improving transient stability margin by using … · distributed static series compensator (dssc)...

http://www.iaeme.com/IJCET/index.asp 31 [email protected]

International Journal of Computer Engineering & Technology (IJCET)

Volume 8, Issue 5, Sep-Oct 2017, pp. 31–41, Article ID: IJCET_08_05_005

Available online at

http://www.iaeme.com/ijcet/issues.asp?JType=IJCET&VType=8&IType=5

Journal Impact Factor (2016): 9.3590(Calculated by GISI) www.jifactor.com

ISSN Print: 0976-6367 and ISSN Online: 0976–6375

© IAEME Publication

IMPROVING TRANSIENT STABILITY MARGIN

BY USING DISTRIBUTED STATIC SERIES

COMPENSATOR

M. D. Patil

Department of Electrical Engineering, Y.T.I.E.T, Karjat, India

Gopal Chaudhari

Department of Electrical Engineering, Y.T.I.E.T, Karjat, India

ABSTRACT

Series-Flexible AC transmission controllers can be used to improve transient

stability of power system but their high cost and lumped nature have restricted their

use worldwide. Hence, distributed Flexible AC transmission system (D-FACTS) is

introduced to overcome limitation of FACTS controller. In this work, effect of

Distributed Static Series Compensator (DSSC) on power angle and response of DSSC

during severe three phase fault condition is also studied by using two-machine system.

It is found that DSSC is capable of maintaining adequate transient stability margin

during severe faults.

Key word: Flexible AC Transmission system (FACTS), Distributed Static Series

Compensator (DSSC), D-FACTS, power angle, transient stability.

Cite this Article: M. D. Patil, Gopal Chaudhari, Improving Transient Stability

Margin by using Distributed Static Series Compensator. International Journal of

Computer Engineering & Technology, 8(5), 2017, pp. 31–41.

http://www.iaeme.com/ijcet/issues.asp?JType=IJCET&VType=8&IType=5

1. INTRODUCTION

India, in case of energy consumption, is the fourth largest country in the world [1]. Old power

system in India is pressing problem and it is required to be revamped immediately. Among

the challenges faced by electric utilities the most crucial is the problem of eliminating

transmission constraints and transmission congestion. Basically there are three limitations

thermal limits, voltage limits and stability limit [2].

In India the total requirement of electrical power can cross 950,000 MW by year 2030 [3]

hence, with increase in generation of electricity there is a need to develop simple and reliable

transmission system. But in case of developing countries like India, it is not always possible

to construct new transmission lines. 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 time in India

M. D. Patil, Gopal Chaudhari


due to lengthy process and involvement of different levels of permissions[4]. Under such

circumstances, it became crucial to utilize the existing transmission system in more efficient

manner.

When it comes to grid utilization the most important issues are controlling active power

flow and getting rid of network congestion. In India transmission and distribution (T&D)

losses and Aggregate Technical and Commercial losses (AT&C losses) are 23.04% and 25.38

% respectively [5]. The congestion occurs because of deficiency in transmission capacity to

supply power to waiting consumers. Congested network hampers the reliability of system and

cost of supply. The situation gets worse when few lines are running below capacity, but still

cannot be used to full capacity and at the same time other lines are overloaded [6]. Hence, in

order to use the existing system to its full capacity we should improve the system with the

help of new devices.

To control the active power flow through transmission line series compensation is

effective. The relation between transmission reactance and active power flow is inversely

proportional. Therefore, by using series compensation one can alter the value of transmission

reactance, and thereby controlling the power flow through line. While improving power

transfer capability of transmission line, one should maintain adequate transient stability

margin. This is necessary, to ensure transient and dynamic stability and to ensure that the

system does not collapse following the forced outage [2]

To achieve two objectives of increasing power transfer capability and transient stability,

technically proven FACTS series controllers can be deployed [7]. Along with the great

flexibility and advantages, FACTS controllers have some limitations associated with them,

which has restricted their use worldwide.The limitations are discussed in [6]. To overcome

the issues related to FACTS technology a new concept from the family of D-FACTS is

introduced [6].

Recently proposed distributed flexible ac transmission system has same capabilities that

of the FACTS. The difference between FACTS and D-FACTS is FACTS devices are lumped

in nature whereas the D-FACTS devices are distributed along the length transmission line.

