Protection of transformers (autotransformers) with high voltage winding 3 kV and above and shunt reactors 500 kV

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 3. Protection and automation

Relay protection. Protection of transformers (autotransformers) with high voltage winding 3 kV and above and shunt reactors 500 kV

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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3.2.51. For transformers¹⁾ relay protection devices against the following types of damage and abnormal operating modes should be provided:

1) multi-phase short circuits in windings and terminals;

2) single-phase ground faults in the winding and on the terminals connected to the network with a solidly grounded neutral;

3) turn short circuits in windings;

4) currents in the windings due to external short circuits;

5) currents in the windings due to overload;

6) lowering the oil level;

7) partial breakdown of the insulation of 500 kV bushings;

8) single-phase earth faults in 3-10 kV networks with isolated neutral, if the transformer feeds the network in which the disconnection of single-phase earth faults is necessary for safety reasons (see 3.2.96).

In addition, it is recommended to use protection against single-phase earth faults on the 6-35 kV side of autotransformers with a higher voltage of 220 kV and higher.

1. Here and below in Sec. 3, the term "transformers" also applies to autotransformers (of appropriate voltages and powers), unless a special clause is made in the text.

3.2.52. For 500 kV shunt reactors, relay protection devices should be provided against the following types of damage and abnormal operating modes:

1) single-phase and two-phase earth faults in windings and terminals;

2) turn short circuits in windings;

3) lowering the oil level;

4) partial breakdown of the insulation of the inputs.

3.2.53. Gas protection against damage inside the casing, accompanied by the release of gas, and from a decrease in the oil level must be provided:

for transformers with a capacity of 6,3 MVA and more;
for shunt reactors with a voltage of 500 kV;
for in-shop step-down transformers with a capacity of 630 kVA and more.

Gas protection can also be installed on transformers with a capacity of 1-4 MVA.

The gas protection must act on a signal in case of weak gas formation and a decrease in the oil level and on shutdown in case of intensive gas formation and a further decrease in the oil level.

Protection against damage inside the transformer housing, accompanied by gas evolution, can also be carried out using a pressure switch.

Low oil level protection can also be implemented as a separate level switch in the transformer expander.

To protect the on-load tap-changer with arc breaking in oil, a separate gas and pressure switch should be provided.

To protect the tap changers located in a separate tank, a separate gas relay should be provided.

It should be possible to transfer the action of the tripping element of the gas protection to a signal and to perform separate signaling from the signaling and tripping elements of the gas relay (which differ in the nature of the signal).

It is allowed to perform gas protection with the action of the disconnecting element only on the signal:

on transformers that are installed in areas prone to earthquakes;
on intrashop step-down transformers with a capacity of 2,5 MVA or less, which do not have switches on the higher voltage side.

3.2.54. To protect against damage to the terminals, as well as from internal damage, the following must be provided:

1. Longitudinal differential current protection without time delay on transformers with a capacity of 6,3 MVA and more, on shunt reactors of 500 kV, as well as on transformers with a capacity of 4 MVA when the latter operate in parallel in order to selectively disconnect a damaged transformer.

Differential protection may be provided for transformers of lower power, but not less than 1 MVA, if:

the current cutoff does not meet the sensitivity requirements, and the maximum current protection has a time delay of more than 0,5 s;
The transformer is installed in an area prone to earthquakes.

2. Current cutoff without time delay, installed on the supply side and covering part of the transformer winding, if differential protection is not provided.

These protections must act to open all transformer breakers.

3.2.55. Longitudinal differential current protection must be carried out using special current relays, detuned from magnetizing current surges, transient and steady unbalance currents (for example, saturable current transformers, brake windings).

On transformers with a power of up to 25 MVA, it is allowed to perform protection with current relays detuned in terms of operating current from magnetizing current surges and transient values of unbalance currents (differential cutoff), if the required sensitivity is provided.

Longitudinal differential protection must be designed so that the connection of the transformer with the busbars is included in its coverage area.

It is allowed to use current transformers built into the transformer for differential protection, if there is protection that provides disconnection (with the required speed) of short circuit in the connections of the transformer with busbars.

If a reactor is installed in the low voltage circuit of the transformer and the transformer protection does not provide the sensitivity requirement in case of a short circuit after the reactor, it is allowed to install current transformers on the side of the low voltage terminals of the transformer to protect the reactor.

3.2.56. The differential and gas protection of transformers, autotransformers and shunt reactors should not be assigned the functions of start sensors of a fire extinguishing installation. The start of the fire extinguishing circuit of these elements must be carried out from a special fire detection device.

3.2.57. The 500 kV bushing insulation monitoring device (KIV) must be designed to act on the signal in case of a partial breakdown of the bushing insulation, which does not require immediate shutdown, and to turn off when the bushing insulation is damaged (before a complete breakdown of the insulation occurs).

A blocking must be provided to prevent false operation of the KIV device in case of breaks in the circuits for connecting the KIV to the terminals.

