FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A control system and method for modifying a reference waveform to an inverter.
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Valsan Simi Paul, Vaidya Tushar Gopalkrishna, Chaudhary Mukeshkumar of Crompton Greaves Ltd, EDC, CG Global R&D, Kanjurmarg (E), Mumbai 400042; Maharashtra, India, an Indian National.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed.

Field of the Invention:
This invention relates to the field of Inverters.
Particularly, this invention relates to the field of Sine-PWM Inverters.
This invention envisages a control system and method for modifying a reference waveform to an inverter according to the harmonics to be eliminated.
Background of the Invention and Prior Art:
Inverters find ever increasing presence in a variety of applications like variable speed AC motor drives, interfacing renewable energy source with grid, uninterrupted power supplies (UPS), etc. Among the different inverter control strategies used, Pulse Width Modulated (PWM) inverters are the most commonly used. These inverters are capable of producing AC voltages of variable magnitude as well as variable frequency. It is expected that the output voltage would result in a good quality sinusoidal voltage waveform of desired fundamental frequency and magnitude after some filtering. One of the major issues with PWM is the presence of harmonics. The higher order harmonics around the carrier frequency are relatively easier to filter out, but the lower order harmonics, though smaller in magnitude, often cause problems in certain applications. A variety of techniques have been proposed to mitigate the lower order harmonics to some extent.
Systems involving selective harmonic elimination-pulse width modulation (SHEPWM) techniques offer a tight control of the harmonic spectrum of a given

voltage waveform. This is achieved by solving transcendental equations characterizing the harmonics to obtain switching angles. The SHEPWM based methods can theoretically provide the highest quality output amongst all the PWM methods. SHEPWM is normally done in two steps. First, the switching angles are calculated offline, by solving many nonlinear equations simultaneously and then, these angles are stored in a look-up table to be read in real time. The main drawback here is the difficulty in solving the SHEPWM equations which are nonlinear in nature and may produce simple, multiple, or even no solutions for a particular value of modulation index.
Another commonly used technique is cascaded multilevel inverter wherein the output waveform has several voltage levels leading to a better and more sinusoidal voltage waveform. As a result, a lower total harmonic distortion (THD) is obtained. The advantage is that the harmonics in the output waveform can be reduced without increasing switching frequency or decreasing the inverter power output. As the number of voltage levels reach infinity, the output THD approaches zero. The number of achievable voltage levels, however, is limited by voltage unbalance problems, voltage clamping requirement, circuit layout, and packaging constraints.
Some other systems use synchronized phase shifted parallel PWM inverters with current-sharing reactors for harmonic reduction. This technique reduces the inverter output distortion significantly. It is necessary to find the optimum phase shift between parallel inverters that yields minimum total harmonic distortion (THD) and the current-sharing reactors should be of equal values. A centroid based

determined by making the area under the PWM signal equal to the area under the reference waveform and the position of PWM pulse is aligned with the center of integration (COI) of the area of the reference waveform. This divides the total area under the sampled reference waveform into two equal areas.
Walsh function harmonic elimination method is proposed by yet other systems. By using the Walsh domain waveform analytic technique, the harmonic amplitudes of the inverter output voltage can be expressed as functions of switching angles. Thus, the switching angles are optimized by solving linear algebraic equations instead of solving nonlinear transcendental equations.
A non-patent literature reference titled "A Modified Reference Approach for Harmonic Elimination in Pulse-width Modulation Inverter Suitable for Distributed Generations " by R. N. Ray et al. seems to disclose an online harmonic elimination technique of the output voltage for a pulse-width-modulation-based voltage source inverter by using a reference waveform of sine-triangle pulse-width modulation which is modified using a separate signal of proper phase, frequency, and amplitude to reduce the total harmonic distortion of the output. However, it lacks the means and method to correct the angle of the reference wave.
Another prior art document JP9205773A discloses the concept of filtration of noise in PWM-controlled self-exciting rectifier by angle signal computing element and PLL circuit to a preset phase correcting amount angle. However, it lacks the means and method to incorporate the use of carrier wave signals for generating appropriate switching signals.

