In this section, we describe the O&M practices that can enhance the life of a transformer.
Transformer Operation
The following factors need to be kept in mind while operating a transformer:
A. Overloading of distribution transformer should be avoided.
•Inadvertent burden of extra load due to unauthorized connections or loads should be identified by periodic testing of current in the distribution transformer. The Tong Tester may be used for this purpose at peak hours and other times of the day. Alternatively,maximum demand ammeter may be connected to exactly determine the maximum load current drawn from the transformer. In the event of extra load, transformer failure can be prevented and its life enhanced by
− transferring the load to the nearby transformer;
− enhancement of the existing transformer capacity; or
− installing a new transformer.
Unequal loading in three phases may also cause overloading in one phase. In such a case, redistribution of the loads, as far as possible equally, among the 3 phases, will prevent transformer failure.
B. Fuse wires (HG fuse and feeder fuses) should be of proper size.
• HG fuse is the only reliable protection for distribution transformers under conditions of fault in LT distribution. The LT fuses should be of a heavy size since these are meant for high currents on the LT side.Then the chances of LT fuses blowing in short time decrease.
• Higher size HG fuses are sometimes used in distribution transformers due to non-availability of proper size HG fuse wire and also because the seriousness of the consequences is not realized. THIS SHOULD NEVER BE DONE because if the HG fuse is of higher size, the fault will sustain for a longer period until the heavy size fuse blows. This can result in increased chances of transformer failure. Even if the fault lies within the transformer, using HG fuse of proper size will minimize the damage to the transformer.
• Notwithstanding the use of HG fuse protection, the use of proper size of fuses on LT feeder depending upon the load has to be ensured to avert transformer failure.
C. Two phasing in rural areas should be prevented.
•Often, two phase supply is maintained on rural distribution feeders to prevent operation of three phase motors for staggering the peak hour loads. However, agriculturists/consumers have invented many methods to start the motor and run it under 2-phase conditions. The total power intake of the motor under the 2-phase condition will be approximately the same as under the 3-phase, contributingunbalance overload in 2 phases. In turn, the distribution transformer will also be subjected to overload in these 2 phases and the core of the transformer will have unbalanced magnetic field in the region of saturation point. This will also cause transformer failure and needs to be checked.
•Similarly, in all the areas covered by the distribution lines with 2-phase arrangements, all the single phase lighting loads are dumped in these 2 phases so that supply is available under all conditions. This leads to heavy unbalanced current through the neutral conductor and the transformer is likely to be overloaded in these 2 phases. Further, the flow of unbalanced current in the neutral conductor will raise the potential of the neutral with respect to the earth which is dangerous to the consumers.In both these cases, adopting 3-phase balance load scheme should enhance both the life and efficiency of the transformer as it avoids unnecessary overloading of 2 phases.
D. Thin breakable diaphragm should be used in the explosion vent. Use of metallic diaphragm will result in the explosion of the transformer itself due to the development of high pressure. To avoid this, a thin breakable diaphragm should be used in the explosion vent. This will cause the explosion of only the vent under high pressure conditions. It will avert transformer failure on this account.
E. The correct diversity factor for loads should be adopted.The value of diversity factor (DF) is assumed for different categories of load to decide the capacity of the transformers. Due to reduced hours of supply in the hours of critical power generation, the actual DF is less than the assumed value. This leads to overloading of the transformers and results in their failure. Transferring load from an overloaded transformer to another transformer or replacing it with a new higher capacity transformer will avert such situations and enhance the life and efficiency of the transformer.
F. Non-standard methods should be avoided.
Avoiding the use of non-standard methods can avert transformer failure.We now describe some of these.
• Use of ACSR conductor, bare or enclosed in PVC pipe from the transformer bushing as against insulated PVC cables.
• Use of open type fuse for the secondary control of the transformer as against the standard porcelain fuses.
• Use of single fuse to control more than one feeder.
• Use of Aluminium strands of ACSR conductors as fuses in LT open type fuses and HG fuses as against tinned copper.
Having learnt about the correct ways of transformer operation, you may like to know: What methods are used for transformer maintenance?
Maintenance Methods
We describe these methods, in brief.
• Prevention of Tree Fouling on LT Lines: Sustained tree fouling with LT conductors may result in conductor snapping or cracked LT Pin Insulators. This causes heavy earth fault current, which may lead to transformer failure. Regular tree cutting and trimming will avoid such failures.
