investigation ofa shunt active power filter with pi …

D. M.Soomro, S. K.Alswed and M. A. Omran. / International Journal of New Technologies in Science and Engineering Vol. 2, Issue. 6, 2015, ISSN 2349-0780

Available online @ www.ijntse.com 49

INVESTIGATION OFA SHUNT ACTIVE POWER FILTER WITH PI AND FUZZY LOGIC CONTROLLERS UNDER DIFFERENT LOADS

D. M.Soomro, S. K.Alswed and M. A. Omran Department of Electrical Power Engineering, Faculty of Electrical and Electronic Engineering,

UniversitiTun Hussein Onn Malaysia (UTHM) 86400 Parit Raja, BatuPahat, Johor, Malaysia

E-Mail: [email protected], [email protected],[email protected]

Abstract: Now a day non-linear electrical devices are found in wide applications. These devices led to cause power quality (PQ) problems. One common of these problems is a distortion in the output sine waveforms of source current and voltage. It may lead equipment to overheat and sometimes cause failure of equipments’ function. This paper presents the design and application of three-phase shunt active power filter (SAPF) by using p-q theory with PI and fuzzy logic controller to mitigate the harmonics caused by nonlinear loads. The MATLAB / Simulink was used as a simulation tool. The achieved results are within the recommended IEEE-519 standard i.e. less than 5% for both controllers and also the improvement of power factor (PF) of the system to almost unity. Keywords: Power Quality (PQ) Shunt Active Power Filter (SAPF) Total Harmonic Distortion (THD) Power Factor(PF) PI controller Fuzzy logic controller (FLC)

I. INTRODUCTION Recent years, PQ distortionproblems have becomemost serious problems in electrical power systems (PSs) due to the increaseduse of nonlinear loads drawing non sinusoidal currents. APF have been widely used for harmonic mitigation well as reactive power compensation, load balancing, voltage regulation, and voltage flicker compensation in three-phase and three phase four-wire systems with nonlinear loads a high level of harmonic currents in both the three line conductors and more significantly in the neutral wire has been enrolled. Unbalanced load also results in further declination of the supply quality [1]-[2]. Various harmonic mitigation techniques have been proposed to reduce the effect of harmonics. These techniques include phase multiplicationpassive filters (PPF), active power filters (APFs), and harmonic injection. One of the most popular APFs is the shunt active power filter (SAPF). It is mainly a current source.It is connected in parallel with the non-linear loads. Conventionally, a SAPF is controlled in such a way as to inject harmonic and reactive compensation currents based on calculated reference currents. The injected currents are meant to cancel the harmonic and reactive currents drawn by the nonlinear loads [3]. Recently, fuzzy logic controller (FLC) has generated a great deal of interest in various applications and has been introduced in the power electronics field [3]-[4]. The advantages of FLCs over the conventional PI controller are that they do not need an accurate mathematical model; they can work with imprecise inputs, can handle nonlinearity, and may be more robust than the conventional PI controller. Use of FLC for improvement of PQand of reduction harmonics is not a new issue rather various authors have introduced some innovative methodologies using FLCthesetools [5]. The most important observation from the work reported by various researchers for PQ improvement is the design of APF under ‘fixed R-L load’ conditions or for R-L loads with additional unbalanced load [6].

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This paper, presents anAPF using two controllers PI and FL controllers to control the harmonics under two R-L balanced and unbalanced load conditions. To investigate how APF will deal with additional unbalanced load is connected.

II. SHUNT ACTIVE POWER FILTER Effect of harmonics Harmonics in PS can turn into the wellspring of a mixture of unwelcome impacts. For example, harmonics can cause signal interference, overvoltage, data loss, and circuit breaker failure, as well as equipment heating, breakdown, and harm. Any dissemination circuit serving modern electronic gadgets will contain some degree of harmonic frequencies. The greater the power drawn by nonlinear loads [6], cause higher the level of voltage and current distortion as shown in Fig. 1.

