LONGITUDINAL INDUCTION
VOLTAGE MEASUREMENT
ON
COMMUNICATION CABLES RUNNING PARALLEL TO
OVERHEAD LINES
IEEE PES Transmission and Distribution
Conference_ Chicago April 2008
Dean Sharafi
Introduction
ƒ Electro-magnetic field is created as a
result of current passing through the
conductor.
ƒ This field induces a voltage on
adjacent conductors depending on:
•
•
•
•
Distance
Angle
Screening
Bonding.
1
Power Engineer's Considerations
ƒ
ƒ
ƒ
ƒ
Safety
Compliance with standards
Commissioning Tests
Collaboration of power and
communication industries
Standards
ƒ International Telecommunication Union
ƒ Normal Conditions
• Accessible by public, 60V r.m.s
• Accessible by technicians, 150V r.m.s
ƒ Fault Conditions
• 430V r.m.s
• Probability of faults
• Protection clearance time
2
LFI Voltage Limits
Description
LFI
Volta
ge
Limit
A
High
Reliability line
with
protective equipment that would
clear an earth fault within 0.35
secs
1500
Vrms
B
High
Reliability line
with
protective equipment that would
clear an earth fault in, from 0.35
secs to 0.5 secs
1000
Vrms
C
Line not classed as a High
Reliability
as
protective
equipment would not clear an
earth fault within 0.5 secs
430
Vrms
All
Normal
operating conditions
where the cable can be accessed
by technicians
150
Vrms
All
Normal
operating conditions
where the cable can be accessed
by the public
60
Vrms
LINE
CATE
GOR
Y
LFI Hazards
ƒ Hazards to humans (Physiology and
probabilistic Analysis)
ƒ Voltage stress on Telecom Cables
ƒ Damage to equipment
3
Human Effects
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
Current Magnitude
Current path, H-H, H-F
Duration
Instant of occurrence during heart cycle
Body impedance
Individual sensitivity
Frequency
Fault Conditions
ƒ Earth faults
• Zero sequence current in earth-wires
ƒ information needed during the design stage:
• Phase conductor, earth-wire conductor and tower
impedances,
• Tower dimensions and spanning distances,
• Fault currents at the substation,
• Mutual impedances between
• phase conductors and communication cables,
• earth-wire conductor and communication cables
4
Induced Voltage
ƒ Difference between voltage induced
faulted phase conductor and earth-wire.
ƒ V = CLiK
by
• where: V
= induced longitudinal voltage [V]
• C
= mutual impedance per unit length
[ohm/km]
• L
= length of exposure (between o/h line
and communication cable) [km]
• i
= fault current [A]
• K
= shielding factor {K=1 for no shielding}
Mutual Impedance
ƒ
C = 2πf log e (1 +
6 × 10 5 ρ
) × 10 − 4
d2 f
ohm/km
ƒ d = geometric separation between
earth return circuits in meters
ƒ ρ = earth resistivity in ohm-meter
ƒ f = system frequency in Hz
5
Mitigation Measures
ƒ Increasing shielding pairs in
communication cables,
ƒ Latent shielding for the communication
cables via gas-filled protectors,
ƒ Additional shielding conductor parallel
to the power lines/cables,
ƒ Fiber optic interface
ƒ Reducing the fault level.
Commissioning Tests
ƒ Injection in the overhead line and
measuring the induced voltages.
ƒ Not practical to inject high currents
ƒ Scaled to maximum fault current
assuming system linearity.
ƒ At or close to system frequency (50 Hz
in Australia),
ƒ Existence of background noise
“standing voltages”.
6
Test Difficulties
ƒ Inject higher currents and varying the
frequency as much away from system
frequency as possible.
ƒ Difficulty in adjusting generators to operate in
a range, which they are not primarily
designed for
•
•
•
•
•
Phase Angle Meters
Double Beam Oscilloscopes
Selective Voltmeters
Chart Recorders
Spike Transient Voltmeters
Test Difficulties
ƒ
ƒ
ƒ
ƒ
ƒ
Heavy equipment
logistic difficulties
Great amount of manpower
Time required for performing tests
Test in a large area
7
New Concept
ƒ Primary
test
equipment
capable
of
generating voltages and currents at various
frequencies and connectable to GPS devices.
ƒ Current injection is made with various
frequencies, and induced voltage is
measured simultaneously with the same
applied frequency.
ƒ The results are then extrapolated to show the
response of the system for 50Hz frequency.
Test Difficulty
ƒ Difficulty is that in some occasions the
injection location is far away from the
location where induced voltages have to
be measured.
8
Solution
ƒ Two primary test units,
ƒ Global positioning system synchronising
devices
ƒ Simultaneous initiation of two sets
located at different locations.
Test Set-up
9
Test Method
ƒ Injection unit at injection location,
ƒ Measuring unit measures the induced
voltages at measuring point.
ƒ Measurement
with
same
injected
frequency
ƒ Two devices synchronised by GPS
synchronising clocks.
Test Method
ƒ Off system frequency injection
ƒ Interpolation of values measured from 30
Hz and 70 Hz injection.
ƒ Extrapolated to maximum fault current
ƒ Checked against allowable limits.
10
Case Study
ƒ On Friday July 15 2005, Western Power
performed the measurement of
longitudinal voltage induction for the
new Thornlie Railway Line Extension on
behalf of Public Transport Authority of
Western Australia.
Design Assumptions
ƒ A previous report had determined that
induction depending on soil resistivity
was:
• 7.8V – 8.6V under normal train operation,
• 166V – 204V under traction fault
conditions.
• Both values within allowable range
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Test Results
ƒ With injection current of 50A
ƒ Voltage rise for two measurements
were 1.02 and 1.05v
ƒ Extrapolated to a calculated fault level
of 4000A, which was 83V.
Test Benefits
ƒ Eliminating need for heavy equipment.
ƒ More accurate and meaningful results
by filtering out system frequency
noise.
ƒ Answer to the problem of testing in a
large area.
12
Noise Filtration
Voltage (V)
Induction Voltage Vs Frequency For 50A
Current Injection
2
1.5
1
0.5
0
0
10 20 30 40 50 60 70 80 90 10
0
Frequency (Hz)
Induction Voltage Vs Frequency For 50A Current
Injection
Test Configuration
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2 950
2.905
2.860
2.815
2.770
2.725
2.680
2.635
2.590
2.545
2.500
2.455
2.410
2.365
2.320
2.275
2.230
2.185
2.140
2.095
2.050
2.005
1.960
1.915
1.870
1.825
1.780
1.735
1.690
1.645
1.600
1.555
1.510
1.465
1.420
1.375
1.330
1.285
1.240
Ref km
separation (m)
Line Separation
High Voltage Overhead Line
Separation
Com. Cable
Line Separation
45
40
35
30
25
20
15
10
5
0
Km reference
14
Thank You for Your Attention!
Any Questions?
15