Relay protection only connected to two-phase CT

Relay protection can operate with only two-phase CTs using an open-delta or two-CT scheme, but careful consideration of phase compensation and fault detection is required.Overview of Two-Phase CT ConnectionsIn three-phase systems, it is sometimes impractical or costly to install a CT on every phase. In such cases, relay protection can be implemented using only two CTs, commonly referred to as an open-delta CT connection. The third phase current is mathematically derived from the two measured phases, allowing the relay to monitor the system effectively . This method is often used in retrofit scenarios, legacy installations, or where physical space limits CT installation .ApplicationsBusbar Protection: Two CTs can be used in an open-delta configuration to detect faults on a busbar. The relay measures the difference between the two CT secondary currents and calculates the missing phase to identify fault conditions .Transformer Differential Protection: In some transformer connections, two CTs on the delta side can be used to compensate for phase shifts introduced by the transformer vector group. The relay interprets the secondary currents with a phase adjustment to maintain accurate differential protection .Metering and Protection Trade-offs: While two-CT schemes reduce installation costs, they may introduce accuracy limitations. The derived third-phase current can lead to errors if the system is unbalanced or if CT saturation occurs during high fault currents .Key ConsiderationsPhase Compensation: When using two CTs, the relay must account for phase shifts, especially in transformer differential protection where wye-delta or delta-wye connections introduce a 30° displacement .Relay Settings: Relay settings must be adjusted to account for the √3 factor and the derived third-phase current to avoid false trips or missed faults .CT Accuracy: Protective CTs must maintain accuracy under fault conditions. Class C CTs are preferred for protection relays because they provide reliable performance across a wide range of currents .Limitations: Open-delta CT connections may not detect certain zero-sequence or unbalanced faults as effectively as full three-CT schemes. Careful design and relay selection are essential to ensure selective and reliable protection .ConclusionUsing only two-phase CTs for relay protection is feasible and widely applied in open-delta or retrofit configurations. However, proper phase compensation, relay calibration, and understanding of system limitations are critical to ensure accurate fault detection and reliable protection. This approach balances cost and practicality while maintaining acceptable protection performance in many industrial and utility applications .