Ceiling Current
Low ceiling voltage exciters, normally
less than 150% of rated value, can usually be allowed to attain their
ultimate ceiling current. Where high ceiling voltages are employed
for improved transient performance, the ceiling current, if
unrestricted, may reach high values and require excessive exciter
capacity.
An inclusion of a field current limiter
should be considered to limit the ceiling current to a specified
value. Ceiling voltage would then still be available to force the
rapid change in current.
The ceiling current of the excitation
system should have a transient time capability equal to or greater
than the short time overload capability of the synchronous machine to
which it is connected. ANSI C50.13-1989 [3] and ANSI C50.14-1977 [4]
give the field winding short-time thermal overload requirements.
Note that these overloads are based on
the voltage (rather than current) applied to the field windings.
Presently there is no corresponding requirement in ANSI C50.12-1982
[2] for salient pole machines.
Ceiling Voltage
The ceiling voltage of an excitation
system is normally not specified directly but is a function of the
excitation system's nominal response requirement. This is one area
where it is easy to specify conflicting requirements, and the
specification writer is cautioned to be sure that some other
reference to ceiling voltage does not conflict with the response
requirement.
The response should be specified by the
user and the selection of the ceiling voltage left up to the
manufacturer. For systems that obtain their energy from an ac source,
the per unit voltage and (if applicable) current values of this
source at which the nominal response requirement shall be met should
be specified.
Present standards base the rating of an
exciter on its continuous output parameters and its time response to
transient change. It is understood that the equipment must function
in the transient mode and achieve ceiling output conditions without
any detrimental effects.
The ratio between ceiling and normal
operation voltages will increase as higher nominal response systems
are specified. For certain special cases, a negative ceiling voltage
may be required to control machine overvoltage conditions. Due to
firing angle margin requirements of thyristor exciters, the negative
ceiling is normally specified to be not more than
Fault and Pole-Slipping Duties
The excitation system must withstand,
without damage, any faults or abnormal operation of the synchronous
machine. Faults on the synchronous machine ac terminals will induce
large positive currents into the field (adding to the normal field
current).
In addition, the induced current will
have an ac component at the power frequency. This is important when
rectifier exciters supplied at the power frequency are involved since
the peak current occurs at the same point each cycle and tends to
overload one phase of the rectifier.
The magnitude and time duration of this
induced current is a function of machine and system reactances. Refer
to IEEE C37.18-1979 (ANSI) [5] for a table of suggested values of
induced currents for various types of machine construction.
In addition to the positive induced
field current under faults, there can be negatively induced currents
(subtracting from the normal field current). These negative currents
can be induced into the field circuit during pole-slipping events.
When the negative induced current is so
large that the total current becomes negative, and if the negative
current is not allowed to flow, then the resulting voltage may become
excessive. Excitation systems that employ solid-state rectifiers
normally conduct current only in the positive direction.
Some machines are inherently
self-protecting due to additional current paths in the rotor. These
may be damper windings or a solid steel structure. In machines where
there is a possibility of large voltages, protective equipment may be
supplied to protect both the exciter and machine field circuit.
While the magnitude of the induced
negative field current is a function of the machine design, the time
the current flows is a function of the number of pole-slipping
cycles, the system operating procedures, and the protective relay
settings involved.
The maximum time that any potentially
damaging negative field current will flow should be specified in
order to ensure there is sufficient energy capacity in any protective
equipment.
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