Nr 59

Prace Naukowe Instytutu Maszyn, Napędów i Pomiarów Elektrycznych Politechniki Wrocławskiej Nr 59

Studia i Materiały

Nr 26

2006

AC/DC converter, PWM rectifier, intelligent power module, digital signal processor, sensors, protections, prototype, design, experimental verification

Michał KNAPCZYK * , Krzysztof PIEŃKOWSKI* F

F

HARDWARE APPLICATION OF AC/DC CONVERTER BASED ON INTELLIGENT POWER MODULE AND DSP CONTROLLER

The paper presents the hardware application of the AC/DC converter based on 3.3 kW intelligent power module (IPM) and 32-bit digital signal processor (DSP). The AC/DC converters called also PWM rectifiers provide synchronous rectification or active filtering improving electrical power quality. The design process of the PWM rectifier has been described. The practical realization of the voltage and current sensors as well as control electronics and the protection systems has been presented. The complete laboratory setup has been assembled and demonstrated. Voltage Oriented Control has been implemented into the DSP to control the AC/DC converter prototype. A simulation model of the synchronous rectifier has been pre-examined to verify parameters of the linear PI controllers used in the experimental setup. Simulation and experimental results have been presented and discussed.

1. INTRODUCTION The increasing number of the modern power electronics converters is controlled by microprocessor-based systems [1, 2, 3]. The dynamic development of the microprocessor techniques results from the numerous advantages of the digital systems. Stability of the parameters, easy modifications of the control algorithms, the possibility of implementation of advanced control techniques as well as providing diagnostic functions are the major features of digital control devices. Nowadays modern digital control units have overtaken all control tasks. In contrary to diffuse analog electronics, digital processing devices are integrated on one small evaluation board. Besides the fast digital signal processor (DSP), the evaluation boards include analog-to-digital converter, input/output systems and memory. The DSP-based kits are specialized to realize control algorithms __________ *

Politechnika Wrocławska, Instytut Maszyn, Napędów i Pomiarów Elektrycznych, 50-370 Wrocław, ul. Smoluchowskiego 19, [email protected], [email protected]

mostly for the intelligent motion control applications. Hence they are equipped with the peripheral PWM modules to provide the firing pulses for the power converters. Because of their small size and high functionality the DSP boards have become the integral part of the controlled device, being the unified, self-sufficient, embedded system [6, 7]. In the range of the small and medium power ratings the semiconductor structures are usually produced in a form of the modules containing complete IGBT bridges. Further improvement in designing of power modules is the integration of protective and measuring electronics directly with the semiconductor structure inside the module or in its close vicinity in case of very small dimensions. Such intelligent power module provides overcurrent, under-voltage and over-temperature protection. The electronic control interface of the intelligent power module (IPM) transmits firing signals for the power transistors and simultaneously provides fault feedback information for the superior control unit scanning the routine work of the power module [8]. This paper presents the design process of the laboratory setup with the AC/DC converter and the DSP-based control unit. 2. DESIGNING OF THE EXPERIMENTAL SETUP OF THE PWM RECTIFIER For its proper operation the AC/DC converter requires the exact information about all available state variables [5]. The position of the grid voltage vector must be currently computed to provide the correct synchronization of the line current vector.

Fig.1. Diagram of the experimental setup of the AC/DC converter

In general two AC voltage sensors, two line current sensors and the DC-link voltage sensor are necessary to implement the basic control system of the synchronous rectifier. Fig.1 presents the diagram of the experimental setup of the PWM rectifier with its measurement, protection and control systems. The following sections describe the design process of the power module with its electronic components and the control unit. 2.1.INTELLIGENT POWER MODULE

The power unit of the proposed prototype of the AC/DC converter is based on the 3.3 kW power IGBT module by EUPEC® with the electronic interface EiceDRIVER™ 6ED003E06-F [8]. The reason of the widespread use of the IGBT transistors is the ease of their control since practically no power is taken during the switching process. The crucial problem in realization of the driver system for the three-phase IGBT bridge is the step change of the reference potential for the upper-side transistors. This inconvenience has been overcome by the application of the IGBT driver IR2136S by International Rectifier™ and the boot-strap technique.

