FORM2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. TITLE OF THE INVENTION: SYSTEM AND METHOD TO PREVENT NUISANCE
TRIPPING OF RELAY THROUGH SECOND HARMONIC BLOCKING PROTECTION
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:

TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to a system and method to prevent nuisance tripping of relay under inrush conditions and more particularly, to the second harmonic blocking protection once the inrush condition is detected and thus blocking all overcurrent protection elements.
BACKGROUND AND THE PRIOR ART
Presently there are a number of mechanisms available for the detection of the inrush current. This is a critical protection feature present in the relays of all the competitors in the market. In a power system, protection relay plays an important role for proper and uninterrupted functioning of the healthy section of the system. The main function of the protection relay is detection and tripping/blocking of the breaker, depending upon the application, in order to isolate the faulty section of the system from the healthy part.
Ep 2 110 935 A2 titled "DC/DC Converter1 comprises the first and second switches SW1, SW2 arranged in series between the power source Vdd and the ground Vss, and the switch control unit 1 which controls the on- off operation of the switches SW1. SW2. When the signal Scl indicates that the output node potential Vnd goes below the first reference potential Vrl which is the reference to detect the occurrence of the inrush current while the first witch SW1 is in the ON state, the control circuit 10 turns off the first switch SW1. Thus, the detection of the inrush current is conducted by making use of a voltage drop due to the on resistance of the first switch SW1. so that it is unnecessary to provide a resistance element for detecting the inrush current.
US 6,483,680 Bl titled ''Magnetizing inrush restraint method and relay for protection of power transformers" provides a method and relay for magnetizing inrush restraint in protective relays for power transformers. The method utilizes the phase angle relation between the fundamental component and the second harmonic in addition to the magnitude ratio. This is accomplished by calculating and utilizing the complex second harmonic ratio. The specially shaped operating characteristic for the complex second harmonic ratio is made dynamic providing for better security without jeopardizing dependability of the transformer protective relay.

US 7,626,794 B2 titled "Systems, methods, and apparatus for indicating faults within a power circuit utilizing dynamically modified inrush restraint" comprises a faulted circuit indicator comprises a processor, which dynamically determines a conductor de-energization level based on a monitored average current. This dynamically- determined de-energization level and/or monitored average current may be used to provide dynamic inrush restraint and/or dynamic backfeed restraint (reset restraint). If current within a power conductor falls below the determined de-energization level, the processor determines that the conductor has de-energized, and accordingly ignores high current spikes less than a predetermined duration that occur during re-energization.
US 7,738,221 B2 titled "Transformer inrush current detector" provides a differential protection system for power transformers using Rogowski coils as current sensors can support an inrush current detection method based on sensing lows in the derivative of the sensed current. Effective detection of power transformer inrush conditions can enable blocking of a protection relay during inrush where the differential current may exceed a differential threshold value indicative of a fault without the presence of an actual fault. The outputs of the Rogowski coils, being proportional to the first time derivative of the sensed current, may be useful in the inrush detction method. Also, with reduced saturation concerns, the Rogowski coil protection system may employ a single slope response with increased sensitivity. A discrete time sampling technique for identifying low di/dt portions within the sensed current also may be useful in detecting power transformer inrush conditions.
US 20090147412 Al titled "Transformer inrush current detector" provides for a differential protection system for power transformers using Rogowski coils as current sensors can support an inrush current detection method based on sensing lows in the derivative of the sensed current. Effective detection of power transformer inrush conditions can enable blocking of a protection relay during inrush where the differential current may exceed a differential threshold value indicative of a fault without the presence of an actual fault. The outputs of the Rogowski coils. being proportional to the first time derivative of the sensed current, may be useful in the inrush detction method. Also, with reduced saturation concerns, the Rogowski coil protection system

may employ a single slope response with increased sensitivity. A discrete time sampling technique for identifying low di/dt portions within the sensed current also may be useful in detecting power transformer inrush conditions.
WO 2009073407 Al titled "Transformer inrush current detector'' provides for a differential protection system for power transformers using Rogowski coils as current sensors can support an inrush current detection method based on sensing lows in the derivative of the sensed current. Effective detection of power transformer inrush conditions can enable blocking of a protection relay during inrush where the differential current may exceed a differential threshold value indicative of a fault without the presence of an actual fault. The outputs of the Rogowski coils, being proportional to the first time derivative of the sensed current, may be useful in the inrush detection method. Also, with reduced saturation concerns, the Rogowski coil protection system may employ a single slope response with increased sensitivity. A discrete time sampling technique for identifying low di/dt portions within the sensed current also may be useful in detecting power transformer inrush conditions.
Although readily available the prior art fails to provide an integrated solution combining arrangement for detection of the inrush current on one hand and phase overcurrent., earth fault. sensitive earth fault(SEF) & negative sequence overcurrent on the other hand. Also, the prior art fails to differentiate between the general fault conditions and inrush current which too may be above the pickup level of the overcurrent elements and cause them to trip. Consequently the inrush current though not a desired condition for tripping still leads to tripping of the overcurrent elements.
Therefore, there is a need to develop a system and method to prevent nuisance tripping of relay under inrush conditions through the second harmonic blocking protection once the inrush condition is detected and thus blocking all overcurrent protection elements.

