by Jeon Myeong-ryeal, Oh Sei-ill, Lee Hee,Shin Chang-gyun, Electric Power Research Institute, Korea
Line Constants
Wide Are Monitoring
36
Analysis of
Measured
Transmission
Line Constants
The accuracy of line impedance data
has great impact on system analysis
The transmission line constants are the most important
element of data needed for the operation of an electric power
network. It comprises positive-sequence impedance (Z1),
zero-sequence impedance (Z0) and admittance.
The parameters of line constants are conventionally
computed by calculation programs, and the measured values
of transmission line constants have been utilized as reference
data when a newly built generating plant or substation
undergoes a commissioning test.
Notwithstanding, it is known that the conventional
method of reading voltage drop after applying voltage to
the transmission would not work effectively in energized
substations due to the influence of induction voltage.
However, the new type of measurement equipment
introduced hereon is unique in terms of injecting electric
1 Line Constants
Measuring Circuit
Impedance
measurements setup
current through the circuits and measuring the voltage
raised from the loaded test current. Because this new
measurement device is equipped with an additional feature
for selecting variable frequency for the source current, it
could advantageously perform measurement of transmission
constants without receiving any interference from induction
voltage of the frequency in use.
The measurement of transmission constants as described
below has been conducted with the help of the new
sophisticated measurement equipment to verify and analyze
the deviation between calculated and measured values of
transmission constants.
Representation of Line Constants Measurement
The transmission line constants are defined as the
constants showing electrical impedance values between
busbars of transmission lines in electric power networks.
These data are crucial in the electrical interpretation of power
networks. The transmission constants are also utilized in the
construction or expansion of power facilities as basic data
for areas such as the simulation review of load flow and fault
2 Measuring Schematic
for Transmission Line Constants
Captured
data was used
Positive Sequence Impedance Z1 Circuit
A S/S
(Measuring Point)
Line PT
CB
Line DS
to examine
instantaneous phase
Line EDS
Zero Sequence Impedance Z0 Circuit
Injection
B S/S
angles
CB
Line DS
between
Rockport and
Line EDS
Marysville
PAC.WINTER.2008
T/L
by Jeon Myeong-ryeal, Oh Sei-ill, Lee Hee,Shin Chang-gyun, Electric Power Research Institute, Korea
37
current, voltage stability and the protection relay settings.
While the calculated values have been conventionally
used in specifying transmission constants for the reasons
of physical obstruction to the field measurement of
transmission line constants, there has been growing support
for adopting the new measuring equipment featured with a
frequency-dependent device.
Therefore, we have responded by demonstrating
measurement of transmission line constants with the new
equipment as described in this article.
The new data obtained by this measurement will be
utilized as basic materials for future management of and
policies for transmission line constants by analyzing and
comparing calculated and values measured.
Measurement of Transmission Line Constants
There are three methods of applying test voltage and
current to transmission lines, i.e., phase-to- ground,
line-to-line, and 3-phase combined-to-ground. The positive
sequence impedance was measured by the line-to-line
method, while zero sequence impedance was measured by
3-phase combined-to-ground method.
The impedance was computed by measuring the results of
test voltage applied by the test equipment, and the admittance
by measuring the transmission charging current at the time of
initial energization.
Measurement Equipment
The measurement equipment used is as follows:
A compact multifunctional primary test set capable
of applying up to 2000V and 800 A, with a frequency range
of 10-400 Hz
Coupling unit
We need to highlight that measurement is impossible
with induction voltage exceeding 500V (as is the case with
345kV overhead transmission lines.) See Figure 3.
Biographies
Measurement Schematic for Line Constants
The transmission constants were measured, as shown
in Figure 3, by connecting the measurement equipment to
the EDS terminal behind the line DS at measuring end of
Substation “A”, and grounding 3-phase combined via EDS
at the other end of Substation “B”. See Figure 2.
Computation of Transmission Line Constants
The transmission line constants were computed, as
shown in Figure 4, by using a frequency-dependent method
at frequencies of 20, 40, 80 and 100 Hz, respectively to
find corresponding R and X values, which were averaged to
produce mean values.
However, the 60-Hz setting was excluded taking
into account noticeable errors due to the surrounding
electro-magnetic induction.
Figure 6 clearly shows that the resistance component
remains almost constant with variation of frequency, while
the reactance linearly increases as frequency rises.
Measurement of Transmission Line Constants
Approximately 5% of all transmission lines have been
selected for measurement of transmission constants for
analysis of deviation between calculated- and measuredvalues. These data will be used as basic material when
management and policies are established for overall
transmission constants in the future.
