Protection Application Handbook (англ. яз.)
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HIGH RESISTANCE EARTHED SYSTEMS
Earth Fault Protection
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Due to the low current magnitude, high resistance earth faults will
not quickly develop to lower resistance so a high fault resistance
need to be detected. In Sweden a fault resistance of 5000 Ω must
be detected for some types of overhead lines. In order to detect
such high values it is often necessary to use a resonance earth-
ing in order to achieve reasonable high neutral point voltages.
(>5V at 5000 Ω).
Due to the high sensitivity required it’s required to use cable cur-
rent transformers surrounding the three phases to do the mea-
suring. The current transformer is then selected with a suitable
ratio independent of load current and no current will flow during
normal services. A sensitivity down to 1-2A primary is required at
high resistance earthed systems.
4.3 NEUTRAL POINT VOLTAGES.
In order to detect high values of fault resistance up to 5000 Ω“it’s
often necessary to use a resonance earthing to achieve accept-
able high neutral point voltages (>5V at 5000Ω). The neutral point
voltage is required to give the directional criteria for the direction-
al earth fault protection and is also used to provide a back-up
earth fault protection at the busbar or the transformer bay.
A small neutral point voltage exists, during normal service of the
network due to unsymmetrical capacitance of the phases to earth
and the resistive current leakage at apparatus such as surge ar-
resters. Normally the occurring levels of unbalance currents is
0,2 - 5%. Neutral point detection relays and directional relays
should therefor never be allowed to have sensitivities below
5-10% to compare with the requirements of very low levels in sol-
idly earthed systems.
The neutral point voltages are calculated with the fault resistance
“R
f
” in series with the neutral reactance “Z
N
”. The neutral imped-
ance consists of the earthing resistance “R
N
” and the capacitive
HIGH RESISTANCE EARTHED SYSTEMS
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reactance of the network “X
C
”. The factor “R
f
/R
f
+ Z
N
”, gives the
occurring, per unit, voltage.
The “Z
N
” impedance can be calculated “U
n
/( xl
f
)”, where I
f
is
the total fault current.
It must be remembered that “Z
N
”, as well as the vectorial sum
have to be calculated.
4.4 DIRECTIONAL EARTH FAULT PROTECTION
Due to the infeed of capacitive currents from healthy objects dur-
ing an earth fault it’s usually required to use directional earth fault
relays. Directional relays are used when the infeed of capacitive
current from an object during a fault in an other object is higher
than 60% of the required sensitivity.
The directional relays will measure the active component of the
fault current only, i. e. the current generated by the earthing resis-
tance. The principle for fault current and capacitive current gen-
eration is shown in Figure 15.
3
RESONANCE EARTHED SYSTEMS
Earth Fault Protection
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Figure 15. The fault and capacitive current distribution in a high resistance
earthed network.
4.5 CALCULATION OF EFFICIENCY FACTOR.
The efficiency factor must be calculated when sensitive earth
fault relays are used (see later sections).
5. RESONANCE EARTHED SYSTEMS
5.1 PRINCIPLE
The resonance earthed networks are earthed at the source side
of the network only. Normally the earthing is restricted to one
point only, or two with parallel transformers, and an arrangement
I
UN>
I
R
UN>
Open
I
c
L2
I
c
L3
L1
L
2
L
3
IR
Ic2+IcL
2
I
c
3
+
I
c
L3
Ic2
I
c
3
I
R
Ic2
I
c
3
Ic2
I
c
3
Fault in L1
Ic3Ic
2
U
N
UL1
UL
2
UL3
I
c
2
+
I
c
L2
Ic3+IcL
3
Ictot
Note: CT does not
measure Ic2 and Ic3
UN
UN
Note: UN has always the
angle of the faulty phase
(+180 deg)
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is provided to allow connection with the earthing point. This var-
ies with the service conditions. The earthing consists of a tapped
reactor tuned to the network capacitance compensating the cur-
rent and gives a fault current close to zero Amperes at the fault.
In many cases high ohmic resistors is connected in parallel to
generate a resistive fault current for the protection system to
measure.
Alternatives without this type of resistor and with transient mea-
suring relays as described below are used.
Figure 16 shows the earthing principle and the flow of earth fault
current components.
5.2 FAULT RESISTANCE AND FAULT CURRENT
LEVELS
The earthing resistor is mostly set to give an earth fault current of
5-15A. The trend to decrease the current level gives lower re-
quirement of earthing grid and higher neutral point voltages at
high resistive earth faults.
Due to the low current magnitude, high resistance earth faults will
not quickly turn into low resistance. This means that a high fault
resistance need to be detected. In Sweden for overhead lines
with insulation a fault resistance of 5000 Ω, must be detected and
up to 20 kΩ alarmed.
For other overhead lines the value for detection is 3000 Ω.
In order to detect these high values it’s often necessary to use a
resonance earthing to achieve acceptable high neutral point volt-
ages (>5V at 5000Ω).
Due to the high sensitivity required it’s advantageous to use cable
current transformers surrounding the three phases to do the
measuring. The current transformer is then selected, with a suit-
able ratio independent of load current, and no current will flow
during normal services.
RESONANCE EARTHED SYSTEMS
Earth Fault Protection
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A sensitivity down to below 1A primary is required with reso-
nance earthed systems.
5.3 NEUTRAL POINT VOLTAGES
In order to detect high values of fault resistance, up to 5000 Ω, it’s
often necessary to use a resonance earthing to achieve accept-
able high neutral point voltages (>5V at 5000 Ω). The neutral
point voltage is required to give the directional criteria for the di-
rectional earth fault protection and is also used to provide a
back-up earth fault protection at the busbar or the transformer
bay.
