PHASE ANGLE AND VOLTAGE DROP BETWEEN
SENDER AND RECEIVER
EE 455: Power/Machines Laboratory
Contributors:
Dr. A.A. El-Keib
Mr. Clifton Black
Mr. Johnny Carlisle
Mr. Neil Hutchins
Objectives



To regulate the receiver end voltage of a transmission line.
To observe the phase angle differences between the voltages at the sending and
receiving ends of a transmission line
To observe transmission line voltage drop when the sending and receiving end
voltages have the same magnitude.
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
In the previous experiment, we saw that a resistive or inductive load at the end of a
transmission line produced a very large voltage drop, which would be quite intolerable
under practical conditions. Motors, relays, and electric lights work properly under stable
voltage conditions, close to the potential for which these devices are rated.
We must, therefore, regulate the voltage at the receiver end of a transmission line in
some way so as to keep the voltage as constant as possible. One approach which appears
promising, is to connect capacitors at the end of the line because, as we saw in the
previous experiment, the capacitors produce a very significant voltage rise. This, indeed,
is one way by which the receiving end voltage is regulated in some practical instances.
Static capacitors are switched in and out during the day, and their value is adjusted to
keep the receiver end voltage constant.
For purely inductive loads, the capacitors should deliver reactive power equal to that
consumed by the inductive load. This produces a parallel resonance effect in which
reactive power required by the inductance is, in effect, supplied by the capacitance and
none is furnished by the transmission line.
For resistive loads, the reactive power, which the capacitors must supply to regulate
the voltage is not easy to calculate. In this experiment, we shall determine the reactive
power by trial and error, adjusting the capacitors until the receiver end voltage is equal to
the sender end voltage.
Finally, for loads which draw both real and reactive power (they are the most
common) the capacitors must be tailored to compensate first, for the inductive component
of the load and second, for the resistive component.
Pre-Lab Assignment
1) How is it possible to have a voltage difference between the sending and receiving
ends of a transmission line if the voltages at each end have the same magnitude?
2) For step 2 of the laboratory procedure, calculate the expected values of W1, VAR1,
V2, W2, VAR2, and angle.
Equipment List
Quantity
Description
Three-Phase Power Supply
Three-Phase Resistive Load
Three-Phase Transmission Line Module
Three-Phase Voltmeter
Three-Phase Watt/VAR Meter
Phase Angle Meter
Three-Phase Capacitive Load
Three-Phase Synchronous Generator/Motor
DC Metering Module
1
1
1
1
2
1
1
1
1
Number
EMS 8821
EMS 8311
EMS 8329
EMS 8426
EMS 8446
EMS 8451
EMS 8331
EMS 8241
EMS 8412
Procedure
Caution: High voltages are present in this Laboratory Experiment! Do not make
any connections with the power on!
1) Set the impedance of the transmission line (EMS 8329) to 120 ohms and connect
instruments as shown in Figure 1. The load will be modified during the course of the
experiment. The circuit should be connected to the three-phase variable voltage
supply (EMS 8821).
1

