As you are aware, the distribution sector has a large number of distribution transformers of various capacities. Any failure of these transformers is bound to cause great inconvenience to the consumers and huge financial losses to the utilities. It is therefore extremely important to avoid transformer failure.
We list below some important reasons for distribution transformer failure.
•Poor Performance
This could be due to
− low oil level;
− draining of oil due to leakage/theft of oil;
− improper earthing;
− frequent faults on LT lines due to loose spans leading to short circuit;
− mechanical failure of winding;
− improper tree clearance of LT lines;
− defective breather and consequent ingress of moisture;
− low electric strength of oil/winding insulation; and
− corrosion of core laminations.
•Improper Protection
This could be the result of
− using defective or over rated fuses;
− consistent overloading; and
− not providing Lightning Arrestors (LAs).
Manufacturing Defects These result from
− improper / inadequate design;
− poor quality of material;
− bad workmanship; and
− poor short circuit withstand capacity.
All these reasons for transformer failure can be classified under the following heads (Fig.):
Ageing,
Manufacturing defects,
Improper structure/erection of distribution transformer,
Improper operation and maintenance, and
Natural calamities.
We now discuss each one of these briefly.
Ageing
The expected life span of the distribution transformers above 100 kVA capacity is about 35 years and that of up to 100 kVA capacity is about 25 years. But experience shows that most of transformer failures begin to occur even before 20 years of its life.Until a few years ago, distribution transformer manufacturers incorporated many more safety factors in design. In recent years, manufactures have adopted the cost-benefit approach in the design of transformers, which just about manages to satisfy the requirements of IS specification. The result is that they compromise on both quality and reliability requirements of IS specification. While the transformers so manufactured meet the requisite standards when tests are conducted before and immediately after installation, they fail to do so after a few years of being in operation due to ageing.
Thus, many transformers are unable to serve the expected full life period and even if they are in service, they are quite likely to fail before the expected full life due to lower reliability. Hence, giving top priority to the replacement of those in-service transformers that have served their full-life period will reduce transformer failure.
Manufacturing Defects
In the past, distribution transformers served for more than 60 years, which is double the life expectancy. But now many distribution transformers fail after a few years of service and have to be repaired twice or thrice during their life time. Many reasons for pre-mature failure of the distribution transformer are related to manufacturing defects. We describe them, in brief.
A. INADEQUATE/POOR DESIGN
The trend of design is now towards lowering the manufacturing costs per unit even if it is at the expense of quality. Moreover, the tender appraisal is mostly confined to the initial cost, with no consideration for maintenance of the transformer up to the end of its fair life period. Consequently the safety factor is affected adversely. The following aspects of transformer design impact transformer failure:
• Transformer tank size: Inadequate clearance for free circulation of oil can lead to abnormal temperature rise, causing great damage to the HV winding insulation and, consequently, premature failure of transformers.
• Percentage impedance (mechanical strength of coil): Most of the distribution transformers are located in remote areas and many a times it is not possible to give special attention to the operating conditions. Harsh conditions can also lead to failure. The solution to this problem lies in designing transformers with large impedance so as to increase theirshort circuit withstand
capacity .
Percentage impedance depends upon the following factors.
Size of wire used in HV coils − Economical size of coil yields lower size gauge wire, but this reduces the mechanical capability of coils. As a result, the coils may not be able to withstand higher current densities which occur during the short circuit conditions.
− Radial distance between HV and LV coils − Increasing the radial distance between HV and LV coils increases the percentage impedance. It also leads to better mechanical strength of the coil to withstand higher short circuit stresses developed during short circuit conditions. But this will lead to higher cost.
− Effect of impedance on the short circuit stresses −The short circuit stresses are proportional to the square of the short circuit current. If the impedance is increased from 4.5 % to 5 % - 5.5%,the effect on the short circuit stresses developed in the transformer is reduced considerably.
