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Design and Implementation of a Pure Sine Wave Single Phase Inverter for Photovoltaic Applications

Mohamed A.Ghalib1, Yasser S.Abdalla 2, R. M.Mostafa3 1 Automatic Control Department, Faculty of Industrial Education, Beni-suef University, Egypt.

master_bsu@yahoo.com 2 Electrical Department, Faculty of Industrial Education, Suez University, Egypt.

3 Electronics and Automatic Control Department, Beni-Suef University, Egypt

Abstract This paper aims at developing the control circuit for a single phase inverter which produces a pure sine wave with an output voltage that has the same magnitude and frequency as a grid voltage. A microcontroller, based on an advanced technology to generate a sine wave with fewer harmonics, less cost and a simpler design. The technique used is the sinusoidal pulse width modulation signal (SPWM) which is generated by microcontroller. The designed inverter is tested on various AC loads and is essentially focused upon low power electronic applications such as a lamp, a fan and chargers etc. The proposed model of the inverter can improve the output wave forms of the inverter and the dead time control reduced to 63µs. The finished design is simulated in Proteus and PSIM software to ensure output results which is verified practically.

Keywords: A microcontroller; Sinusoidal Pulse Width Modulation (SPWM); Dead time; Analog to digital converter (ADC), Inverter.

1. Introduction Nowadays, the world needs the electricity to be increased. The main reasons for the energy increase demand are the population, the economy growth and the rapid depletion of fossils based on energy reserve and rapid growth of energy demand. Then, it must research for an alternative source of power generation. One of these sources is a renewable energy which possibly has no harm on the environment [1]. The need of the power rating inverter is required to operate electrical and electronic appliances smoothly. Most of the available commercially uninterruptible power supplies (UPSs) are actually square wave inverters or quasi sine wave inverters. Electronic devices, managed by these inverters will be damaged due to the contents of the harmonics [2, 3]. Available pure sine wave inverters are too expensive and the output non sinusoidal, but the sine wave generation is

extremely important in power electronics. For getting a pure sine wave, the SPWM switching technique is applied. This method involves a certain pattern of switching used in the DC-to-AC inverter bridges [4, 5]. The SPWM is a powerful technique. It's mainly widely used in power electronics applications such as the motor driver, UPS, and the renewable energy system [6]. SPWM switching techniques are characterized by constant amplitude pulses with a different duty cycle for each period. The most common method to generate this signal is to compare a sinusoidal with a triangular waveform [7, 8, 9]. The purpose of this work is to replace the convention method with the use of peripheral interface controller (PIC) microcontroller. Microcontroller is capable of storing commands to generate the necessary pulse width modulation waveform due to the built in PWM module. The microcontroller provides the variable frequency pulse width modulation signal that controls the applied voltage on the gate drive by using the system of PIC16f877A. The microcontroller is more simple and flexible to change control algorithms in a real time without further changes in a hardware with it's low cost and reduces the complexity of the control circuit for the signal phase inverter bridge [10]. The application of this inverter is to be either for stand-alone or for grid connection from a direct supply of photovoltaic cells. The microcontroller has built control circuit in dead time.

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2. Problem statement

1- The inverter is one of the power conversion device that is widely used in the world to convert DC input voltage to AC output voltage. The output voltage wave form of ideal inverters should be sinusoidal. However, the waveform of practical inverter is non-sinusoidal and contains harmonics [11, 12]. The electronic devices, managed by this inverter will be damaged due to the contents of the harmonic. Harmonics contents in inverter output depend on the number of pulses per cycle. [2, 13, 14, 15, 16, 17] Many researchers investigated that the output signal wave is distorted. 2- In switching the losses problem, the number of pulses per cycle is also affected. The use of high switching technique will contribute to the high power losses. The following factor is to be considered in order to meet the following requirements.

i. The Cost of the equipment ii. The Size of the filter iii. The Power loss in switching the element

3- The most important problem to be considered is the dead time control. Dead time period must be suitable to avoid the problem of damaging the switch and harmonic problem. If the dead time is short, it will cause damage to the switches and if it is long, it will cause increase in the total harmonic distortion, as studied in [6 ,7].

