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 DIY Chatroom Home Improvement Forum Problem with 11 kV motor
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07-10-2008, 02:16 PM   #1
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## Problem with 11 kV motor

When we start motor at 11kV, just after closing, the motor feeder breaker as well as utility breaker both tripped. Then as another breaker between motor feeder and utility is intacted. Relay MICOM P220 indicated that there are two faults 1) unbalanced current and 2) Earth faults
The values of current are as under:
Ia=304.62A
Ib=302.5A
Ic=314.2A
In=49.47A
Further to this it is also pointed that the capacity of utility is 2 MWe
What are the causes? How they can be rectified? Lastly can the said utility bear such load.

07-10-2008, 02:40 PM   #2
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Quote:
 Originally Posted by subhpoto When we start motor at 11kV, just after closing, the motor feeder breaker as well as utility breaker both tripped. Then as another breaker between motor feeder and utility is intacted. Relay MICOM P220 indicated that there are two faults 1) unbalanced current and 2) Earth faults The values of current are as under: Ia=304.62A Ib=302.5A Ic=314.2A In=49.47A Further to this it is also pointed that the capacity of utility is 2 MWe What are the causes? How they can be rectified? Lastly can the said utility bear such load.
Interesting question when you consider that this is a DIY site

Anyway, can you provide the motor HP and rated voltage (if different from 11KV).

Is this an industrial or power generation station?

You say that the motor breaker as well as the utility tie breaker both open which makes me suspicious that you have a coordination issue with the protective relays on these two breakers (more of an aside; not addressing the question at hand). Is the utility breaker the only connection between them and you, i.e. when it trips does you facility lose all power?

Ok, what kind of CT arrangement do you have connected to the protective relay? Is it 3 CT's per phase and one residual ground fault CT or are you using a different scheme? I'm assuming that the I(n) = ground current and 50A to ground would certainly constitute an issue. And in this case, it is not necessarily surprising that you get both imbalance and earth fault trips. Is this a new or rebuilt motor? Has the CT wiring been modified or is it new (or is this a new relay perhaps?)?

Your question as to whether the utility can support the operation of this motor cannot necessarily be answered without more information. Even with the HP on the motor, you need to take into consideration whatever other loads are present. If you're concerned about the utility's capacity because you tripped the utility breaker as well as the motor breaker, I'd be looking at the coordination settings between the two breakers, as I mentioned earlier.

Back to the motor though. The first thing that I'd do is disconnect the t-leads and megger it to determine the fitness of the motor's insulation.

Thanks,
Jim

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Last edited by BigJimmy; 07-10-2008 at 05:18 PM. Reason: Read the P220 manual and thought about things a bit more

 07-10-2008, 03:51 PM #3 Licensed Electrical Cont.     Join Date: Feb 2004 Location: NY State Posts: 7,737 Rewards Points: 1,824 This stuff is greatly out of place on this DIY site. This should be taken to a professional site. subhpoto, are you in the US? __________________ Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
07-10-2008, 05:23 PM   #4
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Quote:
 Originally Posted by Speedy Petey This stuff is greatly out of place on this DIY site.
Well Speedy, ya got to admit that it does break the monotony of the same old "my receptacle suddenly stopped working" posts!
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 07-11-2008, 12:35 AM #5 " Euro " electrician     Join Date: Apr 2006 Location: WI & France { in France for now } Posts: 5,369 Rewards Points: 2,000 The way of the OP mention 11 KV system I am sure it is either UK or Aussie system which I know they used there and yeah France used that voltage for transmmion and hevey local spurs. Merci,Marc
 07-11-2008, 01:31 AM #6 Newbie   Join Date: Jul 2008 Posts: 2 Rewards Points: 10 Thanks Bigjimmy for ur reply this is an 800 kW motor for process industry. the scheme is used 3 CT"s one per phase and one one residual ground fault CT.and about utility, we have 2MWe electrical capacity and at the time of running this motor all load have been switched off.
 07-11-2008, 11:25 AM #7 Member   Join Date: Jun 2007 Posts: 3,699 Rewards Points: 58 So we know you have a 1072 Hp motor. Your posted currents must be no load correct? The question that begs an answer is how are you starting this beast. This initial inrush current is extremely high, probably higher than the KW rating of the complete system. Whats the voltage? And what kind of motor is it. (Induction)? Who designed and built it? This is a special build motor as it is out of the realm of NEMA and IEC. Tell us how you are starting the motor. Reducing the inrush current will be the only option you have to prevent breakers from tripping.
07-11-2008, 11:45 AM   #8
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Quote:
 Originally Posted by J. V. So we know you have a 1072 Hp motor. Your posted currents must be no load correct? The question that begs an answer is how are you starting this beast. This initial inrush current is extremely high, probably higher than the KW rating of the complete system. Whats the voltage? And what kind of motor is it. (Induction)? Who designed and built it? This is a special build motor as it is out of the realm of NEMA and IEC. Tell us how you are starting the motor. Reducing the inrush current will be the only option you have to prevent breakers from tripping.
J.V. makes some good points and he's got a ton of experience with motors and starters and whatnot.

BUT---

1. Barring any miswiring of the CT secondaries and/or protective relay, it sounds like they've got a ground fault. First I'd recommend pulling the T-leads and meggering the motor (assuming that this is a simple SC induction machine). This is the first issue.

2. I still question whether the protective devices in the motor feeder breaker and utility tie breaker are properly coordinated.
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 07-11-2008, 11:56 AM #9 Member   Join Date: Jun 2007 Posts: 3,699 Rewards Points: 58 Yes, I agree as he is getting a ground fault warning. I wonder how he is getting the current values. The motor must be running for him to measure these values. You know me, I'm always looking for a quick fix LOL (SS).
 07-11-2008, 12:38 PM #10 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 I'm thinking this is a test question not a real life problem. He is also from the UK.
07-11-2008, 01:37 PM   #11
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Quote:
 Originally Posted by J. V. Yes, I agree as he is getting a ground fault warning. I wonder how he is getting the current values. The motor must be running for him to measure these values.
You can look up the protective relay they're using (http://www.areva-td.com/solutions/li...alGuide-EN.pdf) (I did because I'm such a friggin' nerd). It's one of those "every ANSI device function imaginable" in a solid state protective device. This thing has a built in recorder (as most do) what will tell you not only the cause of the trip but the value(s) that were out of range. Since you need to actually measure something to determine that any problem exists, the values that he reports are likely from a start. I'm a little rusty on my 3-ph calcs, but an 800HP motor (assuming SC. induction) should consume about 60A per phase at 6350V(phase) assuming a nominal 90% p.f. That being said, the 350A per phase that he reports seems well in the ball park for startup inrush. And when you consider that there is typically little, if any TD on ground fault measurements, the sucker is probably tripping right as it's starting.

We used to start 4160V 350Hp and 400Hp motors across the line with no problem (MultiLin 280 motor protection relays and GE 5KV vacuum bottle CB's) and at the OP's higher voltage, the current would be even easier on the system for a given HP-rated motor. Then again, I've heard a lot of good questions asked that have gone without any answers from the OP. This type of thing is a bear to troubleshoot without a whole heck of answers and information.

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