A Modular Simulink-Based Controlled Three- Phase ...

A Modular Simulink-Based

Anawach

Sangswang

Controlled Three- Phase Switch Mode Inverter George

Rest

C.O. Nwankpa

Centerfor ElectricPowerEngineering Departmentof Electricaland ComputerEngineering DrexelUniversity Philadelphia,Pennsylvania

Abstract: Theprimary goal ofthispaper istopresent the design and construction of a Digital Signal Processor (DSP) controlled three-phase inverter based on Simulink and Real Time Workshop Toolbox from MathWorks. Traditionally various switching schemes, such as Pulse Width Modulation (PWM) and Square Wave Switching, in addition to, reduction ofharmonics through power electronic switching are studied in class, however, their tangible effects cannot be seen without simulation and/or laboratory experiments .The purpose of this inverter is to provide a modular educational platform with minimal setup time to study the operation of DC-AC converters. Access to the inverter is provided through a variety of measurement sensors for data acquisition purposes, which emphasizes hands-on laboratory experience. An actual detailed setup and experimental procedure is presented that outlines the platform’s ease of use and accuracy. 1. Introduction Because advances in semiconductor of enormous technology, power electronics devices with high power handling capabilities are commercially available and inexpensive, Power electronics are playing an important role in today’s technology; they have an increasing number of applications in the industrial and domestic areas. The major fimction of power electronics is to control the flow of power by shaping the supplied power from the source. Power electronics also introduces distortion of the output waveform and injects harmonics into the supplier system. These harmonics can be reduced by using appropriate filter circuits. In addition, the advancement of computer simulation and microprocessor technology makes various control schemes involving power electronics to become more flexible and easier. From educational side, computer simulation has facilitated laboratory teaching in power engineering related fields like, power electronics. The wide variety of computer simulation programs such as PSpice, Matlab, and Simulink allow students access to circuit waveforms, dynamic and steady-state behavior descriptions among others [1]. In conjunction with this, laboratory setup in power electronics can also be a valuable experience, which highlights what students have learned in class. In general, with computer simulation it is much easier to study the inftuence of a parameter on system behavior than a simulation through a hardware laboratory. While in industry, combination between computer simulations and hardware implementation has been used to shorten the

overall design process. This method becomes ideally suitable in today’s curricular agenda where limited time is allocated for laboratory sessions. Commercial training laboratory systems available tend to be rather inflexible and expensive [2]. In recent years, many universities have developed laboratory setups for power electronics courses directed towards the use of computer simulation or computers interacting with hardware. For example, a software tool for power electronics circuits at Hong Kong Polytechnic has stressed the necessity for realistic time-domain simulations [3]. The University of Missouri-Rolls has implemented a facility for computer control of electrical drives and machines [4]. Similar facilities have been set up at Montana State University [5]-[6]. The primary goal of this paper is to present the design and construction of a Simulink-based digital signal processor controlled three-phase inverter. The purpose of this inverter is to provide an educational platform for students to study the operation of DC-AC Converters in a laboratory environment. The Simulink-based controlled laboratory setup allows students to adjust parameters and quickly iterate to achieve required results. The Simulink toolbox in Matlab is used to simulate switching schemes and the C code for a DSP board is generated by Real Time Workshop. Using these technologies along with a readily available DSP board from Texas Instruments Inc. (TI), a customized code is directly generated and downloaded to the control board. The marked differences between this three-phase inverter and commercial inverters are: ●

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●

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Measurement sensors: Provide access to a diversity of measurements to basically every part of the inverter. Data acquisition: Measurements can be taken directly horn these sensors for analytical purposes. Experimental Model: Assorted types of switching schemes and loads and can be easily added and tested. Sirnulink-based control: It employs graphical user interface (GUI) for building and simulating models stressing quick setup times.

