WWW.IJITECH.ORG ISSN 3-8665 Vol.03,Issue.09, October-05, Pages:74-78 Space Vector VSI with Paralleled DC DC Boost Converters Parallel Current Sharing NOOTHPALLY RAJU, D.SREEDEVI PG Scholar, Vijay Rural Engineering College, Nizamabad, TS, India, Email: nraj64@gmail.com. Asst Prof, Vijay Rural Engineering College, Nizamabad, TS, India, Email: sreedevi.d87@gmail.com. Abstract: Inverters used in many applications have a special interest in modern technology, inverting the DC voltage to three phase AC voltages. In this paper we introduce a new topology to boost the input DC voltage from any of the DC sources to attain high gain output voltage. The proposed topology is a parallel current sharing DC-DC booster converter topology with a VSI (Voltage Source Inverter) at the output. Due to the parallel operation of the converters the reliability of the converter increases as an available substitute for any switch failure. The stresses on the converters are also minimized as the current is divided between the converters to avoid emphasis on the power electronic devices. The complete design and analysis is carried out in MATLAB Simulink software with all parametric graphs. design on the case of a double-input H-bridge-based buckboost buckboost converter. A last method which can be underlined is presented in where the sharing scheme is used to cancel circulating current between parallel inverters. Keywords: DC-DC Converter, VSI. I. INTRODUCTION From past few years DC-DC converters have versatile applications where the boosting and bucking operations are mandatory for many applications. In order to reduce the stress on the power electronic switches of the DC-DC converter, the booster converters are split into parallel converter where the current is also paralleled reducing the passage of total current through a single converter. This modification of the DC-DC converter minimizes the stress on the switches reducing the voltage spikes caused during switching operation. Not only the stress the efficiency of the converter is also increased and the most concerned is the reliability. The parallel DC-DC booster converter can be seen in fig. below. Current sharing is an important functionality for parallel power converters to ensure reliable and efficient operation. A good review of current-sharing techniques has been proposed by Chen et al. in on different interconnection schemes. A first method for controlling parallel boost converters is to define one converter as the master which imposes output voltage while others are slave and only current regulated. Unfortunately, this control strategy is not suitable for a good reliability of the global system since it might suffer from fault tolerance against the failure of the master converter. Another method found in the literature is to use current sharing to manage the output voltage error as done in by using synergetic control. In, current sharing is directly integrated in a nonlinear controller Fig.. Proposed paralleled booster converter with N modules. However, in most of the applications, current reference is equally divided between the parallel elements as done. Equal power repartition has for example been used, where the control is realized with a classical PID. Many studies propose an equal repartition among many different structures, as for example on paralleled dual active bridge dc dc converters. This is also the case for classical control techniques of paralleled inverters as done. Equal repartition is also used while thinking in voltage mode controlled structures with no current sensors as done on multiphase buck converter structures.in this paper, a new current-sharing method based on the knowledge of estimated loss parameters is proposed. As previously mentioned, compared with the sharing techniques found in the literature, the proposed method presents real advantages. In most of the literature, the current repartition is only designed in order to allow the system to work. However, as it will be underline in this paper, the repartition can influence on the global structure efficiency. Classically, this is not taken into account. In this aim, the new sharing scheme proposed in this paper allows us to maximize the global efficiency of the overall structure. Copyright @ 05 IJIT. All rights reserved.
