FORM 2
THE PATENT ACT 1970
&
THE PATENTS RULES, 2003 COMPLETE SPECIFICATION
(section 10 and rule 13)
1. TITLE OF THE INVENTION:
"Apparatus and Method of Filtering Air Core Sensor Signals for Protection and Control Units of Circuit Breakers"
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(c) ADDRESS: Larsen & Toubro Limited
Electrical & Automation North Wing, Gate 7, Level 0, Powai Campus, Saki Vihar Road, Mumbai 400 072, INDIA
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification
particularly describes the invention and the manner in which it is to be performed

Apparatus and Method of Filtering Air Core Sensor Signals for Protection and Control Units of Circuit Breakers
Field of invention
This invention relates generally to a signal conditioning circuit, and more particularly to a filtration and amplification of the signal conditioning circuit.
Background of the Invention
An electronic trip unit is an intelligent device that is used in conjunction with an electro-mechanical circuit breaker to measure system parameters such as voltage, current, and power. These electronic trip units protect the system against faults such as overload, short circuit, and ground fault. The circuit breaker with electronic trip units employ current sensors and specifically air core sensors that produce an output signal which is proportional to the time derivative of the primary current present in the system. The electronic trip unit measures this signal, integrates it to make it proportional to the primary current and then uses the output of integration for measurement. The output of these sensors needs to be effectively processed for efficient performance of these trip units. However, the output of these current sensors is noisy which may lead to nuisance tripping and improper functioning of the circuit breakers.
Accordingly, there exists a need for an apparatus and a method that prevents nuisance tripping and improper functioning of the circuit breaker.
Objects of the present invention

An object of the present invention is to provide an apparatus and a method of filtering an air core sensor signal of an electronic trip unit of a circuit breaker that is free from a high frequency and/or a low frequency noise.
Another object of the present invention is to provide an apparatus and a method of filtering the air core sensor signal for protection and control units of circuit-breakers, which enables reaching response at a very fast rate.
Yet another object of the present invention is to provide a apparatus and method of filtering air core sensor signals for protection and control units of circuit breakers, which eliminates high frequency board noise.
Summary of the Invention
The present invention discloses an electronic trip unit adapted for filtration and amplification of noise signals. The electronic trip unit comprises a two stage passive low pass filter that receives an output of an air core sensor. The two stage passive low pass filter is having a lowest cut off frequency that offers large impedance to the input signal for filtering a high frequency noise therefrom. The two stage passive low pass filter produces an attenuated output that is fed to an operational amplifier gain stage. The operational amplifier is having a comparator that sets an appropriate gain for amplifying the attenuated output of the two stage passive low pass filter for producing an amplified output that is fed to a single stage passive low pass filter. The single stage passive low pass filter is having a substantially higher cut off frequency than a frequency of the amplified output of the operational amplifier gain stage for filtering a low frequency noise therefrom. The single stage passive low pass filter produces an unpolluted output that is fed to a controller after analog to digital conversion via an analog to digital converter.
Brief Description of the Drawings

Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:
FIG.l shows a block diagram of an electronic trip unit constructed in accordance the present invention; and
FIG. 2 illustrates a circuit diagram for one of the phases of the electronic trip unit of FIG. 1.
Detail description of the invention
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
The present invention discloses an electronic trip unit that uses an air core sensor for filtering and amplification process wherein the electronic trip unit employs air core current sensors that produce an output signal that is proportional to the first time derivative of the primary current present in the system.
The output of the current sensor generally includes noise which needs to be dealt with, in order to ensure proper functioning of the circuit breaker and avoid nuisance tripping.
Referring to FIG.l, a block diagram representation of a signal conditioning circuit 100 is shown. The signal conditioning circuit 100 includes an air core sensor 111. The air core sensor 111 is Rogowski that is configured to measure alternate current pulses or high speed current pulses. The Rogowski type air core sensor 111 has an output that is proportional to the first time derivative of primary current di/dt. The output of the air core sensor 111 is fed to a two stage passive

low pass filter 112 that acts as an integrator in this one particular embodiment. The two stage passive filter 112 is such that it keeps a cut off frequency at minimum level in order to offer large impedance to high frequency signals. The two stage passive filter 112 is configured to filter out external high frequency noise from the signal to give an output signal which is proportional to the primary current. The output of the two stage passive filter 112 is fed to an operational amplifier gain stage 113. The output signal of the two stage passive filter 112 is attenuated and weak that is unable to be read by an analog to digital converter. Hence, the attenuated output of the two stage passive low passive filter 112 is fed to the amplifier gain stage 113 for amplification. The amplifier gain stage 113 includes a comparator that sets an appropriate gain to produce an amplified output. The signal conditioning circuit 100 includes a single stage low pass passive filter 114 that receives the amplified output from the amplifier gain stage 113 to filter remaining noises induced by the board or due to a plurality of parameters of the operational amplification in the gain stage 113, such as for example temperature in this one particular embodiment. The single stage low pass passive filter 114 is having a cut off frequency that is substantially higher than a frequency of signal under measurement in order to filter such noises. The filtered signal output of the single stage low pass passive filter 114 is fed to an analog to digital converter 115 (ADC 115, hereinafter) that is free from unwanted board noise. The ADC 115 sends unpolluted signals to a controller after analog to digital conversion.
Referring now to FIG. 2, a schematic illustration of a circuit 200 adapted for the filtration and amplification of the signal from the air core sensor 210 is shown that includes a two stage low pass passive filter 211, an operational amplifier gain stage 212 and a single stage low pass passive filter 213. It is understood here that the schematic illustration of the circuit 200 may vary in other alternative embodiments of the present invention. The circuit 200 illustrates a signal path only for one phase of the circuit breaker. However, it is understood that similar schematics may be used for the other phases of the circuit breaker that can be

