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ELECTRICAL DISTRIBUTION SYSTEM (EDS)
Electricity powers almost everything on board the Ohio
class. Despite plenty of non-electrical back-up
systems, using them most likely means the submarine is
stranded on the ocean floor and will eventually be
abandoned. To try to prevent ever getting to this
point, the EDS is designed to maintain electrical
continuity with a clever setup.
GENERATING POWER
Generating electrical power is pretty much the same as
any heat-based power plant, nuclear or otherwise.
The process by which the reactor produces high pressure
steam was described in the propulsion
section. Unlike the main engines, the turbine
generators have their output shafts connected to an AC
generator.
The turbine end of the generator acts to convert the
molecular momentum of steam (pressure) and kinetic energy
of steam (flow) into rotational kinetic energy on the
shaft by means of turbine blades on a rotor. These
blades are shaped to achieve these energy transformations
and perform work. There are two basic blade types
and the turbine generator has a combination of both.
Both types take advantage of a fluid's changes in velocity
and pressure when passed through a nozzle-shaped
area. The basic function of any nozzle is to
restrict flow in order to increase velocity at the expense
of pressure. The energy conversions caused by the
blades are described in the following image, starting with
an analogy at the top. Note that "velocity" refers
to velocity of steam, not velocity of the rotor. For
the generators, rotor velocity is constant during steady
state operation.
IMAGE
SOURCE: Mech
4 Study
IMAGE SOURCE: Diesel
& Gas Turbine Worldwide
The rotating shaft of the turbines are connected to AC
generators. The shaft of the generators are
connected to rotors which are constructed to be
electromagnets. Keep in mind the requirements for
generator action: a magnetic field, a current-carrying
conductor, and relative motion between the two. So,
the first requirement is met by the electromagnet.
The conductor requirement is satisfied by a stator that
surrounds the rotor and the relative motion is achieved
given that the rotor rotates and the stator is
stationary. A voltage is induced in the stator and
outputs an AC current that supplies the majority of the
ship's loads. It is AC due to the voltage
oscillating between positive and negative in response to
the rotating magnetic field. Though the entire AC
system is three-phase to minimize the effect of these
oscillations.
IMAGE SOURCE: waitbutwhy.com
The turbine generators connect directly to switchboards
designated as non-vital, since they have a high
probability of being tripped off in response to an
emergency, either procedurally or automatically as
self-protection. While there is a connection from
these non-vitals to the vitals so that the generator can
power everything, this connection is setup to
automatically cut itself and switch the vital loads to
battery power in the event of a trip.
MOTOR GENERATORS
The requirements for generator action were mentioned
above. The requirements for motor action are nearly
the same with the main difference being that instead of
creating voltage in the stator, the current is already
supplied, creating a rotating magnetic field due to AC
oscillations. This magnetic field pushes against the
magnetic field of the rotor, causing the rotor to rotate,
driving any device that requires rotational input.
This is how most motors work, but keeping things basic
allows a unique application called the motor generator.
IMAGE SOURCE: waitbutwhy.com
The similarities between generator action and motor
action mean that it is possible to construct a device that
is capable of doing both. Put two of them together,
and the result is a machine that can be a motor on one end
driving a generator on the other end, with the freedom to
swap which end is driving and which is generating.
These are the machines that connect the AC side of the EDS
to the DC side. Generating a DC current with a
rotation-based device is a bit more complicated and the DC
side of the motor generator has extra components to do the
job. Having a DC side in the EDS is necessary
because it is impossible for a battery to be AC. The
normal mode of operation, with steam turbines, the diesel
generator, or with shore power is to have the motor
generator to be AC-motoring/DC-generating to charge the
battery. In the event that the AC source of
electricity is lost (much more common than you would
think), the motor generator is setup to automatically
switch it's operating mode to DC-motoring/AC-generating,
discharging the battery to keep vital loads powered.
Due to the nature of a battery serving as a backup in case something goes wrong, there are a few loads placed on the DC side of the EDS to deliver bare essentials just in case even the motor generators fail to operate. Two important examples include a separate circuit for low-level lighting and the emergency propulsion motor, capable of engaging with the propeller shaft with a clutch.
DIESEL GENERATOR
The battery is a very limited resource of electricity,
allowing only about half a day's worth of power.
That's why if steam power is lost for whatever reason and
it has been determined that restoring it will take a
while, the response is to start the diesel
generator. There's enough fuel on board to last a
few days at least, depending on usage.
Note that this image shows a smaller
diesel engine on a different submarine class. IMAGE SOURCE: Jonesblog
The generator side of the diesel generator works exactly
the same as those on the steam-powered turbine
generators. The diesel side works similar to any
other internal combustion engine. There is plenty of
reading elsewhere online for that. Instead, here are
a few facts:
- Cylinder layout: Straight 12 with 24 pistons using an opposed-piston design.
- Source of air intake is the internal air of the submarine with replacement air coming from the snorkel mast.
- Exhausts directly into the ocean. Exhaust piping is internally sprayed with seawater to cool the gases and prevent cavitation.
- Since a starter motor for this engine would draw too
much power, the engine is instead started with
compressed air.