NOOTHPALLY RAJU, D.SREEDEVI II. SPACE VECTOR PWM Space vector PWM technique is an advanced technique to control the VSC (Voltages Source Converters) with digitalized generation of control signals depending on the sector operation. The conventional technique (Sinusoidal PWM) is an analog control switching which creates slightly higher THD (Total harmonic Distortion) as compared to space vector PWM. Considering a simple six switch S-S6 VSC comprising of IGBTs (Insulated Gate Bipolar Transistor) with an anti-parallel diode to the IGBT to avoid circulating currents in the switch. The schematic of the three phase full bridge inverter is shown below in fig. Fig. 3: Space vector trajectory sector operation Fig.. Three phase VSI with six IGBTs The upper region three switches are denoted with S, S3 and S5 and the lower region three switches are denoted with S, S4 and S6. The input has a capacitor so as to reduce the ripples in the input DC voltage. The VSI comprises of three legs which can be considered as three phase AC PWM outputs with a certain fundamental frequency. The possible switching states (ON or OFF) of the IGBTs are calculated as 3 = 8 states. With three switches in each region the switching states are given as in table. Table : SWITCH S S3 S5 ST MODE 0 0 0 ND MODE 0 0 3 RD MODE 0 0 4 TH MODE 0 5 TH MODE 0 0 6 TH MODE 0 7 TH MODE 0 8 TH MODE In the above table it can be clearly observed that the st mode and the last mode are either all ON or all OFF which are not possible switching pattern for the VSI and neglected for the operation. The remaining six switching states are possible generating control signals to the IGBTs, which can be arranged in a hexagonal format for clear understanding. The space vector trajectory can be seen below in fig. 3. The control signal generation depends in the above sector selection operation V-V6. The Vref signal is a three phase sinusoidal signal with fundamental frequency of 50Hz and an amplitude of pu. The voltage angle with reference the normal is α degrees produced as a resultant of the two adjacent vectors V and V. V, V3 and V5 are the vectors 0 degrees phase shift to each other and the other V4, V6 and V are the opposite of the previous vectors respectively. The MATLAB simulink modeling of the simple space vector PWM is shown below in fig. 4. 3-phase input Sine Wave Sine Wave Sine Wave Three Phase to Two Phase K*u em 3 4 5 6 SPACE VECTOR PULSE WIDTH MODULATION A Cartesian to Polar B v ab SECTOR sequencing factors C [ 0.5 ;0 0.5;0 0 0.5] [ 0 0.5 ; 0.5; 0 0 0.5] [0 0 0.5 ; 0.5; 0 0.5] [0 0 0.5 ; 0 0.5; 0.5] [0 0.5 ; 0 0 0.5; 0.5] [ 0.5; 0 0 0.5; 0 0.5] sector SECTOR DETERMINATION 3 4 5 *, 6 sector selector modulation index m sector # amplitude phase Matrix Multiply pi/3 sector # mod pi/3 alpha sin f(u) x sin (sqrt(3)*cos(u)-sin(u))/(sqrt(3)*cos(u)+sin(u)) Fixed frequency carrier x y z control signals > overmodulation u<0 linear modulation PWM SIGNALS Fig.4: Space vector PWM technique in MATLAB Simulink As a reference signal generator three sin generators are considered with an amplitude of pu, and a phase shift of 0degrees (A-phase = 0deg, B-phase = 0deg, C-phase = - 0deg) to each other. To the three phase sinusoidal wave for a clarke's transformation is applied converting abc to αβ components. The formula for the clarke's transformation is given below. y 0 SVM Volume.03, Issue No.09, October-05, Pages: 74-78
Space Vector VSI with Paralleled DC DC Boost Converters Parallel Current Sharing The α and β components are in 90degress phase shift deciding the sector selection of the space vector trajectory. The complex formats of the waveforms are converted to magnitude and phase with the use of Cartesian to polar converter generating modulation index and phase degrees. The magnitude of the converter is fed to the gain m (Modulation index gain) to change the modulation index as per the user requirement. A Modulus after division block is connected to the phase output with pi/3 (0degress) divisor. The output phase is fed to three trigonometric sin functions to generate three reference signals of 0, 0 and -0 degrees phase shifted signals x, y and z. The three reference signals are further multiplied to the sector matrix multiplier to generate