Connecting an Alternative Energy Source to the Power Grid by a DSP Controlled DC/AC Inverter

Transcription

1 Inaugural IEEE PES Conference and Exposition in Africa Durban, South Africa, - July Connecting an Alternative Energy Source to the Power Grid by a DSP Controlled DC/AC Inverter Yuval Beck, Bishara Bishara, Dror Medini Department Interdisciplinary studies, Faculty of Engineering, University of Tel-Aviv P.O. Box 9, Tel-Aviv 9978, Israel Phone: , Fax: , beck@post.tau.ac.il Abstract In times when environmental issues such as global warming and air pollution are in focus, there are more objections to the use of conventional and nuclear power stations. The use of alternative energy sources such as water turbines, solar cells and wind turbines becomes essential for producing clean energy. In this paper, a novel design of a kw DC/AC Inverter is presented. The inverter is versatile, allowing for the connection of any DC alternative power source such as solar cells, wind turbines, etc, to the AC. Therefore, in peak load times, every household can perform as a clean micro power station. The inverter is implemented as a phase shift synchronous and it is controlled by means of a DSP. Sampling the voltage makes it possible for the Inverter to synchronize with the at a zero angle. Then the DSP synthesizes the AC voltage wave form to be shifted, with respect to the, with a software controllable leading angle. This configuration makes it possible to transfer the clean energy omitted by the alternative power source to the. Energy contribution to the is possible by connecting synchronous s that have 8% efficiency. In addition, the inverter is designed for low cost and its parameters are easy to control via computer software. I. INTRODUCTION In recent years the cost of electricity has risen. Forecasts show that the price will continue to rise drastically. The various reasons for these price increases include the prices of oil, gas and coal, the population growth and manufacturing costs. At times when an electric company has to provide peak power, the costs are even higher. This is due to the increasing residual production costs of the operation of non effective reserve power stations and the losses in distribution lines and transformers that rise in the second power of the current. Moreover, there are rising objections to the operation of conventional and nuclear power station for environmental reasons. The solution to the above mentioned is clean energy produced by solar cells, wind turbines, fuel cells or water turbines. These various energy sources can be stationed in every household and can contribute clean energy to the main power. Connecting a large number of small "clean power stations" to the results in:. reducing the manufacturing costs to the power station;. as a result of the return energy, the power station deals with a reduced load, which means reduction of power losses on the lines;. using a large scale of distributed generation, results in redundancy that increases the reliability of the system;. decreasing the consumers electric bill. These clean energy sources can be connected to a kw DC/AC inverter directly or through a DC bus. The inverter can be connected simply to one of the power sockets in the house and the system operates as a that contributes power to the main. In this paper, a novel design of a kw DC/AC Inverter is presented. The inverter is designed to work as a phase shift synchronous controlled by a DSP. The inverter can be connected to any DC alternative power source. The paper presents the theory, technology and the circuit of the inverter. II. THEORETICAL CONSIDERATIONS Designing a circuit that perform as an interface between a general DC source and the AC power, demands understanding of DC/DC switched mode converters theory, AC/AC conversion and synchronization theory and techniques. The system is based on an H-bridge of power MOSFETs that is synchronized to the power. The control of the bridge is done by a Motorola DSP board DSPF8. The DSP samples the voltage of the and produces a synchronized PWM modulated signal to the MOSFETs gates, generating phase shifted sine wave at the MOSFETs bridge terminals. The synchronization process is based on a well known standard theory. For proper and smooth synchronization three conditions must be satisfied. The voltage of the (clean energy source) must be equal to the s voltage;. The frequencies of the two sources must be equal and. The phase shift between the sources must be zero (for a three phase application phase order must be satisfied too). All these conditions are checked and the generated voltage satisfies these conditions so the inverter can be synchronized to the. After synchronization occurred the phase is shifted as shown in Fig : //$. IEEE

