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Grounding Transformer sizing
- Thread starter Dmobley
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rcwilson
Senior Member
- Location
- Redmond, WA
Re: Grounding Transformer sizing
Here?s some things to consider:
Which voltage needs the ground bank? 138 kV or 34.5?
If it is 138 kV, look at using the neutral of the step up transformer with a single phase grounding transformer or reactor. If its 34.5 kV, select the type of grounding transformer: zig-zag, wye-broken delta, high-resistance grounded wye, or other?
What is the allowable or required ground fault current from the bank or from the other generators and loads on the system?
What is the time rating? If the fault is supposed to clear in 3 seconds, can you use a transformer with a 10 second rating? Per IEEE Std- 32 "Test Procedure for Neutral Grounding Devices", a grounding transformer's continuous rating is 3% of its 10 second rating. A 1200 kVA transformer can handle a 40 MVA fault current for 10 seconds. (1200 kVA = 40 MVA x 3%)
The transformer's time rating and KVA may also depend on the type and speed of the primary and backup relaying.
A good reference is Lew Blackburn?s ?Protective Relaying?. He has a chapter on grounding and grounding transformers.
Good Luck
What is the time rating needed? If the fault is supposed to clear in 3 seconds, can you use a transformer with a 10 second rating. Per IEEE Std- 32 "Test Procedure for Neutral Grounding Devices", a grounding transformer's continuous rating is 3% of its 10 second rating. A 1200 kVA transfomrer could handle a 40 MVA fault current for 10 seconds.
The transformer's time rating and KVA may also depend on the type and speed of the primary and backup relaying.
Here?s some things to consider:
Which voltage needs the ground bank? 138 kV or 34.5?
If it is 138 kV, look at using the neutral of the step up transformer with a single phase grounding transformer or reactor. If its 34.5 kV, select the type of grounding transformer: zig-zag, wye-broken delta, high-resistance grounded wye, or other?
What is the allowable or required ground fault current from the bank or from the other generators and loads on the system?
What is the time rating? If the fault is supposed to clear in 3 seconds, can you use a transformer with a 10 second rating? Per IEEE Std- 32 "Test Procedure for Neutral Grounding Devices", a grounding transformer's continuous rating is 3% of its 10 second rating. A 1200 kVA transformer can handle a 40 MVA fault current for 10 seconds. (1200 kVA = 40 MVA x 3%)
The transformer's time rating and KVA may also depend on the type and speed of the primary and backup relaying.
A good reference is Lew Blackburn?s ?Protective Relaying?. He has a chapter on grounding and grounding transformers.
Good Luck
What is the time rating needed? If the fault is supposed to clear in 3 seconds, can you use a transformer with a 10 second rating. Per IEEE Std- 32 "Test Procedure for Neutral Grounding Devices", a grounding transformer's continuous rating is 3% of its 10 second rating. A 1200 kVA transfomrer could handle a 40 MVA fault current for 10 seconds.
The transformer's time rating and KVA may also depend on the type and speed of the primary and backup relaying.
rcwilson
Senior Member
- Location
- Redmond, WA
Re: Grounding Transformer sizing
Data Required:
System Voltage.
Amps on ground fault- If there is not a requirement to limit the ground fault, this value will be the phase-to-ground fault current from your short circuit study of the system.
Time duration of fault - 3 seconds? Lowest standard rating for grounding equipment is 10 seconds, next is 1 minute.
Rough approximation of KVA rating is V x I x f, where V is phase-ground voltage in kV, I is the fault current and f is the continuous rating factor: f= 3% for 10 sec rating, 7% for one minute rating.
For a 1000 amp ground fault on the 34.5 kV system, with a 10 second rating, the kVA would be:
(34.5/1.732) x 1000A x .03 = 597.5 = 600 kVA .
So, if 1000 A is the phase-ground fault current, the grounding transformer should be at least 600 kVA.
Data Required:
System Voltage.
Amps on ground fault- If there is not a requirement to limit the ground fault, this value will be the phase-to-ground fault current from your short circuit study of the system.
Time duration of fault - 3 seconds? Lowest standard rating for grounding equipment is 10 seconds, next is 1 minute.
Rough approximation of KVA rating is V x I x f, where V is phase-ground voltage in kV, I is the fault current and f is the continuous rating factor: f= 3% for 10 sec rating, 7% for one minute rating.
For a 1000 amp ground fault on the 34.5 kV system, with a 10 second rating, the kVA would be:
(34.5/1.732) x 1000A x .03 = 597.5 = 600 kVA .
So, if 1000 A is the phase-ground fault current, the grounding transformer should be at least 600 kVA.
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