A Novel PWM Technique for Multilevel VSI fed Vector Controlled Drives based on Universal Offset Time Expression

(1)

Advances in Engineering, Management and Sciences J. Mech. Cont.& Math. Sci., Special Issue, No.-3, September (2019) pp 82-96

Copyright reserved © J. Mech. Cont.& Math. Sci.

P. Rama Mohanet al.

82

A Novel PWM Technique for Multilevel VSI fed Vector Controlled Drives based on Universal Offset Time Expression

P. Rama Mohan

¹

, Neeli Mallikarjuna

²

, Puli Obulesu

³

A. Suresh Kumar

⁴

, D Lenine

⁵

1,2,4,5

Department of Electrical and Electronics Engineering,RGMCET, Nandyal, A.P., India

3

Department of Electrical and Electronics Engineering, SVREC, Nandyal, A.P., India

Corresponding Author: Dr. P. Rama Mohan Email: [email protected]

https://doi.org/10.26782/jmcms.spl.3/2019.09.00007

Abstract

This paper presents a novel generalized scalar Pulse Width Modulation (PWM) technique based on universal offset time expression for Multilevel Voltage Source Inverter (VSI) fed Vector Controlled Drives In this technique, by varying a constant between 0 and 1, various PWM techniques have been derived. These PWM techniques don’t require information of angle and sector. Also, these techniques are simple to implement because, reference vector calculation and sector identification is not required. So, there is less complexity. The Multilevel inverter uses level shifting carrier signals. The proposed concept is simulated and evaluated.

Keywords :

PWM Algorithm,Vector Control, Induction Motor Drive, Multilevel Inverter, Voltage Source Inverter.

I. Introduction

Nowadays, multilevel inverters are used for large power and medium power induction motor applications, instead of two-level inverters because of high power handling capacity and good quality of output voltages. In literature[V],[VI],[VII],[IX], to control the frequency and voltage of inverter configurations, different PWM techniques are proposed. The implementation of the PWM techniques is carried out based on space vector method [III],[X] and carrier- comparison method [I],[VIII]. Though both methods give identical results, but carrier based PWM techniques don’t use complex calculations. Moreover, in multilevel inverters, complexity in control algorithm also increases. Hence, in this paper, the preference is given to carrier based techniques for multilevel inverter configurations.

To get fast transient response in electric trains and ship propulsion applications, the vector controlled or decoupled controlled [II],[IV] induction motor drives are gaining importance because of less weight to volume ratio and less maintenance.

(2)

J. Mech. Cont.& Math. Sci.,

P. Rama Mohanet al.

The regular two level inverter fed vector cont reduced efficiency of drive system because

also high current ripple and torque ripple. Moreover the drive also has limitations on drive power ratings. Hence, in this paper, a seven

configuration is employed

modulating signals have been proposed

comparison approach is the base for the proposed techniques

II. PWM Technique

The seven-level inverter circuit is is shown in the figure. Here the capacitors number of levels. From the theory,

switching devices of 2(N

and the voltage V_ab is known as the line voltage. In carrier comparison generate control signals,

seven-level inverter configuration.

Fig.1.

Advances in Engineering, Management and Sciences J. Mech. Cont.& Math. Sci., Special Issue, No.-3, September (2019) pp

83

two level inverter fed vector control based induction motor drive d efficiency of drive system because the output voltage has poor quality and high current ripple and torque ripple. Moreover the drive also has limitations on drive power ratings. Hence, in this paper, a seven-level multilevel inverter employed. Also, different PWM techniques based on discontinuous modulating signals have been proposed for switching losses reduction. The carrier comparison approach is the base for the proposed techniques.

PWM Technique for Multilevel Inverter Configuration

level inverter circuit is provided in Fig.1. Here, one leg of inverter is shown in the figure. Here the capacitors divide the bus voltage into required number of levels. From the theory, for N level inverter, capacitors of

2(N-1) are used. The voltage V_ao is known as the pole voltage is known as the line voltage. In carrier comparison

generate control signals, six carrier waves are compared with reference wave level inverter configuration. The generation of pulses are provided in Fig.2.

Fig.1. Circuit Topology of Seven-level inverter

Advances in Engineering, Management and Sciences (2019) pp 82-96 rol based induction motor drive gives the output voltage has poor quality and high current ripple and torque ripple. Moreover the drive also has limitations on level multilevel inverter different PWM techniques based on discontinuous . The carrier

, one leg of inverter the bus voltage into required of (N-1) and is known as the pole voltage is known as the line voltage. In carrier comparison method, to reference wave in The generation of pulses are provided in Fig.2.

(3)

P. Rama Mohanet al.

84 Fig.2.Generation of pulses

The three reference signals to generate control signals is

c and b a, i

&

3 and 1,2 r

3 1) - 2(r - { cos V_ref



  }

Vi

(1).

PWM techniques with these reference signals given in (1) generate high current ripple and torque ripple. For the reduction of these ripples, the reference signal has to be modified. The modified reference signals can be obtained using reference signals given in (1). From these instantaneous reference signals (V_a,V_b,V_c), imaginary switching times are determined from (2).

s dc

i

in T

V

T  V (2).

Based on the reference signals, switching times may be +ve or -ve. That’s why these switching times is as imagine.Hence, these switching times are considered as imaginary. The modulating signals are determined from (3).

(4)

P. Rama Mohanet al.

85





















 



  1

2 2

*

s dc gi

in T

V T

V

^(3).

Where

] [

] )

1 ( ) 1 (

[ _max _min

offset in

s in gi

T T

kT T

k k T T T















^(4).

min

) max

1 ( ) 1

( k k T kT

T

T_offset  _s    

Now, by varying k from 0 and 1 in (4), various discontinuous modulating signals (DPWMMAX, DPWMMIN, DPWWM0, DPWM1, DPWM2 and DPWM3) and continuous modulating signals (SVPWM) can be obtained as provided in Fig.3.

