DESC:TECHNICAL FIELD

The present subject matter described herein, in general relates to circuit breakers and more particularly, to a protection and metering of the circuit breakers or any circuit using RC filter.

BACKGROUND

A resistor–capacitor (RC) circuit is a well known filter used in number of applications. One such application is the electronic trip unit of circuit breakers. The general working of the circuits in electronic trip unit is given below.

A current sensor output passes through a signal conditioning stage with certain gain. Also, a resultant signal from this stage is shifted by certain value of DC Offset. The signal is filtered by using this stage. Due to transient response of RC circuit, the DC offset takes some time to stabilize. During transient stage the DC value goes up and then settle to it required value. The output of signal conditioning stage is given to ADC pin of microcontroller. The microcontroller will start working as soon as the power supply is given and the computation of root mean square (RMS) value of current is done that involves sampling of the signal. The DC offset value is subtracted from this signal and RMS value is computed. The DC value subtracted is fixed and it is decided on the basis of the value of the components used in signal conditioning stage. But the actual DC offset at startup is not fixed and follows a curve. This will lead to erroneous computation of current in data acquisition module. The current value computed is given to various other protection modules like overload, short circuit and instantaneous.

Further, any error in computation can lead to error in trip time by protection functions. In case of instantaneous protection, the trip time is decided on the basis of value of current per cycle. So the erroneous current computation can lead to no trip or nuisance trip at startup. In order to overcome this problem, signal conditioning circuit implementation has to be changed but it will lead to increase in cost.

There exists an inherent delay until RC network acts as a filter. This delay depends on the time constant of RC network and condition is called transient response.

In the view of above mentioned drawbacks, and for application such as circuit breaker demanding quick response, there arises a need to develop a method and system that responds faithfully without delay and without any hardware change which is cost effective and user friendly.

SUMMARY

This summary is provided to introduce concepts related to a peak calibration method to compensate the transient response of RC circuit. This summary is not intended to identify essential features of the subject matter nor is it intended for use in determining or limiting the scope of the subject matter.

In one implementation, a method and system that responds faithfully without delay and without any hardware change which is cost effective and user friendly is disclosed.

In one implementation, the present invention relates to circuit breaker protection and metering but it is applicable for any circuit using RC filter.

In one implementation, the present invention relates to the metering and protection in electronic trip unit of circuit breakers. The invention deals with a technique to overcome error in trip time and metering introduced due to delayed response of RC circuit. The technique involves analysis of the signal conditioning circuit and plotting the response curve using simulation tools like Monte Carlo. Also, the analysis is done using practical data for a range of current input to the electronic trip unit. On the basis of data available, a median value of curve is taken and used in the firmware for current computation in initial few cycles until the output of RC circuit is stabilized. On the basis of analysis, it is observed that the peak value obtained at startup for output of signal conditioning stage depends on the stabilized DC Offset value. This stabilized DC Offset value can vary because of the component tolerances. Hence, DC Offset calibration is done and the value of the offset is saved in memory. This offset is taken as base to compute the points of the startup curve in firmware.

In one implementation, the analysis of the signal conditioning circuit and plotting the response curve is performed using simulation tools like Monte Carlo. Also, the analysis is done using practical data for a range of current input to the electronic trip unit.

In one implementation, on the basis of data available, a median value of curve is taken and used in the firmware for current computation in initial few cycles until the output of RC circuit is stabilized.

In one implementation, a DC offset calibration is done and the value of the offset is saved in memory. This offset is taken as base to compute the points of the startup curve in firmware.

Accordingly, in one implementation, a method to enhance working of an electronic trip unit by no trip or nuisance trip at startup of said electronic trip unit due to erroneous current computation, using a firmware is disclosed.
The method comprises of
· obtaining at least one output of at least one signal conditioning circuit at startup;
· analyzing said at least one output obtained to generate at least one DC offset curve analyzed;
· generating at least one DC offset curve generated based on said at least one output obtained;
· comparing at least one DC offset curve analyzed with at least one DC offset curve generated thereby plotting a median value; and
· calculating a current based on said median value.

In one implementation, an electronic trip unit of circuit breaker is disclosed. The electric circuit breaker comprises of a current transformer (1), a Rogowski coil (2), a signal conditioning module (3), a power supply module (4), a trip circuit (5) in an electronic trip unit, a flux shift device (FSD) (6), a regulator (7), a microcontroller (8), a low pass filter circuit (10), and an operational amplifier circuit (11). When current is passed through said electric circuit breaker, the current output is available at secondary of said current transformer (1) and corresponding voltage is available at output of said Rogowski coil (2). The current transformer output is given to said power supply module (4) and said Rogowski output is given to said signal conditioning module (3). The signal conditioning module (3) has certain gain and the signal is offset by a DC value. The output of said Rogowski coil (2) of the individual phases for single phase or three phase is fed to said passive low pass filter circuit (10), and said output of the passive low pass filter circuit (10) is fed to said operational amplifier circuit (11).

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to refer like features and components.

