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

Polarization and Depolarization Current (PDC) technique is an effective tool to assess the condition of oil-paper insulation system in power transformers. So far the PDC behaviors of mineral oil-paper insulation have been widely investigated. However, with the increasing number of transformer choosing natural ester as its insulation oil, it is important to investigate the PDC characteristics of natural ester-paper insulation to see whether the PDC technique can also be used to assess the condition of new insulation system using natural ester in transformers accurately. In this research, natural esterpaper insulation sample and mineral oil-paper insulation sample were subjected to thermally accelerated ageing experiment at 110°C for 120 days. The PDC characteristics of natural ester-paper insulation sample and mineral oil-paper insulation sample were compared over the ageing process. A new method for assessing the ageing condition of the oil-paper insulation in terms of the depolarization charge quantity was proposed. Results show that the polarization/depolarization current of natural ester-paper insulation sample is higher than that of mineral oil-paper insulation sample with the same ageing intervals. The depolarization charge quantity of both kinds of oil-paper insulation sample is very sensitive to their ageing conditions. There is an exponential relation between the stable depolarization charge quantity of both kinds of oil-paper insulation sample and the degree of polymerization (DP) of paper. The depolarization charge quantity can be used to predict the ageing condition of oil-paper insulation providing the measurement temperature is kept the same.

/pdf/quantitative-analysis-ageing-status-of-natural-ester-paper-511vrn5ot0.pdf

Quantitative analysis ageing status of natural ester-paper insulation and mineral oil-paper insulation by polarization/depolarization current

Frequency domain spectroscopy (FDS) has been used to assess the ageing condition of oil-paper insulation used in transformers. To further understand the ageing process, the reduction in degree of polymerization (DP) of cellulosic paper in itself on the dielectric response was investigated first. It has been found that the reduction in DP itself can alter the dielectric characteristics. The oil-paper insulation was thermally accelerated aged at 110 °C for up to 154 days according to the procedure described in IEEE Guide. Then dielectric characteristics of the thermally aged samples were obtained at different measurement temperatures. The results reveal that the relative permittivity (er') plots of oil impregnated pressboards increase with the reduction in cellulose polymer chain length. er' and tanδ values of oil impregnated pressboards shifts upwards in the lower frequency range (10-3 Hz -10-1 Hz) with different ageing condition, indicating the possibility of utilizing the characteristics of er' and tanδ values at lower frequencies (10-3 Hz-10-1 Hz) to quantitatively characterize ageing condition of oil impregnated pressboard. The parameters including DP, er' and tanδ values at the three characteristic frequencies (10-3 Hz, 10-2 Hz, 10-1 Hz) have been analyzed as a function of the ageing time, showing an exponential relationship based on the best fit. In addition, the er' and tanδ curves of aged oil impregnated pressboard have been observed to be shifted upwards to higher values at lower frequencies when the measurement temperature is increased. A shifting factor αT which describes the relationship between frequency and temperature has been defined. It allows one to shift the master curve of impregnated pressboard with DP obtained at reference temperature to other testing temperatures.

Quantitative analysis of ageing condition of oil-paper insulation by frequency domain spectroscopy

Dielectric response measurements are non-invasive and promising diagnostic methods that are being gradually used for assessing the insulation and aging condition of transformer oil-paper insulation system. It is known that moisture content in oil-impregnated insulation has significant effect on its dielectric response phenomena, and whereas at present there is few research about the characteristics of aged cellulose without the impact of water. In this work, a series of experiments are designed and carried out in well-controlled laboratory. Accelerated thermal aging is performed on oil-immersed pressboard samples with different time intervals (0, 120, 250 and 400 h). Then these pressboard samples with different aging states are put into petroleum ether to remove the residual oil inside, and dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) and scanning electron microscope (SEM) are implemented to characterize the aging status of pressboard samples. Two kinds of dielectric response tests, i.e., the polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), are performed on each oil/water-removed pressboard sample in vacuum and new-oil-impregnated ambient, respectively. Samples with different aging states show quite different dielectric response characteristics in both ambients. With the aging status more serious, the pressboard sample exhibits higher values of polarization/depolarization currents and complex capacitances. It is confirmed that, besides the influence of water induced by aging, the deterioration of cellulose itself also causes the increasing of pressboard conductivity, and hence greatly affects the PDC and FDS test results. Dielectric response diagnostic techniques have a potential to reflect the aging information of solid insulation.

Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

The water tree is one of the key factors leading to the insulation property deterioration in cross-linked polyethylene (XLPE) cables. An accelerated ageing experiment has been performed to produce water trees with the purpose of ageing the full-size XLPE cable under the high-frequency electric field. Based on Polarization and Depolarization Current (PDC) method, the diagnostic tests are employed to evaluate the ageing extent of the cable subjected to water tree at different ageing time. Results show that the average length of water trees is proportional to the ageing time, and thus the experiment is effective to accelerate the growth of water trees. Furthermore, such various diagnostic indicators as the I∗t∼lnt curves, the loss factor at different frequency, have their rules to assess the ageing condition of the cable with the duration of ageing time. Therefore, information obtained from PDC current can be used for determining the ageing extent of the cables.

Condition assessment of XLPE insulated cables based on polarization/depolarization current method

Non-destructive dielectrometry measurement techniques such as polarisation and depolarisation current (PDC) measurement is gaining popularity in power industry to evaluate the insulation condition of transformers. Due to nature of the polarisation species involved in transformer insulation, a perfect PDC measurement would take a very long time, to allow all the interfacial polarisation species to be relaxed. Test duration of 10,000s charging and 10,000s discharging has been considered a reasonable compromise in the context of power transformer insulation. However, in a practical field situation power utilities usually cannot afford such a long measurement time due to economical factors. The authors of this paper explore the possibility of further reducing the PDC measurement time to meet the preference of power industries. Since polarisation and depolarisation are reverse process to each other; it would be possible to estimate oil and paper conductivity from short duration measurements. An experimental investigation has been performed on a distribution transformer and on a model transformer to estimate and compare the accuracy of conductivity computation with different charging and discharging times. These test data helps to decide on the optimal PDC test duration. To help extrapolation of PDC data from short duration to longer duration (i.e. 10,000s), both the current difference and charge difference methods have been adopted. The results from these methods have been critically analysed in this paper.

Optimal Time Selection for the Polarisation and Depolarisation Current Measurement for Power Transformer Insulation Diagnosis

Dielectric testing techniques such as frequency domain spectroscopy (FDS) is currently being widely used by the power utilities for the assessment of the condition of transformer oil-paper insulation (OIP) systems. However, it has been observed that the results of these tests are highly influenced by the operating temperature as well as the ingression of moisture into the paper insulation. The distribution, migration and equilibrium of moisture between oil and paper in transformer insulation system are also highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results due to involvement of these factors. Proper analysis of the dielectric test result is only possible with a thorough understanding of the physical behaviour of the insulation system in response to temperature and moisture. To this end a series controlled laboratory experiments have been performed on a model transformer to study the effects of moisture and temperature on the FDS results. Later on, a possible analytical approach has been discussed to deal with situation in the field.

Effects of moisture and temperature on the frequency domain spectroscopy analysis of power transformer insulation

With the advancement in digital technology, diagnostic techniques based on dielectric relaxation methods e.g. frequency domain spectroscopy (FDS) are currently being widely used by the power utilities for the assessment of the condition of power transformer oil-paper insulation. However, it has been observed that the results of these tests are highly influenced by several factors, more important among them are the temperature of the insulation, moisture, ageing, acidity and even by the geometry of transformer cooling duct. Efforts have been made to study the individual effects of temperature, moisture, ageing and acidity on the FDS measurements. A perusal of current literature reveals that the investigations on the effects of geometrical parameters (dimensions of spacer barrier in oil duct) on FDS characteristics have not been studied extensively. Opportunity has therefore been taken of the current situation by the authors to suggest an alternative experimental method to get this information by conducting experiment on a proto-type insulation model.

Influence of the geometrical parameters of power transformer insulation on the frequency domain spectroscopy measurement

Preventative diagnosis of transformers has become an important issue in recent times in order to improve the reliability of electric power systems. A number of diagnostic techniques such as Return Voltage (RV), Polarization/Depolarisation Current (PDC) and Dielectric Dissipation Factor (tan .) measurements at low frequency are currently available for practicing engineers. This paper outlines the summary of these techniques and a case study with all these measurements. Then the difficulties with the interpretation techniques are highlighted along with the recommendation of an expert system previously developed by the first author of this paper.

J.H. Yew

Papers

Optimal Time Selection for the Polarisation and Depolarisation Current Measurement for Power Transformer Insulation Diagnosis

Effects of moisture and temperature on the frequency domain spectroscopy analysis of power transformer insulation

Influence of the geometrical parameters of power transformer insulation on the frequency domain spectroscopy measurement

How Confident are we With Polarisation Based Diagnostics for Transformer Condition Assessment