With 27 levels of voltage, a three-stage converter can follow a sinusoidal waveform in a very precise way. It can control the load voltage as an M dev

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1 High Power Machine Drive, ased on Three-Stage onnection of H onverters, and ctive Front End Rectifiers. Juan Dixon, lberto retón, Felipe Ríos Department of Electrical Engineering Pontificia Universidad atólica de hile asilla 36, orreo 22, Santiago, hile fax jdixon@ing.puc.cl Luis Morán Department of Electrical Engineering Universidad de oncepción asilla 53-, oncepción, hile fax lmoran@renoir.die.udec.cl bstract. three-stage inverter using H converters is being analyzed for high power machine drive applications. The great advantage of this kind of converter is the minimum harmonic distortion obtained at the machine side. The drawbacks are the isolated power supplies required for each one of the three stages of the multiconverter. In this paper this problem has been overcome in two ways: 1) by using independent windings for each phase of the motor, or 2) by using independent input transformers. Special configurations and combinations of passive rectifiers and active front end rectifiers for one of the stages of the drive are used to eliminate all input harmonics. The topology can also keep unity power factor at the input terminals. Simulation results are shown and some experiments with small four-stage prototypes are displayed. The control of this multi-converter is being implemented using DSP controllers, which give flexibility to the system. I. INTRODUTION Power Electronics devices contribute with important part of harmonics in all kind of applications, such as power rectifiers, thyristor converters, and static var compensators (SV). On the other hand, the PWM techniques used today to control modern static converters such as high power machine drives, strongly depend on switching frequency of the power semiconductors. Normally, voltage (or current in dual devices) moves to discrete values, forcing the design of machines with good isolation, and sometimes loads with inductances in excess of the required value. In other words, neither voltage nor current are as expected. This also means harmonic contamination, additional power losses, and high frequency noise that can affect the controllers. ll these reasons have generated many research works on the topic of PWM modulation [1-4]. More recently, multilevel converters [5-7] have permitted to have many levels or steps of voltage to reduce the THD levels. Multi-stage converters [8, 9] work more like amplitude modulation rather than pulse modulation, and this fact makes the outputs of the converter very much cleaner. This way of operation allows having almost perfect currents, and very good voltage waveforms, eliminating most of the undesirable harmonics. nd even better, the bridges of each converter work at a very low switching frequency, which gives the possibility to work with low speed semiconductors, and to generate low switching frequency losses. The objective of this paper is to show the advantages of multi-stage converters for high power machine drive applications. The drawbacks of requiring isolated power supplies is solved using different /3/$ IEEE. 226 techniques, based on the fact that the first converter, called, takes more than 8% of the total power delivered to the machine. three-stage converter using H power modules, which gives 27 different levels of voltage amplitude is studied. The current and voltage waveforms for a standard 4 kv, 2 MW induction machine is simulated. There are also some experiments with a small laboratory prototype, using a four-stage three-phase converter. II. SIS OF MULTI-STGE ONVERTERS. asic Principle The circuit of fig.1 shows the basic topology of one converter used for the implementation of multi-stage converters. It is based on the simple, four switches converter, used for single phase inverters or for dual converters. These converters are able to produce three levels of voltage in the load: Vdc, -Vdc, and Zero. Vdc LOD Fig. 1. Three-level module for building multiconverters The Fig. 2 displays the main components of a three-stage converter which is being analysed in this work. The figure only shows one of the three phases of the complete system. s can be seen, the dc power supplies of the four converters are isolated, and the dc supplies are scaled with levels of voltage in power of three. The scaling of voltages in power of three allows having, with only three converters, 27 (3 3 ) different levels of voltage: 13 levels of positive values, 13 levels of negative values, and zero. The converter located at the top of the figure has the biggest voltage, and will be called. The other two modules will be the Slaves. The works at a lower switching frequency and carries more than 8% of the total power, which is an additional advantage of this topology for high power machine drives applications.

