Claims:WE CLAIM:

1. A 3-stage threshold method in an electronic trip unit, for instantaneous current protection, wherein said method comprising:
receiving, by a signal conditioning circuit module, current sensor output signal from a current sensor means;
stage I: determining, by said signal conditioning circuit module, an instantaneous protection threshold to default-HIGH threshold value for handling high fault current;
initializing an analog to digital converter unit (ADC) to enable sampling of ADC output signals at a high sampling frequency;
processing of said ADC signals and determining if said signals is greater to said threshold value for issuance of trip command;
Stages 2: updating said default-HIGH threshold to a default-RATED threshold based on pre-stored current rating, for handling low fault current;
Stage 3: setting a threshold value based on a pre-stored calibration data, for handling marginal faults.

2. The method as claimed in claim 1, wherein at power up, performing, by said trip unit, system clock initialization and memory initialization.

3. The method as claimed in claim 1, wherein said current rating is pre-stored in a memory module is pre-stored based on a soft-rating calibration means.

4. The method as claimed in claim 2, wherein after determining of said threshold value, resetting, by using a timer trigger module, said sampling frequency to said normal sampling frequency.

5. An electronic trip unit for instantaneous current protection, wherein said electronic trip unit comprising:
a signal conditioning circuit to amplify and filter a current sensor output signal received from a current sensor means, and generates an output signal;
wherein, said signal conditioning circuit saturates the current sensor output signal above a minimum fault current for instantaneous protection, to determine:
a default-HIGH threshold for handling high fault current at power up
updating said default-HIGH threshold to a default-RATED threshold based on current rating, for handling low fault current;
a threshold value based on a pre-stored calibration data, for handling marginal fault current;
an analog to digital converter unit, receives said output signal from said signal conditioning circuit, to perform sampling of said output signal at a sampling frequency higher than normal sampling frequency;
a processor means receives and processes said output digital signal to generate a trip command at said sampling frequency.

6. The electronic trip unit as claimed in claim 5, wherein said current rating is pre-stored in a memory means.

7. The electronic trip unit as claimed in claim 6, wherein said memory means pre-stores said calibration data to enable setting of said threshold value for said instantaneous current protection.

Dated this 29th day of March 2019

Abhishek Sen
Of S. Majumdar & Co.
(Applicant’s Agent)
Registration No. 980
, Description:TECHNICAL FIELD

[001] The present subject matter described herein, in general, relates to switchgears such as circuit breakers. More particularly, the invention relates to electronic trip units in a circuit breaker.

BACKGROUND

[002] Circuit breakers are used to break the current passing through it in case the current goes above a certain threshold value. The trip units provide a variety of protections such as current protection, voltage protection, frequency protection, power protection etc. The instantaneous protection is one of the most critical among all the current protections as the fault current values are typically very high in the range of 15 to 20 times the rated current but can also go up as high as 100 times the rated current due to different short circuit conditions. Such high fault currents cause a lot of electrical stress on the feeder. If the circuit breaker is not tripped quickly the energy would cause catastrophic damage to the system and the surroundings. Hence the trip unit needs to sense the fault current as fast as possible and issue the trip the command. In the case where the fault currents are in the range near the threshold for instantaneous protection, the sensing accuracy becomes an important factor. The trip unit needs to decide whether the fault current lies above the instantaneous protection threshold or below it where delayed trip is expected.

[003] The electronic trip unit is an embedded system where one or multiple microcontrollers are deployed along with different functional hardware modules such as current sensors, signal conditioning circuit, Flash memory storage, power supply, etc. The electronic trip is designed to derive power from the current through the feeder, hence is called as self-powered electronic trip unit. If the circuit breaker faces a make-on short circuit fault, the trip unit needs to power on, initialize the microcontroller, sense the fault and issue the trip command to the breaker. These activities at power up take up more time and puts the system under a lot of electrical stress. High-end microcontroller-based trip units have features such as communication and display interfaces and often the microcontroller works on real-time operating systems. Such systems take even higher times to complete the initialization processes and then sense the fault current.

[004] Another issue arises when trip units have soft-rating plug feature where the rated current value is configurable. Instantaneous protection threshold depends on the rated current value. One way is to store rated current value in a Flash memory and the trip unit reads the value at power up to set the threshold and various other parameters which depend on the rated current value. These processes at power-up add to the delay which again causes problems in the case of a make-on short circuit fault.

