Oilimmersed Transformer Protection Devices & Schemes: A Complete Guide

　　Oilimmersed transformers are core equipment in power distribution networks, industrial plants, power stations, and renewable energy systems. Their stable operation directly affects power supply reliability and equipment safety. However, internal faults, overloads, overheating, external short circuits, and insulation degradation can lead to transformer damage or even safety accidents. Therefore, complete oilimmersed transformer protection devices & schemesare essential to detect faults early, issue alarms, and isolate failures quickly. This article explains the key protection devices, working principles, and complete protection schemes for oilimmersed transformers to help engineers, purchasers, and maintenance teams make better selection and operation decisions.

　　Why OilImmersed Transformer Protection Is Indispensable

　　Oilimmersed transformers use insulating oil for cooling and insulation. When internal faults occur, the oil decomposes into gas, causing pressure and temperature rises. Common faults include winding interturn short circuits, phasetophase short circuits, core overheating, oil leakage, overload, and external shortcircuit shocks. Without effective protection, minor faults can escalate into tank rupture, fire, or largescale power outages. Modern protection systems achieve early warning, rapid isolation, and pressure relief, maximizing equipment lifespan and reducing operational risks.

　　Core Protection Devices for OilImmersed Transformers

　　1. Buchholz Relay (Gas Relay)

　　As the most important protection device for oilimmersed transformers, the Buchholz relay is installed between the main tank and conservator. It monitors gas accumulation and oil flow caused by internal faults.

　　Light Buchholz: Activates an alarm for slight faults (early overheating, insulation aging, slow oil leakage).

　　Heavy Buchholz: Triggers circuit breaker tripping for severe short circuits, preventing fault spread.

　　It is a mandatory device for transformers above 500 kVA and provides sensitive protection for internal faults that other electrical protections cannot detect.

　　2. Differential Protection Relay

　　Differential protection is the main electrical protection for internal short circuits. It compares current values on the primary and secondary sides. Under normal operation, the differential current is nearly zero. When an internal fault occurs, the current balance breaks, and the relay trips instantly.

　　It effectively defends against phasetophase faults, winding short circuits, and bushing faults, widely used in medium and large transformers.

　　3. Overcurrent & Overload Protection

　　Instantaneous overcurrent protection: Quickly responds to large current surges from external short circuits.

　　Timedelay overcurrent protection: Avoids malfunction from inrush current while protecting against longterm overload.

　　It acts as backup protection for differential and Buchholz relays, ensuring fullscope coverage.

　　4. Restricted Earth Fault (REF) Protection

　　REF protection focuses on singlephase earth faults inside transformers. It uses residual current detection to locate ground faults accurately and trip rapidly, improving protection sensitivity.

　　5. Temperature Protection (OTI & WTI)

　　Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) monitor realtime temperature. When exceeding thresholds, they start cooling fans or trigger alarms/tripping.

　　Overheating accelerates insulation aging; temperature protection maintains safe operating conditions and extends service life.

　　6. Pressure Relief Device (PRD)

　　Severe internal faults produce massive gas, causing sharp pressure spikes. The PRD opens automatically to release pressure, preventing tank rupture or explosion.

　　It works with the Buchholz relay to form a dual safety barrier.

　　7. Oil Level Monitoring & Silica Gel Breather

　　Oil level sensors alarm for oil leakage, avoiding insulation degradation and overheating.

　　The silica gel breather absorbs moisture and impurities, keeping oil insulation stable.

　　8. Lightning Arrester & Overvoltage Protection

　　Lightning and switching surges damage winding insulation. Surge arresters and overvoltage relays limit voltage spikes, defending against insulation breakdown.

　　Complete OilImmersed Transformer Protection Schemes

　　A reliable protection system combines main protection, backup protection, and nonelectrical protectionfor full coverage.

　　1. Main Protection Scheme

　　Buchholz relay: Detects all internal faults (gas, oil flow, oil level drop).

　　Differential protection: Clears internal short circuits within milliseconds.

　　REF protection: Targets singlephase earth faults with high sensitivity.

　　2. Backup Protection Scheme

　　Overcurrent protection: Handles external short circuits and overloads.

　　Overvoltage protection: Defends against lightning and system overvoltage.

　　3. NonElectrical & Auxiliary Protection

　　Temperature monitoring (OTI/WTI)

　　Pressure relief device

　　Oil level alarm

　　Silica gel breather

　　DGA (Dissolved Gas Analysis) for predictive maintenance

　　4. Typical Protection Configuration by Capacity

　　Small transformers (≤630 kVA): Buchholz relay + fuse + temperature + oil level protection.

　　Medium transformers (630–2500 kVA): Add differential and overcurrent protection.

　　Large transformers (>2500 kVA): Full configuration including REF, PRD, and online monitoring.

　　How to Select & Optimize Protection Schemes

　　1. Match transformer capacity and voltage level: Small units use simplified configuration; large units need complete main and backup protection.

　　2. Comply with IEC, IEEE, and GB standards: Ensure certification and safety.

　　3. Combine online monitoring: DGA, temperature, and pressure monitoring enable predictive maintenance.

　　4. Regular testing: Verify relay action, sensor accuracy, and wiring reliability.

　　5. Coordinate with system protection: Set reasonable time limits to avoid widespread outages.