Design and Implementation of Micro-inverter for Photovoltaic Application

Transcription

1 Volume 118 No ISSN: (on-line version) url: Design and Implementation of Micro-inverter for Photovoltaic Application Seeranga Nandhini.S, Dr.R.Seyezhai, Sowmya.V, Ms.D.Umarani Department of Electrical and Electronics SSN College of Engineering Chennai, Tamilnadu May 10, 2018 Abstract The objective of this work is to implement a two stage micro-inverter with reduced Total Harmonic Distortion(THD) and increased efficiency. The first stage consists of a DC- DC boost converter, followed by the DC-AC inverter stage. A typical solar panel converts only 30 to 40 percent of the incident solar irradiation into electrical energy. To increase the conversion efficiency, Maximum Power Point Tracking(MPPT) algorithms (Perturb & Observe, Incremental Conductance) have been simulated in PSIM software and better tracking efficiency was observed with the Incremental Conductance algorithm. To reduce the harmonics of the output voltage of the inverter in the second stage, different inverter topologies namely H5, HERIC and push-pull have been simulated and H5 is found to have higher efficiency 1

2 and lesser distortion. In order to control the switching pattern of the inverter, the pulse width modulation strategies such as Unipolar, Bipolar, Sawtooth and Inverted Sine have been analysed and the micro-inverter with lesser harmonic distortion have been suggested for power quality improvement. Simulation studies are carried out in PSIM software and the results are validated. Key Words:Boost converter, H5, HERIC, Push-pull, P&O, InC, micro-inverter 1 Introduction The generation of electric power from solar PV has gained momentum and is emerging as the most useful form of renewable energy sources. It is fast growing due to lesser complexity in implementation, relatively high efficiency and less maintenance. Among the various photovoltaic inverters, the micro-inverter[1] is a low power inverter with a rating up to 350W. In the micro-inverter system, every panel has its own inverter and the output of each can be directly connected to the grid. The individual units are termed as micro-inverter. Each panel can operate at its MPP, thereby increasing the power output of the entire system. The micro-inverter is gaining popularity in residential application with advantages such as higher efficiency, reliability and safety. The work focuses on implementing a two-stage microinverter with lesser harmonics and increased efficiency. Two-stage microinverter comprises of DC-DC converter in the first stage and a DC-AC converter in the second stage. The output of the solar panel is fed as input to the boost converter to raise the level of the output voltage. This output is then given to the inverter and its output drives the load. In this work, a 40Wp panel is taken as the source. Various analysis have been done to determine the best topology for implementation. 2 MICRO-INVERTER The block diagram of the micro-inverter system is shown in Fig.1. 2

3 Fig 1. Block diagram The dual stage topology[2] of the micro-inverter is performed using a DC-DC stage and a DC-AC stage connected in cascade obtaining a conversion chain of the type DC-DC-AC. In this case, the DC-DC converter is connected to the PV module and the DC-AC converter is connected to the load. To ensure maximum power is tracked from the panel under varying irradiance and temperature levels, the maximum power point tracking(mppt) [3] algorithms have been incorporated to control the switching pattern of the converter. The inverter is controlled using the pulse width modulation techniques[4]. These techniques have the advantages of low THD outputs and effectiveness. 3 PV MODELLING The modeling of a 40Wp solar panel has been carried out using the single diode model.the schematic of the modelling is given in Fig 2. Fig 2. Schematic of PV modelling The specifications of the 40Wp panel taken from the datasheet are shown in Table I. 3

4 TABLE I : PANEL SPECIFICATIONS The various cell parameters such as Iph, Is and ideality factor(n) have been calculated using the appropriate formulas given in [5].The I-V and P-V characteristics obtained for varying irradiation levels is shown in Figs.3& 4 respectively. Fig 3. I-V Characteristics of PV Panel 4

5 Fig 4. P-V Characteristics of PV Panel As it can be seen from the above figures, the power varies correspondingly to the irradiance. Tthe terminal voltage of the module varies between 0-Voc while the current value changes between Isc- 0 when the module s operation point moves between short-circuit and open-circuit conditions, respectively. The panel provides the maximum power of 40W at the standard conditions of 1000W/m2. Parameters like cell s working temperature, irradiance, diode ideality factor, series and shunt resistances have all significant effects on cell s I-V and P-V characteristics. 4 MAXIMUM POWER PEAK TRACK- ING METHODS The irradiance of the sun incident on the PV panel tends to change throughout the day. But there occurs peak in the PV graph. In order to obtain maximum efficiency, the panel has to be operated at this peak value. By employing suitable algorithms, the converter is given gate pulses to operate with maximum power. The converter used in this circuit is a boost converter. The switch of this converter has to be controlled using the MPPT [6] algorithm. The converters input is from the PV panel and it powers the inverter. 5

