rcw retired EE
Electrical
- Jul 21, 2005
- 907
One of two new, identical, 2 MVA, 4.16kV-480/277V delta-wye, KNAN transformers has unusual excitation current tests results prior to energization. Both transformers passed factory tests and most of the usual NETA field tests: insulation power factor (Doble), insulation resistance, TTR, winding resistance, oil test etc.
Following standard Doble procedures, the single phase excitation current was measured. One transformer had no issues but the test set could not build current on unit #2 when trying to energize H1-H3 or H3-H2. Test set would trip. H1-H2 was OK.
A transformer degaussing test set was used to demagnetize the core with limited success. The Doble excitation test set was only able to build up to 100V before tripping. Prior to demagnetization, it could not get even get to 10V.
Our testing company thinks the transformer is going to blow when energized. The manufacturer, a very reputable supplier, says to energize the transformer and that the excitation current test is of no concern. They point out that individual excitation current tests were not specified and are not part of the typical IEEE standard tests. The factory engineers say they have seen this before where two sister units have different excitation currents but were placed in service with no problems. They do not have an explanation as to why there is a difference.
There is no indications of physical impact or damage that might have moved the core or caused magnetic anomalies.
On the good unit, typical excitation current readings at 1.25kV:
H1-H2: 0.49 A, 3.6 kW. H2-H3: 1.05A, 7.97kW. H3-H1: 1.1 A, 7.9kW
Suspect unit tests at 1.25 kV and 0.10 kV:
@1.25 kV: H1-H2: 0.47 A, 3.5 kW. H2-H3: tripped. H3-H1: tripped
@0.100kV: H1-H2: 1.13 A, 4.37 kW. H2-H3: 5.96A, 52.7 kW. H3-H1: 2.56A, 8.61 kW
Factory tests results show 3-phase no load loss at 2kW and excitation current at 0.2% = 4.8A @ 4.16 kV (about 1.45 A @ 1.25 kV)
I tend to go with the manufacturer, they are guaranteeing it. But it would be nice to see an explanation before closing the breaker.
Have you seen any anomalies like this on new units? Could single phase versus three phase tests be skewing the results?
Following standard Doble procedures, the single phase excitation current was measured. One transformer had no issues but the test set could not build current on unit #2 when trying to energize H1-H3 or H3-H2. Test set would trip. H1-H2 was OK.
A transformer degaussing test set was used to demagnetize the core with limited success. The Doble excitation test set was only able to build up to 100V before tripping. Prior to demagnetization, it could not get even get to 10V.
Our testing company thinks the transformer is going to blow when energized. The manufacturer, a very reputable supplier, says to energize the transformer and that the excitation current test is of no concern. They point out that individual excitation current tests were not specified and are not part of the typical IEEE standard tests. The factory engineers say they have seen this before where two sister units have different excitation currents but were placed in service with no problems. They do not have an explanation as to why there is a difference.
There is no indications of physical impact or damage that might have moved the core or caused magnetic anomalies.
On the good unit, typical excitation current readings at 1.25kV:
H1-H2: 0.49 A, 3.6 kW. H2-H3: 1.05A, 7.97kW. H3-H1: 1.1 A, 7.9kW
Suspect unit tests at 1.25 kV and 0.10 kV:
@1.25 kV: H1-H2: 0.47 A, 3.5 kW. H2-H3: tripped. H3-H1: tripped
@0.100kV: H1-H2: 1.13 A, 4.37 kW. H2-H3: 5.96A, 52.7 kW. H3-H1: 2.56A, 8.61 kW
Factory tests results show 3-phase no load loss at 2kW and excitation current at 0.2% = 4.8A @ 4.16 kV (about 1.45 A @ 1.25 kV)
I tend to go with the manufacturer, they are guaranteeing it. But it would be nice to see an explanation before closing the breaker.
Have you seen any anomalies like this on new units? Could single phase versus three phase tests be skewing the results?
