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control strategy for AC fault ride through, including voltage droop, power reduction and DC chopper control, in order to enhance the system ac fault ride through ...
2014 International Conference on Power System Technology (POWERCON 2014)

Chengdu, 20-22 Oct. 2014

Improved AC Fault Ride Through Control Strategy for MTDC System with Offshore Wind Farms Wenting Li, Jing Lv, Gang Shi, Xu Cai, Yongning Chi Abstract— Based on a five-terminal HVDC system connected with offshore wind farms, this paper investigates a three-level control strategy for AC fault ride through, including voltage droop, power reduction and DC chopper control, in order to enhance the system ac fault ride through capability. The restrictions of individual control level are respectively investigated before a trigger mechanism of the three-level control strategy is defined. With the improved AC fault rid through control, different protection measures will take actions according to the fault severity. The proposed control strategy is validated through PSCAD/EMTDC simulation and the results prove the feasibility of these control strategy in different situations. Index Terms—voltage droop control, mulit-terminal VSC-HVDC, offshore wind farm, fault ride through

I. INTRODUCTION

T

ypical applications of Multi-terminal HVDC(MTDC) grids are presented by the European HVDC Study Group in reference[1], Offshore wind farms integration is one of the main applications, for MTDC system has the advantages of high flexibility, increased redundancy and greater stability[2] -[3]. Particularly, grid codes require strictly wind turbines to have a reliable capability of (FRT) fault ride through [4]. However, when offshore wind farms are connected to the mainland ac grids through a MTDC link, it provides a greater capability of achieving grid code requirements, for it decouples wind farms and the mainland AC grid and has a higher flexibility of adjusting active power distribution [5]. In this sense, the MTDC system with wind farm is also expected to have FRT capability. In fact, the crucial point of FRT requirements for the MTDC system is to avoid dc overvoltage during ac fault [6]. Therefore, power reduction during ac fault period is necessary to be Manuscript received July 25, 2014. This work was supported by National 863 program of China (2013AA050601), the Science and Technology Commission of Shanghai, China (13dz1200202). W. Li, J. Lv, G. Shi, and X. Cai are with the Wind Powe Research Center of Shanghai Jiao Tong University, 200240 Shanghai, China (e-mail: [email protected]). Y. Chi is with the China Electric Power Research Institute.

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achieved by coordinated control between MTDC system and wind farms[7]. Various control strategies to reduce active power in point-to-point HVDC system are evaluated in [8]. Some strategies are based on fast communication t, while others are able to trigger the power reduction control merely using local controllers, such as voltage reduction and frequency increase methods[9]-[10]. When these control strategies are adopted to MTDC system, a feasible control scheme of multi-VSC converters should be firstly studied and the voltage droop control is highly promising in wide applications [11]-[14]. When an ac grid fault occurs in the MTDC system, suitable operational modes and corresponding switch process to reduce power are discussed in [15]-[17]. However, the boundaries of these power reduction methods are rarely studied, especially when different characteristics of wind farms and their protection equipment are considered. Moreover, the relationships between these methods and voltage droop control of MTDC system also need a further investigation. This paper presents a coordinated control strategy with voltage droop control and communication-free power reduction methods to improve FRT capability of the MTDC system. Their boundaries are deduced. Therefore, the coordinated control involving three-level of protection is defined within these restrictions and these protection measures are triggered according to the severity of ac grid fault without communication. The remaining of the paper is organized as follows. Section II outlines a five-terminal HVDC system, and Section III analyzes the voltage droop and power reduction control together with their restrictions. Section IV gives out an explanation of the proposed control strategy. And the corresponding simulations based on PSCAD/EMTDC are presented in Section V. Finally, Section VI draws some conclusions. II. DESCRIPTION OF MULTI-TERMINAL HVDC SYSTEM WITH WIND FARMS

Figure 1 displays a single line diagram of a five-terminal HVDC system integrating into ac grid. In the dc system, three

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onshore converters apply voltage droop control to keep dc voltage stable while the two offshore converters control ac voltage constant to ensure all the wind power is delivered to the dc network. In the ac system, three ac grids are connected by simplified reactance Z1 and Z2. And Z1 is relatively large, representing a remote electrical distance between ac grid 1 and ac grid 2, while Z2 is rather small representing a close electrical distance between ac grid 2 and ac grid 3. Therefore, two kinds of fault points are studied. Fault 1 will only influence ac grid 1 , however, fault 2 will inevitably affect ac grid 2 and ac grid 3 to different extent due to their small electrical distance.

