In this paper, a fuzzy logic based space vector PWM controlled hybrid active power filter proposed for compensating harmonics, increasing the distribution power improving the power factor of the three phase distribution system. In the proposed control filter three control circuits are used such as PI control unit, fuzzy unit SVPWM control unit. Fuzzy arithmetic's are used for adjusting proportional -integral coefficients timely. The desired output voltage is generated based on generated reference voltage by fuzzy based SVPWM .the purpose controlled filter shows shorter response time and higher control precision. Simulations are carried out using MATLAB .It is found that the %THD has been improved from 21.67 to 1.57and power factor is improved to 0.9718 .The simulation results shows that the effectiveness and feasibility of the proposed filter.
Key Words: Hybrid Active Power Filter, Fuzzy Logic Controller, IGBT Inverter, Space Vector PWM, Total Harmonic Distortion (THD).
1.Introduction
Power quality is the main problem that the industry is facing today. The quality of power has been deteriorating with the presence of various current and voltage harmonics, low power factor, voltage sags and swells, flicker and many other disturbances. Among the various disturbances, Harmonic distortion is one of the most serious power quality problems. Particularly, in the distribution systems, harmonics are the major concerned problem. The growing use of electronic equipments is one of the major causes to impute the harmonics, which led to distortion of voltage and current waveforms and increased reactive power demand in ac mains as they pass through the system impedance.
However, in the present situation various power quality improvement solutions are available; Isolate harmonic loads on separate circuits (with or without harmonic filters), Harmonic mitigating transformers, Phase shifting (zig-zag) transformers, Filter capacitor backs, Line Reactors, K-Rated / Drive Isolation Transformers, Harmonic Mitigating / Phase Shifting Transformers, Passive parallel / series tuned Filters and Active Filters.
Passive filtering is the simplest conventional solution to reduce the harmonics. But they have many demerits such as; a) the number of passive filters installed would depend on the number of harmonic component to be compensated, this demands for the information of harmonic content to be know in advance. b) These cannot function under the saturated conditions, c) At some frequencies, these filters may lead to resonance. All the above demerits of the filters are overcome by the use of active filters. But, for high-power applications, the Active filters are not cost effective due to their large rating and high switching-frequency requirement of the pulse width modulation inverter.
For harmonic current tracking controls, there are two schemes [11]-[14]: One is the linear current control and the other is nonlinear current control. Hysteresis nonlinear control method is simple but leads to a widely varying switching frequency. This limitation has been improved with variable hysteresis band switching strategies but it requires a complex controller to achieve satisfactory performance. Predictive current control offers the best potential for precise current control, but the implementation of a practical system can be difficult and complex. In this paper, Fuzzy based SVPWM controller was proposed. The proposed controller filter shows shorter response time and higher control precision. The simulation and experimental results also show that the new control method is not only easy to be calculated and implemented, but also very effective in reducing harmonics
II) Principal Of Operation
In each switching cycle the controller samples the supply current ia ,ib & ic are supply current is calculated with equation
-ia= ib + ic
The summation of three supply currents is zero .these three phase supply currents are measured & transformed into direct & quadrature axis components of two dimensional plane. The fundamental component of supply current transformed intod-q axis & supply current amplitude is generated by the fuzzy controller with Vdc & vref . reference value of dc bus voltage . In place of PI controllers with pulse width modulation approach two fuzzy controllers, With SVPWM scheme are employed -> i) For direct axis component another for quadrature axis component .The output two Fuzzy controller are equivalent to reference voltage vector .
By using Fourier magnitude block, voltage magnitude & angle is calculated. From the obtained signal, these values are fed to developed code & compare to the relative sequence .The generated switching actions are applied to & balancing of the filter takes place.
III) Hybrid Active Power Filter
The fig - shows the hybrid APF with non linear load consist of both active & Passive Filter .The Passive filter connected shunt with the distribution system and is tuned to present low impedance at a particular harmonic current. The shunt active passive filter takes a three phase voltage source inverter as the main circuit & uses capacitor C as the Voltage storage element on the dc side to maintain the DC side to maintain the DC bus voltage Vdc constant .