There is a need to eliminate the concerns of the prior art, and provide a more reliable, economic, and modular system.
Objects of the Invention:
An object of the invention is to provide a control system and method which mitigates the lower order harmonics in a Sine-wave PWM inverter system.
Another object of the invention is to provide a control system and method for an inverter which is reliable and economical.
Yet another object of the invention is to provide a control system and method to correct the angle of the reference wave of an inverter.
Still another object of the invention is to provide a control system and method to incorporate the use of carrier wave signals for generating appropriate switching signals.
Summary of the Invention:
According to this invention, there is provided a control system for modifying a reference sine wave to an inverter according to the harmonics to be eliminated, said control system comprises:
a. inverter output voltage means adapted to output inverter voltage;

b. delay means adapted to delay said output inverter voltage by 90° to obtain a
delayed inverter voltage;
c. reference sine wave output means adapted to output a reference sine wave;
d. phase lock loop means adapted to apply phase lock loop to said reference
sine wave in order to obtain angle information in relation to the harmonic to
be eliminated;
e. transformation means adapted to transform said output inverter voltage and
said delayed inverter voltage, using said angle information, to obtain a
plurality of DC voltage outputs;
f. first low pass filter, having a pre-defined cut-off frequency, adapted to
extract DC quantities from said first DC output voltage;
g. second low pass filter, having a pre-defined cut-off frequency, adapted to
extract DC quantities from said second DC output voltage;
h. first proportional integral controller adapted to provide a first feedback
control for driving the system; i. second proportional integral controller adapted to provide a second feedback
control j. reverse transformation means adapted to transform said first DC voltage
output and said second DC voltage output, using said angle information, to
obtain an a component of voltage; k. carrier signal output means adapted to output carrier signal; 1. subtractor means adapted to subtract said reverse transformed a component
of voltage with reference sine-wave to obtain a difference signal; and m. comparator means adapted to compare said carrier signal with said
difference signal to generate switching pulses for said inverter which
eliminate said harmonics.

Typically, said system includes harmonics' selection means adapted to select harmonics to be eliminated, said harmonics selected from a bank of harmonics consisting of odd harmonics.
Typically, said first low pass filter includes means to tune cut-off frequency to 1 Hz.
Typically, said second low pass filter includes means to tune cut-off frequency to 1 Hz.
Typically, said transformation means includes computation means for performing computations in accordance with the following equations in order to obtain said transformation:
Vde = KX sin(n8) - Vp x cos(n6) Vqe = Va* cos(nd) - Vp x sin(n6)
Typically, said reverse transformation means includes computation means for performing computations in accordance with the following equations in order to obtain said reverse transformation:
K = Vde x sin(n6) -Vqe x cos(n6)
According to this invention, there is provided a control method for modifying a reference sine wave to an inverter according to the harmonics to be eliminated, said control method comprises the steps of: a. obtaining inverter voltage;

b. delaying said output inverter voltage by 90° to obtain a delayed inverter
voltage;
c. obtaining a reference sine wave;
d. applying phase lock loop to said reference sine wave in order to obtain angle
information in relation to the harmonic to be eliminated;
e. transforming said output inverter voltage and said delayed inverter voltage,
using said angle information, to obtain a plurality of DC voltage outputs;
f. extracting DC quantities from said first DC output voltage, using a first low
pass filter, having a pre-defined cut-off frequency;
g. extracting DC quantities from said second DC output voltage, using a
second low pass filter, having a pre-defined cut-off frequency;
h. providing a first feedback control for driving the system using a first
proportional integral controller; i. providing a second feedback control for driving the system using a second
proportional integral controller; j. reverse transforming said first DC voltage output and said second DC
voltage output, using said angle information, to obtain an a component of
voltage; k. obtaining a carrier signal; 1. subtracting said reverse transformed a component of voltage with reference
sine-wave to obtain a difference signal; and m. comparing said carrier signal with said difference signal to generate
switching pulses for said inverter which eliminate said harmonics.

