A Comparison of Instrumentation for Measuring the Losses of Large Power Transformers

The results of a comparison of several different types of equipment and techniques for measuring the short circuit (copper) and open circuit (iron) losses of a large power transformer are presented. The measurements were made on a 233-MVA, 735-kV, single phase, 60-Hz power transformer with a short circuit power factor of 1.3%, at currents up to rated, and open circuit voltages to 115 percent of the rating. The short circuit tests were performed using two current comparator type high-voltage capacitance bridges, a thermal wattmeter, an electro-dynamic wattmeter system with conventional instrument transformers and a new semi-automated digital wattmeter system. For the open- circuit tests only the three wattmeters were employed. The average discrepancy between the five measuring systems used in the short circuit tests was less than 1% and between the three systems in the open circuit tests less than 0.5 %.