Analysis of Causes for 110kV Power Transformer Load Test Results Exceeding the Limit

1. Introduction

The load test is one of the most important routine tests for power transformer. The paper analyzes and discusses why load test results of two 110kV power transformers exceed the limit. The causes for load loss and short circuit impedance data exceeding the limit are worked out. The manufacturer is required to improve the power transformer. Finally, improved transformers pass the delivery tests.  

When the transformer operates, the current passing through the winding can generate many kinds of load loss, including DC resistance loss of winding and lead, eddy-current loss generated by the lead in the leakage field, unbalanced current loss caused by leakage field, eddy-current loss caused by leakage field in the elements (such as clamp, plate and oil tank etc) and additional loss. As the capacity of power grid increases, the power supply department is becoming stricter with load loss. If the measuring result of delivery test exceeds a limit, the power supply department would refuse to accept the product. Hence, it is evident that the load test of power transformer, one of the most important tests, is very essential.  

The purpose of load test is to determine the parallel operation and calculate the efficiency, thermal stability and dynamic stability of power transformer through measuring the short circuit loss and impedance voltage of power transformer. Moreover, the load test can verify whether the power transformer conforms to technical code and the requirement stated in the contract.