The Design Of Embedded PV Inverter Grid-Connected System

Transcription

1 Advanced Materials Research Online: 0-- ISSN: -9, Vols. 0-09, pp -9 doi:0.0/ 0 Trans Tech Publications, Switzerland The Design Of Embedded PV Inverter Grid-Connected System Zheng Enrang,a, Li Mingyong, b College of Electric and Information Engineering, Shaanxi University of Science and Technology, China, 700 a zhenger@sust.edu.cn, b limingyong00@.com, Keywords: PV, Inverter Grid-Connected, STM, Embedded system, SPWM. Abstract. An embedded PV inverter Grid-Connected system based on STMF0 whose main program is built with Keil uvision is designed here, which is consisted of control core unit, SPWM wave modulating unit, dead-band and optical isolating unit,ir0 driver unit, three-phase inverter unit, RC filter unit, Grid signal acquisition unit and LCD0x0 unit. The dead time of the designed dead-band unit is adjustable, which can effectively prevent inverter subversion. The experimental results show that the system can not only meet the design requirements, but also make the Grid-Connected effect ideal. Introduction The development and utilization of new energy has become the trend of energy development, solar energy, as part of new energy, has won widespread attention because of its advantages, such as enormous reserves, long-lasting, green. It is greatly significant to research on solar power and inverter Grid-connected. At present, three-phase grid-connected photovoltaic inverter has been studied in the literatures: the parameters of main components are calculated in literature [], but how to design the hardware of three-phase grid-connected photovoltaic inverter is not mentioned. A PV inverter Grid-Connected system based on TMS0F is designed in literature [], but the cost is expensive, The paper discusses how to design PV inverter and a PV inverter Grid-connect system based on STMF0 is designed here, which has dead zone circuit to prevent the inverter subversion. In addition, the designed system has a lower cost and can meet the Grid-connect requirements. Main Structure Embedded PV inverter Grid-connect system includes control core unit, SPWM wave modulating unit, dead-band and optical isolating unit,ir0 driver unit, three-phase inverter unit, RC filter unit, Grid signal acquisition unit, LCD0x0 unit, STMF0 is used as the controller of control unit, which accomplishes data processing and control functions, such as sending out relative sine data, Grid signal acquisition, showing system information and communications. the other units finish corresponding functions. The connection relationship between the various units is shown in Figure : U d Fig. Embedded PV inverter Grid-connect system structure diagram Hardware Design () STMF0 introduction and circuit design. STMF0 is a new -bit embedded microprocessor based on Cortex-M core, clock up to 7MHz, 0 I/O ports, 7 general-purpose DMA, support for standard 0-pin JTAG and serial wire debug (SWD) for emulation debug, timers, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-0/0/,::)

