SVPWM Based Step-Up Inverter

SVPWM Based Step-Up Inverter

Abstract

In a normal inverter the output voltage is boosted by means of a transformer so as to meet the load voltage. In the present topology an inverter has been designed which increments the voltage at the input itself, so the use of transformer is eliminated at output. To reduce the level of harmonic content Space Vector Pulse Width Modulation (SVPWM) is used. To describe the operating principle and control the paper focuses on running a 3-phase induction motor fed from the output of the inverter.

Keywords: Switched Capacitor Step-up Converter, Space Vector Pulse Width Modulation, Fast Fourier transform.

1. Introduction

   A conventional square wave inverter has the disadvantage of getting lower order harmonics at a high level. To counter this disadvantage a number of pulse width modulation techniques like bipolar sinusoidal pulse width modulation, unipolar sinusoidal pulse width modulation, selective harmonic elimination and

   The first section of this paper gives introduction to the topic followed by the section describing about the switched capacitor step-up converter. The third section is about SVPWM technique, fourth section deals with the design of inverter using the above technologies to drive a 3-phase induction motor. Fifth section contains the simulation results and final conclusion in the sixth section.

2. Switched Capacitor Step-up Converter (SCSC):

   A switched capacitor step up converter uses the capacitor switching to boost the output voltage to required value. This is done by charging capacitors in parallel and discharging them in series. The circuit of SCSC is shown in figure 1.

   S3 S1

   harmonic injection were implemented since decades, S4 C

   but the harmonic content is not reduced completely. To V1

   minimize the harmonic content approximately to zero R

   S2

   S2

   Space Vector Pulse Width Modulation technique has been developed. The proposed inverter uses SVPWM technique.

   In a normal H-Bridge inverter, for an applied dc voltage of a certain magnitude we get the ac voltage of same magnitude at the output of inverter. A step-up transformer is needed to boost the voltage to the level that is required by load. This increases the cost and size of the inverter. To rout out this disadvantage switched capacitor topology has been used that raises the voltage at the input of inverter. Hence ac voltage that is greater than the applied dc voltage is obtained without the use of transformer.

   Figure 1: Step up converter

   When the switches S1, S2 are closed the capacitor is charged to a voltage of Vd, for a time t1.

   Vc = Vd (1)

   During this time S3, S4 are open .When the switches S3, S4 are closed the voltage across the load is Vd + Vc for a time t2 as shown in figure 2.

   Figure 2: Output of Step up converter

   VL = Vc + Vd (2)

   VL = Vd + Vd =2Vd (3)

   1. Reference voltage magnitude and its angle with respect to axis are calculated as below.

      2 2

      2 2

      Vref = (V + V ) (4)

      = Tan-1(V / V) (5)

   2. Reference angle in a particular sector is calculated as

      = – k (600); k such that < 600 (6)

   3. Sector number = k + 1

   Thus the voltage is boosted. Here we observe that as the number of capacitors increase, the output voltage also increases.

   2) Computation of T1, T2

   and T0

   using

3. Space Vector Pulse Width Modulation (SVPWM)

   By using SVPWM technique harmonics are eliminated in the output wave form, another advantage of this SVPWM technique is dc bus utilization can be increased up to 15%. Thus it proves itself to be more reliable over sinusoidal pulse width modulation.

   3.1 Algorithm for SVPWM:

   1) The sector in which the tip of the reference sector is situated is to be determined from the instantaneous phase references (Va, Vb and Vc) as shown in figure 3.

   T1 = (Ts) (Vref) (Vdc) (7)

   T2 = (Ts) (Vref) (Vdc) (8)

   T0 = T s- (T1 + T2) (9)

   1. Based on the time calculations given in step 2 firing pulses should be given to switches S1 to S6 as shown in figure (4-9).

      A Phase

      B Phase

      C Phase

      Figure 3: Three Phase reference voltages

      1. Va, Vb, Vc are converted to V, V.

   Figure 4: Switching sequence of sector 1

   Figure 5: Switching sequence of sector 2

   Figure 6: Switching sequence of sector 3

   Figure 7: Switching sequence of sector 4

   Figure 8: Switching sequence of sector 5

   Figure 9: Switching sequence of sector 6

4. Design of proposed Inverter:

   The novel inverter is a blended version of switched capacitor step-up converter and SVPWM inverter as shown in figure 10.

   DC

   Voltage Source

   SCSC

   Filter

   SVPWM

   Inverter

   3-phase Induction motor

   DC

   Voltage Source

   SCSC

   Filter

   SVPWM

   Inverter

   3-phase Induction motor

   Figure 10: Block diagram of proposed system

   The switched capacitor step-up converter which is connected to the input of inverter consists of two capacitors connected as shown in figure 11, using MOSFETs as input devices.

   S1

   S2 S5

   S6 S3

   Simulation has been done using MATLAB (Simulink) for the proposed inverter feeding an induction motor with the following specifications:

   Line voltage: 400V VA Rating: 4000 Frequency: 50Hz

   Vdc

   S4

   C1

   S7

   C2 R1

   RPM: 1430

   The complete Simulink model developed is given in Figure 13.

