India’s manufacturing sector, heavy industries, various global service centres are all dependent on reliable power supply. This underlines the principle that ‘No Power-No Business’. This makes it utmost important that ‘Transformers’, the heart of substation, must function reliably. Failures of critical transformer-assets not only impact industries but other consumer categories also affect the economy of the country and have social and political ramifications.
We’ll study the transformers, which come for repairs after failure in the industry and analyse the various causes of failure of power transformer and distribution transformer in detail. By knowing the causes of failure, we can take steps to overcome it and thus reduce the fault occurrence in transformers and make the power supply more reliable.
A transformer is a vital equipment that connects the generation to various types of loads. The right kind of design, manufacture, test, operation and protection increase normal life of transformer.
All power utilities are much worried these days due to high rate of failure of distribution transformers and service. The failure rate of transformers in India is in the order of 12 to15% as against less than 1% in developed countries. No one wants to share responsibility of failure. Manufacturers often blame the users for running the transformers in overload for single phasing or unbalancing. Users are of the opinion that the cause of failure is due to faulty design or bad materials or poor workmanship. But the fact is – responsibility should be shared equally by both.
The manufacturer should accept the feedback from the utilities without any prejudice – and take remedial measures, while the users, on their part, should ensure that the equipment is not abused, and correct feedback on the product’s performance is passed to the manufacturer.
Collection of failure data is the first major task. In free repair service or in repair contract, very little effort is made by utilities to find out the root cause of failure, which could be one of the reasons why a damaged transformer is replaced by a new one without removing the cause of damage, leading to failure immediately or within a very short period.
The projects has been done at Royal Electricals Pvt. Ltd. plot no 1405, phase 5, GIDC Estate,Vithal Udhyognagar 388121, Dist.Anand, Under the IDP Scheme Gujarat Technological University.
Classification of failures of a transformer
For the purpose of discussion, we have divided faults into three classes:
Failure attributed by users
- Prolonged over loading
- Single phase loading
- Un-balanced loading
- Faulty terminations
- Power theft by hooking
- Faulty earth connection to tank body as well as LV terminal
- Failures due to external short-circuit
- Less maintenance
- Improper installation
Failure causes at the manufacturer’s end
- Faulty design
- Poor quality of material
- Bad workmanship
- Improper transportation
- Sharp edges of conductor
- Incomplete drying
- Bad insulation covering on conductor
- Improper joints or connection
Failure during working condition
- Deterioration of oil
- Faults in magnetic circuit
- Inadequate pre shrinkage of the winding
- Inter turn faults
Major failures in power transformers
- Oil leakage
- Deterioration of oil
- Ventilation failure
- Loose clamping
- Bushing flashover
- Fault in OLTC
- Inter-turn fault
Failure due to bad insulation of conductor
Various types of insulation are used as coverings of conductors. The type of covering depends upon the type of use and basic insulation of the transformer. It is very common to see a conductor with lost wrapping. Also, the conductors have single covering instead of double covering. The quality of paper is also one of the causes of failure.
The workmen should be properly trained for proper insulation of winding – and identity bad material of insulation during the process of coil making.
Fig. 1: Winding insulating process…
Improper joints or connections
The local heating generated by improper joints or connections may slowly lead to a deterioration of the oil – if the joints are oil immersed. The oil temperature indicator and/or winding temperature indicator (both with alarm contacts) can be used to insulate such problems. Gas operated relay, can also be used to sound an alarm – and actuate the trip circuit if the condition calls for it.
Case study: (improper joints or connections)
11kV/415V, distribution transformer: in distribution transformer, the linemen sometimes make improper terminal connection. This results in to overheating of transformer near the joints, resulting in to failure.
Fig. 2: Improper joint…
Deterioration of the insulating oil
The insulating oil deteriorates gradually with use. The main cause is the absorption of the moisture in the oil. Each time the moisture is doubled in a transformer, the life of the insulation is cut by one-half. Failures due to moisture are the most common causes of transformer failures.
Reasons for moisture influx
- Moisture can be in the insulation when it is delivered from the factory. If the transformer is opened for inspection, the insulation can absorb moisture from the atmosphere
- If there is a leak, moisture can enter in the form of water or humidity in air. Small oil leaks, especially in the oil cooling piping, will also allow moisture ingress
- Moisture is also formed by the degradation of insulation as the transformer ages
- Most water penetration is flow of wet air through poor gasket seals due to pressure difference caused by transformer cooling. The most common moisture ingress points are gaskets between bushing bottoms and the transformer top and the pressure relief device gasket
- If in breather the moist silica is not replaced by dry silica gel. Then moisture can enter from atmosphere.
Effect of moisture
Paper insulation has a much greater affinity for water than does the oil. The water will distribute itself unequally, with much more water being in the paper than in the oil. The paper will partially dry the oil by absorbing water out of the oil. Moisture and oxygen cause the paper insulation to decay much faster than normal.
Due to moisture oxidation takes place. Oxidation results in the formation of acids in the insulating oil, which in turn, contributes to the formation of sludge.
The rate of oxidation also depends on the temperature of the oil; the higher the temperature, faster is the oxidative breakdown. Sludge settles on windings and inside the structure, causing transformer cooling to be less efficient, and slowly over time temperature rises.