Fig. 1: Effect of harmonics on voltage or current waveform [5]

THD in power distribution system can be measured by using equation (1)

1001

I

2nI

THD I%

(1)

Where n = 2, 3, 4, 5,... ∑In is the total current I1is the fundamental component of the current

APFs are becoming a viable alternative to passive filters and are gaining market share speedily as their cost becomes competitive with the passive variety, one of the most common type of APF to reduce the harmonic current distortion is SAPF [1]. Principle of SAPF The shunt-connected APF, with a self-controlled dc bus, has a topology similar to that of a static compensator (STATCOM) used for reactive power compensation in power transmission systems. SAPF compensates load current harmonics by injecting equal but opposite harmonic compensating current. In this case, the SAPF operates as a current source injecting the harmonic components generated by the load but phase shifted by 180° [7], [8]. Fig.2 shows the connection of a SAPF compensating the harmonic load currents.


Fig. 2: Connection of a SAPF Design of SAPF based on instantaneous PQ theory The instantaneous active and reactive power theory or simply the p-q theory is based on a set of instantaneous values of active and reactive powers defined in time domain. There are no restrictions on the voltage or current waveforms, and it can be applied to three-phase systems with or without a neutral wire for three-phase generic voltage and current waveforms. Thus, it is valid not only in the steady state, but also in the transient state [9]. This theory is very efficient and flexible in designing controllers for power conditioners based on power electronics devices. Other traditional concepts of power are characterized by treating a three-phase system as three single-phase circuits. The p-q theory first uses Clarke transformation to transforms voltages and currents from the a,b,c to αβ0 coordinates and then defines instantaneous power in these coordinates. Hence, this theory always considers the three- phase system as a unit, not a superposition or sum of three single-phase circuits [9], [10]. The p-q theory can be defined in three-phase systems with or without a neutral conductor. Three instantaneous powers: the instantaneous zero-sequence power P0, the instantaneous P, and the instantaneous q are defined from the instantaneous phase voltages and line currents on the αβ0 axes as represented in equation (2).

푃푃푞

=푉 0 00 푉 푉0 −푉 푉

퐼퐼퐼

(2)

Let us consider a three phase system voltages푉 ,푉 and푉 that are the instantaneous phase voltages and퐼 , 퐼 and퐼 which are the instantaneous line currents. Since zero sequence power in three phase three-wire system is always zero, then equation (2) becomes:

푃푞 =

푉 푉−푉 푉

퐼퐼 (3)

In the proceeding discussion, the α and β currents will be set as functions of voltages and the real power (P) and imaginary power(Q) respectively to explain the physical meaning of the power defined in the p-q theory. Equation (3) can be written as:

퐼퐼 =

푉 푉−푉 푉

ퟏ 푃푞

푃 = 푣 푖 + 푣 푖 + 푣 푖 (4)


If current and voltages from α and β variables are replaced to their equivalent a, b, and c variables in equation (3), the instantaneous q will be:

푞 = 푉 퐼 − 푉 퐼 =1√3

[(푉 − 푉 )퐼 + (푉 − 푉 )퐼 + (푉 − 푉 )]퐼 ]

푞 = √

[푉 퐼 + 푉 퐼 + 푉 퐼 ] (5)

This expression is similar to that implemented in some instruments for measuring the three-phase Q. The difference is that voltage and current phases are used in those instruments. Here, instantaneous values of voltage and current are used instead [11].According to p-q theory, real and reactive powers can be written as:

푃 = 푃 + 푃, 푞 = 푞 + 푞,푃 = 푉 퐼 (6) Where: P = the active power for a three phase system with or without neutral conductor in steady state or during transients and it is representing the total instantaneous energy flow/second between source and load. 푃= alternating value of the instantaneous real power exchanged between the power source and the load through the a-b-c coordinates. Since alternating value of the instantaneous real power does not involve any energy transference from the source to load, it must be compensated. It is due to harmonic currents. 푃 = the average value of the instantaneous real power and is transferred from the power source to the load. It is the only desired power component to be supplied by the power source and due to fundamental active current. q = the imaginary power and proportional to the quantity of energy that is being exchanged between the phases of the system. It does not contribute to energy transfer between source and load at any time. 푞= alternating value of the instantaneous imaginary power exchanged between system phases and does not imply transfer of energy between power source and load. Since alternating value of the instantaneous imaginary poweris unwanted, it must be compensated. It is also due to harmonic currents. All these powers are explained in Fig.3. 푞= average value of the instantaneous imaginary power, exchanged between system phases and does not imply transfer of energy between power sources and load. The choice of compensation of average value of the instantaneous imaginary power depends on reactivepower compensation and is due to fundamental reactive current. P0= the active power due to zero sequence components.

Fig. 3: Theconcept of different powers between power source and load [6]

The basic idea behind the shunt current compensation isillustrated in Fig. 4. It shows a source supplying power to nonlinear load that is being compensated by the filter. Actually SAPF is shunt


compensator. We assumed that the SAPF behaves as three phase controlled current source that can generate harmonics in phase opposition depending upon current reference 푖∗ ,푖∗ and 푖∗ [11], [12].The calculated P of the load can be separated into its average (푃) and oscillating (푃) parts. Likewise, the load Q can be separated into its average (푞) and oscillating (푞) parts. Then, undesired portions of the p and q of the loads that should be compensated are selected.The reason for incorporating minus sign in the compensating power is to emphasize that the compensator must inject harmonics in perfect phase opposition. Remember that convention of current is selected as shows in Fig. 4 that source current is the sum of load currents and filter current. Inverse Clarke transformation from α, β, and 0 coordinates are applied to calculate the compensating current references푖∗ , 푖

∗ and 푖∗ instantaneously.

Fig. 4: SAPF in 3-phase system [6]

III. METHODOLOGY

In this paper, MATLAB/SIMULINK is used as a tool to implement the proposed SAPF with PI and fuzzy logic controllers in different load conditions. Mainly, there are two simulations; first simulation is with non-linear load while the second one is with SAPF connected.

First simulation The simulation diagram is as shown in Fig. 5.Three-phasesupply, which is feeding non-linear load with R-L parameters along with 3-phase full bridge diode rectifier, is considered for simulation. The values of simulation parameters are shown in the Table 1.

Fig. 5: First simulation setup in MATLAB/Simulink with nonlinear load



Second simulation To investigate the working of designed SAPF, it is connected to Fig. 5 under different load conditions ofsuddenly connected R-L load of90 Ω, 0.5Hand unbalanced R-L load of 5, 10, 15 Ω on each phase respectively as shown in Fig.6.Signal of the comparator is used to activate the inverter power switching. As shown in Fig. 8.

Fig. 6: Second

simulation setup with SAPF P-Q and current compensation calculation

This is the heart of APF.PQ theory algorithm for calculation of P, Q and compensation current are implemented as given in Fig.7. Embedded MATLAB function block is used to implement all the mathematical operations involved in the algorithm. Fig.7shows all the blocks required for calculation of compensation current. Inthat, Clarke transform is used to calculate P, Q, zero sequence power, and stationary frame at compensated reference current generate by using PI controller [13]. The SAPF using DC link capacitor, DC link reference voltage, and actual voltage are compared to the voltage to be regulated. Then inverse Clarke transform is used to calculate three phase compensated reference current.

Fig.7: Blocks required for calculation of compensation current




The hysteresis band current controller The principle of hysteresis current control is very simple. The purpose of the current controller is to control the load current by forcing it to follow. It is achieved by switching the action of an inverter to keep the current within the hysteresis band [13].The load currents are sensed and compared with respective command current by hysteresis band. There are two inputs for the hysteresis band, which is the reference current and the new injection current, the error between reference current and the new injection current are fed to the six relays. This signal will guide the inverter’s switching to compensate the harmonic current [14].

