MODELING AND SIMULATION OF Z- SOURCE INVERTER

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1 From the SelectedWorks of suresh L 212 MODELING AND SIMULATION OF Z- SOURCE INVERTER suresh L Available at:

2 MODELING AND SIMULATION OF Z-SOURCE INVERTER 1 SURESH L., G.R.S. NAGA KUMAR, and M.V. SUDARSAN Abstract Z source inverters have been recently proposed as an alternative power conversion concept as they have both voltage buck and boost capabilities. These inverters use a unique impedance network, coupled between the power source and converter circuit, to provide both voltage buck and boost properties, which cannot be achieved with conventional voltage source and current source inverters. To facilitate understanding of Z source inverter, this paper presents a detailed analysis, showing design of impedance network, implementation of simple Boost control PWM technique and simulation of Z source inverter for different values of modulation indices. Index Terms PWM Technique, SBC, Z source inverter. I. INTRODUCTION There exists two traditional converters, voltage-source (or voltagefed) and current-source (or current-fed) converters, either rectifier or inverter depending on power flow directions. There are some limitations in those two inverters. An output LC filter is needed, which causes additional losses and control complexity. B. Current inverter (CSI) source CSI is 3-Ø bridge inverter fed from current source i.e a voltage source in series with large inductor as shown in fig 2. Six switches are used; each composed of Insulate Gate Bipolar Transistor (IGBT) or Metal Oxide Semiconductor Field Effect Transistor (MOSFET) with series diode to provide unidirectional current flow and bidirectional voltage blocking. Unlike VSI, CSI has nine switching states in those six are active states and three are zero states. The AC output voltage is greater than DC input voltage. A.Voltage source inverter (VSI) VSI is a 3-Ø bridge inverter fed from DC voltage source (or) AC voltage source with diode rectifier as shown in fig 1. A large capacitor is connected at the input terminals tends make the input DC voltage constant. Six switches are used in the main circuit; each composed of power transistor and an antiparallel diode to provide bidirectional current flow and unidirectional voltage blocking capability. It has eight switching states. In those eight states, six are active states and two are zero states. VSI can be operated as a stepped wave inverter or pulsewidth modulated (PWM) inverter. Fig 2: Current Source Inverter However, the current-source inverter has the following conceptual and theoretical limitations: It is a boost inverter i.e, the output AC current is greater than the input current it cannot be used as buck inverter. The cost of CSI is high. The operating power factor is poor on line side. CSI is vulnerable to EMI noise in terms of reliability. The dynamic response is slow. Fig.1:Voltage Source Inverter It has the following conceptual and theoretical barriers. The AC output voltage is limited below and cannot exceed the DC input voltage. External equipment is needed to boost up the voltage, which increases the cost and lowers the overall system efficiency. There is a possibility for the occurrence of short through which destroys the devices. II Z-SOURCE INVERTER The main objective of static power converters is to produce an AC output waveform from a dc power supply. Impedance source inverter is an inverter which employs a unique impedance network coupled with the inverter main circuit to the power source. This inverter has unique features in terms of voltage (both buck & boost) compared with the traditional inverters. A twoport network that consists of a split-inductor and capacitors that are connected in X shape is employed to provide an impedance source (Z-source) coupling the inverter to the dc source, or another converter. The DC source/load can be either a voltage or a current source/load. Therefore, the DC source can be a battery, diode rectifier, thyristor converter, fuel cell, PV cell, an inductor, a capacitor, or a combination of those [1]. Switches used in the converter can be a combination of switching devices and anti-parallel diode as shown in Fig. 3 Mr. SURESH L. is with the Vignan s Lara Institute of Technology& Science, Vadlamudi, INDIA (phone: ; suresh.21@gmail.com). Mr. G.R.S. NAGA KUMAR is with Vignan s Lara Institute of Technology & Science, Vadlamudi, INDIA. He is now with the Department of EEE.( naga113ee22@gmail.com). Mr. M.V. SUDARSAN is with the Electrical Engineering Department, Vignan s Lara Institute of Technology & Science, Vadlamudi, INDIA ( mvsudarsan.eee@gmail.com).

