ISSN 2348–2370 Vol.07,Issue.01, January-2015, Pages:0001-0006 www.ijatir.org A Renewable Grid for Cascaded Multi Cell Z-Source Inverter with Boost Inversion for ASD Drives V. JANAKI RAM1, A. PURNA CHANDRA RAO2 1 PG Scholar, Dept of EEE, Prasad V. Potluri Siddhartha Institute of Technology, Vijayawada, India. 2 Dept of EEE, Prasad V. Potluri Siddhartha Institute of Technology, Vijayawada, India. Abstract: This paper presents a cascade multi cell Z-source inverter used to control the speed of an BLDC motor. The proposed cascade multi cell Z -source inverter (ZSI) can effectively reduce the voltage stress across the capacitors in the impedance network. This reduces the voltage range of the capacitors used, and also the cost of the proposed topology which is in turn used to control the speed of a BLDC motor. This concept is used in many industrial applications. The cascade multi cell Z-Source inverter is a combined inverter with an additional buck-boost feature and the proposed topology increases the efficiency of the circuit by reducing the voltage stress across the capacitors. This topology finds its applications in a number of renewable energy sources, where the input voltage is unreliable in nature and keeps varying from time to time. The simulation results indicate that the proposed topology is a promising technique that can be applied to improve the overall inverter efficiency. Keywords: Z-Source Inverter, Brushless Dc Motor, Reduce Voltage Stress. I. INTRODUCTION Many modern powers electronic applications usually demand some amount of voltage boosting, especially those directly connected to the grid. Traditional voltage-source inverters (VSIs) alone are, therefore, not satisfactory since they only step down voltages. To introduce additional boost functionality, dc–dc boost converters can be placed before the VSIs or current-source inverters (CSIs) can be used instead. Both inverters have some amount of boost inductance added to their dc circuits, which certainly is a common modification introduced to inverters with boost functionality (if switched-capacitor technique is not used). The inductance added to a CSI is usually larger to keep its dc input current constant. This, together with other disadvantages like tougher control and lack of standard semiconductor modules for implementation, usually limits the use of CSIs, as compared to VSIs. The Z source inverter is a single stage converter that can either buck or boost the ac output voltage from a Dc supply. This topology overcomes the shortcomings of the traditional voltage source and current source inverters, where the output ac voltage is either respectively less or more than the input dc voltage. This combined operation of the z source inverter eliminates the need of a separate dc-dc converter, thus reducing the cost and increasing the efficiency of the circuit. Z source inverter also allows two switches of the same leg to be gated in the circuit, thus eliminating the shoot through fault that occurs in traditional converters. This feature of the inverter provides the elimination of dead time in the circuit, thus increasing the reliability and reducing the output distortion of the inverter. Saying that does not free the Z source inverter from few of its operating problems the voltage across the capacitors in the traditional Z source inverter is equal to the input voltage which increases the volume and cost of the capacitors used; and also the start up current and voltage in the circuit is very much higher which may destroy the devices at one time or the other. So to overcome the above said problems, a new topology of the Z source inverter is used, that can be used to drive a BLDC motor and speed control. A recently developed new inverter, the Z-source inverter has a for ASD systems to overcome the four mentioned problems. A Z-source inverter based ASD system can: 1. Produce any desired output ac voltage, even greater than the line voltage, regardless of the input voltage. 2. Thus reducing motor ratings; 3. Provide ride-through during voltage sags without any additional circuits. 4. Improve power factor and reduce and harmonic current and common-mode voltage. This paper presents the Z-source inverter ASD system configuration, its equivalent circuit, analysis, and control. Simulation results are included to prove the concept and the features of the new ASD system. II. Z SOURCE INVERTER To overcome the problems of the traditional V-source and I-source converters, this paper presents an impedancesource (or impedance-fed) power converter (abbreviated as Z-source converter) and its control method for implementing dc-to-ac power conversion. Fig. 1 shows the general Z-source converter structure proposed. It employs a unique impedance network (or circuit) to couple the converter main circuit to the power source, load, or another converter, for providing unique features that cannot be Copyright @ 2015 IJATIR. All rights reserved. V. JANAKI RAM, A. PURNA CHANDRA RAO observed in the traditional V- and I-source converters where iL> ii (2) a capacitor and inductor are used, respectively. The Zsource converter overcomes the above-mentioned Again, because of the symmetry of the circuit, capacitor conceptual and theoretical barriers and limitations of the currents iC1and iC2 and inductor currents iL1 and iL2 should be traditional V-source converter and I-source converter and equal to each other, respectively. In this mode, the input provides a novel power conversion concept. current from the dc source becomes iin= iL1+ ic1= iL1+ (iL2− ii)= 2iL− ii >0 (3) Therefore, the diode is conducting, and the voltage across the inductor is VL = Vo − VC Which is negative (the capacitor voltage is higher than the