With respect to the experiment discussed in this paper, it is desirable to show how the inverter is used for the purpose of demonstrating and analyzing the inverter waveforms and

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characteristics. The problems that had to be resolved to accomplish the goal, is discussed in the next section. Section 3 presents the methodology to the solution for these problems, Experimental set-up and results of Square Wave and pulse-width modulation (PWM) switching schemes are shown in section 4. Conclusion of this paper is in section 5. 2. Problem Formulation The objective of the project was to design a three-phase inverter as an educational platform to garner student appreciation of inverter behaviors, applications and effects. In order to realize the objective, it was necessary to build an inverter that is able to run with different types of switching schemes and loads, A DSP board and computer interface are required to accomplish control schemes and data acquisition.

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of the average current, rms current, peak current and peak inverse voltage. 2.1.2 Filters The only filter that will be used in this project is the rectifier output filter. Since the output of the rectifier is dc, a low-pass filter is desired. A dc filter is designed to smooth out the dc output and filter out harmonic contents on the input of the inverter. L.

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Fig.1 InverterSystemSetup As shown in figure 1, the overall system consists of a threephase input ~ower’ supply, rec&3er, filter, full bridge inverter, and load. The three-phase input power supply is fkom the utility. A diode rectifier converts an ac input to de output in an uncontrolled manner. A low-pass filter makes the dc output to be as ripple free as possible. A three-phase full bridge inverter changes a dc input into a symmetrical ac output. Since dc input is not controllable, the ac output of the inverter is controlled by pulse-width-modulation (PWM) and fed to the load. From the inverter setup block diagrams, relevant hardware and software issues are discussed as follows. 2,1 Hardware Features There are seven main issues that are included in the hardware features. 1) Obtaining dc input for the inverter by using a rectifier circuit. 2) A filter circuit is used to reduce voltage ripple. 3) Inverter 4) Measurement sensors for controlling switching 5) Gate drive circuitry operation 6) DSP controller 2.1.1 Rectifier Circuit The finction of a rectifier is to convert an ac signal into a unidirectional signal. The input is directly supplied from the utility. The rectifier draws highly distorted current during turn off time in each diode and generates harmonics to the load at the same time. In order to obtain the dc input voltage, a three-phase diode bridge and a filter are used to achieve lower ripple content in the waveforms and higher power. The ratings of diodes are to be determined in terms

The figure above shows that the higher order harmonic contents are attenuated by the inductor, while the capacitor helps keeping the dc voltage constant, In addition, the capacitor also acts as a snubber for switches in the inverter. It is necessary to make the load impedance much greater than the impedance of the capacitor; to allow the nth harmonic current to pass through the filter capacitor. 2.1.3 Inverter The fimction of the inverter is to change a de input voltage to a symmetrical ac output voltage with controllable magnitude and fi-equency. The dc input of the inverter is obtained from output of the low-pass filter of the rectifier. Each switch of the inverter is operated by a small control signal. Switches in the same leg are not turned on at the same time, to prevent short circuit in a leg. Blanking time must be added to make sure that there are no two switches in the same leg turning on at the same time. Switches in this inverter are fully controllable power devices with fast switching speed, because this inverter is expected to handle various kinds of switching schemes, including ones that require high switching speeds. High negative voltage blocking capability is also necessary for preventing the switch ffom damage. The voltage and current rating of power devices in the inverter depends on the types of load and switching schemes. For unidirectional switching, as in this inverter, an anti-parallel connected diode is needed to provide a path for the current in the event that there is an inductive load. If an attempt is made to open switch the energy stored in the inductor will be transformed in order to maintain the direction of the current. This may cause damages to the switch. Such a diode is called a fi-eewheeling diode. 2.1.5 Drive circuits The function of a drive circuit is to turn the switch from an off state to on state and vice-versa. The power rating of the drive circuit may vary depending on the type of switches being used. A drive circuit also creates a blanking time for switches. Therefore, a high-speed drive circuit is desired for such an inverter that is able to operate with various switching

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schemes including high speed switching. A drive circuit must electrically isolate the control signal fi-om power switch, for safety reason and undesirable interferences. Overcurrent protection for the power switch is also essential, in which a communication between a drive circuit and control board is required.

