Transformer OTI and WTI Protection: Working Principle, Settings, Alarm and Trip Logic

Temperature monitoring is one of the most important aspects of transformer protection and operation. The Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) form the backbone of thermal protection systems in power transformers. They ensure safe operation and prevent transformer failures due to overheating.

In this technical guide we will discuss how OTI and WTI functions, what settings they require, why they are important for transformer protection, how to do settings in dial, and how modern systems integrate these devices into protection schemes that ensure reliable, long-term transformer operation.

Why Temperature Monitoring is Important for Transformers

Transformers operate by converting electrical energy and during this process a significant portion of energy is lost as heat. This heat is generated primarily in the windings due to ohmic losses (I²R losses) when current flows through the copper windings. If this heat accumulates without proper dissipation it degrades the insulation system. This may lead to insulation breakdown and transformer failure.

The core principle is simple: transformer insulation has a maximum safe operating temperature limit. Exceeding this limit reduces the transformers lifespan. Every 8-10°C increase above the rated temperature can cut the insulation life in half.

Insulation Classes and Temperature Ratings

Before discussing OTI and WTI protection, it’s important to understand the insulation classes that define transformer temperature limits.

These classifications are based on the insulation materials used in the transformer. For example, a Class A insulation system (using cotton, paper, and mineral oil) can safely operate up to 105°C continuously, while a Class C system (using synthetic materials) can withstand up to 220°C.

The temperature rating assumes an ambient (surrounding air) temperature of 40°C.

The actual hotspot winding temperature is calculated as:

\(\text{Hotspot Temperature} = \text{Ambient Temperature} + \text{Temperature Rise} + \text{Hotspot Allowance}]\)

For instance, a Class A transformer with a 55°C temperature rise operating in a 40°C ambient environment would have a hotspot temperature of approximately 95°C (40 + 55), leaving only a 10°C safety margin before reaching the 105°C limit.

Oil Temperature Indicator (OTI)

The Oil Temperature Indicator (OTI) monitors the temperature of the transformer insulating oil at the hottest point inside the transformer tank, typically near the top. While oil itself doesn’t generate heat directly, it serves as both the cooling medium and insulating medium. By monitoring oil temperature, we can assess whether:

1. The transformer is operating within its safe thermal limits
2. The cooling system is functioning properly
3. An abnormal condition (like an internal fault) is occurring

How OTI Works

The OTI operates on a simple but effective principle of liquid expansion:

1. Sensing Bulb: A sealed bulb filled with a temperature-sensitive fluid is immersed in the transformer oil, typically placed in a thermometer pocket at the top of the tank.
2. Capillary Tube Connection: The sensing bulb is connected to the indicator dial through a thin capillary tube, also filled with the same fluid.
3. Thermal Expansion: As temperature increases, the fluid inside the bulb expands and is pushed through the capillary tube into the dial mechanism.
4. Mechanical Indication: This expansion drives a mechanical linkage that moves the needle on the dial, displaying the current temperature.
5. Electrical Contacts: The OTI contains mercury switches (or modern solid-state equivalents) that close at preset temperature thresholds:
   • Alarm Contact: Closes when oil temperature exceeds the alarm setting (e.g., 80°C for Class A)
   • Trip Contact: Closes when oil temperature exceeds the trip setting (e.g., 90°C for Class A)
6. Maximum Temperature Indicator: A separate red needle shows the maximum temperature reached since the last reset, helping operators identify temperature trends and anomalies.

Typical OTI Settings

For Class A Insulation (105°C max):

• Alarm Setting: 80°C
• Trip Setting: 90°C

For Class C Insulation (220°C max):

• Alarm Setting: 100°C
• Trip Setting: 110°C

These settings are conservative, designed to give operators time to take corrective action before the transformer approaches critical temperatures.

Winding Temperature Indicator (WTI)

Unlike the oil, which changes temperature slowly due to its large thermal mass, the transformer windings heat up rapidly when the load increases. The windings are where the heat is actually generated through I²R losses. Therefore, measuring winding temperature directly provides:

1. More Accurate Overload Detection: WTI responds quickly to load changes
2. Tighter Protection Margins: Allows precise control of cooling systems
3. Better Insulation Protection: Windings age based on their temperature, not the oil temperature

The key difference: OTI measures the symptom (oil temperature), while WTI directly monitors the source (winding temperature).

