BL20A0501
Electricity distribution technology
Earth fault protection
Earth fault
In Finland, the medium-voltage network is either neutral isolated or earthed
via an arc-suppression coil.
a)
b)
c)
d)
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single-phase
earth fault or
conductor fall
two-phase-toearth short circuit
double earth fault
broken conductor
and single-phase
earth fault on the
load side
2
Earth fault
Earth fault –small currents, dangerous touch voltages
In a neutral isolated and suppressed (resonant earthed) network, there is no
low-impedance path for the fault current of a single-phase earth fault to
”short-circuit”the circuit.
The fault current has a path only via the earth capacitances of the
conductors.
Fault current is very low (1–200 A)
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Earth fault
Earth fault current
I f = U v ⋅ 3ωC0
where Co is the earth capacitance of the network / phase
Earth capacitance depends thus on the total length of the network
downstream from the main transformer and the earth capacitances of the
conductors.
When there is fault resistance at the point of fault
3ω C o
=
I
f
1 + (3ω C o Rf )
2
U
v
In other words, the fault resistance reduces the fault current that is very
small already.
Therefore, it is extremely difficult to detect faults that occur through a large
fault resistance.
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Earth fault
Earth fault current depends on the line length of the entire network
and the earth capacitance of the conductor type.
Approximations:
U l
⋅
kV
km A
– Overhead line: I f ≈ 300
20
Example
U l
⋅
kV
km A
Underground cable: I f ≈
5
110/20
60 km 20 kV
20
overhead line 4 A
20
cable 240 A
Above, we have the total length (not the length of main lines)
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Voltages during an earth fault
Voltages during an earth fault
– during an earth fault, the voltages
in all phases and at the neutral
point will change.
– during the fault, the voltage of the
faulted phase will decrease. If Rf
is zero, the voltage of the faulted
phase is zero.
– the voltages of healthy phases will
rise. If Rf is zero, the voltages are
equal to the line-to-line voltage.
– the voltage at the neutral point will
rise during the fault. If Rf is zero,
the neutral point voltage equals to
phase voltage Uo = Uv
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Voltages during an earth fault
During an earth fault, the voltages of all phases and the neutral point
voltage will change.
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Voltages during an earth fault
During a fault, the voltage of the faulted phase decreases. If Rf is zero, the
voltage of the faulted phase is zero.
The voltages of healthy phases will rise. If the fault resistance is zero, the
voltages equal to the line-to-line voltage
U = 3 ⋅U v
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Neutral point voltage
Consequently, the higher the fault resistance, the lower
the zero-sequence voltage.
– when Rf = ∞ , Uo = 0
U
o
=
1
1 + (3ω C o R f )
2
⋅U v
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Earthing voltage, touch voltage
When flowing to ground, the earth fault current always meets a certain
earthing resistance Rm. As a result, compared to the real earth potential
at an ”infinite”distance, the voltage at the point of earth fault will rise to
a value (earthing voltage Um).
Um = Im Rm
The maximum permissible values for earthing voltage have been
determined in electrical safety regulations. For instance, if the system
ground and protective ground of the transformer substation are
combined, the maximum permitted earthing voltage is
U mmax =
500
[V]
t
where t is the duration of the earth fault.
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Earthing voltage,
touch voltage
Earthing voltage is not the same as the touch voltage experienced by a
person or animal at the point of earth fault.
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Suppressed network
In suppressed (resonant earthed) network, an arc-suppression coil (an
inductance) is connected between the neutral point of the main transformer
and earth to compensate capacitive earth fault current.
If the reactance ωL of the arc-suppression coil is equal to the reactance
1/3ωCo formed by the earth capacitances, then the earth fault current Im ≈ 0.
In practice, there is always some imperfection in tuning; however, also in
these cases the earth fault current is very low.
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Suppressed network
Thanks to the low earth fault current, the requirements concerning the
earthing voltage are easier met; furthermore, a low-current earth fault arc
often extinguishes itself (without any high-speed auto-reclosing function).
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Suppressed network
Neutral isolated network
Suppressed network
L1
L1
L2
I
L2
ΣΙ
I
L3
L3
ΣΙ
C1
C1
110/21kV
110/21kV
L1
L2
a
ΣΙ
II
L
IL
L1
R
L2
IR
a
L3
If
C2
ΣΙ
II
L3
If
C2
Rf
Rf
b
b
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Ic
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Suppressed network
1 

