Please explain transformer protection

The terms you are using are not familiar ones for protection of power transformers. Is that what you are interested in? Please explain further.

hi jwperry,

just i want to know the basic difference between transfromer e/f protection like

1. restricted e/f

2. residual e/f

3. senstitive earth fault

also how will be CT connections to that protective relays.

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see answer #1

Referring to the IP title, could this be a question regarding "tranformer protection" of something else? Maybe decoupling of circuits?

These are three distinct types of earth fault protections for power transformers.

a)Restricted earth fault-unit protection wherein percentage of the winding remains unprotected.So for all faults occuring within the transformer secondary upto the CT point (for REF),restricted earth fault relay would operate.Instantaneous operation

b)For SEF,a CBCT is used.Not generally used for power transformers.

c)Residual earth fault:This is something like a back up protection using IDMTL principle.

Dear Majumderneil,

thank u for ur valuable explanation.

can u give me the CT connection for E/F relays.

As per my knowledge,

1. REF relay will be connected between secondary CT's and neutral CT.

2. Residual E/f relay will be directly connected from neutral CT.

3. and as u have mentioned earlier, SEF connected with CBCT.

Are these right? please give ur feedback.

1)Restricted earth fault

This kind of protection applies only for wye connected transformers.

The protection will be of the differential type( any type) and will be connected between the residual connection of the phase CTs and the neutral CT.

2)sensitive earth fault

Is a kind of connection to detect very small current to earth. In this case, the connection must be made through a residual connection or, better, by using a toroid transformer, embracing the three phases. This protection will certainly have a very ow setting in order to detect smalll faults to ground.

3)residual earth faults

Is justa a way to connect three CTs in a three phase circuit when you join together one side of the CTs and the others be connected to a relay.

For further information, please refer to www.selinc.com or www.abb.com

If any doubt still remains. please d not hesitate to contact me.

Kind regars

Are you referring to voltage under-frequency protection?

In power stations where the main generator transformer is direct connected to the generator, a condition of low frequency with resulting saturation (and damage) of the transformer can occur during start up of the generator. Generally the generator field excitation is removed until the generator is up to near synchronous speed. The transformer has a voltage/frequency saturation curve that is monitored by the E/F relay.

Large power substation transformers are protected by a transformer differential relay scheme where the currents into and out of the transformer are summed. The result should be zero except for the known losses of the transformer.

If a oiled filled transformer the has had an internal flash over or over heated windings, it can be detected by a sample of the gas above the oil. It will contain traces of acetylene gas.