International Journal of Advanced Engineering Science and Technological Research (IJAESTR) ISSN: 2321-1202, www.aestjournal.org @2015 All rights reserved.
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Space Vector Pulse Width Modulation Technique
Simran Bhalla, Dr. Jagdish Kumar ,Department of Electrical Engineering PEC university, Chandigarh, India E-mail: simran7693@gmail.com
jagdishkumar@pec.ac.in
Abstract- With advances in solid-state power electronic devices and microprocessors, various inverter control techniques employing pulse-width-modulation (PWM) techniques are becoming popular in AC motor drive applications. These PWM-based drives are used to control both the frequency and the magnitude of the voltages applied to motors .The most widely used PWM schemes for three-phase voltage source inverters are carrier-based sinusoidal PWM and space vector PWM (SVPWM). This review paper gives the idea of SVPWM applied to the two level inverter , its advantages over conventional PWM method and different types of switching sequence used in SVPWM.
I. INTRODUCTION
SVPWM was first introduced in the mid-1980s and was greatly advanced by Van Der Broeck in 1988. There is an increasing trend of using space vector PWM (SVPWM) because of their easier digital realization and better dc bus utilization.
The Modulation Index is higher for SVPWM as compared to SPWM.
The output voltage is about 15% more in case of SVPWM as compared to SPWM.
The current and torque harmonics produced are much less in case of SVPWM.
The maximum peak fundamental magnitude of the SVPWM technique is about 90.6% of the inverter capacity
In 1991, Holtz proposed a classical over-modulation technique based on SVPWM , which divided the over- modulation range into two modes of operation and increased the utilization rate of the DC voltage to that of a six-step wave.
In the SVPWM technique, the duty cycles are computed rather than derived through comparison as in SPWM. The SVPWM technique can increase the fundamental component by up to 27.39% that of SPWM. The fundamental voltage can be increased up to a square wave mode where a modulation index of unity is reached.
SVPWM is accomplished by rotating a reference vector around the state diagram, which is composed of six basic non-zero vectors forming a hexagon. A circle can be inscribed inside the state map and corresponds to sinusoidal operation. The area inside the inscribed circle is called the linear modulation region or under-modulation region. As seen in Figure, the area between the inside circle and
outside circle of the hexagon is called the nonlinear modulation region or over-modulation region. The concepts in the operation of linear and nonlinear modulation regions depend on the modulation index, which indirectly reflects on the inverter utilization capability.
Fig. 1 Space Vector Diagram
II. PRINCIPLE
SVPWM is based on the principle that three phase set of voltages can be represented by a space vector of constant magnitude, equal to the amplitude of the voltages, and rotating with angular speed ω= 2π fRef.
In this method of modulation the inverter is treated as single unit, unlike SPWM where separate modulators are applied to each phase.
A typical two-level inverter has 6 power switches (labelled S1 to S6) that generate three phase voltage outputs. The six switching power devices can be constructed using power BJTs, GTOs, IGBTs, etc. The choice of switching devices is based on the desired operating power level, required switching frequency, and acceptable inverter power losses.
When an upper transistor is switched on, the corresponding lower transistor is switched off.
International Journal of Advanced Engineering Science and Technological Research (IJAESTR) ISSN: 2321-1202, www.aestjournal.org @2015 All rights reserved.
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Fig.2 Three phase inverter
A switching state is an instantaneous voltage vector composed of three phase–voltage levels according to the well-known space vector theorem.
combinations of switching patternns can be mapped into coordinates:
The task of the modulator is to select such switching states that meet the reference.
The reference voltage vector Vref rotates in space at an angular velocity w = 2pf , where f is the fundamental frequency of the inverter output voltage.
When the reference voltage vector passes through each sector, different sets of switches in will be turned on or off.
It is possible to decompose the reference vector into vectors that lie on two adjacent active vectors and two zero vectors, which are located at the center of the hexagon.
Fig. 3 Vector sectors
III. SVPWM FOR TWO
INVERTER
The space vector can be also represented
Fig.4 Vector decomposition
International Journal of Advanced Engineering Science and Technological Research (IJAESTR) ISSN: 2321-1202, www.aestjournal.org @2015 All rights reserved.
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LEVEL
in another
International Journal of Advanced Engineering Science and Technological Research (IJAESTR) ISSN: 2321-1202, www.aestjournal.org @2015 All rights reserved.
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reference frame with two orthogonal axes (p and q). We assume that the p axis is aligned in the horizontal direction and that the q axis is vertical. Then the abc three-phase voltage vector given in can be transformed into a vector with p-q coordinates. The p-q vector is used to find the sector of the abc plane in which the reference voltage
IV. SVM SCHEME
The choice of the null vector determines the SVM scheme.
There are a few options:
the null vector V0 only, the null vector V7 only, or a combination of the null vectors.
1. A popular SVM technique is to alternate the null vector in each cycle and to reverse the sequence after each null vector. This is referred to as the symmetric 7-segment technique.The total switching period is divided intro 7 parts, the zero vector is applied for 1/4th of the total zero vector time first followed by the application of active vectors for half of their application time and then again zero vector is applied for 1/4th of the zero vector time. This is then repeated in the next half of the switching period. This is how symmetrical SVPWM is obtained.
2. In 7 segment SVM line-to-line voltage waveform contains even order harmonics. To make the three- phase line-to-line voltage half-wave symmetrical, Type-A(starts and ends with (000))and Type-B (starts and ends with (111))switching sequences can be alternatively used each sector in the space vector diagram is divided into two regions. Type- A sequence is used in the non-shaded regions, while type-B sequence is employed in the shaded regions.
Fig.6 even order harmonic elimination
3. In the five-segment sequence, one of the three inverter output terminals is clamped to either the positive or negative dc bus without any
switching’s during the sampling period Ts.
Fig. 7 five segment sequence
V. CONCLUSIONS
By varying the magnitude of the input reference different modulation index can be achieved.
SVPWM can be applied to any multilevel inverters.
It was reviewed that the 7-segment SVM scheme performs better in terms of THD of the output line voltage. It is also observed that the THD of the 5- segment SVM lies between the THD of 7-segment SVM and &7-segment SVM with even order harmonic elimination.
SVM scheme to reduce common-mode voltage for cascaded multilevel inverters.
Fig.5 seven segment sequence
International Journal of Advanced Engineering Science and Technological Research (IJAESTR) ISSN: 2321-1202, www.aestjournal.org @2015 All rights reserved.
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VI. UNDERGOING RESEARCH
SVM with unbalanced system ie. Vao + Vbo +Vco not equal to 0.
Reduce losses with a filter system within the scheme.
International Journal of Advanced Engineering Science and Technological Research (IJAESTR) ISSN: 2321-1202, www.aestjournal.org @2015 All rights reserved.
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