WYE-WYE Transformer

[Donalddc]

Has anyone tested a wye-wye transformer? What type of issues did you run into? Since the neutrals are connected together inside the transformer what kind of testing can you do without draining and separating the neutrals? Any information on testing a wye-wye would be appreciated.

[MakinThatBacon]

Has anyone tested a wye-wye transformer? What type of issues did you run into? Since the neutrals are connected together inside the transformer what kind of testing can you do without draining and separating the neutrals? Any information on testing a wye-wye would be appreciated.

Yes, sometimes they have a primary neutral bonding switch to disconnect the primary from the secondary. If they don't you test and test voltages are limited to secondary voltages, besides excitation. Typically you can only perform voltage tests to frame (power factor and polarizing index tests). These are typically the cheapest of liquid filled transformers since they have fewer options. But, other tests like excitation, TTR and winding resistance will work fine. If the neutral is internally bonded to ground then you cannot perform PF or PI tests. Most customers don't want to drain a tank to perform these tests. Hopefully this is helpful.

[Donalddc]

Yes, sometimes they have a primary neutral bonding switch to disconnect the primary from the secondary. If they don't you test and test voltages are limited to secondary voltages, besides excitation. Typically you can only perform voltage tests to frame (power factor and polarizing index tests). These are typically the cheapest of liquid filled transformers since they have fewer options. But, other tests like excitation, TTR and winding resistance will work fine. If the neutral is internally bonded to ground then you cannot perform PF or PI tests. Most customers don't want to drain a tank to perform these tests. Hopefully this is helpful.

Thank you so much. That was a great help.

[TestingProToBe]

Has anyone tested a wye-wye transformer? What type of issues did you run into? Since the neutrals are connected together inside the transformer what kind of testing can you do without draining and separating the neutrals? Any information on testing a wye-wye would be appreciated.

I have been running into the same issues. And what stinks is if the N is grounded internally the nameplate ususally doesn't tell you! Only after about an hour of scratching your head wondering why your insulation tests (PF and Megger) aren't working, you realize the N is internally grounded. My thinking at this point is if you see a Wye-Wye transformer, don't perform any insulation testing, you just end up wasting a bunch of time.