No abstract

This study presents a novel phase current reconstruction strategy for switched reluctance machines (SRMs) using two cross-winding current sensors. The phase currents are reconstructed by solving the linear equations associated with two adjacent phase currents in the different turn-on regions. The effect of current sensor offset and power transistor fault on the proposed reconstruction method is analysed. On the basis of the current difference at the rising edge of each drive signal, an offset sensor identification method is presented and two online compensation schemes are adopted. For power transistor short-circuit fault, the logic-judgment-based and freewheeling-time-based diagnostic methods are investigated and a virtual current sensor is introduced to ensure the effectiveness of the reconstruction process. The proposed phase current reconstruction strategy is free from power transistor open-circuit fault. In addition, the current reconstruction method is easily extended to SRMs with higher number of phases without additional current sensors. Simulations and experiments validate the effectiveness and flexibility of the proposed reconstruction strategy.

Winding insulation degradation may lead to phase‐to‐phase fault in switched reluctance machines (SRMs). First, the fault modes are described considering the connected sequence of the coils and the fault behaviours are analysed in details. Second, the equivalent circuit of three‐phase windings is represented and the faulty SRM is modelled based on the derived electromechanical equations. Third, the locations of three current sensors are optimised to calculate the incoming line current and outgoing line current of each phase without utilisation of any extra hardware. The phase‐to‐phase fault can be detected and located by monitoring the residuals between the two line currents during motor operation. Forth, to improve the post‐fault operation performance of the motor, a triple closed‐loop control scheme aiming at suppressing the short‐circuit current is conducted by modifying the switching states of power transistors. Finally, simulations and comparative experiments on a three‐phase 12/8 structure SRM validate the effectiveness of the proposed methods.

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.INDEX TERMS HVDC transmission, low voltage ride-through, modular multilevel converters, offshore wind farm.

No abstract