Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Enhancement of Voltage Profile and Minimization of Transmission Line Losses Using Unified Power Flow Controller M. Senthil Kumar1, Dr. P. Renuga2

1

Sona College of Technology/ EEE Department, Salem-5, Tamil Nadu, India senthileed@yahoo.com 2 Thiagarajar College of Engineering/ EEE Department, Madurai-15,Tamil Nadu, India preee@tce.edu State estimation in power system can be formulated as a nonlinear weighted least squared error (WLSE) problem representing the zero injections of buses and the zero active power exchange between the power system and FACTS devices. There are several kinds of FACTS devices. Thyristor-Controlled Series Capacitors (TCSC), Thyristor Controlled Phase Shifting Transformer (TCPST) and Static Var Compensator (SVC) can exert a voltage in series with the line and, therefore, can control the active power through a transmission line[3][18]. On the other hand UPFC has a series voltage source and a shunt voltage source, allowing independent control of the voltage magnitude, and the real and reactive power flows along a given transmission line. The UPFC was proposed for real-time control and dynamic compensation of ac transmission systems, providing the necessary functional flexibility required to solve many of the problems which are being facet by the industry. The UPFC consists of two switching converters are operated from a common dc link provided by a dc storage capacitor [5][19]. Particle swarm optimization (PSO) is a computational intelligence based technique that is not largely affected by the size and nonlinearity of the problem, and can converge to the optimal solution in many problems where most analytical methods fail to converge. PSO is easier to implement and there are fewer parameters to adjust [11]. In this paper Particle Swarm Optimization (PSO) technique is introduced to optimize the location of UPFC. The proposed technique was tested on the IEEE 5 bus system and IEEE 14 bus system and UPFC was installed at any of the weakest voltage at load buses. For practical and economic considerations, the number of UPFC units is limited to one. Here UPFC is connected in between buses 3 and 4 in IEEE 5 bus system and buses 4 and 9 in IEEE 14 bus system to perform the test.

Abstract—Loss minimization in power system is an important research issue. Transmission line losses in a power system can be minimized by means of reactive power compensation. After the establishment of power markets with transmission open access, the significance and use of Flexible AC Transmission Systems (FACTS) devices for manipulating line power flows to relieve congestion and optimize the overall grid operation have been increased. This paper presents a technique to provide simultaneous or individual controls of basic system parameters like transmission voltage, impedance and phase angle, there by controlling the transmitted power using Unified Power Flow Controller (UPFC). The Particle Swarm Optimization (PSO) technique is used to compute the power flow. The performance of this technique is tested using IEEE 5 bus and IEEE 14 bus systems. The test result showed that the location of UPFC improves the voltage profile of the system and also minimize the transmission line losses. Index Terms— Flexible AC Transmission Systems (FACTS), Real and Reactive Power, Unified Power Flow Controller (UPFC), Particle Swarm Optimization (PSO)

I.

INTRODUCTION

Most large power system blackouts, which occurred worldwide over the last twenty years, are caused by heavily stressed system with large amount of real and reactive power demand and low voltage condition. When the voltages at the system buses are low, the losses will also be increased. This study is devoted to develop a technique for improving the voltage and minimizing the losses and hence eliminate voltage instability in a power system [4]. State estimation is the process of estimating the values of an unknown system variable based on the measurement system according to some criterion. The basic idea is to "fine-tune" state variables by minimizing the sum of the residual squares and it is the well-known least squares (LS) method. State estimation is a widely used tool in power system energy management systems. The essence of state estimation is that the measurements taken are of active and reactive power, and system voltage magnitudes and phase angles (i.e, the ‘states’) are estimated [1]-[2]. Application of FACTS devices are currently pursued very intensively to achieve better control over the transmission lines for manipulating power flows.

II.

