Claims:1. A system for ground current calibration, said system comprising:
a computing device comprising a processor operatively coupled with a memory, said memory storing instructions executable by the processor to:
inject, into the system, a first current, the first current having a first value in a predetermined range for the system;
inject, into the system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current;
determine, at the computing device and from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter;
determine, at the computing device, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test; and
determine, at the computing device, an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current,
wherein, when the error is within an allowable range, a calibration factor for ground current is determined as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients.
2. The system as claimed in claim 1, wherein, when the error exceeds the allowable range, the computing device is configured to split the predetermined range int two or more ranges and then the processor is configured to execute, again, the instructions stored in the memory for each of the split ranges until the determined error is within the allowable range.
3. The system as claimed in claim 1, wherein the ground current is determined as a function of vector sum of phase ground current from each phase of the injected current and a corresponding calibration factor for the ground.
4. The system as claimed in claim 1, wherein the determined calibration factor is stored into the meter under test.
5. A method for ground current calibration, said method comprising the steps of:
injecting, into a network system, a first current, the first current having a first value in a predetermined range for the network system;
injecting, into the network system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current;
determining, at the computing device and from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter;
determining, at the computing device, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test; and
determining, at the computing device, an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current,
wherein, when the error is within an allowable range, a calibration factor for ground current is determined as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients.
6. The method as claimed in claim 5, wherein, when the error exceeds the allowable range, the computing device is configured to split the predetermined range into two or more ranges and then repeat the method for each of the split range until the determined error is within the allowable range.
7. The method as claimed in claim 5, wherein the ground current is determined as a function of vector sum of phase ground current from each phase of the injected current and a corresponding calibration factor for the ground.
8. The method as claimed in claim 5, wherein the determined calibration factor is stored into the meter under test.
, Description:TECHNICAL FIELD
[1] The present disclosure relates, in general, to the field of calibration of electronic trip units. In particular, the present disclosure relates to calibration of ground current in an electronic trip unit.

BACKGROUND
[2] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[3] Circuit breaker is used to isolate the faulty bus from main system in case of unhealthy condition. Electronics trip unit is a microprocessor provided in the circuit breaker based numerical relay which provide different functionality like metering, protection and trip functions. Using current and voltage based sensor, electronics trip unit measures the input values of current and voltage and executes appropriate protection if input current and voltage exceeds a threshold value and provides the trip command to circuit breaker to isolate the faulty line from rest of system.
[4] The electronics trip unit provides a variety of current based protection like instantaneous, overload, long inverse, short circuit, thermal. The electronics trip unit also provides multiple voltage based protection like overvoltage, under voltage, frequency as well as power based directional protections.
[5] In the remote areas where auxiliary power supply is not available, there is a big challenge to energize the electronics trip unit in the event of a power cut. For this purpose, current based self-powered supply source is used to power up the electronics trip unit.
[6] A limitation with this type of application is protection trip time. If the input measurement value of current and voltage exceeds from a pickup value (a specific threshold), electronics trip unit sends a trip command to FSD and trip the circuit breaker to isolate the unhealthy section from rest of system. Electronics trip unit itself has some latency (delay) to calculate the current and voltage and applying trip command to circuit breaker. If this latency is more, than the total time for tripping the circuit breaker would also increase.
[7] Another feature of electronics trip unit is the metering and measurement. Same trip unit can be used for energy metering purpose. For energy metering application, accuracy of input current and voltage itself has an important role. Current sensor (CT or Rogowski) plays important role in metering accuracy. Due to large dynamic range of AC current (>20 times of rated current), it is very difficult of maintain linearity of sensor. Nonlinearity of current sensor is higher for lower current due to low signal to noise ratio (SNR). Due to non-linearity of sensor, calculated ground current will also be non-linear and resultant error of ground current will be absolute sum of error of individual phase.
[8] In circuit breaker application, for measuring the ground current, ground fault sensors are used. These additional sensors increase the wiring and cost. For avoiding these limitations, it is suggested to calculate ground current internally instead of measuring from outside sensor. However, if individual phase current sensor error is large, then derived ground current error will also be large.
[9] There is, therefore, a requirement in the art for an approach to accurately determine ground current in an electronic trip unit.
[10] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

OBJECTS OF THE INVENTION
[11] A general object of the present invention is to provide a system and method for ground current calibration in an electronic trip unit.
[12] Another object of the present invention is to provide a system for ground current calibration in an electronic trip unit that compensates for non-linearity.
[13] Another object of the present invention is to provide a system for ground current calibration in an electronic trip unit that is economical.
[14] Another object of the present invention is to provide a system for ground current calibration in an electronic trip unit that can be easily implemented.