Figure 1illustrates a block diagram of an electronic trip unit is shown, in accordance with an embodiment of the present subject matter.

Figure 2 illustrates a circuit diagram for signal conditioning stage is shown, in accordance with an embodiment of the present subject matter.

Figure 3 illustrates an output of signal conditioning stage at startup is shown, in accordance with an embodiment of the present subject matter.

Figure 4 illustrates a method a method to enhance working of an electronic trip unit is shown, in accordance with an embodiment of the present subject matter.

Figure 5 illustrates a flow chart of the invention is shown, in accordance with an embodiment of the present subject matter.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Preferred embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.

The terms and words used in the following description 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.

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.

In one implementation, the present invention relates to circuit breaker protection and metering but it is applicable for any circuit using RC filter.

In one implementation, the present invention relates to the metering and protection in electronic trip unit of circuit breakers. The invention deals with a technique to overcome error in trip time and metering introduced due to delayed response of RC circuit. The technique involves analysis of the signal conditioning circuit and plotting the response curve using simulation tools like Monte Carlo. Also, the analysis is done using practical data for a range of current input to the electronic trip unit. On the basis of data available, a median value of curve is taken and used in the firmware for current computation in initial few cycles until the output of RC circuit is stabilized. On the basis of analysis, it is observed that the peak value obtained at startup for output of signal conditioning stage depends on the stabilized DC Offset value. This stabilized DC Offset value can vary because of the component tolerances. Hence, DC Offset calibration is done and the value of the offset is saved in memory. This offset is taken as base to compute the points of the startup curve in firmware.

In one implementation, an electric circuit breaker is disclosed. The electric circuit breaker comprises of a current transformer (1), a Rogowski coil (2), a signal conditioning module (3), a power supply module (4), a trip circuit (5) in an electronic trip unit, a flux shift device (FSD) (6), a regulator (7), a microcontroller (8), a low pass filter circuit (10), and an operational amplifier circuit (11). When current is passed through said electric circuit breaker, the current output is available at secondary of said current transformer (1) and corresponding voltage is available at output of said Rogowski coil (2). The current transformer output is given to said power supply module (4) and said Rogowski output is given to said signal conditioning module (3). The signal conditioning module (3) has certain gain and the signal is offset by a DC value. The output of said Rogowski coil (2) of the individual phases for single phase or three phase is fed to said passive low pass filter circuit (10), and said output of the passive low pass filter circuit (10) is fed to said operational amplifier circuit (11).

In one implementation, a power supply signal is passed through said regulator (7) and used for powering said microcontroller (8). The said power supply is made available to flux shift device (FSD) (6) through trip circuit (5).

Referring now to figure 1, a basic block diagram is given of electronic trip unit. When current is passed through the circuit breaker, the current output is available at secondary of current transformer (1) and corresponding voltage is available at output of Rogowski coil (2). The current transformer output is given to power supply module (4) and the Rogowski output is given to signal conditioning module (3). The signal conditioning module has certain gain and the signal is offset by a DC value. The power supply is also made available to flux shift device (FSD) (6) through trip circuit (5). The power supply signal is passed through regulator (7) and used for powering the microcontroller (8).

Referring now to figure 2, a basic circuit diagram for signal conditioning module is given. The output of the Rogowski of the individual phases for single phase or three phases is fed to the passive low pass filter circuit 10, whose functionality is similar to that of an integrator.

The integrator is used because the measured voltage is proportional to the first time derivative of the primary current. In order to obtain the equivalent value of current that is proportional to the primary current, the Rogowski output voltage is integrated. The cut off frequency of the passive low pass filter circuit is kept as low as possible. The integrator filters out the external noises that are high frequency in nature.

The output of the passive low pass filter circuit is fed to the Operational Amplifier Circuit 11. The operational amplifier acts as an amplifier with the help of gain setting resistors. A DC offset is provided in order to accommodate the entire signal within Vcc and GND.

Referring now to figure 3, an output of signal conditioning unit when input is zero and the circuit is powered up is shown.

Referring now to figure 4, a method to enhance working of an electronic trip unit is shown, in accordance with an embodiment of the present subject matter. In one implementation, a method to enhance working of an electronic trip unit by no trip or nuisance trip at startup of said electronic trip unit due to erroneous current computation, using a firmware is disclosed. The method comprises of

At block 402, at least one output of at least one signal conditioning circuit is obtained.

At block 404, said at least one output obtained is analyzed to generate at least one DC offset curve using simulation technique like Monte Carlo. The various output possibilities at startup for components value varying according to their tolerance range is noted down.

At block 406, at least one DC offset curve is generated based on said at least one output obtained. The practical data is obtained by testing for certain number for signal conditioning circuit of electronic trip unit. The curve value for these circuits observed at startup is noted down.

At block 408, at least one DC offset curve analyzed by using simulation software with at least one DC offset curve generated using practical data is compared and thereby a median value is plotted.

At block 410, a current based on said median value is calculated.

In one implementation, said firmware comprises of software configured to generate said at least one DC offset curve analyzed.

