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Hysteresis current controller for harmonic reduction in grid connected Solar Pv system.

INTRODUCTION

Demand of renewable energy in India is increasing day by day due to less generation of power than demand and environmental concerns. Among several renewable sources of energy off-grid and grid connected solar photovoltaic system technology is highly available resources. It requires less maintenance and is capable of generating outputs from few microwatts to megawatts. The performance of grid-connected solar photovoltaic system (SPV) depends on many factor such as local climate, inverter efficiency, overall system losses, photovoltaic (PV) technology used, environmental parameters such as global irradiance and ambient temperature [3]. Grid connected photovoltaic power systems are energized by PV panels which are connected to the utility grid via an inverter can upload the excess energy to the grid. Integrating solar PV effects the functional operation of the power system network like load/frequency control, load following, unbalancing of voltage and current levels in the network and power quality issues including voltage disturbance, poor power factor, reactive power compensation flicker and harmonic distortions [7]. When integrating solar power into the grid harmonics produced due to use of power electronics converter. The application of power electronics devices such as arc furnaces, adjustable speed drives, computer power supplies etc. are some typical non-linear characteristic loads used in most of the industrial applications and are increasing rapidly due to technical improvements of semiconductor devices, digital controller and flexibility in controlling the power usage. The use of the above power electronic devices in power distribution system gives rise to harmonics and reactive power disturbances. The harmonics and reactive power cause a number of undesirable effects like heating, equipment damage and Electromagnetic Interference effects in the power system.

Harmonic distortion is one of the main power quality disturbances frequently encountered by the utilities. The harmonic disturbances in the power supply are caused by the non-linear characteristics of the loads. The harmonic currents flow through the power system impedance, causing voltage distortion. The distorted voltage waveform causes harmonic current to be drawn by other loads connected at the point of common coupling (PCC). The presence of harmonics leads to transformer heating, electromagnetic interference and solid state device malfunction. Hence, it is necessary to reduce the dominant harmonics below 5% as specified in IEEE 519-1992 harmonic standard. Active filter injects harmonic current into the grid connected PV system with the same amplitude but opposite phase than that of the load. The principal components of the active power filter are the Voltage Source Inverter (VSI), DC energy storage device, 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 compensation current in to the line [4].

II. Grid Connected Pv System:

A grid-connected photovoltaic system is a power generating solar photovoltaic system that is directly connected to inverter to grid. A grid-connected photovoltaic system consists of solar panels, inverter, power conditioning unit and grid connection equipment. In PV panels, solar cells are the basic components and it is made of silicon. A solar cell is generally a P-N junction which is made of silicon. Photons from the sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. The photons are absorbed by the semiconductor atoms, freeing electrons from the negative layer. This free electron finds its path through an external circuit toward the positive layer resulting in an electric current from the positive layer to the negative one. The PV cell circuit model consists of an current source in parallel with the diode as shown in figure 1. In this circuit, Iph is the photon generated current and Id is the shunt current through the diode.

[FIGURE 1 OMITTED]

The circuit above can be described by the following equations. The output current (I) is given by

I = [I.sub.ph] - [I.sub.d] (1)

Where

[I.sub.ph] -Current through PV array

[I.sub.d] -Diode current which is proportional to the saturation current

The Diode current Id can be written as

[I.sub.d] = [I.sub.0] [exp (V/[AN.sub.s][V.sub.t])-1 (2)

Where

V--Voltage imposed on the diode

[I.sub.0]--Reverse saturation current flow through the diode

[V.sub.t]--Thermal voltage

[N.sub.s]--Number of cells connected in series

A--Ideality factor

For grid connected system grid synchronization plays an important role. Phase locked loop (PLL) is used to synchronise the phase sequence and the frequency of the grid with the inverter. It is used to reduce the error between the output current and the reference current obtained from the controller. Phase Locked Loop is a feedback signal which locks the two input signal with same frequency and shifted in a single phase. It is used to compare two frequencies and results the input frequency is equal to the output frequency. Also it is used to provide rotational frequency at direct and quadrature components. At the point of common coupling (PCC) the abc components are transform into d q component and then force the q component to zero which is used to minimize the error.

