Nuisance Tripping of Variable Speed Drives on External Earth Fault

Not knowing anything specific, you don't give much detail, I'd say your best course is to hire a technician to make it right.

You can search for troubleshooting guides on the internet, if you know what make, model the VFD's are.

One of the first things for the technician to do is to go through the downstream circuits with test equipment, to locate and eliminate earth faults.

You are probably expecting too much from your VSDs, they are not UPSs. You have not supplied any essential engineering data such as the magnitude and duration of the dip, motor ratings, etc. One has to wonder why the generator's AVR has allowed the bus voltage to drop so low unless these are extremely close-in faults. You also make it sound like these are common occurrences, so it's time for a complete review of the plant's electrical system and the configuration of the surrounding grid.

A review of your previous postings indicates that this plant is plagued with voltage dip problems which must be addressed on a system-wide, not case-by-case, basis. Hire a professional.

Not so common, but it happens 2 to 3 times in a year, due to flash over at some MV cables or breakers.

Flashovers should not occur that often unless you are in an area with high pollution levels, high humidity, and/or very dusty/dirty air. They are an indication that you are not following good maintenance procedures, the equipment is not specified or installed correctly, very old, dirty, in disrepair, not being operated correctly, etc., etc.

You can fool your management by not spending money to stay under budget, but the equipment will suffer and retaliate by breaking down over time. Posting here will not solve your problems, proper maintenance might.

Problem is with other electrical circuit and not with VFD, check thoroughly the electrical system with reference to earthing circuit, voltage dip, unbalanced load etc.

Of course the problem is with external electric system, but it is beyond our control. We can only manage our VFDs by providing some additional support like DC link chocks for DC bus stability, separate earthing for VFDs etc.

I once investigated a system that was acting much like this. First there was a problem in the isolation between the reference inputs. There was a measurable voltage difference between the signal grounds of the various drives.

Also had the joy of working on a system that was Y fed. With the center of the Y grounded & bonded the (6) Yaskawa drives attached would run up to about 40Hz then fault??- The gate signal to the IGBTs would become unstable and misfire. Took the Ground off the Y input and they ran like a charm...

Dear Mr.satendrakumart,

Your posting does not contain technical details. If possible, try to post under what conditions - tripping occurs. Pl. examine the following.

1. Is the VFD Room temp. is OK with Air Conditioners. What is the level of Heat Dissipation provided.?

2. Whether the Air-Blower in the VFD Panel found tripped. Is so, on account of excessive heat accumulation, tripping will occur and you may not get any indicator. I faced this problem 3 years back.

3. Pl. do HARMONIC ANALYSIS by a competent person and contain it below 5%.

4. Severe Earth Fault is possible,

It happens at sever earth fault only, but in external circuits (at user end). I want to know the phenomena of tripping. Whether it is due to

1. harmonic generated during earth fault or

2. Voltage sag at DC bus of VSD panel during external earth fault or

3. .......

If it happens only during severe earth / oc faults it may be due to the voltage dips associated, what more your power is sourced internally meaning it is causing a drastic and longer dip. this dips can be beyond the vsds operational settings or if the input voltage is through a magnetic contactor than the contactor is getting disengaged momentarily.