EE342 - Electrical Power Lab
Experiment PS2
Interconnected Power Systems and
Operation of Transmission Lines
I Objectives
•
•
•
To investigate the characteristics of interconnected power systems.
To investigate the action of Excitation and Speed control in an interconnected system.
To investigating the effect of load values at the receiving end on the performance of an
overhead transmission line.
II Background
2.1 Advantages and Disadvantages of Interconnection
Power systems are usually interconnected since such a scheme has many advantages such as:
¾ Having a higher degree of reliability: If a forced outage occurs inside the system, it
can compensate the power deficit from the interconnected system.
¾ Having fewer Stand-by Units: Due to the fact that loads may be supplied from any
place in the network, fewer standby units are required in an interconnected system.
¾ Having Lower Voltage Deviations: Loads can be affected too much by voltage
variation. For example if a 3φ induction motor is operated at higher voltage, its life is
reduced. Also if it operates at lower voltage, its efficiency will be reduced.
¾ Having Lower Frequency Deviations: The variation of frequency may also cause
many problems for computer systems, some chemical processes, communication
systems, medical equipment, defense critical systems and many other applications
which must operate at a certain specified frequency.
¾ Energy can be bought and sold, therefore, it is more cost affective.
The main disadvantage of the interconnected system is increasing the short circuit level of the
system so that if a short circuit occurs at any point in the system, it may spread through out the
whole system, which may lead to a system black-out.
2.2 Synchronization
Before two synchronous generators can be connected in parallel, they must have:¾ equal frequency.
¾ equal terminal voltage.
¾ equal phase angle.
¾ same phase sequence.
If any of the above four conditions is not satisfied, very large currents can flow in generators
which can cause mechanical or electrical damage to both machines. Therefore, these four
conditions must be satisfied before parallel connection of generators is attempted. Normally, in
power systems, many generators are running and new generators have to be connected to the
system during the periods when loads are increasing. Thus, a generator is “synchronized” to the
system and then interconnected to it. When we synchronize it, we make sure that the four
conditions for parallel operation are met. This operation is achieved with the help of suitable
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instruments (synchroscope, frequency meter, set of lamps, etc). Figure 1-a, shows the
connection of a synchronizing switch. While Figure 1-b demonstrates the phasor diagram that
represents this process.
Figure 1:
Synchronization of Two Systems
2.3 Controllers of Synchronous Machine
In large generators, there are two main automatic control circuits. One is responsible to maintain
constant terminal voltage, which is known as automatic voltage regulation (AVR). The other is
responsible for keeping frequency at constant level by controlling the amount of mechanical
power input to the machine. This is called automatic frequency control (AFC).
This power system simulator is equipped with a speed (frequency) controller only; i.e. the AVR
is not available. To discuss these control systems, the equations of active and reactive power will
be discussed in the following section.
2.4 A Synchronous Machine Connected to an Infinite Bus
Consider the simple model of a synchronous machine connected to an infinite bus of Figure 2.
jXe
P
Q
+
E∠δ
Figure 2:
V∠0
A Synchronous Machine Connected to Infinite Bus
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Where V∠0 is the Voltage of infinite bus
E∠δ is the Machine internal e.m.f
Xe
is the Equivalent total series reactance
Xe
= generator reactance + transformer reactance + line reactance
It can be shown that active and reactive powers supplied by the machine to the infinite bus are
given as:
EV
(1)
P=
sin δ
Xe
V
(2)
Q=
(E cos δ - V)
Xe
The value of Q depends on the operating condition as follows:
E cos δ > V
→ Q>0
E cos δ = V
→
Q=0
E cos δ < V
→ Q<0
Equation (2) shows that the reactive power may be positive, negative, or zero. i.e. Q may flow
from the generator to the system, in this case we say the machine is over-excited working at
lagging power factor. Alternatively, Q may flow from the system to the generator when it is
under-excited and working at leading power factor. The sign (direction) and value of Q depends
upon system requirements.
2.5 Characteristics of Interconnected Systems
When a single generator is connected to a large power system whose rating is much larger than
that of the generator, the voltage and frequency controllers of the small machine can not affect
the large system voltage nor its frequency. In this case, the system can be considered as an
infinite bus relative to the generator. The equivalent circuit of a single generator connected to the
infinite bus is shown in Figure 2 and its e.m.f. is given by E=4.44 k ϕ f.
Attempting to change the speed of the prime mover (induction motor in the lab) will not affect
the system frequency. But the synchronous generator speed is directly proportional to its
frequency; therefore the speed can not change if the frequency can not change. Therefore the
speed will return to a constant value which the synchronous speed of the infinite system.
