Distribution Transformer Failure : Causes, Analysis and Prevention
Transformers plays a very important role in the power system. Though they are some of the most reliable component of the electrical grid they are also prone to failure due to many factors both internal or external. There could be many initiators which cause a transformer failure, but those which can potentially lead to catastrophic failure are the following :
- Mechanical Failure
- Dielectric Failure
In both cases, the transformer is no longer able to perform its intended function of carrying load and stepping down (or up) the voltage. The main point of concern in ageing and the life expectancy of transformers is the condition of the insulation system, which is typically based on organic products.The organic products in a transformer degrade over time and finally they lose the capability to withstand the stresses a transformer might see in daily life
Some of the most common causes of transformer failure are :
- Lighting Surges
- Poor Workmanship-Manufacturer
- Inadequate Maintenance
- Line Surges/External Short Circuit
- Deterioration of Insulation
- Sabotage, Malicious Mischief
The Bathtub Curve
The following graph describes the relative failure rate of a whole group of similar products like transformers. During their life cycle, transformers will go through three different periods of failure rates, which explains the “bathtub” shape.
- Infant Mortality : Failures are the least expected. Design, manufacturing or material defects are common causes and requires from the manufacturer a deep analysis of the incidents.
- Normal Life : Also called useful life where random failures may occur. This is the lowest constant failure.
- End of Life Wear-out : Wear and tear make products fail more often and signal the end of life.
Transformer Turns Ratio Tester
types of transformer failures
Transformer Failure appear in different ways, depending on the type of construction. Some modes of failure can occur regardless of construction type. These might include tap changer failures, bushing failures, tank failures, moisture ingress, and other forms of dielectric fluid contamination. Sometimes the failure could be purely due to lack of regular maintenance or lack of awareness. And sometimes it could be due to natural causes like lightning which causes electrical surge in the power lines. In more interesting fashion it sometime also happens due to snakes, squirrels etc.
According to a latest 'Technical Paper' and transformer failure report published by OMICRON, up to 26% of the major transformer failure is related to tap changer.
Up to 26% of major transformer failures are related to the tap changer !
An example of a detailed transformer failure report of a 315MVA which failed at Bamnauli Substation, Delhi-India can be found here.
Mechanical failures can be the result of shipping damage, seismic activity, and thru-faults. The obvious result of a mechanical failure is the displacement of winding turns or damage of the turns by the forces exerted during the damaging event. Mechanical failure can result in scalloped conductors (beam failure), conductors which have been looped over adjacent turns by the hoop stress (hoop failure), or in rare cases, conductors which have been severed by the tension applied by the hoop force. That’s why it’s highly recommended to perform a SFRA (Sweep Frequency Response Test) test on site to observe any change in the SFRA test result in comparison to the factory results. Doble's M5400 Sweep Frequency Response Analyzer well renowned for carry out the SFRA tests.
Indications for Mechanical Failure
Moreover change in low voltage excitation current, a change in impedance, and sometimes, the presence of partial discharge (PD) during an induce voltage test can also give valuable indications about a mechanical failure in the transformer. Mechanical failure is often discovered by electrical failures which are the result of mechanical deformation.
Electrical failures are the result of insulation degradation. This can be caused by thermal degradation over the life of the transformer, by thermal degradation due to excessive or frequent fault current, or by dielectric breakdown due to high voltage stress. A dielectric breakdown can also be the result of mechanical forces tearing the insulation. The result of a electrical failure can be a turn to turn failure. The consequences can be arc from the energized winding to an adjacent winding or to ground. It is important to note that overloads rarely result in transformer failures, but do cause thermal aging of winding insulation.
According to a detailed article at Electrical Engineering Portal, when a transformer becomes hot, the insulation on the windings slowly breaks down and becomes brittle over time. The rate of thermal breakdown approximately doubles for every 10°C. 10°C is referred to as the “Montsinger Factor” and is a rule of thumb describing the Arrhenius theory of electrolytic dissociation. Because of this exponential relationship, transformer overloads can result in rapid transformer aging. When thermal aging has caused insulation to become sufficiently brittle, the next fault current that passes through the transformer will mechanically shake the windings, a crack will form in the insulation, and an internal transformer fault will result.
