Operation and Coordinated Control of Multi-terminal VSC-HVDC Transmission System for Wind Power Generation

Yuan Fu1,a, Yi Wang1,b, Yingli Luo2,c, and Xiangyu Zhang1,c,* 1North China Electric Power University, Yonghua North Street 619, Baoding, 071003, China

2North China Electric Power University, Beijing, 102206, China afuyuan8000@163.com, byi.wang@ncepubd.edu.cn, czh.xy.sq@163.com

*Xiangyu Zhang

Keywords: High voltage dc transmission (HVDC), Multi-terminal DC (MTDC), Voltage source converter (VSC), Wind power.

Abstract. The multi-terminal DC (MTDC) is anticipated as a flexible and reliable solution for the large scale wind power transmission. This paper investigates the use of voltage source converter (VSC) based MTDC system for outward transmission of the large scale wind power. A MTDC system for aggregating and dispatching wind power is proposed firstly. And then the coordinated control strategy for the proposed MTDC system is discussed. Finally, a four-terminal MTDC system connecting two DFIG-based wind farms, the local and remote grids is simulated for demonstrating the performance of the MTDC system during wind speed variations and DC line faults. The results show that the proposed MTDC system and control strategy can reallocate the wind power in the DC network to achieve a constant power for long distant transmission to the load center.

1. Introduction HVDC transmission system is a feasible and economical solution for long distance power transmission. Compared to the classical line commutated converter based HVDC (LCC-HVDC), the voltage source converter based HVDC (VSC- HVDC) has advantages such as independent control of active and reactive power, no commutation failures, black-start capability, easy extension to multi-terminal DC(MTDC) system, etc.

Many types of MTDC system have been proposed with different operation objectives, terminal numbers and DC network configurations. MTDC systems for transmitting power between conventional AC networks and connecting wind farms based on induction generators, DFIG, and synchronous generators were proposed in [1-3]. In [4], a three-terminal VSC-HVDC connecting an offshore wind farm, an oil and gas platform load and an onshore ac grid was proposed and analyzed. However, only one grid side VSC was considered in these studies. In [5-6], a simple four-terminal MTDC system was proposed for integrating two offshore wind farms into two onshore ac grids situated in various geographical location and the power sharing and DC grid management strategies were outlined. However, the effects of the wind power fluctuation and intermission on the line transfer capacity need further investigation.

In this paper, a novel four-terminal DC network, which connects two wind farms to the local grid and dispatches wind power to the remote load center, is simulated using MATLAB/Simulink to verify the performance and stability of the proposed MTDC system, and the coordinated control strategies between the wind farm side VSCs and the grid side VSCs are investigated.

2012 International Conference on Power and Energy Systems Lecture Notes in Information Technology, Vol.13

978-1-61275-011-8/10/$25.00 2012 IERI ICPES2012

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2. Model and control of VSC The VSCs in the proposed MTDC system have the same configuration but different control modes. The schematic diagram and the vector-based equivalent circuit of a VSC station in the synchronous dq reference frame is shown in Fig. 1.

(a) Schematic diagram of VSC-HVDC (b) Complex vector equivalent circuit of a VSC station in the synchronous

frame Fig. 1 Schematic diagram and equivalent circuit of a VSC station

According to Fig. 1(b), the voltage, current, power of the converter can be expressed as

c s c

djd

R L Lt

cc s c c c IV V I I (1)

s f f

djd

C Ct

sc s s VI I V (2)

dcdc dc L

ddVC I It (3)

s s s s

3 j V I2

P Q (4)

dc s c c dc dcP P I R V I (5)

f s fd jd

C Ct

s s c sV I I V (6) where Rc represents the total loss of the VSC, Lc is the filter inductor, Cf is the filter capacitor.

The VSC instantaneous power control is typically based on vector control with dual closed loop structure. For the vector control using grid voltage orientation, the active power and reactive power are proportional to the d-axis and q-axis currents respectively. During power transmission, at least one VSC terminal should control the DC link voltage, and the other terminal controls active power flow so as to achieve automatic active power balance of the DC network [7-8]. Whilst for the wind farm side VSC station not only collects energy from the wind farm but also needs to control the wind farm AC network voltage and frequency. Therefore, there can be three different active power control modes for different VSC operations, i.e. DC voltage control, power flow control, and voltage source control modes, which are discussed as follows.

2.1 Mode I: DC voltage control For the normal operation of a MTDC system, its common DC link voltage must be maintained at a constant value to balance the transmitted power under all conditions. To achieve this balance, the VSC connected to the local grid with fast energy storage is assigned to control the dc voltage.

