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

Shunt Active Power Filter For Current Harmonic Cancelation 1

P.Anjalee kumari,

V.Sarayu

Y.Suribabu

G.Sambasiva Rao

Sr.Lecturer Lecturer Sr.Lecturer Sr.Lecturer R.V.R &J.C.College of Engineering(affiliated to Nagarjuna University), Department of EEE, Guntur. anjali_pati@yahoo.co.in, sarayu.vunnam@gmail.com, ysuribabu@gmail.com, sr_gudapati@yahoo.com

Abstract--In this paper models for three-phase active power filter controller for balanced and unbalanced nonlinear load is made and is simulated using Matlab/simulink software. The proposed active power filter can largely reduce the total harmonic distortion of current and correct the power factor to unity with balanced and unbalanced nonlinear load. The advantage of this Active power filter are simplicity of control circuits and low implementation cost.

Fig.1 3-phase shunt active power filter

I. INTRODUCTION

canceling the current harmonics contained in the nonlinear load current. This will thus result in sinusoidal line currents and unity power factor in the input power system. The load current can be subdivided in to the fundamental and harmonic components. If the harmonic components are filtered out, the fundamental component of load current is there. The fundamental component can be subdivided in to real component and reactive component For good performance of the active power filter, the supply current must be equal to the real component of the fundamental component of the load current.

The power quality problems are manifested in voltage, current or frequency deviations and result in failure or miss operation of customer and utility apparatus. Power electronic equipment usually introduces current harmonics. These current harmonics result in problems such as a low power factor, low efficiency, power system voltage fluctuations and communications interference. Traditional solutions for these problems are based on passive filters due to their easy design, simple structure, low cost and high efficiency. These usually consist of a bank of tuned LC filters to suppress current harmonics generated by nonlinear loads. Passive filters have many disadvantages, such as resonance, large size, fixed compensation character and possible overload. To overcome these dis-advantages, active power filters have been presented as a current-harmonic compensator for reducing the total harmonic distortion of the current and correcting the power factor of the input source. This will thus result in sinusoidal line currents and unity power factor in the input power system. At present, calculation of the magnitude of the compensating currents of an active power filter is based either on the instantaneous real and reactive powers of nonlinear loads or the integrative methods of Fourier analysis Both these approaches neglect the delay time caused by low pass, high pass filters when compensating current calculations. The inductor is used to perform the voltage boost operation in combination with the DC-link capacitor and functions as a low pass filter for the line current of an active power filter. The principle of operation of an active power filter is to generate compensating currents into the power system for

II.CONTROL STRATEGIES FOR CURRENT HARMONICS COMPENSATION: The reference current for Shunt Power Active Filter (SPAF) is determined by calculating the fundamental harmonic of the nonlinear load current and subtracting it from the total current which can be obtained by different methods like [7,8]: - by digital low pass-band filters; - by real time Sliding Fast Fourier Transformer (SFFT); - by determining it in a synchronous rotating frame generated by the fundamental harmonic of the voltage supply, obtained by the means of a Digital Phase Lock Loop (DPLL). Fig.2 shows the block diagram of proposed 3-phase shunt active power filter. The simplest solution for current controllers is to operate in the fixed frame by comparing the reference harmonic currents with the output of the Active Filter. The three errors ÎľÎš applied to three current regulators, the outputs representing the reference voltages for the PWM inverter in order to produce the contra phase current harmonics. We have to take into account the fact that the reference is represented by a signal comprising. 5th, 7th, 11th, 13th, etc harmonics representing the harmonics of the load plus a first harmonic in phase with the mains voltages in order to charge the filter capacitor and to keep the dc voltage 274

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

constant (to cover the switches losses). A more efficient solution is to calculate the control signals in a rotating frame with pulsation ω1. This transformation is equivalent to multiply all the variables with e≗ jЫ1t. In such a case, the perturbation and the first harmonic current references become constants and the rest become signals of the 6th, 12th, 18th and so on of 6k frequencies: A delay of real time reference can be seen as an equivalent virtually advance. In such applications synchronization between the mains and the chosen PWM switching frequency (10 kHz) is not easy to be done, as 10 kHz is not a multiple of 300 Hz (6 x 50). For the easiness of the control, the switch frequency can be chosen to be multiple of 2N: 300 2N 300 32 9600 sw f = Ø = Ø = Hz instead of 10 kHz. One step in virtual advance means that the reference to be shifted back by M = N ≗1 = 31 samples and for two steps advance a shift back by M = N ≗ 2 = 30 samples; It is clear that one step advance improves dramatically the form of the compensated current but for two steps the situation deteriorates again. In conclusion, this type of control improves accuracy of the transient and steady state responses of the control system when reference input signals and disturbances are periodic, consisting of the harmonic components of a common fundamental frequency. This direct repetitive control is less per formant in the case of non-periodic or non-harmonic disturbances and at the limit bringing even instability.

