also used for protection of transmission lines and feeders with slight; The modifications are required to cope with the protection problems greater length of lines. Further differential protection can ea used, Bit itb expensive for longer lines due to greater length of pilot wires required. The protection of bus-bars is very important since they form a vital part of the: system. Necessity : Bus bar protection is needed for the following reasons : (a) In case of fault on a bus bar, the fault current and power may be very large ani cause serious damage to the system. (©) Faulton bus bars may affect continuity of power supply and cause d of power to a large portion of the system (c) The stability of the system is affected by fault in bus zone. In the case of large Power systems protection faulty sections are of bus bar sections is necessary $0 solated before the fault Spreads over the whole system. For the purpose of protectiion, tof causes for bus-zone faults. too (a) Failure of support insulator resulting in earth fault less {b) Flash over across support insulator duting over voltages tl he (c) Heavily polluted insulator causing flash over (a) Failure of connected equipment a} {e) Due to earth quake, mechanical damage ete. ply §RUNETYPES OF SCHEMES FOR BUS-BAR PROTECTION a | There are two main schemes of bus bar protection namely, 1. Differential Protection. 2. Frame Leakage Protection and 1. Differential Protection : It is based on simple circulating current principle that during normal load conditions or external fault conditions, the sum of currents entering a bus jon equals the sum of those leaving LC) Generators hat ers all on on Lines Differential Current Protection of Bus-zone FIG 5.1:Frame Leakage Protecti: develops are mostly earth faults, by providing earthed metal barrier s conductor in the bus structure. With this arrangement every fault that might oe involve a connection between a conductor and an earthed metal part. | Outgoing Feeders» the switch g h the primary of a CT Th e leakage relay. In the eve nt of earth ; ork this rely will Operate. fault o The aim of any particular arrangement of bus-bars is to achieve sufficient ‘operation, sufficient reliability and minium cost. The following are the typical bus-bar arrangements. 1. Single bus bars } 2. Duplicate bus bars. i tations. The basic scheme 1. Single Busbars : This type of layout is meant only for small st 8 shown in Fig. 5 his and following figure G, C, B and S denote generators Se os couch ectively while T denotes a step-up Gitcuit breaker and switch (or isolator) resp transformer], such a large number of circuit breakers. Prese t favour ee Be transformers may, therefore, be a in yenerator one i n unit system (where one g Some cases. The preferred practice © nant ‘oauey atl it to hv. voltage former are connected as @ unit f° sof blah ollage cael ne se ‘ Gear altogether and have only © « bus-barsitbecomes necessary ected to the lines conn t cause a complete € generators and oa ‘on one section does not lize the layout so andis This ircuit breaker (called bus couplet) can be done by a¢! © bus-bar sections sO tied uP Ui breakers on low voltage side are shown in Fig: 5:4:cB} FIG 5.3: Single Bus-bar Bus Section Bus Coupler FIG 5.4: Two Bus-Bar Sections Bus Section 2. Duplicate Busbars ; In this layout the plant is split into two entirely se which can be worked at different voltages, if desired. Both Lu and h.v. b busbars alone may be duplicated, This is the ost flexible layout and is mean generating and/or transforming stations, Fig. 5.5 shows this type of layout In the case of a large generating station hé generators and two Outgoing lines connected through step-up transformers.| HL Bus a i / 8 fine Outgoing Lines FIG 5.5: Duplicate Busbars DERSTANDING PROTECTION OF TRANSMISSION t DERS 3 ri i The transmission lines are used for power transmission over longer distances, so the type of protective devices used are of different nature. Also, the transmission lines operate at much higher voltages than the feeders. The cable feeders are generally used for distribution system and they may operate as radial feeders or parallel feeders or in a ring main, Before any protection fora particular feeder can be decided, it is necessary to realize its importance in maintaining supply. IRE PROTECTION Pilot relaying schemes are used for the protection of transmission line sections. In these schemes, some electrical quantities at the two ends of the transmission line are compared and hence they require some sort of inter connecting channel over which information can be transmitted from one end to the other. Such an interconnecting1s which are used anced voltage Principle. Jating Current Differenti angement for the circulating CUT arr ne at each end. Under healt 1 and circulate in pilot wires, schematic Fig. 5.6 shows the Pe crs ‘are connected in each protected lin aes : re fault conditions the secondary currents are eq) 7 Protected Line Pilot Wires FIG 5.6: Schematic Diagram of Circulating Current Principle Under normal conditions and in case of external faults, differential current of is zero and relay does not operate. During internal faults this balance is distur differential current