Introduction
1.1 Scope
This guide describes
the issues which should be considered when it is desired to install neutral earthing
resistors or reactors (NER's) (including resonant reactances) on power systems having a phase to earth voltage less than 20.5kV to limit the magnitude of phase to earth fault currents, for the purpose of controlling the EPR and induced voltages on nearby telecommunication systems to below hazard levels.
Nothing in this Guide shall relieve all those who own and operate electric power lines
from complying with the provisions of the Electricity Regulations or any other statutory
Act or Regulation.
The guide does not describe all the conditions to be met for systems operating at higher
phase to earth voltages than 20.5kV, because such systems may incorporate plant with
graded insulation or may be designed for less than the basic insulation levels (BIL)
needed when using an NER.
The effect of insulation ageing and consequent reduction of basic insulation level has not been taken into account in this guide.
1.2 Background
There have been many instances of telecommunication system damage due to
excessive impressed current and voltage from the nearby power system (usually during
earth faults on the power system). These present a hazard to telecommunication system
users and staff, as well as telecommunication system plant.
The mechanisms through which this hazard or damage arises are:
Induction (Magnetic Coupling)
Capacitive (Electric Coupling)
Direct Contact
Earth Potential Rise
Induction (Magnetic Coupling)
When an earth fault occurs in a power supply system, the net unbalanced fault
current that flows in the power line (and returns via earth) creates a magnetic
field. If a telecommunication cable runs parallel to the power line for a sufficient
length, a high voltage may be induced by this magnetic field onto any metallic
conductors in the telecommunication cable.
1.2.2 Capacitive (Electric Coupling)
When a power system earth fault occurs, a significant unbalanced voltage to
ground (zero-sequence) will be developed on the power system conductors. A
small unbalanced voltage can also occur under steady state conditions as a
result of unbalanced phase impedances, or unbalanced 3 phase loading. The
unbalanced voltage (AC or DC) sets up an electric field between the conductors of the power system and the ground beneath. Aerial metallic telecommunication circuits and plant situated in this electric field can have voltages “capacitively coupled” onto them.
A live, i.e. a charged conductor has an associated electric field. Any unearthed
conductor within such an electric field (or a conductor connected to earth via a
high impedance) will be charged by the electric field to a voltage determined by the capacitance between the source conductor and the secondary conductor, and the capacitance between the secondary conductor and ground.
Such capacitive coupling is independent of exposure length, but the source
impedance behind the capacitively established voltage (i.e. the ability to deliver a hazardous current) is dependant on exposure length. Length determines the
total capacitance of the affected conductor and hence the total charge which
can be stored on it.
Note: The source impedance is inversely proportional to the capacitance
between the two lines.
As the impedance (capacitance) to ground of the affected conductor also affects the energy which can be stored on that conductor, problems caused by capacitive coupling can be resolved by burying the power and/or
communications line, thereby eliminating the capacitive relationship. It should be noted that burying lines will have no benefit in the case of magnetic coupling.
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