Brief Description of the Accompanying Drawings:
Figure 1 shows a basic single phase inverter system topology, referred to as the H-bridge;
Figure 2 illustrates inverter output voltage, obtained with fm = 50Hz, modulation index m = 0:8 and fc = 4kHz, without the dead band and DC ripple effect; and
Figure 3 shows the harmonic content of output voltage of inverter i.e. of Vab.
The invention will now be described in relation to the accompanying drawings, in which:
Figure 4 illustrates a block diagram model of the system for nth harmonic elimination.
Figures 5, 6, 7, and 8 illustrate simulated results of the system and method of the control system of Figure 4 for the inverter system of Figure 1.
Figure 5 illustrates the effect of compensation of 3rd and 5th harmonic elimination presented along with the values before compensation;
Figure 6 illustrates the results in presence of ripple on DC voltage;
Figure 7 illustrates the considerable reduction in the targeted harmonic even in presence of deadband; and

Figure 8 illustrates the performance of the proposed control system when the effect of ripple in DC voltage as well as presence of deadband are considered.
Detailed Description of the Accompanying Drawings:
Figure 1 shows a basic single phase inverter system topology, referred to as the H-bridge. The various parameters used for studies done in accordance with the svstem of this invention are listed in Table I.

Parameter Symbol Value Unit
DC link voltage Vdc 800 V
Modulating liequency fm
Jtm 50 Hz
Carder frequency fc 4000 Hz
Modulation Index m 0.8 -
Dead hand t-db 2.2 μ s
Table 1
All systems incorporating PWM techniques generate output voltages resulting in a sinusoidal voltage waveform of desired fundamental frequency and magnitude. Though filtering is used to achieve a reduction in the harmonic content of the signal, it is not always possible to remove the lower order harmonics. By an appropriate pre-defined switching control, the magnitudes of lower order harmonic voltages can be reduced, often at the cost of increasing the magnitudes of higher order harmonic voltages, which theoretically is easier to filter out.
Amongst all systems including various PWM techniques, sinusoidal pulse width modulation (SPWM) is the most popular as it has many advantages; best being the linearitv in controllinc the fundamental comnonent.

In such systems, there is included a sinusoidal pulse width modulation means adapted to generate switching pulses, wherein the switching pulses are generated by comparing a sinusoidal reference of frequency fm with a triangle carrier wave of frequency fc.
A. Inherent to PWM Technique:
The switching is done in such a way as to get a three level output voltage (unipolar switching). The inverter output voltage, obtained with fm = 50Hz, modulation index m = 0:8 and fc = 4kHz, without the dead band and DC ripple effect is shown in Figure 2 of the accompanying drawings.
The output voltage equation for three level single phase inverters can be given as:

where Jn(x)'s is Bessel function of order n and argument x.
As is evident from the figure as well as Equation 1, the output voltage has a fundamental voltage component as well as some harmonics superimposed on the fundamental due to the switching. Figure 3 shows the harmonic content of output voltage of inverter i.e. of Vab.
Referring to Figure 1, theoretically, switching pulses generated by PWM technique are applied to switches SI and S3, while their inverted versions are applied to

switches S2 and S4. But in reality, a small dead band is introduced between the switching of SI and S2, so as to ensure that the DC source is not shorted by simultaneous conduction of SI and S2. The dead band so introduced makes the actual inverter output voltage to be slightly different from the expected voltage thereby affecting the harmonic profile.
In many systems of the prior art, the DC source to the inverter is obtained by rectifying AC supply. The DC derived in such a manner has a second harmonic ripple on the DC value. This second harmonic ripple gets reflected at the output voltage. The expression for which can be obtained by modifying Equation 1, replacing Vdc by Vdccos(2w0t). To study the effect of DC ripple, a 100Hz signal of peak to peak magnitude approximately equal to 4% Vdc is injected on the DC bus.
It is usually believed that the ratio of fc to fm affects the output voltage harmonic content of the PWM inverter. But it can be practically seen that the effect of this ratio comes into picture only when this ratio is small. A sufficiently high ratio,
which is usually the case in practical applications, i.e., fc/fm>9, guarantees that the effect of this ratio on harmonics is negligible.
According to this invention, there is provided a control system (100) for modifying a reference sine wave to an inverter according to the harmonics to be eliminated. Typically, the harmonics to be eliminated are lower order harmonics. Typically, the inverter is a Pulse Width Modulated Inverter system. Particularly, the inverter is a Sine-wave Pulse Width Modulated Inverter system. Typically, said system includes harmonics' selection means adapted to select harmonics to be eliminated, said harmonics selected from a bank of harmonics consisting of odd harmonics.

