Accepted Manuscript

Title: Enervate Carrier LS-PWM for Reducing Harmonic Distortions in SCHB-MLI

Author: Rohith Balaji Jonnala Ch Sai Babu

PII: DOI:

Reference:

S2213-0209(16)30005-2 http://dx.doi.org/doi:10.1016/j.pisc.2016.03.005 PISC 153

To appear in:

Received date: 1-1-2016

Accepted date: 24-3-2016

Please cite this article as: Jonnala, R.B., Babu, C.S.,Enervate Carrier LS-PWM for Reducing Harmonic Distortions in SCHB-MLI, Perspectives in Science (2016), http://dx.doi.org/10.1016/j.pisc.2016.03.005

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Enervate Carrier LS-PWM for Reducing Harmonic Distortions in SCHB-MLI

Rohith Balaji Jonnalaa*, Ch Sai Babub Electrical and Electronics Engineering Department, a*Shri Vishnu Engineering College for Women (Autonomous), Bhimavaram - 534202, W. G. Dt., bUniversity College of EngineeringKakinada, JNTUK, Kakinada - 533003, E. G. Dt.,

A. P., India. a *rohithbalajijonnala@yahoo.com

Abstract: Over the different Modulation Techniques, Pulse Width Modulation is one of the major renowned modulation strategies, because the pulses are generated based on reference signal and the reference signal is sinusoidal. So many modifications are made in this strategy since PWM era starts in the area of Power Electronics, the modifications are like with the reference or carrier. This paper also discussed about the analysis with performance parameters when the carrier is modified with the impact of Hysteresis Band structures. The Modified Carrier is abridged towards the Zero line of actual carrier, this type of carrier is supports to reduce the switching losses at the edge of pulse train and also reduce the total harmonic distortions from the converter output signal. The lower switching losses and better harmonic profile elevates the inverter performance, in this paper Symmetrical Cascaded H-Bridge Multilevel Inverter is analysed with the proposed Enervate Carrier associated to Level Shift Pulse Width Modulation.

Keywords: Symmetrical Cascaded H-Bridge Multilevel Inverter, Level Shift Modulation, Modified Carrier Modulation, Better Harmonic Profile, Enervate Carrier Modulation.

1 Introduction:

In the end of third quarter of 19th century Hasmukh S. Patel et al. describes the generalized strategy for the harmonic elimination in inverter [9], enhancement of this strategy is later entitled as Pulse Width Modulation strategy. The pulses are generated by comparing the triangular with the reference signal (sinusoidal), the width of the pulses are different based on the amplitude of reference signal and intersectional area of triangular wave. This strategy gives the better operating conditions to inverter with low distorted outputs. Modulation strategy is the key or sole of the inverter, so many variations of modulation techniques are introduced, modified and hydrides the few techniques [1,7]. These modulation strategies are classified based on the operating strategy, pulse pattern, computation and references.

Among all of these modulations, Level Shift Pulse Width Modulation get more influence on the multilevel inverter's scope of applications. Because LS-PWM has a simple computational complexity for any level of inverter but not for the others, simple unit triangular waves are compared with a sine wave in positive and negative sides then the comparision gives the pulses train to operate the inverter [2,11]. These pulses are directly transferred to inverter's positive and negative thyristor groups.

There is a need of optimal coordination between the carrier frequency, switching losses and harmonic profile of converter, so many modifications and optimal arrangements are introduced [3,6,8,10,13,14]. Another type of proposed modification with the optimal arrangement of these parameters is discussed in next section.

2 Proposed Modified Modulation Strategy:

For an effective inverter, the switching losses and harmonic content must be low [5,14], but these are interlinked with each other. In nominal operation of inverter, switching losses are depends on switching frequency and switching frequency is based on carrier frequency so carrier frequency is maintained at low. Similarly the harmonic content level is low when the carrier frequency is high, but harmonic distortions are the most effective parameter on inverter operation than switching losses. If the switching losses are reduced without changing the switching frequency it will effects to reduce the harmonic distortions. These correlated parameters are optimized with a modification and the key idea is enervating the carrier magnitude.

