Advanced Pulse Width Modulation Techniques for Z Source Multi Level Inverter

— This paper proposes five level diode clamped Z source Inverter. The existing PWM techniques used for ZSI are restricted for two level. The two level Z Source Inverter have high harmonic distortions which effects the performance of the grid connected PV system. To improve the performance of the system the number of voltage levels in the output waveform need to be increased. This paper presents comparative analysis of a five level diode clamped Z source Inverter with different carrier based Modified Pulse Width Modulation techniques. The parameters considered for comparison are output voltage, voltage gain, voltage stress across switch and total harmonic distortion when powered by same DC supply. Analytical results are verified using MATLAB.


I. INTRODUCTION
wide research has taken place on two level Z Source Inverters [ZSI] in the past few years. The ZSI has the advantages of performing buck and boost operations and required conversion is achieved in a single stage. The drawback of conventional voltage source inverter is two switches in the same leg should not be gated on simultaneously. This state of operation is hailed in ZSI which is called as shoot through state. By applying shoot through state the output voltage of the inverter is boosted which improves the reliability of inverter. This property of ZSI is considered favorable for PV system [1] where available DC voltage is less & varying in magnitude.
The two level ZSI is shown in Fig. 1. On DC side of ZSI, a diode D and impedance network connected in "X" shape, composed of two inductors L 1 and L 2 & two capacitors C 1 and C 2 [2]. This Z circuit couples the inverter circuit to the power source which allows the single stage conversion.
The buck boost operation of ZSI depends on boost factor. To get a boost factor null state are reimbursed with shoot through state without disturbing the active states [2]. For placing shoot through states, the PWM methods that is simple boost, maximum constant boost and maximum constant are B used as explained in [3], [4]. In simple boost method, two constant reference signals are used in addition to sinusoidal reference signal for inserting shoot through states. In maximum boost method, two sine wave envelopes are used to produce shoot through states. In maximum constant boost, sine waves with an offset value varying around peak value of reference sine wave are used to generate shoot through pulses.
The fore mentioned PWM techniques uses only one carrier wave for generating gate pulses which restricts the output voltage of the ZSI to two level only. Due to this the Total Harmonic Distortions [THD] in the output voltage increases. To increase the output voltage magnitude the shoot through period need to be increased, which leads to increase in voltage stress across the switches. To overcome these difficulties, the output voltage level of ZSI is increased by using diode clamped multi level inverter topology. The number of levels in the output voltage is increased by taking (m-1) carriers, where m stands for number of levels in the output voltage. In this paper, five level diode clamped Z Source inverter is simulated using various multi carrier PWM techniques [5], [6]. The performance of ZSI is evaluated by considering output voltage, voltage gain and voltage stress across switch and THD.

II. IMPEDANCE TYPE INVERTER
The Conventional Inverter will operate in two states: active state & null state. In ZSI along with active state & null State, the third state i.e., shoot through state is allowed. In six active states and in zero-state (null state) mode, the ZSI operates as conventional inverter. In the shoot-through state, all the switches in one or more legs are turned on. The operating states of conventional inverter & ZSI are explained in Fig. 2. Null states are represented as "0" & shoot through state is represented as "1". From Fig. 2  The Equivalent circuit of ZSI in active states is shown in Fig. 4. In this state the inverter will operate as normal inverter the active states. By controlling the shoot through duration the DC link voltage is varied, which in turn modifies the output [7], [8]. The Equivalent circuit of ZSI in active states is shown in Fig. 4. In this state the inverter will operate as normal inverter but the input voltage to the inverter is of boost depends on shoot through duty ratio. When circuit is in a shoot the two capacitors voltage is greater than (V C1 +V C2 > V 0 ), will appears across the diode. Thus reverse biased, and the capacitors ch voltage across both the inductors are: The five level Diode clamped Z source inverter is shown in Fig. 6. This inverter configuration following constraints, (M-1) sources, 2(M devices, (M-1) (M-2) clamping diodes for each leg, where M stands for number of level in the output voltage. In this paper a five level ZSI is simulated which requires 4 DC so same value, 8 switching Device & 12 clamping diodes. To generate shoot through state one of the following switching combination is used.
To get complete shoot through state, the Switches S S a3 , S a4 , S a1 1 , S a2 1 , S a3 1 & S To get shoot through state in Z source 1 alone, the switches S a1 , S a2 , S a3 , S a4 , To get shoot through period in Z source 2 alone, the switches D a1 , S a2 , S a3 , S a4 To get shoot through period in Z source 3 alone, the switches D a2 , S a3 , S a4 , S a1 To introduce shoot through period in Z source 4 alone, the switches D a3, S a4 , S a1 1 , S a2 To get upper shoot through state (ZSI1 & ZSI2), the switches S a1 , S a2 , S a3 and S but the input voltage to the inverter is boosted. The magnitude of boost depends on shoot through duty ratio. 5 Equivalent circuit in shoot through state When circuit is in a shoot through state, the summation of ors voltage is greater than input DC voltage appears across the diode. Thus diode is reverse biased, and the capacitors charge the inductors. The the inductors are:  Fig. 8 shows the implementation of maximum constant boost control method. By using this method one can achieve the maximum voltage gain & keeps the shoot ratio constant. In this control technique five modulation curves are used out of which three are reference signals and other two are shoot-through sine waves with an offset value varying around peak value of reference sine wave shown as +K & in Fig. 8. Here all the carrier wave forms are in phase. This type of carrier distribution is called as Phase Disposition (PD). When the carrier triangle wave is greater than the upper shoot through envelope or lower than bottom shoot envelope the inverter is turned into a shoot through state. In between states the inverter will operate as tradit

