A PV powered shunt active power filter for power quality improvement

1 Introduction

Over the past few decades,power demand rose tremendously,leading decision makers to look for other power resources such as renewables.Mainly,solar energy has been known a growing market especially through direct conversion into electricity.The main objective was to power up remote areas or sustain weak power grid.Nowadays,other applications are introduced giving these renewable energies more importance and attractiveness.

The use of power electronics systems in all ways of life has put the burden on power grids in terms of power quality and standards satisfaction.Most receivers are nonlinear loads,leading therefore to a tremendous harmonic currents which could cause equipment overheating,an increase of line losses,and distribution transformers saturation and may interfere with adjacent communication systems.If the load or the power system voltages are unbalanced,the situation worsens [1].In order to fix these problems,active power filters are widely used to compensate for harmonics injection in power transmission/distribution systems [2].

In literature,different topologies of shunt active power filters have proven their effectiveness in various applications where a variety of control strategies were implemented such as instantaneous active and reactive power control [3]and direct power control [4-5].The latter is based on direct control of power introduced by Noguchi et al [6].This technique presents several advantages such as simple control implementation and robustness.

Of course,the main purpose of all these techniques is to compensate for reactive power and can also mitigate harmonics and distortions.

In the present study,the inverter utilization is two folds:first,it injects active power to the grid; secondly it compensates line current harmonics,thus reduces the cost of shunt active power filter [4].Beside that,since the output of solar arrays vary with solar radiation and climate conditions,a maximum power point tracking algorithm is implemented on a DC/DC converter to enforce PV arrays to run at the maximum power point.

Thus,a simulation of direct power control is suggested for controlling a shunt active power filter,in order to improve the power quality,compensate reactive power and mitigate harmonics and distortion,and inject additional active power to the grid.The DC link capacitor connected to the inverter,charges and discharges through the grid.A PI controller is used to keep its terminal voltage near its reference value of active power.In addition,instead of using the well-known tracking techniques mainly perturb & observe [7]and incremental conductance [8],a new artificial intelligence method called fuzzy logic control is used to guarantee the maximum power point tracking

In overall,and from control strategy point of view,one can assume that there are two nested loops.The APF is the inner one and the PV chopper is the outer loop.This will reflect the adequate chopping frequencies choices.In case of PV operation with the APF,the frequency of the PV Boost converter would be lower than the frequency of the APF.

2 System Description

The structure in Fig.1 describes the general configuration of the proposed system,it contains a three-phase grid connecteds nonlinear load which includes a diode rectifier feeding a resistive load.

An inverter,connected to the grid acts as a shunt active power filter.The dc link capacitor is associated to a photovoltaic array through the boost converter.

Fig.1 Overall configuration of the proposed system

3 Shunt active power filter

3.1 Direct power control

Direct power control approach was proposed by Noguchi [6],and this strategy was inspired from the direct control of the torque applied in electrical machine.This method is based on the instantaneous active and reactive power dp and dq,and angular position control of the source voltage vector,therefore the selection of the optimum switching state is made from the switching table.

The dc side is regulated in order to get the reference value of the active power pref,whereas the reactive power reference qref is imposed equal to zero,as shown in Fig.2.

3.2 Regulation of dc link capacitor

To maintain the dc bus voltage fixed and near its reference value,a PI controller with an anti-windup configuration is used [9],as in Fig.3.

In order to determine the PI parameters (kp,ki) the subsequent transfer function is written in the following form:

Fig.2 Direct power control strategy

From (1),the relation between Vdc and Vdc_ref is a second order transfer function in the form:

Fig.3 Controller block diagram

3.3 Instantaneous power calculation

The instantaneous active and reactive powers are obtained by calculating ps(t) and qs(t) [3-4],after measurement of the source currents and voltages:

3.4 Selector selection

Based on instantaneous errors dp,dp phase angle θ,given by (5)-(7),a switching table is used to select the switching states Sa,Sb and Sc of the inverter.

Fig.4 Sectors on α,β stationary coordinates

4 Photovoltaic system

4.1 Modelling of the photovoltaic panel

The one diode equivalent circuit of the photovoltaic cell is adopted,and is as shown in Fig.5

Fig.5 PV cell modeled as a diode circuit

According to the above circuit,one can write:

where:

Ipv:PV array current (A).

