21.1. Overexcited (boosting voltage).-A
capability curve is defined as a curve which shows
boundaries of the area on the kilowatt-kilo-var
diagram within which a machine may be operated
continuously. Capability curves are furnished for
each installation which show kVA capability
throughout the expected range of operation and
which include the range of permissible operating
voltages. One such curve sheet for a generator
rated 65 789 kVA, 13 800 volts, and 95 percent
power factor is shown in
capability curve is defined as a curve which shows
boundaries of the area on the kilowatt-kilo-var
diagram within which a machine may be operated
continuously. Capability curves are furnished for
each installation which show kVA capability
throughout the expected range of operation and
which include the range of permissible operating
voltages. One such curve sheet for a generator
rated 65 789 kVA, 13 800 volts, and 95 percent
power factor is shown in
operating voltage is limited to 105 percent of rated
terminal voltage in order to meet voltage
limitation requirements of the Standards for
Synchronous Generators. Capability curves for
machines in categories a and b in paragraph 7 not
operating under manufacturer's warranty will
normally show limitations for 115 percent of
nameplate ratings. Limitations for new machines
(post 1982) will normally be the nameplate rating,
terminal voltage in order to meet voltage
limitation requirements of the Standards for
Synchronous Generators. Capability curves for
machines in categories a and b in paragraph 7 not
operating under manufacturer's warranty will
normally show limitations for 115 percent of
nameplate ratings. Limitations for new machines
(post 1982) will normally be the nameplate rating,
For uprated generators, the kilowatt rating rather
than the kVA rating is set at a maximum fixed
value which should not be exceeded. The
Capability curve will be based on a constant
kilowatt load.
than the kVA rating is set at a maximum fixed
value which should not be exceeded. The
Capability curve will be based on a constant
kilowatt load.
21.2. Underexcited (bucking voltage).-
Generator capability when operating underexcited
is limited by the permissible armature current and
steady-state stability limit of the machine. A
capability curve sheet for underexcited operation
is shown in
Generator capability when operating underexcited
is limited by the permissible armature current and
steady-state stability limit of the machine. A
capability curve sheet for underexcited operation
is shown in
a function of system at this point are above
terminal voltage. As the kilovar load is increased
at zero-kilowatt load, the generator will eventually
become unstable and start to slip poles. The kVA
terminal voltage. As the kilovar load is increased
at zero-kilowatt load, the generator will eventually
become unstable and start to slip poles. The kVA
reactance, and generator and armature current
rated values so that continuous operation near
this point is out of the generator reactances,
external
rated values so that continuous operation near
this point is out of the generator reactances,
external
question because of armature heating.
Most modern generators may be operated
continuously at zero field current and rated
voltage except for a few machines with special
electrical characteristics such that stator heating
becomes a problem. Modern, continuously acting
voltage regulators are equipped with
underexcited reactive ampere limiters which
permit machine operation at underexcited
capability determined by thermal limitations
without danger of the machine becoming
unstable. These limiters respond to underexcited
reactive stator current and to terminal voltage.
As a part of the regulating function, limiters
automatically hold the underexcited reactive
amperes at lower values as kilowatt load on the
machine is increased. Limiter settings are
adjustable so that their operating characteristics
may be coordinated with the thermal capability
curve and the steady state stability curve. The
Denver Office, should be consulted when any
operating problems arise concerning
underexcited operating capability either under
load or strictly as a synchronous condenser.
Additional underexcited synchronous condenser
capacity is sometimes made available by
operating with negative field current under
automatic control of voltage regulators.
However, any changes in limiter settings to
accommodate this operation must be
coordinated with machine thermal limitations and
with the settings of the loss of excitation relays.
continuously at zero field current and rated
voltage except for a few machines with special
electrical characteristics such that stator heating
becomes a problem. Modern, continuously acting
voltage regulators are equipped with
underexcited reactive ampere limiters which
permit machine operation at underexcited
capability determined by thermal limitations
without danger of the machine becoming
unstable. These limiters respond to underexcited
reactive stator current and to terminal voltage.
As a part of the regulating function, limiters
automatically hold the underexcited reactive
amperes at lower values as kilowatt load on the
machine is increased. Limiter settings are
adjustable so that their operating characteristics
may be coordinated with the thermal capability
curve and the steady state stability curve. The
Denver Office, should be consulted when any
operating problems arise concerning
underexcited operating capability either under
load or strictly as a synchronous condenser.
Additional underexcited synchronous condenser
capacity is sometimes made available by
operating with negative field current under
automatic control of voltage regulators.
However, any changes in limiter settings to
accommodate this operation must be
coordinated with machine thermal limitations and
with the settings of the loss of excitation relays.
13 (FIST 1-4 3/91)