Journal of Power Electronics

The Korean Institute of Power Electronics (전력전자학회)
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Frequency Stabilization Method for Grid Integration of Large-scale Centralized Wind Farms via VSC-HVDC Technology

  • Peng, Yanjian (College of Electrical and Information Engineering, Hunan University) ;
  • Li, Yong (College of Electrical and Information Engineering, Hunan University) ;
  • Liu, Fang (School of Information Science and Engineering, Central South University) ;
  • Xu, Zhiwei (Hunan Provincial Key Laboratory of Wind Generator and Its Control, Hunan Institute of Engineering) ;
  • Cao, Yijia (College of Electrical and Information Engineering, Hunan University)
  • Received : 2017.09.19
  • Accepted : 2017.12.09
  • Published : 2018.03.20
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Abstract

This work proposes a control method of frequency stabilization for grid integration of large-scale wind farms via the voltage source converter-based high-voltage direct current (VSC-HVDC) technology. First, the topology of grid integration of a large-scale wind farm via the VSC-HVDC link is provided, and simple control strategies for wind turbines, wind farm side VSC (WFVSC), and grid side VSC are presented. Second, a mathematical model between the phase angle of WFVSC and the frequency of the wind farm is established. The control principle of the large-scale wind power integrated system is analyzed in theory in accordance with the mathematical model. Third, frequency and AC voltage controllers of WFVSC are designed based on the mathematical model of the relationships between the phase angle of WFVSC and the frequency of the wind farm, and between the modulation index of WFVSC and the voltage of the wind farm. Corresponding controller structures are established by deriving a transfer function, and an optimization method for selecting the parameters of the frequency controller is presented. Finally, a case study is performed under different operating conditions by using the DIgSILENT/PowerFactory software. Results show that the proposed control method has good performance in the frequency stabilization of the large-scale wind power integrated system via the VSC-HVDC technology.

Keywords

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Fig. 1. Single-line diagram of a large-scale wind farm and VSC-HVDC transmission system.

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Fig. 2. Power？wind speed curve of a real 2 MW DFIG.

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Fig. 3. Structure of the frequency controller.

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Fig. 4. Diagram of WFVSC and a large-scale wind farm.

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Fig. 5. Frequency control mechanism of WFVSC.

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Fig. 6. Control scheme of WFVSC and GSVSC.

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Fig. 7. Frequency control structure in the complex frequencydomain.

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Fig. 8. Frequency response of the designed controller with different control gains. (a) K = 0.9, (b) K = 0.7, (c) K = 0.5, and (d) K = 0.1.

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Fig. 9. Dynamic response of the large-scale wind power integrated system under wind speed variation. (a) Generator speed. (b) Node voltage. (c) Phase angle of node voltage. (d) Modulation index. (e) Active power. (f) System frequency.

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Fig. 10. Dynamic response of the large-scale wind power integrated system consisting of two different wind farms under wind speed variation.(a) Generator speed. (b) Voltage. (c) Phase angle of voltage. (d) Modulation index. (e) Active power. (f) System frequency.

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Fig. 11. Dynamic response of the large-scale wind power integrated system under single-phase-to-ground fault. (a) Generator speed. (b) Node ( ) u.cy p. 1.02 1.02 Time (s)voltage. (c) Phase angle of node HV voltage. (d) Modulation index. (e) Active power. (f) K=0.5, (g) K=0.7. (h) K=0.9. en ( (g)qu cy 1.0 (c)1.0re en .)f qu 0.98 0.98 -0.69 .uA. Case 1: Variation of Wind Speed m re farm side is 0.006 Hz when the active power of the large- p 1.02te f 0.96 x (ys 0.96 e -0.72 scale wind farm decreases suddenly at t=10 s. Frequency of cy 1.0In this situation, wind speed is varied within 12 m/s. Fig. 9 S m d nte 0.94 0.94 i nys 2.0 2.4 2.8 3.2 3.6 4.0 n the large-scale wind farm is recovered at 50 Hz. The e ushows the dynamic respo se of the large-scale wind power S o -0.75 q 0.980 6.0 8.0 10.0 0.92 ti Time (s) relationship between the phase angle of the MV-side’s reme (s) integrated system via the VSC-HVDC link. The wind speed 2.0 2.4 2.8 3.2 3.6 4.0 la f 0.96.0 6.0 8.0 10.0 u -0.78 Time (s) (g) voltage (see Fig. 6) and the PWM modulation index of m(c) decreases from 12 m/s to 9 m/s at t=10 s, and the active d eTime (s) o st 0.94power of the large-scale wind farm is reduced, which may ) u. M -0.81 WFVSC is consistent with Eq. (16) when the DC voltage of y(d) p. 1.02 (h) Slead to frequency fluctuation. When the active power ( the VSC-HVDC link remains constant. 0.92cy 1.0 -0.84 2.0 2.4 2.8 3.2 3.6 4.0decreases suddenly, the frequency controller adjusts the phase en 0.0 2.0 4.0 6.0 8.0 10.0 The two wind farms that are integrated into the main gridTime (s)angle of WFVSC. Hence, less active power is transferred qu 0.98 Time (s) via the VSC-HVDC link are also studied. One wind farm isrefrom the large-scale wind farm to the VSC-HVDC link. Thus, f 0.96 (d) composed of 100 DFIGs, and the other wind farm is (h)m tethe active power balance between the large-scale wind farm ys 0.94 composed of 200 DFIGs. The initial wind speed of each windand the VSC-HVDC link is reached, which means the S 0.92 farm is 12 m/s, but the wind speeds of the former and latter2.0 2.4 2.8 3.2 3.6 4.00 6.0 8.0 10.0 frequency can be kept stable effectively. Under frequency Time (s) wind farms decrease to 2 m/s at t=10 s and 3 m/s at t=25 s.ime (s) control, the frequency fluctuation of the large-scale wind Fig. 10 shows the simulation results of the two large-scale(d) (h)

TABLE I FREQUENCY STABILITY TIME AND OVERSHOOT VALUE DISTRIBUTION AT DIFFERENT K VALUES

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TABLE II PARAMETERS OF THE VSC-HVDC LINK

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