 Open Access
 Total Downloads : 614
 Authors : Mr. B. V. S. Acharyulu, Dr. K. B. Madhu Sahu
 Paper ID : IJERTV2IS1373
 Volume & Issue : Volume 02, Issue 01 (January 2013)
 Published (First Online): 11022013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Novel Control of ThreeArm AC Automatic Voltage Regulator with Fuzzy logic Technique
Mr.B.V.S.Acharyulu1 Dr. K.B.Madhu Sahu2
1(Student, Department of EEE, Aditya Institute of Technology and Management,Tekkali
2 (Professor, Department of EEE, Aditya Institute of Technology and Management,Tekkali
Abstract This paper proposes a novel threearm AC automatic voltage regulator (AVR) with fuzzy logic technique. A fuzzy controller based AVR has been developed and simulated to reduce the switching losses. The proposed fuzzy controller based AVR has capability of delivering sinusoidal output current with good output voltage regulation. This threearm power converter acts as an ac boost converter when the utility voltage is lower than the specified voltage. On the contrary, this threearm power converter acts as an ac buck converter when the utility voltage is higher than the specified voltage. Hence, the output voltage of the AVR can be maintained at the specified voltage. Moreover, there is no need to use a large dc capacitor in sustaining a constant dc voltage. Hence, the size can be decreased, the cost can be reduced, and the life of the power convertor can be extended to analysis Total Harmonic Distortion (THD).
Key Words– AC boost converter, ac buck converter, automatic voltage regulator (AVR), Fuzzy logic controller, THD
1 INTRODUCTION
Currently the Quality of supplied power is important to several customers. Power quality (PQ) is a service and many customers are ready to pay for it. In the future, distribution system operators could decide, or could oblige by authorities to supply their customers with different PQ levels and at different prices. The proposed threearm AVR acts as an ac boost converter when the utility voltage is lower than the specified voltage, and it acts as an ac buck converter when the utility voltage is higher than the specified voltage. Hence, the output voltage of the AVR can be maintained at the specified voltage. The power demanded by the load is directly supplied by the conversion results of the power converter (ac to ac). In comparison with the conventional three arm power converter which requires double conversion (ac to dc and dc to ac), the proposed threearm AVR requires only a single conversion[13]. Moreover, the power electronic switches in only one arm of the threearm power converter are switched in high frequency, while those of the other arms are switched in low frequency. The switching power loss is reduced, and no transformer is required. In comparison with the conventional threearm AVR with a constant dc bus voltage, the dc bus voltage of the proposed threearm AVR is a fullwave rectified voltage[46]. Hence, the use of a large dc capacitor in sustaining a constant dc
voltage is avoided, and only a small dc capacitor is employed to act as a snubber and filter circuit. Consequently, the proposed threearm AVR has the advantages of reduced installation cost and volume, as well as increased reliability and power efficiency. To verify the performance of the proposed threearm AVR, a prototype is developed and tested.

FUZZY SET THEORY

Definition of a fuzzy set: assuming that X is a collection of objects, a fuzzy set A in X is defined to be set of ordered pairs.
A = {(x, ÂµA (x))/x X} (1)
Where ÂµA(x) is called the membership function of x in A. The numerical interval X is called Universe of Discourse. The membership function ÂµA(x) denotes the degree to which x belongs to A and is usually limited to values between 0 and 1

Fuzzy set operation: Fuzzy set operators are defined Fuzzy set operators are defined based on their corresponding membership functions. Operations like AND, OR, and NOT are some of the most important operators of the fuzzy sets. It is assumed that A and B are two fuzzy sets with membership functions ÂµA(x)and ÂµB(x)respectively. Then, the following operations can be defined:

The AND operator or the intersection of two fuzzy sets The membership function of the intersection of these two fuzzy sets (C = A B ) , is defined by
Âµc(x)= min{ ÂµA (x), ÂµB(x)}, x X (2)

The OR operator or the union oftwo fuzzy sets The membership function of the union of these two fuzzy sets(D
=AB), is defined by
ÂµD(x)=max{ ÂµA (x), ÂµB(x)}, x X (3)



FUZZY LOGIC CONTROLLER
Fuzzy control systems are rulebased systems in which a set of fuzzy rules represents a control decision mechanism to adjust the effects of certain system stimuli . The aim of fuzzy control systems is normally to replace a skilled human operator with a fuzzy rule based system. The fuzzy logic controller provides an algorithm, which can convert the linguistic control strategy based on expert knowledge into an automatic control strategy. Figure 1 illustrates the basic configuration of a fuzzy logic controller, which consists of a fuzzification interface, a knowledge base, a decisionmaking logic, and a defuzzification interface[7].
Figure 1: Design of Fuzzy Logic Controller