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* 2.1.6 Measurement sensors Voltage and current sensors will be following points. ● Input of the rectifier ● Output of the rectifier (DC bus) ● Output of the inverter

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The measurements are used for analytic and protection purposes. Because this laboratory setup is designed for students, the safety requirement with regard to isolation from high voltage equipments is an obligation. All voltage and current measurements at each part must be accessible through measurement point terminals. Measurement sensors must have excellent accuracy and high immunity to external interference. Wide frequency bandwidth is also necessary for the sensors to handle dc signals in the dc link to ac signals at the output of the inverter. Hall effect devices are perfectly fit for this setup. 2.1,7 DSP Controller This laboratory setup is designed for real-time control. This requires such technology as a floating-point DSP, which offers opportunities to develop computationally intensive applications that were previously not feasible by microprocessors. For flexibility reasons, a control board must be a self-contained system, without occupying a host PC. The board can communicate with the host PC by using a standard RS232 serial port. This allows any set of readily available computers in the laboratory to interface with the control board.

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Fig 3. A modularSimulink-basedcontrolscheme 2.2.2 Real-Time Workshop Real-Time Workshop is a toolbox equipped in Matlab Simulink. It allows C code to be generated from block diagrams in Simulink. Once a system has been designed and simulated in Simuhnk, code for digital signal processors can be generated, compiled, linked, and downloaded to the DSP board. By providing a device driver block in Simulink, the generated executable code will be able to run on a specific hardware [8]. With Real-Time Workshop, various switching schemes can be easily added and implemented with a minimal setup time. 3. Methodology 3 - # Full Bridge

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2.2, Software Features As an educational platform, it is desired that this setup must be able to demonstrate how a variety of switching signals are generated. In order to appreciate the result of the computer simulation, it must allow for implementation of the generated signals on the real inverter.

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2,2,1 Matlab Simulink Simulirik is a software package for creating, editing and simulating dynamical systems on Matlab fkom Mathworks, Inc [7]. It is widely used in industry and academic institutions, Simulink utilizes a graphical user interface (GUI) for building models where models are created as block diagrams. Simulink also allows complete models to be simulated by a variety of integration solvers. In addition, users can Ghange parameters and immediately see what happens for “what it” exploration. While the simulation is running, users can see the result via display blocks, such as scopes.

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Fig.4DSP ControlledThree-PhaseInverter The experimental setup consists of a rectifier, full bridge inverter, gate drive circuitry, control circuit, filtering and protection circuits. Protection circuits sense overcument ffom the switches and feedback a signal to the controller. The inverter was designed to provide accessibility of sensors for

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data acquisition, Measurements parts of the setup. All hardware discussed separately.

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3.1 Hardware Features The entire package was designed to handle 60A load current, which gives it a three-phase power rating 20KVA. The electrical design considerations and processes are given as follows. 3.1.1 Measurement sensors In order to obtain current signals, LA 1OO-P current transducers from LEM are used with a 100A primary current limit and a 1:2000 conversion ratio. These Hall effect current sensors are installed at various points in the inverter. A 12V dc power supply is required for these current transducers. Their outputs are accessible through measurement points placed on the extender of encased box, which are clearly labeled. This setup does not require special voltage sensor for voltage measurement, but a voltage differential or high voltage probe is used to avoid short circuit to mutual ground of the oscilloscope, Both input and output voltages can be subsequently obtained directly from their terminals. 3.1.2 Rectifier Circuit A three-phase full bridge rectifier was chosen because of its high power capability, lower ripple content and less distortion compared to a single-phase rectifier. From calculation with the line-to-line input voltage of 208 Volts and input current of 60 Amps at the ffequency of 60 Hz, a three-phase diode bridge rectifier with peak inverse voltage of 300 Volts and average current limit through each diode of 65 Amps was selected.

measurement point. In the rectification, the rate of change of the voltage across the diode (dv/dt) is not harmfid to the diode and the rectifier noise is very small compared with switching noise created by the inverter. Because of this snubber circuits are not used. 3.1.3 Filter The inverter is designed to handle a range of loads, therefore only the relationship between the inductor and capacitor can be determined[Rashid]. For our anticipated loads this relationship is: LX C=10.051X10-6 Since the inductors are not commercially available in a wide range of properties, it is a good idea to acquire or build an inductor fwst and then size the capacitor. An inductor was wound and tested, using a steel core and #6 wire. It has an inductance ranging fi-om 5 .5mH to 14.3mH over a varying load current. Due to availability, a 450V, 1700uF capacitor was chosen therefore, the Lx c relationship ranges from 9,35x 10-cto

