output 1860V, Rated Power 500 kVA

Valves: 8 power modules connected serieson DC side, DC output 15 kV

Fig. 3: 8 MVA / 15 kV MVDC project

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DC reactor: 0.5 mH, DC 15 kV / 400 A

Dimension: Two containers, each 6m x 5m x 4.5m

Inverter Substation (14 3A)

Converters: 4MVA, DC 15 kV / AC 400 V, AC35 kV

Transformer: 400 V x 14 / 400 V x 1, 35 kV x1, 400 V x 1 (auxiliary), 4MVA

Power module: DC Input 1100 V, AC Output 400V, Rated Power 285 kVA

Valves: 14 power modules connectedseries on DC side, AC output 400 V

DC reactor: 0.5 mH, DC 15 kV / 400 A

Dimension: Two power module containers,each 6m x 2.7m x 2.9m; onetransformer container, 9.6m x2.8m x 2.3 m; one reactorcontainer, 9.6m x 2.8m x 2.1 m

Performance Feedback

The MVDC installation has been operatingsuccessfully since its installation in 2013, with goodavailability and reliability, and the project hassatisfied the client requirements.

CNOOC has subsequently deployed a secondMVDC system to link an additional two platformson another group. A technology demonstratorproject has also recently been implemented usingthe same technology for an distribution utilityproviding MVDC infeed to a constrained urbansubstation.

4. MVDC CASE STUDY DCDISTRIBUTION TECHNOLOGYDEVELOPMENT PROJECT

There is a China National Project currently underwayto develop and trial next generation DC DistributionNetwork technology. This is to undertake keytechnology research and application for VSC-DCintelligent distribution grids. The project forms partof the national high-tech research and developmentplan (863 plan) Sub-topics 4. The responsiblepartners are: RXPE, Tsinghua University, Shenzhenpower supply bureau, Zhejiang University, and CSGAcademy of Sciences.

The topic purpose can be summarised as follows:

Improve power supply capacity, solvingdistribution corridor limiting problem

Fig. 4 : 8 MVA / 15kV MVDC project SLD

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Improve power quality, solving the problem ofsensitivity of emerging industries for voltagequality

Renewable energy through DC connected intogrid, improve reliability and utilisation.

Key technology and application research to beundertaken on DC Distribution grid as part of thisproject includes:

1. Research of intelligent DC distribution networkbasic topology and optimisation.

2. DC Distribution grid grounding method research

3. LV DC Distribution network voltage levelselection

4. DC distribution network energy storageequipment optimal selection and capacityconfiguration research.

Some of the specific tasks being undertaking are todevelop and test some of the new equipment thatwill be required if DC distribution networks are tobecome a reality. The specific equipment beingdeveloped and tested as part of this project are:

4.1 MVDC Circuit Breaker

This is to undertake the theoretic research anddevelop an engineering prototype of an MVDC circuitbreaker. The specified parameters are a rated voltageof 7.5kVdc with a 5ms interrupt time and 1kArated current.

Two different operating principles are beinginvestigated as part of this project, one based on apower electronic switch, and the other based onartificial zero-crossing.

4.2 MVDC Secondary Transformer

This is to undertake the theoretic research anddevelop an engineering prototype of a secondarysubstation to provide step-down from MVDC toLVDC.

The specified parameters are rated DC voltages of7.5kV/400V, and a rated power of 200kW. This isonly required to be uni-directional at this stage.

Two technology proposals being considered, the firstusing a high frequency transformer and MV inverterwith a simple diode bridge LV stage. The secondapproach is to use a multi-branch topology withbridges in series on the MV side, and parallel on theLV side and multiple smaller isolation transformers.

4.3 MVDC Application Standard

A final important part of the project is to develop aninitial standard of MVDC for the china powerindustry. This is an important step in developing thesupply chain as well as industry acceptance in thetechnology and application.

