DESC:TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to de-rating in a circuit breaker. More particularly, the present invention provides a method of de-rating using air core sensor, preferably rogowski rather than conventional core balance current transformer, achieved through analog circuitry.

BACKGROUND AND THE PRIOR ART

A Circuit Breaker is a manually or automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. One variant of Circuit breakers are microprocessor based circuit breakers. These Circuit Breakers make use of an Electronic Trip unit as the nucleus of sensing the fault condition.

An electronic trip unit is a device that is conventionally used in conjunction with a circuit breaker to control the circuit breaker's current (and, or voltage) versus trip time response. Hence, the trip unit typically receives information relating to current through the circuit breaker through the conventional current sensors, such as Core balance current transformer and/or Air core sensors, preferably Rogowski and processes the information to provide feedback to a user or to provide trip functionality to the associate circuit breaker. When the sensed current exceeds a pre-defined threshold, the trip unit issues a trip signal, in some cases after a predetermined time delay. Generally, the trip signal is communicated to a solenoid, such as a circuit breaker flux shift device, which is an electromechanical device, configured to co-operate with a tripping mechanism to separate the circuit breaker contacts and interrupt the current in the protected circuit.

The time versus current trip characteristics are, in part, a function of a maximum continuous current permitted by the circuit breaker and follows predefined curves. The predetermined time delay of issuance of the trip command is an inversely related to the magnitude of the sensed current. For example, very large magnitude currents (generally produced by a fault), instructs the microcontroller or the processing unit in the electronic trip unit to issue a trip signal instantaneously.
The circuit breaker may, of course, also be used to monitor voltage, and trip in a case of any disturbance in predetermined voltage conditions such as under-voltage, overvoltage, and voltage imbalance conditions.

Conventional circuit breaker includes tripping parameters, for example, the maximum continuous current permitted in the protected circuit. The maximum continuous current the circuit breaker is designed to carry is known as a frame rating or current rating of the circuit breaker. As long as the sensed current remains below a predefined protection rating such as long-time, short-time, ground fault, or instantaneous, the breaker will remain in the not trip condition.

Typically, in a configuration of a combination of a Current transformer and an Air Core sensor, the Core balance current transformer is used to provide power to the trip unit where as the Air Core sensor, preferably the Rogowski is employed in the sensing domain of the Electronic Trip Unit. The ETU records the current flowing through the circuit breaker or motor overload relay via the Rogowski coil, phase amplifiers and an A/D converter section, comprising largely of the Microcontroller unit. Conventional current sensing systems for the ETU employ a current sensor in each phase and in the neutral, if used.

In conventional trip units a rating plug having a rating resistor, or set of resistors, comprising predetermined resistor values which set a current rating (i.e., gain) which is the maximum continuous current permitted in the electronic circuit is employed. The rating resistor provides an analog voltage gain proportional to the sensed current in the protected circuit. The rating resistor value is selected to provide a predetermined voltage when a current proportional to the maximum, continuous current permitted in the protected circuit passes through it. Thus, the value of the rating resistor accordingly sets the ampere rating of the corresponding circuit interrupter. While analyzing the feasibility of the circuit under practical conditions, it is observed that by simply altering the gain resistor( rating resistor) value, critical parameters like DC offset values also get altered, which needs to be compensated for.

Another noteworthy observation is that the conventional rating plug changes the operating curve for actuation of a breaker having an electronic circuit interrupter, thus changing the ampere rating of the breaker. For safety purposes, the circuit breaker must be properly configured to provide the type of protection judged by the customer or plant engineer to be appropriate. Therefore, modification of the protection rating or the current vs. trip time response curve is very serious matter and should be handled appropriately and in a way that prohibits errors. Not all rating plugs are compatible with all electronic trip units. Therefore, a known problem is to ensure that a rating plug is compatible with the electronic trip unit into which it is to be inserted.

In order to deal with this problem, a need is felt for a system having Design alterations catering to only two or three crucial ratings, example 100% rating, 80% rating and 62.5 % rating. The alteration in the design to respond to the rating must not alter the operating curve thus maintaining a certain level of accuracy as promised by the circuit with 100% rating. The original sense signal characteristics must not be altered while incorporating the de-rating function.

Hence it is mandatory to implement the de-rating of the sense signals, keeping in mind the above mentioned design criteria.

There are various existing mechanisms and patented disclosures of circuit breakers having electronic trip units ("ETU"s) and removable rating plugs for setting the circuit breaker ampere rating. However, the existing mechanisms are complex to implement and also have the above mentioned drawbacks.

Therefore, the present invention provides de-rating of the original ETU by the means of an easy to implement technique of gain compensation. The method provides a reliable, easy to implement and robust proposal for the implementation of de-rating in a circuit Breaker.