3 2 Fig. 3: ZSI Using the Antiparallel Combination of Switch and Diode Six switches are used in the circuit; each is traditionally composed of a power transistor and an antiparallel (or freewheeling) diode to provide bidirectional current flow and unidirectional voltage blocking capability. The commonly used switches are Metal Oxide Semi-Conductor Field Effect Transistor (MOSFET), Insulated Gate Bipolar Transistor (IGBT), Bipolar Junction Transistor (BJT), Silicon Controlled Rectifier (SCR), and Gate Turn off Thyristor (GTO) etc. Here we employed IGBT as the switch as it combines the advantages of both BJT and MOSFET. A. Impedance Network The Z-source concept can be applied to all DC-to-AC, AC-to- DC, AC-to-AC and DC-to-DC power conversion. It consists of voltage source from the DC supply, Impedance network, and three phase inverter and with AC motor load. AC voltage is rectified to DC voltage by the three phase rectifier. In the rectifier unit consist of six diodes, which are connected in bridge way. This rectified output DC voltage fed to the Impedance source network which consists of two equal inductors (L 1, L 2) and two equal capacitors (C 1, C 2).The network inductors are connected in series arms and capacitors are connected in diagonal arms.the impedance network is used to buck or boost the input voltage depends upon the boosting factor.this network also act as a second order filter.this network should require less inductance and smaller in size. Similarly capacitors required less capacitance and smaller in size. This impedance network, constant impedance output voltage is fed to the three phase inverter main circuit. Depending upon the Gating signal, the inverter operates and this output is fed to the 3-phase AC load or AC motor. B. Equivalent Circuit and Operating Principle The Z-source inverter is analyzed using voltage source inverter. The unique feature of the Z-source inverter is that the output ac voltage can be any value between zero and infinity regardless of the input DC voltage. That is, the Z-source inverter is a buck boost inverter that has a wide range of obtainable voltage. The traditional V- and I-source inverters cannot provide such feature. The main feature of the Z-source is implemented by providing gate pulses including the shoot-through pulses. Here how to insert this shootthrough state becomes the key point of the control methods. It is obvious that during the shoot-through state, the output terminals of the inverter are shorted and the output voltage to the load is zero. The output voltage of the shootthrough state is zero, which is the same as the traditional zero states, therefore the duty ratio of the active states has to be maintained to output a sinusoidal voltage, which means shoot-through only replaces some or all of the traditional zero states. Let us briefly examine the Z-source inverter structure. In Fig. 3, the three-phase Z-source inverter bridge has nine permissible switching states (vectors) unlike the traditional three-phase V-source inverter that has eight. The traditional three-phase V-source inverter has six active vectors when the DC voltage is impressed across the load and two zero vectors when the load terminals are shorted through either the lower or upper three devices, respectively. However, three-phase Z-source inverter bridge has one extra zero state (or vector) when the load terminals are shorted through both the upper and lower devices of any one phase leg (i.e., both devices are gated on), any two phase legs, or all three phase legs. This shoot-through zero state (or vector) is forbidden in the traditional V-source inverter, because it would cause a shoot-through. We call this third zero state (vector) the shoot-through zero state (or vector), which can be generated by seven different ways: shootthrough via any one phase leg, combinations of any two phase legs, and all three phase legs. The Z-source network makes the shoot-through zero state possible. This shoot-through zero state provides the unique buck-boost feature to the inverter. The Z-source inverter can be operated in three modes which are explained in below. Mode I: In this mode, the inverter bridge is operating in one of the six traditional active vectors; the equivalent circuit is as shown in figure 4. Fig.4: Equivalent Circuit of the ZSI in one of the Six Active States The inverter bridge acts as a current source viewed from the DC link. Both the inductors have an identical current value because of the circuit symmetry. This unique feature widens the line current conducting intervals, thus reducing harmonic current. Mode II: The equivalent circuit of the bridge in this mode is as shown in the fig. 5 Fig. 5: Equivalent Circuit of the ZSI in one of the Two Traditional Zero States The inverter bridge is operating in one of the two traditional zero vectors and shorting through either the upper or lower three device, thus acting as an open circuit viewed from the Z-source circuit. Again, under this mode, theinductor carry current, which contributes to the line current s harmonic reduction as shown in below fig 6.

