Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 7/220
1. INTRODUCTION
1.1 Protection of overhead lines and cable circuits
Overhead lines are amongst the most fault susceptible items of plant in a modern power
system. It is therefore essential that the protection associated with them provides secure
and reliable operation. For distribution systems, continuity of supply is of para mount
importance. The majority of faults on overhead lines are transient or semi-permanent in
nature, and multi-shot autoreclose cycles are commonly used in conjunction with
instantaneous tripping elements to increase system availability. Thus, high speed, fault
clearance is often a fundamental requirement of any protection scheme on a distribution
network. The protection requirements for sub-transmission and higher voltage systems must
also take into account system stability. Where systems are not highly interconnected the
use of single phase tripping and high speed autoreclosure is commonly used. This in turn
dictates the need for high speed protection to reduce overall fault clearance times.
Underground cables are vulnerable to mechanical damage, such as disturbance by
construction work or ground subsidence. Also, faults can be caused by ingress of ground
moisture into the cable insulation, or its buried joints. Fast fault clearance is essential to limit
extensive damage, and avoid the risk of fire, etc.
Many power systems use earthing arrangements designed to limit the passage of earth fault
current. Methods such as resistance earthing make the detection of earth faults difficult.
Special protection elements are often used to meet such onerous protection requirements.
Physical distance must also be taken into account. Overhead lines can be hundreds of
kilometres in length. If high speed, discriminative protection is to be applied it will be
necessary to transfer information between the line ends. This not only puts the onus on the
security of signalling equipment but also on the protection in the event of loss of this signal.
Thus, backup protection is an important feature of any protection scheme. In the event of
equipment failure, maybe of signalling equipment or switchgear, it is necessary to provide
alternative forms of fault clearance. It is desirable to provide backup protection which can
operate with minimum time delay and yet discriminate with the main protection and
protection elsewhere on the system.
1.2 MiCOM distance relay
MiCOM relays are a range of products from T&D EAI. Using advanced numerical
technology, MiCOM relays include devices designed for application to a wide range of power
system plant such as motors, generators, feeders, overhead lines and cables.
Each relay is designed around a common hardware and software platform in order to
achieve a high degree of commonality between products. One such product in the range is
the series of distance relays. The relay series has been designed to cater for the protection
of a wide range of overhead lines and underground cables from distribution to transmission
voltage levels.
The relay also includes a comprehensive range of non-protection features to aid with power
system diagnosis and fault analysis. All these features can be accessed remotely from one
of the relays remote serial communications options.