III. Proposed Shunt Active Filter In Grid Connected Pv System:

The Active Power Filter consists of three principal parts, a three phase voltage source inverter, a DC side capacitor and the coupling inductance Lf. The capacitor is used to store energy and the inductance is used to smoothen the ripple present in the harmonics current injected by the active power filter. The schematic representation of a Shunt Active Filter connected in a three phase system feeding a non-linear load is shown in Figure 2. A three phase power supply with 415V, 50Hz, which is connected to a full bridge diode rectifier as the nonlinear load.

[FIGURE 2 OMITTED]

Due to the nonlinear characteristics of the diodes, the sinusoidal current waveform for the supply is distorted. The distorted current is fed into the reference current calculated portion to produce the desired compensated harmonics current which is the inversed of the original harmonics in the line. The reference current calculated algorithm is based on the instantaneous active and reactive theorem in time domain.

Voltages [V.sub.a], [V.sub.b], [V.sub.c] and current [I.sub.a], [I.sub.b], [I.sub.c] indicate the phase voltages and currents at the load side respectively. The active filter is connected in parallel with the load to suppress the harmonics. The shunt active filter generates the compensating currents [I.sub.fa], [I.sub.fb], [I.sub.fc] to compensate the load currents [I.sub.fa], [I.sub.fb], [I.sub.fc] so as to make the current drawn from the source as sinusoidal and balanced. The performance of the active filter mainly depends on the technique used to compute the reference current and the control strategy followed to inject the desired compensation current into the line.

a. Harmonics Extraction Technique:

Derivation of compensation current is the important part of active filter control. The time domain methods are mainly used due to its speed with less calculation compared to the frequency domain methods. Instantaneous p-q theory, one of the time domain methods is followed in this work. The details of instantaneous p-q theory are presented here.

The p-q theory is based on a-P the transformation, and is known as the Clarke Transformation. It transforms the three phase voltage and current into a-P stationary reference frame using the following equations:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

Since in a balanced three-phase three wire system, neutral current is zero, the zero sequence current does not exist. The power components p and q are related to the [alpha]-[beta] voltages and currents, and can be written as,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where p is the instantaneous real power and q is the instantaneous imaginary power. The instantaneous real power includes AC and DC values and can be expressed as follows:

P = [bar.p] + [??] (4)

Where [??] is the alternating value of the instantaneous real power which is exchanged between the source and load and [bar.P] is the average real power.

The alternating (AC) value of instantaneous real power is calculated back to a-b-c frame which represents the harmonic distortion, given as reference for the current controller. The mean (DC) value of the instantaneous real power is usually the only desirable power component. The other quantities have to be compensated using the shunt active filter. To calculate the reference compensation currents in the a-p coordinates, the expression 3 is inverted as given below,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Where [??] is the alternating value of the instantaneous real power which is exchanged between the source and load and q is the instantaneous imaginary power corresponding to the power that is exchanged between the phases of the load.

In order to obtain the reference compensation currents in the a-b-c coordinates the inverse of the transformation is applied to eqn 2 and is given as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

The eqn 6 gives the reference compensation current to be injected to the system.

b. Hysteresis Current Controller:

Hysteresis current controller derives the switching signals of the inverter power switches in a manner that reduces the current error. The switches are controlled asynchronously to ramp the current through the inductor up and down so that it follows the reference. The current ramping up and down between two limits is illustrated in Figure 3. When the current through the inductor exceeds the upper hysteresis limit a negative voltage is applied by the inverter to the inductor. This causes the current in the inductor to decrease. Once the current reaches the lower hysteresis limit a positive voltage is applied by the inverter to the inductor and this causes the current to increase and the cycle repeats.

[FIGURE 3 OMITTED]

The current controllers of the three phases are designed to operate independently. Hysteresis current controller determines the switching signals to the inverter. The switching logic for phase A is formulated as below,

If [i.sub.fa] < ([i.sub.fa]* -HB) upper switch (G1) is OFF and lower switch (G4) is ON

If [i.sub.fa] < ([i.sub.fa]* +HB) upper switch (G1) is ON and lower switch (G4) is OFF