Acids cause an increase in the rate of decay, which forms more acid, sludge, and moisture at a faster rate. This is a vicious cycle of increasing speed forming more acid and causing more decay.
Case study : (Deterioration of the insulating oil)
Moisture content in the oil increases, and when the transformer is energized, water begins to migrate to the coolest part of the transformer and the site of the greatest electrical stress. This location is normally the insulation in the lower one-third of the winding. Paper insulation has a much greater affinity for water than does the oil. The water will distribute itself unequally, with much more water being in the paper than in the oil. The paper will partially dry the oil by absorbing water out of the oil. Temperature is also a big factor in how the water distributes itself between the oil and paper.
There is almost twice the moisture near bottom as there is at the top. So, this transformer failed in the lower one-third of the windings due to paper insulation breakdown.
Fig. 3: Failure due to moisture content in oil…
Case study : (Inter turn fault)
50 KVA, 11kV/440 V
Cause of failure
Failure of transformer took place due to shorting of few turns of winding of the same phase. This was due to overloading of transformer, which results into insulation failure due to overheating.
Fig. 4: Interturn faults…
Case study: (Fault in magnetic circuit)
Rating: 11kV/440V, 30 KVA
Cause of failure
Insulation between lamination got damaged, which resulted into local overheating and due to which many laminations got short circuited. Thus, winding temperature got increases and its insulation failed.
Fig. 5: Core insulation failure…
Inadequate pre shrinkage of the winding
Insulating paper blocks used for horizontal and vertical supports of coil are bound to shrink during service due to generation of heat. This shrinkage is more in case of disk coil. Shrinkage may cause looseness in coil assembly, which may result in failure due to short circuit forces.
To overcome such failure, it is recommended that good quality of insulating bolt must be used. After heating coils to the required level, coils are compressed judiciously by a skilled workman till it reaches the required shrunk height.
Case study 🙁 Inadequate pre shrinkage of the winding)
Cause of failure
As shown in the figure (below), due to shrinkage of paper blocks, the coil got loose. During external short circuit fault, failure of the transformer occurred due to movement of discs.
Fig.6: Failure due to paper block shrinkage…
Oil in addition to serving as insulating means serves to transfer the heat generated in the windings and the core toward the walls of the tank and the radiators. Due to this it has:
- High dielectric breakdown
- Low viscosity
If the oil leaks from the transformer tank due to some reason, the oil level in the tank will drop. In the worst case, the connections to bushings and parts of the winding will get exposed to air. This will increase the temperature of the windings. This in turn, would damage the insulation of the winding. Apart from this moisture can get in through the leak, and degrade the transformer oil – leading to an overheated transformer.
In power transformer, the conservator tank is provided with an oil level indicator having an alarm facility. If the oil level drops below a predetermined level, the alarm will ring. It allows the operator to initiate necessary actions.
But in distribution transformer, oil level indicator with alarm facility is not provided. Only a small transparent window (gauge) placed conservator tank to see the oil level is provided. It shows the level of the oil directly, being able to see from the outside. When the crystal is dirty, you can wipe it off with a rag. Periodic checking of this window is not done in India. So, many of small transformers fail due to decrease in oil level.
Fig. 7: Oil level window…
Causes of oil leakage
Oil leaks can occur from many parts of transformer tank:Radiator fins, Bad welds, Cracked voltage bushings, Gaskets, Butterfly valves controlling the flow of oil between the radiator and main oil tank.
Oil leaks in gaskets
Oil leaks from transformer gasket, if it has aged, lost elasticity and cracked as it cooled much further than its normal operating range. This is often a slow weeping rather than a catastrophic failure; however, over a time, the weeping can cause a serious amount of oil loss.
It is better to use the gasket without a joint, but it is not possible as the gasket is too large. There are round, square, rectangular and oval-shaped gaskets, but in any case try to join the gaskets by using a flat portion of the gasket. If this is not done, then there are more chances of gasket failure. If the element (or a component to seal that) adopted for the gasket, is not of thin layer or not dried with air – than the oil can leak through the gasket.
Many times even after correct adjustment, the gasket oil leak is not stopped, then the gasket will have to be replaced with a new one. A gasket with low elasticity such as lead type must always be changed with a new. Do not use the old one again.
Fig. 8: Gaskets…
Oil leakage from radiator
Radiator fins are a common area for oil leaks. Fins are made from thin metal to aid heat transfer. The downside of that thinness is that internal corrosion due to water ingress and separation or external environmental corrosion can quickly penetrate and spill the oil.
This leakage must be repaired by welding, to make sure that the heat from the welding is not going to produce an explosive gas mixture.
(There is no need to take any cautionary steps in the case of nonflammable oil).
Fig. 9: Oil leakage from radiator…
From case studies of Royal Transformers Pvt. Ltd. on transformers, we conclude that major failures that occur are mainly on distribution transformer of rating 11kV/433V.The major causes of failure on this range of transformers are unbalanced loading, single phasing, overloading and some user attributed reasons. The major failures on power transformers are due to insulation damage, deterioration of oil, leakage of oil and also due to bushing failures.