Synchronous Motor & Need for Rotor Earth Fault Protection 1

[RRaghunath]

Is this required!
What is the practice generally!
I find Shell standard doesn't recommend Rotor earth fault protection for Synch motors (it recommends Loss of excitation protection).
The Steel industry in India specifies in their purchase requistion.
Siemens motor unit says, 90% of Synch motors supplied by them world over are without Rotor earth fault protection.

Look forward to hearing from you.

Thanks.

 

[slavag]

Hi Raghun.
i don't know if it's some standard.
What is size of motor and volatges.
function 40( loss of excitation)is must and 55( loss of synchronizm) not bad practic. 64R, we provided on all big synch motors.
Between us, is not so expensive.
best Regards.
Slava

 

[LionelHutz]

I usually deal with motors <5000hp and 5kV or less. There, it is rare to see ground fault on the rotor. However, it would not be hard to do by just adding a CT around the AC conductors feeding the field bridge, assuming you ground the supply transformer.

It could also be possible to resistively ground the supply transformer feeding the field bridge to limit the fault current.

 

[edison123]

A single earth fault in synch machine rotors is not a problem. When the second earth fault, it could bypass a part of the field winding leading to severe vibration and related mechanical damages.

 

[slavag]

Yes, Edison.
This is true!!!
Regards.
Slava

 

[ScottyUK]

I deal with fairly big synchronous machines - generators, although they're not fundamentally different from motors - and we always use a rotor earth fault detector, even on the brushless units where the rotor E/F detection is tricky. I guess it depends on the size of the machine involved, the revenue loss if the motor fails, and whether a spare is available which can be put in service. The rotor E/F detector gives an early warning of trouble before the second E/F occurs, buying time to plan a graceful shutdown and either repair or swap the rotor.


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If we learn from our mistakes I'm getting a great education!

 

[RRaghunath]

Thanks all for the responses.

Scotty UK,
Thanks. I agree with that.
The issue is to do with the Synch motor with the brushless excitation and vendors are not recommending the same saying it is not much of use and affects integrity due to the need to increase the shaft length to accommodate additional slip ring.

 

[ScottyUK]

You might want to contact Brush Electric Machines at Loughborough and enquire about their Rotor Earth Fault Monitor for the BDAX series of generators. It is a wireless unit which draws power from the field and does not need additional slip rings. We have these units on our gas turbine sets and they are 'reasonably' reliable. They have picked up a rotor earth fault while it was confined to a single pole on a couple of occasions, and have given a few false positives too. If your motor runs at less than 3000 rpm I'm pretty sure the reliability of the electronics would improve: sometimes we see a unit die on us without warning and we suspect that it is the centrifugal effect which is damaging the electronics. It's potted in resin so we've not managed a full stripdown. The guy to talk to from a technical perspective is called Chris Lewis, switchboard is +44 (0)1509 611511.




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If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

Found the data sheet: the REFM is the little light alloy housing with a yellow label on it shown in Fig. 1.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Just opinion.
Scotty, for the big generators (like that you have) I think must use 64R protection with level of signal( about 10kOhm) and level of trip( about 1kOhm). More from that, only injection principle.
It's additional cost, but you sleep good.
From bad exp of one nuclear power plant with generator 1000MW, rotor e/f---half year of repair.
Regards.
Slava