4 3 Where V o is the DC source voltage and Fig. 6: Equivalent Circuit of the ZSI in the Non Shoot-Through States. Mode III: The inverter bridge is operating in one of the seven shoot-through states. The equivalent circuit of the inverter bridge in this mode is as shown in the below figure 7. In this mode, separating the dc link from the ac line. This shoot-through mode to be used in every switching cycle during the traditional zero vector period generated by the PWM control. Depending on how much a voltage boost is needed, the shoot-through interval (T ) or its duty cycle (T /T) is determined. It can be seen that the shoot-through interval is only a fraction of the switching cycle. The average voltage of the inductor over one switching period should be zero in steady state, thus, We have Similarly the average DC link voltage across the inverter bridge can be found as follows. From equation 4: From equation 6: Fig. 7: Equivalent Circuit of the ZSI in the Shoot-Through State. C. Analysis of Impedance Network The equivalent circuit of the impedance network [3] is shown in fig. 8 The peak DC-link voltage across the inverter bridge is Where B is a boost factor The output peak phase voltage from the inverter Fig. 8: Equivalent Circuit of Impedance Network Where M is the modulation index In this source For simplicity, assuming that the inductors L 1 and L 2 and capacitorsc 1 and C 2 have the same inductance and capacitance respectively, the Z-source network become symmetrical. From the symmetry and the equivalent circuits, we have (1) Given that the inverter bridge is in the shoot-through zero state for an interval oft, during a switching cycle, T and from the equivalent circuit, Fig. 8, one has Now consider that the inverter bridge is in one of the eight non shoot-through states for an interval of T1, during the switching cycle. From the equivalent circuit, Fig. 8, one has (2) The output voltage can be stepped up and down by choosing an appropriate buck - boost factor B* B*= B.M (it varies from to α) (11) The capacitor voltage can be expressed as The boost factor B is determined by the modulation index M. The boost factor B can be controlled by duty cycle of the shoot-through zero state over the non-shoot through states of the PWM inverter. The shoot-through zero state does not affect PWM control of the inverter. Because, it equivalently produces the same zero voltage to the load terminal, the available shoot- through period is limited by the modulation index. D. Advantages of Z-source Inverter The following are the advantages of Z-source inverter when compared to the two traditional inverters i.e. voltage source inverter and current source inverter. )

5 4 Secures the function of increasing and decreasing of the voltage in the one step energy processing. (lower costs and decreasing losses) Resistant to short circuits on branches and to opening of the circuits. Improve resistant to failure switching and EMI distortions. Relatively simple start-up (lowered current and voltage surges). Provide ride-through during voltage sags without any additional circuits. Improve power factor reduce harmonic current and common-mode voltage. Provides a low-cost, reliable and highly efficient single stage for buck and boost conversions. Has low or no in-rush current compared to VSI. III PWM TECHNIQUES The number of control methods to control Z-source inverter, that include the sinusoidal PWM techniques, three types of PWM control algorithms: simple boost control (SBC), maximum boost control (MBC), constant boost control (CBC). The modulation index also called as amplitude modulation ratio (M) which is the main control factor is defined as the ratio of amplitude of reference wave to the amplitude of carrier wave same high frequency triangular signal. Comparator compares these two signals and produces pulses (when V sin>v tri, on and V sin<v tri, off). These pulses are then sent to gates of the power IGBT s through isolation and gate drive circuit. Figure 1 shows the pulse generation of the three phase leg switches (S 1, S 3 and S 5-positive group/upper switches and S 2, S 4 and S 6- negative group/lower switches).this method is much uncomplicated; however, the resulting voltage stress across the device is relatively high because some traditional zero states are not utilized either partially or fully. This characteristic will restrict the obtainable voltage gain because of the limitation of device voltage rating. For a complete switching period, Tis total switching period, T is the zero state time period and Dois the shoot-through duty ratio. In this paper, the control of ZSI is done by this control technique (SBC). The linearity between the modulation index and the output voltage is achieved by under modulation index (M < 1). A. Simple Boost Control [5, 8] Actually, this control strategy inserts shoot through in all the PWM traditional zero states during one switching period. This maintains the six active states unchanged as in the traditional carrier based PWM. The implementation of simple boost control method [7] is illustrated in Fig. 9. Two straight lines are employed to realize the shoot through duty ratio (Do). The first one is equal to the speak value of the three-phase sinusoidal reference voltages while the other one is the negative of the first one. When the triangular carrier waveforms is greater than the upper envelope, V p, or lower than the bottom envelope, V n, the circuit turns into shoot-through state. Otherwise it operates just as traditional carrier-based PWM. Fig 1: PWM Signals from Simple Boost Control Important mathematical expressions are: (12) (13) Where G is inverter voltage gain; M is modulation index; B is boost factor. (15) Fig 9: Implementation Diagram of SBC The voltage stress across the inverter devices is given by Shoot-through pulses are inserted into the switching waveforms by logical OR gate. To produce switching pulses, three phase reference wave forms having peak value with modulation index (M) are compared with the IV SIMULATION & RESULTS