Figure 4 illustrates a block diagram model of the system for nth harmonic elimination.
In accordance with an embodiment of this invention, there is provided an inverter output voltage means (10) adapted to output inverter voltage. Typically, said output inverter voltage is termed as an a component.
In accordance with another embodiment of this invention, there is provided a delay means (20) adapted to delay said output inverter voltage by 90 to obtain a delayed inverter voltage. Typically, said delayed voltage is termed as a (3 component.
In accordance with yet another embodiment of this invention, there is provided a reference sine wave output means (30) adapted to output a reference sine wave. Typically, the reference since wave output is termed as Vref.
In accordance with still another embodiment of this invention, there is provided a phase lock loop means (40) adapted to apply phase lock loop to said reference sine wave in order to obtain angle information, nO, where n is the harmonic order which is to be eliminated.
In accordance with an additional embodiment of this invention, there is provided a transformation means (50) adapted to transform said output inverter voltage and said delayed, inverter voltage, using said angle information, to obtain a plurality of DC voltage outputs. Typically, said first DC voltage output is termed as Vdc. Typically, said second voltage output is termed as Vqe.

Typically, said transformation is performed in accordance with the following equations (Equation 2) and by employing mathematical operating means in order to obtain said transformation:
Vde = Va x sin(nӨ) - Vβx Cos(nӨ) (2)
Vqe=Vaxcos(nӨ)-Vβxsin(nӨ) (2)
In accordance with yet an additional embodiment of this invention, there is provided a first low pass filter (60), having a pre-defined cut-off frequency, adapted to extract DC quantities from said first DC output voltage.
Typically, said first low pass filter has a cut-off frequency of 1 Hz.
In accordance with yet an additional embodiment of this invention, there is provided a second low pass filter (70), having a pre-defined cut-off frequency, adapted to extract DC quantities from said second DC output voltage.
Typically, said second low pass filter has a cut-off frequency of 1 Hz.
In accordance with still an additional embodiment of this invention, there is provided a first proportional integral controller (80) adapted to provide a first feedback control for driving the system to be controlled with a weighted sum of the error (difference between the output and desired set-point) and the integral of that value.

In accordance with still an additional embodiment of this invention, there is provided a second proportional integral controller (90) adapted to provide a second feedback control for driving the system to be controlled with a weighted sum of the error (difference between the output and desired set-point) and the integral of that value.
In accordance with another additional embodiment of this invention, there is provided a reverse transformation means (110) adapted to transform said first DC voltage output and said second DC voltage output, using said angle information, to obtain an in-phase or a component of voltage.
Typically, said reverse transformation is performed in accordance with the
following equation (Equation 3) and by employing mathematical operating means
in order to obtain said transformation:
• Va = Vde x sin(nӨ)-Vqe. x cos(nӨ) (3)
In accordance with yet another additional embodiment of this invention, there is provided a carrier signal output means (120) adapted to output carrier signal.
In accordance with still another additional embodiment of this invention, there is provided a subtracter means (130) adapted to subtract said reverse transformed a component of voltage with reference sine-wave to obtain a difference signal.
In accordance with an additional embodiment of this invention, there is provided a comparator means (140) adapted to compare said carrier signal with said difference signal to generate switching pulses. Though the block diagram shows the