Therefore, the magnitude of ‘E’ depends mainly upon ϕ (generator excitation). Also power
angle ‘δ‘, i.e. angle of ‘E’ with respect to terminal voltage of the infinite bus, depends upon the
prime mover output power; refer to equation (1). Thus, the current ’I’ supplied by the generator
to the infinite bus, depends on the active power ‘P’ supplied by the generator to the system, and
the reactive power ‘Q’ supplied or absorbed by the generator.
2.6 Modeling of Transmission Lines
The main parameters of a line are as follows:
1. The resistance, which depends upon material, length and area of conductor.
2. Series inductive reactance, which depends upon conductor dimensions and distances
between lines.
3. Shunt capacitive reactance which depends upon conductor dimensions and distances
between lines and distances between lines and ground.
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The transmission lines can be simulated either by a) R,L (short model), b) R,L,C in form of
nominal π or T network, and c) exact model depending on the length of transmission lines. The
power flow, the voltage regulation and the efficiency of transmission lines are affected by the
line parameters. The relationship between different terminal quantities can be expressed by the
following equations:
VS = A VR + B IR
IS = C VR + D IR
Where
VS, IS:
VR, IR:
A, B, C & D :
Where
are the sending end voltage (line to neutral) & line current respectively.
are the receiving end voltage (line to neutral) & line current respectively.
are the generalized constants and are functions of line parameters.
VS = VS ∠δ,
A = D = A ∠α,
VR = VR ∠0
B = B ∠β
Hence receiving end power PR
and QR are given as:
PR =
VS VR
A
2
cos(β − δ) −
VR cos(β − α)
B
B
QR =
VS VR
A
2
sin(β − δ) −
VR sin(β − α)
B
B
2.7 The performance of the Line
Two measures are always use to evaluate the performance of a transmission line; the voltage
regulation and the efficiency.
Changing the load value during the day will change the voltage at receiving end assuming that
the voltage at sending end is constant. In case of long line operating al light loads (almost no
load), the receiving end voltage may be greater than sending end voltage due to line capacitance.
This phenomenon, which is called Ferranti effect, can be shown from the phasor diagram of
Figure 3.
Figure 3:
π - Model for Transmission Line and its Phasor Diagram
On the other hand, at heavy loads the voltage at receiving end is less than the sending end
voltage by a significant value. The difference between the voltage at no load and at load
determines the voltage regulation of the line at the studied load as follows:
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Line Voltage Regulation % = 100*(|VRnl| –| VR|) / |VR|
The efficiency of the transmission line is simply the percentage ratio between the receiving
end and the sending end, thus
Line Efficiency % = (PR / Ps ) *100
III Experimental Procedure
3.1 Synchroniation of G1 to Grid System
This procedure is for connecting Generator 1 to the main, or GRID supply at the GEN 1
Busbar. The synchronizing switch is therefore CB8 that is duplicated in the central control
panel.
1) Before switching the supply on, check that the excitation and power pots are on
minimum position.
2) Above the Generator 1 Control Panel there is a synchroscope that has two inputs:
‘Reference Bus’ and ‘Incoming Bus’. The synchroscope also has an on-off switch that
should be normally in the 'off' position except when synchronizing.
3) To synchronize Generator 1 to the main or GRID supply the Grid red and yellow
terminals should be connected to the REF bus terminals of the synchroscope. Similarly
the red and yellow terminals of GEN 1 should be connected across to the INCOMING
bus terminals of the synchroscope.
4) Link sockets S1 to S3 directly.
5) Switch on the Mains Supply MCB on the left of the Simulator panel.
6) Close circuit breakers CB2, CB3, and CB5.
7) Press the green START button for the motor.
8) Quickly bring up the speed to 1800 rev.min-1 (if you are too slow, the under/over
frequency system will trip).
9) Close the circuit breaker CBFb in the Generator 1 Control panel. Increase the excitation
to give a voltage equal to that of the Grid supply.
10) Switch on the synchroscope. Watching the synchroscope, gently alter the speed so that
the red LEDs of the synchroscope are indicating slow clockwise rotation. Just before top
dead centre of the synchroscope (at 11 o'clock), positional indication changes to the
green LEDs. Close the duplicate circuit breaker control switch CB8b, in the Generator 1
Control panel when the green LED illumination approaches top dead centre. Circuit
breaker CB8 closes to connect the Generator 1 to the GEN 1 BUS.
11) Generator 1 is now synchronized to the Grid supply. Record the values of the
speed/power pot and the generator excitation pot in Table 1. The speed/power pot now
controls the power output of the generator. Generator excitation pot controls reactive
power.
3.2 Effect of Varying Generator Excitation Reference
1) With the generator synchronized to the grid system, ensure that the excitation pot and the
speed/power pot are adjusted on the values recorded in Table 1. If not, readjust them.