The control system is usually defined in the synchronous d-q reference frame fixed to the grid voltage. As shown in Fig. 2, the outer DC voltage and AC voltage/reactive power control loops generate the reference d and q axis current orders respectively for Mode I. For the inner current loop, the converter current PI regulators and the decoupling voltages are used to follow the relevant current references. According to (1), the required converter output voltage is

417

* ' * *p i

'c s

( ) ( )d

j

k k t

R L

c c c c cs c c c

V V I I I I

V V I I (7)

where 'V is the decoupling voltage, kp and ki are the proportional and integral gains of the current controller.

2.2 Mode II: Power flow control After aggregating wind power flow and mitigating wind power fluctuations at the swing bus of the DC network, a constant power flow is dispatched to the remote load center, and the RGVSC is the receiving end.

For Mode II, the inner current control loop and the outer reactive power control loop are the same with Mode I, but the d-axis current reference is given by the power flow control in the outer control loop as shown in Fig. 2. The RGVSC operates at power flow control mode normally, and switches to DC voltage control only when LGVSC reaches power limitation due to sudden changes of wind speed or disturbances of the local AC grid.

2.3 Mode III: Voltage source control The wind farm side VSC stations operate as a voltage source with constant AC voltage amplitude and frequency, and collects energy from the wind farms. The outer control loop to keep wind farm side voltage constant is shown in Fig. 2. For the outer ac voltage loop, a PI controller and feed forward decoupling item are adopted and the output of the controller determines the current reference of the inner control loop. According to (6), the current references for the inner control loop can be given as

*p1 i1 s f( ) ( )d jk k t c * *c s s s s sI V V V V V (8)

where kp1 and kis are the proportional and integral gains of the AC voltage controller. The frequency and phase of the converter output voltage Vs are determined solely by the VSC control system, and the s used for reference frame transformation is provided by integrating a constant frequency rather than using conventional phase-locked-loops (PLL).

s

Fig. 2 Schematic control diagram of the VSC

3. Coordinated control of MTDC system The proposed MTDC system is constructed for aggregating wind farms to mitigate the power fluctuations, and dispatching wind power between the local grid and the remote load center. The local grid may have the ability of balancing a certain degree of wind power fluctuations while it has reserve power supply or fast energy storage. However, the regulation capacity of the local grid only considers covering the short term wind power fluctuations rather than the total wind generation capacity. Due to the limitation of the LGVSC capacity, the operation mode of the MTDC system can

418

be divided into three cases: Case I: _ max _ _ maxLG WF total RG LGP P P P Case II: _ _ maxWF total RG LGP P P Case III: _ _ maxWF total RG LGP P P

where PWF_total is the total aggregated wind power, PRG is the dispatching power for outward transmission, and PLG_max is the maximum regulation capacity of the LGVSC. The coordinated control strategy of the VSC terminals and wind farms is depicted in Fig. 3.

Fig.3 Simplified coordinated control strategy of the MTDC

3.1 Coordinated control under Case I Under Case I, the VSC station connected to local grid is assigned as the swing bus, and operates at Mode I to maintain DC voltage constant. Due to the LGVSC can balance the total wind power fluctuations, the WFVSC operates at Mode III to collect all the generated wind power, while the RGVSC operates at Mode II to transmit a constant power flow to the load center. The DC link voltage must be controlled within acceptable limits to ensure the stability of the DC network.

3.2 Coordinated control under Case II If the wind speed sudden decrease or there is a DC line fault of the wind farm side VSCs, the aggregated wind power is less than the rated transmitting power to the RGVSC, which causes the DC voltage droop sharply and may reach the lower limit. Under this condition, the LGVSC supply all the reserve capacity for the RGVSC, but it is still not enough to balance the power of DC network. And then the LGVSC switches to current limit control mode. Meanwhile the RGVSC switches the control mode form Mode II to Mode I to control the DC voltage at a low value. The receiving power flow by the RGVSC is reduced consequently.

