The two major equipments in a power system are the generators and the power transformers. Faults occur less frequently on these apparatus, than on transmission lines. But the damage caused by the kinds of faults usually takes much more time and money to rectify than are required to rectify the damage done by faults on lines. Rapid reclosing of circuit breakers if feasible in lines and it helps in saving the amount of damage. Gas actuated relay is the simplest form of transformer protection.
A fault in a generator or transformer on the other hand, always needs some attention of the supervisory staff. Fast clearing of faults, however helps in minimizing the damage to the apparatus and also reduces the interruption of power service resulting from reduced voltage and from stability.
Nature of Faults or Causes of Faults in Transformer
Power Transformer being static, totally enclosed and oil immersed develop faults only rarely but consequences of even a rare fault may be serious unless the transformer is quickly disconnected from the system. The faults can be in general divided into three main categories;
Fault in Auxiliary Equipment
- Transformer Oil
- Oil pump and fans
- Gas cushion pressure
- Core and winding insulation
Fault in windings
- Phase to phase
- Phase to earth
- Inter turn faults
Over Load and External Short Circuit
The choice of protection for any given power transformer, depend upon a number of factors such as size, importance and whether it has on-load or off-load tap changer.
Safety Devices with Power Transformer
In addition to electrical relays, a power transformer can be divided with the following safety and monitoring devices: Oil level gauge, Vacuum gauge, pressure / vacuum switch, sudden pressure relay, pressure relief valve, oil temperature indicator, hot spot temperature indicator.
Low oil level is a harmful condition, because internal insulation clearance, creepages etc, between leads, bushings and tanks are expected to air. This low oil level could result from: Initial mistake to fill sufficient oil up to the mark. 2. Leakage of the oil through the tank.
The oil level indicator has a float and an arm and the float is suspended in oil. When the oil level drops down, the float tilts the arm, thereby closing the alarm contacts. Both low and high level alarm contacts are provided
Gas Actuated Device (Buchholz Relay)
During internal faults, below oil level, the heat of arc causes decomposition of oil. The gases formed by decomposition are gathered in the air cushion and the conservator of the transformer. The rate of gas generation depends upon the fault current and arc voltage, The fault may be inter turn fault, earth fault or phase to phase fault.
The gases generated by the arc, are used for detecting the above faults. Pressure relief devices and rate of rise of pressure relay, besides gas accumulator relay called buchholz relay are used for the purpose. Of the above devices, buchholz relay is the most important devices which is shown in the figure. Buchholz relay gives an alarm so that the transformer can be disconnected before the incipient fault grows into a serious one.
Buchholz relay is a gas actuated device, installed in oil immersed transformers against all kinds of faults. It is used to give an alarm in case of incipient, i.e., slow developing faults in the transformer and to disconnect the transformer from the supply in the event of severe internal faults. It is usually installed in the pipe connecting the conservator to the main tank. It is the universal practice to use buchholz relay in transformer in excess of 750 KVA capacity.
The relay has two mercury switches, mounted on hinges. The upper mercury switch attached to a hinged flap and the lower switch is fixed on a hinge located in the direct path of the flow of oil from the transformer to the conservator. The upper element closes an alarm circuit, during incipient faults, whereas the lower one is designed to trip the breaker in case of severe faults.
Whenever incipient faults within the transformer occur, they cause the decomposition of some transformer oil in the main tank. The products of decomposition contain more than 70% of Hydrogen gas, which being light, tries to go into the conservator and in the process, gets entrapped in the upper part of the relay chamber. When a pre-determined amount of gas is accumulated, it exerts sufficient pressure to the float to cause it to tilt and close the contacts of the mercury switch attached to it. This completed the alarm circuit to sound an alarm.
If a serious fault occurs in the transformer an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the buchholz relay and tilts the flap to close the contacts of the lower mercury switch. This completes the trip circuit to open the circuit breaker and protects the transformer.
The following types of faults in transformer can be protected by the buchholz relay and are indicated by alarm as a warning.
- Short Circuit in Winding
- Earth Fault in winding
- Phase to phase fault in winding
- Failure of insulators
- Low oil level
- Failure of core bolt insulation
- Short circuited laminations
Advantages of Gas Actuated Device
- This relay is the simplest form of transformer protection.
- It detects the incipient faults at a stage, much earlier than that is possible with other forms of protection.
Disadvantages of Gas Actuated Device
- It can only be used with oil immersed transformers equipped with conservator tanks
- The device can detect only faults below oil level in the transformer. Therefore, separate protection is needed for connecting cables
- The relay is slow, minimum operating time is 0.1 sec and average time is 0.2 sec