3. The System and Characterization of the Proposed Design.

Figure 1 shows the basic block diagram of the proposed system. The range of the inverter circuit is to obtain a desired output voltage of 220 V ac and a frequency of 50 Hz. The contents of the designed system are:- 1- Power module of the inverter. 2- The microcontroller circuit and programming

software. 3- Testing the inverter circuit.

The full H-bridge inverter circuit is used to

convert a DC voltage to a sinusoidal AC voltage at a desired output voltage and frequency.

Fig.1 Block diagram of the proposed system.

Fig.2 The Full H-bridge single phase inverter.

Generating a sin wave centered on zero

voltage requires both positive and negative voltage across the load. This can be achieved from a single source through the use of H-bridge inverter circuit as shown in Fig. 2. In standard H-bridge circuit, switches S1, S3, S2 and S4 are arranged in this configuration [18]. Both gating signal GS1 and GS3 are switched simultaneous at one half of cycle while both gating signal GS2 and GS4 are simultaneous switched at the other half [13, 19, 20]. The difference is only at GS1 and GS3 signals leading GS2 and GS4 by half cycle or 180 degree of the switching signal. The output of the circuit has a periodic waveform that is not sinusoidal [20]. The PIC microcontroller is used to generate the required sinusoidal PWM signals to drive and switch the H-bridge MOSFET transistor.

The basic circuit of this system is a PIC microcontroller which is developed to generate a sinusoidal PWM with the dead time controller. The dead time control is useful to reduce the

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cost and components. The extremely important problem to be considered is the dead time control. The timing diagram of the dead time is shown in Fig. 3.

Fig.3 Observation timing diagram of dead time.

There is a potential overlapping signals

between ON period switches pair S1, S4 and S2, S3 pair in H-bridge inverter. This overlap can cause the short circuit of DC bus [6]. The dead time can be controlled through programming by using PIC microcontroller. The period of dead time must be suitable to avoid the problem of switch damage and harmonic.

3.1 Software Algorithm

Fig.4 The flow chart for programming the signal phase

inverter SPWM signal. Pseudo code contains the following steps:

1- Initialize the variables and all peripheral microcontrollers.

2- Setting the address of the look up table.

3- Calculating the PWM module. 4- Calculating the output voltage

amplitude. 5- If the output voltage is equal to Vreference

then returns step 2. 6- If the output voltage is greater than

Vreference then decreases the address of the look up table and returns to step 3.

7- If the output voltage is less than Vreference then increases the address in look up table and return to step 3.

8- Going to step 1.

3.2 Gate Driver The driving of the MOSFET gate is

dependent on two basic categories, a low-side and a high-side configured, in the full H-bridge circuit. The high-side of the MOSFETs (Q1,Q2) can float between the ground and the high voltage power, the low-side of the MOSFET (Q3,Q4) is connected between the power source and is constantly ground [7], the TLP250 driver, has an input and output stage and power supply. This driver is an optically isolated driver. The gate driver circuit is shown in Fig. 5.

Fig. 5 the gate driver TLP250

The input forward voltage will typically be between 1.6 V and 1.8 V, the propagation delay time will typically between 0.15µs and 0.5µs and the maximum operating frequency is to be 25 kHz in datasheet. When designing circuits with TLP250 a 100nf bypass capacitor (ceramic capacitor) is in output of the driver, this capacitor called boost strip capacitor used to protect the driver of dv/dt, a capacitor stabilizes the operation of the high gain linear amplifier in the TLP250.

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3.3 Results and Discussion The circuit diagram of the full H-bridge and

driver circuits is shown in Fig. 6. In this section the design of the hardware setup. The illustrated full H-bridge inverter consists of four IRL540N MOSFET switches rated 100V, 36A and fast switching. This switch has ultra-low resistance 44 mΩ, resulting in less power dissipation and higher efficiency.

Fig.6 Circuit diagram of full H-bridge and driver circuits.

