Posted By: Shraddha Raut

June 18, 2016
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Overview

Generator has to be protected from faults occurring within the generator stator or rotor and also from external faults/abnormal operating conditions in the grid which affected the generator. Stator Inter Phase/Earth Faults: Inter phase and earth faults could be the result of moisture or due to ageing or foreign matter etc. Single phase to earth faults may occur between any phase conductor and core whereas inter-phase faults are more likely to occur in the overhangs. Inter-phase faults involving earth faults could be the result of single phase to earth fault which may simultaneously result in phase to earth fault in other phase due to increase of voltage with respect to earth caused because of the first phase to earth fault. In direct water cooled stator winding generators, phase to core fault may occur due to insulation failure, due to over-heating caused by failure of water circulation through the stator conductor. Inter phase fault in the generator if allowed to continue can cause non repairable damage, can even result in fire due to high short circuit currents. There is a tendency for the generator to over speed since the power output is drastically reduced. If the faulty generator is not immediately isolated the system may be thrown out of gear due to instability. For high capacity generators, it is usually a practice to limit the earth fault current to avoid extensive damage to the stator core. Earth fault current of even 100 amperes for a few seconds can cause extensive damage. The earth fault current is generally limited to about 15/20 amps. Inter Turn Faults: Inter turn faults are the faults, involving two turns of the same phase. Higher capacity Turbo-generators for Thermal Power Stations are so designed to have only one turn in one slot, as such the chances of inter turn faults are almost ruled out. However, in two layer winding generators, two conductors belonging lo same phase may be shorted, causing damage to the machine. Field Failure/Loss of Excitation: Loss of excitation may be caused by fault in the AVR or tripping of field breaker. Generator when running in normal condition and connected to the grid, shares the reactive demand of the system. On loss of excitation the generator starts drawing reactive power from the system instead of supplying reactive power. In case the other generators cannot meet the requirement of reactive power this shall result in large voltage drop which may ultimately result in instability: Generator rotor also gets heated due to induced currents in the rotor winding. Stator also heats up due to high stator currents due to the increase in reactive current from the system Rotor Earth Faults: Since rotor circuits operate ungrounded, a single earth fault is caused by insulation failure due to moisture, ageing of insulation or vibration in the rotor etc. But the existence of single ground fault increases the chances of a 2nd ground fault. The occurrence of 2nd earth fault can cause large fault currents causing serious damage. Because of bypassing of certain part of rotor, flux pattern is disturbed producing unbalanced forces resulting in vibrations. Although a machine can continuously run on a single earth fault but second rotor earth fault, if allowed to occur, should be detected immediately and generator should be tripped. Over Voltage: Over voltage at the generator terminals mainly cause’s higher flux in the transformer connected to it and may ultimately burn the core. Overvoltage can be caused by AVR malfunctions, sudden loss of load on generator or switching on of a long unloaded transmission line. Under Frequency: For the system to operate at specified parameters a balance between the load and generation is maintained by individual governors, centralized frequency controllers and load dispatch stations. An outage of a major generating unit, if not accompanied by load shedding or the other machines peaking up the loads (which would pick up in case sufficient spinning reserve is available) would result in drop of frequency. A low frequency operation is harmful in general for motors and transformers. In a turbo-generator unit, low frequency would result in high rotor currents, over-fluxing and core vibrations due to over excitations. In the turbine, low frequency has to be checked to avoid resonant frequencies and consequent turbine blade fatigue. It has, however, to be kept in mind that tripping of unit under low frequency shall result in further drop in frequency of the system to which the generator is connected. Alarm is generally provided to warn the operators on occurrence of low frequency. The tripping of the unit may be carried out depending upon the manufacturer’s instructions. External Inter-Phase/earth Fault: These faults may occur outside the generator, in bus ducts, transformers, bus bars or feeders etc. The cause can be different depending upon the affected area. Negative Sequence Current: A fault in the system due to not closing or closing of all the three poles of generator circuit breaker or a fault in a system would result in unbalancing current being drawn from the generator. The unbalanced current results in a negative sequence current in the stator windings. This negative sequence current causes a double frequency current induced in the rotor. This current causes serious damage to the rotor in case generator continues to supply unbalance currents. A relation I22t = Constant, between negative sequence current I2 and time’t’ in second, specifies the operating limits of generator. A generator can supply a certain amount of negative sequence current continuously but as the component of negative sequence current increases, the time for which this current can be supplied so reduces as per I22t = Constant , which is around in the range of 8 for steam turbo-generators. Over Voltage/Under Frequency in the System: Simultaneous over voltage/under frequency in the system can be caused by bad load dispatch techniques and mainly causes over fluxing in the transformers. It may be mentioned here that effect / cause of over voltage & under frequency needs to be studied separately depending upon whether the generator is running isolated (i.e. the before synchronizing or after it is isolated from the grid) or when it is synchronized with the grid. Over Current in the System: Grid disturbance leading to pole slipping and/or high active/reactive power demand from the generator may cause over current in the generator. Reserve Power: When instead of generator supplying power to grid, it receives power, the operating condition is termed as reverse’ and is likely to occur as follows: (i) When immediately after synchronizing control valves are not operated which may happen due to some fault in the system or some delay by the operating personnel. (ii) In case of sudden closer of stop valves or control valves when the generator unit is still connected to the grid. Reverse power operation is harmful to the turbine since without steam flow in the turbine if turbine continues to rotate, it will result in heating of turbine blades due to churning action. However, the period for the turbine to overheat may vary from a few seconds to minutes depending upon the turbine and operating conditions.

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