Received November 19, 2018, accepted December 24, 2018, date of publication January 1, 2019, date of current version January 23, 2019.

Digital Object Identifier 10.1109/ACCESS.2018.2890406

AC Grids Characteristics Oriented Multi-PointVoltage Coordinated Control Strategyfor VSC-MTDCZHOU LI 1, (Member, IEEE), YAZHOU LI 1, RUOPEI ZHAN1, YAN HE 1,AND XIAO-PING ZHANG 2, (Senior Member, IEEE)1School of Electrical Engineering, Southeast University, Nanjing 210096, China2Department of Electronic, Electrical and Systems Engineering, University of Birmingham, Birmingham B15 2TT, U.K.

Corresponding author: Zhou Li (lizhou1985@163.com)

This work was supported by the National Natural Science Foundation of China under Grant 51707031.

ABSTRACT Meshed dc grid technology is an attractive solution to improve the operational reliability ofa voltage source converter-based multi-terminal HVDC transmission (VSC-MTDC) system; in addition,the VSC-MTDC becomes widely accepted for the integration of large-scale renewable energy and powersupply to passive ac grids. Hence, the performance requirements of the dc voltage control and the powercontrol of the VSC-MTDC system become higher, and this is the key issue of coordinated control strategiesfor the VSC-MTDC system. Considering that the mainstream coordinated control under the above newsituations is facing challenges in terms of dc voltage dynamic control performance or dynamic powerdeviation, this paper proposes an ac grids characteristic-oriented multi-point voltage coordinated controlstrategy for the VSC-MTDC system to provide satisfactory dynamic dc voltage control performance withminimized dynamic and steady state power deviations of the converter stations. The theoretical analysis andthe simulations results based on PSCAD/EMTDC are to verify the effectiveness of the proposed controlstrategy.

INDEX TERMS HVDC transmission, VSC-MTDC, AC grids characteristics oriented, coordinated controlstrategy, voltage control, power control.

I. INTRODUCTIONVoltage source converter based high voltage direct current(VSC-HVDC) technology, as an important technology in thesmart grid [1]–[3], can supply power to passive networksand be used for the integration of large-scale isolated renew-able energy sources. These properties make the technologyideal for constructing multi-terminal HVDC (VSC-MTDC)systems connecting with various types of AC grids [3]–[6].And with the development of the technology, VSC-MTDCsystems are facing new challenges:

1) Besides being used to connect the AC grid with strongsystem strength, the VSC-MTDC is also used for the integra-tion of large-scale renewable energy and power supply to pas-sive AC grid as well. When the converter station connectedto an AC grid with large-scale renewable energy or a passiveAC grid, in order to maintain stable AC voltage, the convertermust operate under amplitude-phase control mode. And

with amplitude-phase control, active power fluctuations oflarge-scale renewable energy or the load varying of the pas-sive AC grid will affect the voltage stability of the DCgrid [7]–[9].

2) As meshed HVDC grids can be developed basedon radial networks to further enhance the reliability andeconomic operation of HVDC grids [10], more meshedHVDC grids will be applied to improve the operationalreliability of MTDC systems. However, the DC voltagestability control of the MTDC systems will become moredifficult [11].

Considering the above challenges, the VSC-MTDC coor-dinated control aspect, especially the control of DC voltage,is of great importance [2]. So far, the master-slave controlstrategy, voltage-margin control strategy and voltage-droopcontrol strategy are considered as mainstream control strate-gies [12].

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For the master-slave control strategy, only the masterstation is responsible for the constant DC voltage controlwhile other stations are responsible for the active powercontrol [13], and once the master station quits operation dueto a fault, the system voltage will be out of control whichseriously affected the system reliability [14], [15].

For the voltage-margin control strategy, the VSC stationscan switch between fixed power and fixed direct-voltageoperation, according to the voltage of the DC grid and thepower rating of stations [12], [15], [16]. But the operatingrange matching for the VSC stations under voltage-margincontrol will become more complicated with the increasingnumber of VSC-MTDC terminals [17], [18].