2 Progress in Renewable and Sustainable Energy USART interfaces, two IC interfaces, SPI interface, CAN interface and USB interface. Its Powerful function can meet real-time control and real-time communication requirements for the PV inverter grid system, With the help of STMF0, It is much easy to design system to be cost-effective, debugging-friendly and convenient. The main controller circuit connection diagram is shown as follows, which is consists of a JTAG debug interface, oscillator circuit, reset circuit, the STMF0 processor. The STMF0 accomplishes data processing and control functions, such as sending out relative sine data, Grid signal acquisition, showing system information and communications. Besides, PB0-PB7 is used as data interface. STMF0 DPV DPV TDI TCK TDO R9 R70 R7 R7 TCK TDI JTAG JTAG TMS R77 DPV S TNTRST PA0 PA PA PA PA PA PA PA7 TNTRST PA TMS PA9 PA0 PA PA TMS TCK TDI 0P C Y C M R7 0P BOOT0 C7 00nF DPV APV U 0 PA0-WKUP PA PA PA PA PA PA 7 PA PA PA9 PA0 PA PA PA/JTMS/SWDIO PA/JTCK/SWCLK PA/JTDI OSC_IN/PD0 OSC_OUT/PD BOOT0 NRST VBAT VDD_ VDD_ VDD_ VDDA STMF0CT PB0 PB PB/BOOT PB/JTDO PB/JNTRST PB PB PB7 PB PB9 PB0 PB PB PB PB PB PC-TAMPER-RTC PC-OSC_IN PC-OSC_OUT Fig. STMF0 circuit diagram VSS_ VSS_ VSS_ VSSA PB0 PB PB PB TNTRST PB PB PB7 TEST INT0 INT PB PB CUT USART_TXD USART_RXD BUZZ TEM_IN OSC_TEST () SPWM wave modulation circuit. SPWM wave is generated by comparing sine wave and triangular wave. A table with -point sine wave data is stored in STMF0, and STMF0 time regularly transmits sine wave data to the digital to analog chip(da) AD7 to converse, in this way three-phase sine wave is generate. In SPWM wave modulation circuit, two AD7 are used to generate sine wave. As we known, every AD7 has two DA output channels, so there are channel s DA output, three of DA channels are used for three-phase sine wave output, the rest one s output connects to the reference voltage pin of three DA channels, in this way adjusting the output voltage amplitude of the three DA channels easily comes true. The follow figure shows how the circuit connects. Triangular wave is generated by self-excited circuit. Adjusting the size of the R or capacitor C in the figure can change the frequency of the triangular wave, when choosing R=0kΩ while C=0.0µF, the frequency of generated triangular wave is. KHz. Then by comparing Three-phase sine wave and triangular wave, the three SPWM waves are generated. Because of noise existing in sine wave and triangular wave, repeating comparison maybe occur. In order to avoid repeating comparison, the anti-jamming circuit is necessary. VCC U 7 VDD RfB DATA0 DB0 OUTB DATA DB DATA DB DATA DB DATA 0 DB DATA 9 DB VrefA DATA DB VrefB DATA7 7 DB7 PA CS PA WR PA7 DACA/B AGND RfA DGND OUTA GND AD7 9 0 Vref UC TL0BCN UA TL0BCN R7 R9 0K R 0K R C 7 R 0K + - UB TL0BCN 7 - C 7 R + UD TL0BCN A phase B phase D 0V D N D N Triangular wave D N R D R N K C9 0 R K DGND AGND 0k 7 R 0k C A B UA TL0N R 0k UB TL0N Fig. Three-phase sine wave generation and triangular wave generation circuit diagram

3 Advanced Materials Research Vols () Dead-band delay and optical isolation circuit. In order to prevent inverter subversion, dead-band delay circuit is needed. Figure shows how the dead-band delay circuit is designed. SPWM A 9 C D D R 0k R 0k N R 0k UD CD09UBCN R 0k VCC U7 R 00 + VCC U9A VE VO B - GND C 0 N7 VCC U R9 00 C7 + VCC U9B VE VO D - GND 0 N7 7 R 0 PWM R7 0 7 R0 0 PWM R 0 Fig. Dead-band delay and the optical isolation circuit diagram With the help of the follow figure, how the circuit works is discussed as follows: SPWM signal is input signal, when SPWM signal coming, it has two way to go, one way connects to R, the another connects to CD09 s input pin. When the voltage level of point A is low, in other word, logic 0,point B s level is also low. The diode D is cut-off. When the level of A jumps from low to high, diode D is still cut-off, but the level of B is going to rise because of a charging loop (R and C )existing, the charging time t is: t = R C () When charging completes, both point B and point A s level are high(logic ). However, when A s level jumps from high to low, B s level is higher than A, in this case, diode D becomes conductive, then C turns to discharge, the discharging loop is C, R,D, or C,R, the discharge time is: t = ( R// R) C () Take advantage of the different charge and discharge circuit, it is easily to explain why time difference exists in the process of charging and discharging. If reasonably adjusting resistance size of R and R, different time difference is generated. A dead time occurs on point B and D because of time difference. The signal of Point B and D is respectively wave-shaped by CD0, then it goes through the fast optical isolation chip N7, the PWM waves for three-phase bridge are generated. The next figure reflects waveform diagram of the circuit, t is the dead time. VDD VDD C 0 C 0 Fig. Waveform diagram of Dead-band delay circuit () Drive circuit. IR0 is applied in driver circuit, who owns bootstrap circuit inside, which can reduce the number of power and make it easy to design drive circuit. IR0 is widespread used in both small and medium-sized power conversion device. The follow figure is the diver circuit. VDD U +V 9 VDD Vcc D PWM 0 7 G HER0 HIN H0 PWM G C C C C LIN L0 SD 0 0uF 0 0 SD C0 00uF Vss IR0 VB Vs COM XGND S Fig. Drive circuit XGND

