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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