• Improper use of aluminium wires: Improper use of aluminium wires leads to HV coil failure. The use of aluminium conductors has been recommended for windings up to 200 kVA transformers. However, the super enamel covering the aluminium wire tends to crack during asymmetrical conditions and leads to coil failure. Hence, the use of higher cross-section conductors with double paper covering would be desirable.
• Improper use of interlayer papers: Coil failure is usually seen as an electrical failure. This generally occurs when interlayer insulation breaks down at the end of the turn and creeps to the next layer. This type of insulation failure can be avoided by using folding papers and reinforcing the end turn insulation with proper sleevings. Uniform separation of HV coil along with the inner coil, using spacers helps to avoid pressing of end turns as well as any further shrinkage during service.
• Use of inferior quality materials: Use of inferior quality wires for coils, poor quality of oil and other insulation material, etc., to bring down the cost of the transformer also increases the probability of failure of the transformer before full life of the transformer. The design calculation of the tenderers should conform to the quantity and grade of input materials of core and windings furnished in the tender. For ensuring this, the transformer should be subjected to strip test. This will make sure that the losses and impedance furnished in the tender have been actually achieved by the transformer.
B. IMPROPER WORKMANSHIP
Apart from poor design, sub-standard execution of a good design also becomes a reason for transformer failure. We now briefly describe some such reasons.
•Improper alignment of HV windings: When a transformer is loaded,the primary and secondary ampere-turns act in magnetic opposition
but are in complete alignment with respect to the core and coils. When current flows through the coils, magnetic field is set up around them,which has an associated magnetic flux. Even a small error in the alignment of either coils, i.e., an asymmetrical ampere-turn balancing,leads to production of cross magnetic fluxes. This results in lower impedance and hence mechanical failure of the coils.
• Improper clamping arrangement: Inadequate clamping arrangements of the HV coils lead to vibrations and movement of the coils during short circuit conditions resulting in failure of HV Coils.
• Improper connections: In many cases, the connecting delta leads to the bushing are not properly supported on the framework, resulting in breakage during trans-shipment or at the time of the first charge of transformer. Moreover, improper soldering of leads will result in open circuit even at normal full load conditions. Also, such transformers may fail while encountering the first fault or after a few faults.
• Inadequate tightening of core: Even with proper fuse protection on the HV side, inadequate tightening will result in failure of transformer due to collapse of the windings. The transformer can fail due to this fault even under minor fault condition in the LT distribution due to mechanical vibration in the core and windings.
We list below some important reasons for distribution transformer failure.
•Poor Performance
This could be due to
− low oil level;
− draining of oil due to leakage/theft of oil;
− improper earthing;
− frequent faults on LT lines due to loose spans leading to short circuit;
− mechanical failure of winding;
− improper tree clearance of LT lines;
− defective breather and consequent ingress of moisture;
− low electric strength of oil/winding insulation; and
− corrosion of core laminations.
•Improper Protection
This could be the result of
− using defective or over rated fuses;
− consistent overloading; and
− not providing Lightning Arrestors (LAs).
Manufacturing Defects These result from
− improper / inadequate design;
− poor quality of material;
− bad workmanship; and
− poor short circuit withstand capacity.
All these reasons for transformer failure can be classified under the following heads (Fig.):
Ageing,
Manufacturing defects,
Improper structure/erection of distribution transformer,
Improper operation and maintenance, and
Natural calamities.
We now discuss each one of these briefly.
Ageing
The expected life span of the distribution transformers above 100 kVA capacity is about 35 years and that of up to 100 kVA capacity is about 25 years. But experience shows that most of transformer failures begin to occur even before 20 years of its life.Until a few years ago, distribution transformer manufacturers incorporated many more safety factors in design. In recent years, manufactures have adopted the cost-benefit approach in the design of transformers, which just about manages to satisfy the requirements of IS specification. The result is that they compromise on both quality and reliability requirements of IS specification. While the transformers so manufactured meet the requisite standards when tests are conducted before and immediately after installation, they fail to do so after a few years of being in operation due to ageing.
