Protection Of Overhead Lines And Cable Circuits; Micom Distance Relay; Protection Features; Introduction - GE MiCOM P40 Agile Technical Manual

Application Notes
MiCOM P40 Agile P442, P444
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 provide secure and
reliable operation. For distribution systems, continuity of supply is of paramount 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. Therefore, 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 to avoid the risk of fire.
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 is necessary to
transfer information between the line ends. This puts the onus on the security of signalling
equipment and on the protection in the event of loss of this signal. Therefore, 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 General Electric. 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 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.
1.2.1

Protection Features

The distance relays offer a comprehensive range of protection functions, for application to
many overhead line and underground cable circuits. There are 3 separate models available,
P441, P442 and P444. The P442 and P444 models can provide single and three pole
tripping. The P441 model provides three pole tripping only. This manual is applicable just for
P442 and P442 relays. The protection features of each model are summarised below:
• 21G/21P: Phase and earth fault distance protection, each with up to 6 independent
zones of protection. Standard and customised signalling schemes are available to give
fast fault clearance for the whole of the protected line or cable.
• Load Blinding (Load Avoidance) for both phase and ground distance elements
• 50/51: Instantaneous and time-delayed overcurrent protection - Four elements are
available, with independent directional control for the 1
P44x/EN AP/Hb6
st nd
and 2 element. The 3
(AP) 5-7
rd