Page 60 - Energize October 2022

TECHNICAL

at different stages of the conversion to produce a clean sinusoidal waveform. to maintain inertia.
Figure 1 shows the typical inverter stages. The AC output is achieved by turning on/ Generator sets selection should
off the IGBTs at a pre-defined sequence and speed which generates pulses of currents consider the fuel available at the project
which is known as modulation, see Figure 2. There are multiple modulation techniques location, the expected usage, emissions
such as pulse-width (PWM), pulse-frequency and pulse-amplitude modulation. PWM requirements, noise limitations, and
inverters are the most common as they eliminate lower order harmonics and decrease the microgrid’s configuration. There are
the total harmonic distortion (THD) content in the output AC voltage. many different generator technologies
Inverters can be grid-forming or grid-following. In grid-following, a voltage reference available on the market that can
must be established by another AC energy source such as the utility or a generator set meet the needs to microgrids. The
before the inverter is able to synchronise and connect to the network. two most common concerns when
In grid-forming inverters, the inverter does not require a reference voltage and can using generator sets for microgrids
establish the voltage of the network independently. Solar inverters are most commonly and prime applications are noise and
grid-following, whereas storage inverters can be either grid-forming, grid-following or emissions, with increasing pressure
both. to be environmentally and socially
responsible.
Synchronous machines Modern generator sets are available
Generator sets with exceptionally low emissions due to
While PV panels and batteries are DC power devices, most electrical equipment uses AC advancements in combustion and after-
power. A generator set, by its nature, generates AC power and can directly be used to treatments technologies. Additionally,
power AC loads on demand, and can be paralleled with other AC power sources such as low carbon fuels such as renewable
the utility grid. natural gas (RNG) and biogas can be
Incorporating a synchronous machine into a microgrid application has many benefits. used for carbon neutral operation.
A generator set is a dependable, dispatchable energy source which, if fuel is available Noise can be addressed through
can be used to meet the electrical load demand. Generators, with their high-power highly sound-attenuated enclosures and
density, can start quickly, accept large block loads, and provide a reference to grid buildings. Noise and emissions at the
following components such as PV or battery inverters. site level can also be minimised through
With intelligent controls in modern generator sets, autonomous synchronisation intelligent controls which optimise
and load sharing can be done at the equipment level and there are a wide variety the use of each asset to minimise the
of generator sets available at various power levels that accept different fuel types. duration and timing of the operation.
Generator sets provide inertia to a system that inverter-based generation cannot
provide, contributing to the overall stability of the system. Paralleling AC power sources
Some microgrids, consisting of synchronous generators and PV energy for example, To parallel or connect alternating power
will require a minimum capacity of synchronous generators to be operating at all times sources, several characteristics of the
AC output voltage must be the same or
within an acceptable range. It is essential
that the waveform and phase sequence
are the same. Also, the frequency, phase
angle and voltage amplitude must be
within the acceptable range to safely and
effectively parallel the AC sources.
Figure 1 Synchronous generator sets produce
AC voltage and can be either in load
sharing (analogous to grid-forming)
or in load governing mode (analogous
to grid-following) which are the terms
used to describe the generator set
paralleling functions. Load sharing is the
proportional division of kW and kVAR
total load among multiple generators in
a paralleled system.
Load sharing is essential to avoid
overloading and stability problems on
the paralleled generator sets. In load
sharing the generator sets regulate their
Figure 2 voltage and frequency. Load sharing can

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