Implementation of Single Phase Voltage Source Inverter Control Algorithm for Photovoltaic System

IJSRD - International Journal for Scientific Research & Development| Vol. 3, Issue 08, 2015 | ISSN (online): 2321-0613

Implementation of Single Phase Voltage Source Inverter Control Algorithm for Photovoltaic System Mrs.Kiruthiga.B1 Dr. Narmatha Banu.R2 1 Assistant Professor 2Professor 1,2 Department of Electrical Engineering 1,2 Velammal College of Engineering and Technology, Tamilnadu, India Abstract—This paper presents an implementation of single phase voltage source inverter (VSI) control algorithm for grid connected photovoltaic (PV) system. The maximum power point tracking (MPPT) is capable of extracting maximum power from PV array connected to DC link voltage level. The MPPT algorithm is solved by incremental conductance method (INC). Sinusoidal pulse width modulation (SPWM) uses a simple mapping to generate gate signals for the inverter. A new digital proportional resonant (PR) current control algorithm is used to remain the current injected into the grid sinusoidal and to achieve high dynamic performance with low total harmonic distortion (THD). The validity of the system is verified through MATLAB/Simulink and results are compared with proportional integral (PI) algorithm for PV system in terms of THD. Key words: Photovoltaic (PV) System, Maximum Power Point Tracking (MPPT), Voltage Source Inverter (VSI), Total Harmonic Distortion (THD) I. INTRODUCTION Renewable energy sources demand is increasing in these days as they derived the energy from sun either directly or indirectly and serving energy to humankind for years together. A small PV system provides enough AC electricity to power a single home. It has many applications for example in military and civilian Earth observation satellites, street lights, traffic signs, electric cars, solar powered tents and electric aircraft has integrated photovoltaic systems to provide a primary or auxiliary power source in the form of AC or DC power, depending on the design and power demands.

Current distortion is an important performance index for grid connected PV inverters [1], [2]. To mitigate lower order harmonics using a large output filter inductance is not a good strategy as it increases losses in the system along a larger fundamental voltage drop and with a higher cost. Hence the output current of inverter requires an advanced closed loop control to interact with utility.

Different methods of MPPT are discussed in [3] shows a very good result for incremental conductance algorithm is suitable for digital applications and less complex. MPPT generates duty ratio given as pulse to the IGBT in the boost converter. It is important to cope that the inverter output corresponds to the changes in dc input voltage, to regulate voltage of inverter and to satisfy the constant volts and frequency control requirement. It is difficult to design the inverter controller with proper gain to improve performance at the fundamental frequency. There are various techniques to vary the inverter gain. The most efficient method of controlling the output voltage is by using PWM control within the inverters [4]. The commonly used techniques are single pulse width modulation, multiple pulse width modulation, sinusoidal pulse width modulation,

modified pulse width modulation. The PWM control strategy used here is sinusoidal pulse width modulation.

This type of modulation is commonly used in industrial applications. The gating signals given to inverter are obtained by comparing a sinusoidal current reference with a triangular carrier wave. The frequency of the reference wave determines the output of the inverter frequency and the amplitude controls the modulation index and also depends of the rms value of the output voltage. When PWM control is used, maximum power can be realized to VSI [5].

Digitally PI control system can be implemented and it also offers many advantages over their analog counterparts. An important advantage offered by digital control is in the flexibility in the controller characteristics [6]. In the digital PI control method, the frequency of the ripples in inverter output current is the same as the frequency of triangular waveform that results to constant switching frequency operation. PI has a drawback that it has inability in tracking reference wave [7].

The power from MPPT block is fed as power reference to the input power control to obtain a nonzero average current reference [8]. The power reference keeps on changing due to fast MPPT action. Due to this a dc offset is generated in the inverter terminal voltage. The reference current is generated from the MPPT block and the sine of the unit vector from the phase locked loop (PLL) is given as input to current controller.

Different types of PLL structures for single phase grid connected inverters are discussed [9]. The function of PLL is to achieve unity power factor while operating that includes synchronization of the inverter output current with the grid voltage and to have sinusoidal current reference.

The amplitude and the frequency values of the grid voltage are monitored directly using PLL structure. A single phase PLL structure based on generalized integrator is discussed [10]-[12].

