A survey on performance evaluation of low impedance numeric and high impedance busbar protection schemes

National Grid, UK is currently under the process of replacing the conventional high impedance (HiZ) busbar protection systems in their substations with modern low impedance (LI) numeric busbar protection schemes. They are also planning to deploy the same schemes in their new substations and substation extensions. This calls for an extensive changeover from the conventional operation, protection and management philosophy adopted by National Grid which is mainly dictated by HiZ busbar protection system. In this regard, National Grid is carrying out a thorough review of performance of LI numeric busbar protection schemes and comparison of their performance with the HiZ systems in matters of security and dependability and familiarity of schemes with the operating personnel. This paper reports a part of the review process in the form of performance comparison of HiZ and LI busbar protection in terms of security, dependability, compatibility with existing protection systems and protection management. The study draws on IEEE and CIGRE reports on deployment of numeric busbar protection, manufacturers´ literature, and National Grid´s own reliability studies, assessment of their policy for deployment of numeric busbar protection and operational experience of the operating personnel. The paper also highlights some of the key issues, resulting from this study, which should be considered by National Grid before full commissioning of the modern busbar protection.