Furthermore, themaster-slave control strategy and voltage-margin control strategy all belong to the category of singlepoint voltage control strategies. Only one station participatesin voltage regulation of DC grid at any time, where powerfluctuations in DC grid can easily cause DC voltage fluctu-ations. Hence the DC voltage dynamic control characteristicis poor [1], [12], [16].

For the voltage-droop control strategy, as more than onestation works as the droop control station and participates involtage regulation of DC grid at the same time [21]–[23],it can achieve good dynamic DC voltage control perfor-mance for the VSC-MTDC with large number of termi-nals [19], [20]. However, once there is a large change of DCpower flow, it would cause large dynamic power deviation atthe droop control stations; in some extreme conditions, it maycause the current or transmission power exceed the limits andthus affect the stability of the VSC-MTDC [23], [24].

Therefore, the mainstream control strategies forVSC-MTDC system are facing challenges in terms of DCvoltage dynamic control performance or dynamic powerdeviation. Considering the new challenges, this paper isto propose an AC grids characteristics oriented multi-pointvoltage coordinated control strategy for VSC-MTDC system:1) the AC grids characteristics oriented selection criteriaare proposed to identify the coordinated control require-ments of each VSC terminal; 2) main and auxiliary DCvoltage control stations are selected to improve DC volt-age dynamic control performance; 3) a power deviationself-elimination technology is proposed and applied on themain DC voltage control stations to reduce dynamic powerdeviation of the converter stations, while the steady-statetransmission power deviation of the DC grid is minimized aswell.

The paper is organized as follows: Section 2 discussesthe coordinated control requirements of VSC-MTDC undernowadays’ new situations. Section 3 proposed an AC gridscharacteristics oriented multi-point voltage coordinated con-trol strategy for VSC-MTDC system, and then analyzedits control characteristics. Section 4 presented simulationresults, based on a six-terminal meshed VSC-MTDC sim-ulation system in PSCAD/EMTDC, to verify the effective-ness of proposed control strategy. And Section 5 draws theconclusions.

II. NEW CONTROL DEMAND OF VSC-MTDCA. NEW SITUATIONS OF VSC-MTDC SYSTEM1)With the development of power systems, VSC-MTDCwillbe increasingly used for the integration of large-scale renew-able energy, such as wind power and photovoltaic powergeneration, and for power supply to passive AC grids as well.

The large-scale renewable energy sources whose powerstations are normally located at remote regions, as well as thepassive AC grids, cannot get strong support from the mainAC grids. When the converter station connected to an ACgrid with a large-scale renewable energy source or a passiveAC grid, in order to maintain stable AC voltage, the convertermust operate under amplitude-phase control mode.

For amplitude-phase control, active power and reactivepower transmitted by the VSC connected to wind farms can-not be adjusted independently. As a result, the ability of thetransmitted active power control cannot be achieved or imple-mented, and then, due to the randomicity and volatility of therenewable energy and the load of passive AC grids, activepower fluctuations of large-scale renewable energy or theload varying of the passive AC grid will cause the change ofDC voltage, and thus significantly affect the stability of theDC grid.

Therefore, the integration of large-scale renewable energyas well as power supply to passive AC grid via VSC-MTDCplaces high rigidity requirements on the active power balanc-ing capability of the VSC-MTDC system.

2) For a meshed HVDC grid with n converter stations, thispaper takes VSC1 as an example. Assuming that there arem stations connected to VSC1, the relationship between thepower transmitted from VSC1 to the DC grid (Pdc1) and theDC voltage at each converter station (Vdci) is:

Pdc1=Vdc1

(Vdc1

r12//r13// . . . //r1m−Vdc2r12−Vdc3r13−. . .−

Vdcmr1m

)(1)

where r1j is the equivalent resistance of the DC line fromVSC1 to VSCm, j = 2, 3, . . . ,m.Equation (1) indicates that the power transmitted by the

VSC1 is affected by the DC voltage of each adjacent sta-tion. Therefore, if the transmitted active power of a certainstation is changed by adjusting its DC bus voltage, it willinevitably affect the transmitted active power of other stationsin the system, thereby affecting the DC voltage of other con-verter stations. Compared with the radial structure commonlyused in existing VSC-MTDC engineering, the DC voltage ofeach VSC in meshed HVDC grids is closely related, whichincreases the complexity of DC voltage control.