• Prevention of Tree Fouling on HT Lines: Tree fouling on HT lines may cause failure of transformers due to flow of earth fault current. This is because the primary of all the transformers are delta-connected and all the 3 windings from cluster of transformers connected to this HT distribution line will feed the fault apart from the source of EHT substation (EHTSS). Monitoring and regular tree cutting is the solution to this problem for enhancing the life of the transformer .
• Maintenance of Breather: Non-provision of fly-nuts for the breather container will create a gap through which moisturised air will enter into the transformer tank. To arrest this gap, neophrine gaskets should be provided instead of rubber gaskets. Rubber gets damaged if it comes in contact with transformer oil. If oil is not filled in the breather, then the dust particles will not be absorbed from the air entering the transformer tank,causing transformer failure. Hence, proper maintenance of breather will
prevent such failures.
• Removal of Water Condensate in the Transformer: Due to absorption of moisture from atmosphere over a long period of time, a large quantity of water may collect in the transformers. Since water has higher density it gets collected at the bottom of the transformer. Indeed water level can even reach the bottom level of the windings, resulting in failure of transformers. Sometimes it leads to bursting as well. This is because of silt formation at the bottom of oil, which prevents escape of gas formed, resulting in bursting of the bottom of the tank. Occasional draining of oil from the bottom of the transformer will check collection of such large quantity of water. The conservator tank also acts as the collector of water-condensate of moisture entering through the breather. This should be removed before it contaminates the oil and causes transformer failure.
• Prevention of Oil Leakage in Bushings or Any Other Weak Part of the Transformer: Oil leakage may be due to excessive heating or pressure that may develop in the bushing. This will bring down the oil level in the tank. Moreover, moisture will find its way into the tank through the aperture from which oil is oozing. This can result in the contamination of the oil in the tank and hence in the deterioration of HV/LV insulation and ultimately lead to transformer failure. To avoid this, bimetallic clamps with proper size of bolt and nuts connected to the LT bushing may be used, which will reduce excessive heating and damage to bushing rods.Reduction in heating will lead to higher life and efficiency of the transformer.
• Avoiding Low Oil Level: Poor visibility of the oil level in the glass level gauge due to accumulated dust, etc., may not show the exact level of oil in the tank. Oil is likely to go below the core level and the jumper wire from core winding assembly to the bushing rod will not be covered with oil. This leads to excessive temperature rise and the failure of inter-turn insulation as well as flashover of the windings. Transformer life and efficiency can be improved by avoiding such excessive temperature rises.
•Avoiding High Oil Level: Oil should be filled upto the marking in the conservator tank. There should be space in the conservator tank for expansion of oil when the transformer is loaded. If the conservator tank is completely filled with oil, the transformer may fail due to high pressure resulting in explosion of vent pipe. Maintaining correct oil level will, thus, enhance transformer life by avoiding such failure.
• Prevention of Low BDV of the Oil: The Breakdown Value/Voltage (BDV) of the transformer oil may become very low due to oil contamination. This results in the increase of the carbon content and decrease of resistance in the oil. Since the temperature of the oil remains the same, acidity of the oil increases resulting in deterioration of insulation of the windings and transformer failure. To prevent this, oil has to be filtered in order to remove the dust and processed through the reclamation plant to reduce the acidity and improve the BDV value of the oil. The minimum BDVs for different voltage ratings of transformers are given in Table
The following factors need to be kept in mind while operating a transformer:
A. Overloading of distribution transformer should be avoided.
•Inadvertent burden of extra load due to unauthorized connections or loads should be identified by periodic testing of current in the distribution transformer. The Tong Tester may be used for this purpose at peak hours and other times of the day. Alternatively,maximum demand ammeter may be connected to exactly determine the maximum load current drawn from the transformer. In the event of extra load, transformer failure can be prevented and its life enhanced by
− transferring the load to the nearby transformer;
− enhancement of the existing transformer capacity; or
− installing a new transformer.
Unequal loading in three phases may also cause overloading in one phase. In such a case, redistribution of the loads, as far as possible equally, among the 3 phases, will prevent transformer failure.
B. Fuse wires (HG fuse and feeder fuses) should be of proper size.
• HG fuse is the only reliable protection for distribution transformers under conditions of fault in LT distribution. The LT fuses should be of a heavy size since these are meant for high currents on the LT side.Then the chances of LT fuses blowing in short time decrease.