Fig.3. ModulatingSignals

(5)

P. Rama Mohanet al.

86

To achieve fast transient response, vector control or decoupled control is employed for the drives as shown in Fig.4. Here, decoupled control can be achieved by resolving stator current vectors (i_s) as i_dsfor direct axis and i_qs for quadrature axis.

Fig.4.Vector control based drive

In the above figure, V_a, V_b, V_care the reference signals and they are derived first.

Using these three reference signals, various new discontinuous and continuous modulating signals are derived and compared with carrier signals to generate control signals (S_a, S_b, S_c).

III. Results and Discussion

Effectiveness of various techniques i.e. SVPWM, DPWMMAX, DPWMMIN, DPWM0, DPWM1, DPWM2 and DPWM3 on vector control based seven-level inverter fed motor drive is verified in MATLAB based studies &the results are presented. The induction motor is Ls=Lr=0.17H, Rs=1.57Ω, Rr=1.21Ω and J=0.089Kg-m². The switching frequency of 3 kHz and input voltage of 540V DC are used in simulation studies. The steady state results of vector control based seven-level inverter fed drive with all the proposed techniques are given in Fig.5 to Fig.11. Each figure shows modulating signal (V_a^*), Line Voltage (V_ab), Line Current, speed and torque for various techniques. The line current harmonic spectrum and THD are given in Fig.12.

(6)

P. Rama Mohanet al.

87

Fig.5. Steady state results with SVPWM technique

(7)

P. Rama Mohanet al.

88

Fig.6. Steady state results with DPWMMINtechnique

(8)

P. Rama Mohanet al.

89

Fig.7. Steady state results with DPWMMAXtechnique

(9)

P. Rama Mohanet al.

90

Fig.8. Steady state results with DPWM0technique

(10)

P. Rama Mohanet al.

91

Fig.9. Steady state results with DPWM1 technique

(11)

P. Rama Mohanet al.

92

(12)

P. Rama Mohanet al.

93

(13)

P. Rama Mohanet al.

94 (i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

Fig.12. Line Current Harmonic spectrum with (i)SVPWM (ii)DPWMMIN (iii)DPWMMAX (iv)DPWM0 (v)DPWM1 (vi)DPWM2 (vii)DPWM3 techniques

(14)

P. Rama Mohanet al.

95

From the above results, the speed is controlled and maintained constant by using vector control. The modulating signals are clamping for 120^oduration either to +ve or -ve bus with discontinuous modulating signals (DPWMMIN, DPWMMAX, DPWM0, DPWM1, DPWM2,DPWM3). Hence,33% of switching losses are reduced with these PWM techniques. Moreover, size and cost of cooling equipment get reduced. The THD is also reduced to a great extent by using the proposed techniques to the industrial drives.

V. Conclusion

To get fast transient response, vector control based seven-level inverter fed drive with various PWM techniques are discussed in this paper for high power and medium power applications. Here, reduced switching losses and reduced THD can be obtained with discontinuous techniques when compared with continuous techniques.

All the proposed techniques based on carrier comparison method to reduce complexity are implemented and verified for the drive.

References

I. A.R.Beig, “Application of three-level voltage-source-inverters to voltage- fed & current-fed high-power inductionmotor drives”, Ph.D. dissertation, Indian Institute of Science, Bengaluru,India,2004

II. D.Casadei, F.Profumo, G.Serraand A.Tani, “FOC and DTC: Two Viable Schemes for Induction Motors Torque Control”, IEEE Transactions on Power Electronics, Vol.:17, Issue:5, pp.779-787, 2002.

III. D.W.Kang, “Improved carrier-wave based SVPWM method for generalised cascaded multi-level-inverter topology”, Proceedings of IEEE-APEC, pp.542-548, 2000.

IV. J.A.Santisteban and R.M.Stephan, “Vectorcontrol methods for inductionmachines: an overview”, IEEE Transactions on Education, Vol.:44, Issue:2, pp.170-175, 2001.

V. M.A.Pérez, J.Rodriguez, K.Gopakumar and M.Malinowski, A Survey on Cascaded Multi-level-Inverters”, IEEE Transactions on Industrial Electronics, Vol.:57, Issue:7, pp.2197-2206, 2010.

VI. P. Rama Mohan, T. Bramhananda Reddy and M. Vijaya Kumar, “Simple and Efficient High Performance PWM Algorithm for Induction Motor Drives”, Journal of Electrical Engineering, Vol.: 11, Issue: 4, pp. 23-30, 2011.

VII. P. Rama Mohan, T. Bramhananda Reddy and M. Vijaya Kumar, “A Simple Generalized PWM Algorithm for Three Phase Voltage Source Inverter fed AC Drives”, International Review of Electrical Engineering, Vol.: 10, Issue: 2,pp. 180-188, 2015.

(15)

P. Rama Mohanet al.

96

VIII. S.Das and G.Narayanan, “Novel Switching Sequences for a Space- Vector-Modulated Three-Level-Inverter”, IEEE Transactions on Industrial Electronics, Vol.:59, Issue:3, pp.1477-1487, 2012.

IX. T.G.Habetler, F.Z.Peng and L.M.Tolbert, “Multi-level converters forlarge electricdrives”, IEEE Transactions on Industry Appications,Vol.:35, Issue:1, pp.36-44, 1999.

X. Z.Pan and F.Z.Peng, “A Sinusoidal PWM Method With Voltage- Balancing Capability for Diode-Clamped-Five-Level Converters”, IEEE Transactions on Industry Applications,Vol.:45, Issue:3, pp.1028-1034, 2009.