Thus, many transformers are unable to serve the expected full life period and even if they are in service, they are quite likely to fail before the expected full life due to lower reliability. Hence, giving top priority to the replacement of those in-service transformers that have served their full-life period will reduce transformer failure.
Manufacturing Defects
In the past, distribution transformers served for more than 60 years, which is double the life expectancy. But now many distribution transformers fail after a few years of service and have to be repaired twice or thrice during their life time. Many reasons for pre-mature failure of the distribution transformer are related to manufacturing defects. We describe them, in brief.
A. INADEQUATE/POOR DESIGN
The trend of design is now towards lowering the manufacturing costs per unit even if it is at the expense of quality. Moreover, the tender appraisal is mostly confined to the initial cost, with no consideration for maintenance of the transformer up to the end of its fair life period. Consequently the safety factor is affected adversely. The following aspects of transformer design impact transformer failure:
• Transformer tank size: Inadequate clearance for free circulation of oil can lead to abnormal temperature rise, causing great damage to the HV winding insulation and, consequently, premature failure of transformers.
• Percentage impedance (mechanical strength of coil): Most of the distribution transformers are located in remote areas and many a times it is not possible to give special attention to the operating conditions. Harsh conditions can also lead to failure. The solution to this problem lies in designing transformers with large impedance so as to increase theirshort circuit withstand
capacity .
Percentage impedance depends upon the following factors.
Size of wire used in HV coils − Economical size of coil yields lower size gauge wire, but this reduces the mechanical capability of coils. As a result, the coils may not be able to withstand higher current densities which occur during the short circuit conditions.
− Radial distance between HV and LV coils − Increasing the radial distance between HV and LV coils increases the percentage impedance. It also leads to better mechanical strength of the coil to withstand higher short circuit stresses developed during short circuit conditions. But this will lead to higher cost.
− Effect of impedance on the short circuit stresses −The short circuit stresses are proportional to the square of the short circuit current. If the impedance is increased from 4.5 % to 5 % - 5.5%,the effect on the short circuit stresses developed in the transformer is reduced considerably.
• Improper use of aluminium wires: Improper use of aluminium wires leads to HV coil failure. The use of aluminium conductors has been recommended for windings up to 200 kVA transformers. However, the super enamel covering the aluminium wire tends to crack during asymmetrical conditions and leads to coil failure. Hence, the use of higher cross-section conductors with double paper covering would be desirable.
• Improper use of interlayer papers: Coil failure is usually seen as an electrical failure. This generally occurs when interlayer insulation breaks down at the end of the turn and creeps to the next layer. This type of insulation failure can be avoided by using folding papers and reinforcing the end turn insulation with proper sleevings. Uniform separation of HV coil along with the inner coil, using spacers helps to avoid pressing of end turns as well as any further shrinkage during service.
• Use of inferior quality materials: Use of inferior quality wires for coils, poor quality of oil and other insulation material, etc., to bring down the cost of the transformer also increases the probability of failure of the transformer before full life of the transformer. The design calculation of the tenderers should conform to the quantity and grade of input materials of core and windings furnished in the tender. For ensuring this, the transformer should be subjected to strip test. This will make sure that the losses and impedance furnished in the tender have been actually achieved by the transformer.
B. IMPROPER WORKMANSHIP
Apart from poor design, sub-standard execution of a good design also becomes a reason for transformer failure. We now briefly describe some such reasons.
•Improper alignment of HV windings: When a transformer is loaded,the primary and secondary ampere-turns act in magnetic opposition
but are in complete alignment with respect to the core and coils. When current flows through the coils, magnetic field is set up around them,which has an associated magnetic flux. Even a small error in the alignment of either coils, i.e., an asymmetrical ampere-turn balancing,leads to production of cross magnetic fluxes. This results in lower impedance and hence mechanical failure of the coils.
• Improper clamping arrangement: Inadequate clamping arrangements of the HV coils lead to vibrations and movement of the coils during short circuit conditions resulting in failure of HV Coils.
