In electrical transformer while external faults are dealt with the main protection system, individual items of the transformers should also be protected against the internal faults.
Faults within the main tank or tap changer selector compartment normally gives rise to gas evolution which is slow in the case of minor and incipient internal faults in the transformers and violent for more severe internal faults of the transformer. Other transformer protection devices also ensures against over-heating in parts of the windings or tap changers. Such protection devices includes: gas and oil actuated (Buckholz) relays, pressure relief valves and winding temperature indicators
Gas and Oil Actuating Relays (Buchholz Relays):
Buckholz type gas and oil actuated relays are mounted in the oil pipework leading to the conservator. These relays are mounted in the sloping pipework connecting the transformer tank to their conservator. Buckholz relay comprise a metal container inside which are two, an upper and lower, hinged float elements.
The upper float drops when rising bubbles, produced from the generation of gas resulting fro a slow or incipient faults. These bubbles pass along the pipe leading to the conservator and are trapped in the relay chamber causing its oil level to fall. As the float drops with the oil level, it causes a normal open switch contacts to close and complete a circuit usually connected to an alarm. The switch will also acts to give a low oil level alarm.
The lower element is sensitive to rapid oil flows and oil surges. When a severe internal fault occurs it will produce a rapid evaluation of gas and violent displacement of oil along the conservator pipe thus operating the lower surge element. The switch contacts operated by the float are normally open and close on operation. They are usually connected to a circuit to perform the trip function to DE-energize the transformer.
Pressure Relief Devices:
Pressure relief devices are fitted to the transformer and reactor tanks to protect them from increasing pressure due to a violent oil surge which may otherwise cause splitting of oil or ejection of bushings. Small and medium sized transformers are often fitted with explosion vents, comprising a chimney structure capped by a bursting disc. However this device is not suitable for large power transformers above 220kV since the tanks are such a large size and the oil surge created by a fault is unlikely to rupture the diaphragm in time to save the tank against explosive shock waves created locally.
The alternative is to use one or more spring operated pressure relief device around the tank which are actuated when the pressure rises to a predominate value (typically 830 mbar) and reseals again when the pressure has reduced (typically 550mbar). There is a minimum spillage of oil from the operation of such a devices and the spillage is contained within the bound around the transformer.
These devices are equipped with a switch devices with a set of switch contacts which may be used to operate an alarm.
Winding Temperature Indicators:
Winding temperature indicators are provided for protecting windings against overheating. The protection system consists of a dial type instrument connected to a capillary system and bulb sensor. The bulb is enclosed in the oil thermometer pocket in the tank cover at highest oil temperature. A thermal image in the form of a separate heater coil is mounted either around the bulb in the thermometer pocket or around the operating bellow inside the instrument case. The heater is supplied from the secondary of the current transformer monitoring the winding current. The instrument is calibrated first as oil thermometer and second as hot spot temperature indicator. The supply to the heater is adjusted so that it adds to the temperature of the top oil, a thermometer equal to the winding hot spot gradient as determined from the factory temperature rise measurement. A separate instrument is provided for each winding carrying load current. These instruments are equipped with the up to four sets of switch contacts for cooler control, winding temperature alarm and trip functions. The alarm and fixed contacts have fixed differentials whilst the cooler control contacts have differentials that are adjustable
Alarm -110o C
Cooler ON – 90oC
Cooler OFF – 65oC