Fig.2. Evaluation board EiceDRIVER™ with 3.3kW power module by EUPEC® [8]

Fig. 2 presents the evaluation board with the three-phase IGBT power module. Its electronic interface provides short-circuit, under-voltage, over-temperature and overcurrent protection by the shunt resistor, driver and comparator. The FS10R06VL4 power module by EUPEC® provides the maximal DC-link voltage Udcmax=300V and the maximal DC current Idcmax=10A. The minimum dead-time for the input control signals of the power module is 1.8μs. The evaluation board allows for interfacing with the DSP controller directly without using optocouplers. Hence the DSP controller may work on the converter negative DC-link node. However due to precautions the galvanic separation between the power module and the DSP-based control unit is recommended.

2.2. DSP-BASED CONTROL UNIT

For the realization of the control tasks in the experimental setup of the AC/DC converter the evaluation board eZdsp™ F2812 by Spectrum Digital® based on the TMS320F2812 Digital Signal Processor by Texas Instruments® has been chosen. Fig.3 presents the DSP-based control unit applied to the proposed laboratory setup of the PWM rectifier. The block diagram shows the signal processing routine.

Fig.3. eZdsp™ board based on TMS320F2812 DSP by Texas Instruments® [7]

The TMS320F2812 DSP stems from the C28x family of TI® microprocessors and has been design to execute programs written in C/C++. This is the fixed-point, 32-bit data word microprocessor with two overlapping data and program address spaces. The major features of the presented DSP are: 18K words on-chip RAM, 128K words on-chip FLASH, 64K words off-chip SRAM, 30MHz clock (operating frequency up to 150MHz), 56 multiplexed digital Inputs/Outputs, 12-bit 16-channel Analog-to-Digital converter (80ns) with the input voltage range from 0 to 3V, 45 interrupts divided into 8 levels of priority, 5V of supplying voltage. Besides the DSP includes two Event Manager systems for the applications to power electronics devices (PWM modulator to control three-phase two-level and three-level power converters) [4]. Before the TMS320F2812 DSP processes and executes a program it is required to configure manually all necessary registers and the ranges of the used memory since the microprocessor does not provide the ready-implemented CLS-type libraries. There is a large variety of the free environments helpful by configuring and programming the TMS microprocessors. The advanced programming environment for TI® microprocessors is CodeComposerStudio™. There are also the coupling platforms for rapid prototyping and coding like Embedded Target for TI C2000 Toolbox™. The executable C code based on the Simulink™ graphical model is then generated automatically.

2.3. VOLTAGE AND CURRENT SENSORS

For the low-power AC/DC converters it is admissible to couple the control system with the power unit throughout non-insulated measurement amplifiers for the voltage sensing. Despite the direct galvanic connection there is the large impedance (>1MΩ) between the power circuit and the microprocessor defined by values of measuring resistors and the input impedance of the operational amplifier itself. The amplifier operates in differential mode thus its output signal is proportional to the measured input voltage according to the ratio of the feedback and input resistance. Fig.4 presents the application of the two AC line-to-line voltage sensors based on the differential amplifier solution.

Fig.4. AC voltage sensor: (left) overview, (right) scheme

Fig.5 presents the application of the sensor designed for the DC-link voltage measurement. The principle of operation of the DC-link voltage sensor is based also on the differential routine. Both the AC and DC voltage sensor use TL072 operational amplifiers by TI®. For the maximal reduction of the voltage interferences as spikes or noises a number of small capacitors have been applied as the input voltage filters.

Fig.5. DC voltage sensor: (left) overview, (right) scheme

In the proposed control system of the PWM rectifier the current transducers have been applied. The operation of the current sensors is based on Hall effect thus the implementation of the shunt resistors in series to the power circuit is not necessary. These Hall devices provide galvanic separation between control electronics and the power unit enhancing the safety of the measuring and the control system. Fig.6 presents the threephase line current sensor based on the LA 50-P current transducers by LEM®. The measuring range can be easily adjusted by setting the appropriate number of “turns” of the current cable round the transducer’s core.

