Trans Stab Paper

7/28/2019 Trans Stab Paper

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Power System Transient Stability Analysis Using

ETAP Software

Jignesh S. Patel

P.G. Student of Electrical Power SystemB.V.M. Engineering College

V.V.Nagar, Gujarat

[email protected]

Manish N. Sinha

Assistant Professor in EE DepartmentB.V.M. Engineering College

V.V.Nagar, Gujarat

[email protected]

Abstract Power-system stability is a term applied toalternating-current electric power systems, denoting a condition

in which the various synchronous machines of the system remain

in synchronism, or "in step," with each other. Conversely,

instabilitydenotes a condition involving loss of synchronism, or

falling "out of step". Occurrence of a fault in a power system

causes transients. To stabilize the system load flow analysis is

done. Actually in practice the fault generally occurs in the load

side. As we controlling load side which will lead to complex

problem in order to avoid that we are controlling the generator

side. This paper covers the transient stability analysis of 400 kV

substation of Soja. A three phase fault is located at specified bus

to analyze the effect of fault location in critical clearing time on

the system stability.

Keywords- critical clearing time, ETAP, three phase fault,

transient, transient stability.

I. INTRODUCTIONSuccessful operation of a power system depends largely on

the engineer's ability to provide reliable and uninterrupted

service to the loads. The reliability of the power supplyimplies much more than merely being available. Ideally, the

loads must be fed at constant voltage and frequency at all

times. The first requirement of reliable service is to keep the

synchronous generators running in parallel and with adequate

capacity to meet the load demand. Synchronous machines do

not easily fall out of step under normal conditions. A second

requirement of reliable electrical service is to maintain the

integrity of the power network. The high-voltage transmission

system connects the generating stations and the load centers.

Power-system stability is a term applied to alternating-current

electric power systems, denoting a condition in which the

various synchronous machines of the system remain in

synchronism, or "in step," with each other. Conversely,instability denotes a condition involving loss of synchronism,

or falling "out of step."

For convenience of analysis, stability problems are

generally divided into two major categories:

Steady-state stability Transient stability

Steady-state stability refers to the ability of the power

system to regain synchronism after small and slow

disturbances, such as ground power changes. An extension of

the steady-state stability is known as the dynamic stability.

Transient stability studies deal with the effects of large,

sudden disturbances, such as the occurrence of the fault, the

sudden outage of a line [1] [2].Transient stability entails the evaluation of a power

systems ability to withstand large disturbances, and to survive

transition to a normal operating condition. These disturbances

can be faults such as: a short circuit on a transmission line,

loss of a generator, loss of a load, gain of load or loss of aportion of transmission network. Large number of simulationsis carried out regularly during planning stages to gain

knowledge of this system. Yet, even a well designed and

normally operated system may face the threat of transient

instability [3].

On the platform of ETAP, a worldwide-used simulation tool

for power system analysis, the electric grid is settled up for themodeling of generators, transformers, lines, cables, loads, the

external equivalent grid, etc. Eleven typical load flow

operations, including operation structures under normal,

maintenance and extreme conditions, are set up as base cases

for the detailed study, including inter-connected situation and

isolated operation, or some key electric equipment out ofservice. With the units number increasing, the system

presents rotor angle stability. With the multiple voltage levels

in the system, voltage profile is sensitive to the system

structure and operation point. It is critical for the case that

most of the loads are large motors. Although the system is

connected to the bulk system under normal condition, it may

operate isolated at extremely fault contingency. Then the

system faces the demanding requirement of frequency stability

and control [4].

The aim of the investigation is to analyze the behavior of

the synchronous machine in particular the angular position of

the rotor with respect to time after the fault occurs in the

system. Section II is the study of transient stability analysis.Section III describes the development of system model.

Section IV is short circuit analysis of the model. Section V

describes the results for transient stability. Section VI

concludes the paper.

