Paper acccpted for prcscntation at PPT 2001
2001 IEEE Porto Power Tech Conference
1 OLh-1 3thScptember, Porto, Portugal
Power System Transient Stability Analysis
Including Synchronous And Induction Generators
Claudio L.Souza, Lucian0 M. Neto, Gerald0 C. Guimarles, Adelio J. Moraes
Federal Universiy of Uberl6ndiu
Faculty of Electrical Engineering
38400-902 Uberldndiu MG Brazil
Tel./Fax: +55 - 34 323 9 - 4191
clsouza@ujiu.br or gcaixeta@ufir. br
Abstract -This article aims to analyze the transient stability
of electrical power systems including the influence of
induction generators driven by prime movers whose primary
fuel is the industrial wastes of sugar cane alcohol plants. In
steady state, these machines work with the synchronous
generators attending part of the active power demand
(cogeneration), and during disturbances they also act to
improve the system transient stability. Using an existing
transient stability program, some simulations are run to
compare the performance of a typical electrical system with
and without the presence of induction generators.
Index Terms - Electrical power systems, transient stability,
induction generators, cogeneration.
I. INTRODUCTION
The continuous demand growth of power systems
associated with the decrease in investments on new
generation units have caused these systems to work more
stressed and with increased stability problems. Thus, it is
evident the necessity of studies in this area aiming to the
determination of new techniques andor devices which can
result in an improvement of the global system stability
[1,2]. A possible solution would be the use of the energy
accumulated in the industrial wastes as an alternative
energy source to feed part of load demand of the electrical
system. As an example of such industrial waste in Brazil is
that one resulted by the productive processes of the sugar
cane alcohol plants. In this case, an appropriate device
would be the induction generators driven by prime movers
using the sugar-cane leftovers as the primary fuel. Such
machines, when compared to synchronous generators of
same capacity and similar conditions, hold the following
advantages: robustness, reduced size, decreased cost, no
synchronization operations required and high reliability.
However, the major feature of the induction generator, in
which it becomes even more advantageous, is that its
transient response is extremely fast. Thus, the power
transference, during the transient process after a
disturbance, occurs extremely rapid and with reduced
0-7803-71 39-9/01/$10.0002001 IEEE
frequency oscillations. Therefore, this article intends to
extract conclusions regarding the performance of the
induction generators working in an electrical power system
where most of the energy is supplied by synchronous
generators. The overall system stability is evaluated when a
major disturbance, such as the loss of a synchronous
generator, is simulated with and without the presence of
the induction generators.
11. OVERVIEW OF INDUCTION GENERATORS
The operation of the induction generator can best be
understood by first reviewing the basic principles of the
induction motor [3,4]. When the stator windings of an
induction motor are connected to an alternating current
power source, reactive power flows from the source to the
stator windings and establishes a magnetic field which
rotates around the stator at synchronous speed. The
rotating magnetic field of the stator produces a magnetic
flux in the air gap between the rotor and stator. This flux
cuts across the bars in the squirrel cage rotor, inducing a
voltage across the rotor bars. The induced voltage produces
a current flow in the rotor bars, since they are short
circuited by the end rings of the rotor. The rotor currents,
in turn, produce a magnetic field which interacts with the
rotating stator field to produce a torque on the rotor and
thus drive the load. The rotor must turn at less than
synchronous speed, since the production of torque depends
upon relative motion between the rotor and stator fields to
generate a changing flux in the rotor bars. The production
of torque on the turning rotor represents a power output
from the motor wherein energy is transferred across the air
gap from the stator to the rotor and thus to the load. In this
mode both real and reactive power flow into the motor, and
energy is transferred from the stator to the rotor.
If the same machine is again connected to an external ac
power source and driven above syncrhonous speed, it will
function as a generator and supply energy to the power
system [4]. Reactive power will still flow from the ac
power source to the stator windings to establish the
rotating magnetic field of the stator. However, since the
rotor is now driven above of the synchronous speed, the
relative motion between the rotating fields is opposite to
that of the motor operation. As a result, the energy is
transferred from rotor to stator through the air gap. In this
mode of operation as a generator, the active power flows
from machine to the system, while the necessary reactive
power for its excitation comes from the network.
