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Neutral earthing in an industrial HV network n
°
62
Neutral earthing in
an industrial HV
network
photographie
François Sautriau
Having graduated from the Ecole
Supérieure d'Electricité in 1968, he
joined Merlin Gerin in 1970. After
working on the design of networks
and protective devices, he was head
of the design office for industrial
projects and then for naval
equipment projects.
He is now, consultant in the
Marketing Division of the Protection
and Control Department
E/CT 62 up dated April 1996 Merlin Gerin Technical Specification n
°
62 / p.2 Merlin Gerin Technical Specification n
°
62 / p.3
neutral earthing in an industrial HV network
contents
1. Introduction
p. 4
2. Earthing
Direct earthing
p. 4
Earthing through a reactor
p. 4
Earthing through a resistor
p. 4
3. Requirements imposed by
Earthing through a current
p. 5
overvoltages
limiting reactor
Earthing through a resistor
p. 4
4. Requirements imposed by networks
p. 5
5. Requirements imposed by receivers
p. 6
6. Calculating fault currents
p. 6
7. Earth protection mode
Earth protection adjustment
p. 7
Earthing with accessible neutral
p. 8
Earthing with an artificial neutral
p. 8
Appendix 1: Comments on determining
p. 10
network capacitance values
Appendix: Bibliography
p. 11
HV electrical networks can be earthed
in different ways. This document
analyzes the constraints imposed by
the different parameters of the
installation (overvoltages, network,
receivers) and calculates the fault
currents.
Different protection modes are
described along with the settings and
adjustments suggested according to
the requirements. Merlin Gerin Technical Specification n
°
62 / p.4
1. introduction
[1] See bibliography
When designing an industrial HV
network, a suitable neutral earth
arrangement must be selected: the
neutral can either be insulated, or it can
be connected to earth. The use of an
insulated neutral in an HV network has
the advantage of ensuring operational
continuity since it does not trip on the
first fault, however the network
capacitance must be such that an earth
fault current is not likely to endanger
personnel or damage equipment.
On the other hand, an insulated neutral
2. earthing
neutral generally implies mandatory
tripping on the first fault, however:
s
it reduces overvoltages,
s
it provides a simple, reliable,
selective means of protection,
s
it allows the use of equipment, and in
particular cables, with lower insulation
levels than for an insulated neutral.
implies the following:
s
the risk of high overvoltages likely to
favourize multiple faults,
s
the use of superinsulated equipment,
s
compulsory monitoring of the
insulation,
s
protection against overvoltages,
which will become compulsory in the
near future,
s
the need for complex, selective
protection against earth faults which
cannot usually be ensured by simple
current-measuring relays. An earthed
earthing through a resistor
This is often the most satisfactory
solution.
A study is necessary to choose
between these two earthing, (through a
reactor or through a resistor) :
accurate determining of these earthing
modes depends on the voltage level,
the size of the network and the type of
receivers.
Depending on the earthing mode, a
criterion then determines a maximum
impedance value corresponding to the
overvoltage problem.
Next, it is necessary to check its
compatibility with the requirements of
the network and the receivers.
However, in the event of an earth fault,
the current is not limited, damage and
interference occur and there is
considerable danger for the personnel
during the time the fault persists.
This solution is not used for HV
distribution.
earthing through a reactor
Tuned reactor (Petersen coil)
This solution is sometimes used for
public HV networks. It is rarely used for
industrial distribution.
Protective relays sensitive to the active
component of the residual current must
be used to obtain selectivity.
Current limiting reactor
This solution can result in serious
overvoltages, as demonstrated by Le
Verre (the Research and Development
division of the E.D.F.) [1]. It can be
used only where there are low limiting
impedances.
The purpose of this study is not to
compare the different neutral earth
arrangements, but rather, once the
neutral earth solution has been
adopted, to determine the earthing
mode by finding a compromise
between three often contradictory
requirements:
s
to sufficiently damp overvoltages,
s
to limit damage and disturbances
caused by an earth fault,
s
to provide simple, selective protective
devices.
Earthing can be of different types:
s
direct (without impedance-dependent
current limiting),
s
through a reactor,
s
through a resistor.
direct earthing
This type of earthing is the most
efficient in limiting overvoltages;
protection selectivity presents no
difficulties. Merlin Gerin Technical Specification n
°
62 / p.5
3. requirements imposed by overvoltages
4. requirements imposed by networks
its path and in particular to the cable
shields. The maximum current
withstood by the cable shields may be
specified by the constructors. As a
general rule, the value used is between
500 and 3 000 A for 1 second.
The above criterion is used to define
the lower limit of the phase to earth
fault current.
To determine the upper limit, it is
necessary to check that the fault
current does not cause damage along
fig. 1 : a zigzag or neutral point coil provides
an earth fault current limiting reactor.
current I
C
in the event of an earth fault.
Hence the relation
I
L
2
I
C
.
Determination of the cable capacitance
values depends on their design (see
appendix for this calculation).
earthing through a current
limiting reactor
(see fig. 1)
The study of overvoltages that occur
when short-circuits are eliminated from
networks with the neutral earthed
through a reactor gives the following
results:
s
let,
I
0