The Fig.1 shows the power circuit of a UPFC which has an Excitation Transformer (ET), a Boosting transformer (BT), two three phase GTO based voltage source converters (VSCs), and a dc link capacitor. This arrangement functions as an ideal ac to ac power converter in which the real power can freely flow in

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UPFC MATHEMATICAL MODEL

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

the best value in the group (gbest) among pbests. Each agent tries to modify its position using the current velocity and the distance from pbest and gbest. The modification can be represented by the concept of velocity. Velocity of each agent can be modified by the following equation.[11]-[13] Vi=Vi + rand x (pbest i – Si) + rand x (gbest – Si) where, Vi : velocity of agent i, rand : uniformly distributed random number between 0 and 1, Si : current position of agent i, pbesti : pbest of agent i, gbest : gbest of the group. Figure. 1. Basic circuit arrangement of the Unified Power Flow Controller

Using the above equation, a certain velocity that gradually gets close to pbest and gbest can be calculated. The current position (searching point in the solution space) can be modified by the following equation.

either direction between the ac terminals of the two inverters and each inverter can independently generate (or absorb) reactive power at its own ac output terminal . Inverter 1 can also generate or absorb controllable reactive power, if it is desired, and thereby it can provide independent shunt reactive compensation for the line. Inverter 2 provides the main function of the UPFC by injecting an ac voltage Vw with controllable magnitude Vm and phase angle (ө ) at the power frequency. This injected voltage can be considered essentially as a synchronous ac voltage source [6]. The transmission line current flows through this voltage source resulting in real and reactive power exchange between it and the ac system. The real power exchanged at the ac terminal (i.e., at the terminal of the insertion transformer) is converted by the inverter into dc power which appears at the dc link as positive or negative real power demand. The reactive power exchanged at the ac terminal is generated internally by the inverter [7]-[10]. III.

s i− = s i + v i

The particle swarm optimization concept consists of, at each time step, regulating the velocity and location of each particle toward its pbest and gbest locations according to the following two equations respectively. Vidn+1 = wvidn + c1r1n ( pidn-Xidn) + c2r2n ( pidn-Xidn)

(2)

Xidn+1 = Xidn + Vidn+1

(3)

where w is inertia weight; c1, c2 are two positive constants, called cognitive and social parameter respectively ;d=1, 2, …, D; i=1, 2, …, m, and m is the size of the swarm; r1, r2 are random numbers, uniformly distributed in [0,1]; and n=1, 2, …, N, denotes the iteration number, N is the maximum allowable iteration number.

STEADY STATE MODEL OF UPFC

A. Particle Swarm Optimization Technique for Reactive Power and Voltage Control Reactive Power and Voltage Control (Volt/Var Control: VVC) determines an on-line control strategy for keeping constant voltages profile of target power systems under normal and up normal situations. VVC can be formulated as a mixed-integer nonlinear optimization problem with continuous state equipment. The objective function can be varied according to the power system condition. For example, the function can be loss minimization of the target power system for the normal operating condition [4][17].

The steady state model of UPFC consists of two ideal voltage sources, one in series and one in parallel with the associated line. Neglecting UPFC losses, during steadystate operation it neither absorbs nor injects real power into the system. No real-power is exchanged between the UPFC and the system. The two sources are mutually dependent. The real and reactive power going through line can be formulated by the equations [3]. IV. PARTICLE SWARM OPTIMIZATION TECHNIQUE PSO is basically developed through simulation of bird flocking in two dimension space. The position of each agent is represented by XY-axis position and the velocity is expressed by vx (the velocity of X-axis) and vy (the velocity of Y-axis). Modification of the agent position is realized by the position and velocity information. PSO procedures based on the above concept can be described as follows. Namely, bird flocking optimizes a certain objective function. Each agent knows its best value so far (pbest) and its XY position. Moreover, each agent knows

Active and reactive power losses occur in the transmission lines depending upon the power to be transmitted. The active power loss equation of the kth line, between buses i and j (Fig 2) can be written as,

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(1)

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

C. VVC Flowchart using PSO

Start Figure 2.Transmission Line

PL-k=Gk(Vi2+Vj2-2ViVjcos(δi-δj))

Read agents, velocities

(4)

The series reactive power loss equation of the kth line, between buses i and j can be written as, QL-k=Bk(Vi2+Vj2-2ViVjcos(δi-δj))

Calculate Ploss

(5)

Where, Gk; is kth line conductance Bk; is kth line susceptance Vi;voltage magnitude of ith bus δi;phase angle of ith bus

If constraint is violated, evaluate value= Ploss+Penalty

In power system, the total active power loss of all the lines of the system is PL=∑P L-k

k=1…………nl

Set pbest for agent. For best evaluated value, gbest=evaluated value

(6)

and the total series reactive power loss of all the lines of the is QL=∑Q L-k

k=1…………nl

Calculate velocities and agents

(7)

