A New Multicarrier SPWM Technique for Five Level ... - IEEE Xplore

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[2] Samir Kouro, Pablo Lezana, Mauricio, Angulo, and José Rodríguez,. “Multicarrier PWM with DC-link ripple feedforward compensation for multilevel inverter” ...
A New Multicarrier SPWM Technique for Five Level Cascaded H-Bridge Inverter Abhishek Paikray Dept. of Electrical Engineering Veer Surendra Sai University of Technology, Burla, India e-mail:[email protected] Abstract—Many researchers have investigated modulation techniques applied to the cascaded multilevel inverter to improve its harmonics performance. Different methods have been proposed for cascaded multilevel inverter modulation that one of them is level-shifted PWM method (LS-PWM). In this paper, a novel multicarrier SPWM technique which uses a trapezoidal triangular carrier is proposed for a five level cascaded multilevel inverter. This carrier waveform is being implemented with different LS-PWM techniques such as phase disposition (PD), phase opposition disposition (POD) and alternative phase opposition disposition (APOD). The line voltage and total harmonics distortion (THD) obtained in various techniques are compared with the outputs of triangular carrier wave. To validate the improvement, these PWM techniques are simulated for a 1KW, 3φ five level inverter with 2 KHz switching frequency using MATLAB/SIMULINK. The effect of modulation index on the line voltage and harmonics are also analyzed. The proposed switching technique enhances the fundamental component of the output voltage and lowers total harmonic distortion. Keywords—Cascaded Multilevel Inverter (CMI); Sinusoidal PWM (SPWM); Trapezoidal Triangular Multicarrier PWM(TTMC PWM); Triangular Multicarrier PWM (TMC PWM); Total Harmonics Distortion (THD).

I.

INTRODUCTION

The multilevel inverter has drawn tremendous interest in the power industry and is highly attractive converter topology for use with medium- or high-voltage and high-power applications [1]. It has got attractive features like high voltage capability, reduced power device stress, nearly sinusoidal output, low dv/dt’s and smaller or even no output filter, making them suitable for high power applications [2]. The multilevel inverters are being used in various applications such as flexible ac transmission systems, renewable energy sources, active power filter, uninterruptible power supplies and electrical drives [3], [4]. Multilevel inverter (MLI) offers several advantages that make it preferable over the conventional voltage source inverter (VSI). These include the capability to handle higher DC link voltage, reduced power device stress and improved harmonics performance [5]. By using a multilevel structure, the stress on each switching device can be reduced in proportional to the higher voltages. In some applications, it is possible to avoid expensive and bulky step-up transformer [6]. Another significant advantage of a multilevel output is better and more sinusoidal voltage waveform as well as lowers THD [7].

Banaja Mohanty Dept. of Electrical Engineering Veer Surendra Sai University of Technology, Burla, India e-mail:[email protected] Further, high dv/dt of semiconductor devices increases the electromagnetic interference (EMI) problem, resulting more common mode voltage and hence the stresses on the motor bearings are increased leads to possibilities of failure of motor. Thus by increasing the number of levels in the output waveform, the switching dv/dt stress is reduced in the multilevel inverter [2], [5]. Since its inception in early 1980’s first proposed by Nabae, multilevel voltage source inverter was a three-level neutral point clamped (NPC) Inverter [8]. At later stage, several multilevel topologies have evolved, such as the Diode Clamped Multilevel Inverter (DCMLI) also known as Neutral Point Clamped (NPC) Inverter, Flying Capacitor Multilevel Inverter (FCMLI) and Cascaded Multilevel Inverter (CMLI) [9], [10]. II.

CASCADED MULTILEVEL INVERTER

A cascaded multilevel inverter consists of a series of Hbridge (single-phase, full-bridge) inverter units [11]. Each inverter unit can generate three different output voltage levels +Vdc, 0 and –Vdc, by connecting the dc source to the output terminals utilizing various switching combinations of the four semiconductor switches in each inverter. The five level cascaded multilevel inverter is having two separate dc sources and produces five level output, they are +2Vdc, +Vdc, 0, -Vdc and -2Vdc. One leg of a three phase five level cascaded inverter topology is shown in Fig.1.

