FAILURE ANALYSIS AND PERFORMANCE ENHANCEMENT OF DISTRIBUTION TRANSFORMERS

Abstract

Distribution transformers (DTs) are the vital links in the chain of components constituting a power distribution network. In general, failure of the DTs adversely affects the activities in the fields of agriculture, education, commerce and industry apart from inconvenience to the public at large. In principle, ensuring the reliable DTs is a two step process; first, technical assessment based on critical component monitoring and diagnostics to identify failure causes and second, taking preventive measures against failure. In this thesis, a comprehensive study has therefore been made to identify causes of failure of the DTs and enhancing their performance. The literature survey revealed that failure of the DTs may be caused by many reasons such as the poor design, poor workmanship during manufacturing (loosely fitted coils, spacers, end insulating rings and tap contacts), inadequate maintenance (loss of coolant, corrosion, accumulation of dirt and oil), substandard material used in manufacturing, ageing, lightening surges and short circuit, etc. The DTs may also fail due to insulation deterioration of winding conductors, magnetic core laminations and transformer oil. The insulation of the winding conductors may get damaged by their sharp edges, badly made joints between coils, mechanical and thermal stresses. The insulation between laminations of the magnetic core may be damaged by the sharp edges of the core and the yoke, metallic fillings or small turning between the laminations, magnetic core saturation, aging of the insulating material, etc. The insulation of transformer oil may be deteriorated by moisture contents in the oil, excessive oil temperature, ageing of the oil, etc. In India, electrical utilities in power deficient, poorly designed and haphazardly expanded power distribution networks have reported high premature failure rate of frequently energized DTs. A Delhi electrical utility has reported failure rate of DTs very less where almost 24-hours power supply is maintained to the customers i.e., rare energization of DTs is required. Therefore, it is envisaged that high premature failure rate of DTs is due to their frequent switching. In poor power distribution networks, the electrical supply trips frequently due to various reasons such as planned and unplanned load shedding, maintenance and extension works, faults, etc. Thus, DTs are required to energize repeatedly for restoring the power in such distribution networks which in turn causes frequent cold load pick up (CLPU) and inrush current in DTs. Abstract The inrush current is generated during energization of the DTs due to magnetic saturation of the core containing residual flux. The inrush current is found to be nonsinusoidal in nature and its magnitude may vary from 10 to 20 times of the rated current. Owing to the slow attenuation of the transients, the effects of inrush current may persist for several seconds before attaining the steady state condition. The CLPU condition is caused by loss of diversity among thermostatically controlled electrical devices during restoration of power in distribution networks. The CLPU condition produces several times higher load current than the rated value which may last upto several hours. The CLPU and inrush current produce more than 3 times electro-magnetic forces on the HV winding conductors of frequently energized DTs for long duration than due to short circuit condition. The thermal time constant of HV windings of the DTs is very low i.e., around 10 minutes, that causes temperature rise in them to be fast. Thus, the high electro-magnetic and thermal stresses may cause elongation/strain due to creep in the HV winding conductors of the frequently energized DTs. The creep is defined as the progressive deformation of a material under constant stress at elevated temperature. The creep in metals occurs at the temperature above 0.3Tm {Tm is the melting point temperature of metal in degree Kelvin) due to plastic deformation as a function of time at constant load. Creep curve has three distinct stages i.e., primary, secondary or steady state and tertiary stage. The slope of the creep curve exhibits creep rate i.e., elongation of the material with time. The steady state creep rate is the most significant design parameter derived from the creep curve which can be used for assessing the performance (life and switchings withstand capability) of the DTs. If strain due to creep in the HV winding conductors between radial spacers of adjacent windings on the same limb of the core exceeds the critical creep strain then these conductors may get short circuited which eventually can results in failure of the frequently energized DTs. The critical creep strain is the minimum strain that may cause contact and short circuit between winding conductors of adjacent windings. Thus, creep in HV winding conductors can be considered as one of the causes for failure of the frequently energized DTs. Failure mechanism of the frequently energized DTs due to creep was studied experimentally by conducting the creep tests on HV winding