According to the ANSI guide, the IEEE standard specifies the current transformer grounding practices. The grounding of the current transformer is very important in the perspective of safety and the correct operation of the protective relays. As per the grounding standard of the current transformer, the current transformer secondary circuit should be connected to the station ground at only one point. This holds true irrespective of the number of current transformer secondary winding connected to the circuit.
If the current transformer has sets of three secondary winding, the individual winding should not be connected to the ground, the circuit formed with the sets of the secondary winding of the current transformer must be connected at only a single point, at the neutral formation point of the CTs. The single grounding of current transformers eliminates the problems of the voltage generated at different ground points of the current transformer.
According to IEEE C57,13.3-2014-IEEE Guidelines for grounding of instrument transformer
- The secondary of the current and potential transformer must be grounded
- The secondary of the CTs must be grounded at a single point.
How the CT Grounding at Multiple Points Creates Problem?
If the current transformers are grounded at multiple ground points, the normal current does not create any problem. However, during a fault condition, the magnitude of potential rise at the different ground points of the current transformer will be different. The rise in ground potential may not be an exact representation of the primary current and the relay may trip even there is no fault in the protection zone. Tripping a differential relay due to a fault external to the zone of protection is one of the more popular nuisance trips.
If each current transformer is grounded separately, during fault conditions the potential rise for CT1 and CT2 secondary may be different and the relay may get tripped with this even there is no fault in the protection zone. To avoid this problem, the recommended method of grounding is to install a single ground point at the one point either at the switchboard or at the relay panel.
The point of grounding in the instrument transformer secondary circuit should be at the control board or the first point of application. If the protection relay is mounted in the breaker the neutral point of both the CTs must be grounded at one point. If the protection relay is mounted in a separate location, the star connection can be made in the breaker but the neutral points of the CTs must be grounded at a single point at the relay end.
The differential relay must trip only when the fault is internal to the protective zone. If the CTs are grounded at different points, the fault external to the protective zone may raise the ground potential of both the current transformers used for differential protection. In this situation, the ground potential rise at one of the CTs may be more than the ground potential rise at the other CT, and this difference of potential at secondary of the current transformer may cause spurious tripping of the differential protection relay.
In case of an external fault, there would be a slight difference in the secondary voltage of CTs as the rise in the ground potential would be the same for both the CTs & there would be no spurious tripping.
The reliable operation of the differential protection can be ensured if the CTs are grounded at the one point that is the star point of CTs.
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