OBJECTS OF THE INVENTION

A basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.

Another object of the present invention is to provide a method of de-rating the sense signal in circuit breakers employing air core sensors by gain compensation technique.

Another object of the present invention is to provide a method which is reliable, easy to implement and robust for of de-rating in a circuit breaker.

These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY

This summary is provided to introduce concepts related to method of de-rating the sense signal in circuit breakers employing air core sensors by gain compensation technique and the concepts are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.

Accordingly, in one implementation, a circuit breaker employing a gain compensation technique for use in electronic trip unit using a rating plug having a rating resistor, or set of resistors is disclosed. The circuit breaker characterized in that compensating gain by dynamically altering gain setting resistor value of a gain setting resistor and a voltage divider resistor value of a voltage divider resistor in said electronic trip unit .

In one implementation, the circuit breaker further comprises of one or more air core sensor configured to measure an alternate current or high speed current pulses to generate an output. The signal conditioning circuit comprises of one or more passive low pass filter circuit and one or more operational amplifier circuit, wherein said passive low pass filter circuit is configured to receive said output and integrate said output to generate an integrated signal. The operational amplifier circuit comprises of said gain setting resistor and configured to amplify said integrated signal, wherein said integrated signal comprises gain and thereby accommodate said integrated signal in a format readable by an analog to digital converter (ADC) of a controller comprising said voltage divider resistor. Further, the breaker also comprises of one or more trip mechanism configured to receive said gain setting resistor value from said gain setting resistor and said voltage divider resistor value from the controller.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.

Figure 1 illustrates a block diagram of the electronic trip unit, in accordance with an embodiment of the invention.

Figure 2 illustrates an exemplary two stage system consisting of a low pass passive filter, in combination with the operational amplifier gain stage.

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

Accordingly, the present invention provides a method of employing de-rating in a circuit breaker using air core sensor, preferably Rogowski rather than conventional core balance current transformer, achieved through analog circuitry. The invention employs the technique of gain compensation, by altering two different values of resistors, namely the gain setting resistor and the voltage divider resistor in the signal conditioning circuit.

Figure 1 is a block diagram representation of the electronic trip unit, consisting of an air core sensor, Rogowski, which is a device that is used for measuring alternate current or high speed current pulses. The Rogowski consists of a helical coil of wire with the lead from one end returning through the centre of the coil to the other end. Thus both terminals of the Rogowski coil are at the same end of the coil. There is no iron core and thus the drawback of saturation faced by Current Transformers is overcome.

Since the voltage that is induced in the coil is proportional to the first time derivative of primary current, the output of the Rogowski coil is usually connected to an electronic integrator circuit to provide an output signal that is proportional to the current.

The output of the Rogowski is proportional to the first time derivative of primary current (di/dt).
E=Ldi/dt
where ,
E is the output voltage of the Rogowski coils measured in volts
L is the mutual inductance of the Rogowski coil measured in henry.
I is the primary current measured in amperes.

The output of the air core sensor of the individual phases namely R, Y, B and N denoted by 101 is fed to the signal conditioning section 102. The signal conditioning section102 comprises of two individual circuits, namely the passive low pass filter circuit103 and the operation amplifier circuit 104. The passive low pass filter circuit 103 has its functionality 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 103 is kept as low as possible. The integrator offers large impedance to high frequency signal and hence filters out the external noises that are high frequency in nature.

The output of the passive low pass filter circuit 103 is fed to the operational amplifier circuit 104. 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, a format that is readable by the ADC of the controller, 105. In case of a fault condition, the controller 105 gives an output signal to the trip mechanism 106, which comprises of an electromechanical device which in turn propagates the mechanical section of the circuit breaker to open the electrical circuit.

The Operational Amplifier Circuit 104 consists of the Gain setting resistors Ri and Rf, which are responsible for the Gain of the sense signals, whereas the Offset circuit added in the Operational Amplifier Circuit 104 needed to accommodate the entire signal within VCC and GND, a format that is readable by the ADC of the controller, 105, consists of the voltage divider resistors.

The air core sensor 101, passive low pass filter circuit,103 and operational amplifier circuit, 104 are best seen by referring to fig.2, which depicts a schematic illustrative of an embodiment of a passive low pass filter circuit 103, operational amplifier circuit104, as depicted in fig. 1. However, it will be appreciated if the schematic of fig. 2 is for illustration purposes only, and that embodiments of the invention may be practiced using alternative arrangements of electronic components.