FUZZYBASED AVR
The first step in designing a fuzzybased AVR is to choose which state variables, representative of system dynamic performance, must be taken as the input signals to the controller. The second step is to choose the linguistic variables, keeping in mind that the number of linguistic variables specifies the quality of the control. As the number of the linguistic variables increases, the computational time and required memory also increase. Therefore a compromise between the quality of control and computational time is needed to choose the number of linguistic variables. For the test systems, following seven linguistic variables for each of the input and output variables are used to describe them: (i)LP (Large Positive), (ii) MP (Medium Positive), (iii)SP (Small Positive), (iv) ZR (Zero), (v) SN (Small Negative),
(vi) MN (Medium Negative) and (vii) LN (Large Negative). The normalization of the input variables is done by dividing the input values by maximum of the corresponding value of the input variable obtained by open loop simulation. Thirdly, it is required to determine the membership functions for the fuzzy sets. In this paper, the authors have used Fuzzy control systems are rulebased systems in which a triangular membership functions to define the degree of membership as shown in Figure 2.
In designing the AVR, the rules are defined using linguistic variables. The two inputs, deviation of error and its derivative, result in 49 rules for each machine. A proper way
to show these rules is given in Table 1. A typical rule has the following structure:
Rule 1. IF voltage error is LN (Large Negative) AND derivative of error is LN (Large Negative) THEN VAVR (Output of fuzzy AVR) is LP (Large Positive)
.
Figure 2: Triangular Membership Function of Input and Output Variables
In this paper, the MmnMax method is used to find the fuzzy region for each fuzzy rule. Fuzzy rules are connected using AND operator, where the AND operator means finding minimum between two membership functions[1].
Table 1: Fuzzy Rules Table
Fig.3 Circuit conguration of the proposed threearm AVR.

SYSTEM CONFIGURATION AND OPERATION THEORY
The circuit configuration of the proposed threearm AVR is shown in Fig. 3. This AVR comprises a threearm power converter, an input inductor, a small dc capacitor, and an output filter. The proposed AVR acts as an ac boost converter when the utility voltage is lower than the specified voltage, and it acts as an ac buck coverter when the utility voltage is higher than the specified voltage[8]. Hence, the output voltage of the AVR can be maintained at the specified voltage. Since the power converter operates as an ac boost converter or an ac buck converter, the dc bus voltage of the power converter is a rectified utility voltage where the amplitude can be controlled. The dc bus voltage of the proposed threearm AVR with a fullwave rectified voltage is different from that of the conventional threearm AVRs with a constant dc voltage.

AC Boost Mode
When the utility voltage is lower than the specified load voltage, the threearm power converter operates as an ac boost converter. In this situation, the first and third arms are controlled by a square signal with the fundamental frequency of utility, and the second arm is controlled by a highfrequency pulse width modulation (PWM) signal. Fig. 4 shows the operating circuit of the proposed AVR under the ac boost mode. The inductor LA is applied as the energy storage element when the threearm power converter operates as an ac boost converter. Fig. 4(a) shows the operating circuit of the ac boost converter when the utility voltage is in the positive halfcycle. As shown in Fig. 4(a), G1 and G6 are always on, and G2 and G5 are always off. When G3 is on and G4 is off, the inductor LA is energized through the utility, G1 and G3. In this duration, the inductor voltage (vLA) is given by
vLA = vs (4)
Where vs is the utility voltage. The current of the inductor LA is increased. The energy stored in the inductor LA will be released through G1 and G4 to the dc capacitor of the three arm power converter when G3 is off and G4 is on, and the inductor voltage becomes
vLA = vs vc (5)
Where vc is the dc bus voltage of the threearm power converter. Since the dc bus voltage of the threearm power converter will be higher than the utility voltage under the ac boost mode, the current passing through the inductor LA is decreased. When the current passing through the inductor LA is continuous, by applying Faradays law, the voltagesecond balance can be represented as
VsT VcT (1D) =0 (6)
Where D and T are the duty ratio and the switching period ofG3, respectively. From (3), the amplifier gain can be derived as Mv = 1/ 1D (7)
Fig. 4. Operating circuit of the proposed AVR under the ac boost mode. (a) Positive halfcycle. (b) Negative halfcycle[2]
The operation of the threearm power converter is similar to the dc/dc boost converter during the positive halfcycle. Fig. 4(b) shows the operating circuit while the utility voltage is in the negative halfcycle. As shown in Fig. 4(b) and the amplifier gain is also the same as (7). As shown in (7), the dc bus voltage of the threearm power converter is a rectified ac voltage which is higher than the utility voltage when serving as an ac boost converter, and the amplifier gain is determined by the duty ratio D. The efficiency of the dc/dc boost converter is dependent on the duty ratio[9]. The ripple of the input current can be derived as
iLA = vsDT/ LA (8)
Where f is the switching frequency. the ripple of the input current is dependent on the duty ratio, switching frequency f,
and inductor LA. In the continuous conduction mode, the minimum product of LA and f can be derived as
(LA) min=D(1D)2Z/2f (9)
Where Z is the load. Hence, the inductor LA can be determined by the switching frequency, specified ripple current, range of the duty ratio, and load. As shown in Fig. 4, the dc capacitor CA and output filter (LB, CB) form as a thirdorder lowpass filter to filter out the switching harmonic in the output voltage. Hence, a lower capacitance dc capacitor CA of several tens of microfarads can be selected[1012].