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Fig 2 onshore converter control system

B. Restrictions of Voltage Droop Control Power transmission capability of each onshore converter station is constrained by the maximum ac current Iac_max of the converter. Thus when the ac grid i is faulted and the ac current of onshore converter i reaches its limit Iac_max, the onshore converter i will switch to current limit mode automatically and the deliverable active power to it will be consequently reduced [19]. Meanwhile, the surplus power will naturally flow to other normal onshore converters according to the voltage current characteristics curve, shown in Fig 3.

Pmax = 3Vac I ac _ max cos ϕ Fig 1 Multi-terminal HVDC network integrates into AC system network

III. CONVENTIONAL AC FAULT RIDE THROUGH METHODS AND THEIR CONSTRAINTS

When an ac grid fault occurs at the PCC (common connecting point) of one grid, the AC voltage will instantly dip. Active power will not be able to be delivered and dc current will abruptly increase to exceed the limit. Therefore, different control modes should be switched to instantly.

(2)

Where, Pmax ,Iac_max are respectively the maximum active power the maximum ac phase current and dc current of onshore converter, cos ϕ is power factor. If the normal converter j operates at the red point when fault occurs, the active power delivered to it is Pj , which is related to the wind power at that time. It still has the ability of absorbing more active power until Pj max . The maximum power absorbed by other converters is ΔPj . However, if it exceeds this limit, dc voltage will increase.

A. Voltage Droop control of onshore converter In normal operation mode, assuming that all the onshore converters have the same initial voltage U dc 0 , the relationship

n −1

ΔPj = ∑ ( Pj max − Pj )

(3)

j =1

between active power and dc voltage can be expressed as: * = U dc 0 + k dc I dc U dc

Pdc = Edc I dc

(1)

Where U dc , Idc ,Pdc are the dc voltage and dc current and active power of the inverter[18]; Due to the advantages of voltage droop control, power transmitted to the ac grid can be distributed according to the droop coefficient k dc . Fig 3 Voltage-current characteristics of onshore converter

C. Power reduction control of offshore converters There are three operation modes of offshore converter,

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namely, normal mode, dc current limit and the power reduction operation [15]. Once dc voltage exceeds the threshold value U dcth , offshore converter will come into power reduction operation mode immediately, shown in Fig 4. Two methods can be applied in this mode. One method is to reduce ac voltage magnitude reference. Another way is to add ac grid frequency reference. In this way, dc voltage can be controlled by power reduction method of voltage reduction or frequency increase methods by (4)-(5). Detailed control strategies are presented in [20][21].

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At first, wind farms without fault ride through equipment, power reduction can be achieved by storing wind energy as kinetic energy in the wind turbine rotor during fault. Thus rotor speed will be accelerated by reducing electromagnetic torque. From equation (6), electromagnetic torque can be reduced by decreasing voltage. If stator and rotor impedances are seen as unchanged and the variation of synchronous speed is trivial, electromagnetic power changes in direct ratio of k1 to the square of voltage, shown in equation (7).

Pe = Teω =

3V 2 R2 2 ⎡ 2⎤ R s ⎢⎛⎜ R1 + 2 ⎞⎟ + ( X 1 + X 2 ) ⎥ s ⎠ ⎣⎝ ⎦

(6)

If the variation of slip is ignored, we can obtain by

ΔPv = ΔPe ≈ k1ΔV 2 Fig 4(a) voltage reduction method (b) frequency increase method

Δf = k f (U dc − U dcth ) Δ V = kv (U dc − U dcth )

(4) (5)

Where, U dc is the DC voltage of offshore converter, kf ,kv, are frequency and voltage droop coefficient . D.Restrictions of power reduction methods Regarding the restrictions of power reduction, two key points should be noted. One is to avoid the protection equipment of wind farms cutting wind turbines off. Another is to maintain the stability of wind turbines without over speed. One possible way to avoid the over speed of wind turbine is to adapt fast pitch control. However, the maximum pitching speed could be within 10º-20º per second. This is too slow to ensure a fast response to satisfy the LVRT requirements [22].