6 The converter has an inductor with a value of 674H, a capacitor on the input side with a value 50F and on the output side with a value 500uF. This converter is made to operate with a duty cycle of 50%. The schematic of boost converter[7] with PV panel is shown in Fig 5. Fig 5. Boost Converter MPPT algorithms are used to operate the PV system in the peak of the PV graph. By operating it that way, the problems due to load mismatch can also be avoided apart from increasing the efficiency.. The two types of MPPT algorithms taken for investigation in this paper are, Perturb & Observe and Incremental Conductance [8]. A. Perturb and Observe MPPT algorithm This algorithm perturbs the voltage at two consecutive points and the observes the power at that voltage. This algorithm is similar to climbing a mountain. It compares the power first. Then the comparison of voltage is done. Accordingly, the duty cycle is altered in ordered to move towards the maximum power peak. The flowchart of the algorithm is illustrated in fig 6. 6

7 Fig 6. Flowchart of P&O algorithm This algorithm has been implemented using blocks in PSim. The voltage and current from the PV panel have been taken using the voltage sensor and current sensor and given as inputs. The ouput of the algorithm is gate pulses which is given to the switch of the boost converter.the irradiance to the PV panel is differed and the power across both input and output sides of the converter are noted along with the voltage. Fig 7. Schematic diagram of P&O Algorithm Fig 8. Waveform of a) Power of PV panel and Power tracked due to MPPT and b) Input and Output Voltage of the converter 7

8 TABLE II : PARAMETERS OBSERVED DUE TO P&O MPPT B. Incremental Conductance MPPT Algorithm This algorithm reaches the peak using the slope of the PV graph as the left side slope is negative and the right side one is positive. The slope at the peak is zero. This method compares the slope of each point and reaches the peak. It found to be efficient than P&O method. Unlike P&O it does not oscillate on reaching the maximum power peak. So, the losses due to oscillations are reduced. The fig 9 depicts the flowchart of InC and the fig 10 shown below is the schematic subsystem of InC. Fig 9. Flowchart of Incremental Conductance According to the algorithm, the slope is checked first and later the current change is also checked. Based upon the results of the analysis of the slope of the PV graph, accordingly the duty cycle is altered to reach the maximum power peak. 8

9 Fig 10. Schematic diagram of Incremental Conductance Fig 11. Waveform of a) Power of PV panel and Power tracked due to MPPT and b) Input and Output Voltage of the converter TABLE III : PARAMETERS OBSERVED DUE TO INC MPPT As there are few drawbacks and losses due to oscillation in P&O, the tracking efficiency is found to be lesser than the InC algorithm. Since our aim is to track maximum power possible, we propose the InC algorithm for triggering the switch of the converter. 9

10 5 INVERTER TOPOLOGIES In order to reduce the THD, various inverter topologies [9] such as H5, HERIC and Push-pull have been simulated in PSIM software. A. H5 Inverter The H5 inverter [10] comprises of five active switches. The basic circuit diagram of the H5 is shown in Fig. 12 Fig 12. H5 inverter Fig.12 shows the inverter configuration with five switches, LCL filter at the output side and R load. The switches S2, S3, S4 and S5 operate in unipolar switching pattern. The switch S1 operates when on pulse is generated by comparing rectified sinusoidal reference wave with triangular signal. The output of the inverter is a sinusoidal wave varying between +Vdc and -Vdc. The output of the boost converter is fed as input to the inverter and acts as the second stage of the micro-inverter. The various parameters obtained from the entire system simulation has been tabulated below in Table. IV TABLE IV : MICRO-INVERTER WITH H5 TOPOLOGY B. HERIC Inverter The six active switch HERIC [11] topology is a H-bridge inverter with two MOSFETS connected in series in between the two legs of the bridge. The schematic of the HERIC is shown in Fig

11 Fig 13. HERIC inverter The unipolar modulation is given as the gating pattern for the switches in the H-bridge. The on-off square pulse is given to the switches in between the legs in complementary manner. This inverter acts as the second stage of the micro-inverter and the values obtained from simulation are shown in Table V TABLE V : MICRO-INVERTER WITH HERIC TOPOLOGY C. Push-pull Inverter The push-pull inverter [12] is a transformer based isolated six switch inverter topology. The circuit is shown in fig 14. Six switches are employed and also requires a transformer with a center tapped primary and secondary. The two switches are connected in antiparallel in the upper and lower legs of the secondary of the transformer. Two more switches are connected to the primary side. The load is connected between the upper leg and the centre tap of the secondary side of the transformer. 11

12 Fig 14. Push-pull inverter The Table VI shows the various values obtained from simulation. TABLE VI : MICRO-INVERTER WITH PUSH-PULL TOPOLOGY D. Comparison of the inverter topologies. The three inverter topologies are compared on the parameters like Efficiency, THD and switching losses. The inverter with best results is chosen for the implementation. The Table VII shows the comparison of the complete microinverter with three topologies of the inverter. TABLE VII : COMPARISON OF INVERTER TOPOLOGIES From the above table, it can be inferred that the H5 topology has reduced harmonic content and increased efficiency. It can also be seen that the switching losses for H5 is lesser than those of HERIC and Push-pull. Thus, the H5 inverter topology is chosen as the second stage in the micro-inverter system. 12