Three-phase balanced as shown below:
Xsa =Xs Sin(wt) (1)
Xsb = Xs Sin(wt-2Ï€/3 ) (2)
Xsc = Xs Sin(wt+2Ï€/3) (3)
Where Xs is the supply voltage or supply current
Eq (1),(2),(3) are the three -phase voltage [ Xsa Xsb Xsc ] a-b-c can be expressed as two-phase representation in d-q reference frame by Clark's transformation and it is given by eq (4)
Xs= = (4)
Fig:3.1. Hybrid active power filter construction for non linear load
Compensation principle:
Is
Laf Lpf
Vs Caf Cpf IL
Iinv
Equivalent circuit model
Zl
Zpf
Zaf
Zs Ish
Iaf Ipf Ilh
Vsh Iinv Vinv
Hybrid active power filter
Ish = ……………………(1)
Ilh = - Iaf ……………….(2)
IV) Control strategies of Hybrid Active Power Filter
a) Control block diagram of Fuzzy based SVPWM
Fig:4.1. shows a fuzzy logic controller has been designed to implement the space vector pulse width modulation for harmonic compensation. FLC is the best control method of currents . In each switching cycle the controller sample the supply currents ia ,ib & ic . these three phase supply currents are measured & transformed into synchronous dq axis reference frame .so that the fundamental component of supply currents is transformed into dc quantity in the dq axis & the supply current amplitude is is generated by the PI controller with Vdc and Vref ,the reference value of DC bus voltage .the obtained dq axis components will generate voltage command signal. voltage magnitude and angle is calculated from the obtained block.From block, these DC values are fed to developed code and compared to the repeating sequence . then time constant t1,t2, to on time of v1,v2 and v0 are calculated and the generated switching actions are applied to active power filter and inverter of active power filter will generate the desired compensating harmonics .The Active Power Filter inject an equal but opposite distortion harmonics back into the power line and cancel with the original distorted harmonics on the line.
Switching
gate
signals
S
V
P
WM
abc
dq
abc
dq
ia ia id id
ib
ic
iq
FLC
VDC
Vref
Fig: 4.1.Control block diagram of Fuzzy based SVPWM
V) FUZZY LOGIC CONTROLLER
Once the fuzzy controller were developed and incorporated into the simulated system, the simulation performances helped in the iteration of the controllers. The Fuzzy controller were fine tuned in several stages of that iteration.
Controller surface, where the horizontal axis are e and ce and the vertical axis is the output signal kp and ki . Fig: 4.1. and Fig 4.2.are the surface view that kp and ki are positive high when both e and ce are positive high. on the other hand ,when e and ce are negative high. The output signals are also negative high.
Fig : 4.1.Surface view of kp Fig: 4.2.Surface view of ki
Fig:4.3. Fuzzy controller as well as the PI -controller with a stepped speed command, as indicated. The responses are found to be essentially identical .The correspondence response for the ids loop indicates robustness, eliminating transient in the PI control.
DATA BASE
PI
CONTROLLER
DE
FUZZIFIER
RULE EVALUATOR ΔkI
FUZZIFIER e Δkp
ce Δkp
RULE BASE
Fig: 4.3.Block diagram of Fuzzy Logic Controller
The fuzzy control rule design involves defining rules that relates to the output model properties.For designing the control rule base for tuning ΔKp and ΔkI , the following important factors have been taken into account.
1) For large values of /e/, a large Δkp is required, and for small values of /e/,a small Δkp is required.
2) For e, ec >0, a large Δkp is required and for e, ec >0 a small Δkp is required.
3) For large values of /e/ and /ec/ , Δkp is set to zero, which can avoid control saturation.
4) For small values of /e/, Δkp is effective ,and Δkp is larger when /e/ is smaller ,which is better to decrease the steady state error .so the tuning rule of Δkp and ΔkI can be obtained.
Table: Adjusting parameters of ΔKp Table: Adjusting parameters of ΔkI
VI) SIMULATION RESULTS:
The proposed methods are implemented by simulation and simulation results are obtained by using MATLAB circuit design. simulation results of wave forms of source voltage ,load current and source currents Fig:6.1.shows wave form of fuzzy based SVPWM.
(a)
(b)
(c)
Fig: 6.1.Wave Forms Of Fuzzy SVPWM (a)Source Voltage, (b)Load Current and (c)Source Current
%/THD values at load current and source current wave forms are shown in below ,they re Fig:6.2.oad current harmonic spectrum for fuzzy based SVPWM ,Fig: 6.3.source current harmonic spectrum for fuzzy based SVPWM .
Fig:6.5. load current Harmonic representation for Fuzzy SVPWM
Fig:6.6. source current harmonic spectrum of Fuzzy SVPWM
VII) CONCLUSION
In this paper , a control methodology for the APF using FUZZY LOGIC CONTROL based SVPWM proposed. The performance of APF with these methods done in MATLAB / Simulink. Comparison of %THD and POWER FACTOR of WITH SVPWM and FLC based SVPWM.
% THD
Power factor
WITH SVPWM
2.67
0.9678
FLC BASED SVPWM
1.57
0.9718
VIII) REFERENCE
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