Transformer Protection Devices and Their Functions
In the event of an abnormal transformer fault, the protection system immediately disconnects the transformer from the power supply, protecting it. Transformer protection comes in two types: electrical and mechanical. These include OTI, WTI, Buchholz relay, voltage, and current. A transformer is an electrical static device used for high-voltage transformations, such as from 11 kV to 400 kV. It is a very expensive device, and once a fault occurs, the maintenance process is lengthy and expensive.
The two most commonly used transformer types are oil-type and dry-type. A transformer has two windings, the primary and secondary windings, which are essential and expensive components. Several safety devices are used to protect both windings.
Why Transformer Protection Is Needed
Transformers must be protected against three major hazards: internal faults, external faults, and abnormal conditions.
- Internal faults such as winding and core problems,
- External faults such as short circuits and overloads,
- Abnormal conditions such as overheating and overvoltage.
If protection is not in place, the result is fire and transformer failure.
What are the types of oil transformer protection or safety?
Electrical Protection:
- Overcurrent and Earth Fault Protection
- Fuse Protection
- Surge and Fire Protection
- Differential Protection
- Restricted Earth Fault Protection
- Overvoltage Protection.
- Earthing
Mechanical Protection:
- Buchholz Relay or Gas Relay
- Pressure Relief Device (PRD)
- Explosion Vent
- Silica Breather
- Oil Label Indicator
- Terminal Bushing
Thermal Protection
- OTI
- WTI
Electrical Protection
The tank is the main body part of the transformer. Both windings are housed in the tank, and oil is used to cool them. This tank body also serves as a base for mounting other accessories, such as terminals, bushings, conservators, breather vent pipe diagrams, remote tap changers, Buchholz Relays, etc.
Overcurrent and Earth Fault Protection
Overcurrent and Earth Fault Protection protect transformers from overload, short circuits, and earth faults. They are installed on the transformer incoming panel or the HT feeder panel.
Differential Protection
Differential protection works on the principle of current comparison. The primary and secondary currents are compared. If the differential exceeds a set value, the relay trips. This is the best protection against internal phase and inter-turn faults.
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Surge and Fire Protection
Lightning arresters are installed at transformer terminals to protect against lightning and switching surges. Large power transformers have a nitrogen injection fire protection system that automatically isolates the oil tank by injecting nitrogen as soon as a fire is detected.
Fuse Protection
A fuse is the simplest protection device for a transformer. When the current exceeds a safe limit, the fuse melts and disconnects the supply. It is used for small distribution transformers but is ineffective against internal faults.
Restricted Earth Fault Protection
Restricted earth fault protection is used for the incoming and outgoing zones of a transformer. It also protects specific zones.
Differential Protection
This is a very sensitive relay for transformers, so it is used for internal faults. Differential protection is a high-speed, unit-type, selective protection scheme for power transformers (typically > 5 MVA) that detects internal faults by comparing input and output currents. Utilising Current Transformers (CTs) on both sides, it trips circuit breakers if the differential current exceeds a set threshold due to winding faults or short circuits.
Overvoltage Protection
Overvoltage protection prevents excessive voltage levels in a transformer. Surge arrester devices are used for this purpose.
Earthing
Earthing is a safety protection system that discharges fault currents to ground and neutral-to-ground reverse currents.
Body Earthing
Two earthing points are connected to the body to protect against leakage current.
Neutral Earthing
Two earthing points are connected to the transformer’s secondary star-point, which is connected to the neutral. Any harmonic current received at this point is grounded.
Mechanical Protection
The tank is the main body part of the transformer. Both windings are housed in the tank, which is filled with oil to cool them. This tank body also serves as a base for mounting other accessories, such as terminals, bushings, conservators, breather vent pipe diagrams, remote frames, tap changers, Buchholz relays, and more.
Buchholz Relay or Gas Relay
The Buchholz relay, also known as the oil- and gas-operated protection relay, is a safety device used on transformers with capacities exceeding 500 kVA. It is installed between the conservator and the main tank. Smaller transformers with ratings less than 500 KVA do not require it. When a fault occurs within the transformer, gas builds up in the oil tank and flows to the conservator. A Buchholz relay is installed.
- If a major fault occurs in the transformer tank and gas builds up, it will trigger an alarm and trip protection.
