ower transformers are one of the most expensive elements in a power system, and their failure is a very costly event. Power transformers are mainly involved in energy transmission and distribution. Unplanned power transformer outages have considerable economic impact on the operation of electric power networks. To have reliable operation of transformers, it is necessary to identify problems at an early stage before a catastrophic failure occurs. In spite of corrective and predictive maintenance, preventive maintenance is gaining due importance in the modern era, and it must be taken into account to obtain the highest reliability of power apparatus such as power transformers. The well-known preventive maintenance techniques such as dissolved gas analysis (DGA), thermal monitoring, partial discharge measurement, capacitance and tan delta measurements, frequency response analysis, etc. are performed on transformers for a specific type of problem. DGA is a very efficient and reliable tool for the earliest detection of inception faults in transformer and other electrical equipment using insulating oil. Condition monitoring of electrical equipment, such as transformers, helps users in many ways such as planning of maintenance schedule, obtaining knowledge of the health of equipment, estimating the remaining service life of equipment, finding areas of further improvement, refining product specification, etc. Kraft paper (cellulose) immersed in mineral oil is used as the insulation system for the copper windings in large power transformers. As the system ages under load, the paper and oil can degrade, potentially leading to catastrophic failure. Partial discharge (PD) is an important tool for improving the reliability of HV insulation systems. It is a very sensitive and nondestructive method of evaluation of the health of the insulation of any HV equipment. PD always is associated with the degradation of insulation systems in HV equipment. Therefore, PDs need to be detected, measured, located, and reduced to a safe value so that the quality of the insulation system is not affected.

Condition Monitoring of Power Transformers - Bibliography Survey

Power transformer plays a vital role in electrical power system. Many of the transformer failures reported are due to insulation failure in windings. The sources of failures are mainly due to impulse overvoltages of different waveshapes and also due to repeated application of impulses. In this paper an attempt has been made to analyze the degradation of transformer insulation for repeated applications of different impulse voltages. As oil impregnated paper (OIP) constitute major insulation of transformer winding, detailed experimental analyses are carried out for OIP insulations using standard test cell for impulse waves of varying front times from 0.064 μs to 1.4 μs and tail times from 6.0 μs to 67 μs. Exponential mathematical models for the voltage-number (V-N) characteristics are derived and used for predicting the number of impulses that OIP can withstand for any wavefront and wavetail. The accuracy of the models are checked with the experimental values.

Effect of Repeated Impulses on Transformer Insulation

One of the cellulose insulation in high voltage power transformers is paper, and it is widely used in transformers due to low cost and excellent physical/electrical properties. However, the performance of paper could be affected by ageing. Ageing of paper is a complex phenomenon and can be influenced by many factors. This PhD thesis aimed to examine the ageing of paper in transformers based on Tensile Index (TI), dielectric strength and Low Molecular Weight Acid (LMA) measurements.The effect of ageing on the dielectric strength of paper was examined, through studying the influence of paper and oil ageing by-products such as LMA and HMA on the AC breakdown voltage. Meanwhile, the end of life for transmission transformer population in the UK was examined by studying the profile of LMA and TI of paper in scrapped transformers, with phase, winding, and layer locations taken into consideration. The relationship between TI, LMA and transformer age was modelled using previously published formulae in literature. The partitioning of LMA and HMA between mineral oil and paper was also investigated. Since TI is not directly measureable when a transformer is in-service, a TI prediction model was developed based on LMA measured in oil.The results indicated that there is no reduction effect of ageing (moderate towards severe) on the dielectric strength of paper, whereas the dielectric strength of paper is mainly influenced by the oil condition. LMA significantly reduces the breakdown voltage and accelerates the ageing of paper in mineral oil. The end of life based on TI is best represented by the 20 % retention of TI. Based on the case studies, the estimated end of life of transmission transformer population in the UK is around 43 years. The vertical distribution profile of LMA in paper and TI of paper along a winding is mainly influenced by vertical temperature profile. As paper ages, the amount of LMA in paper tends to increase as TI reduces. The percentage of LMA in service aged oil is about one fourth of Total Acid Number (TAN). The partition of LMA in mineral oil and paper favours its stay in paper and is heavily influenced by the weight ratio of paper and oil.The performance of the natural ester under high temperature ageing in open or sealed condition was evaluated based on TAN and viscosity measurements. The ratio of LMA and HMA in aged natural ester was examined. The effect of LMA and HMA on the AC breakdown voltage of natural ester and ageing of paper was also investigated. It was suggested that natural ester is suitable for application in hermetically sealed transformers. High temperature oxidation has significant effect on the physical and chemical properties of natural ester. Due to the polar nature of natural ester, the stay of LMA in natural ester is favoured over in paper which could be the reason why the breakdown voltage of natural ester and the ageing of paper are not significantly affected by LMA as in mineral oil.

