IEEE Power & Energy Magazine - March/April 2022 - 62

the power-electronic switching signals for inverter-based renewable
sources and voltage and frequency control of synchronous
generators for the SMR. The primary layer deals with powersharing
among different energy sources. The secondary layer
regulates the overall microgrid frequency and the voltage levels
for dynamic stability and reactive power support. The tertiary
layer deals with the overall operation of the microgrid for longterm
viability, such as set-point adjustments for seasonal variations,
maintenance scheduling, and economic optimization.
At the lower layers, both centralized and distributed mechanisms
have been investigated. Distributed power/frequency
droop-based approaches are being adopted to minimize the
dependency on any high-bandwidth communications among
physically dispersed energy resources. In these approaches, the
system frequency " droops " lower as the power demand increases.
Suitable power-sharing among different power sources can be
achieved through different droop ratios. This approach has been
extended to a multisegment adaptive droop approach to accommodate
battery charge limits and PV curtailment. A further
extension to incorporate multi-unit SMRs can be considered,
where the operational decisions for each unit may depend on
factors such as safety, economy, and regulatory guidelines.
At the secondary layer, system setpoints can be adjusted
infrequently over low-bandwidth communication links to
maintain the desired voltage profiles and recommended sharing
of real and reactive power. The tertiary level deals with energy
management issues, such as optimal dispatch of generators in
SMRs, management of the charging status of storage devices,
and the coordination of demand-response. This includes making
use of weather forecasts to predict future renewable power
production levels and planning for maintenance outages of
SMRs to improve the power availability.
Within this framework, selecting when and how much to
change the SMR output power level is the responsibility of the
microgrid energy management system (EMS). The EMS may
consider multiple objectives and constraints to determine the
optimal operation strategy for the whole system. These considerations
can include fuel costs, efficiency, reserve capacity,
reliability, and equipment stress. For example, the predicted
overnight load demand can be used to select the output power
level for the SMR over this period. The battery can balance out
any short-term variations in the load demand. Microgrid EMSs
typically include simplified models of the power system components
used in an optimization formulation. The optimization
determines the operating schedule and may also incorporate
demand-response mechanisms to manage loads in addition to
generation and storage resources.
The Electric Power Research Institute Utility Requirements
Document referenced by Ingersoll and colleagues for lightwater
type reactors has been updated to set out minimum performance
expectations for these types of SMRs. It specifies a
24-h load cycle of 100% down to 20% and back to 100%, a
ramp rate of 40% per h, and a step change of 20% in 10 min.
Given these ramp rate requirements, and other design-specific
constraints, accurate forecasts for renewable energy production
62
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are needed so the power outputs of the SMR can be adjusted
appropriately. The mechanism for adjusting the power output
involves control actions that adjust the reactivity of the reactor
core, thus changing the heat output. This may involve motorized
movement of neutron-absorbing control rods in and out of
the core to control the neutron density.
The control strategies for traditional nuclear power plants
can be classified either as a turbine-led or a reactor-led mode of
operation. To achieve load-following capabilities, the turbineled
mode is used. If small, but rapid power adjustments are
needed for fast actions, such as frequency regulation, the previously
mentioned steam-bypass mechanism could be employed.
It provides an adjustment band of 5 to 10% of the full power rating
of the module. The ramp rates for larger core power adjustments
are limited by the buildup of neutron-absorbing nuclear
reaction by-products that need time to decay. In some cases,
mechanical stresses on the fuel cladding and piping can result
from thermal shock if recommended ramp rates are exceeded.
The control actions and modes of operation must follow safety
specifications strictly regulated by local nuclear safety regulators,
such as the Nuclear Regulatory Commission in the United
States and the Canadian Nuclear Safety Commission.
An alternative approach to adjusting the SMR power level
is to run the reactor at near-full power and redirect a portion of
its output to another process. Several variations have been considered,
including a combination of electrical and thermal outputs
for hydrogen production, which can then support a more
efficient high-temperature electrolysis process. Since the electrolysis
process can be started and stopped on demand, it can be
a desirable way for absorbing excess reactor output, converting
this energy into hydrogen. Stored hydrogen can later be reverted
into electricity by a fuel-cell-powered system, used in fuel-cellpowered
vehicles in remote communities and industrial sites,
or burned directly for process heat. Related approaches have
also been proposed to use the excess heat energy to generate
synthetic gas and operate desalination plants.
Some design concepts include a molten-salt secondary cooling
loop that includes thermal storage tanks. This effectively
decouples the SMR from the turbine-generator, though at the
expense of some thermal efficiency. This approach leverages
the existing technologies developed for solar thermal power
plants. In situations where these alternative heat applications
are desirable, both electricity generation and thermal applications
have to be considered to achieve the most efficient and
effective energy management strategies for the microgrid.
Multi-unit SMRs can potentially offer higher degrees of
flexibility in operation. However, the corresponding control
strategies can be more complex depending on the system configurations.
If different modules have independent steam turbines
and generators, each module can be treated as a relatively
separate unit. The control, in this case, is relatively straightforward.
However, for systems in which different modules share
a common steam header, interactions among the reactors can
be more complex. These systems may require additional control
actions to achieve the desired balance in power outputs
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