input voltage during boost operation when there are shoot through states); thus, the inductor current linearly decreases, assuming that the capacitor voltage is constant. As time goes on, the inductor current keeps decreasing to a level wherein the condition of (2) can no longer be met and the input current iin or the diode current is decreased to zero; mode 2 ends. Fig1. General structure of the Z-source converter. In Fig. 1, a two-port network that consists of a splitinductor L1 andL2 and capacitors C1and C2connected in X shape is employed to provide an impedance source (Zsource) coupling the converter (or inverter) to the dc source, load, or another converter. The dc source/or load can be either a voltage or a current source/or load. Therefore, the dc source can be a battery, diode rectifier, thyristor converter, fuel cell, an inductor, a capacitor, or a combination of those. Switches used in the converter can be a combination of switching devices and diodes such as the anti parallel combination as shown in Fig. 1. The inductance and can be provided through a split inductor or two separate inductors. The Z-source concept can be applied to all dc-toac power conversion. The diode in series with the fuel cell in Fig.1is usually needed for preventing reverse current flow. III. OPERATING PRINCIPLE OF Z-SOURCE INVERTER Mode 1: The circuit is in a switch shoot-through zero state when the two switches in any of the three phase legs are turned on at the same time, the sum of the two capacitors voltage isgreater than the dc source voltage VC1+ VC2> V0, the diode is reverse biased, and the capacitors charge the inductors. The voltages across the inductors are VL1= VC1VL2= VC2 (4) Fig2. Equivalent Circuit of the Z-Source Inverter Viewed From The Dc Link When The Inverter Bridge Is In The Shoot-Through Zero State. (1) The inductor current linearly increases, assuming that the capacitor voltage is constant during this period. Because of the symmetry L1= L2= L and C1= C2= C of the circuit, one has vL1=vL2=vL, iL1=iL2=iL, and VC1= VC2= VC. Mode 2: The inverter is in a non shoot-through state one of the six active states and two traditional zero states and the inductor current meets the following inequality: Fig3. Equivalent Circuit of the ZSI When the Inverter Bridge Is In One of the Eight Non shoot-Through Switching States. International Journal of Advanced Technology and Innovative Research Volume.07, IssueNo.01, January-2015, Pages: 0001-0006 A Renewable Grid for Cascaded Multi Cell Z-Source Inverter with Boost Inversion for ASD Drives should also be noted that the smallest capacitance endures the highest voltage stress, which might unintentionally create a single point of failure. Balancing resistors for capacitors (and diodes), together with their losses, are therefore almost always added to the circuit for long term usage. To avoid direct series connection, an alternate cascading technique is discussed after describing the generic Z-source cell shown in Fig. 5. Moreover, it is intentionally drawn with an X-shaped structure that resembles the original Z-source network proposed in . With this X-shaped cell, the alternate cascading technique can be performed based on the following few steps: 1. Begin with cell 1 with its windings labeled as W11 and W2. 2. Duplicate a copy of cell 1, and name it as cell 2.Windings of cell 2 are labeled asw12 andw3 with their turns ratio marked as γ3. 3. Flip cell 2 vertically and place it below cell 1. 4. Merge cell 1 and cell 2 with W2 of cell 1 replacing W12 Fig4. PWM control with shoot-through zero states. of cell 2. 5. Shift W12 of cell 2 to be in parallel with W11 of cell 1. IV.CMC Z-SOURCE INVERTERS 6. Duplicate cell k with windings W1k and W(k + 1), and Instead of a transformer with high turns ratio as in turns ratio γk+1. multiple smaller transformers with lower turns ratios are 7. Repeat the flipping and merging until all N cells are used. Their W1 windings are connected in parallel to share cascaded (until k = N). the extreme high instantaneous current stress, while their W2 windings are connected in series to withstand the higher The resulting CMC Z-source inverter is shown in Fig. 6, voltage demanded. Turns ratios of these smaller which clearly does not have any direct series connection. No transformers should be chosen based on available core and balancing resistors and losses are therefore needed, meaning wire sizes that can more readily produce better coupling. At that the inverter in Fig. 6 is likely more efficient than the times, layout and packaging of the application considered direct series-connected circuit shown in Fig. 4. The CMC might also have a role in deciding the transformer sizes. inverter would however still require parallel connections of Besides transformers, the circuit multiple diodes and windings W1k (k = 1 to N) and capacitors (not shown in Fig. capacitors connected in series and parallel instead of using 6 for clarity) to manage the flow of high instantaneous single higher rated entities. Such connections are not strictly current during shoot through. Such parallel connections will necessary, but might at times be needed if higher rated not be a concern in practice, unlike series connections. components are not readily available, are too costly or do not fit nicely to the layout of an application (e.g., height of an electrolytic capacitor). Fig5. Generic trans-Z-source cell. When attempting series connection though, it is necessary to be doubly cautious especially for cases where component parameters drifted greatly. With capacitors, it Fig6. Proposed CMC Z-source inverter. International Journal of Advanced Technology and Innovative Research Volume.07, IssueNo.01, January-2015, Pages: 