conductors (electric conductor grade Aluminum and electrolytic Copper) under different temperature and stress conditions which typically exist under frequent CLPU and inrush current conditions in 11/0.433 kV, 25 kVA DTs. The general procedures for creep tests were adopted according to the ASTM Specification E139-70. The creep test setup consists of Abstract temperature controlled electric heating based furnace in which the stressed test wire is kept coaxially. The strain in the test wire is measured using digital dial gauge which is further recorded as a function of time elapsed in Microsoft Excel sheet in a computer using data acquisition system. Creep curves were obtained for the HV winding conductors at different temperature and stress conditions by performing the creep tests. Creep rates during steady state stage of creep curves i.e., steady state creep rates were determined for the HV winding conductors. The steady state creep rate is the important parameter in view of life of the DTs. Dependence of the steady state creep rates on temperature, stress and materials of HV winding conductor were observed. It was found that steady state creep rates of both the electric conductor (EC) grade Aluminum and electrolytic Copper HV winding conductors rise with increasing the stress and temperature. At the same temperature and stress conditions, the steady state creep rates of EC grade Aluminum HV winding conductor have been found higher than electrolytic Copper HV winding conductor. This fact suggests that EC grade Aluminum HV winding conductor elongates more than electrolytic Copper HV winding conductor under similar operating conditions and hence it can be noted that EC grade Aluminum HV winding conductor is lesser creep resistant than electrolytic Copper HV winding conductor. Life and switchings withstand capability of the frequently energized DTs were estimated by developing algorithms and using the steady state stage of experimentally obtained creep curves for HV winding conductors. The life and switchings withstand capability of the Aluminum wound DTs have been found lesser than the Copper wound DTs. This may be understood due to the fact that EC grade Aluminum HV winding conductor is lower creep resistant than electrolytic Copper HV winding conductor. The life and switchings withstand capability of the DTs were found to lessen at higher operating temperature and stress conditions which in turn suggests that life of the DTs is reduced by energizing them frequently. Elongation due to creep in HV winding conductors of the DTs can further lead to loosening of the conductors. The HV winding conductors may also be left loose during manufacturing due to poor workmanship. It is expected that loose HV winding conductors may be susceptible to vibrations or cyclic loading due to alternating nature of electro magnetic stresses produced in the DTs. The repetitive cyclic loading initiates fatigue in loose HV winding conductors. Fatigue may be defined as a process of progressive and permanent internal damage in a material subjected to repeated loading. iii Abstract Fatigue in loose HV winding conductors may be brutally harmful for the life of frequently energized DTs as the CLPU and inrush current produce alternating electro magnetic stresses of high magnitude on the HV winding conductors during each energization. Fatigue may cause premature failure of DTs when the winding conductors fracture before the design life being exposed to a large number of repetitive cycles loading. Thus, the fatigue in loose HV winding conductors has been identified as one of the causes for failure of the frequently energized DTs. Fatigue tests under axial and flexural stress were performed on HV winding conductors at different values of stress that can be produced during energization of the 11/0.433 kV, 25 kVA DTs. Fatigue life cycles (A/) corresponding to particular stress (S) i.e., S-N curves were plotted for HV winding conductors. The life and switchings withstand capability of the DTs were estimated by developing algorithms and using experimentally obtained S-N curves for HV winding conductors. The life and switchings withstand capability of the DTs have been found to be proportional to the fatigue life (number of cycles) of HV winding conductors. Both the life and switchings withstand capability of the DTs considering fatigue have been found lower for Aluminum wound DTs than Copper wound DTs. The literature revealed many methods for enhancing performance of the DTs. The design and specifications of manufacturing procedures for DTs must be correct, concise and unambiguous. The transformer tank can be made without the chances of wet spot using vapor-phase drying systems. Transformer oil can be made free from moisture and gas contents by oil impregnation systems. Dissolved gas analysis is available as diagnostic and maintenance tool to detect an internal fault. But, the literature does not reveal any preventive measure to reduce failure rate of Aluminum wound DTs due to creep in HV winding conductors. Further, it is proposed that performance of the frequently energized DTs can be improved by using more creep resistant