•Commutation Methods for SCR-Based Square-Wave Link Inverter

Abstract

A silicon controlled rectifier (SCR) based square-wave link inverter (SqLI) is an interesting and useful topology for applications like photovoltaic and battery energy systems because it can provide dc-to-ac power conversion with galvanic isolation but without requiring a line-frequency transformer and eliminates the need for a dc bus, hence eliminate the need for failure-prone aluminum electrolytic capacitors. Furthermore, SCRs are robust, reliable and cost-effective semiconductor switch devices that needs only a simple gate-pulse to turn on. The SCR device is a rugged device, so the SCR device is well suited for the connection to the electrical power systems, where a metal oxide semiconductor field effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT) are susceptible to failure from indirect lightning strikes and electrically fast transients. However, the control of the SCR-based square-wave link converter was previously thought to be difficult due to relatively large number of switches which must be carefully coordinated since there is no dc-link capacitor to buffer the switch action of the output converter from the switch action at the input converter. If an incorrect switching sequence is attempted, or there is error in the timing of the attempted switching, the resulting commutation failure in the converter could lead to distortion of the waveforms or even catastrophic failure of the converter.

In my Ph.D research, I investigated and developed commutation methods of four different SCR-based square-wave link inverters: a single-phase voltage-sourced inverter, a three-phase voltage-sourced inverter, a single-phase current-sourced inverter, and a three-phase current-sourced inverter. The commutation methods can be applied when these four inverters are connected to either a passive load or the ac electrical power grid. The thorough analysis of the operations of these inverters and the details of the new commutation methods that I have developed in my research are provided in this dissertation. The new commutation methods are demonstrated and verified through simulations using Matlab/Simulink.