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PROTECTION OFGENERATORSubmitted by:Name: Subham BhattacharjeeCollege: Techno India College of TechnologyStream: Electrical Engg. 6th semRoll no: 09187016021
INTRODUCTION : It is imperative need to install some protective system to protect the expensive elements of modern power system such as generators, transformers, station bus-bars, transmission lines etc. from different types of faults which are likely to occur sooner & later. In generating station, as a continuous operation of generators is much more necessary so the faulty part has to be cleared very quickly for uninterruptable power supply. Unlike other apparatus, opening a breaker to isolate the faulty generator is not sufficient to prevent further damage. The basic electrical quantities those are likely to change during abnormal fault conditions are current, voltage, phase angle and frequency . Protective elements utilizes one or more of these quantities to detect abnormal conditions in a power system for taking further essential steps to isolate the faulty equipment to keep the healthy part in normal working condition.
GENERATOR FAULTS AND PROTECTION TECHNIQUES : Generator Faults Can be Considered Under The Following Heads :i. Stator Winding Faultsii. Field Winding or Rotor Circuit Faultsiii. Abnormal Operating Conditions Followings Are The Different Process of Protecting Generator :i. Stator Protectionii. Overload or Overcurrent Protectioniii. Overvoltage Protectioniv. Overspeed Protectionv. Protection Against Motoringvi. Rotor Fault Protectionvii. Negative Sequence Protection Against Unbalanced Loadsviii. Protection Against Vibrationix. Bearing Overheating Protectionx. External Fault Back-Up Protection
STATOR FAULTS AND ITS PROTECTION : Stator faults include the following-i. Phase-to-earth faultsii. Phase-to-phase faultsiii. Inter-turn faultsPhase-to-phase faults & Inter-turn faults are less common, these generallydevelop into an earth-faults. Inter-turn faults are more difficult to be detected. The Different Protection Schemes for Stator Protection are :i. Differential Protectionii. Modified Differential Protectioniii. Biased Circulating Current Protectioniv. Self Balance Protection Systemv. Balanced Earth-Fault Protectionvi. Stator Inter-Turn Protectionvii. Stator Overheating protection
DIFFERENTIAL PROTECTION : Two sets of identical CTs, each set is mounted on either side of stator phase winding. The secondaries of these current transformer sets are connected in star & their ends are connected through pilot wires (shown in fig). At normal operating conditions, the currents at the two ends of the protected section are same (I1=I2). So relay will not operate. When the fault occurs the balance is disturbed and differential current (I1-I2) flows through the operating coil of the relay causing relay operation and the trip circuit of the circuit breaker is closed. The relays employed in this protection scheme are generally of electro-magnetic type & are arranged for instantaneous operation as faults are expected to be cleared as quick as possible
MODIFIED DIFFERENTIAL PROTECTION : Generally protection is made for 80 to 85% of the winding in Differential Protection scheme. If any fault occurs near the neutral point then the fault current is very small and relay does not operate. Modified differential protection scheme is used to over come this. Two phase elements (PC and PA) and balancing resistor(BR) is connected in star and the earth relay(ER) is connected between the star point and neutral pilot wire.
RESTRICTED OR BALANCED EARTH FAULT PROTECTION : In case of small size generators the neutral end of 3 phase winding is not available because it is made inside the generator and grounded through some low resistance then percentage differential relay for ground fault is provided and is known as restricted earth fault protection. This scheme can be used only for ground faults but not for phase faults.
