DESC:FIELD OF THE INVENTION
The present invention relates to improving the usage of the thermal memory modeling of the current flowing conductor using R-C circuit with the help of microcontroller for overload or over current protection of electrical devices and conductors in an electrical system. The innovation work presented here, uses new efficient way of implementing thermal memory by mapping charging and discharging of the capacitor with heating and cooling of the conductor, respectively.
BACKGROUND OF THE INVENTION
Usage of electrical protection devices like circuit breakers, relays, fuses is mandatory in any electrical system to protect the devices and conductors in case of fault condition due to over current flowing through the conductors. The damage is caused mainly due to the heating of conductors and components as the fault current flows. In order to protect devices and conductors, protection devices are used which trip the path of current. The conventional circuit breakers have a bimetallic conductor which on over heating because of over current bends and trips. As the current flowing comes down, the bimetal starts cooling down, the contact is reestablished.
With introduction of electronic trip units in circuit breakers, the bimetal function is replaced by a capacitor and resistor circuit. The capacitor holds charge proportionate to the temperature of the conductor. However, many existing circuits are implemented by mapping the discharging of the capacitor to the cooling characteristic of the conductor. This requires persistent or non-volatile memory or external power for volatile memory, in order to retain the thermal data of the conductor when unit is tripped and there is no power.
US Patent no. 4616324, US Patent no. 5418677 and US Patent no. 5850330 talk about unpowered thermal memory implementation without controlling the charging process. Here the cooling characteristics of conductor is mapped to the discharge characteristic of capacitor. They use non volatile memory to store thermal data when power goes off.
In contrast to the prior patents the present innovation work presented here, uses new efficient way of implementing thermal memory by mapping charging and discharging of the capacitor with heating and cooling of the conductor, respectively. This removes the requirement for volatile/non-volatile memory for storing thermal data. Also it removes use of thermal equations or timer which are used currently to implement overload and thermal memory. Thus reducing the computation and improving the efficiency.
As seen in present cases, if the current is continuously increasing then irrespective of whether current is e.g. 1kA or 1000kA the Thermal Capacitor charges to full value and discharge time is same in both cases. But the conductor temperature is not same in both the cases. So if charging rate is controlled then each voltage value across the Thermal Capacitor can be mapped onto some fault current and hence giving a better indication of actual conductor temperature, thus modeling the thermal memory closely to the conventional bimetallic.
OBJECTS OF THE INVENTION
One object of the invention is to overcome the disadvantages/drawbacks of the prior art.
A basic object of the present invention is to provide a method for implementing thermal memory by mapping charging and discharging of the capacitor with heating and cooling of the conductor.
Another object of the present invention is to provide a system for implementing thermal memory by mapping charging and discharging of the capacitor with heating and cooling of the conductor.
These and other advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
SUMMARY 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.

In an aspect of the present invention, there is provided a method for thermal memory modeling of a current carrying conductor in a circuit breaker, said method comprising steps of:
measuring the voltage across a capacitor of a RC Circuit using a controller means,
charging said capacitor equivalent to increase value of the current of said conductor resulting into heating of said conductor, by closing a normally closed switch,
controlling the charging of said capacitor using a means for controlling the charging rate and discharge rate of said capacitor,
tripping said circuit breaker when the voltage across said capacitor greater than a threshold voltage,
discharging said capacitor equivalent to the decrease value of current of said conductor resulting into cooling of said conductor by closing a normally open switch,
controlling the discharging of said capacitor using said means for controlling the charging rate and discharge rate of said capacitor.

