Background
A wastewater treatment plant was established on northern Taiwan. After around several months initial running, 14 static frequency converters （SFCs） which drive 460V induction motors （range from 7.5HP-15HP）were destroyed at the same time. It costs about NTD 5 million. No lightning surges happened at that time, but having switching operation at high voltage side. From the recording of other SFCs’, overvoltage data had been recorded, and there are large amount of power electronic devices installed in this plant, therefore it was identified that such failures were caused by harmonics or switching surge.
Investigation and Finding
The wastewater treatment plant and the power utility met and aimed at finding the best solution to reduce the impact. The first thought was measurement of harmonics and voltage surges.
The SFCs are voltage source inverter, and have neural grounding port at DC side. The grounding ports are connected with power system grounding before the failure.
The measurement results show that total voltage harmonic distortions were less than 5%, so harmonics is not the cause.
Then a POWER QUALITY ANALYZER （sample rate at 1.8MHz）was applied to measure the voltage surge at source side of SFC, the largest voltage surge was measured about 2.1p.u., those voltage surge will be suppressed by the metal oxide varistor（MOV）, which was installed in source incoming side of SFC. From the destroying situation of SFC, we found some diodes were reversing breakdown and some control parts burned, indications of large current through SFC. Because the MOVs are in normal condition, voltage surge can’t from source side, and it is from SFC’s ground point（located between two 10 nF capacitors）. Seeing that high switching voltage surge can go through cable sheath to SFC grounding point and caused the failure, we proposed to remove SFC’s grounding lead.