3.2.58. In cases of connection of transformers (except intra-workshop) to lines without switches (for example, according to the line-transformer block diagram), one of the following measures must be provided to disable damage in the transformer:

1. Installation of a short-circuit device for artificial grounding of one phase (for a network with a dead-earthed neutral) or two phases between each other (for a network with an isolated neutral) and, if necessary, a separator that automatically turns off during a dead pause of the AR line. The short circuit breaker must be installed outside the differential protection zone of the transformer.

2. Installation of open fuse-links on the higher voltage side of the step-down transformer, which act as a short-circuiter and separator, in combination with the automatic reclosure of the line.

3. Transmission of a trip signal to the circuit breaker (or circuit breakers) of the line; at the same time, if necessary, a separator is installed; to reserve the transmission of a trip signal, a short-circuiting device can be installed.

When deciding on the need to apply the transmission of a trip signal instead of the measures of paragraphs 1 and 2, the following should be taken into account:

the responsibility of the line and the admissibility of the artificial creation of a metal short circuit on it;
transformer power and admissible time of elimination of damage in it;
the remoteness of the substation from the supply end of the line and the ability of the circuit breaker to disconnect non-remote short circuits;
the nature of the consumer in terms of the required speed of voltage recovery;
probability of short circuit failures at low temperatures and ice.

4. Installation of fuses on the high voltage side of the step-down transformer.

The measures of paragraphs 1-4 may not be provided for line-transformer units, if the transformer is protected by the general protection of the unit (high-frequency or longitudinal differential special purpose) with a two-way supply, as well as with a transformer power of 25 MVA or less with a one-sided supply, if the protection the supply line also provides protection for the transformer (high-speed line protection partially protects the transformer and backup line protection with a time of no more than 1 s protects the entire transformer); in this case, gas protection is carried out with the action of the disconnecting element only on the signal.

In the case of applying the measures of paragraph 1 or 3, the following must be installed on the transformer:

if there is a transformer on the higher voltage side (110 kV and higher) of built-in current transformers - protection according to 3.2.53, 3.2.54, 3.2.59 and 3.2.60;
in the absence of built-in current transformers - differential (in accordance with 3.2.54) or maximum current protection, made using clamp-on or magnetic current transformers, and gas protection according to 3.2.53.

Damage to the higher voltage terminals of transformers can be eliminated by line protection.

In some cases, in the absence of built-in current transformers, it is allowed to use remote current transformers, if the required protection characteristics are not provided when using clamp-on or magnetic current transformers.

To protect transformers with a higher voltage of 35 kV, in the case of applying the measure of paragraph 1, remote current transformers should be provided; at the same time, the feasibility of installing a short circuit and remote current transformers or a circuit breaker with built-in current transformers must be justified by a feasibility study.

If open fuse-links are used (see clause 2), then in order to increase the sensitivity, the effect of gas protection can be carried out by mechanically performing an artificial short circuit on the fuse-links.

If the substation transformer loads contain synchronous motors, then measures must be taken to prevent the separator from disconnecting (in the event of a short circuit in one of the transformers) the current from synchronous motors passing through other transformers.

3.2.59. On transformers with a power of 1 MVA and more, as protection against currents in the windings caused by external multi-phase short circuits, the following protections with a tripping action should be provided:

1. On step-up transformers with double-sided supply - negative sequence current protection against unbalanced short circuits and overcurrent protection with minimum voltage inrush from symmetrical short circuits or overcurrent protection with combined voltage inrush (see 3.2.43).

2. On step-down transformers - maximum current protection with or without combined voltage start; on powerful step-down transformers in the presence of a two-way supply, it is possible to use negative sequence current protection against asymmetrical short circuits and maximum current protection with a minimum voltage start from symmetrical short circuits.

When choosing the operating current of the overcurrent protection, it is necessary to take into account possible overload currents when disconnecting transformers operating in parallel and the self-starting current of electric motors powered by transformers.

On step-down autotransformers of 330 kV and above, remote protection should be provided for operation in case of external multi-phase short circuits in cases where this is required to ensure long-range redundancy or coordination of protection of adjacent voltages; in these cases, the specified protection may be installed on 220 kV autotransformers.

3.2.60. On transformers with a power of less than 1 MVA (step-up and step-down), as protection against currents caused by external multi-phase short circuits, overcurrent protection acting on tripping must be provided.

3.2.61. Protection against currents caused by external multi-phase short circuits should be installed:

1) on two-winding transformers - from the side of the main power supply;

2) on multi-winding transformers connected by three or more switches - from all sides of the transformer; it is allowed not to install protection on one of the sides of the transformer, but to carry it out from the side of the main power supply, so that it turns off the circuit breakers with a shorter time delay from the side on which there is no protection;

3) on a step-down two-winding transformer supplying separately operating sections - from the supply side and from the side of each section;

4) when using overhead current transformers on the higher voltage side - from the low voltage side on a two-winding transformer and from the low and medium voltage side on a three-winding transformer.

It is allowed to provide protection against currents caused by external multi-phase short circuits only for redundant protection of adjacent elements and not to provide for action in case of failure of the main protection of transformers, if the implementation for such an action leads to a significant complication of protection.