6 5 The Z-source inverter can be operated in both boost and buck operations depending on values of M. If M is greater than.5 it acts as boost inverter, if M is less than.5 then it acts as buck inverter. The following block diagram figure 11 shows the SIMULINK implementation of Z Source inverter. Here a 3-phase RLC parallel load is connected to ZSI. The 3-phase output voltage across load is shown in fig Three phase load voltage for M=.8 phase 'a' phase 'b' phase 'c' Fig 15: Three Phase Load Voltage across Load for M=.8. B. Buck Operation Results voltage...(v) Fig. 11: Implementation Diagram of Z source inverter A. Boost Operation Results By considering inverter output voltage we can say boost or buck operation. The inverter output voltage is shown in fig 12, for M= Inverter line voltage for M= Fig 12: Inverter Output Voltage for M=.8 The corresponding input to inverter circuit is output of diode bridge rectifier is fig Rectifier output voltage for M= Fig 13: Diode Bridge Rectifier Output Voltage for M=.8 The voltage across the capacitor is shown in fig 13. Initially the capacitor voltage rises to maximum value after it reaches to constant value. 9 8 Voltage across capacitor for M=.8 In this mode of operation the modulation index is reduced to.4 we get by varying the amplitude of the carrier wave. The inverter output voltage for M-=.4 is shown in fig 16. In this figure we observe that the inverter output voltage decreases when comparing as in the case M= Inverter llline voltage for M= Fig 16: Inverter Output Line Voltage for M=.4 The corresponding diode rectifier output voltage is shown in fig Rectifier output voltage for M= Fig 17: Diode Bridge Rectifier Output Voltage for M=.4 The voltage across capacitor reaches maximum voltage at the time of starting, for the application of high starting torque. The typical wave form is shown in fig Fig 14: Voltage across Capacitor for M=.8.

7 voltage across capacitor for M= Fig 18: Voltage across Capacitor for M=.4. The output three phase load voltage wave forms is shown in fig Three phase output load voltage for M=.4 phase 'a' phase 'b' phase 'c' Fig 19: Three Phase Voltage across Load for M=.4. The inverter line voltages for different values of modulation index are tabulated as follows which shows that both boost and buck operations are possible in Z-source inverter. Table 1 Load Voltage Profile for Different values of M S.NO. Modulation Index (M) Inverter Output Peak Voltage (volts) REFERENCES [1] F. Z. Peng, "Z-source inverter", IEEE Transactions on Industry Applications, vol. 39, pp , Mar-Apr 23. [2] Rabi BJ and Arumugam R, Harmonics study and comparison of z- source inverter with traditional inverters, American Journal of applied science, vol-2, no.1, pp [3] Bindeshwar Singh, S. P. Singh, J. Singh, and MohdNaim, Performance evaluation of three phase induction motor drive fed from z-source inverter, International Journal on Computer Science and Engineering (IJCSE). [4] AtulKushwaha, Mohd. Arif Khan, AtifIqbal and Zakir Husain, Z- Source Inverter Simulation and Harmonic Study, Global Journal of Advanced Engineering Technologies-Vol1-Issue [5] B.Y. Husodo, M. Anwari, and S.M. Ayob, Analysis and Simulations of Z-Source Inverter Control Methods, IEEE Transactions on Industry Applications, vol. 42, pp , May-Jun 26. [6] Ogbuka, C.U. and M.U. Agu. 29. A Generalized Rectified Sinusoidal PWM Technique for Harmonic Elimination. Pacific Journal of Science and Technology. 1(2): [7] S. Thangaprakash and A. Krishnan, Implementation and Critical Investigation on Modulation Schemes of Three Phase Impedance Source Inverter, Iranian Journal of Electrical & Electronic Engineering, Vol. 6, No. 2, June 21. [8] PankajZope, K.S. Patil, Dr. PrashantSonare, Z-source Inverter Control Strategies, International Journal of Computational Intelligence and Information Security, August 211 Vol. 2, No. 8. V CONCLUSION This paper presents, the theoretical analysis and design of Z-source inverter is studied. The Z-source inverter employs a unique impedance network to couple the inverter main circuit to the power source and thus providing unique feature. The control methods with the insertion of shoot-through states of Z- source inverter have been studied. The proposed scheme under simple boost control is simulated with the help of MATLAB/SIMULINK and the simulation results are obtained for different values of modulation indices. The simulation results shows that both buck and boost operations can be obtained in Z-source inverter by varying Modulation index (M) or Boost factor (B)