IEEE Electrification - September 2022 - 66

subject to their specific application, can have different
compositions and configurations.
As IPSs integrate increasing penetrations of renewable
energy (RE), they face technical challenges in hosting a
greater capacity of inverter-based generation. Inverter-based
technologies are unable to supply the full range of service
commonly provided via synchronous thermal generation,
and new applications and approaches must be developed to
ensure system security and reliability. Known issues include
the limited ability for fault current contribution, typically
1.5 times inverter rating or less and the inability of synthetic
inertia to fully replicate mechanical inertia. The issues
become particularly acute in systems with high penetration
of inverter-based generation based on intermittent
resources (i.e., wind and solar). In response to these issues,
the Center for Renewable Energy and Power Systems has
developed a range of flexible supply-side technologies able
to support high renewable penetrations while preserving
much of the ancillary service traditionally sourced from
synchronous generation. A range of these applications are
discussed in this article including low-load diesel (LLD),
variable-speed diesel (VSD), and different types of storage.
This article discusses the common characteristics seen
in the legacy IPSs and trends and challenges of system
transformation to a clean and sustainable way of
operation. While discussions and illustrations mostly center
around IPSs located on islands, similar trends can be
observed in other remote coastal and inland locations
(e.g., mines, military bases, remote communities such as
Alaska, and other rural settlements).
Conventional IPSs
Traditionally IPSs, with some exceptions of those based on
hydro or geothermal resources, are predominantly dependent
on diesel or oil fuels for electricity generation. The fuel is
burned by the reciprocating internal combustion engines
(ICEs) which convert the chemical energy of the fuel into useful
kinetic energy. This kinetic energy is then used to spin the
rotor of a synchronous generator producing electrical energy.
Utilization of diesel ICEs is a common electricity generation
method in many isolated minigrids (illustrated in Figure
1). Diesel engines have the following characteristics that
make them attractive in remote applications as follows:
x Low capital costs: The technology is mature and has
been on the market for a long time. There is a variety
of manufacturers that offer competitive prices.
x Robustness: Diesel engines are extremely reliable and
can operate under extreme weather conditions and in
different climates.
x Easy to maintain: Maintenance is of a repetitive nature,
well documented and is executed periodically once
the engine reaches certain hours of operation. Highly
specialized skills are normally not required.
Diesel Generation
Unit 1
Unit 2
Unit n
IPS
Grid
Community
Load
Load 1
Load n
x Easy to operate: Technology is mature and does not
require highly specialized skills to operate. Multiple
units can be integrated and work in synchronism.
x Start-up simplicity: Engines can be quickly switched
into operation with no significant warm-up delay.
x Responsiveness: Diesel engines can quickly and reliably
respond to load variations.
x Efficiency: High efficiency is at nominal loading (80%
loading) and acceptable efficiency is at partial loading.
Figure 1. Conventional isolated power system.
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1
Gen A (1.5 p.u.)
Diesel Generation Under 1 p.u. Loading
IPS Electrical Load
Figure 2. The diesel generator operation under a 1 p.u. loading. p.u.: per unit.
66
IEEE Electrification Magazine / SEPTEMBER 2022
x Installation simplicity: New diesel units can be easily
installed and integrated into the existing grid.
There are two main parameters
Diesel Engine Installed Capacity
Available Spinning Reserve
Under 1 p.u. Loading
Diesel Generation = Load (1 p.u.)
Diesel Low Load Limit
(40% of Installed Capacity)
of diesel generators that are important
to consider, namely minimum
load factor and spinning reserve. The
minimum load factor is the ratio of
the engine capacity to its low-load
limit (LLL). In most cases, it is
advised to avoid operating diesel
assets below 40%. Failure to comply
may result in problems related to
" wet stacking, " " blow up, " and " cylinder
glazing, " which can lead to
premature wear of equipment. In
addition, the engine operation is
optimized around its rated output
where it has the highest efficiency.
Spinning reserve is the extra
Power (p.u.)
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