Dc Side Ripple Voltage Analysis and Fuzzy Pi Control Strategy Research for Apf

Owing to the problems of DC side capacitor value choice in the application of Active Power Filter (APF), this article presents a technique based on the analysis of the ripple voltage in DC side by graining appropriately the undulation control rate within the range of settings and control DC voltage so that the voltage fluctuation rate can reach minimum value under the condition of specific capacitance which is aimed at getting good compensation effects. In order to achieve the stability of the DC side capacitor voltage control, we introduce a new technique on the DC output voltage deviation square values of adaptive fuzzy PI control. Meanwhile, a Space Vector Pulse Width Modulation (SVPWM) is used to conduct optimization design for the DC side voltage waveform. The simulation results show that using this method reduces the DC voltage ripple and achieves steady DC output voltage.


INTRODUCTION
Active Power Filter (APF) is highly controllable and responsive.It can not only compensate harmonics but also inhibit flicker and react power compensation [1,2].However, due to time-varying in compensation current and the inverter's own consumption as well as its failure, an appropriate control measures need to be taken to avoid DC capacitor voltage decay or occurrence of large fluctuations ,not all but also, accompanying large number of ripples which may occur in APF [3,4].In fact, stability of voltage across the capacitor in DC side of APF has significant impact on APF compensation performance [5][6][7].Therefore its ripple voltage should be controlled carefully within permissible range.Several factors have impacts on the ripple voltage such as the frequency of compensation current, effective value of source voltage, DC voltage across the capacitor and so on.In most cases, the capacitance needs to be determined according to the range of ripple voltages.After reading a lot of the relevant literature, we find few people research on the ripple voltage on the DC side.For example, in the literature [8][9][10][11][12], the authors mentioned it but they did not specify a solution.
In this paper we put forward a fuzzy-PI control of a new DC-side output voltage deviation squared value which uses the DC voltage deviation squared value !U 2 dc as the input of fuzzy PI control.This method has the following advantages: • Effectively reduces the no-load current • Improves on both ends of the dc side voltage rise time and • Improves the dynamic characteristic of the system In addition to SVPWM pulse width modulation, a lot of reviewed literature mostly adopted SPWM modulation or PWM adjustment.But however, this paper verifies the latter by simulation and experiment.

THE ANALYSIS OF APF DC SIDE RIPPLE VOLT-AGE
Fig.
(1) depicts a system block diagram of the APF in which e a , e b , and e c are APF access phase supply voltage grids; i a , i b and i c are three-phase power supply currents; i ca , i cb and i cc are the required compensation currents; i la , i lb and i lc are the load currents; C is the DC side capacitance; u dc and i dc are the voltage and current flowing through capacitor respectively, and L is a inductor [13][14][15].Three-phase shunt active filter APF is obtained from Fig. (1) and the AC side voltage is three-phase current equation: Set k a , k b and k c as the inverter switching functions and Wherein a, b, c are three inverter bridge arm APF.Bridge work alternately open arms off, but the steady-state operation can be APF inverter input voltage and current of the DC side, which is expressed as: Putting equations ( 1) and (2) into equation (3), the DC side current can be expressed as: The formula: Considering the DC capacitor voltage and current relationship, we have: Where W can be taken as an arbitrary constant which in- dicates the initial energy stored in DC capacitor.
As seen from equation ( 5), 2W c is the constant term is the integral term and a function of time representing the DC capacitor voltage fluctuations.When the three-phase grid voltage is balanced and without distortion, we have: ( ) Regardless of the compensation current is symmetrical or not, compensation current can be decomposed into: The formula: n=1,3,5,7,9,11,13,15,17,19。 i an + , i bn + , i cn + , i an !, i bn !, i cn !, i an 0 , i bn 0 , i cn 0 shows the corresponding phase sequence, n denotes harmonic currents of positive sequence components and negative sequence components and zero sequence components.
Joining equations ( 5) and ( 6), vertical DC side ripple voltage can be obtained as follows: Where: Assuming that compensated nonlinear load is a large inductive load on the uncontrolled rectifier bridge of the DC side, then the phase current on dc side is given by: The formula: d I is ac side inductor current.Let , here for 20 times less harmonic compensation will yield: Collating from equation ( 10), we have: Where ( ) Thus, it can be seen from the aforementioned equation that under certain conditions, a proper control of the capacitance of DC link voltage can rate the smaller the value of the voltage fluctuation, hence resulting in better compensation effect of APF.