B. Maximum Constant Boost Method
given by Fig. 8 shows the implementation of maximum constant boost control method. By using this method one can achieve the maximum voltage gain & keeps the shoot-through duty ratio constant. In this control technique five modulation curves ree are reference signals and other two sine waves with an offset value varying around peak value of reference sine wave shown as +K & -K . Here all the carrier wave forms are in phase. This as Phase Disposition (PD). When the carrier triangle wave is greater than the upper shootthrough envelope or lower than bottom shoot-through envelope the inverter is turned into a shoot through state. In between states the inverter will operate as traditional inverter. భሻ ‫כ‬ (6) Boost Factor = The phase output peak voltage is given by Voltage across capacitor V c is given by   Fig. 9 shows the implementation of maximum boost control strategy. In this control method, the six active states unchanged and turn all zero states into shoot Thus maximum shoot through time interval, Factor and maximum voltage gain are obtained for any given modulation index M without distorting the output waveforms. It can be seen from Fig. 9 that, circuit is in shoot when the carrier wave is either larger than the maximum value of the references or lesser than the minimum of the references. All carrier waveforms above the zero reference are in phase and are out of phase with those below zero reference by 180°. This type of carrier distribution is called as Phase Opposition Disposition (POD). , circuit is in shoot-through state than the maximum value than the minimum of the references. above the zero reference are in phase and are out of phase with those below zero reference by 180°. This type of carrier distribution is called as Phase Opposition  Fig. 10 shows the voltage gain versus boost factor source inverter for all fore mentioned control methods. Fr Fig. 10, it can be concluded maximum boost control method has higher voltage gain when compared to simple boost method & slightly higher voltage gain than the maximum constant boost control method. The voltage stress across the s same voltage gain with different control methods is shown in Fig. 11. From this figure, maximum boost control method will reduces the voltage stress across the switches when compared to simple & ma constant boost control methods.    Fig. 10 shows the voltage gain versus boost factor B of a Zsource inverter for all fore mentioned control methods. From d that for any boost factor, the control method has higher voltage gain when compared to simple boost method & slightly higher voltage gain than the maximum constant boost control method.

Voltage gain Vs boost factor for different control method
The voltage stress across the switching device for obtaining same voltage gain with different control methods is shown in it can be concluded that the maximum boost control method will reduces the voltage stress across the switches when compared to simple & maximum constant boost control methods.   Fig. 12, it can be concluded that maximum boost control method will have high voltage gain when compared to other methods for same shoot through duty ratio. From 10-12, it is clear that ZSI with maximum boost control technique gives better performance.

V. SIMULATION RESULTS
To study the performance of Z source inverter, simulations were performed in MATLAB/SIMULINK.

A. Three Level Z Source Inverter
The boost factor B is taken as 2 for comparing voltage gain, Fundamental voltage & THD of ZSI with different control methods. The parameters considered for the simulation are as follows: the input dc voltage source is V o = L 2 = 3.3m H and capacitor C 1 = C 2 =500 frequency is considered as 10 kHz. Fig. 13 to 15    From Table I it is clear that system is giving better performance with Phase disposition carrier when compared to phase opposition disposition carrier.

B. Five level Z Source Inverter
For comparing the performance of five level ZSI, the modulation index is selected as 1. For this modulation index the fundamental output voltage, boost factor, THD & voltage gain are compared with all boosting technique and with different carriers. In simple boost method, if shoot through modulation index is one, there is no boost (B=1) in the output voltage & hence ZSI will operate as normal inverter. Line voltage of simple boost method is maximum constant boost control method with modulation index of one, then boost factor will be 1.362. The Line to Line voltage of maximum constant boost method is shown in Fig.  17. Finally for maximum boost control with modulation inde one, boost factor will be 1.529. line voltage of ZSI for all three methods with phase disposition carrier & comparison of results are shown in II.  it is clear that system is giving better disposition carrier when compared to phase opposition disposition carrier.

Inverter
For comparing the performance of five level ZSI, the modulation index is selected as 1. For this modulation index the fundamental output voltage, boost factor, THD & voltage all boosting technique and with different carriers. In simple boost method, if shoot through here is no boost (B=1) in the output voltage & hence ZSI will operate as normal inverter. Line to Line voltage of simple boost method is shown in Fig. 16. In maximum constant boost control method with modulation hen boost factor will be 1.362. The Line to Line voltage of maximum constant boost method is shown in Fig. inally for maximum boost control with modulation index one, boost factor will be 1.

VI. CONCLUSION
This paper investigates the voltage gain, THD and voltage stress with three major types of boosting techniques used for ZSI using MATLAB/SIMULINK software. Based on the availability of input & requirement of load method is chosen. THD is controlled by increasing the number of levels in the output voltage. Simple boost method is easy for the implementation but available output voltage is limited and voltage stress across the switches increases. Maximum boost method will have good voltage gain and voltage stress across the switches is reduced when compare to simple & maximum constant boost method. it is clear that system is giving better performance with Phase disposition carrier when compared to Alternate Phase opposition investigates the voltage gain, THD and voltage three major types of boosting techniques used for ZSI using MATLAB/SIMULINK software. Based on the requirement of load, proper control THD is controlled by increasing the number Simple boost method is easy able output voltage is limited increases. Maximum voltage gain and voltage stress when compare to simple &