Study of full load test method for large VSDS driven by non-regenerative VSI

Full load back-to-back test of VSDS's (Variable Speed Drive Systems) are sometimes required for oil and gas projects. VSI (Voltage Source Inverter) technology is becoming the drive system of choice for super large compressor drives, up to 100 MW. Special consideration /facilities are necessary to perform back-to- back tests for VSDS driven by VSI, because the type of converter used for such large VSI's is usually a diode rectifier without regenerative capability. To realize a back-to-back test for this type of VSDS, a VSI with regenerative converter, (usually adopting PWM converter topology), is necessary. But it is a heavy burden to prepare such super large VSI with regeneration capability purely as a test facility. This paper proposes a test method to increase the load test capacity of a back-to-back test facility up to two times the regenerative converter capacity. This idea is to apply a VSI with regenerative converter as a soft starter during start-up of back-to-back connected motor and generator system and as a constant voltage and constant frequency power supply during testing of VSDS. The proposed test facility is configured by direct connection of the load machine output to the input transformer of tested VSDS to circulate the load power directly. Only loss power and reactive power is supplied by VSI with regenerative converter.