- Transformer Definition: A transformer is an electrical device that transfers electrical energy between circuits through electromagnetic induction.
- Transformer Protection: Transformer protection schemes are essential to prevent damages from faults and include devices like Buchholz relays and differential protection systems.
- Common Faults: Transformer faults, such as overloads, winding issues, and short circuits, generate heat and stress that can deteriorate insulation and lead to equipment failure.
- Fault Management: Managing transformer faults involves understanding the impact of winding connections and the use of earthing transformers to handle earth faults effectively.
- Incipient Faults: Incipient faults in transformers, while not immediately dangerous, can develop into major issues if overlooked, highlighting the importance of regular maintenance and monitoring.
Protection must match the construction and duty of the transformers. Two- and three-winding electrical power transformers, an auto transformer, regulating units, earthing transformers and rectifier transformers present different fault and operating conditions. A transformer protection scheme must account for rating, winding connections, earthing methods, tap changers, cooling and system importance.
A Buchholz relay can detect gas accumulation and sudden oil flow in a suitable conservator-type liquid-filled transformer. Smaller distribution transformers may use fuses or overcurrent devices, while larger or more important units often add differential, pressure, thermal and earth-fault functions. The choice cannot be set by voltage or MVA rating alone. A restricted earth fault protection function is one option when the winding and current-transformer arrangement supports it.
Differential protection is commonly applied to important power transformers, but 5 MVA is not a universal threshold. The decision depends on the transformer rating and voltage, cost, system role, winding arrangement, available current transformers and the owner’s protection standard.
Select the transformer protection scheme from the normal loading duty, credible internal and external faults, permissible overload, tap-changing arrangement, grounding, cooling system and required clearing time.
Nature of Transformer Faults
A power transformer has no continuously rotating parts, but abnormal electrical and thermal conditions can still create severe winding, core, insulation and mechanical stress.
A transformer can experience the fault and abnormal-condition groups in the following list.
- Over current due to overloads and external short circuits,
- Terminal faults,
- Winding faults,
- Incipient faults.
These transformer faults can impose high electromagnetic forces and thermal stress on windings, bushings and terminals. Overheating accelerates insulation ageing and can turn a developing defect into a winding fault. A failed transformer cooling system can also raise oil and winding temperatures without an electrical fault. The transformer protection system therefore combines fast fault clearing with alarms or trips for slower thermal and mechanical conditions.
Transformer impedance limits external short-circuit current, but low impedance can still permit severe current and mechanical force. Modern design checks must use the transformer’s specified through-fault capability and applicable current standards. The table below reproduces historical values attributed to BSS 171:1936 and must not be used as a current coordination rule.
| Transformer % reactance | Permitted fault duration in seconds |
| 4 % | 2 |
| 5 % | 3 |
| 6 % | 4 |
| 7 % and over | 5 |
Internal faults include winding-to-earth, inter-turn and phase-to-phase faults. Bushings, leads and tap changers can also fail. A confirmed internal fault normally requires prompt isolation of the transformer, but the trip logic must remain secure during magnetising inrush, current-transformer saturation and external through-faults. Correct fault clearing limits equipment damage and disturbance to the electrical power system.
A developing internal defect may first appear as local heating, gas generation, oil loss or restricted oil flow. Core-lamination insulation failure and winding insulation damage can worsen if the condition continues. A transformer protection scheme may use gas, pressure, oil-level and temperature devices to detect these conditions before extensive damage occurs. A star-winding earth fault close to a resistance-earthed neutral can also produce a small current that some phase-differential or overcurrent elements may not detect.
Winding connections and earthing determine the path and magnitude of earth-fault current.
For current to flow during a winding-to-earth fault, the network needs a complete return path and the transformer magnetic circuit must support the resulting ampere-turn balance.
- A current exists for the current to flow into and out of the winding.
- Ampere-turns balance is maintained between the windings.
Winding earth-fault current depends on the fault position, winding connection, zero-sequence network and grounding impedance. A star point may be grounded solidly or through a resistor. A delta system does not always need to be grounded, but an earthing transformer can create a neutral when grounding is required. A Grounding or earthing transformer is designed to provide a zero-sequence path while presenting high impedance to balanced positive- and negative-sequence operation.
Star Winding with Neutral Resistance Earthed
When a neutral grounding resistor dominates the fault-loop impedance, the voltage that drives a winding-to-earth fault falls as the fault point approaches the neutral. The local fault current and the current reflected into other windings therefore vary with fault position. A simplified winding model may produce a nonlinear relationship, but an actual study must use the winding arrangement, fault location, grounding impedance and transformer data for the specific transformers. Protection near the neutral often needs a sensitive restricted earth-fault or neutral-current function.
Star Winding with Neutral Solidly Earthed
With a solidly earthed neutral, source and transformer impedances limit earth-fault current. Fault position still affects the available winding voltage and the current seen by each protection element, so the impact is not independent of location. The study must also include the external zero-sequence source and return path.
