Chapter 5-2-3 Protection of Transformers and Shunt Reactors

Article 449. Relay Components of Transformers

The protection system of ransformerst should be equipped against the following items other than items stipulated in the technical regulations.

-Partial discharge at 500kV bushings.

-One-phase ground fault in isolating neutral power grid of -6 10kV behind transformer which is interrupted when facing one-pole ground fault for the safety (refer to the article 476).

-One-phase ground fault protection at 6 - 35kV side for autotransformer which has voltage equal to or higher than 220kV at high-voltage side.

-For 500kV transformer, mechanical relays which operate to trip a circuit breaker, have a possibility of incorrect operation, if an earthquake happens. So, an earthquake detector should be equipped in the tripping circuit of the above mentioned relay for the prevention of incorrect operation.

Article 450. Relay Components of 500kV Shunt Reactors

The protection system of 500kV shunt reactors should be equipped against the following items other than items stipulated in the technical regulations.

-Partial discharge at bushings.

-Mechanical relays which operate to trip a circuit breaker, have a possibility of incorrect operation, if an earthquake happens. So, an earthquake detector should be equipped in the tripping circuit of the above mentioned relay for the prevention of incorrect operation.

Article 451. Application of Buchholz Relays

Regarding the Buchholz relay, the relay shall sendalarm signals when amount of generated decomposed gas is low and oil level is decreased. It also has to trip for interruption in case ofamount of the gas is high and oil level continues being decreased.

It is possible to use pressure relay to protect against faults inside of transformer. Protection of oil level decrease can be performed by particular oil level checking relay which is built in oil conservator of transformer.

In order to protect contact on-load tap changer with arc extinguishment in oil, it is necessary to equip particular oil flow relay and pressure membrane.

It is necessary to separate the alarm signal from the tripping signal of the Buchholz relay at the time of the relay operation, and separate the alarm circuit from the tripping circuit of that.

Buchholz relay is only allowed to send thealarm signal (not send the tripping signal) in the following cases:

-For transformer installed in earthquake areas.(For 500kV transformer, see the article 450)

-For step-down transformer with capacity up to 2.5MVA without circuit breaker at high-voltage side.

Article 452. Short circuit protection incidents directly inside of Transformers and Shunt reactors

Regarding the installation of differential relay for transformers exceeding 6.3MVAand 500kV shunt reactors as well as transformers of 4MVA which operate in parallel, the relay can be installed in lower capacity transformer but not lower than 1MVA if:

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-Quick-break protective relay’s sensitivity is not sufficient, and time over-current protective relay’s trip is longer than 0.5 second.

-Transformer is installed in the earthquake areas.

Vertical differential protection shall be carried out by using special current relays whichare calibrated

to avoid theinrush current, transient unbalanced current and to keep stability (e.g. using saturated current transformer, relays with current restrain).

For transformers with capacity up to 25MVA, it is allowed to use current relays which have starting current calibrated to avoid theinrush current, transient unbalanced current (quickbreakdifferential protection) if such relays secure sufficient sensitivity.

Vertical differential protection shall operate so that connected parts of transformer are also located in the protected zone.

It is allowed to use built-in current transformer of transformer for differential protection in casethat other relays secure to trip short circuit (at required time)of the connection part of transformer to bus-

Differential protection relays and Buchholz relay of transformer, autotransformer and shunt reactor do not have toequip sensors with function to start up firefighting equipment. Startup of firefighting equipment shall be performed by specific fire detectors.

500kV-input insulation tester shall trip for sending signals in case that partial discharge at input occurs (without necessity to interrupt immediately) and the trip for interruption in thatcasethe input insulators are damaged (before the insulators are punctured completely).

It is necessary to equip interlock to keep input insulation tester from wrong operation in casethat the circuit connected input insulation tester to input is broken.

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When selecting starting current over-voltage protection, it is necessary to pay attention to possible overload current in case that interrupting transformers are in parallel operation and-startingself current of motor which is supplied with electricity from the transformer.

For 500kV step-down autotransformers, distance protection should be installed for tripping external multi-phase short circuit in case of requiring to secure remote backup protection, or coordination with protective devices of neighboring power grids. It is also allowed to install the above mentioned protective devices for 220kV autotransformers.

For transformer with capacity lower than 1.6MVA, it is necessary to use over-current protection which trips for interruption in case that external multi-phase short circuit happens.

For transformer of 35kV downwards, and capacity of 1.6MVA downwards, it is possible to install fuse instead of quick-break and over-current protection.