SUMMARY
[15] The present disclosure relates, in general, to the field of calibration of electronic trip units. In particular, the present disclosure relates to calibration of ground current in an electronic trip unit.
[16] In an aspect, the present disclosure provides a system for ground current calibration, said system comprising: a computing device comprising a processor operatively coupled with a memory, said memory storing instructions executable by the processor to: inject, into the system, a first current, the first current having a first value in a predetermined range for the system; inject, into the system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current; determine, at the computing device and from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter; determine, at the computing device, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test; and determine, at the computing device, an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current, wherein, when the error is within an allowable range, a calibration factor for ground current is determined as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients.
[17] In an embodiment, when the error exceeds the allowable range, the computing device is configured to split the predetermined range int two or more ranges and then the processor is configured to execute, again, the instructions stored in the memory for each of the split ranges until the determined error is within the allowable range.
[18] In an embodiment, the ground current is determined as a function of vector sum of phase ground current from each phase of the injected current and a corresponding calibration factor for the ground.
[19] In an embodiment, the determined calibration factor is stored into the meter under test.
[20] In an aspect, the present disclosure provides a method for ground current calibration, said method comprising the steps of: injecting, into a network system, a first current, the first current having a first value in a predetermined range for the network system; injecting, into the network system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current; determining, at the computing device and from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter; determining, at the computing device, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test; and determining, at the computing device, an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current, wherein, when the error is within an allowable range, a calibration factor for ground current is determined as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients.
[21] In an embodiment, when the error exceeds the allowable range, the computing device is configured to split the predetermined range into two or more ranges and then repeat the method for each of the split range until the determined error is within the allowable range.
[22] In an embodiment, the ground current is determined as a function of vector sum of phase ground current from each phase of the injected current and a corresponding calibration factor for the ground.
[23] In an embodiment, the determined calibration factor is stored into the meter under test.
[24] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF DRAWINGS
[25] The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain the principles of the present invention.
[26] FIG. 1 illustrates an exemplary block diagram for a system for ground current calibration in a circuit breaker, in accordance with an embodiment of the present disclosure.
[27] FIG. 2 illustrates an exemplary flow diagram for determining DC offset calibration, in accordance with an embodiment of the present disclosure.
[28] FIG. 3 illustrates an exemplary flow diagram for determining AC gain calibration, in accordance with an embodiment of the present disclosure.
[29] FIG. 4 illustrates an exemplary flow diagram for determining ground current in a circuit breaker, in accordance with an embodiment of the present disclosure.
[30] FIG. 5 illustrates an exemplary flow diagram for a method for ground current calibration in a circuit breaker, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[31] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[32] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[33] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[34] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. These exemplary embodiments are provided only for illustrative purposes and so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. The invention disclosed may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure). Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[35] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non – claimed element essential to the practice of the invention.
[36] The present disclosure relates, in general, to the field of calibration of electronic trip units. In particular, the present disclosure relates to calibration of ground current in an electronic trip unit.
[37] In an aspect, the present disclosure provides a system for ground current calibration, said system comprising: a computing device comprising a processor operatively coupled with a memory, said memory storing instructions executable by the processor to: inject, into the system, a first current, the first current having a first value in a predetermined range for the system; inject, into the system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current; determine, at the computing device and from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter; determine, at the computing device, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test; and determine, at the computing device, an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current, wherein, when the error is within an allowable range, a calibration factor for ground current is determined as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients.
[38] In an embodiment, when the error exceeds the allowable range, the computing device is configured to split the predetermined range int two or more ranges and then the processor is configured to execute, again, the instructions stored in the memory for each of the split ranges until the determined error is within the allowable range.
[39] In an embodiment, the ground current is determined as a function of vector sum of phase ground current from each phase of the injected current and a corresponding calibration factor for the ground.
[40] In an embodiment, the determined calibration factor is stored into the meter under test.
[41] In an aspect, the present disclosure provides a method for ground current calibration, said method comprising the steps of: injecting, into a network system, a first current, the first current having a first value in a predetermined range for the network system; injecting, into the network system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current; determining, at the computing device and from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter; determining, at the computing device, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test; and determining, at the computing device, an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current, wherein, when the error is within an allowable range, a calibration factor for ground current is determined as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients.
[42] In an embodiment, when the error exceeds the allowable range, the computing device is configured to split the predetermined range into two or more ranges and then repeat the method for each of the split range until the determined error is within the allowable range.
[43] In an embodiment, the ground current is determined as a function of vector sum of phase ground current from each phase of the injected current and a corresponding calibration factor for the ground.
[44] In an embodiment, the determined calibration factor is stored into the meter under test.
[45] FIG. 1 illustrates an exemplary block diagram for a system for ground current calibration in a circuit breaker, in accordance with an embodiment of the present disclosure. The system 100 can include a current and voltage source 102; a control circuit 104; a meter under test 106, a reference meter 108; and a graphical interface (GUI) 110. The meter under test 106 can further include a current gain stage 112; an analogue to digital converter (ADC) 114; a microprocessor 116; and a serial communications bus 118.
[46] In an embodiment, current transformers (120-1, 120-2, 120-3) step down input current and pass them to the current gain stage 112 to step up the input voltage to a desired level of the ADC 114. Voltage attenuation is also required to suppress input voltage at desired level of ADC 114. The microprocessor 116 is used to for sampling and amplitude calculation. Meter under test 106 communicates with the GUI 110 through a serial communications bus118.
[47] In another embodiment, a reference meter 108 with CT_Ref (120-4) is also connected in the system 100, which provides analogue parameters to the GUI 110. The GUI 110 provides control command to current and voltage source 102 during injection.
[48] Ground current is determined as,