1 + R  3ωC0 −

ωL 

2
2
I mf =
(Rf + R ) + R R  3ωC0 − 1 
ωL 

2
2
f
2
2
⋅
U
3
A low-current earth fault arc will
extinguish itself.
1
Rated
voltage
kV
<20
30
45
2
3
4
5
Earth fault curr. of
Resid. earth fault c.
neutral isolated netw. of suppressed netw.
A
A
5
25
20
50
30
40
-
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Earth fault
Example: There are five feeders from a primary substation. The feeder
1 is an overhead line feeder with a total length of 80 km. The feeders 2,3
and 4 are underground cable feeders with total lengths of 10, 15 and 0.5
km, respectively. The earth capacitance/phase of the overhead lines is 6
nF/km, and for the cables 320 nF/km.
a) Determine the earth fault current, when a direct earth fault occurs at the beginning of the
feeder 1
b) Calculate the maximum earth fault current, when in the case of a backup supply, 100 km
of overhead line and 5 km of underground cable has been connected to the feeder 4.
c) How does the fault current and zero-sequence voltage corresponding to a normal
connection change, when the fault resistance varies between 0 and 100 kΩ ?
d) An earth fault occurs in the protective earthing, the earthing resistance of which is 20 Ω.
Determine the earthing voltage acting over the earthing. How does the earthing voltage
change, if the earthing is improved so that its resistance is 5 Ω.
e) Determine the size of an inductance required at the neutral point, if the target is to
compensate the capacitive earth fault current of the network (suppression)
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Two-phase earth fault, fault current circuit
Two-phase ”short circuit”via ground
Fault current is usually high (difficult to calculate), and flows in the ground
where the conductivity is at highest (water pipes, cable sheaths etc.)
Significant voltage differences in the ground e.g. between water pipes and
communication cables; will result in breakdowns and other serious damages
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Two-phase earth fault, occurrence
Points of earth fault at different places, possibly far away from each other
First, there is usually a single-phase earth fault, as a result of which the
voltages of healthy phases rise to the level of the line-to-line voltage
As a result of the above, a breakdown will occur in the faulted valve
protection or cable terminal
A two-phase-to-earth short circuit (one fault point) does not usually cause
problems (two-phase short circuit)
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Two-phase earth fault, occurrence
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Two-phase earth fault, occurrence
* earth fault current, fault resistance, earthing voltage
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Two-phase earth fault, occurrence
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Earth fault protection of medium voltage
network
Earth fault detection criteria:
– fundamental-frequency neutral point voltage
open delta measurement
– fundamental-frequency sum current
sum connection of current transformers or a cable-type current transformer
– change in phase voltage
power-frequency overvoltage
– high-frequency transient currents
the earth capacitance of the faulted phase will discharge
the earth capacitances of healthy phases will charge
fast, but tripping also in the case of a momentary fault
– current and voltage harmonics
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Earth fault directional relays are applied
Operating conditions:
– sum current > set value
– neutral point voltage > set value
– angular phase shift between current and voltage
90o ± 75o in neutral isolated network
0o ± 75o in suppressed network
Also some power direction relays are used:
Uo*Io> Qas (Uo*Ir > Pas)
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Earth fault protection
To arrange selective earth fault protection, the measurement of both the
zero-sequence current and zero-sequence voltage is required.
Measurement of zero-sequence
current Iv:
Iv =
3C0 − 3C0 j
3C0
⋅ I mf
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Earth fault protection
terminal
sheath
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Earth fault protection
L1
L1
L2
I
L2
ΣΙ
I
L3
L3
ΣΙ
C1
C1
110/21 kV
110/21 kV
L1
L2
a
ΣΙ
II
L
IL
L1
R
L2
IR
a
L3
If
C2
ΣΙ
II
L3
If
C2
Rf
Rf
b
b
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Ic
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Suppressed network
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Neutral isolated network
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Design of earth fault protection
From the operational perspective, the most interesting issues in the
design of the protection are the lowest
– zero-sequence currents (high fault resistance, a small-scale network)
– neutral point voltages (high fault resistance, a large-scale network)
Considering the earthing voltage requirements, significant factors
are
– the highest earth fault current
– the duration of earth fault (the durations are not summed up in
reclosings)
In order for the earth fault to extinguish itself, and for the fault
location
– the duration of the earth fault should be as long as possible
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Calculation formulas
Neutral isolated network:
I
mf
=
3
1 + (3
C
CR
o
o
f
)
2
U
U
v
o
1
=
1 + (3
CoR f )
2
⋅U v
Suppressed network:
I
mf
=
1 + Ro2 (3
C
o
−
1 2
)
L
1 2
( R f + Ro ) + R R (3 C o −
)
L
2
2
f
2
o
U
v
U
o
1
=
2
(1 / Ro ) 2
⋅ I mf
1
+ (3 C −
)
o
L
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Neutral point voltage
Neutral isolated network
earth fault current
Suppressed network
non-compensated earth fault current
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Example
Example: Design the set values of the earth fault protection for the
network illustrated below. The protection has to operate selectively in an
earth fault occurring via the 500 Ω fault resistance always when at least two
feeders are switched on. The earth capacitance of the overhead line is 6.0
nF/km, phase. The earth capacitance of the underground cable (AHXAMKW 120 mm2) is 230 nF/km, phase.
Overhead line 80 km
Undergr.cable 14 km
Undergr.cable 21 km
110/20 kV
Undergr.cable 0.7 km
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Earth fault protection
Earth fault directional relays are applied:
Operating conditions:
– sum current > set value
– neutral point voltage > set value
– angular phase shift between current and voltage
90o ± 75o in neutral isolated network
0o ± 75o in suppressed network
Also some power direction relays are used:
Uo*Io> Qas (Uo*Ir > Pas)
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High-impedance earth faults
When the fault resistance is high (e.g. a tree on the line or the ”cold
end”of the line touches ground), Rf > 10 kohm, both the earth fault
current and the neutral point voltage are very low.
The detection of faults is difficult even with the present technology!
Indication methods:
–
–
–
–
monitoring the changes in neutral point voltage
partial discharge measurements
transient relays
performing the measurements deep in the network (easier to detect a
slight current asymmetry)
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Zero-sequence voltage
Example of monitoring zero-sequence
voltage. A tree on the line, 100 % corresponds
to 5 V voltage (Hämeen Sähkö Oy).
Example of monitoring zero-sequence
voltage. Heavy snow storm, 100 % corresponds
to 5 V voltage (Hämeen Sähkö Oy).
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Zero-sequence voltage
Example of monitoring zero-sequence voltage. A tree on the line, 100 % corresponds to a 5 V voltage (Hämeen Sähkö Oy).
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Zero-sequence voltage
Phase currents in a faulted feeder, when the earth fault occurs at time 10 ms.
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Zero-sequence voltage
Earth fault current from the faulted R phase to ground
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