What is the primary purpose of a zero-sequence CT?

The primary purpose of a zero-sequence current transformer (CT) is to detect ground faults. This type of CT is designed to measure the unbalanced current, which typically occurs in the system when there is a ground fault. In a normal operating condition where the system is balanced, the three phase currents cancel each other out, resulting in a zero sequence current. However, when a ground fault occurs, the current returning through the ground path becomes unbalanced, and the zero-sequence CT will pick up this anomaly.

By accurately measuring this unbalanced current, the zero-sequence CT enables the detection of ground faults quickly and reliably, which is crucial for protecting electrical systems and ensuring safety. This capability is essential in both industrial and utility applications where ground faults can lead to equipment damage and safety hazards.

The other options, while related to electrical systems, do not accurately describe the primary function of a zero-sequence CT. Measuring phase currents pertains to line or phase CTs rather than zero-sequence types, limiting voltage surges relates more to surge protection devices, and providing electrical isolation is a function associated with isolation transformers rather than current transformers like the zero-sequence CT.