• Improper connections: In many cases, the connecting delta leads to the bushing are not properly supported on the framework, resulting in breakage during trans-shipment or at the time of the first charge of transformer. Moreover, improper soldering of leads will result in open circuit even at normal full load conditions. Also, such transformers may fail while encountering the first fault or after a few faults.
• Inadequate tightening of core: Even with proper fuse protection on the HV side, inadequate tightening will result in failure of transformer due to collapse of the windings. The transformer can fail due to this fault even under minor fault condition in the LT distribution due to mechanical vibration in the core and windings.
Improper Structure of Distribution Transformer
IE Rules, 1956 specify various standard clearances to be maintained when distribution transformers are to be erected. The standard clearances adopted for transformer structures will avert its failure (Table ).Non-adherence to these standards makes DTRs prone to failures.
IE Rules, 1956 specify various standard clearances to be maintained when distribution transformers are to be erected. The standard clearances adopted for transformer structures will avert its failure (Table ).Non-adherence to these standards makes DTRs prone to failures.
 |
| Clearances for Transformer Structures |
Impact of Natural Calamities
Heavy lightning: If the HTLAs (HT Lightning Arrestors) fail to divert direct lightning strikes or surges due to discontinuity in the earthing system, the HV winding can fail due to surge voltage or the HT Lightning Arrestor itself may burst .
Bushing flashover: Dust and chemicals carried with air and deposited on the bushings reduce the electric leakage distance and cause flashover.To avoid this, the bushings (both HT and LT) should be cleaned properly at regular intervals. However, cotton waste should not be used for cleaning,as this may cause scratches in the bushing and subsequently lead to flashover of bushing.
Failure due to contact with birds and other animals: To avoid failure of the distribution transformer due to a squirrel crossing it or due to birds sitting on it, the HT/LT bushing and HT/LT jumper leads from the bushing should be covered with yellow tape insulation. This yellow tape insulation will also indicate the overloaded operation of the transformer by the change of colour of the tape from Yellow to Black.
Improper Operation and Maintenance (O&M)
Transformer failure can also stem from poor O&M practices. For example, in addition to normal full load, continuous over-heating and higher no-load losses may reduce the life of the transformer due to reduction in the life of insulating papers, oil, etc. Other improper O&M practices leading to transformer failure are discussed later in the section on enhancing transformer life and efficiency. In Table, we summarise some reasons for transformer failure.
Heavy lightning: If the HTLAs (HT Lightning Arrestors) fail to divert direct lightning strikes or surges due to discontinuity in the earthing system, the HV winding can fail due to surge voltage or the HT Lightning Arrestor itself may burst .
Bushing flashover: Dust and chemicals carried with air and deposited on the bushings reduce the electric leakage distance and cause flashover.To avoid this, the bushings (both HT and LT) should be cleaned properly at regular intervals. However, cotton waste should not be used for cleaning,as this may cause scratches in the bushing and subsequently lead to flashover of bushing.
Failure due to contact with birds and other animals: To avoid failure of the distribution transformer due to a squirrel crossing it or due to birds sitting on it, the HT/LT bushing and HT/LT jumper leads from the bushing should be covered with yellow tape insulation. This yellow tape insulation will also indicate the overloaded operation of the transformer by the change of colour of the tape from Yellow to Black.
Improper Operation and Maintenance (O&M)
Transformer failure can also stem from poor O&M practices. For example, in addition to normal full load, continuous over-heating and higher no-load losses may reduce the life of the transformer due to reduction in the life of insulating papers, oil, etc. Other improper O&M practices leading to transformer failure are discussed later in the section on enhancing transformer life and efficiency. In Table, we summarise some reasons for transformer failure.
 |
| Failure of Distribution Transformers |
Thus far, you have studied about the selection criteria of transformers, their placement and the reasons for transformer failure. If you are able to prevent these causes of transformer failures, the battle is more than half won. The rest is taken care of by transformer testing prior to its installation.
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