The frequency in a power system should be maintained within a narrow band at all times. When a frequency event occurs in a power system, kinetic energy (KE) from the synchronous generators is intrinsically released as a result of an inertial response. If the frequency decreases beyond the governor deadband of the synchronous generators, primary frequency control is activated to arrest the frequency decline and stabilize the frequency, which is called the settling frequency (fset), by deploying primary reserve based on the droop characteristic. Then, automatic generation control (AGC) is activated to restore fset to the nominal value.
To avoid an unexpected outage of load, the frequency nadir (FN) should be higher than the threshold of an underfrequency load-shedding relay. In addition, to activate AGC, fset should be more than the maximum steady-state frequency deviation. Thus, FN and fset are critical metrics for evaluating the frequency stability of a power system. Further, to incorporate high penetrations of variable renewable energy (VRE), the frequency stability should be ensured for the reference event in the synchronous area. Otherwise, VRE should be curtailed to maintain the frequency stability.
To capture the maximum energy from the wind, a variable-speed wind turbine generator (WTG) performs maximum power point tracking (MPPT) operation. However, this might jeopardize the frequency stability when a frequency event occurs in a power system.
To support the frequency stability, upon a frequency event, a WTG can temporarily release the KE stored in the rotating masses of a WTG, thereby improving the FN. This is called inertia control of a WTG.
Inertia control of a WTG employs additional control loops relying on the rate of change of frequency and/or frequency deviation in conjunction with the MPPT loop. For inertia control of a wind power plant (WPP) consisting of multiple WTGs, special attention should be paid on determining the inertia control gain setting for each WTG. This is because the wind speed for each turbine is different because of the wake effects. The control gain of additional control loops for each WTG should be set depending on the stored KE so that inertia control scheme can arrest the FN while preventing over-deceleration (OD) of the rotor speed.
For fixed-gain inertia control schemes, a large gain setting can provide a large contribution to supporting system frequency stabilization. However, this might cause OD of the rotor speed for a WTG which has a small amount of KE because the large gain releases the excessive KE of WTGs from the rotating masses. Further, if the wind speed decreases while performing inertia control, even a small gain may cause OD for some WTGs. The OD of a WTG should disable inertia control of a WTG, thereby causing a subsequent frequency drop, which is called the secondary frequency drop (SFD).
This dissertation proposes a stable inertia control scheme using a rotor-speed dependent adaptive gain for a doubly-fed induction generator (DFIG)-based WPP. The proposed scheme produces the additional active power to a power system to improve the FN while preventing OD of all DFIGs, even in the decreasing wind condition while performing inertia control. To achieve these, the proposed scheme uses a frequency deviation based inertia control loop in association with the adaptive control gain. In the proposed scheme, the adaptive gain is updated depending on the amount of releasable KE in each DFIG during inertia control, which is spatially and temporally dependent. To elevate the FN, upon detecting a frequency drop, large gains are assigned to be proportional to the releasable KE of a DFIG. In addition, to ensure stable operation, the adaptive gains decrease with the releasable KE. The proposed scheme can elevate the FN while ensuring stable operation of a WTG.
The simulation results clearly present that the proposed scheme using the adaptive gain can elevate the FN while ensuring stable operation of all DFIGs in a WPP under various wind and system conditions. Further, even in the decreasing wind condition, in which the releasable KE in DFIGs decreases, the proposed scheme can continuously support the frequency stability, whereas the conventional fixed-gain schemes cause OD, thereby causing the SFD because of the rapid output power reduction.
The advantages of the proposed adaptive gain scheme are that it can improve the frequency-supporting capability of a WPP by employing the adaptive gain. In addition, the proposed scheme can avoid OD of the rotor speed, thereby preventing the SFD. Consequently, the proposed scheme provides a promising solution to arrest the FN, especially in a power system which has a high penetration of a wind power.