Power System Oscillaton Damping using Unified Power Flow Controller

Power System Oscillaton Damping using Unified Power Flow Controller

Laxmidhar Sahu1, Sudhansu Kumar Samal2, Prof.P.C.Panda 3

1Department of EEE,HIT, Bhubaneswar, Odisha

2Department of EEE, PIET, Rourkela

3Department of EE, Emeritus Professor, KIIT University. BBSR

Abstract-The rapid evolution on the area of power electronics has enabled the development of new equipment to be able to use more conveniently from the power systems. Power electronic equipment established under the name of Flexible Alternating Current Transmission Systems (FACTS) is very much popular in last few years. Unified Power Flow Controller (UPFC) is the most popular and widely used FACTS device to control the power flow and to enrich the power system stability in the transmission line. This paper presents the model of a Unified Power Flow Controller which connected to a 500KV line for the improvements of power system stability and damping effect of an on line power system with occurrence of fault. A power system network is designed and simulated by using phasor simulated method and the network is inspected in two steps; without UPFC, with UPFC but not externally controlled. The simulation result shows that without UPFC, the system parameters become unstable during faults. When UPFC is connected in the system, then system parameters become stable. Again, when the UPFC is externally controlled by the proposed controller, the power system parameters (P, Q, and V) become stable very rapidly.

Key words: FACTS,PID,POD, Power System Controller, UPFC model

1. INTRODUCTION

   In recent years, greater demands have been placed on the transmission network, and this demand will continue to increase day by day. While the expansion of power

   stability of the power system. Out of these, UPFC is the most flexible multi-functional FACTs device. UPFC performs the function of a shunt reactive current injection to control bus voltage and inject series reactive voltage to control power flow in transmission line. For many years power system stabilizer(PSSs) have been one of the most common controls used to damp out power system oscillations and to offset the negative damping of the automatic voltage regulators[4]. The major role of PSS is to introduce modulating signal acting through the excitation system to add rotor oscillation damping Even though power system stabilizers the main damping control, during some operating conditions, this device may not produce enough damping especially to inter-area mode and therefore, there is an increasing interest in using FACTS devices like STATCOM and UPFC to aid in damping of these Oscillations and it can contribute to improve the voltages profile in the transient state [5].This paper presents the thyristor based UPFC controllers to improve the performance of multi-machine power system.

2. CONTROL CONCEPT OF UPFC

   The Unified Power Controller (UPFC) is the most popular and versatile member of the Flexible AC Transmission System (FACTs) family which is using high Power electronics

   generation

   and transmission has been severely limited due to limited resources and environmental restrictions. With the boost in electrical power demand, power systems are becoming very much complex to operate and at the same time security proble-

   ms are also becoming the major issue. This type of Stressed systems is always going through the stability problems due to disturbances [1].Therefore; it is very difficult to build new generation units and transmission system due to environment and energy issues. Hence it is very essential to increase the power transfer capability of the existing transmission lines instead of constructing new facilities. In

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   recent time, the most versatile high power semiconductor devices used in the power system applications for secure loading, power flow control and damping the power system oscillations is Flexible AC Transmission System (FACTs) [2][3].There are a number of FACTs devices that control power system parameters to utilize the existing power system and also to enhance the dynamic performance and

   Fig.1.Connection diagram of UPFC with transmission line

   components to control power flow on the power grids. The classical connection of UPFC with transmission line shown on the figure.1. The UPFC uses a combination of static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC) which are coupled

   via a common dc link, to allow bi-directional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM and are controlled to provide concurrent real and reactive series line compensation without an external electrical energy source. Both the STATCOM and SSSC are similar solid- state back-to back voltage converters coupled to the ac means through power transformers, and coupled to the DC links through the capacitor. Both the series and shunt converter use a Voltage Sourced Converter (VSC) connected on the secondary side of a coupling transformer. The VSCs use forced commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage from a DC voltage source. A circuit equivalent of the UPFC within two front end voltage source connected by single transmission line is shown in figure.2.

3. UPFC BASED CONTROL SYSTEM

   In UPFC the shunt converter operates as a STATCOM, and it controls the AC voltage at its terminal as well as at the DC bus. It works on a dual voltage regulation loop. A UPFC the two degree of freedom are used to control the active and reactive power. The series converter operates on either Power flow control (automatic control mode) or in manual voltage injection mode [6].A simplified block diagram of the series converter is shown in fig.3.

   component of the voltage to be synthesized by the VSC. (Vq in quadrature with V1control active power and Vd in phase with V1 controls reactive power). In manual voltage injection mode the regulators are not used. The reference value of injected voltage Vdref and Vqref are used to synthesize the converter voltage.