This advanced technology provides novel approach in controlling the active power flow by

using D-FACTS devices in the existing system [9].

In this paper two-machine system is considered to examine the effect of DSSC on power

angle and transient stability with and without fault. S. Golshannavaz and et al.[8] have

discussed in detail impact of series D-FACTS device on transient stability.

2. CONCEPT OF DISTRIBUTED STATIC SERIES COMPENSATOR

The Distributed Static Series Compensator (DSSC) belongs to D-FACTS family. Concept of

DSSC is derived from Static Synchronous Series compensator(SSSC) which is the FACTS

device. Both devices have same working principle [2,6]. The difference between SSSC and

DSSC is, SSSC is large and connected at a specific point in transmission line whereas DSSC

is connected in distributed manner along the full transmission line length. DSSC gives ability

to control the output of system with demand. It bolsters the step by step capital investment.

As D-FACTS devices are distributed and can be connected as per requirement, there is no

need of over rating of the device to accommodate future demand, which assures better return

on the investment. Unlike SSSC, DSSC does not require insulation to ground as it is directly

attached on to the transmission line. [6]

Improving Transient Stability Margin by using Distributed Static Series Compensator


Figure 1 Circuit schematic of DSSC system

DSSC consists of single phase voltage source inverter which is energized through line

itself. DSSCs can be controlled in group to control active power flow through line hence

these devices are equipped with the wireless communication or power line communication

technique [10]

Where,

Vs = Sending end voltage

Vr = Receiving end voltage

δ = Power angle

XL = Line impedance

The power flow through transmission line can be realized by equation (1). It is evident

from equation that the power flow through transmission line can be influenced by the change

in δ or XL. δ can be changed by using phase shifting transformer, but most economical and

effective method is to control XL by using series compensation. Depending on need DSSC

can be operated in either capacitive or inductive mode to decrease or increase XL respectively

[11].

Phase displacement between output voltage of the DSSC and line current is 90 degrees

and the output voltage is not dependent on the magnitude of the line [12]. The purpose of

series voltage injection is to vary the overall reactive voltage drop of transmission line and

thereby controlling the transmitted electric power [13]



3. CONTROL CIRCUIT OF DSSC

Figure 2 Control system for DSSC

The main aim of series controllers including DSSC is to control the real power flow

through the transmission line. To achieve this main objective either direct control or indirect

control method is needed. Among these two methods the indirect method is more suitable and

feasible, as the direct control method is associated with some demerits, such as higher losses,

harmonics etc. Hence, in present work indirect control method is employed. Here the angular

position of output voltage is controlled. The magnitude of the output voltage is directly

proportional to the dc voltage terminal [14]. The main objective of control system of DSSC is

to maintain charge of dc capacitor of single phase inverter. Also to inject voltage in

transmission line, in such a way, that phase displacement between line current and injected

voltage is 90 degrees. The error signal is generated by comparing Vdc(ref) and Vdc. The phase

angle(Φ) of the line current is obtained by Phase Locked Loop (PLL). The phase angle

controller (-90 or +90) establishes the angle between injected voltage and line current.

Therefore, signal generated is Φ = -90 (capacitive mode) or Φ = +90 (inductive mode). The

resultant of all these three signals is used to generate gate pulses for IGBT [8].

4. POWER SYSTEM UNDER STUDY

Figure 3 Two-machine power system

The two-machine power system is shown in fig 3 is considered under study. It consists of

two generators. The three-phase generators rated with 1000 MVA and 4000 MVA

respectively, are connected to 500 kV network through Delta-Y transformer. Details of power

system parameters are given in Appendix.



5. SIMULATION RESULTS

In this section response of the DSSC is studied under different conditions. The different cases

considered are as follows-

Power angle of power system under steady state without DSSC

Effect of DSSC on power angle under steady state condition

Response of system during three phase fault

Effect of DSSC during three phase fault without auxiliary controller

Effect of DSSC during three phase fault with auxiliary controller

5.1. Power Angle of Power System under Steady State without DSSC

Under normal steady state operation without DSSC power angle of system is sustained

sinusoidal and varying across 53°.

Figure 4 Power angle vs. Time

5.2. Effect of DSSC on Power Angle under Steady State Condition

To study effect of DSSC on power angle two groups of DSSC is added to the two machine

power system. Initially the DSSCs are bypassed by breaker (Sn) showed in fig 1. At t=4

DSSCs are energized from transmission line and as a result power angle is also decreased

from 53° to approximately 50°. This result verifies the ability of DSSC to enhance transient

stability margin.




