FORM2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention. - APPARATUS AND METHOD FOR EARTHFAULT DETECTION USING AIR CORE SENSOR SIGNALS FOR PROTECTION AND CONTROL UNITS OF CIRCUIT BREAKERS
2. Applicant(s)

(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY: An Indian Company.
(c) ADDRESS: L & T House, Ballard Estate, Mumbai 400 001,
State of Maharashtra, India
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The present invention generally relates to a fault detection system and method thereof. More particularly the present invention relates to an improved apparatus for earth fault detection thereby protecting circuit breakers and a method thereof.
BACKGROUND AND THE PRIOR ART
This invention relates to earth fault current computation using air core sensors rather than conventional core balance current transformer through analog circuitry.
An electronic trip unit is an intelligent device used in conjunction with an electromechanical circuit breaker to measure system parameters such as voltage, current, power etc and protect the system against faults such as overload, short circuit, earth fault, etc and control electro mechanical device in cases of occurrences of faults. Earth fault occurs when one of the phases or two phases or ail the phases of either the transmission lines or cables get shorted with the ground of the system. This fault is hazardous because it affects human safety. Moreover the fault current is much lower than the threshold set in the conventional over current releases for it to detect and isolate faulty system. Therefore it is necessary to have an additional circuitry for the detection of earth fault in switchgear to offer earth fault protection.
The earth fault current is defined as the vector sum of the individual phases in a power system. One way of calculating the vector sum is by using a core balance current transformer. The individual current sensed by the current transformer is passed through a core balance current transformer. These individual phases produce flux that gets cancelled out with other flux produced by respective phases, producing zero current output in the absence of earth fault. In case of typical earth fault, the core balance current transformer (CBCT) produces an output current proportional to the magnitude of the earth fault current present in the system. This output is then processed and read by the analog to digital converter (ADC) of the micro controller. The controller issues a trip command to

the breaker if the fault current is beyond the set threshold value for the set duration.
In Circuit breaker with electronic trip units, that employ current sensors preferably an air core sensors that produce an output signal that is proportional to the time derivative of the primary current present in the system cannot use a core balance transformer for earth fault detection because the output of the air core signal is in voltage. A CBCT needs current to generate the flux and carry out the vector sum to calculate earth fault current.
Some of the prior arts in the present field of invention are as follows:
US7309993 teaches a three or four pole low-voltage power switch is disclosed wherein the switch is partly provided with a device for detecting ground faults. For this purpose, the current vectorial sum must be produced in the three or four conductors of a monitored network. For the switches of this type, output signals received from Rogowski coils are directed via resistances to an integration capacitor whose voltage forms an input signal of another measuring amplifier representing the current sum of a monitored network. The output signal of the measuring amplifier is, afterwards processed in a known manner in the microprocessor of an excess-current trip
US7791353 provides for a ground loop locator. A read out on the ground loop locator indicates the presence of a ground loop when a ground loop exists in conductors linked by a current transformer of an exciter portion and a Rogowski coil of a detector portion. Also described are how to make and use the ground loop locator and methods of use for the ground loop locator.
US8203814 provides an electrical switching apparatus that includes a plurality of poles each having a Rogowski coil and a conductor passing through an opening thereof, and a processor circuit including a sensor circuit including a plurality of inputs each electrically interconnected with an output of the Rogowski coii of a corresponding pole. The sensor circuit further includes a number of outputs having values each corresponding to current flowing through the conductor, a memory including for each corresponding pole an offset value and a gain correction factor for the sensor circuit, and a gain correction factor for the

