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Real Time Implementation of Shunt Active Power Filter (SAPF)for Harmonic suppression and Power
Quality Improvement
B. Babes,A. Bouafassa, L. Rahmani,N. Hamouda
ISSN:2335-1209
The International Journal on Advanced Electrical Engineering
Volume 1, 2013 Vol 1, N 3
Pages 159-168 September 30, 2013
Abstract
In this paper, A Shunt Active Power Filter (SAPF) is implemented using a dSPACE DS1104 processor to compensate harmonics and reactive power produced by nonlinear load. The reference source current is computed based on the measurement of harmonics in the supply voltage and load current. A hysteresis based current controller has been implemented in a DSP processor for injecting the compensating current into the power system, so that SAPF allows suppression of the harmonics and reactive power component of load current, resulting in a supply current that is purely sinusoidal. Simulation and experimental results of the proposed SAPF to meet the IEEE-519 standards are presented.
Key words: Harmonics, Power quality, Active power filter, Hysteresis comparator, Real- time control.
1 Introduction
Harmonics contamination is a serious and a harmful problem in Electric Power System. Active Power filtering constitutes one of the most effective proposed solutions. A shunt active power filter that achieves low current total harmonic distortion (THD), reactive power compensation and power factor correction is presented. Hence, it is necessary to reduce the dominant harmon- ics below 5%as specified in IEEE-519-1992 harmonic standard [1]. Harmonic Amplification is one the most serious problem. It is caused by harmonic resonance between line inductance and power factor correction (PFC) capacitors installed by consumers. Active filters for damping out harmonic resonance in industrial and utility power distribution systems have been researched [1]-[2]. Traditionally based, passive L-C filters were used to eliminate line harmonics in [3]-[4].
B. Babes,A. Bouafassa:Department of Electrical Engineering, Setif University, Algeria, elect_babes@
yahoo.fr,amar.bouafassa@gmail.com.
L. Rahmani:Automatic Laboratory of Setif, Setif University, Algeria,lazhar_rah@yahoo.fr.
N. Hamouda:Centre National de Recherche Scientifique en Soudage et Contrôle CSC Route Dely Brahim Chéraga, Algerie,hammouda.nourou@yahoo.fr.
Corresponding autho. B. Babes:elect_babes@yahoo.fr
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However, the passive filters have the demerits of fixed compensation, bulkiness and occurrence of resonance with other elements. The recent advances in power semiconductor devices have resulted in the development of Active Power Filters (APF) for harmonic suppression. Various topologies of active filters have been proposed for harmonic mitigation. The shunt APF based on Voltage Source Inverter (VSI) structure is an attractive solution to harmonic current prob- lems. The SAF is a pulse width modulated (PWM) VSI that is connected in parallel with the load. It has the capability to inject harmonic current into the AC system with the same ampli- tude but opposite phase than that of the load [1]-[3]. The principal components of the SAPF are the VSI, a DC energy storage device that in this case is capacitor, a coupling transformer and the associated control circuits. The performance of an active filter depends mainly on the technique used to compute the reference current and the control method used to inject the de- sired compensation current into the line. There are two major approaches that have emerged for the harmonic detection [3], namely, time domain and the frequency domain methods. The frequency domain methods include, Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT), and Recursive Discrete Fourier Transform (RDFT) based methods. The frequency do- main methods require large memory, computation power and the results provided during the transient condition may be imprecise [4]. On the other hand, the time domain methods require less calculation and are widely followed for computing the reference current. There are several current control strategies proposed in the literature [2]-[5], [6]-[7],[8], namely, PI control, aver- age current mode control (ACMC), predictive current control, sliding mode control (SMC) and hysteresis control. Among the various current control techniques, hysteresis control is the most popular one for active power filter applications. Hysteresis current control [9] is a method of controlling a voltage source inverter so that the output current is generated which follows a ref- erence current waveform. In this paper, a simple and straight hysteresis band current controller is implemented for an active shunt filter to compensate the harmonics and reactive power of a typical non-linear load. A dSPACE DS1104 controller board with five ADC channels and high resolution Pulse Width Modulated (PWM) channels is used to implement the proposed control algorithm. An IGBT based VSI Bridge with common DC bus capacitor is employed to realize the SAPF. A three phase diode bridge rectifier with R-L loading is taken as a non-linear load.
The control algorithm is tested through simulation and thereafter, experimental verification is also carried out.
Figure 1: Shunt active power filter configuration.
