SHORT AND MEDIUM TRANSMISSION LINES
EE 455: Power/Machines Laboratory
Contributors:
Dr. A.A. El-Keib
Dr. Tim A. Haskew
Mr. Clifton Black
Mr. Johnny Carlisle
Mr. Neil Hutchins
Objectives
 To examine the characteristics of a short transmission line by looking at a simple
circuit model of a short line.
 To examine the characteristics of a medium length transmission line by looking at a
simple circuit model of a medium line.
References
Electromechanical Energy Devices and Power Systems, Zia A. Yamayee and Juan L.
Bala, Jr., John Wiley and Sons, Inc., New York, New York, 1989.
- Section 9.5
Discussion
A transmission line, which delivers electric power, dissipates heat because of the
resistance of the conductors. It acts, therefore, as a resistance which, in some cases, is
many miles long. The transmission line also behaves like an inductance because each
conductor is surrounded by a magnetic field which also stretches the full length of the
line. Finally, the transmission line behaves like a capacitor with the conductors acting
more or less like widely-separated plates.
We can picture a transmission line as being made up of thousands of elementary
resistors, inductors and capacitors as shown in Figure 1.
R
L
C
Figure 1. Distributed Equivalent Circuit.
In high-frequency work, this is precisely the circuit which has to be used to explain the
behavior of a transmission line. Fortunately, at low frequencies of 50 Hz or 60 Hz, we
can simplify most lines by lumping the impedances together. Two commonly used
transmission line models are the Pi-model and the T-model. These models are shown in
Figure 2.
R
XL
R
XC
XL
R
Xc
XL
Xc
(b) T Model
(a) PI Model
Figure 2. Pi- and T-Models.
In the nominal pi-model, the inductance from which the inductive reactance is
calculated is equal to the sum of the inductances in the distributed equivalent circuit, and
the same is true for the resistance R. The capacitance from which the capacitive reactance
is calculated is equal to one half the sum of the capacitors in the distributed equivalent
circuit.
In the T model, L and R are equal to one half of the total inductances and resistances
of Figure 1. The capacitance C is equal to the sum of the capacitors shown in Figure 1.
The inductance L and capacitance C are replaced by their equivalent reactances.
The relative values of R, XL and XC depend upon the transmission line length. Lines
under 50 miles in length have negligible resistance and capacitance compared with
inductance and are classified as short lines. This model is shown in Figure 3(a). Lines
which are between 50 and 150 miles in length have negligible resistance, but the
capacitive reactance is appreciable and must be included. These lines are classified as
medium lines and can be modeled using the pi-model of Figure 3(b) or the T-model of
Figure 3(c).
A good understanding of transmission line behavior can be obtained by using these
models. The model of Figure 3(a) will be used in this laboratory exercise to study short
lines, while the model of Figure 3(c) will be used to study medium lines. As a matter of
interest, typical 60 Hz lines have a series reactance of about 0.8 ohms per mile per phase
and a shunt capacitive reactance of about 200,000 ohms per mile*.
*
From “Regulation and Losses of Transmission Lines” Electrical Transmission and Distribution
Reference Book, pp. 279-280, Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania.
XL
XL
XL
XC
(a)
XL
Xc
Xc
(b)
(c)
Figure 3. Various Length Line Models.
EE 455: Power/Machines Laboratory
Short and Medium Transmission Lines
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Pre-lab Questions
1) Using Figures 4 and 5, calculate expected values for Tables 1 and 2.
2) What length transmission lines are considered short and medium lines? Explain why.
3) Compute the nominal ABDC constants for the transmission line modeled in Figure 4.
4) Compute the nominal ABCD constants for the transmission line modeled in Figure 5.
Equipment List
Quantity
1
1
1
2
1
1
1
1
1
Description
Power Supply Module
Resistance Module
Inductance Module
Three-Phase Transmission Line Module
Capacitance Module
Three-Phase Ammeter
Three-Phase Voltmeter
Three-Phase Watt/VAR Meter
Phase Angle Meter
Number
EMS 8821
EMS 8311
EMS 8321
EMS 8329
EMS 8331
EMS 8425
EMS 8426
EMS 8446
EMS 8451
Procedure
Caution: High voltages are present in this Laboratory Experiment! Do not make
any connections with the power on!
