Application of a multifunctional distance protective IED in a 15KV distribution network

Distance and over-current (OC) protection functions are the two main relaying principles applied to protect power system transmission and distribution networks. Both principles are considered non-unit schemes in that time coordination with downstream protective devices is required to ensure selective and secure operations. Traditionally OC protection has been applied on medium voltage distribution networks due to the typical radial system configuration, simple relay design, lower cost and easier coordination with fuses. However, OC protection is known to have deficiencies such as variation of sensitivity under different source impedance and long operation time for faults close to the substation due to the coordination requirement. On the contrary distance relays, conventionally applied on medium to long, high voltage transmission lines, provide a constant reach and time delay proportional only to the physical length of the conductor. In this paper, an application combining the advantages of both relaying principles in a modern multifunction intelligent electronic device (IED) on the utility distribution system was adopted. The goal of this project was to improve: • Operation time for close-in faults to the substation • Auto-reclose blocking for faults on the cable section of the circuit • Sensitivity and safety on detecting fallen conductors • Safety of personnel performing hot-line work The project execution was separated into two stages. The first stage involves IED functional verification, procurement, engineering studies, parameter settings and configuration and fma system integration and functional test in the laboratory. The second stage involves online monitoring of live events and alarms of the IEDs applying IEC 61850 client/server services with dedicated gateways installed in the substations. In this paper the discussion emphasizes only on the planning and engineering considerations of Stage 1. Detailed system models leading to the parameter settings and functional configuration of the distance IEDs are presented. The relay application and coordination principles to accomplish the above targets will be explained. The test method used to obtain accurate cable impedance parameters for the distance relay setting is discussed. Detailed network simulation and test results verifying the relay settings and functionality are also provided. Finally an overview of the substation automation system (SAS) envisioned by the utility to produce live and real-time monitoring of this substation will be briefly discussed.