Lightninarrester
Member
400KVA, 13.8KV/300V transformer. Can we tab the primary to a 13.8KV generator bus without providing a primary OCP per NEC. The Tab length is 8 feet. Thanks
15KV Transformer Tab
- Thread starter Lightninarrester
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Re: 15KV Transformer Tab
An interesting question. But not enough information is given, and I'm not sure I could answer this even with all the info.
Per Table 450.3(A), your tranformer needs primary OCP at not more than 600% of its rating. Does the generator or generator bus have OCP that would satisfy this? If not, the answer is no. Otherwise the answer would be maybe.
You also have to satisfy the rules in article 240. 240.21 are the rules that normally allow feeder taps. In particular, 240.21(B)(3) is the one that usually allows taps on the primary side of a transformer.
But I think Part IX or article 240 - Over 600 volts - may not permit the tap rules to be used.
Steve
An interesting question. But not enough information is given, and I'm not sure I could answer this even with all the info.
Per Table 450.3(A), your tranformer needs primary OCP at not more than 600% of its rating. Does the generator or generator bus have OCP that would satisfy this? If not, the answer is no. Otherwise the answer would be maybe.
You also have to satisfy the rules in article 240. 240.21 are the rules that normally allow feeder taps. In particular, 240.21(B)(3) is the one that usually allows taps on the primary side of a transformer.
But I think Part IX or article 240 - Over 600 volts - may not permit the tap rules to be used.
Steve
rcwilson
Senior Member
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- Redmond, WA
Re: 15KV Transformer Tab
Is this an excitation transformer connected directly to the generator terminals? (300V secondary is a typical excitation system supply voltage.)
If that?s the case, the excitation transformer is usually included in the generator's differential protection zone. A fault in the excitation transformer is detected by the differential relays that trip the generator and the generator breaker, removing power from the transformer. The transformer is typically small enough compared to the generator rating that separate CT?s aren?t even required on the transformer. This is for short circuit protection, not OCP.
Sometimes, the excitation transformer is considered part of the generator supplier?s system with inherent overload protection.
One way of adding OCP is putting CT's on the cables feeding the transformer or on the transformer HV bushings. Connect the CT's to an overcurrent relay that trips the generator and generator breaker. This is cheaper and more reliable than a fused switch or circuit breaker.
IMO, it might be possible to use the tap rule and not have individual OCP. The generator overcurrent protection would have to be set for less than 400% of 400 kVA (1600 KVA) (NEC 240.21 (B)(3). Also, the generator should not be capable of paralleling with the utility otherwise there would be two sources of power to the transformer: generator and utility.
The 240.21 requirements for secondary conductors also need to be considered.
Is this an excitation transformer connected directly to the generator terminals? (300V secondary is a typical excitation system supply voltage.)
If that?s the case, the excitation transformer is usually included in the generator's differential protection zone. A fault in the excitation transformer is detected by the differential relays that trip the generator and the generator breaker, removing power from the transformer. The transformer is typically small enough compared to the generator rating that separate CT?s aren?t even required on the transformer. This is for short circuit protection, not OCP.
Sometimes, the excitation transformer is considered part of the generator supplier?s system with inherent overload protection.
One way of adding OCP is putting CT's on the cables feeding the transformer or on the transformer HV bushings. Connect the CT's to an overcurrent relay that trips the generator and generator breaker. This is cheaper and more reliable than a fused switch or circuit breaker.
IMO, it might be possible to use the tap rule and not have individual OCP. The generator overcurrent protection would have to be set for less than 400% of 400 kVA (1600 KVA) (NEC 240.21 (B)(3). Also, the generator should not be capable of paralleling with the utility otherwise there would be two sources of power to the transformer: generator and utility.
The 240.21 requirements for secondary conductors also need to be considered.
Lightninarrester
Member
Re: 15KV Transformer Tab
Yes, it is an excitation job and the transformer is a PPT. We are replacing the excitation system with new equipment including the PPT. Thanks for your input guys.
Yes, it is an excitation job and the transformer is a PPT. We are replacing the excitation system with new equipment including the PPT. Thanks for your input guys.
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