A great advantage of the induction machine in certain
applications is the ability to alternate between motor
operation and generator operation without requiring
additional equipment or controls. The kW output rating of
an induction machine when operating as a generator will
be 0.746 times the horse power rating of the machine
operating as motor. A 2000-hp induction motor, for
example, would have a rating of about 1500 kW as a
generator [4].
111. MAJOR DIFFERENCES BETWEEN lNDUCTION
AND SYNCHRONOUS GENERATORS
In this section, these two machines will be compared with
respect to: excitation, efficiency, power factor, fault
contribution, speed and load characteristics, and harmonics
141.
A. Excitation
The induction generator requires a external source of
reactive power for excitation. The excitation can be
supplied by two different forms: through the ac power
system and through a bank of capacitors installed at its
terminals when occurs the phenomenon of the selfexcitation. The excitation is required to create the rotating
magnetic field and it is independent of the generator load.
As to the synchronous generator, it requires a continuous
excitation source which is applied to the field windings of
the rotor. When the rotor magnetic field rotates it cuts the
windings of the stator (armature) generating a sinusoidal
voltage. The excitation, as well as the voltage induced on
the windings of the stator, can be varied by changing the
direct current supplied to the field windings.
B. Ef3ciency
The efficiency of an induction machine is slightly lower
when operating as generator than when operating as a
motor. In the generating mode the internal generated
voltage is higher that the terminal voltage, occurring
increases of the core losses and certain stray load losses.
There is also a slightly larger windage loss due to higher
rotor speed. The change in full load efficiency for a twopole machine in the range of 0.4-7.0 MW would typically
be on the order of 0.1-0.3 % [4].
Normally the synchronous generators as well as the
induction generators have higher efficiencies in the larger
ratings. For a 3.0 MW two-pole machine an efficiency of
around 96.3 % would be typical for both the induction and
synchronous generator [4].
C. Power Factor
As the induction generator demand always the same
amount of magnetizante reactive power, its power factor
gradually becomes dependent of the active power
demanded by the load. The power factor is directly
influenced by the size of the generator. If its power
represents a substantial portion of the system, the machine
power factor will influence the resultant power factor of the
network. To improve the power factor it is enough to
install capacitors next to the generators. A typical
induction generator of 3.0 MW operating at full load
would have a power factor of 92-93 % [4].
The active and reactive power ratings of synchronous
generators are usually based upon an 80% lagging power
factor. By adjusting the excitation, however, the machine
can be operated at virtually any power factor: lagging, unit
and leading, within its operating range. In fact, the
synchronous generator can be used to improve the system
power factor while the induction generator always tends to
lower it.
D. Fault Contribution
The induction generator contributes very little to a short
circuit in the system. When a bolted three-phase fault
occurs in its terminals, there is an initial fault current peak
according to the subtransient reactance of the machine.
With a typical subtransient reactance of 0.17 pu, the initial
maximum symmetrical fault current would be about six
times the full load current of the generator. The fault
contribution normally dies out within a few cycles. When
the terminal voltage collapses the reactive power supplied
to the machine ceases and the generator is taken off
operation.
The synchronous generator contributes substantially to a
short circuit in a power system. A typical synchronous
generator has a subtransient reactance around 0.1 pu, and
its initial maximum symmetrical fault contribution is
around ten times its full load current. From approximately
5 to 200 cycles, the fault current is determined by the
transient reactance, which is typically around 0.15 pu.
After this, the fault contribution will be determined by its
synchronous reactance which is typically around 1.5 pu
[4]. However, if the generator is fitted with an automatic
voltage regulator the sustained fault current will be much
higher than this value.
w4
E. Speed and Load Characteristics
SYNCHRONOUS GENERATORS
The output frequency of the induction generator is
determined entirely by the frequency of the electrical
system to which it is connected. Changing the speed of the
prime mover, changes the active power output of the
generator proportionally, but has no effect upon the
frequency. The terminal voltage of the induction generator
is set by the voltage of the connected power system.
The output frequency of the synchronous generator is
determined by the speed of the prime mover. The
synchronous generator needs to be driven at constant
synchronous speed to keep synchronized with the electrical
system. The frequency need to be kept within very narrow
limits when the machine is connected to main grid. The
output power is determined by the torque applied to the
prime mover.
