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
Frequency Variable Capacitor Based Passive Filter for Matching Grid Frequency
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to a frequency variable capacitor based passive filter for matching grid frequency.
BACKGROUND OF THE INVENTION
Harmonics are one of the major concerns in a power system. Harmonics cause distortion in current and voltage waveforms resulting into deterioration of the power system. The first step for harmonic analysis is the harmonics from non-linear loads. The results of such analysis are complex. Over many years,, much importance is given to the methods of analysis and control of harmonics. Harmonics present in power system also has non-integer multiples of the fundamental frequency and have aperiodic waveform. The harmonics are generated in a power system from two distinct types of loads.
The application of sinusoidal voltage does not result in a sinusoidal flow applied sinusoidal voltage for non-linear devices. The non-linear loads draw a current that may be discontinuous. Harmonic current is isolated by using harmonic filters in order to protect the electrical equipment from getting damaged due to harmonic voltage distortion. They can also be used to improve the power factor. The harmful and damaging effects of harmonic distortion can be evident in many different ways such as electronics miss-timings, increased heating effect in electrical equipments or capacitor overloads. There can be two types of filters that are used in order to reduce the harmonic distortion i.e. the active filters and the passive filters. Active harmonic filters are electronic devices that eliminate the undesirable harmonics on the network by inserting negative harmonics into the network. The active filters are normally available for low voltage networks. The active filters consist of active components such as IGBT-transistors and eliminate many different harmonic frequencies.

The signal types can be single phase AC or three phase AC. On the other hand, passive harmonic filters consist of passive components such as resistors, inductors and capacitors. Unlike the active filters which are used only for low voltages, the passive filters are commonly used and are available for different voltage levels.
A typical passive filter includes a series band pass filter connected in series with the nonlinear load and a shunt band stop filter connected in parallel with the non-linear load. The series band pass filter comprises an inductor connected in series with a capacitor. The shunt band stop filter comprises an inductor connected in parallel with a capacitor. The inductance and capacitance of the respective series band pass filter and shunt band stop filter are chosen such that both the filters are tuned to the source or grid frequency. Accordingly, the series band pass filter attenuates the harmonic currents and allows only the fundamental current to pass through it. Similarly, the shunt band stop filter attenuates the fundamental current and allows only the harmonic currents to pass through it.
The inductance and capacitance of the respective filters are constant and in accordance with the grid frequency and said filters are also constantly tuned at one particular grid frequency. However, due to adopting of a large number of power electronic devices, grids are often rich in harmonic content, and suffer to serious waveform distortion. The grid frequency also fluctuates along with the load variations or power supply fluctuations. Furthermore, operation of power grids is defined by certain rated operating parameters, normally voltage and frequency. The existence of imbalances between generated power and consumed power at a given time causes deviations in grid operating frequency. In particular, when generated power exceeds consumed power, grid frequency increases above its rated value. If. on the contrary,

generated power is less than consumed power, frequency decreases in relation to its rated value. All these factors make filtering of harmonics difficult.
One of the options to tune the respective filters to match the grid frequency is to use a capacitor bank along with switches. Accordingly, particular capacitor from the bank may be switched to match the grid frequency. However, such implementation will attract a complex circuitry increasing the loss and will be expensive on the other hand. There has thus been a persistent need to develop a circuit which may be implemented in a passive filter to accurately tune the filters at different source or grid frequency.
SUMMARY OF THE INVENTION
The present invention provides a frequency variable capacitor based passive filter for matching grid frequency comprising a series band pass filter connected in series with a load comprising a first inductor connected in series with at least two first capacitors which are connected in parallel to each other; and a first semiconductor switch connected in series with one of the capacitors; a shunt band stop filter connected in parallel with a load comprising a second inductor connected in parallel with at least two second capacitors which are connected in parallel to each other; and a second semiconductor switch connected in series with one of the capacitors; and at least one control circuitry for varying the duty ratio of the semiconductor switches wherein said control circuit is configured to monitor the grid frequency and to determine the capacitance required for matching the frequency of the series band pass filter and shunt band stop filter with that of grid frequency and wherein, in response to required capacitance, the control circuit determine and adjust the duty ratio of said semiconductor switches to achieve the required capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of an illustrative embodiment of the present invention
DETAILED DESCRIPTION
Referring to Fig. 1, a non-linear load 212 is connected to a power source or a grid 213. Load 212 draws current from the grid 213. The current drawn by the load 212 comprises of fundamental as well as harmonic currents. Accordingly, to segregate the fundamental current and the harmonic currents, passive filters 200. 206 are being implemented. The passive filter 200 is a series band pass filter connected in series with the load 212 for providing high impedance to harmonic currents and allowing fundamental current to flow. The passive filter 206 is a shunt band stop filter connected in parallel with the load 212 for providing high impedance to fundamental current and allowing harmonic currents to flow.
It is known in the art that the fundamental current has a frequency that of the grid 213 frequency whereas the harmonic currents are either the odd or even multiples of the frequency of that of fundamental current or of grid frequency. Accordingly, as discussed before, it is a prerequisite to have the passive filters 200, 206 tuned at the grid frequency.
The series band pass filter 200 comprises an inductor 201 having inductance LI connected in series with at least two capacitors 202, 203 having capacitance CI and C2 respectively connected in parallel to each other; a semiconductor switch 204 connected in series with one of the said capacitors 203 for varying the capacitance; and a control circuitry 205 for varying the duty ratio D1 of said semiconductor switch 204.

