TRANSIENT STABILITY ANALYSIS USING SVC IN IEEE-9 BUS SYSTEM
Ramakanta Jena*,Shaswat Chirantan**,Dr.P.C.Panda***Dr.S.C Swain
*PhD.S O. E. E,KIIT University,Bhubaneswar,India
**M.Tech Scholar, S O. E. E,KIIT University,Bhubaneswar,India
*** Professor, S O. E. E,KIIT University,Bhubaneswar,India
Abstract The Power system Transmission networks are becoming complexity due to growing demand and restrictions on building new lines. Losing stability is one of the major problem of such a modern power system following a disturbance. FACTS technologies are found to be very effective in a power system transmission network for better controllability and increase power transfer capability without sacrificing the desired stability margin.Also FACTS (SVC) technology provides new opportunity for contolling power and enhancing stability at present as well as building upgraded lines for normal and fault conditions
Keywords—Transient stability,FACTS,SVC
Introduction
The power system operation consists of Three stages of operation i.e Generation ,Transmission And Distribution.The important thing in Power system is to maintain continuous Power supply,which retains stability of voltage but due to unexpected events such as faults, the generators are disturbed causing lack of proportion between generation and load demand.So Fault must be cleared within specific time in order to protect damage to the line Equipment ,which leads to power loss and system outage.
POWER SYSTEM STABILITY
This is the ability of a Power system to regain its normal operating point after being subjected to a physical disturbance .
Stability may be classified depends on the
1. Instability of system i.e
Voltage, Frequency instability
2. Size of the disturbance
Small, large disturbance
3. Time frame of stability
Short and Long term.
In the other hand, stability broadly classified as
1. A. Steady state stability
B. Dynamic stability.
A. Steady State Stability .It is the capability of the system to retain in regular or continuous from one operational state to another under the condition of small load changes.
B. Dynamic stability It is the rotor angle stability after being subjected to a disturbances .
2. Transient Stability. Transient instability occurs when a large disturbance such as loss of load, fault and loss of generation etc., occurs the power balance is upset and the generating units rotors experience either acceleration or deceleration. or
Transient stability is the capaability of the power system to retain the state of operational equilibrium under large disturbances.Large disturbances may be considered as L-G,LL-G,LLL-G,line to line fault or sudden change in heavy loads.These cause the power system to oscillates & swings around stability limits.If power system cannot achieve stability limit then system loses synchronism.Alternator will draw power from the power system & deteriorates the functionality of the electrical grids.
OBJECTIVES OF SHUNT COMPENSATION
The Voltage Profile and Steady state Power transfer can be increased by using Shunt Compensating devices. Shunt compensation improves improves transmission characteristic as well as improves stability of the system.Var compensation is used at mid point for voltage regulation and at the end voltage instability is prevented to damp power system oscilltations.
An Ideal Shunt Compensator
1.Improves Voltage Profile
2. Improving Power-Angle Characteristics
3.Improving Stability Margin
4. Improving Damping of Power Oscillations
FACTS CONTROLLERS.
The FACTS controllers are static and flexible controllers which provides controllability and improve power transfer capability.
SVC(STATIC VAR COMPENSATOR)
SVC is basically a shunt based FACTS controller whose output is varied to maintain specified parameters such as bus voltage by exchanging inductive and capacitive current.It consists of TCR/TSR for absorbing reactive var & TSC for injecting reactive vars.As we know voltage instability is caused by reactive power mismatch.So Static Var Compensator can compensate the need of reactive var to the system.It has also the capability of being used as certain filter by tuning capacitor & inductor switching circuit modeled as filter elements[13].
Figure.1-Static Var Compensator
CONCEPT OF TRANSIENT STABILITY IMPROVEMENT:
Compensation using FACTS change the power flow during dynamic disturbances in order to improve the Transient stability and damp power system oscillation. This is described by the equal area criterion.This criteria is explained below by taking a two machine two line system. The corresponding power (P) and Transmission angle(δ) curve is shown below.
Figure-2 Figure3
The P versus delta (δ) curve “a” is operated at an angle delta1 δ1 before the fault . During the fault the system is characterized by curve b and during this period the transmitted power decreases and mechanical input power to the machine remains constant which is shown by the line P1 .So the generator accelerates and (δ) increases delta1 to delta2 from where the the faulted line is disconnected . A1 represents the accelerating energy which is absorbed by the sending end generator. After clearance of the fault the system is characterized by the curve c. From delta2 the transmitted power exceeds P1 and sending end machine(generator ) starts to decelerate, but the transmission angle increases due to stored kinetic energy. At delta3 ,the decelerating energy is given by the area A2 become equal to accelerating energy A1.The transient stability limit is reached at delta3=delta critical beyond which decelerating energy would not balance the accelerating energy and synchronism between sending end receiving end could not be restored. Amargin represents the transient stability margin.