Fig. 1. Model of one leg of three phase five level CMLI

The five level CMLI utilizes two independent dc sources per phase and consequently will create an output phase voltage with five levels. In general, if Ns is the number of independent dc sources per phase, then the following relations apply [9]:

the carrier signal, then the active device corresponding to that carrier is turned on, and if the reference is less than the carrier signal, then the active device corresponding to that carrier is turned off [14].

m = 2Ns + 1

(1)

p = 2 ( m − 1)

(2)

The main advantage of pulse width modulation (PWM) control strategies is to reduce the total harmonic distortion (THD) of the output voltage. Any deviation in the sinusoidal wave shape will result in harmonic currents in the load and their effects in case of motor drives are electromagnetic interference (EMI), power losses and torque pulsation. PWM is considered to be an efficient modulation technique as it does not require additional components and also the lower harmonics can be eliminated leaving higher order harmonics which can be easily filtered out[15].

where m is the number of levels and p is the number of switching devices in each phase. CMLI topology is the most attractive, since it has got modular structure (each level has same structure) so packaging and circuit layout is easier. It requires no clamping diodes as in DCMLI and no voltage balancing capacitor as in FCMLI. With increase in number of levels in the inverter without requiring high ratings on individual devices and the power rating of the CMLI is also increased [5]. It can also work at reduced power level when one of its cells is damaged. Soft switching techniques can be applied to CMLI which results in less switching losses [12]. Additionally, CMLI type of topology is free of DC voltage balancing problem, which is a common issue facing in the DCMLI and FCMLI topologies as separate DC sources eliminate the need of the voltage balancing circuits[8]. This topology is suitable for applications where separate dc voltage sources are available, such as photovoltaic (PV) generators [6], fuel cells and batteries. The phase output voltage is generated by the sum of two output voltages of the full bridge inverter modules. Fig.2 shows the simulation model of a three phase five level CMLI and is developed using MATLAB/SIMULINK. The simulation results are obtained for the output phase voltage and line voltage of the three phase five level CCMLI with 1kW, 3φ resistive loads with various PWM techniques.

Fig. 2. Simulation model of a three phase five level CMLI

III.

MODULATION TECHNIQUES

Sinusoidal Pulse Width Modulation (SPWM) is the simplest technique that can be implemented in both two level and multilevel inverters [13]. Basically, in SPWM, two signals - a sinusoidal reference signal and a high frequency carrier signal (triangular signal) are compared to give two states (high or low). The reference is continuously compared with the carrier signal. If the reference is greater than

The switching frequency of the inverter is the frequency of the carrier signal. In multilevel inverters, the amplitude modulation index, Ma and the frequency modulation index, Mf are defined as [16],

Mf =

Ma =

fc

fm

Am (m − 1) * Ac

(3)

(4)

where Am and Ac are amplitude of modulating and carrier signal respectively, fm and fc are frequency of modulating and carrier signal respectively. For multilevel applications multiple carriers PWM techniques are used. The Multicarrier Modulation techniques, can be divided in to the following categories [17] such as, ¾

Phase disposition (PD) where all the carrier waves are in phase with each other [18].

¾

Phase opposition disposition (POD) where all the carrier waves are in phase above and below the zero reference, however there is a 180 phase shift between the ones above and below zero respectively [19].

¾

Alternative phase opposition disposition (APOD) where each carrier wave is shifted by 180 degrees from the adjacent carrier wave [19].

In this paper, trapezoidal triangular multicarrier sinusoidal PWM (TTMC SPWM) technique is developed. Each trapezoidal triangular carrier is to be compared with the corresponding modulating sine wave. The TTMC PWM is applied with various LS-PWM techniques such as PD, POD, and APOD. The line voltage waveforms are obtained for different modulation techniques by simulation using MATLAB/SIMULINK for a cascaded five level inverter. The results obtained are compared with triangular multicarrier waveforms to evaluate the harmonics performance of the proposed trapezoidal triangular multicarrier PWM technique.