Where V is the ac voltage of offshore converter connected with the wind farm, R1, R2 and X1, X2 are resistance and reactance of stator and rotor respectively, ω is rotor speed and s is slip. Te and Pe is the electromagnetic torque and electromagnetic power. ΔV is the voltage variation and Δ Pv is the active power reduced . Reduced power is in proportion of k1 to the square of voltage, shown in equation (7). Thus the maximum reduced power can be calculated by the maximum voltage variation. If the wind farms have fault ride through equipment, another part of reduced power related to the crowbar or dc chopper in wind farms should be considered, details are explained in [23]. 2) Frequency limitation:

Fig 6 frequency increase controller

1) AC voltage limitation

ΔV

M*

Fig 5 voltage reduction controller

Using voltage reduction method to reduce power, ac voltage limitation should be considered. (1) If there is no fault ride through equipment installed in the wind farms, it is necessary to ensure the ac voltage not to be lower than its threshold of protection facilities. (2) If the wind farms have fault ride through equipment, however, a lower ac voltage limitation is allowed, for crowbar and dc chopper can dissipate more active power. In fact, there is an ac voltage limitation in both kinds of wind farms.

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If frequency increase method is applied to reduce wind power during fault period, frequency limitation is supposed to be satisfied. As the rotor speed of variable speed wind turbine is decoupled from the frequency of PCC due to power electronic converters, it is necessary to install an extra frequency response controller in the wind farm, such as a PI regulator, to adjust the output power reference as a function of the variation of frequency. Thus the output power reference can be set to be proportional to the difference of frequency, referring equation (8). However, this controller will not be activated until dc voltage exceeds U dcth [24]. Detailed design of the controller can be found in [25]-[26]. From equation (8), it is found that the reduced power is related the coefficient kf , which actually represents the variable

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parameter in PI regulator. It is possible to change active power reference to an arbitrary value by adjusting the parameter kf , without exceeding the limitation of Δf in grid codes. However, another constraint of rotor speed should also be noted before designing the kf, referring equation (9)(10).

ΔPf = k f Δf Where,

(8)

ΔPf is the reduced power by frequency increase

method. 3) Rotor speed limitation Rotor speed should also be examined to avoid triggering protection equipment of wind farms due to overspeed in both kinds of power reduction methods. From equations of (9) and (10), it is apparent that the variation of rotor speed Δω will influence the difference between the mechanical and electromagnetic torque ΔT and consequently limit the variation range of active power ΔP .

ΔTmax

Δω ω − ω0 = 2H = 2 H max Δt Δt Δ Pmax = Δ Tmax ω

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extra controller, the maximum power reduction Δ Pmax related to rotor speed can be the restriction of

ΔPf .

Before making use of power reduction methods, it is essential to set boundaries of voltage, frequency and rotor speed according to grid codes [27]. From equations (7) and (10), the total power by these methods can be expressed as follows: (12) ΔPwind = Δ Pv + Δ Pf = Δ Pv + Δ Pmax However, in order to ensure the reliability of the whole system, dc chopper should be a back-up protection to dissipate the remaining energy, for if the power reduced exceeds ΔPwind , dc voltage will be uncontrolled. C. Control strategies at the level of dc chopper protection The dc voltage threshold of triggering dc chopper should be ′ th than that of power reduction chosen at a higher value U dc method U dcth , so that only when the reduced wind power of

(9)

ΔPwind is not enough and DC voltage continues to increase,

(10)

this equipment will take actions. The resistance can be calculated by (13), but certain safety margin is necessary.

Where, Δ Tmax is the maximum difference between the mechanical and electromagnetic torque, Δt is the fault duration time. ω 0 , ω max are respectively the initial and maximum rotor speed.

ΔPchop = ΔPexcess − ΔPwind − ΔPdroop =

2 U ′dcth Rchopper

(13)

Rchopper is the dc chopper resistance, U dc′ th is the dc voltage ′ th > U dcth ). threshold( U dc

IV. P ROPOSED PROTECTIVE MEASURES FOR AC FAULT RIDE THROUGH

Considering all these restrictions, a series of protection measures on three levels are designed to ensure the security of the whole system. A. Control strategies at the level of onshore converters If the ac fault is minor and only affects the onshore converter i, the other converters are able to absorb the surplus power within the restriction of

ΔPdroop , which can be obtained by (11).

Then power unbalance will not appear and dc voltage will not trigger the power reduction control.

