National Grid Electricity System Operator
National Grid TO Innovation Team
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Gas Distribution Networks
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
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
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.
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.