Yechun Xin

The controllable line-commutated converter (CLCC), which achieves forced commutation and effectively prevents commutation failures, has seen initial engineering applications in high-voltage direct current (HVDC) transmission systems. This paper analyzes fault characteristics and develops a valve control strategy for single-phase-to-ground faults on the valve side of the CLCC converter transformer. First, mathematical models of the DC-side circuit and its control system are established. Next, based on the fault location and changes in current paths during the fault, the transient characteristics of the valve current are analyzed, accounting for the effects of the control system. Furthermore, a relationship describing how faults affect the extrema of the valve current during commutation is derived. Finally, a method for optimizing the valve control strategy is proposed based on predicting the times at which the valve current reaches its extrema. This prediction is obtained using a current transfer time model derived from an equivalent circuit describing current transfer between the main and auxiliary branches of the CLCC. Simulation results show that the proposed method can effectively reduce the current stress imposed on the arrester by fault-induced abnormal commutation, improve the current waveform during commutation, and enhance the operational stability of the system during faults.

To solve the problems of weak inertia support and poor frequency regulation capability of power system, a novel coordinated control strategy for frequency regulation of MMC-HVDC connecting offshore wind farms (OWFs) is proposed to improve system frequency stability. In terms of inertia support, based on analyzing the available energy margin of MMC-HVDC, a virtual inertia control method based on decoupling control of DC voltage and capacitor voltage is proposed, which can fully utilize the energy margin of capacitors and improve its inertia support capability. The coupling relationship between the rotor speed of wind turbines and the frequency is directly established through DC voltage, an inertia support coordinated control strategy for MMC-HVDC and OWFs is designed to realize the rational utilization of available energy. In terms of primary frequency regulation, according to the local DC voltage, a primary frequency regulation control strategy is proposed for OWFs based on the variable speed control and pitch angle control. The overall coordination is realized through the reasonable design of control links and parameters, so as to improve the frequency stability of the system. Finally, the simulation model of MMC-HVDC connecting OWFs is built to verify the validity of the proposed control strategies.

Aiming at the problem of broadband stability of voltage source converter based high voltage direct current (VSC-HVDC) system, a control strategy of converter is designed to improve its small signal stability. Based on the modeling of the flexible HVDC system, the overall structure of the internal and external loop control of the VSC-HVDC converter is established. DC control and AC control damping controller are designed respectively in phase locked loop and PI control. The state space equation of the control system is established to analyze its performance and design its parameters reasonably. The stability of small signal of the system is improved effectively through optimization of several links.

Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission is the prospective technology for large-scale wind farms integration. In this paper, a comprehensive coordinated control strategy for DC fault ride-through (FRT) of wind farms connected to the grid through bipolar MMC-HVDC transmission is proposed. To reduce the requirement of breaking speed and capacity of DC breakers, an active current-limiting control method is proposed to limit the rise rate of DC fault current, and the principle of parameter selection is designed. After DC breakers isolate fault, the control mode switching methods of bipolar MMC are designed to make the fault pole MMC operate as a STATCOM to support the AC grid and improve the active power transmission capability of the healthy pole MMC, which can reduce the impact range of unbalanced power between wind farms and converter station. In addition, a coordinated control strategy for wind farms load-shedding considering speed constraints is designed to realize the optimal distribution of load-shedding power among wind turbines, which can regulate unbalanced power together with the healthy pole MMC to realize DC FRT. Finally, the effectiveness and feasibility of the proposed control methods are verified by simulation results.

A static var compensator (Static Synchronous Compensator, STATCOM) is compensated reactive power and three phase unbalance load for the distribution grid. According to analyzing the positive sequence equivalent circuit and negative sequence equivalent circuit under the three phase unbalance load of distribution grid, using the symmetrical component method and superposition principle, a new cascade loop control strategy of positive sequence current and negative sequence current is proposed. Using δ-θ control, the positive sequence loop is compensating reactive power of distribution grid, and maintains stability of STATCOM DC voltage. Using ε-θ control, the negative sequence loop is used to compensating three phase unbalance load of distribution grid. Simulation results are presented to verify the validity of the proposed control strategy.