Scholarly article on topic 'Micro-controlled Pulse Width Modulator Inverter for Renewable Energy Generators'

Micro-controlled Pulse Width Modulator Inverter for Renewable Energy Generators Academic research paper on "Materials engineering"

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{"Pulse Width Modulation" / "Renewable Energy" / Inverter / "HVDC bus" / Simulation / "Prototype Development"}

Abstract of research paper on Materials engineering, author of scientific article — M. Arab, A. Zegaoui, H. Allouache, M. Kellal, P. Petit, et al.

Abstract A new controller for pulse-width modulation (PWM) inverter based on a circuit integrating a microcontroller and a symmetric output stage composed of two switching transistors which operate as switches for PWM has been developed. A model of operation was developed followed by the simulation and the realization of a prototype. The inverter output is filtered and a pure sine wave signal was generated. The PWM inverter adopting this PWM technique shows a simple method allowing to obtain pure sinusoidal output signal. In the context and framework of renewable energy, when associated in a generator architecture based on an intermediate HVDC bus, it is appropriated for a wide-range of power systems and applications.

Academic research paper on topic "Micro-controlled Pulse Width Modulator Inverter for Renewable Energy Generators"

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Energy Procedia 50 (2014) 832 - 840

The International Conference on Technologies and Materials for Renewable Energy, Environment

and Sustainability, TMREES14

Micro-controlled Pulse Width Modulator Inverter for Renewable

Energy Generators

M. Araba, A. Zegaouia b, H. Allouachea, M. Kellal a, P. Petitb c, M. Aillerieb c

aGEER Laboratory, UHBB University, Algeria bLorraine University, LMOPS-EA 4423, 57070 Metz, France cSupelec, LMOPS, 57070 Metz, France

Abstract

A new controller for pulse-width modulation (PWM) inverter based on a circuit integrating a microcontroller and a symmetric output stage composed of two switching transistors which operate as switches for PWM has been developed. A model of operation was developed followed by the simulation and the realization of a prototype. The inverter output is filtered and a pure sine wave signal was generated. The PWM inverter adopting this PWM technique shows a simple method allowing to obtain pure sinusoidal output signal. In the context and framework of renewable energy, when associated in a generator architecture based on an intermediate HVDC bus, it is appropriated for a wide-range of power systems and applications.

© 2014ElsevierLtd. Thisisanopenaccessarticleunder the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) Keywords: Pulse Width Modulation; Renewable Energy; Inverter; HVDC bus; Simulation; Prototype Development

1. Introduction

Nowadays, the application of photovoltaic (PV) systems in production and electrical transport is growing. Despite all still present related problems of energy efficiency, optimal operating, the impact on the environment and network stability [1, 2], in the field of photovoltaic, a lot of researches exist and the number of scientific publications increase in specialized literature. In particular, many questions are currently under study, as the prediction of photovoltaic energy, optimum choice and design of DC-DC or DC-AC power converters that interface between the PV source and payload and the study of all issues related to the control of these electronic converters with high efficiency power transmission [3, 4].

* Corresponding author. Tel.: +213-557-929-992; fax: +0-000-000-0000 .

E-mail address:abd.zegaoui@gmail.com.

1876-6102 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi:10.1016/j.egypro.2014.06.102

The downward trend in the price of photovoltaic modules and their increasing efficiency in terms of energy conversion has put inverters as the only technology for integrating photovoltaic systems in the network. PV systems connected to the electric distribution network are required to fulfill two criteria: to maximize the transfer of energy extracted from the PV modules [1] and to use a high efficiency topology of converters able to inject current into the active network, i.e. a pure sinusoidal current in phase with the network voltage.

By else, environmental constraints will play an important role in the evolution of the power system [1, 4]. However, with regard to the security of the system, specific problems need to be considered when renewable energies, such as large wind farms, should be connected. Especially, this is the case when the AC connection links are weak and/or the ability to sufficient reserves in neighboring systems is not available [5]. In the future, an increasing share of the installed capacity will be linked to levels of distribution (dispersed generation), which poses additional challenges for planning and safe operation for systems. Power electronics are needed to control the flow of charge, reduce transmission losses and to avoid congestion, flow loop and voltage problems [6, 7]. High Voltage Direct Current Systems (HVDC) and Flexible AC Transmission Systems (FACTS) provide essential functionalities to avoid technical problems in power systems; they increase transmission capacity and system stability in a very efficient way and assist in the cascading prevention of disturbances [8].

