The transformer is an important part of the power distribution system, the distribution transformer is mostly used to do the transmission and distribution of electrical power. Transformers are really expensive and without the transformer, the distribution of electrical power is not possible. The repairing and the replacement of the transformer would take time so we need to make sure that the transformer is protected. There are different types of transformers like distribution, power, auto, instrument, isolation, and auto. So all these transformers play a major role in the power system so we must make sure that the transformer is protected. It would take a lot of time to repair or replace a transformer, and due to this the operation of the power system or power transmission could be interrupted, and to avoid this problem the transformer must be protected.
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What are the major faults in a transformer?
Transformer faults can be categorized as internal and external faults
INTERNAL
- Winding phase to phase ground faults
- Winding inter-turn faults
- Tank faults
- Over flexing
- Core insulation failure
EXTERNAL FAULTS
- Overloads
- Overvoltage
- Over-fluxing
- External system short flexing
What are the major reasons for the transformer faults?
The faults can be caused due to many reasons faults can be caused due to the mistake in selecting the insulation type. It can also happen because of not having the proper capacity for the transformer. Transformer faults can also happen due to environmental factors too like temperature, dampness, etc.
Transformer faults due to the lack of perfection in the maintenance and operation of the equipment
- Insulation oil deterioration
- Components of the transformer aren’t maintained properly
- Operation mistakes or mistakes in the protection circuit arrangement
So in order to prevent all these faults, we must protect the transformer and thus the time and maintenance cost can be saved.
What are the preventive measures that should be done in a transformer?
Transformer failure can be avoided by proper maintenance, the preventive maintenance in a transformer must be done by regular inspections and also by the component replacement.
- Check the radiators in the liquid cool units for dirt, dust, or for rust and all these could affect the oil flow
- Bushings and the insulators should be clean
- Electrical connections should be proper
- Tap changers must be inspected
- The power factor of the windings, bushings, and arrestor must be checked
What are the factors that must be considered while doing the transformer protection?
Protection should be done according to the
- Size and rating of the transformer
- Vector configuration
- Source and neutral earthing
- It must be done according to the type of transformer like two winding, three winding, autotransformer etc
- OLTC range should be considered
- We must consider the infeed conditions like radial, parallel, and interconnecting
What are the different types of transformer protection?
MECHANICAL PROTECTION
BUCHHOLZ RELAY
This type of relay would provide protection for oil-filled transformers this relay is a gas-actuated relay. This is the best type of protection for internal faults. A slow-developing fault in a transformer is called the incipient fault and this fault would cause the relay to operate. So due to the incipient fault, the transformer oil would decompose and thus gases are created. So the gas will be accumulated in the relay chamber, due to the gas formation there will be a high velocity of oil flow and due to these high energy faults, the relay will trip. So in case of small faults, this relay would operate by accumulating the gas and after that, there will be an alarm. The relay would trip for the large faults by detecting the velocity of the oil flow.
Detects
- Joints with high resistance
- High eddy current between laminations
- Low and high energy arcing
The alarm is for – Winding and core overheating, partial discharge, bad contacts or joints, and also for core bolt insulation failure.
Relay trips for
The relay would trip if there is a loss of reduction of oil and this could happen in case of leaky pipe joints, tank faults, and also because of contraction of oil under low temperature and low loads.
SUDDEN PRESSURE RELAY
The reaction time of this type of relay would be really quick compared to the Buchholz relay. This relay would easily detect if there is a sudden rise in the pressure and also if it has inverse time characteristics. If a high current passes through a shorted turn then there would be a lot of heat. This heat along with the arcing would cause oil breakdown into combustible gases and this gas generation would increase the tank pressure. So this quick increase in gas pressure would be detected by the relay so before and damage to the transformer the relay would operate.
PRESSURE RELIEF VALVE
This valve is used in a transformer to remove the overpressure in the transformer so that we can prevent the explosion of the transformer tank. In case if the transformer is subjected to extreme pressure due to faulty operations then the pressure relief valve would operate by releasing the excessive pressure and also the transformer will be disconnected from the lines. This valve is a spring-loaded type and this valve would operate if the tank pressure reaches a set value and the transformer will be tripped.
TEMPERATURE PROTECTION
Due to the thermal inertia of the transformer, there will be slight overloading in the oil temperature. So because of this temperature protection will be provided in the winding. This temperature can be calculated indirectly by using a replica resistance and also by using a CT in the secondary current.
ELECTRICAL PROTECTION
DIFFERENTIAL PROTECTION
This type of protection is done to prevent the phase and earth faults. Two current transformers are used one of them would be connected to the secondary side and the other will be connected to the primary side of the transformer. The relay would differentiate the current in the primary and the secondary side. This comparison is done with the help of the CT’s matched ratio such that the secondary current will be balanced in magnitude and phase for an external fault on load. The magnitude balance can be done by selecting the proper CT ratio for the primary and the secondary, it can also be achieved by interposing current transformers. The phase balance could be done by doing the proper vector connections of the selected CT’S
OVERCURRENT & OVERLOAD PROTECTION
The overcurrent in the transformer can be protected by using a protective relay. The relay would pick up if the current magnitude would exceed a certain value, this type of relay would be connected to the system with the help of the current transformer. So this type of protection would provide protection from overloads. We can use a thermal relay to do the overcurrent protection of the overloads. The over-current protection includes the short circuit protection. The short circuit could be phase fault, earth fault, or winding fault.
The overload could cause due to an increase in copper loss or in case if there is an increase in temperature. The overload won’t be harmful for a limited period. In case if there is a large fault current then we can see severe mechanical stress in the transformer.
OVERVOLTAGE PROTECTION
Overvoltage can be caused by switching or by lightning and it would cause inter-turn faults. The overvoltage conditions are of two types and they are transient surge overvoltage and power frequency overvoltage. The transient overvoltage is formed by switching or lightning. These over-voltages are limited by the LA’s.
Power frequency over-voltages would affect the insulation, the flux density will be affected if there is a decrease in frequency. The operation of the transformer can be done if there is a small amount of overvoltage and also an increase in frequency corresponding to it. The transformer shouldn’t operate during a high voltage input with a low frequency.
THROUGH FAULT PROTECTION
The transformer and the source impedance can decrease the fault current through the transformer. There could be incorrect relay operation or transformer failure because of the through current. Mostly these faults occur outside the protection zone and they can’t be detected by the differential protection. In order to prevent the through current faults in a transformer, we can use an overcurrent relay with under-voltage blocking, zero sequences, and negative sequence protection.
OVER FLUXING PROTECTION
The transformer over-fluxing can be caused because of very high voltage or due to very low frequency. This would increase the magnetizing current, due to the increase in the magnetizing current there will be iron loss and also the insulation will be affected by this. So in order to protect the transformer from this fault, we can use a Voltz/hertz relay.
