A petrochemical company started getting a KVAR fault on their emergency CAT diesel generator set driving a critical pump system with a Danfoss FC302 VFD. The user generally runs off of plant power, but the system is designed to switch automatically to an emergency generator, but the unit keeps tripping during testing.

The solution

We strongly believe the issues are related to the harmonics created by the VFD’s and a possible common mode current through the ground back to the generator.  We recommended a harmonic mitigation filter in line with the incoming line power to the VFD and a second motor filter on the output of the VFD.  We confirmed with CAT and MTE and they agreed with us on the solution.

Line side:
MTE Harmonic Filter: MAPW0066D002

Load side:
MTE DCS DV sentry motor Filter DVS W0080E and an ABB contactor to pull the capacitors above 25% load.

Cap Hookup


These kinds of applications are always tricky as many times, the assessment points to not always needing a solution. We have customers using a similar set-up that have no issues at all. Most of the time it is about sizing the generator to the VFD.  In our humble opinion, good engineering practice requires that reactors and filters are used in all generator/VFD combinations. Critical means reliable.

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