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THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention: A SYSTEM AND A METHOD FOR GAIN CALIBRATION IN ELECTRONIC TRIP UNIT IN CIRCUIT BREAKERS
2. Applicant(s):

(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY : An Indian Company
(c) ADDRESS : L & T House, Ballard Estate, Mumbai
400 001, State of Maharashtra, India
3. 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
The present invention relates to the system used for gain calibration for electronic trip unit in circuit breakers. Particularly, the present invention relates to the method for finding out the gain value which is saved in nonvolatile memory of the microcontroller. More particularly, the present invention relates to system and method for gain calibration in electronic trip unit using PC-HMI.
BACKGROUND OF THE INVENTION AND PRIOR ART
The architecture of electronic trip unit has various blocks like Power Supply, Signal Conditioning and so on. The signal conditioning block takes the output of voltage sensor and filters it. In order to increase the range of values converted by ADC of microcontroller, this signal conditioning circuit has certain gain value and it provides DC shift if the ADC used is unipolar. Since the voltage sensor output signal is passed through the gain stage, the gain value has to be multiplied in the microcontroller firmware to get the actual voltage sensor output.
This gain value will change due to component tolerance. Though the gain value will remain the same throughout the range of current passed through the circuit breaker, it might vary from one ETU to other. Therefore, it is necessary to devise a procedure for gain calibration. This procedure should accurately compensate for change in

voltage sensor output value due to gain stage and compute RMS (Root Mean Square) in the firmware using actual signal.
This value of gain plays a very vital role because it will be used for RMS computation in the microcontroller program and RMS value will be used for metering. So, inaccurate gain value will lead to inaccuracy in metering value. Also RMS value is passed to protection module like Overload, Short Circuit, Earth Fault and so on. Therefore, inaccurate RMS value will lead to error in trip time.
Hence, it is necessary to calibrate gain very accurately so that we achieve high accuracy in metering and trip time.
OBJECTS OF THE INVENTION
One object of the present invention is to overcome the
disadvantages / drawbacks of the prior art.
A basic object of the present invention is to provide
system used for gain calibration for electronic trip unit
in circuit breakers.
Another object of the present invention is to provide a
method for finding out the gain value which is saved in
the non-volatile memory of the microcontroller.
These and other advantages of the present invention will
become readily apparent from the following detailed

description taken in conjunction with the accompanying drawings.
SUMMRRY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
According to one of the aspect of the present invention there is provided a system for gain calibration in electronic trip unit in circuit breakers, said system comprising:
an electronic trip unit in circuit breaker, said electronic trip unit comprises : a coil means;
a current transformer;
such that said current passed through the circuit breaker is converted to voltage output by said coil and set down to current output by said current transformer; a signal conditioning unit operatively connected with said coil means, such that said signal conditioning unit

gets voltage output with certain gain from said coil
means;
a power supply unit operatively connected with said
current transformer, said power supply unit gets current
output from said current transformer ;
a trip unit operatively connected with said power supply
unit, said power supply unit provides voltage supply to
said trip circuit;
regulator means operatively connected with said regulator
means , said power supply provided in regulator from
power supply unit; and
a microcontroller means operatively connected with said
regulator means, said power supply is provided from said
regulator ;
an automated gain calibration unit operatively connected
with said electronic trip unit , said calibration unit
comprises :
an Input-Output module; such that said Input-Output
module operatively connected with said electronic trip
unit;
a processor means operatively connected with said an
input /output module via serial communication; and
a multimeter means operatively connected with said
electronic trip unit.
According to another aspect of the present invention there is provided a method for gain calibration in