The transmission line circuits were tested during a 12
weeks period between 18 September and 15 December
2006 are as follows:
A total of 86 circuits of 154 kV transmission lines
under jurisdiction of 11 KEPCO Power Transmission
District Offices and Jeju Branch Office. (10 out of 96 circuits
are not considered due to suspended power supply)
34 circuits of overhead transmission lines (cable types:
ACSR 330, ACSR 410, and ACSR 410B)
Jeon Myeongryeal received
Bachelor of Science
degree in Electrical
Engineering from
In-Ha University
1983. In 1983 he
joined KEPRI (Korea
Power Research
Institute) in DaeJeon, South Korea.
His current position
is Leader of Power
Facility Technology
Service Group.
Oh Sei-ill received
Bachelor of Science
degree in Electrical
Engineering from
Seoul National University of Technology in 1987.
In 1987 he joined
KEPRI (Korea Power
Research Institute)
in Dae-Jeon, South
Korea and is currently Senior Member of Technical
Staff in the Power
Facility Technology
Service Group.
3 Arrangement
of Equipment for Measurement of T-line Constants
A Phase B Phase C Phase
CP CU1
I_AC
INPUT
FUSE 30 A
POWER TRANS
100/2.5A CT
L2
L1
BOOSTER
BOOSTER
V
I-OUT(0-100A)
CP GB1
V-Meter
VI_AC
INPUT
CPC-100
L3
V_SENSE(0-600V)
600/30V PT
PAC.WINTER.2008
by Jeon Myeong-ryeal, Oh Sei-ill, Lee Hee,Shin Chang-gyun, Electric Power Research Institute, Korea
Line Constants
Wide Are Monitoring
38
46 circuits of underground transmission lines (Cable
types: XLPE, OF, CV, and CNCV)
6 complex circuits
Analysis and Comparison of Calculated
and Measured Values
The results for overhead and complex transmission lines
are summarized in Figure 4.
For positive sequence impedance (Z1) transmission lines
with error rates exceeding 5% included 4 out of 40 circuits
with a maximum error rate of18.4%.
Statistics by error range are shown in row 1 of Table 1
with an average X1/R1 value equal to 6.17.
As protection relay settings assume about 5% of error
rate for transmission line constants values, the use of present
calculated values seems not problematic.
For zero sequence impedance (Z0) transmission lines
with error rates exceeding 5% included 14 out of 40 circuits
with a maximum error rate of 18.9%.
Statistics by error range are shown in row 2 of Table 1
with an average X0/R0 value equal to 4.88.
For the positive sequence admittance (Y1) statistics by
error range are shown in row 1 of Table 2.
The error seems to arise from calculation error as well as
from change of the fringing field underneath transmission
lines, such as change of ground altitudes (growth of bush
and trees, etc.).
Positive effects from improving the accuracy of
admittance measurements are that when formulating
reactive power compensation plan, investment cost for
phase modifying equipment may be reduced.
For the ratio of zero sequence (Z0) to positive sequence
impedance (Z1) the following results were obtained:
Average value of calculated Z0/Z1 = 2.69
Average value of measured Z0/Z1 = 2.66
4 Transmission Lines
Measuring the impegance
of transmission lines is
important for improving the
system model
The results for underground transmission lines are
summarized in Figure 5.
For positive sequence impedance (Z1) transmission lines
with error rates exceeding 5% included 23 out of 46 circuits
with a maximum error rate of18.4%.
Statistics by error range are shown in row 3 of Table 1
with an average X1/R1 value equal to 6.21.
The calculated values are not suitable for application to
protection relay settings as error rates are high (about 10%)
and error ranges vary widely depending on the installation
condition of underground transmission lines.
For zero sequence impedance (Z0) transmission lines
with error rates exceeding 5% included all 46 circuits.
Statistics by error range are shown in row 4 of Table
1 with an average X0/R0 value equal to 1.90. 2) Zero
sequence impedance (Z0)
The transmission district offices having substantial length
of underground transmission lines require procurement
of new test equipment for physical measurement of
transmission lines. Research needs to be tasked to raise
accuracy of calculation program.
For the positive sequence admittance (Y1) statistics
by error range are shown in row 2 of Table 2. The ratio of
5 Transmission Lines
Overhead Lines - Overhead & Compex Lines
Underground Transmission Lines
Measured / Calculated Value Rate of Overhead
Line Constant
[Unit: %]
400
Measured / Calculated Value Rate of Underground
Line Constant
[Unit: %]
1200
1000
300
800
200
600
400
100
200
0
0
R1
Average
Max.