4.4 Project Timeline

Project timeline is to run from August 2013 Dec2015. This project under-pins part of the wider drive

Fig. 5 : Wengchang MVDC project installed (14-1A)

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towards better understanding the application,opportunity and resolving barriers to the deploymentof DC distribution networks.

5. CONCLUSION

This paper has hopefully shown to the reader thetechnology and justifications behind MVDC and thatthere is a place for MVDC within todays distributionnetworks. While still arguably niche applications atthis stage, there is growing awareness and widerindustry movements suggesting that there may be acoming rematch of the classic Battle of theCurrents.

It is also important to note that this is not just atechnology for mature networks, but also hassignificant application and relevance for developingcountries facing rapid demand growth and the needto maximise available network and generationresources.

Electricity demand is growing every day and theSmart Grid must be introduced in the near future.This calls for reliability and flexibility in the

distribution system. At present, much of the majorityof progress in developing DC-based technologies hasoccurred at either the EHV (>100kV) or low voltage(

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RTDS Studies for the TWENTIES OLC

R.A. Rivas1 M.S. Erixon L. Wall J. Martin J. Hidalgo C. Rodriguez J. SotoABB, Sweden ABB, Spain REE, Spain

SUMMARY

An OLC is a new FACTS device for seriescompensation of transmission lines. It consists ofa number of series connected reactor banks, eachof them in parallel with a conventional breaker.For the power system of Red Elctrica de Espaa(REE), within the framework of the TWENTIESproject, ABB will supply and commission a firstOLC device in 2013.

This paper presents the results of real-time powersystem simulator tests performed on the actualcontrol system of the TWENTIES Overload LineController (OLC). The OLC will operate in serieswith a 220-kV transmission line of REE.

The real-time digital simulator (RTDS) modelincluded power equipment connected to 400-kVand 220-kV substations of REE, as well as thedynamic representation of hydroelectric, thermaland wind power generation. Disturbances such asline energization, de-energization, faultapplication and automatic re-closing weresimulated and the different control modes andblocking functions of the OLC were tested.

The tests verified the correct dynamicperformance of the OLC control system underexpected operating conditions and contingenciesin the power system network of REE.

Keywords: Series Compensation, OverloadMitigation, Power Flow Control, FACTS, RTDS.

1. INTRODUCTION

An OLC is a new FACTS device for seriescompensation of transmission lines. It consists of anumber of series connected reactor banks, each of

them in parallel with a conventional breaker. For thepower system of REE, within the framework of theTWENTIES project, demo #6 [1], ABB will supplyand commission a first OLC device in 2013. The OLCwill be connected with the 220-kV network of REEat the Magalln substation, in series with a 24-kmtransmission line between that substation and theEntrerros substation.

The general objective of the OLC is to provide theline with power flow control and an operation closerto its natural limits, thus increasing the transmissioncapability for renewable energy evacuation. Thespecific objectives of the OLC are:

1. Maintain the power flow through the line belowoverloading conditions by inserting the seriescompensation (inductive impedance step)required.

2. Maintain the power flow through the line insidean operating band by connecting anddisconnecting the impedance steps required.

3. Provide the line with series compensation equalto the setpoint () chosen by the systemoperator.

4. In case of emergency (hazardous overload),provide the line with back-up overloadprotection by inserting the amount of seriescompensation required.

This paper presents the results of the OLC/PowerSystem Interaction tests performed for theTWENTIES project, demo #6. The tests were carriedout with an RTDS [2] in Vasteras, Sweden, on Sep 11-13, 2012, and were a second (and final) part of theFactory Acceptance Tests (FAT) for the OLC controlsystem.

The paper is organized as follows. Section 2 givesan overview of the OLC and its possible control1 [email protected]

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modes. Section 3 presents the network under studyand describes the scenario that motivated the OLCsolution. Section 4 provides an overview of theRSCAD [3] model and explains the RTDS setup,depicting the interface among the RSCAD model, abreaker simulator, and the ABB-MACH controlsystem [4]. Section 5 presents the results and analysesof the case studies, specifically network verification,line energization/de-energization, fault applicationand automatic reclosing, possible control modes,emergency control, and degraded mode operation.Finally, Section 6 states the conclusions of the work.

