Ratio, polarity, and impedance measurements are compared with nameplate data to verify their correctness and to ensure that there is no hidden shipping damage, that the transformer field assembly is correct, and that the transformer is ready for service. In addition, these test data reports become a valuable tool when compared with later diagnostic tests used to assess transformer condition.
Single-phase test procedures can be used to measure the ratio and impedance of two-winding transformers, three-winding transformers, autotransformers, and three-phase transformers. Moreover, in the case of three-phase transformers (with a Wye connection) and grounding banks, zero-sequence impedance measurements are made with the single-phase procedure. Comparisons between measurements are useful when single-phase tests are made on three identical transformers or on each phase of a three-phase transformer, as it is unlikely that each single-phase unit or each phase of a three-phase transformer would have sustained the same damage.
If equipment terminals are accessible or if the bus is connected to the transformer terminals, conceivably transferring test potentials to other locations, fence off the exposed areas with guards as required by safety procedures, warn working personnel of test-energized potentials, and if necessary provide a Safety Watcher. If possible, ratio test transformers before terminal connections to buses have been made.
SINGLE-PHASE POLARITY
The polarity designation of each transformer winding is determined by the relative direction of instantaneous current or voltage as seen at the transformer terminals. For example, primary and secondary leads are said to have the same polarity when, at a given instant, current enters the primary lead in question, the instantaneous induced voltage in the secondary is increasing when the impressed voltage on the primary is increasing, or conversely, if they are both decreasing at the same instant.
Transformer polarity relates to how winding leads are brought out to bushing terminals. These connections are determined by transformer design, winding directions and internal lead clearance requirements. Transformer polarity is either subtractive or additive. If the instantaneous polarity (as defined above) of adjacent terminals is the same, transformer polarity is subtractive. If diagonally opposite terminals of a transformer have the same instantaneous polarity, transformer polarity is additive.
Transformer winding polarity locations are important when identical windings on a transformer are to be paralleled, when paralleling transformers with identical ratios and voltage ratings, when determining three-phase connections of transformers, and to establish the correct connections for three-phase transformers that operate in parallel with the power system. Polarity between transformer windings may be determined either by comparison with a transformer of known polarity, DC flashing or the AC method. Only the latter two methods are used by TNE.