Fig.6. AC current sensor based on LEM® module: (left) overview, (right) scheme

Since the transducers output signals change their sign periodically the signal offset has been added with the help of the voltage divider and the current buffer based on LM358 single supplied operational amplifier. Hence the AD converter of the DSP receives appropriately scaled signals within its operational range. For the maximum enhancement of the DSP protection the outputs of voltage and current sensors have been equipped with the 1N5711 Schottky barrier diodes operating as the voltage limiters. 2.4.SUPPORTING ELECTRONICS

As it was mentioned by the discussion about the digital signal microprocessor, the TMS320F2812 DSP controller provides Event Manager modules including the hardware PWM modulator. Active high/low logic and minimum dead-time of the switching signals can be easily set in the software of the DSP. However while prototyping and experimenting with new control schemes it is recommended to provide the hardware deadtime module reliable in case of emergencies. Fig.7 presents the hardware dead-time module that has been designed using SN7414 Schmitt-trigger inverters, SN5406 buffers with open-collector outputs and the RC branches. Despite the high dependence on the values of parameters and the operating temperature the dead-time module provides sufficient functionality in the laboratory setup by the typical ambient conditions.

The module splits the control signal dedicated for the one converter leg into two signals for the upper and lower IGBT transistor and provides the active low operation.

Fig.7. Hardware dead-time, control signal splitting and inversion: (left) overview, (right) scheme

Fig.8 shows the principle of operation of the proposed hardware dead-time module.

Fig.8. Control signals with dead-time: (left) detailed diagram, (right) experimental measurement

For the sake of the safe operation of the digital control system the galvanic separation based on the PC849 Sharp® opto-couplers has been introduced (Fig.9).

Fig.9. Galvanic separation based on PC849 opto-couplers: (left) overview, (right) scheme

The prototype of the AC/DC converter has been assembled and coupled with the host computer equipped with CodeComposerStudio™. Fig.10 demonstrates the overview of the experimental setup with the PWM rectifier and the supporting equipment.

Fig.10. Overview of the experimental setup

3. EXPERIMENTAL INVESTIGATIONS AND SELECTED RESULTS The operation of the proposed AC/DC converter experimental setup has been verified by the implementing into the DSP the control algorithm based on the Voltage Oriented Control (VOC) technique. The vector and block diagram of the VOC have been presented in Fig.11. This technique consists in the appropriate forming of the line current vector ig in the synchronous reference frame aligned with the line voltage vector eg.

Fig.11. Voltage Oriented Control with Carrier-Based Sinusoidal PWM: a) vector diagram; b) block scheme

The DC-link voltage PI controller determines the value of the x-component of the line current vector ig. The igy component is set to zero to provide Unity Power Factor operation of the PWM rectifier. The measured line currents are transformed into the (xy) synchronous coordinates in order to obtain their rectangular components. The voltage vector angle ϕ necessary for the Park transformation is computed upon the AC line-toline voltage measurement. Next the line current errors are calculated and forwarded to the inputs of the two linear PI current controllers. The line current controllers determine the converter input reference voltage that is next transformed into the (ABC) coordinates. Finally the three-phase converter input reference voltages are compared with the carrier triangular signal resulting in the control PWM pattern for the power transistors. Tab.1. Parameters of the experimental setup Source phase voltage eg: Source voltage frequency fg: Line choke resistance Rg: Line choke inductance Lg: DC-link capacitance Cd: DC-link nominal voltage Udc: Nominal load resistance Rload: Carrier frequency fcarrier DSP sample time Tp:

14.4 V 50 Hz 100 mΩ 7.8 mH 1000 μF 60 V 60 Ω 3 kHz 50 μs

Tab.1 presents the values of parameters of the laboratory setup and supplying conditions. The experimental investigations have been carried out by the lowered grid voltages coming form the isolation transformer. Fig.12 shows the line currents by the step change of the converter load. The examined control system of the converter provides the sinusoidal line currents without the intake of the reactive power (Unity Power Factor).