2. THE OVERLOAD LINE CONTROLLER

Figure 1 shows a simplified single-line diagram ofthe OLC.

The OLC is connected with the 220-kV grid via twocircuit breakers, one labeled CB1 to connect it withthe Magalln substation and another labeled CB2 toconnect it with the Magalln-Entrerros transmissionline. It can also be completely bypassed through athird breaker (CB3).

The OLC consists of three Mechanically SwitchedSeries Reactors (MSSRs) rated 12 MVAr (2.6 /phase), 24.5 MVAr (5.2 ?/phase), and 48 MVAr (10.3/phase). The bypass breakers (QS1, QS2 and QS3)are conventional circuit breakers rated 3150 A.

The reactor combinations allow for 8 differentsettings, which provide step-wise control of the power

flow through the line. Figure 2 depicts the possiblesettings.

The OLC is controlled by a microprocessor basedcontrol system. The control system is based on theMACH concept, built around an industrial PC withadd-in circuit boards and I/O racks connected viastandard type field buses. Dedicated voltage andcurrent transformers provide the control system withnetwork variables employed in the OLC control.

The control system provides facilities for OLC controleither from the Operator Work Station (OWS) in theOLC control room or remotely via a Gateway Station(GWS) in communication with the SCADA systemof REE.

The OLC control system is structured in the followingmodes:

1. Automatic Control

2. Manual Control

And the Automatic Control (normal mode ofoperation) is divided into:

Closed Loop Control:

z Power Limitation Mode (P-lim)z Power Regulation Mode (P-set)

Open Loop Control:

z Impedance Mode (Z-set)z Maximum Compensation Mode (Z-max)

Fig. 1: Simplified Single-Line Diagram of the OLC

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In addition, there is Emergency Control to providethe line with back-up overload protection.

line with maximum series compensation (18.1 ) ifdesired by the operator.

E. Emergency Control (P-emerg)

The objective of P-emerg is to provide the line withbackup overload protection. This function can beactivated or deactivated by the operator andautomatically inserts the series compensationrequired whenever the power flow S exceeds athreshold value. When activated, it runs in parallelwith the other operating modes.

P-emerg ensures fast compensation duringcontingencies, particularly when the OLC is on Z-set mode (P-limit mode and P-set mode OFF), anddoes not block the insertion of more impedance afterbeing activated. The operating time (detection timeplus actuation time) is much shorter than 3 seconds,which is the maximum time response of the linebreaker inter-trip employed by REE.

3. NETWORK UNDER STUDY

The power network under study is located in thenortheast part of Spain (Autonomous Communityof Aragn) and is characterized by high wind energypenetration. Due to this, during certain moments ofhigh wind production, overloads may appear in the220-kV Magalln-Entrerros line after the trippingof an alternative path through the 400-kV grid.

Nowadays this overload is reduced, firstly by doingtopological maneuvers in the 220-kV Magallnsubstation so that part of the power flow throughthe 220-kV Magalln- Entrerros line can beredirected to alternative paths. If this measure is notsufficient, re-dispatch of conventional generationand, as a last resort, curtailment of wind energyproduction should be carried out.

The OLC will control the power flow through the220 kV Magalln-Entrerros line, delaying, if notavoiding, taking the measures mentioned above

4. RSCAD MODEL AND RTDS SETUP

To test the operation of the control system an RSCADmodel was created as of a PSSE [5] network model.The RSCAD model included lines, transformers, seriesbranches, loads, and shunt compensators connectedto five 400-kV substations and six 230-kVsubstations of REE, as well as the dynamic

Fig. 2: OLC Settings for Step-Wise Control

A. Power Regulation Mode (P-set)

The objective of the P-set mode is to keep the powerflow inside an operating band (hysteresis band) byautomatically switching in and out the requiredamount of series compensation. If S becomes greaterthan the setpoint (P-set) chosen by the operator, thenthe required compensation is switched in. If Sbecomes lower than P-set min (

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representation of hydroelectric, thermal and windpower generation. The model also contained theexplicit representation of two 400-kV lines, parallelto the 220-kV circuit, which overload the 220-kVMagalln-Entrerros line when getting out of service.

