CONDITION ASSESSMENT OF MEDIUM-POWER TRANSFORMERS USING DIAGNOSTIC METHODS: PDC, FDS, FRA TO SUPPORT DECISION TO MODERNIZE OR REPLACE SERVICE-AGED UNITS

SUMMARY Privatization of distribution companies induced the new owners to assess technical condition of their transformers, and to decide on modernization of older units or sending them to scrap. Such decision is based on an evaluation of the cost of modernization, and operating costs expressed in percents of a new transformer cost. Specific features of the transformers manufactured in Poland some thirty years ago, such as a generous safety margin of HV insulation and a relatively low load of many units, have been taken in consideration. In general, transformers manufactured before 1965 have excessive no-load loss since the core was made of hot-rolled steel. Further operation of such units is not justified considering a high capitalizedcost of no-load loss. Cellulose insulation of some older units has lost its elasticity, and the reduced the initial winding compression. This has rendered the transformer prone to winding displacement by dynamic force of short-circuit current. Such deformation is detected by measurement and analysis of winding frequency response (FRA). Replacement of locally made on-load tap changer (OLTC) by a new one obtained from renomme manufacturer is recommended, but may not be justified in older units, since the old OLTC can be refurbished. The transformer condition assessment involves such factors as ageing of transformer solid insulation, water content in cellulose, as well as oil degradation and sludge in oil ducts. These are determined using dielectric polarization methods: polarization and depolarization current (PDC), frequency dielectric spectroscopy (FDS), recovery voltage measurements (RVM), as well as dissolved gas in oil analysis (DGA), complete physico-chemical oilanalysis and measurement of furans content. Results of the condition assessment allow the utility to dress a ranking list of their transformers, and to plan replacement, modernization or scrapping of older units, depending on their critical importance to the network reliability and the scope of required modernization.