5.3. Response of System during Three Phase Fault

Without DSSC in system a three phase fault with duration 0.085s is created near generator 1.

After occurrence of a fault the two generators losses synchronism as shown in fig. 6, 7 and 8.


Figure 7 Generator 1 angular speed (W1) vs. Time


5.4. Effect of DSSC during Three Phase Fault without Auxiliary Controller

The same fault situation is repeated with DSSC active in circuit. The response of system in

terms of power angle, angular speed of generator 1 and generator 2 is shown in fig. 9, 10 and

11. This verifies ability of DSSC to maintain synchronism of system for short duration faults.





Figure 11 Generator 2 angular speed W2 vs. Time

5.5. Effect of DSSC during Three Phase Fault with Auxiliary Controller

Power oscillation damping (POD) controller is used as auxiliary controller in addition to

control system of DSSC. The main objective of DSSC is to control real power flow through

transmission line. Hence, it cannot damp oscillations occurring after severe fault. POD

controller is used to get additional electrical torque in phase with speed deviation. The

washout circuit is used to avoid auxiliary controller from responding to normal condition

[15]. The parameters of POD controller values are obtained by trial and error method to get

desired signal.

Figure 12 Block diagram of POD controller



If system is subjected to severe three phase fault of duration 0.1 then alone DSSC is

incapable to maintain synchronism hence auxiliary POD controller is added in system. And

the effect on system of DSSC with POD is evident from Fig. 13 to 17.



Figure 15 Generator 1 voltage vs. Time

Figure 16 Generator 2 angular speed W1 vs. Time



Figure 17 Generator 2 voltage vs. Time

6. CONCLUSIONS

D-FACTS technology provides novel and more reliable approach to enhance power transfer

capabilities and transient stability of power system. From two machine power system

simulation results our work it is evident that DSSC can be used to enhance transient stability

without compromising active power flow through transmission line. We have also shown, In

case of short duration three phase fault, if DSSC is not active in system, the two generators

losses synchronism. Whereas, it is also found that, if DSSC is active in system during short

duration fault system stabilizes after few oscillations. For severe faults to maintain

synchronism auxiliary controller should be added. Therefore, more reliable and cost effective

DSSC device can be employed in developing countries like India to fulfill power demand of

growing population simultaneously maintaining adequate transient stability margin

7. APPENDIX

System data- All parameters are in p.u. unless specified otherwise.

Generators

Nominal powers: S1= 1000 MVA, S2= 4000 MVA, Nominal voltage: V=13.8kV, Nominal

frequency= 60 Hz, Reactance: Xd=1.305, Xd’=0.296, Xd”= 0.252, Xq= 0.474,

Xq’=0.243,Xq”=0.18, Time constant Td=1.01s, Td’= 0.053s, Tqo=0.1s, Stator resistance: Rs=

2.8544e-3, Coefficient of inertia and pairs of poles: H=3.7s, p=32

Excitation System

Low pass filter time constant: Tlp=0.02s, Regulator gains and time constants: Ka=200,

Ta=0.001s, Exciter gains and time constant: Ke=1,Te=0,Transient gain reduction:Tb=0,Te=0

Damping filter gains and time onstants: Kf=0.001, Tf=0.1s, Regulator output limits and gains:

Efmin=0, Efmax=7, Kp=0

Hydraulic Turbine and Governor

Servo motor and time constants: Ka=3.333, Ta=0.07, Gate opening limits: Gmin=0.01,

Gmax=0.97518, Vgmin=-0.1s/pu, Vgmax=0.1 pu/s, permanent droop: Rp=0.05, PID regulators

Kp=1.163, Ki=0.105, Kd=0, Td=0.01s, Hydraulic turbines: β=0, Tw=2.67s

Transformer

Nominal powers: St1=1000MVA, St2=4000 MVA, winding connection :D1/Yg

Winding parameters: V1= 13.8kV, V2=500kV, R1=R2=0.02, L1=0, L2=0.12, Magnetizing

resistance Rm= 500, Magnetizing reactance Lm=500



Transmission lines:

Number of phases=3,

resistance per unit lenth: R1=0.02546 ohm/km, R0=0.3864 ohm/km

Inductance per unit length L1=0.9337x10-3 H/km, L0=4.1264x10-3 H/km

Capacitance per unit length=C1=12.74x10-9 F/km, C0= 7.751X10-9 F/km,

Line length=700km.