2 III. THE INVERTER DESIGN The inverter presented here is a DC/AC inverter, controlled by a DSP. The inverter is designed to invert different DC voltages (dependent on the clean energy source it is connected to), to a nominal AC voltage synchronized to the power at first. Then, the phase is shifted so there is power flow to the. At first stage a W inverter was designed and built. Later on a kw inverter is to be implemented. A. Block diagram Fig.. The shifted waveform between the and. The inverter consists of four main sub-systems:. AC/AC converter;. DSP board DSPF8;. H-bridge;. Boost converter. The block diagram is presented in Fig. The phase shift between the voltage of the and the voltage wave form of the results in energy flow from the to the. (See the voltage difference between points A and B). The system can be looked at as a general power flow problem through a series impedance. as seen in Fig. E I E AC Clean power dc AC/AC converter BOOST Converter A/D MOTOROLA DSP F8 PWM H-Bridge Fig.. Block diagram of the circuit. Fig.. The connection of the to the. Then the current flow to the can be easily calculated as: where: E I = E () E = E sin( ωt) () E = E sin( ωt δ) the total power flow back to the is then: E P = sinδ () this is with total agreement with the theory of synchronous machines. The clean power DC source is connected to the boost converter. The converter steps up the voltage to a constant voltage which is determined by the sampled AC voltage. The stepped up voltage is then supplied to the H-bridge unit. This unit produces AC voltage that is synchronized with the AC. The H-bridge performance is controlled by the PWM signal generated by the DSP unit. The AC/AC converter is an interface between the power and the DSP unit. It transforms the voltage from V down to the maximum input voltage allowed by the A/D unit in the DSP i.e. V. this converter is a simple transformer with additional voltage divider. The DSP board in this system is used for sampling the voltage of the power, for data processing and generating a PWM signal for controlling the H-bridge. The DSP is a MOTOROLA F8. The DSP was programmed with Metrowerks Code Warrior software. The main circuits of this system are the boost converter and the H-bridge circuits. In the next sections these circuits and their theory is presented.

3 J In IN C R k D.u u N9 T 8 R 7 TRAN_HM D DIODE ZENER (V) 7 U8 Vcc Drive Output R9 Q IRF8 D MUR Out C u J Out ISNS Current Sense Input 8 R7 K R9. n C R k ISET Gnd Compensation C.8u VFB n C R M R k R k Fig.. the boost converter circuit B. The Boost Converter The input DC voltage from the alternative power source is low in amplitude (much lower than the RMS voltage of the ). Therefore, a switched mode boost converter is used for stepping up the low voltage to the nominal voltage. The boost circuit is presented in Fig.. The converter is based on a U8 controller and the converter is a non isolated one, with efficiency of up to 9% and active power factor correction. C. The H-Bridge The H-bridge is the unit that converts the PWM signal generated by the DSP to a V sinusoidal voltage wave. The unit includes the drivers and operates as follows: The PWM modulated signal enters the drivers. For quick operation of the power MOSFETS it is amplified and distributed to the upper and lower MOSFETs gates. The circuit is designed in a manner that at a given time only two FETs on opposite diagonals are switched on. Moreover, we can look at the circuit in a way that we have a positive circuit when Q and Q are switched on and a negative circuit when Q and Q are switched on. When the positive circuit is activated the voltage is pulled up and when the negative circuit is activated the voltage is pulled down. For a noise free sinusoidal wave form at the output, inductors and capacitor are placed. The switching of the H-Bridge is done in a bipolar switched mode as explained ahead. (See Fig..) Fig.. a schematic standard H-Bridge. In Fig.. a standard H-Bridge configuration is presented. In practice the switches are IRFP/TO power MOSFETS. The trigger for the FETs gates is a PWM signal, generated by a reference voltage and a triangle wave as shown in Fig.. A bipolar mode means that both circuits (the positive and the negative as stated before) are switched simultaneously by opposite phases, this means that the inputs of switches A and A are controlled by the V control signal, while switches B and B are controlled by the inverted signal of V control. The load voltage varies accordingly and is in the range of V CC and V CC. (see Fig..).