1) Construct the circuit in Figure 4. For the load, place a 300 ohm resistor in parallel
with a 300 ohm inductor for each phase (wye-connected). Set the transmission line
impedance to 60 ohms, and adjust the source such that Vs = 200 volts (line-to-line).
VR
5
6
200V
3-phase
phase
4
1
4
IS
60 ohms
2
5
3
6
IR
LOAD
EMS 8329
Figure 4.
EE 455: Power/Machines Laboratory
Short and Medium Transmission Lines
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2) Measure Is, IR, and VR for the circuit, and record the measurements in Table 1.
VS (V)
IS (A)
IR (A)
VR (V)
With Load
Without Load
Table 1.
3) Remove the load from the circuit and repeat all necessary measurements to complete
Table 1. Using your measurements from Table 1, compute the transmission line
voltage regulation. Compare your results with a calculation of voltage regulation
based on analysis of the per-phase equivalent circuit.
4) Reconnect the load and insert the phase angle meter (EMS 8451) to measure the
phase angle of the sending end voltage with respect to the receiving end voltage. Use
the known load impedance and load voltage phasor measurments to compute the
receiving end current phasor. Use the ABDC constants computed in pre-lab question
3 to compute the sending end voltage and current phasors. Compare these results to
the measurements.
5) Construct the circuit in Figure 5. For the load, place a 300 ohm resistor in parallel
with a 300 ohminductor for each phase (wye-connected). Set the impedance of each
EMS 8329 transmission line module to 120 ohms Note: When using the EMS 8329
transmission line model, the capacitor is not part of the model, so add a shunt
capacitance of 300 ohms per phase (wye-connected). Set Vs = 200 volts.
VR
4
1
4
5
IS
2
3
6
200V
3-phase
120 ohms
EMS 8329
4
1
5
2
6
3
120 ohms
EMS 8329
5
LOAD
IR
6
EMS 8331
Figure 5.
6) Measure Is, VR, and IR, and record in Table 2.
VS (V)
IS (A)
IR (A)
VR (A)
With Load
Without Load
Table 2.
EE 455: Power/Machines Laboratory
Short and Medium Transmission Lines
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7) Remove the load and make the necessary measurement to complete Table 2.
8) Reconnect the load and insert the phase angle meter (EMS 8451) to measure the
phase angle of the sending end voltage with respect to the receiving end voltage. Use
the known load impedance and load voltage phasor measurments to compute the
receiving end current phasor. Use the ABDC constants computed in pre-lab question
4 to compute the sending end voltage and current phasors. Compare these results to
the measurements.
9) Remove the phase angle meter and insert the three-phase watt/VAR meter (EMS
8446) to measure the load complex power. Record your measurements. Relocate the
meter to measure the source complex power output. Record your measurements in
Table 3. Use the data in Table 2 and the equivalent circuit to account for the
differences in the load and source complex powers. Also, compare computed versus
measured efficiency.
Measurement Location
Load
Source
P (W)
Q (VAR)
Table 3.
Questions
1) A 35 mile long three-phase transmission line has a total series reactive impedance of
60 ohms. It delivers 40 MW at 200 kV with a power factor at the load of 0.9 lagging.
Find the sending end voltage, current, and power factor. Calculate the voltage
regulation of the line.
2) A 100 mile long 3-phase transmission line transmits 80 MW at 200 kV with a 0.9
lagging power factor at the load. The line has a series impedance of 35+j140 ohms
and Y = j0.000650 S. Find the voltage, current, and power factor at the sending end
of this transmission line using the pi-model. Also compute the efficiency and voltage
regulation of the line. Use per unit analysis.
3) Discuss the difference between Vr with and without a load for the short transmission
line (circuit of Figure 4). Is there any difference? Why or why not?
4) Discuss the difference between Ir with and without a load for the medium
transmission line (circuit of Figure 5). Is there any difference? Why or why not?
EE 455: Power/Machines Laboratory
Short and Medium Transmission Lines
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