3.3 Coordinated control under Case III When the value difference between the infeed wind power and the transmission power is much larger than the maximum regulation capacity of the LGVSC, the DC voltage increases rapidly until reach its upper limit. Therefore, the LGVSC is no longer able to control the DC voltage under this condition, and it will go into current limit mode. Consequently, the WFVSC switches operation mode from Mode III to Mode I to control the DC voltage, and the wind turbines must reduce the output power immediately to balance the power flow of the DC network. An extra droop controller dependent DC voltage will provided a de-loading order of P, which is sent to the wind farm controller. The principle of the coordination control of the varying converter terminals is to maintain the system stability and to transmit a constant power to the remote load center under large disturbance. Therefore, there are two operation modes of the RGVSC acoording to different disturbances. If the wind farms generate maximum power simultaneously, the RGVSC still maintants the power at the rated. On the other hand, when a short circuit fault occurs at the power transmission line, the RGVSC should be temporary blocked during the AC fault and fast recovers to the constant power flow control mode. As shown in Fig.3, the MTDC system by using the coordinated control can be immune from influence of wind power fluctuations or AC fault, and transmit a constant wind power to the remote load center. In addition, the MTDC system can switch operation modes in different cases by detection DC voltage with no need of communication.

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4. Simulation studies Simulation studies of the proposed MTDC system for integrating two DFIG based wind farms with the local and remote grids have been performed using MATLAB/Simulink. The system configuration and parameters are shown in Fig.4. Two DFIG-based wind farms and the WFVSCs are both rated at 500MW. The wind power is collected by WFVSCs, then aggregated by LGVSC, and finally transmitted to a remote grid located 200km away. The main task of the local grid is to mitigate the wind power fluctuations and control DC link voltage. The upper limit and lower limit of the DC voltage are set as 1.02 and 0.98 respectively. The local grid has the ability of fast balancing the power fluctuations within 100MW. The rated transmission capacity of the line 3 to remote grid is 400MW.

Fig.4 Single line diagram of a four-terminal MTDC network

The MTDC system control and operation with wind speed variations were simulated firstly, and the results are shown in Fig.5. The wind speeds are 8m/s and 10m/s for wind farm 1 and wind farm 2 respectively with generated power being around 0.25 p.u (125MW) from WF1 and 0.52 p.u.(260MW) from WF2. As the LGVSC can balance 100MW fluctuatios, the DC voltage is controlled at the set value of 1p.u as shown in Fig.5(a). When the wind speed of WF1 step increase to 10m/s at 2s, the WF1 generated power is changed from 0.25p.u to 0.52p.u accordingly. While the WF2 keeps 0.52p.u at all time. Thus the wind power aggregated to the local grid is changed from 0.77p.u (385MW) to 1.04p.u (520MW) at 2s, while the transmitted power to the remote grid always keeps at 400MW. Due to the LGVSC cannot absorb the more power fluctuation of the DC ntetwork, the DC voltage increases fast until the WFVSCs switch operation mode to Mode I. The WF1 has to be de-loading operation instead of MTTP control mode to achieve the power balance of the DC network.

Fig.5 Performances of the MTDC system

Conversely, the wind speeds of WF1 and WF2 are 10m/s and 8m/s initially. As can be seen in Fig. 5 (b), When the wind speed of WF1 drops to 8 m/s at 2s, the total wind power changes from 0.77p.u

420

(385MW) to 0.5p.u (250MW). Accordingly, the LGVSC cannot control DC voltage as well. Under this case the RGVSC has to operate at DC voltage control mode as Fig. 3(a), and reduce the transmitted power to the remote grid for maintaning the power balance of the DC network.

Further tests on the operation and performance of the system during large disturbance caused by the trip of WF1. Fig.5 (c) shows the MTDC system performance under the condition of WF1 trips. Because of the sudden loss of 125MW infeed power, the DC link voltage drops sharply. Although the LGVSC provides the 100MW reserve capacity to the MTDC system, the transmitted power cannot reach the rated power of 400MW. The LGVSC goes into current limit control mode and no longer controls the DC link Voltage. When the DC link voltage drops to 0.97p.u, the RGVSC swtichs to DC voltage control mode as Fig. 3(a). Finaly, the RGVSC receiving power is reduced to 0.7p.u, meanwhile the DC voltage is maintained at 0.98p.u.

5. Conclusion This paper investigates the potential of the MTDC system for large scale wind power transmission. Compared to the AC network for wind farms interconnection, the MTDC system is more flexible and reliable due to the fast control ability of the VSC. Three control modes of the VSC for different connected terminals, i.e. DC voltage control, power flow control and voltage source control are discussed. The control characteristics of VSC station and a novel coordinated control strategy that operates the VSCs smoothly under the three conditions are depicted. And the simulation results with wind speed variations and transmission line faults show that the advantages of the MTDC system for dispatching wind power as the requirements of the power system operator and solving the bulk wind power within a long distance transmission bottleneck.

6. AcknowledgementS This research was financially supported by the National Natural Science Foundation of China (No. 50977028 and No. 50807015) and by the Fundamental Research Funds for the Central Universities.

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