[005] Reference is made to a prior art CN1009785B, which discloses a solid state instantaneous trip apparatus for current-limiting circuit breaker. It makes use of integrated circuits and dedicated hardware designed to sense the current and provide instantaneous tripping for short circuit faults. Being a completely hardware-based design, it has fixed configuration and no flexibility to change its settings or provide any other feature.

[006] Further, reference is made to a prior art US6167329A, which refers to an electronic trip unit which uses one microcontroller to perform operations in normal load current range and a separate second microcontroller which works for fault currents. The patent provides two-controller architecture to manage different features along with overcurrent protections such as overload and instantaneous tripping. This system requires an auxiliary power supply for its operation and it does not address the issues about make-on faults of high current magnitude. Also, it does not provide features such as soft-rating plug.

[007] However, in view of the above mentioned issues with the existing trip unit, there is a dire need for a method to provide effective instantaneous protection which is accurate enough to differentiate marginal fault currents from the currents that are below the instantaneous protection threshold, and senses the make-on fault without much delay to minimize the electrical stress and clear the fault by tripping the circuit breaker, even with the other important features such as soft-rating plug.

SUMMARY OF THE INVENTION

[008] 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.

[009] An objective of the present invention is to provide an electronic trip unit for circuit breakers providing effective instantaneous current protection required to protect the feeder load in case of short circuit conditions.

[0010] Another objective of the present invention is to provide an electronic trip unit which achieves effective instantaneous protection by a method with 3-stage thresholds which is able to respond very fast to make-on faults with very high fault currents, and also is able to handle the marginal faults with high accuracy.

[0011] Yet another objective of the present invention is to provide a self-powered electronic trip unit which does not need an external power supply.

[0012] Accordingly, in one aspect, in one implementation, the present invention provides a 3-stage threshold method in an electronic trip unit, for instantaneous current protection, wherein said method comprising:
receiving, by a signal conditioning circuit module, current sensor output signal from a current sensor means;
stage I: determining, by said signal conditioning circuit module, an instantaneous protection threshold to default-HIGH threshold value for handling high fault current;
initializing an analog to digital converter unit (ADC) to enable sampling of ADC output signals at a high sampling frequency;
processing of said ADC signals and determining if said signals is greater to said threshold value for issuance of trip command;
Stages 2: updating said default-HIGH threshold to a default-RATED threshold based on pre-stored current rating, for handling low fault current;
Stage 3: setting a threshold value based on a pre-stored calibration data, for handling marginal faults.

[0013] In another aspect, in one implementation, the present invention an electronic trip unit for instantaneous current protection, wherein said electronic trip unit comprising:
a signal conditioning circuit to amplify and filter a current sensor output signal received from a current sensor means, and generates an output signal;
wherein, said signal conditioning circuit saturates the current sensor output signal above a minimum fault current for instantaneous protection, to determine:
a default-HIGH threshold for handling high fault current at power up
updating said default-HIGH threshold to a default-RATED threshold based on current rating, for handling low fault current;
a threshold value based on a pre-stored calibration data, for handling marginal fault current;
an analog to digital converter unit, receives said output signal from said signal conditioning circuit, to perform sampling of said output signal at a sampling frequency higher than normal sampling frequency;
a processor means receives and processes said output digital signal to generate a trip command at said sampling frequency.

[0014] 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 above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:

[0015] Figure 1 illustrates the system block diagram, according to one implementation of the present invention.

[0016] Figure 2 shows a flowchart of the 3-stage method embedded in an electronic trip unit for instantaneous current protection at power up, according to one implementation of the present invention.

[0017] 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 PRESENT INVENTION

[0018] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.

[0019] 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 of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0020] 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.

[0021] It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

[0022] 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.

[0023] Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.

[0024] It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0025] In one implementation, for achieving accuracy in sensing the fault, the trip unit is pre-calibrated and the threshold values to be used are stored in a memory module such as a Flash memory.

[0026] In one implementation, reference is made to figure 1 which shows basic block diagram of the system. In the system, the current sensor such as a current transformer (CT) or a Rogowski coil can be used. The current sensor output is given to the signal conditioning module which processes the signal with amplifier and filter circuits and outputs a signal which is given to the Analog-to-digital converter module. The processor and ADC modules can be separate or integrated on a System-on-Chip (SoC). There is a Flash memory module which is accessed by processor to read or write non-volatile data such as soft-rating and calibration data. The processor can give a tripping command to the circuit breaker if a fault is detected.