be the earth fault current
limiting reactance,
s
and L the network three-phase
short-circuit reactance.
The neutral-to-earth overvoltage
occurring when short-circuits are
eliminated is: V
V
=

1
2
I
0
L for a radial field cable
network, V
V
=

1
2
I
0
L
for all other cases.
In practise, the earth fault current is
limited to at most 10 % of the three-
phase short-circuit current, as applied
by the EDF to its HV power distribution
network.
s
U
3 r is the value of the earth fault
current
I
L
in the earthing connection,
s
3 C U
3
is the network capacitive
earthing through a resistor
As recommended by EDF for
hydroelectric power networks. The
resistance value r is détermined in
order to obtain a total active power
loss : U
2
3 r equal to or greater than the
capacitive power 2 C U
2
in the event
of a phase-earth fault, i.e.:
U
2
3 r
2 C U
2
.
When dividing by U3 , this become
U
3 r 2 . 3 C U
3
where: Merlin Gerin Technical Specification n
°
62 / p.6
5. requirements imposed by receivers
between 3 kV and 15 kV, most
frequently 5.5 kV in France; the earth
fault current should not exceed 20 A in
order to avoid damage to the steel
plating of the machines, for if reworking
a winding is a regular repair, repairing a
machine when the metal plating is
damaged is much more time
consuming and more costly.
6. calculating fault currents
The currents in the different circuits are
easily calculated using a simple
approximative method.
This consists in ignoring the short-
circuit impedance of the source and the
coupled impedances with respect to the
neutral earth impedance and the
network capacitances. In other words,
we consider that earth fault currents are
much lower than three-phase short-
circuit currents (see fig. 2).
To calculate the neutral-to-earth
potential, the sum of the currents
flowing to earth is considered to be
zero (see diagram).
I
N
+
I
rD
+
I
rS
= 0
0 = g V
N
+ [G + j

C] (V
N
+ E)
+ j C (V
N
+ a
2
E) + j C (V
N
+ aE)
0 = V
N
[g + G + 3j C] + GE
+ j CE (1 + a
2
+ a)
Since 1 + a
2
+ a = 0
This gives:
V
N
= GE
g
+
G
+
3 j C
where V
N
= zE
z
+
Z
+
3 j C z Z
fig. 2: Earth fault current calculation parameters.
In HV networks, receivers are
transformers which have no particular
requirements as concerns the neutral
earthing in a power supply network.
However, industrial HV networks can
supply rotating machines with voltages
z = 1
g
Z = 1
G
Neutral earth impedance
Phase to earth fault impedance
C
D
Phase to earth capacitanc e of the faulty outgoing
feeder
C
S
Phase to earth capacitance of a sound outgoing
feeder
C =

C
S
Total phase to earth capacitance of the network
E
Network phase voltage
V
N
Neutral point to earth potential
I
N
Neutral to earth current
I
D
Fault current
I
rD
Residual current of the faulty outgoing feeder
I
rS
Residual current of a sound outgoing feeder
E
V
N
3
2
1
g = 1
z
I
N
G = 1
Z
C
D
I
r D
I
r S
C
S
I
D
aE
a E
2 Merlin Gerin Technical Specification n
°
62 / p.7
In the event of a short-circuit Z = 0, the
above formulae become:
s
s
s
s
s
s
Since we know V
N
, the different
currents (I
N
neutral to earth current, I
D
fault current, I
rD
and I
rS
residual currents
in