AC Buck Mode
When the utility voltage is higher than the specified load voltage, the threearm power converter operates as an ac buck converter. In this situation, the first and second arms are controlled by a square signal with the fundamental frequency of utility, and the third arm is controlled by a highfrequency PWM signal. The inductor LB serves as the energy storage element when the threearm power converter operates as an ac buck converter. Fig. 5(a) shows the operating circuit of the ac buck converter when the utility voltage is in the positive halfcycle. As shown in Fig. 6(a), G1 and G4 are always on, while G2 and G3 are always off. The utility voltage is rectified through the first and second arms of the threearm power converter; thus, a rectified utility voltage appears at the dc bus of the threearm power converter. Both the input inductor LA and the dc capacitor CA performed as a lowpass filter. When G5 is on and G6 is off, the inductor LB is energized from the rectified utility voltage through G4 and G5. In this duration, the inductor voltage (vLB) can be represented as
VLB = vcv0 (10)
Where vo is the load voltage. Since the rectified utility voltage is higher than the load voltage, the current passing through the inductor LB will be increased, and it stores energy in this duration. The energy stored in the inductor LB will be released to the load through G4 and G6 when G5 is off and G6 is on, and the inductor voltage is
VLB = v0 (11)
Hence, the current passing through the inductor LB will be decreased. When the current passing through the inductor LB is continuous and the Faradays law for the inductor LB is used, the voltagesecond balance can be represented as
VcDT voT = 0 (12)
Where D and T are the duty ratio and switching period of G6, respectively. From (7), the dropped gain can be derived as
Since the input inductor LA and the dc capacitor CA form a lowpass filter, the rectified voltage of the threearm power converter is close to the absolute utility voltage. Hence, the operation of the threearm power converter is similar to the dc/dc buck converter under the positive halfcycle. Fig. 5(b) shows the operating circuit when the utility voltage is in the negative halfcycle. As shown in Fig. 5(b), G2 and G3 are always on, while G1 and G4 are always off. The utility voltage is rectified through the first and second arms of the threearm power converter; thus, the dc bus voltage of the threearm power converter is the negative utility voltage. The inductor LB is energized by the rectified utility voltage through G3 and G6 when G5 is off and G6 is on, and the energy stored in the inductor LB will be released to the load through G3 and G5 when G5 is on and G6 is off. The operation of the threearm power converter is also similar to the dc/dc buck converter under the negative halfcycle, and the dropped gain is the same as that in (13). As shown in (13), the load voltage is lower than the dc bus voltage of the threearm power converter [13].
Mv = D (13)
Fig. 5. Operating circuit of the proposed AVR under the ac buck mode. (a) Positive halfcycle. (b) Negative halfcycle.[2]
The dc bus voltage of the threearm power converter is close to the absolute utility voltage. Hence, the relationship between the utility voltage and the load voltage is close to
(4) when serving as an ac buck converter. The dropped gain is determined by the duty ratio D. Hence, the proposed AVR can sustain the load voltage at a specified voltage under the swell utility voltage. Since the voltage across the inductor LA is smaller than that of the conventional threearm AVR, the inductance of the inductor LB in the proposed AVR can be reduced.
In the continuous conduction mode, the minimum product of
LB and f can be derived as
(LB)min = (1 D)Z/2f. (14)
Hence, the inductance of the inductor LB can be determined by the switching frequency, specified ripple current, range of the duty ratio, and load. The ripple of the output voltage can be derived as
vo = (VcVo)D/8LBCBf 2. (15)


TEST SYSTEM
Fig 6 Fuzzy Controller Based ThreeArm Automatic Voltage regulator(test system)
Table 2 Main parameters ofproposed avr[2]
Fig. 6 shows test system of the proposed AVR. It includes a utility voltage processing unit, a load voltage processing unit, and a selecting unit. The utility voltage processing unit is employed to generate a lowfrequency square signal and a selecting signal. The utility voltage is detected by a voltage detector, and then, it is sent to a zerocrossing detector. The output of the zerocrossing detector and its inverted signal are square waves in synchronization with the utility voltage to obtain the driving signals of G1 and G2 of the first arm. The detected utility voltage is also sent to a selecting circuit to generate a selecting signal C1. The selecting signal C1 determines the operation mode of the threearm power converter. If the utility voltage is lower than the specified voltage, the selecting signal C1 is HIGH for operating at the ac boost mode. On the contrary, the selecting signal C1 is LOW for operating at the ac buck mode when the utility voltage is higher than the specified voltage.

