GEN IV Cell DC Bus low faults

Hi Expert house,

We are having GEN IV air cooled VFD with 9 cells w/o cell bypass

4.16 KV input, 100 Amps cell rating.

Since last two years we received at least 4-5 time Cell DC Bus low warning. Each time we checked and found blown fuse. Strangely all are coming on TRAFO SECONDARY delta side phase

A2, B2 and C2. cells. We checked the faulty cells with external VARIAC and found drawing abnormal current with a typical 5 % of rated voltage indicating cell internal Capacitors/semiconductor fault.

Questions are

1. Is there any aging for these power cells even with  INPUT MV ON always

2. We have total 3 air-cooled drives and occurring problem in one drive only? Any thing to cehck.

3. Why the cell faults occurring only on Phase L2..

Thanks

N Vijay

Hi N. Vijay,

Check all the transformer secondary voltages for the A2, B2 and C2 cells. Compare the voltages against some other cells that are not experiencing any issues. There could be a transformer secondary winding issue. 

For the ageing of components in the cells the dc link electrolytic capacitors have a service life time. It is dependent on several factors. Worst cast scenario for a GEN IV drive is 10~12 years in a heavy duty environment ie average operating temperature = 35°C, operating hours > 8500 hours/yr, full load. 

Hi Will

Thanks for immediate response. How to measure secondary voltages live. Are there any test points like VMA, VMB, VMC (TP 5, 6 and 7) for motor voltage. Will it reflect @ VIA,VIB nd VIC , I mean TP 1,2 and 3. Shall I monitor  cyclic variation through Debug tool these values...

Regards,

Vijay

Hi N. Vijay,

There are no PCB test points for the transformer secondary voltages or any feedback to the debug tool. To measure the secondary voltages there are a few options. One option with the drive isolated is that you can remove the necessary covers and connect up the required meters with the leads coming out of the drive to the meters. Put the covers back on energise the drive to get your readings. Measure the 3 phase to phase voltages for each cell experiencing issues and compare against some cells that are not having issues. This will be a good start to find out if the transformer is causing the DC Bus Low faults. 

Good morning Will

OK. Let me check the practicability as LTT norms should allow to charge MV with doors open..

One more question is that in case if it is TRAFO issue what is confusing is that the removed cell shown internal fault. I mean during external checking through 3 phase Variac, the removed cells drawing abnormal current even at 2% nominal voltage. Shall we consider as effect of TRAFO secondary cause.

.Once we replace the fuse and cell, it is running without any issue

Thanks

N Vijay

Hi N. Vijay,

When you inspected the failed cells did you notice anything visually? Were the rectifier diodes in good condition? How did the DC link caps look? A shorted diode on the rectifier section of the cell could cause possibly cause a low DC link voltage if this blows a fuse and a phase is lost. This would also explain the high current when using a variac direct. If you still have the failed cells check the 3 phase rectifier with a meter. Its a process of elimination. Things to check;

• Transformer secondary voltages
• Cabling or bus work from secondaries to cell
• Cell rectifier
• DC link capacitors
• IGBTs
• Cell control board and cable loom to DC link

Keep us posted on what you find

small correction it is phase L1 not L2.

One more thing I forgot is that Phase L1 to DC BUS -ve shown low ohmic value, indicating rectifier fault.

So my question is it is purely cell related issue OR

TRAFO secondary also can cause Rectifier fault (Drawing more current  etc...)

Regards,

Vijay

It is good practice to measure the transformer secondary voltage in this case. Once you have that information then you can figure out if the issue is caused by the transformer, cell, or even a combination of both. 

[snip] ...

Hi Will

Good morning,

We are having planned shutdown and want to measure the voltages. But is it possible after connecting brought out leads, cells can be pushed back intact. OR shall I perform this test without power cells.

Will it work if MV trip interlock from VFD is bypassed or not?

Regards,

N Vijay