Fig. 3: RSCAD model of MACHTM - controlled OLC

Figure 3 shows the RSCAD model of the MACH-controlled OLC. Table 1 lists the RSCAD buildingblocks employed to represent the power systemequipment.

Table 1: RSCAD Building Blocks Employed

Component Building Block [3]

Lines rtds_sharc_sld_TLINE

Transformers rtds_XRTRF3

Shunt resistors, reactors, capacitors rtds_sharc_sld_SHUNTRLC

Series reactors rtds_sharc_sld_SERIESRLC

Thermal and hydro generators rtds_sharc_sld_MACV31

Hydro exciters rtds_sharc_ctl_SCRX

Hydro governors rtds_sharc_ctl_HYGOV

Thermal exciters rtds_risc_ctl_ESST4B

Thermal turbine/governor rtds_GGOV1.def

Wind farms, DFIG (small delta t) rtds_vsc_TRF3, rtds_vsc_BRC3,rtds_vsc_SCL_TRF1, rtds_vsc_TLTERM,rtds_vsc_TLCONS, rtds_vsc_T3PHTW,rtds_vsc_RES1, rtds_vsc_INDM,rtds_vsc_VBUTTER,rtds_vsc_LEV2BB, rtds_vsc_3LGFIR,rtds_vsc_TRIWAV

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Figure 4 illustrates the RTDS setup implemented. Asshown, the actual MACH control system andbypass breaker I/O interface cabinets to install onsite are connected with an RTDS and a bypass breakersimulator. As a result, the actual controller is able toreceive voltage and current signals from the networkmodel running on the RTDS and send back thecorresponding reactor insertion/bypass orders. Toclose the test loop, the RTDS is provided with theopening/closing orders from the bypass breaker I/Ointerface cabinets via the open/closed indicationsignals from the bypass breaker simulator.

5. CASE STUDIES

The following studies were carried out:

z Verification of Network Equivalentz Line Energization and De-energization

z Fault Application and Automatic Reclosingz Impedance Mode (Z-set)z Maximum Compensation Mode (Z-max)z Power Regulation Mode (P-set)z Power Limitation Mode (P-limit)z Emergency Control (P-emerg)z Degraded Mode Operation

z Verification of Network Equivalent

To validate the implementation of the networkequivalent in RTDS, the power flow through the220 kV Magalln-Entrerros line and the voltages atthe 220 kV Magalln and Entrerros substations werecompared with those obtained from the originalPSSE network model. Table 2 lists the power flowsand voltages thus obtained.

Fig. 4: RTDS Setup

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Table 2: Power Flow and Bus Voltage Verification

Quantity PSSE RTDS Diff. [%]

Voltage Magalln [pu] 1.0284 1.0300 0.16

Voltage Entrerros [pu] 1.0265 1.0260 -0.05

P sent from Magalln [MW] 272.5 271.3 -0.44

Q sent from Magalln [MVAr] -32.1 -34.1 6.32

S sent from Magalln [MVA] 274.4 273.5 -0.32

The short-circuit levels were measured by simulatingthree phase faults at the 220-kV Magalln andEntrerros substations, respectively. Table 3 tabulatesthe values of short-circuit level obtained from theRTDS.

Table 3 : Short-circuit Levels

Substation Short-circuit level

Magalln 220 kV 15.40 kA 5868 MVA

Entrerros 220 kV 13.96 kA 5320 MVA

As indicated in the tables, the RTDS networkequivalent has the correct values for the verificationof the dynamic performance of all of the OLC controlmodes and blocking functions.

z Line Energization and De-energization

To verify that the OLC control blocks the automaticswitching in and out of the reactors when the220-kV Magalln-Entrerros line is energized or de-energized, tests with different value of initiallyinserted reactance, P-set setpoint and P-emerg levelwere performed.