THE DESIGN OF DC-SIDE OUTPUT VOLTAGE DEVIATION SQUARED VALUE OF FUZZY-PI CON-TROLLER
By grain section of the DC side voltage undulation rate equation, based on the DC side voltage ripple and analysis of energy conversion, admiral general traditional DC side voltage deviation value dc U Δ as the input amount of PI control will maintain the stability of the DC side voltage.But this method is obviously insufficient, in the process of control since it cannot effectively reduce the no-load current of APF.No-load current will influence the final compensation effect and at the same time, voltage rise time on both ends of the dc side will be slow.Further, it will have negative effect of the dynamic characteristics.Therefore taken the above supposition into consideration, the deficiency of the DC side voltage set point is determined and stability control must be put in place to ensure that the compensation of the shunt active filter effect and also, to ensure its safe and reliable operation.Here, DC voltage deviation squared value is used as the input signal from the controller.The calculation process is shown in equation 13 [16,17], Where U ref is as a given ref- After the square of the voltage deviation is determined, its value will be used as inputs fuzzy-PI control, as shown in Fig. (2).
The design method of fuzzy controller is to use the voltage deviation squared value as the error E and the rate of change is EC.Assuming that !K p and !K i are the fuzzy PI controller parameters derived from the correction as a comparative PI controller integral parameter correction [18,19].Taking E and EC as the inputs, !K p and !K i as outputs, selected input and output variables of the fuzzy language domain is taken as [-6,6].In taking seven languages on the domain variables namely; NB, NM, NS, ZE, PS, PM, PB, representing negative big, negative middle, and negative small, zero *, positive small，positive middle, positive big.Again, considering the coverage on the theory of domain and sensitivity, stability and robustness principle, the language of the fuzzy language variables values NB and PB will depict sshaped membership function curve whereas other variable language values will use triangular membership function curve.!K p and !K i of the membership function curve is shown in Fig. (3).
Enter the amount in accordance with the relationship between output, develop and tune parameters of fuzzy control rules as is shown in Table Where: !K p and !K i are the original setting of good PI pa- rameters, !K p and !K i are the outputs of Fuzzy controller.Traditional closed loop PI control rely too much on precision of mathematical model of the system, poor robustness.Therefore, in the complex system control system, it is difficult to achieve satisfactory effect.Fuzzy control is to make up for the deficiencies of the original control strategy which will make the system achieve stability quickly thereby reducing the overshoot and switching load current shock and also, keep the integral part of PI control's ability to solve system steady-state error.Fig. (3) shows the current module of system instructions.
In Fig. (3), i ah , i bh and ch i are harmonic currents by i p ! i q detection method which is compensated by generating a PWM pulse signal generated by the voltage control signal to control the opening of the switching device off.General ap-  proach is to use a triangular carrier control and hysteresis control.When triangular carrier control following error is large, the switching loss is also large.Hysteresis control will result in a larger pulse, so in order to reduce the DC voltage ripple, SVPWM modulation strategy is used.Space vector pulse width modulation (SVPWM) in the APF is a very effective PWM control techniques [20][21][22].Space vector pulse width modulation voltage space vector contains six equal amplitudes ( 2U dc 3 ), Phase difference 60 o electrical angle of zero vector U 1 !U 6 .This six vectors in the complex plane are divided into six sectors, located in the center of the complex plane amplitude is zeros' zero vector 0 O and 7 O .The use of a linear combination of SVPWM control strategy to control switch between two adjacent nonzero vector ( U 60 and U 120 ) and zero vector ( O 0 and O 7 ) make each control period to approximate the continuous rotation of the reference vector U ref [23-25].
When the reaction time of two nonzero vector synthesis output vectors are equal and the role of time zero vector is zero, the space vector pulse width modulation method can achieve the minimum DC voltage utilization.At any time of Active Power Filter output, vector angle and amplitude of the output voltage is determined by the power grid voltage drop compensation loop vector and the vector sum of the decision [26,27].Therefore, any time DC voltage utilization gets the most hours, minimum value can be determined to meet the required DC side capacitor in order to reduce the DC voltage ripple.Fig. ( 4) is a diagram for the simulation model.

