IEEE Power & Energy Magazine - March/April 2022 - 61

respect to the ratings of the power sources). It is challenging to
attain such a balance unless some form of dispatchable energy
resources, storage, and/or demand-side management schemes
are used. As the proportion of renewable sources increases in
such systems, this problem becomes even more challenging.
Unlike in traditional power systems, the relatively small geographic
area of a microgrid means the effects of local weather
conditions, such as clouds and wind gusts, are not spread out,
leading to potentially large fluctuations in power production.
There are also predictable daily and seasonal variations in
renewable power production that need to be considered. In this
case, an SMR can serve as a reliable source of continuous controllable
power, ramping up and down according to changes in
demand or variations in renewable power sources.
As an example, consider a simple standalone microgrid
shown in Figure 2(a) with the corresponding 48-h load and generation
profiles illustrated in Figure 2(b). The load profile in
Figure 2(b), panel (i) exhibits small peaks in the morning and
larger ones in the evening. The solar photovoltaic (PV) and wind
power outputs in Figure 2(b), panels (ii) and (iii) vary due to
changes in the weather conditions. Notably, the production does
not match the load profile by using PV and wind sources alone.
When an SMR is incorporated into the supply mix, one
operating strategy, as illustrated in Figure 2(b), panel (iv),
adjusts its output power to meet the anticipated peak demands
and to smooth out the variations associated with the renewable
resources. However, the rate of power changes in an SMR
may be constrained by physical and safety limits and may not
Energy Resources
dc to ac
PV
PV
(ii)
dc to ac
Wind
Industrial
(iv)
SMR
Vehicular
(v)
Bidirectional
dc to ac
(vi)
Battery Storage
(a)
Residential
(b)
figure 2. (a) Microgrid configuration; (b) Example operation scenario showing 48-h power profiles: (i) load profile; (ii) PV
profile; (iii) wind profile; (iv) SMR ramping; (v) battery charging/discharging; and (vi) battery SOC: state-of-charge.
march/april 2022
ieee power & energy magazine
61
Charging
Discharging
SMR Output Difference
Battery
Commercial
Wind
(iii)
SMR
be able to match the load demand precisely on its own. Different
power regulation techniques need to be developed. For
example, upon a sudden drop in demand, one option is to use a
steam-bypass mechanism to quickly trim the turbine-generator
output power without ramping down reactor power itself. However,
in circumstances when the reactor is operating at a low
power level, different solutions are needed to accommodate a
sudden large increase in demand.
To deal with this problem, a relatively small capacity battery
energy storage system can be introduced into the microgrid to
balance any remaining mismatches between the demand and
the supply as shown in Figure 2(b), panel (v). In this case, the
battery charges whenever there is surplus power available and
discharges if the combined PV, wind, and SMR outputs are
unable to support the load demand. Sudden unexpected power
imbalances can be compensated by the battery. The battery can
react relatively quickly as long as it operates at an intermediate
state-of-charge level where it can both supply and absorb
energy as needed. This is illustrated in Figure 2(b), panel (vi).
Control and Energy Management
in SMR/Renewable Microgrids
Since microgrids integrate multiple types of energy sources of
wildly different characteristics, storage, and loads, it is challenging
to control them effectively to achieve the desired level of
service quality. Control and energy management strategies for
microgrids are commonly organized in a hierarchical form. The
lowest level contains the real time control loops, which regulate
Loads
(i)
Load
Battery SOC
IEEE Power & Energy Magazine - March/April 2022

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2022