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3.1.4 Inverter The inverter consists of 2 switches in a phase. Its input is from the rectifier output. Each switch is controlled by a gate drive circuit. Gate driver circuits

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There are 3 Hall-effect current sensors placed at the input of the rectifier to measure input current in each line. At the output of the rectifier also has a Hall effect current sensor installed to obtain dc current waveforms. For voltage measurement, each input terminal of the rectifier and both dc output terminal of the rectifier are also connected to the

Insulated gate bipolar transistors (IGBTs) are chosen because of their high speed switching frequency and low switching power loss. In addition, they do not require much power for the gate drive circuits. Because IGBTs have high impedance gates, a small amount of energy is sufficient to turn the IGBT on. A 100A 600V P-Channel half-bridge IGBT with antiparallel diodes inside is used as 2 switches in each phase. They can handle up to 40kHz in hard switching and 200kHz in resonant mode. Hall effect devices are mounted with the inverter output to measure output current.

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3.1.5 Gate driver The major functions of gate drive circuits are to amplifi the control signals fi-om the control board and electrically isolate control signals from high voltage side of switches. ‘?

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CAD, The diode bridge and the IGBTs are mounted on heat sinks in order to help dissipate their heat. Two small fans are also used to draw the hot air out of the box and keep all the components cooled. The control board is mounted flat in an isolated metal box to reduce electromagnetic interference.

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Fig. 7 Gate drivercircuit The control signal voltage is applied, at the point (a) in the figure above, across the base-emitter of a switching transistor, which then turns on and off creating a varying switching signal. Output signals (c) and (d) are connected to source (S) and gate (G) of the IGBT, respectively as shown in figure 6. This signal is optically isolated from the control signal to avoid unwanted interference. This gate drive circuit also has built in overcurrent protection. It senses the current from IGBT’s drain (D) through point (b), and determines whether the inverter is handling too much current or not. If overcurrent is sensed the gate driver turns the IGBT off and then sends a control signal back to the controller, Maintaining electrical isolation between systems, and terminating signal interconnect shields correctly, minimizes the possibility of electromagnetic compatibility problems. 3.1.6 DSP board The TMS320C31 DSP is used as a control board. This board provides a high speed Harvard architecture designed for fast execution of complex signal processing algorithms, The board is supported by the following external peripherals: three 82C54 16 bit timers, two 4:1 multiplexed DSP 102 A/D converters, one Z80C30 serial controller supporting 2 serial ports, two 82C55 parallel peripheral interface supporting 48 bits of 1/0. Application of the DSP and its accompanying peripherals can be programmed with the help of C source code flashed onto an EEPROM chip located on the board. Hardware configuration is further facilitated by on board jumpers and external triggering options [9]. 3.1.7 Mechanical placement Placement of the all circuits inside the laboratory setup is a critical design consideration. Measurements were taken on all of the major components and the layout was drawn up on

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In addition to the metal box, the control board is mounted away fi-om the inductor as far as possible. The gate driver circuits are centrally located since it is wired to power supplies, the control board and IGBTs. The three-phase input and output terminals are compatible with the current laboratory setup. There are measurement points at each terminal so that the voltage can be probed. There are also dc measurement points connected to Hall effect devices. 3.2 Software Features Simulink-based software features are divided into 2 parts as shown in the flow chart in Fig. 3. In addition, the TMS320C3X code generation tools from TI and the SBC31 monitor program ffom Innovative Integration are needed to successfully download the executable output file to the control board. 3.2.1 Simulink There are 2 switching schemes created in Simulink, threephase square wave and PWM switching schemes. A 3-phase square wave is created by, using 3 pulse generators from the source library in Simulink. Each pulse generator was set to a 50 percent of duty ratio and a phase of 120 degrees. The tlequency of the square wave can be entered or changed by users. Simulation results can be seen immediately, via the scope or other display blocks, while the simulation is mnning. A PWM signal is created by comparing a triangular wave with a sinusoidal wave (sinusoidal PWM type). Since Simulink does not provide a triangular signal generator, it has to be created. Users can enter the modulating frequency, tiequency modulation ratio ( m f ) and amplitude modulation ratio (ma

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in Simuhnk.