six pulses that has to be fed to the VSI IGBT switches. The sequencing factors selected with respect to the sector selection are shown below in fig. 4. Depending upon the six sector selections the output of the control signals are generated which are further fed to the triangular carrier waveform with a certain switching frequency. The fundamental reference space vector waveform (fundamental frequency as 50Hz) with carrier waveforms (carrier waveform 0000Hz) is shown in fig. 6. A relational operator is connected to these waveforms (reference and carrier) to generate pulses for the upper region switches where the lower region switches get the gate output of the generated pulses. Fig. 6: fundamental waveform with triangular carrier waveform. III. STRUCTURE MODELING AND ESTIMATION OF DC-DC CONVERTER The system consists in N parallel boost converters with one output capacitor as shown in Fig.. The considered modeling equations are a direct application of Shahin et al. work [7]. In this paper, it is proposed to model losses through a boost converter by adding N serial and one parallel resistors in the conventional ideal model. For the considered application, each individual boost converter model has a serial resistance rsk, while a unique parallel resistor Rp includes all the rest of the losses for the whole structure. This difference with the method proposed in [7] comes from the nature of the structure with only one output capacitor and one output current sensor. The next section details the method to obtain an accurate online estimation of the resistor values where dk represents the duty cycle corresponding to the PWM output signal uk. Fig.5. Sequencing factor with respect to the sector selection. The reference space vector control waveform is a third harmonic content sinusoidal waveform with a frequency of 50Hz compared to the triangular waveform. Even if the adopted loss modeling is only represented through resistors, it is useful to underline that not only ohmic losses are taken into account. In fact, every loss through the converter is taken into account with those of equivalent resistors, such as core hysteresis and eddy current losses, conduction ohmic losses, and switching losses of semiconductors. Particularly, he parallel resistor Rp does not only represent the capacitor Co losses through its ESR. Indeed, it is well known the even under zero power, boost converters still present losses, which will be taken into account through parameter Rp while serial resistor is not able to model this behavior. Finally, parameters rsk and Rp represent the overall losses through the structure.this will be verified in the experimental part by checking that calculated losses correspond to measurement. For a more detail description of this loss modeling technique, the reader is invited to read reference, where analytical study is presented, as well has load dependence behavior of such a modeling in the case of a single-boost converter.it has to be noticed that Volume.03, Issue No.09, October-05, Pages: 74-78
NOOTHPALLY RAJU, D.SREEDEVI the estimation of losses through the converters can also been used for others purpose IV. SIMULINK RESULTS Fig. 0: Three phase Space vector PWM inverter output line voltages Fig.7. DC-DC paralleled booster converter with space vector controlled VSI. Fig. : Three phase Space vector PWM inverter output phase voltages. V. CONCLUSION A new current-sharing technique on parallel dc dc boost converters with implementation of Space vector PWM three phase inverter has been presented in this paper. Through Fig. 8. Pulse inputs to IGBTs S and S online estimation, individual converter losses are deduced and power repartition coefficients are redefined in order to maximize the efficiency of the overall structure. Output voltage and individual input currents are regulated through a two-loop control design based on flatness theory and sliding mode controllers.compared with the most used currentsharing technique consisting in equal repartition, the proposed method shows its interest when one or more converter in parallel present malfunctioning. In these conditions, the presented method allows a gain in efficiency up to 4.5% in the tested cases, depending on