Life estimation of high voltage transformer insulation under composite voltages

Abstract

The HVDC converter transformers are reported to fail generally on the thyristor-valve side, where complex alternating voltage waveforms superimposed with dc voltages would occur. The effect of these waveforms on the failure and endurance of the valve side winding insulation of the transformers is not yet understood, which is investigated here. In this work, the voltage waveforms occurring in a converter transformer are obtained through PSCAD/EMTDC for a typical double pole ±500 kV, 1000 MW DC-link HVDC transmission system based on the CIGRE benchmark system. The waveforms are then amplified using a High Voltage amplifier for experimental breakdown investigations. Using the above experimental setup, a variety of stepped-stress breakdown experiments were conducted, with different step time durations for accelerating life testing, as required by Stepped-stress damage equalization method. Stepped-stress damage equalization method (SS-DEM) has been used for obtaining the lifetime characteristics from failure data. A comparison with life curve under pure sinusoidal and actual alternating voltages with superimposed dc and harmonic voltage has been made to understand why the converter transformers generally failed on the valve side. The results first time give fundamental reasons for the possible failure of the converter transformers and put forth important design considerations. The results also indicate that the insulation between turn to turn and winding to the core of star-connected top transformer on valve side is the most vulnerable, followed by, the insulation between winding to core of delta connected bottom transformer on the valve side. In this work, the normalized v-t characteristics were proposed based on which design stress limit and breakdown stress limit can be decided with adequate safety factors. Life estimation and dielectric characterization of polymers are considered important owing to the widespread use of polymeric insulation worldwide in power cables for electric power transmission. Low density polyethylene (LDPE) is one of the most widely used insulation polymers derived from ethylene. Hence it is important for manufacturers and utilities alike to evaluate the ageing and dielectric response of LDPE. In this work, the evaluation of ageing was done on two different materials (LDPE and Kraft paper). Thus the material assessment requires a conventional constant stress method to obtain v-t characteristics. However, conventional constant-stress accelerated ageing tests for life estimation are time consuming, cumbersome, and suffer from the disadvantage that some samples may not fail even after a long time. This necessitates censoring the data for life estimation, increasing the complexity of data analysis and leaving some amount of uncertainty due to censoring. Stepped-stress tests together with the Damage Equalization Method (DEM) were reported elsewhere, which proposed equalizing the cumulative damages of several types of tests in linear regression for estimation of endurance coefficient (n). However, probabilistic/statistical features were ignored, which may cause errors in estimation, especially when the differences in various stepped-stress tests are small or when the variance of the damages is large. In view of this, the DEM is completely revised with the introduction of damage as a random variable which resulted in establishing a relationship between the Weibull scale, shape parameters, and the endurance coefficient of inverse power law. The value of n obtained through revised DEM turned out to be critical where all the scale parameters of all the Weibull distributions of damages in different stepped-stress tests are nearly equal. Until now, the endurance coefficient of inverse power law and strength constant were treated as constants. In this work, it is shown that these coefficients do have statistical features. Their values and ranges have been put forth. Experiments were conducted on oil impregnated paper and low density polyethylene samples at different accelerated step stresses for life estimation. The results of the estimation of endurance coefficient have been compared with previous methods and reasonable conclusions are drawn in favor of the proposed method, at the end. The challenge of transmission of generated energy from the offshore wind farms effectively and economically over long distances involves huge investments in High Voltage Direct Current (HVDC) systems. Instead of using an expensive and large power transformer station for the entire wind farm, each wind turbine is connected to a highpower isolated DC-DC converter (for instance Dual Active Bridge (DAB) configuration) to reduce the size of the station. The key component inside such a DC-DC converter is a Medium Frequency Transformer (MFT). The MFT delivers the galvanic isolation requirements, higher power density, efficiency, and cost reduction. The leakage inductor is introduced in the primary side of the MFT for higher power density, however, it emits the high-frequency (HF) voltage distortions which deteriorates the MFT insulation systems. The HF voltage distortions waveforms were obtained by a simulation of a dual active bridge model considering the parasitic capacitances and snubber circuits. The obtained waveforms were used as an input for conducting the accelerated aging test. The purpose of conducting the accelerated aging tests was to confirm the effect of the HF voltage distortions on the degradation of the primary side insulation systems. The breakdown tests were performed using a distorted waveform with frequencies of 2 kHz, 300 Hz, 50 Hz. The results indicate that the degradation of the primary insulation progresses faster under HF voltage distortions waveform. A comparison of the life curves under different frequencies has been made to indicate the reduction in the life of the primary side insulation.