Why does the Neutral cable of Transformer is Grounded ?

to eliminate the un balanced voltage,and for safety.

Primarily in balance3-phase system the current of the common neutral point to ground is equal to zero. This neutral point like in the transformer secondary is grounded to stabilized the system during unbalance conditions of the load and moreover to provide relay protection system from the neutral ground CT and enable the upstream breaker to disconnect the transformer during excessive unbalance and ground faults.

Purpose of Neutral Grounding:-

Neutral of a3 Phase Star Connected Transformer is to be Earthed Properly to Enable Earth-Fault  protection for STATOR Windings & associated Systems & also to Limit Transient Over-voltage during Fault in System or Switching, Lightning,or Heavy Load through-off  or long time Unclear fault in the System etc.etc.

In such Faults it limits the Earth fault current to less than Full load Current , for a terminal Earth Fault.

It gives System stability, by providing easy path for Unbalance system Current/Voltage into the ground during fault Condition,thus System got Stabilized..

 

These are process of Grounding the Neutral :-

1. Connect Neutral with Ground / Solid Earthing ( Small Distribution trafo)

2.Resistance Grounding : - Connect Neutral with Ground through Metal-Grid type Resistor. For Large or Medium Voltage Transformer.Resistors are short time tated for30 seconds.

3.High Impedance Grounding:- Neutral is Earthed through Primary winding of a distribution Transformer & A loading Resistor is connected across low voltage secondary. 

4. Ziz-zag- or Star/Delta transformer in the Tertiary Winding to Neutralise Earth fault current into the Ground.

Necessary Relaying  against different faults in System When System Neutral is Grounded:-

1) Connect CT in the Neutral of Proper Rating ( Say100/1,25/1 according System design). Standby-Earth fault Protection, gives100% protection against Earth-Fault.

2) Connect3 PT in Open-Delta , PT Open_Delta is the Sensing of Neutral displacement Relay.

VDG-14. In normal condition Open-Delta point Voltage will be Zero, but in case of Fault There must be some voltage in Open-Delta-point. Typical setting5-20 Volts.

3) Instantaneous Earth Fault Relay.

 

 

The neutral of a transformer depends not only the type of transformer but also the configuration of this transformer. For instance an isolation transformer if being used at the out put will require to have wye-delta configuration. For this the neutral needs to be grounded because the transformer is not getting any reference from source and will need to have its output neutral grounded. Please add more details to your question to get an effective answer

@ safi ullah ........why does the current unbalances ??

For grounding the unbalanced current......................

Some extra current may flow due to fault..............