A Transformer Cooling Control Circuit is designed to monitor and manage the temperature of a power transformer by automatically switching on and off the cooling equipment (like fans, oil pumps, etc.) based on predefined temperature thresholds. This ensures efficient operation, prevents overheating, and extends the lifespan of the transformer.
🔧 Components of Cooling Control Circuit
| Component | Function |
|---|---|
| Temperature Sensor (RTD / Thermostat / PT100) | Senses transformer winding or oil temperature |
| Relay (Control Relay or Timer Relay) | Activates cooling equipment based on temperature signal |
| Cooling Equipment (Fans, Oil Pumps) | Provides forced air or oil circulation for cooling |
| Thermal Protection Relay (Winding Temperature Relay – WTI, Oil Temperature Indicator – OTI) | Provides set point-based control and protection |
| Contactor | Electrically switches cooling equipment on/off |
| Auxiliary Supply (AC/DC) | Powers the control circuit |
| Alarms & Indicators | Visual/Audible indication of high temperature or fan failure |
🔁 Typical Cooling Control Logic
- Temperature Monitoring
- The temperature sensor (PT100 or Thermistor) monitors winding/oil temperature.
- Connected to an indicator (OTI/WTI) with programmable set points.
- Relay Activation
- At a preset temperature (e.g., 65°C), the OTI/WTI triggers a relay.
- Relay energizes a contactor to start cooling fans (ON1).
- At a higher preset (e.g., 80°C), another relay triggers oil pump (ON2).
- Shutdown / Trip Logic
- If temperature exceeds critical level (e.g., 95°C), a trip signal is sent to shut down the transformer.
- Audible/Visual Alarm is activated.
⚙️ Modes of Cooling Control
| Mode | Description |
|---|---|
| Manual | Operator switches cooling system ON/OFF manually |
| Auto | Cooling system controlled by temperature relays automatically |
| Auto-Manual Selector Switch | Enables switching between modes |
Functional Schematic: Cooling Control Circuit for 132/33kV Transformer
⚡ Key Components in the Control Circuit
- OTI (Oil Temperature Indicator) – with alarm and trip contacts
- WTI (Winding Temperature Indicator) – with alarm and trip contacts
- Buchholz Relay – for internal fault detection (not directly for cooling but integrated)
- Cooling Fans – Forced air cooling (3-6 fans)
- Oil Pump(s) – Circulate transformer oil
- Contactor (AC) – For switching pumps and fans
- Auxiliary Relays (Control Relays) – For logic control
- Auto/Manual Selector Switch – Allows operator to switch control mode
- Local/Remote Selector – For SCADA/Bay Control Unit (BCU) integration
- Annunciator Panel / Alarms – For operator alert
- Thermal Protection Relays (WTI/OTI setpoints) – With trip and alarm contact settings
- MCB/Fuse – For protection of control circuit
- Control Supply – 110V/220V AC/DC
[PT-100] or [Sensor from WTI/OTI]
│
+------+------+
| Temperature |
| Indicator |
| WTI / OTI |
+------+------+
│
┌─────────┴─────────────┐
↓ ↓[Alarm Relay] [Cooling Relay (Set at 65–70°C)]
│
↓
[Contactor] ───────► [Cooling Fans ON]
│
[2nd Relay (Set at 80–85°C)]
↓
[Contactor] ───────► [Oil Pump ON]
│
[Trip Relay (Set at >95°C)]
↓
[Trip Circuit Breaker + Alarm]
🔌 Cooling Equipment Types & Control
| Cooling Type | Control Method |
|---|---|
| ONAN (Oil Natural Air Natural) | No active control – natural convection |
| ONAF (Oil Natural Air Forced) | Fans controlled via OTI/WTI |
| OFAF (Oil Forced Air Forced) | Oil pump + fans controlled by relays |
| ODWF (Oil Directed Water Forced) | Water pumps + cooling water system managed by temperature control circuit |
🔐 Protections & Interlocks
- Fan failure alarm
- Over-temperature trip
- Redundant sensors
- Control panel interlock
- Breaker trip on excessive temperature
