Synchronous operation of generators in power system is required to supply continuous electricity to customers. Proper transient stability must be maintained for stable operation of power system. To enhance the transient stability of power system FACTS or D-FACTS technology can be used. In this paper, Distributed Static Series Compensator (DSSC) which, belongs to D-FACTS technology is used to enhance the transient stability of two-machine system with Power System Stabilizer (PSS) as an auxiliary controller and it is found that DSSC along with PSS is able to maintain required transient stability during severe three-phase to ground fault.

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International Journal of Electrical Engineering & Technology (IJEET)
Volume 7, Issue 3, May–June, 2016, pp.40–48, Article ID: IJEET_07_03_004
Available online at
http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3
ISSN Print: 0976-6545 and ISSN Online: 0976-6553
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© IAEME Publication
TRANSIENT STABILITY ENHANCEMENT
BY USING DSSC AND PSS
S. I. Barde
Department of Electrical Engineering,
BRHCET, Vangani, India
ABSTRACT
Synchronous operation of generators in power system is required to supply
continuous electricity to customers. Proper transient stability must be
maintained for stable operation of power system. To enhance the transient
stability of power system FACTS or D-FACTS technology can be used. In this
paper, Distributed Static Series Compensator (DSSC) which, belongs to D-
FACTS technology is used to enhance the transient stability of two-machine
system with Power System Stabilizer (PSS) as an auxiliary controller and it is
found that DSSC along with PSS is able to maintain required transient
stability during severe three-phase to ground fault.
Key words: Distributed Static Series Compensator, D-FACTS, FACTS Power
System Stabilizer, Synchronous operation, Transient stability.
Cite this Article: S. I. Barde, Transient Stability Enhancement by Using
DSSC and PSS. International Journal of Electrical Engineering &
Technology, 7(3), 2016, pp. 40–48
http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3
1. INTRODUCTION
Transient stability in power system can be defined as, the ability of power system to
remain in synchronism during severe fault conditions [1]. It means ability of power
system to regain its original pre-fault equilibrium position or another equilibrium
point which is generally in vicinity to pre fault equilibrium point. In 1920, power
system stability is recognized as crucial and important problem [2].
Demand of electrical power is increasing tremendously and it is expected that, in
India the total requirement of electrical power can cross 950,000 MW by year 2030
[3]. Therefore, to supply electrical power continuously to customers, new generation
and transmission system should be developed. But in India, constructing new
transmission line is not always feasible. In addition to the high capital cost involved in
development of transmission system other hurdles are Right of Way (ROW), scarce
land availability and forest clearance, and getting forest clearance takes considerable

2. Transient Stability Enhancement by Using DSSC and PSS
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time in India due to lengthy process and involvement of different levels of
permissions [4].
Hence, there is need to use existing transmission system efficiently. To enhance
transient stability without compromising active power flow through transmission line
technically proven FACTS controller can be used [5]. But, these FACTS controllers
have some limitations. To overcome these limitations D-FACTS devices were
introduced. The D-FACTS controllers are distributed in nature and these controllers
are connected along the high voltage (HV) or extra high voltage (EHV) transmission
line [6]. Deepak M. Divan and et al. has introduced concept of D-FACTS technology
to relieve congestion and to improve power flow capabilities [6]. S. Golshannavaz and
et al have discussed effect of Distributed Static Series Compensator (DSSC) on
transient stability [7]. In this work, DSSC with fuzzy controller is used to improve the
transient stability of two machine system.
2. SERIES COMPENSATION AND POWER ANGLE
The power system is highly non-linear system and maintaining adequate transient
stability margin is complex phenomenon. Hence, proper power system design and
planning is required to ensure proper system operation during contingencies. In case
of, transient stability contingencies considered are short circuit faults which are
further classified as line to ground fault (LG), line to line ground fault (LLG) and
three phase fault (LLLG) [2].
To ensure stable operation of power system, system operator must maintain proper
transient stability. Practically power angle is maintained at 30° [8]. To enhance the
power flow without compromising transient stability, series FACTS or D-FACTS
controllers can be used. These controllers consist of voltage source inverter [5, 7].
Series controllers inject voltage in series with transmission line and phase angle
between injected voltage and line current is 90°. By operating these controllers in
capacitive mode, inductive reactance of the transmission line can be neutralized
partially [9].
Figure 1 Phasor diagram for uncompensated transmission line
Figure 2 Phasor diagram for series compensated transmission line