Resonant controller has its resonant peaks at the grid frequency for precise fundamental reference tracking and to some low order harmonic frequencies for selective harmonic compensation [13]. The resonant term of the controller can be used as generalized integrator.

Proportional resonant controller used for current regulation in grid tied system and the utility frequency experiences deviation from its nominal value. This controller runs over a larger band of frequency variations [14]. The resonant block of the PR controller is simple to implement. When a harmonic current is drawn from the supply it gives rise to a harmonic voltage drop proportional to the source impedance at the point of common coupling (PCC) and the current.

When harmonic frequency produced by non linear load coincides with natural frequency of system resonance occurs [15].

The photovoltaic system is considered for the application consists of a photovoltaic array, dc to dc boost

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Implementation of Single Phase Voltage Source Inverter Control Algorithm for Photovoltaic System (IJSRD/Vol. 3/Issue 08/2015/195)

converter, inverter, step up transformer, grid and controller.

The PV inverter system considered is simple. The topology of the model has advantages of good suitability for less rated photovoltaic inverters and reliability get improved since lesser amount of components are present. For solving MPPT, incremental conductance algorithm is used.

The controller taken into consideration is PI and PR controller. The system performance and the total harmonic distortion of both controllers are compared from the results obtained. Simulation results show that the behavior of PR is better than PI. The basic block diagram is shown in Figure 1.

Fig. 1: Basic block diagram of the system under consideration.

operates at a constant current source at low values of operating voltage and a constant voltage source at low values of operating current.

III. MAXIMUM POWER POINT TRACKING The MPPT has an important task to perform in PV energy conversion system that it continuously tunes the system to draw maximum power from the source regardless of weather or load conditions. Since the solar array has nonideal voltage -current characteristics and the conditions such as irradiance, ambient temperature, and wind that affect that output of the solar array are unpredictable, the tracker should deal with a nonlinear and time-varying system. If operating voltage is lesser or greater than maximum power then power curve slope lies at that point will be positive or negative. Maximum power point lies at the peak of the curve and hence has zero slopes.

Incremental conductance has many advantages over perturb and observe algorithm. Perturb and observe oscillates around the Maximum power point whereas incremental conductance can track rapidly the variations in irradiance conditions with high accuracy. The disadvantage of this method is it is more complex than perturb and observe and has more computational time. The flowchart of incremental conductance is shown in Figure 3.

II. MATHEMATICAL MODEL OF PV ARRAY In PV array model, a PV cell is represented by a current source in parallel with diode and a series resistance as shown in Figure 2. ( )+ , * (1) In this PV array, mathematical model can be expressed in Equation (1) that shows the non-linear output characteristics of solar cell, where is photo current, is reverse saturation current, is electron charge, is series resistance, is parallel resistance, is dimensionless factor, is Boltzmann constant and is temperature.

Fig. 2: Equivalent circuit of a PV cell model. ( ) ( ) (2) [ ( )] (3) and depends on the cell temperature and solar irradiation and these can be mathematically expressed as ( ) ( )] (4) ( )[ (

)

( (

) )

(5)

where is short circuit current depends linearly on cell temperature, is standard irradiation (1000 W/m2), is temperature coefficient, is standard temperature (298ËšK) and is the thermal voltage. The cell

Fig. 3: Flowchart of Incremental conductance.

IV. INVERTER CURRENT CONTROL A. PI Controller The PI controller is used conventionally for generating reference waveform. PI controller could not able to eliminate the steady state error at the fundamental frequency.

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Implementation of Single Phase Voltage Source Inverter Control Algorithm for Photovoltaic System (IJSRD/Vol. 3/Issue 08/2015/195)

The transfer function of PI controller has a proportional and an integral component.

(6) The bode plot are obtained for open loop transfer function as in Equation 6 are shown in Figure 4.1. The sinusoidal reference could not be tracked properly by the control structure of conventional PI controller in order to for providing a good dynamic response when a gird voltage feed – forward is used.

wider band. Thus the parameters of the controller can be tuned according to the needs. From the results of bode plots, for proper tuning the resonant term gain must be large enough to remove steady state error. The proportional term decides the stability of the system and reduces harmonics in current at other frequencies. The resonant term decides the resonant frequency only at which the controller can track its reference value and it determines the bandwidth at which it must be operated.

