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Discovery ANALYSIS The International Daily journal ISSN 2278 – 5469 EISSN 2278 – 5450 © 2015 Discovery Publication. All Rights Reserved Analysis of an Impedance (Z) Source Inverter Using Modified PWM Technique with Simple Boost Control Publication History Received: 02 September 2015 Accepted: 04 October 2015 Published: 06 November 2015 Page 18 Citation Jitendra Patil, Vikas Kulkarni. Analysis of an Impedance (Z) Source Inverter Using Modified PWM Technique with Simple Boost Control. Discovery, 2015, 48(221), 18-23 Analysis of an Impedance (Z) Source Inverter Using Modified PWM Technique with Simple Boost Control Jitendra Patil Vikas Kulkarni Dept. of Electrical Engg. AISSMS COE Pune, India [email protected] Dept. of Electrical Engg. AISSMS COE Pune, India [email protected] Abstract—This paper presents an impedance source (also called Z-source) power converter (ZSC) with its control technique for converting dc-to-ac voltage, which is source voltage for a small 3 phase squirrel cage induction motor. Generally, we need to convert voltage according to the application requirement, like ac-to-dc, ac-to-ac, dc-to-ac and dc-to-dc conversions respectively i.e. it can be applied to any type of power conversion. Traditionally, for dc-to-ac power conversion we use voltagesource inverter (VSI) or current-source inverter (CSI) where capacitor and inductor are used respectively. Conventional voltage-source inverters and current-source inverters are suffering from no. of major limitations that can be easily overcome with the use of Z-source converter. For many industrial applications in adjustable speed drives we require smooth/step less variation in speed that can be achieved by using modified PWM technique which also has more advantages than the conventional PWM technique. Simulation results are added here for analysis purpose. each phase leg, otherwise a short circuit would take place (also called as shoot-through state) in the circuit and it will destroy the device. In other words, dead-time provision is provided to block upper and lower devices which produce waveform distortion. An additional LC filter is required for smoothing and filtering voltage waveform to get purely sinusoidal waveform compared to current source inverter, but it causes more power loss and control complexity [1]-[3]. Keywords—Voltage source inverter; current source inverter; Z source inverter; modified PWM technique. Fig. 1. Traditional VSI and CSI Ι. INTRODUCTION In VSI we cannot switch ON the gate simultaneously i.e. the upper and lower devices of Its ac output voltage always has to be greater than its dc input rail. It is step-up inverter for dc-to-ac power conversion and vice-versa [1], [13]. At any time at least one device is gated and maintained in ON state from upper side and from lower side. Otherwise an open circuit would occur which will destroy the device. For safe commutation we require an overlap time provision which produces waveform distortion [1]. Current-source inverter works only in close loop control, so it limits multi-motor applications [2]. Apart from the above barriers and limitations, Voltage source inverter and current-source inverter have some common problems, like electromagnetic interference and total harmonic distortion 19 We cannot increase its output ac voltage level than its dc input voltage level. Therefore, it is step-down inverter for dc-to-ac conversion and viceversa. If over drive is required in any specific application then additional circuit will be required which further increases system cost and complexity. Current-source inverter (CSI) consists of dc source in series with inductor and 3 phase 6 pulse inverter bridge as shown in fig. 1[B]. Current source inverter also has some limitations as explained below – Page Previously, there existed only two traditional converters, which are voltage-source inverter (VSI) and current-source inverter (CSI) as shown in fig. (1.A) and (1.B) respectively. To reduce complexity in the circuit, a dc source is directly used rather than an ac source with rectifier bridge. Voltage-source inverter (VSI) consist of a dc source with large capacitor in parallel and 3 phase 6 pulse inverter bridge, in which 6 power transistors with an antiparallel diodes as shown in fig. 1[A]. The function of an antiparallel diode is to provide bidirectional current flow and to block unidirectional voltage capability [1]-[2], [7]-[9]. In many applications voltage-source inverters are preferred over the current-source inverters, however it has following limitations – ΙΙ. Z-SOURCE Generally, voltage-source inverters and currentsource inverters are used to control motor drive system [15]. To