This paper proposes a temporary frequency-support scheme of a doubly fed induction generator (DFIG) that can improve the frequency nadir while ensuring rapid frequency stabilization, particularly for high wind power penetration levels (WPPLs). Upon detecting a disturbance, the power reference is increased by the incremental power and maintained for a preset period. The proposed incremental power varies with the rotor speed and WPPL. Then, to force the rotor speed to converge to a stable operating range, the reference decreases with the rotor speed. During the deceleration period, the proposed scheme releases less kinetic energy, which helps in the rapid recovery of the rotor speed. During the acceleration period, to accelerate the rotor speed recovery, the reference smoothly decreases with time and rotor speed until it reaches the maximum power point tracking curve. The test results, which are based on the IEEE 14-bus system, demonstrate that even though less kinetic energy is released, the proposed scheme can improve the frequency nadir while rapidly recovering the rotor speed under various wind conditions and penetration levels, but it is particularly effective for higher penetration levels. The scheme helps providing a promising solution to the ancillary services of a DFIG in a power system with high WPPLs.

This article describes a positive-sequence model to represent two widely reported droop-controlled grid-forming inverters for transmission system transient stability simulation studies. Methods of how to develop the equivalent voltage source behind impedance to represent inverters with and without inner control loops, modeling of P-f and Q-V droop controls, active and reactive power limiting modeling, and modeling of fault current limiting controls have been described in detail. The model was implemented in commercially available positive-sequence simulation tools and compared with detailed electromagnetic transient models in both a single-grid-forming-inverter infinite-bus system and a modified IEEE 39-bus system. Finally, the model was tested on the U.S. Western Interconnection system. The study results showed that the model has a good level of accuracy compared to the detailed EMT models, and at the same time also achieves a high computational efficiency, making it suitable for large-scale transmission system simulation studies.

This paper proposes a dynamic frequency support scheme of a supercapacitor energy storage system (SCESS) in coordination with run-of-the-river (ROR) -based pumped storage hydropower (PSH) to enhance the short-term frequency stability in a power system that has a high penetration of renewable energy. To achieve this, the proposed coordinated frequency controller (CFC) enables the SCESS and PSH plant to provide the frequency response. The CFC scheme employs a dynamic droop characteristic in parallel with an integral controller and a distribution function. The dynamic droop characteristic determines the power production for frequency regulation employing a variable gain, which varies with the total capacity of the frequency control units and magnitude of the system frequency error with time; the gain increases with the frequency error, and thereby arresting the frequency nadir at a higher level than in conventional droop characteristic. In addition, the distribution function dispatches the power from the dynamic droop and integral controller to the control units in proportion to their headrooms; furthermore, the distribution function considers a sudden loss of generation from a SCESS by its operational constraint during the frequency support. Thus, the proposed dynamic frequency support scheme can enhance the short-term frequency stability for a frequency event.

Growing trends in the deployment of inverter‐based renewable energy will decrease the inertia and frequency control capability of electric power systems by replacing conventional power plants; thus, the frequency of future power systems might be dynamic. This study proposes a capability‐coordinated frequency control (CCFC) scheme of a virtual power plant (VPP) including adjustable‐speed pumped storage hydropower (AS‐PSH), a wind power plant (WPP), and an energy storage system to support the frequency nadir and reduce the steady‐state error of system frequency. The CCFC scheme is based on a hierarchical‐control structure in which a CCFC organises the output of local frequency control units. To support the frequency nadir, the CCFC dispatches weighted frequency errors that are proportional to the available headroom of the units; thus, the errors are forwarded separately with a system frequency error to the primary control loop of each unit and thereby arrest the frequency nadir at a higher value than a system without the CCFC. To reduce the steady‐state error of the system frequency, the CCFC determines a partial active power command by additionally feeding an integrator of the CCFC with a modified frequency error that depends on the unit with the largest control.

This paper proposes a flexible control scheme (FCS) of a photovoltaic (PV) power plant to support the voltage stability and provide fast recovery after the grid faults in a power system with high penetration renewable energy sources (RES). To achieve these goals, the proposed FCS includes two control loops: a reactive current (IQ) injection loop and a frequency-damping control (FDC) loop. The IQ injection loop injects IQ in proportion to the terminal voltage of the PV during a fault. After the fault clearance, the FDC loop provides active and/or reactive power modulation to stabilize system frequency; then, the FDC loop puts the system recovery forward by damping out the remaining synchronous machines' frequency oscillations. The FDC loop comprises active power damping function without PV power curtailment and reactive power damping function. We verified the proposed FDC's performance in a two-area system for various penetration levels of inverter-based resources (IBRs) using PSCAD simulator. We investigated the control schemes' effectiveness by comparing the efficiency and the response in mitigating the post-fault oscillations and restoring the voltage regulation.