B. NEW DEMAND FOR MULTI-POINT VOLTAGECONTROL STRATEGYVoltage droop control is a kind of multi-point DC voltagecontrol, which is a technique that enables the power distri-bution among all droop control converter stations to regulatethe DC voltage. Assuming that a meshedVSC-MTDC systemcontains n converter stations, station 1 tom adopt DC voltage

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droop control, stationm to (m+k) adopt constant active powercontrol, and other stations adopt amplitude-phase control.

In steady state, the relationship between the actual powerflow (Pi) and the DC voltage (Vdci) is:

Vdcrefi = Vdci + Ki(Prefi − Pi) (2)

where Prefi is the active power reference value of VSCi.Vdcrefi is the DC voltage reference of VSCi. Ki is the droopcoefficient. i = 1, 2, . . . ,m.P0i is the initial power of VSCi, and the initial unbalanced

power of the DC grid is:

P06 =∑(

P01,P02, . . . ,P

0m,P0m+1, . . . ,P

0n

)(3)

When the system reaches a stable state, the relationshipbetween the change of the active power of VSC (1Pi) andthe change of the DC voltage (1Vdc) is:

1Vdc = V ′dci − V0dci = Ki(P′i − P

0i ) = Ki1Pi (4)

where V 0dci is the initial voltage of VSCi, and V ′dci is the

stabilized voltage of VSCi, and P′i is the stabilized activepower of VSCi.As the system is stable, the transmitted power of constant

active power control or amplitude-phase control station isunchanged, then the total variation of the transmitted powerof all voltage droop control stations is P06 . Therefore:

P06 =m∑i=1

(P′i − P0i ) = 1Vdc

m∑i=1

1Ki

(5)

Taking (3), (4) and (5) into account, V ′dci and P′i can be

calculated as

V ′dci = V 0dci +

P06m∑j=1

1Kj

P′i = P0i +P06

Kim∑j=1

1Kj

(6)

Equation (6) indicates that, under the voltage droop controlstrategy, all voltage droop control stations participate in thepower balancing and the unbalanced power consumed byeach voltage droop control station is corresponding to theirdroop coefficients. When the system reaches a new steadystate, the unbalanced power will cause the DC voltage ofeach voltage droop control station deviate from the originalstate. As a result, a large change of DC power flow will causelarge dynamic power deviation at the droop control stations;in some extreme conditions, it may cause the current or trans-mission power exceed the limits and thus affect the stabilityof the VSC-MTDC.

FIGURE 1. Main-circuit diagram of a VSC.

C. NEW DEMAND FOR SINGLE-POINT VOLTAGE CONTROLSTRATEGYIn a master-slave control or voltage-margin control strategy,one DC voltage control station acts as master station to main-tain constant DC voltage while other stations act as slave sta-tions and control the transmitted active power. Hence, thesecontrol strategies can be classified as single-point voltagecontrol strategy.

Fig. 1 shows the main-circuit diagram of a VSC, in whichCeq is the lumped capacitance of VSC or the equivalentcapacitance of DC grid [21], 1Pc is the deviation betweenthe actual value and stable value of VSC transmitted activepower.

During short time segment 1t(t1, t2) of dynamic process,assuming that KP

dc is the changing rate of transmitted powerof constant DC voltage control station with time and Pt16 isthe unbalanced power of the DC grid at t1, then:

(Pt16 − KPdc1t)1t ≈ 1Wc = Ceq

∫ Vdc(t2)

Vdc(t1)Vdc(t)dVdc(t)

=12CV 2(t2)−

12CV 2(t1) (7)

where 1Wc is the energy change of Ceq during 1t .According to (7), in single-point voltage control strategy,

the DC voltage at t2 is:

V (t2) =

√21Wc

C+ V 2(t1) ≈

√2(Pt16−1tKP

dc)1t

C+V 2(t1)

(8)

Equation (8) indicates that the DC voltage variation is causedby the unbalanced power charging or discharging the equiva-lent capacitance of the DC grid. And the fluctuation of the DCvoltage increases with the increasing of duration and quantityof disturbance power in the DC grid.