Where, nl is the total number of lines. Calculate Ploss and new evaluated value

B. VVC Algorithm using PSO The proposed VVC algorithm using PSO is expressed as follows: Step 1.Initial searching points (agents) and velocities are generated using the above-mentioned state variables randomly. Step 2. Ploss to the searching point for each agent is calculated using load flow. If the constraints are violated, penalty is added to the loss (evaluation value of agent). Step 3.pbest is set to each initial searching point. The initial best evaluated value (loss with penalty) among pbests is set to gbest. Step 4.Velocities is calculated using (2). Step 5.New searching points are calculated using (3). Step 6.Ploss to the new searching point and the evaluation value is calculated. Step 7.If the evaluation value of each agent is better thanthe previous pbest, the value is set to pbest. If the best pbest is better than gbest, the value is set to gbest. All of gbests are stored as candidates for the final control strategy. Step 8. If the iteration number reaches to the maximum iteration number, then exit otherwise, go to Step 4.

If new evaluated value is better than pbest, agent=pbest. For best pbest,gbest=pbest and store gbest

yes If i<n

no Stop

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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

D. PSO Parameter Control The following parameters are subjected to vary and their values are given in Table I. TABLE I VARIOUS PARAMETERS AND THEIR VALUES Without UPFC Bus.

With UPFC

Voltage

Angle

Voltage

Angle

(pu)

(deg)

(pu)

(deg)

1

1.0600

0.00

1.0600

0.0

2

1.0326

-1.944

1.0475

-5.612

3

1.0242

-4.582

1.0297

-11.843

4

1.0236

-4.901

1.0256

-9.848

5

1.0180

-5.707

1.0195

-8.882

No

RESULTS AND DISCUSSION

V.

Fig. 3 and 4 shows the standard IEEE 5 bus and IEEE 14-bus systems. Based on PSO algorithm the UPFC is included in between the weakest load buses 3 and 4 in IEEE 5 bus system and between the load buses 4 and 9 in IEEE 14 bus system. Table II and III shows the state variables of proposed system (with and without UPFC). Table IV shows the comparative results of proposed system. From the tables it is concluded that the system voltages has improved and the losses are reduced when UPFC is installed.

Figure.4 IEEE 14 bus system TABLE III STATE VARIABLES OF IEEE 14-BUS Bus No

Angle

Voltage

Angle

(pu)

(deg)

(pu)

(deg)

1

1.06

0.00

1.06

0.0

2

1.004

-5.762

1.010

-5.612

3

0.970

-12.268

0.973

-11.843

4

0.973

-10.279

0.981

-9.848

5

0.979

-8.944

0.986

-8.882

6

1.032

-14.449

1.050

-14.716

Value

7

1.022

-13.769

1.020

-13.490

50-100

8

1.066

-13.273

1.050

-13.893

6

9

1.006

-14.901

1.013

-15.054

10

1.003

-15.468

1.006

-15.334

Table II State Variables of IEEE 5 bus system Parameter

With UPFC

Voltage

Figure.3. IEEE 5 Bus System

Sl.No

Without UPFC

1.

Number of Particles

2.

Dimension of Particles

3.

Range of Particles

0.1-0.5

4.

Maximum Velocity

20

5.

Learning Factors C1 & C2

1.4

11

1.016

-15.003

1.018

-15.220

6.

Inertia Weight Wmin & Wmax

0.4& 0.9

12

1.010

-15.660

1.032

-15.335

13

1.004

-15.313

1.023

-15.362

14

0.996

-16.616

0.997

-16.185

248 ÂŠ 2009 ACEEE

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

[7]

TABLE IV COMPARATIVE RESULTS Power Loss

P (MW)

Q (MVAr)

Without UPFC (IEEE 14 Bus System) With UPFC (IEEE 14 Bus System)

12.397

53.55

11.781

48.85

[8]

[9]

VI . CONCLUSION This paper presents the application of particle swarm optimization technique in power system state estimation with and without UPFC. The unified power flow controller provides simultaneous or individual control of basic system parameters like transmission voltage, impedance and phase angle, there by controlling the transmitted power. The Particle Swarm Optimization technique is used to select the optimal location of UPFC in power systems. The power loss occurring in the various branches and state variables of IEEE 5 bus and IEEE 14-bus systems are evaluated using PSO based power flow analysis. From the results it is concluded that the system performs better when the UPFC is connected. The state variables are improved and the Transmission line losses are minimized.

[10]

[11]

[12]

[13]

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