Connecting a CT to a protection relay differs from metering applications in several important ways. Protection relays require CTs with specific accuracy classes designed to perform linearly during fault conditions, when primary currents may be many times the rated value. The CT secondary must be connected to the relay's current input terminals with correct polarity to ensure proper directional and differential protection operation.
The secondary connections S1 and S2 should be connected to the relay terminals marked k and l respectively on the circuit diagram-. For differential protection schemes, polarity alignment is critical. The currents entering both sides of the protected equipment must be considered with respect to their direction. When a fault occurs in actual operation, the relay must be able to discriminate between internal faults and through-faults, and this discrimination depends entirely on correct CT polarity and connection-.
In transformer differential protection, the connection of CTs must account for the phase shift introduced by the power transformer's winding configuration. If the primary transformer winding is delta-connected, the primary CTs are typically connected in star; if the secondary transformer 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-. For more complex situations, external interposing current transformers (ICTs) may be used to achieve the required ratio and phase correction-.
Ground fault protection schemes also rely on specific CT connections. Direct relaying can provide grounded-phase protection using a CT with a ratio such as 50/5 or 10/1. Balance flux relaying detects grounded-phase current using a doughnut-type CT installed around the three phase conductors, where the sum of the currents should normally be zero. Residual trip relaying uses the residual current derived from the three phase CTs-.
Regardless of the protection scheme, the same fundamental safety rules apply: the CT secondary must never be left open while primary current is flowing, and the secondary circuit must be grounded at one point. Before wiring, the CT secondary should be de-energized by shorting it via a shorting block. All external current transformer secondary windings must be grounded to the ground bus, and under no circumstances are the CT secondary windings to be left open-circuited when primary current is flowing. After installation, verify polarity by observing the relay's measured currents under load or by using a phase-angle meter during commissioning.