Relay protection and automation of power oil transformers 110 kV

Power oil transformers are the most expensive equipment for distribution substations. Transformers are designed for a long service life, provided they operate normally and are not subject to unacceptable current overloads, surges, and other undesirable operating modes.

Various protection and automation devices are required to prevent transformer damage, extend its service life and ensure its normal operation.

Consider what protection and automation devices are provided in power oil transformers.

Transformer gas protection

Gas protection is one of the main protections of the transformer. This protection is designed to disconnect the 110 kV transformer from the network in case of internal faults in the power transformer tank.

This protective device is installed in the oil line that connects the transformer tank to its conservator.The main structural element of the gas relay is a float and two pairs of contacts that are connected when the float is lowered. In normal operation, the gas relay is filled with transformer oil and the float is in the up position with both pairs of contacts open.

In the event of a turn-to-turn short circuit in the transformer windings, or in the case of the so-called Steel burning (violation of the insulation of the steel sheets of the magnetic circuit), gases appear in the tank, which are formed during the decomposition of electrical materials under the influence of an electric arc.

The resulting gas enters the gas relay and displaces the oil from it. In this case, the float drops and closes the contacts. Depending on the amount of accumulated gas, the contacts can close, affecting the signal or completely disconnecting the transformer from the network.

Activation of the gas relay can also be due to a significant decrease in the oil level in the power transformer tank, which indicates the complete absence of oil in the conservator. That is, this device also acts as a protection against excessive reduction of the oil level in the transformer.

Load switching tank tank protection

110 kV power transformers usually have a built-in on-load voltage regulator (OLTC). On-load toggle switch located in a separate compartment of the transformer tank, isolated from the main tank by windings. Therefore, a separate protective device - a reactive relay - is provided for this device.

All failures in the on-load tap-changer tank are accompanied by a discharge of transformer oil into the conservator, therefore, in the event of an oil flow, the jet protection is activated immediately, automatically disconnecting the power transformer from the mains.

Oil level switch (RUM)

The gas relay signals the complete absence of oil in the conservator of the power transformer, but it is necessary to detect an unacceptable decrease in the oil level in time - this function is performed by the oil level relay (RUM).

The oil level switch is installed, as a rule, in the conservator of the main tank of the transformer, as well as in the conservator of the load switch. The device is configured in such a way that the float, the main structural element of the relay, closes the relay contacts if the oil level falls below the minimum permissible value for this power transformer.

This safety device provides a signal to activate an alarm, which makes it possible to detect a drop in the oil level in time.

Differential Transformer (DZT) Protection

The differential protection of the transformer (DZT) is the main protection of the transformer and serves to protect against short-circuits of the windings of the transformer and the current conductors that are in the coverage area of ​​this protection.

The principle of operation of this protection is based on comparing the load currents of each of the windings of the transformer. In normal operation, there is no unbalance current at the differential protection relay output.In the event of a two-phase or three-phase short circuit, an unbalance current occurs — the differential current and the relay act to completely disconnect the transformer from the network.

The scope of this protection is the current transformers on each voltage side of the power transformer. For example, in a transformer with three windings 110/35/10 kV, the zone of protective coating, in addition to the transformer itself, includes a bus (cable) that passes from the bushings of the transformer to current 110 kV, 35 kV and 10 kV transformers.

Current step protection of transformers

For greater reliability, in addition to the main protection of the power transformer, backup protection is provided - stepped current protection for each of the windings.

For each of the windings of the transformer, separately overcurrent protection (MTZ) a few steps. Each stage of protection has its own pick-up and drop-off time.

If the transformer feeds many consumers with high inrush currents, then to prevent false operations, the overcurrent protection has a so-called voltmeter blocking — voltage protection blocking.

For the selectivity of the transformer protection operation, each of the protection stages has a different response time, while the above basic transformer protections have the shortest response time. Thus, in the event of a transformer failure or a short circuit in the protection zone, the main protections are triggered immediately, and in the event of a failure or withdrawal condition, the transformer is protected by backup current protections.

Also, the MTZs of the power transformer retain the protection of the outgoing connections fed by that transformer, tripping in the event of a fault.

MTZ provides protection against two- and three-phase short circuits. To protect against single-phase earth faults, the 110 kV high voltage winding has zero-sequence current protection (TZNP).

The medium-voltage winding of the 35 kV power transformer and the low-voltage winding 6-10 kV supply networks with an isolated neutral in which single-phase earth faults are recorded by voltage transformers.

Most 6-35 kV networks with an isolated neutral operate in a mode in which a single-phase earth fault is not considered an emergency and, accordingly, is not automatically excluded from the operation of the earth fault protection. Service personnel receive a signal about the presence of a single-phase earth fault and begin to search for and disconnect from the network the damaged area, since prolonged operation in this mode is unacceptable.

Exceptions are made in cases where the exclusion of single-phase faults in networks is necessary for safety requirements. In this case, the ground fault protection can operate to completely disconnect the transformer or disconnect one of its windings.

Transformer surge protection

To protect the transformer from overvoltage, surge arresters or surge arresters (SPDs) are installed on the bus on each side of the transformer.

If the transformer operates in earthed neutral mode on the 110 kV high voltage side, the neutral is connected to earth through an arrester or surge arrester to protect the winding from damage if the voltage exceeds the permissible values ​​in the event of a fault in the supply network.

Additional protection of the transformer

To protect the power transformer, a number of additional protections are provided to exclude the development of small defects, deviations from normal operation in a larger emergency situation.

Overload protection — acts on the signal to promptly reduce the load on the transformer.

The temperature control relay signals an increase in the temperature of the upper oil layers above the set (allowable) values. This protection automatically includes additional transformer cooling systems, if any. For example, the blow-by fans and the pumps for forced circulation of oil in the coolers are included. If the oil temperature rises even higher, then the relay acts to disconnect the transformer from the grid.

The overvoltage protection switches off the transformer secondary winding breaker in the event of a voltage drop to unacceptable values.

Automation of power transformers 110 kV

If there are two transformers at the substation, then when the voltage drops to unacceptable values ​​or when the transformer is disconnected, the undervoltage protection affects automatic transfer switch (ATS)… This device includes sectional or busbar switches providing power to consumers from a backup power source — a power transformer.

On the medium and low voltage input switches of the transformer can be implemented automatic circuit breaker reclosing (AR), one-time restoration of the power supply of the transformer in case of disconnection from the action of one or another protection.

If the power transformer is constructive on-load voltage regulator (OLTC), then an Automatic Voltage Regulator (AVR) can be installed for it. This device monitors the voltage of the transformer windings and provides automatic switching of the on-load tap-changer to ensure the required voltage level of the windings.