A variable-frequency electric drive with power supply by two independent inputs

Abstract

Excessive sensitivity to interruptions of power supply is a disadvantage of variable frequency drives: short-term power cutoffs or voltage depressions. Within the period of integration of the frequency changing circuit at essential units of a thermal power stations, this can result in increasing risk of electrical power unit blackout. To compensate for this disadvantage, variable frequency drives with electrical supply from two independent inputs have been developed. A frequency-changing circuit with two rectifiers connected on the direct current side has been taken as the basic one. To prevent uncontrolled switching over between inputs, priority of one input should be provided. In this case, increasing the voltage on the input that has been taken as primary is not an efficient solution. Continuous operation of the electric drive should be provided with automatic switching over the reserve input at the loss of supply voltage on the primary input. We propose an arrangement based on two groups of identical frequency-changing circuits connected in parallel on the direct current side. The arrangement also contains a voltage adjuster equipped with an automatic control system, which must provide steady operation of the electric drive with primary input supply. The power unit of the adjuster is constructed based on the active field-bus module (ALM) realized on fully controllable switch contacts. A frequency drive with a simplified main circuit and with a diode rectifier and a group of the active field-bus module ALM has been developed. To carry out experimental investigations of its modes, an experimental prototype of an electric drive with a fast response STATCOM model voltage (power) switch converter has been developed and manufactured. This converter acts as a voltage adjuster of ALM. Experimental investigations of the electric drive with simulation of blackout on one of the inputs and gradual voltage load transfer between inputs and other modes have been conducted. The oscillograms of the main coordinates have been presented. It is shown that the required quality of the transient phenomena is provided in all modes. Thus, automatic switching of the frequency changing circuit over the reserved input practically has no effect on the operation of the electric drive. Thus, we can draw the conclusion that the technological mode of the basic unit will not be affected. Generally, experimental investigations have confirmed that steady operation is maintained of the variable frequency drive under conditions of voltage changes or switching off one of the inputs.