Motor-Generator Set Bolted Fault

[SmokinJoeR]

Hi,

I have a 30hp 39A @ 440V 3 phase AC induction motor turning a 20kVA 58A @ 200V synchronous generator. My question is how could I calculate the line current into the AC motor if a fault was to occur on the generator output? I have asked the manufacturer of this M-G set for this information and they do not have any.

Is there some form of fault current calculation I can do for the generator which will give me a torque value and then use that to calculate the kind of current the AC motor will pull?

Thanks for your help

-Joe

 

[davidbeach]

Current into the induction motor will go down and the motor speed will increase. By how much though could be hard to predict. Worst case would be 3-phase fault on the generator terminals.

 

[electricpete]

Yes, as David stated an bolted fault draws primarily reactive power mechanical load on the motor decreases and we suspect eventually (if fault not cleared) the induction motor will approach sync speed no-load current. What happens in between is a matter for transient analysis.

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[electricpete]

I didn't mean to suggest that a steady state was possible - something will obviously trip pretty darned quick.

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[davidbeach]

Why, what would trip?

Bolted fault on the terminals, voltages collapses, current decays to well below full load. At that size and voltage the only protection is probably a molded case circuit breaker and if it doesn't trip instantaneously (of if the fault is between it and the generator) there will be no trip. You would have to have differential or something that uses voltage. If the excitation system can keep the voltage up you stand a better chance of something tripping, but again the likelihood of that kind of excitation system on this machine is not high.

 

[waross]

I've seen PMG excitation systems down to about 28 kW or 30 kW on special order but not down as low as 20 KVA.
All I would expect on that size set would be a molded case circuit breaker.
I agree with David.

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

I'll take your word for it. Maybe the original poster can clarify the source of the exciation. For all we know there is an externally-supplied excitation.

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[SmokinJoeR]

The source of excitation is a brushless exciter. Unfortunately the name plate on the generator just states that and for Field Voltage and Field Current it states N/A.

Thanks for all your help.

 

[waross]

Yes, with that setup, expect voltage collapse and low current as per David's posts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I don't get it. Why would voltage collapse in this case?

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[electricpete]

I would have thought the field remains constant.

The voltage drop is created by the synchronous reactance which is the same as during normal operation. There is not excessive voltage drop unless there is excessive current... Or is there some flaw in this logic?

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[waross]

I take this to be a set where the Automatic Voltage Regulator is fed from the generator output terminals. With a bolted fault the power to the AVR falls to zero. With no power, the AVR cannot provide excitation to the field of the brushless exciter. So, the excitation drops to zero.
A set with a Permanent Magnet Generator to supply power to the AVR will hold up the current on a bolted fault and hold a better voltage on less than a bolted fault.
There is a special AVR for PMG operation, but in an emergency a failed PMG and or a failed can be bypassed by installing a conventional AVR until spare parts arrive. You take a performance hit and with larger sets there may be issues with protection (Not enough current available to trip the breaker under some fault conditions.)
This discussion pertains to a 20 KVA set. I have never seen a PMGs on a set that small.
A caveat, if the AVR is powered from "A" phase and "B" phase and he set is subject to a fault from "A" phase to "C" phase or from "C" phase to ground on a 4 wire system things get more complicated.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Bill. I will have to refresh my memory on that type of circuit.

It sure doesn’t seem safe to have a generator that doesn’t trip during a fault.


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[ruggedscot]

Its not safe to have a generator that doesnt trip- but this type of set up you should have protection set into the system to pick up any faults. As said a bolted fault causes the voltage to decay a relay can pick this sort of change up and trip the incoming breaker to the motor. Looking at this Id go for voltage monitoring on the output of the generator. The prospective short circuit current being a lot less than the full load current availiable.

 

[ScottyUK]

Bus-fed exciters are a typical application for a 51V voltage-restrained or voltage-controlled overcurrent relay. With voltage restrained overcurrent relays, when the voltage falls below a set value, the operating time of the overcurrent characteristic is continuously reduced with declining voltage. In voltage controlled overcurrent relays, the operating time characteristic is changed from the load characteristic to the fault characteristic when the voltage falls below a set level.


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

 

[waross]

Don't forget folks this is a 20 KVA genset probably built to sell at a competitive price.
In the real world:
Bolted faults are very rare, (Except for the time the crew crossed the connections on a 100KVA distribution transformer, but that's another thread.)
I have had the most serious issues with motor starting overloads on these small sets.
Specifically, (on more than one installation) a set capable of starting two Air Conditioning Units (A/Cs) consecutively trying to start three simultaneously. That's about 2.5 times the heaviest allowable overload.
The load stalled the gen set. But as the speed was droping, the Under Frequency Roll Off was dropping the voltage. At some point the Automatic Transfer Switch (ATS) contactor dropped out on low voltage. With no load, the set accelerated. The voltage went up as the frequency went up. ATS pulled in. Set stalled. Repeat until the contacts in the ATS fail open.
A small set running with zero voltage and little current is an inconvenience but no great danger to people or things.
Protection could be provided but the customer will not pay for it.
I don't believe that under voltage trips are even a standard option on sets below 1 MW. It almost requires battery power for the control gear and that just does not happen on the smaller sets.

It sure doesn't seem safe to have a generator that doesn't trip during a fault.

I think it was in the 60's, when I worked on series street lighting. Increase the load, the current goes up. Current stays the same but high voltages may be present.
A short circuit takes out the lamps affected but if there is no arcing there is no danger. An open circuit IS dangerous. There are no fuses but there are "Bugs" and "Buttons" across devices that will short out line to line and give protection on open circuit conditions.
If a regulator or current transformer was to be left energized without load for any reason, it was made safe by shorting the secondary terminals.
Does anyone else out there remember these systems??
It is experience that leaves you with less prejudice towards situations that entail zero volts and little current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Airfield lighting is still constant current loops; eliminates all concern for voltage drop.

 

[ScottyUK]

Bill,

Yeah - a 51V relay will cost nearly as much as the machine!


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

 

[sibeen]

Airfield lighting is still constant current loops; eliminates all concern for voltage drop.

Although for runway lighting each globe has a step down transformer, each of the primaries are in series.

It would be a tad embarrassing if one blown globe took out the whole lighting system

 

[ruggedscot]

Its not that expensive to add in some undevoltage protection - voltage relay and some form of timer relay - Im sure that this could be added reasonably easily and give you the peace of mind that you are looking for.

Also this has to be weighed against the effects of what a bolted short would create - and what you are trying to protect against. If there was a bolted short on the output how would that stress the generator? what temperature rises would you see ? what effect would that have on the driving motor? again it all adds up - as you say its a cost driven exercise and they will be weighing in probability and effect to this one. And a big consideration would be the effects to the set up. Forgetting the bolted fault what about a partial short ? how does that affect the set up what sort of currents are generated? what about overloads if an overload of 125 or even 150% occurrs how does that affect the system.