While, in the voltage droop control strategy, theDC voltageof droop control station VSCi at t2 is:

Vi(t2) ≈

√2(Pt1i −1tKP

dci)1t

C+ V 2

i (t1) (9)

where Pt1i is the unbalanced power which is undertaken byVSCi at t1. And according to (6), Pt1i can be calculated as

Pt1i = Pt16

/Ki m∑j=1

1Kj

(10)

Different from the voltage droop control strategy in whichall voltage droop control stations participate in the power

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balancing, only the master station participates in the powerbalancing in the single-point voltage control strategy. Theunbalanced power of the DC grid is fully undertaken by theconstant DC voltage control station, resulting in a slow reg-ulation speed of the unbalanced power and a long regulatingtime. And hence, the power fluctuation in the DC grid willcause larger DC voltage fluctuation in VSC-MTDC systemusing the single-point voltage control strategy. As a result, thecontrol capability of DC voltage of the single-point voltagecontrol strategy is insufficient for meshed VSC-MTDC sys-tems with large-scale renewable energy integration or passiveAC grid connections.

III. AC GRIDS CHARACTERISTICS ORIENTEDMULTI-POINT VOLTAGE COORDINATED CONTROLSTRATEGYA. MAIN STRUCTURE OF THE COORDINATED CONTROLSTRATEGYBased on the above analysis, it can be obtained that comparedwith the single-point voltage coordinated control strategy,the multi-point voltage coordinated control strategy has gooddynamic stability of the DC voltage, while the accuracy ofthe transmitted power is sacrificed in large change of the DCpower flow.

In order to provide both satisfactory dynamic DC voltagecontrol performance as well as power control performancefor the VSC-MTDC system. This paper proposes an ACgrids characteristics oriented multi-point voltage coordinatedcontrol strategy for VSC-MTDC system, as shown in Fig. 2.Firstly the AC grids characteristics oriented selection criteriaare proposed to identify the coordinated control requirementsof each VSC terminal according to the characteristics of theconnected AC grids; secondly suitable converter stations areselected as main and auxiliary DC voltage control stationsto improve DC voltage dynamic control performance; andthirdly a power deviation self-elimination technology is pro-posed and applied on the main DC voltage control stationsto reduce dynamic power deviation of the converter stations,and hence the steady-state transmission power deviation ofthe DC grid is minimized as well.

B. AC GRIDS CHARACTERISTICS ORIENTED SELECTIONCRITERIAAs shown in Fig. 3, the AC systems connected to theVSC-MTDC system can be classified into four types accord-ing to the AC grids characteristics oriented selection criteria.And then the coordinated control requirements of each con-verter station can be found.

The first type of AC systems is active networks with goodpower regulating capacity, and the stations connected to thistype of AC systems can be selected as main DC voltagecontrol stations and operated under voltage droop controlwith Pref given from deviation self-elimination technologyto undertake the unbalanced power of the DC grid.

FIGURE 2. Block diagram of AC grids characteristics oriented multi-pointvoltage coordinating control strategy.

FIGURE 3. Block diagram of AC grids characteristics oriented selectioncriteria.

The second type of AC systems is active networks withgeneral power regulating capacity, and the connected sta-tions can be selected as auxiliary DC voltage control stationsand operated under voltage droop control with Pref givenfrom power dispatching system to improve the DC voltagedynamic control performance.

Active networks with specified power transfer referencerequirement are the third type of AC systems and the con-nected stations can be operated under PQ decoupled controlto maintain the accuracy of the power transferred.

