Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel I

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 11 | April 2017 ISSN (online): 2349-6010

Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor Praveen O PG Student Department of Electrical & Electronics Engineering Mar Baselios College of Engineering and Technology, Trivandrum, Kerala, India

Dr. Nisha G. K Associate Professor Department of Electrical & Electronics Engineering Mar Baselios College of Engineering and Technology, Trivandrum, Kerala, India

Abstract Speed control of Induction motor drive is one of the major problem faced in almost all industries and leading sectors. In this paper a technique is implemented for a better speed control for induction motor drives using a multilevel inverter. Multilevel inverter have lower distortion of output voltages and Total Harmonic Distortion (THD) of machine currents under steady state operating conditions can be minimized. The switching angle for the pulse is selected to reduce the harmonic distortion. In this drive system it has advantages like reduced total harmonic distortion and higher torque. The model of the multilevel inverter system is fed with SVM method to control the induction motor. The simulations are performed using MATLAB/SIMULINK software and the results are presented.

Keywords: Neutral Point Clamped Inverter NPC, Space Vector Pulse Width Modulation, Induction Motor _______________________________________________________________________________________________________ I.

INTRODUCTION

Induction motor is the most commonly used electrical machine in almost all level of voltage in industrial applications, because of its low cost and increased reliability. Due to development of high power and low cost power electronic devices in the area has provided a larger area of application for the ac drives. Hence, ac drives like induction motor drives along with power electronic converters have replaced the dc motor drives in industries.

Selection of suitable power electronic converter is the difficulty in using ac drives lies. When using Pulse Width Modulation (PWM) for the control of the power electronic converter, duty ratio input needs to be in a specific range or this can create stability issues. Thus, the power conversion stage is playing a vital role.A multilevel inverter is a good choice for replacing the conventional voltage source inverters or current source inverters. It has many advantages like reduced voltage stress, increased quality of output voltage and increased power rating.

In this paper, a neutral point clamped MLI with three levels is designed for induction motor drives. A three level space vector is used to generate switching pulses for the neutral point clamped MLI. The performance of MLI fed induction motor drive is done for various operating conditions.

II. NEUTRAL POINT CLAMPED INVERTER The neutral point clamped inverter provides multiple voltage levels through connection of the phases to a series bank of capacitors.

In the original invention, the concept can be extended to any number of levels by increasing the number of capacitors in the design [1]-[2]. The additional level was the neutral point of the dc bus, so the name neutral point clamped inverter was introduced. But, with an even number of voltage levels, the neutral point is not accessible, and the term Multiple Point Clamped (MPC) is sometimes applied [3].Early descriptions of this topology were limited to three-levels where two capacitors are connected across the dc bus resulting in one additional level [4]. Due to capacitor voltage balancing issues, the diode-clamped inverter implementation has been mostly limited to the three-level. Because of industrial developments over the past several years, the three-level inverter is now used extensively in industry applications. Although most applications are medium-voltage, a three-level inverter for 480V is on the market.

Although the structure is more complicated than the two-level inverter, the operation is straightforward and well known. Each phase node can be connected to any node in the capacitor bank [5]-[7].Connection of the a-phase to junctions can be accomplished by switching transistors Sa1 and Sa2 both off or both on respectively as shown in Fig. 1. These states are the same as the two-level inverter yielding a line-to-ground voltage of zero or the dc voltage.

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Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor (IJIRST/ Volume 3 / Issue 11/ 023)

Sa 0 1 2

Table – 1 Switching of one leg of NPC Ta2 Ta1 Vag Iadc1 Iadc2 0 0 0 0 0 0 1 Vc1 Ias 0 1 1 Vc1+Vc2 0 Ias

Fig. 1: Three level neutral clamped multilevel inverter.

Connection to the junction is done by gating Sa1 off and Sa2 on. In this representation, the labels Sa1 and Sa2 are used to identify the transistors as well as the transistor logic. Since the transistors are always switched in pairs, the complement transistors are labeled Sa1 and Sa2 accordingly. The switching state, the a-phase current ias will flow into the junction through diode as Da1 if it is negative or out of the junction through diode Da2 if the current is are positive.

III. THREE LEVEL SVPWM Space vector pulse width modulation is quite different from the PWM methods. With PWMs, the inverter can be thought of as three separate push-pull driver stages which create each phase waveform independently. SVM however treats the inverter as a single unit. Specifically the inverter can be driven to eight unique states. Modulation is accomplished by switching the state of inverter [8]-[10].