3. S. I. Barde
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(1)
Where, Vs & Vr = Bus voltages
δ = Power angle
Xl = Line impedance
From equation (1) and phasor diagram (fig. 1 and 2) it is evident that by
decreasing transmission line reactance we can increase the power flow through
transmission line and similarly, effect of series compensation on power angle δ can be
observed. Due to decrease in transmission line reactance the δ angle is also reduced
which means we are transmitting more power with reduced power angle. The limits
for power angle δ are 0° < δ < 90° [10].
3. DTRIBUTED STATIC SERIES COMPENSATOR
A. Voltage Source Inverter
All FACTS and D-FACTS controllers are based on voltage source inverter. The
voltage source inverters are preferred due to their better performance and less cost as
compared to current source inverter. Hence, in case of DSSC single phase voltage
source inverter is used. The circuit schematic of DSSC system is shown in figure 3.
By changing polarity of dc current power flow can be changed in either direction.
Therefore, voltage source inverter uses bidirectional devices such as GTO or IGBT
with parallel diode connected in reverse [5].
B. Single Turn Transformer (STT)
STT is important part of DSSC. STT is designed in such a way that it will clamp on
transmission line. STT has high turn ratio, say 100:1 or 75:1. This design protects VSI
under fault condition. If the fault current is 30 kA or 50 kA, the inverter current is
only 300 A or 500 A respectively. This is well below the current rating of available
GTO or IGBT [11].
Figure 3 Circuit schematic of DSSC system

4. Transient Stability Enhancement by Using DSSC and PSS
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C. Contol System
The main objective of control system of DSSC is to maintain charge of DC capacitor.
Also to inject voltage in transmission line, in such a way, that injected voltage will be
orthogonal to line current [12]. In this work fuzzy controller is developed to achieve
conditions. Rotor angular difference between two generators and power angle is used
as an input to fuzzy controllers. Block diagram of control system is shown in fig 4.
Figure 4 Control system for DSSC
4. SIMULATION RESULTS
To study effect of DSSC on transient stability two-machine system, shown in fig. 5 is
considered. The system considered consist of two generator with 1000 MVA and
5000 MVA rating. Length of transmission line is 700 km. To study effect of DSSC on
transient stability following two different cases are considered.
Figure 5 Two-machine system
A. Without DSSC
The three-phase to ground fault is created at T= 1.1. In this case DSSC is not active in
circuit. Due to three phase fault created near generator one power angle goes on
increasing after T=1.2 and two generators losses synchronism (fig. 6).

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Figure 6 Varitation of power angle during 3-phase fault
As two generators losses synchronism its effect on rotor angular speed and
machine voltage can be observed in fig.7 and 8. generator one accelerates and
generator two decelerate.
Figure 7 Varitation of rotor speed during 3-phase fault.
Figure 8 Varitation of generator voltage during 3-phase fault.

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B. With DSSC
Same fault condition is used to study effect of DSSC on power angle, rotor angular
speed and machine voltage. If system is subjected to severe three phase fault of
duration 0.1 then alone DSSC is unable to maintain synchronism as its primary work
is to enhance power flow capability of transmission line and not transient stability
enhancement. Hence, auxiliary POD controller or power system stabilizer (PSS) is
needed in system to damp oscillation in the rotor speed. In this work, PSS is used to
provide auxiliary damping signal to maintain transient stability.
As shown in fig. 9 power angle oscillates between 73° to 37° and become stable
after T=4 completely and attains the pre fault equilibrium. Similarly effect of fault on
machine voltage and angular speed is shown in fig. 10 and 11. From these results it is
clear that DSSC along with auxiliary controller can maintain transient stability.
Figure 9 Varitation of power angle during 3-phase fault with DSSC and PSS.
Figure 10 Varitation of generator voltage during 3-phase fault with DSSC and PSS.

7. S. I. Barde
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Figure 11 Varitation of rotor speed during 3-phase fault with DSSC and PSS
Voltage and current generated by voltage source inverter is shown in fig. 12 and
13 and output voltage of DSSC is shown in fig. 14.
Figure 12 Output voltage of single phase voltage source inverter
Figure 13 Output current of single phase voltage source inverter.

8. Transient Stability Enhancement by Using DSSC and PSS
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Figure 14 Output voltage of DSSC
5. CONCLUSION
To supply continuous electrical power to the customers, maintaining transient stability
is very important. D-FACTS technology provides more reliable approach to enhance
power transfer capabilities and transient stability of power system than FACTS
technology. From two-machine power system simulation results, it is evident that
DSSC along with auxiliary controller can be used to enhance transient stability
without compromising active power flow through transmission line. We have shown,
In case of three phase fault, if DSSC is not active in system, the two generators losses
synchronism. Whereas, if DSSC is active in system during three phase fault, system
stabilizes after few oscillations and generators do not lose synchronism. DSSC
provides cost effective method to enhance the transient stability. In this work, we
have proved that, DSSC with fuzzy logic controller can be used along with PSS as
supplementary controller to mitigate the transient stability problem and by using
DSSC we can transmit more power without violating the transient stability limits.
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