The last type of AC systems includes passive networks andmassive renewable energy networks without large-scale syn-chronous generator, the connected stations can be operatedunder amplitude-phase control to maintain AC bus voltage.

C. POWER DEVIATION SELF-ELIMINATION TECHNOLOGAssuming a meshed VSC-MTDC system contains n stations,the first m (2≤ m ≤ n) stations are operated under voltage

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FIGURE 4. Block diagram of power deviation self-elimination technology.

droop control, station 1 to s (1≤ s ≤ m − 1) are the firsttype of converter station which act as the main DC voltagecontrol stations and station (s + 1) to m are the second typeof converter stations which act as the auxiliary DC voltagecontrol stations; station (m + 1) to (m + k) are operatedunder the constant active power control; and other stationsare operated under the amplitude-phase control. To reducedynamic power deviation of voltage droop control stations,a power deviation self-elimination technology is proposed asshown in Fig. 4.

Taking the steady state after the nth adjustment as anexample, the total deviation of active power transferred of allthe voltage droop control stations is:

Pn6 =∑(

Pnref 1,Pnref 2 . . .Pnrefs,Pref (s+1)

. . .Prefm,Ps(m+1) . . .Pn)

(11)

In order to avoid adjusting Prefi of the main DC voltagecontrol stations frequently, especially under the situations ofminor power fluctuations in theDCgrid, the threshold voltageVmargin is introduced to the main DC voltage control stations.When the total variation of the active power transferred of theHVDC transmission system satisfies:∣∣∣∣∣∣Pn6

/ m∑j=1

1Kj

∣∣∣∣∣∣ = |1Vdc| > Vmargin (12)

Then the active power references of the main DC voltagecontrol stations can be adjusted to:

Pn+1refi = Pnrefi +1Pni = Pnrefi + Pn6

/Ki s∑j=1

1Kj

(13)

After the system reaches a new steady state, the totalvariation of the power transmitted by all the voltage droopcontrol stations 1Pn+16 is:

1Pn+16 =

m∑i=1

(Pn+1refi − Pn+1i ) = 0 (14)

According to (10) and (14), system operates at standardvoltage after the system reaches a new steady state, and thesteady-state transmission power deviation of the DC grid isminimized as well.

FIGURE 5. Steady-state voltage–power curve. (a) Steady-state voltagepower curve of main DC voltage control station. (b) Steady-state voltagepower curve of auxiliary DC voltage control station.

D. CONTROL CHARACTERISTICS OF THE COORDINATINGCONTROL STRATEGY1) STEADY STATE CONTROL CHARACTERISTICS

Steady-state voltage power curves of the DC voltage con-trol stations are shown in Fig. 5. Considering (12) and (13),in the steady state, the deviation of the active power trans-ferred of the DC voltage droop control station satisfies:

|1Pi| = Ki |1Vdc| < KiVmargin (15)

This means that in steady state, the steady-state transmis-sion power deviation of the DC grid is minimized with a smallvoltage margin value, which can also ensure the DC systemoperates near the rated voltage.

2) DYNAMITIC CONTROL CHARACTERISTICSTaking the (n+ 1)th dynamic process, as shown in shaded

parts of Fig. 5, as an example, the abscissa of green line in theFig. 5 shows the steady state transferred active power of theDC voltage control station corresponding to the different DCvoltages, which can be considered as correction values (P∗refi)of Prefi corresponding to the different DC voltages. In the (n+1)th dynamic process, P∗refi is calculated as:

P∗refi = Pn+1refi +1Ki

(Vdci − Vdcrefi) (16)

Equation (16) indicates that, during the dynamic process,the transferred power of each DC voltage control station ischanged according to the voltage variation.

The DC voltage variation is suppressed by all DC volt-age control stations in the proposed control strategy, whichimproves the dynamic DC voltage control performance.

Hence, the advantages of the proposed control strategycomparedwith themainstream coordinated control strategies,in terms of the dynamic and steady state characteristics,is summarized as follows.