II. TRANSIENT STABILITY ANALYSISA. Transient stability

Each generator operates at the same synchronous speed and

frequency of 50 hertz while a delicate balance between the

13-14 May 2011 B.V.M. Engineering College, V.V.Nagar,Gujarat,India

National Conference on Recent Trends in Engineering & Technology


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input mechanical power and output electrical power is

maintained. Whenever generation is less than the actual

consumer load, the system frequency falls. On the other hand,whenever the generation is more than the actual load, the

system frequency rise. The generators are also interconnected

with each other and with the loads they supply via high

voltage transmission line.

The power system is routinely subjected to a variety of

disturbances. Even the act of switching on an appliance in thehouse can be regarded as a disturbance. However, given the

size of the system and the scale of the perturbation caused by

the switching of an appliance in comparison to the size and

capability of the interconnected system, the effects are not

measurable. Large disturbance do occur on the system. These

include severe lightning strikes, loss of transmission linecarrying bulk power due to overloading. The ability of power

system to survive the transition following a large disturbance

and reach an acceptable operating condition is called transient

stability.Any disturbance in the system will cause the imbalance

between the mechanical power input to the generator and

electrical power output of the generator to be affected. As aresult, some of the generators will tend to speed up and somewill tend to slow down. If, for a particular generator, thistendency is too great, it will no longer remain in synchronismwith the rest of the system and will be automaticallydisconnected from the system. This phenomenon is referred toas a generator going out of step.

B. Elementary view of transient stability analysis

Fig.1.Simple two machine power system

Fig.2.Phasor diagram of the different parameters

Consider the very simple power system of Fig.1, consisting

of a synchronous generator supplying power to a synchronous

motor over a circuit composed of series inductive reactance

XL. Each of the synchronous machines may be represented, at

least approximately, by a constant-voltage source in series

with a constant reactance. Thus the generator is represented by

Eg and Xg; and the motor, by EMand XM. Upon combining

the machine reactance and the line reactance into a single

reactance, we have an electric circuit consisting of two

constant-voltage sources, Eg and EM, connected through

reactanceX=XG +XL +XM. It will be shown that the power

transmitted from the generator to the motor depends upon the

phase difference of the two voltagesEG andEM. Since these

voltages are generated by the flux produced by the field

windings of the machines, their phase difference is the same as

the electrical angle between the machine rotors. The vectordiagram of voltages is shown in Fig.2 Vectorially,

EG =EM + jXI (1)

Hence the current is,

= (2)The power output of the generator and likewise the power

input of the motor, since there is no resistance in the line isgiven by,

=

= (3)Where Re means the real part of and means theconjugate of .Now let,

= 0And

= Then

=

(4)So,

= 090 = 90 90 = cos90 = sin (5)This equation shows that the power P transmitted from the

generator to the motor varies with the sine of the displacement

angle between the two rotors, as plotted in Fig.2.

The curve P versus is known as thepower angle curve and is

shown in fig.3. The maximum power that can be transmittedin the steady state with the given reactance X and the given

internal voltagesEG andEMis,

= And occurs at a displacement angle = 90. The value of

maximum power may be increased by rising either of the

internal voltages or by decreasing the circuit reactance [1].




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Fig.3. Power-angle curve of the system.

C. Swing equationThe electromechanical equation describing the relative

motion of the rotor load angle () with respect to the statorfield as a function of time is known as Swing equation. The

swing equation in terms of the inertia constant becomes,

= (6)Where,

M = inertia constant, it is not really constant when the rotorspeed deviates from the synchronous speed.

Pm = Shaft power input, corrected for windage and friction

losses.

Pe = Pa sin = electrical power output, corrected for electrical

losses.

Pa = amplitude for the power angle curve.

m = mechanical power angle.

Swing equation in terms of electrical angle is,

= (7)

D.Equal-area criterianThe transient stability studies involve the determination of

whether or not synchronism is maintained after the machine

has been subjected to sever disturbance. This may be sudden

application of load, loss of generation, loss of large load, or a

fault on the system. In most disturbances, oscillations are ofsuch magnitude that linearization is not permissible and the

nonlinear swing equation must be solved. A method known as

the equal-area criterion can be used for a quick prediction of

stability. This method is based on the graphical interpretation

of the energy stored in the rotating mass as an aid to determine

if the machine maintains its stability after a disturbance. Themethod is only applicable to a one-machine system connected

to an infinite bus or a two-machine system.

