Xiaohe Wang

T The lightweight design of key equipment is crucial to achieving cost-effective offshore wind power transmission based on modular multilevel converter based high voltage direct current (MMC-HVDC). This study investigates two innovative lightweight arm reactor schemes for offshore applications: a transformer-integrated arm reactor (TIAR) scheme and a distributed arm reactor (DAR) scheme, with complete parameter design methodologies for both approaches. In the TIAR scheme, a three-winding transformer replaces the conventional dual-winding transformer, thereby simplifying the MMC structure. In the DAR approach, the centralized arm reactor is eliminated, and the arm inductance is distributed across individual SMs, which helps reduce the layout area traditionally occupied by the arm reactor room. Through detailed PSCAD/EMTDC simulations of a pseudo-bipolar MMC-HVDC system integrating offshore wind farms, this study compares its steady-state and transient performance with varies designs. Key findings demonstrate: (i) Under DC bipolar short-circuit faults, both proposed schemes limit the maximum arm current to below 1.2 p.u. without significant SM capacitor voltage collapse; (ii) During converter blocking, the transformer valve-side voltage in lightweight designs exhibits lower magnitudes compared to conventional arm reactor schemes.

In this paper, a solid-state circuit breaker (SSCB) suitable for high-voltage DC systems is presented. A single gate driver is used to drive multiple series-connected power semiconductor devices, leading to a compact, reliable, and cost-effective design. An accelerating capacitor in parallel with a diode is used to charge the gate capacitors of the upper devices without an additional gate driving circuit. Simultaneous turn-on and turn-off are realized similarly to the structure with individual gate drivers. It can achieve good static and dynamic voltage balancing with the help of metal oxide varistors (MOV). Compared to the conventional single gate driver structures, negative gate voltages for all the devices are guaranteed during the entire OFF state simply by using two transient-voltage-suppression (TVS) diodes. Thus, the notorious gate voltage oscillation and the severe voltage unbalance problems during the voltage recovery process are eliminated. An 800 V/100 A SSCB simulation and experimental prototype based on two series-connected IGBTs are built to verify the effectiveness of the proposed structure.

The parallel interleaved three-phase inverters are suitable for high-power applications due to the current ripple canceling effect. The power density and efficiency can be further improved with the current ripple prediction (CRP) based high frequency zero-voltage switching (ZVS). However, the variable switching frequency increases rapidly as the power decrease, resulting in higher turn-off loss at light load despite the elimination of turn-on loss. In this paper, a phase-shedding control strategy is proposed along with the CRP based ZVS method to improve the light load efficiency. Only four phase-legs of the two parallel inverters operate at light load to reduce the switching frequency and the circulating current between the two clamping phase-legs. The proposed method can achieve full-range ZVS for all the switches without any auxiliary circuits or high frequency sensors. Current sharing can also be realized between the two clamping phase-legs based on accurate gate signal modulation. A 5 kW simulation and experimental prototype using SiC devices interfacing 400 V dc with three-phase 110 V ac grid is developed to verify the effectiveness of the proposed control strategy.

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission is becoming a trend in offshore wind-farm integration. However, the large DC-side energy dissipation equipment, which is utilized to dissipate surplus wind power under grid fault conditions, will largely increase the cost of MMC-HVDC systems. To reduce the cost, a novel low voltage ride-through (LVRT) strategy is proposed in this paper. When a grid fault occurs and the DC voltage exceeds the limit, the sending end converter is controlled to reduce the AC voltage of the wind farm. The LVRT of the wind generators will be activated, and the output active power of the wind farm is reduced. With this coordination, the DC-side energy dissipation equipment only needs to dissipate surplus power in the early stage of the grid fault before the output active power of the wind farm drops. Therefore, the heat generated by braking resistors can be significantly reduced. On this basis, the braking resistors can be distributed into the submodules of the receiving end converter (REC) station. The LVRT problem can be solved without building an individual energy dissipation station. Using the proposed coordination strategy, the construction cost of the MMC-HVDC system with offshore wind farm integration can be significantly reduced.

The investigation of stochastic optimal power flow (SOPF) is to seek the optimal solution of static stability constrained optimal power flow considering the uncertainty of parameters in power systems. To solve the problem, this paper proposes an approach based on the stochastic collocation method (SCM) and Gram-Charlier expansion to obtain the optimal solution of SOPF. Firstly, the SOPF model to simultaneously consider uncertainties and static stability is formulated. Then, probabilistic chance constraints are reformulated as a set of deterministic constraints using polynomial approximation, which explicitly describe the relationship between the probability of chance constraints and control variables. By applying the primal-dual interior point method to the reformulated SOPF model, the optimal solution can be efficiently obtained. The proposed SCM-based approach has been thoroughly tested on a modified 3-machine 9-bus system and IEEE 118-bus system, which verifies the effectiveness and accuracy of the proposed method.

In the offshore wind farm high-voltage direct-current (HVDC) system, the power delivery capability of the onshore modular multilevel converter (MMC) decreases severely under grid fault, which makes the DC-bus voltage increase rapidly and threatens the safe operation of the system. This paper proposes a low-voltage ride through (LVRT) strategy for MMC with $\text{Y}_{0}/\text{Y}_{0}$ arrangement transformer under single-line-to-ground (SLG) fault. The influence of different transformer arrangements to the MMC under SLG fault is analyzed. On this basis, a power delivery capability enhanced method is proposed for MMC with $\text{Y}_{0}/\text{Y}_{0}$ arrangement transformer to take advantage of its control ability on zero sequence current. In addition, an optimized LVRT strategy based on resonant controller is proposed, which has simple control structure and can ride through the SLG fault without DC chopper. The offshore wind farm MMC-HVDC simulation system is established in PSCAD/EMTDC and simulation studies are conducted to validate the effectiveness of the proposed LVRT strategy.

As the popularity rate of distributed power sources continues to increase, the operational stability of the power grid is severely challenged. The virtual synchronous generator (VSG) technology has been extensively studied because it enables the inverter to imitate the operation mechanism of synchronous generators, which realizes the friendly connection of the distributed generation, and increases the stability of the power grid. However, the traditional VSG control strategy does not have the capability of low voltage ride through (LVRT). The current amplitude is likely to exceed the limit and it is difficult to provide controllable reactive power when the grid voltage drops. Therefore, this paper proposes an improved LVRT control strategy for VSG. The active and reactive power control loops of the VSG are improved and a modified current control loop is introduced to suppress transient current surge and provide reactive power support under symmetrical drop of grid voltage. In addition, the control strategy can realize flexible distribution of active and reactive power to meet the grid requirement during LVRT. Finally, the validity of the proposed strategy is verified by the simulation results.