To address the system operability challenges due to the continuous reduction of system inertia by increasing penetration of renewable power generation, this paper proposes a new synthetic inertia control (SIC) strategy for doubly-fed induction generator (DFIG) based wind turbine generators (WTG). The proposed SIC scheme enables individual DFIG generators to contribute an effective inertial response which helps to stabilise the rate of change of frequency (RoCoF) and minimise the maximum frequency deviations in particular under severe disturbance conditions. To achieve the proposed SIC, Lithium-ion supercapacitors are required in the DC link of the DFIG power electronic conversion system to increase extra energy storage capability. The DFIG's DC voltage operates at various levels following the derived SIC algorithms for energy exchange with the connected AC grid, and the conventional control system for DFIG is modified to implement the algorithms. The functions and system benefits of SIC scheme is verified using a DFIG model connected to a power system and compared with the simulations of conventional DFIG control scheme. Practical issues such as size, cost, weight, connection configuration and charging/discharging rate of lithium-ion capacitors for the SIC are discussed and relevant recommendations are presented.

This paper proposes a novel variable-inertia emulation control (VIEC) scheme that enables voltage-source-converter based high-voltage DC (VSC-HVDC) transmission systems to flexibly support AC grid frequency stability like synchronous generators. The VIEC scheme allows us to extract the energy from the augmented DC capacitance for inertia emulation by controlling the DC voltage without affecting the stability of the AC system connected on the remote side. In particular, with the proposed VIEC scheme, the inertia time constant can be flexibly emulated and its value can be automatically adjusted according to the rate of change of grid frequency as well as grid frequency deviations. This leads to more effective inertial support across different timescales. Modal analysis is carried out to investigate the impacts of inertia and DC capacitance, and to obtain the optimal control parameters. The effectiveness and advantages of the proposed VIEC scheme are demonstrated on an IEEE benchmark system in the presence of faults and load changes.

This article proposes a novel inertia emulation and fast frequency-droop (IEFF) control strategy for a point-to-point voltage source converter-based high voltage direct current (VSC-HVdc) transmission system. It allows asynchronous ac grid synchronization in forms of inertial and fast frequency-droop responses provided to “both” sides of the VSC-HVdc interconnected grids, different from the historically reported schemes only providing these responses to one grid. Under the IEFF scheme, the power-regulating VSC (PR-VSC) station emulates a synchronous generator directly through its active power control, whereas the dc-voltage-regulating VSC (DR-VSC) station employs a dc voltage/frequency droop control to convey its connected grid frequency communication-free to the remote PR-VSC for inertial and fast frequency-droop oriented power variations. To avoid the clash of the PR-VSC IEFF and DR-VSC IEFF control under simultaneous frequency events in the asynchronous grids, a novel frequency support mode selection scheme is designed to effectively prioritize the IEFF support with a higher frequency gradient. The key parameters of the IEFF controller are optimized through small signal stability analysis. The effectiveness of the proposed IEFF scheme is verified through controller hardware-in-the-loop experiments. The results show that the proposed IEFF scheme effectively promotes synchronization and enhances overall frequency stability for the two interconnected asynchronous grids.

This paper investigates the stability of diode rectifier based HVDC (DR-HVDC) connected offshore wind farms. The impedance model of the DR-HVDC connected offshore wind farms in dq frame is analytically developed and is validated by comparing its impedances to those obtained from frequency scanning of time domain EMT model in PSCAD/EMTDC. Based on the impedance model, the impacts of the sizes of DR-HVDC DC smoothing reactors and AC filters on offshore system stability are analyzed. It shows reduced DC smoothing reactance degrades the d-axis stability margin and can potentially lead to system instability. Higher bandwidth of offshore AC voltage control of wind turbine line side converter is proposed to minimize such instability potential. The analysis also shows that reduced AC filters size helps to increase q-axis stability margin and leads to a more stable system. Moreover, stability analysis considering the interaction between WTs is conducted and it reveals that the WT P-V control has a significant impact on the stability of the DR-HVDC connected WTs.

The integration of full converter-based wind power generation (FCWG, e.g. permanent magnet synchronous generator (PMSG)) not only introduces the PMSG oscillation modes (POMs) but also might excite severe resonances with electromechanical oscillation modes (EOMs) of the power system. In this paper, a two-open-loop-subsystem dynamic model is firstly established to investigate the interactions between the PMSG and the rest of the power system. On this basis, a modal shift evaluation (MSE) method by using bilateral damping torque analysis is proposed to accurately quantify the interaction effect of POMs and EOMs on each other and effectively explain their complex interaction process. Then two important concepts, i.e., modal shift sensitivity (MSS) with respect to various PMSG controller parameters and resonance excitation index (REI) according to a per unit open-loop modal distance indicating the intensity of modal interactions, are derived to dig the essential modal resonance mechanisms. Furthermore, by using MSS and REI as two tools, the modal interaction optimization (MIO) is conducted through POM tuning in order to prevent potential system modal resonance and enhance resonance mode damping for the first time. The optimized modal interaction is validated to be beneficial and effective for the improvement of power system resonance stability.

This paper mainly focuses on transient stability issue of phase-locked-loop synchronization (PLL-Syn) in a weak-grid connected VSC (WG-VSC) system. First, the linear matrix inequality (LMI) optimization method has been applied to construct the largest estimated domain of attraction (LEDA) of the WG-VSC system for the first time. Compared with the frequently-used methods such as direct energy function method, Takagi-Sugeno (TS) method and Sum-of-Square Programming (SOS) method, the proposed method has the lowest conservation in constructing LEDA. Second, to ensure transient stability of PLL-Syn in severe faults, the concept of the feasible region of active/reactive current reference is proposed. Furthermore, based on the LEDA obtained by LMI optimization method, a simple algorithm to obtain this feasible region is presented. And, the influence of typical parameters on this feasible region is analyzed. Third, for transient instability scenarios, a simple variable step size iterative algorithm has been proposed to search the estimated critical action time with the derived LEDA. Moreover, two associated emergency control methods including adjusting the current reference values back to the feasible region and clearing grid fault have been considered to verify the estimated critical action time. Time-domain simulation results based on detailed switching model in PSCAD/EMTDC have been provided to verify the above theoretical analysis.