- Definition of Transformer Faults: Faults in transformer refer to issues like insulation breakdowns and core faults that can occur inside or outside the transformer.
- External Faults: These include short circuits and high voltage disturbances that cause heating and mechanical stress.
- Transient Surge Voltage: Surge voltages, caused by events like lightning, can travel through the power system and damage transformer insulation.
- Power Frequency Overvoltage: Overvoltage from sudden load disconnections increases stress on insulation, leading to heating and potential damage.
- Internal Earth Faults: The fault current in star-connected windings depends on earthing impedance and distance from the neutral point.
High-capacity transformers must be protected from both external and internal electrical faults. External faults include through short circuits and overvoltages. Internal faults include winding-to-earth, phase-to-phase, inter-turn and core faults.
External Faults in Power Transformer
External Short Circuit of Power Transformer
Short circuits can happen in two or three phases of the electrical power system. The fault current is usually high, depending on the short-circuited voltage and circuit impedance up to the fault point. This high fault current increases copper loss and heats the windings. It also produces large mechanical forces, strongest in the first cycle of the fault current.
High Voltage Disturbance in Power Transformer
High voltage disturbance in power transformer is of two kinds,
- Transient Surge Voltage
- Power Frequency Over Voltage
Transient Surge Voltage
A high-voltage, high-frequency surge may arise in the power system from any of the following causes,
- Arcing ground if neutral point is isolated.
- Switching operation of different electrical equipment.
- Atmospheric Lightening Impulse.
Whatever the cause, a surge voltage is a travelling wave with a steep front and high frequency. That wave travels through the electrical power system network. When it reaches the power transformer, it can break down the insulation between turns next to the line terminal and create a short circuit between turns.
Power Frequency Over Voltage
System overvoltage can follow a sudden disconnection of a large load. The amplitude of this voltage is higher than its normal level, but the frequency stays the same as in normal service. Overvoltage increases stress on the transformer insulation. From voltage 
, a higher voltage causes a proportionate increase in the working flux.
Iron loss then rises, and magnetizing current rises by a larger factor. Extra flux is diverted from the transformer core into other steel structural parts. Core bolts which normally carry little flux may then carry a large component of flux diverted from the saturated region of the core alongside. The bolts can heat quickly and destroy their own insulation and the winding insulation.
Under Frequency Effect in Power Transformer
As voltage and the number of turns in the winding is fixed,
If frequency falls, flux in the core rises. The effects are similar to those of overvoltage.
Internal Faults in Power Transformer
The main faults that occur inside a power transformer are categorized as,
- Insulation breakdown between winding and earth
- Insulation breakdown in between different phases
- Insulation breakdown in between adjacent turns i.e. inter – turn fault
- Transformer core fault
Internal Earth Faults in Power Transformer
Internal Earth Faults in a Star Connected Winding with Neutral Point Earthed through an Impedance
In a star-connected winding with the neutral point earthed through an impedance, fault current depends on the earthing impedance and the distance from the fault point to the neutral. The voltage at the fault point is higher if it is further from the neutral, so fault current is higher. Fault current also depends on the leakage reactance of the winding portion between the fault point and the neutral, but this is usually low compared with earthing impedance.
Internal Earth Faults in a Star Connected Winding with Neutral Point Solidly Earthed
In this case, earthing impedance is ideally zero. The fault current depends on the leakage reactance of the portion of winding between the fault point and the neutral of the transformer. The fault current also depends on the distance between the neutral point and the fault point in the transformer. As in the previous case, the voltage across these two points depends on the number of winding turns between the fault point and the neutral. So in a star-connected winding with the neutral solidly earthed, the fault current depends on two main factors: the leakage reactance of the winding between the fault point and the neutral, and the distance between those points. The leakage reactance of the winding varies in a complex way with the position of the fault. Reactance falls very rapidly as the fault point approaches the neutral, so fault current is highest for a fault near the neutral end. At that point the voltage available for fault current is low, but the reactance that opposes the fault current is also low, so the fault current is still high. Away from the neutral, the voltage available for fault current is high, but the reactance of the winding between the fault point and the neutral is also high. Fault current therefore stays high throughout the winding, almost regardless of where the fault sits on the winding.
Internal Phase to Phase Faults in Power Transformer
Phase-to-phase faults in the transformer are rare. If such a fault does occur, it will give rise to substantial current to operate instantaneous over current relay on the primary side as well as the differential relay.
Inter Turns Fault in Power Transformer
A Power transformer connected to an extra-high-voltage transmission system is likely to see high-magnitude, steep-fronted, high-frequency impulse voltage from a lightning surge on the line. Voltage stress between winding turns can then exceed what the insulation can hold, and insulation fails between turns at some point. The LV winding is also stressed by transferred surge voltage. A large share of power transformer failures start as a fault between turns. Inter-turn faults may also come from mechanical forces between turns caused by an external short circuit.
Core Fault in Power Transformer
If any part of the core lamination is damaged or bridged by a conducting material, it can cause eddy current and local overheating. The same can happen if the insulation of bolts used to tighten the core laminations fails. These faults cause severe local heating but do not change the transformer input and output current enough for standard electrical protection schemes to see them. Excess heat can break down transformer oil, releasing gases that accumulate in the Buchholz relay and trigger an alarm.
