This dissertation proposes a frequency regulation scheme for a variable-speed wind turbine generator (WTG) that can smooth out the variable-speed WTG’s fluctuating power caused by varying wind speeds and improve the frequency drop for a generator trip and load connection/disconnection. Thus, the proposed scheme can keep the system frequency within a narrow range and improve the frequency nadir and maximum rate of change of frequency. The proposed scheme employs an additional loop based on the system frequency deviation that operates in conjunction with the maximum power point tracking (MPPT) control loop. Unlike the conventional, fixed-gain scheme, in the proposed scheme, the control gain is modified by considering the ratio of the output of the additional loop to that of the MPPT loop. To improve the contribution of the scheme toward maintaining the frequency while ensuring the stable operation of a variable-speed WTG, in the low rotor speed region, the ratio is set to be proportional to the rotor speed; in the high rotor speed region, the ratio remains constant. If the ratio for the operating region of the rotor speed is determined, the gain can be obtained by simply multiplying PMPPT function by the ratio.
The IEEE 14-bus system was chosen to investigate the performance of the power smoothing schemes. It includes five synchronous generators (SGs), static loads, and three aggregated DFIG-based wind power plants (WPPs). It was simulated using an EMTP-RV simulator and a real time digital simulator. In this dissertation, the system frequency for performing the power smoothing schemes is calculated in the variable-speed WTG controller from the measured WPP terminal voltage at the sampling frequency of 3,840 Hz using a phase-locked loop and the measured voltages are passed through a second-order, anti-aliasing, low-pass filter with a cutoff frequency of 1,920 Hz to the controllers.
The performance of the power smoothing scheme of a variable-speed WTG is affected by the wind speed variation, load connection/disconnection and generator trip. The performance of the proposed scheme is investigated under the scenarios by varying the wind speeds for wind power penetration levels of 15% and 30%: 30 MW and 60 MW of the rated power of each WPP, respectively. In this paper, the wind power penetration level was defined based on the installed capacity.
The simulation results indicate that the proposed scheme can smooth the output power fluctuation of the WPPs and improve the frequency drop by adjusting the control gain of the frequency deviation loop with the rotor speed, and thereby improve the frequency-regulating capability of a DFIG in an electric power grid.
In a long-term small disturbance, the advantages of the proposed scheme are that it can mitigate the fluctuation of the variable-speed WTG output power, and thereby improve the power smoothing capability, especially in a high level of wind penetration into the grid. In addition, it can utilize the operating speed range of a variable-speed WTG by absorbing or releasing the kinetic energy stored in a variable-speed WTG with the rotor speed. Further, it helps regulate the frequency deviation into a narrow range even in a power grid that has a high penetration level of wind power. The small energy loss is to be expected in exchange for the higher reliability of power system operation by maintaining the frequency within a narrow range. Moreover, in a short-term large disturbance, the proposed scheme can improve the frequency nadir. This is because the output power for the proposed scheme is larger than that for the convetional scheme since the output of the frequency deviation loop for the proposed scheme is lager than that for the conventional scheme.