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 prevent nuisance tripping of relay under inrush conditions.
Another object of the present invention is to provide for the second harmonic blocking protection.
Another object of the present invention is to provide for blocking of all overcurrent protection elements.
Another object of the present invention is to detect the inrush condition.
Another object of the present invention is to utilize the second harmonic content for detecting the inrush condition.
Another object of the present invention is to provide an integrated solution combining arrangement for detection of the inrush current on one hand and phase overcurrent. earth fault, sensitive earth fault(SEF) & negative sequence overcurrent on the other hand.
Another object of the present invention is to ensure proper and uninterrupted functioning of the healthy section of the system.
Another object of the present invention is to provide HMI settings and the firmware logic to be incorporated in Mew Feeder Protection relay.
Yet another object of the present invention is to apply the same invention as a firmware built in the new feeder protection relay.

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 OF THE INVENTION
There is provided a system and method to prevent nuisance tripping of relay under inrush conditions. According to one embodiment of the present invention, there is provided a system for the second harmonic blocking protection.
Other embodiment of the present invention provides for blocking of all overcurrent protection elements.
Other embodiment of the present invention provide for detecting the inrush condition.
Other embodiment of the present invention provides for utilizing the second harmonic content for detecting the inrush condition.
Other embodiment of the present invention provides an integrated solution combining arrangement for detection of the inrush current on one hand and phase overcurrent, earth fault, sensitive earth fault(SEF) & negative sequence overcurrent on the other hand.
Other embodiment of the present invention further provides for ensuring proper and uninterrupted functioning of the healthy section of the system.
Yet other embodiment of the present invention provides for HM1 settings and the firmware logic to be incorporated in New Feeder Protection relay.
In addition, the circuit may be used in a numerical relay as a part of LV/MV switchboards. The arrangement given in the present invention can be used as a firmware built in the new feeder protection relay.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1 illustrates the flow of the inputs /commands to the corresponding hardware of the numerical relay.
Fig 2 illustrates the second harmonic protection.
DETAILED 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.
Reference is first invited to Fig 1 where the HMI setting and the flow of the inputs/commands to the corresponding hardware of the numerical relay are shown. The protection algorithm logic is installed in the CPU controller, in a coded form which is embedded inside the relay.
Fig 2 shows the application space and concept of second harmonic blocking and illustrates the status of the second harmonic blocking feature, fundamental component of three phase current and second harmonic component of three phase current.
The invented system is thus an electronic circuit arrangement for combining both the active and passive residual current device (RCD) in a single unit with a selector switch.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention provides a system and method to prevent nuisance tripping of relay under inrush conditions through the second harmonic blocking protection once the inrush condition is delected and thus blocking all overcurrent protection elements.

In a power system, protection relay plays an important role for proper and uninterrupted functioning of the healthy section of the system. The main objective of the protection relay is detection and tripping/blocking of the breaker, depending upon the application, in order LO isolate the faulty section of the system from the healthy part.
Figure 1 depicts the flow of the inputs /commands to the corresponding hardware of the numerical relay. There are numerous components in terms of embedded systems (hardware) in the numerical relay of which Data acquisition & metering and CPU controller are the two main sub sections of the relay. Initially the current transformer secondary inputs i.e. phase currents which are analog inputs are fed to the Analog/Digital (A/D) converter located inside the relay. The A/D converter captures the current analog samples per cycle and converts into digitized form. 64 samples / cycle are captured by the A/D converter. These digitized samples from A/D converter go into the Data acquisition and metering section of the numerical relay and further to the protection algorithm logic.
The protection algorithm logic is installed in the CPU controller, in a coded form, which is embedded inside the relay. Through Data acquisition and metering section, the current inputs advance to CPU controller where the protection algorithm logic is processed.
In second harmonic blocking protection, as soon as this feature picks up, depending upon the conditions to be met, the overcurrent elements are blocked by the flex logic operand.
The second harmonic feature is required in FPR for feeders connected to transformers. Whenever a transformer connected to a feeder is energized, it draws a large initial magnetizing current which may be several times the rated current of a transformer. This current is known as the magnetizing inrush current. Due to the heavy magnitude of the inrush current, it may be above the pickup level of the overcurrent elements and cause them to trip. However, this is not a fault condition and hence not a desired condition for tripping. So once the inrush condition is detected. all the overcurrent elements are blocked.