[49] In case of non-linearity in sensor of any phase current, an error in ground current is introduced. For getting better accuracy in case of ground current, individual phase current calibration is performed before vector sum in case of ground current. Hence,

[50] In another embodiment, phase current calibration procedure is performed in two steps – first is DC offset calibration; and second is AC gain calibration.
[51] In another embodiment, DC gain calibration is also performed in two steps – first is power-up DC offset; and second is dynamic DC offset.
[52] In microprocessor based relays, for the purpose of metering, current and voltage sensors are used. Output of sensors go to signal condition electronics circuit, then to ADC. Due to tolerance of electronics component and sensors, final input may be clamped with some DC offset value. This DC offset can be calibrated by power up DC offset calibration method. Power up DC offset value is used during power up the electronics trip unit. In case of switching load condition, DC offset (sensor output) might change. For calibrating this DC offset, dynamic DC offset calibration method is used. In dynamic DC offset calculation, DC offset can be calculated during run time by averaging instantaneous samples of one cycle.
[53] The sensor is a current transformer, which steps down current in transformer secondary. The secondary current passes through a low resistance, which develop voltage. This developed voltage is used for further signal conditioning. In case of nonlinear sensor, gain and phase error of current at secondary terminal vary for different value of primary current. Due to poor signal to noise ratio (SNR) for lower current values, large DC offset and magnitude measurement error is introduced in secondary point of sensor.
[54] The ADC is used to convert analogue signal into digital counts. Effective number of bits (ENOB) of ADC defines the distinguishing capability of lower amplitude signal. For higher accuracy ENOB should be better.
[55] In another embodiment, using DC offset, AC gain and phase calibration, accuracy of phase current and voltage can be improved up to 0.2%, considering a sensor error of up to 4% and a signal condition hardware error of up to 1%. In same case, accuracy of ground current can be improved up to 1%.
[56] DC offset can be calculated by averaging the number of samples captured during one complete cycle. This offset includes signal conditioning as well as sensor offset error.

Xi – instantaneous samples of current and voltage
V0 – corresponding DC offset value
[57] AC magnitude error can be performed by least square regression method. This method is used for calibrating nonlinear sensor. Nonlinear sensor provides different gain error corresponding to the different input. SNR decreases if input current range is low. So overall error of measurement is increase if input current is lower. Hence, it is very difficult to achieve better accuracy with single point calibration.
[58] The least squares regression line is the linear fit that minimises the sum of squares error.
To find error ei, where i=1, 2…n

Sum of squares error,

For least error,

Values of a and b are calculated as,


[59] During calibration, known sequence of input (current or voltage) are injected in sensor primary and corresponding output samples are stored. For example, if measurement current range is 0.1 to 1.2 In, then input current values can be 0.1 to 1.2 In, in step sizes of 0.05 In or less. Ideally all measurement sensor characteristic should be linear, which means it should follow a straight line equation , where y is the desired input value with respect of sensor output x.

m – number of input and output values for specified range, which depends on step size of input current or voltage sample.
[60] During calibration, user can read desired input value from any reference meter and sensor measurement value from meter under test and can store the number of input and output values, depending on the step size of input current or voltage sample. The curve fitting technique can be applied on the stored samples. On behalf of stored X, Y matrix, coefficients a and b can be calculated. After, calculating coefficients a and b, Y’ (modified value of Y) is calculated as,

The calibration error matrix,

[61] Value of Em depends on claimed measurement accuracy. In case the error of any element of the matrix Em is higher than desired value, then the range is split in two and a and b values are again recalculated for that new range and subsequently, corresponding y’ and Em values are calculated. Again, it is checked weather error is below desired error value. If no, the process is repeated. If the error is coming within band, then user can store all related information like number of split ranges, a and b values for each range inside the meter under test. Finally, reverification process can occur.
[62] Any graphical user interface can be used for calibration purpose. From above suggested method, calibration factor (a, b) is calculated for individual current phase.