2 With 27 levels of voltage, a three-stage converter can follow a sinusoidal waveform in a very precise way. It can control the load voltage as an M device (mplitude Modulation). The Fig. 3 shows the voltage modulation of each one of the Three H converters, for 1% amplitude modulation. 9xVdc. Power Distribution One of the good advantages of the strategy described here for multiconverters is that most of the power delivered to the machine comes from the. The example of Fig. 4 shows the power distribution in one phase of the three-stage converter, feeding a pure resistive load with sinusoidal voltage. little more than 8% of the real power is delivered by the converter, and only 2% for the Slaves. Even more, the last slave only delivers 5% of the total power. That means, the dc power sources needed by the second Slave is small. 5% POW ER IN 2 ND SLVE Machine phase 3xVdc 1 st Slave 15% POW ER IN 1 ST SLVE 8% POW ER IN MSTER Vdc 2 nd Slave Fig. 4. ctive power distribution in a four-stage converter. Fig. 2. Main components of the three-stage converter. The Fig. 3 shows the voltage modulation of each one of the three converters of the chain of Fig ND SLVE 1 ST SLVE This characteristic makes possible to feed the second Slave with low power dc supplies. However, as in some levels of low voltage regulation the power goes through the system, the sources need to be bi-directional. There are three solutions for this problem: 1) active front-end rectifiers, 2) bi-directional dc-dc power supplies, or 3) passive rectifiers with dissipative resistors. In the last case it should be required to evaluate the power losses. nother attribute of this configuration, which is possible to see in Figs. 3 and 4, is the very low switching frequency of each converter, specially the, which carries most of the power. Then, the larger the power of the unit, the lower its switching frequency. In the case analized here, the has been implemented with GTOs, and the Slaves with IGTs. MSTER Fig. 3. Voltage modulation in each converter 227 III. INPUT POWER TOPOLOGY s it was already mentioned, isolated power supplies for each converter are required. In Fig. 5 the electrical schematic of the complete power part including rectifiers and inverters is displayed. The three s are fed with standard rectifiers, each one in 6-pulse configuration. These rectifiers are isolated from the supply by a four winding transformer, to create three secondary voltage systems, one for each of the three s, and shifted in 2, and 2. With this configuration, a very low harmonic distortion from the mains point of view is obtained [1]. Each one of the first Slaves ( Slaves 1, and in Fig. 5), which carries 15% of the total power, needs

3 bidirectional power supplies because at some low voltage operation the power goes from the machine to the mains. To solve this problem, three PWM active rectifiers are used. The advantage of using this type of rectifier at this stage is that they work as power factor compensator and active power filters from the mains point of view, allowing to have almost perfect current waveforms at the supply side. Finally, the second Slaves ( Slaves 2, and in Fig. 5), are fed with simple Graetz bridges with a dissipative resistors, which are necessary when the machine operates with very low voltage (less than 15%) during starting. However, they can also be implemented with PWM rectifiers like Slaves 1. 6-pulse conv H bridges N pulse conv PWM conv H bridges M 2-2 PWM conv 8% Power 6-pulse rect 15% Power Fig. 6. nother topology using independent motor windings 6-pulse rect III. SIMULTED WVEFORMS 5% Power The following simulations were performed using PSIM, a special simulator for power electronics circuits [11]. The Fig. 7 shows the output voltages and the motor current produced by the three-stage converter. The converter voltage, the phase-to-phase voltage, the phase-to-neutral voltage, and the motor current are displayed. In the case of Fig. 6 topology, the output voltage of the three-stage converter is the same as the phase-voltage of the machine, because the windings are independent and isolated. The machine is a 2MW, 4kV induction motor. M Fig. 5. One of the proposed topologies for high power drives The drawback of the configuration of Fig. 5 is that the power rectifiers of the s need a good filter at the dc link, because each represents a single-phase load. To avoid this problem, the three s can be fed in parallel, keeping the transformer configuration with the rectifiers connected in series as shown in Fig. 6. However, the three windings of the machine have to be fed independently (no electrical connection between them). Fig. 7. converter voltage, phase-to-phase voltaje, c) phase-to-motor neutral voltage and current. c) 228

4 The Fig. 8 shows the harmonic spectrum of machine voltages for the case of Fig. 5 and Fig. 6. It can be noticed that the spectrum is cleaner for the case of Fig. 5 (machine with neutral connection), but in both cases the amplitude of the higher harmonics is less than 1%, and hence the amplitude of current harmonics are absolutely negligible % % c) Fig. 8. Voltage harmonics spectrum at the motor windings Fig. 6 topology, Fig. 5 topology The Fig. 9 shows the three phase-voltages generated by the thre-stage converters, and the machine current, which looks perfectly sinusoidal. On the other hand, the Fig. 1 shows the current distortion when the voltage of the machine varies from 1% to 1%, for the case of Fig. 6 topology (independent no neutral connection windings), which is the worst of the two systems from the machine point of view. It can be noticed that the current remains almost sinusoidal even with 25% voltage amplitude, without the need of PWM modulation. For this simulation the frequency and the slip of the machine have been kept constant. d) e) Fig. 1 current waveform distortion at the machine 1% voltage, 75% voltage, c) 5% voltage, d) 25% voltage, and e) 1% voltage Fig. 9. phase voltajes at the three phases of the converter, and winding voltage and current for Fig. 6 topology It is also important to show the power distribution in each stage of the power converter, particularly in some cases where the power is reversed with the voltage variations. The Fig.11 shows the particular case when the Slave 1 is returning power from the motor to the system, and this situation happens because the system is trying to keep the current sinusoidal. This reason justifies the fact of using active front end rectifiers at the first Slave level. Otherwise, the power could not be returned to the mains. s it was 229