RESULTS
In order to verify the performance of AVR a fuzzy based AVR model has been developed and simulated. Fig 7 shows the results of the proposed AVR under a utility voltage of 143 V and resistive load. This is test system 1.The load voltage is sinusoidal which is regulated 110V. Fig 8 shows the results of the proposed AVR under a utility voltage of 77 V and resistive load. This is test system 2.The load voltage is sinusoidal which is regulated 110V.Fig 9 shows the results of the proposed AVR under a utility voltage of 121 V and nonlinear load. In this test system 3 the load current not completely distorted and load voltage is regulated at 110V. Fig.10 shows the results of the proposed AVR upon changing the utility voltage abruptly from 143 to 77 V.In this test system 4 the load voltage is regulated at 110V. Fig. 11 shows the results of the proposed AVR upon changing the utility voltage abruptly from 77 to 143 V.In this test system 5 also the load voltage is regulated at 110V.By using fuzzy controller based AVR we can reduce the %THD in the load current, which is shown in the table 3.
Specified load voltage
110V,
60Hz
PWM
switching frequency
20kHz
DC capacitor CA
20ÂµF
Output filter capacitor CB
20 ÂµF
Input inductor LA
0.4 mH
Output filter Inductor LB
0.4 mH
Specified load voltage
110V,
60Hz
PWM
switching frequency
20kHz
DC capacitor CA
20ÂµF
Output filter capacitor CB
20 ÂµF
Input inductor LA
0.4 mH
Output filter Inductor LB
0.4 mH
Fig. 7. Results of the proposed AVR under a utility voltage of 143 V and resistive load. (a) Utility voltage. (b) Utility current.

Load voltage.(d) Load current
Fig. 8. Results of the proposed AVR under a utility voltage of 77 V and resistive load. (a) Utility voltage. (b) Utility current.
(c) Load voltage. (d) Load current
Fig. 11. Results of the proposed AVR upon changing the utility voltage abruptly from 77 to 143 V. (a) Utility voltage. (b) Utility current. C) Load voltage. (d) Load current
Fig. 9. Results of the proposed AVR under a utility voltage of 121 V and nonlinear load. (a) Utility voltage. (b) Utility current.
(c) Load voltage. (d) Load current
Fig. 10. Results of the proposed AVR upon changing the utility voltage abruptly from 143 to 77 V. (a) Utility voltage. (b) Utility current. (c) Load voltage. (d) Load current.
S. NO
NO OF TEST SYSTEM
WITH PI CONTROLLER
WITH FUZZY CONTROLLER
1
Test system 1
11%
2%
2
Test system 2
21%
2%
3
Test system 3
70%
0.1%
4
Test system 4
2.3%
0.05%
5
Test system 5
2.3%
2%
S. NO
NO OF TEST SYSTEM
WITH PI CONTROLLER
WITH FUZZY CONTROLLER
1
Test system 1
11%
2%
2
Test system 2
21%
2%
3
Test system 3
70%
0.1%
4
Test system 4
2.3%
0.05%
5
Test system 5
2.3%
2%
Table 3 % THD in load current


CONCLUSION
A novel fuzzy based AVR configured by a threearm power converter has been proposed in this paper. The proposed AVR is operated as an ac boost under overvoltage of the utility, and it is operated as an ac buck when the utility is undervoltage. Moreover, there is no need to use a large dc capacitor in sustaining a constant dc voltage. Hence, the size can be decreased, the cost can be reduced, and the life of the power converter can be extended.
The closed loop control schemes of direct current control, for the proposed AVR with fuzzy controller have been described. A suitable mathematical have been described which establishes the fact that in both the cases the compensation is done but the response of fuzzy controller is faster and the THD is minimum for the both the voltage and current. Proposed model for the AVR is to compensate input voltage harmonics and current harmonics caused by non
linear load. The work can be extended to compensate the supply voltage and load current imperfections such as sags, swells, interruptions, voltage imbalance, flicker, and current unbalance. By using fuzzy controller instead of PI controller we can reduce %THD in the load current shown in table 3.
ACKNOWLEDGEMENT
The authors are thankful to the management of Aditya Institute of Technology and Management, Tekkali, Srikakulam (Dist), JNTU, Kakinada for providing facilities and to publish this work
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