C I R E D 18 th International Conference on Electricity Distribution Turin, 6-9 June 2005 THE IMPACT OF REPETITIVE COMBINED VOLTAGES WITH LOW AND HIGH FUNDAMENTAL FREQUENCIES ON THE AGEING OF CAST RESIN Tom SELS † , Jan DECLERCQ ‡ , Jose LOPEZ-ROLDAN ‡ , Daniel VAN DOMMELEN * , Ronnie BELMANS * † Tuc Rail nv – Belgium ‡ Pauwels Trafo nv – Belgium * K.U.Leuven ESAT - Belgium tom.sels@tucrail.be jan.declercq@pauwels.com daniel.vandommelen@esat.kuleuven.ac.be SUMMARY Due to the increased unbalanced loading of electrical grids, the penetration of power electronics and an increased number of switching phenomena in low and medium voltage grids, it is obvious that electrical insulation systems are no longer subjected to only a 50 Hz, purely sinusoidal voltage. The existence of fast transients, switching impulses or in general combined voltages with at least two clearly distinguishable frequencies affects the ageing mechanisms and expected lifetime of electrical insulation systems. Some typical fast transients superimposed on the main 50 Hz voltage are generated in the laboratory by using a Tesla transformer and supplied to a commonly used cast resin insulated system. One of the most important conclusions confirms an accelerated accumulated ageing mechanism of the resin due to the combined fast transient pulses. This specific ageing is also modelled by an equivalent ageing effect using only a 50 Hz sinusoidal voltage having a crest value of 127 % of the highest peak value ever measured for the superimposed fast transients. INTRODUCTION During the last decades problems related to power quality issues as voltage dips and swells, and even more high frequency (HF) pulses, fast transients superimposed on the 50 Hz main voltage, Figure 1, appears in both the low and medium (MV) or high (HV) voltage grids [1]. -20 -15 -10 -5 0 5 10 15 20 25 30 0 2,5 5 7,5 10 12,5 15 17,5 20 time [ms] Voltage [kV] 0 5 10 15 20 25 30 3,95 4 4,05 4,1 4,15 4,2 4,25 time [ms] Voltage [kV] Figure 1 : Fast transient superimposed to the 50 Hz main voltage - Reproduction in the laboratory These transients not only have a major impact on for instance automated production centres, but also affects the electrical equipment itself, more specifically the electrical insulation materials. This statement is reinforced by several reports of different study committees: many unusual failures of electrical machines, motors and transformer in the MV-grid were reported. Closer investigation mostly points towards the presence of fast transients in these grids. Overvoltages due to switching operations mainly caused by vacuum circuit breakers and –to a lesser extent– SF 6 -circuit breakers are typical examples of fast transient MV power quality problems. The use of those circuit breakers may introduce resonance’s in the MV-grid, resulting in extremely high voltages at a high frequency superimposed on the 50 Hz main voltage (Figure 1). Since the major part of electrical insulation is not developed for this voltage waves, unexpected problems like dielectric breakdown and accelerated deterioration may occur. Until now, no tests are executed to evaluate the insulation subject to such combined HF-HV waveforms. Therefore, a MV test circuit is developed in the laboratory to generate the combined voltage shape and to investigate the ageing effect on insulation materials. The aim is to give a first move in covering the gap between the classical standardized tests at DC or 50 Hz and the impulse tests at higher frequencies. The effect of such kind of waveforms on the development and growing speed of partial discharges in a typical cast resin is already reported in [2]. This paper focuses mainly to the comparison of the ageing mechanism under the combined HF- HV pulses and the 50 Hz ageing of the same cast resin. TEST CIRCUIT & TEST SAMPLES Extended Tesla-Transformer To produce the required waveform of Figure 1 in order to investigate the ageing effect on insulation materials subject to combined HF-HV voltages, an extended Tesla transformer is developed and built in the laboratory [2]. A simplified drawing is shown in Figure 2. The basic element of the circuit is a Tesla transformer TT [3] supported by 4 additional blocks: the 50 Hz main voltage U 50Hz and its coupling capacitor C C to the Tesla circuit (TT), a measuring unit and the test sample C T . A detailed description of the test circuit is given in [2]. Test Samples In order to get a general test object, which can act as a reference through all the measurement, a typical needle-plate set-up is chosen, Figure 3. To avoid unwanted discharges and disruptions of the electric field between needle and plate, the plate is manufactured as a well-shaped aluminium electrode CIRED2005 Session No 1