This paper studies the dynamic behaviors of weak-grid-tied VSCs with simplified transfer functions, which provides an accurate stability analysis and useful indications for tuning system parameters. A reduced-order multi-input multi-output (MIMO) transfer function that contains four single-input single-output (SISO) transfer functions for the weak-grid-tied VSC is first presented. It is found that the four SISO transfer functions share the same equivalent open-loop transfer function, i.e., the same stability conclusion. The Bode plots of the equivalent open-loop transfer function show that the inner current loop behaves as a band-pass filter whose maximum gain is approximately at the frequency of the PLL's bandwidth. By stability criterion, the harmonic amplification and instability occur when its maximum gain exceeds 0dB caused by high PLL's bandwidth, large grid impedance or high active power. It is also found that the target system is less stable when it works as an inverter than as a rectifier, due to the risk of the local positive feedback in the inverter mode. An effective criterion is further proposed to guide the selection of a proper PLL's bandwidth to ensure the stability of the VSC system. Simulation results validate the correctness of the analysis and the efficacy of the criterion.

This letter analyzes the mechanism of the AC bus overvoltage at the LCC-HVDC rectifier station caused by the sending-end AC faults. Firstly, the influence of commutation failure on relevant electrical quantities is analyzed. After the commutation failure disappears, the inverter switches to constant current control. Under the joint action of the constant current control of the rectifier, the DC current and the trigger angle of the rectifier decrease rapidly, which reduces the reactive power absorbed by the rectifier. With the recovery of HVDC transmission capacity, the sending end AC bus voltage sharply rises, which increases the reactive power output of the filter. Reactive power redundancy occurs in the sending end AC bus leading to overvoltage. The suppression method proposed in this letter can effectively increase the reactive power absorbed by the LCC-HVDC rectifier during fault recovery. Besides, the method can also significantly enhance the minimum value of the inverter extinction angle and avoid possible commutation failure. The effectiveness and applicability of the strategy are confirmed through Electromagnetic Transient simulation.

Modern bulk power systems are developing into AC-DC hybrid systems with high penetration of renewables. There has been a lack of representative benchmarks to study stability and security problems of modern power systems. This paper presents an attempt to provide a set of four test models, addressing problems of voltage stability, rotor angle stability, frequency stability, and power frequency overvoltage under large disturbance. On the four stability/security problems, the models consider both their conventional features and distinct features induced by renewables. All the data of the models are extracted from real power systems, with modifications to establish a near-future scenario with over 50% renewable penetration. The test models are proposed by China Electric Power Research Institute, and acknowledged by Chinese Society for Electrical Engineering (CSEE). They are expected to serve educators, researchers and practitioners in related areas.

This paper proposes advanced control strategies to enhance fault ride-through capability of modular multilevel converter-based high voltage direct current (MMC-HVDC) connected doubly-fed induction generator (DFIG)-based wind farms. A stepwise voltage drop control is proposed for the sending-end MMC to suppress dc line voltage fluctuations by demagnetizing DFIGs. Meanwhile, a modified transient current control is proposed for the DFIG, where a voltage-dependent active current control is integrated into the rotor-side converter to improve synchronization stability of the converter, and a feedforward transient stator current control structure is designed to eliminate dc component in the stator current. The proposed control strategies are validated by comparing with three typical methods through time-domain simulations.

Based on the Monte Carlo method, this paper simulates, predicts the load, and considers the travel chain of electric vehicles and different charging methods to establish a predictive model. Based on the results of electric vehicle simulation prediction, an optimal scheduling model of the distribution network considering the demand response side load is established. The firefly optimization algorithm is used to solve the optimal scheduling problem. The results show that the prediction model proposed in this paper has a certain reference value for the prediction of an electric vehicle load. The electric vehicle is placed in the optimal scheduling resource of the distribution network, which increases the dimension of the scheduling resources of the network and improves the economics of the distribution network operation.

This paper investigates the synchronization stability of voltage source converter (VSC) connected to infinite bus system with frequency limiter of phase-locked loop (PLL). First, three different types of PLL frequency limiters are modelled based on piecewise-smooth dynamical system (PSDS). Then the effects of different frequency limiters and frequency limit values on the region of attraction (ROA) of the system are studied and the properties of the PSDSs are analyzed. It is found that PLL with anti-windup limiter with back calculation can achieve global stability with appropriate limit value, and the principle of setting limit value based on a specific ROA estimation is proposed. Based on ROA estimation with Lyapunov function, a method of setting frequency limit value which guarantees global stability is proposed. The validity of the proposed strategy is verified through MATLAB/Simulink simulation.