y’m – calibrated individual phase current
a, b – calibration factors of individual phase current
CF – final calibration factor for individual phase current of ground
[63] FIG. 2 illustrates an exemplary flow diagram for determining DC offset calibration, in accordance with an embodiment of the present disclosure. In the method 200, to calibrate DC offset, GUI injects rated value through current and voltage source and waits for stabilization. Stabilization time can be different according to the calibration setup. Now, the GUI reads offset value from meter under test and validates if the value lies between specified range. If yes, then the value is written again in the meter under test.
[64] FIG. 3 illustrates an exemplary flow diagram for determining AC gain calibration, in accordance with an embodiment of the present disclosure. In the method 300, measurement range is assumed 0.1 to 1.2 In. During calibration,
• GUI calibrator writes default calibration factor inside the MUT;
• GUI calibrator injects 0.1 rated value and waits for stabilization time;
• GUI calibrator reads the measured value from MUT and reference meter and stores it;
• GUI calibrator increases the injected value with step size (.05 or less) and stores the new measured values, and the process repeats till injected value reaches at 1.2In;
• GUI calibrator has all measured (x) and reference values (y) for range 0.1 rated to 1.2 rated
• GUI calibrator uses the least square error method for calculating coefficients of straight line equations for this particular range;
• GUI calibrator, after calculating coefficient, applies these coefficients on current measured samples (x) and calculates calibrated values (y’);
• GUI calibrator, on behalf of reference meter value (y) and calibrated value (y’), calculates error;
• If still error is large as desired value, then the range is split in two slabs, for example, a) 0.1 rated to 0.6 rated and b) 0.6 rated to 1.2 rated;
• The previous step is repeated until the error is not within range;
• If error is within range, GUI calibrator writes all calibration details in MUT, where calibration data includes a) Number of split slabs, b) Specific threshold values of each slab, c) Calibration coefficients for each slab; and
• The calibration is verified.
[65] FIG. 4 illustrates an exemplary flow diagram for determining ground current in a circuit breaker, in accordance with an embodiment of the present disclosure. In the method 400,
• Calibration factor (a1, b1), (a2, b2), (a3, b3), (a4, b4) for R,Y,B,N phase current respectively, are calculated using AC gain calibration method;
• All instantaneous samples of one cycle of individual phase (32 samples if sampling rate is 1600 samples/sec) are passed by a FIR low pass filter for phase compensation;
• RMS value of last cycle is calculated and calibration factor (a, b) of respective phase are applied;
• Using calibrated RMS value, calibration factor (CF1, CF2, CF3, CF4) of respective phase is calculated; and
• The calibration factor is applied for determining ground current.
[66] FIG. 5 illustrates an exemplary flow diagram for a method for ground current calibration in a circuit breaker, in accordance with an embodiment of the present disclosure. The method 500 includes the steps of:
• 502 – injecting, into a network system, a first current, the first current having a first value in a predetermined rated range for the network system;
• 504 – injecting, into the network system, a plurality of second currents, each of the plurality of second currents being within the predetermined range, a first of the plurality of second currents being a predetermined step higher than the first current and each of the remaining plurality of second currents being the predetermined step higher than a previous current;
• 506 – determining, from a meter under test, upon stabilisation of the injected currents, an offset value for each of the injected currents based on a comparison of corresponding metered values of the injected current from the metre under test and a reference meter;
• 508 – determining, based on corresponding metered values of the plurality of injected currents and based on straight-line coefficients, a corresponding plurality of calibrated metered values of injected current from the meter under test;
• 510 – determining an error in calibration based on each of the plurality of metered values of injected current and a corresponding calibrated metered value of injected current;
• 512 – determining, when the error is within an allowable range, a calibration factor for ground current as being a function of the metered value of an instant injected current, the corresponding calibrated metered value of the instant injected current and the straight-line coefficients; and
• 514 – when the error exceeds the allowable range, splitting the predetermined range into two or more ranges and then repeating the method for each of the split ranges until the determined error is within the allowable range.
[67] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive patent matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “includes” and “including” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C ….and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practised with modification within the spirit and scope of the appended claims.
[68] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE INVENTION
[69] The present invention provides a system and method for ground current calibration in an electronic trip unit.
[70] The present invention provides a system for ground current calibration in an electronic trip unit that compensates for non-linearity.
[71] The present invention provides a system for ground current calibration in an electronic trip unit that is economical.
[72] The present invention provides a system for ground current calibration in an electronic trip unit that can be easily implemented.