In this way, the capability and cost of dc chopper can be less than it is used solely. And it avoids wasting too much wind energy if the fault is not too serious. D.Control logic of these three-level protection measures From Fig 7, it is illustrated the control logic of these protection measures. In this diagraph, Pexcess is the total power failing to be injected into onshore converters due to ac grid fault; When Pexcess is less than

ΔPdroop , voltage droop control is

adopted; when it is between

ΔPdroop and Δ Pwind , power

reduction methods are also utilized; when Pexcess exceed the

n

ΔPdroop = ∑ ΔPj

(11)

j =1 j ≠i

B. Control strategies at the level of offshore converters

sum of Δ Pwind and

ΔPdroop , DC chopper is triggered to be the

last protection line. In this way, these protective measures are fairly reliable to get power balance and keep DC voltage stable.

As for voltage reduction method, the limitation of is mainly voltage constraint, for the permitted variation range of voltage is so small that the energy stored in kinetic is not enough to cause overspeed, the maximum reduced power can be obtained by (7). However, as for frequency increase method with an

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Fig 7 Logic graph of the control strategy

V. SIMULATION RESULTS AND ANALYSIS (a) (b) Fig 8 (a) RMS value of the ac voltage of the three onshore converter stations; (b) Onshore DC link voltage 0.4

0.3

Idc (kA)

In order to validate the proposed control strategies, a five-terminal HVDC system based on PSCAD/EMTDC is built. Wind farm 1 of SCIG (squirrel cage induction generator) without FRT equipment utilizes voltage reduction method to reduce power, coordinated with offshore converter station 1. Wind farm 2 of DFIG (doubly fed induction generator) as well as offshore converter station 2 adopts frequency increase method to reduce active power, the main simulation parameters of the MTDC system model are shown in Table I.

0.2

3 2 1

0.1

TABLE I THE MAIN PARAMETERS OF THE SIMULATION MODEL

U dcth

Rated Value 320kV 200MW 150MW 150MVA 150MVA 150MVA 100uF 336kV

U′dcth

340kV

A. Case 1: A minor fault In this case, a three-phase ground fault , lasting 500ms, occurs at ac grid 1, shown as “fault 1” in Fig 1. It causes the ac grid voltage to drop to 0.5 p.u while the other grid voltages are not affected. This fault is not severe that the power failed to be injected to ac grid 1 is less than ΔPdroop and dc voltage does not exceed the U dcth , thus power reduction methods are not triggered. From Fig 8 (a), it is shown that the ac voltage of ac grid 1 drops approximately to 0.5 p.u while the others are maintained at 1 p.u during the fault period. Consequently, more power is delivered to onshore converter 2 and 3 to reestablish power balance, shown in Fig 9(a). Thus DC voltage is kept stable in Fig 8(b). After the fault is cleared at 3.5s, the DC current gradually recovers to its normal level with an acceptable current increasing, shown in Fig 9(b).

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0.0 3.0

3.2

3.4

3.6

Time s︶ ︵

Parameter DC voltage Wind farm1 capability Wind farm2 capability Capability of onshore converter 1 Capability of onshore converter 2 Capability of onshore converter 3 DC capability

(a) (b) Fig 9(a) active power of the three onshore converter stations; (b) DC current of the three onshore converter stations

B. Case 2: A severe fault In this case, two kinds of severe faults are compared and analyzed to validate the power reduction methods and DC chopper measures. In the first scenario, a severe fault occurred at the same place as in case 1, but the short circuit impedance is smaller , causing the ac voltage of ac grid 1 to drop to 0 p.u while the other girds are not affected, shown in Fig 10(a). In this situation, voltage droop control is not enough to control dc voltage and the slight overvoltage triggers power reduction methods to take actions, shown in Fig(11) and Fig(12). The power reduced does not reach its maximum value

ΔPwind

and dc chopper is not

triggered, shown in Fig 10(c) In the second scenario, a sever occurred in ac grid 2, shown as “fault 2”in Fig 1, causing its ac voltage to drop to almost 0 p.u, meanwhile the voltage of ac grid 3 is also influenced to drop to 0.3 p.u, for their electrical distance is small. In this situation, the reduced power reaches its maximum value and dc voltage continues to increase to U′dcth . So dc chopper is triggered to reduce power, shown in Fig 10(d).

