Application of distance protection for transformers in Eskom transmission.
Eskom is South Africa’s state owned utility who is responsible for the generation, transmission and distribution of electricity. The transmission network in Eskom consists of thousands of kilometres of lines operating at voltages from 220kV to 765kV. Three winding transformers, two winding transformers and autotransformers are employed in Eskom’s transmission network. High Voltage (HV) Inverse Definite Minimum Time (IDMT) overcurrent protection and Medium Voltage (MV) IDMT overcurrent protection are employed to provide backup for these transformer’s differential protection and for uncleared through faults. Eskom’s Transmission setting philosophy states “that the HV and MV IDMT overcurrent elements must be stable at 2 x full load current of the transformer”. This has resulted in MV and HV over-current protection not detecting MV multiphase busbar faults in substations with low MV fault levels which are located far away from generating stations. In such cases a possible solution is to use distance protection for transformers. This research study investigates how the different vector groups of the power transformer affect the impedance measured by the protection relay, and how standard distance algorithms in protection relays respond to faults located on the MV side of the power transformer. The study consists of a literature review of current practices of transformer distance protection. Autotransformers and distance relays are discussed with manual fault calculation examples. The effects of tap-changers and transformer inrush current on transformer distance protection are also discussed. DigSilent Power Factory software version 15.0.2 was used in the modelling and simulation of faults. The response of distance elements for various faults located on the MV side of transformers are analysed and summarised in two tables which indicate which loops will measure the precise distance to fault through the various transformers and which loops will measure the distance to fault with a slight error. Multifunction Intelligent Electronic Devices (IEDs) with both distance and differential functions are now being commissioned in the Eskom transmission network for the protection of transformers. The distance elements employed in transformer IEDs are similar to distance elements found in a line distance relay. These distance functions can be set to provide local backup protection for uncleared HV and MV busbar faults in the Eskom Transmission network.