SPACE VECTOR PULSEWIDTH MODULATION CONTROLLED MULTILEVEL CURRENT SOURCE INVERTER

International Research Journal of Computer Science (IRJCS) ISSN: 2393-9842 Issue 5, Volume 2 (May 2015) www.irjcs.com

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SPACE VECTOR PULSEWIDTH MODULATION CONTROLLED MULTILEVEL

CURRENT SOURCE INVERTER B.ARULVANI Dr.V.PADMATHILAGAM, M.E. (Power System) Asst. professor / Electrical Engg Annamalai University Annamalai University

ABSTRACT: Space Vector Pulse Width Modulation (SVPWM) scheme is chosen to control the multilevel current source inverter. Among different modulation techniques, SVPWM is popularly accepted in recent trend, as it is easier for digital realization. SVPWM reduces the harmonics and the switching losses and has better dc bus utilization. Multilevel inverter have capability to deliver high output power with lower dv/dt (or) lower di/dt with less distorted output waveforms resulting in reduction of harmonics injected into the system. Multilevel current source inverter is one of the effective solutions for better output power quality (ie) less harmonic currents and less total harmonics distortion of the output current. This paper focuses on step by step development of Space Vector Pulse Width Modulation controlled multilevel current source inverter to reduce the harmonics in the output in MATLAB/SIMULINK platform. Results are presented and analysed.

KEYWORDS: Pulse Width Modulation (PWM), Space Vector Pulse Width Modulation (SVPWM), Voltage Source Inverter(VSI), Current Source Inverter(CSI),Total Harmonic Distortion(THD). 1. INTRODUCTION Recent developments of high performance semiconductor power switches, such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and Insulated Gate Bipolar Transistors (IGBTs) increases the research interest in high power converters such as multilevel voltage source inverters and its dual multilevel current source inverters. Multilevel inverters have the capability to deliver high output power with lower harmonic distortion and reduce the size of the filter. Multilevel inversion is a power conversion strategy in which the output voltage is obtained in step, thus bringing the output closer to a sine wave and reduces the total harmonic distortion [1], [2]. There are many applications for multilevel inverters, such as Flexible AC Transmission System (FACTS), high voltage direct current lines and electrical drives. Multilevel current source inverter has additional advantages like short circuit production and over current production. Among several modulation techniques like selective harmonic elimination, space vector control, space vector pulse width modulation, space vector pulse width modulation (SVPWM) techniques stand a step ahead. The SVPWM is more sophisticated technique for generating sine wave that provides a higher voltage with lower harmonics distortion [6]. 2. WHY CURRENT SOURCE INVERTER

The pulse width modulated voltage source inverter is the most common power converter topology for adjustable speed induction motor drives. In high power areas, CSI using Gate Turn-off Thyristors (GTOs) are of special interest. The power circuit of the CSI is simpler and more robust than the VSI due to no freewheeling diodes with unidirectional current flow. The CSI can provide higher reliability related with a dc-link inductor than a capacitor for the VSI and inherent over current production by current regulation of the controlled rectifier. The CSI permits four quadrant operations transferring the electric power in both directions using the controlled rectifier with closed-loop current control [7]. In addition, the CSI is more efficient because of the quasi-square wave mode operation, which turns ON and OFF only once per cycle of the output current. The multilevel CSI opens the possibility of operation with high power levels as well as elimination of harmonics generated by the converter. The higher the level lower the harmonic content at the output current. An infinite number of levels ensures zero harmonic distortion at the output current waveform. In order to suppress the source of the harmonics (ie the harmonic current on the dc side) the dc inductor is usually of large value. So that the dc input current can be kept constant and the output current waveform is purely determined by the PWM signals [12]- [14]. 3. CURRENT SOURCE INVERTER

The main objective of there static power converters is to produce ac output current waveforms from a dc source. For sinusoidal ac outputs, its magnitude, frequency and phase should be controllable. Due to the fact that the ac line currents feature high di/dt, a capacitive filter should be connected at the ac terminals in inductive load applications



(such as adjustable speed drives).Thus, nearly sinusoidal load voltages are generated that justifies the use of these topologies in medium voltage industrial applications, where high quality voltage waveforms are required [8] [11]. In a Current Source Inverter (CSI), the current from the dc source is maintained at an effectively constant level, irrespective of the load or inverter conditions by inserting a large inductance in series with the dc supply to enable changes of the inverter voltage to be accommodated at low values of di/dt. The input to the CSI is obtained from a fixed voltage ac source, battery, photo voltaic cell. 3.1. SINGLE PHASE H-BRIDGE CURRENT SOURCE INVERTER The basic configuration of an H-bridge CSI is shown in Fig.1. This inverter generates three-level current waveform, i.e. level +I ,0 andI, as listed in the Table.I.[2].

