FORM 2
THE PATENT ACT 1970
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION:
Zone Selective Interlocking System for Circuit Breakers
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(c) ADDRESS: LARSEN & TOUBRO LIMITED,
L&T House, Ballard Estate, P. 0. Box: 278, Mumbai 400 001, India
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

Zone Selective Interlocking System for Circuit Breakers
Field of the invention
The present invention relates generally to zone selective interlocking system for circuit breakers, and more particularly, to the zone selective interlocking system for circuit breakers through electrical power line communication method.
Background of the invention
Zone selective interlocking (ZSI) of circuit breakers is a means by which two or more selectively coordinated electronic trip devices can communicate with each other and alter their preset tripping mode to provide faster response time for upstream fault conditions. This allows a nearest upstream breaker from the faulted zone to clear the circuit in the shortest time possible and reduce system damage level potential.
Conventionally, the zone selective interlocking is achieved by using shielded or twisted pair cables or using programmable logic controllers. However, both of these methods use a wired communication method which requires extensive cabling in a substation or factory.
Specifically, the zones can coordinate their tripping in the instance of a fault. The ZSI feature is useful in case of faults such as Short Circuit or Ground Fault. When a short circuit or ground fault occurs on a feeder, the nearest upstream circuit breaker senses the fault and is supposed to disconnect the affected feeder. However, when the nearest upstream circuit breaker fails to detect the fault or issue a trip command, the next nearest upstream breaker issues a trip. But there can occur a mismatch between the settings of the two circuit breakers, which might result in the trip of the higher level circuit breaker resulting in disconnection of even the healthy feeder. Accordingly, to prevent the unwanted

trip and disconnection of healthy feeder, the zone selective interlocking (ZSI) is required so as to coordinate the communication of the circuit breakers.
The zone selective interlocking (ZSI), causes a downstream circuit breaker to detect fault on the feeder, and a restraining signal is issued to the upstream circuit breaker to delay tripping thereof. But when the downstream circuit breaker is unable to detect the fault, the upstream circuit breaker disconnects the feeders. Specifically, limited interlocking can be provided by time discrimination using the settings of the trip units. To reduce the let through energy, the zone selective interlocking (ZSI) is beneficial.
Figure 1 shows a block diagram of the zone selective interlocking system for the circuit breakers in accordance with the prior art. The system is explains by considering three circuit breakers CB1, CB2 and CB3. All the three circuit breakers have electronic trip units as Trip Unit 1, Trip Unit 2 and Trip Unit 3. Here, CB1 and CB 2 are installed at zone 1 (as downstream circuit breakers) and CB3 is installed at zone 2 (as upstream circuit breaker). If fault such as short circuit or earth fault occurs at point A, CB1, which is nearest to the fault, detects the fault. CB1 detects this fault with the help of Electronic Trip Unit 1 fitted inside it. This electronic trip unit has microcontroller based circuitry which helps to detect the fault. This fault signal is communicated to ZSI of CB1 by Trip Unit 1. This ZSI of Trip Unit 1 issues a restraining command to the ZSI connected in the Trip Unit 3 of CB3 to prevent it from tripping. CB1 now clears the fault. In case CB1 does not clear the fault, CB3 will clear the fault.
In US patent No. 7570471B2 which is for circuit breaker trip unit with zone selective interlock and system monitoring, external wiring is used for sending ZSI signals as shown in figure 1. This increases cost of external wires and complexity of the design.

Another US Patent No. 8280653B2 discloses a zone selective interlocking test method and apparatus and circuit interrupter apparatus and power distribution system including the same as shown in figure 2. The '653 patent further includes the external wiring for transferring ZS1 signals.
However, all the zone selective interlocking (ZS1) units of each circuit breaker of the prior art are connected by using wired network. This involves complexity of the wired network and also the cost of the wires since these should be the shielded or twisted pair cables.
Accordingly, there exist a need to provide the zone selective interlocking (ZSI) for circuit breakers which overcomes drawbacks of the prior art.
Objects of the invention
An object of invention is to provide a zone selective interlocking system for circuit breakers using power line communication through existing electrical power lines, in order to reduce complexity of the external wired network in providing the zone selective interlocking (ZSI) for circuit breakers and to reduce cost of the zone selective interlocking (ZSI).
Summary of the invention
Accordingly, the present invention provides a zone selective interlocking system for circuit breakers using electrical power lines. The zone selective interlocking system comprises one or more first circuit breakers configured downstream to a feeder. Each of the first circuit breakers includes an electronic trip unit with a Power Line Communication (PLC) transceiver configured therein. The zone selective interlocking system further comprises one or more second circuit breakers configured upstream to the feeder. Each of the second circuit breakers includes an electronic trip unit with a Power Line Communication (PLC)

