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# Problem with starting 11kV 800kW motor

Discuss Problem with starting 11kV 800kW motor in the Commercial Electrical Advice area at ElectriciansForums.net

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#### subhpoto

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When we start motor 800 kW 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.

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R

#### rumrunner

When we start motor 800 kW 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.
Hi subhpoto,
looking at the figures i think you will find you have a low resistance on phase c,which using ohms law (for simplicity ),is whats causing a higher current on that phase ,either that or you have a high resistance on the other 2 phases,the current on the neutral is the indicator ,it wasnt there when the thing was built ,and i think im correct in saying its only there because of the inballance between phases,
I would turn it off ,and check all connections at the motor end,if you find 2 loose ones you could be very lucky if there all nice and tight ,it might be a long night,have a check the control end as well but i think its at the motor ,or rather "in" the motor,

so if all connections are nice and tight ,dissconect them all at the motor end and carry out an insulation resistance test on each winding ,if one is lower than the rest ,you have a break down in the insulation to earth ,as idicated by the control relay
problem is im not sure if even at 1000volts you will find it ,when the normal operating voltage is 11000,maybee you nead special kit to detect it,but i bet ya a pound to a pinch of **** fella ,thats what it is.

to rectify it if it is you might need a rewind,as it wont get better,it will get worse ,i would monitor it running and i think it will trip more ,often ,could **** up the relay as well but thats a risk you will need to weigh up in your own head ,but dont worry about that just yet,if your in charge you might be pumped by then.

Shakey might have a different approch mind,i know hes got good experiance of motors and gennys,but i think by the time you have checked the connections he might reply.and dont panic ,and remember Electricians dont have problems ,we have solutions,

Please let me know what it turns out to be ,
all the very best in the whole wide and wonderfull world

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M

#### Moog

Hi subhpoto,
looking at the figures i think you will find you have a low resistance on phase c,which using ohms law (for simplicity ),is whats causing a higher current on that phase ,either that or you have a high resistance on the other 2 phases,the current on the neutral is the indicator ,it wasnt there when the thing was built ,and i think im correct in saying its only there because of the inballance between phases,
I would turn it off ,and check all connections at the motor end,if you find 2 loose ones you could be very lucky if there all nice and tight ,it might be a long night,have a check the control end as well but i think its at the motor ,or rather &quot;in&quot; the motor,

so if all connections are nice and tight ,dissconect them all at the motor end and carry out an insulation resistance test on each winding ,if one is lower than the rest ,you have a break down in the insulation to earth ,as idicated by the control relay
problem is im not sure if even at 1000volts you will find it ,when the normal operating voltage is 11000,maybee you nead special kit to detect it,but i bet ya a pound to a pinch of **** fella ,thats what it is.

to rectify it if it is you might need a rewind,as it wont get better,it will get worse ,i would monitor it running and i think it will trip more ,often ,could **** up the relay as well but thats a risk you will need to weigh up in your own head ,but dont worry about that just yet,if your in charge you might be pumped by then.

Shakey might have a different approch mind,i know hes got good experiance of motors and gennys,but i think by the time you have checked the connections he might reply.and dont panic ,and remember Electricians dont have problems ,we have solutions,

Please let me know what it turns out to be ,
all the very best in the whole wide and wonderfull world
Just to check this fault trips the breakers before the switch to delta, yes?, what happens if the motor is already running? it sounds like a problem with either the start winding or the switch from star to delta. If you disconnect the start windings and start the motor up manually and without a load, powering only run windings, do you still get earth fault and or current imbalance

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#### benji

##### -
Arms
where did the neutral come from? 11KV ?
yours benji

M

#### Moog

A bit more info about problem form another forum Problem with 11 kV motor - DIY Chatroom - DIY Home Improvement Forum " Thanks Bigjimmy for ur reply this is an 800 kW motor for process industry. the scheme is used 3 CT&quot;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."

W

#### wattsup

I would disconect the motor, leave flying leads and test each line live. I would presume there is an imballence, thru the starter or from source. Is it auto star delta or manual? Is it old, oil filled starter?

S

#### Shakey

Just to check this fault trips the breakers before the switch to delta, yes?, what happens if the motor is already running? it sounds like a problem with either the start winding or the switch from star to delta. If you disconnect the start windings and start the motor up manually and without a load, powering only run windings, do you still get earth fault and or current imbalance
there are no 'start' and 'run' windings, the is only one set of windings. The starter just reconfigures them for star and delta

i would go with Rum on this, there is clearly an imbalance, which is beng felt on the neutral, so either low or high resistance windings. My money is on low resistance, maybe even a partial short on one bank of coils.

W

#### wattsup

Surely the fault could be with the starter and not the motor?

#### benji

##### -
Arms
only ever worked on high voltage motors three times all of them 3.3kv fed by a 3c 70mm connected in delta i think.Not allowed to touch HV now unless youve been on a HV course and have been made an HV/AP
yours benji

I hope all relevant documentation is in place and all the procedures are logged ie.do you have a switching schedule wrote out are your isolation methods approved and have you asked for a sanction to test and is it on an official document .The reason for the above is if somebody gets hurt somebody will be to blame and the HSE will be all over you.This drive has been of line sometime now i'am begining to think it's a figment of somebodies mind.
yours benji

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