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International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-1, March.- 2015]

Enhancement of UPFC performance using Fuzzy Logic based intelligent controller Sudip Halder 1, Debanjan Mukherjee2, S. Saha3, Aniruddha Mukherjee4 Electrical Engineering Department, University of Engineering and Management, Jaipur, Rajasthan, India Abstract—In today's highly complex and interconnected power systems, sometimes made of thousands of buses and hundreds of generators, there is a great need to improve electric power utilization while still maintaining reliability and security. In order to meet the ever-growing power demand, utilities prefer to rely on already existing generation instead of building new transmission lines that are subject to environmental and regulatory policies. On the other hand, power flows in some of the transmission lines are well below their thermal limits, while certain lines are overloaded, which has as an overall effect of deteriorating voltage profiles and decreasing system stability and security. In addition, existing traditional transmission facilities in most cases are not designed to handle the control requirements of complex highly interconnected power systems. This overall situation requires the review of traditional transmission methods and practices, and the creation of new concepts which would allow the use of existing generation and transmission lines up to their full capabilities without reduction in system stability and security.

If the oscillatory response of a power system during the transient period following a disturbance is damped and the system settles in a finite time to a new steady operating condition, we say the system is stable. If the system is not stable, it is considered unstable. This primitive definition of stability requires that the system oscillations should be damped.

Damping control function of a static synchronous series compensator (SSSC) installed in power systems has also been analyzed [3]. The linearized model of the SSSC integrated into power systems is established and methods to design the SSSC damping controller are proposed.

Control strategy for suppressing undesirable electromechanical oscillations in power system with FACTS devices [4] with several FACT devices like STATCOM, SSSC and Unified Power Flow Controller (UPFC). Also their performance in power system has been compared which shows that most effective and flexible control can be achieved with the use of UPFC. Several references in technical literature can be found on development of UPFC steady state, dynamic and linearized models. Steady state model referred as an injection model is described in [5]. UPFC is modeled as a series reactance together with the dependent loads injected at each end of the series reactance.

A nonlinear fuzzy PI controller is proposed reference [6], which has the advantage of fuzzy controller while maintaining the simplicity and robustness of classical PI controller.

To overcome these difficulties better control techniques have been proposed in this dissertation to make the power system more reliable and more stable

Keywords—UPFC, Fuzzy logic controller and

The UPFC is placed between two busses referred to as the UPFC sending bus and the UPFC receiving bus. It consists of two voltage sourced converters (VSC) as mentioned before with a common DC link. For the fundamental frequency model, the VSCs are replaced by two controlled voltage sources as shown in the figure bellow. The voltage source at the sending bus is connected in shunt and will therefore be called the shunt voltage source. The second voltage source i.e. the series voltage source is placed between the sending and the receiving busses. The UPFC is placed on high voltage transmission lines. This arrangement requires step-down transformer in order to allow the use of power electronics devices for the UPFC.

Proportional-Integral (PI) Controller I.

INTRODUCTION

Much of industry effort and interest related to system stability since the 1960’s has been considered on transient stability.

Significant improvement in transient stability performance have been achieved through use of high speed fault clearing device, high initial response exciters , series capacitors and FACT devices [1].

A STATCOM can also be used in stabilizing sub synchronous resonance oscillation of a large turbine generator set [2]. Here a STATCOM along with a PI controller is used to effectively damp all SSR (Sub Synchronous Resonance oscillation) and to improve system stability. The proposed control system is simple cost effective easy to implement and technically sound.

II.

UPFC DESCRIPTION AND OPERATION

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International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-1, March.- 2015]

The series converter injects an AC voltage at an angle in series with the transmission line. Series voltage magnitude and its phase angle with respect to the sending bus are controllable in the range of and III.

CONTROL OF CONVERTERS

Fig. 1: Fundamental system model with UPFC Applying the pulse width modulation (PWM) technique to the two VSCs the following equations for magnitudes of shunt and series injected voltage are obtained as:

Where 

is amplitude modulation index of the shunt VSC control signal



is amplitude modulation index of the series VSC control signal



is shunt transformer turns ratio.



is series transformer turns ratio.



is the system side base voltage in kV



DC link voltage in kV. The phase angles of

and

are

Where 

:firing angle of the shunt VSC with respect to the phase angle of the sending bus voltage



:firing angle of the series VSC with respect to the phase angle of the sending bus voltage

Fig. 2: Control of shunt and series converter The circuit arrangement of two coupled converters offers series voltage injection along with independently controllable reactive power exchange and facilitates several control modes for the UPFC. Reactive shunt compensation and free control of series voltage injection are the options to the approach selected for power flow control.

