VFD explosions and question marks.

As a general rule of thumb a THD above 15% needs the transformer to be derated. Is the transfer derated? How much?

Above 15% it's also a good rule to be using a K rated transformer? Are you using a K rated transformer?

Also what is the environment Like? What's the heat load like in the enclosure and the ambient?

I have seen this happen a lot in environments where you had steel and other metal particulates in the air and certain non metallic dusts. It takes less than you think to cause a problem.

we had a Powerflex 700 blow up on us several times much like you are talking about, after it had been running fine for years. After changing the drive twice ourselves and it exploding both times, we called A/B to come in and verify everything. He installed the drive and checked all parameters... same result, the drive blew up. The resolution was that the X0 on the transformer was not bonded to ground and another motor that was fed off the same Xfmr was actually shorted. After bonding X0, the other drive faulted "ground fault" and we found a shorted split bolt inside of that fans junction box. The A/B service tech said that if we had removed the MOV's then it would have never blown up but code does not approve of doing that and the entire machine would have been running as an electrocution hazard. Just an idea, incase you havent checked it yet

Do you perhaps have an automatic power factor correction system? They can induce severe transients in some cases.

Peter Laing said:

Do you perhaps have an automatic power factor correction system? They can induce severe transients in some cases.

Click to expand...

+1

Drives don't like cap banks so that is something to check.

Read this thread:
http://www.plctalk.net/qanda/showthread.php?t=101306

Check power quality by means of power analyzer and recorder it could be presence of harmonics

As Mprance has mentioned grounding is extremely important. I've seen that exact problem causing VFD issues; A connection between the local MCC ground and supply transformer ground wasn't explicitly shown on the electrical prints and the installation electrician missed it; the only ground path was a steel electrical conduit. VFD trips and failures went on for months, at great expense to the customer, OEM, and VFD supplier. Luckily there were no safety issues. By the time the problem was finally solved the company was nearly bankrupt.

And you might want to consider installing line reactors if you don't already have them. Line reactors reduce dv/dt and harmonic currents with the result of lower peak current through the input rectifier and reduced high voltage 'ringing'. Line reactors are often 'built in' to larger drives, but it's worth looking into anyway.

Is the output of your transformer a wye (star) configuration, or delta? If it is wye, is the neutral point solidly grounded (earthed)?

You already have a 12 pulse drive, so that should be dealing with the harmonics and provides impedance, therefore the suggestions for investigating harmonics and adding reactors would not be relevant to your situation.

It's far far more likely this is a grounding (earthing) issue or a capacitor issue, meaning you have capacitors somewhere else in your system that are causing problems for this drive. You need a professional with experience in power quality issues as it relates to VFD installations to do a thorough investigation before you purchase and destroy any more hardware.

PBuchanan said:

As a general rule of thumb a THD above 15% needs the transformer to be derated. Is the transfer derated? How much?

Above 15% it's also a good rule to be using a K rated transformer? Are you using a K rated transformer?

Also what is the environment Like? What's the heat load like in the enclosure and the ambient?

I have seen this happen a lot in environments where you had steel and other metal particulates in the air and certain non metallic dusts. It takes less than you think to cause a problem.

Click to expand...

Transformer has Dyn5 connections , it is not a K rated transformer. 3 inputs and 3 outputs - 6 pulse transformer.
The ambient is dry , there is no humidity in the ambient. The servise measured the humidity over and over again , they found nothing notable. The room temperature is almost 18-20 C and we dont think this problem is coming from ambient.

Mprance said:

we had a Powerflex 700 blow up on us several times much like you are talking about, after it had been running fine for years. After changing the drive twice ourselves and it exploding both times, we called A/B to come in and verify everything. He installed the drive and checked all parameters... same result, the drive blew up. The resolution was that the X0 on the transformer was not bonded to ground and another motor that was fed off the same Xfmr was actually shorted. After bonding X0, the other drive faulted "ground fault" and we found a shorted split bolt inside of that fans junction box. The A/B service tech said that if we had removed the MOV's then it would have never blown up but code does not approve of doing that and the entire machine would have been running as an electrocution hazard. Just an idea, incase you havent checked it yet

Click to expand...

The X0 on the transformer is connected to the common grounding system.

What was done before :

ABB ACS800-07

- The motor had been changed. It blew up.
- All the grounding system checked and it was strengthened. It blew up.
- All the modules inside the VFD and control cards changed. It blew up.

We turned to SINAMICS G150 and it run 3 months properly. After the first blew what was done :
(There is no line and motor reactor in the drive)
- All the ground system was checked again. All the cables from transfo to VFD and from VFD to motor had been changed. It blew up again.

What is being done now :
- MV cable is checked with high voltage isolation test , normal.
- We are changing the transformer with another one 6 pulse transformer (We don't have 12 pulse transformer right now).
- We will put the line reactors inlet and outlet side.
- We are earthing the X0 of transfo seperately from common earthing.
system.
- We are suppling the 230V auxilaries from a UPS system.

We will see the results.

Sorry, don't know what Dyn5 means, we don't use those terms here and I'm too busy to look it up.

So on your original transformer, were the secondaries for the 12 pulse both Star configured with the X0 terminals grounded? Saying that you are NOW going to ground the X0 terminals implies to me that they were not. That would be the likely cause of the repeated failures.