Figure 7 shows that proposed hardware setup of the implementation inverter circuit to obtain sinusoidal wave AC output voltage with a rated voltage magnitude of 221 V AC and frequency 50Hz. These values are agreeing with the voltage and frequency of the grid, the PIC16F877A microcontroller is operated at clock speed of 20 MHz and the control signal sinusoidal PWM is set to be 16 KHz. The PIC16F877A is operating 5V, generated by a LM7805 linear voltage regulator. The compiler editor has used mikroC PRO for PIC and PICKit 2 programmer V1.10.

The operation of the circuit is as follows: The MOSFET driver TLP250 is used to apply the switching pulses coming from the microcontroller to the MOSFET switches. The sinusoidal PWM signals are produced by the microcontroller and used to drive the MOSFETs. For generating sinusoidal pulse width modulation signal we divide each half cycle of sine wave into 32 changes in the PWM signal a half cycle which takes 10ms. The duty cycle can be calculations from the following equation: Yi = PWMmax*sin (i*180/n), where: PWM=0-255, (PWMmax=255) i = 0, 1, 2 ,….., n.

Figure.8 illustrates the output signal of the simulation and experimental work. The simulations have been performed using PSIM and Proteus software to investigate the validity of the switching strategy.

Fig.7. Observation hardware setup of the pure sine wave inverter with load 11W.

Figure 8 shows the simulation and experimental results of the four control signals operation of the H-bridge inverter, an oscilloscope numbered 54600B with 100MHz, two channels are used to measure the experimental results. The PIC16f877A microcontroller is used to generate the required PWM signals to drive and switch the H-bridge MOSFET transistor.

The two level PWM operation of the H-bridge inverter will have the high sides of the circuit switching at the sinusoidal PWM (SPWM) control frequency of 16 kHz at a half cycle which takes 10ms the different is only SPWM1 signal leading SPWM2 by half cycle and the low sides switching at the sine wave lower frequency 50 Hz at a half cycle which takes 10ms also PWM1 control signal leading PWM2 by half cycle or 180 degree of the switching control signals out of phase.

The two signals are then connected to essential conditioning elements to be able to switch ON the MOSFET at the full bridge. The signals PWM going to the MOSFET (IRL 540N) will switch two diagonal of the full H-bridge simultaneously at one of the two halves and the other two diagonal MOSFET (IRL 540N) at the other halves.

LC filter

Output a pure sine wave

The output value 221 V ac

DC power supply

H-bridge inverter PICKit 2 programmer

Gate driver TLP250

Load with connection output

A2

K3

V+ 8

VO 6/7

V- 5

U1

TLP250

Q1IRL540N

Q2IRL540N

Q4IRL540N

Q3IRL540N

18 VDC

A2

K3

V+ 8

VO 6/7

V- 5

U2

TLP250

A 2

K 3

V+8

VO6/7

V-5

U3

TLP250

A 2

K 3

V+8

VO6/7

V-5

U4

TLP250

18 VDC

18 VDC

Resistor

Resistor

R1

22

Resistor

Resistor

R2

22

Resistor

Resistor

R3

22

Resistor

Resistor

R4

22

C1100nF

C2100nF

C3100nF

C4100nF

C5

47uF

C647uF

C747uF

C8 47uF

INVERTED_SPWM

50 Hz Square Wave ,

SPWM_CONTROL SIGNAL

50 Hz Square Wave ,

CONTROL SIGNAL

0 degree ofiset180 degree ofiset

Load

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Fig.8 illustrates microcontroller signal waveform generated by simulation at (a) and by experimental at

(b). Figure 9 shows the simulation and the experimental results of output waveform of the full bridge single phase inverter. The output voltage of H-bridge inverter has a periodic waveform that is not a sinusoidal wave but to achieve a desired output AC sine wave signal by connecting the LC filter. The frequency of the output wave from of the simulation and the experimental results is 50 Hz. This frequency is exactly equal to grid frequency.

Fig.9 The signal simulation (a) and experimental

signal results (b) of output waveform of full bridge single phase inverter.

It has been shows [6, 7] that the dead time measured period is of 180 µs, Fig. 10 shows the dead time measurement of the waveform in the H-bridge inverter from the simulation. The period of the dead time must be suitable to avoid the problem of switch damage and harmonic. The measurement period of the dead time in this work is reduced to 63 µsec.

