GENERATOR PROTECTION
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.
Generator Over-Current
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.
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