4. POWER SYSTEM MODEL WITH UPFC

   In this study a model of a simple transmission system containing 2-hydrulic power plants connected to a power grid is illustrated [7]. The complete Simulink model is shown in figure.4.A UPFC is used to control the power flow in a 500/230 kV transmission system. The power system is connected in a loop configuration, and it consists of five buses (B1, B2, B3, B4, B5) interconnected with each other via three transmission lines (L1, L2, L3).Two 500/230 KV transformer banks are connected. Two power plants located on the 230 KV systems generate total of 1500 MW which is again transmitted to a 500 KV,1500 MVA and a 200 MW load connected at bus B3. The plant model also included a speed regulator, an excitation system as well as a power system stabilizer (PSS). The complete single line diagram of 2-machine power system with UPFC controller is shown in figure.4.

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   Fig.2.A general circuit equivalent of UPFC

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   Fig.4. Single line diagram of 2-machine power system with UPFC controller.

   The UPFC located at the right end of line L2 is used to control the active and reactive power at the 500 KV bus B3, as well as the voltage at bus B_ UPFC. It consist of

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   two VSCs (one connected in shunt and another connected in series and both are interlinked through a DC bus on the DC side of a AC power system, through a coupling reactors and transformers).

5. TEST CASES AND SIMULATION RESULTS

The simulation is carried out for two different types of fault condition.

Case A. Single line to ground fault Case B. Three phase fault

Fig.3. UPFC based control system

In power control mode, the measured active power and reactive power are compared with reference values to produce P and Q errors. The error P and error Q are then given to two voltage regulator to compute the Vd and Vq

Case A.1: Single line to ground fault (without UPFC)

When single line to ground fault occurred at 0.1s and the fault breaker opened at 0.2s (3-phase 4-cycle fault), if there is no UPFC used then whole system voltage and power becomes unstable. The simulation results are shown in fig.5andfig.6.

CaseA.2: Single line to ground fault (UPFC without power system controller)

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If UPFC is applied then system voltage and power becomes stable with in 1.5s and 1.6s. The simulation results are shown in fig.7 and fig.8.

Case B.1: Three-Phase faults (without UPFC)

During 3-Phase faults, If UPFC is not applied, then again system voltage and power becomes unstable. The simulation results are shown in figure.9 and figure.10.

Case B.2: Three-Phase fault (UPFC without power system controller)

During Three-Phase faults, If UPFC is applied then system voltage becomes stable at 1.5s and power becomes stable at 2.1s.Simulation results are shown in the figure.11 and figure.12.

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Fig.7. Bus voltage in p.u. (UPFC without power flow controller)

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Fig.8. Bus power in MW (UPFC without power flow controller)

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Fig.9. Bus voltage (B1) in p.u. without UPFC

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Fig.5. Bus voltage (B1) in p.u. without UPFC

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Fig.6. Bus power (B1) in MW without UPFC

Fig.10. Bus power (B1) in MW without UPFC

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Fig.11. Bus voltage in p.u. with UPFC

VIII. REFERENCES

1. S. Tiwari, R. Naresh, R. Jha, Neur network predictive control Predictive control of UPFC forimproving transient stabilityPerformance of power system, Applied Soft Computing, Volume 11, Issue 8,December 2011, Pages 4581-4590, ISSN 1568- 4946.

2. Y.-H. Song, A.T. Johns (Eds), Flexible A.C. Trans mission Systems (FACTS) IET, 1999,Chapter 7,Pages1-72.

3. Hingorani NG, Gyugyi L (2000). Understanding FACTS, IEEE Press, pp., 323-387.

4. F. L.Gyugyi, C.D. Schauder. S.L. Torgenon and A. Edris, TheUnified Power Flow Controller:A new Approach toPowerTransmission Control, IEEE Trans. On Power Delivery,Vol. 10, No. 2, 1995, pp.1088-1097.

5. Z. Huaang, Y.X. Ni, C. M. She, F. F. Wu, S. Chen, and B.Zhang, Application of Unified Power Flaw controller in interconnected Power Systems modeling, interface, control, strategy, and

   Bus Power(MW)

   Bus Power(MW)

   3000

   2000

   1000

   Pa(MW)

   Pb(MW)

   Pc(MW)

   casestudy, IEEE Trans.On Power Systems. Vol.15, No.2, pp. 817- 824, May 2000.

6. " MATLAB Math Library User's Guide", by the Math Works. Inc.

7. Gyugyi, L.; "Unified power-flow control concept for Flexible AC Transmission Systems, "Generation,Transmission and Distribution, IEEE Proceedings, vol. 139, no.4, pp.323-331, Jul 1992.

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0 0.1 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

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Fig.12. Bus power in MW with UPFC

1. RESULT & DISCUSSION

   The performance of UPFC with Power Flow Controller having same 500KV transmission line is summarized in table below.

   		Stability Time
   		1-Phase Fault		3-Phase Fault
   Status	UPFC	Volt	P	Volt	P
   No UPFC	NO	inf	inf	inf	inf
   UPFC	100MVA	1.5s	1.6s	1.5s	2.1s

2. CONCLUSIONS

This paper presents the effectiveness of the UPFC on power system stability improvement i.e. machine oscillation damping, real & reactive power, voltage level for different type of fault conditions such as single line to ground fault and three phase fault .in future we can emphasis on the effectiveness of the UPFC by different controllers like PSS, POD, Fuzzy ,etc..