Search

This paper presents an improved H8 power converter for common-mode voltage (CMV) reduction and electromagnetic interference suppression. The proposed H8 converter can realize zero CMV variation when entering and leaving the zero state, which is mainly achieved by both the improvement of the structure and the control strategy. For the system structure, two switches are added on the dc bus to float the ac part of the three-phase converter in the zero state. Besides, additional capacitors are used to realize controllable CMV. For the control strategy, a simple control strategy is proposed for the H8 converter. It can automatically adapt to the flowing orientation of the load current to realize synchronous switching of the power switches, which effectively eliminates the impact of the dead time. Through analysis, simulations, and experiment, a comparison between the proposed H8 converter and the conventional H6 converter is performed. Results validate the effectiveness of the H8 topology.

In this paper, the influence of transformer winding deformation on the equivalent electrical parameters is studied, and the proximity effect and skin effect caused by AC current in the wire are simulated. It is verified that the current distribution will be affected by the proximity effect and skin effect. This effect will lead to the change of the AC resistance of split conductor. An equivalent area method for the quantitative calculation of the high frequency resistance of windings is presented. It contributes to the analysis and diagnosis of transformer winding faults, especially the diagnosis of transformer winding deformation.

In an islanded inverter-interfaced microgrid (IIM), fault current is generally controlled at the same level of the normal operation current due to the limited capacities of inverters, rendering the conventional overcurrent protection methods impractical. To solve this issue, this paper presents a novel protection strategy, which incorporates flexible control of the inverter and setting of microprocessor-based relay (MBR). Once a short-circuit fault occurs at distribution line, a current limiting strategy combining with hysteresis control and maximum current root-mean square (RMS) control (MCRC) is applied for the inverter to limit the fault current within permissible range. In the MCRC mode, a percentage of fifth harmonic is injected to the fault current, facilitating the MBRs to identify the fault according to the harmonics components. Additionally, the fault current versus fault impedance ( $I_{{\rm{fault}}}- Z_{{\rm{fault}}}$ ) droop control is synthesized to make the inverters output larger fault currents when the faults are closer to the inverters, and hence, the MBRs can be coordinated with the inverse-time principle. The main advantages of this method are that it neither relies on high magnitude of the fault current to identify the fault nor needs communications among protective devices to ensure the coordination of protections. Experiments have been conducted to demonstrate the feasibility of this method.