To reduce the frequency deviation and the rate of change of frequency (RoCoF) in a low inertia power system, some converters are required to provide the frequency response (FR) power normally associated with the frequency deviation and/or the RoCoF, by droop/inertia/PD control. In this paper, a rapid power compensation (RPC) based FR strategy is developed to optimize the ability to compensate grid imbalance power, by fully exploiting the converter idle capacity. To this end, first, mathematical proof demonstrated the improved performance of the RPC strategy in terms of frequency deviation suppression versus droop control, and in terms of RoCoF suppression versus inertia control, with identical converter capacity limit. Moreover, it is proved that the RPC strategy can achieve consistent FR performance w.r.t. the optimal PD control, i.e. it can maximize the suppression of frequency deviation and RoCoF simultaneously, yet avoiding the limitations due to unknown grid parameters. Finally, by analyzing the operation modes and identifying the pertinent switching logic, detailed implementation of the proposed RPC strategy is developed. Its superb FR performance is verified by experiment results in a 2-converter low-inertia system, and simulation results in an IEEE 4-machine 2-area system.

Active power filters are conventionally utilized to compensate steady-state harmonic currents and reactive power in the utility, yet their capabilities are usually limited if the elimination of undesirable effects associated with decaying DC components during transients is the target, especially under weak grid condition. In this paper, active cancellation of the typical first-order decaying DC-mode transients is explored. To this end, a real-time detection algorithm of decaying DC-component is firstly developed for a generic single-phase distorted AC current signal. Furthermore, fast and robust elimination of both decaying DC-transient and harmonics can be performed simultaneously in 3-phase grids, by adoption of moving average filters in $d-q$ frame and subsequent calculations. Hardware-in-the-loop experiment results verified the effectiveness of the proposed technique.

It is of great significance for voltage source converter (VSC) based multi-terminal DC (MTDC) grid to isolate the DC fault lines within several milliseconds after DC fault. Existing protection schemes and fault analysis methods are mainly based on the numerical simulations, which are lack of the theoretical analysis. In this paper, a high-frequency equivalent model of VSC based MTDC grid that is utilize for initial DC fault current calculation is firstly proposed. In the proposed model, the parallel connected capacitors of VSCs are regarded as short-circuited, and the initial fault current calculation of the fault DC line and the healthy DC line can be reduced as a simplified RL and RLC circuit. Accordingly, a novel DC fault detection method for VSC based MTDC system is further proposed. In the proposed approach, the primary detection utilizes the transient average value of line current and the fault DC line can be identified quickly with only one-end information. In addition, the errors between the transient fault current calculation based on the distributed parameters line model and the lumped parameters line model are also evaluated. Numerous simulation studies carried out in PSCAD/EMTDC have demonstrated that the proposed high-frequency equivalent model can be utilized for initial DC fault analysis of VSC-MTDC system and the proposed protection scheme is effective under different fault locations and high fault resistances. Compared with the traditional protection schemes based on rate of change of current (ROCOC), the proposed one requires relatively low sampling frequency, low computation burden, and has high fault resistance tolerant ability, and high robustness with respect to the data missing.

It is of great importance for Multi-terminal DC (MTDC) system to immediately isolate the DC fault line within several milliseconds after DC fault due to VSC or half-bridge based MMC converters with limited ability to control DC fault current during DC fault. Failing of the DC fault isolation during “Capacitor Discharging Stage” may lead to the overall MTDC system out of operation with the fast blocking of IGBTs. Therefore, initial DC fault current analysis is essential for system protection design, especially for fault detection. Accordingly, a transient simplified model is first proposed with only high-frequency components of the fault DC network reserved. As a result, the original complex DC network is largely reduced and the fault DC current with several milliseconds can be clearly expressed based on the proposed model. According to the distinct differences between the current of faulty cable and that of healthy cable, a non-unit DC fault detection method utilizing the transient average value of line current with low computational complexity and high-sensitivity is further proposed. Numerical simulations based on PSCAD/EMTDC have been carried out, which well-demonstrate the accuracy of the proposed transient simplified model for initial DC fault analysis and the effectiveness of the proposed transient average current based fault detection scheme. Compared with the traditional change of rate of current based protection method, the proposed one stands out for relatively low sampling frequency, high fault resistance tolerant ability, and robustness against outside noise and data missing.

Droop control, inertia control, and PD control are three typical frequency response (FR) strategies of grid-tied inverters interfaced renewable energy sources (RES) to restrain the power system frequency indicators, i.e., frequency deviation ( $\Delta \omega $ ) and the rate of change of frequency (RoCoF). Due to the highly limited inverter capacity, the FR performance in power systems with high RES penetration should be optimized. To this end, this paper first analyzes the impact of high-penetration renewable energy injection on power system frequency stability, and derives the system frequency expressions in the absence of an FR scheme, and in the presence of droop/inertia/PD controls that are representative for various virtual synchronous generator-based FR schemes. Under the same constraint of available FR capacity, control parameter ranges of these FR schemes are obtained, and their optimal conditions for restraining the maxima of $\Delta \omega $ and RoCoF are identified and compared analytically. Comparison conclusions are finally proved by the provided experiment and simulation results.