IEEE Electrification - September 2020 - 42

the energy is transferred via the power converter, with
adverse impact on the system performance and the voltage waveform distortions. Sometimes the shaft generator
is directly connected to the ship's main bus bars. However,
this simple low-cost solution leads to instantaneous frequency fluctuation in waves comparable or even greater
than in ships with electric propulsion.

Standard Framework
The standards related to maritime microgrids usually do
not concern the instantaneous frequency fluctuations,
except for the previously mentioned transient variation.
The sole exception is the IEEE Standard 45 series. This
phenomenon was mentioned in IEEE Recommended Practice for Electrical Installations on Shipboard, IEEE Standard
45, 2002, and IEEE Recommended Practice for Electrical
Installations on Shipboard-Design, IEEE Standard 45.1, 2017.
These standards define the frequency modulation as
The permitted periodic variation in frequency during normal operation that might be caused by regularly and randomly repeated loading. For the
purpose of definition, the periodicity of frequency
modulation should be considered to not exceed 10 s.
The severity of frequency modulation is calculated as
the ratio of the difference between maximum and

125-kW
Bow
Thruster

M
~
~

M1
~

...

Mn
~

Load

Finally, the resulting instantaneous frequency fluctuations depend on the interaction between both generation
and load with varying instantaneous characteristics. The
most important factors behind the fluctuations can be
summarized as
xx
the characteristic of load modulation
xx
the diesel engine (generator's prime mover) governor
type and settings
xx
the number of diesel engine cylinders and flywheel
xx
the generator and its automatic voltage regulators
characteristics
xx
the number of generators working in parallel and
mean load.
Nevertheless, the resulting instantaneous frequency
fluctuation is a combination of components related to
torque angular pulsation and components related to
load modulation.
Finally, there is another reason for instantaneous frequency fluctuation in maritime microgrids: sometimes, to
save fuel, the shaft generator is used. This means that the
main engine used for ship propulsion also will be the
prime mover of the generator. The changes of this engine
rotational speed related to sea conditions obviously lead
to fluctuations of the frequency on the shaft generator's
terminal. To stabilize the frequency in the entire system,

3 × 400 V,
50 Hz
Bus-Bars

BR1
376 kVA
1,500 rpm

BR2
376 kVA
1,500 rpm

(a)

376 kVA
1,500 rpm
G2
~

G1
~

357 kW
1,500 rpm

BR3

357 kW
1,500 rpm

G3
~

357 kVA
1,500 rpm
(b)

Figure 1. (a) The research-training ship under analysis with mechanical propulsion. (b) A simplified diagram of the electric power system of the
ship with mechanical propulsion without shaft generator (the main engine is not presented in the figure). The generator prime movers are fourstroke diesel engines with 10 cylinders. M: motor; G: generator.

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020

IEEE Electrification - September 2020

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