BUS-BARS FORM A LINK BETWEEN THE INCOMING AND OUTGOING

CIRCUITS AT THE GENERATING STATIONS OR SUB STATIONS. IF A FAULT

DEVELOPS IN THIS PART OF THE POWER SYSTEM, CONSIDERABLE

DAMAGE AND DISRUPTION OF SUPPLY WILL OCCUR. TO REDUCE THE

EFFECT OF FAULT, VARIOUS BUS-BAR ARRANGEMENTS ARE EMPLOYED.

STILL PROPER PROTECTION SCHEME HAS TO BE ADOPTED TO

IMPROVE THE RELIABILITY OF SUPPLY. ALTHOUGH, THE VARIOUS

SCHEMES HAVE BEEN DEVELOPED FOR THE PROTECTION OF BUS-BARS

BUT THE MOST COMMON SCHEME IS DIFFERENTIAL PROTECTION

THE SCHEMATIC DIAGRAM OF CURRENT DIFFERENTIAL PROTECTION

SCHEME EMPLOYED FOR THE PROTECTION OF SUB- STATION BUS-BARS IS

SHOWN IN PREVIOUS SLIDE.

THE SECONDARY'S OF ALL THE CT’S CONNECTED IN INCOMING &

OUTGOING FEEDERS ARE CONNECTED IN PARALLEL AS BEFORE. THE CTS

DESIGNED IN SUCH A WAY THAT UNDER NORMAL CONDITION , THE EMF’S

INDUCED IN SECONDARY’S OF THE CT’S PLACED ON OUTGOING FEEDER.

THEN NO CURRENT FLOWS THROUGH THE OPERATING COIL OF THE RELAY

WHICH IS CONNECTED ACROSS THE CONNECTING WIRES.

OPEARTION

UNDER NORMAL CONDITIONAL OR EXTERNAL FAULT CONDITIONS, THE SUM

OF THE CURRENT ENTERING THE BUS BAR IS EQUAL TO THE SUM OF CURRENT

LEAVING IT. THEREFORE, NO CURRENT FLOWS THROUGH THE OPERATING COIL.

HOWEVER, WHEN FAULTS OCCURS WITHIN THE PROTECTED ZONE ( BUS- BAR),

THE CURRENT ENTERING THE BUS-BAR WILL NO LONGER BE EQUAL TO THOSE

LEAVING IT. THUS, A DIFFERENTIAL CURRENT FLOWS THROUGH THE OPERATING

COIL OF THE RELAY WHICH CLOSES THE TRIP CIRCUIT.

HIGH BUS FAULT CURRENTS DUE TO LARGE NUMBER OF CIRCUITS CONNECTED:

CT SATURATION OFTEN BECOMES A PROBLEM AS CTS MAY NOT BE

SUFFICIENTLY RATED FOR WORST FAULT CONDITION CASE

LARGE DYNAMIC FORCES ASSOCIATED WITH BUS FAULTS REQUIRE FAST

CLEARING TIMES IN ORDER TO REDUCE EQUIPMENT DAMAGE

FALSE TRIP BY BUS PROTECTION MAY CREATE SERIOUS PROBLEMS:

SERVICE INTERRUPTION TO A LARGE NUMBER OF CIRCUITS

SYSTEM-WIDE STABILITY PROBLEMS

WITH BOTH DEPENDABILITY AND SECURITY IMPORTANT, PREFERENCE IS ALWAYS

GIVEN TO SECURITY.

INTERLOCKING SCHEMES

OVER-CURRENT (“UNRESTRAINED” OR “UNBIASED”) DIFFERENTIAL

OVER-CURRENT PERCENT (“RESTRAINED” OR “BIASED”)

DIFFERENTIAL

LINEAR COUPLERS

HIGH-IMPEDANCE BUS DIFFERENTIAL SCHEMES

LOW-IMPEDANCE BUS DIFFERENTIAL SCHEMES

BLOCKING SCHEME TYPICALLY USED.

SHORT COORDINATION TIME

REQUIRED .

CARE MUST BE TAKEN WITH

POSSIBLE SATURATION OF FEEDER CTS.

BLOCKING SIGNAL COULD BE SENT

OVER COMMUNICATIONS PORTS.

TECHNIQUE IS LIMITED TO SIMPLE

ONE-INCOMER DISTRIBUTION BUSES.

50

50 50 50 50 50

BL

OC

K

DIFFERENTIAL SIGNAL FORMED BY

SUMMATION OF ALL CURRENTS

FEEDING BUS.

CT RATIO MATCHING MAY BE

REQUIRED.

