Capacitors and SA for the MV motors 3

[slavag]

Dear All.
This issue was discussed few times on the Forum ( include Motor&GeneratorForum).
I would like understanad this issue for my self.

We have a few 6MW 11.5kV motors.
SWG 11.5 with vacuum CB and SA on the cable termination. In additional in the terminal box of motors installed kit SA and capacitors. Distance from CB to motors from 300 up to 600 meters. From my point of view we don't need this kit on the motors. SA in the SWG is enough. I don't know, but I think 240mm^2 300-600 cable is good instead capacitors on the motor.
I know what is a reason of capacitor installation, but I know tens instalations of MV motors, tens years in operation w/o capacitors.

What is my problem. I have some leakages in the motor start and wrong operation of earth fault protection (51). All protection is connected to ZS CT. Protection issue I will solve, no problem. I try understand if this capacitors is problem.

Hope, I was clear .

Thanks in advance.
Best Regards.
Slava

 

[rbulsara]

Is this a new installation? or a new problem on an existing installation which was working fine? This makes a big difference in troubleshooting approach.

Focusing just on the E/F issue, if the protection is separate for each motor, I would disconnect the caps and SA on that motor and see what happens. (Assuming you have verified protection wiring and settings).

I assume these are capacitors for power factor correction. They are not essential for motor operation, but helpful for power factor correction and in reducing inrush/starting current.

You also said there a 'few' motors. Does this happen on all motors? Does it happen only during starting? Are caps delta or wye connected? If wye- is wye point it grounded?

SA on motors is a subject in itself, but perhaps unrelated to the E/F issue. I would only consider SA on motors if they are outside and too far away from the switchgear. If the cables are underground, need for SA at motor diminishes.






Rafiq Bulsara

http://www.srengineersct.com

 

[slavag]

Thanks a lot Rafiq.
It's a new installation.
Protection is separated for each motors, operated only infeed protection, nothing strange in the motors E/F protection.
Operated only in the motor start time.
Operated in both infeeds ( each one feeded 5 6MW motors with DOL).
Problem was started before two mounth with test of pumps.
I was checked what happend and we was found problems with MV cable shielding. Now problem back.

I was recommended yerstaday to end user disconnect cpacitors on all motors and check again.
As Im understand, those capacitors for the damping issue, front of overvoltages in case of CB disconnection.

I can solve this problem with 3U0 module in protective relays, as directional elemnt or as "AND" gate with 3I0 protection function.

Capcitors are wye connection and grounded in parallel with SA.

Relays wiring and setting I was verified few times, Im not like this setting, but isn't something complitly wrong.


PF isn't problem too, we have a cpacitor banks on the busbar 2.5MVar per bus. PF about 0.93-0.95.

Best Regards.
Slava

 

[rbulsara]

Thanks, Slava.

I would just start from the basics and try to zero in on the issue by method of elimination of the suspects. Probably, in the following sequence.

1. Set the E/S setting higher and see if that helps. This begs another question: Do you know the E/F current magnitude compared to the settings?

2. Disconnect the SA/cap Kit and re-test.

3. Check the cable shielding and ZS CT wiring again, you need to run the shield ground back through the CT. See thread238-262855.

4.Unbalance cap current (perhaps due to mismatched or bad caps) would flow its grounded neutral and that may cause a E/F trip?

5. Are motors wye connected and grounded at wye? If so I would remove grounding of wye point at the motors. Or would need to set the EF setting higher. (Just a thought. Other may have something to say on this.)

See if you can narrow down the suspect causes.

As for the switching over voltages, newer motor insulation should not be susceptible to OV due to VCB operation. It used to be an issue when VCBs were newer (in 70's, early 80's) and being applied to old motors.

I had found a discussion on that. I will post a link if I can find it again.





Rafiq Bulsara

http://www.srengineersct.com

 

[magoo2]

It sounds like these capacitors are surge capacitors not power factor correction capacitors. Surge capacitors are often applied on motors rated above 4 kV to slope the incoming surge. The arresters will clip the magnitude, but the surge capacitors will slow down the surge (i.e., reduce the rate of rise).