A small neutral point voltage exists, during normal service of the
network, due to unsymmetrical capacitance of the phases to
earth and current leakage at apparatus as surge arresters. Nor-
mally the occurring levels, of unbalance currents, is 0,2 - 5%.
Neutral point detection relays and directional relays should there-
for never be allowed to have sensitivities below 5-10%. Compare
with the requirements of very high sensitivity in solidly earthed
systems.
The neutral point voltages are calculated with the fault resistance
“R
f
”, in series with the neutral reactance “Z
N
”. The neutral imped-
ance consists of the earthing resistance “R
N
”, the capacitive re-
actance of the network “X
C
” and the reactance of the earthing
reactor (normally tuned, “X
L
”).
The factor “R
f
/(R
f
+ Z
N
)” gives the occurring, per unit, voltage.
The “Z
N
” impedance can be calculated “U
n
/( xl
f
)”, where I
f
is
the total fault current.
It must be remembered that “Z
N
”, as well as the vectorial sum
have to be calculated. This means that the capacitive and reac-
tive components “X
C
” and “X
L
” are in opposition and normally will
end up close to zero. This means that the fault current is lower
and the total reactance also will be close to zero.
3
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5.4 DIRECTIONAL EARTH FAULT PROTECTION
Due to the infeed of capacitive currents, from healthy objects,
during an earth fault, it’s usually required to use directional earth
fault relays. Directional relays are used when the infeed of capac-
itive current from an object, during a fault in an other object, is
higher than 60% of the required sensitivity.
The directional relays will measure the active component of the
fault current only, i.e. the current generated by the earthing resis-
tance (the principle for fault current and capacitive current gener-
ation is shown in Figure 16).
Figure 16. A Directional earth fault relay on a resonance earthed system.
I
UN>
IR
UN>
Open
IcL2
IcL3
L1
L2
L3
IR
Ic2+IcL2
Ic3+IcL3
Ic2
Ic3
IR
Ic2
Ic3
Ic2
Ic3
Fault in L1
Ic3
Ic2
UN
UL1
UL2
UL3
Ic2+IcL2
Ic3+IcL3
Ictot
Note: CT does not
measure Ic2 and Ic3
UN
UN
Note: UN has always the
angle of the faulty phase
(+180 deg)
IL
IL
IR
IL
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Earth Fault Protection
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5.5 NEUTRAL POINT CONTROL WITH SPECIAL
EQUIPMENT
A new principle for earthing has been discussed in Sweden and
some other countries during the last couple of years. This princi-
ple includes a neutral point control, where the unsymmetrical
voltage occurring in the neutral during normal service is mea-
sured, and compensated for in all phases by reactive elements.
The earthing is done with a neutral reactor in combination with a
movable core (other solutions exists) able to continuously regu-
late the reactance and compensate for the capacitive current in
the network.
The unsymmetry occurring due to e.g. unsymmetrical capaci-
tances at the different phases to earth as shown in figure 14 will
cause a neutral point voltage. This can be compensated for in dif-
ferent ways but phase-wise reactor/resistors as shown in figure
15, will with an intelligent measuring, enable a regulation and a
compensation for the unbalance. This means that the neutral
point voltage during normal service is zero.
Earth fault currents down to tenth of an ampere can then be
achieved and its possible to have the network in service during
an earth fault until it is convenient to take the line out of service
for reparation. Earth fault protection must here be arranged,
based on other principles than measuring of fundamental earth
fault currents. A transient measuring relay as described below is
one possible solution.
UN
UL1
UL2
UL3
IcL2
IcL3
Ico
Note: UN has always the
angle of the faulty phase
(+180 deg)
IL
IR0
∆C0
C0 C0
C0
R0 R0 R0
L1
L2
L3
L - ∆L
UN
∆Ico
-∆IL
-IR0
∆IL
RESONANCE EARTHED SYSTEMS
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Figure 17. Unsymmetrical capacitances giving neutral point voltages during
normal service.
Figure 18. A compensation equipment for neutral point unsymmetry.
5.6 TRANSIENT MEASURING RELAYS
In the high resistive, resonance and unearthed systems, an earth
fault measuring based on the transient occurring at an earth fault
can be used. The transient is caused by the sudden change of
voltages and the sudden inrush of capacitive currents into the
healthy phases of the power lines.
This current can be measured with a residual sum measurement
or with cable current transformers.
The transients occur with a frequency of 100-5000 Hz and is
damped out very quickly. Normally within the first half cycle after
the fault, see figure 19. The measurement is therefore combined
with a measurement of neutral point voltage and the relay will
seal-in the direction of the transient as long as the neutral point
voltage is available.
Forward as well as reversed direction can be detected by the di-
rection of the transient.
Transient measuring relays can for resonance earthed system be
set to be sensitive to high resistive earth faults if no resistor is
used, as the occurring neutral point voltages will be rather high.
L1
L2
L3
UN
Harmonic
compensation
Regulation &
supervision
Course ad
j
.
MEASURING EARTH FAULT CURRENT
Earth Fault Protection
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The transient in the current is independent of the neutral device
as this is caused by the transient change of voltages.
Figure 19. The neutral point voltages and the high frequency transient oc-
curring at an earth fault.
6. MEASURING EARTH FAULT CURRENT
6.1 INTRODUCTION
At earth faults in a three phase system the residual sum of the
three phase currents will not end up to zero as during normal ser-
vice. The earth fault current can be measured by a summation of
the three phase currents.
The summation can either be made by a summation of the three
phase current transformers, in a residual sum connection or by a
MEASURING EARTH FAULT CURRENT
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cable current transformer surrounding the three phase conduc-
tors. Figure 20 shows the alternatives).
Figure 20. Measurement of earth fault current can be made with a residual
sum measurement of a cable CT surrounding all three phases.
I
I