Figure 5 shows the waveforms for the case of lineenergization with P-set=230 MVA, P-emerg=300MVA, and X initially inserted=10.3 . As depicted,there was no change of inserted impedance duringthe transient. Neither was there any change ofimpedance after reaching the steady state(approximately after 1.8 s) as the power flowremained below the setpoint (230 MVA).

z Fault Application and Automatic Reclosing

To verify that the OLC control blocks the automaticswitching of the reactors when there is short-circuiton the 220-kV Magalln-Entrerros line, single- andthree-phase faults were applied on both ends of it.

As before, the tests were carried out with P-set andP-emerg enabled.

Figure 6 depicts the waveforms for the case withsingle-phase fault at Magalln and three-phaseautomatic reclosing. As illustrated, the control systemavoided changes of inserted impedance during thetransient. At steady state the power flow was alsobelow the emergency level, so changes of insertedimpedance were not carried out either.

z Impedance Mode (Z-set)

To verify that the impedance steps chosen by theoperator are carried out by the OLC control in termsof inserted reactors, the setpoint was changed from0 to 18.1 using the steps depicted in Figure 2.Figure 7 illustrates the waveforms obtained for thecase in which the setpoint went from 2.6 to 5.2 and later to 7.8 . As shown the OLC control is ableto achieve the insertion of the amount ofcompensation requested by the operator. As can alsobe noticed, it is necessary in some cases to first stepup in impedance and later step down to reach thedesired setpoint. For example, to switch from 2.6 to 5.2 the OLC has to close QS1 and open QS2.However, to avoid a sudden increase of power flow,it first opens QS2 (transiently inserting 7.8 intothe line) and later closes QS1 .

z Maximum Compensation Mode (Z-max)

The purpose of this test was to check that all thereactors are inserted when maximum compensationis requested by the operator. Figure 8 shows thewaveforms obtained with X initially inserted=0.

z Power Regulation Mode (P-set)

To verify that the power setpoints (MVA) chosen bythe operator are carried out by the OLC control in

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terms of power regulation, the power flow throughthe 220-kV line was first increase and later decrease.To increase and decrease the power flow throughthe line, the power output of the equivalentgeneration located at the 220-kV Entrerrossubstation was ramped down and up, respectively.

Figure 9 depicts the waveforms obtained for theincrease of power flow when the following settingswere used: P-set=130 MVA, hysteresis=10% (P-setmax=130 MVA, P-set min=117 MVA), P-emergactivated, X initially inserted=2.6 and initialpower flow=123 MVA (inside hysteresis band). Asshown, the OLC control is able to perform theautomatic insertion and bypass of the reactorsrequired to keep the power flow inside the chosenhysteresis band. Note, however, that afterapproximately 52 s compensation greater than18.1 would be needed to keep the power flowbelow 130 MVA.

z Power Limitation Mode (P-lim)

To verify that the power setpoints (MVA) chosen bythe operator are carried out by the OLC control interms of power limitation, the power flow throughthe 220-kV line was also varied. As before, the powerflow through the line was modified by changing thepower output of the equivalent generation locatedat the 220-kV Entrerros substation.

Figs. 10 and 11 illustrate the waveforms obtainedfor a case with the following settings: P-lim max=210MVA and P-lim min=100 MVA. As shown in Figure10, the OLC control is able to perform the automaticinsertion and bypass of the reactors required to limitthe power flow. Note also (see Figure 11) that theinserted reactors are switched out only afterobtaining power flows below P-lim min (100 MVA),and that at that point the power reference becomestemporarily equal to P-lim max (210 MVA) to avoidswitching in reactors during the transient.

z Emergency Control (P-emerg)

To verify that the OLC inserts the compensationrequired as soon as the threshold (MVA) foremergency control is exceeded by the power flowthrough the line, the system was subjected to linecontingencies at 400-kV level. Specifically, theoverload was simulated by switching out the two400-kV lines parallel to the 220-kV Magalln-Entrerros line.