While figure 2 depict the signal path in only one phase of the circuit breaker, it will be appreciated and understood by one skilled in the art that similar schematics may be used for the other phases, and coupled appropriately at the output side.
The output of the air core sensor in fig.2 is the input to the signal conditioning circuit, whose output is supplied to the microcontroller circuit, preceded by the trip mechanism. For 100% rating, we can hence state
Vout = G100 * Vin
where
Vout = output of the signal conditioning circuit
Vin = output of the rogowski coil and input to the signal conditioning circuit
G100 = gain of the signal conditioning circuit with a 100% rating.
Hence,
G100 = Vout/Vin Eq(1)
Now, to deduce the gain requirement for de-rating the sense signal to a factor of 80%,
G80= G100/0.8 Eq(2)
where G80 = gain of the signal conditioning circuit with 80% rating, and 0.8 is the de-rating factor.

Similarly,
G 62.5= G100/0.625 Eq(3)
where G 62.5 = gain of the signal conditioning circuit with 62.5% rating, and 0.625 is the de-rating factor.

It is worth noting that the constant de-rating factors of 80% and 62.5 have been chosen only for illustrative reasons and that the embodiment of the invention is not limited to these values, in fact it can be extended to practice using alternative arrangements of electronic components.

With a perspective of reverse engineering, having designed the gain values for 80% de-rating and 62.5% de-rating, the necessary alterations in the signal conditioning circuit to receive the required results can be deduced by the ac gain analysis and the dc offset analysis of the signal conditioning circuit.

AC analysis of the signal conditioning circuit

In order to perform AC analysis of the signal conditioning circuit illustrated in fig. 2, all the dc sources are reduced to zero. Hence Vdc = 0. This leaves us with an equation for AC gain depicted by

eq(4)
where
Rint is the resistance of the integrator circuit
Xcint is the equivalent impedance of integrator capacitor Cint given by
Xcint = 1/(2*pi*f*c)
where
Cint is the capacitance in farad
f is the operating frequency in hertz
Rf is the gain setting feedback resistor
Ri is the gain setting input resistor to the inverting terminal of the operational amplifier.

DC offset analysis of the signal conditioning circuit

In order to perform dc analysis of the signal conditioning circuit illustrated in fig. 2, all the ac sources are open circuited. This leaves us with an equation for dc offset depicted by
Eq(5)
where
R1 and R2 are the voltage divider resistors, required to set the dc offset value
Rp is the parallel input resistance to the signal conditioning circuit
R0 is the internal impedance of the Rogowski coil.

It can be observed from Eq (2) that in order to set the gain for de-rating value of 80% or 62.5%, a gain value, greater than the G100 needs to be set. Now, in order to set a greater gain value than G100, the gain setting resistor Ri or Rf needs to be modified. Let us assume that resistor Ri is reduced, reducing Ri, would in turn increase the value of Rf/Ri in Eq(4), thus the ac gain would be increased as desired.

Although by reducing only the value of the gain setting resistors, the desired ac gain is achieved, it may easily go unnoticed that the dc offset value from eq(5) is also distorted by this change. By analyzing Eq(5), we notice that reduction in Ri would lead to an increase in value of the dc offset, thus disorganizing the sense signal. Hence, a compensatory change needs to be made in one of the voltage divider resistors R1 or R2, to balance the dc offset value altered by the change in the gain setting resistor Ri. By reducing the value of R2, the required compensation can be achieved. It can thus be concluded that in order to incorporate de-rating of the sense signal in circuit breakers by gain compensation technique, along with altering the value of the gain setting resistor, it is mandatory to alter the voltage divider resistance value in order to achieve the following:
· Design alterations catering to 100% rating, 80% rating and 62.5 % rating with minimal circuit changes
· The alteration in the design to respond to the rating with no alteration to the operating curve thus maintaining a certain level of accuracy as promised by the circuit with 100% rating.
· The original sense signal characteristics remain untouched while incorporating the de-rating function.

To summarize the Gain compensation technique,
· Increase of the Overall System Gain by reducing the value of Ri which is the Operational Amplifier Gain Setting resistor.
· Perform AC analysis to get a gain equivalent to G100/(Derating Factor)
· Since a change in the Gain setting resistor has a direct effect on the DC Offset, Perform DC analysis again with the revised Gan setting resistor.
· Correspondingly change the Voltage divider resistor to compensate for the change in the Offset value due to the Gain Setting Resistor.

Accordingly, in one implementation, a circuit breaker employing a gain compensation technique for use in electronic trip unit using a rating plug having a rating resistor, or set of resistors is disclosed. The circuit breaker characterized in that compensating gain by dynamically altering gain setting resistor value of a gain setting resistor and a voltage divider resistor value of a voltage divider resistor in said electronic trip unit .

