Following a frequency event in a power system, synthetic inertia control (SIC) of a wind turbine generator (WTG) can improve the frequency nadir by instantly releasing the stored kinetic energy in the rotating masses. However, SIC might lower the settling frequency below the maximum steady-state frequency deviation, thereby disabling automatic generation control (AGC). This delays frequency recovery to the nominal value. This paper proposes a closed-loop SIC scheme for a WTG in association with slightly over-speeded deloading operation (SODO) that can improve both the frequency nadir and settling frequency. To achieve this, prior to an event, the proposed SIC scheme performs SODO, which can store more releasable energy into the rotating masses of a WTG while minimizing the energy loss. The proposed scheme can improve the frequency nadir by releasing more energy, improve the settling frequency, which enables AGC, and avoid over-deceleration of the rotor speed of a WTG in the low wind speed region. Simulation results clearly demonstrate that the proposed SIC scheme can improve both the frequency nadir and settling frequency under various wind conditions, event sizes, and penetration levels, thereby supporting the frequency stability in a power system that has a high penetration of wind power.

This paper proposes a power-smoothing scheme for a variable-speed wind turbine generator (WTG) that can maintain the frequency within a narrow range under normal operations. The proposed scheme uses an additional control loop based on the frequency deviation that operates in conjunction with maximum power point tracking (MPPT). To improve the energy-absorbing capability in the overfrequency section (OFS) and energy-releasing capability in the underfrequency section (UFS), the scheme suggests separate control gains, which are determined based on the ratio of the output of the additional loop to that of the MPPT loop. The ratio in the OFS is set to decrease with respect to the rotor speed whereas the ratio in the UFS is set to increase with respect to the rotor speed. The simulation results demonstrate that the proposed scheme can significantly lessen the WTG output power fluctuations by adjusting the control gains of the UFS and the OFS even though the WTG output power is severely fluctuating, thereby mitigating the frequency fluctuations. The scheme effectively uses the heavy rotating masses of a WTG as an energy storage system for enhancing the power grid flexibility in terms of frequency, particularly in high penetration levels of variable renewable energy.

Correct and fast detection of a micro-grid (MG) islanding is essential to the MG since operation, control, and protection of the MG depend on its operating mode i.e., an interconnected mode or islanding mode. This study describes the design, evaluation and implementation of an islanding detection method for an MG, which includes a natural gas-fired generator, a doubly fed induction generator type wind generator, a photovoltaic generator, and some associated local loads. The proposed method is based on the instantaneous active and reactive powers at the point of common coupling (PCC) of the MG. During the islanding mode, the instantaneous active and reactive powers at the PCC are constants, which depend on the voltage of the PCC and the impedance of the dedicated line. The performance of the proposed method is verified under various scenarios including islanding conditions for the different outputs of the MG, and fault conditions by varying the position, type, inception angle and resistance of the fault, using the PSCAD/EMTDC simulator. This paper also concludes by implementing proposed method into a TMS320C6701 digital signal processor. The results indicate that the proposed method successfully detects islanding for the MG in islanding conditions, and remains stable in fault conditions.

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.

This paper proposes a frequency-smoothing scheme of a doubly-fed induction generator (DFIG) that can significantly eliminate the frequency fluctuations caused by continuously fluctuating wind speeds. To achieve this, a proportional-integral (PI) control loop depending on the frequency deviation instead of a proportional control loop used in conventional schemes is added to the maximum power point tracking loop. The P control loop in the proposed frequency-smoothing scheme eliminates the alternating component in the frequency deviation, whereas the I control loop eliminates the slowly-varying component. The gains for the P control loop and I control loop are suggested for effectively using the rotating masses of a DFIG while avoiding over-deceleration of the rotor speed. The simulation results evidently demonstrate that the proposed scheme nearly removes the frequency deviation under various wind conditions, by using the proposed PI control loop, especially in high penetration levels of wind.

A high wind-power penetration level causes increased uncertainty in power system operation because of the variability and limited predictability of wind generation. This paper proposes a novel type of unit commitment (UC) considering spinning reserve and interruptible load (IL) as operating reserve facilities to increase system flexibility for reliable, economical operation. Two uncertainty sources, load and wind generation, were modeled via autoregressive moving averages (ARMA). The formulation of interruptible load was considered in the implementation of unit commitments. Lagrangian relaxation-dynamic programming (LR-DP) was used to solve the unit commitment problem efficiently. The expected energy not supplied (EENS) was regarded as a probabilistic reliability criterion. The effectiveness of the proposed unit commitment was evaluated using an IEEE 118-bus system. The simulation results clearly demonstrated that with demand-side participation, the operating cost was significantly reduced when handling the increased uncertainty due to wind power integration within the required reliability criteria.