Design of Chopper Fed Z Source PWM Inverter

Transcription

1 Volume 119 No , ISSN: (on-line version) url: ijpam.eu Design of Chopper Fed Z Source PWM Inverter 1 K. Vibha and 2 K. Sudha 1 Department of Electronics and Instrumentation Engineering, SRM IST, Chennai, India. 2 Department of Electronics and Instrumentation Engineering, Vellammal Engineering College, India. Abstract This paper focuses on chopper control method, which is fed to an impedance source (Z-source network) power converter. Chopper control method is employed for obtaining constant DC voltage from variable DC voltage sources (fuel cell, solar cell). The Z-source converter employs a unique impedance network to couple the converter main circuit to the power source, thus providing unique feature that cannot be achieved in traditional voltage-source and current source converter where capacitors and inductors are used respectively. The control strategies of Z-source converter include simple boost control, where the traditional PWM technique is modified to suit the insertion of shoot through states. The Z- source concept can be applied to all conversion such as DC-to-AC, AC-to- DC and DC-to-DC conversion. Simulink model was developed using Matlab/Simulink. Index Terms:Z-source Inverter, simple boost PWM control, chopper Fed Z-source inverter

2 1. Introduction Inverter is widely being used in industrial application. The input source may be fuel cell, solar cell, battery or other dc sources. Conventional inverter pulses are generated for triggering thereby the shoot-through in which both power switches in a leg are at once turned on must be avoided because it causes short circuit This problem can be overcome by the Z-source inverter. The DC-DC boosted PWM inverter topology can alleviate the stresses and limitations however suffer problems such as high cost and complexity associated with the two-stage power conversion. The inverter, which is of voltage fed and current fed inverter have following limitation. They are either a boost or a buck converter and cannot be a buck-boost converter [14]. Their main circuits cannot be interchangeable. In other words, neither the V-source converter main circuit can be used for the I-source converter and nor vice versa. The above limitations can be overcome in Z source inverter. Input voltage to the inverter can be of constant or variable dc voltage. In case of variable dc voltage, constant dc voltage is achieved by using control method. 2. Z-Source Inverter Fig 1: Z-source Inverter The Z-source inverter is a recently proposed converter topology that utilizes the shoot through zero states to boost dc voltage and produce an output voltage greater than original voltage [1]. At the same time, the Z-source inverter enhances the reliability of the inverter bridge because of the special Z-source network, with which the shoot through can be the operating state. Fig.1 shows the main circuit configuration of the Z- source inverter, where a unique Z-source inverter is coupled between the load and dc source. The Z-network is implemented by a split-inductor (L1 and L2) and capacitors (C1 and C2) connected in X shape. By controlling the shootthrough duty cycle, the ZSI produce any desired output ac voltage, even greater than the line voltage; provide ride-through during voltage sags without any additional circuits; reduce in-rush and harmonic current. Pulse width 15166

3 modulation (PWM) control for the Z-source inverter has to be modified to utilize the shoot-through states for voltage boost. In the traditional PWM technique of the voltage source inverter, there are eight permissible switching states: six active and two zero states. During the two zero states, the upper three or the lower three switches are turned on simultaneously, thus shorting the output terminals of the inverter and producing zero voltage to the load. During one of the six active states, the dc voltage is impressed across the load, positively or negatively. In addition to the eight traditional switching state, the Z-source inverter has several shoot-through zero states, during which both the upper and lower switches of one or multiple same phase legs are turned on. It is obvious that during a shoot through zero state, the output terminals of the inverter are shorted and the output voltage to the load is zero. Therefore, the shoot through states has the same effect (i.e., zero voltage) to the load as the traditional zero states; however, these shoot through states can boost the dc voltage. The active states have to be kept unchanged to maintain the output voltage waveform, and the traditional zero states can be replaced partially or entirely by the shoot through zero states depending on how much voltage boost is needed. Several modified PWM control method for the Z-source inverter based on traditional control methods was presented in [2]. Comparison of VSI, CSI, ZSI Current Source Inverter Voltage Source Inverter 1. As inductor is used in the d.c link, As capacitor is used in the source impedance is high. It acts the d.c link. It acts as a as a constant current source. low impedance voltage source. 2. A current source inverter is capable of withstanding short circuit across any two of its output terminals. Hence momentary short circuit on load and mis-firing of switches are acceptable. 3. This is used in only buck or boost operation of inverter. A VSI is more dangerous situation as the parallel capacitor feeds more powering to the fault. This is also used in only a buck or boost operation of inverter. 4. The main circuits cannot be The main circuit can be interchangeable. interchanged here also. 5. It is affected by the EMI noise It is affected by EMI noise 3. Analysis of Equivalent Circuit Z source Inverter As capacitor and inductor is used in the d.c link it acts as a constant high impedance voltage source. In ZSI mis-firing of the switches sometimes are also acceptable. This is used in both buck or boost operation of inverter. Here the main circuits are interchangeable. It is not affected by EMI noise The equivalent circuit of Z-Source inverter is shown in Figure2. There are two operating mode in Z-source inverter

