Analysis of Transformer Insulation under High-frequency Transients

Abstract

In recent times, there have been an increase in installation of wind turbines for generation of electricity through renewable energy sources. The wind energy is a clean and cost effective source of generating power, but integrating the wind farm to the grid has brought additional concerns for the insulation of the installed step-up transformers. The wind-turbine step-up transformers are subjected to high-frequency high dV/dt transient voltage due to the operation of power electronic converters and vacuum circuit breakers. These high-frequency transients are responsible for the premature failure of the transformer’s turn-to-turn and layer-to-layer paper-oil insulation. Thus, the current research work addresses the effect of the transient voltage parameters on the degradation of the transformers paper-oil insulation.

The distribution of the transient voltage along the transformer winding as a function of transient voltage parameters including rise time (290 ns and 1000 ns), duty cycle (10% and 50%) and amplitude (100 V and 1000 V) are analysed to obtain the transient voltage that can cause the highest tap-to-tap voltage amplification. The switching frequency of the transient voltage (square wave) is fixed at 1 kHz. The investigation of the transient voltage parameters that lead to the highest tap-to-tap voltage level will help the industry to design the insulation level for the windings of the step-up transformers based on the transient stresses generated in a wind farm chain. The installation of RC snubbers and surge arresters at the transformer terminal will help reduce the transient voltage amplitude, rise time, and switching frequency, but it is very difficult to design an arrestor or an RC snubber for a complicated wind turbine topology, and the installation cost of these RC filters are high. Thus, the transformer winding behaviour as a function of the transient voltage parameters are obtained. The transient voltage with an amplitude of 100 V, rise time of 290 ns, and duty cycle of 50 % had the highest tap-to-tap transient voltage level along the transformer winding. The mitigation technique used to minimize the transient voltage level are designed to reduce the transients at the transformer terminal but the internal voltage of the transformer under the transient voltage stresses can be high due to inter-winding resonance.

In order to analyse the importance of inter-winding resonance, the thesis presented three types of transfer function frequency response for two transformers, T1 and T2. Transformer T1 had been aged under the application of the sinusoidal voltage, and transformer T2 had been aged under the application of the repetitive transient voltage. The transfer function frequency responses obtained were magnitude of voltage difference response in dB, impedance difference response, and voltage ratio difference response. The tap-to-tap difference response for each of the transfer functions were obtained to analyse the inter-winding voltage distribution and respective resonance frequency. All three transfer functions were compared to obtain the sensitivity of these transfer function to distinguish between the two aged transformers. The transformer windings can be modelled as a network of resistance (R), inductance (L), and capacitance (C). The RLC network is due to the transformers inductance (self and mutual), capacitance and resistance of the winding structure, and the dielectric resistance and capacitance of the paper-oil insulation. Various transfer function frequency responses can emphasize on certain elements of these RLC network and ignore certain other elements of the RLC network. These lead to differences in sensitivity for the above mentioned transfer function frequency responses.

Furthermore, the influence of the transient voltage parameters on the ageing of transformer paper-oil insulation is analysed to obtain the degradation level of the insulation under repetitive transient voltages. The dissipation factor, polarisation current, and depolarisation current are the ageing responses used to obtain the degradation level under transient voltage ageing of the transformer paper-oil insulation. The paper-oil insulation was aged under repetitive transient voltage for a long duration of 130 hours with Voltesso 35 as the insulating oil. Two-level, two-factor design of experiments were used to obtain the effect of transient voltage parameters on the ageing of the paper-oil insulation. Rise time and duty cycle were the two factors (parameters) of the transient voltage selected for the ageing experiments. The two levels of rise time were 220 ns and 650 ns, and two levels of the switching frequency were 1 kHz and 3 kHz. The amplitude and the duty cycle of the transient voltage (square wave) are fixed at 6 kV and 50 % respectively. The study of transient voltage parameters that lead to the highest degradation of the transformer paper-oil insulation will help the transformer manufacturer decide the quality of the insulation that needs to be used on the step-up transformer based on the measured transient voltage signature in the wind farm. Additionally, the installation cost of the RC filters are high, thus the above analysis can be used to evaluate the requirement of such expensive filters to be installed at the transformer terminals. The research work obtained that a faster rise time (220 ns) and higher switching frequency (3 kHz) of the transient voltage led to a higher ageing degradation level for the paper-oil insulation. The transient voltage ageing of paper-oil insulation are carried out and compared for two mineral oils, they are, LuminolTM TRi oil and Voltesso 35 oil. The two oils have difference in their electrical and chemical properties, thus their ageing responses under repetitive transient voltage are studied. The ageing of the two mineral oils under transient voltage are studied to obtain their ability to withstand the transient voltage, and to select the best oil that does not degrade/age the paper-oil insulation under the repeated voltage stress. For the transient voltage ageing studies conducted in the current research work, the paper-oil insulation with Luminol TRi as insulating oil performed better compared to the paper-oil insulation with Voltesso 35 as insulating oil. The transient voltage ageing of transformer paper-oil insulation at room temperature is compared with the transient voltage ageing at elevated temperature of 50° C with Voltesso 35 A as the insulating oil. The Voltesso 35 A oil is used to replicate the transformer that has been in service for long duration. The wind turbine step-up transformers undergo thermal stress due to varying load cycle depending on the wind speed. The thermal stress can also be caused due to harmonics generated during the power electronic converter operation. Thus, the transient voltage ageing with additional thermal stress is studied in the current research work. The ageing degradation level of paper-oil insulation stressed under transient voltage at elevated temperature of 50° C was significantly higher compared to the ageing degradation level of paper-oil insulation stressed under transient voltage at room temperature.