Min.
87.9
99.8
71.9
Standard
Deviation %
PAC.WINTER.2008
6.2
X1
Z1
R0
X0
Z0
Y1
100.1
112.8
82.0
99.7
111.7
81.6
87.6
132.0
66.8
100.2
118.6
84.8
99.5
118.9
84.9
148.0
279.1
70.3
4.8
4.8
13.4
7.6
7.6
62.4
R1
Average
Max.
Min.
Average
Deviation %
87.9
99.8
71.9
37.2
X1
Z1
R0
X0
Z0
Y1
100.1
112.8
82.0
99.7
111.7
81.6
87.6
132.0
66.8
100.2
118.6
84.8
99.5
118.9
84.9
148.0
279.1
70.3
7.9
8.0
217.4
333.1
283.7
61.3
by Jeon Myeong-ryeal, Oh Sei-ill, Lee Hee,Shin Chang-gyun, Electric Power Research Institute, Korea
Table 1 Statistics by Error Range and by Average
row
5%
5
10%
10
15%
15
29%
20
25%
Total
1
2
3
4
35
25
22
0
2
6
14
14
1
7
6
12
2
2
2
3
o
o
2
17
40
40
46
46
Table 2 Statistics by Error Range and by Average
row
10%
20
50%
50
100%
100
200%
200
300%
Total
1
2
17
5
9
28
5
9
9
2
o
2
46
46
6 T-line Constants
Line Constants
T-line Constants with Variable Sequences
Impedance versus Frequency
3.0
2.5
Impedance [0 hm]
average to calculated values is 131.9±61.3% .
The measured values of admittance of underground
transmission lines were found to be lower than those of
overhead transmission lines.
For the ratio of zero sequence (Z0) to positive sequence
impedance (Z1) the following results were obtained:
Average value of calculated Z0/Z1 = 0.48
Average value of measured Z0/Z1 = 1.87
Transmission line constants per unit length for different
conductors for admittance are shown in Figure 7 and for
positive and zero sequence impedance - in Figure 8.
The analysis of the measured data obtained in this
research clearly shows that:
The constants of overhead transmission lines are
excellent as they stay well within acceptable error range.
The constants of underground transmission lines are
remarkably high, particularly in zero sequence impedance
due to underground cable grounding system, such as
whether the close bond grounding is provided at one end
or at both ends of cable spans. Further research must be
conducted to review the accuracy and application problem
with the calculation program for constants of underground
transmission lines.
Also, as the importance of transmission line constants
is expected to be emphasized with the innovation of
power network operations and techniques, measurement
equipment will be broadly introduced to enable extensive
measurement and analysis of transmission line constants,
so that an expanded data base can be effectively utilized.
Building an accurate data base of transmission line
constants will greatly improve the quality of interpretation
of electric power networks, and will further contribute
to stabilization and optimum operation of electric power
systems.
2.0
1.5
1.0
0.5
0.0
0.0Hz
20.0Hz
40.0Hz
60.0Hz
80.0Hz
100.0Hz 120.0Hz
Frequency [Hz]
R (x)
X (f)
Rcalc(60.0Hz)
Xcalc(60.0Hz)
8 Impedance
7 Admittance
For different conductors
Positive Sequence and Zero Sequence Impedance
Measured Line Constant per 1 km
[unit: /km]
1.5
Measured Admitance per 1 km
[unit: Mho/km]
1.0
400
300
0.5
200
0.0
0
Average
Max.
Min.
ACSR
330
5.41
9.30
2.44
ACSR
410
ACSR
410B
XLPE
OF
CNCV
5.2320
9.4066
3.7320
6.3307
9.7944
5.0557
100.03
173.43
16.46
210.70
329.42
153.17
138.04
207.97
83.85
A C S R
Average
100
330
410
410B
XLPE
OF
CNCV
R1
0.0861
0.0702
0.0375
0.0256
0.0261
0.0281
X1
0.4616
0.4690
0.3323
0.1676
0.1573
0.1321
Z1
0.4696
0.4742
0.3344
0.1696
0.1598
0.1351
Wide Are Monitoring
39
R0
0.2478
0.2566
0.1818
0.1544
0.1450
0.0546
X0
1.2296
1.1855
0.9606
0.3009
0.2513
0.1008
Z0
1.2544
1.2135
0.9757
0.3412
0.2950
0.1149
PAC.WINTER.2008