Fig.12. Line currents: (left) simulation (with line phase voltage), (right) experiment

The dynamics of the DC-link voltage control loop depends on the performance of the PI voltage controller. The converter output DC voltage presented in Fig.13 tends to its reference value with the neglectable small overshoot.

Fig.13. Transient of DC-link voltage at step change of the converter load: (left) simulation (including charging the DC-link capacitor at start-up), (right) experiment

Fig.14 demonstrates the converter input line-to-line PWM voltage. The resulting PWM voltage drop over the line chokes forces the flow of the sinusoidal line currents.

Fig.14. Converter input line-to-line PWM voltage: (left) simulation, (right) experiment

4. CONCLUSIONS

The paper presents the practical approach to design of the experimental setup of the AC/DC converter. The proposed prototype of the PWM rectifier is based on the 3.3kW intelligent power module (IPM) and the DSP-based control unit. For the proper opera-

tion the PWM rectifier requires the feedback information about all state variables. Hence the necessary voltage and current sensors have been constructed and examined. The supporting and protective electronic devices have been designed to enhance the safety of the AC/DC converter and its control system during the different conditions. Voltage Oriented Control of the PWM rectifier has been implemented into the DSPbased control system. The advanced programming environment has facilitated coding in C language and provided the user-friendly interface between the host computer and the DSP evaluation board while executing the control programs. The experimental results have confirmed the proper approach to the design process showing the precious advantages of the AC/DC converter in the real applications. REFERENCES [1] BOSE B., Modern Power Electronics and AC Drives, Prentice Hall PTR, 2002 [2] BORISAVLEVIC A., IRAVANI M. R., DEWAN S. B., Digitally Controlled High Power SwitchMode Rectifier, IEEE Transactions on Power Electronics, vol.17, no.6, November 2002. [3] BINGSEN W., CATHEY J. J.,DSP-controlled space-vector PWM current source converter for STATCOM application, Electric Power Systems Research 67, March 2003. [4] eZdsp™ F2812 Technical Reference, Spectrum Digital Inc., February 2003. [5] QIU D. Y., YIP S. C., CHUNG H., HUI S. Y., On the Use of Current Sensors for the Control of Power Converters, IEEE Transactions on Power Electronics, vol.18, no.4, July 2003. [6] MIHALANCHE L., A High Performance DSP Controller for Three-Phase PWM Rectifiers With Ultra Low Input Current THD Under Unbalanced and Distorted Input Voltage, 40th IEEE IAS Annual Conference, October 2005. [7] Texas Instruments - www.ti.com [8] ZHOU K., WANG D., Digital Repetitive Controlled Three-Phase PWM Rectifier, IEEE Transactions on Power Electronics, vol.18, no.1, January 2003. [9] ZHIHONG L., KEGGENHOFF R., EiceDRIVER™ 6ED003E06-F - Evaluation Board for EasyPACK750 – Datasheet and Application, EUPEC GmbH, July 2003.

LABORATORYJNY UKŁAD PRZEKSZTAŁTNIKA AC/DC Z ZASTOSOWANIEM INTELIGENTNEGO MODUŁU MOCY I PROCESORA SYGNAŁOWEGO Artykuł przedstawia proces projektowania i wykonania układu laboratoryjnego przekształtnika AC/DC z zastosowaniem inteligentnego modułu mocy 3,3kW i procesora sygnałowego DSP. Przekształtniki AC/DC zwane również prostownikami PWM zapewniają pobór sinusoidalnych prądów z sieci zasilającej lub aktywną filtrację zniekształconych prądów odbiorników nieliniowych zainstalowanych w bezpośrednim sąsiedztwie. Przedstawiono proces projektowania poszczególnych elementów składowych układu eksperymentalnego prostownika PWM: modułu mocy, czujników napięcia i prądu oraz elektronicznych układów wspomagających i zabezpieczających. Zaprezentowano charakterystykę mikroprocesora sygnałowego DSP zastosowanego do realizacji zadań sterowania. Zaimplementowano metodę Voltage Oriented Control (VOC) do sterowania układem przekształtnika AC/DC. Przedstawiono i omówiono wybrane wyniki badań symulacyjnych i eksperymentalnych.