Fault current flow in Generator winding 1

[prc]

Normally LV side of large Generator Transformers(GSU) are connected in delta and HV side in YN. It is claimed by some experts that this is done to avoid flow of zero sequence currents through generator stator windings during an LG fault on HV side lines of GSU. Why Generator windings cannot stand zero sequence currents? Is this statement correct?

 

[dpc]

Delta-wye xfmr provide a ground source for the transmission side while allowing limiting ground fault current on the generator via high-resistance or low-resistance grounding. The fault current needs to be limited in the generator to minimize core damage in the event of a ground fault. Also generator zero sequence reactance can be less than the subtransient reactance - resulting in high fault current sourcing if solidly grounded. With the delta-wye xfmr, the generator will not source any zero sequence current for a ground fault on the high side of the GSU. Since the majority of faults will be ground faults, it is good protection of the expensive generators.

I'm not aware of any uniquely destructive impact of zero sequence current compared with equivalent amount of good old-fashioned positive sequence fault current. Negative sequence current is different, of course.

Cheers,

Dave

 

[Hoxton]

Current is current.

A generator generates current, not KVA or KW or volts.

When I was a student engineer, my chief engineer explained it to me this way: if your generator is rated to generate 100 Amps of current, that it is it, no more.

If the load is unbalanced so that the phase currents are different, then no phase current may exceed rated current, say 100 A. Otherwise that stator phase will overheat as the excess current will result in excess heat. This is not engineering, this is arithmetic. So the KVA capability of the generator is reduced.

Does the generator care if there is zero sequence current (ie current in the neutral, assuming the generator is star connected)? In simple terms, no, current is current, but the phase current limit must not be exceeded, so permanent zero sequence current reduces the KVA capability of the generator.

 

[waross]

No mention of rotor overheating from zero sequence currents?
In our small system the balance between phases shifted depending on the time of day.
We were not able to maintain a balanced current on our generators at all times.
Thus we had zero sequence current for a number of hours every day.
The generators were never overloaded.
Every few years we had to have the rotors serviced due to the cumulative damage effects of the zero sequence current.

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

 

[prc]

Thank you dpc. I thought if zero sequence current goes in to stator winding(in case of GSU LV is star instead of delta) it can create disturbance in magnetic field causing damaging vibrations.

 

[dpc]

prc,

I've not heard of that being a concern. But I'm not really a machine guy. IEEE guides strongly recommend resistance-grounding of all medium-voltage generators to limit fault current. But, as I understand it, this is to limit damage in the event of an internal generator ground fault.

 

[ScottyUK]

Rotor heating is a problem from negative phase sequence currents circulating a double line frequency in the rotor forging. Zero sequence currents would also cause heating, but aren't seen with the generally-accepted delta winding. With the delta wound transformer, zero-sequence currents are faults.

From a practical perspective, the delta transformer winding doesn't require a fourth heavy-current connection to be made through an IPB to the generator neutral point, so a delta transformer is cheaper to connect to and the design of the machine is simplified a little.

A shorted turn on the rotor can cause a thermal bend which results in vibration. I haven't heard of vibration from zero-sequence currents though.

 

[edison123]

I thought the GSU trafos were designed for delta primary to handle current (lower phase current) and wye secondary to handle high voltage (lower phase voltage). Never heard of zero sequence current playing a role for primary delta connection in trafo or causing heating in generator stator.

Muthu

http://www.edison.co.in

 

[prc]

Thank you Scotty and Edison for your inputs.
1) The point mentioned by Edison is relevant for simplifying transformer design. But Delta connection is not for the benefit of transformer maker. They can make windings with still higher current values eg: Furnace and rectifier transformers.
2) My reference is a Transformer book from US(John Winders,2002)
He mentions two reasons for adopting delta for LV of large GSUs. (page 36; chpater2) First Ground Currents on HV side do not affect ground fault protection of generator. Second rotating machines can literally be shaken apart by mechanical forces resulting from zero sequence currents. The delta connected winding blocks zero sequence currents on the transmission system from the generator.

I thought rotating machine experts can confirm whether these are correct statements.

 

[waross]

With a wye/wye a line to neutral load or fault is carried by one generator winding.
With a wye/delta the line to neutral currents become phase to phase currents and are shared by at least two generator windings.

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

 

[edison123]

prc

AFAIK, there is no zero sequence current protection for generators, only negative sequence protection to prevent rotor damages. And I have no anecdotal story of generators being destroyed by zero sequence currents. Does Winders cite any such incident or even a study?

Muthu

http://www.edison.co.in

 

[prc]

Winders just made that statement. Normally with delta LV, zero sequence current will not come on generator for LG fault on HV side. For LG fault on LV side or generator ground fault, current is limited by generator neutral grounding resistor. Probably this may be the reason for not having any Z0 protection for generator. I believed Winder as I thought in case I0 start flow in stator winding ( in three phases) vibration is logical from unbalancing.