Based on construction of transformers
Transformer Failure modes for shell form construction
Shell form construction is resistant to winding deformation due to thru-faults.This is because the coil “pancakes” are arranged in multiple groups to limit force magnitude. The exposure to conductor bending is limited by many support spacers to avoid beam bending. Form fit tank and core prevent movement of the core and winding groups.
The area which is vulnerable is the edge of the coil where “pancake” windings are connected together. There is a relatively high voltage in the kV range at this point between the same end of odd or even number “pancakes”. Failure can be prevented by adequate support of the outer turns.
Winding failure modes for core form construction
Core form construction can exhibit failure in several ways. Radial tension failure, also known as hoop tension failure, can occur;conversely, radial compression failure can also occur. Radial failure can collapse the inner winding unless the winding support structure is strong.
Axial failures can occur in both compression and tension. Sometimes the windings telescope, caused by uneven axial forces.The result is typically a tangled mess of winding conductors which eventually arc to ground or each other, and disrupt the force vectors in ways that were not accounted for in the original design.
Core form designs have subsets consisting of numerous winding configurations such as layer windings, helical (or spiral-type)windings, continuous disc, and interleaved disc windings. Each has different voltage stress applied to different components of the winding (individual strand to-strand, conductor-to-conductor, layer to-layer or disc-to-disc, etc.)
Indications for Electrical Failure
Electrical failures will manifest themselves as a source of dissolved gas products. Diagnostic tests will typically show deterioration and the results will provide clues where the failure occurred or where it is about to happen. Winding turns ratio, winding insulation tests, insulation power factor will all give indications and should confirm the DGA results. They frequently show results observed during an internal inspection or tear-down as some form of insulation “burning” which appears as discoloration or carbonisation of cellulose. Those where cellulose is not involved typically show as points of contact on core steel or tank steel and winding or leads where the conductor is bare.
Common Distribution Transformer Failure Analysis
Other causes of distribution transformer failure modes can be the result of a grounded core or core clamping structures (such as through-bolts) that develop shorts. These result in a shorted turn (the core) and produce high currents which are often detected by dissolved gas analysis.Transformer can also fail due to poor maintenance. Especially when there are leakages in the transformer tank and the transformer oil level drops below a certain level which gives rise to local heating in the windings which will eventually fail if there is no transformer oil to cool down the temperature. Thus it's very essential to carryout regular oil filtration of the transformer and leak arrests before it becomes very critical. Preventive maintenance is the key to avoid such kinds of disasters.
Here is a short video the showing the devastating consequences when an electrical transformer explodes :
Other Miscellaneous Transformer Failure
Loose connections : Due to long term vibration, connections can loosen in the transformer. If this goes unnoticed it might lead to excessive buzzing and overheating.
Excessive Harmonics : If a higher temperature is observed at the neutral termination then it's an indication of the presence of harmonics. Usually the harmonics on the third and fifth level indicates interference from the electronics loads.
Unbalance Loads : Due to the presence of the unbalance load in the power system a higher temperature on the one of the phase may occur resulting in local heating.
Cooling issue : Some times the cooling fans are out of order or the transformer is suffering a severe oi leakage resulting in the increase of temperature inside.
Solution : Preventative Maintenance
A preventive maintenance schedule consists of regular inspections and component replacements according to the product specific maintenance schedule. Regular preventive maintenance helps facilitate forward budget planning. A number of case studies by Electrical India just demonstrates how important it is to have good maintenance of transformers. Maintenance is the key to avoid transformer failure: a planned program of maintenance, inspection and testing can significantly reduce the number of transformer failures, and the unexpected interruption of power.
Additional measures may include:
- On liquid-cooled units, check the radiators for leaks, rust, accumulation of dirt and and mechanical damage that would restrict the oil flow.
- Keep the porcelain bushings and insulators clean
- Keep electrical connections tight
- Inspect tap changers on a regular basis
- Transformer windings, bushings and arresters should have a power factor test on a 3 year basis
- Check the ground connection on the surge arrestor annually