How WTI Works

Since it’s dangerous and impractical to place sensors directly inside high-voltage windings, WTI uses an indirect measurement technique called thermal image simulation:

1. Sensing Bulb Location: A bulb is placed in a heated pocket located in the hottest oil area inside the transformer tank (not in the winding itself).
2. Heater Coil: Surrounding the sensing bulb is a heating resistance coil that simulates the winding temperature rise.
3. Current Transformer (CT) Connection: The secondary winding of a current transformer connected to the main transformer’s loading winding supplies current to the heater coil. As the load current increases, the heater coil generates more heat.
4. Temperature Representation: The total temperature shown by WTI is:
   \(\text{WTI Temperature} = \text{Top Oil Temperature} + \text{Winding Temperature Rise Above Oil}\)
5. Thermal Simulation: The heater coil is made from the same material as the main transformer winding. This ensures that when load current flows through it, the temperature rise is proportional to the actual winding temperature rise.
6. Mercury Switches for Control: The WTI contains multiple mercury switches for:
   • First cooling stage (around 60-65°C)
   • Second cooling stage (around 70-75°C)
   • Alarm stage (around 85°C)
   • Trip stage (around 95-105°C, depending on insulation class)

Example WTI Reading

Imagine a transformer with these conditions:

• Ambient temperature: 25°C
• Top oil temperature (measured by OTI): 70°C
• Transformer load: 80% of rated capacity
• The heater coil in WTI generates heat proportional to this load

The WTI would show approximately 85°C (70°C oil + 15°C winding temperature rise above oil).

If the load increases to 100% (full load), the heater coil generates more heat, and WTI might read 95°C (70°C oil + 25°C winding temperature rise above oil).

This relationship demonstrates why WTI settings are always higher than OTI settings  because the winding is naturally hotter than the oil.

Typical WTI Settings

For Class A Insulation (105°C max):

• Fan Start Setting: 60-65°C
• Alarm Setting: 85°C
• Trip Setting: 95°C

For Class C Insulation (220°C max):

• Fan Start Setting: 80-85°C
• Alarm Setting: 130°C
• Trip Setting: 140°C

Temperature Rise Comparison: Why WTI is Always Higher Than OTI

This chart illustrates a critical principle: during load variations, WTI temperature always rises faster and reaches higher values than OTI. This is because:

1. Heat is generated in the windings first
2. This heat takes time to dissipate through the oil
3. The WTI’s heater coil responds immediately to load current changes
4. The OTI’s response is delayed because it measures the bulk oil temperature

For this reason, the WTI protection acts as the first line of defense against overheating, while OTI serves as a backup protection and overall system health indicator.

Protection and Control Actions Triggered by OTI and WTI

When a transformer has automatic cooling equipment (fans and pumps), the WTI controls these devices in stages:

Practical Example: Cooling System Operation

Consider a 500 kVA distribution transformer with fans and a cooling pump:

Scenario: Load Increases Gradually

• At WTI = 55°C: Transformer operating normally; cooling system is on standby
• At WTI = 65°C: (First Mercury Switch Activates): First fan group starts automatically. This increases air circulation, cooling the oil.
• At WTI = 70°C: Temperature stabilizes due to increased cooling. No further action needed.

Scenario: Persistent High Load or Cooling System Failure

• At WTI = 65°C: Fan Group 1 starts
• At WTI = 75°C: (Second Mercury Switch Activates): Since temperature continues rising, Fan Group 2 starts, providing maximum air cooling
• At WTI = 80°C: Oil pump (if equipped) starts, forcing hot oil through external coolers
• At WTI = 85°C: (Alarm Mercury Switch): A relay closes, sending an alarm signal to control room. An audible/visual alarm alerts the operator.
• At WTI = 95°C: (Trip Mercury Switch): A second relay closes, sending a trip signal to the circuit breaker. The transformer is automatically disconnected from the grid.

OTI Control Logic

While OTI primarily serves a protective and monitoring function, it also has associated controls:

OTI-Based Settings (Class A Transformer):

• Normal Range: 40-80°C  System operates normally
• Warning Range: 80-90°C  OTI alarm triggers; operator should investigate
• Critical Range: Above 90°C  OTI trip signal; transformer should be isolated

OTI typically controls cooling in transformers without WTI, particularly in older designs or simpler cooling systems. Modern designs use OTI as a redundant check on WTI.

Real-World Example: OTI and WTI in Operation

Let’s examine a practical scenario at a substation:

Transformer Specifications:

• Capacity: 100 MVA, 132/33 kV
• Insulation Class: Class F (180°C)
• Cooling Type: OFAF (Oil Forced Air Forced)
• OTI Alarm/Trip: 90°C / 100°C
• WTI Alarm/Trip: 115°C / 125°C

Operating Scenario:

Time 09:00 AM Normal Load (60% capacity)

• Ambient Temperature: 35°C
• OTI Reading: 75°C
• WTI Reading: 90°C (75°C oil + 15°C winding rise)
• Status: Normal operation; all cooling systems on standby

Time 11:00 AM  Increased Load (95% capacity)

• Ambient Temperature: 40°C (peak daytime heat)
• OTI Reading: 85°C (rising but within safe limit)
• WTI Reading: 110°C (85°C oil + 25°C winding rise)
• Status: WTI has reached near alarm; first fan group starts automatically
• Oil pump activates to enhance cooling

Time 12:30 PM  Sustained High Load + Cooler Malfunction

• Cooling efficiency reduced due to cooler tube blockage
• OTI Reading: 92°C (exceeds alarm at 90°C)
• WTI Reading: 120°C (exceeds alarm at 115°C)
• Status: Both OTI and WTI alarms triggered
• Operator receives alert; checks cooler, discovers blockage
• Maintenance team clears cooler; cooling restored
• Temperatures return to normal levels by 14:00

Time 14:00 PM  Post-Maintenance

• OTI Reading: 78°C
• WTI Reading: 95°C
• Status: System back to normal operation

This example shows how OTI and WTI work together to provide layered protection: WTI provides early warning of overload conditions, allowing manual or automatic cooling response before OTI alarm is reached.

