IEEE Electrification Magazine - June 2015 - 56

When the electric machine is operating as a generator,
the converter must control the system's voltage and frequency, acting in place of the common AVRs and SGs, so
appropriate regulators need to be adopted. The prime
mover's SG is still present, but it operates following a reference that is totally independent from the IPS power and
frequency management. As an example, in a shaft generator, the prime mover's speed will follow the propeller's
needs and not the IPS's. In the inverter-based generators,
AVRs can be either present or not, depending on the alternator technology or the designer choices. Indeed, having a
power converter that is capable of managing the variations in the frequency and voltage on the alternator side,
converting them in the ranges better suited for the IPS, it
is possible to work with a fixed excitation system [e.g., a
permanent magnet (PM) generator].
It is important to note that the converter could control its
electric output variables with a bandwidth that is far greater
than a common generator set. In fact, the power-electronic

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Figure 8. The inverter-based generator responses due to voltage
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Figure 9. The diesel generator responses due to inverter-based generator references step variations.

converter can impose, almost instantaneously, voltage and
current vector variations at its terminals. (The time delay
depends on the converter's commutation frequency.) Accordingly, it is necessary to implement voltage and frequency regulators that are tuned to the other generators of the system; otherwise, some issues can arise. If the inverter-based generator is
set with a too-high regulation bandwidth (compared to the diesel generators), it will bear all the regulation actions necessary
to withstand the IPS voltage and frequency transients. This
behavior may be desired to some extent, giving as a result the
partial freeing of the diesel generators from the fast transient
responses, but it has an impact on the inverter generator itself,
both on the prime mover and converter. In fact, these devices
must be designed to bear the increase in solicitation without
suffering. However, it must be noted that the prime mover, particularly in the case of shaft generators, is already heavily solicited by the mechanical stress due to the propeller operation;
therefore, the addition of other solicitations due to the electric
plant transients may be critical.
As an example, Figures 8 and 9 show the simulation
results regarding an entire IPS supplied by a shaft generator
paralleled to a diesel generator. The system considered is the
one shown in Figure 7. The shaft generator's voltage and frequency controls have been set in accordance to the diesel
generator's AVR and SG settings to realize an equal sharing
of active and reactive power between the two power sources.
Figure 8 shows the voltage and power responses of the
inverter-based generator due to references step variations.
Figure 9 shows the corresponding responses of the paralleled
diesel generator. As can be seen, the controllers' settings permit the same responses from both the generators, despite
their different architecture.
Nevertheless, the differences in the voltage and frequency
cycles between generators directly connected to the power system and inverter-based generators are relevant, even if they are
tuned so that they respond with the same equivalent time constant. In fact, the direct-connected generator responds with an
electromechanical-based cycle, while the inverter-based generator responds with a cycle, which is the composition of the electric machine's electromechanical cycle and the total electric
cycle of the converter. These differences could cause unexpected interactions between the control cycles when the two types
of generators are paralleled, unearthing instabilities and abnormal variations in the electric variables.

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2015

The previously explained voltage droop function, besides its
utility in permitting a stable parallel operation of the generators and the definition of the VAR sharing between them, leads
to a lowering in the main switchboard voltage when the reactive load increases. This reduction can usually reach a value of
around 5% of the rated voltage at the rated reactive load,
depending on the IPS designer choices. To avoid this, a master
AVR (MAVR) could be introduced in the system. The MAVR is
an additional AVR that senses and regulates the main switchboard's voltage. It works by varying the voltage references of all

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