MIL-STD-1399-3008
3. DEFINITIONS
3.1 Electric power system. The electric power system is the electric power generation and distribution system (excluding electric propulsion systems) including generation, cables, switchboards, switches, protective devices, converters, transformers, and regulators up to the user equipment interface.
3.1.1 Electrical interface. The interface is the boundary between the electric power system and the user equipment where the electric power system characteristics and the user equipment compatibility requirements apply.
3.2 Electric power system ground. Ground is a plane or surface used by the electric power system as a
common reference to establish zero potential. Usually, this surface is the metallic hull of the ship. On a nonmetallic hull ship, a special ground system is installed for this purpose.
3.2.1 Ungrounded electric power system. An ungrounded electric power system is a system that is intentionally not connected to the metal structure or the grounding system of the ship, except for test purposes. An ungrounded electric power system can continue to perform normally if one line conductor becomes solidly
grounded. However, an ungrounded system may be subject to over-voltages greater than five times nominal voltage as a result of an inductive arcing ground between one line and ground.
3.2.2 High resistance-grounded electric power system. A high-resistance grounded electric power system is a system that employs an intentional high resistance between the electric system neutral and ground. High-resistance grounding provides the same advantages of ungrounded systems (i.e., the system can continue to perform normally with one line grounded) yet limits the severe transitory over-voltages associated with ungrounded systems.
3.2.3 Solidly-grounded electric power system. A solidly-grounded electric power system is a system in which
at least one conductor or point (usually the neutral point of the transformer or generator winding) is intentionally and effectively connected to system ground. A single ground fault from one line to ground will produce high fault
current that should cause selective tripping of protective circuit breakers interrupting power service continuity.
3.3 Frequency. Units are in Hertz (Hz).
3.3.1 Nominal frequency. Nominal frequency (fnominal) is the designated frequency in Hz.
3.3.2 Frequency tolerance. Frequency tolerance is the maximum permitted departure from nominal frequency during normal operation, excluding transients and modulation. This includes variations such as those caused by load changes, the environment (temperature, humidity, vibration, inclination), and drift. Tolerances are expressed in percentage of nominal frequency.
3.3.3 Frequency modulation. Frequency modulation is the permitted periodic variation in frequency during normal operation, calculated by Equation 1 and shown in Figure 2. For purposes of definition, the periodicity of frequency modulation should be considered as greater than 1 cycle but not exceeding 10 seconds.
§ − ·
fmaximum fminimum
Frequency modulation (percent) = i ix100
⎝ 2 xfnominal )
EQUATION 1
4
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