Why Earthing Transformer are used?
Earthing transformer is an auxiliary transformer and a part of an electrical earthing system, which is used to provide neutral connection for earthing to a power system where the neutral connections are unavailable. Earthing transformer is generally used in a power system is delta connected. It provides a low impedance path to the neutral and also limits the transient over voltage when the ground faults occur in the system.
In Power plant, it is mainly used during the Import time or drawing power from the grid when the star connected alternator or generator not in the line. i.e if alternator present means, the earthing transformer does not require to be in line, the star alternator’s neutral point does the earthing transformer’s function.
The earthing transformer can be used to create a neutral point for a network. ZN (Zig-Zag-Neutral) connection is usually applied. Z connection provides linear and specified zero sequence impedance. YN+d can also be applied.
Also see: Why tertiary Winding used.
Why earthing transformer is required: Case study
We have installed one number of earthing transformer. We had a problem, whenever the fault occurs inside of our plant (11kV), then our main grid power transformers 110kV breaker got tripped on earth fault. We have checked everything, relay co-ordination and relay calibration everything was fine. but we do not know the real problem. Also our distribution transformers are Star-Delta configuration which means delta is 11kV side and star is 440 Volts side, in 110kV side the same configuration but star was primary and delta was secondary. Hence our secondary feeder side of the all transformer was delta connected, that’s why if any fault occur is the 11kV side it directly goes to the 110kV side. Afterwards we decided to install a new earthing transformer, we installed, then the problem was rectified.
We have used Zig-Zag transformer…
Why we Have Chosen Zig-Zag Transformer:
- A zig-zag transformer is used for providing grounding on the transformer.
- It provides insulation between the ground and the component so that the system component may not be affected by the fault currents.
- We have lot of VFD, since Zigzag transformer terminates the harmonics of the power system.
- Reduces voltage stress under a fault condition.
- It has three limbs and has no secondary winding. One limbs contain two set of winding and both are identical. In this one set is connected in the star to provide the ground point. Under normal operating conditions the total flux in each limb is negligibly small. Therefore, the transformer draws very small magnetizing current. Under fault conditions, the impedance of the grounding transformer is very low.