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Earth fault protection of Generators

The neutral point of the generator is usually earthed, so as to facilitate the protection of the stator winding and associated system. Impedance is inserted in the earthing lead to limit the magnitude of the earth fault current.

Lecture notes on switchgear and protection devices for students
Lecture notes on switchgear and protection devices for students

Generators which are directly connected to the transmission or distribution system are usually earthed through a resistance which will pass approximately rated current to a terminal earth fault. In case of generator-transformer unit, the generator winding and primary winding of a transformer can be treated as an isolated system that is not influenced by the earthing requirements of the transmission system.

Modern practice is to use a large earthing transformer (5-100 KVA) – the secondary winding which is designed for 100-500V is loaded with a resistor of a value, which when referred through the transformer ratio, will pass a suitable fault current. The resistor is therefore of low value and can be of rugged construction.

It is important that the earthing transformer never becomes saturated, otherwise a very undesirable condition of ferro resonance may occur.

Restricted earth fault protection
Restricted earth fault protection

Earth fault protection can be obtained by applying a relay to measure the transformer secondary current by connecting a voltage measuring relay in parallel with the load resistor.

Title:Lecture notes on switchgear and protection devices for students  – VEER SURENDRA SAI UNIVERSITY OF TECHNOLOGY
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Lecture notes on switchgear and protection devices for students
Lecture notes on switchgear and protection devices for students

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More Information


    Nov 04, 2019

    We have medium voltage switchgear room in an environment where the maximum temperature is 30degC
    Is it advisable to install air conditioners in the room when all switching systems are remote and through a SCADA system?.

    • Edvard
      Nov 04, 2019

      Hi Olice, you should be fine with 30C, but it is advisable to implement some automatic ventilation or air conditioning. Connecting it to the SCADA is just an option. In this case, you must have also a measurement of temperature and humidity in the room – connected to the SCADA. A bit complicated, but it can be done.

  2. Aakash Tiwari
    Oct 20, 2018

    You are the best. Thank you for all your efforts.

  3. Moody ganiyu
    Oct 19, 2018

    Thanks for a great job you’re doing. I want to receive your lecture weekly

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