SIMULATION STUDY
In order to verify whether the above methods are reasonable, we use MATLAB / SIMULINK to simulate it.Threephase voltage supply of circuit simulation is 380V/50Hz, non-linear load resistance R=30 !,L=10e-3H, APF module AC side resistance R=0.02 !,L=2mH,inverter DC capacitor value is C=6800uF,given voltage value ref U =600V.
For comparison control effect, we set switch module in   After simulation, it was found out that there is obvious current ripple in the use of simple and conventional PI controller.When the square of the voltage deviations is used as conventional controller, the DC voltage ripple is reduced significantly, but it still exists.However, when square of the voltage deviations is used as the fuzzy PI controller, it was ascertained that the voltage is equal to the given voltage and the ripple is greatly reduced.
In order to better illustrate the simulation results, the capacitance values selected here were 10000uF and 4700uF to compare the simulation results, as is shown in Figs . (6 and  7).
The simulation results show that when the capacitor value is selected 4700uF, DC voltage fluctuation rate is large.Measured the data from the simulation diagram, the voltage value is from 590.8v to 619.2v by using traditional PI method, it is from 600.4v to 610.7v by adopting the deviation of traditional PI method.The voltage value from 640V decreased to 590V after reaching the given voltage value 600V if by using the deviation of fuzzy PI method.Obviously, the capacitance value is too small to lead to the waveform is very unstable.When 10000uF of the capacitance value is selected, DC voltage ripple is small, maximum ripple voltage is 610v by using traditional PI method whilst maximum ripple voltage is 603v by adopting the deviation of traditional PI method.When using the deviation of fuzzy PI method, the ripple voltage and the impulse voltage disappear and the voltage value is directly proportional to the given voltage 600v.According to the simulation results and considering the actual application, choosing 6800uf DC voltage for capacitor, the voltage values are faster, tend to be stable and lead to elimination of ripple components.

EXPERIMENTAL VERIFICATION
The experiment is based on 380V/66KVA APF prototype.Experimental conditions: Three-phase AC voltage is 380V/50Hz，the IGBT switching frequency is 6.4 kHz, nonlinear load resistance is 30 Ω, inductance value is 10mH,   inverter dc side capacitor value is 6800uF, DC side given voltage value is 650V.Control links adopt the deviation of Fuzzy -PI regulating method, harmonic detection algorithm uses the p q i i − detection method based on the theory of in- stantaneous reactive power and the current tracking control uses modulation method based on space vector pulse width.Three-phase shunt APF control system block diagram is shown in Fig. (8).Three-phase load current, bridge arm current and DC side voltage signal through the sensor meas-urement circuit into A/D conversion chip to sample, sampling chip uses the AD574A.This is experimented by using deviation Fuzzy -PI control method to reduce the DC voltage ripple.
As shown in Fig. (9) for the physical prototype and nonlinear load harmonic source.

CONCLUSION
Through MATLAB simulation and APF prototype experiment, it can be found out that DC capacitor directly affects the efficiency of the APF but current measurements are influenced by the DC side capacitor ripple voltage.In addition, Adopting DC-side output voltage deviation squared value of fuzzy-PI control method as voltage control and also, adopting SVPWM as pulse width modulation can effectively solve the problem of DC side ripple voltage and achieve the purpose of the stability of the desired dc side voltage.

.
Through the above analysis available DC voltage fluctuation rate is expressed as:
Fig. (3), so as to switch between fuzzy PI control and simple PI control.Simulation results Pictured DC voltage value U dc , where figure (a) is a direct result of the simulation PI con-

Fig. ( 10
Fig. (10) depicts the supply voltage waveforms (blue line) and DC side capacitor voltage waveform (yellow line) comparison chart.

Fig
Fig. (10a) is the traditional PI control method.The waveform tends not to be smooth due to the present of ripple.

Fig
Fig. (10b) is the deviation of Fuzzy -PI control.The waveform is improved in full compliance with the system to eliminate the DC voltage ripple requirements.As seen from the experimental results, in the use of voltage deviation square values of Fuzzy -PI controller, DC side voltage ripple is very small and almost stable.The stable voltage waveform is to achieve a given voltage 650v.Adopting the square voltage deviation value as the input signal of fuzzy PI controller-PI controller can effectively improve the DC side voltage ripple.

Table 2 .
After a fuzzy controller in real time, calculate 1 and p K Δ and i K Δ .Using Center of gravity approach for solving Fuzzy algorithm, the actual value p K Δ and i K Δ after the operation of Fuzzy solution is used to modify the traditional parameters as indicated in equation (14):

Table 1 .
p K Δ fuzzy control rules.