output of the PI controller and the sinusoidal reference per phase are fed to an analogue multiplier to create desired magnitude of the supply current. The controller of the shunt active filter is concise and requires less computational efforts than many others found in the literature. It is formed by a dc voltage regulator, a synchronizing circuit (DPLL) and a compensating current. Here, the PWM current control is considered as part of the power converter. The dc voltage is used in the voltage regulator to generate the control signal. It forces the shunt active filter to draw additional active current from the network, to compensate for losses in the power circuit of the shunt active filter. In several cases, it is possible to eliminate the low-pass filter and to build a dc voltage regulator consisting only of a PIController. The dc voltage regulator realizes a slower feedback control loop that is useful to correct compensation errors that arise during transients. The intrinsic dynamic of the synchronizing circuit (DPLL circuit) and of some low-pass filters included in the controller introduces temporary compensation errors that affect the dc voltage. III. SIMULATION RESULTS:

Fig.3 Simulation diagram of 3-phase shunt active power filter with non linear load

The simulation of 3-phase shunt active power filter is carried out using simulink / Matlab program. The following figures shows the results of load currents, source currents, dc load voltages and compensating current.

Fig.2 Block diagram of proposed 3-phase shunt active power filter

The proposed APF has simple control circuit, it consists of detecting the supply current instead of the load current. In the proposed APF, three parameters has to be detected, the dc bus voltage, the 3-phase supply voltage and three phase supply current. The signals of the three phase supply voltage are used to create three sinusoidal reference waves shifted by 1200 with unity magnitude. The detected bus voltage is compared with the setting voltage. The difference between the signals is fed to a PI controller to create the desired magnitude of the supply current. The

Fig.4 DPLL output

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

The Total Harmonic Distortion (THD) in the source current of the proposed three phase system of non linear load with Active Power Filter is 1.54%. IV. CONCLUSION: The proposed APF has a simple control circuit and low cost of implementation. From the above analysis and simulation results it can be found that the proposed APF has all the performance of conventional active power filters. The effectiveness of the proposed method is demonstrated in the quality of the supply current after filtering and the reduction of THD of the supply current. Therefore the proposed APF can suppress the current harmonic to force the supply current to be sine wave. The simulation result indicates that the total harmonic distortion (THD) of the source current reduced and the power factor is improved.

Fig.5 Load currents

Fig.6 Compensated current

REFERENCES: 1. A novel and analytical model for design and implementation of active power filter’ IEE proc –electr., power appl.,vol.148,no.4,july 2001 2. Nassar Mendalek and Kamal Al-Haddad Modeling and Nonlinear Control of Shunt Active Power Filter in the Synchronous Reference Frame. 2000 IEEE. 3. FUJITA, H., and AKAGI, H.: ‘A practical approach to harmonic compensation in power system-series connection of passive and series active filters’, IEEE Trans. Ind. Appl., 1991, 27, (6), pp. 1020-1025. 4. MORAN, L.A., DIXON, J.W., and WALLACE, R.R.: ‘A three phase active power filter operating with fixed switching frequency for reactive power and current harmonic compensation’, IEEE Truns Ind Electron., 1995, 42, (4), pp. 402408. 5. R. Magureanu, S. Ambrosii, D. Creanga, L. Bratosin, A. Draghici University ”Politehnica” Bucharest – Department of Electrical Engineering, Splaiul Independentei 313, Bucharest Romania,“Active power filters advanced control”. 6. Maurício Aredes and Luís F. C. Monteiro, UFRJ − Federal University of Rio de Janeiro COPPE/POLI – Electrical Engineering Department “A Control Strategy for Shunt Active Filter”. 7. Power Systems Harmonics Fundamentals, Analysis and Filter Design by Wakileh, George J. 8. Active power filter for reactive power compensation and Harmonic suppression by Hurng-Liahng jou

Fig.7 Source voltage and source current

Fig.8 FFT analysis for source currents

Fig.9 dc load voltage and dc load current