The harmonics content in the inrush current are different from those of a normal fault current. The dc offset range varies from 40 to 60%, the second harmonic content varies from 15 to 70% and the third harmonic content is from 10 to 30%.Other harmonics are progressively less. The third harmonic and its multiples do not appear in the CT leads as these harmonics circulate in the delta winding of the transformer and the delta connected CTs connected on the star winding of the transformer. Hence the second harmonic content is utilized for detecting the inrush condition. The second harmonic level may not be the same for all phases during inrush and therefore phase inrush blocking is being used where blocking signal will be sent if any one of the phases detects second harmonic content above the threshold value. The ratio of the 2nd harmonic current/ fundamental current is being used for detection.
Scttings:-
Thc following settings are required on the HMI:-
Second harmonic blocking feature:-Enabled/Disabled
Second harmonic ratio setting:- 10% to 75% in steps of l%.(Default=20%)
Elements to be blocked:-
Ail overcurrent protection elements (Phase. Earth. SEP. Negative sequence overcurrcnt)
Inputs
Status of the second harmonic blocking feature (enabled/disabled) Fundamental component of the Three Phase currents (la, lb, Ic) Second harmonic component of the Three Phase currents (la, lb, Ic)
Outputs
The blocking signal becomes high / low depending on the gate logic output.
Abbreviations used:
CPU: central processing unit A/D: analog to digital

FPR: feeder protection relay
CT: current transformer
HM1: human machine interlace
SEP: sensitive earth fault
Ta(2nd): second h armonic component of la
lb (2nd): second harmonic component of lb
Ic (2nd): second harmonic component of Ic
la(fund): fundamental component of la
Ib(fund): fundamental component of Tb
Ic(fund): fundamental component of Ic
To suppress the magnetizing inrush current occurring when supplying power of three phases of the transformer are performed simultaneously using three single- phase circuit breakers or a non-phase segregated operation- type circuit breaker, without providing a circuit breaker with a resistor or other equipment. A magnetizing inrush current suppression method for transformer suppresses a magnetizing inrush current occurring at the start of energizing of a three- phase transformer, when a three- phase power supply is input to a terminal of each phase by means of a three- phase circuit breaker. In the method, by integrating phase voltages or line- to- line voltages on the primary side or the secondary side or the tertiary side when three- phase AC voltages are applied in a steady state to the transformer, steady- state magnetic flux for each phase of the transformer is calculated, and the polarity and magnitude of the residual magnetic flux of each phase of the transformer after the circuit breaker shuts off the transformer are calculated, and the three- phase circuit breaker is caused to close simultaneously in a region in which three phases overlap, each of the three phases having the polarity of the steady- state magnetic flux equal to the polarity of the residual magnetic flux for each phase of the transformer.
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. A system to prevent nuisance tripping of relay by second harmonic blocking protection
using a numerical relay, said system comprising:
a controller means loaded with a protection algorithm logic,
an analog/digital converter for capturing current analog samples per cycle and converting into digitized form;
a data acquisition and metering section taking said digitized inputs from said analog/digital converter
wherein said controller means adapted to check whether ratio of 2" harmonic value and fundamental phase current value from Data Acquisition & metering exceeds the threshold value and the 2" harmonic blocking is enabled through controller (ON/OFF); wherein said controller updating a flag value inside controller register for respective phase, which acts as a blocking signal for respective phase whereby said blocking signals of each phase after getting simulated through a digital OR gate logic generates a final blocking signal in case of 2nd harmonic occurrence in any phase greater than the set value when the function is enabled.
2. System as claimed in claim 1 wherein said controller means is embedded with said numerical relay.
3. System as claimed in claim 1 wherein said controller means is adapted to block the over-current elements on receiving abnormal current signals.
4. A method to prevent nuisance tripping of relay through the second harmonic blocking protection using a numerical relay comprising the steps of:
capturing current analog samples per cycle and converting into digitized form using an analog/digital converter; said digitized samples from said analog/digital converter fed to a data acquisition and metering section; output from said data acquisition and metering section further fed to a CPU controller where the protection algorithm logic is processed:

said CPU controller further generating the flex logic operand to block the over-current elements.
5. Method as claimed in claim 4 wherein said CPU controller during inrush on detecting different second harmonic level use phase inrush blocking.
6. Method as claimed in claim 5 wherein said inrush blocking is used on detecting second harmonic content above the threshold value in any one of the phase.
7. A method as claimed in claim 4 wherein the ratio of a second harmonic current and fundamental current is used by said CPU controllers for over-current detection.
8. A system to prevent nuisance tripping of relay through the second harmonic blocking protection using a numerical relay as herein described and illustrated with reference to the accompanying drawings.
9. A method to prevent nuisance tripping of relay through the second harmonic blocking protection using a numerical relay as herein described and illustrated with reference to the accompanying drawings.