Testing UST, GST, GST-G, IPF/DF

Can't help you -- but you get props for defining your acronyms.

Ungrounded specimen test(ust), grounded specimen test(gst), grounded specimen test with guard(gst-g)

All of those tests are required to do a complete battery of insulation power factor tests on a transformer. Each one tests a different component of the leakage currents between transformer windings and from them to ground. The manufacturer's instructions of the test equipment you would be using should explain in detail which test connection provides which component.

Briefly, GST test on a winding gives its leakage current, watts loss, and power factor to the other winding and ground. UST test gives same between windings but not to ground, and GST-guard test gives values from a winding to ground but not to the other winding. Together these tell you the condition of all the different parts of the insulation system.

THIS DEPENDS ON THE SYSTEM CONFIGURATION AND EQUIPMENT vector group viz

For UST _(H-L)selection -The supply side should be U/earthed/High Impedance grounded-Transformer Vector GroupDy11 -and test between HV wdg-LV wdg.

GST(grounded specimen)H-G -This is the case of EHV syatems higher than 66KV where neutral is solidly grounded.

GST-g(grounded specimen test with guard)-This is applicable for HV bushing Tandelta test.

I beg to differ. System configuration and vector group have no bearing on the tests, unless the transformer is a wye-wye with internal neutral bond that cannot be disconnected. In that case the windings cannot be separated from each other to do the complete set of tests.

GST, UST, and GST-g are all needed to perform tests of a transformer's winding insulation. The transformer is disconnected from all outside bus, conductors, etc. for this testing. UST and GST-g are used for testing the C1 and C2 capacitance layer tests on a condenser-type bushing (not solid porcelain or polymer).

Contact Doble (www.doble.com). They are the de facto experts in power factor testing. Their equipment comes with good manuals, and they have a lot of information on their web site. Interpreting the data is an art, and their engineers can help out with that too. If you do not have a contract with them (they rent the equipment), contact the manufacturer of your test equipment. Some things to keep in mind:

1. Safety! Always make sure the transformer is completely disconnected (but not the core ground bushing, if present), the supply and load side lines grounded, the bushings shorted per winding (H1-H2-H3; X1-X2-X3-X0, e.g.), and the transformer tank grounded to station ground.

2. Whenever possible, compare test results to previous results or test results of sister units.

3. Always enter the nameplate data (transformer and bushings) in the test software.

4. Bushing tests are tricky. Typically, the 69 kV and below bushings do not have a designed C2 capacitance, and it will test differently in the transformer and in a test rack.