OVERCURRENT OVERVOLTAGE PROTECTION PROTECTION Overloading of the generators Overvoltage protection is required may be caused either due to in case of hydro-electric or gas turbine generators but not in casei. Partial breakdown of of turbo generators. winding insulation Over voltage may be caused due to-ii. Excessive load on the power supply system i. Transient over voltage in the transmission line due In occurrence of such to lightening. faults, the generator can be ii. Defective operation of disconnected from the system the voltage regulator. manually. iii. Sudden loss of load due In modern alternators it has to line tripping. not so much importance though The protection is provided with an unnecessary overloading over voltage relay. interfere with the continuity of It is usually of induction pattern the supply with an IDMT Characteristic
ROTOR FAULTS AND ITS PROTECTION : Faults in the rotor circuit may be either earth faults or between the turns of the field winding . Field circuits are normally operated un-earthed. So a single earth fault will not affect its operation. But when a second fault arises then field winding is short circuited and produce unsymmetrical field system which leads to unbalanced forces on rotor and results in excess pressure and bearing and shaft distortion. The Different Protection Schemes for Rotor Protection arei. Rotor Earth Fault Protectionii. Loss of Excitation Protectioniii. Protection Against Rotor Overheating because of Over excitationiv. Rotor Temperature Alarmv. Automatic Field Suppression & Use of Natural Circuit Breakervi. Alternative Arrangement of Field Suppression
ROTOR EARTH FAULT PROTECTION : A high resistance is connected across the rotor circuit and its mid point is grounded through a sensitive relay. The another method of rotor earth fault protection is done by i. DC injection method ii. AC injection method The dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor(in case of ac). A single earth fault in rotor circuit will complete the path and the fault is sensed by the relay. DC injection method is simple & has no problems of leakage current.
LOSS OF EXCITATION PROTECTION : It is caused by accidental tripping of field breaker, short-circuit in the field circuits, poor brush contact or operating errors. In absence of field current alternators runs as an induction generator & currents are inducted in the rotor teeth and wedges & also draws reactive power from the system. Under-current moving coil relay is connected in series with field winding for loss of excitation protection. The relay mostly used for this type of protection is Directional distance type relay.
ROTOR OVERHEATING PROTECTION BY TEMPERATURE ALARM : This type of protection scheme is installed generally in large alternators. It indicates the level of temperature but not the actual hot spot temperature. The relay measures the temperature by measuring the resistance .(as shown in fig). The relay measures the ratio of voltage & current.
AUTOMATIC FIELD SUPPRESSION AND USE OF NEUTRALCIRCUIT BREAKER : In case of a fault in the generator and though the circuit breaker is tripped ,the fault continues to fed as long as excitation will exist because emf is induced in the generator itself. Hence all protection system not only trip the generator circuit breaker but also trip the “automatic field discharge switch “
NEGATIVE PHASE SEQUENCE PROTECTION : Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current . This current in rotor causes rotor overheating and damage to the rotor. This can be protected by negative sequence current filter with over current relay.
EXTERNAL FAULT BACK UP PROTECTION : Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection against external phase-to-phase faults & earth faults. Induction type IDMT relay is used for this purpose. High se, definite minimum time, induction type, inverse over-current, earth fault relays are required for back-up protection of generators.
PROTECTION AGAINST VIBRATION AND BEARINGOVERHEATING PROTECTION : Rotor earth fault protection & Negative sequence protection of generator against unbalanced loads prevent vibration under those circumstances. A vibration detector mounted on one of the bearing pedestals & on one of the upper guide bearing in case of horizontal & vertical shaft generating set respectively. Bearing overheating can be detected by a relay accounted byi. A thermometer-type bulb inserted in the hole of the bearingii. A resistance-temperature-detector relay If lubricating oil is circulated through the bearing under pressure, the oil temperature should be taken into account for an alarm if oil circulation is stopped
CONCLUSION : Switchgears & other protective devices are used in the electrical machines & in the power system elementsi. To detect the abnormal faulty conditionii. To notify this information to the maintenance officer of that factory or generation stationiii. To disconnect the faulty elements from the healthy system or shutdown the faulted apparatus for continuous operation of the machines, for safety of this expensive electrical devises as well as for the safety of the workers The protective device does not prevent the cause of fault or does not prevent ta occurrence of fault , it is used after the occurrence of fault for quick action to remove those faulty section for further normal operation .
REFERENCES :A Course In Power Systems by J.B.Gupta Modern power System Analysis by D.P.Kothari & I.J.Nagrath www.wikipedia.com www.google.co.in