PI controller To control DC bus voltage, it is required to take care of little amount of power flowing into DC capacitor, thus compensating for switching and conduction losses. The dc link voltage control loop does not require being as fast as it responds to steady state operating condition. The actual DC link voltage is compared with a reference DC link voltage and passed through a PI controller [15],[16]. To maintain dc-link voltage at a fixed reference value, the dc-link capacitor requires a certain amount of real power, which is directly proportional to the difference between the reference and actual voltages. The control signal coming from PI controller to regulate DC link voltage can be expressed as:

푃 = 퐾 푉 − 푉 + 퐾 ∫ 푉 − 푉 푑푡(12) Where퐾 and퐾 are proportional and integral gains of the PI controller. By increasing proportional gain퐾 reduces rise time and steady state error. It also causes increase in the overshoot and settling time [17]. Similarly, increase of integral gain 퐾 reduces steady state error, and increases overshoot and settling time. The control diagram of PI controller is as shown in Fig. 9.

Fig. 8: Hysteresis current control

Fig. 9: PI controller




Fuzzy logic controller In this paper conventionally PI controllers have been used to maintain the DC bus

voltage.The followed control strategy employs the decision making for the control of DC bus by using Fuzzy Logic controller. The concept of FLC was offered in 1965 by Professor LotfiZadeh. Over the past few decades, the use of fuzzy logic, in control systems has gained widespread popularity. FLC are an excellent choice when precise mathematical formula calculations are impossible. The FLC system comprises mainly of four components the fuzzifier, the rule base, the inference engine and the defuzzifier.

1. Fuzzification

The process of converting numerical variable to linguistic variable is done in fuzzification. Here seven triangular shaped membership functions are used and their linguistic variables are Negative Big (NB), Negative Medium (NM), Negative Small (NS), Zero Error (Z), Positive Small (PS), Positive Medium (PM) and Positive Big (PB). Each membership function is defined using three vertices a,b,c which represents the values corresponding to the left minimum, peak and right minimum of a triangle representing the membership function [18], [19]. This is shown in Figure 10 and 11 respectively.

Fig. 10: Membership function of fuzzifier Fig. 11: Triangular membership function input variables and defuzzifier output variable

2. Fuzzy Rule Base

The rule base, stores the linguistic control rule base, needed byrule evaluator. Large errors in transient state need coarsecontrol and in small errors need fine control in steady state.Based on these elements[18], 49 rules of the Fuzzy rule table,are used in this paper,as shown in Table 1.

TABLE 1: FUZZY RULES OF THIS PAPER

e /∆푒

NS

NM

NB

Z

PS

PM

PB NB PB PM PS PS PS PS Z NM PM PS PS PS PS Z NS NS PS PS PS PS Z NS NS Z PS PS PS Z NS NS NS PS PS PS Z NS NS NS NS PM PS Z NS NS NS NS NM PB Z NS NS NS NS NM NB




3. Inference Engine and Defuzzification

To determine a specific or crisp value, the rule base has to be used with an inference method or engine followed by defuzzification. Mamdani’s fuzzy implication and max-min composition rule are used for inference. Centroid method is used for defuzzification. This fuzzy rule can be designed based on experience. Here the error (e) and change in error (∆e) are considered as input to FLC and the output of FLC is the control current Imax. This is the required active current needed to maintain the dc link voltage. This current is used as the part of the reference current (Isa*,Isb*,Isc*) to the current controller which controls the inverter to provide the required compensation current[20]. This estimated reference current (Isa*,Isb*,Isc*) and the actual sensed current (Isa,Isb,Isc) are compared in the hysteresis current controller and the error signal controls the operation of the converter switches as in [21]. Each switch of the converter is controlled independently.

