engleski

Basic Research for Fibre Optic Application in HVDC Transformer

especially in the valve winding. This is due to the fact that the insulation system of the valve winding has to deal with both AC field distribution and DC field distribution. While the AC field distribution is characterized by the electrostatic displacement field the DC field distribution is characterized by the stationary flow field [1]. Consequently in case of DC stress the electrical field is in the material which has the lowest conductivity. In addition to that the polarity reversal (PR) test generates transition effects in the insulation system. The behaviour of these transition effects are unknown by insertion of FO in the insulation system. Researches on FO have shown that the conductivity of FO is much lower compared to oil, paper or pressboard. According to theory the FO will be stressed with a high electrical field which leads to a reduction of the material breakdown withstand of the FO and also in the reduction of the creepage withstand. However, the dimension of the FO is small and simulations according to theory have shown that its influence on field distribution is not high. Due to this fact more researches are necesarry to ensure a save operation of FO used in valve windings. FO of two different manufacturers were used for the researches. First a basic research was done to determine the material parameters. Basic materials in FO are Perfluoralkoxy-Polymere (PFA) and Polytetrafluorethylen (PTFE). The electrical conductivity of these materials was investigated as sheet material and it was shown that the conductivities of PFA and PTFE were similar to each other and always lower than oil, paper and pressboard. In addition to that the conductivity of PFA and PTFE depends on the electrical field strength of the surrounding field and temperature. Further investigations were done to find out the allowable tangential and absolute field strength of the FO. For this purpose the FO were inserted once vertical and once horizontal into the electrodes. Two different types of experiments were done: 1. Rise-in test where the voltage was continuously increased until the electrical breakdown of the FO 2. Endurance test where the voltage was held constant over several hours. After these basic researches first design criteria were defined. For the verification of the design criteria a winding assembly model was developed. The model enables testing of the FO in the electrical highly stressed area in an upper winding assembly. The design of the model is conceived so that during testing similar field strength appears as in a big HVDC transformer. Three models were made for testing: the first model without FO to check the design of the model, the second with FO from the manufacture 1 and the third with FO from the manufacture 2. The models have the same drying and impregnation process like a transformer. Models were tested with DC and PR (according to IEC Standard) at various temperatures. In addition, long-duration tests with DC of at least 6h and PR of at least 12h 45min were performed. The criterion for a successful test was the adherence of the PD requirement according to IEC. All three models have passed the tests successfully.

HVDC-transformer; Fibre optic; Temperature measurement; Winding assembly; DC field

nije evidentirano