FIG. 6.6 : MERZ-PRICE PROTECTION ALTERNATOR 1 winding (armature winding) is healthy and when there is no internal fault equal currents \dary windings of CT's. So no current flow through any winding of the relay. phase or when there is phase ‘0 phase ker is tripped. ces of the pilot wire When the stator flow through the secon: put te current balance is disturbed when there is earth fault in one fu current flows through one or more windings of the relay and the circuit brea points of the pilot wires so that the resistan elay coils are connected across the mid on each side of the relay are equal. ar the alternator and CT, is near the switch gears and the distance between the alternator Normally CT, is ne: and switch gears is more. $0 itis not possible to connect the relay aoross the mid-points of the pilot wires. Balancing resistances are onpected as shown to overcome this difficulty pilot wires are balanced by the balancing resistors. The balancing resistors Due to this the resistances of the able, Fig. 6.7 shows this type of connes are Kept adjust ct alee ction.PROTECTION FOR ALTERNATOR WHICH IS EARTHED THROUGK [RESISTANCE : .f alternator is earthed through high resistance. Value of the earth faut, a ali re Besse iad tetaance is high This means that some portion of the aerate = will not get protection, To overcome his difficulty, some change 's made in the above arrangemen. circuit is shown in Fig. 6.8. In this two relay coils are used for the ‘elements and one balancing resistance are’ connect point and the neutral wire. coil is used for earth fault. Two phase fault and the third c ult relay is connected across Fe May ted in star. Earth fa a Alternator ch a SL ne LAR, TOOT ce. a % OS} Lan FTTTTD- eat Or=teae R = [ Balancing resistors Phase fault Barth, relay fault relay \ il FIG. 68 : DIFFERENTIAL PROTECTION FOR ALTERNATOR WHOSE NEUTRAL IS EARTHED THROUGH HIGH RESISTANCE In the healthy condition the currents on the two sides of the stator winding will be equal. So the curl balance ‘is obtained as the secondary currents.of current transformers are equal and no current will flow throug the relay winding. Current flows through the carth fault relay when there is earth fault in any phase Tf there is fault between two phases, the fault current will circulate in the secondary windings of two transformet through two coils and no current will flow through the earth fault relay. Thus the sensitive earth fault ey ‘can be uscd so no difficulty is encountered due to the high earthing resistance 6.7 BALANCE EARTH FAULT PROTECTION : * Some times for the alternators having low capacity. culating current protection system. * So it is not possible to make use of the ci * Balanced earth fault protection system is used in such circumstances. * Such an arrangement is shown in Fig. 6.9 Star point of alternator is to grounded through a resistor. A current transformer is connected in the neu CT's on the line side are connected as shown. There is only one winding of the relay coil In the healthy condition, sum of current of CT; at X and Y is zero. There is no current through the #2 also. So the secondary current of CT, also is zero. So there is no current flow through the relay But when an earth fault takes place in the alternator winding or there is earth fault at F, current {10wS t the secondary of CT, and current flows through the relay and the relay operates‘ifferential protection system cannot be used. It is also used as back up protection for the lffeentia protection system Syllabus Topic : Restricted earth fault protection 14, 2016; Dec. 2013, 2 1. Explain restricted carth fault protection in an alternator, [December-2013, 7-Marks] 4 Restricted earth fault protection is provided to alternators though it does not protect the complete wiring | against earth faults. Justify this type of protection, [May-2014, 7-Marks} | 4 Discuss the type of protection used for the stator phase and ground faults on the armature winding of a generator. [December-2014, 7-Marks] | 4. Draw circuit diagram for restricted earth fault protection. State its application, What is the effect of neutral | founding resistance on percentage winding protected. (May-2016, 7-Marks] A ‘restricted earth fault” relay works by measuring the actual current flowing to earth from the frame of the unit. If that current exceeds a certain preset maximum value of milliampears then the relay will tip to Cut off the power supply to the unit. ae Here neutral is earthed through resistance which is called “earth resistance” to limit the earth In this protection, some percent of winding is protected, depends on the value of earth Relay setting are such that, it does not operate for earth fault on EHV side, At neutral Voltage is Jess so that the earth fault at this point does