Where, HB is the hysteresis band and G1, G4 is the gate switches illustrated in Figure 3. In the same fashion, the switching of phase B and C devices are derived. The Hysteresis limits relate directly to an offset from the reference signal and are referred to as the Lower Hysteresis Limit and the Upper Hysteresis Limit. The current is forced to stay within these limits even while the reference current is changing. Conventional hysteresis current control operates by comparing a current error between the reference and the current being injected by the inverter against fixed hysteresis bands. When the error exceeds the upper hysteresis band, the inverter output is switched low, and when the error falls below the lower hysteresis band, the inverter output switches high.

c. Voltage controller for DC voltage maintenance:

Another important task in the development of active filter is the maintenance of constant DC voltage across the capacitor connected to the inverter. This is necessary because there is energy loss due to conduction and switching power losses associated with the diodes and IGBTs of the inverter in active power filter, which tend to reduce the value of voltage across the DC capacitor. Generally, PI controller is used to control the DC bus voltage. The PI controller based approach requires precise mathematical model which is difficult to obtain. Also, it fails to perform satisfactorily under parameter variations, non-linearity, and load disturbances.

[FIGURE 4 OMITTED]

The controller adjusts the real power (Preg) requirement for voltage regulation to keep the constant voltage across the capacitor.

IV. Simulation Model:

[FIGURE 5 OMITTED]

The grid connected PV system simulation diagram is shown in the figure 5. In this simulation the non-linear load is connected in the grid connected PV system. The non-linear loads generate the harmonic current. By using of shunt active power filter to reduce the harmonic current. In this shunt active power filter is based on PQ theory calculations, PI controller, PWM techniques and Inverter circuits.

a. Simulation Result without Shunt Active Filter:

The harmonic current compensation is implemented in a grid connected PV system using a shunt active power filter. The rms value of grid voltage of the system is set as 415V and a combination of 3-phase universal bridge rectifier with an RLC load across it constitutes the nonlinear load which introduces the harmonics into the system.

[FIGURE 6 OMITTED]

Figures 6 present simulation using Matlab/Simulink for a grid connected PV system without a shunt active filter. And the Figures 7 present simulation results for a grid connected PV system without a shunt active filter. In this simulation model the circuit breaker is placed in the shunt active power filter. The circuit breaker is open condition the load current is without connected the filter current. The harmonics current is present on the load side. The waveform of harmonics in the load side is shown in the figure.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The grid connected PV system without shunt active filter are shown in Figures 7. It can be seen that the harmonic has severely disturbed the grid currents. Figure 8 shows the harmonic spectrum of grid current without shunt active filter. It can be found that the THD of grid current, load current and PV current are 19.58%, 15.79% and 1.06% respectly.

b. Simulation Results with Shunt Active Filter:

[FIGURE 9 OMITTED]

The figure 9 evaluate the Simulation model of grid connected PV system with shunt active filter using Matlab/Simulink and the Figure 10 presents a simulation results of grid connected PV system with shunt active filter. While the circuit breaker is closed the shunt active filter connected with circuit in order to reduce the harmonics in grid current.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Figure 11 shows the harmonic spectrum of grid current with shunt active filter. It can be seen that the harmonic reduced the grid current. It can be found that the THD of grid current, load current and PV current are 3.90%, 3.20% and 1.02% respectly.

The grid connected PV system are assumed to be balanced and sinusoidal. The DC side of the inverter is connected to a capacitor. The development of active filter is used to maintain constant DC voltage across the capacitor connected to the inverter. This is necessary because there is energy loss due to conduction and switching power losses associated with the diodes and IGBTs of the inverter in Active Power Filter, which tends to reduce the value of voltage across the DC capacitor. Generally, PI controller is used to control the DC bus voltage which is shown in figure 12.

[FIGURE 12 OMITTED]

In this work by using shunt active power filter, the output voltage of the inverter is controlled by changing the switching pulses. This causes a flow of instantaneous power into the inverter which charges/discharges the inverter DC bus capacitor. Despite the resultant DC bus voltage fluctuations, its average value remains constant in a loss less active filter. However, the converter losses and Active Power Filter exchange causes the capacitor voltage to vary. Hence the DC bus capacitor must be designed to achieve two goals, i.e., to comply with the minimum ripple requirement of the DC bus voltage and to limit the DC bus voltage variation during load transients. The THD value is the vector sum of all harmonics in the power system. The less value of THD is most important, it is clear that less harmonics and improve power quality.