Our research is focusing on network topology dedicated to renewable electrical energy production. The global schema of such topology is shown in Fig. 1. Renewable energy plays an important role in the future of the energy policy. Recently a large number of scientists have focused their research work on the transport of renewable and sustainable energy on a HVDC bus [8-10]. Then it is important to use distributed architectures of PV sources, wind turbines or even both, based on the connection of high efficiency DC-DC converters to a HVDC bus. High-electrical power transport through HVDC bus powered by renewable sources is an alternative that deserves consideration.

HVDC Bus

AC distribution network

DC S / DC

Fig. 1 : HVDC bus transport network topology based on distributed architecture renewable sources feeding.

In the framework of our global research on network technology, the main purpose of this paper is to optimize the operation of the pulse width modulator (PWM) of PV inverters, which are implemented between the HVDC bus and

the AC grid, as represented in Fig. 1. There are three types of DC/AC inverters available on the market, which are classified by their output type: square wave, modified-sine wave and pure sine wave. Pure sine wave inverters offer more accuracy and less unused harmonic energy delivered to a load, but they are more complex in design. Pure sine wave inverters will power devices with more accuracy, less power loss, and less heat generation. In the specific aim of the present contribution, and in order to have a pure controllable sinusoidal AC signal at the output, we present a theoretical analysis of the driver operations of the PWM, starting by the presentation of the classical model and based on a new analytical methodology, we suggest a new PWM technique suitable for an ac drive system. We extend this study by the development and the realization of the corresponding driving code is applied to this inverter that, finally, we have implemented in the microcontroller of an inverter prototype developed in our laboratory.

2. Theoretical approach of the Pulse Width Modulation

The conventional PWM control theory uses sinusoidal modulating signal types, as reference signal and generally a triangular signal, as a carrier signal. The carrier wave frequency must be sufficiently large compared to that of the reference signal. This method is represented in Fig. 2. The level intersections of the two signals over a period of reference determine the fearing angles of the inverter switches [9-11].

Carrier signal

Fig. 2: Diagram of conventional PWM control

The sinusoidal signal that should be sampled must obey to the law:

S(8)= Asin( 8) (1)

where A is the magnitude of the signal and 0 is the phase signal. We consider now the discretization of the PWM signal. For that purpose, we consider the sampling frequency, Fe (i.e. a sampling period Te) and the frequency of the desired signals for sampling, F (i.e. a reference period T). With sampling resolution defined byn = Te/T, the number of period Te corresponds to a period T then

S(i)=Asin(2u i/n) (2)

with i, an integer varying from 1 to n.

For each value of i correspond a point of the inverter output that should be reproduced n times. Therefore S(i) is a discretized sinusoidal signal (digitalized) and reproduced in the output of the inverter with the desired magnitude. Using the sampling resolution or, equivalently the frequency ratio or digital frequency f = F/Fe, Eq. 2 becomes:

S(i)=Asin( 2-nfi) (3)

As indicated in Fig. 3, each point of a digitalized signal S(i) will be represented by a square signal of periodTe and a magnitude B with a specific pulse width

T0ni = a{Te (4)

where oq is the specific duty cycle corresponding to the point S(i). oq will serve in the system for the command of the inverter.

Toni Ton2

T/2=nTe/2

Te 2Te 3Te

Fig. 3: Different square signals according to each point of the digitalized signal.

For a given i, the average value of the corresponding signal is: S(i)moy = f Jo"6 S,(t)dt= ttjB

Thus, from equations (3) and (5), we can write:

aj = - sin( 2-rcfi)

This equation allows us to compose the table of the various available duties that can be used for the generation of the sinusoidal signal with the desired magnitude. Thus, the maximum amplitude that the sinusoidal signal can achieve is defined by the two specific values for sin( 2-nfi) = 1 and aj = 1. The only possible solution of Eq.6 satisfying these conditions gives A = B. Let A' the other possible amplitude of the desired sinusoidal signal at the output inverter terminals, (i.e. A' < A), its specific duty cycle equation is

aj' = — sin( 2-rcfi)

yielding with Eq. 6 to aj'/aj = A'/A that we can write as

< = (A'/AH = r cq (8)

where 0 < r <1.