Load:5000 MW

REFERENCES

[1] http://www.eia.gov/beta/international/analysis.cfm?iso=IND

[2] Hingorani, Narain G., and Laszlo Gyugyi. Understanding FACTS: concepts and

technology of flexible AC transmission systems. Wiley-IEEE press, 2000.

[3] Manoj kumar Gupta. Power Plant Engineering. PHI Learning pvt. ltd. 2012.

[4] http://www.cea.nic.in/reports/powersystems/nep2012/transmission_12.pdf

[5] http://www.cea.nic.in/reports/monthly/executive_rep/jan15.pdf

[6] Divan, Deepak, William Brumsickle, Robert Schneider, Bill Kranz, Randal Gascoigne,

Dale Bradshaw, Michael Ingram, and Ian Grant. "A distributed static series compensator

system for realizing active power flow control on existing power lines." In Power Systems

Conference and Exposition, 2004. IEEE PES, pp. 654-661. IEEE, 2004.

[7] Gyugyi, Laszlo, Colin D. Schauder, and Kalyan K. Sen. "Static synchronous series

compensator: a solid-state approach to the series compensation of transmission lines."

Power Delivery, IEEE Transactions on 12, no. 1 (1997): 406-417.

[8] Golshannavaz, Sajjad, M. Mokhtari, Mojtaba Khalilian, and Daryoosh Nazarpour.

"Transient stability enhancement in power system with distributed static series

compensator (DSSC)." In Electrical Engineering (ICEE), 2011 19th Iranian Conference

on, pp. 1-6. IEEE, 2011.

[9] Divan, Deepak, and Harjeet Johal. "Distributed facts—A new concept for realizing grid

power flow control." Power Electronics, IEEE Transactions on 22, no. 6 (2007): 2253-

2260.

[10] Hadjsaïd, Nouredine, and Jean-Claude Sabonnadière, eds. Electrical Distribution

Networks. John Wiley & Sons, 2013.

[11] Johal, Harjeet, and Deepak Divan. "Design considerations for series-connected distributed

FACTS converters." Industry Applications, IEEE Transactions on 43, no. 6 (2007): 1609-

1618.

[12] R.M. Mathur and R.K. Varma, Thyristor-Based FACTS Controllers for Electrical

Transmission Systems, IEEE Press and Wiley Interscience, New York, USA, Feb. 2002.

[13] Fajri, Poriya, and Saeed Afsharnia. "A PSCAD/EMTDC model for distributed static

series compensator (DSSC)." In Electrical Engineering, 2008. ICEE 2008. Second

International Conference on, pp. 1-6. IEEE, 2008.

[14] Fajri, Poria, Morteza Farsadi, Daryoush Nazarpour, and Arash Tavighi. "A novel strategy

for controlling a group of distributed static series compensators." In Electrical and

Electronics Engineering, 2009. ELECO 2009. International Conference on, pp. I-119.

IEEE, 2009.

[15] Kundur, Prabha. Power system stability and control. Edited by Neal J. Balu, and Mark G.

Lauby. Vol. 7. New York: McGraw-hill, 1994.



[16] Prof. P. Ramanjaneyulu, Prof. Dr. V.C. Veera Reddy, An enhanced genetic algorithm

approach for optimal Placement of facts devices to enhance ATC, International Journal of

Electrical Engineering and Technology (IJEET), Volume 5, Issue 4, April (2014), pp. 96-

103

[17] Naimul Hasan, B.B.Arora, J.N.Rai, Comparison Of Facts Devices For Two Area Power

System Stability Enhancement Using Matlab Modelling, International Journal of

Electrical Engineering and Technology (IJEET), 5 (8), (2014), pp. 42-51

[18] H.Lakshmi, M.Sadhama. Design Of A Mode Decoupling Facts Device For Voltage

Control Of Wind-Driven Induction Generator Systems. International Journal of Advanced

Research in Engineering and Technology (IJARET), 4 (3), (2013), pp. 55-61

improving transient stability margin by using … · distributed static series compensator (dssc)...

Documents