Real-Time Workshop (RTW) Real-Time Workshop (RTW) generates source code as defined by the Simulink model. A device driver block must be included in Simulink model in order to run with a specific model, The following steps in RTW are needed: 1) Create main program for the control board to execute the generated code. 2) Create a system target tile. This allows the Target Language Compiler (TLC) to transform the model into generated code. 3) Create a template make’iile to build the real-time executable code. 4) Create device drivers to communicate between a real-time program and an 1/0 device. A main program and the generated model code are compiled and linked together with the device drivers by a template makefile to build an executable code. This code is downloaded to the control board.

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4. Experimental setup and results The entire system has a rating of 20kVA. It has been used as a three-phase inverter experimental setup for the power electronics course at Drexel University. During the experiment, students create and edit the model and then run an offline simulation for each switching scheme, in Simulink. The simulation result can be seen via the scope block. After the required simulation result is obtained, the RTW is invoked to generate an executable code. This code is downloaded to the control board via the monitor program. Students can change parameters in the model such as, frequency and modulation ratio and then, see the result.

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It makes such computational intensive algorithms as advanced real-time control of the inverter feasible, by use of a DSP board. This setup was designed based on ease of use and access for measurement data. In Simulink, students are introduced to an effective design process, which allows immediate results to be seen. Together with Real-Time Workshop, the computer simulation result can be easily implemented with a minimal setup time. 6. Acknowledgement This research was supported by the National Science Foundation under Project Number ECS-9453407. The authors would also like to thank Dr. Amadi Nwankpa, Ann Arbor, for his valuable University of Michigan, suggestions on the software design of this setup. 7. References [1]

[2] [3]

[4]

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The plots in fig. 9 and 10 show the output screen shots of the 3-phase square wave and PWM, respectively. The square wave frequency is set to 60 Hz and the PWM setting is Ma = 0.6 and Mf = 15. They are obtained from measurement point terminals through the oscilloscope. 5. Conclusion The proposed experimental setup emphasizes a correlation among computer simulation and real-time data acquisition.

[5]

Mohan, Ned Power Electronics: Computer Simulation, Analysis and Education Using Pspice (Release 9), Minnesota Power Electronics

Research& Education, 1999, dSpaceInc., Solutionsfor control,Nortbville,Michigan,1999. K.Chauf, “A SoftwareTool for Learning the DynamicBehaviorof Power ElectronicsCircuits” Vol. 39, No. 1, February 1996,pp. 5055. J, D, Morgan and M. H, Tranter, “Industry-Unh’ershyCooperation with Emphasis on a Machines and Drives Laboratory”, IEEE Transactions on Education, Vol. E-29, No, 2, May 1986, pp. 115119, M. H. Nehrir,A. J. Odermann,and B. D. Bowen, “A Microcomputer-

Microprocessor-Based DC Motor Speed Controller for Undergraduate Electric Machinery Laboratory”, IEEE Transactions on Educafion, Vol. 33, No, 4, November 1990, pp. 341-345. M. H, Nehrir, V. Gerez, and A. J. Odermann, “A Microcomputer[6] Controlled Thyristor Bridge Rectifier Experiment for Undergraduate Electric Machinery Laboratory”, IEEE Transactions on Education, Vol. 37, No. 1, February 1994, pp. 101-106, MathWorks Inc., Simulink, Dynamic System Simulation for Matlab, [7] Natick, Massachusetts, 1998. MathWorks Inc., Real-Time Workshop User’s Guide, Natick, [8] Massachusetts, 1998, [9] Innovative Integration, SBC31 C Language Supplement, Westlake Village, California, 1994, [10] S.P, Carullo, R, Bolkus, J. Hartle, J. Fey, C.O. Nwankpa, and R. Fischl, “Interconnected Power System Laboratory: Fault Analysis Experiment”, IEEE Transactions on Power Systems, Vol. 11, No. 4, November 1996, pp. 1913-1919.

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