Sabapathi et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945
Research Paper
ANALYSIS OF POWER SYSTEM STABILITY FOR
MULTIMACHINE SYSTEM
D. Sabapathia and Dr. R. Anitab
Address for Correspondence
Research Scholar, Department of Electrical & Electronics Engineering, M.P.N.M.J. Engineering College,
Chennimalai India
ABSTRACT
a
This paper proposes Power System Stabilizer (PSS) in addition to the existing AVR and Governor for power system
stability. The variations of rotor angle, voltage and frequency of TNEB system are taken as comparison parameters. The
system is simulated with the existing controllers and the proposed controllers for three phase fault and single phase fault
using ETAP software. The combination of AVR, Governor and PSS maintains synchronism during all kinds of faults.
KEYWORDS: AVR, Governor, PSS, Power system stability and ETAP.
1. INTRODUCTION
In recent years, there has been considerable interest
in designing excitation controllers, Governors along
with Power System Stabilizers (PSS) which are
expected to give better dynamic performance for
large scale power system over a wider range of
system and operating conditions. Voltage and
frequency is a very important index of power supply
in power system operation. Both utility equipment
and consumer equipment are designed to operate
within a certain voltage and frequency range. A
system is said to be synchronously stable (i.e., retain
synchronism) for a given fault if the system variables
settle down to some steady-state values with time,
after the fault is removed [1]. Prolonged operation of
the equipment at voltages and frequencies outside the
allowable range could adversely affect its
performance and possibly cause damages to the
equipment. The generator excitation system and the
governor system are the most important means of
voltage and frequency control in a power system. It
should maintain the generator terminal voltage and
frequency at a constant value under normal operating
conditions and regulate to its prefault steady value
quickly and effectively once the fault occurs. Many
drawbacks of AVR/PSS have already been discussed
in [2, 3]. The delineates alternative designs that use
differential geometric control theories, namely, exact
and partial feedback linearizing techniques [4]. The
intent of that was to explore the potentiality of
nonlinear exciters.
Any synchronous generator in a power system is
traditionally equipped with an Automatic Voltage
Regulator (AVR), to sustain the generator terminal
voltage, and a PSS, to provide damping torque and
this combination does not meet out the stability
criteria in all aspects.
Analysis of nonlinear
voltage regulators [5] in an SMIB system over a wide
range of system and operating conditions and the
synchronizing and damping torques analysis have
been shown only for SMIB system. The proposed
controller aims to improve the voltage control,
stability and frequency control for a multi-machine
infinite bus system under small signal disturbances
and transient conditions. Simulation is performed for
different cases using above software and results are
analysed and compared for transient and small signal
disturbance [6]. In the First Case, Large Fault (three
phase fault) at 400 kV is considered to verify the
transient stability. In the Second Case, Small Fault
(single line to ground fault) at 400 kV is considered
for analysis.
2. SYSTEM DATA
Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/116-120
Tamilnadu Electricity Board (TNEB) system was
considered to evaluate the performance of the
proposed Automatic Voltage Regulator (AVR),
Governor and PSS.
2.1 System
TNEB 400 kV Grid system consists of 57 Buses out
of which 40 are 400 kV buses and 52 transmission
lines. The entire generators which are directly
evacuated at 400 kV are modelled with their
generator transformer. Totally 11 generating stations
with one or more units at each generation plant is
modelled. The generator transformer is also modelled
with its leakage reactance.
Generator is modelled with transient model,
considering direct axis sub transient reactance, direct
axis transient reactance, direct axis synchronous
reactance, armature resistance, and open loop time
constants for both transient and sub-transient model
and inertia constant for the generator. Koodankulam
1000 MW Nuclear Power Plant, Coastal Energen 660
MW, Mettur and North Chennai, Neyveli are the
important generating stations. The above power
plants are located at Tamilnadu, India.
Generator Transformer is modelled with its leakage
reactance, X/R ratio and OLTC is modelled with its
minimum and maximum tap and tap step.
Transmission line is modelled with nominal Pi model
with series resistance and reactance and shunt half
line charging susceptance.
All the loads are lumped at 400 kV and the load is
modelled as a combination of constant impedance,
constant current and constant power loads.
The generator consists of AVR, governor and PSS
which plays crucial role in stability. Since the
generating stations of various locations are installed
in various years, the very old power plant consists of
dc excitation system and (remove with here) less than
a decade generating station consists of either static or
brushless excitation. All the AVRs are modelled as
per manufacturer block diagram and transfer
function.
