NG TR(E) 399 PDF

INTRODUCTION

Cable cooling is installed on a significant number of National Grid Transco's (NGT) electricity cable circuits, particularly those in London, to prevent the thermal backfill surrounding the cable drying out during extended periods of high load. This also enables increased ratings to be available for these circuits.

As demands in England and Wales have increased, and the location of generators has changed, NGT has been required to operate its transmission assets harder. As a result the benefit of cable cooling preventing the backfill from drying out has enabled some cable ratings to be increased further.

The design of cable cooling differs from route to route, although the majority of cooling systems consist of either three or four water pipes buried with the cables. The size of these pipes differs from route to route, as does the material they are constructed of, although the majority are plastic. The water being passed through the pipe is cooled by the use of either air to water or water to water cooling banks. The different designs tend to mean that components differ from system to system. In addition, the design of water to water systems varies considerably depending on where and when they were installed. For example, there are at least three different water to water cooling systems taking water from either lakes, canals or bore water holes to provide the coolant for the cooling station.

Having so many different designs makes holding spares complex and expensive and means that maintenance and fault finding are complex as knowledge on each system is held locally. With NGT moving to centrally despatched field staff, maintaining detailed knowledge of the different cooling systems will become increasingly difficult. In addition to the cooling system design being different, the control systems used to operate the cooling system also differ from circuit to circuit. Again this creates problems with holding spares and maintenance, especially given the rate at which telecoms and electronic systems have changed over recent years. Many of these systems, although not necessarily as old as the cooling system they control, are now obsolete or excessively complex for the role they carry out.