Relay protection open delta

Open delta connections use two single-phase transformers to provide three-phase voltage inputs to relays, primarily for earth fault and vector sum protection, with specific testing and measurement considerations.Overview of Open Delta ConnectionAn open delta connection (also called V-V connection) uses two single-phase transformers to supply a three-phase system instead of the usual three transformers in a closed delta. This configuration is often used in emergency situations or for relay protection purposes when full three-phase transformers are not available . In relay protection, open delta PTs are commonly employed to:Detect earth faults in delta-connected windings.Measure the vector sum of phase voltages, which is zero under balanced conditions.Provide inputs to natural displacement relays or other protective relays for delta winding protection .How Relays Interpret Open Delta VoltagesIn an open delta PT setup:The relay receives phase-to-phase voltages from the two transformers.One PT end is often grounded near the relay for safety.The relay defines a reference voltage (VNRELAY = 0 V), and measures VARELAY and VCRELAY relative to this reference.From these two measured voltages, the relay calculates the remaining phase-to-phase voltages (VBC, VCA) using phasor relationships . This allows the relay to simulate a full three-phase measurement even though only two transformers are used, enabling detection of faults such as line-to-ground or earth faults in ungrounded or delta systems .Applications in ProtectionOpen delta PTs are particularly useful for:Earth fault protection in delta-connected generator windings or ungrounded systems.Natural displacement relays, which rely on the vector sum of phase voltages to detect abnormal conditions.Overvoltage detection in ungrounded systems, where single line-to-ground faults cause the unfaulted phase-to-ground voltages to rise, which can be monitored via the open delta PT configuration .Testing and SimulationTesting relays connected to open delta PTs requires careful simulation of phase-to-phase voltages:Inject phase-to-phase voltages into the relay inputs using a test set.Ensure the jumper between VBRELAY and VNRELAY is short and without isolation devices.The relay calculates the missing phase voltages from the measured ones to verify correct operation under balanced and fault conditions .Modern relay testing tools, such as Omicron or SEL test sets, can simulate open delta PT connections by setting appropriate phase angles and magnitudes to replicate normal and faulted conditions .Key ConsiderationsOpen delta PTs provide less power capacity than a full delta system but are sufficient for relay inputs.Proper phasor relationships must be maintained to ensure accurate relay operation.Grounding and safety measures are critical during testing and operation.Relays must be programmed to interpret open delta voltages correctly, especially for earth fault and overvoltage protection . In summary, open delta PT connections are a practical solution for relay protection in delta or ungrounded systems, allowing relays to detect earth faults and monitor phase voltages accurately using only two transformers, with careful attention to phasor calculations and testing procedures.