Governor also ranges from old governors in the
power plants which are operating more than 20 years
to recent governors in the recently commissioned
power plants. Electronic governors are used in the
recent power plants.
PSS is not present in the old power plants. Whereas
separate PSS is available in some power plants and
PSS is a part of AVR in the very recent power plants.
3. Modelling of Power System
AVR, Governor & PSS are modelled for transient
stability in ETAP software as shown below:
3.1 AVR
Sabapathi et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945
AVR with its rectifier time constant Trec and gain
constant Ka and associated time constant Ta, exciter
gain Ke and its associated time constant Te are
modelled as shown in Fig. 1.
Figure 1. Block diagram for AVR
The output of the rectifier is compared with the
reference voltage and then multiplied by gain. The
output of the rectifier is fed to a limiter, which
restricts the minimum and maximum output to
prevent the components failure. The exciter gain and
time constant is given to field limiter which restricts
the field voltage within its minimum and maximum
limit so that the field current is limited to prevent the
rotor damage, as in Eqs. (1) and (2).
The modelling problem of a truly multi-machine,
multi-order representation is not a simple matter and
the algebra behind it will be quite cumbersome as the
number of machines increase. Depending on the
chosen model for a given machine, several
coefficients associated with the differential and
algebraic equations may or may not exist depending
on the kind of model used for the rest of the
machines.
In fact, the whole mathematical representation will
differ for each study case [7].
3.2 Governor
Governor works based on the changes in speed, as in
Eqs. (3) and (4). In thermal power plants, the
governor consists of three stages of turbine (High
Pressure – HP, Intermediate Pressure - IP and Low
Pressure – LP) for normal technology and four stages
(Very High Pressure – VHP, High Pressure – HP,
Intermediate Pressure IP and Low Pressure – LP) for
critical technology.
If there exists a difference between total generation
and load, the mismatch of these will reflect as change
in frequency and hence the change in speed of the
machines. Governor will adjust the turbine valve to
reduce the mismatch between generation and load.
3.3 Power system stabilizer
Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/116-120
Figure 2. Block diagram for thyristor excitation system
Fig. 2. shows the block diagram representation for
thyristor excitation system with PSS. PSS is the most
widely used device for resolving oscillatory stability
problems [8]. The development of general concepts
associated with applying power system stabilizers
utilizing shaft speed, ac bus frequency and electrical
power inputs and root locus involved shifting of
Eigen values related to the power system modes of
oscillation by shifting the poles and zeros of the
stabilizer [9]. Stabilizer consists of three blocks such
as, PSS Gain, Washout Time constant and Phase
compensation. This limits the speed and magnitude
of AVR to ensure that the oscillations are damped out
quickly.
PSSs utilizing shaft speed, ac bus frequency and
electrical power inputs to regulate the field voltage
are given in Eqn. (6).Voltage dependence on angular
speed changes is demonstrated. These changes
contribute to changes in the electric power supplied
by the machine. This effect enhances the stability of
the system. This is true if the field voltage is
constant. However, machine terminal voltages are the
negative feedback to the exciters. Therefore, change
of voltages due to angular speed increments tend to
reduce the field voltage and vice versa. This has
negligible effects on machines connected to infinite
bus as the speed changes will eventually vanish.
However, for multi-machine isolated systems, the
angular speed changes should settle to a final value
depending on the perturbation [10]. Therefore, the
voltage frequency dependence tends to destabilize the
system. A positive feedback from angular speed
changes to the exciter can overcome this problem. A
feedback gain in the range of unity can be sufficient
to stabilize the uncontrolled (without PSS) system
[11].
Sabapathi et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945
4. SIMULATION AND RESULTS
Various contingencies are simulated on the TNEB as
given below. The contingencies include large
transient event like three phase fault and
contingencies like single line to ground fault are
considered. The system has been modeled with
existing AVR, governor and PSS and studies are
carried out. Rotor angle, voltage and frequency
oscillations are considered as key criteria
(parameters) to compare the results. Then this system
with various AVR, Governor and PSS model and
parameters are considered to evaluate their behavior.
Best suitable model and parameters of AVR,
Governor and PSS is identified based on the above
said criteria.
4.1 Three phase fault
Though there are rare, very severe faults in power
system, the fault, which in general used to find out
the critical clearing time, is three phase faults. In
general rotor angle stability is taken as index, but the
concept of transient stability, which is the function of
operating condition and disturbances, deals with the
ability of the system to remain intact after being
subjected to abnormal deviations.