Protection against external multi-phase short circuit shall be installed as follows: 1. For 2-winding transformer: installed at main feeding source side.

2. For multi-winding transformer with 3or more circuit breakers: installed at every sides of transformer. However, it is allowed not to install protection in one among sides, but protection at main feeding source shall have 2-level time, and shorter level time is for interrupting breaker at the side without 2-level time protection.

3.For 2-winding step-down transformer which feeds separate operating sections: installed at the supplying side and at each section’s sides.

Protection of external ultim-phase short circuit shall follow theabove point 2, moreover, it is necessary to consider the necessity to supplement quick-break current protective relay to break the short circuit on busbar at low-voltage and medium-voltage sides with shorter time (based on short circuit current level, availability of busbar individual protective relay, ability to coordinate with protective relay of outgoing feeder).

In case that external short circuit protective relay of step-up transformer is not sensitive and selective enough, it is allowed to use current relay of corresponding protection of power generator to protect transformer.

Article 455. Back-up Protection for Outside Short Circuits

For step-up transformer exceeding 1MVA and transformer with 2 and 3 supply sources,and auto transformers, as required, backup protection should be equipped for ground fault short circuitin neighboring components. For autotransformer, apart from the above-mentioned protection, it is necessary to secure the selectivity of ground fault protective relay of power grid at various voltages by installing zero sequence relay for external ground fault short circuit at winding side connected to power grid with high grounding current.

In case that power grid which has some transformers with insulatedlscoiat step-down neutral terminal is in operation with neutral isolation, it is necessary to have measure to prevent the prohibited operating mode with neutral of this transformer mentioned in the article 432. Therefore, power plants or transmission substations which own earthed neutral and isolated neutral transformers, it is necessary to ground their neutral automatically, or install breaking protective relay for isolated neutral transformer before interrupting earthed neutral transformer which shares thesame busbar or same section of power grid.

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For autotransformers and multi-winding transformers which are fed by some sources, external short circuit protective relays shall trip directionally if the selectivity requires such trip.

For 220 - 500kV autotransformer in transmission substation, or for generator-transformer of 500kV and 220kV500kV connecting autotransformer of power plant, it is necessary to install external short circuit protective relay with fast trip acceleration, when busbar differential protection relay is not put in operation, to secure to break remaining faulted components without quick-break relay in 0.5 second.

Article 456. Protection against Earth Faults for Transformers

For step-down transformer and transformer-main line bock, which have voltage ta high-voltage side lower than 35kV and winding at low-voltage side is connected in star earthedto neutral, it is necessary to equip single-phase earth fault protection shall be equippedat low-voltage network by using:

1. External short circuit over-current protection installed at high-voltage side of transformer, if necessary, sound sensitivity shall be secured for readiness of using 3-relay diagram.

2. An automatic circuit-breaker or fuse at the output of low-voltage side.

3.Aspecial zero sequence protective relay installed on neutral conductor of transformer (in casethat the sensitivity of protective relay as mentioned in point 1 and 2 cannot be ensured).

It may be not necessary to install protective relay as mentioned in point 3 for industrial electric equipment in case that low-voltage cubicles are equipped with protective devices for outgoing feeders nearby transformer (distance up to 30 m from transformer), or if-phase3 cable is used to connect transformer to the above-mentioned cubicles.

In case that protective relay is installed according to point 3, it is allowed not to coordinate it with protective relay of outgoing feeder from low-voltage cubicle.

For line-transformer diagram, if protective relay is used as mentioned in point 3, it only needsottrip automatic circuit-breaker at low-voltage side without needing installation of secondary cable so that the mentioned protective relay trip circuit breaker at high-voltage side.

In case that high-voltage side of the above-mentioned transformer is equipped with fuse, the stipulations in the above-mentioned point can be applied for it.

For step-down transformer at highvoltageside of 6- 10kV, low-voltage side will feed outgoing feeder’s cubicle with fuse protection, master fuse or automatic circuit-breaker should be installed.

If fuses at the low-voltage cubicle side and fuses (or protective relays) at the high-voltage side are watched over by one operator, master fuse or automatic circuit-breaker may not need being installed at the low-voltage side.

Article 457. Overvoltage Relay for Overload Protection

For transformers exceeding 0.4MVA, depending on rate and overloadwithstanding capability, overcurrent protective relay for overload protection by sending alarm signals should be equipped.

For transmission substation without on-duty staff, the above-mentioned protective relay is allowed to trip automatically to reduce load or interrupt (in case that overload cannot be fixed by other solutions).

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