Simulation Results with the Fuzzy Logic Based Coordination Control

Simulation is done in MATLAB/SIMULINK environment for 1.5 MW PMSG based wind turbine. The simulation is run for 5 second. Time series simulation results are obtained for symmetrical and asymmetrical grid faults.

5.4.4.1. Under Symmetrical Faults

The profile of variable wind speed is presented in Fig.5.18(a). Rated wind speed is 12m/s and pitch angle remains zero until wind speed is equal or less than 12.1m/s. It increases to limit the output power and rotor speed according to wind speed variation. A three line to ground fault is occurred at 1s. The fault duration is 0.2s. During the fault period, the grid voltage is decreased from 1 p.u. to 0.35 p.u. As the power imbalance occurs voltage of the DC link capacitor is increased. Without any LVRT control, the voltage is beyond its limit. It is observed from Fig.5.18(c) that the DC link over voltage is minimized.

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Moreover, the voltage transients as well as fluctuations are also less for the proposed LVRT control.

Generator speed and grid real power in this period are shown in Fig.5.18(d)-Fig. 5.18(e). According to Fig. 5.18(f) without LVRT controller the reactive power injection was 0.05 p.u. As soon as the proposed FL based coordinated control applied the reactive power injection is increased to 0.16 p.u. As per Fig. 5.18(g) the grid voltage profile before LVRT control was 0.32 p.u. during fault period. While it has increased to 0.49 p.u. after fuzzy coordinated control applied to the system.

(a)

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(c)

(d)

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(f)

(g)

Fig.5.18. Simulation result for symmetrical fault a) Variable wind speed b) Pitch angle c) DC link voltage d) Generator speed e) Grid real power f) Grid reactive power g) Grid voltage

5.4.4.2. Under Asymmetrical Faults

A double line to ground fault is occurred at line at 1s. The fault is cleared at 1.2s. A medium dip is occurred. So, the grid voltage is decreased from 1 p.u. to 0.6 p.u. DC link voltage of capacitor is increased from 1p.u. to 1.6 p.u. without LVRT control as shown in Fig.5.19(c). It is beyond the acceptable limit. From Fig.5.19(b) It is found that the pitch angle becomes zero during fault period without LVRT control but it is increased to 5.7 degree to limit DC link voltage with proposed control. Referring to Fig.5.19(c) the

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fluctuation of DC link voltage is minimized as well as it is well regulated with the proposed fuzzy coordinated control. Fig.5.19(d)-Fig.5.19(e) illustrated the scenario of generator speed and grid real power in the fault period without LVRT control and with proposed control. As per Fig. 5.19(f) without LVRT controller the reactive power injection was 0.045 p.u. The reactive power injection is enhanced to 0.22 p.u as soon as the proposed fuzzy coordinated control is applied. According to Fig. 5.19(g) the grid voltage profile before LVRT control was 0.53 p.u. during fault period. While it has increased to 0.69 p.u. after the proposed coordinated control is applied to the grid connected WECS.

(a)

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(c)

(d)

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(f)

(g)

Fig.5.19. Simulation result for asymmetrical fault a) Variable wind speed b) Pitch angle c) DC link voltage d) Generator speed e) Grid real power f) Grid reactive power g) Grid voltage

5.5. Summary

A PI based coordinated control method is proposed in this chapter to improve the LVRT performance of a variable speed PMSG based grid connected WECS. Then the proposed approach is verified under different scenarios during the symmetrical and asymmetrical grid faults. From the simulation results, it has been demonstrated that the proposed method can enhance the LVRT performance by injecting enough reactive power to

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support the grid voltage during the grid faults with the help of the GSC and the STATCOM. Due to the simplicity and effectiveness of the proposed control, it can be a better choice compared to the conventional control with the BC during the LVRT. Next, a Fuzzy Logic based coordinated control method is also proposed to improve the LVRT performance of a grid connected WECS. The proposed method is tested for a 1.5 MW wind turbine for symmetrical and asymmetrical grid faults. From the simulation results, it is found that the proposed coordinated control method can limit the DC-link overvoltage as well as can inject the reactive power for supporting the grid voltage under grid faults.

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[7] H. Gaztanaga, I. Etxeberria‐Otadui, D. Ocnasu, and S. Bacha, “Real‐time analysis of the transient response improvement of fixed‐speed wind farms by using a reduced‐scale STATCOM prototype,” IEEE Transactions on Power System, vol. 22, no. 2, pp. 658–666, May 2007.

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[8] Wang Li, Chia-Tien Hsiung, "Dynamic stability improvement of an integrated grid-connected offshore wind farm and marine-current farm using a STATCOM", IEEE Transactions on Power Systems, vol. 26, no. 2, pp. 690-698, 2011.

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CHAPTER SIX

FREQUENCY REGULATION BY THE GRID

CONNECTED WECS UNDER NORMAL AND FAULT

RECOVERY CONDITIONS

6.1. Introduction

Frequency regulation by using the grid connected WECS during usual load frequency deviations and the frequency deviations after a grid fault recovery is described in this chapter. The following two approaches such as the physical inertia-based control and the flux linkage control with a Superconducting Magnetic Energy Storage (SMES) system are proposed. Performance of the proposed controllers is validated by dynamic simulations under different scenarios.