Simulation Parameters

TABLE 2: SIMULATION PARAMETERS USED IN THIS PAPER

IV. RESULTS AND DISCUSSION

The simulation results of voltage, current and THD are obtained with MATLAB tool to analyse the performance of SAPF without and with compensation as shown in Fig. 12, 13, 14, 13 and 15.

1) Case 1 (R-L load)

Fig. 12: Source voltage without and with SAPF

Parameter Value Source voltage (rms) 480V Source frequency (f) 50 Hz Source resistance (RS) 1 mΩ Source inductance (LS) 1e-3mH Case 1: Load (R-L) 90 Ω, 0.5H Case 2: Additional unbalanced load ofR-L

90 Ω, 0.5H

DC link capacitor (Cdc) 4500 µF Inductor filter 5mH DC link PI controller Kp = 25, Ki = 20




Fig. 13: Source voltagewithout SAPF

Fig.12is source voltage waveform. The distortion of voltage is negligible, THD of voltage is 1.13% as shown in Fig. 13. In the waveform of current, distortion is present cussed by nonlinear loads there as shown in Fig. 14. In that case, THD of currentwas measured as 19.30 % as shown in Fig. 15. On the other hand, power factor noticed was 0.887.

Fig. 14: waveform of current without SAPF

Fig. 15: THD of current without SAPF With the current after applying SAPF with PI controller , the resultant source current harmonic was mitigated and THD was reduced from 19.30% to 3.63% as shown in Fig. 16,the THD for phases ‘A’, ‘B’,’C’ are 3.10%,3.81%,3.9% respectively as shown in Table 3.But with fuzzy logic controller, the THD is less than with PI i.e. THD for each phases are 1.87%,1.02%,1.29% respectively.Fig. 17shows the result with fuzzy logic controller.

Fig. 16: THD after SAPF with PI




Fig. 17: THD after SAPF with fuzzy logic

Fig. 18 presents the waveform of source and load current after the SAPF was applied. The current waveform was highly distorted, but after compensated current waveform is almost sinusoidal. And also the power factor of system is nearby 1.

Fig. 18: waveform of current with SAPF

2) Case 2 (Additional unbalancedR-L load with R-L load)

Fig. 19 and Fig. 20 exhibit the simulation results obtained in the THD of the source current with nonlinear R-Land suddenly connected additional unbalanced R-Lloads at 0.2 sec. The THD is 18.13%. THD of each phases ‘A’,’B’,’C’ were 18.04 % ,18.2 %,17.64 % respectively .The highest harmonics were the 5thand the 7threpresenting 13.66 %and 7.56 % respectively of the fundamental.

Fig. 19:Current waveform without SAPF




Fig. 20: An analysis of THD without SAPF

The SAPF was connected to the harmonic distorted system at time t=0.1sec to mitigate the effect. It can be seen that the filter with PI controller takes only 0.025 sec for the shunt APF to follow the change of the load current as shown in Fig. 21. But with fuzzy logic controller it takes only 0.020 sec as shown in Fig.22.

Fig.21:current waveform with PI controller

Fig. 22: current waveform with fuzzy logic controller

In this case, SAPF was investigated with R-L load and with additional R-L loads.The THD with PI controller was reduced from 18.13% to 3.41%. Otherwise, THD with fuzzy logic controller is 1.21% which matches the IEEE 519 standard limits as shown in Fig.23 and Fig.24 respectively.The power factor is improved 0.889 to 0.999 also.




Fig.23: Analysis of THD after SAPF with PI

Fig. 24: Analysis of THD after SAPF with fuzzy logic

The resulting THD of the current and PF beforeand after SAPF with PI controller and fuzzy logic controller are summarised in Table3.While Table 4gives a glance by describing the harmonic orders of all simulations without and with bothcontroller.