flows through the relay operating coils Fig. 5.7 she # ®:7 shows a schematic diagram of the balanced voltage principle. Ident are connect bes mt 'n each protected line, one at each end. The pair of C.1's in eae ected in series with a relay in such a way tl secondar hat under normal conditio Ny voltages are equal and in opposition i ., they balance each othe scheme, current * does not normally circulate throu ugh pilot wires.Pilot Wites OC : Operating Coit Relay RC : Restraining Coit Relay FIG 5.7 : Schematic Diagram of Balanced Voltage Principle CT's Pilot Wires a ‘ae we SREY FIG 5.8: Merz-Price Voltage Balance Scheme for all 3-phases of the Line Under normal conditions and in case of external faults, current entering the line at one end is equal to that leaving it at the other end. Therefore induced voltages in the | secondaries of C.T’s are equal and opposite hence no current flows through the relays. | In case of internal faults, secondary voltages of C.T’s become un equal and circulating current will flows through the pilot wires and operating coils of the relays. Fig. 5.8 shows the connections of merz-price voltage balance scheme for all the three Phases of the line.system can be used for ring mains as well as paral Pb. 7 tO} This system provides instantaneous protection for one Disadvantages : (a) Accurate matching of C.Ts is very essential (b) If there is a break in the pilo-wire circuit, the system will not opersid (c)_ This system is very expensive owing to the greater length of pilot wires (a) The system does not Provide back up protection or over load Protection. (e) Because of current transformer equality difficulty the system is not used 33 kV, X71 _ Translay Scheme of Pilot Wire Protection This system is similar to voltage balance system except that here balance or opposition | is between the voltages induced in the secondary windings wound on the relay magnets. and not between the secondary voltages of the line current transformers. cr, cr Power Flow Jon} cB. Distance protection is widely used protective scheme for the protection of high and extra high voltage (EHV) transmission and sub-transmission lines. Vv j A distance relay measures the ratio ‘7 at relay location which gives the measure distance between the relay and fault location. The impedance (resistance/repoint of fault. v If the ratio Tis less Peng (the impedance of the line section being protected) the relay will trip instantly. If “7 is greater than Z it indicates that the fault is beyond the protected line and the relay will not trip. There are two types of distance relays : L Definite distance. relay. 2. Time-distance relay, fe finite Distance Relay : Application of distance relays for the protection of transmission lines is shown in Fig. 5.10. G is generating station, substations S}, Sz and Sq are in series. Let the impedance of line GS; be Z1, $1 Sa be Zp, S2 S3 be Z3. The relays at G, 51, Sz and $3 are set to operate for impedance less than Z1, Zz, Z3 and Zq respectively. zy (e ) pee FIG 5.10 : Distance or Impedance Protection Suppose a fault takes place at point P between Substations Sz and Ss. The total fault loop impedance from generating station up to fault pointigZ; + Zp +Z, from substation S; to fault is Zp + Z and from substation Sz to fault is Z. Therefore the relays at generating station and substation $1 will not operate, but the fault loop impedance for relay at substation Sp becomes less than Zs (because Z is less than Zs), therefore, relay at substation Sp will operate. Similarly, in case of fault occurring between S1 and S2y telay at S; will operate In this manner, instantaneous protection can be obtained for all conditions of operation, 2. Time Distance Relay : Fig. 5.11 represents a simple power system with time distance telays at power station and substations A, B and C.FIG 5.11; Time Distance Relay Fig. 5.12 illustrates the characteristics curves of the relays installed at the each section. Let there be a fault in between sub-station B and C at point distance relay at sub-stations B will trip in 0.5 sec. While the relay at sub at power station will operate in 1.05 and 1.53 sec. respectively. In order may be cleared by the nearest circuit breaker first the circuit breaker oper should also be included; and if the current breaker opening time is assumed to’ sec.after its trip contacts energize, the breaker operation time fi represented by dotted lines for sub-stations A and B. F Power Sub-Station ane on Sub-Station Station A B . FIG 6.12 : Characteristic Curves of the Relays Installed at the Beginning of Each Gail Further it will be obvious that the distance relays at sub-station A and at power: ull act as back up relays for fault at point F, Le,, lf the relay at statiog a a the relay of sub-station A will tip the circuit after 13 sec (including time o operation) and even relay at substation A fails to operate, then the relay at station will trip the circuit in 1.53 sec. 10 PROTECTION OF FEEDERS ‘ 2 The feeder means the connecting link between two circuits. The feeder could bet form of transmission line, short, meclium orlong, or this could be a distributio{a) Radial feeders (b) Parallel feeders, (c) Ring mains. $10.1 Protection of