The control circuitry 205 is configured to monitor the grid frequency and to determine the capacitance CRequired required for matching the frequency of series band pass filter 200 with that of grid frequency and wherein, in response to required capacitance CRequired, control circuitry 205 determine and adjust the duty ratio of the semiconductor switch 204 to achieve the required capacitance CRequired- Since capacitors 202 and 203 are in parallel to each other and the semiconductor switch is connected to the capacitor 203, the required capacitance of the series band pass filter is given as follows:
CRequired = C1+D1*C2
Accordingly, the duty cycle of the semiconductor switch 204 will be given as
D1 = [CRequired-C1] / C2
Thus by varying the duty cycle of semiconductor switch 204, the effective capacitance of series band pass filter 200 may be varied to reach the required capacitance CRequired accurately tuning the series band pass filter 200 at grid frequency.
The shunt band stop filter comprises an inductor 207 having inductance L2 connected in parallel with at least two capacitors 208, 209 having capacitance C3 and C4 respectively connected in parallel to each other; a semiconductor switch 211 connected in series with one of the said capacitors 209 for varying the capacitance; and a control circuitry 210 for varying the duty ratio D2 of said semiconductor switch 211.

The control circuitry 210 is configured to monitor the grid frequency and to determine the capacitance CRequired required for matching the frequency of shunt band stop filter 206 with that of grid frequency and wherein, in response to required capacitance CRequired, control circuitry 210 determine and adjust the duty ratio of the semiconductor switch 211 to achieve the required capacitance CRequired Since capacitors 208 and 209 are in parallel to each other and the semiconductor switch is connected to the capacitor 209, the required capacitance of the shunt band stop filter is given as follows:
CRequired = C3+Dl*C4
Accordingly, the duty cycle of the semiconductor switch 211 will be given as
D2 = [CRequired-C3]/C4
Thus by varying the duty cycle of semiconductor switch 211, the effective capacitance of shunt band stop filter 206 may be varied to reach the required capacitance CRequired accurately tuning the shunt band stop filter 206 at grid frequency.
Accordingly, to match the grid frequency to that of the filters, one is required only to adjust the duty ratio of the respective semiconductor switches. Therefore, it may be noted that the present invention eliminates the need of a capacitor bank and also eliminates the need of any additional circuitry for switching to respective capacitor. Accordingly, the present invention provides a cost effective simple solution to match the frequency of passive filters with that of the grid frequency. It may also be noted that a single control circuitry may be configured to

vary duty ratios Dl and D2 of both the semiconductor switches connected in series band pass filter and shunt band stop filter.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. Accordingly, the protection sought herein is as set forth in the claims below. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications.

CLAIMS
1. A frequency variable capacitor based passive filter for matching grid frequency
comprising:
a series band pass filter (200) connected in series with a load (212) comprising a first
inductor (201) connected in series with at least two first capacitors (202; 203) which are
connected in parallel to each other; and a first semiconductor switch (204) connected in
series with one of the said capacitors (203);
a shunt band stop filter (206) connected in parallel with a load (212) comprising a second
inductor (207) connected in parallel with at least two second capacitors (208; 209) which
are connected in parallel to each other; and a second semiconductor switch (211)
connected in series with one of the said capacitors (209); and
at least one control circuitry (210) for varying the duty ratio of said semiconductor
switches (204; 211);
wherein said control circuit (205; 210) is configured to monitor the grid frequency and to
determine the capacitance required for matching the frequency of said series band pass
filter (200) and shunt band stop filter (206) with that of grid frequency and wherein, in
response to required capacitance, said control circuit (205; 210) adjusts the duty ratio of
said semiconductor switches (204: 211) to achieve the required capacitance.
2. A frequency variable capacitor based passive filter for matching grid frequency as
claimed in claim 1, wherein duty ratio Dl of said first semiconductor switch of said series
band pass filter is calculated by:
Dl = [CRequired-Cl]/C2;
wherein CRequired is the capacitance required for matching the frequency of said series
band pass filter and C1 and C2 are the capacitance of the first two capacitors.

3. A frequency variable capacitor based passive filter for matching grid frequency as
claimed in claim 1, wherein duty ratio D2 of said second semiconductor switch of said
shunt band stop filter is calculated by:
D2 = [CRequired-C3]/C4;
wherein CRequired is the capacitance required for matching the frequency of said shunt band
stop filter and C3 and C4 are the capacitance of the second two capacitors.
4. A frequency variable capacitor based passive filter for matching grid frequency as
claimed in claim 1 comprises a first control circuitry connected to said series band pass
for varying the duty ratio of first semiconductor switch and a second control circuitry
connected to said shunt band stop filter for varying the duty ratio of second
semiconductor switch.