PR controller is used to generate the inverter voltage reference. The PR controller is so that the output current of the system does not contain any dc offset. This is necessary because the presence of a dc offset would result in flow of even harmonics. If PR controller the dc offset will propagate through the system and the inverter terminal voltage will have a non zero average value.

V. SIMULATION RESULTS

Fig. 4: Bode plot obtained for open loop transfer function of PI controller B. PR Controller At the selected frequency to be operated, proportional resonant (PR) controller introduces a infinite gain at that resonant frequency to eliminate steady state error. The resonant term has presence of double integrator that forces the dc steady state error to zero. The PR controller is a combination of proportional term and resonant term.

The transfer function of PR controller is (7) Bode Diagram

Fig. 6: Current reference waveform for a step change in power reference.

300

kr=100

200

(Voltage(volts)

kr=10 kr=1 100

Phase (deg)

0 90

100 0 -100 -200

45

-300

0

0.05

0

0.1 Time(sec)

0.15

0.2

200

-45 -90 1

10

2 3 Frequency (rad/sec) 10 10

4

10

Fig. 5: Bode plot obtained for PR controller for different values of integral gain in the resonant term From the result of bode plot of PR controller for open loop transfer function as in Equation 7 is shown in Figure 4.2 for parameters =1,ω=100π and =1, 10 and 100 shows that the width of the frequency band depends on resonant constant. Increase in the resonant term leads to

urrent(ampere)

Magnitude (dB)

300

200

The general structure of single phase grid connected PV inverter is depicted in Simulink environment to obtain a simulation model performed in MATLAB. The solar panel is connected to grid through two arm four pulse single phase inverter to produce an inverted output voltage from dc voltage. The inverter output voltage is controlled by using SPWM technique. The current reference is generated by multiplying unit sine vector from phase locked loop with computed reference from PV power. Figure 5 shows the variation in current reference to step change in power reference.

100 0 -100 -200

0

0.02

0.04

0.06

0.08 0.1 0.12 Time(sec)

0.14

0.16

0.18

0.2

Fig. 7: Inverter response of voltage and current with PI controller.

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Implementation of Single Phase Voltage Source Inverter Control Algorithm for Photovoltaic System (IJSRD/Vol. 3/Issue 08/2015/195)

The inverter output current is fed as feedback signal and the error is obtained from difference of current reference and inverter output current. The error is given as input to current controller block. The output signal from controller is compared with triangular wave for generating gating pulses to the inverter. The inverter current is synchronized with grid voltage using a closed loop control technique that has MPPT, single phase PLL, current controller and comparator. MPPT is used for power reference and also to provide duty ratio command to the switch in boost converter, single phase PLL produces a in phase unit vector of the grid voltage and a comparator for performing pulse width modulation.

300

(Voltage(volts)

200 100 0 -100 -200 -300

0

0.05

0.1 Time(sec)

0.15

0.2

150

Current(ampere)

100 50 0 -50

Fig. 10: Inverter current harmonic spectrum for PR controller.

Figure 5.4 show the harmonic spectrum of inverter current waveform in Figure 5.2. The summary of total harmonic distortion (THD) is given in Table I for current waveforms shown in Figure 5.3 and Figure 5.4. The harmonic distortion with PI controller is 5.90% that is declined to 3.98% when using with PR controller. In Table I the THD in current is brought to less than 5% when provided with PR controller. The proposed controller improves inverter response to be within limits of distortion.

The wave shape is improved due to the attenuation of harmonic content.

With PI With PR Quantity controller controller Inverter current 5.90% 3.98% THD Table 1: Comparison of inverter current thd

-100

VI. CONCLUSION -150

0

0.02

0.04

0.06

0.08 0.1 0.12 Time(sec)

0.14

0.16

0.18

0.2

Fig. 8: Inverter response of voltage and current with PR controller.

The inverter response to current reference is obtained using simulation. The inverter response of both voltage and current waveforms with PI controller is shown in Figure 5.1 and with PR controller is shown in Figure 5.2.