3. Experimental results and discussions

For the development of the application, we have taken a concrete situation with a sampling frequency Fe equal to 20 kHz (i.e. a sampling period Te = 50p.s) and the frequency of the desired signal for sampling F equal to 50Hz (i.e. a reference period T =20ms). It is obvious that with a sampling resolution, n = Te/T = 400, the Shannon's theorem is fully satisfied.

The software application that we have developed to control the reference signal on its half-period is based on the chart presented in Fig. 4.

Fig. 4: Flowchart to generate a half-period of the signal

We have simulated the electrical PWM controller circuit using the Proteus-Isis software environment. The schema of the simulation is represented in Fig. 5. The circuit integrates a push-pull inverter (half-bridge) controlled by a microcontroller PIC16F876A. The code included in the microcontroller corresponds to the translation in C language of the flowchart in Fig. 4. We note that in this circuit, we have chosen a high frequency transformer as long as its primary weeding is in series with the switches.

According to the parameter values stored in an internal table and as calculated by the program itself, microcontroller generates the PWM signal having the shape of a square variable pulse as shown in Fig. 6

Fig. 5: Inverter control by microcontroller generating the PWM signal.

Within the command signal, Fig. 6, the inverter is able to generate a pure sinusoidal signal at its output terminals. The so-generating wave is represented in Fig. 7. This signal shows huge ripples and needs to be filtered before connected to the AC grid. Thus, and as shown in Fig. 5, we have integrated in the circuit a filtering stage at the output. The output-filtering signal is shown in Fig. 8.

PWM signa l

a) ra as

0,0005 0,0010 0,0015 0,0020 0,0025

Time (s)

Fig. 6: Variable duty cycle generated by microcontroller for inverter controlling.

^ 100'

Output voltage

0,10 0,11 0,12 0,13 0,14 0,15 0,16

Time (se

240-, 220 200 180 > 160

140 • 120 • 100 • 80' 6040

Zoom of the output voltage

0,122 0,124 0,126 0,128 0,130

Time (s)

Fig. 7: Output voltage inverter terminals according to the proposal PWM approach. Signal without filtering.

Time (s) Time (s)

Fig. 8: Output voltage inverter terminals according to the proposal PWM approach after filtering.

The experimental results of the inverter signal command recorded at an oscilloscope screen are represented in Figs. 9 and 10. Figs. 9 (with time scales equal 5 ms / div and 2 ms / div for the left and the right parts of Fig. 9, respectively) show the experimental signals for the two transistors operating in a complementarity mode, as recorded at the microcontroller output with magnitude of 5 volts and a period of 2ms.

RIGOL STOP » -! f O 4.40U

■r ■

U- ■

1 1

CHI- 2.0002.000 Time 5.000ms O»0.0000s

CHI- 2.00UI3 HÜffl 2.00U Time 2.000ms O»0.0000s

Fig. 9: Experimental duty cycle generated by microcontroller for inverter controlling and its zoom in the right side.

Finally, we report in Fig. 10, the filtered output signal of the inverter driven by the PWM signals.

Fig. 10: Output voltage inverter according to the proposal PWM approach, the filtered PWM signal.

We can observe that the signals providing from the two transistors are fully complementary along the cycle. The combination of the two parts of the duty cycle provides a pure sinusoidal signal. By else, the specific choice of F and Fe for this prototype realization allows the generation of a signal fully compatible with grids and equipment working with a 50 Hz - AC voltage.

4. Conclusion

Issues discussed in this article are to provide a inverters connected to micro-grids and powered by renewable energy sources such as solar and wind. These DC/AC converters are to be fed through a HVDC bus and must have a high efficiency with low cost.

In this study we have formulated a new theoretical and detailed approach to digital pure sine wave command for inverters controlling. The proposed method has been implemented in a microcontroller according to a dedicated control algorithm that we have developed. Experimental results were very conclusive and in full agreement with the simulation results. Finally, we have successfully tested the feasibility of this theoretical approach. The various tests have showed that our design is able to output a pure sine wave with a simple circuit based on a microcontroller driving its output variable duty cycles. Through the addition of a HVDC stage fed by solar and wind sources, our system and application could be inserted in the command of various types of inverters, such as the half bridge or the H-bridge inverters.

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