Consider a synchronous machine connected to an infinite

bus. The swing equation with damping neglected is given by,

= = (8)

Where Pa is the accelerating power.

From the above equation, we have

= (9)

Fig.4. Equal-area criterion (sudden change of load).

Consider the machine operating at the equilibrium point 0,corresponding to the mechanical power input Pm0 = Pe0 as

shown in figure 4. Consider a sudden step increase in input

power represented by horizontal line Pm1. Since Pm1 > Pe0, the

accelerating power on the rotor is positive and the power angle

increases. The excess energy stored in the rotor during the

initial acceleration is

= = (10)With increase in , the electrical power increases, and when

= 1, the electrical power matches the new input power Pm1.

Even though the acceleration power is zero at this point, therotor is running above synchronous speed; hence, and

electrical power Pe will continue to increase. Now Pm < Pe,

causing the rotor to decelerate toward synchronous speed until

= max.According to Equation for stability,

= 0 (11)The rotor must swing past point b until an equal amount of

energy is given up by the rotating masses. The energy given

up by the rotor as it decelerates back to synchronous speed is,

=

= (12)The result is that the rotor swings to point b and theangle ,at which point| | = | | (13)This is known as the equal-area criterion. The rotor angle

would then oscillate back and forth between 0and maxat its

natural frequency. The damping present in the machine willcause these oscillations to subside and the new steady state

operation would be established at point b [5].




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III. SYSTEM MODELThe Gujarat Energy Transmission Corporation has

established a 400kV SOJA sub-station. it is 1.5 km between

from Gojariya-Gandhinagar highway. It has a land of higher

bearing capacity. Due to the higher requirement of agricultural

as well as bulk power requirement of industrial area the soja

center is so chosen. It covers total 50acres land, in which theconstruction of 400kV, 220kV and 33kV switchyard, control

room and also staff quarters etc. The incoming line of 400kV

at Soja s/s is from Wanakbori and PGCIL 400kV s/s which is

single circuit type transmission line .The tower required for

erection of 400kV transmission line which is coming from

Wanakbori and PGCIL s/s are of three type i.e., A, type C and

type D tower. The total number of tower required between

Wanakbori & PGCIL and soja s/s is 412. The line has charged

since 28th January 1987 In single line diagram two incoming

lines from Wanakbori and PGCIL of 400kV, and two

incoming line from Gandhinagar of 220kV.

Fig. 5 One line diagram of 400 kV substation of Soja

A power system must be modeled as a nonlinear system

for large disturbances. Although power system stability may

be broadly defined according to different operating conditions,an important problem which is frequently considered is the

problem of transient stability. It concerns the maintenance of

synchronism between generators following a severe

disturbance. By the excitation control in a generating unit

transient stability can be greatly enhanced. Another important

issue of power system control is to maintain steady acceptable

voltage under normal operating and disturbed conditions,

which is referred as the problem of voltage regulation [5].

IV. SHORT CIRCUIT ANALYSIS OF SYSTEM MODELA. System model in ETAP

The case example is modeled in ETAP and shown in

fig.6. ETAP is chosen as the simulation tool, which isdeveloped by OTI, a comprehensive analysis platform for the

design, simulation, and operation of generation, transmission,

distribution, and industrial power systems [5]. It supplies acalculations for load flow, short circuit, transient stability, etc,

which are beneficial to explore the characteristics of

simulated systems.

Fig. 6 system model in ETAP

B. Short circuit study

The short circuit view of the system in ETAP is shownin figure 7. In ETAP, the report can be generated for LLL,

LL, LG, LLG LLLG (symmetrical and asymmetrical both)fault.

Fig. 7 Short circuit view in ETAP




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V. TRANSIENT STABILITY ANALYSIS OF THE SYSTEMMODEL IN ETAP

The ETAP Transient Stability Analysis program is designed

to investigate the system dynamic responses and stability

limits of a power system before, during, and after system

changes or disturbances. The program models dynamic

characteristics of a power system, implements the user-definedevents and actions, solves the system network equation and

machine differential equations interactively to find out system

and machine responses in time domain.

