IEEE Electrification Magazine - June 2015 - 51

In an AES, the electric power is generated in two or more
separate power stations, each of them presenting at least two
generator sets. Each set is composed of a prime mover (typically
a diesel engine), a synchronous machine, and all the subsystems required for their operation (fuel pumps, heat exchangers,
automation, etc.). As can be seen in Figure 1, which shows the
typical IPS layout of a cruise ship, the power stations feed separate busbars, which can be operated separately as well as connected by means of a conjunction breaker. The ship's loads are
fed by the busbars, directly or by means of transformators, and
are connected to one or another depending on considerations
about fault tolerance and load balancing.
The subdivision of the power-generating capability into
several parts, each completely separate from the others, is
mandatory to assure the fault-tolerance level required by
the current marine classification rules. Cruise or merchant
ships conventionally adopt the simpliest subdivision,
implementing two separate power stations with two or
three generator sets each. This solution allows for compliance with the rules with the least possible economic
impact. In military ships, where the costs are not a primary
issue, it is preferred to give priority to the survivability of
the unit in combat condition, spreading multiple power stations all over the ship hull.

Power Quality
Ships' IPSs are a particular case of an islanded grid since
they combine generation, distribution, and utilization of

electric power in a single system without external energy
inputs. AESs' power systems are characterized by high
installed power and the presence of loads whose power can
match the power of a single generator. Managing the variations in those loads' absorbed power while, at the same
time, ensuring a high quality of power is challenging.
Moreover, in the common ship's IPS practice, generators
are switched on and off following a logic that tries to minimize the fuel consumption. Doing that, frequent connection and disconnection transients are produced. An example of this practice is reported in Figure 2, where the startup, load, unload, and shut-down of a single diesel
generator during a maneuvering phase are shown. The
most relevant traces are the green one, which is the generator's terminals voltage, and the orange one, which is the
generator's active power. Observing the abscissa, where the
time is reported at which the measure was made, it can be
seen that the entire procedure occurs in less than 20 min.
These continuous connection and disconnection transients act on a system where a lack of power-generating
capability is present due to the nonsimultaneous presence
online of all the generators. These facts, combined together, could lead to severe variations in the voltage and frequency when high power loads are used.
To respond to these solicitations and, at the same time,
ensure the correct operation of the generating units, each
generator set is equipped with a group of controllers. These
range from security systems, which are essential to avoid

DG1
16 MVA

DG2
12 MVA

DG3
16 MVA

GTG
22 MVA

DG5
12 MVA

DG4
16 MVA

11 kv-60 Hz
AFT MSWB

11 kv-60 Hz
FWD MSWB

Port
Cycloconverter

Starboard
Cycloconverter

Low-Voltage Hotel Load

Bow Thruster
1,900 kW
Bow Thruster
1,900 kW
Air-Conditioning Compressor
1,060 kW
Air-Conditioning Compressor
1,060 kW

Air-Conditioning Compressor
1,060 kW
Air-Conditioning Compressor
1,060 kW
Bow Thruster
1,900 kW

Low-Voltage Hotel Load

M 17.4 MW
M 17.4 MW
Port Propulsion
Starboard Propulsion
Motor
Motor
Figure 1. The typical IPS layout of an AES.
IEEE Electrific ation Magazine / j une 2 0 1 5

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