   Figure 11: Switched Capacitor Step-Up Converter

   The switching sequences of SCSC for feeding a normal two level inverter are:

   1. Switches S2, S4, S5 and S7 are in closed condition and S1, S3 and S6 are in open condition for 10 milliseconds to charge the capacitors C1, C2 in parallel.

   2. Switches S1, S3 and S6 are in closed condition and S2, S4, S5 and S7 are in open condition for next 10milliseconds to discharge the capacitors C1, C2 in series.

      In this way the output voltage of SCSC is boosted to thrice the input. The obtained voltage consists of ripples which are suppressed by the use of filter shown in figure 12.

      D1 L1

      Figure 13: Simulink Model

      5.1 Results and Discussion:

      The proposed model has been simulated using MATLAB /Simulink toolbox. The variation of rotor speed with respect to time obtained from simulation is shown in figure 14.

      Rad/Sec

      Rad/Sec

      Vin

      C3 Vout

      Figure 12: Filter Circuit

      The diode D1 is used to obstruct the negative peak from the inverter side such that the SCSC converter is protected. LC circuit acts as normal filter and gives pure dc voltage across the capacitor C3.

      The obtained dc voltage is applied to a three-phase inverter which is triggered by a SVPWM control circuit. Thus we get a three-phase ac voltage with Vdc as peak, across the inverter. The obtained ac voltage is given as input to an induction motor whose rated voltage is greater than or equal to the obtained ac voltage at the inverter output. The performance of inverter and mtor are explained in the next section.

5. Simulation of the proposed system:

   Time

   Figure 14: Variation of rotor speed w.r.t time

   It is observed that speed became constant at 158 rad/sec

   i.e. 1508 rpm approximately.

   The input voltage given to SCSC is 133.33 volts which is boosted to 400 volts (i.e. thrice the input). The

   Va

   Va

   waveforms of voltages Va, Vb, Vc obtained from the SVPWM inverter are shown in figure (15-17).

   Time

   Vb

   Vb

   Figure 15: Phase A Voltage (Va)

   Time

   Vc

   Vc

   Figure 16: Phase B Voltage (Vb)

   Time

   Figure 17: Phase C Voltage (Vc)

   The waveform of three-phase current drawn by three- phase induction motor measured at the output of the SVPWM inverter is shown in figure 18.

   Figure 18: Three-Phase Currents

   Iabc

   Iabc

   The reduction of harmonic content in Three-phase voltage is observed by doing Fast Fourier transform (FFT) analysis. The frequency Vs magnitude plot is shown in figure 19.

   Figure 19: Frequency Vs magnitude plot

   Total Harmonic Distortion: 53.62% Third Harmonic: 0.11%

   Fifth Harmonic: 0.25%

   Seventh Harmonic: 0.08%

   Eleventh Harmonic: 0.20%

6. Conclusion:

   The combined performance of the switched capacitor step up converter and SVPWM inverter is presented in this paper. The harmonic content of the output voltage is greatly reduced by employing SVPWM technique and the voltage at the output of inverter is boosted by using switched capacitor technique. The analysis provides the information for effective use of proposed system.

7. References:

1. W. vander Broek, H. C. Skudelny, and G. V. Stanke

   Analysis and realization of PWM based on voltage space

   vectors, IEEE Trans. Ind. Applicat., vol. 24, no. 1, pp. 142 150, 1988.

2. Grahame Holmes and Thomas A. Lipo, Pulse Width Modulation for Power Converters Principles and Practices, IEEE Press, Wiley Publications2000.

3. Blasko, A hybrid PWM strategy combining modified space vector and triangle comparison methods, in IEEE PESC Conf. Rec., 1996, pp.18721878.

4. S. Patel and R. G. Hoft, Generalized techniques of harmonic elimination and voltage control in thyristor inverters: Part IHarmonic elimination, IEEE Trans. Ind. Applicat., vol. 9, pp. 310317, May/June 1973.

5. J. Kerkman, B. J. Seibel, D. M. Brod, T. M. Rowan, and

   D. Leggate, A simplified inverter model for on-line control and simulation, IEEE Trans. Ind. Applicat., vol. 27, no. 3, pp. 567573, 1991.

6. H.W. Van Der Brock, H.C. Skundelny and G.V. Stanke,

   Analysis and realization of a pulse width modulator based on voltage space vectors, IEEE Trans Ind. Appl., 24. pp. 140-150, Jan/Feb 1998.

7. O.Ogasawara, H. Akagi, and Nabel, A novel PWM scheme of Voltage source inverters based on space vector theory, in proc. EPE European conf. Power Electronics and Applications pp. 1197-1202, 1989.

8. S.R. Bowes and Y.S. Lai, The relationship between space vector modulation and regular-sampled PWM, IEEE Trans. Power Electron, Vol. 14, pp. 670-679, Sept. 1997.