SVM is a digital modulation technique where the objective is to generate PWM load line voltages. This is done in each sampling period by properly selecting the switching states of inverter and calculation of the appropriate time period for each state [11]-[12].

SVM can be implemented through the following steps: Switching States For a three-level three-phase inverter there are 27 switching states. These states represent the connection to the different DC-link points. If there is a load connected to the output of these states the inverter will generate a output phase voltage. This can be calculated as follows: Va0= (2S1a−S1b−S1c) + (2S2a−S2b−S2c) Vb0= (2S1b−S1a−S1c) + (2S2b−S2a−S2c) (1) Vc0= (2S1c−S1b−S1a) + (2S2c−S 2b−S2a) These are the line-to-neutral voltages. To receive the line-to-line voltage: Vab= Va0−Vb0 Vbc=Vb0−Vc0 (2) Vca= Vc0−Va0 There is requested to generate five levels of outputs, so the three-level can be created. These levels are 2VDC, VDC, 0, -VDC and -2VDC (for the line-to-line voltage). Fig. 2 shows space vector diagram for a three-level inverter demonstrating 19 voltage vectors and 27 switching states.

As for the two-level inverter the reference vector is given with the help from three voltage vectors. For the three-level converter each sector also is divided into 4 regions, specifying the output even more.

Table - 2 Voltage vectors Value of voltage vectors Redundant switching states Zero Voltage Vectors (ZVV) V=0 Small Voltage Vectors (SVV) V1, 4,7,10,13,16

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Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor (IJIRST/ Volume 3 / Issue 11/ 023)

Medium Voltage Vectors (MVV) Large Voltage Vectors (LVV)

V3, 6,9,12,15,18 V2, 5,8,11,14,17

Fig. 2: Space vector diagram for a three-level inverter demonstrating 19 voltage vectors and 27 switching states.

Table 2 shows voltage vector. Based on the magnitude the voltage vectors can be defined as: The principle of SVPWM method is that the command voltage vector is approximately calculated by using three adjacent vectors [13]-[15]. The duration of each voltage vectors obtained by vector calculations; V*Ts=(T1V1+T2V2+T3V3) (3) T1+T2+T3=Ts (4) Where V1, V2, V3 - vectors that define the triangle region in which V*is located.

T1,T2,T3 - corresponding vector durations.

Ts - sampling time.

In a three-level inverter similar to a two-level inverter, each space vector diagram is divided into 6 sectors. Forsimplicity here only the switching patterns for Sector A will be defined so that calculation technique for the other sectors will be similar [16].

Sector A is divided into 4 regions as shown in Fig. 3, where all the possible switching states for each region are given as well.

SVPWM for three-level inverters can be implemented by using the steps of sector determination, determination of the region in the sector, calculating the switching times, T a, Tb, Tc and finding the switching states[16].

Fig. 3: Sector A and its switching states for three-level inverter

Determining the sector The sector in which the command vector V* as shown in Table 3.

Table - 3 Sector selection α V* in sector 0° ≤ α < 60° A 60° ≤ α < 120° B 120° ≤ α < 180° C 180° ≤ α < 240° D 240° ≤ α < 300° E 300° ≤ α < 360 F

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Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor (IJIRST/ Volume 3 / Issue 11/ 023)

Determining the region in the sector Table - 4 Value of m1and m2 Value of m1, m2 V* Region m1+m2>0.5 1 m1> 0.5 2 m2> 0.5 3 m1 and m2< 0.5 4

Fig. 4: Calculation of m1 and m2

Table 4 shows value m1 and m2. The m1 and m2can be calculated as: đ?&#x2018;?

2 2 a = m2 = đ?&#x153;&#x2039; = b = mn.sinÎą đ?&#x2018; đ?&#x2018;&#x2013;đ?&#x2018;&#x203A;

3

â&#x2C6;&#x161;3

m1 = mn.cosÎą â&#x20AC;&#x201C; ( m1=mn(đ?&#x2018;?đ?&#x2018;&#x153;đ?&#x2018; đ?&#x203A;ź â&#x2C6;&#x2019;

(5)

â&#x2C6;&#x161;3

2

â&#x2C6;&#x161;3 đ?&#x2018; đ?&#x2018;&#x2013;đ?&#x2018;&#x203A;đ?&#x203A;ź â&#x2C6;&#x161;3

đ?&#x153;&#x2039;

mn. sinÎą).cos( ) 3

)

(6)

Calculating the switching times, Ta, Tb, Tc Table 5 shows the switching times Ta, Tb, Tc for Sector A.