ON EXTERNAL FAULTS, SATURATED

CTS YIELD SPURIOUS DIFFERENTIAL

CURRENT.

TIME DELAY USED TO COPE WITH

CT SATURATION.

51

ZC = 2 – 20 - typical coil impedance

(5V per 1000Amps => 0.005 @ 60Hz )

59

If = 8000 A

40 V 10 V 10 V 0 V 20 V

2000 A

2000 A 4000 A

0 A

0 VExternal

Fault

ESEC= IPRIM*XM - SECONDARY VOLTAGE ON RELAY TERMINALS

IR= IPRIM*XM /(ZR+ZC) – MINIMUM OPERATING CURRENT

WHERE,

IPRIM – PRIMARY CURRENT IN EACH CIRCUIT

XM–LINER COUPLER MUTUAL REACTANCE (5V PER 1000AMPS => 0.005

@ 60HZ ),

ZR – RELAY TAP IMPEDANCE

ZC – SUM OF ALL LINEAR COUPLER SELF IMPEDANCES

59

If = 8000 A

0 A

0 V 10 V 10 V 0 V 20 V

40 V

2000 A

2000 A

4000 A

0 A

Internal BusFault

FAST, SECURE AND PROVEN.

REQUIRE DEDICATED AIR GAP CTS, WHICH MAY NOT BE USED FOR ANY

OTHER PROTECTION.

CANNOT BE EASILY APPLIED TO RECONFIGURABLE BUSES.

THE SCHEME USES A SIMPLE VOLTAGE DETECTOR – IT DOES NOT

PROVIDE BENEFITS OF A MICROPROCESSOR-BASED RELAY .

(E.G. OSCILLOGRAPHY, BREAKER FAILURE PROTECTION, OTHER

FUNCTIONS)

OPERATING SIGNAL CREATED BY CONNECTING ALL CT SECONDARY'S IN PARALLEL.CTS MUST ALL HAVE SAME RATIO.MUST HAVE DEDICATED CTSOVERVOLTAGE ELEMENT OPERATES ON VOLTAGE DEVELOPED ACROSS RESISTOR CONNECTED IN SECONDARY CIRCUIT.REQUIRES VARISTORS OR AC SHORTING RELAYS TO LIMIT ENERGY DURING FAULTS.ACCURACY DEPENDENT ON SECONDARY CIRCUIT RESISTANCE.USUALLY REQUIRES LARGER CT CABLES TO REDUCE ERRORS HIGHER COST

CANNOT EASILY BE APPLIED TO

RECONFIGURABLE BUSES AND OFFERS

NO ADVANCED FUNCTIONALITY

59

PERCENT CHARACTERISTIC USED

TO COPE WITH CT SATURATION AND

OTHER ERRORS.

RESTRAINING SIGNAL CAN BE

FORMED IN A NUMBER OF WAYS.

NO DEDICATED CTS NEEDED.

USED FOR PROTECTION OF RE-

CONFIGURABLE BUSES POSSIBLE.

5187

nDIF IIII ...21

nRES IIII ...21 nRES IIII ...,,,max 21

INDIVIDUAL CURRENTS SAMPLED BY PROTECTION AND SUMMATED

DIGITALLY.

CT RATIO MATCHING DONE INTERNALLY (NO AUXILIARY CTS).

DEDICATED CTS NOT NECESSARY.

ADDITIONAL ALGORITHMS IMPROVE SECURITY OF PERCENT

DIFFERENTIAL CHARACTERISTIC DURING CT SATURATION.

DYNAMIC BUS REPLICA ALLOWS APPLICATION TO RECONFIGURABLE

BUSES.

DONE DIGITALLY WITH LOGIC TO ADD/REMOVE CURRENT INPUTS

FROM DIFFERENTIAL COMPUTATION.

SWITCHING OF CT SECONDARY CIRCUITS NOT REQUIRED.

LOW SECONDARY BURDENS.

ADDITIONAL FUNCTIONALITY AVAILABLE.

DIGITAL OSCILLOGRAPHY AND MONITORING OF EACH CIRCUIT

CONNECTED TO BUS ZONE.

TIME-STAMPED EVENT RECORDING.

BREAKER FAILURE PROTECTION.

IMPROVE THE MAIN DIFFERENTIAL ALGORITHM OPERATION.

A) BETTER FILTERING B) FASTER RESPONSE

C) BETTER RESTRAINT TECHNIQUES D)SWITCHING TRANSIENT BLOCKING

PROVIDE DYNAMIC BUS REPLICA FOR RECONFIGURABLE BUS BARS.

