High-Speed Generators for Power-Dense, Medium-Power, Gas Turbine Generator Sets

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T E C H N I C A L P A P E R
High-Speed Generators for Power-
Dense, Medium-Power, Gas Turbine
Generator Sets
& R. M. Calfo, M. B. Smith, and J. E. Tessaro
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NEJ 20
Abstract
Concepts to improve turbine generator (TG) set power density (PD) are identified, developed, and
evaluated. The evaluation focuses on the TG set as a part of overall system optimization, with weight
and volume penalties for special auxiliaries. PD is given priority, while other attributes such as effi-
ciency, acoustics, and total system cost are also assessed. The strengths and weaknesses of each
concept are assessed based on technical viability, potential for PD improvement, and risk. Sixty-one
generator concepts over four generator types—air-cooled wound field, water-cooled wound field,
permanent magnet (PM), and high-temperature superconducting (HTS)—are evaluated with three
ship-level electrical distribution architectures—high-frequency AC and active- and passive-rectified
DC. A prime mover of 14 MW at 7,000 r.p.m
. is assumed. The study assesses generator size enabled
by directly coupling the generator to the prime mover, eliminating the typical gear required in TG
sets with engine speeds greater than 3,600 r.p.m. Technology approaches are described and trends in
the design data are identified. Advanced PM and superconducting rotor technologies are evaluated.
For the power, speed, and system requirements studied, these technologies do not differ significantly
from water-cooled designs in size and weight. A water-cooled generator would be compatible with a
range of future power distribution systems such as high-frequency AC, rectified DC, or 60 Hz AC
(using a rectifier and an inverter).
Introduction
Interest in increasing the power density (PD) of
TG sets motivated an investigation by Curtiss-
Wright Electro-Mechanical Corporation (EMD)
of high-speed generators coupled directly to gas
turbines. This eliminates gearing between the
turbine and generator and allows a smaller gen-
erator for a given power. Distribution options
including non-standard AC distribution (120 or
240 Hz), DC distribution, or inverter systems
converting DC generator output to 60 Hz AC
distribution all free the generator from the re-
quirement to operate at 3,600 r.p.m. for a 60 Hz
output.
NAVAL ENGINEERS JOURNAL Several fundamental approaches exist for in-
creasing the PD of a generator. The general
expression for generator power output is
Pa ¼ Kðpf ÞABD2LN; ðkWÞ ð1Þ
K is the unit conversion factor, pf is the power
factor (unit-less), A is the armature surface cur-
rent density (A/in.), B is the peak air gap flux
density (T), D is the rotor diameter (in.), L is the
stator core length (in.), and N is the rotational
speed (r.p.m.).
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