Particularly with a vacuum breaker, which can produce chop the current and produce a nasty voltage spike (L di/dt), you probably need both.

I would check with the motor manufacturer and see what they recommend. I'd be inclined to have both arresters and surge capacitors installed at the motor terminals.

 

[slavag]

Hi.
Rafoq.
1. I was recommended pot setting about 4-5 times, designer of object don't want.
Yes of course I know what is a "fault" current.
We try understand what is a reason.

2. Hope we check it.
3.Yes of course, it first thing what I was checked , and of course founded problems.
4. I think so.
5. I must check it again.

Magoo2, motors mnf not saied: not or yes, yet, I will recommend to end user ask again.

Your point is exectly why I was asked this Q on the Forum.
Lot of guys saied installed SA and capacitors near to motors. I don't understand this point, I have SA after VCB, what is difirence, isn't lighting protection, SA near to VCB is also good. Second point, capacitors., we will disconnect them and check what happened. But I don't know, if it's good solution for the long time, Swiching on those motors tens per day.

Best Regards.
Slava

 

[rbulsara]

salvag:

I meant how much (in amps) is the fault current and what is the setting ( in amps) and time delay, etc. I know that you know what the "fault current" is.
Cheers.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

The surge arresters and capacitors are there to protect the motor windings. They need to be as close to the motor as possible, preferably in the motor terminal box. This is a standard practice (around here) for medium-voltage motors, regardless of any other surge arresters in the system. The same goes for generators (and surge arresters for transformers for that matter). An increase of even a few feet in distance can cause a surprisingly large increase in the surge voltage that the windings can see.



David Castor

http://www.cvoes.com

 

[slavag]

Hi.
Rafiq.
Both Xfr's Y/D. On the bus ( every one) is connected zigzag Xfr. Ground fault is limited up to 300A. On the all bays are installed ZC CT's 100/1A.
Settings ( Im not remeber exactly)are :
12A very inverse and 30A 0.04s for outgoing bays,
15A very inverse and 30A 0.2s for infeeds.
In the situation of 3 motors running and all VFD and aux Xfr's are connected, current in the ZC CT of infeed is about 1-2A , max 4A.
about 0-1A in the outgoing bays.
Im monitoring sum phase current ( residual current) too.

Max E/F current with two motors running and one is started ( not important combination from 5 motors and what bus) current pick-up up to 70-80A for the 6-7s. BUT only in infeed, in the motor up to 15-16A.

I will attach here DR of starts. on the next week.

David, I was readed about it, in this case SA and capacitors are installed in the terminal box. Maybe, I have check this, is a motor mnf kit. From other hand, I know hungres MV motors that running w/o capacitors with VCB or vacuum contactors for many years.

OK. Thanks a lot again. Will wait for opinion of other members.
Firstly, I will recommned ask againf motor mnf.
Secondary, try disconnect SA and capacitors in the 2-3 motors on the one bus and check once-twice, of course with motor mnf approval.
Depending on the results, will decide, what will do, increase setting is simple and maybe right way.

Problem, is not my object, is not my project, Im only not official consult in this case )))).

Good Luck.
Have a nice week end.
Slava

 

[rbulsara]

Slavag:

Remote troubleshooting is always iffy, but your data only strengthens the suspicions I had mentioned earlier, and perhaps a combination of a few items therein.

When you know, please post as to what they find and the eventual solution.



Rafiq Bulsara

http://www.srengineersct.com

 

[slavag]

Rafiq.
Of course, I will post.
I push on them test all on this week.
Thanks a lot for your attention and help.
Good Luck.
Slava

 

[slavag]

Hi Rafiq.
Designer and end used was decided increase setting w/o some tests.
Now is work, will see.
Thanks again.
Best Regards.
Slava

 

[rbulsara]

Slava:

Great! Thank you for the feedback. Let us know how it fares in the long run.

Regards,

Rafiq Bulsara

http://www.srengineersct.com