DEPENDABLY DETECT CT SATURATION IN A FAST AND RELIABLE MANNER,

ESPECIALLY FOR EXTERNAL FAULTS.

IMPLEMENT ADDITIONAL SECURITY TO THE MAIN DIFFERENTIAL

ALGORITHM TO PREVENT INCORRECT OPERATION.

EXTERNAL FAULTS WITH CT SATURATION.

CT SECONDARY CIRCUIT TROUBLE (E.G. SHORT CIRCUITS).

DATA ACQUISITION UNITS (DAUS)

INSTALLED IN BAYS.

CPU PROCESSES ALL DATA FROM

DAUS.

COMMUNICATIONS BETWEEN DAUS

AND CPU OVER FIBRE USING

PROPRIETARY PROTOCOL.

SAMPLING SYNCHRONISATION

BETWEEN DAUS IS REQUIRED.

PERCEIVED LESS RELIABLE.

DIFFICULT TO APPLY IN RETROFIT

APP.

5 2

DA U

5 2

DA U

5 2

DA U

CU

co pp er

f i be r

ALL CURRENTS APPLIED TO A

SINGLE CENTRAL PROCESSOR

NO COMMUNICATIONS,

EXTERNAL SAMPLING

SYNCHRONISATION NECESSARY

PERCEIVED MORE RELIABLE (LESS

HARDWARE NEEDED)

WELL SUITED TO BOTH NEW AND

RETROFIT APPLICATIONS.

52 52 52

CU

co pp er

THE CHANCES OF FAULTS OCCURING ON THE FEEDER (TRANSMISSION

LINE) IS MUCH MORE DUE TO THEIR GREAT LENGTH AND EXPOSURE TO

THE ATMOSPHERIC CONDITIONS. THEREFORE, VARIOUS PROTECTION

SCHEMES HAVE BEEN DEVELOPED WHICH MAY BE CLASSIFIED AS:

A) TIME-GRADED OVER CURRENT PROTECTION

B) DIFFERENTIAL PROTECTION

C) DISTANCE PROTECTION

IN TIME GRADED OVER CURRENT PROTECTION SCHEME, THE TIME

SETTING OF RELAY IS SO GRADED THAT IN THE EVENT OF FAULT, THE

SMALLEST POSSIBLE SECTION OF THE SYSTEM POSSIBLE SECTION OF THE

SYSTEM IS ISOLATED. THIS SCHEME IS APPLIED FOR THE PROTECTION OF

(A) RADIAL FEEDERS

(B) PARALLEL FEEDERS

(C) RING MAINS

• THE TIME-GRADED PROTECTION FEEDER IS OBTAINED BY EMPLOYING

INVERSE DEFINITE MINIMUM TIME LAG RELAYS. THE RELAYS ARE SO SET

THAT THE MINIMUM TIME OF OPERATION DECREASE FROM THE POWER

STATION TO THE REMOTE SUB-STATION AS SHOWN IN FIG. IN NEXT

SLIDE.

• THE OPERATING TIME OF INVERSE DEFINITE MINIMUM TIME LAG

RELAYS IS INVERSELY PROPRTIONAL TO THE OPERATING CURRENT, BUT

IS NEVER LESS THAN THE MINIMUM DEFINITE FOR WHICH IT IS SET.

IF A FAULT OCCURS BETWEEN STATION E AND F, IT WILL BE CLEARED IN

0.1 SECOND BY THE RELAY AND CIRCUIT BREAKER OF SUBSTATION E

BECAUSE ALL OTHER RELAYS HAVE HIGHER OPERATING TIME. IF THE

RELAY AT SUB STATION E FAILS TO TRIP, THE RELAY AT D WILL OPERATE

AFTER A TIME DELAY OF 0.5 SECONDS I.E. AFTER 0.6 SECONDS FROM THE

OCCURRENCE OF FAULT.

WHERE CONTINUITY OF SUPPLY IS ABSOLUTELY NECESSARY, TWO

FEEDERS ARE RUN IN PARALLEL. IF A FAULT OCCURS ON ONE FEEDER, THE

SUPPLY CAN BE MAINTAINED FROM THE OTHER FEEDER, DISCONNECTING

THE FAULTY FEEDER. FOLLOWING FIG. SHOWS THE SYSTEM WHERE TWO

FEEDERS ARE CONNECTED IN PARALLEL BETWEEN GENERATING STATION &

SUB-STATION.

AT THE GENERATING STATION, NON-DIRECTIONAL OVER CURRENT

RELAYS ARE CONNECTED WHEREAS DIRECTIONAL OVER CURRENT

INSTANTANEOUS RELAYS ARE CONNECTED AT SUB-STATION END.

