08-02-2013, 09:32 AM
Control of Voltage Source Inverters using PWM/SVPWM for Adjustable Speed Drive Applications
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Abstract
Pulse Width Modulation variable speed drives are increasingly applied in many new industrial applications that require superior performance. Recently, developments in power electronics and semiconductor technology have lead improvements in power electronic systems. Hence, different circuit configurations namely multilevel inverters have become popular and considerable interest by researcher are given on them.Variable voltage and frequency supply to a.c drives is invariably obtained from a three-phase voltage source inverter. A number of Pulse width modulation (PWM) schemes are used to obtain variable voltage and frequency supply. The most widely used PWM schemes for three-phase voltage source inverters are carrier-based sinusoidal PWM and space vector PWM (SVPWM). There is an increasing trend of using space vector PWM (SVPWM) because of their easier digital realization and better dc bus utilization. This project focuses on step by step development SVPWM implemented on an Induction motor. The model of a three-phase a voltage source inverter is discussed based on space vector theory. Simulation results are obtained using MATLAB/Simulink environment for effectiveness of the study
Introduction
Three phase voltage-fed PWM inverters are recently showing growing popularity for
multi-megawatt industrial drive applications. The main reasons for this popularity are easy
sharing of large voltage between the series devices and the improvement of the harmonic quality
at the output as compared to a two level inverter. In the lower end of power, GTO devices are
being replaced by IGBTs because of their rapid evolution in voltage and current ratings and
higher switching frequency. The Space Vector Pulse Width Modulation of a three level inverter
provides the additional advantage of superior harmonic quality and larger under-modulation
range that extends the modulation factor to 90.7% from the traditional value of 78.5% in
Sinusoidal Pulse Width Modulation.
Voltage Source Inverters
The main objective of static power converters is to produce an ac output waveform from a dc power supply. These are the types of waveforms required in adjustable speed drives (ASDs), uninterruptible power supplies (UPS), static var compensators, active filters, flexible ac transmission systems (FACTS), and voltage compensators, which are only a few applications. For sinusoidal ac outputs, the magnitude, frequency, and phase should be controllable.
According to the type of ac output waveform, these topologies can be considered as voltage source inverters (VSIs), where the independently controlled ac output is a voltage waveform. These structures are the most widely used because they naturally behave as voltage sources as required by many industrial applications, such as adjustable speed drives (ASDs), which are the most popular application of inverters. Similarly, these topologies can be found as current source inverters (CSIs), where the independently controlled ac output is a current waveform. These structures are still widely used in medium-voltage industrial applications, where high-quality voltage waveforms are required. Static power converters, specifically inverters, are constructed from power switches and the ac output waveforms are therefore made up of discrete values. This leads to the generation of waveforms that feature fast transitions rather than smooth ones.
Single-Phase Voltage Source Inverters
Single-phase voltage source inverters (VSIs) can be found as half-bridge and full-bridge
topologies. Although the power range they cover is the low one, they are widely used in power
supplies, single-phase UPSs, and currently to form elaborate high-power static power topologies,
such as for instance, the multicell configurations.
Half-Bridge VSI
Fig.2 shows the power topology of a half-bridge VSI, where two large capacitors are required to
provide a neutral point N, such that each capacitor maintains a constant voltage (Vi)/2. Because
the current harmonics injected by the operation of the inverter are low-order harmonics, a set of
large capacitors (C+ and C-) is required. It is clear that both switches S+ and S- cannot be ON
simultaneously because a short circuit across the dc link voltage source Vi would be produced.
There are two defined (states 1 and 2) and one undefined (state 3) switch state as shown in Table
1. In order to avoid the short circuit across the dc bus and the undefined ac output voltage
condition, the modulating technique should always ensure that at any instant either the top or the
bottom switch of the inverter leg is on.
Full-Bridge VSI
Fig. 3 shows the power topology of a full-bridge VSI. This inverter is similar to the half-bridge
inverter; however, a second leg provides the neutral point to the load. As expected, both switches
S1+ and S1- (or S2+ and S2-) cannot be on simultaneously because a short circuit across the dc
link voltage source Vi would be produced. There are four defined (states 1, 2, 3, and 4) and one
undefined (state 5) switch states as shown in Table 2. The undefined condition should be avoided
so as to be always capable of defining the ac output voltage. It can be observed that the ac output
voltage can take values up to the dc link value Vi, which is twice that obtained with half-bridge
VSI topologies. Several modulating techniques have been developed that are applicable to fullbridge
VSIs. Among them are the PWM (bipolar and unipolar) techniques.
Conclusion and Future work
As seen from the above discussion Space Vector PWM is superior as compared to Sinusoidal pulse width modulation in many aspects like :
1) The Modulation Index is higher for SVPWM as compared to SPWM.
2) The output voltage is about 15% more in case of SVPWM as compared to SPWM.
3) The current and torque harmonics produced are much less in case of SVPWM.
However despite all the above mentioned advantages that SVPWM enjoys over SPWM, SVPWM algorithm used in three-level inverters is more complex because of large number of inverter switching states.
Hence we see that there is a certain trade off that exists while using SVPWM for inverters for Adjustable speed Drive Operations. Due to this we have to choose carefully as to which of the two techniques to use weighing the pros and cons of each method.