HV Motor Testing - 6.6kV

Hi, If the protection is not tripping, what is? Give details on this device please.

Is this squirrel cage motor or slipring?

Motor drawing normal current.

Motor protections not tripping.

but incoming voltage dropping - Are you sure it is not an input supply problem?

Probable Faults

1) Wrong connection

2) wrong supply Voltage

3) Open rotor bars...

4) Your protection may have a fault or incorrectly wired..hence machine not tripping

5) Circuit Breaker single phasing ( this should trip motor with unbalanced fault)

But most importantly.......GET A QUALIFIED PERSON TO LOOK & TEST THE PROTECTION CIRCUITS & TO VERIFY MOTOR...6,6KV IS DANGEROUS

ALSO.. Check the supply Grid.... If all is well and you are getting a VD of 500Volts... check cable sizing, and supply capacity...3300 kw is a fairly big motor

There is always problems of that nature as far as i know in electrical engineering.some big problems or faults may take short time to solve,and some very little problems or faults may take much time to solve.

What u have to do is to continue trouble-shooting and u will get the fault.Remember u have not exhuasted areas to be checked for probable cause of the problem and why it drops 500v.

Patrick Whowha

hi, it seems from your data that all electrical measurement are ok . you should make visual check inside the motor to winding, rotor and bearing condition.

for 500v voltage drop don't exceeds 10% of rated voltage. it can't be the reason

Well first I'd like to thank you all for your constructive responses.

The Baker test on Monday was clear - 3 traces perfectly overlaying each other - no shorted turns, open windings or earth faults - tested up to 11kV pulses.

I convinced the management that short of opening the motor for a visual inspection on slots, wedges, end rings, rotor etc, ( which due to warranty's would probably be done by the manufacturer & would cause certain inconveniencies ) we should run the drive uncoupled again.

This time we would ensure the DCS volt / current trace capture scan times were set as small as possible & that any control interlocks ( logic stops within the pump start sequences due to changing load between a pump going into service & one coming out of service ) were overridden.

We also set up a recorder locally on the bus wired VT 110v secondary to capture board volts a second time.

I note at this time that prior to running the motor, with a secondary injection set, I pushed 10 amps through the core balance CT to prove earth fault CT wiring, & forced an alarm & trip. This was a concern to me, as on Saturday, one of our gang who spent time with the construction contractor, pointed out a hand written log, in which 2 entries were about this drive tripping on ground fault ! The motor was allegedly sat a while with no heaters on, & some effort went into conditioning it. I cannot say for certain that the ground fault trips were not due to inrush imbalances or incorrect settings, or due to a true fault, just that they were recorded. No repairs have been done to the motor that anyone knows of.

The option to run the drive was agreed, the motor was run yesterday, witnessed & seen to run up to full speed in 4.5 secs as normal, pulling normal current - 1650 A ( ish )

The switchboard voltage dipped some 500v !

Another point which became evident, was that reports suggested the drive struggled when one or more of the other large pumps were already running - there are 3 in total.

I can hear your cogs from here - you're ahead of me !!!

The drive was coupled & ran again, with no obvious signs raising concern - but with obviously more inrush was evident in listening to the drive locally at a safe distance. It accelerated in a normal time & with normal currents.

The board again dipped a simillar amount of volts.

Another simillar pump was run up - dipping the board again.

The pump at hand was again run with more load already on the switchboard & ran up as before - ditto board volt comments.

We too started to talk about concerns with electrical design - this plant is brand new & just handed over to us.

The 40MVA transformer is some 250 metres from the switchboard. It is fed via 18 400mm cables - 6 per phase. The drive being tested is around 100m from the board & fed via 3 x 240mm cables. ( the cable csa's are correct, but I estimate the lengths )

In summary, I sugest I have been trying to find a fault which is not where I tested - the motor.

My main lesson learned is that just becxause someone ( with electrical experience ) said the motor was single phasing - or appeared to be - doesn't mean it was doing this. It turned out that the motor ran up to some speed on Dec 30th, the time of the defect being raised ( shown by the Bently Nevada monitoring gear ) but was stopped shortly after run up by something other than electrical protection.

Unfortunately for us, the bently gear is only on the pump, so offered no back up info when the drive was described to stall when started uncoupled on Wednesday last week ! Hence much testing.

The suspect pump is still running with no problems 24 hrs on.

My last motor related comparison will be when we run the pumps redundant partner tomorrow. I want to compare the amount of thermal capacity used on a cold start - figures from the GE Multilin SR469 relay.

I will move my area of concern to the system design & what is acceptable volt drop for this board & what are the consequences to other drives & transformers fed from the board.

To be honest the supply should be as good as it gets, with a frame 9FB GE Turbine / 300 MW + generator sat very close !!

I thank you all again & will of course feed back my findings as & when they materialise.

Regards,

CTB

This is an excellent chap OP who cares to keep us updated. Unlike most who ask a question and then disappear.

Hope to find more of them around.