4 Progress in Renewable and Sustainable Energy ()Three-phase bridge inverter circuit. Three-phase full-bridge circuit is adopted in DC-AC inverting, the switching device is, -channel PWM signals G~G respectively connect to the s gate, diodes D~ D in figure 7 are use for freewheeling, which can effectively protect the s. Ud G D D D N Q N Q N R 0 G R 0 G R 0 Q R7 R R9 L L L XGND G D D D N Q N Q N R 0 G R 0 G R 0 R0 R Fig.7 Three-phase bridge inverter circuit () Grid signal acquisition circuit. GPT-0B, a kind of voltage transformer, is used to collect Grid voltage signal, while GCT-0A, a kind of current transformer, is used to the collect Grid current signal. GPT-0B has pins, two pins are input that are called primary side, while the other two are output that are called secondary side. When the input current of primary side is ma,the output coupling current of secondary side is also ma, the phase difference is 7. The secondary side of GPT-0B connects to op amp NE. Adjusting R 7 to set the peak value of U in to.v,then adding a.v DC bias signal to U in, the final signal inputs to analog to digital conversion chip MAX97. Grid current signal acquisition is similar to the principle of Grid voltage signal acquisition. In order to ensure consistency of inverter, zero-crossing detection circuit for Grid is necessary. R Q Fig. Grid signal acquisition circuit Software Design The Software of embedded PV inverter Grid-connect system is designed with Keil uvision, C language is used for programming. System program consists of four parts, the Grid signal acquisition, data processing, sending out sine wave data and LCD display. When system startups, what to do first is initialization. Next, STMF0 begins to collect Grid signal and check whether Grid signal is normal, if lack of phase, power failure or overload occurs, STMF0 blockades signal output. If the Grid is normal, system keeps waiting until zero-crossing trigger on phase A, once the zero-crossing is detected, STMF0 recollects Grid signal, including A, B, C phase s voltage and current and calculates the sine data by look-up a sine table stored in STMF0, then it sends sine data of phase A, B, and C to SPWM wave modulation circuit. The modulated SPWM signal successively goes through dead-band delay and optical isolation cirduit, drive circuit, inverter main circuit. In this way, making the frequency, amplitude, phase of the system track the Grid comes true. Finally, repeat the above procedure. In addition, in order to make it convenient to debug the system, LCD display is necessary. The flow chart is as follows:

5 Advanced Materials Research Vols Conclusion Fig.9 The flow chat of main program for PV inverter Grid-connected system This paper have researched on how to design the hardware and software of PV inverter Grid-connected system, the system can track the Grid s frequency, amplitude and phase. The PV inverter Grid-connected system based on STMF0 meets the grid-connected requirements, proofing the hardware and software of system is reasonable. Next, the research should focus on reducing harmonic in the inverter process, improving inverter efficiency, and improving the stability of the system. The following figure shows modulated PWM waveform of A phase and the PV inverter Grid-connected system s output of Phase A, C waveform. Fig.0 Modulated PWM waveform of A phase and the PV inverter Grid-connected system s output of Phase A, C waveform References [] HU Xiaohang, YANG Jingchang: Grid-connection device based on TMS0F07 for photovoltaic power generation, Electric Power Automation Equipment, 0,():7-0. [] Liang Ma: Nonlinear PID control for three-phase PWM rectifier based on predictive current control, ICIEA, -,June 00, pp. 9-. [] Che Yanbo, LiuBin: Design of Three-phase Photovoltaic Grid-connected Inverter Based on DSP, Electric Power Automation Equipment, 00,():-7. [] Yao Chen, Xin Minjin: Modeling and Control of Three-phase Voltage Source PWM Rectifier, IPEMC, 00,vol., pp. -. [] Bonvin J. The Swiss choice:consumers fund green electricity in Switzerland, Renewable Energy Word, 00,pp. 9-0 [] F. Blaabjerg, C. Zhe, and S. B. Kjaer: Power electronics as efficient interface in dispersed power generation systems, IEEE Trans. Power Electron, 00, vol 9(), pp. -9

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