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Transformer Differential Protection System – Merz Price Protection of Transformer

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In the last article, we discussed about “Merz Price Protection or Differential Protection of An Alternator“. In this article, we will discuss in detail about, the transformer protection system and Merz Price or Differential Protection System of Transformer. Let’s get started.

Transformer Protection System

Among the electrical machines, a transformer is a machine of simple and simple structure. A transformer is a stationary electrical machine whose core with windings is usually immersed in oil in a steel tank. Because of this transformer faults can occur and can be fatal. For this, the transformer should be isolated from the entire system as soon as possible. An automatic transformer protection system is required to protect the transformer from these fatal faults.

Protective equipment and protection systems used in transformer differential protection:

(a) Gas relay (Buchholz relay). It gives an alarm for primary-level faults.

(b) The transformer is protected against phase-to-earth and inter-turn faults by means of a Merz price protection system.

(c) Protection against insulation degradation faults due to high voltage (lightning surge and arcing earth etc.) with other relays and lighting arresters.

Also used are core balance leakage protection, combined leakage, and overload protection systems. The type of protection to be chosen for a particular power transformer depends on various factors, such as size and importance of the transformer, cooling method, on-load or off-load, tap changer, etc.

What are protection system required for transformer?

(1) Details of transformer-

Namely- (a) KVA rating. (b) Voltage ratio. (c) Connection system of windings. (d) Percentage reactance. (e) Whether neutral point is earthed, value of earthing resistance. (f) Whether to be used indoors or outdoors. (g) Whether there is a conservator.

(2) Length and cross section of connecting lead used between CT and relay panel etc.

(3) Amount of fault at the terminals of the power transformer.

(4) Network diagram with location of transformers.

(5) Information specifically required for any protection procedure.

Transformer Differential Protection System – Merz-Price Protection Of Transformer

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Merz-price or Transformer differential protection system based on circulating current principle is very useful for transformer protection system as it works effectively on phase-to-ground and phase-to-phase faults. Adequate precautions should be taken before applying this system. Because the number of turns of the primary and secondary windings of the transformer is not the same. Moreover, the connection system on both sides of the transformer is not the same. Power transformers may have star-star, star-delta, delta-star, delta-delta etc connections. Figure three Connection of -phase delta-star power transformer is shown below.

merz price protection of Transformer ‍ eeepedia.com

Figure: Differential Protection of Transformer (3 Phase Delta-Star)

Again, the transformation ratio of a transformer is never unity. Therefore, the current transformer (CT) connections and the number of turns should be such that equal currents are produced in each phase under normal conditions and their flow direction through the pilot wire is opposite to each other. For this condition, the CTs secondary is five numbers reversed and the transformer will have delta to star connection and delta to CT star connection.

Under normal conditions, the relay is silent as the current flowing in the secondary of the CTs is equal and opposite to each other. But if there is a current imbalance in the secondaries of the CT due to an internal fault, the relay will energize or activate and trip the circuit breaker and disconnect the supply.

Relays connected in this manner must have the required time delay characteristics. Because of switching surges, transient voltages, etc., there must be a time delay arrangement to prevent the circuit breaker from tripping at momentarily high voltages.

All precautions to be taken during merz price or Differential Protection System of Transformer:

Transformer protection is provided in the same way as generator or Alternator protection is provided in a merz price system, but certain precautions have to be taken. For example –

(1) Turn ratio of CT varies or is not unity: Generally, the transformation ratio in a transformer is never unity. As a result, the current in the primary and secondary of the transformer is also different. so that equal current is produced in each phase in normal condition and their direction of flow is reversed in the pilot wire.

(b) Reverse connection of transformer to be connected to CT: In a three-phase power transformer, there is a phase difference between the primary and secondary currents. So in this condition, even if the CT connection with the correct turn ratio is connected, some amount of differential current can flow in the relay coil and operate the relay in normal conditions. For this, the CT connection has to be reversed to the transformer connection.

That is if the connection of the power transformer is star/delta, the connection of CT will be delta/star. Again the connection of the power transformer is delta/delta and the connection of CT is star/star.

(3) To change the turn ratio of the CT used, there should be a tap changing arrangement: As different transformers have a tap changing arrangement and changing the tap causes problems. Therefore, under normal conditions, the differential current relay operates even if the taps of the transformer are changed. Therefore, to solve this problem, similar tap-changing arrangements have to be made in CT as well.

(4) CTs used should be of harmonic resistance and harmonic blocking type: Magnetizing in-rush current during transformer protection causes problems. The amount of magnetizing current is very small under normal load conditions. But when the transformer is turned off, this current increases very rapidly for a short period of time. This current can operate the relay, so the CTs have to be of the mentioned religion.

Hope you have got a complete understanding of the transformer differential protection system and transformer protection in merz-price protection of transformer.

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