Protection and Control Circuit Integration

Simple Manual Control (Traditional Design)

In older transformers without automatic controls, OTI and WTI contacts are wired to light indicators on the control panel:

• OTI alarm contact lights a red lamp
• WTI alarm contact sounds an audible alarm
• Operator manually starts cooling fans

Automatic Control (Modern Design)

Modern transformers integrate OTI/WTI with programmable controllers:

1. Temperature Sensor Input: OTI and WTI signals are fed to a PLC or relay
2. Conditional Logic:
   • If WTI < 50°C: All cooling OFF
   • If WTI 50-65°C: Periodic fan testing (every 72 hours)
   • If WTI 65-75°C: Fan Group 1 ON
   • If WTI 75-85°C: Fan Group 2 ON
   • If WTI 85-95°C: Oil Pump ON + Alarm
   • If WTI > 95°C: Trip Signal to CB
3. Redundancy: OTI provides a secondary check; if OTI > 100°C, trip signal is issued regardless of WTI
4. SCADA Integration: Temperature data is transmitted to control centers for remote monitoring

How to Set WTI and OTI Settings in the Dial

Both OTI and WTI contain mercury switches positioned inside the dial mechanism. The OTI has two mercury switches: S1 for alarm activation and S2 for trip activation. The WTI has four mercury switches: S1 and S2 for cooling control (fan and pump), S3 for alarm, and S4 for trip.

Step-by-Step Procedure for Setting Dial

Step 1: Preparation and Safety

Before beginning any adjustment work, ensure the transformer is completely isolated and de-energized. Remove the front glass cover of the temperature indicator by carefully unscrewing the retaining clips.

Step 2: Identify the Setting Scales

Inside the dial, you’ll see two concentric setting scales marked as S1 and S2. These scales are calibrated in temperature degrees (Celsius) and have adjustment screw positions around them.

S1 controls the alarm contact, while S2 controls the trip contact. For WTI dials, you may see four switches (S1, S2, S3, S4) controlling fan, pump, alarm, and trip functions respectively.

Step 3: Locate Adjustment Screws

Each switch has an adjustment screw positioned behind its setting scale. These screws are small and require careful handling. Use a fine screwdriver appropriate to the screw head.

Step 4: Set the Alarm (S1) Temperature

Slowly turn the pointer of the dial manually toward the temperature value where you want the alarm to trigger.

For example, if you’re setting an OTI alarm at 80°C for a Class A transformer, rotate the pointer until it reaches the 80°C mark on the dial. Once the pointer reaches the desired temperature, locate the corresponding S1 adjustment screw. The switch should activate (closing the NO contact to become NC) exactly when the pointer reaches this temperature mark.

If the contact doesn’t activate at the correct temperature, carefully adjust the S1 setting screw clockwise or counterclockwise until the switch operates precisely at the desired temperature setting.

Step 5: Set the Trip (S2) Temperature

Repeat the same procedure for the trip setting using S2.

For instance, if you’re setting WTI trip at 95°C, rotate the pointer to 95°C and adjust the S2 screw until the switch activates exactly at this point. Remember that trip settings must always be higher than alarm settings to allow time for corrective action before disconnection occurs.

Step 6: Verification and Testing

After adjustment, verify the settings by slowly moving the pointer across the temperature range:

• Below alarm point: Contact should be open (NO)
• At alarm point: Contact should close (NC)
• Below trip point: Trip contact should be open
• At trip point: Trip contact should close

Step 7: Reassemble and Document

Once verified, reinstall the front glass cover and secure the retaining clips. Document all settings made, including the exact temperatures for alarm and trip points, the date of adjustment, and any discrepancies found during testing. This creates a maintenance record for future reference.

Maintenance of OTI and WTI

Regular Inspections

• Monthly: Check dial readings during normal operation; compare OTI and WTI readings for consistency
• Quarterly: Inspect sensing bulb for corrosion; verify mercury switches are functioning (if applicable)
• Annually: Calibrate instruments against a portable temperature gauge in the same location

Preventive Maintenance

1. Sensing Bulb: Ensure it remains in contact with oil. If pocket corrosion occurs, consider replacing the pocket
2. Capillary Tube: Check for any kinks or damage
3. Mercury Switches: Modern transformers often use sealed contacts; verify they operate smoothly
4. Protection Logic: Periodically test alarm and trip circuits to ensure they function

Recalibration

Professional calibration should be done:

• Every 2-3 years for normal operation
• Immediately if readings seem inconsistent with actual load

Conclusion

The Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) represent the defense against thermal damage in power transformers. OTI provides a measure of the overall transformer thermal condition and oil quality degradation, while WTI directly monitors the most vulnerable component the windings by simulating their temperature rise based on load current.