IEEE Electrification Magazine - June 2015 - 13

IRRORING THE TERRESTRIAL POWER SYSTEM, NAVAL WARSHIPS
have employed electrical power systems for over 100 years. The
design philosophy for naval power systems is expressed well by
the Naval Sea Systems Command (NAVSEA) Design Practices and
Criteria Manual, Electrical Systems for Surface Ships, Chapter 300:
The primary aim of the electric power system design will be for survivability and continuity of the electrical power supply. To insure continuity of
service, consideration shall be given to the number, size and location of
generators, switchboards, and to the type of electrical distribution systems
to be installed and the suitability for segregating or isolating damaged sections of the system.

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naval Power System Characteristics
While much of the equipment and many of the methods and standards are
common between terrestrial grid-based power systems and naval power systems, naval power system characteristics are more closely matched to islanded microgrids.
x Variable frequency: The frequency cannot be assumed to be constant aboard
ship. The limited rotational inertia of the prime movers and generators
allows for rapid accelerations and decelerations of the shaft and corresponding frequency fluctuations in response to load changes. Frequency fluctuations can be expected to last up to 2 s.
x Lack of time-scale separation: For naval power systems, the principal time
constants of controls, machine dynamics, and electric dynamics all fall
within the same general range of milliseconds to seconds. The practice of
decomposing the problem by time-scale separation, which is often used in
analyzing commercial power systems, becomes much more difficult.
x Load sharing instead of power scheduling: The commercial power utilities
operate by scheduling the power delivered by each of the generating units.
The mismatch between scheduled power generation and the actual load is
met by the equipment acting as a swing generator. Aboard the ship, however,
both real and reactive power are shared equally among all paralleled generators through the fast exchange of load-sharing information. This fast
exchange of information strongly amplifies the dynamics of all the paralleled generators.
x Short electrical distances: The distances on board a ship are short (typically
under 350 m), making the modeling of transmission lines unnecessary for
many applications and trivializing the load-flow problem, which is important to the commercial power sector. The short electrical distances also
strengthen the dynamic coupling of the various subsystems making up the
electrical power system.
x Load dynamics: Commercial utilities usually assume loads are either consuming constant real and reactive power or are constant impedances.
Shipboard systems, however, must account for the dynamics of loads such
as propulsion motors, large pumps, pulsed loads, propeller dynamics, and
ship dynamics.
x Tighter control: Because a ship is relatively small, a higher level of centralized
control can be exercised over the shipboard power system than can be exercised in the commercial power industry.
x Ungrounded or high-impedance grounded systems: Naval power systems are
designed to enable continued operation with a single line to ground fault.

Digital Object Identifier 10.1109/MELE.2015.2413434
Date of publication: 29 May 2015

IEEE Electrific ation Magazine / j une 2 0 1 5

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2015