Public perception towards OHLs 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 CTs for 275kV and 400kV cable systems. However, CT installation and maintenance on cable circuits 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 the conventional CT based protection scheme.
In addition, to increase the operational reliability and reduce down time it is imperative to detect and discriminate transient faults i.e. lightning strikes on OHLs of hybrid systems to enable the Delay Auto Re-close as stated in TS 3.24.7 and PS (T) 10.
Through work with CIGRE and contacts with other utilities, an alternative non conventional current sensor i.e. Rogoswki coil base cable protection system by Cooper Power Systems has been identified as a potentially ideal solution which could offer greater operational, safety, and construction benefits over conventional protection systems due to the following features:
Linearity and no saturation even at high fault currents and magnetising inrush reducing the likelihood of protection mal operation. This characteristic could also be used for monitoring and profiling of insulation degradation which could lead to better asset management practice.
Light weight and compact size to address the space constraint issue.
Increased safety as opening secondary wiring during operation does not result in hazardous voltages.
Installation does not require opening of the primary conductors owing to the splitcore design which could reduce outage time for installation and hence has the potentiality to be deployed as ERTS system.
Objectives
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 Current Transformer (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.