From the results of various simulations and after the comparison of %THD for grid connected PV system with and without filter is tabulated in Table 1. The THD value is the vector sum of all harmonics in the power system. The less value of THD is most important, it is clear that less harmonics and improve power quality.

Conclusion:

The performance of the grid connected PV system with shunt active power filter is analyzed using Hysteresis Current Controller technique for minimizing harmonics, in the grid connected PV system. The p-q theory is used to generate reference current from the distorted load current, also PI controller is used to maintain the DC side voltage nearly constant. The performance of the Hysteresis Current Controller in shunt active power filter are verified with the simulation results. The simulation results show that the proposed technique is effective in current harmonic filtering. Further the proposed technique has quick response time and it keeps the good quality of filtering.

REFERENCES

[1.] Mosazadeh, S.Y., S.H. Fathi, A.R. Sheykholeslami, 2012. 'Adaptive Hysteresis Band Controlled Grid connected PV System With Active Filter Function' IEEE Conference on Power Engineering and Renewable Energy, pp:1-6.

[2.] Munirah Ayub, Chin Kim Gan, Aida Fazliana Abdul Kadir, 2014.'The Impact of Grid-Connected PV Systems on Harmonic Distortion', IEEE Innovative Smart Grid Technologies-Asia (ISGT ASIA). pp: 669-674.

[3.] Singh, Suresh, Rakesh Kumar and Vivek Vijay, 2014. "Performance analysis of 58 kW grid-connected roof-top solar PV system", Power India International Conference (PIICON).

[4.] Rathika Ponpandi, Devaraj Durairaj, 2011. "A Novel Fuzzy Adaptive Hysteresis Controller Based Three Phase Four Wire-Four Leg Shunt Active Filter for Harmonic and Reactive Power Compensation", Energy and Power Engineering, pp: 422-435.

[5.] Prakash K. Ray, Soumya R. Mohanty and NandKishor, 2013. 'Classification of Power Quality Disturbances Due to Environmental Characteristics in Distributed Generation System,' IEEE Transactions on sustainable energy, 4(2): 302-313.

[6.] Pratiksha Gupta, Surendra Kumar Tripathi, 2014. 'Analysis of Grid-Tied Hybrid Wind PV Generation System,' International Conference on Innovative Applications of Computational Intelligence on Power, Energy and Controls with their Impact on Humanity, pp: 447-451.

[7.] Maung Than Oo, G M Shafiullah and Alex Stojcevski, 2014. "Modelling and power quality analysis of a grid-connected solar PV system", Australasian Universities Power Engineering Conference (AUPEC).

[8.] Anita Choudhary and Prerna Gaur, 2015. "A Study of Hysteresis Band Current Control Scheme For Shunt Active Power Filter Used For Harmonics Mitigation" International Journal of Advanced Research In Computer Engineering & Technology (IJARCET) 4(6).

[9.] Dr. Rathika P. and Dr. D. Devaraj, 2010. "Artificial Intelligent Controller Based Three Phase Shunt Active Filter For Harmonic Reduction And Reactive Power Compensation", International Multi Conference For Engineers And Computer Scientists (IMECS-2010), 17-19.

[10.] Ekhlas Mhawi, Hamdan Daniyal and Mohd Herwan Sulaiman, 2015. "Advanced Techniques in Harmonic Suppression via Active Power Filter", A Review International Journal of Power Electronics and Drive System (IJPEDS).

(1) M. Jebastin and (2) Dr.P. Rathika

(1) PG Scholar, Department of EEE, V V College of Engineering, Tirunelveli, TamilNadu.

(2) Professor and Head, Department of EEE, V V College of Engineering, Tirunelveli, TamilNadu.

Received 27 May 2016; Accepted 28 June 2016; Available 12 July 2016

Address For Correspondence:

M. Jebastin, PG Scholar, Department of EEE, V V College of Engineering, Tirunelveli, TamilNadu.
Table 1: Performance Analysis

              THD%

Current(A)    Without Filter    With filter

Grid          19.58%            3.90%
PV            1.06%             1.02%
Load          15.79%            3.20%
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Author:Jebastin, M.; Rathika, P.
Publication:Advances in Natural and Applied Sciences
Date:Jun 30, 2016
Words:3198
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