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2 Active power filter principle
Active power filters (APFs) are basically power electronic devices that are used to compensate the current or voltage harmonics and the reactive power flowing in the power grid. The APFs may be used as a controlled current source and it has to supply a current wave as close as possible to current reference. [10], [11] As it is shown in the Fig.1, the active filter is composed of a three phase voltage source inverter with an ac inductor (Lf) and a dc bus capacitor (Cdc) to provide a constant dc voltage and the real power necessary to cover the losses of the system. A three-phase AC supply system (Vsa,Vsb andVsc) with line impedance (RsandLs).[12],[13]
3 Control scheme
The control scheme consists of PI controller, limiter, and three phase sine wave generator for reference current generation and generation of switching signals. The peak value of reference currents is estimated by regulating the DC link voltage. The actual capacitor voltage is com- pared with a set reference value. The error signal is then processed through a PI controller, which contributes to zero steady error in tracking the reference current signal. The output of the PI controller is considered as peak value of the supply current (Imax), which is composed of two components: (a) fundamental active power component of load current, and (b) loss com- ponent of APF; to maintain the average capacitor voltage to a constant value. Peak value of the current (Imax) so obtained, is multiplied by the unit sine vectors in phase with the respective source voltages to obtain the reference compensating currents. These estimated reference cur- rents (Isa∗, Isb∗, Isc∗) and sensed actual currents (Isa, Isb, Isc) are compared at a hysteresis band, which gives the error signal for the modulation technique. This error signal decides the oper- ation of the converter switches. In this current control circuit configuration the source/supply currentsIsabcare made to follow the sinusoidal reference currentIabc, within a fixed hysteretic band. The width of hysteresis window determines the source current pattern, its harmonic spec- trum and the switching frequency of the devices. The DC link capacitor voltage is kept constant throughout the operating range of the converter. In this scheme, each phase of the converter is controlled independently. To increase the current of a particular phase, the lower switch of the converter associated with that particular phase is turned on while to decrease the current the upper switch of the respective converter phase is turned on. With this one can realize, potential and feasibility of PI controller. The conventional hysteresis band current control scheme used for the control of active power filter line current is shown in Fig.2
Figure 2: Control scheme for APF.
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3.1 Hysteresis current controller
The schematic diagram of hysteresis controller is shown in Fig.3. The error between reference current If∗ and real compensating current If is the input to hysteresis comparator. And then the PWM signals are generated by the hysteresis comparator. The semiconductors of SAPF are controller by those PWM signals and then the control of compensating current If is realized.
Figure 3: Block diagram of periodical sampling constant Hysteresis band.
Defining HB as the width of hysteresis comparator, when | ∆If |< HB the output of hysteresis comparator will remain invariable, and when| ∆If |≥ HB the output of hysteresis comparator will reverse. Then the direction ofIf will change. Based on the analysis mentioned above, the∆If will change between−HB andHB, and If will change betweenIf∗−HB and If∗ +HB with sawtooth-shape. Fig.4 shows the tracking process of compensating current.
Figure 4: The upper and lower bands of the reference compensation current.
From Fig. 4, the below relations can be obtained:
di+f(t) dt = 1
Lf
(Vf(t)−Vs(t)) (1)
di−f(t) dt = 1
Lf(Vf(t)−Vs(t)) (2) Wheredi+f anddi−f are the rising current and the falling current, respectively. Furthermore, the following relations can be form:
di+f(t)
dt .t1− di∗f(t)
dt .t1 = 2HB (3)
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di−f(t)
dt .t2− di∗f(t)
dt .t2 = 2HB (4)
f = 1
t1+t2 (5)
Wheret1 andt2 are switching intervals andf is the switching frequency. By substituting (1), (2) and (5) in (3) and (4), the hysteresis band (HB) can be achieved as follow:
HB = Vdc
4f Lf − Lf
4f Lf(Vs(t)
Lf +di∗f(t)
dt )2 (6)
3.2 DC Voltage controller
When the SAPF is compensating the harmonic and reactive power components, the dc capacitor voltage Vdc varies. Hence Vdc is also sensed and regulated at a reference value in order to establish a self-sufficient energy at the dc bus. The regulation loop consists of the comparison of the measured voltage with the reference voltage, admitting that the function of the system to be controlled is given by [14]-[15]:
G(s) = Vs2
Vdc−ref ∗Cdc∗s (7)
The closed loop transfer function using a PI regulator is given by:
Vdc Vdc−ref
=
(kp+ ksi)(V Vs2
dc−ref∗Cdc∗s) 1 + (kp+ksi)(V Vs2
dc−ref∗Cdc∗s) (8)
The development of this equation gives:
Vdc Vdc−ref
=
kp∗Vs2
Vdc−ref∗Cdc ∗s+V ki
dc−ref∗Cdc
s2+ V kp∗Vs2
dc−ref∗Cdc ∗s+ V ki
dc−ref∗Cdc
(9) A second order characteristic equation of the closed loop system is deduced:
s2+ 2ξωns+ωn = 0 (10)
where:
ωn =
s 2ξki
Vdc−ref ∗Cdc , ξ = kp∗Vs 2kip
Vdc−ref ∗Cdc (11)
From (12) the proportional and integrator coefficientkp,ki of the controller can be deduced:
kp = 2ξki
ωn , ki = ωn∗Vdc−ref ∗Cdc
Vs2 (12)
The expression of the current Isc to compensate the inverter losses and maintain the constant DC-link voltage is given by:
Isc =kp∗∆Udc+Ki
Z
∆Udcdt (13)
To obtain optimal dynamic performance for the system, the value of the damping rationξmust be equal a 0.707.