2) Change the setting of the excitation pot in steps of +5% according to Table 2. For each
value take the readings of excitation current ‘If’ (from G1 measurement panel) and the
real power ‘P’, the reactive power ‘Q’, the current ‘I’, the power factor ‘pf’ and the
frequency ‘f’ from MD.
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3.3 Effect of Varying Generator Speed/Power Reference
1) With the generator synchronized to the grid system, ensure that the excitation pot and the
speed/power pot are adjusted on the values recorded in Table 1. If not, readjust them.
2) Change the setting of the speed/power pot in steps of +5% according to Table 3. For each
value take the readings of excitation current ‘If’ (from G1 measurement panel) and the
real power ‘P’, the reactive power ‘Q’, the current ‘I’, the power factor ‘pf’ and the
frequency ‘f’ from MD.
3) When switching off, reduce the power output and the reactive power to as near zero as
possible before opening the circuit breaker CB8.
3.4 Transmission Line Performance
1) Use the routing diagram of figure 4 to route the system shown in Figure 3 on the power
simulator. Connect line capacitance#2 to the two terminals of line#2, with the help of
two different links.
2) Connect the resistive loads R2 & R3 to obtain different values of loads from 0% to 125%
in steps of 25% as shown in Table 4. For each load value take readings of the sending
end Ps, Qs, Vs& Is using MD and readings of the receiving end Pr, Qr, Vr& Ir using ML.
3.5 Effect of Load Power Factor on Line Performance
1) For the system of experiment 3.4, adjust the resistive load at 100%. Add the inductive
loads L2 & L3 at different values from 0% to 100% in steps of 25% as shown in
Table 5. Take readings of the sending end Ps, Qs, Vs& Is using MD and readings of
the receiving end Pr, Qr, Vr, Ir in addition to the power factor pfr using ML.
2) Connect capacitive load C2 at 25% to obtain the leading power factor case study.
Figure 3:
Study system for Experiments 3.4 & 3.5
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Report
Your report should contain the following
a- A one-page summary of the objectives and theoretical background.
b- The experiment data sheets (Tables and/or graphs).
c- The answers to the questions that will follow.
d- Comment on results and a general conclusion.
1)
Use the data of Table 2 to plot If, V, I, f, P, Q and pf against the variations
of the excitation reference.
2)
From (1), identify the variables that are effectively changing with
increasing Vref, and determine the mode of change (increase/decrease).
3)
Use the data of Table 3 to plot If, V, I, f, P, Q and pf against the variations
of the speed/power reference.
4)
From (3), identify the variables that are effectively changing with
increasing Pref, and determine the mode of change (increase/decrease).
5)
Comment on the variation of Q in Table 3 with increasing Pref (use
equations 1&2) .
6)
Complete the data of Table 4; i.e., calculate the values of the line
efficiency and line voltage regulation for each case.
7)
Comment on the values of Qr as compared to Qs.
8)
The load at receiving end is pure resistive, however, the reactive power at
receiving end is not equal to zero. Comment.
9)
Use the data of Table 4 to plot the voltage regulation of the line against
load variations (voltage regulation% against P in W). Comment.
10) Use the data of Table 4 to plot the efficiency of the line against load
variations (efficiency% against P in W). Comment.
11) Complete the data of Table 5; i.e., calculate the values of the line
efficiency and line voltage regulation for each case.
12) Use the data of Table 5 to plot the voltage regulation of the line against
load variations (voltage regulation% against P in W). Comment.
13) Use the data of Table 5 to plot the efficiency of the line against load
variations (efficiency% against P in W). Comment.
.
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Table 1:
References at Synchronization Instant
Excitation Reference Pot ‘Vref’
Speed/Power Reference Pot ‘Pref’
Table 2: Effect of Varying Generator Excitation Reference
ΔVref %
0
5
10
-5
-10
If
V
I
f
P
Q
pf
Table 3: Effect of Varying Generator Speed/Power Reference
ΔPref %
0
5
10
15
If
V
I
f
P
Q
pf
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Table 4: Transmission Line Performance
Reasistive Load
R2 + R3
Ps
Qs
Vs
Is
Pr
Qr
Vr
Ir
Voltage
Regulation%
Efficiency%
0%
25%
50%
75%
100%
Table 5: Effect of Load Power Factor on Line Performance (R2+R3=100%)
Inductive Load
L2 + L3
Ps
Qs
Vs
Is
Pr
Qr
Pf r
Vr
Ir
Voltage
Regulation%
Efficiency%
0%
25%
50%
75%
100%
L2+L3=0%,
C2=125%
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