1) Appropriate converter stations are selected to undertakethe unbalanced power of the DC grid based on the AC gridscharacteristics oriented selection criteria.

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FIGURE 6. Six-terminal VSC-MTDC simulation system.

2) The dynamic DC voltage control performance can beimproved by the multi-point voltage control.

3) Dynamic power deviation of the converter stations isreduced by power deviation self-elimination technology, andhence the steady-state transmission power deviation of theDC grid is minimized as well.

IV. SIMULATION RESULTSA. SIMULATION SYSTEM OF VSC-MTDC WITHRENEWABLE ENERGY INTEGRATIONA six-terminalmeshedHVDCgrid, as shown in Fig. 6, is usedto illustrate the multipoint voltage control technology basedon the scenario selection presented in previous sections.The six-terminal grid consists of two wind farm interfacingconverters (VSC1 and VSC4) and four AC-grid interfac-ing converters (VSC2, VSC3, VSC5 and VSC6), in whichVSC3 are connected to a passive network while other gridside converters are connected to AC synchronized powergrids. The six-terminal HVDC grid has two power-inputnodes, three power-output nodes, a power balance node andseven branches representing the DC transmission lines link-ing the converters.

In order to verify the validity of the proposed control,the master-salve control strategy, the voltage droop controlstrategy and the proposed control strategy are set up to makecomparisons under the same simulation environment.

In master-slave control strategy, VSC2 acts as master sta-tion, and VSC5 and VSC6 act as slave station; in voltagedroop control strategy, VSC2, VSC5, and VSC6 all adoptDC voltage droop control. In the proposed control strategy,VSC2 acts as the main DC voltage control station, andVSC5 and VSC6 act as the auxiliary DC voltage controlstations. The parameters of each VSC are shown in Table 1.

At the start of the simulation,Wind farm 1 (WF1) and windfarm 2 (WF2) are rated at 1450 and 550 MW, respectively.The initial active load connected to VSC3 is 600MW. Theinitial Pref of other stations are shown in Table 2.

TABLE 1. Parameters of the VSC-MTDC system.

TABLE 2. Pref of Each VSC.

Three scenarios are analyzed. In the first scenario,VSC5 replaces VSC2 to be the master station after VSC2 isblocked due to the fault. In the second scenario, a sudden loadchange in passive network is analyzed. In the third scenario,due to a fault in AC grid4, VSC4 is out of service and theload-side converters extract power from the HVDC grid asnormal.

B. SCENARIO 1: MASTER STATION OUT OF SERVICEScenario 1 is set up to verify the control performance of theproposed control strategy when the master station is out ofservice. As shown in Fig. 7, at the start of the simulation, allthe stations are in steady state, working under the operatingpoint listed in Table 1.

VSC2 is blocked due to a fault at 1.5s. Considering com-munication delay, VSC5 is transformed to the master stationafter 10ms. Simulation waveforms are shown in Fig. 7.

As VSC1, VSC3 and VSC4 are all operated under theamplitude-phase control to maintain the AC voltage of PCC,the transferred power of VSC1, VSC3 and VSC4 is the real-time output power of the AC grids, which is less affected bythe DC voltage.

For master-slave control strategy, after VSC2 is blocked,there is an active power surplus in the DC grid, which causes aDC voltage swell by about 12%. After 10ms, the control strat-egy of VSC5 is switched to a constant DC voltage control,resulting in an active power overshoot in VSC5 by -810MWwhich eventually exceeds Pmax . This indicates that there areproblems such as the inefficiency on capability of DC voltagecontrol and power overshoot of the master station.

For voltage droop control strategy, after VSC2 is blocked,the surplus of the transferred power in theDC grid is passivelydistributed by the remaining droop control stations, trackingto the relationship in (6). After the system reaches a newsteady state, the DC voltage is stabilized at 530kV and trans-mission power of the remaining voltage droop control stationsdeviates from the pre-specified power control reference.