electronic trip unit in circuit breakers, said method comprising steps of:
configuration of UART,ADC and Timer ;
initialization of variables to zero and calibration complete status flag is low;
checking of calibration complete status flag is high, such that if calibration complete status flag is high then the checking of calibration complete status flag is continued or else UART started and query is sent to the meter;
checking of whether meter data is received and such that meter value is converted from array of string to a float number ;
starting of timer and after timer interrupt occurs, ADC conversion is started with channel number 1 in 29;
checking if the ADC is unipolar then the DC Offset is subtracted from each sample;
determination of RMS for each channel by taking square of sample values, adding them, dividing by N (where N is the number of samples per cycle) and take square root and the cycle number is incremented;
determining RMS for C number of cycles and RMS value for each cycle of a particular channel is added to sum of RMS variable of that channel and when RMS is calculated for C

number of cycles, the average RMS for a particular channel is computed by dividing by C ;
further determining gain for each channel by dividing average RMS for each channel by Average Meter Value ;
determination of gain for all the channels; and
saving the gain value in non-volatile memory and then calibration complete flag is made high .
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention- The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Figure 1 illustrates block diagram of Electronic Trip Unit.

Figure no. 2 illustrates block diagram of Automated Gain Calibration Setup.
Figure no. 3 illustrates flowchart for computation of gain for all phases (part 1).
Figure no. 4 illustrates flowchart for computation of gain for all phases {part 2).
Figure no. 5 illustrates flowchart for computation of gain for all phases (part 3).
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a system and a method for gain calibration in electronic trip unit in circuit breakers.

The present invention gives a description of the setup used for automated gain calibration procedure which will be controlled using a PC-HMI (Human Machine Interface). It deals with application firmware which gives analog and digital signal through serial communication to Input-Output Interface for analog signal generation depending on the value sent by application firmware. The analog signal is given as voltage sensor output to the various phases (channels) and it acts as secondary injection. The digital output is given to power on the microcontroller and other module involved in gain calibration. And the Input-Output interface takes digital input from the Electronic trip unit which gives the status of calibration. This is passed to application firmware using serial port and the status of calibration is displayed.
The present invention deals with firmware design for microcontroller to compute the value of gain and write it in non-volatile memory. Detailed description is given about the hardware setup and algorithm used- When the metering and protection firmware is executed then it will retrieve the gain value from the memory and use it for computation of RMS.
The present invention deals with making the gain calibration process automated using a PC-HMI. The signal given to phase as voltage sensor output is generated by Input-Output module when it receives command from the PC using serial communication. Hence, by using a start button on PC-HMI the calibration process is started.

The firmware computes the gain for each phase accurately as the analog signal generated by Input-Output module is measured by 61/2 digital multimeter and this takes care of fluctuation in the source if there is any. Certain numbers of meter values are saved and average is taken.
The present invention also gives status of the gain calibration. It gives feedback about error in meter communication, non volatile memory write fail and gain is out of predefined range in certain phases. This will help to makes changes in the certain part of the setup or reject the ETU if gain value is out of range.
It is a novel idea of automated system for gain calibration where the status of calibration is displayed using PC-HMI to show whether it has passed or failed and also display reason for failure.
Advantages:
1. The gain calibration procedure is controlled using PC-HMI. The command is given from PC interface to the Input-Output module to generate sine wave to be given to all the phases of electronic trip unit and supply is given to power up the electronic trip unit.
2. The firmware design for computation of gain value for each phase takes average of the meter values acquired through serial communication with the meter as well as

average of the RMS value of the signal received by ADC of the microcontroller.
3. The PC-HMI is also used to display the status of calibration. The microcontroller pins are used to give output to the Input-Output module which checks the value of each pin and depending on the combination of these values, the status of calibration is known. It is displayed whether the calibration procedure is passed or failed. Also, the reason for failure like meter communication failure, non volatile memory write fail and gain out of range in one or more phases is displayed.
4. The timer is started at the back end of application firmware when the gain calibration procedure is started. If the port pins does not change within predefined time then it is displayed that the calibration procedure to be done again.
Features
The features of the invention include automated process for gain calibration controlled using a PC HMI. The power supply given to the electronic trip unit and sine wave input given at each phase is generated by Input-Output module on receiving command from PC using serial