elimination of one harmonic, this scheme can be extended to include other harmonics also in a similar fashion which eliminate said harmonics.
The inverter of Figure 1 along with the control system of this invention, of Figure 4, was simulated and tested using MATLAB simulation progam.
The results for various case studies described and illustrated herein. In Figure 5, the effect of compensation of 3rd and 5th harmonic elimination is presented along with the values before compensation. It can be seen that the 3rd harmonic compensation resulted in 80% reduction of 3rd harmonic whereas 5th harmonic reduction resulted in 90% reduction of 5th harmonic. Figure 6 shows the results in presence of ripple on DC voltage. Again, it can be seen that there is considerable reduction in the magnitude of the targeted harmonic. In presence of a deadband, usual SHEM techniques fail as they are highly dependant on the switching instant and any delay in that can cause detrimental effects. But in this system and method, as is evident from Figure 7, there is considerable reduction in the targeted harmonic even in presence of deadband. Figure 8 shows the performance of the proposed control system when the effect of ripple in DC voltage as well as presence of deadband is considered.
The technical advancement of this system lies in using a control system for a Sine-PWM inverter systeni, said control system adapted to reduce any chosen dominant lower order harmonic with a change in the control flow means of conventional Sine-PWM inverter control. This system is reliable and economical. It is relevant in the current scenario where lot of importance is given to power quality as well as interfacing of renewable sources to the power grid.

We claim :
1. A control system for modifying a reference sine wave to an inverter according to the harmonics to be eliminated, said control system comprising:
a. inverter output voltage means adapted to output inverter voltage;
b. delay means adapted to delay said output inverter voltage by 90° to obtain a
delayed inverter voltage;
c. reference sine wave output means adapted to output a reference sine wave;
d. phase lock loop means adapted to apply phase lock loop to said reference
sine wave in order to obtain angle information in relation to the harmonic to
be eliminated;
e. transformation means adapted to transform said output inverter voltage and
said delayed inverter voltage, using said angle information, to obtain a
plurality of DC voltage outputs;
f. first low pass filter, having a pre-defined cut-off frequency, adapted to
extract DC quantities from said first DC output voltage;
g. second low pass filter, having a pre-defined cut-off frequency, adapted to
extract DC quantities from said second DC output voltage;
h. first proportional integral controller adapted to provide a first feedback
control for driving the system; i. second proportional integral controller adapted to provide a second feedback
control j. reverse transformation means adapted to transform said first DC voltage
output and said second DC voltage output, using said angle information, to
obtain an a component of voltage; k. carrier signal output means adapted to output carrier signal;

1. subtractor means adapted to subtract said reverse transformed a component of voltage with reference sine-wave to obtain a difference signal; and
m. comparator means adapted to compare said carrier signal with said difference signal to generate switching pulses for said inverter which eliminate said harmonics.
2. A system as claimed in claim 1 wherein, said system includes harmonics'
selection means adapted to select harmonics to be eliminated, said harmonics
selected from a bank of harmonics consisting of odd harmonics.
3. A system as claimed in claim 1 wherein, said first low pass filter includes means to tune cut-off frequency to 1 Hz.
4. A system as claimed in claim 1 wherein, said second low pass filter includes means to tune cut-off frequency to 1 Hz.
5. A system as claimed in claim 1 wherein, said transformation means includes computation means for performing computations in accordance with the following equations in order to obtain said transformation:


6. A system as claimed in claim 1 wherein, said reverse transformation means includes computation means for performing computations in accordance with the following equations in order to obtain said reverse transformation:

7. A control method for modifying a reference sine wave to an inverter according to the harmonics to be eliminated, said control method comprising the steps of:
a. obtaining inverter voltage;
b. delaying said output inverter voltage by 90° to obtain a delayed inverter
voltage;
c. obtaining a reference sine wave;
d. applying phase lock loop to said reference sine wave in order to obtain angle
information in relation to the harmonic to be eliminated;
e. transforming said output inverter voltage and said delayed inverter voltage,
using said angle information, to obtain a plurality of DC voltage outputs;
f. extracting DC quantities from said first DC output voltage, using a first low
pass filter, having a pre-defined cut-off frequency;
g. extracting DC quantities from said second DC output voltage, using a
second low pass filter, having a pre-defined cut-off frequency;
h. providing a first feedback control for driving the system using a first
proportional integral controller; i. providing a second feedback control for driving the system using a second
proportional integral controller; j. reverse transforming said first DC voltage output and said second DC
voltage output, using said angle information, to obtain an a component of
voltage;

k. obtaining a carrier signal;
1. subtracting said reverse transformed a component of voltage with reference
sine-wave to obtain a difference signal; and m. comparing said carrier signal with said difference signal to generate
switching pulses for said inverter which eliminate said harmonics.