Rogowski coil, a number of routines, and a processor cooperating with the sensor circuit and the routines to provide for each pole a corrected current value as a function of a corresponding one of the values, the sensor circuit offset value and gain correction factor, and the Rogowski coil gain correction factor.
US4446420 provides a device and a method for detecting and locating faults and/or partial discharges in a gas-insulated electrical equipment, such as gas-insulated bus or cable, a gas-insulated substation or a gas-insulated switchgear. The detecting device comprises a plurality of field coupled current sensors located at given intervals inside the housing of the equipment for detecting any variation of the current flowing through the conducting element extending inside this housing. Each sensor consists of a toroidal helix pick up coil, preferably a differentiating Rogowski coil, located in an annular cavity machined in the wall of the equipment housing and extending all around the conducting element. Each cavity is connected along its entire length to the inner surface of the housing wall by a slit having opposed side walls sufficiently spaced apart to be insulated from each other. Each sensor gives a signal which is proportional to the detected variation of the current. A control circuit is connected to each of the sensors for comparing their signals and measuring the time interval between all the signals given by the sensors of a given variation current, and thereby determining where is located the fault or partial discharge having caused this given variation of current. The main advantages of this device are that it is not electrically connected to the circuit carrying the main current and it is not influenced by the voltage drop associated with the flow of current on the inside surface of the annular cavities. Moreover, the sensitivity of the device is a function of the number of turns in the pick up coil, and therefore can be very high.
US6414475 provides a fiscal electricity meter for measuring the energy supplied to a load. The load current flows through the primary winding of a transformer and induces an EMF indicative of the current flowing in the secondary winding. The secondary winding comprises a sense coil, arranged to couple more strongly to the primary, and a cancellation coil which have equal and opposite turns area products so as to provide a null response to extraneous magnetic fields. The coils are arranged so that their magnetic axis are co-located and aligned together so

that they also provide a null response to extraneous magnetic fields having a field gradient.
CN202183595 provides a utility model that discloses a rapid monitoring system of a breaker fracture insulating breakdown fault, characterized in that the output end of an Rogowski coil is connected with the two input ends of an operational amplifier A via a breaker normal closed auxiliary switch QF, the light emitting diodes in a first optocoupler IC1 and a second optocoupler IC2 are in head and tail parallel connections, and phototransistors in the first optocoupler IC1 and the second optocoupler IC2 are in parallel connections; the emitter of the phototransistor in the second optocoupler IC2 is grounded via a time relay J, the collector of the phototransistor in the first optocoupler IC1 is connected with a power supply VCC, and the R end of an RS trigger is connected between a button FA and a resistor R2; the S end of the RS trigger is connected between the emitter of the phototransistor in the second optocoupler IC2 and the time relay J via an auxiliary switch J of the time relay, and the output Q end of the RS trigger is connected with a tripping device. By using the rapid monitoring system of the breaker fracture insulating breakdown fault, the breaker fracture insulating breakdown fault can be eliminated rapidly, and the harm brought by using a failure protection long time delay to eliminate the fault is avoided.
CN102323503 relates to a method for detecting inrush current distortion of a transformer based on a Rogowski coil, which belongs to the technical field of relay protection of electric power systems. The method comprises the following steps of: measuring primary side current of the transformer to acquire a differential voltage analog signal; acquiring a corresponding differential voltage digital signal; judging whether abnormal current occurs or not; performing cosine window-added Fourier transform on the differential voltage digital signal to solve effective values of every-time harmonic waves; calculating the degree of distortion, namely judging whether inrush current occurs or not according to the degree of distortion, and the like. By the method, the conditions that incorrect tripping of differential protection during on-Ioad switch-on of the transformer is prevented and exciting current and fault current are strictly distinguished are ensured, a criterion value has extremely high redundancy, and the degree of distortion is accurately and reliably determined.

CN102435827 relates to a Rogowski coil-based method and a Rogowski coil-based device for detecting a direct-current short-circuit current fault. The device comprises a Rogowski coil. A Rogowski coil measurement differential output signal is divided into two paths, wherein one path of the signal is communicated with a high-pass filtering unit, a first voltage comparison unit, a logic gate circuit and a circuit time delay unit in sequence; the other path of the signal is communicated with a low-pass filtering unit and a second voltage comparison unit in sequence; and a Rogowski coil measurement integration output signal is communicated with a third voltage comparison unit. The Rogowski coil-based method and the Rogowski coil-based device for detecting the direct-current short-circuit current fault are based on a protecting strategy of combining two criterions of a short-circuit current amplitude and a short-circuit current rate of rise, and under such a manner, the action reliability of a protecting device is improved, good electromagnetic compatibility can still be kept under the condition of stronger electromagnetic interference, the false tripping possibility of a tripper is reduced to a maximum degree, and simultaneously, the device can be ensured to still accurately and normally work under the condition of stronger electromagnetic interference. Simultaneously, the Rogowski coil-based method and the Rogowski coil-based device for detecting the direct-current short-circuit current fault are capable of well reducing the impact on a system under the short-circuit condition and relieving the load of a break.
Conventional earth fault detection involves:
• Use of a Core balance current transformer (CBCT) for detection.
• It uses a CBCT, amplifier circuit for detection. A CBCT is used in addition to the three current transformer used for current measurement of individual phases.
• Amplifier uses a dual power supply to amplify the CBCT output or uses a bridge rectifier to convert it into a pulsating dc, which is read by the controller.
The prior art provides for different ground fault detecting method which involves complex mechanism and lacks the efficiency to detect the ground loops. The cost