not occur. | St Gear ent Proseetion 12021 / 14. | alee Gu flows through the relay coil and the relay operates. To overcome this difficulty perc relay is used. Fig. 6.13 shows the circuit of a percentage biased ial relay for the protection connected altemator. ® Stator winding of alternator is connected in star. Is star point is earthed. CT's on the two sides are connected in star. Their star points are earthed. ‘As explained previously, there are two types of winding in percentage biased differential relay. One is the main winding or the operating coil and the other is the restraining coil. cr, Alternator ch To tip circuit FIG. 6.13 : CIRCUIT OF PERCENTAGE BIASED DIFFERENTIAL, RELAY FOR THE PROTECTION OF ALTERNATOR + Restraining coils are connected in pilot wire as shown. Operating coils are connected across the mid-point and the neutral of the current transformers. + When the alternator feeds the external fault currents in secondary windings of CT's on both the sides of the alternator winding increase. Differential current increases if the characteristics of the CT's are not identical. + But at the same time the restraining current also increases. So the relay cannot operate, But if there is internal fault in the stator winding, the relay operates as the value of the differential current becomes more than the restraining current. + Fig, 6.14 shows this type of protection system for the delta connected alternator. + * stator winding is connected in delta. Secondary windings of CT's connected in phase circuit are in delta while the secondary windings Of CT's comected inline are connected in stat, The sar point is Silich Gear and Protection /2021 (15: _(a — cia Sar i FERENTIAL PROTECTION sc. i s PERCENTAGE BIASED DIFFERENTIAL ae ae TIE DELTA CONNECTED ALTERNATOR 6.12 EARTH FAULT PROTECTION ¢ nace Tooke valve of the pickup curent for the over current relay is mote so. the relay cannot operate with value of the earth fault current. So to give earth fault protection, an earth fault relay is connected in neutral circuit through CT as shown in Fig. 6.11. Or the earth fault relay is connected in the neutral of relay. NEGATIVE PHASE SEQUENCE PR’ OTECTION AGAINST UNBALANCED LOAD : ee eye Negative phase sequence protection. Explain the negative phase sequence and field failure protection of generator. [May-2013, 2018, 7-Marks| | ‘Due to the unbalanced Joad on the alternator. 1. Stator winding is over heated and - 2. Vibrations are produced : ; Panu! TOTO A + Rotor winding is overheated due to induction ‘of double frequency currents in the rotor. So : it is necessary to provide protection against the over loading. The negative sequence component is produced when the load becomes unbalanced, This component rotates with synchronous speed in FIGS615 =, PROTECTION the direction opposite to that of the rotor AGAINST UNBALANCED LOADING«Asa result the stator and rotor windings get over heated, Moreover other generators connected to the bus bars are overloaded as this generator cannot supply power. + There is no immediate damage to the alternator when the excitation fails, So the operator in the control room can isolate the faulty generator from the bus bar even though the automatic operation can be used. Such circuit is shown in Fig. 6.16. + For this, an undercurrent relay, time lagged tripping relay and time lagged reinforcing relay are employed. » These are connected as shown in Fig. 6.16. Under current relay is of moving coil type + Thisrelay is driven by the shunt connected: in the field circuit. In the healthy condition the relay carries current and is in the energized condition. Time lagged reinforcing FIG. 6.16 : PROTECTION AGAINST FIELD FAILURE * When due to some reason the field current is reduced too much or field current becomes zero, the relay gets deenergized. Duc to this the contacts ofthe time lagged tripping relay and time lagged reinforcing relay are closed. * Now there is time lag in the reinforcing relay that means the contacts remain closed even if the supply is off, The arrangement is done so that the tripping relay gets supply through these contacts * So the reinforcing relay and tipping relay both energize when the under current relay operates. Now immediately if the due to transient the under current relay is energized, the coniacts change over and supply to both these relay is cut * But the contacts of the reinforcing relay are in the closed condition due to the time lag feature so the supply to the tripping relay is continued through these contacts. Thus the tipping will not become on and off due to the changes resulting due to the transiemt condition. 615 FIELD SUPPRESSION * When the fault occurs in outside the protected zone the circuit breaker trips and the altemator stops feeding the outside fault.