Radial } Feeders b by Time Graded Over Current Protective Relays The main characteristic of a radial sytem is that power an dow ony in one direction, that continuity of lt from generator or supply end to the load. It has the disadvantage i supply cannot be maintained at the receiving end in the event of faul Time-graded protection of a radial feeder can be achieved by using definite time relays. Fig. 5.13 shows the over curren t protection of a radial feeder by definite time relays Here a generating station supplies three sub stations. At all substations an over current relay has been used with a definite minimum time setting. The time setting is such that it is least for the relay of the last substation and is maximum for that of the generating station. The relay 4 has an operating time of 0.8 second while for other relays, time delay is successively increased by 0.4 second. Normally the time delay step lies between 0.3 sec and 0.5 sec. ee: = : > [16 See a FIG 5.13: Time Grading for Raidal Feeder Swann: xx0e oracorme or senor suesie sfesfeudtocaure in the pection GD, then 000 We eeoRa No. 4 will operate because time of operation of this relay is the mini only section CD of the system will be isolated. If the relay No. 4 fails to tip No. 3 wil operate after a tme delay, of 0:4 second Ney after 1.2 second occurrence of fault ‘The disadvantage of this system is that if there are @ number of feeders in tripping time for faults near the supply end becomes high (2 seconds in this main application is in systems where the fault levels at the various locations do not greatly. In addition to the discrimination of timings, another important requirement of the vs that it should posses inverse-time characteristics [¢., mo severe the fault, quickly the relay should operate. The relays used in a particular system should be that they should not cross each other no matter what the severity of the fault may} 5.11 PROTECTION OF PARALLEL FEEDERS BY DIRECTIONAL RI up: AB: cp: When continuity of supply is required two parallel feeders may be installed. Ifa fa occurs on one feeder, it can be isolated from the system and continuity of supply be maintained through another feeder. Feeder-1 Non directional relays Directional relays FIG 5.14: Protective Scheme for Parallel Feeder Fig. 5.14 shows an over current protective scheme for parallel feeders. At the $2 end of the feeders (at A and B), time graded non directional over oil relays required. The symbol <> indicates a non-directional relay. At the other end of (at C and D), directional relays (reverse power relays) are required. The directional relays should be instantaneous type and operate only when P flows in the reverse direction ie,, in the direction of arrow at C and D.Fa B until its over load relay 'y Operates thus completely isolatin ipplying g the faulty feeder and su _ power through healthy feeder. This method of protection is satisfactory when fault is heavy. and trips the circuit breaker, Necessity of Directional Over Current Relays at C and D : If non-directional relays are used at C and D, both relays placed at C and D will trip for a fault at F. This is not desired as the healthy feeder is also tripped. Due to this very reason relays at C and D are directional over current relays. For faults at feeders, the direction of current at A and B does not change and hence: relays used at A and B are non-directional ROTECTION OF RING MAIN FEEDERS AP The system of interconnection between a series of power stations by an alternative route is called ring main system. In case of damage of any section of the ring, that section may be disconnected for repairs and power will be supplied from both ends of the ring, thereby maintaining continuity of supply. Fig. 5.15 shows an over current scheme for the protection of a ring feeder. Compared. with radial feeders, the protection of ring feeders is costly and complex. 1.5 Seconds. (05 Second | 10secona fiona 1.5 Seconds 0.5 Second FIG 5.15: Protection of Ring Feeder In Fig. 5.12, G is the generating station and A, B, C and D are substations. At the erating station, the power flows only in one direction ie., away from the bus-bars, Non-directional time lag over load relays are used. At both the ends of the substationsaway from the substation. In the network shown while going round the ring in the direction on the further side of each sub-station are set with decreasing time lag re example, at generating station 2 seconds, at substations A, B, C and D are sec, 0.5 sec and instantaneous respectively. Similarly when going round the opposite direction, the relays on the outgoing sides would be as under : Generating station, G = 2sec, Substations D = 15sec. C = 10sec, B = 0.5sec and A = Instantaneous Suppose a fault occurs at point F. In that case the power is fed into the fault th two paths ABF and DCF. The relays between B and F & C and F will operate. way the fault on any section will cause the relays on that section only to operate the healthy sections shall be operating un interruptedly. The time grading can not be crosser than 0.33 or 0.5 second and as the time that can be fed is 2 or 2.5 seconds, the maximum number of sectio can be protected in this way are six.