The valve-side single-phase-to-ground (SPG) faults in half-bridge modular multilevel converter (HB-MMC) based bipolar high-voltage direct-current (HVDC) systems induce some special fault consequences: overvoltage on the submodule (SM) capacitors in converter upper arms and non-zero-crossing currents in the grid-side AC circuit breaker (ACCB). In this letter, a protection strategy based on converter embedded double-thyristors is proposed to address these issues. The double-thyristors are installed in parallel with each SM in the lower arms of the converter. By triggering the double-thyristors, the SPG fault is converted into a three-phase short-circuit which ensures current zero-crossings in the grid-side ACCB. Moreover, as the voltages of converter ac buses are clamped by the ground through the triggered double-thyristors, the upper arm overvoltage is mitigated. The effectiveness of the proposed protection strategy has been verified in a bipolar HB-MMC HVDC link built in PSCAD/EMTDC.

Voltage source inverters (VSIs) with vector control based on phase-locked loop (PLL) suffer instability when connecting to a very weak AC grid (short circuit ratio (SCR) $ < $ 1.3). The conventional inductive grid impedance compensation for the PLL by virtually reducing the grid impedance can stabilize this connection. However, the analysis in this paper indicates that its stabilization effectiveness is sensitive to grid impedance variance and, indeed, overcompensation causes the PLL instability. Therefore, in this paper, an improved grid impedance compensation for the PLL is proposed to achieve the same stabilization for very-weak-grid connection and possess a good tolerance of grid impedance variance and overcompensation. A comprehensive small-signal model of the VSI using the proposed PLL’s grid impedance compensation is derived for stability analysis and parameter design. The time-domain simulation for this VSI is built to validate the stability analysis. Comparison studies for both proposed and conventional PLL’s grid impedance compensation are conducted including the stability effectiveness, VSI performance and grid impedance variance.

Medium-voltage direct-current (MVDC) technology exhibits advantages over ac transmission due to its enhanced capacity, controllability, and flexibility. A practical example is the ANGLE-DC project from Scottish Power Energy Networks, which converts existing 33 kV ac circuits into dc operation to improve the system transmission capacity in North Wales, U.K. Specially designed cascaded power converters are employed to build up the ±27 kV dc link voltage. Within the converter stations, converter-side single-phase ac faults may threaten the safe operation of the converters. This paper presents an analysis of such faults for an MVDC link based on ANGLE-DC. It is shown that the capacitors of the converters will be overcharged without suitable protection in place. A protection strategy using a bypass thyristor-based branch is presented to relieve overvoltage issues following converter-side faults, and it is verified through simulations conducted in PSCAD/EMTDC. For completeness, case studies considering post-fault operation are also included.

The comprehensive resonance analysis and soft-switching design of the isolated boost converter with coupled inductors are investigated in this paper. Due to the resonance participated by the voltage doubler capacitor, clamping capacitor, and leakage inductance of coupled inductors, the reverse-recovery problem of the secondary diodes is restrained within the whole operation range. By choosing appropriate magnetic inductance of the coupled inductors, zero-voltage switching on of the main mosfets is obtained collectively at the same working conditions without any additional devices. Moreover, the range of duty ratio is enlarged to achieve soft switching and an optimal operation point is obtained with minimal input current ripple, when duty ratio approaches 0.5. Additionally, two kinds of resonances are analyzed and an optimized resonance is utilized to achieve better power density. The prototype is implemented for the vehicle inverter requiring a 150 W output power, input voltage range varying from 10.8 to 16 V, and 360 V output voltage. Experiment results verify the design and show that the minimum efficiency is about 93.55% and 90.53% at low load and full load, respectively.

The protection and current flow regulation of high-voltage direct-current (HVDC) grids requires the deployment of additional semiconductor-based equipment including dc circuit breakers (DCCBs) and current flow controllers (CFCs). However, the inclusion of multiple devices could significantly increase the total cost of an HVDC system. To potentially reduce costs, this paper presents an innovative multi-function integrated DCCB (MF-ICB). The proposed device exhibits a reduced number of semiconductor switches and can fully block dc faults at different locations while regulating dc currents. The configuration of the integrated solution and its operating principle are assessed, with its performance being examined in PSCAD/EMTDC using a three-terminal HVDC grid. Simulation results demonstrate the capability and effectiveness of the MF-ICB to regulate grid current and isolate dc faults.