With the proposed control strategy applied, the controlstrategy of VSC5 is switched to a constant DC voltage

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FIGURE 7. Simulation results of VSC-MTDC system during the passivenetwork power step change. (a) DC bus voltage of main DC voltagecontrol station (Vdc2) and auxiliary DC voltage control station (Vdc5).(b) Transmission power of each converter station.

control, with 10ms delay, after VSC2 quits operation. Thepower flow in the DC grid can be actively controlled and dis-tributed into the main DC voltage droop control stations. Thevoltage peak is 515kV during the whole dynamic process, andthe DC voltage is restored to standard voltage after the systemreaches a new steady state. Furthermore, the overshoot ofthe transferred power is avoided effectively in the dynamicprocess.

Hence, the simulation results have demonstrated that, withthe proposed control strategy applied to the VSC-MTDCsystem, the better dynamic and steady performance can beachieved in comparison to the other two control strategieswhen the master station is out of service.

C. SCENARIO 2: SUDDEN LOAD CHANGEScenario 2 is set up to verify the performance of the pro-posed control strategy under sudden load changes. As shownin Fig. 8, at the start of the simulation, all the stations arein steady state, working under the operating point listedin Table 1. A load increase by 300MW is applied at thepassive network at 1.5 s.

As VSC1, VSC3 and VSC4 are all operated under theamplitude-phase control, the fluctuation of the passive loadin AC grid3 is passively transmitted to the DC power grid,

FIGURE 8. Simulation results of VSC-MTDC system during the suddenload change of passive network. (a) DC bus voltage of main DC voltagecontrol station (Vdc2) and auxiliary DC voltage control station (Vdc5).(b) Transmission power of each converter station.

resulting in the fluctuation of the DC voltage. The transferredpower of VSC1 and VSC4 is the real-time output of windfarms, which is less affected by the DC voltage.

For the master-slave control strategy, as shown in Fig. 8(a),the increase of the passive load causes the power shortage inthe DC grid which leads to a DC voltage sag. The minimumvalue of DC voltage is 445kV. Fig. 8(b) presents the increaseof the load in the receiving system is supplemented by the DCvoltage control station.

For the voltage droop control strategy, the change of theload in the passive grid is passively distributed among all thevoltage droop control stations. The changes of the transferredpower of VSC2, VSC5 andVSC6 are 75, 75, 150MW, respec-tively, tracking to the relationship in (6). After the systemreaches a new steady state, the DC voltage is stabilized at480kV and the transferred power of all the droop controlstations deviates from the reference value.

With the proposed control strategy applied, it can be seenfrom the curves of Ps2, Ps5 and Ps6 that all the droop controlstations participate in the power regulation of the DC gridto stabilize the DC voltage, and the transferred power ofVSC5 and VSC6 is restored to the pre-specified power con-trol reference after the system becomes stable. The minimum

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FIGURE 9. Simulation results of VSC-MTDC system during the passivenetwork power step change. (a) DC bus voltage of main DC voltagecontrol station (Vdc2) and auxiliary DC voltage control station (Vdc5).(b) Transmission power of each converter station.

value of the DC voltage is 490kV in the dynamic processand the DC voltage is restored to the normal voltage after thesystem reaches a new steady state.

D. SCENARIO 3: WIND FARM OUT OF SERVICEThis scenario is designed to verify the performance of theproposed control strategy when the wind farm is out of ser-vice. As shown in Fig. 7, at the start of the simulation, all thestations are in steady state, working under the operating pointlisted in Table 1. WF2 is out of service at 1.5 s

For the master-slave control strategy, the transferred powerof VSC2 is reversed to 250 MW to compensate the activepower shortage in the DC grid after WF2 is out of service.The minimum value of the DC voltage is 445 kV during thedynamic process. While the DC voltage is restored to pre-specified power control reference after the system becomesstable.

For the droop control strategy, the changes of the trans-ferred power of VSC2, VSC5 and VSC6 are 138, 138,274MW, respectively. After the system reaches a new steadystate, the DC voltage is stabilized at 480kV which deviatesfrom the DC voltage control reference.