of constructing the instruments is also very high. There is therefore a need for a device that detect earth fault current in a more accurate way that overcome the drawbacks in conventional core balance current transformer.
In contrast, the present invention involves use of an air core sensor preferably rogowski for earth fault detection. CBCT, based on iron core, has a tendency to saturate (higher input currents, short circuit conditions, higher harmonics etc), which leads to erratic results. Rogowski, which is based on air core, does not saturate. Further, in the present invention a single CBCT is replaced by three rogowski coil. The same rogowski is used for current measurement as well as earth fault current calculation. Thus, a direct reduction of the use of CBCT helps to reduce the cost of the entire unit. Also the entire inventory of CBCT is eliminated.
Additionally, present invention uses a single power supply for the active two stage integrator. By providing a dc-offset, it takes care of the signal to be between Vcc and ground. Thus the summation amplifier output can be read by the microcontroller, which will subtract the known offset from the signal to get the required output. This leads to elimination of dual power supply and also rectifier, in case when we use single power supply. The present design is therefore optimized for a low cost solution.
OBJECT OF THE INVENTION
A primary object of the invention is to overcome the drawbacks/ disadvantages of the prior art.
Another object of the invention is to provide a method for computing earth fault detection using air core signal, particularly to circuits using rogowski as current sensor.
Another object of the present invention is to provide for earth fault current computation using air core signals rather than conventional core balance current transformer achieved through analog circuitry.

Another object of the present invention is to implement same rogowski for current measurement as well as earth fault current calculation.
Yet other object of the present invention is to provide a mechanism that is optimized for a low cost solution.
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 accordance with one aspect of the present invention there is provided an improved apparatus for earth fault detection thereby protecting circuit breakers, said apparatus comprising a plurality of air core sensors for detecting signals; a plurality of integrator means connected to said air core sensors; a power supply means for providing a dc-offset thereby level shifting to said signals; a summation amplifier means for summing output of said integrator means of said individual phases; a microcontroller means whereby output of said summation amplifier is fed; wherein said dc offset provided using said power supply means such that said signal is super imposed on said dc offset; wherein said voltage fed to said microcontroller configured for detecting dc offset and subtracting the known dc offset from said signal thereby providing for an earth fault current.
Other aspect of the present invention provides a method for earth fault detection thereby protecting circuit breakers, said method comprising the steps of detecting signals using a plurality of air core sensors; integrating said signals using a plurality of integrators; providing a dc offset using a power supply means during said integration for level shifting of said signal; summing the level shifted signal with other phases using a summation amplifier; said voltage read by a microcontroller for detecting dc offset and subtracting the known dc offset from said signal to calculate earth fault current.

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 illustrates a block diagram representation of the electronic trip unit.
Figure 2 illustrates a circuit diagram of the trip unit
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 INVENTION
Accordingly, the present invention relates to earth fault detection circuits using an air core sensor, and more particularly to circuits using rogowski as a current sensor.
FIG.1 is a block diagram representation of the electronic trip unit, consisting of an air core sensor, 111, rogowski, which is a device that is used for measuring

alternate current or high speed current pulses. 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 111,112, 113,114 respectively is fed to the two stage active low pass filter, 115,116,117,118 respectively., whose functionality is similar to that of an integrator.
The integrator is used because the measured voltage is proportional to the first time derivative of the primary current. In order to obtain the equivalent value of current that is proportional to the primary current, the rogowski output voltage is integrated. The cut off frequency of two stage active filter, 115, is kept as low as possible. The integrator, 115, offers large impedance to high frequency signal and hence filters out the external noises that are high frequency in nature.
The active integrator which is also an active filter is deployed using an operational amplifier. Apart from the previous mentioned function, the operational amplifier also acts an amplifier with the help of gain setting resistors, the paraphernalia of the op-amp. The op-amp also introduces a dc offset. The sinusoidal signal is now super imposed on the dc offset. The output of the analog active filter of R, Y, B and N phases are summed using an operational amplifier 119, that acts as an summation amplifier. A DC offset is again provided in order to accommodate the entire signal within Vcc and GND, a format that is readable by the ADC of the controller, 120. The output of the summation amplifier, 119, is proportional to the vector sum of the individual phase voltages which are in turn proportional to the individual phase currents. Hence the vector sum is crystallized using the summation amplifier. 119. The microcontroller which reads the output pin of the