4 K REACTANCE
(100 MVA BASE POWER)
REGION
?
'b6
REGION
rq
STATICS
STATICS
LOADS
INDUCTION
MOTORS
INDUCTION
MOTORS
I
F. Harmonics
STATICS
The induction generators do not introduce additional
harmonic voltages into the power system. In fact, the
squirrel cage rotor tends to dampen out harmonic
disturbances [4]
The output voltage of the synchronous generators
contains odd harmonic voltages. The third harmonic is that
with greater importance and which can reach up to ten
percent at full load.
IV. TEST SYSTEM AND GENERAL, DATA
A 10-bus electrical system shown in the schematic
diagram of figure 1 was used for the simulations. It has 3
busses with synchronous generators, 2 busses with
induction generators, 3 busses with loads and 2
transference busses. The synchronous generators can be
understood as belonging to a hydroelectric plant, the
regions A and B can represent two nearby consumers and
region C can represent an alcohol plant which operates in
co-generation with the electrical energy utility. The system
active losses was ignored.
When the system operates without induction generators,
each one of the synchronous generators on busses 1, 2 and
3 supplies the same initial active power of 22 MW to
attend a total installed load of 66 MW. When the two
induction generators (total of 6 MW) are connected to the
system, keeping the same load demand of the previous
case, the initial power of each synchronous generator has
to be reduced by 2 MW in order to achieve the power
balance. Thus, it becomes 20 MW as it shown in table 1,
The tables 1, 2 and 3 present, respectively, the test
system input data, the parameters of the synchronous
machines and the parameters of the induction generators,
used for the transient stability simulations shown in the
next section.
INDUCTION
MOTORS
INDUCTION
GENERATORS
Fig. 1 Schematic diagram of 10-bus test system
Bus
No.
Generated
Power
MW
MVAr
I
20,000
20,000
2
9
10
Consumed
Power
MW MVAr
Terminal
Voltaee
pu
degree
13,000
0,000
0,000
1,000
0,000
13,000
0,000
0,000
1,000
-0,000
I 3,O I 1,0 I0,Ol I 0,009 I 0,098 1 0,098 I 3,9
I 3,O I 1,0 I 0 , O I I 0,009 I 0,098 I 0,098 1 3,9
V. SIMULATIONS
The main objective of the simulations is to evaluate the
stability of the electrical power system, when induction
generators are connected to the system, worlung in parallel
with synchronous generators. For comparison, two distinct
cases are considered.
A. Case I :
The responses indicated in figures 2 and 3 do not show
any significant change between the two cases, because the
total system load demand remained practically the same
with equal overload applied to the synchronous generators.
-
36
IG-OUT
System running without induction generators
(curves IG-OUT)
This is considered as the base or reference case. In this
stage all the demand of active power required by the
system is supplied by the synchronous generators. In this
and in the following case, the same type of disturbance is
analyzed, that is, a loss of 33% of the total synchronous
generation. When studying the impact of this disturbance
on the synchronous machines, only the curves
corresponding to one of the synchronous generators
(located on bus 1) are considered here, since all
synchronous machines are identical and with similar
voltage and speed controllers.
LL
0
a
6
4
2
10
Time ( s )
Fig. 2 Bus-1 synchronous machine active power response
20
B. Case 2: System running with induction generators
(curves IG-IN)
In this case two induction generators are connected to
busses 9 and 10 of the test system (see figure 1). Each one
has its active power specified in 3 MW. Altogether the two
units comprises 9.1 % of the total active power supplied by
the three synchronous generators. To carry out the stability
analysis, besides the synchronous machine curves of case
1, it is also included some results concerning to one of the
identical induction generators. The one connected to bus 9
(see figure 1) was chosen.
VI. RESULTS
A . Bus-1 synchronous machine results
The curves of figures 2 to 5 show the dynamic response
of the synchronous machine on bus 1 for both cases,
absence 'and presence of induction generators. Since the
two remaining synchronous generators are equal, each one
share 50 % of the imposed overload.