Three phase to ground fault at very important 400 kV
substations (Sriperumpudur and Salem located in
Tamilnadu, India) in Tamil Nadu Electricity Board
(TNEB) system are simulated. Rotor angle
oscillations of generators and frequency are verified.
The graph shows the rotor angle oscillation of
various generators for a fault at 400 kV
sriperumpudur substation for the duration of 100 ms.
with existing system parameters and proposed
parameters. Results show that the proposed
parameters reduce the rotor angle oscillation to great
extent as shown in Figs. 3 - 8.
Figure 3. Rotor angle Variations of TNEB systems with existing controllers during 3 phase fault.
Figure 4. Rotor angle Variations of TNEB systems with proposed controllers during 3 phase fault.
Figure 5. Voltage at 400 kV bus in TNEB systems with
existing controllers during 3 phase fault.
Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/116-120
Figure 6. Voltage at 400 kV bus in TNEB systems with
proposed controllers during 3 phase fault.
Sabapathi et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945
Figure 7. Frequency at 400 kV bus in TNEB systems
with existing controllers during 3 phase fault.
4.2 Single line to ground fault
Quite frequent fault in power system is single line to
ground fault. In general, single line to ground fault
does not have much to do with the critical clearing
time and so rotor angle oscillation was compared.
Single line to ground fault at few 400 kV substations
(Udumalpet, Almati, Sriperumpudur and Salem) in
TNEB system is simulated. Rotor angle oscillations
of various generators and frequency are compared
with existing system and proposed controllers. The
graph shows the rotor angle oscillation of various
generators and frequency for a fault at 400 kV
Sriperumpudur substation for the duration of 100 ms.
with existing system parameters and proposed
parameters. Classical approach to solution of stability
analysis problems in power system is based on
analysis of properties of a system of equations,
representing dynamic behavior of a power system as
a whole. This general approach being adequate and
efficient in application to dynamic analysis of small
and medium-size power systems meets with
difficulties when applied to solution of these
problems in large power interconnections. Results
show that the proposed parameters reduce the rotor
angle oscillation to great extent as shown in Figs. 9
and 10.
Figure 8. Frequency at 400 kV bus in TNEB systems
with proposed controllers during 3 phase fault.
Figure 9. Rotor angle variations of TNEB systems with existing controllers during single line to ground fault.
Figure 10. Rotor angle variations of TNEB systems with proposed controllers during single line to ground fault.
Graph shows that the rotor angle oscillation is greatly
reduced and suppressed with proposed controllers.
For a three phase fault the system is unstable (i.e
Rotor angle goes beyond 1800 ) with the existing
controllers which is made stable with proposed
controllers and the maximum rotor angle oscillation
is limited to 1700. In addition, the oscillation occurs 5
Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/116-120
seconds after clearance of the fault with proposed
controllers, whereas the oscillation persists with
existing controllers. Since the system considered is
large, the impact of the frequency and voltage by the
proposed controllers is not significant, but still that
improves the voltage profile as tabulated below in
Table 1.
Sabapathi et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945
Since 400 kV system is solid earth system, the fault
current is almost close to three phase fault current.
For a Single line to ground fault the system is stable
with the existing controllers however the angle of
rotor reaches maximum of 1720 which is reduced to
1600 with proposed controllers. It clearly indicates
that the proposed controllers are able to improve
stability and reduce the angle of oscillation. The
rotor oscillation continues for larger time after
clearance of the fault with existing controllers,
whereas proposed controllers suppress the oscillation
quickly. Since the system considered is large the
impact of the frequency and voltage by the proposed
controllers are not significant but still that improve
the voltage profile as tabulated in Table 2.
Table 1. Three phase fault
Existing
Proposed
Parameters
controllers
controllers
Rotor angle in
Unstable
170
degree
Voltage in volts
0.37
0.43
Frequency in hz
50.5
50.4
Table 2. Single line to ground fault
Existing
Proposed
Parameters
controllers
controllers
Rotor angle in degree
172
160
Voltage in volts
0.7
0.82
Frequency in hz
50.3
50.2
5. CONCLUSIONS
In this paper, the variations of rotor angle, voltage
and frequency of TNEB system have all been taken
as comparison parameters. The system has been
simulated with ETAP software for a fault of 100
milliseconds duration from 3 to 3.1 seconds. System
is analyzed over the duration of 10 seconds. The
system has been simulated with the existing
controllers and the proposed controllers for three
phase fault and single phase fault. From the
simulation results, it has been ascertained that, the
existing controller was not able to maintain the
synchronism of the system during the above said
faults but the proposed controller maintains the
synchronism.
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