HV winding conductors than those are presently used. However, the life of the Copper wound DTs considering creep have been found more than Aluminum wound DTs because the electrolytic Copper conductor is more creep resistant than EC grade Aluminum conductor. But, the Aluminum wound DTs are universally used due to their lower manufacturing cost than Copper wound DTs. Therefore, an experimental study has been performed on electrical resistivity, creep characteristics and cost of thermo-mechanically processed (TMP) Aluminum alloy (AA) 6060 wire to assess its suitability as winding conductor in place of electric conductor (EC) grade Aluminum wire for prolonged life and increased reliability of the DTs. IV Abstract Resistivity of the TMP AA 6060 was found marginally differ than that of EC grade Aluminum HVwinding conductor which would results into slightly different power losses in the DTs for both the materials. The TMP AA 6060 was found more creep resistant than the EC grade Aluminum HV winding conductor. Cost of the DTs using TMP AA 6060 as HVwinding conductor is found marginally higher than the Aluminum wound DTs but lower than Copper wound DTs. The Aluminum alloy 6060 wire was thermo-mechanically processed by heating the wire at 125 °C for 2 minutes followed by wire drawing operation at ambient conditions until wire diameter was reduced from 2.5 mm to 0.8 mm (similar to the EC grade wire used as winding conductor). The temperature 125 °C and duration 2 minutes for thermomechanical processing are sufficient to ensure successful wire drawing operation for Aluminum alloy 6060 wire of diameter 2.5 mm. This process relieves the residual stresses in the wire that may enhance the creep resistance of the wire. In addition, a new methodology was developed for designing energy efficient DTs of enhanced performance. The energy efficient grading is a cost effective approach for improving efficiency of the DTs by designing them for lower power losses. Technical and economical aspects have been taken into account for designing the energy efficient DTs. The energy efficient DTs have low lifetime cost despite their large capital cost due to low cost of losses over life span. Energy efficient grades from Grade-1 to Grade-5 were assigned to the designed DTs of ratings 16 kVA to 200 kVA by taking two constraints into account; first, maximum allowable power losses in DTs and second, operation of DTs near maximum efficiency at weighted average loading during service. By taking load factor to be 50% in the Indian context, the weighted average loading of the DTs (considering power losses) in service has been found to be 55% offull load. Lower grade DT refers to higher losses than higher grade DT. The high energy efficient grade DTs were designed to have low power losses by using large diameter of the HV winding conductors in construction than the low energy efficient grade DTs. Moreover, the use of more winding material in high energy efficient grade DTs makes them costlier than low energy efficient grade DTs. The additional cost of high energy efficient grade DTs was taken into account by estimating payback period by considering total owning cost (purchase price and cost of power losses) of the DTs, interest rate on extra investment and energy tariff. The payback period can simply be defined as the ratio of the additional cost on account of higher energy efficient grade DTs to the decrease in annual operating expenditures. Abstract The use of large diameter of the HV winding conductor in high energy efficient graded DTs can further reduce the creep and fatigue in HV winding conductors which in turn may further help to increase the life and performance of the DTs. The energy efficient graded DTs were also compared with the Star-labeled DTs (specified by the Indian Bureau of Energy Efficiency) in respect of energy savings and power losses. The energy and power losses were found lesser in the energy efficient graded DTs than the Star-labeled DTs. The various contributions made through this thesis are summarized as follows: i. The creep and fatigue in high voltage winding conductors were identified as the causes for failure of frequently energized distribution transformers that were verified by experimental results. ii. The life and switchings withstand capability of distribution transformers were calculated by analyzing experimental creep and fatigue results of high voltage winding conductors. iii. The life and switchings withstand capability of Aluminum wound distribution transformers were found lower than Copper wound distribution transformers. iv. A new thermo-mechanically processed Aluminum alloy 6060 was developed for using as high voltage winding conductor alternative to EC grade Aluminum high voltage winding conductor for enhancing performance of the Aluminum wound DTs. v. A new methodology has been developed for designing the energy efficient distribution transformers of improved performance. Energy efficient grades have been assigned to the distribution transformers on the basis of maximum allowable power losses and their operation near maximum efficiency at weighted average loading during service.