communication. The firmware design for computation of gain value for each phase takes average of the meter values as well as average of the RMS value of the input received by the microcontroller. So the gain computation accuracy is increased. The microcontroller pins are used to give output to the Input-Output module which checks the value of each pin and depending on the combination of these values, the status of calibration is known. It is displayed whether the calibration procedure is passed or failed. Also, the reason for failure like meter communication failure, non volatile memory write fail and gain out of range in one or more phases is displayed. This will specify the gain circuitry of which phase has to be worked upon and the components to be replaced.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill

in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.
By the term "substantially" it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.

With reference to block diagram in Figure no. 1, it is shown that electronic trip unit consists of signal conditioning unit 3, power supply unit 4 and so on. The current passed through the circuit breaker is converted to voltage output by Rogowski coil and it is step down to current output by current transformer. The voltage output is given signal conditioning circuit with certain gain and current output is given to power supply unit which gives voltage supply to trip circuit 5 and power supply to microcontroller 8 through regulator 7. The signal conditioning circuit has a gain to increase the range of input sample by ADC in microcontroller.
The block diagram in Figure no. 2 is the setup used for calibration. The PC is connected to Input-Output module 10 using serial-communication. It gives command to the Input-Output interface to generate sine wave and give digital output. The sine wave is given to phases A, B, C and D in Electronic Trip Unit in 11. It consists of DIP switches to configure the ETU for certain protection settings. The power supply is also given to ETU by Input-Output module. The microcontroller pins are used to give output to the Input-Output module which checks the value of each pin and depending on the combination of these values, the status of calibration is known. It is displayed whether the calibration procedure is passed or failed. Also, the reason for failure like meter communication failure, non volatile memory write fail and gain out of range in one or more phases is displayed. 6 1/2

digital multimeter 13 is used for measurement of signal given as secondary injection to the ETU.
The firmware design/ method is explained using Figure no.3, Figure no. 4 and Figure no. 5.
The peripherals are initialized in the 15, 16 and 17. In 15, the UART configuration involves baud rate, parity bit and so on. In 16, ADC configuration is done and In 17, Timer configuration is done. In 18, all the variables are initialized to zero and the Calibration Complete status flag is low.
It is checked whether the Calibration Complete Flag is high in 19. If yes, the program will be continuous loop or else UART is started by sending query to the meter in 20. After checking whether meter data is received in 21, meter value is converted from array of string to a float number in 22. This value is added to Sum Of Meter Values variable. The query is sent X times and meter values are added up. Then Sum Of Meter Values is divided by X to get Average Meter Value in 25. Timer is started in 26. After timer interrupt occurs, ADC conversion is started with channel number 1 in 29. On completion of ADC conversion, sample value is saved in 31. The step number 27, 28, 29, 30 and 31 is repeated for all the channels. Then sample number is increased in 33. Then it is checked whether all samples in a cycle are saved. If yes go to 35 or else go to 27. If the ADC is unipolar then the DC Offset is

subtracted from each sample. Compute RMS for each channel by taking square of sample values, adding them, dividing by N (where N is the number of samples per cycle) and take square root as shown in 38, 39 and 40. The cycle number is incremented in 42. And RMS value is computed for C number of cycles and RMS value for each cycle of a particular channel is added to Sum Of RMS variable of that channel. When RMS is calculated for C number of cycles, find average RMS for a particular channel by dividing by C as shown in 44. Compute gain for each channel by dividing Average RMS for each channel by Average Meter Value in 46. After gain is computed for all the channels, save the gain value in non-volatile memory. Then Calibration complete flag is made high in 49 and goes to step 19.

