Transformer Faults causes

Major Causes of Transformer Faults and Protection Mechanisms

Transformers are critical assets in power systems, but they are also vulnerable to faults due to electrical, mechanical, and thermal stresses. A fault in a transformer not only affects reliability but may also lead to system instability and fire hazards. Below are the major causes of transformer faults and the protective schemes used to mitigate them.

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Major Causes of Faults in Transformers

1. Insulation Failure

   • The most common cause of transformer faults is insulation breakdown in windings. Ageing, overheating, moisture ingress, and over-stressing weaken insulation, leading to turn-to-turn short circuits or winding-to-core faults.

2. Tap Changer Failures

   • On-load and off-load tap changers are prone to mechanical wear, carbonization, and contact failures, which may lead to arcing and insulation damage.

3. Oil Leakage

   • Large transformers are typically oil-immersed. Leakage not only reduces insulation strength but also creates a fire hazard.

4. Inrush Currents

   • Switching an unloaded transformer generates high inrush currents rich in harmonics. These may cause misoperation of protective relays and local heating.

5. Inter-Turn Faults

   • Short circuits between turns in the same winding create localized hot spots, which accelerate insulation ageing and may escalate into severe internal faults.

6. Over-Fluxing

   • Operation at rated voltage with reduced frequency (under-frequency) or application of over-voltage at rated frequency causes magnetic core saturation, leading to excessive core losses and heating.

7. Sustained Overloads

• Prolonged overloading increases winding temperature beyond design limits, accelerating insulation deterioration and potentially causing catastrophic failure.

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Protection Schemes for Power Transformers

(A) Differential Protection

• Purpose: Detects internal short circuits.

• Principle: Compares input and output currents of the transformer. A difference indicates an internal fault.

• Challenges:

  • Magnetizing inrush current: Can be 8–30 times rated current during energization, rich in harmonics.

  • Over-excitation: Core saturation due to over-voltage or under-frequency operation increases excitation current.

  • CT Saturation: External faults may cause CT distortion, leading to false differential current.

Solution:

• Percentage differential relays (1–5% slope) with harmonic restraint are used. Harmonic restraint blocks operation during inrush by detecting high harmonic content.

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(B) Restricted Earth Fault (REF) Protection

• Protects against low-magnitude earth faults within the transformer winding.

• Especially useful for high-resistance winding-to-core faults that may not trigger differential protection.

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(C) Overcurrent Protection

• Provides backup protection for large transformers (≥5 MVA).

• Typically includes two-phase fault relays and one earth-fault relay in star-delta configurations.

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(D) Overfluxing Protection

• Principle: Flux (Φ) ∝ Voltage / Frequency (E/f).

• If E/f exceeds a set threshold, flux density rises beyond safe limits.

• Protection: Relays monitor E/f ratio and initiate tripping after a time delay (to prevent tripping for transient over-voltages).

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(E) Overheating Protection

• Thermal Image Protection: Uses temperature detectors and thermal models to monitor winding heating.

• Implementation:

  • Resistance Temperature Detectors (RTDs) or thermistors in oil.

  • CT-fed heaters simulate winding heating effect.

  • Trips when winding exceeds ~95 °C.

• Cooling fans and pumps can be automatically controlled.

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(F) Protection Against Incipient Faults

1. Buchholz Relay (Gas-Actuated Relay):

   • Installed between the conservator and transformer tank.

   • Detects slow-developing faults (incipient faults) that release gases from oil decomposition.

   • Provides alarm for small gas accumulation and tripping for severe oil surges.

2. Gas Analysis:

   • Dissolved Gas Analysis (DGA) identifies specific gases (H₂, CH₄, C₂H₂, etc.) to indicate the type of fault.

3. Pressure Relief Device:

   • Operates when internal oil pressure rises above ~10 psi.

   • Prevents tank rupture by releasing oil.

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(G) Fire and Lightning Protection

1. Fire Protection:

   • Causes: insulation failure, arcing, overheating, or oil leakage.

   • Mitigation: regular maintenance, lightning arresters, grounding, suppression systems, and oil fire protection systems.

2. Lightning Protection:

   • Lightning overvoltage surges can reach several times the rated current within microseconds.

   • Solution: Surge arresters placed close to transformer bushings.

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Conclusion

Transformer faults are inevitable due to ageing, operating stresses, and external disturbances. However, with comprehensive protection schemes—such as differential relays, REF protection, overcurrent, overfluxing relays, Buchholz relays, and fire/lightning protection systems—transformer reliability and safety can be significantly enhanced.

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Transformer On load Tap changer

ON Load Tap Changer

There is always requirement of variation of voltage at secondary side even when load is connected then on load tap changer is used for the same. In case of on load tap changer tapping’s on the winding are brought out through a terminal board to separate oil filled compartment in which the on-load tap changer switch is housed. This tap changer is in the form of switch with motorized control. Handle is also provided in case of emergency.

The essential feature of an on-load tap changing gear is the maintenance of circuit continuity throughout the tap changing operation. The circuit must not be broken otherwise there will be discontinuity of supply to load. Selector switch must not break current an additional separate oil filled compartment is used to house the diverter switch which breaks the load current by an interrupted arc. During that process carbon will be formed therefore oil in the diverter switch compartment must be prevented from mixing with oil in the main tank.  Oil in the selector switch tank might be connected to main transformer tank oil through conservator.

In case of on load tap changer continuity with next tap must be made before one tapping is left opened. Therefore, selector switch is on-load tap changers is a make before a break switch and during the period of transition from one tap to another, momentary connection must be made between adjacent taps. This will lead to short circuiting of turns between the adjacent tappings. Therefore, the short circuit current must be limited by including resistors or reactors.    

On Load tap changer winding connection are shown below where there is one Selector switch S1 is at Tap no.1 and other S2 is on Tap 2. The Diverter switch S3 is shown connecting tap 1 to the neutral point of the transformer winding and the Switching for the changeover to Tap2 is as follows:-

1.      Contacts “a”and “b” are closed Resistance R1 shorted as shown. The load current flows from tap through contact b. This is the running position at Tap1.

2.      An External mechanism moves the diverter switch S3, contact b opens. The load current from Tap1 now flows through resistor R1 and contact a.

3.      As the moving contact of S3 continues it travels to the left, contact d closes and resistance R1 is open circuited. Both the resistances R1 and R2 are connected across taps 1 and 2 and the load current flows through these resistances to their mid point.

4.      When diverter switch S3 moves still further to left contact a is opened. The load current flows from Tap2 through resistance R2 and contact d.

5.      Finally as the Switch S3 reaches to extreme left position, contact c Closes and resistance R2 is short circuited. The load current from tap2 flows through contact a. This is running position for tap 2.

From above we have seen that there isn’t any movement of S1 and S2 and only there was movement of S3.

However , a further tap change is the same direction i.e. from tap 2 to tap 3 , is required, the selector switch S1 is moved to tap 3 before selector switch S1 is moved to Tap 3 before the diverter Switch S3 Moves. The diverter switch then follows the sequence as described above but in the reverse order.

To limit energy loss due to resistance, resistance should be kept for as minimum time in line as possible.