• Higher size HG fuses are sometimes used in distribution transformers due to non-availability of proper size HG fuse wire and also because the seriousness of the consequences is not realized. THIS SHOULD NEVER BE DONE because if the HG fuse is of higher size, the fault will sustain for a longer period until the heavy size fuse blows. This can result in increased chances of transformer failure. Even if the fault lies within the transformer, using HG fuse of proper size will minimize the damage to the transformer.
• Notwithstanding the use of HG fuse protection, the use of proper size of fuses on LT feeder depending upon the load has to be ensured to avert transformer failure.
C. Two phasing in rural areas should be prevented.
•Often, two phase supply is maintained on rural distribution feeders to prevent operation of three phase motors for staggering the peak hour loads. However, agriculturists/consumers have invented many methods to start the motor and run it under 2-phase conditions. The total power intake of the motor under the 2-phase condition will be approximately the same as under the 3-phase, contributingunbalance overload in 2 phases. In turn, the distribution transformer will also be subjected to overload in these 2 phases and the core of the transformer will have unbalanced magnetic field in the region of saturation point. This will also cause transformer failure and needs to be checked.
•Similarly, in all the areas covered by the distribution lines with 2-phase arrangements, all the single phase lighting loads are dumped in these 2 phases so that supply is available under all conditions. This leads to heavy unbalanced current through the neutral conductor and the transformer is likely to be overloaded in these 2 phases. Further, the flow of unbalanced current in the neutral conductor will raise the potential of the neutral with respect to the earth which is dangerous to the consumers.In both these cases, adopting 3-phase balance load scheme should enhance both the life and efficiency of the transformer as it avoids unnecessary overloading of 2 phases.
D. Thin breakable diaphragm should be used in the explosion vent. Use of metallic diaphragm will result in the explosion of the transformer itself due to the development of high pressure. To avoid this, a thin breakable diaphragm should be used in the explosion vent. This will cause the explosion of only the vent under high pressure conditions. It will avert transformer failure on this account.
E. The correct diversity factor for loads should be adopted.The value of diversity factor (DF) is assumed for different categories of load to decide the capacity of the transformers. Due to reduced hours of supply in the hours of critical power generation, the actual DF is less than the assumed value. This leads to overloading of the transformers and results in their failure. Transferring load from an overloaded transformer to another transformer or replacing it with a new higher capacity transformer will avert such situations and enhance the life and efficiency of the transformer.
F. Non-standard methods should be avoided.
Avoiding the use of non-standard methods can avert transformer failure.We now describe some of these.
• Use of ACSR conductor, bare or enclosed in PVC pipe from the transformer bushing as against insulated PVC cables.
• Use of open type fuse for the secondary control of the transformer as against the standard porcelain fuses.
• Use of single fuse to control more than one feeder.
• Use of Aluminium strands of ACSR conductors as fuses in LT open type fuses and HG fuses as against tinned copper.
Having learnt about the correct ways of transformer operation, you may like to know: What methods are used for transformer maintenance?
Maintenance Methods
We describe these methods, in brief.
• Prevention of Tree Fouling on LT Lines: Sustained tree fouling with LT conductors may result in conductor snapping or cracked LT Pin Insulators. This causes heavy earth fault current, which may lead to transformer failure. Regular tree cutting and trimming will avoid such failures.
• Prevention of Tree Fouling on HT Lines: Tree fouling on HT lines may cause failure of transformers due to flow of earth fault current. This is because the primary of all the transformers are delta-connected and all the 3 windings from cluster of transformers connected to this HT distribution line will feed the fault apart from the source of EHT substation (EHTSS). Monitoring and regular tree cutting is the solution to this problem for enhancing the life of the transformer .
• Maintenance of Breather: Non-provision of fly-nuts for the breather container will create a gap through which moisturised air will enter into the transformer tank. To arrest this gap, neophrine gaskets should be provided instead of rubber gaskets. Rubber gets damaged if it comes in contact with transformer oil. If oil is not filled in the breather, then the dust particles will not be absorbed from the air entering the transformer tank,causing transformer failure. Hence, proper maintenance of breather will
prevent such failures.
