On methods of measuring magnetic properties of power transformers

Sammanfattning: The core of a power transformer can weigh hundreds of tons, and represents a significant part of the total ownership cost of the transformer, in terms of both the construction cost and of the monetary cost of losses. It is therefore desirable to have a method of diagnosing the core to ensure optimal performance, and to enable the operator to plan repairs of damaged or defective cores. This study shows that measuring the magnetizing current gives an amplification effect, compared to measuring just the no-load loss, that allows even small variations in the state of the core to be detected. The measurement is performed on several large power transformers, demonstrating that the method is easy to use and that it gives reliable results that are comparable between different transformer designs, though further experiments are necessary in order to fully validate it. Experiments performed on small-scale transformer models show that different types of core faults give different characteristic changes in the magnetizing current, thus making it possible to diagnose the core.The large scale does, however, introduce other phenomena that must be taken into account. The most important of these is that, at high voltages, the capacitances in the transformer become noticeable and must be compensated for. This study describes a method of approximating the capacitive part of the no-load current using naturally occurring harmonics in the supply voltage.Additionally, there is, at present, no satisfactory method of describing the magnetizing current of a three-phase transformer. Because of the non-linearity of the core steel, there is a large difference between the measured no-load current and the actual magnetizing current. Three methods for measuring the magnetizing current in a three-phase transformer are described. However, these methods are only applicable under very special circumstances, and are generally not useful under real conditions. As an alternative, a simulation method is outlined. Measurement results from a transformer can then be compared to the simulation result, and any differences between the two can be interpreted to make a diagnosis of the transformer core. The method is used to simulate a large three-phase transformer, and produces a qualitative agreement. An improvement to the hysteresis model used in the simulation, which could potentially make it possible to make quantitative predictions as well, is discussed.Much work remains before the methods described in this thesis can become fully functional diagnostic tools, but the work shows the feasibility of the project, and the potential benefits. By measuring the magnetizing current it is possible to obtain valuable information about the status of the core. This makes it easier for the transformer operator to plan maintenance, or for the manufacturer to assess the quality of the core, and increases operational reliability and efficiency.