Table.I OPERATION MODES OF H BRIDGE CSI

Q1 Q2 Q3 Q4 OUTPUT CURRENT 1 0 1 0 +I 1 0 0 1 0 0 1 1 0 0 0 1 0 1 -I

Fig.1 THREE-LEVEL CURRENT SOURCE INVERTER

3.2 CASCADED H BRIDGE CURRENT SOURCE INVERTER The configuration of the five-level CSI is obtained by connecting two H-bridge CSI in parallel.

Fig.2 FIVE LEVEL CURRENT SOURCE INVERTER

Fig.2 shows the configuration of the chosen Five level current source inverter composed of eight unidirectional power switches Qc1-Qc8. Preferably IGBTs are used. The output current levels of the five level CSI are +I, +2I, 0, I and 2I. In positive half cycle four switches are ON and another four switches are OFF. In negative half cycle the switching sequence is reversed. Therefore the output is like a stepped waveform [2] [4]. 4.DC CURRENT GENERATION CIRCUITS In the H-bridge based five level CSI, the dc current source is obtained by employing an auxiliary circuit. The circuit works as a regulated dc current source for the inverter. The circuit simply consists of a controlled power switch (Q), a smoothing inductor (L) as shown in fig.3. The switch regulates the dc current flowing through the smoothing inductor and reduces the smoothing inductor size owing to the high switching frequency operation. Free wheeling diode is used to keep continuous current flowing through the smoothing inductor.



Fig.3 DC CURRENT MODULE

5.SPACE VECTOR PULSE WIDTH MODULATION

Space Vector Pulse Width Modulation(SVPWM) is a modulation algorithm which translates phase voltage reference, coming from the controller into time/duty cycles to be applied to PWM peripheral. SVPWM is an advanced computation intensive PWM method and possibly the best PWM technique for three phase inverters. SVPWM refers to a special switching sequence of the upper three power switches of a three phase inverter [1]. It is a type of modulation technique used for changing the pulse width according to the given reference vector. In this technique, all possible switching states are represented as vectors in a two dimensional voltage space which are obtained by transforming three phase dependent vectors to three phase independent vectors The reference vector is assumed to be constant for the corresponding sampling time. Ideally the sampling frequency should be infinity but the sampling frequency is limited by the turn-on and turn-off times of the device used. Therefore the maximum possible sampling frequency is chosen for minimum total harmonic distortion.

5.1 PRINCIPLE OF SVPWM The circuit of a typical three phase voltage source inverter is shown in Fig.4. S1 to S6 are six power switches that shape the output, which are controlled by the switching variables x,x,y,y,z and z. When an upper switch is switched ON , i.e., when x, y or z is 0, the corresponding lower switch is switched OFF, i.e., the corresponding x y or z is 1. Therefore, the ON and OFF states of the upper switches S1,S3 and S5 can be used to determine the output voltage [5].

Fig.4 THREE PHASE VOLTAGE SOURCE PWM INVERTER.

The operational principle of the PWM can be explained more clearly with the space vector representation. Fig.5. displays the relationship between abc and d-q reference frames. The voltage vector can be expressed in the vector form as

Fig.5 RELATIONSHIP BETWEEN abc AND STATIONARY d-q REFERENCE FRAMES

cbas VVVV 2 (1)

where

3

2exp j



)120sin()120sin(

sin

tVVtVVtVV

mc

mb

ma

(2)

and Vm is the amplitude of the fundamental component. The relationship between the switching variable vector [x, y, z]t and the line-to-line voltage vector [Vab Vbc Vca] is given by as follows

zyx

V101110

011

VVV

dc

ca

bc

ab

The relationship between the switching variable vector [x, y, z]t and the phase voltage vector [Van Vbn Vcn] can be expressed as

zyx

Vdc

211121112

3VVV

cn

bn

an

(3)

Fig.6 SECTORS AND SWITCHING VECTORS IN THE COMPLEX PLANE

Fig.6. shows the sectors and vectors in complex plane. The angle between any adjacent two non-zero vectors is 60 degrees. Meanwhile, two zero vectors (V0 and V7) are at the origin and apply zero voltage to the load. The eight vectors are called the basic space vectors and are denoted by V0, V1, V2, V3, V4, V5, V6 and V7. The same transformation can be applied to the desired output voltage to get the desired reference voltage vector Vref in the d-q plane. There are eight possible combinations of ON and OFF states in the three phase VSI configuration as shown in Fig.7 .The ON-OFF sequences of the six switches should respect the following two conditions [5]. Three of the six switches must be always ON and the other three always OFF. The upper and lower switches of the same leg are driven with two complementary pulse signals. In this way, no

vertical short through could happen.