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Fig. 1. LS-PWM related Pulses. (a) Comparision of reference and carrier. (b) Pulse pattern with

ineffective zones.

The pulse train of LS-PWM is analyzed and the ineffective areas are identified based on the switching losses. This ineffective area is indicated with the dotted circle and shown in Fig. 1, at middle of the pulse train the width is very high almost its looks like an ON pulse so the switching states can't affects the output then the switching losses are high. With the knowledge of Over-modulation strategies [4], the ineffective zone of pulse train is treated as ON pulse. The amplitude of the triangular is enervated towards Zero line, and the pulse are generated based on

Pulse Pattern of Proposed Modulation.

3 Analysis of Proposed Strategy with SCHB-MLI:

The proposed modulation strategy is analyzed with Symmetrical Cascaded H-Bridge Multilevel Inverter (SCHB-MLI); surely the switching losses are reduced when the switching states are reduced so the proposed strategy has the low switching losses due to the switching states based on modified carriers. Finally the analysis is concentrated on the harmonic distortion values and the fundamental voltage values. For the analysis purpose single phase, 7-level, 3-cell, symmetrical DC voltage cascaded H-Bridge inverter is used with R load. For the comparision and validation purpose the same inverter is simulated with the basic LS-PWM and the obtained V-THD is 15.59%.

(a) (b)

Fig. 3. Graph between (a) Magnitude of carrier and VThD at modulation index=1, (b) Modulation Index and VTHD at magnitude of carrier =0.5 for Proposed LS-PWM.

For the analysis purpose, first the basic LS-PWM is simulated with respect to varying the modulation index. It gives the basic idea and the performance evaluated parameters are come in to scope, the proposed idea is to vary the amplitude of carrier then there are a number of combinations between the variations of amplitudes of carrier and reference. So here the Progressive Methodology is used, in this the generalised way analysis is propagated with the reference of best results and then with that best result another variation is to be analyzed.

Table 1. Voltage THD for the Basic LS-PWM (modulation index = 1)

Magnitude of the Carrier VTHD (%) Fundamental Voltage (V)

0.1 16.03 258.7

0.2 15.16 256.5

0.3 14.49 254

0.4 14.05 251.3

0.5 13.91 248.4

0.6 14.08 245.3

0.7 14.58 241.8

0.8 15.42 238

0.9 16.64 233.7

By the progressive methodology basic LS-PWM is analyzed with varying the magnitude of carrier from 0.1 to 0.9, these harmonic distortion analysis is graphically illustrated in Fig. 3(a),

and the linearly modulated to over-modulation range of 1.2 of analysis is illustrated in Fig. 3(b). By the observation of Figs. 3 (a) & (b), the best values are 0.5 and 1 respectively to the Magnitude of carrier and Modulation index, and for that analysis the fundamental voltages with the THD values are tabulated in Table 1.

4 Simulation Result:

At the state of optimal condition of inverter with proposed modulation is simulated and the results of Output Voltage waveform with Harmonic Profile is shown in Figs. 4.

Harmonic order

Fig. 4. (a) Output Voltage waveform and (b) FFT analysis for Proposed LS-PWM. 5 Conclusion:

As per the discussion on previous sections, the way to overcome the specified snags of a LS-PWM associated multilevel inverter is reduced with Modified Carrier. With the analysis and comparision of performance parameters, it is clear that the inverter operates with the efficient condition with the reduction of THD from 15.59% to 13.91% and switching losses are also reduced with the proposed logic.

Acknowledgment

The authors gratefully acknowledge the support provided by Jawaharlal Nehru Technological University Kakinada, Andhra Pradesh, TEQIP Centrally Sponsored Scheme through Ministry (HRD) of Govt. of India and Shri Vishnu Engineering College for Women, Bhimavaram.

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