6. Frequency Domain Spectroscopy

Frequency domain spectroscopy (FDS) is a non-destructive method of assessing the insulation condition of high voltage equipment, especially oil-paper insulation system of transformers. It is a reliable tool for predictive maintenance of such equipment in the field. In this method each test object is considered as a black box accessible only through its terminals. Consequently this method identifies the global changes in the insulation condition and is not suitable for identifying any local defect in the insulation system. FDS method involves the application of a sinusoidal voltage across the terminals of the test object and the measurement of the amplitude and phase of the response current flowing through the insulation. From the measured voltage and current, the complex capacitance, complex permittivity and dielectric dissipation factor of the insulation are determined over a wide frequency range. Based on the fundamentals of the methodology, it is expected that the presence of polar substance such as moisture could be detected by FDS. It is also sensitive to other polar by-products of insulation aging like low molecular weight acids. FDS results are affected by the geometry of the insulation system, i.e. the relative dimensions of spacers, barriers and oil ducts. The external factors that affect FDS most are operating temperature and weather conditions like rain. For on-site testing significant errors can be introduced in FDS measurement, if certain precautions are not taken, particularly when different equipments are connected in parallel with the transformer. Mathematical modeling is important for the interpretation of FDS results for the estimation of moisture content in solid insulation. It is also relevant to analyze the relationships between time domain and frequency domain test results under practical conditions in order to determine the condition of the transformers and also to establish general validity of the test results.