VFD input current measurement

[Jordan82]

Hi everyone, this is my first post here. I am very impressed with the forum.

We are in the process of installing a 5KV medium voltage VFD for a cooling water pump at an industrial site (800hp). We have the unique situation of having to use the existing motor starter relay (Multilin 269) to feed the VFD. After connecting the VFD, we noticed that the current reading on the Multilin is not showing up. We do have complete voltage, current, and power readings on the VFD keypad.

The motor starter relay is equipped with 3 phase CT's with 5A. secondaries, and a zero-sequence ground fault CT. I am concerned that the Multilin is unable to process the current signal because of the distorted waveform caused by the VFD. That also makes me doubt the integrity of the ground-fault measurement, which is being used to trip the contactor if there should be a fault between the starter and VFD.

Any thoughts? I hope this post was somewhat clear. Thanks in advance.

 

[jraef]

The old original 269's mP is not sophisticated enough and/or the sampling rate is probably too slow so it likely gets stuck trying to interpret the distorted waveform. 269's were developed long before VFDs were so prevalent. You may need to upgrade to the 269 Plus version, it has filtering for use in VFD systems. But if you are going to change, I would go for the 369 or even the 469 relay. The 369 is going to cost you roughly the same and the 469, although a lot more expensive, is a superior relay, especially for VFD applications. It comes with full metering capability and is capable of monitoring distortion on the input, which may come in handy some day even if you don't see a need right now.

 

[Jordan82]

Thank you for the informative response jraef.

I should have specified, but we actually do have the Multilin 269 plus. I'm trying to get in touch with the manufacturer to see if this may yet be the problem.

We have put current through the CT primaries, and the Multilin reads correctly (with a clean sine wave)..so I really cannot think of any other possible causes.

I think I'm going to suggest installing the 469 for best performance, and in the event that they would like to monitor harmonic distortion in the future. I'll have to check the specs to make sure the existing CT's and RTD's will function with the 469.