2 1.5 1 Magnitude

A. Triangular Multicarrier Sinusoidal PWM (TMC SPWM) The performance of the multilevel inverter is based on the multi-carrier modulation technique used. Two level to multilevel inverters are made using several triangular carrier signals and one reference signal per phase. Carrara [20] developed multilevel sub harmonic PWM (SH-PWM) is as follows. For an m level inverter, (m-1) triangular carrier waves are required. And all the carrier waves should have the same frequency and the same peak to peak magnitude [15]. They are defined as

0 -0.5 -1 -1.5 -2

0 ≤ t < t3

0

t3 ≤ t < t6

1

2 3 Time (sec)

4

5 -3

x 10

Fig. 4b) Carrier arrangement for TMC POD PWM

(5) 2 1.5

It is shown that using symmetrical triangular carrier generates less harmonic distortion at the inverters output [21, 22] and is shown in Fig. 3 1

1 Magnitude

⎧t ⎪ t ( Am ) ⎪3 u (t ) = ⎨ ⎪ t6 − t ( A ) ⎪⎩ t 6 − t 3 m

0.5

0.5 0 -0.5

0.8

Magnitude

-1 0.6

-1.5 -2

0.4

1

2 3 Time (sec)

4

5 -3

x 10

Fig. 4c) Carrier arrangement for TMC APOD PWM

0.2

0

0

0

1

2

3

4

5

Time

-4

x 10

Fig.3. Triangular carrier wave

The multicarrier modulation techniques (PD, POD, and APOD) are implemented using triangular multicarrier signals are shown in Fig. 4(a), 4(b) and 4(c) respectively.

B. Trapezoidal Triangular Multicarrier Sinusoidal PWM (TTMC SPWM) Trapezoidal Triangular wave is a combination of two waveforms viz, a trapezoidal and a triangular waves as shown in the fig.4. The upper half is a triangular wave and the lower half is a trapezoidal wave. 1

2 0.8

Magnitude

1.5

Magnitude

1 0.5

0.6

0.4

0 0.2

-0.5 0

-1

0

-2

1

2

3

4

Time

-1.5

Fig. 5. 0

1

2 3 Time (sec)

4

5 -4

x 10

Trapezoidal Triangular carrier wave

5 -3

x 10

Fig. 4a) Carrier arrangement for TMC PD PWM

The Trapezoidal Triangular wave can be estimated with the following function:

The frequency modulation index and amplitude modulation index will be same as that of triangular waves as described in equations (3) and (4) respectively. The multicarrier modulation techniques (PD, POD, and APOD) are implemented using trapezoidal triangular multicarrier waves are shown in Fig. 6(a), 6(b) and 6(c) respectively. 2

2 1.5 1 Magnitude

⎧t ⎪ t ( Am ) ⎪1 0 ≤ t < t1 ⎪ Am ⎪ t1 ≤ t < t 2 ⎪{ t − t 2 ( Am )} + Am (6) t2 ≤ t < t3 ⎪⎪ t3 − t 2 u (t ) = ⎨ t3 ≤ t < t4 ⎪{ t 4 − t ( A )} + A m m t4 ≤ t < t5 ⎪ t 4 − t3 ⎪ t5 ≤ t < t6 ⎪ Am ⎪ t −t 6 ( Am ) ⎪ ⎪⎩ t5 − t6 where Am is the amplitude of modulating signal divided into two equal halves each for the trapezoidal and the triangular waves. Time between 0-t6 is equally divided in six intervals.

0.5 0 -0.5 -1 -1.5 -2

0

1

2 3 Time (sec)

4

5 -3

x 10

Fig. 6c) Carrier arrangement for TTMC APOD PWM

IV.

SIMULATION RESULTS

The feasibility of the proposed PWM strategy has been investigated and verified through computer simulations for the three-phase five level cascaded multilevel inverter model using MATLAB/SIMULINK software. Phase disposition, phase opposition disposition and alternative phase opposition disposition techniques are used for the various multicarrier SPWM techniques such as 1. Triangular Multicarrier Sinusoidal PWM 2. Trapezoidal Triangular Multicarrier Sinusoidal PWM

1.5

The line voltage waveform at fundamental frequency of 50Hz and switching frequency of 2 KHz is obtained for the proposed CMLI. For comparison, the total harmonic distortion (THD) was chosen to be evaluated for all the modulation techniques for various modulation indices. Fast Fourier Transform (FFT) is applied to obtain the spectrum of the output voltage to find out THD. The THD is calculated using the following equation,

Magnitude

1 0.5 0 -0.5 -1 -1.5 -2

0

1

2 3 Time (sec)