overcome aforementioned problems regarding VSI and CSI, this paper presents an impedance source (also called Zsource) inverter as a solution. The Z-source inverter is able to produce any output voltage range that might be greater than the ac input voltage by controlling the boost factor [1],[14]. With the help of shoot through state (which is forbidden in traditional converters) output voltage of the inverter can be varied. This (shoot through) state is an additional state (ninth state) in Z-source inverter while a traditional converter has only eight switching states [1]-[3]. called as self- boost type inverter. Diode (D) is used in Zsource inverter circuit to prevent the discharging of overcharged capacitor through the source [1]-[2]. The advantage of shoot-through zero state is, it does not affect pulse width modulation scheme of inverter. It equivalently produce same zero voltage at the load terminals, hence, it greatly reduces voltage stress. Available shootthrough period is limited by the zero state periods that can be determined by modulation index. ΙΙΙ. PWM TECHNIQUE There is multiple PWM techniques available like1.Single PWM. 2.Multiple PWM. 3.Sinusoidal PWM. Unlike the traditional inverters Z-source inverter does not requires any dead-time or overlap time provision which reduces waveform distortion greatly. The in-rush current and harmonics in the current can be reduced due to the inductor. It is buck-boost type inverter, so no need to change circuit configuration during operation. Z-source inverter can provide ride-through capability where voltage sags are major problems. Z-source inverter requires small capacitors and inductors compared to traditional voltage-source inverters and current-source inverters respectively [1][6]. In Z-source inverter there are basically nine permissible states and three modes of operation. In first mode inverter bridge generates one of the six traditional active vectors. In second mode inverter bridge produces one of the two traditional zero vector. Mode three is the shoot-through mode which is forbidden in traditional converters. Depending on how much voltage boost is required in the circuit, shootthrough interval (T0) and duty cycle (T0/T) is determined and varied. During this shoot-through state energy is transferred from capacitors to inductors and as a consequence Z-source inverter gains the voltage boosting capability and hence it is Fig. 3. Traditional PWM without shoot-through. Fig. 4. Modified PWM with shoot-through. PWM switching is based on sawtooth carrier wave. In this method at any time, total time period of one switching cycle will not change the shoot-through zero states are introduced in 20 Z-source inverter consist of two split inductors L1 and L2 and two split capacitors C1 and C2 which are connected in X shape to provide an impedance source to main circuit. IGBT’s or MOSFET’s are used to design main circuit i.e. 3 phase inverter. Z-source network is connected in between dc side or source and ac side or load. Z-source inverter has no. of advantages over a traditional converter, they are as follows – Page Fig. 2. Z-source network Most of the time a sinusoidal PWM technique is used due to its no. of advantages. In PWM techniques there are two different waves of different frequencies which are compared in PWM comparator. The traditional pulse width modulation is used when the dc input voltage is sufficient to produce an ac output voltage level shown in fig. (3). In other words we don’t need voltage boosting. In this method shoot-through state is not distributed in zero states. Fig. (4) shows the modified PWM technique in which shoot-through state is distributed in every switching cycle to achieve desired output voltage level [1],[7]-[9],[12]. each phase linearly. It should be noted that the active states remain unchanged. OBTAINABLE OUTPUT VOLTAGE WITH SIMPLE BOOST CONTROL It should be noted that there are no. of control methods which includes simple boost control, maximum boost control, constant boost control and traditional and modified space vector based pulse width modulation controls. Boost factor is the key factor in buck-boost operation of Z-source inverter. The boost factor also depends on the distribution of the shoot through duty intervals into the non shoot-through states. The simple boost technique is described in this paper and it is used widely in applications because of its simplicity and easy implementations. It uses two straight lines one from upper side and one from lower side as shown in fig. (4). Straight lines are compared with saw tooth waves and by varying reference of straight lines, shoot-through duty interval can be controlled. When the carrier wave (saw tooth) is greater than