Hence It is clear from the above that the transient stability is determined by the P and delta curve of post fault system.
SVC works in a superior manner when installed at middle of the transmission line which connects two systems.Thus SVC operates as a midpoint shunt compensator & accordingly enhances the maximum power transfer,nodal voltage support,transient stability limit & power oscillation damping.From the concept of equal area criterion we can clearly analyze the improvement of transient stability margin bu shunt compensation in a two area system[10] as depicted in figure.4.Static Var Compensator has two modes of control strategy,one is Var control mode & other is Voltage regulation mode.Var control mode can be activated by varying susceptance(Bref) value & Voltage regulation mode is controlled by a reference voltage(Vref) & a voltage regulator unit.Transient stability improvement & power oscillation damping is operated by voltage regulation mode.
(a) (b)
Figure.4-Transient stability margin of a two machine system (a)without compensation(b)with midpoint compensation
A1 & A2 are accelerating & decelerating energy represented by areas without compensation.Ap1 & Ap2 are accelerating & decelerating areas with midpoint compensation.Similarly we can see the transient stability margin Amargin(uncompensated system) & Apmargin(midpoint compensated system).
CHARACTERISTIC OF SVC
SVC is composed of a shunt capacitor(thyristor switched capacitor) and a shunt reactor thyristor switched reactor.
SVC operates in two modes i.e in Var control mode(where susceptance is fixed) and in Voltage control mode (where voltage can be controlled within limits)
Figure-5 Figure-6
Generally, A SVC gives near about constant voltage, and possess maximum VAR absorption/generation capability . The performance of the SVCcan be seen in Figure (6).
In below fig the characteristics of SVC is shown in Figure (7).The compensating current decreases linearly and maximum var output decreases with square of AC voltage,as shown in fig below
Figure-7 Figure-8
POWER OSCILLIATION DAMPING
The Below Fig. shows Power oscillation damping of transmission angle and steady state value .
Figure-9
MATHEMATICAL INTERPRETATION
The swing equation of a electrical machine indicates the dynamic behavior.The Dynamic behavior of an n-generator power system can be described by the following equations
……………….1
………………………2
……………..3
………………4
where
M-moment of inertia of machine
– rotor angle of machine
= 1- 2
Pm- Mechanical Power output
Pe- Electrical Power output
Pa- Accelerating Power
– synchronous speed/Rotor speed in electrical degrees
DATA SHEET FOR THE STUDY
Table-1 Generator, Transformer, & SVC Parameters
Table-2-Transmision Line and Load parameters
Table-3 Bus data
POWER SYSTEM STUDY
The Matlab simulink is used to analyze the transient stability of IEEE 9-bus multi machine power system .The Operation is carried out for two generators at nominal loading and one at Heavy loading condition.The base MVA and system frequency are considered to be 100 MVA and 60 Hz, respectively. The generator M1 is connected to slack bus 1, generators 2 (M2) and 3 (M3) are connected to bus bars 2 and 3, respectively. Loads A, B and C are connected in bus bars 5, 6 and 8 respectively. The transient stability analysis is being carried out by monitoring the performance of the generators (M1, M2 and M3) and at different buses. The analysis of this system have been considered when three phase fault occurs in the network.
A.ANALYSIS WITHOUT FAULT
The fig below shows a simulink diagram of IEEE-9bus system. First the Rotor speed is analyzed without fault and found very less fluctuation initially then the system becomes steady quickly.
Figure-10
Relative Rotor angle of Machine 2 w.r.t Machine 1, Machine 3 w.r.t Machine 2 and Machine1 w.r.t Machine3.(Before fault)
B. ANALYSIS WITH THREE PHASE FAULT AT LINE 6(BETWEEN BUS 8 AND 9)
Figure-11
Here a Three phase fault (LLL-G) is considered at Bus -9 in Line 6 for a duration of 0.1 second.and the fault is cleared at the same 0.1 second so the system stability is being Affected.The rotor speed of Machine 3 w.r.t Machine 2 and machine 1 fluctuates more as fault is occcured near the Machine 3 .Then the system becomes steady.
The relative rotor angle delt2_1, delt3_2and delt1_3 of IEEE 9 bus system with fault at line 6 and without SVC
C.ANALYSIS WITH THREE PHASE FAULT AT LINE 6(BETWEEN BUS 8 AND 9) WITH SVC AT BUS 8-9
The relative rotor angle delt2_1, delt3_2and delt1_3 of IEEE 9 bus system with fault at line 6 and with SVC.
Here SVC is connected in line-6 i.e between Bus 8 and 9 and the variation of Rotor speed is shown in fig above.
COMPRASION
Conclusion
From the overall analysis we can see that power system stabilizer & static var compensator are invariably designed for system stability enhancement.In the other way SVC enhances the system stability by regulating the bus voltage specified by voltage regulation mode.From the comparision it is clear that SVC gives the better Result.
References
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