summation amplifier, subtracts the known dc offset from the signal and calculates the earth fault current.
The air core sensor,111, two stage integrator, 115, and summation amplifier, 119 are best seen by referring to FIG.2, which depicts a schematic illustrative of an embodiment of a two stage low pass active filter 211, summation amplifier 212, 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 FIG.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.
In the given schematic of FIG. 2, the two stage low pass active filter 211 has a -3db point at 15Hz with a -40db/decade roll-off. The two stage low pass active filter 211 provides a steep slope, hence faster response to the input signal, with a cut-off frequency of 15Hz.
The two stage low pass active filter 211, the operational amplifier gain stage is defined by using the gain setting resistor R65 and the feedback resistor R72 represented by,
Av = - (R72/R65)
The output of the two stage low pass active filter is coupled with other phases through a resistor and summed using an operational amplifier 212. The output of the summation amplifier is directly proportional to the earth fauft current in the system. This output voltage is fed as input to the ADC of the micro controller, 120. The micro controller reads this value and checks whether the value exceeds the set pick up and persist more than set duration. If so, the micro controller issues trip command to the trip mechanism, 121.
As disclosed, the invention mentioned here within, may include some of the following advantages: the signal entering the ADC is made free from high frequency as well as low frequency noise with the help of the two stage low pass active filter 211, the response is reached at a faster rate due to the use of a two

stage active filter which provides us with a roll-off of -40db/decade, the signal is integrated with the help of two stage low pass active filter. The summation amplifier helps to perform the vector sum of individual phases, the output of which is directly proportional to the earth fault current in the system.
Therefore, in the present invention air core signals are first integrated using a two stage active integrator. During integration, a dc offset is provided so that the signal is level shifted. The level shifted sinusoidal signal is then summed with other phases, using a summation amplifier to calculate the earth fault current. This arrangement of the analog circuitry to achieve earth fault detection is therefore unique.
In the present invention, integration is carried out using a two stage integrator and a dc offset is introduced in such a way that the sinusoidal signal is super imposed on the dc offset. Then the sinusoidal signal with dc offset of individual phase is summed with other phases, using a summation amplifier to get an output, which is proportional to the earth fault current present in the system. This voltage is read by the microcontroller which understands the presence of dc offset and it subtracts the known dc offset from the input signal to get the required earth fault current present in the system.
Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM:
1. An improved apparatus for earth fault detection thereby protecting circuit
breakers, said apparatus comprising:
a plurality of air core sensors for detecting signals;
a plurality of integrator means connected to said air core sensors;
a power supply means for providing a dc-offset thereby level shifting to
said signals;
a summation amplifier means for summing output of said integrator
means of said individual phases;
a microcontroller means whereby output of said summation amplifier is
fed;
wherein said dc offset provided using said power supply means such that
said signal is super imposed on said dc offset;
wherein said voltage fed to said microcontroller configured for detecting
dc offset and subtracting the known dc offset from said signal thereby
providing for an earth fault current.
2. Apparatus as claimed in claim 1 wherein said air core sensors are optionally rogowski for earth fault detection.
3. Apparatus as claimed in claim 1 wherein said integrator means is a two stage active low pass filter.
4. Apparatus as claimed in claim 1 to 3 wherein cut off frequency of said two stage active low pass filter is kept substantially low.
5. A method for earth fault detection thereby protecting circuit breakers, said method comprising the steps of:
detecting signals using a plurality of air core sensors;
integrating said signals using a plurality of integrators;
providing a dc offset using a power supply means during said integration
for level shifting of said signal;

summing the level shifted signal with other phases using a summation
amplifier;
said voltage read by a microcontroller for detecting dc offset and
subtracting the known dc offset from said signal to cafcufate earth fault
current.
6. An improved apparatus for earth fault detection thereby protecting circuit breakers as herein described and illustrated with reference to accompanying drawings.
7. A method for earth fault detection thereby protecting circuit breakers as herein described and illustrated with reference to accompanying drawings.