IF AN EARTH FAULT OCCURS ON FEEDERS AT POINT F AS SHOWN IN FIG.

THE FAULT IS FED;

(A) DIRECTLY FROM FEEDER 2 VIA RELAY B.

(B) FROM FEEDER I VIA A , P AND SUB-STATION Q AS SHOWN IN FIG. BY THE

DOTTED ARROWS.

THIS CLEARLY SHOWS THAT DIRECTIONAL RELAY P CARRIES THE CURRENT IN

NORMAL DIRECTION WHERE AS DIRECTIONAL RELAY Q CARRIES THE

CURRENT IN REVERSE DIRECTION MOMENTARILY. THIS OPEARATES THE

RELAY Q INTANTANEOUSLY. THE RELAY B HAVING INVERSE TIME

CHARACTERISTICS ALSO OPERATES BECAUSE OF HEAVY FLOW OF CURRENT .

THE SYSTEM IN WHICH VARIOUS POWER STATIONS OR SUB-STATIONS

ARE INTER-CONNECTED BY THE NUMBER OF FEEDERS FORMING A CLOSED

CIRCUIT IS CALLED A RING- MAIN SYSTEM.

IN THIS SYSTEM OF PROTECTION, NON-DIRECTIONAL OVER CURRENT

RELAYS HAVING INVERSE TIME CHARACTERISTIC ARE EMPLOYED. WHEREAS

DIRECTIONAL OR REVERSE POWER ARE EMPLOYED ON BOTH THE SIDES OF

EACH SUBSTATION. THE MINIMUM DEFINITE TIME OF ALL THE RELAY ARE

SET PROPERLY AS SHOWN IN FIG.

WHENEVER THE FAULT OCCURS ON ANY OF THE SECTION ONLY

CORRESPONDING RELAYS WILL OPERATE WITHOUT DISTURBING THE

OTHER RELAYS OF THE NETWORK, THUS, THE FAULTY SECTION IS ISOLATED

AND SUPPLY IS MAINTAIN.

THE TRANSLATION SCHEME IS BASICALLY A VOLTAGE BALANCE

DIFFERENTIAL PROTECTION SCHEME. BUT IN THIS SCHEME, VOLTAGES

INDUCED IN THE SECONDARY WINDINGS WOUND ON THE RELAY

MAGNETS IS COMPARED IN PLACE OF SECONDARY VOLTAGES OF THE

LINE CURRENT TRANSFORMERS.

THE SCHEMATIC DIAGRAM OF A TRANLEY SCHEME FOR THE

PROTECTION OF 3-PHASE FEEDER IS SHOWN IN FIG . ON NEXT SLIDE.

THE RELAYS USED IN THE SCHEME ARE ESSENTIALLY OVERCURRENT

INDUCTION TYPE RELAYS.

THE CENTRAL LIMB OF THE UPPER MAGNET (U.M.) CARRIES A WINDING

(A OR A’) WHICH IS ENERGISED BY THE SUM OF SECONDARY CURRENTS OF

CT’S PLACED ON FEEDER TO BE PROTECTED.

THE CENTRAL LIMBS OF UPPER MAGNET ALSO CARRIES A

SECONDARY WINDING (B OR B’) WHICH IS CONNECTED IN SERIES WITH

THE OPERATING WINDING (C OR C’) PLACED ON THE LOWER MAGNETS

(L.M). IN BETWEEN THE TWO MAGNETS, AN ALUMINIUM DISC D IS

PLACED WHICH IS FREE TO ROTATE. SPINDLE OF DISC CARRIES MOVING

CONTACT WHICH CLOSES TRIP CIRCUIT UNDER FAULT CONDITIONS.

• UNDER NORMAL CONDITIONS, THE CURRENTS AT TWO ENDS OF THE

FEEDER ARE EQUAL SO THAT THE SECONDARY CURRENT IN BOTH SETS

OF CT’S ARE EQUAL. CONSEQUENTLY, THE E.M.F’S INDUCED IN THE

SECONDARY WINDINGS C AND C’ ARE EQUAL AND OPPOSITE AND NO

CURRENT FLOWS THROUGH THE CLOSED CIRCUITED SECONDARIES.

HOWEVER, WHEN FAULT OCCURS ON FEEDER SYSTEM SAY AT POINT F

THE VOLTAGE INDUCED IN C AND C’ WILL NO LONGER REMAIN EQUAL.

THEREFORE, CURRENT FLOWS THROUGH THIS WINDING AND TORQUE

IS DEVEOLPED IN THE DISC.