In one implementation, working of said RC circuit is enhanced by reducing error in trip time and metering introduced due to delayed response of said RC circuit.

In one implementation, said at least one output is at least one waveform generated by the at least one signal conditioning circuit.

In one implementation, at least one DC Offset is the mean value of the at least one waveform.

In one implementation, said at least one DC Offset is generated based on component tolerances of the at least one signal conditioning circuit.

In one implementation, said median value depends on a stabilized DC Offset value.

In one implementation, a mechanism is provided to stabilize said DC offset after startup and saves a stabilized value of said DC offset in memory.

In one implementation, said current is calculated based on said median value till an output of RC circuit is stabilized. The median value may be obtained by the analysis of output of the signal conditioning circuit is done using simulation software. The software generates different startup DC Offset curves taking component tolerances into consideration. Also, the circuit may be tested practically and data is generated. On the basis of theoretical and practical values, a median value of curve is plotted. Further analysis showed that the curve value also depends on the stabilized DC Offset value. Hence, the additional step of saving DC offset is added

In one implementation, said stabilized DC offset value is fetched from said memory and a curve is plotted on the basis of median value and stabilized DC offset value.

In one implementation, a root mean square (RMS) value of said current is calculated by using at least two points on said curve computed.

Referring now to figure 5, a flowchart for firmware implementation is given. The implementation intends to overcome the limitations of using RC circuit by compensating the delay using firmware. The analysis of output of the signal conditioning circuit is done using simulation software. The software generates different startup DC Offset curves taking component tolerances into consideration. Also, the circuit is tested practically and data is generated. On the basis of theoretical and practical values, a median value of curve is plotted. Further analysis showed that the curve value also depends on the stabilized DC Offset value. Hence, the additional step of saving DC offset is added. An offset calibration code is added in firmware which waits for DC offset to stabilize after startup and saves that stabilized value in memory. During normal working (non-calibration mode), the stabilized DC offset value is fetched from the memory and the curve is plotted on the basis of median value and stabilized DC offset value. US6515464, a prior-art discusses about method for offset calibration, and hence a person skilled in the art will be able to understand the method of calibration. The output of signal conditioning unit is given to ADC of microcontroller and samples are saved. Instead of subtracting a constant DC offset value, the points on the curve computed are subtracted to find RMS value of the current. Hence, the delay in the response of the filter is compensated by the present invention.

Exemplary embodiments discussed above may provide certain advantages. Though not required to practice aspects of the disclosure, these advantages may include those provided by the following features:

One feature of the invention is that, the present invention compensates the delay due to transient response of the RC circuit by theoretical and practical analysis of the curve.

Another feature of the invention is that, the present invention can use passive filter circuit without additional components for metering and protection.

Yet another feature of the invention it that, as the initial delay of RC circuit is compensated, the nuisance tripping in Instantaneous protection is eliminated for given RC circuit.

Still another feature of the invention it that, the proposed invention includes an optimized code, that does not include any math library for exponential functions.

Although an a peak calibration method to compensate the transient response of RC circuit has been described in language specific to structural features and/or methods, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific features or methods or devices described. Rather, the specific features are disclosed as examples of implementations a peak calibration method to compensate the transient response of RC circuit.
,CLAIMS:1. A method to enhance working of an electronic trip unit by no trip or nuisance trip at startup of said electronic trip unit due to erroneous current computation, using a firmware, comprising:
obtaining at least one output of at least one signal conditioning circuit;
analyzing said at least one output obtained to generate at least one DC offset curve analyzed;
generating at least one DC offset curve generated based on said at least one output obtained;
comparing at least one DC offset curve analyzed with at least one DC offset curve generated thereby plotting a median value;
calculating a current based on said median value.

2. The method as claimed in claim 1, wherein said firmware comprises of software configured to generate said at least one DC offset curve analyzed.

3. The method as claimed in claims 1 and 2, wherein said at least one DC offset curve generated is based on data generated by testing said RC circuit.

4. The method as claimed in claims 1-3, wherein working of said RC circuit is enhanced by reducing error in trip time and metering introduced due to delayed response of said RC circuit.

5. The method as claimed in claims 1-4, wherein said at least one output is at least one waveform generated by the at least one signal conditioning circuit.

6. The method as claimed in claims 1-5, wherein at least one DC Offset is the mean value of the at least one waveform.

7. The method as claimed in claims 1-6, wherein said at least one DC Offset is generated based on component tolerances of the at least one signal conditioning circuit.

8. The method as claimed in claims 1-7, wherein said Offset value at startup depends on a stabilized DC Offset value.

9. The method as claimed in claims 1-8, wherein a mechanism is provided to stabilize said DC offset after startup and saves a stabilized value of said DC offset in memory.

10. The method as claimed in claims 1-9, wherein said current is calculated based on said median value till an output of RC circuit is stabilized.

11. The method as claimed in claims 1-10, wherein said stabilized DC offset value is fetched from said memory and a curve is plotted on the basis of median value and stabilized DC offset value.