coupled appropriately at an output side. The circuit 200 includes a plurality of resistors, a plurality of capacitors, a plurality of operational amplifiers and a plurality of voltage sources.
Accordingly, the circuit 200 includes a first resistor R25, a second resistor R26, a third resistor R27, a fourth resistor R28, and a fifth resistor R52 that are of 47 kilo-ohm (kohm, hereinafter) respectively. The circuit 200 includes a sixth resistor R29 and a seventh resistor R33 that are of 100 kohm respectively. The circuit 200 includes an eight resistor R30 and a ninth resistor R31 that are of 100 ohm respectively. The circuit 200 includes a tenth resistor R32 that is of 890 kohm. The circuit 200 includes an eleventh resistor R83 that is of 820 ohm. The circuit 200 includes a first capacitor C12, a second capacitor C18, a third capacitor C20 that are of 10 pico-Farads (pF, hereinafter) respectively. The circuit 200 includes a fourth capacitor C16, a fifth capacitor C17 that are of 220 nano-Farads (nF, hereinafter) respectively. The circuit 200 includes a sixth capacitor C19 that is of 220 pF. The circuit 200 includes a seventh capacitor C51 that is of 100nF. The circuit 200 includes an operational amplifier (op amp, hereinafter) U5. The circuit 200 includes a first DC voltage source V7 and a second DC voltage source V16 that are of 3.3 Vdc respectively. However, it is understood that alternative values of the resistors, capacitors, operational amplifiers and the voltage sources may be used for the aforementioned electronic components depending on the desired accuracy of the circuit 200.
The two stage low pass passive filter 211 is having a -3db point at a 12Hz frequency with a -40db/decade roll-off. The two stage low pass passive filter 211 provides a steep slope in order to response to the input signal in a faster manner with the cut-off frequency of 12Hz. The output of the two stage low pass passive filter 211 is fed to the operational amplifier gain stage 212 which is defined by the R26 and R32 wherein R26 acts as the gain setting resistor and R32 acts as the feedback resistor R32 to represent the amplified gain stage as-Av = -(R32/R26)

The single stage low pass passive filter 213 filters out few noises induced by the board and/or the parameters of the operational amplifier in the gain stage 212, such as temperature. The single stage low pass passive filter, 213 is having a cut off frequency Fc defined by using the resistor R83 and capacitor C51 to filter out these noises and which is represented by a formula-FC-1/{(2Π*R83*C51)Λ(1/2)}
The cut-off frequency value Fc of the single stage low pass passive filter 213 is calculated as 1.94KHz in this one particular embodiment. The cut-off frequency Fc is preferably maintained at a higher value such that the original signal is not interfered alongside the removal of the high frequency board noise. The filtered signal output from the single stage low pass passive filter 213 is adapted to be fed to the ADC 15 (refer FIG. 1) that is free from high and low frequency noises followed by sending unpolluted signals to a controller after analog to digital conversion.
Advantages of the present invention
1. The circuit 200 facilitates the signal entering the ADC 115 to be made free from high frequency as well as low frequency noises using the two stage low pass passive filter 211 and the single stage low pass passive filter 213 respectively.
2. The response is reachable at a very faster rate due to use of a two stage passive filter 211 having a roll-off of-40db/decade and a cut-off frequency of 12Hz.
3. The output signal of the two stage passive filter 112 is amplified by the amplifier gain stage 113 to set an appropriate gain to be comfortably read by the analog to digital converter 115.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

We claim:
1. An electronic trip unit adapted for filtration and amplification of noise
signals, the electronic trip unit comprising:
a two stage passive low pass filter receiving an output of an air core sensor, the two stage passive low pass filter having a lowest cut off frequency offering a large impedance to the input signal for filtering a high frequency noise therefrom, the two stage passive low pass filter producing an attenuated output;
an operational amplifier gain stage receiving the attenuated output of the two stage passive low pass filter, the operational amplifier having a comparator, the comparator setting an appropriate gain for amplifying the attenuated output of the two stage passive low pass filter, the operational amplifier gain stage producing an amplified output; and
a single stage passive low pass filter receiving the amplified output of the operational amplifier gain stage, the single stage passive low pass filter having a substantially higher cut off frequency than a frequency of the amplified output of the operational amplifier gain stage for filtering a low frequency noise therefrom, the single stage passive low pass filter producing an unpolluted output configured to be fed to a controller via an analog to digital converter.
2. The electronic trip unit as claimed in claim 1, wherein the cut off frequency of the two stage low pass passive filter is 12Hz with a -40db/decade roll-off
3. The electronic trip unit as claimed in claim 1, wherein the cut off frequency of the single stage low pass passive filter is 1.94 KHz