Vpv:PV array voltage (V).

Table1 Switching Sequences

dp dq θ1 θ2 θ3 θ4 θ5 θ6 θ7 θ8 θ9 θ10 θ11 θ12 1 0 101 111 100 000 110 111 010 000 011 111 001 000 1 110 111 010 000 011 111 001 000 101 111 100 000 0 0 101 100 100 110 110 010 010 011 011 001 001 101 1 100 110 110 010 010 011 011 001 001 101 101 100

ISH :Current flowing through the resistor Rsh.

ID :Current in the diode D.

q :Electron charge=1.6*10-9C.

k :Boltzmann constant.

n :Ideality factor of the diode it is between 1 and 2.

T :Operating temperature of the PV cell.

Usually,the value of Rsh is very large and that of Rs is very small,hence they may be ignored in order to simplify the analysis.

The PV panel under study includes 3 series connected solar panels which are characterized by the following parameters:

Table2 PV panel Electrical parameters

Maximum power Pmax 55 w Maximum power point voltage VMPP 17.8 v Open circuit voltage VOC 21.7 v Maximum power point current Impp 3.1 A Short circuit current ISC 3.2 A

4.2 Fuzzy logic MPPT controller

Fuzzy logic control,widly used in various applications,depends mainly on expressing the operational laws governing a system in linguistic terms instead of mathematical equations [9,10].

The proposed control law is shown in Fig.6,and consists of three stages:fuzzification,rule evaluation,and defuzzification.

The inputs,e and Δe,and the output du are designed by the trapezoidal and triangular methods respectively as shown in Fig.7.accordingly,they are given by:

Fig.6 Principal of fuzzy logic MPPT controller

Table3 Rules bases in the fuzzy logic controller

e Δe NB NM NS ZE PS PM PB NB NG NG NGM NM NMP NP ZE NM NG NGM NM NGM NP ZE PP NS NGM NM NMP NP ZE PP PMP ZE NM NMP NP ZE PP PMP PM PS NMP NP ZE PP PMP PM PMG PM NP ZE PP PMP PM PMG PG PB ZE PP PMP PM PMG PG PG

5 Simulation result and discussion

In order to verify the performance of the proposed control scheme which involves the shunt active power filter associated with the photovoltaic system,the simulation is carried out using MATLAB/Simulink under various climate conditions.

Different tests were performed at various stages,before and after filtering as well as for active power injection into the main supply from the PV source through the shunt active power filter.

Fig.7 Fuzzy input and output membership

Firstly,before operating the filter,the source current is not sinusoidal and contains harmonics because of the nonlinear load,for example,the three phase diode rectifier,Fig.8 (f) and (g).

Fig.8 Simulation results under varying solar radiation

At t= 0.05 s,the filter is activated.It is shown that the source current becomes sinusoidal.Also,reactive power,replicated by the injected harmonic current,is sent back to the three phase system.The dc capacitor stabilizes near its power reference; this reflects the efficiency of the controller,Fig.8 (c) to (g).

At 0.3 seconds,solar power generation started,at two different insolation levels.Of course,the active power increases and decreases accordingly.The source current amplitude follows inversely these variations.The higher the solar radiation,the higher the active solar power produced,the higher contribution of the solar source to power the load,the lower main source power supply to the load.This proves that the power is injected by the PV system,Fig.8 a to g.

The PV system gives its maximum power point when the illumination becomes 1000 w/m2,thus,the fuzzy logic control tracks the maximum power point and the overall results show the effectiveness of the proposed control scheme.

Fig.9 (a) DC bus voltage (b) Injected current

The judicious choice of the chopping frequency can be verified through the figure bellow,Fig.9 (a) and (b).Fig.9 (a) illustrates the voltage at the output of the PV boost converter and Fig.9 (b) illustrates the injected current of the inverter.The commutation frequency is noticeably shown.

6 Conclusion

The simulation results show that the proposed control method combining a DPC for the active power filter associated with a fuzzy logic controller for MPPT has proven its effectiveness and robustness to accurately compensate reactive power and inject solar power available at its maximal value.This choice will reduce the burden of bulky configuration in view of a twofold action of the active filter which acts as a filter by injecting required harmonic currents to the main supply and as an inverter by injecting available DC solar power.