IEEE Electrification Magazine - September 2017 - 56

According to Doerry from the U.S Navy, there are
many reasons for employing a medium-voltage dc
(MVdc) system in a military application, which include
the following:
xx
Smaller transformers and electromagnetic devices are
needed because the power conversion equipment
operates at high frequencies.
xx
Transmission of power uses the full cross section of a
dc conductor. Also, dc systems may be able to use
lighter cable weights.
xx
Only voltage matching is required by parallel power
sources, not phase matching.
Further advantages include the reduction of the power
system weight-to-space ratio, reconfigurability in case of
fault, and enhanced power quality.
In Figure 1, an MVdc system proposed for a U.S. Navy
all-electric ship is depicted. Generators and energy storage devices are shown on the left side, and loads, including propulsion motors, actuators, sensors, and pulsed
power weapons, appear on the right side. The main characteristics of this system are that it is enabled by power
electronics, it has limited generation capability and low
rotating mass, and it must accommodate fast-changing
load levels. Notice that there are other dc electrical distribution systems with similar characteristics, such as a terrestrial dc microgrid, more electric aircrafts, submarine

System-Level Stability Challenges
In a dc ISPS, the individual load (or groups of loads) is normally fed through power electronic converters directly connected to the distribution MVdc-bus. A power converter has
a nonlinear dynamic because of its switching behavior. It
can be considered a variable structure system. Power converters typically operate under closed-loop control, and
their dynamic behavior is largely determined by that control, which can be either linear or nonlinear. This adds an
additional element of complexity to the overall system
dynamics. It can be said that the interconnection of these
devices also results in a nonlinear system. When observing
the system from the dc-bus, load converters operating
under closed-loop output voltage control exhibit a constant
power load (CPL) behavior, which is also nonlinear behavior.

MVdcBus

Circuit
Breaker

~
~
=

PulseCharging
Circuit
=

Gas Turbine
=

Shore Power
Interface

~
~
=
Transformer

Dedicated
MVdc-bus
Energy
Storage
Capacitor
Banks,
Fuel Cells

Auxiliary
Generator

=
=

Port
Propulsion
Motor
Point-of-Load
Converters
=

UPS
Batteries

~
~

LVacLoad

LVdcLoad

Starboard Longitudinal
dc-bus
=

~
~

Port Longitudinal
dc-bus
=

Pulsed
Load

~
~

Ship Service
Load Centers

Main
Generator

vehicles, and space stations. In this context, the developed and presented concepts can be a vital contribution
to terrestrial dc microgrids and dc distribution systems in
general, since ships offer a greenfield deployment platform for new technology.
In dc systems, voltage stability must be guaranteed,
which is a hard requirement. One possible cause of instability, and consequently a challenge to global voltage
control, comes from the actively controlled power electronics devices used in MVdc distribution systems.

Starboard
Propulsion
Motor
Radar
Load

Diesel Generator
Figure 1. The proposed MVdc power-distribution system for the U.S. Navy all-electric ship (simplified). LVac: low-voltage ac; LVdc: low-voltage dc;
UPS: uninterruptible power system.

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2017

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2017