Table.II OUTPUT VOLTAGES FOR THE VARIOUS SWITCHING PATTERNS



Fig.7 THE INVERTER VOLTAGE VECTORS (V0 TO V7)

Space voltage vector and corresponding states SVPWM refers to a special switching sequence of the upper three power transistors of a three phase power inverter. 5.2 SVPWM ALGORITHM Five steps can be identified to implement the SVPWM technique: Step1 : Definition of the possible switching vectors in the output voltage space. Step 2 : Identification of the separation planes between the sectors in the output voltage space. Step 3 : Identification of the boundary planes in the output voltage space. Step 4 : Obtaining decomposition matrices. Step 5 : Definition of the switching sequence 6. SIMULATION AND RESULT DISCUSSION The Matlab Simulink software is used to simulate five level current source inverter, where all parameters and blocks are modeled based on basic concepts explained in above sections. One of the key feature is that it allows the user to simulate the design over a specified period of time. 6.1 SIMULATION OF FIVE-LEVEL CASCADE CURRENT SOURCE INVERTER Five level current source inverter is modeled based on the theoretical concept as explained. The modulating signal is generated by space vector algorithm. Among chosen four carrier signals, two carrier signals are applied across the positive half cycle of the modulating signal and remaining two are applied across the negative half cycle of the modulating signal. Gate signals are generated when the carrier signals intersect with the reference signal. SVPWM generated gate pulses are applied to the switches. Here in CSI each switch is paired with other switch so there are eight switches in CSI but four pulse generations is sufficient because of its paired nature. These pulses are given to one subsystem in Simulink model from where all switches of gate terminal are connected. Based on the concepts explained in modulation techniques, four pulses are generated. Remaining four pulses are generated by shifting the phase angle. PARAMETERS: No of switches - IGBTs : 8 Diodes : 8 Number of ideal switches : 2 DC input voltage : 160 V Input inductance : 500H Load resistance : 2

Fig.8 Represents the Space Vector Pulse Width Modulation for Five Level Current Source Inverter Where Space Vector is Compared to Triangular Carrier Wave.



Fig.8 SIMULATED SPACE VECTOR PULSE WIDTH MODULATION FOR FIVE LEVEL CURRENT SOURCE INVERTER.

SIMULATION RESULTS: The simulated output current and voltage waveform of multilevel current source inverter is displayed in Fig.8 and Fig.9. respectively. This current flow through the load and the five level voltage waveform is obtained across the load. Fig .11.(a) and Fig.11.(b) shows the FFT spectrum of output current generated by unoptimized switching pulse controlled and SVPWM controlled five level current source inverter respectively. Fig .12.(a) and Fig.12.(b) shows the FFT spectrum of output voltage generated by unoptimized switching pulse controlled and SVPWM controlled five level current source inverter respectively.

Fig.9 SIMULATED OUTPUT CURRENT OF SVPWM CONTROLLED CHOSEN FIVE LEVEL CURRENT SOURCE INVERTER WITH R=2 LOAD.

Fig.10 SIMULATED OUTPUT VOLTAGE OF SVPWM CONTROLLED CHOSEN FIVE LEVEL CURRENT SOURCE INVERTER

WITH R=2 LOAD.

Table.III THD OF OUTPUT CURRENT AND VOLTAGE UNOPTIMIZED SWITCHING PULSE CONTROLLED AND SVPWM CONTROLLED CHOSEN FIVE LEVEL CSI.

Parameters

%THD Unoptimized switching Pulse

signal SVPWM switching signal

Current 45.35 35.77 Voltage 43.00 20.83



Table.III displays the THD of output current and voltage unoptimized switching pulse controlled and SVPWM controlled chosen five level CSI.

Fig.11 (a) FFT SPECTRUM OF OUTPUT CURRENT OF Fig.11 (b) FFT SPECTRUM OF OUTPUT CURRENT OF UNOPTIMIZED SWITCHING PULSE CONTROLLED SVPWM CONTROLLED CHOSEN FIVE LEVEL CSI CHOSEN FIVE LEVEL INVERTER

Fig.12 (a) FFT SPECTRUM OF OUTPUT VOLTAGE Fig.12 (b) FFT SPECTRUM OF OUTPUT VOLTAGE OF UNOPTIMIZED SWITCHING PULSE CONTROLLED CONTROLLED CHOSEN FIVE LEVEL CSI CHOSEN FIVE LEVEL INVERTER From the Table.III, Fig.10 and Fig.11 The total harmonic distortion of output current and Voltage of SVPWM controlled chosen five level Inverter is lesser than the unoptimized switching pulse Controlled five level inverter. CONCLUSION:

Space Vector Pulse Width Modulation (SVPWM) is a more sophisticated technique for generating sine wave that provides a higher voltage with lower harmonic distortion and advanced computation intensive PWM method. SVPWM technique controlled five level CSI is simulated in simulink block. The output voltage waveform and current waveform and FFT spectrum of current and voltage is presented and analyzed. The THD of output current and voltage in SVPWM controlled CSI compared to the un optimized switching pulse controlled CSI is lesser. REFERENCES : [1] Wei-dong jiang, Shao-wu Du,Liu chen Chang, Yi Zhang, and Qin Zhao, Hybrid PWM Strategy of SVPWM and

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SPACE VECTOR PULSEWIDTH MODULATION CONTROLLED MULTILEVEL CURRENT SOURCE INVERTER

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