- If the oil level drops, it will provide a protection alarm and trip condition.
Pressure Relief Device (PRD)
A PRV is a pressure relief valve, also known as a Pressure Relief Device (PRD). It is installed in oil-filled transformers and serves primarily to safely reduce excessive pressure that builds up in the transformer’s internal tank.
When a critical situation occurs within the transformer, such as an internal fault, excessive heat, partial discharge, or severe arcing, the oil and gas pressure suddenly increases. If this pressure is not released, the tank risks rupture or explosion.
The PRV operates automatically in such situations. As soon as the pressure inside the tank exceeds a predetermined safe limit (Set Point) and exceeds the valve’s spring force, the PRV immediately opens. Upon opening, the excess pressure and gas are released into the atmosphere, and the internal pressure continues to be released until it subsides. Once the pressure subsides, the pressure valve clasp returns to its original position and fully closes, reducing tank pressure and protecting the transformer.
Explosion Vent
This is known as a safety device for protecting oil-type transformers. It is a vent pipe diaphragm made of aluminium. A thin plate is attached to the top end to prevent the tank from bursting in the event of a significant fault. It acts as a shield against internal faults and excessive gas release. It is installed on top of the tank.
Silica Breather
A breather is installed on a conservator to remove moisture and dust particles from the air. A breather in a transformer allows air to enter and exit the transformer body during oil thermal expansion.
During operation, when a power load is applied, and the outside temperature is higher than the tank oil temperature, the oil temperature will rise, increasing the oil level. This rise in the oil level will cause the tank oil to flow into the conservator, and then the conservator air will escape through the silica gel in the breather.
When the tank oil level is unloaded or lowered, the conservator oil will flow downward, then the conservator will draw air from the silica gel breather. This process is called transformer breathing.
Silica
This is used inside the breather bottle. When air passes through the silica gel into the breather, the silica gel absorbs moisture and dust. After absorbing moisture from the air, the silica gel will change colour from blue to pink. This type of silica gel should be replaced immediately.
Colour of Silica Gel:
- Dark Blue: Dry-type silica gel.
- Light Blue: Dry-type silica gel.
- Pink: 100% moisturized silica gel – should be replaced urgently.
Oil Label Indicator
This is mounted on the vertical side of the conservator tank to measure the oil level in the conservator. The oil level should be at least 70% of the conservator tank. This allows for clear monitoring and topping up if the level is low.
Terminal Bushings
Terminals and bushings are installed on transformers to connect power supply cables to the input and output sides. They are used to connect the power supply to the windings. Two types of terminals are used, one for input and the other for output. The terminals are mounted on bushings. They are installed on the tank and the terminals as insulators. Made of porcelain, epoxy resin, or ebonite, they are placed on top of the tank.
- High Voltage Bushings: These are used on the primary side for voltages between 11 kV and 33 kV.
- Low Voltage Bushings: These are used for low-voltage external supplies, such as 433 kV.
Thermal Protection
Thermal protection in transformers is a safety mechanism that protects the transformer from excessive temperatures (overheating). When a transformer overheats, its windings (coils), insulation, and oil can deteriorate. The thermal protection system provides timely warnings or transformer shutdown.
Oil Temp Indicator Device: (OTI)
The oil temp indicator measures the temperature of the transformer oil in the oil tank. If the oil temperature in the main tank exceeds a set limit due to a fault, such as a short circuit or insulation damage, an alarm and trip circuit will be activated. This will cause the oil temperature indicator (OTI) device to trip the breaker. The OTI alarm and tripping limits can be set according to site requirements.
OTI Default Limit Settings:
- Oil temp alarm: 80°C–85°C
- Oil temp tripping alarm: 85°C–90°C
Winding temp indicator device (WTI)
The temp indicator measures the temperature of the transformer windings. If the winding temperature exceeds the set limit due to a fault, such as a short circuit or insulation damage, an alarm and trip circuit will be activated. This will cause the winding temperature indicator (WTI) device to trip the breaker. The WTI alarm and tripping limits can be set to suit site requirements. Standard limits are listed below.
WTI Default Limit Settings
- Winding Temp Alarm: 85-90°C
- Winding Temp Tripping Alarm: 90-95°C
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