Both relay will be connected to current transformer having a primary current rating equal to that of the earthing resistor. The relay will respond to earth faults occurring over most of the rotor circuit. In some cases a setting is chosen, such that the relay will not operate for a system fault but will only respond when fault current is fed into the generator, in this way it only acts as a back-up to the main generator protection.
This condition can tolerated for a short time but clearly there will be increased heating of the rotor because of the slip- frequency currents which flow. Loss of field can be detected by undercurrent relay connected to a shunt in the field circuit.
CT Red Yellow Blue. These current, which have a phase rotation in the opposite direction to the normal phase rotation, produce a magnetic field which induces currents in the rotor at twice the system frequency. This causes considerable heating in the rotor and would cause damage.
The actual NPS currents is difficult to determine. If the circuit breaker is tripped the set will begin to accelerate and although the governor is designed to prevent over-speed a further centrifugal switch is arranged to close the steam valve.
There is still a risk, however, that the steam valve not close completely and even a small gap can cause over-speed and so where urgent tripping is not required. A sensitive under-power relay is used to detect when this value is reached. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next.
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Download now. Related titles. Carousel Previous Carousel Next. A means of detecting the first ground fault provides protection against the effects of a second fault to ground on the rotor. A ground fault occurring anywhere within the excitation system and rotor winding will cause current to flow through the limiting resistor the voltage at the fault point will add to the bias voltage and cause a current flow through the GFD circuit , the GFD relay, the bias supply to ground and then back to the fault location.
Current flow through the GFD relay brings in an alarm. If a generator is subjected to an unbalanced load or fault, the unbalance will show up as ac current in the rotor field. With the 4-pole rpm generators used in nuclear stations, this current will be at twice line frequency or Hz.
Continued operation with a phase imbalance will cause rapid over-heating of the rotor due to the additional induced circulating currents these currents will also cause heating of other internal components of the generator. This will result in rapid and uneven heating within the generator and subsequent damage to insulation and windings hence, reduced machine life and thermal distortion could occur. Also the unbalanced magnetic forces within the generator due to these currents will cause excessive vibration.
A specialized relay to detect these circulating currents, called a negative sequence current relay, is used to detect the phase imbalance within the generator. The term negative sequence is just a mathematical term to describe the effects of unbalancing a symmetrical three phase system.
The most critical phase unbalance would come from an open circuit in one of the windings and may not be detected by any other protection. Other causes of phase imbalance include unequal load distribution, grid faults and windings faults. When a generator develops insufficient excitation for a given load, the terminal voltage will decrease and the generator will operate at a more leading power factor with a larger load angle. If the load angle becomes too large, loss of stability and pole slipping will occur and the turbine generator will rapidly go into over-speed with heavy ac currents flowing in the rotor.
A loss of field could be caused by an exciter or rectifier failure, automatic voltage regulator failure, accidental tripping of the field breaker, short circuits in the field currents, poor brush contact on the slip-rings or ac power loss to the exciters either from the station power supply or from the shaft generated excitation current.
A relay that sense conditions resulting from a loss of field, such as reactive power flow to the machine, internal impedance changes as a result of field changes or field voltage decreases, may be used for the detection of the loss of field. A field breaker limit switch indicating that the breaker is open also gives an indication that there is no field to the generator.
If the generator is required to produce greater than rated voltage at rated speed or rated voltage below rated speed , the field current must be increased above normal generated voltage is proportional to frequency and flux.
The excess current in the rotor and generated voltage will result in over-fluxing of the generator stator iron and the iron cores of the main and unit service transformers.
Damage due to overheating may result in these components. This problem may occur on generators that are connected to the grid if they experience generator voltage regulation problems. It may also occur for units during start-up or re-synchronizing following a trip the field breaker should open when the turbine is tripped.
When the field breaker opens, a field discharge resistor is inserted into the rotor circuit to help prevent terminal voltage from reaching dangerous levels. Over-excitation on start-up may be a result of equipment problems or operator error in applying excessive excitation prematurely excitation should not be applied to the generator until it reaches near synchronous speed.
While connected to a stable grid, the grid frequency and voltage are usually constant. If the system frequency drops excessively, it indicates that there has been a significant increase in load. This could lead to a serious problem in the grid and it is of little use to supply a grid that may be about to collapse. In this case, the generator would be separated from the grid. The grid or at least portions of it may well collapse. The system can slowly rebuild with system generators ready to restore power to proper, pre-collapse operating conditions.
As mentioned above, if a generator connected to the grid has sufficient excitation applied below synchronous speed since grid frequency has dropped for it to produce rated voltage, the excitation level is actually higher than that required at synchronous speed. Overexcitation and the problems described above may result. This protects the generator from continuing operation when the generator is pole slipping. Pole slipping will result in mechanical rotational impacts to the turbine, as the generator slips in and out of synchronism.
This can be the result of running in an under excited condition see the section on loss of field or a grid fault that has not cleared. Relays that detect changes in impedance of the generator can be used to detect the impedance changes that will occur when the unit slips poles.
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