Test-bench for very high power variable frequency drives working under constrained grid conditions

This paper introduces a test-bench concept for high power variable frequency drives that can work reliably also with a weak and power limited grid. Experimental results are shown for a 10MW+ drive test bench with only a remote 1.25MVA feeding transformer supply. Common “pump-back” configurations circulate the test bench power via the supply grid. This produces huge distortions and potential instabilities with weak grids. Hence, we circulate the power via a virtual 50 or 60 Hz grid that is not connected to the feeding grid. Only the losses are fed from the grid directly to the dc-link of one test-bench VFD via a simple diode rectifier. Since the total system losses are typically only about 10% of the rated powers, the corresponding grid disturbances are significantly reduced. The implemented test-bench is utilizing a virtual machine but the concept can also be combined with a real motor-generator set.