Proofs
4 System modeling and simulation
To simulate the proposed control strategy of the SAPF, a model is developed in Matlab/Simulink˝o environment using SimPower Systems Blockset. Fig.5 depicts the test bench to estimate the per- formance of the SAPF with proposed control scheme. The complete SAPF system is composed mainly of a three-phase source, a nonlinear load which is a three phase rectifier feeding an inductive load, a PWM voltage source inverter, and a control bloc.
Figure 5: Main block of proposed control scheme with SAPF under MATLAB.
First simulation is carried out with a fixed load and the SAPF is switched on at t=0.05s. Fig.6 shows the source voltageVsa(V), source currentisa(A), DC side capacitor voltageVdc(V)and filter currentif a(A). The instant the filter is switched on the source current becomes sinusoidal from the stepped wave shape, the DC capacitor voltage achieve quickly the reference value V*dc (142V) after practically only one cycle (about 10ms). In Fig.7 one can see that the active power Ps(W)joined its nominal value and that reactive powerQs(V AR) becomes null when the active filter is activated at this moment.
Figure 6: Source voltage, Source current, DC side capacitor voltage and filter current wave- forms. Filter switched on at 0.05s.
Proofs
Figure 7: Active and reactive powers source. Filter switched on at 0.05s.
Fig.8 shows the source current spectrum analysis before and after filtering. Before filtering;
onecan see the current harmonics distortion value was 23.07%= THDi and after filtering it will be 3.59% =THDi, which proves that the proposed SAPFcontrol strategy has the capability of compensating for current harmonics successfully
Figure 8: Source current and its spectrum before filtering .
Figure 9: Source current and its spectrum after filtering .
Proofs
5 Experimental validation
The experimentation of this work is done using the test bench which was developed in LAS Laboratory, University of Setif.1 (Fig.10).
Figure 10: Photography of the APF system prototype.
The input step-down transformer (12KVA, 380/220 V) is connected to the mains. The three phase parallel active filter is achieved with a voltage source inverter of 20 KVA. This VSI contains a three phase IGBT 1200 V, 50 A (SKM 50 GB 123D). To ensure the insulation and the dead time of control signals a developed card based on the IXDP630 component and a special driver circuit (SEMIKRON, SKHI 22) are used. The control strategy is implemented using a dSPACE card DS1104 developed under Matlab/SimulinkTM RTW environment. The sampling time using in practical tests for the proposed systems is 45µs. In this control type, switching frequency is variable, although in our design this frequency is limited to 20 kHz to not reach the IGBTs maximum switching frequency. The same experimental test bench parameters are usedfor simulation: Vs = 50V (RMS), Rs = 0.1Ω,Ls = 0.1mH,Rc = 0.8Ω, Lc = 2mH,Rf = 0.1Ω,Lf = 10mH, RL = 30Ω,LL = 50mH, Vdc = 142V, C = 1100µF, HB = (hysteresisband) = 0.01,ki = 18.86andkp = 0.154
Figure 11: Steady state response of the SAPF with a current-source type of nonlinear load.
Proofs
Fig. 11 shows steady state response of the SAPF for harmonic elimination with a current source type of nonlinear load. In this ?gure, all the quantities are shown for phase a and top to bottom waveforms are the load currentica, supply currentisa,active filter currentif aand DC side capacitor voltage Vdc. These waveforms show the capability of the proposed SAPF to compensate harmonic currents of the load,the DC-bus voltage of the SAPF is regulated at its reference value (142v). The frequency analysis of supply current before and after compensation in phaseaare shown in Fig. 12 and Fig. 13. The SAPF is able to reduce harmonics in the supply currents from 26.4%before compensation to 4.6%after compensation.The THD of the supply voltage is 3%.
Figure 12: Source current spectrum and vector diagram before filtering .
Figure 13: SSource current spectrum and vector diagram after filtering.
6 Conclusion
A shunt active power filter based on a hysteresis current control algorithm has been studied inthis paper to determine the reference current for the SAPF in order to improve the power quality and compensate reactive power required by nonlinear load. With the advanced control system designed in this paper the proposed SAPF can attenuate harmonics well and has a good dynamic performance, various simulation and experimental results of the proposed control al- gorithm are presented to confirm his validity and effectiveness. The THDi of the supply current after compensation is 4.60%which is less than 5%; the harmonic limit.
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