0001-0006 V. JANAKI RAM, A. PURNA CHANDRA RAO V. ASD DRIVE CONTROL VI. SIMULATION RESULT In servo applications position feedback is used in the Simulations have been performed to confirm the above position feedback loop. Velocity feedback can be derived analysis. Above Figures shows simulation waveforms when from the position data. This eliminates a separate velocity the fuel-cell stack voltage is V0 =50V and the Z-source transducer for the speed control loop. A BLDC motor is network parameters areL1=L2=L=3e-6H and C1=C2=C= driven by voltage strokes coupled by rotor position. The 500F. The purpose of the system is to produce a three-phase rotor position is measured using Hall sensors. By varying output line-to-line was 400-V power from the fuel-cell stack the voltage across the motor, we can control the speed of the whose voltage changes 50~400 V dc depending on load motor. When using PWM outputs to control the six switches current. From the simulation waveforms are shown. When of the three-phase bridge, variation of the motor voltage can the fuel-cell voltage is low, as shown in Fig.8, the shootbe obtained by varying the duty cycle of the PWM signal. through state was used to boost the voltage in order to The speed and torque of the motor depend on the strength of maintain the desired output voltage. The waveforms are the magnetic field generated by the energized windings of consistent with the simulation results With the help of the the motor, Which depend on the current through them. designed circuit parameters, the MATLAB simulation is Hence adjusting the rotor voltage and current will change done and results are presented here. Speeds are set at 1500 motor speed. Commutation ensures only proper rotation of rpm and the load torque disturbances are applied at time the rotor. The motor speed depends only on the amplitude of t=.06 sec. The speed regulations are obtained at set speed the applied voltage. This can be adjusted using PWM and the simulation results are shown. Figure 9 shows the technique. The required speed is controlled by a speed inverter input voltage (Vin =50V) waveform respectively. controller. This is implemented as a conventional Figure 10 shows the boosting inverter output voltage waveform (Vout=400V) .The waveforms of the back EMF proportional-Integral controller. are shown in Fig.12. The stator current waveforms are The difference between the actual and required speeds is shown in Fig 12.Figure.13 shows the speed waveform of given as input to the controller. Based on this data PI BLDC Motor. controller controls the duty cycle of the PWM pulses which correspond to the voltage amplitude required to maintain the desired speed. When using PWM outputs to control the six switches of the three-phase bridge, variation of the motor voltage can be achieved easily by changing the duty cycle of the PWM signal. In case of closed loop control the actual speed is measured and compared with the reference speed to find the error speed. This difference is supplied to the PI controller, which in turn gives the duty cycle. PMBLDC motor is popular in applications where speed control is necessary and the current must be controlled to get desired torque. Figure 7.shows the basic structure for closed loop control of the PMBLDC motor drive. It consists of an outer speed control loop, an inner current control loop for speed and current control respectively. Speed loop is relatively Fig8. Simulation waveform of dc Input voltage of each slower than the current loop. cell voltage. Fig7. Speed Controller. Fig9. Simulation waveform of combined cell dc Input voltage. International Journal of Advanced Technology and Innovative Research Volume.07, IssueNo.01, January-2015, Pages: 0001-0006 A Renewable Grid for Cascaded Multi Cell Z-Source Inverter with Boost Inversion for ASD Drives Fig10. Simulation waveform of Inverter Output voltage (Vout=400v). Fig13. Controlled Speed of the motor using CMC Zsource inverter. VII.CONCLUSION This paper has proposed a cascade multi cell Z-source inverter used to control the speed of an BLDC motor. The drive offers the advantages of both Z-source inverter and BLDC motor. The existing inverter scheme suffers from shoot-through reliability problem. This topology provides better performance than the traditional inverter topology for an identical load and speed conditions. The feasibility of Zsource inverter fed BLDC motor drive is proved by the simulation results. From the results obtained, it is clear that the Z-source inverter fed PMBLDC motor drive is very promising for various industrial applications. The drive response can be improved by using PWM technique. Fig11. Simulation waveform of Inverter Output current. Fig12. Stator Current and Back EMF of BLDC motor. VIII. REFERENCES [1] J. Kikuchi and T. A. Lipo, ―Three phase PWM boostbuck rectifiers with power regenerating capability,‖ IEEE Trans. Ind. Appl., vol. 38, no. 5, pp. 1361–1369, Sep/Oct. 2002. [2] G. Moschopoulos and Y. Zheng, ―Buck-boost type ac-dc single-stage converters,‖ in Proc. IEEE Int. Symp. Ind. Electron, Jul. 2006, pp. 1123–1128. [3] F. Z. Peng, ―Z-source inverter,‖ IEEE Trans. Ind. Appl., vol. 39, no. 2, pp. 504–510, Mar./Apr. 2003. [4] P. C. Loh, D. M.Vilathgamuwa,Y. S. Lai, G. T. Chua, andY.W. Li, ―Pulsewidth modulation of Z-source inverters,‖ IEEE Trans. Power Electron., vol. 20, no. 6, pp. 1346–1355, Nov. 2005. [5] J. Liu, J. Hu, and L. Xu, ―Dynamic modeling and analysis of Z-source converter—Derivation of ac small signal model and design-oriented analysis,‖IEEE Trans. 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International Journal of Advanced Technology and Innovative Research Volume.07, IssueNo.01, January-2015, Pages: 0001-0006