V. CONCLUSION PI and fuzzy logic controllers based SAPF based on instantaneous p-q theory is simulated in MATLAB /SIMULINK by using different load conditions i.e. R-Lload, and additional unbalance R L load. Its application with both controllers are successfully proved and the validity achieved by minimizing the harmonics and improving PF as summarised in Table 3. Consequently, the supply current is almost pure sinusoidal. THD observed is fund to be within the prescribed limits of 5% as recommended by IEEE-519 standard.

TABLE 3: RESULTS OF SIMULATIONS

Operation condition Without SAPS With SAPF PI Fuzzy logic

THD % PF THD % THD % PF

R-L load

19.30

0.887

3.63

1.87

1 R-L with additional unbalanced R L loads

18.13

0.889

3.41

1.21

0.999

TABLE 4: HARMONIC ORDERS OF ALL CASES WITHOUT AND WITH SAPF

Case

Phases

Harmonic orders %THD

ퟑ풓풅 ퟓ풕풉 ퟕ풕풉 ퟗ풕풉 ퟏퟏ풕풉

Case 1

R-L load

Without filter

A 0.12 16.47 9.15 0.04 3.11 19.30 B 0.32 16.41 9.22 0.10 3.07 19.40 C 0.20 16.45 9.19 0.06 3.11 19.33

With PI controller

A 0.40 1.45 0.97 0.12 0.61 3.10 B 0.59 1.60 1.0 0.42 0.48 4.37 C 0.27 1.27 1.32 0.29 0.30 3.90

With fuzzy A 0.66 0.74 0.47 0.21 0.24 1.87 B 0.32 0.38 0.37 0.10 0.09 1.02




controller C 0.44 0.62 0.25 0.12 0.18 1.29

Case 2

R-L load With

Additional unbalanced

R-L load

Without filter

A 0.37 15.37 8.45 0.17 2.96 18.04 B 0.92 13.66 7.56 0.39 2.52 17.84 C 0.88 13.97 8.33 0.47 2.57 17.98

With PI controller

A 0.34 2.87 0.90 0.59 0.34 3.41 B 0.91 3.0 1.23 0.60 0.33 3.83 C 1.04 2.61 1.0 0.23 0.37 3.79

With fuzzy controller

A 0.97 0.35 0.31 0.05 0.07 1.21 B 0.47 0.30 0.48 0.18 0.06 0.90 C 0.75 0.28 0.31 0.29 0.11 0.93

VI. FOR FUTURE RESEARCH Simulation of SAPF for current and voltage harmonics compensation can be made by using universal PQ conditioner (UPQC).Experimental investigations can be made on SAPF by developing a prototype model in the laboratory to verify the simulation results for both PI and hysteresis controllers.

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Dr.Dur Muhammad Soomro (S'90-M'02) received the B.Sc. and M.Sc. degrees from MEHRAN University of engineering and technology in Pakistan, and the Ph.D. degree from University Teknologi Malaysia (UTM), in Malaysia, all in electrical power engineering. Currently, he is a senior lecturer in the department of electrical power engineering at University Tun Hussein Onn Malaysia (UTHM). His research interest includes power electronics and power quality. He is a member for Institute

of Electrical and Electronics Engineers (IEEE), Institute of Engineers Pakistan, and Pakistan Engineering Council. Tel:+60-74537359, E-mail: [email protected].

Mohamed A.M Omran received his B.S. degree in 2003 and the M.S degree in 2015 from University Tun Hussein Onn Malaysia (UTHM), in Malaysia, both in electrical power engineering. Currently, he is a PhD candidate. His major interests are in power electronics, power quality and high voltage. E-mail: [email protected].

Sager K. Alswed received his B.S. degree in 2009 from Elmergib University, in Libya. And the M.S degree in 2014 from University Tun Hussein Onn Malaysia (UTHM), in Malaysia.Both in electrical power engineering. Currently, he is a PhD candidate. His major interests are in power electronics, and power quality. E-mail: [email protected].


mailto:[email protected].



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