08.Importance of CT Star Point Representation

Importance of CT Star Point

1. Correct Polarity and Direction of Current Flow

   • The CT star point ensures that all CTs are aligned with the system's current flow.

   • Incorrect star point placement can lead to protection relay malfunction (e.g., relay trips during normal load or fails during faults).

2. Relay Reference Point

   • The star point is often the reference point for protection relays (like differential or earth fault relays).

   • If placed incorrectly, the relay will "see" the wrong differential or residual current.

3. Earth Fault Protection

   • For residual current measurement (e.g., neutral overcurrent or restricted earth fault protection), CTs must be connected properly with the star point aligned away from the protected equipment.

   • If CTs are star-pointed on the wrong side, it can cause false trips or no trip during actual faults.

• Single Busbar Configuration

• Single Busbar with Bus section Configuration

• Single Busbar with Transfer busbar configuration

• Single Busbar with Bus section + Transfer bus coupler + Section Isolator configuration

• Double Busbar with Bus coupler Configuration

• Double Busbar with Bus coupler + Bypass Isolator Configuration

• Double Busbar with Bus coupler + Transfer Buscoupler configuration

• Mesh Busbar configuration

• One and Half Breaker Busbar Configuration

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Relay -C

In this case of Relay C, the current flows from the busbar into the transmission line. The CT star point is connected on the side of the protected line. This configuration ensures that the relay accurately senses the direction of current flow and operates correctly during fault conditions. This type of configuration widely used in Line / Transformer HV side. 

Relay -D

In this case of Relay D, the current flows to the busbar From the transmission line. The CT star point is connected on the side of the protected line. This configuration ensures that the relay accurately senses the direction of current flow and operates correctly during fault conditions.This type of configuration widely used in Line / Transformer HV side.

Relay -A

In this case of Relay A, the current flows from the busbar into the transmission line. However, the CT star point is connected away from the protected line. Due to this reversed star point connection, the relay interprets the current as leaving the line, even though it is actually entering.This type of configuration widely used in Busbar / Transformer LV side.

Relay -B

In this case of Relay B, the current flows to the busbar From the transmission line. The CT star point is connected away from the protected line. Due to this reversed star point connection, the relay interprets the current entering to the line, even though it is actually leaving. This type of configuration widely used in Busbar / Transformer LV side.

There are some methods to avoid such mistakes, which we will discuss shortly. Stay connected for more details.