Comparing Figure 5.1 and Figure 5.2, it is seen that PR can track reference sine wave easier than PI. The harmonic spectrum of inverter current waveform in Figure 5 is shown in Figure 5.3

In this paper, a single phase VSI for grid connected PV system is presented with PI and PR control technique and the results are discussed. The configuration for the proposed system was designed and simulated using MATLAB/Simulink. The proposed inverter control scheme futures several advantages such as generation of high quality currents and the capacity to operate at lower switching frequency. This control scheme is simulated digitally with grid. The simulation result for PI shows that 5.90% of THD that get reduced to 3.98% while using PR control scheme.

The result is suitable for inverter applications such as PV generation systems with grid connected.

REFERENCES

Fig. 9: Inverter current harmonic spectrum for PI controller.

[1] S.B. Kjaer, J.K.Pedersen and F. Blaabjerg. “A review of single –phase grid-connected inverters for photovoltaic modules,” IEEE Trans. Ind. Appl., vol.41, no.5, pp.1292-1306, Sep./Oct. 2005.

[2] M. Calais, J. Myrik, T. Spooner, and V.G. Agelidis, “ Inverters for single-phase grid connected photovoltaic systems-an overview,” in Proc. of PESC, vol.4, pp.1995-2000,2006.

[3] T. Esram and P.L. Champman, “Comparison of photovoltaic array maximum power point tracking techniques,” IEEE Trans. Energy Conver., vol. 22, no.2, pp. 439-449, Jun. 2007.

[4] D.N.Zmood and D.G.Holmes, “Stationary frame current regulation of PWM inverters with zero steady-state error,” IEEE Trans. Power Electron., vol.18, no.3, pp.814-822, May 2003.

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Implementation of Single Phase Voltage Source Inverter Control Algorithm for Photovoltaic System (IJSRD/Vol. 3/Issue 08/2015/195)

[5] V.Verma, A.Kumar, “Grid connected single phase rooftop PV system with limited reactive power supply,” Proc. of ICPEC, pp.39-43, 2013.

[6] J.Selvaraj, N.A.Rahim, C.Krismadinata, “Digital PI Current Control for Grid Connected PV Inverter,” IEEE Trans on power Electronic, pp.742-746, 2008.

[7] P.Sivaraman, R.Desanayagi, “Performance analysis of PV fed single phase T-source inverter,” Proc. of ICPEC, pp.266-271, 2013.

[8] M.Ciobotaru, R.Teodorescu and F.Blaabjerg, “Control of single –stage single-phase PV inverter,” EPE Journal, vol.16, no.3, pp.20-26, 2006.

[9] M.Ciobotaru, R.Teodorescu and F. Blaabjerg, “Improved PLL structures for single-phase grid inverters,” Proc. of PELINCEC, pp.6, 2005.

[10] M.Ciobotaru, T.Kerekes, R.Teodorescu and A.Bouscayrol, “PV inverter simulation using MATLAB/Simulink graphical environment and PLECS blockset,” Proc. of IECON, pp.5313-5318, 2006.

[11] M.Ciobotaru, T.Kerekes, R.Teodorescu and A.Bouscayrol, “A new single phase PLL structure based on second order generalized integrator,” Proc. of PESC, pp.1-6, 2006.

[12] X.Yuan, W.Merk, H.Stemmler and J.Allmeling, “Stationary frame generalized integrators for current harmonics of concern under unbalanced and distorted operating conditions. IEEE Trans. Ind., vol.38, pp.523 – 532, 2006.

[13] R.Teodorsescu, F.Blaabjerg, M.Liserre and P.C.Loh, “Proportional–resonant controllers and filers for grid – connected voltage–source converters,” IEE Proc. Electr.

Power Appl., 153, no.5, September 2006.

[14] A.V.Timbus, M.Ciobotaru, R.Teodoresu, and F.Blaabjerg, “Adaptive resonant controller for gridconnected converters in distributed power generation systems,” Proc. of APEC, pp.6, 2006.

[15] J.A.Orr, and A.E. Emanuel,“On the need for strict second harmonic limits,” IEEE Trans Power Del., vol.15, no.3, pp.967-971, Jul. 2000.

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