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3 2 1

0

3.0

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3.4

(d) are respectively the trigger signal of DC chopper without DC chopper and with DC chopper; (e) and (f) are respectively onshore DC link voltage without DC chopper and with DC chopper; (g) and (h) are respectively active power of the three onshore converter stations without DC chopper and with DC chopper; (i) and (j)are respectively the DC current of the three onshore converter stations without DC chopper and with DC chopper;

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(a)

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(d)

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60 40

3 2 1

20 0

40

3 2 1

20

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(g) 0.5

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DC current (kA)

0.5

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0.0

-0.1 3.4

Time (s)

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(h) 0.6

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Time (s)

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0 3.0

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0 2.8

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Time (s)

60

-20 3.4

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0

-20

Time (s)

100

(c) (f) Fig 11(a)(b)(c) are respectively AC grid voltage, frequency and active power of offshore converter 1 using voltage reduction method without DC chopper; (d)(e)(f) are those quantities with DC chopper

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Time (s)

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(e)

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340

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Time (s)

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Time (s)

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(b)

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(d)

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DC voltage (kV)

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3.0

Active power (MW)

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DC current (kA)

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Frequency of offshore converter 1 (Hz)

0.2

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(a) Frequency of offshore converter 1 (Hz)

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96 3.0

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Chopper signal

Chopper signal

112

1.0

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250

Ac voltage of offshore converter 1 (MW)

250

RMS value of ac voltage (kV)

RMS value of ac voltage (kV)

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52.0

51.5

51.0

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(a)

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(d)

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3 2 1

0.0

-0.1 3.0

3.2

3.4

3.6

Time (s)

(i) (j) Fig 10 (a) and (b) are respectively the RMS value of the ac voltage of the three onshore converter stations without DC chopper and with DC chopper ; (c) and

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Active power of offshore converter 2 (MW)

Active power of offshore converter 2 (MW)

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VI. CONCLUSION

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Time (s)

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Rotor speed of wind farm 2 (pu)

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Time (s)

5

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5

6

(c) (f) Fig 12 (a)(b)(c) are respectively frequency , rotor speed of wind turbine and active power of offshore converter 1 using voltage reduction method without DC chopper; (d)(e)(f) are those quantities with DC chopper

From the comparison of the two scenario, shown in Fig 10 to Fig 12, it can be known that the fault in the first scenario causes about 110MW active power fails to transmit to ac grid. About 10MW of this part of power is absorbed by other converter station, shown in Fig 10 (g), 40MW are reduced by offshore converter 1 with voltage reduction method, shown in Fig 11(c), and 60MW are reduced by offshore converter 2 with frequency increase method, shown in Fig12(b). Moreover, from Fig 11(a)(b) and Fig 12(a)(b), it is apparent that the voltage, frequency of the two offshore converters do not exceed their limits. The rotor speed of wind farm 2, reaching 1.24 p.u, is also less than its maximum of 1.25 pu. Therefore, the dc voltage is controlled, shown in Fig 10(e), and dc chopper is not triggered , shown in Fig10 (c). However, in the second scenario, there is about 180MW active power fails to transmit to ac grid, shown in Fig 10(h). Though other onshore converters absorb 20MW, shown in Fig10 (h) , the voltage reduction method decreases about 40MW power and the frequency increase method absorbs 90MW with rotor speed of wind farm 2 reaching its maximum value of 1.25p.u, there is still some active power causing dc voltage to increase. Hence, when dc voltage increases to U′dcth (340kV), shown in Fig 10(f) , dc chopper is triggered, shown in Fig10(d). Consequently, a power balance is again established and dc voltage is controlled. Therefore, the proposed protective measures are feasible and effective to keep active power balance and avoid large overvoltage under various conditions. Thus the FRT capability of multi-terminal HVDC system with wind farms is enhanced.