5 mentioned before, these rectifiers also allow to keep the input currents of the system free of harmonics. c) Fig. 14. Single-phase current and three-phase voltages Fig. 11 Power distribution in the three stages of the converter., Slave 1, c) Slave 2 IV. EXPERIMENTL RESULTS The Fig. 12 shows the voltage steps waveforms obtained with a 3 kw four-stage prototype. The figure shows only half wave. On the other hand, the Fig. 13 shows the phase voltage and currents in one of the three phases of the multiconverter when it feds an induction machine. Finally, in Fig. 14, the voltages of the three phases, and the current in one of them are observed. It is noticed again that the voltages are very good. The prototype used for the experiments is shown in Fig.14. It can be observed that the voltages are quite sinusoidal and the resultant current is also very clean. These results justify the research developed with this kind of converter because, as was shown in figures 5 and 6, they are specially suited for very large machine drives, which can be implemented with GTOs at the level, and with IGTs at the Slaves levels. Fig. 12. Voltage steps waveforms in a four-stage converter Fig. 15. Four-stage multiconverter prototype V. ONLUSIONS Fig. 13. Voltage and current waveforms in a four-stage converter three-stage inverter using H converters has been analyzed for high power machine drive applications. The great advantage of this kind of converter is the minimum harmonic distortion obtained at the machine side. The need of isolated power supplies required for each one of the three stages of the multiconverter has been solved in three ways: passive rectifiers at the level (8% of the power), active front-end PWM rectifiers (which act as a power filters and var compensators) at the Slave 1 level (15% of the power), and passive rectifiers with dissipative power resistors during very low voltage operation at the Slave 2 (only 5% of the total power). Simulation results were shown and some experiments with a small four-stage prototype was displayed. The control of this multi-converter is being implemented using DSP controllers, which will give flexibility to the system. 23

6 VI. KOWLEDGEMENTS The authors want to thank onicyt through Projects Fondecyt 1246 and 12982, for the support given to this work. VII. REFERENES with Floating apacitor Technology, European Power Electronics onference, EPE 21. [11] Powersim Technologies. PSIM Version 4.1, for Power Electronics Simulations, User Manual, Powersim Technologies, Vancouver, anada, Web page: [1] H. kagi, The State-of-the-art PowerElectronics in Japan, IEEE Transactions on Power Electronics, Vol.13, Nº 2, February 1998, pp [2]. ose, Power Electronics and Motion ontrol- Technology status and recent trends, IEEE Transactions on Industry pplications, Vol. 29 Nº 5, 1993, pp [3] D. hung, J. Kim, and S. Sul, Unified Voltage Modulation Technique for Real Time Three-Phase Power onversion, IEEE Transactions on Industry pplications, Vol. 34, Nº 2, 1998, [4] J. Holtz and. eyer, Fast urrent Trajectory Tracking ontrol ased on Synchronous Optimal Pulse Width Modulation, IEEE Transactions on Industry pplications, Vol. 31, Nº 5, 1995, pp [5]. Draou, M. enghanen, and. Tahri, Multilevel onverters and VR ompensation, hapter 25, Power Electronics Handbook, Muhamad H. Rashid, Editor-in hief, cademic Press, 21, pp [6] F. Zheng Peng, Generalized Multilevel InverterTopology with Self Voltage alancing, IEEE Transactions on Industry pplications, Vol. 37, Nº 2, March-pril 21, pp [7] K. Matsui, Y Kawata, and F. Ueda, pplication of Parallel onnected NP-PWM Inverters with Multilevel Modulation for Motor Drive, IEEE Transactions on Power Electronics, Vol. 15, Nº 5, September 2, pp [8] J. Dixon and L. Morán, Multilevel Inverter, ased on Multi-Stage onnection of Three-Level onverters, Scaled in Power of Three, Industrial Electronics onference, IEON-2, Sevilla, Spain, 5-8 Nov. 22. [9] O. Gaupp, P. Zanini, P. Daehler, E. aerlocher, R. oeck, J. Werninger, remen s 1 MW Static Frequency Link, Issue Nº 9, 1/96, 1996, pp.4-17, M42 [1] G. einhold, R. Jakob, M. Nahrstaed, New Range of Medium Voltage Multilevel Inverter Drives 231