Fig.10 The simulation signal results of the dead time measurement of the output waveform of full bridge

single phase inverter. The output of the H-bridge inverter is shown in Fig. 11 passed to the step up transformer and the output of the transformer is connected to the load through an LC filter to achieve the desired output AC sinusoidal waveform.

Fig. 11 The simulation signal results (a) in PSIM and (b) in proteus

The generation of the pure sinusoidal wave from the photovoltaic cells is the main objective of this work. But actually, the output signal is

63 µs

b b

a

b

PWM2

PWM1

SPWM1

SPWM2

a

b

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distorted and investigated in many researches works [2, 13, 14, 15, 16, 17], as shown in figures [12, 13, 14, 15, 16, 17].

Fig. 12 represent reference No.2

Fig. 13 represent reference No.13

Fig. 14 represent reference No.14

Fig. 15 represent reference No.15

Fig. 16 represent reference No.16

Fig. 17 represent reference No.17 Fig.11 shows the simulation results of PSIM and Proteus output software sine waveform of the full H-bridge single phase inverter and the experimental results of output waveforms of the inverter to ensure the output waveform results a pure sine wave is practically verified. The inverter is tested on various ac loads. As shown in Figs. 18, 7, 19, 20. The following conditions: Loads 11W and 15W (lamp) connected in parallel. DC input voltage of 18V Input current of 3.5A AC output voltage 221.3V and frequency of 50Hz

Fig.18 Observation hardware setup of a pure sine Wave inverter without load.

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Fig.19 Observation hardware setup of a pure sine wave inverter with load 15W.

Fig.20 Observation setup of the pure sine wave inverter with two load 26W

4. Conclusion

1- The main task of this work is to develop and improve the control circuit for a single phase inverter which has been implemented using PIC microcontroller.

2- The used method to control the inverter switch is the SPWM technique. This method is superior to other methods because improve the quality of the output waveform.

3- The simulation results to are performed at PSIM and Proteus software and compared the experimental results to performed by the LAB-module

4- The dead time is reduced to 63 µs in the proposed comparing with the research which reach 180 µs.

5- The tested inverter is loaded at various ac loads such as 11 W, 15 W and 26W.

5. References

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[3] A Qazalbash, A Amin, A Manan, M Khalid, "design and implementation of microcontroller based PWM technique for sine wave inverter" International Conference on power Engineering Energy and Electrical Drives, , P 163-167, March 2009, IEEE.

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[11]A Akkaya, A.A Kulaksiz, " A microcontroller-based stand- alone photovoltaic power system for residential appliances" Science direct, Vol. 78, pp. 419–431, 2004.

[12] S Daher, J Schmid, L.M Fernando, "Multilevel Inverter Topologies for Stand-Alone PV Systems" IEE Transactions On Industrial Electronics, VOL. 55, NO. 7, pp. 2703-2711, JULY 2008.

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M. A. Ghalib was born in Egypt. He received the B.Sc. degree in Process Control Systems from Beni Suief University, Egypt, in 2008. Currently, he is a demonstrator at the Faculty of Industrial Education, Beni Suief University, Beni Suief, Egypt. His research activities include photovoltaic applications and renewable energy.

Y. S. Abdalla was born in 1971. He received the B.Sc. and M.Sc.degrees in Electronics and Communication Engineering from the Faculty of Engineering, Cairo University, Egypt, in 1994, and 1999 respectively and the Ph.D. in Electrical Engineering from the University of waterloo, Canada in 2006,. Currently, he is an assistant Prof. in the Faculty of Industrial Education, Suez University, Egypt. His research interests are in the area of power electronics applications, VLSI and renewable energy.

R. M. Mostafa was born in 1949. He received the B.Sc. degree in Electrical Engineering from Military Technical Collage, Cairo, Egypt, and the Ph.D. in Electrical Engineering from Universite / Pierre et Marie Curie (Paris 6), Paris, in 1972, and 1983 respectively. Currently, he is a full time professor in the Faculty of Industrial Education, Beni Suief University, Beni Suief, Egypt. His research interests are in the area of Power Electronics applications in industry, Control of Electrical Machines, Embedded System design, Switch Mode Power Supplies, Photovoltaic Energy Conversion.