Ageing assessment of insulation paper with consideration of in-service ageing and natural ester application

In comparison with the AC and DC working voltages, the effect of switching impulse voltage on insulation is very different because of its fast-rising rate and large amplitude. This paper mainly focuses on the characteristics of PD in oil-impregnated paper insulation under switching impulse voltage. First, a robust experimental platform was set up including the impulse power source, test samples with uniform gas and oil gap, and the signal acquisition and processing system. PD characteristics in the air and oil gap are analyzed and compared involving the time-domain waveforms, the variation tendency of the average amplitude and numbers of PD pulses, the occurrence time and the instantaneous value of impulse corresponding to the first PD. Based on the dielectric barrier discharge (filament) theory, a parallel microcircuit model was established to explain the similarities and dissimilarities, and to evaluate the gap discharge intensity. The occurrence and development of PD under switching impulse voltage of typical oil-impregnated paper defect were studied systematically with simulation, experiments, and modeling.

Partial discharge characteristics of uniform gap in oil-impregnated paper insulation under switching impulse voltage

Oil impregnated cellulose paper (OIP) constitute major insulation of transformer windings The cellulose insulation determines the lifespan of a transformer Many of the transformer failures are mainly due to frequent application of over-voltages It is necessary to analyze the degradation of OIP with respect to time interval between the impulse overvoltages Cellulose degradation is usually characterized in terms of scission fraction which is given by degree of polymerization (DP) In this paper, an attempt has been made to analyze the degradation of OIP for different impulse voltages and time interval between the successive impulse voltages Detailed experimental analyses are carried out for impulse overvoltages of 12/50µs, 04/50µs and 0064/50µs by maintaining the time interval of 1 second, 3 seconds and 5 seconds between the successive impulses

Effect of repeated impulses on OIP

Impulse testing of transformer involves a defined sequence of impulse applications at the specified voltage levels. However, in case of failure/damage during these tests, the winding gets subjected to repeated impulse shots, after repair. In this paper the effect of such repetitive impulses on transformer insulation has been investigated. Typical thicknesses of insulating paper as well as disc-to-disc winding models have been subjected to repetitive impulse stresses as encountered in actual service. The performance has been monitored at regular intervals by measurement of tensile strength and degree of polymerization (DP) for paper and partial discharge (PD) parameters of winding models.

Effect of repetitive impulse voltage on models of EHV transformer windings to determine ageing of paper insulation

Transformers play an important role in power transmission. It is, therefore, necessary to know the condition, once the transformer becomes aged. Presently, residual life assessment (RLA) methods are being extensively used to know its condition. The insulation system of a transformer consists of mainly oil and paper; and hence, the conditions of these two materials are critical for reliable operation of transformer. Methodologies are available for RLA studies using different parameters like dissolved gas analysis (DGA), furfural content, and degree of polymerization (DP) etc. However, it is also necessary to study the impulse withstandability of cellulose paper and establish its relationship with DP. In this paper, the effect of ageing of cellulose paper on DP, dielectric strength and impulse withstandability has been studied and a methodology has been developed to know the condition of paper vis-a-vis the impulse withstandability and dielectric strength.

T.S.R. Murthy

Papers

Effect of repetitive impulse voltage on models of EHV transformer windings to determine ageing of paper insulation

Study on the effect of impulse voltage withstandability on the thermally-aged transformer paper