Investigation of 3 terminal differential protection using standard-based numerical relays
Transmission lines are a vital part of the electrical distribution system, as they provide the path to transfer power between generation and load. Factors like de-regulated market environment, economics, etc. have pushed utilities to operate transmission lines close to their operating limits. Any fault, if not detected and isolated quickly will cascade into a system wide disturbance causing widespread outages for a tightly interconnected system operating close to its limits. Current differential criterion is used with success to protect various elements in power systems, i.e. transmission lines, power transformers, generators and busbars. The alpha plane differential relaying system provides sensitive protection for transmission lines, security and dependability for external faults. This thesis focuses on three terminal alpha plane differential protection with the aim to develop a complete test method using OMICRON test universe software essentially defining security, dependability and sensitivity of the alpha plane characteristic. The research analyses the three terminal alpha plane characteristic and existing primitive test methods and develops an improved test method using IEC 61850 standard. The primitive methods are time consuming and result in unnecessary prolonged outages. These methods have been discussed and improved in the thesis by implementing IEC 61850 standard. First the standard IED Capability Description (ICD) file is modified by developing new logical nodes using AcSELerator Architect and XML Maker software. Then the developed logical nodes, three terminal differential protection alpha plane characteristic with its additional infeed/outfeed check logic, and the developed test method are tested simultaneously using Test Universe software. A laboratory test bench is built using three SEL311L relays, two CMC 356 Omicron injection devices, PC, MOXA switch, CMIRIG-B time synchronising unit, SEL 2407 satellite synchronised clock, and a DC power supplier. The test method developed in this research vindicates benefits of IEC 61850 standard over hard wired systems. Prolonged outage times due to test set preparation using hard wires are drastically reduced. The thesis findings and deliverables will be used as a solution to industrial problems, postgraduate studies of other students and research project.