4 Fig.. wave forms of a bipolar switching. IV. EPERIMENTAL SETUP AND RESULTS A W model was built and the above mentioned units were integrated. The circuit was powered with external power supplies. These power supplies contributed the necessary voltage to the inverter's circuits and emulated the alternative DC clean power source. The inverter's output was connected to a socket of the power. The first experiment was to check whether there is power flow from the inverter to the power. A current meter was connected to the line and it showed current flow, which means that power is flowing to the. Next to control this power flow a program was written for the DSP to control the phase shift between the s voltage and the inverter generated voltage. To calculate this power from the resulting voltage and current measurements we first define the s voltage as in eq(). Then the current is defined as: OUT C u OUT VR U 8 9 7u Hin NC HIN SD HO VB VS NC 7 C u D MBR Q IRFP/TO L INDUCTOR Q IRFP/TO L INDUCTOR C u D MBR LO COM U NC N VSS LIN SD Lin Lin LIN VSS COM C u Q IRFP/TO Q IRFP/TO C u VS VB HIN 9 Hin N LO 7 8 HO NC 7u R R IN IN Fig. 7. The H-bridge circuit The DSP generates a single PWM signal. To switch the H- bridge in a bipolar mode a logic circuit was designed for generating two inverted PWM signals. These signals are injected to the HIN and LIN inputs of the drivers. The logic circuit was also designed to generate dead time between switching to avoid the problem of overlapping between the positive and the negative circuits. This overlapping can cause a substantial power loss and synchronization difficulties. The H-bridge circuit with the supporting drivers is shown in Fig.7. I = I sin( ωt δ) The average power is then: ()

5 T P = i() t v() t dt = T T = EI sin( t)sin( t ) dt T ω ω δ = () = EI The results of the experiments are shown next. First a phase shift was programmed. The resulting voltage waves are presented in Fig. 8. Table I. The measured voltages and currents in respect to different phase shift angles. power current voltage Phase shift 8W.A V. W.A V 77W.9A V 7. 7W.9A V W 8.A 88V Fig. 8. voltage wave of the inverter and with phase shift. V.DISCUSSION AND CONCLUSIONS The AC/DC Inverter concept, design and experimental results are presented in this paper. The research approach was to use clean energy sources such as, solar cells, wind or water turbines, fuel cells etc. for contributing power to the main. The circuit was designed and built to generate AC voltage synchronized and phase shifted to the. The circuit is versatile due to the fact that the control over the phase shift parameter, the sample rate and the PWM, are all controlled by computer software. The controller of these parameters is the DSP. The results show that power was flowing from the inverter to the. The power got to up to W at a angle. In the future a -.kw inverter is to be built and clean energy sources will be connected to it. ACKNOWLEDGMENT The authors wish to thank Freescale semiconductors for their technical support and providing the DSP evaluation board. REFERENCES Fig. 9. voltage wave of the and the current with phase shift. From Fig.8 it is clearly shown that the generated synthesized inverter voltage wave is synchronized with the voltage of the power. Next the current at phase shift was measured. The results are shown in Fig.9. The current measured in these conditions was.a. other voltages and phase shifts were measured and some of the results are summarized in table I. [] Sutanto D. Alternative energy resource from electric transportation. [Conference Paper] st International Conference on Power Electronics Systems and Applications. Proceedings (IEEE Cat. No.E98). Hong Kong Polytechnic Univ., pp.9-. Hong Kong, China. [] Poh Chiang Loh, Holmes DG. A variable band universal flux/charge modulator for VSI and CSI modulation. [Journal Paper] IEEE Transactions on Industry Applications, vol.8, no., May-June, pp.9-7. Publisher: IEEE, USA. [] Twining E, Holmes DG. Grid current regulation of a three-phase voltage source inverter with an LCL input filter. [Journal Paper] IEEE Transactions on Power Electronics, vol.8, no., May, pp Publisher: IEEE, USA.