[0027] In one implementation, reference is made to figure-2 which shows the basic flow diagram of the 3-stage method at power up executed by the electronic trip unit.

[0028] In one implementation, at power up, the electronic trip unit first performs the essential activities such as system clock initialization and RAM initialization for certain necessary variables. The instantaneous protection threshold is initialized with a Default-HIGH threshold value to begin with. After that the ADC peripheral can be configured and sampling can be started at a frequency 10 times higher than normal sampling frequency using a timer trigger module.

[0029] In one implementation, the instantaneous protection method is executed in a high priority interrupt routine where ADC samples are processed and tripping decision is taken based on the current threshold value. The main process continues further to initialize Flash memory module and firstly reads the rated current value from the memory which is pre-stored based on the soft-rating calibration process. Now the processor knows the exact current rating and hence the Instantaneous protection threshold is updated from Default-HIGH to Default-RATED. The protection method keeps running in background with high frequency and it updates its working threshold as soon as the main process updates it. Now the main process reads the pre-stored calibration data which contains the information of the actual gains of the signal conditioning module and obtains the accurate threshold for Instantaneous protection. After that the main process reinitializes the timer trigger module for normal sampling frequency and hands over the main process control to the scheduler or the operating system in case of higher end trip unit.

[0030] In the implementation, the signal condition circuit module is designed in a way that the signal given to the ADC saturates above the instantaneous protection threshold for 100% current rating. If a very high current fault occurs at power up, the processor will get saturated sample values. Hence the trip unit begins the instantaneous protection algorithm with a default-HIGH threshold corresponding to the saturated samples values which is carefully designed considering the hardware tolerances of signal conditioning. Hence for a make-on fault of very high magnitude, the trip unit is able to achieve a minimum trip time as it skips time-consuming tasks such as Flash memory operations. The protection algorithm works on the samples of current signal and runs in the background in a high priority interrupt routine. The main process then starts Flash memory operations. Firstly, the rated current data is read and the threshold is updated to Default-RATED which corresponds to a default threshold depending on the current rating. This threshold is able to take care of the fault levels which well above the minimum fault current but not high enough to saturate all the samples. Finally, the trip unit reads the pre-stored calibration data and obtains accurate instantaneous protection threshold so that the trip unit can respond to marginal faults.

[0031] This method of 3-stage thresholds is based on the trade-off between the sensing accuracy and trip time. For very high fault currents, minimum trip time is a priority over the accuracy of the sensing because the samples are going to saturate anyway. The middle stage balances the accuracy and trip time to accommodate the faults that are well above the minimum fault current but not high enough to saturate all the samples. The third stage is where the fault is marginal and it can afford a little higher trip time but accuracy of the sensing is of the most importance. Hence the pre-stored calibration data is used to cater to the marginal faults and avoid nuisance tripping.

[0032] In the implementation, the instantaneous protection method works on samples of the current signals. The algorithm presented in the figure 2 works on moving-window two samples which proves to be a very simple but effective algorithm. It does not require large processing power. Further in higher end trip units, with more powerful processors, additional algorithms are used to improve the accuracy even further such as averaging, filtering, regression, etc.

[0033] Some of the noteworthy features of the present invention, are mentioned below:
• An electronic trip unit which achieves effective instantaneous protection by a novel method with 3-stage thresholds which is able to respond very fast to make-on faults with very high fault currents, and also is able to handle the marginal faults with accuracy.
• The trip unit has a signal conditioning circuit for amplification and filtering of current sensor output. It saturates the signal above the minimum fault current for instantaneous protection, which helps in deciding the default-HIGH threshold for very high fault current at power up.
• The trip unit provides instantaneous protection in accordance with a soft-rating plug where rated current is stored in Flash memory. The trip unit updates the threshold to default-RATED based on current rating and it is more accurate than the first default- HIGH threshold to take care of lower fault currents which still well above the minimum fault current.
• The trip unit has pre-stored calibration data in the Flash memory which is used to accurately set the threshold for instantaneous protection to handle the marginal faults in make-on condition as well as runtime condition where the scheduler or the operating system takes over the main process control.
• At power up, the trip units runs the ADC sampling at a frequency that is 10 times of the normal sampling frequency used during runtime. The instantaneous protection method with 3-stage thresholds works at this high frequency to get the best trip time for make-on faults.

[0034] Although a method in an electronic trip unit for instantaneous current protection have 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 of the method in an electronic trip unit for instantaneous current protection.