transceiver configured therein. Upon occurrence of a fault such as short circuit or ground fault, a first circuit breaker from downstream which is nearest to the fault detects the fault, and the electronic trip unit of this circuit breaker communicates a signal to the power line communication (PLC) transceiver section of the said first circuit breaker from downstream. This PLC transceiver of the said first circuit breaker from downstream nearest to the fault in turn sends a restraining signal to PLC transceivers of the other circuit breakers located upstream the feeder, using the electrical power lines and, also trips the said first circuit breaker from downstream. If the said first circuit breaker from downstream nearest to the fault does not trip then the immediate upstream breaker trips.
Brief description of the drawings
Figure 1, 2 and 3 shows a block diagram zone selective interlocking of circuit breakers of the prior art; and
Figure 4 shows a zone selective interlocking system for circuit breakers using power line communication, in accordance with the present invention.
Detailed description of the invention
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
The present invention provides a zone selective interlocking (ZSr) system for circuit breakers using electrical power lines. Specifically, using power line communication, a ZSI signal is transferred on an existing power grid hence minimizing the wired network. The power line communication transfers the

signal using high frequency (that is of the order of Kilohertz) hence it does not interfere with the circuit breakers' existing signals to maintain the synchronization between upstream and downstream circuit breakers.
Referring now to figure 4, a block diagram of a zone selective interlocking (ZSI) system (100) (hereinafter "the system (100)) for circuit breakers using electrical power lines, as per the present invention, is shown. The system (100) includes a one or more of first circuit breaker(s) (20) and a one or more of second circuit breaker(s) (40).
Each circuit breaker of the one or more of first circuit breakers (20) includes an electronic trip unit (12) with a Power line Communication (PLC) transceiver (hereafter referred as PLC) (14) configured therein. Further, each circuit breaker of the one or more of second circuit breakers (40) includes an electronic trip unit (32) with a PLC transceiver (34) configured therein.
For the purpose of explanation, the system (100) is shown to includes two circuit breakers as a representative circuit breakers of the one or more of first circuit breakers (20), for example the circuit breakerl (20a) ((hereinafter 'CB1 (20a)') and the circuit breaker 2 (20b) (hereinafter ' CB2 (20b)'). The CBI (20a) includes an electronic trip unit 1 (12a) (hereinafter 'the ETU1 (12a)') and the CB2 (20b) includes an electronic trip unit 2 (12b) (hereinafter 'the ETU2 (12b)'). Further, the ETU1(12a) includes a PLC transceiver 14a) and the ETU2(12b) includes a PLC programmable logic controller transceiver (14b) configured therein. Specifically, the CB1(20a) and the CB2 (20b) are installed at zone 1 i.e. downstream to the feeder (not shown).
Further, the system (100) is shown to includes one circuit breakers as a representative circuit breaker for the one or more of second circuit breaker (40) for the purpose of explanation, for example, the circuit breaker 3 (40a) ((hereinafter 'the CB3 (40a)'). The CB3 (40a) includes an electronic trip unit 3

(32a) (hereinafter 'the ETU3 (32a)'). Further, the ETU1 (32a) includes a power line communication (PLC) transceiver (34a). Specifically, the CB3 (40a) is installed at zone 2 i.e. upstream to the feeder (not shown).
Specifically, if fault occurs at point A of zone 1 as shown in figure 4, the CB1 (20a) which is nearest to the fault, detects the fault. The fault is categorized as either short circuit or earth fault. The fault is detected by the ETUI (12a) which is fitted inside the CB1 (20a). The ETU1 (12a) communicates a signal to the programmable logic controller (PLC) transceiver (14a). The programmable logic controller transceiver (PLC) (14a) then sends a restraining signal using the electrical power lines (not shown) present. In an embodiment, the restraining signal is sent using high frequency carrier signals. Thereafter, the circuit breaker configured upstream, which is away from the fault, for example , the CB3 (40a) receives the restraining signal through the programmable logic controller (PLC) transceiver (34a) thereof and waits for the CB1 (20a) to trip. The CB1 (20a) trips and the faulty feeder is disconnected from the circuit.
Advantages of the invention
1. The system (100) reduces wiring complexity, thereby minimizing the cost of existing system.
2. The system (100) requires minimum hardware since the PLC is mounted on board to electronic trip unit circuit.
3. The PLC ZSI of the system (100) has minimum let through energy, thereby improving the circuit breakers life.
4. In the system (100), since higher frequency signal is used for PLC, the system (100) does not interfere with the circuit breaker signal hence error free system.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms

disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient. but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

We Claim
1. A zone selective interlocking system for circuit breakers using
electrical power lines, the zone selective interlocking system comprising:
one or more first circuit breakers configured downstream to a feeder, each of the first circuit breakers having an electronic trip unit with a power line communication transceiver (PLC transceiver) configured therein; and
one or more second circuit breakers configured upstream to the feeder, each of the second circuit breakers having an electronic trip unit with a PLC transceiver configured therein;
wherein, upon occurrence of a fault a first circuit breaker which is nearest to the fault location, detects the fault, and the electronic trip unit of that circuit breaker communicates a signal to the PLC transceiver thereof, which in turn sends a restraining signal using the electrical power lines to all of the one or more second circuit breakers, through the PLC configured in the electronic trip unit thereof, and wherein any of the second circuit breakers does not trip unless and until the first circuit breaker nearest to the fault fails to trip and disconnect the feeder from the circuit.
2. The zone selective interlocking system as claimed in claim 1,
wherein the electrical power lines send a restraining signal using carrier signals
having frequency in the range of 100 to 150 kilohertz.