3
2
4
EMS 8451
0-250V
0-250V
V1
V2
4
1
4
1
5
2 W1, VAR1 5
2
6
3
200V
3-phase
phase
6
EMS 8446
EMS 8329
4
W2, VAR2 5
6
3 EMS 8446
LOAD
Figure 1.
EE 455: Power/Machines Laboratory
Phase Angle and Voltage Drop Between Sender and Receiver
2
2) Adjust V1 to 200 volts and keep it constant for the remainder of the experiment.
Increase the resistive load in steps, keeping all three phases balanced. Take readings
of V1, W1, VAR1, V2, W2, VAR2 and the phase angle between V1 and V2. Note: V1
is chosen as the reference voltage for the phase-angle meter. Record your results
in Table 1, and graph V2 as a function of the load power W2, in watts. On this curve,
indicate the phase angle corresponding to each measured point.
Table 1.
R ()
open ckt.
1200
600
400
300
240
200
171.4
V1
W1
VAR1
V2
W2
VAR2
ANGLE
CAUTION: Always remove a capacitive load before removing a resistive load. A
severe overload is otherwise expected.
3) Connect a three-phase balanced capacitive load (EMS 8331) in parallel with the
resistive load (EMS 8311). Repeat step 2, but for each resistive load adjust the
capacitive load so that the load voltage V2 is as close as possible to 200 V (V1 must
be kept constant at 200 V.). Record your results in Table 2. Graph V2 as a function of
W2 and superimpose it on the previous graph which you created in step 2. Note that
the addition of static capacitors has yielded a much more constant voltage, and
furthermore, the power W2 , which can be delivered, has increased. On this graph,
indicate the phase angle difference between V2 and V1 as well as the reactive power
VAR2, which was used for the individual resistive load settings.
Table 2.
R ()
open ckt.
1200
600
400
300
240
200
171.4
XC (ohms)
V1
W1
VAR1
V2
W2
VAR2
ANGLE
4) Remove the capacitive load (EMS 8331) and connect a synchronous machine (EMS
8241) in parallel with the resistive load (EMS 8311). Connect the field of the
machine to the fixed 120 VDC supply. A DC ammeter (EMS 8412) should be used to
monitor the field current. It may be necessary to adjust V1 above 200 V in order to
start the machine. Once the machine is close to synchronous speed, set the field
rheostat for minimum current and close the field switch. Return V1 to 200V. Turn the
EE 455: Power/Machines Laboratory
Phase Angle and Voltage Drop Between Sender and Receiver
3
field rheostat to each limit and notice the effect on V2. Return the field rheostat to the
position for minimum field current. Repeat step 2, but for each resistive load adjust
the field current of the synchronous machine so that the load voltage V2 is as close as
possible to 200 V (V1 must be constant at 200 V.). Record your results in Table 3.
Table 3.
R (ohms)
open ckt.
1200
600
400
300
240
200
171.4
If
V1
W1
VAR1
V2
W2
VAR2
ANGLE
5) In this step, we shall observe a significant voltage drop along a transmission line even
when the voltages V1 and V2 at the sender and receiver ends are equal in magnitude.
How is it possible to have a voltage drop when the voltage magnitudes at the two ends
are equal? The answer is that the drop is due to the phase angle between the two
voltages. Using the circuit shown in Figure 2, set the per-phase load resistance at
171.4 ohms, and with V1 =200 volts, adjust the capacitive reactance until the load
voltage is as close as possible to 200V . Measure V1, W1, VAR1, V2, W2, VAR2, V3
and the phase angle.
V1 =
V
V2 =
V
W1 =
W
W2 =
W
VAR1 =
VAR VAR2 =
1
0-250V
V1

V3 =
V
VAR  =
deg.
3
2
4
EMS 8451
V2 0-250V
R
XC
4
1
4
1
4
5
2
5
2
5
6
200V
3-phase
phase
3
6
EMS 8446
3
EMS
8329
V3
R
XC
6
EMS 8446
R
XC
0-250V
EMS 8311
EMS 8331
Figure 2.
EE 455: Power/Machines Laboratory
Phase Angle and Voltage Drop Between Sender and Receiver
4
6) Using the results of step 5, calculate the voltage, current, real power, and reactive
power per-phase, draw a phasor diagram of the sender and receiver-end voltages, and
verify the voltage drop against the measured value.
Questions
1) A three-phase transmission line has a reactance of 100 ohms per phase. The sender
end voltage is 100 kV and the receiver end voltage is also regulated to be 100 kV by
placing a bank of static capacitors in parallel with the receiver load of 50 MW at unity
power factor. Calculate:
(a) the reactive power furnished by the capacitor bank,
(b) the reactive power supplied by the sender,
(c) the voltage drop in the line per phase,
(d) the phase angle between the sender and receiver voltages,
(e) and the apparent power supplied by the sender.
2) If the 50 MW load in question 1 were suddenly disconnected, calculate the receiver
voltage which would appear across the capacitor bank. What precaution, if any, must
be taken?
3) If a transmission line were purely resistive, would it be possible to raise the receiver
end voltage by using static capacitors? Explain your response.
EE 455: Power/Machines Laboratory
Phase Angle and Voltage Drop Between Sender and Receiver
5