For the proposed control strategy, it can be seen from thecurve of Ps2 that the transferred power of VSC2 is reversed

to 250 MW to compensate the active power shortage in theDC grid. The curves of Ps5 and Ps6 indicate that VSC5 andVSC6 participate in the power regulating of the DC grid tosuppress the fluctuation of the DC voltage. The minimumvalue of the DC voltage is 485 kV and there is no overshoot ofthe active power transmitted of the droop control stations indynamic process. The results demonstrate that the proposedcontrol strategy maintains the DC-grid voltage closer to thecontrol reference and at the same time reduces the dynamicpower deviation of the converter stations of the auxiliary DCvoltage control stations.

The simulation results have demonstrated the stabilityanalysis of the proposed control strategy in Section 5.In comparison to the other two control strategies, theVSC-MTDC system adopting the proposed control strategyhas a better performing voltage dynamic stability. Further-more, the dynamic power deviation of the converter stationsis reduced and at the same time the voltage of DC grid is muchcloser to the pre-specified control reference.

V. CONCLUSIONThis paper has proposed an AC grids characteristics ori-ented multi-point voltage coordinated control strategy forVSC-MTDC system, which can provide satisfactory dynamicDC voltage control performance with minimized dynamicpower deviation of the converter stations and minimizedsteady-state transmission power deviation of the DC grid.Then a 6-terminal meshed VSC-MTDC simulation systemhas been built in PSACD/EMTDC to verify the validity ofthe proposed control strategy.

Theoretical analysis as well as simulation results haveshown the effectiveness of the AC grids characteristics ori-ented multi-point coordinated control strategy:

1) According to the proposed AC grids characteristics ori-ented selection criteria, the coordinated control requirementof each VSC-MTDC terminal can be adjusted to match thecharacteristics of the connected AC grid.

2) Appropriate converter stations are selected as the mainand auxiliary DC voltage control stations, based on the ACgrids characteristics oriented selection criteria, and are oper-ated under multi-point voltage control to improve DC voltagedynamic control performance.

3) The proposed power deviation self-elimination technol-ogy can be applied to the main DC voltage control stations toreduce dynamic power deviation of the converter stations, andfurthermore to minimize the steady-state transmission powerdeviation of the DC grid.

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ZHOU LI received the B.S. and Ph.D. degrees inelectrical engineering from the School of Elec-trical Engineering, Southeast University, China,in 2007 and 2014, respectively, where he is cur-rently a Lecturer. His research interests includepower system operation and control, ac/dc hybridpower transmission, and renewable energy gener-ation and control.

YAZHOU LI received the B.Eng. degree inelectrical engineering from Zhengzhou Univer-sity, China, in 2016. He is currently pursu-ing the M.Eng. degree in electrical engineeringwith Southeast University. His research interestsinclude the control and operation of the powersystems and voltage source converter-based highdirect current.

RUOPEI ZHAN received the B.Eng. degree inelectrical engineering from Southeast University,China, in 2018, where he is currently pursuingthe M.Eng. degree in electrical engineering. Hisresearch interests include flexible high voltage dctransmission technology.

YAN HE received the B.Eng. degree in electri-cal engineering from the Nanjing University ofScience and Technology, China, in 2016. She iscurrently pursuing the M.Eng. degree in electri-cal engineering with Southeast University. Herresearch interests include the control and operationof the power systems and voltage source converter-based high direct current.

XIAO-PING ZHANG (M’95–SM’06) is currentlya Professor of electrical power systems with theUniversity of Birmingham, U.K. He is also theDirector of Smart Grid, Birmingham Energy Insti-tute, and the Co-Director of the BirminghamEnergy Storage Center. He has co-authored thefirst and second editions of the monograph Flexi-ble AC Transmission Systems: Modeling and Con-trol [Springer, edition (First), 2006, edition (Sec-ond), 2012]. He has co-authored the book Restruc-

tured Electric Power Systems: Analysis of Electricity Markets with Equilib-rium Models (IEEE Press/Wiley, 2010).

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