Connecting multiple CTs in a three-phase system requires systematic attention to phase identification, polarity, and the specific requirements of the connected equipment. Three-phase systems may be configured as 3-phase 4-wire Wye, 3-phase 3-wire direct connection, or 3-phase 3-wire open delta, and the CT connection arrangement depends on the system configuration-.
In a standard three-phase system, one CT is installed on each phase conductor-L1, L2, and L3. The CTs are often designated as CT1, CT2, and CT3. Each CT has its primary side connected in series with the corresponding phase conductor, and its secondary side connected to the measuring or protection equipment.
For energy metering, the S1 conductor from each CT connects to the corresponding current input terminal on the meter. The S2 conductors from all three CTs are typically connected together and grounded at a single point. The S1 conductor (often white) connects to the input terminal, and the S2 conductor (often black) connects to the adjacent terminal for each phase. It is essential that the CT monitoring each phase is matched with the voltage input from that same phase. CT1 must monitor the same phase as the voltage connected to the meter on L1, CT2 must monitor the same phase as L2, and CT3 must monitor the same phase as L3-.
For protection relays, the connection arrangement may be more complex. In transformer differential protection, the CT connections must account for the phase shift between the primary and secondary sides of the power transformer. If the transformer primary winding is delta-connected, the primary CTs are connected in star; if the transformer secondary winding is star-connected, the secondary CTs are connected in delta-. This compensates for the 30-degree phase shift and ensures that the currents entering the relay are in phase under normal conditions.
Ground fault protection in three-phase systems may use a dedicated zero-sequence CT installed around all three phase conductors. In this configuration, the CT detects the residual current that flows only when a ground fault occurs. For protection applications where the CT is connected to an earth leakage relay, the neutral conductor and all of the live conductors must be passed through the CT aperture, while the earth connection must not be passed through the CT.
Regardless of the specific application, all CTs in a three-phase system must monitor current in the same direction. When summation CTs are used, the first CT should be connected to P1 and P2 while the second CT should be connected to P3 and P4 on the summation CT. It is important that all CTs monitor current in the same direction. The secondary circuit must be grounded at one point, and the polarity of each CT must be verified to ensure correct operation of both metering and protection functions.