4 [Mode 1]: The circuit is in a shoot-through zero state, the sum of the two capacitors voltage is greater than the dc source voltage (V C1 +V C2 > V O ), the diode is reverse biased, and the capacitors charge the inductors. Figure 3 shows the equivalent circuit of ZSI when there is Shoot-through. The voltages across the inductors are: Fig.2: Equivalent circuit of ZSI V L1 = V C1, V L2 = V C2. (1) The inductor current increases linearly assuming the capacitor voltage is constant during this period. Because of the symmetry (L 1 = L 2 =L and C 1 = C 2 =C) of the circuit, one has V L1 =V L2 =V L, I L1 =I L2 =I L &V C1 = V C2 = V C.. (2) Fig.3: Equivalent Circuit of ZSI When There is Shoot-Through State [Mode 2]: The inverter is in a non-shoot through state (one of the 2 active states and 2 traditional zero states) and the inductor current meets the following in equation, I L = ½ Ii (3) In this mode, the input current from the dc source becomes: Iin = I L1 + I C1 = I L1 + (I L2 - I i ) = 2I L - Ii > 0 (4) Therefore, the diode is conducting and the voltage across the inductor is V L = V O V C (5) 15168

5 which is negative (the capacitor voltage is higher than the input voltage during boost operation when there is shoot through states), thus the inductor current decreases linearly assuming the capacitor voltage is constant. The Fig.4 shows the equivalent circuit of ZSI when there is non-shoot-through. As time goes on, the inductor current keeps decreasing to a level that no longer the condition of (2) can be met. At this point, the input current I in or the diode current is decreased to zero [3]. Fig.4: Equivalent Circuit of ZSI When There is Non Shoot-Through State From the above equivalent circuit of fig.2, V L = V C, V d = 2V C & V i = 0 (6) During the switching cycle T, V L = V O -V C, V d = V O, V i = V C -V L = 2V C - V O (7) Where V O is the dc source voltage and T= T 0 +T 1. The average voltage of the inductors over one switching period (T) should be zero in steady state, thus we have V L = (T O.V C + T 1 (V O V C )) / T=0 V C / V O = T 1 / (T O T 1 ) (8) Similarly the average dc link voltage across the inverter bridge can be found as follows. V i = (T O *0+ T 1 (2V C V O )) / T = [T 1 / (T 1 -T O )] *V O V i = V C (9) The peak dc-link voltage across the inverter bridge is expressed in (7) & can be re-written as: V i = V C - V L = B.V O (10) where B (boost factor) = T/(T 1 -T O ) i.e

6 The output peak phase voltage from the inverter V ac = M.V i /2 (11) where M is the modulation index. Using (10), equation (11) can be expressed as V ac = M.B.V O /2 (12) For the traditional V-source PWM inverter, the well-known relationship is V ac = M.V O /2.Equation (12) shows that the output voltage can be stepped up and down by choosing an appropriate buck-boost factor, B B = B.M (it varies from 0 to α). The voltage gain is determined by the modulation index m and the boost factor B. the boost factor B can be controlled by duty cycle of 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 produce the same zero voltage to the load terminal. 4. Control Strategies The unique feature of Z-source inverter is that it allows the shooting through an inverter phase leg, which gives rise to an ac output voltage boost, controlled by varying the duty cycle (To/T). This feature is achieved by modifying the traditional PWM techniques [4]. This section presents some of the modified control strategies. As described in [1], the voltage gain of the Z-source inverter can be expressed as V ac /[V o /2]=M.B (13) Where V ac is the output peak phase voltage, V o is the input dc voltage, M is the Modulation index, and B is the boost factor, which is determined by B=1/[1-2To/T] (14) Where T0 is the shoot through time interval over a switching cycle T, or (T0/T) =D0 is the shoot through duty ratio. In [1], a simple boost control method was used to control the shoot through duty ratio. If three phase references to control shoot through duty ratio in a traditional sinusoidal PWM. The Z-source inverter maintains the six active states unchanged as the traditional carrier based PWM control. For this simple boost control, the obtainable shoot through duty ratio decreases with the increase of M. The maximum shoot through duty ratio of this control method is limited to (1-M), thus reaching a modulation index of one. In order to produce an output voltage that requires a high voltage gain, a small modulation index has to be used. However, small modulation index result in greater voltage stress on the devices. Based on (13) and (14), define the [12] voltage gain G as G = M.B = Vac/[Vo/2] = M/[2M-1] (15) 15170

