Mar 2013
Electricity Transmission
Non Conventional Current Sensors
Mar 2013
National Grid Electricity System Operator
National Grid TO Innovation Team
Click here to send a question to the contact.
Innovation Funding Incentive
Gas Distribution Networks
Public perception towards Overhead Lines (OHL) and limitations on rights-of -ways in populated areas could potentially lead to an increase in construction of Cable only and Cable & OHL hybrid circuits in future. This situation presents a unique challenge for cable protection systems especially on the Hybrid one as protection systems must differentiate between cable and OHL faults to ensure the greater reliability of the power system.

Current practice in National Grid is to implement two main unit protection schemes sourced from two different suppliers by using conventional Current Transformers (CT) for 275kV and 400kV cable systems. However, CT installation and maintenance on cable circuit is immensely cumbersome due to bulk structure of CTs and space constraints associated with cable tunnels and trenches.

Moreover, CTs on cable circuits are subject to a high magnitude of charging/discharging current during switching on and off, a condition which could lead to potential CT core saturation and mal-operation of the protection relays. For green field application these constraints may be addressed by an appropriate design solution; however on refurbished circuits especially where part of the OHL circuit is undergrounded by using cables, it becomes challenging to achieve the prescribed selectivity and security with conventional CT based protection scheme.

This R&D project aims:

  • To evaluate the practicability, reliability and benefits of implementing alternative non conventional current sensors (i.e. Rogowski coil) based differential unit protection for Cable systems (i.e. Cable only and Cable & overhead line OHL) hybrid installations) over conventional CT based protection. To carry out the preliminary evaluation a pilot installation is recommended on Pitsmoor-Wincobank cable circuit in April 2012 as a monitoring unit.
  • To determine the system’s suitability to be utilised as Emergency Return to Service (ERTS) system.

This will help to formulate a technical and operational knowledge base for Non Conventional Instrument Transformer (NCIT) protection systems which could lead to evaluation of future technical and procurement strategy to deploy as replacement and/or new Cable system protection.