Differential Protection for Delta-Star Transformers

Differential protection is a common and effective method used to protect power transformers from internal faults. It works by comparing the current entering the transformer on one side with the current leaving on the other side. Under normal operating conditions, the net current flowing into the protective relay should be zero (ignoring magnetizing current and tap changer effects for simplicity).

However, power transformers, especially those connecting systems with different voltage levels or winding configurations like delta-star (Dyn), introduce a phase shift between the primary and secondary voltages and currents. For a delta-star transformer, there is typically a 30-degree phase shift between the primary line currents and the secondary line currents.

CT Connection and Phase Shift Compensation

Current Transformers (CTs) are used to step down the high primary and secondary currents of the transformer to levels suitable for the protection relay. When applying differential protection to a transformer with a phase shift, the CT connections must compensate for this shift. This is achieved by connecting the CTs on one side in a configuration that introduces an opposing phase shift.

For a delta-star transformer, which introduces a phase shift (e.g., secondary current lagging primary current by 30 degrees), the CT connections should be chosen as follows:

• On the transformer's primary (delta) side, the CTs are connected in Star. This connection connects the phases directly to the relay and helps in compensating for the delta connection on the transformer primary.
• On the transformer's secondary (star) side, the CTs are connected in Delta. Connecting the CTs in delta on the star side of the transformer introduces a 30-degree phase shift that counteracts the 30-degree phase shift introduced by the transformer itself.

By connecting the CTs in this manner (Primary CTs in Star, Secondary CTs in Delta) for a Delta-Star transformer, the currents fed to the differential relay from both sides will be in phase during normal operation. This ensures that the relay only operates when there is an actual internal fault causing a significant difference in the currents, and not due to the normal phase shift of the transformer.

Summary of CT Connections for Delta-Star Transformer Protection

To summarize, for a Delta-Star transformer requiring differential protection, the CT connections should compensate for the transformer's internal phase shift. This is achieved by:

Therefore, for the differential protection of a delta-star transformer, the required CT connection is Primary Star, Secondary Delta.