7 for any desired voltage gain G,[10-11] the maximum modulation index can be used is M=G/[2G-1] (16) From [1], the voltage stress Vs across the switches is BV 0. The voltage stress [13] under this modulation method can be calculated by Vs=BV 0 =(2G-1) V 0 (17) Using this control method, shoot through have been inserted in Z-source inverter. 5. Chopper FED Z-Source Inverter Fig.5: Block Diagram of Chopper Fed Z-Source Inverter The input fed to Z-source can be of fuel cell, solar cell. These sources provide variable DC voltage. To obtain constant DC voltage, chopper is used as front end conversion, in which the output voltage is controlled by PI controller [5-8]. This obtained constant DC voltage is then fed to Z-source inverter. PI controller is tuned so that the pulse width gets changed so as to produce the constant DC voltage. Fig.5. Shows the block diagram of chopper fed Z-source inverter. The input to the chopper is fuel cell (variable source). This variable source is converted to constant dc voltage from the chopper and its control method (PI controller), which is then fed to Z-source inverter. Thus constant DC voltage is given to Z-source inverter, in which buck or boost process is achieved. 6. Results and Discussion Simulations have been performed to confirm the above analysis. Figure6 shows the circuit configuration and Fig 7-8 shows simulation waveform for single phase chopper fed Z - source inverter. The input to the chopper is 12 volts and the Z-source network parameter are, L1=L2=L=57.58uH and C1=C2=C=0.417mF.The output voltage boosted to 30Volts

8 Amps Volts(v) International Journal of Pure and Applied Mathematics Fig.6: Simulation of Single-Phase Chopper fed Z-Source Inverter 7. Conclusion Time(sec) Fig 7: Output Voltage Waveform Vac = M.B.Vo/2 = 30 V Time(sec) Fig.8: Output Current Waveform In this paper, chopper fed single-phase Z Source Inverter is proposed. The Z

9 source converter employs a unique impedance network (or circuit) to couple the unique features that cannot be observed in the traditional voltage-source and current-source converters where capacitors and inductors are used respectively. The Z-source converter overcomes the conceptual, theoretical barriers and limitations of the traditional voltage-source converter and current-source converter and provides a novel power conversion concept. Analytical and simulation results have been presented. The Z-source inverter can boost-buck voltage, minimize component count and increase efficiency. References [1] Seragi S.A., Review on Z-Source Inverter, In International Journal of Computer Applications, National Conference on Advances in Communication and Computing (2014). [2] Lai J.S., Peng F.Z., Multilevel converters-a new breed of power converters, IEEE Transactions on industry applications 32(3) (1996), [3] Pankaj Zope K.S. Patil Prashant S., Z-source Inverter Control Strategies, International Journal of Computational Intelligence and Information Security 2(8) (2011). [4] Thangaprakash S., Krishnan A., Implementation and critical investigation on modulation schemes of three phase impedance source inverter, Iranian Journal of Electrical and Electronic Engineering 6(2) (2010), [5] Peng F.Z., Z-source inverter, IEEE Transactions on industry applications 39(2) (2003), [6] Loh P.C., Vilathgamuwa D.M., Lai Y.S., Chua G.T., Li Y., Pulsewidth modulation of Z-source inverters, IEEE Conference of the 39th IAS Annual Meeting Industry Applications 1(2004). [7] Rajakaruna S., Jayawickrama Y.R.L., Designing impedance network of Z-source inverters, The 7th International Conference on Power Engineering (2005), [8] Shen M., Wang J., Joseph A., Peng F.Z., Tolbert L.M., Adams D.J., Maximum constant boost control of the Z-source inverter, IEEE Conference of the 39th IAS Annual Meeting Industry Applications 1(2004). [9] Ding X., Qian Z., Yang S., Cui B., Peng F., A PID control strategy for DC-link boost voltage in Z-source inverter, IEEE Twenty Second Annual conference on Applied Power Electronics (2007), [10] Jung J.W., Keyhani A., Control of a fuel cell based Z-source converter, IEEE Transactions on Energy Conversion 22(2) (2007),

10 [11] Shen M., Peng F.Z., Operation modes and characteristics of the Z-source inverter with small inductance, IEEE Industry Conference on Applications 2(2005), [12] Peng F.Z., Yuan X., Fang X., Qian Z., Z-source inverter for adjustable speed drives, IEEE power electronics letters 1(2) (2003), [13] Peng F.Z., Shen M., Qian Z., Maximum boost control of the Z- source inverter, IEEE Transactions on power electronics 20(4) (2005), [14] Peng F.Z., Joseph A., Wang J., Shen M., Chen L., Pan Z., Huang Y., Z-source inverter for motor drives, IEEE transactions on power electronics 20(4) (2005),

11 15175

12 15176