Moses Kang

Frequency Control Support of a Doubly-Fed Induction Generator Based on the Torque Limit

This paper proposes a torque limit-based inertial control scheme of a doubly-fed induction generator (DFIG) that supports the frequency control of a power system. If a frequency deviation occurs, the proposed scheme aims to release a large amount of kinetic energy (KE) stored in the rotating masses of a DFIG to raise the frequency nadir (FN). Upon detecting the event, the scheme instantly increases its output to the torque limit and then reduces the output with the rotor speed so that it converges to the stable operating range. To restore the rotor speed while causing a small second frequency dip (SFD), after the rotor speed converges the power reference is reduced by a small amount and maintained until it meets the reference for maximum power point tracking control. The test results demonstrate that the scheme can improve the FN and maximum rate of change of frequency while causing a small SFD in any wind conditions and in a power system that has a high penetration of wind power, and thus the scheme helps maintain the required level of system reliability. The scheme releases the KE from 2.9 times to 3.7 times the Hydro-Québec requirement depending on the power reference.

Inertial response of wind power plants: A comparison of frequency-based inertial control and stepwise inertial control

The frequency regulation capability of a wind power plant plays an important role in enhancing frequency reliability especially in an isolated power system with high wind power penetration levels. A comparison of two types of inertial control methods, namely frequency-based inertial control (FBIC) and stepwise inertial control (SIC), is presented in this paper. Comprehensive case studies are carried out to reveal features of the different inertial control methods, simulated in a modified Western System Coordination Council (WSCC) nine-bus power grid using real-time digital simulator (RTDS) platform. The simulation results provide an insight into the inertial control methods under various scenarios.

Stable Adaptive Inertial Control of a Doubly-Fed Induction Generator

This paper proposes a stable adaptive inertial control scheme of a doubly-fed induction generator. The proposed power reference is defined in two sections: the deceleration period and the acceleration period. The power reference in the deceleration period consists of a constant and the reference for maximum power point tracking (MPPT) operation. The latter contributes to preventing a second frequency dip (SFD) in this period because its reduction rate is large at the early stage of an event but quickly decreases with time. To improve the frequency nadir (FN), the constant value is set to be proportional to the rotor speed prior to an event. The reference ensures that the rotor speed converges to a stable operating region. To accelerate the rotor speed while causing a small SFD, when the rotor speed converges, the power reference is reduced by a small amount and maintained until it meets the MPPT reference. The results show that the scheme causes a small SFD while improving the FN and the rate of change of frequency in any wind conditions, even in a grid that has a high penetration of wind power.

Power Smoothing of a Variable-Speed Wind Turbine Generator in Association With the Rotor-Speed-Dependent Gain

This paper proposes a power-smoothing scheme for a variable-speed wind turbine generator (WTG) that can smooth out the WTGs fluctuating power caused by varying wind speeds, and thereby keep the system frequency within a narrow range. 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, its control gain is modified with the rotor speed. In the proposed scheme, the control gain is determined 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 WTGs, 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. The performance of the proposed scheme is investigated under varying wind conditions for the IEEE 14-bus system. The simulation results demonstrate that the scheme successfully operates regardless of the output power fluctuation of a WTG by adjusting the gain with the rotor speed, and thereby improves the frequency-regulating capability of a WTG.

Modified stepwise inertial control using the mechanical input and electrical output curves of a doubly fed induction generator

A stepwise inertial control (SIC) scheme of a wind turbine generator (WTG) rapidly increases its constant output to support the frequency control if a large disturbance such as a generator trip occurs in a power grid. This helps to significantly arrest the frequency nadir, but causes significant reduction in the kinetic energy (KE) of a WTG. Thus, the output of a WTG should be rapidly reduced to restore the rotor speed. This might cause a second frequency dip. This paper presents a modified SIC scheme of a doubly fed induction generator (DFIG) to prevent a second frequency dip. The modified SIC scheme generates a reference signal that consists of the constant reference and the reference for maximum power point tracking operation. The former is analytically determined by considering the mechanical input power and electrical output power curves of a DFIG. The output of the proposed SIC scheme decreases with the KE of the DFIG and thus no second frequency dip occurs. The performance of the proposed SIC is investigated for a 100MW aggregated DFIG wind power plant using an EMTP-RV simulator. The results clearly show that the proposed SIC scheme significantly increases the frequency nadir without causing a second frequency dip.

Frequency support of PMSG-WTG based on improved inertial control

With increasing integrations of large-scale systems based on permanent magnet synchronous generator wind turbine generators (PMSG-WTGs), the overall inertial response of a power system will tend to deteriorate as a result of the decoupling of rotor speed and grid frequency through the power converter as well as the scheduled retirement of conventional synchronous generators. Thus, PMSG-WTGs can provide value to an electric grid by contributing to the systems inertial response by utilizing the inherent kinetic energy stored in their rotating masses and fast power control. In this work, an improved inertial control method based on the maximum power point tracking operation curve is introduced to enhance the overall frequency support capability of PMSG-WTGs in the case of large supply-demand imbalances. Moreover, this method is implemented in the CART2-PMSG integrated model in MATLAB/Simulink to investigate its impact on the wind turbines structural loads during the inertial response process. Simulation results indicate that the proposed method can effectively reduce the frequency nadir, arrest the rate of change of frequency and mitigate the secondary frequency drop while imposing no negative impact on the major mechanical components of the wind turbine.

Assessment of system frequency support effect of PMSG-WTG using torque-limit-based inertial control

To release the “hidden inertia” of variable-speed wind turbines for temporary frequency support, a method of torque-limit based inertial control is proposed in this paper. This method aims to improve the frequency support capability considering the maximum torque restriction of a permanent magnet synchronous generator. The advantages of the proposed method are improved frequency nadir (FN) in the event of an under-frequency disturbance; and avoidance of over-deceleration and a second frequency dip during the inertial response. The system frequency response is different, with different slope values in the power-speed plane when the inertial response is performed. The proposed method is evaluated in a modified three-machine, nine-bus system. The simulation results show that there is a trade-off between the recovery time and FN, such that a gradual slope tends to improve the FN and restrict the rate of change of frequency aggressively while causing an extension of the recovery time. These results provide insight into how to properly design such kinds of inertial control strategies for practical applications.