WE CLAIM:
1. A system for gain calibration in electronic trip unit in circuit breakers, said system comprising : an electronic trip unit in circuit breaker, said electronic trip unit comprises : a coil means; a current transformer;
such that said current passed through the circuit breaker is converted to voltage output by said coil means and set down to current output by said current transformer; a signal conditioning unit operatively connected with said coil means, such that said signal conditioning unit gets voltage output with certain gain from the coil means ; a power supply unit operatively connected with said current transformer, said power supply unit gets current output from said current transformer ;
a trip unit operatively connected with said power supply unit, said power supply unit provides voltage supply to said trip circuit; regulator means operatively connected with said regulator means , said power supply provided in regulator from power supply unit; and a microcontroller means operatively connected with said regulator means, said power supply is provided from said regulator ;

an automated gain calibration unit operatively connected with said electronic trip unit , said calibration unit comprises :
an Input-Output module; such that said Input-Output module operatively connected with said electronic trip unit;
a processor means operatively connected with said an Input-Output module via serial communication; and
a multimeter means operatively connected with said electronic trip unit.
2. System as claimed in claim 1 wherein said signal conditioning circuit has a gain to increase the range of input sample by ADC in microcontroller.
3. System as claimed in claim 1 wherein said processor means gives command to the Input-Output interface to generate sine wave and give digital output.
4. System as claimed in claim 1 wherein said electronic trip unit comprises plurality of phases.
5. System as claimed in claim 1 wherein said Input-Output module comprises DIP switches to configure the electronics trip unit (ETU) for certain protection settings.

6. System as claimed in claim 1 wherein said electronics trip unit (ETU) receives power supply from said Input-Output module.
7. System as claimed in claim 1 wherein said microcontroller pins are used to give output to the Input-Output module which checks the value of each pin and depending on the combination of these values, the status of calibration is known.
8. System as claimed in claim 1 wherein said processor displays whether the calibration procedure is passed or failed, also for failure like meter communication failure, non volatile memory write fail and gain out of range in one or more phases is displayed.
9. System as claimed in claim 1 wherein said digital multimeter is 6 1/2 and is used for measurement of signal given as secondary injection to the ETU.
10. System as claimed in claim 1 wherein said coil means is a rogowski coil.
11. A method for gain calibration in electronic trip unit in circuit breakers, said method comprising steps of :
configuration of UART,ADC and Timer ;

initialization of variables to zero and calibration complete status flag is low;
checking of calibration complete status flag is
high, such that if calibration complete status flag
is high then the checking of calibration complete
status flag is continued or else UART is started
and query is sent to the meter;
checking of whether meter data is received and such
that meter value is converted from array of string
to a float number ;
starting of timer and after timer interrupt occurs,
ADC conversion is started with channel number 1 in
29;
checking if the ADC is unipolar then the DC Offset
is subtracted from each sample;
determination of RMS for each channel by taking
square of sample values, adding them, dividing by N
(where N is the number of samples per cycle) and
take square root and the cycle number is
incremented;
determining RMS for C number of cycles and RMS value
for each cycle of a particular channel is added to
sum of RMS variable of that channel and when RMS is
calculated for C number of cycles, the average RMS
for a particular channel is computed by dividing by
C ;

further determining gain for each channel by dividing average RMS for each channel by Average Meter Value;
determination of gain for all the channels; and saving the gain value in non-volatile memory and then calibration complete flag is made high. By taking average of the meter values, the calibration firmware has become more robust.
12. Method as claimed in claim 10 wherein said configuration of UART comprises baud rate, parity bit and the like.
13. Method as claimed in claim 10 wherein said meter value is added to Sum Of Meter Values variable and the query is sent X times and meter values are added up, then Sum Of Meter Values is divided by X to get Average Meter Value.
14. Method as claimed in claim 10 wherein said ADC conversion when completed, the sample value is saved.
15. A system for gain calibration in electronic trip unit in circuit breakers as herein described and illustrated with reference to the accompanying drawings.

16. A method for gain calibration in electronic trip unit in circuit breakers as herein described and illustrated with reference to the accompanying drawings.