As with electrical motor protection, generator protection schemes have some similarities and overlap. This is advantageous, since not all generators have all of the protection schemes listed in this section. In fact, there are many protection schemes available; only the more common ones are discussed here.
Over-currents in the windings due to over-loads or faults will cause extensive damage. The generator must be separated from the electrical system and field excitation removed as quickly as possible to reduce this damage to a minimum.
During run-up and shutdown, the field may accidentally be applied while the frequency is below 60Hz. Under these conditions normal protections may not work or may not be sensitive enough. A sensitive over-current protection called supplementary start over-current is usually provided when the frequency is less than about 56Hz.
Generator Differential Protection
Differential protection can be used to detect internal faults in the windings of generators, including ground faults, short circuits and open circuits. Possible causes of faults are damaged insulation due to aging, overheating, over-voltage, wet insulation and mechanical damage.
Generator Ground Fault Protection
Generators are usually connected to the delta winding of a delta-star main transformer. This allows the generator to produce nearly balanced three phase currents even with unbalanced loading on the
primary of the main transformer. This minimizes stress, vibration and heating of the stator windings during unbalanced system conditions and electrical system faults.
However, with the generator connected to a delta winding, a separate protection has to be used to protect against stator faults. Any resistance to ground will pull the delta towards ground and may initially go undetected by the differential relay. The stator ground relay will trip the generator before severe damage results. Often the ground relay has a low-set alarm included to allow possible correction before a trip condition exists.
Possible causes of ground faults are insulation damage due to aging, overheating, over-voltage, wet insulation and mechanical damage. If the faults are not cleared, then the risk of insulation damage will occur due to overheating (as a result of high currents) or damage from arcing if the insulation has already been damaged.
Rotor Ground Fault Protection
The windings on the rotor of an ac generator produce the magnetic field at the poles. In four pole generators (typical of 60 Hz, 1800 rpm units), the occurrence of a single ground fault within the rotor generally has no detrimental effects.
A second ground fault, however, can have disastrous results. It can cause part of the rotor winding to be bypassed which alters the shape of the otherwise balanced flux pattern. Excessive vibration and even rotor/stator contact may result. 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.
Generator Phase Unbalance Protection
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 1800 rpm generators used in nuclear stations, this current will be at twice line frequency or 120Hz.
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. This may result in bearing wear/damage and reduced machine life and may result in a high vibration trip.
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.
Generator Loss of Field Protection
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.
Generator Over-Excitation Protection
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. Over-voltage may also cause breakdown of insulation, resulting in faults/arcing.
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).
A specialized volts/hertz relay is used to detect this condition and will trip the generator if excessive volts/hertz conditions are detected.
Generator Under-frequency Protection
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.
A specialized volts/hertz relay compares voltage level and frequency and will trip the generator if preset volts/hertz levels are exceeded.
Generator Out of Step Protection
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. Another method to provide this protection is to detect the loss of excitation, using the loss of field protection and trip the unit if excitation is too low (i.e., trip the generator when pole slipping is imminent). This has been discussed in the loss of field section of this module.
Generator Reverse Power Protection
Motoring refers to the process of an ac generator becoming a synchronous motor, that is, the device changing from a producer of electrical power to a consumer of it. Following a reactor trip or setback/stepback to a very low power level, it is beneficial to enter the motoring mode of turbine-generator operation. However, this is not a desirable mode of operation for standby or emergency generators. They are not designed to operate in this manner and can be seriously damaged if power is allowed to flow in the wrong direction.
A means of indicating when the transition from exporter to importer of power occurs is provided by a device known as a reverse power relay. As its name suggests, it is triggered by power flowing in a direction opposite to that which is normally desired.