When performing protection against currents caused by external multi-phase short circuits, according to 3.2.59, clause 2, the need and possibility of supplementing it with a current cutoff, designed to trip short-circuit faults on medium and low voltage buses (based on the level of currents) should also be considered short circuit, the presence of a separate busbar protection, the possibility of matching with the protection of outgoing elements).

3.2.62. If the protection of step-up transformers against currents caused by external multi-phase short circuits does not provide the required sensitivity and selectivity, then it is allowed to use a current relay of the corresponding generator protection to protect the transformer.

3.2.63. On step-up transformers with a capacity of 1 MVA and more, on transformers with two- and three-sided power supply and on autotransformers, according to the condition of the need to reserve the disconnection of earth faults on adjacent elements, and on autotransformers, in addition, and according to the condition of ensuring selectivity of protection against short circuits on ground of networks of different voltages, a zero-sequence current protection against external earth faults should be provided, installed on the side of the winding connected to the network with high earth fault currents.

In the presence of a part of transformers (among those with incomplete insulation of the winding on the side of the zero terminal) with an isolated neutral, it must be ensured that the unacceptable neutral mode of these transformers is prevented in accordance with 3.2.28. For this purpose, in cases where transformers with grounded and insulated neutral, supplied from the low voltage sides, are installed at the power plant or substation, protection must be provided to ensure the disconnection of the transformer with isolated neutral or its automatic grounding before disconnecting the transformers with grounded neutral operating on the same buses or network section.

3.2.64. On autotransformers (multi-winding transformers) fed from several sides, protection against currents caused by external short circuits must be directional, if required by selectivity conditions.

3.2.65. On autotransformers 220-500 kV of substations, generator-transformer units 330-500 kV and communication autotransformers 220-500 kV of power plants, it should be possible to quickly accelerate protection against currents caused by external short circuits, when the differential protection of tires or busbars is disabled, which ensures shutdown damage on elements left without high-speed protection with a time delay of about 0,5 s.

3.2.66. On step-down transformers and transformer blocks - a main line with a higher voltage up to 35 kV and a low voltage winding connected to a star with a grounded neutral, protection against single-phase earth faults in the low voltage network should be provided, carried out by using:

1) overcurrent protection against external short circuits, installed on the high voltage side, and, if required by the sensitivity condition, in a three-relay version;

2) circuit breakers or fuses on low voltage terminals;

3) special protection of the zero sequence, installed in the neutral wire of the transformer (in case of insufficient sensitivity of the protections according to clauses 1 and 2).

For industrial electrical installations, if the assembly on the low voltage side with connection protection devices is located in close proximity to the transformer (up to 30 m) or the connection between the transformer and the assembly is made with three-phase cables, it is allowed not to apply protection according to clause 3.

When applying the protection according to clause 3, it is allowed not to coordinate it with the protection of the elements extending from the assembly on the low voltage side.

For the line-transformer circuit, in the case of applying the protection according to clause 3, it is allowed not to lay a special control cable to ensure the action of this protection on the circuit breaker from the higher voltage side and to perform it with the action on the circuit breaker installed on the low voltage side.

The requirements of this paragraph also apply to the protection of these transformers by fuses installed on the higher voltage side.

3.2.67. On the low voltage side of step-down transformers with a higher voltage of 3-10 kV, supplying assemblies with fuse-protected connections, a main fuse or circuit breaker should be installed.

If the fuses on the low voltage connections and the fuses (or relay protection) on the higher voltage side are serviced and maintained by the same personnel (for example, only power system personnel or only consumer personnel), then the main fuse or circuit breaker on the low voltage side of the transformer may not be installed.

3.2.68. Protection against single-phase earth faults according to 3.2.51, clause 8, must be carried out in accordance with 3.2.97.

3.2.69. On transformers with a capacity of 0,4 MVA and more, depending on the probability and value of a possible overload, maximum current protection against currents caused by overload with an effect on the signal should be provided.

For substations without constant duty of personnel, it is allowed to provide for the effect of this protection on automatic unloading or shutdown (if it is impossible to eliminate the overload by other means).

3.2.70. If there is a separate additional transformer on the neutral side of the transformer for voltage regulation under load, it is necessary to provide, in addition to those specified in 3.2.51 - 3.2.57, 3.2.59, 3.2.63, the following protections:

gas protection of additional transformer;
overcurrent protection with braking in case of external short circuits against damage in the primary winding of the additional transformer, except for cases when this winding is included in the area of the differential current protection of the low voltage side circuits of the autotransformer;
differential protection, which covers the secondary winding of the additional transformer.

3.2.71. The protection of a linear additional transformer installed on the low voltage side of the autotransformer should be carried out:

gas protection of the additional transformer itself and protection of the contactor device of the on-load tap-changer, which can be performed using a pressure switch or a separate gas switch;
differential current protection of the circuits of the low voltage side of the autotransformer.

See other articles Section Rules for the installation of electrical installations (PUE).

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