In any electrical system, we do the neutral grounding at the power source e.g. the star-points of generators or transformers. By keeping the grounded neutrals at the power source, earth fault current will have a return path from the point of short-circuit at downstream to the source. In this way the direction of earth fault current flow can be easily identified and the earth fault protection relays in the distribution system can easily be coordinated.
After reading the above paragraph, I am sure that a question will strike your smart brain, why do we do neutral grounding as there won’t be any earth fault current in absence of neutral grounding?
Yes, it is correct that there will not be any earth fault current in absence of neutral grounding but neutral grounding has many advantages, they are as follows:
- Voltage of the phases is limited to phase to Ground Voltage
- The high voltage due to arching ground or transient line to ground fault are eliminated.
- Sensitive protective Relays for earth fault protection can be used.
- Over voltage due to lightening are discharged to ground otherwise there would have been a positive reflection at the isolated neutral of the system.
Hope your doubt is clear now. This is the reason Neutral grounding is done in a system.
Now coming to grounding of Star Point of Induction Motor,
Grounding a motor star point will create an earth path for earth fault current to flow through that motor’s star point. If there are 10 motors in a process plant and their star points are all grounded then obviously there are 10 additional paths for earth fault currents to flow through.
If all the motors’ star points are grounded in this way the earth fault current detections by the protection relays will be complicated and it is most likely that Relay will trip at the incorrect locations because earth fault currents are flowing in many directions toward multiple grounded neutral points.
Therefore the electrical consumers i.e. the load, including the capacitor banks, even if they are star connected are not to be grounded.
Motor is a balanced 3-phase load. However when the system supply voltage is unbalanced caused by unbalanced loads somewhere else or due to network conductors problem, the motor operating under unbalance voltage will result in unbalance current in the 3 windings. The same is true for the generator windings under that condition. The design engineer may then decide that individual machines should be fixed with negative phase sequence current protection.
Even if there is a neutral voltage shift in the induction motor, we should not ground the motor’s neutral point. If we ground the induction Motor, it may create nuisance trip on earth fault protection relays.