13 6 MODULATION STRATEGIES The main objective behind adopting control strategies is to generate good quality controllable AC voltage and to minimize the harmonic distortion, switching losses and the filtering requirements. Various modulation techniques for VSI are reported in the literature. The modulation strategy discussed in this paper is Pulse Width Modulation (PWM). Pulse width modulation is the process of modifying the width of the pulses in the pulse train in direct proportion to the control voltage. The different techniques are discussed in this paper by varying the high frequency carrier signal, keeping the reference signal as the sinusoidal wave. The different modifications include unipolar, bipolar, sawtooth and inverted sine signals. The different PWM techniques are analyzed on their THD and efficiency to determine the technique for power quality improvement. Since the chosen topology of inverter is H5, the gating patterns for the five switches of the inverter will be illustrated in the upcoming sections. A. Unipolar Modulation The unipolar modulation[11] normally requires two sinusoidal modulating waves Vref1 and Vref2 of same magnitude and frequency but 180 degree out of phase. The two modulating wave are compared through a common triangular carrier wave Vcarrier generating two gating signals Vg2 and Vg3 for the upper two switches S2 and S3. The gate pulse generation is shown in Fig 15. Fig 15. Comparison of reference and carrier signal for unipolar strategy B. Bipolar Modulation The sampling of SPWM bipolar switching is that the reference sinusoidal waveform having magnitude Vref is compared with triangular carrier signal having amplitude Vcarrier. The upper and the lower switches in the same inverter leg work in a complimentary 13

14 manner which means the gating signals are generated for only one of the switches in each leg and the compliment of the same is given to the other switch belonging to the same leg. The gating pattern is shown in fig 16. Fig 16. Comparison of reference and carrier signal for bipolar strategy C. Sawtooth Modulation The switches work in the unipolar switching pattern with the high frequency carrier wave replaced by the sawtooth signal. The on pulse is generated when the amplitude of sine is greater than that of the sawtooth wave. The graph is shown in Fig 17. Fig 17. Comparison of reference and carrier signal for sawtooth strategy D. Inverted sine Modulation The rectified sine wave with the diodes reversed to obtain negative polarity is given as the carrier signal to the comparator. This modulation is also unipolar. Fig 18. depicts the pattern. 14

15 Fig 18. Comparison of reference and carrier signal for inverted sine strategy E. Comparison The above mentioned PWM strategies were simulated and their THD and efficiency was obtained. The tabulation of the results is shown in Table VIII TABLE VIII : ANALYSIS OF PWM STRATEGIES Figure 1 Subdivision of System Reliability From the results of simulation, it can be inferred that the inverted sine PWM technique has lesser THD. Since, harmonics play a vital role in the power quality, a compromise is made with the 1% lesser efficiency of inverted sine compared to the higher unipolar efficiency. Therefore, the inverted sine PWM technique is taken for implementation. The output voltage waveform is of H5 inverter is shown in fig.19 15

16 Fig 19 Output voltage of H5 inverter 7 SIMULATION RESULTS The final simulation of the proposed micro-inverter is done with the PSIM software. The source is the 40Wp PV module modelled using the single diode model. Incremental conductance is the MPPT algorithm to track the maximum power from the panel and controls the switching of the converter switch. The H5 inverter with the inverted sine PWM technique completes the second stage of the micro-inverter system. The inverted sine PWM strategy is implemented. The complete schematic is shown in Fig

17 Fig 20. Schematic of the complete simulation The simulation parameters are tabulated below. TABLE IX : SIMULATION PARAMETERS The gating pattern for the five inverters of H5 inverter is shown in fig

18 Fig 21. Gating pattern of the H5 inverter The simulation waveform of the complete micro-inverter system is shown in fig 22. Fig 22. Waveform of a) Input of converter and output of converter b)output of H5 inverter. 18

19 The simulation results of the micro-inverter system is tabulated in Table X. TABLE X : RESULTS FROM SIMULATION 8 CONCLUSION This paper has analyzed a two-stage micro inverter for photovoltaic applications. For obtaining maximum power from PV, this paper has explored incremental conductance algorithm whose tracking efficiency is higher compared to the conventional P&O method. Various inverter topologies were analysed for the micro inverter and from the simulation results, it is found that H5 inverter resulted in reduced total harmonic. To control the operation of the inverter, different modulation techniques were investigated and the technique using inverted sine wave as carrier was efficient than the other techniques. Hence, finally H5 was interfaced with PV based DC-DC boost converter with INC MPPT algorithm and overall, this twostage micro-inverter resulted in reduced total harmonic distortion thereby improving the power quality. Therefore, the proposed micro inverter is an appropriate topology for PV applications. 9 ACKNOWLEDGMENT The project is internally funded by the management of SSN College of Engineering. We thank the management for the continuous encouragement and the support they have given us in pursuing this project. 19

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