What is the impact of improper grounding on the accuracy of differential protection schemes in a substation?

Differential protection is a protection scheme used to detect internal faults within a defined zone, such as a transformer or a busbar. It operates by comparing the current entering and leaving the protected zone. Under normal operating conditions or external faults, the current entering should equal the current leaving (after accounting for any transformer turns ratios or phase shifts). If there's an internal fault, the currents will be significantly different, causing the differential relay to trip. Proper grounding is critical for the accurate operation of differential protection. Improper grounding can lead to circulating currents in the ground system, which can then induce spurious currents in the CT (current transformer) circuits used by the differential relay. These spurious currents can cause the relay to operate incorrectly, either by falsely tripping during normal operation or by failing to trip during an internal fault. For example, if the substation ground grid has high impedance or is not properly connected, ground fault currents can take alternate paths through the CT circuits, creating a false differential current. Similarly, if multiple grounding points are used and not properly coordinated, circulating ground currents can flow through the CT circuits. Furthermore, improper grounding can also affect the accuracy of the CTs themselves. If the CTs are not properly grounded, they may be susceptible to saturation during fault conditions, leading to inaccurate current measurements and potentially maloperation of the differential relay. Therefore, a well-designed and maintained grounding system is essential for the reliable and accurate operation of differential protection schemes.