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MTDC system is promising for offshore wind farms integration, so it is essential to require it to satisfy the FRT provisions. Though a diversity of control strategies have been proposed, three communication-free control strategies draw more attention, namely, voltage reduction, frequency increase and dc chopper control methods. Thus this paper provides a detailed derivation of their maximum value within various restrictions. Within these boundaries, a three-level control strategy is worked out to avoid power unbalance during ac fault and thus enhances the system FRT capability. The trigger mechanism of the proposed control strategy is related to the ac fault severity. If the ac fault is not serious, causing a little surplus power, only voltage droop control of MTDC system can solve the power unbalance. If the ac fault is serious and plenty of active power fails to flow to the ac grid, the power reduction control of offshore converters will be stimulated. However, if the ac fault is more severe and even causes several ac grids to be affected, the third level control of dc chopper will take actions. In this way, active power during fault period can be made full use of and the dc voltage is controlled in different situations. The achievement of this control strategy is practical for the future MTDC system with offshore wind farms, as it provide theoretical foundations for the design of a reliable and economic FRT control system. Further research following will focus on the solution of dc fault. REFERENCES [1]

[2]

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G. DKE/German Commission for Electrical, Electronic and InformationTechnologies, “Technical guidelines for first HVDC grids,” EuropeanHVDC Grid Study Group Sept. 24, 2012, p. 102. [Online]. Available: http://www.ptd.siemens.de/121213_EU_HVDCGridStudyGroup.pdf S. Cole, J. Beerten, and R. Belmans, “Generalized dynamic VSC MTDC model for power system stability studies,”IEEE Trans. Power Syst., vol. 25, no. 3, pp. 1655–1662, Aug. 2010. Ackermann T. Transmission systems for offshore wind farms[J]. Wind power in power systems, 2005: 479-503. Tsili M, Papathanassiou S. A review of grid code technical requirements for wind farms[J]. IET Renewable Power Generation, 2009, 3(3): 308-332. Xu L, Andersen B R. Grid connection of large offshore wind farms using HVDC[J]. Wind Energy, 2006, 9(4): 371-382. Teixeira Pinto R, Rodrigues S F, Bauer P, et al. Grid code compliance of VSC-HVDC in offshore multi-terminal DC networks[C]//Industrial Electronics Society, IECON 2013-39th Annual Conference of the IEEE. IEEE, 2013: 2057-2062. Shi G, Wu G, Cai X, et al. Coordinated control of multi-terminal VSC-HVDC transmission for large offshore wind farms[C]//Power Electronics and Motion Control Conference (IPEMC), 2012 7th International. IEEE, 2012, 2: 1278-1282. Feltes C, Wrede H, Koch F W, et al. Enhanced fault ride-through method for wind farms connected to the grid through VSC-based HVDC transmission[J]. Power Systems, IEEE Transactions on, 2009, 24(3): 1537-1546. T. D. Vrionis, X. I. Koutiva, N. A. Vovos, and G. B.

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POWERCON 2014

Paper No CP3165

Chengdu, 20-22 Oct. 2014

Wengting Li received the B.S. degree of electrical engineering from the Harbin Institute of Technology University, Harbin, China, in 2013. She is currently working toward the Ph.D. degree in Wind Power Research Center of Shanghai Jiao Tong University, Shanghai, China. Her interests are in the field of application of power electronics in power system, and particularly, the interaction between offshore wind farms and MTDC transmission system. Jing Lv (S’14) was born in 1985. He received the B.Eng. Degree in electrical engineering from China University of Mining and Technology, Jiangsu, China, in 2009, the M.Sc. degree in electrical engineering from Shanghai Jiao Tong University, Shanghai, China, in 2011, and is currently pursuing the Ph.D. degree in electrical engineering at Shanghai Jiao Tong University. His main research interests include wind power generation and grid integration, high-voltage dc (HVDC) technology, modular multilevel converter (MMC), and control and protection of multiterminal dc system. Gang Shi (S’12) received the M. Sc degree from Shanghai Jiao Tong University, Shanghai, China in 2009, where he is currently working toward the Ph.D degree. In 2010, he was a Guest Researcher with Aalborg University, Aalborg, Denmark. His research interests are in DC grid for wind power collection and transmission. Xu Cai received the B.S. degree from Southeast University, Nanjing, China, in 1983, M.S. degree and Ph.D. from China University of Mining and Technology, in 1988 and 2000 respectively. He was with the Department of Electrical Engineering, China University of Mining and Technology, as an associate professor from 1989 to 2001. He joined Shanghai Jiao Tong University, as a professor from 2002 and is director of Wind Power Research Center of Shanghai Jiao Tong University from 2008 and vice director of State Energy Smart Grid R&D Center (Shanghai) from 2010 to . His special fields of interest lie in power electronics and renewable energy exploitation and utilization, including wind power converters, wind turbine control system, large power battery storage systems, clustering of wind farms and its control system and grid integration

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