1. PROTECTION scheme for bus bar / feeder
PRESENTED BY:
MR. G.P. SINGH
H.O.D., ELECTRICAL ENGG.
GOVT. POLY. COLLEGE,
AMRITSAR
Contact no:

2. Introduction
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

4. Differential Protection Scheme
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.

5. Differential Protection Scheme cont…….
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.

6. Protection Requirements
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.
Protection of power system busbars is one of the most critical relaying applications. Busbars are areas in power systems where fault current levels may be very high. In spite of that, some of the circuits connected to the bus may have their Current Transformers (CTs) insufficiently rated. This creates a danger of significant CT saturation and jeopardizes security of the busbar protection system.
A false trip of a distribution bus can cause outages to a large number of customers as numerous feeders and/or sub-transmission lines may get disconnected. A false trip of a transmission busbar may drastically change system topology and jeopardize power system stability. Hence, the requirement of a maximum security of busbar protection.
On the other hand, bus faults generate large fault currents. If not cleared promptly, they endanger the entire substation due to both dynamic forces and thermal effects. Hence, the requirement of high-speed operation of busbar protection.
With both security and dependability being very important for busbar protection, the preference is always given to security.

7. Bus Protection Techniques
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
Power system busbars vary significantly as to the size (number of circuits connected), complexity (number of sections, tie-breakers, disconnectors, etc.) and voltage level (transmission, distribution).
The above technical aspects combined with economic factors yield a number of solutions for busbar protection.

8. Interlocking 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.
A simple protection for distribution busbars can be accomplished as an interlocking scheme. Overcurrent (OC) relays are placed on an incoming circuit and at all outgoing feeders. The feeder OCs are set to sense the fault currents on the feeders. The OC on the incoming circuit is set to trip the busbar unless blocked by any of the feeder OC relays. A short coordination timer is typically required to avoid race conditions.
When using microprocessor-based multi-functional relays it becomes possible to integrate all the required OC functions in one or few relays. This allows not only reducing wiring but also shortening the coordination time and speeding-up operation of the scheme.
Modern relays provide for fast peer-to-peer communications using protocols such as the UCA with the GOOSE mechanism. This allows eliminating wiring and sending the blocking signals over the communications.
The scheme although easy to apply and economical is limited to specific (simple) busbar configurations.

9. Over-current (unrestrained) Differential
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.
Typically a differential current is created externally to a current sensor by summation of all the circuit currents. Preferably the CTs should be of the same ratio. If they are not, a matching CT (or several CTs) is needed. This in turn may increase the burden for the main CTs and make the saturation problem even more serious.
Historically, means to deal with the CT saturation problem include definite time or inverse-time overcurrent characteristics.
Although economical and applicable to distribution busbars, this solution does not match performance of more advanced schemes and should not be applied to transmission-level busbars.
The principle, however, is used as a protection function in an integrated microprocessor-based busbar relay. If this is the case, such unrestrained differential element should be set above the maximum spurious differential current and may give a chance to speed up operation on heavy internal faults as compared to a percent (restrained) bus differential element

10. Linear Couplers ZC = 2  – 20  - typical coil impedance
(5V per 1000Amps => 60Hz ) 59
If = 8000 A
40 V
10 V
0 V
20 V
2000 A
4000 A
0 A
External
Fault
A linear coupler (air core mutual reactor) produces its output voltage proportional to the derivative of the input current. Because they are using air cores, linear couplers do not saturate.
During internal faults the sum of the busbar currents, and thus their derivatives, is zero. Based on that, a simple busbar protection is thus achieved by connecting the secondary windings of the linear couplers in series (in order to respond to the sum of the primary currents) and attaching a simple voltage sensor.
Disadvantages of this approach are similar to those of the high-impedance scheme

11. Linear Couplers 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 => 60HZ ),
ZR – RELAY TAP IMPEDANCE
ZC – SUM OF ALL LINEAR COUPLER SELF IMPEDANCES

12. Linear Couplers Internal Bus Fault 59 If = 8000 A 0 A 0 V 10 V 20 V

13. LINEAR COUPLERS 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)

14. HIGH IMPEDANCE DIFFERENTIAL
Operating signal created by connecting all CT secondary's in parallel.
CTs must all have same ratio.
Must have dedicated CTs
Overvoltage 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

15. PERCENT DIFFERENTIAL
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.
Percent differential relays create a restraining signal in addition to the differential signal and apply a percent (restrained) characteristic. The choices of the restraining signal include “sum”, “average” and “maximum” of the bus currents. The choices of the characteristic include typically single-slope and double-slope characteristics.
This low-impedance approach does not require dedicated CTs, can tolerate substantial CT saturation and provides for high-speed tripping.
Many integrated relays perform CT ratio compensation eliminating the need for matching CTs.
This principle became really attractive with the advent of microprocessor-based relays because of the following:
Advanced algorithms supplement the percent differential protection function making the relay very secure.
Protection of re-configurable busbars becomes easier as the dynamic bus replica (bus image) can be accomplished without switching secondary current circuits.
Integrated Breaker Fail (BF) function can provide optimal tripping strategy depending on the actual configuration of a busbar.
Distributed architectures are proposed that place Data Acquisition Units (DAU) in bays and replace current wires by fiber optic communications.

16. Low Impedance Percent Differential
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.

17. Low Impedance Percent Differential
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.

18. Digital Differential Algorithm Goals
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).

19. Low Impedance Differential (Distributed)
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.

20. Low Impedance Differential (Centralized)
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.

21. FEEDER PROTECTION
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

22. TIME GRADED OVER-CURRENT 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

23. TIME-GRADED PROTECTION FOR RADIAL FEEDERS
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.

24. TIME-GRADED PROTECTION FOR RADIAL FEEDERS CONT..
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.

25. TIME-GRADED PROTECTION FOR PARALLEL FEEDERS
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.

27. TIME-GRADED PROTECTION FOR PARALLEL FEEDERS CONT….
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 .

28. TIME GRADED PROTECTION FOR RING-MAINS
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.

29. TIME GRADED PROTECTION FOR RING-MAINS cont……
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.

30. DIFFERENTIAL PROTECTION (TRANSLAY SCHEME)
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.

31. DIFFERENTIAL PROTECTION (TRANSLAY SCHEME) cont…
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.

32. DIFFERENTIAL PROTECTION CONT.
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.

33. DIFFERENTIAL PROTECTION CONT.
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.

34. THANK YOU