The different plot for generator 1&2 when fault on bus-6 at

0.5sec and cleared at 1sec are shown below in fig.8 (a), (b),

(c), (d), (e), (f), (g) & (h).

Fig. 8 (a) Generator Exciter current

Fig.8 (a) shows the result of the exciter current (Per unit vs.

Time (sec)) for generator 1&2.

Fig. 8 (b) Impedance Z

Fig.8 (b) shows the impedance plot (X in % machine base vs.

R in % machine base).

Fig. 8 (c) Generator Reactive power

Fig. 8 (c) shows the reactive power plot (Mvar vs. Time (sec))

for generator 1&2.

Fig. 8 (d) Generator Electrical Power

Fig. 8 (d) gives the plot of electrical power (MW vs. Time

(sec)) for generator 1&2.

Fig. 8 (e) Generator Speed

Fig. 8 (e) shows the plot for speed variation (Rpm vs. Time

(sec)) for generator 1&2.




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Fig. 8 (f) Generator Relative Power Angle

Fig. 8(f) is the plot for relative power angle (Degree vs. Time

(sec)). We can see the result for generator 1&2.

Fig. 8(g) Generator Terminal Current

Fig. 8(g) is the plot of Generator Terminal Current (Amp vs.

Time (sec)) for generator 1&2.

Fig. 8 (h) Generator Absolute Power angle

Fig. 8(h) shows the plot of Generator Absolute Power Angle

(Degree vs. Time (sec)) for generator 1&2.

VI. CONCLUSIONDynamic performance of a power system is significant in

the design and operation of the system. The transient stabilitystudy determines the machine power angles and speed

deviations, system electrical frequency, real and reactivepower flows of the machines, power flows of lines and

transformers, as well as the voltage levels of the buses in the

system. These system conditions provide indications for

system stability assessments. The results are displayed on theone-line diagram, and also can be printed or plotted. For

transient stability studies, you should model particular groups

of machines in the system , which are known to have

important influences on the system operation. The total

simulation time for each study case should be sufficiently

long to obtain a definite stability conclusion.

Power system stability is the property of a power system

that insures the system remains in electromechanical

equilibrium throughout any normal and abnormal operating

conditions.

Because the power system stability is an electromechanical

phenomenon, it is thus defined as the ability of designated

synchronous machines in the system to remain in

synchronism with one another following disturbance such as

fault and fault removal at various locations in the system.

This paper presents the study of transient stability analysis

and also the transient stability analysis using ETAP. It shows

the different graph of voltage, current, power angle and speed

in the ETAP.

ACKNOWLEDGMENT

I would like to thank to B.V.M Engineering College for

allowing the project work and procure the licensed package of

ETAP software and kind support during project work.

REFERENCES

[1] Ankit Jha, Lalthangliana Ralte, Ashwinee Kumar, Pinak Ranjan PatiTRANSIENT STABILITY ANALYSIS USING EQUAL AREA

CRITERION USING SIMULINKMODEL, Department of Electrical

Engineering National Institute of Technology Rourkela, 2008-09.[2] Pranamita Basu, Aiswarya Harichandan, POWER SYSTEM STABILITY

STUDIES USING MATLAB, National Institute of Technology Rourkela-

769008, Orissa..[3] P.K. Iyambo, R. Tzonova, Transient Stability Analysis of the IEEE 14-

Bus Electrical Power System, IEEE Conf. 2007.

[4] Liang Wang, Li Li, Shanshan Shi, Yiwei Zhang, Zongxiang Lu, JunliangZhangG. Eason, B. Noble, and I. N. Stability and Security Assessment for an

Industrial Electric Grid with Enterprise-owned Power Plants,DRPT2008 6-9April 2008 Nanjing China 1563.[5] Hadi Saadat, Power System Analysis, Tata McGraw-Hill Publishing

Comp. Ltd, New Delhi, Sixteenth reprint 2009.

[6] ETAP version 7.5.0.



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