Fig. 5: Line Energization

Fig. 6: Impedance Mode (Z-set): QSn_IX indicates high level ifQSn is closed and QSn_OX indicates high level if QSn is open

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Fig. 7: Single-Phase Fault at Magalln with Three-PhaseAutomatic Reclosing

Fig. 8: Maximum Compensation Mode (Z-max)

Fig. 9: Power Regulation Mode (P-set)

Fig. 10: Power Limitation Mode (P-lim), P-lim min

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Figure 12 depicts the waveforms for the case inwhich the two lines were switched out consecutivelyand not reconnected. The following settings wereused: P-emerg=300 MVA, and X initially inserted=0 . As shown, in no more than 3 s the OLC controlis able to perform the automatic insertion of thereactors required to mitigate the overload.

z Degraded Mode Operation

To verify that the OLC continues operating withoutthe impedance band(s) affected in case of faultybypass breakers, conditions of open failure, closefailure and lock-out were simulated.

Figure 13 illustrates the waveforms obtained for thecase in which the power flow decreases and QS2fails to open, thus blocking the insertion of the5.2- reactor. As shown, P-set is equal to 170 MVAand the OLC control is able to continue operatingon power regulation mode. The impedance, however,changes from 10.3 to 12.9 and later to 2.6 given the absence of the 7.8- and 5.2- steps.

Fig. 11: Power Limitation Mode (P-lim), P-lim max

Fig. 12: Emergency Control (P-emerg)

Fig. 13: Degraded Mode Operation, Open Failure QS2

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6. CONCLUSIONS

The results of the OLC/Power System Interaction testsperformed for the TWENTIES project, demo #6,have been presented. The tests verified the correctdynamic performance of the OLC control system andthe bypass breaker I/O interfaces under differentcontrol modes and contingencies in the power systemnetwork of REE. The tests also confirmed that theOLC control blocks the change of insertedcompensation when the 220-kV Magalln-Entrerrosline is subjected to disturbances such as de-energization, energization, short-circuit andautomatic reclosing. In addition, the tests confirmedthe feasibility of degraded mode operation in caseof faulty bypass breakers in the OLC.

The OLC is a simple and effective FACTS solution toprovide power flow control, power flow limitationand overload mitigation, particularly in those caseswhere the transmission lines are part of powernetworks with high penetration of wind energysources.

BIBLIOGRAPHY

1. J. C. Snchez et al, OLC Conceptual Design andEquipment Specification, Twenties Transmittingwind, EC-GA no 249812, March 2011.[Online]. Available: http://www.twenties-project.eu

2. RTDS Technologies, Circuit RTDS HardwareManual, Winnipeg, Canada: RTDS Technologies,January 2009, rev 00.

3. RTDS Technologies, Real Time Digital SimulatorTutorial Manual (RSCAD VERSION), Winnipeg,Canada: RTDS Technologies, November 2010.

4. MACH Knowledge Based Control System forFACTS, ABB Power Technologies AB.[Online]. Available: http://www.abb.com/FACTS

5. PSSE Version 32.0.5, Vol I ProgramApplication Guide, Schenectady, NY, USA:Siemens PTI-Software Solutions, RevisedOctober 2010.

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Overview of the World First Five-Terminal VSC-HVDCTransmission Project

Gao Peng Shao Zhenxia Xu Long Wu DianfengNR Electric Corporation (NR), China

SUMMARY

Due to the advantages of VSC technology andpulse width modulation (PWM), the VSC-HVDChas a number of potential advantages as comparedwith CSC-HVDC, such as being easy to realizemulti-terminal HVDC transmission system, rapidand independent control of active and reactivepower, etc. On July of 2014, the world first5-terminal VSC-HVDC project located inZhoushan islands was put into commercialoperation in China. For convenient reference forthe other engineers or researchers, the systemdesign and engineering application, such ascontrol & protection technology and systemanalysis, have been introduced in the paper.Finally, the system field on-site tests and sometest cases are introduced for better understanding.

