To ensure the proper functioning of transformers, transformer voltage ratio and polarity are essential parameters that need to be measured accurately. Transformer voltage ratio and polarity are essential parameters that need to be measured accurately. It is required for efficient power transmission and distribution, and for safe operation of equipment that measures the voltage ratio and polarity accurately.
Methodology for Voltage Ratio and Polarity measurements is the same for single-phase as for two- and three-phase machines Before energizing the test circuit, everyone in the work area must be informed and advised to stay in the clear while electrical tests are being performed. When selecting the
actual voltages to be measured and recorded on the datasheet for the ratio test, make sure all voltages will stay below dangerous levels (there may be a step-up voltage hazard). It is not practical, and usually not possible, to attempt these tests at full-rated voltages with the
available test apparatus.
Also, note that one winding is connected phase to phase and the other is the phase to neutral. Be aware that the expected voltage is based on the transformer winding turns ratio and not the kV ratio (primary to secondary). Exercise caution, because misapplication of voltage could generate lethal voltage at the transformer terminals. Because of the hands-on nature of this test, apply test voltages to the highest voltage winding rather than the
other way around. From a practical standpoint, once single-phase ratios have been determined, it is usually not necessary to Ratio test three-phase transformers by applying low-magnitude three-phase voltage.
The polarity of the windings is easily checked at the same time the voltage ratio is being
tested. The important concern when performing a polarity check is to tie the non-polarity ends of the two separate windings together and measure across the polarity ends. The resultant voltage will either be the sum or difference of the two separate winding voltages, a sum indicating additive and a difference indicating subtractive polarity. Transformers above 500 kVA are built with subtractive polarity.