The application of let-through energy protection to the main and back-up protection elements on high voltage overhead feeders

摘要

High voltage feeders are generally protected using main (e.g. impedance) and back-up protection. The main protection is generally set to operate as fast as possible for faults on the feeder. Operating time delays are introduced to ensure that selectivity is maintained between protection devices and this improves system security. The back-up protection elements are normally current based inverse definite minimum time elements (IDMT). These IDMT elements are not bound to certain reach limits like the main protection elements. Hence they are set much slower than the main protection elements. During network faults, the fault current will heat up the conductor based on the magnitude of fault current, the resistance of the conductor and the time that the conductor is exposed to this fault current. The conductor exposure energy limit can be calculated by setting the I~2Rt heating effect equal to the heat gained in the conducting material and assuming it is an adiabatic process. The i~2t rating of the conductor is also referred to as the let-through energy (LTE) limit or joule integral of the conductor. If the conductor is exposed to this for an extended period of time the conductor can get damaged (plastic deformation). The traditional simplified IDMT equations cannot be used in an interconnected network because the measured current can change before a trip is issued. To evaluate the LTE application on high voltage feeders, three case studies were used. The first a general evaluation of 48 actual high voltage feeders, secondly a hypothetical feeder was used and lastly a detailed evaluation on a problematic feeder. The results indicated that the probability of conductor thermal damage is unlikely when the main impedance protection is considered. However, the probability of conductor damage due to back-up protection is high. Especially for faults close to the busbar and when the back-up protection is set to initiate an auto-reclose cycle. The recommendation is to speed up the IDMT elements and apply instantaneous curves where possible. This evaluation allows the protection engineer to not only provide good selectivity, sensitivity and speed to the network protection, but that he is actually protecting the feeder and improving network availability in the long run.