The shunt inverter is operates under a closed loop current control structure whereby shunt real and reactive power components are independently controlled. Shunt reactive power responds directly to and input demand however shunt real power is dictated by another control loop that acts to maintain a preset voltage level at the dc link. The shunt converter operates to draw a control current from the line.

One component of this current is automatically determined by the requirement to balance the dc link voltage.

The other current component is reactive and can be set to any desired reference level.

The magnitude and angle of the voltage injected in series with the line is controlled by the series converter. The series inverter generates a voltage vector (across the line-side terminal of the insertion transformer) with magnitude and phase angle requested by the reference input. In automatic power flow mode, series injected voltage is determined automatically and continuously by a vector control system to ensure that the desired values of real and reactive power (P and Q) are maintained despite system changes. The basic function of series compensation is to control the magnitude of the injected voltage in proportion to the line current, so that series insertion emulates reactive impedance when viewed from the line. The injected voltage is controlled with respect to the input bus voltage so that the output bus voltage is phase shifted relative to the input voltage by an angle specified by the reference input.

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International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-1, March.- 2015]

To control active and reactive power with series converter by calibrating modulation index and the phase of series voltage injected the control action is generally done with the help of conventional PI controller as follow:

Fig. 6: Linguistic variables for Fuzzy Controller

Fig. 3:Active and reactive power control using PI controller To achieve better performance these PI controllers have been replaced with fuzzy logic based controller. Because of system’s high non linearity property fuzzy logic based PI controller has always been preferred over conventional PI controller. This can reduce the settling time and is more robust against strong uncertainties IV. THE FUZZY PI CONTROLLER

Generally values of L1 and L2 are taken as L1=8Ki, L2=8Kp. Range of H is taken such a way that it covers the most variation of U. RULES: The action of fuzzy controller is guided by a set of if-then rules. The rules can be controlled as following: Rule 1: If E is N and EC is N then U is N. Rule 2: If E is N and EC is P then U is Z. Rule 3: If E is P and EC is N then U is Z. Rule 4: If E is P and EC is P then U is P. V.

DATA ANALYSIS AND RESULT

Performance of a two machine infinite bus system has been studied in MATLAB simulink

Fig. 4: Fuzzy logic based PI controller The membership functions for input and output have been defined as shown follow: The linguistic variable for two numeric input and one output are E EC and U respectively.

Fig. 7: System model The proposed fuzzy logic controller performance is compared with the conventional PI controller used in series inverter.

Two cases of disturbances, one is a three phase fault of duration 0.1 sec and another is sudden load variation of 100MW is introduced in a two machine infinite bus system and the system behavior is studied. Rotor speed deviation of two machines has been taken as the parameter to analyze system performance.

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International Journal of Engineering Research & Science (IJOER)

[Vol-1, Issue-1, March.- 2015]

FOR THREE PHASE FAULT OF 0.1 SECOND Comparison between PI controller and fuzzy logic controller

Fig. 8:Rotor speed deviation of machine one

Fig. 11:Rotor speed deviation of machine two

How fast the machines in the power system get synchronized that has been observed. Its seen that the rotor speed deviation of first machine is largest for the system having no UPFC. Comparing the effects of the UPFC having PI controller in series inverter and fuzzy logic controller in series inverter it is clear that use of fuzzy logic based intelligent controller has to be preferred than conventional controller as in the previous case rotor of machine gets stabilized quicker.

REFERENCES [1] Fig. 9:Rotor speed deviation of machine two

[2]

FOR SUDDEN LOAD VARIATION OF 100 MW Comparison between PI controller and fuzzy logic controller has been shown in the following figures

[3]

[4]

[5]

[6]

P. Kundur (1993),“Power System stability and control” , McGraw –Hill Inc. New York.

A. F. Abdou, A. Abu-Saida, and H. R. Pota (2011) “Application of a STATCOM for Damping Subsynchronous oscillations and Transient Stability Improvement” IEEE ,Universities Power Engineering Conference (AUPEC),pp.1-5.

Wang, H.F. (1999)” Design of SSSC damping controller to improve power system oscillation stability”, Africon, IEEE vol.1, pp.495 - 500.

Sukumar Mishra, (March 2006) “Neural Network Based Adaptive UPFC for Improving Transient Stability Performance of Power System”, IEEE transaction on neural networks. VOL. -17, no. -2.

Chuan –Te Chang, Yuan –Yih Hsu (2003) “Design of an ANN tuned adaptive UPFC supplementary damping controller for power system dynamic performance enhancement” Sience Direct, Electric Power System Research 66 pp.259-265.

Ka Ho Chan, Xiaowen Chu (2006), “Design of a Fuzzy PI controller to Guarantee Delay Differentiation on Web Servers” Department of Computer Science, Hong Kong Baptist University.

Fig. 10: Rotor speed deviation of machine one

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