upper straight line or lower than bottom straight line at that time only the circuit turns into shoot-through mode, otherwise it operates just like traditional PWM control as shown in fig. (4). The method is very simple ; however the resulting voltage stress across device is more because some zero vectors are not utilized properly [1],[4]-[6]. Before going through the simulation results, theoretical calculations are added which is very important for comparing purpose. TABLE I PARAMETERS USED FOR 0.18 kW INDUCTION MOTOR S.No. If TS is the total switching period, T0 is the zero state time period and D0 is the shoot-through duty ratio which is given by, = = S. No. Parameters 1 2 3 4 5 6 7 Dc supply = = = = Switching frequency Modulation index, Shoot through duty ratio, Initial capacitor voltage, and (2) = .B= = . B . ( ) (3) And, Where voltage and (4) is the ac output voltage , is the input dc is the modulation index which is given by, = (1- ) index. In other words for high voltage gains small modulation index should be used [1], [3]. 150 V 160 1000 μ 10 kHz 0.642 0.358 335 V With the help of duty ratio, the boost factor B is Calculated as follows, = (5) In simple boost control as shown in equation (5) shoot through duty ratio is inversely proportional to the modulation Value = = (6) × ×( . ) = 3.52 . B . = (0.642) x (3.52) x (150/2) = 169.5 V V. PARAMETERS USED AND SIMULATION RESULTS Traditional voltage-source inverters and current-source inverters use capacitors and inductors to store energy and (7) (8) The equation (8) shows peak phase voltage which in turn helps to calculate the line-line peak voltage which is 21 / Specification 0.18 kW 400 V 50 Hz 4 0.119 Ω 0.0997 Ω 0.07142 H 0.07142 H 0.762 H 0.001 kgm2 TABLE II Z-SOURCE NETWORK PARAMETERS and (G) is the voltage gain of inverter which is given by, G= Output power RMS line voltage Input frequency No. of poles Stator resistance, Rotor resistance, Stator inductance, Rotor inductance, Mutual inductance, Moment of inertia, J (1) Boost factor is given by, B= Parameters 1 2 3 4 5 6 7 8 9 10 Page ΙV. filtering element to suppress voltage and current ripples respectively. Here Z-source inverter is used, which uses both two split capacitors and inductors to store energy which boosts voltage and acts as a second order filter. , , and has same values because of circuit symmetry [1]-[2]. Table I and II shows the parameters used for 0.18 kW induction motor and Z-source inverter respectively. Also the simulation graphs are given for analysis purpose. All reference values of Z-source network are tabulated below [1]. calculated by multiplying that phase voltage by √3 and which is equal to x √3 = (9) = 169.5 x√3 (10) = 293.58 V (11) By using equation (11), the rms voltage is calculated which is, = = Fig. 7. 3 phase IM current (12) √ . √ = 207.59 V (13) (14) Equation (11) and (14) shows that the output ac voltage is boosted up to 293.58 V (peak) or 207.59 V (rms). Simulation results are also shown below to confirm above calculations. Fig. (5) and (6) shows the performed simulation results of line-to-line and phase voltage respectively for 150 V dc input voltage with 0.642 modulation index and 0.358 shootthrough duty ratio. Waveform shows that the output ac voltage of inverter is nearly about 298 V for 0.642 modulation index. Fig. 8. Total harmonic distortion in voltage profile Fig. (7) shows three phase motor currents individually for Phase A, Phase B and Phase C. Initially during the transient period distortions are visible. Phase currents attains stability during steady state operation. Fig. (8) shows Total Harmonic Distortion in phase voltages, carried out using FFT analysis. VI. CONCLUSION Fig. 6. Phase voltage of 3 phase induction motor REFERENCES [1] Fang Zheng Peng, “Z-source inverter”,IEEE transaction on industry applications., vol.39,no.2, march/april, 2003 22 The Z-source inverter can produce any desired output voltage which is required for application which may be greater than the input voltage level with the help of modulation index and shoot-through duty ratio. This feature is forbidden in traditional voltage-source inverters and current source inverters, hence now a day’s researchers are moving towards this technology. In this work theoretical analysis and simulation results are described. The results obtained by simulations have close agreement with results obtained by theoretical calculations which proves Z-source inverter concept. Modified PWM technique with simple boost control is used because of its no. of advantages over the traditional PWM control. Page Fig. 5. Simulation waveform of 3 phase line-to-line voltage [4] [5] [6] [7] [8] [9] [10] S. 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