Ta Tb Tc

Ta Tb Tc

Table - 5 Switching time of Ta,Tb,Tc Region I Region II Ts* 1.1*m*Ts*sin((Ď&#x20AC;/3)-Îą) (1-1.1*m*sin(Îą+Ď&#x20AC;/3)) Ts/2* 1.1*Ts*m*sinÎą (1-(2*1.1*sin(Îą+Ď&#x20AC;/3)) Ts/2((2*1.1*m* 1.1*Ts*sinÎą sin(Ď&#x20AC;/3-Îą))-1) Region III Region IV Ts/2* Ts/2* (1-2*1.1*m*sinÎą) (2*1.1*m*sin(Îą)-1) Ts/2* 1.1*m*Ts* (2*1.1*m*sin(Ď&#x20AC;/3+Îą)-1) sin((Ď&#x20AC;/3)-Îą) Ts/2* Ts* (1+2*1.1*m*sin(Îą-Ď&#x20AC;/3)) (1-1.1*m*sin(Îą+Ď&#x20AC;/3))

IV. INDUCTION MOTOR The induction motor has very wide range of industrial applications because of its simple construction, ruggedness & low cost.

These advantages are superseded by control problems when using in industrial drives with high performance demands [17].

Induction motors are used in a very large scale in industries because of its robust construction, higher efficiency, easy for maintenance, low price and easy availability. The principle of operation of Induction Motor is developed to drive the steady state equivalent circuit and parameter calculation. The dynamic model is used to obtain the transient and steady state behavior of induction motor.

Analysis of the dynamic behavior of Induction Motor are described the equation of induction motor. A 3-phase winding can be reduced to 2-phase winding set by using this approach, with the magnetic axis being formed in quadrature. The stator and rotor variable (voltage, current, and flux linkages) of an induction motor may rotate at an angular velocity or remain stationary, when transferred to a reference frame [18]-[20].

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Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor (IJIRST/ Volume 3 / Issue 11/ 023)

Fig. 5: Equivalent Circuit diagram of induction motor in dqo.

This frame of reference is generally called as arbitrary reference frame in generalized machine analysis. Direct Torque Control (DTC) and Field Oriented Control (FOC) have emerged as standard industrial solutions for high dynamic performance operation of these machines.

V. FILTER DESIGN A filter is required to obtain an approximate sine wave for the load, since the output of multilevel inverter is not sinusoidal. The filter is designed as per the equation: fc=â&#x2C6;&#x161;đ?&#x2018;&#x201C;đ?&#x2018;&#x;. đ?&#x2018;&#x201C;đ?&#x2018; (7) fc- corner frequency fr-reference frequency fs-switching frequency fc=

1

(8)

2đ?&#x153;&#x2039;â&#x2C6;&#x161;đ??żđ??ś

VI. RESULTS AND DISCUSSIONS SVPWM fed three level Neutral point clamped multi-level inverter with filter has been done using MATLAB/SIMULINK. The input for inverter is taken as 1000 V. The switching of inverter was done by using SVPWM total harmonic distortion was taken for both output voltage. Table 6 shows the simulation parameters. Fig. 6 shows space vector modulation Neutral Point Clamped Inverter model. Fig. 7 shows output voltage from inverter. Fig. 8 shows Induction Motor model and Fig. 9, 10, 11 shows the stator current waveform, torque of Induction Motor at 30 Nm and Speed in rpm at 30 N m torque.

Table - 6 Simulation parameters Parameters Value Vdc 1000V Switching Frequency 10kHz Output voltage 311V Rload 1Ί Lload 1mH Cfilter 10 ΟF Lfilter 5mH

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Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor (IJIRST/ Volume 3 / Issue 11/ 023)

Fig. 6: SVPWM fed NPC

Fig. 7: Output voltage from inverter

Fig. 8: Induction Motor Model

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Open Loop Control of Three Level Space Vector Pulse Width Modulation of Neutral Clamped Multilevel Inverter Fed Induction Motor (IJIRST/ Volume 3 / Issue 11/ 023)

Fig. 9: Stator current waveform of induction motor

Fig. 10: Induction Motor Torque at 30 Nm

Fig. 11: Speed in rpm at 30Nm torque

VII. CONCLUSION The simulation of three level Neutral clamped multi-level inverter fed induction motor was carried out using space vector pulse width modulation. The performance of the inverter and induction motor has been done using MATLAB/SIMULINK.From the results it is obtained thatthe space vector pulse width modulation fed Neutral clamped inverter fed induction motor has lower total harmonic distortion and torque ripple of induction motor is also very much reduced.

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