In one implementation, the circuit breaker further comprises of one or more air core sensor configured to measure an alternate current or high speed current pulses to generate an output. The signal conditioning circuit comprises of one or more passive low pass filter circuit and one or more operational amplifier circuit, wherein said passive low pass filter circuit is configured to receive said output and integrate said output to generate an integrated signal. The operational amplifier circuit comprises of said gain setting resistor and configured to amplify said integrated signal, wherein said integrated signal comprises gain and thereby accommodate said integrated signal in a format readable by an analog to digital converter (ADC) of a controller comprising said voltage divider resistor. Further, the breaker also comprises of one or more trip mechanism configured to receive said gain setting resistor value from said gain setting resistor and said voltage divider resistor value from the controller.

In one implementation, said gain compensation is achieved by dynamically altering gain setting resistor value of said gain setting resistor and said voltage divider resistor value of said voltage divider resistor in said electronic trip unit.

In one implementation, said air core sensor is preferably a Rogowski sensor.

In one implementation, said output of said air core sensor is proportional to the first time derivative of primary current.

In one implementation, said output of said air core sensor is in an electrical resistivity of phases namely R, Y, B and N.

In one implementation, said passive low pass filter circuit is an integrator configured to integrate said output as to obtain an equivalent value of current that is proportional to the primary current.

In one implementation, said passive low pass filter circuit offers large impedance to high frequency signal and hence filters out said external noises that are high frequency in nature.

In one implementation, said output integrated is in is in an electrical resistivity of phases namely R, Y, B and N.

In one implementation, a DC offset is provided in order to accommodate said output integrated within Vcc and GND, said format readable by said ADC of said controller.

In one implementation, when a fault is detected, said controller feeds signal integrated within the format readable to said trip mechanism which comprises of an electromechanical device which in turn propagates a mechanical section of said circuit breaker to open the electrical circuit.

The practical aspects of the pros and cons of change in the gain setting resistors have been widely explained in this invention, with a probable solution to the drawback of only changing the gain setting resistor being the highlight of this invention.

The technique disclosed here within modulates the sense signal in a manner such that no software changes in the microcontroller section are required to incorporate the de-rating of the circuit breaker

While the embodiments illustrated in the figures and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. for example, while the method of designing de-rating concept chosen here is by altering the gain setting resistors as well as the voltage divider resistors, other methods of de-rating namely by modifying the software downloaded in the micro controller of the ETU have also been contemplated upon. The invention is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims.
,CLAIMS:1. A circuit breaker employing a gain compensation technique for use in electronic trip unit using a rating plug having a rating resistor, or set of resistors, said circuit breaker characterized in that compensating gain by dynamically altering gain setting resistor value of a gain setting resistor and a voltage divider resistor value of a voltage divider resistor in said electronic trip unit.

2. The circuit breaker as claimed in claim 1 comprises of:
one or more air core sensor configured to measure an alternate current or high speed current pulses to generate an output;
said signal conditioning circuit comprising one or more passive low pass filter circuit and one or more operational amplifier circuit, wherein
said passive low pass filter circuit is configured to receive said output and integrate said output to generate an integrated signal; and
said operational amplifier circuit comprising said gain setting resistor and configured to amplify said integrated signal, wherein said integrated signal comprises gain and thereby accommodate said integrated signal in a format readable by an analog to digital converter (ADC) of a controller comprising said voltage divider resistor; and
one or more trip mechanism configured to receive said gain setting resistor value from said gain setting resistor and said voltage divider resistor value from the controller.

3. The circuit breaker as claimed in any of the preceding claims wherein said gain compensation is achieved by dynamically altering gain setting resistor value of said gain setting resistor and said voltage divider resistor value of said voltage divider resistor in said electronic trip unit.

4. The circuit breaker as claimed in any of the preceding claims wherein said air core sensor is preferably a Rogowski sensor.

5. The circuit breaker as claimed in any of the preceding claims wherein said output of said air core sensor is proportional to the first time derivative of primary current.

6. The circuit breaker as claimed in any of the preceding claims, wherein said output of said air core sensor is in an electrical resistivity of phases namely R, Y, B and N.

7. The circuit breaker as claimed in any of the preceding claims, wherein said passive low pass filter circuit is an integrator configured to integrate said output as to obtain an equivalent value of current that is proportional to the primary current.

8. The circuit breaker as claimed in any of the preceding claims, wherein said passive low pass filter circuit offers large impedance to high frequency signal and hence filters out said external noises that are high frequency in nature.

9. The circuit breaker as claimed in any of the preceding claims, wherein said output integrated is in is in an electrical resistivity of phases namely R, Y, B and N.

10. The circuit breaker as claimed in any of the preceding claims, comprises a DC offset in order to accommodate said output integrated within Vcc and GND, said format readable by said ADC of said controller.

11. The circuit breaker as claimed in any of the preceding claims, wherein when a fault is detected, said controller feeds signal integrated within the format readable to said trip mechanism which comprises of an electromechanical device which in turn propagates a mechanical section of said circuit breaker to open the electrical circuit.