• Removal of Water Condensate in the Transformer: Due to absorption of moisture from atmosphere over a long period of time, a large quantity of water may collect in the transformers. Since water has higher density it gets collected at the bottom of the transformer. Indeed water level can even reach the bottom level of the windings, resulting in failure of transformers. Sometimes it leads to bursting as well. This is because of silt formation at the bottom of oil, which prevents escape of gas formed, resulting in bursting of the bottom of the tank. Occasional draining of oil from the bottom of the transformer will check collection of such large quantity of water. The conservator tank also acts as the collector of water-condensate of moisture entering through the breather. This should be removed before it contaminates the oil and causes transformer failure.
• Prevention of Oil Leakage in Bushings or Any Other Weak Part of the Transformer: Oil leakage may be due to excessive heating or pressure that may develop in the bushing. This will bring down the oil level in the tank. Moreover, moisture will find its way into the tank through the aperture from which oil is oozing. This can result in the contamination of the oil in the tank and hence in the deterioration of HV/LV insulation and ultimately lead to transformer failure. To avoid this, bimetallic clamps with proper size of bolt and nuts connected to the LT bushing may be used, which will reduce excessive heating and damage to bushing rods.Reduction in heating will lead to higher life and efficiency of the transformer.
• Avoiding Low Oil Level: Poor visibility of the oil level in the glass level gauge due to accumulated dust, etc., may not show the exact level of oil in the tank. Oil is likely to go below the core level and the jumper wire from core winding assembly to the bushing rod will not be covered with oil. This leads to excessive temperature rise and the failure of inter-turn insulation as well as flashover of the windings. Transformer life and efficiency can be improved by avoiding such excessive temperature rises.
•Avoiding High Oil Level: Oil should be filled upto the marking in the conservator tank. There should be space in the conservator tank for expansion of oil when the transformer is loaded. If the conservator tank is completely filled with oil, the transformer may fail due to high pressure resulting in explosion of vent pipe. Maintaining correct oil level will, thus, enhance transformer life by avoiding such failure.
• Prevention of Low BDV of the Oil: The Breakdown Value/Voltage (BDV) of the transformer oil may become very low due to oil contamination. This results in the increase of the carbon content and decrease of resistance in the oil. Since the temperature of the oil remains the same, acidity of the oil increases resulting in deterioration of insulation of the windings and transformer failure. To prevent this, oil has to be filtered in order to remove the dust and processed through the reclamation plant to reduce the acidity and improve the BDV value of the oil. The minimum BDVs for different voltage ratings of transformers are given in Table
 |
| Minimum Breakdown Voltage Ratings for Transformers of Different Voltage Ratings |
•Preventing Low Insulation Resistance (IR) Value: The insulation resistance may be lowered due to moisture content in the oil and in the winding insulation, and may cause transformer failure. To prevent this, the entire transformer core with windings should be placed in hot air chamber until the moisture content is removed from the core and winding insulation resistance can be measured with the help of a Megger. The minimum safe insulation resistance for different voltage ratings of windings is given in Table.
 |
| Minimum Safe Insulation Resistance in MΩ Ω (Mega-ohm) |
Ensuring appropriate insulation resistance enhances the life of transformer.
• Preventing Loose LT Lines: If the LT lines are very loose or they sag,shorting of LT lines could occur and cause frequent blowing of feeder fuses. It could also cause conductor snapping if proper size of fuses are not used or if the structure is not properly earthed. To prevent this fromhappening, phase separators should be used or lines should be restaged. Loose LT lines should be checked to reduce short circuit stresses on transformer and hence enhance its life.
• Proper Maintenance of Fuse Gaps in HT/LT Side of the Transformer: Improper maintenance of the fuse gap setting results either in frequent blowing of fuses or non-blowing of fuses when required.The desirable fuse gap setting is given in Table . Adequate gap setting will avert transformer failure and hence enhance its life.
 |
| Appropriate Fuse Gap Setting |
In this section, we have discussed major O&M measures required to prevent transformer failure and enhance its life. It is your job to ensure that these measures are implemented on a regular basis.
At this point, you may like to pause and review these measures.
With this discussion on various ways of enhancing transformer life by paying proper attention to its operation and maintenance, we end this unit. In this unit you have studied important aspects related to distribution transformer, reasons of transformer failure, transformer testing and ways of enhancing life and efficiency of transformer by paying attention to the operation and maintenance aspects of these transformers. Let us now present the summary of its contents.
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