Figures 2 and 3 describe, respectively, the behavior of the
active and reactive powers. After the loss of the generation,
the curves of figure 2 show that the active power, for both
situations, in the absence and presence of the induction
generators, increases 11 and 10 MW, respectively, as
expected. As to the reactive power supplied by the
synchronous generator, shown in the figure 3, it also
presents the same increase of 50 % during the loss of
generation in both cases, (that is, increases of 6.0 and 6.5
MVAr, respectively) to compensate the loss.
10
"
'
~
"
'
~
"
'
~
"
'
~
"
'
4
10
Time ( 5 )
Fig. 3 Bus-I synchronous machine reactive power response
Figure 4 gives the terminal voltage behavior of the
synchronous machine on bus 1, and, as it was expected, it
shows a sudden fall (7.0 %, approximately), just after the
disturbance, in both cases. As the synchronous generators
control the voltage of this bus, there is not any appreciable
alteration of the voltage behavior when the induction
generators were included in the system.
1 .M
IG-IN
Time ( 5 )
Fig. 4 Bus-1 synchronous machine voltage response
Figure 5 indicates that the frequency stabilizes in a level
0.75 % and 1.08 % below its initial value, respectively, in
the presence (curve IG-IN) and absence (curve IG-OUT) of
the induction generators.
59.4
59.2
1
.....
~~~~~
................
"
~~~~~
'
I
"
disturbance (figure 7) due to the voltage drop on its
terminals (figure 8) caused by the generation loss. It
returns to its initial value as soon as the voltage recover its
initial steady state value.
Figures 6 and 7 points out that the induction generator
do not possess stability problems, since it returns quickly to
its initial steady state if its terminal voltage comes back to
is normal value. This represents a major advantage of
induction generators with respect to synchronous
generators on stability viewpoint.
.... ........ ................ ................. ...........
~~~
'
"
~~~
"
"
~~~
'
"
'
10
Time ( s )
Fig. 5 Bus-1 synchronous machine frequency response
The curves of figure 5 show that, just after the loss of one
third of the total synchronous generation, the damping of
frequency oscillations of the synchronous generator on bus
1 is larger for the case with induction generators than
without these machines.
1 . J
'
'
0
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
I
10
Time (s)
Fig. 7 Bus-9 induction generator reactive power response
B. Bus-9 induction generator results
The curves of active power, reactive power and voltage,
respectively, shown in figures 6, 7 and 8, give the behavior
of the induction generator on bus 9, for the loss of 33% of
the total synchronous generation.
Figure 6 evidences the effect of damping introduced by
the induction generator, following the generation deficit,
since this machine tries to compensate this power
unbalance through a sudden initial increase of its active
power. After this, the power oscillations reduce rapidly and
stabilize practically on the same initial level.
-2.5
0.9''
"
I
'
"
I
'
'
'
I
"
'
I
"
'
1
10
Time ( s )
Fig. 8 Bus-9 induction generator voltage response
VII. CONCLUSION
-4
10
4
Time (s)
Fig. 6 Bus-9 induction generator active power response
The reactive power absorbed by the induction generator
is reduced during the interval of 0.5 second after the
The results given above show that, for the same system
load, the supply of a small portion of the demand (9.1%)
for the two induction generators, does not significantly
affect the steady state system operation. However, with the
event of a system disturbance, such as a loss of a generator,
which tends to cause transient stability problems, the
presence of these induction generators acts as a stabilizing
factor. Thus, these machines causes the electrical system to
become stiffer from the point of view of the stability.
VIII. REFERENCES
Books:
Anderson, P.M. and Fouad, A.A., “Power System
Control and Stability”, The Iowa State University
Press, Ames, USA, 1977.
[2] Kundur, P., “Power System Stability and Control”,
McGraw-Hill Inc., EPRI, USA, 1994. May/June, pp
[l]
497-503,1984.
[3] Fitzgerald,
A.E., Kingsley Jr., C., “Electric
Machinery - The Dynamics and Static of
Electromechanical Energy Conversion”, McGrawHill Book Company, 196 1.
[4]
Periodicals:
Parsons .Jr.,. R., “Cogeneration Application of
Induction Generators”, IEEE Transactions on
Industry Applications, Vol. 1A-20, No.3.