Keywords: VSC-HVDC, Multi-terminal, coordinatedcontrol, five-terminal DC system, Current SourceConverter.

1. INTRODUCTION

A. Advantages of VSC-HVDC

Due to the advantages of VSC (Voltage SourceConverter) technology and PWM (Pulse WidthModulation), the VSC-HVDC has a number ofpotential advantages as compared with CSC-HVDC(Current Source Converter-HVDC). HVDCtechnology based on VSC, especially for multi-terminal, has been an area of growing interest recentlybecause of its suitable for forming a flexible powertransmission link [1-6].

The advantages of VSC-HVDC are as follows.

zzzzz Independent control of P and Q with four

quadrants operation

zzzzz Generally, no need of filters

zzzzz Self-commutation with high controllability

zzzzz Small footprint compared with CSC-HVDC

zzzzz Fast active power reverse

zzzzz Has no influence of receiving AC ending system

zzzzz Dynamic support for AC grid as STATCOM

zzzzz Possibility of connection to the weak andpassive grids

zzzzz Possibility of fault ride-through and black startcapability.

B. Applications of VSC-HVDC

VSC-HVDC is developed based on fully controlledsemiconductor, so VSC-HVDC can be considered asa generator without inertia and with reactive andactive power being controlled independently in fourquadrants. Due to outstanding flexibility andcontrollability in a wide range of applications, so farabout 13 projects of VSC-HVDC systems have beensuccessfully designed and put into operationworldwide since the first Hellsjon VSC-HVDC projectwith the capacity of 10kV/3MW was put intooperation in Sweden in 1997. The main applicationfields of VSC-HVDC can be summarized as follows.

zzzzz Asynchronous interconnection of AC system

zzzzz Long distance DC power transmission

zzzzz Underground or underwater cables transmission

zzzzz Integration of renewable energy

zzzzz Power delivery to large urban areas

zzzzz Power transmission as multi-terminal DC system

zzzzz Power service to isolated islands or offshoreplatform

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2. BACKGROUND OF ZHOUSHANISLANDS POWER GRID

A. Overview of Zhoushan islands powe grid

renewable energy generation development for China.But because of unavailability of efficienttransmission corridor, it was unable to undertakesuch enormous amount of wind power through theexisting AC grid. So it is urgent to build newtransmission for improving the stability andreliability of the Zhoushan power grid.

B. Main reasons of choosing VSC-HVDC asextension

The main reasons of choosing VSC-HVDC as thesolution for Zhoushan grid interconnection are asfollows.

zzzzz Zhoushan islands grid is a typical weak powersystem.

zzzzz For the space limitation of the islands, the VSC-HVDC is more environmental friendly comparedwith CSC-HVDC. Such as, the Daishan converterstation with capacity of 300 MW, shown inFigure 2, is really small footprint comparatively.

zzzzz Undersea cable can be used for interconnection.

zzzzz Flexible power control is suitable for mutualpower supply among islands.

zzzzz Flexibility of VSC-HVDC is very suitable forwind power integration.

zzzzz For the possibility of total collapse of the wholeislands grid, the blackout ability is required.

zzzzz Independent control of P and Q in 4 quadrantsfor supporting AC system and improving thewhole system stability.

Fig. 1 : Map of Zhoushan islands

As the world first 5-terminal VSC-HVDC project, itis designed to transmit electric power from themainland to Zhoushan islands, located in theHangzhou bay at southeast coast of China. For goodunderstanding, the background of Zhoushan grid isintroduced firstly.

There is no big power source within the islands everbefore. And the total installed generation capacityof Zhoushan is 765.3 MW till 2013. And the peakload of Zhoushan in 2013 is 818 MW according togrid operation data. And it is estimate

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