the load power. Another benefit of the proposed repartition is the fact that it leads to uniform ageing between the paralleled elements. This should have an effect on the structure health improving its long-term reliability and facilitating its maintenance. Longterm experimentations are required to attest this last assumption and will be part of future works.the presented current sharing has been discussed, verified, and tested both by simulation and experiment. The validation has been realized on the case of a three-parallel boost converter Fig. 9: Output voltage of DC-DC converter structure, but the concept can easily be scaled to any number of phases. Indeed, theoretical study has been led on the Volume.03, Issue No.09, October-05, Pages: 74-78
Space Vector VSI with Paralleled DC DC Boost Converters Parallel Current Sharing Author s Profile: general case of N converters in parallel. Practically, each phase will need its own current regulation, its own serial resistor estimation, and the repartition coefficients calculation will require only a few more time as the number of converter increase. VI. REFERENCES [] Y. Zhang, M. Yu, F. Liu, and Y. Kang, Instantaneous current-sharing control strategy for parallel operation of ups modules using virtual impedance, IEEE Trans. Power Electron., vol. 8, no., pp. 43 440, Jan. 03. [] R. Ayyanar, R. Giri, and N. Mohan, Active input-voltage and load-current sharing in input-series and output-parallel connected modular dc-dc converters using dynamic inputvoltage reference scheme, IEEE Trans. Power Electron., vol. 9, no. 6, pp. 46 473, Nov. 004. [3] J. Kimball, J. Mossoba, and P. Krein, Control technique for series input-parallel output converter topologies, in Proc. -IEEE 36th Power Electron. Spec. Conf., 005, pp. 44 445. [4] F. Garcia, J. Pomilio, and G. Spiazzi, Modelling and control design of the six-phase interleaved double dual boost converter, in Proc. 9th IEEE/IAS Int. Conf. Ind. Appl., 00, pp. 6. [5] P. Thounthong and B. Davat, Study of a multiphase interleaved step-up converter for fuel cell high power applications, Energy Convers. Manag., vol. 5, pp. 86 83, 00. [6] C.-H. Cheng, P.-J. Cheng, and M.-J. Xie, Current sharing of paralleled DC DC converters using GA-based PID controllers, Expert Syst. Appl., vol. 37, pp. 733 740, 00. [7] J. Shi, L. Zhou, and X. He, Common-duty-ratio control of input-parallel output-parallel (ipop) connected DC DC converter modules with automatic sharing of currents, IEEE Trans. Power Electron., vol. 7, no. 7, pp. 377 39, Jul. 0. [8] A. Roslan, K. Ahmed, S. Finney, and B. Williams, Improved instantaneous average current-sharing control scheme for parallel-connected inverter considering line impedance impact in microgrid networks, IEEE Trans. Power Electron., vol. 6, no. 3, pp. 70 76, Mar. 0. [9] D. Sha, Z. Guo, and X. Liao, Control strategy for inputparallel-output-parallel connected high-frequency isolated inverter modules, IEEE Trans. Power Electron., vol. 6, no. 8, pp. 37 48, Aug. 0. [0] Y. Cho, A. Koran, H. Miwa, B. York, and J.-S. Lai, An active current reconstruction and balancing strategy with DClink current sensing for a multi-phase coupled-inductor converter, IEEE Trans. Power Electron., vol. 7, no. 4, pp. 697 705, Apr. 0. [] R. Foley, R. Kavanagh, and M. Egan, Sensorless current estimation and sharing in multiphase buck converters, IEEE Trans. Power Electron., vol. 7, no. 6, pp. 936 946, Jun. 0. Noothpally Raju Completed B.Tech.in Electrical &Electronics Engineering in 03 from Global Institute of Engineering And Technology, Chilkur, Affiliated to JNTUH, Hyderabad and M.Tech in Power Electronics in 05(pursuing) from VIJAY RURAL ENGINEERING COLLEGE Affiliated to JNTUH, Nizamabad, Telangana, India. Area of interest includes research in Power Electronics and RES. D.SREEDEVI Completed B.Tech in Electrical &Electronics Engineering in 008 from Sridevi Women s Engineering College, Hyderabad, Affiliated to JNTUH, Hyderabad and M.Tech in Power Electronics in 0 from Auroras Engineering College, Bhongiri. Working as Asst Professor at VIJAY RURAL ENGINEERING COLLEGE,Nizamabad, Telangana, India. Area of interest includes research in Power Electronics Volume.03, Issue No.09, October-05, Pages: 74-78