4 x 10

Fig. 6a) Carrier arrangement for TTMC PD PWM

THD =

2 1.5

Magnitude

1 0.5 0 -0.5 -1 -1.5 -2

0

1

2 3 Time (sec)

80

5 -3

4

5 -3

x 10

Fig. 6b) Carrier arrangement for TTMC POD PWM

∑v n= 2

v1

2 n

(7)

where n is the harmonic order, vn is the RMS value of the nth harmonic component and v1 is the RMS value of the fundamental component. Here the %THD is calculated up to a harmonic order which is twice the switching frequency. For 2 KHz switching frequency up to 80th order harmonics is taken in to account for calculating THD. The line voltages waveforms obtained with triangular multicarrier (PD, POD, and APOD) PWM techniques are shown in Fig. 7(a), 7(b) and 7(c) respectively. The line voltages waveforms obtained with trapezoidal triangular multicarrier (PD, POD, and APOD) PWM techniques are shown in Fig. 8(a), 8(b) and 8(c) respectively.

500 Vab

Vab

500 0 -500

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0 -500

0.04

Vbc

Vbc

0 -500

-500 0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04

0.01

0.015

0.02 0.025

0.03

0.035

0.04

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04

0

0.005

0.01

0.015

0.02 0.025 Time

0.03

0.035

0.04

500 Vca

Vca

0.005

0

500 0 -500

0

500

500

0 -500

0

0.005

0.01

0.015 0.02 0.025 Time (sec)

0.03

0.035

0.04

Fig. 7a) Line Voltage Waveforms for TMC PD PWM

Fig. 7e) Line Voltage Waveforms for TTMC POD PWM 500

Vab

Vab

500 0 -500

-500 0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

Vbc

Vbc

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

Vca

Vca

0 -500

0.005

0.01

0.015 0.02 0.025 Time (sec)

0.03

0.035

0.015

0.02 0.025

0.03

0.035

0.04

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04

0

0.005

0.01

0.015 0.02 0.025 Time (sec)

0.03

0.035

0.04

0 -500

0

0.01

500

0.04

500

0.005

0 -500

0

0

500

0.04

500

-500

0

0.04

Fig. 7f) Line Voltage Waveforms for TTMC APOD PWM

Fig. 7b) Line Voltage Waveforms for TMC POD PWM

Fig. 8(a), 8(b) and 8(c) shows the THD comparison between TMC and TTMC using (PD, POD, APOD) PWM techniques respectively.

Vab

500 0 -500

50

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04 40

-500

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04

Vca

500 0 -500

0

0.005

0.01

0.015 0.02 0.025 Time (sec)

0.03

0.035

Vab

0

0

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04

40

0

0

0.005

0.01

0.015

0.02 0.025

0.03

0.035

0.04

500

% THD

Vbc

1.0

0.9 0.8 0.7 Modulation Index

0.6

THD analysis for line voltages of PD PWM

50

500

-500

20

Fig. 8a)

500

-500

30

10

0.04

Fig. 7c) Line Voltage Waveforms for TMC APOD PWM

Vca

TMC PWM TTMC PWM

0

% THD

Vbc

500

TMC PWM TTMC PWM

30

20

0 10

-500

0

0.005

0.01

0.015 0.02 0.025 Time (sec)

0.03

0.035

0.04 0

Fig. 7d) Line Voltage Waveforms for TTMC PD PWM Fig. 8b)

1.0

0.9 0.8 0.7 Modulation Index

0.6

THD analysis for line voltages of POD PWM

[9]

50

40

TMC PWM TTMC PWM

% THD

[10] 30

[11]

20

10

[12] 0

Fig. 8c)

1.0

0.9 0.8 0.7 Modulation Index

0.6

THD analysis for line voltages of APOD PWM

V.

CONCLUSION

This paper present simulation results for a three phase cascaded five level inverter which use trapezoidal triangular carrier wave as novel multicarrier modulation technique is implemented in MATLAB/SIMULINK environment. In trapezoidal triangular carrier waveform, different techniques such as phase disposition (PD), phase opposition disposition (POD) and alternative phase opposition disposition (APOD) are implemented. The output quantities like fundamental line voltage and percentage THD of line voltage are being found. For TTMC PWM in PD technique gives the lowest THD of 16.83 % among all other. The proposed methods offer better harmonic performance in compare with the triangular carrier. REFERENCES [1]

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