In another aspect of the present invention, there is provided a system for thermal memory modeling of a current carrying conductor in a circuit breaker, said system comprising:
a RC circuit;
a controller means for measuring a voltage across a capacitor of said RC circuit;
a means for controlling the charging and discharge rate of said capacitor;
wherein said means controlling the charging or discharging of said capacitor according to increase or decrease of the current in said current carrying conductor.
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: Flow chart for thermal memory modeling of conductor using R-C circuit
Figure 2: Circuit Diagram 1
Figure 3: Circuit Diagram 2
Figure 4: Circuit Diagram 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 numerals are used to depict the same or similar elements, features and structures.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
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.
Accordingly, present invention provides a system and method for thermal memory modeling of a current carrying conductor in a circuit breaker.
An RC circuit in conjunction with a microcontroller 6 is used to model the temperature of current carrying conductor. The charging and discharging characteristic of the Capacitor 1 is mapped onto heating and cooling characteristics, respectively, of the conductor. The charge rate of the Capacitor 1 is decided by RC time constant in which R 3 is the resistor in charging path. While discharge rate of the Capacitor 1 is decided by the RC time constant in which R 2 is the resistor in discharge path. At any given instant, Voltage across Capacitor 1 indicates present thermal state of the conductor.
Voltage across Capacitor 1 is used by the software to model the temperature across the conductor. The charging of the capacitor 1 is done to simulate rise of temperature of the conductor due to rise in current. Thus as the current in the conductor increases causing heating, the charge across the capacitor also increases proportionally. Microcontroller 6 issues a trip when the voltage across capacitor exceeds the threshold voltage across capacitor which is equivalent of the steady state temperature of conductor.
When the breaker trips the power to the circuit is cut-off. The Capacitor 1 stores the charge across it indicating the current temperature of the conductor. As the power off time passes by, the capacitor loses its charge equivalent to cooling of temperature.
On power up the voltage across Capacitor 1 indicates the thermal state of the conductor. The capacitor 1 voltage increases or decreases in accordance with the current flow and in turn indicates temperature of the conductor.
Figure 1: Flow chart for thermal memory modeling of conductor using R-C circuit

Initially both transistor T1 and T2 are open.
1. Measure the voltage across the Thermal Capacitor 1 using ADC in the microcontroller 6. (Step 1)
2. In step 2, it is checked whether current is increasing or decreasing.
3. If the current is increasing, charge the Thermal Capacitor 1 , by closing transistor T1, according to the equivalent of the current flowing through the conductor. This is achieved by controlling the charging rate of Capacitor 1 using programmable resistors array or digital potentiometer. Maintain this voltage across Capacitor 1. (Step 3,4).
t = - RC loge(1-Vf/Vi)
where –
t is charging time
RC is charging path
Vf is final voltage across Capacitor 1
Vi is initial voltage across Capacitor 1
4. The charging rate is decided by the rate of rise of current which is directly linked to the rise of conductor temperature.
5. When the current starts decreasing, Transistor T2 is closed and T1 is opened. The voltage across capacitor thus discharges according to reduction in current. (Step 5,6)
t = - RC loge(-Vf/Vi)
where –
t is discharging time
RC is discharging path
Vf is final voltage across Capacitor 1
Vi is initial voltage across Capacitor 1
6. If the voltage across the capacitor 1 is greater than the Threshold voltage Vth which is equivalent to the Steady State temperature, then the microcontroller 6 issues a trip. (Step 7,8)
7. On next power up, the voltage across Capacitor 1 is rechecked and corresponding to the current flowing, it again starts charging.
8. The programming resistor can be programmed using I2C, SPI or normal digital I/Os.

Figure 2: Circuit Diagram 1
Capacitor 1 is the thermal Capacitor in parallel to the Programmable Resistor 2 which provides the discharge path to the Capacitor 1. The RC time constant of the setup is selected so that the discharge characteristic of the setup matches with the cooling curve of the current carrying conductor. Programmable resistor 3 is the charging resistor. This controls the charging of capacitor 1 according to the current flowing through conductors. A programmable resistor is required in both charging and discharging path to vary the resistor in accordance with the actual current flowing through the conductor A normal resistor would have constant charging and discharging curve and will not simulate the current fluctuations in the conductor. So if normal resistors are connected they would give same trip time for various currents. The on-chip ADC of microcontroller 6 continuously monitors the voltage across Capacitor using I/O line 8. When a fault is detected the microcontroller 6 turns on the transistor 3 using I/O line 7. The transistor 1 allows the source to charge the Capacitor 1. When the current decreases as compared to its previous value, Transistor T2 is closed and T1 is opened. I/O line 7 and 9 are used to control the opening and closing operation of the transistor T1 and T2. So now discharge of voltage starts to simulate actual conductor temperature by both charging and discharging alternately and appropriately.
After the breaker trips, the Capacitor 1 discharges according to its RC time constant this in turn gives direct indication of the cooling of the conductor. When the breaker is closed the voltage across Capacitor 1 indicates the thermal state of the conductor thus helping the controller to take a decision whether breaker can withstand fault current or not.
Switch T1 is normally closed and T2 is normally open.
Figure 3: Circuit Diagram 2
Its working is similar to Figure 2. Just the source to charge the capacitor is not an I/O line but Vcc.
Figure 4: Circuit Diagram 3
Its working is similar to Figure 2. Just the discharge path has normal resistor and transistor T2 is removed. In this case discharge path comes into picture only when breaker is disconnected. The entire charging is controlled by Programmable resistor 3.
Advantages:
1. No external power required for retaining thermal data.
2. Thermal modeling is much closer to bimetalic
3. No need of volatile or non-volatile memory.
4. Very useful for fluctuating and Ramp up/down current.
5. No timer required for maintaining trip time elapsed.
Features of the present invention:

1. Not only discharging, but charging of Thermal Capacitor is also mapped onto thermal state of conductor.
2. No external power required for retaining thermal data.
3. No need of volatile or non-volatile memory.
4. For fluctuating current , now the voltage across Thermal Capacitor gives a direct indication of the conductor temperature. So if the current increases to a small value then Thermal Capacitor also charges to a small value and not to maximum voltage. If now the current decreases the Thermal Capacitor discharges accordingly.
5. As seen in present cases, if the current is continuously increasing then irrespective of whether current is for eg. 1kA or 1000kA the Thermal Capacitor charges to full value and discharge time is same in both cases. But the conductor temperature is not same in both the cases. So if charging rate is controlled then each voltage value across the Thermal Capacitor can be mapped onto some fault current and hence giving a better indication of actual conductor temperature. ,CLAIMS:1. A method for thermal memory modeling of a current carrying conductor in a circuit breaker, said method comprising steps of:
measuring voltage across a capacitor of a RC Circuit using a controller means,
charging said capacitor equivalent to increase value of the current of said conductor resulting into heating of said conductor, by closing a normally closed switch,
controlling the charging of said capacitor using a means for controlling the charging rate and discharge rate of said capacitor,
tripping said circuit breaker when the voltage across said capacitor becomes greater than a threshold voltage,
discharging said capacitor equivalent to the decrease value of current of said conductor resulting into cooling of said conductor by closing a normally open switch,
controlling the discharging of said capacitor using said means for controlling the charging rate and discharge rate of said capacitor.
2. The method as claimed in claim 1, wherein said means for controlling the charging rate and discharge rate of said capacitor is a programmable resistors array or digital potentiometer and the like.
3. The method as claimed in claim 2, wherein said programmable resistors array is programmed by using I2C, SPI or normal digital input /output and the like.
4. The method as claimed in claim 1, wherein said controller means is a microcontroller.
5. The method as claimed in claim 1, wherein said measuring of the voltage across a capacitor by using an analog to digital converter means inbuilt in said microcontroller.
6. The method as claimed in claim 1, wherein said normally closed switch and normally open switch are transistor T1 and T2 respectively.
7. The method as claimed in claim 1, wherein charging and discharging of said capacitor is mapped onto heating and cooling of said conductor.
8. The method as claimed in claim 1, wherein charging and discharging rate of said capacitor is provided by a RC time constant of said RC circuit.
9. A system for thermal memory modeling of a current carrying conductor in a circuit breaker, said system comprising:
a RC circuit;
a controller means for measuring a voltage across a capacitor of said RC circuit;
a means for controlling the charging and discharge rate of said capacitor;
wherein said means controlling the charging or discharging of said capacitor according to increase or decrease of the current in said current carrying conductor.
10. The system as claimed in claim 9, wherein said means for controlling the charging rate and discharge rate of said capacitor is a programmable resistors array or digital potentiometer and the like.
11. The system as claimed in claim 10, wherein said programmable resistors array is programmed by using I2C, SPI or normal digital input /output and the like.
12. The system as claimed in claim 9, wherein voltage across a capacitor of said RC circuit is measured using a controller means.
13. The system as claimed in claim 9, wherein said capacitor is charged or discharged according to increase or decrease value of the current of said conductor, which corresponds to heating or cooling of said conductor, during closing a normally closed switch or normally open switch.
14. The system as claimed in claim 9, wherein said circuit breaker is tripped when the voltage across said capacitor greater than a threshold voltage.
15. The system as claimed in claim 9, wherein said controller means is a microcontroller.
16. The system as claimed in claim 9, wherein said measuring of the voltage across a capacitor by using an analog to digital converter means inbuilt in said microcontroller.
17. The system as claimed in claim 9, wherein said normally closed switch and normally open switch are transistor T1 and T2 respectively.
18. The system as claimed in claim 9, wherein charging and discharging of said capacitor is mapped onto heating and cooling of said conductor.
19. The system as claimed in claim 9, wherein charging and discharging rate of said capacitor is provided by a RC time constant of said RC circuit.