Areva MiCom P120 Technical Manual Download

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Application Guide

P12x/EN AP/A96

MiCOM P120/P121/P122/P123

Page 5/80

1. INTRODUCTION

1.1

Protection of Underground and Overhead Lines

The secure and reliable transmission and distribution of power within a network is heavily
dependent upon the integrity of underground cables and overhead lines, which link the
various sections of the network together. Therefore the associated protection system must
also provide both secure and reliable operation.

The most common fault conditions, on underground cables and overhead lines, are short
circuit faults. These faults may occur between the phase conductors but will most often
involve one or more phase conductor becoming short-circuited to earth.

Faults caused by short circuits require the fastest faulted conductor clearance times but at
the same time allowing for suitable co-ordination with other downstream protection devices.

Fault sensitivity is an issue common to all voltage levels. For transmission systems, tower-
footing resistance can be high. Also, high resistance faults might be prevalent where lines
pass over sandy or rocky terrain. Fast, discriminative faulted conductor clearance is required
for these fault conditions.

The effect of fault resistance is more pronounced on lower voltage systems, resulting in
potentially lower fault currents, which in turn increases the difficulty in the detection of high
resistance faults. In addition, many distribution systems use earthing arrangements designed
to limit the passage of earth fault current.

Earthed methods as such as using resistance, Petersen coil or insulated systems make the
detection of earth faults arduous. Special protection equipment is often used to overcome
these problems.

Nowadays, the supply continuity in the energy distribution is of paramount importance.

On overhead lines most of faults are transient or semi-permanent in nature.

In order to increase system availability multi-shot autoreclose cycles are commonly used in
conjunction with instantaneous tripping elements. For permanent faults it is essential that
only the faulted section of the network is isolated. High-speed, discriminative fault clearance
is therefore a fundamental requirement of any protection scheme on a distribution network.

Power transformers are installed at all system voltage levels and have their own specific
requirements with regard to protection. In order to limit the damage incurred by a transformer
under fault conditions, fast clearance of the windings with phase to phase and phase to earth
faults is a primary requirement.

Damage to electrical plant equipment such as transformers, cables and lines may also be
incurred by excessive loading conditions, which leads directly to overheating of the
equipment and subsequent degradation of their insulation. To protect against such fault
conditions, protective devices require thermal characteristics too.

Uncleared faults, arising either from the failure of the associated protection system or of the
switchgear itself, must also be considered. The protection devices concerned should be
fitted with logic to deal with breaker failure and relays located upstream must be able to
provide adequate back-up protection for such fault conditions.

Other situations may arise on overhead lines, such as broken phase conductors.
Traditionally, a series fault has been difficult to detect.

With today's digital technology, it is now possible to design elements, which are responsive
to such unbalanced system, conditions and to subsequently issue alarm and trip signals.

On large networks, time co-ordination of the overcurrent and earth fault protection relays can
often lead to problematic grading situations or, as is often the case, excessive fault
clearance times. Such problems can be overcome by relays operating in blocked overcurrent
schemes.

Areva MiCom P120, Technical Manual

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