IEEE Power & Energy Magazine - July/August 2021 - 59

dispatch combinations of the DERs considered in a given
aggregation portfolio. This eventually results in the
" capability " envelope.
The aggregated active-reactive power potential of
the aggregated DERs subject to network constraints, indicated
here as feasibility, is a smaller NOE and is illustrated in
Figure 7(b), along with the DER-level operating envelopes
assigned by the DSO, as discussed previously. To properly
account for network constraints and potential active and reactive
power management strategies, building the " feasibility "
envelope requires power flow or optimal power flow studies.
A comparison between capability and feasibility NOEs
(shown in Figure 8) reflects the significant reduction in aggregated
operational space because
of network constraints, with the
red region denoting the operating
region that is infeasible either
because of thermal or voltage
violations. However, voltage
constraints can be alleviated by
injecting/absorbing reactive power,
as illustrated in the figure, so that
network capacity and,
2
1.5
1
0.5
therefore,
upstream provision of active power
can be unlocked through reactive
power management.
Grid and Market Services:
Temporal Aspects of
Flexibility Deployment
A feasibility NOE reflects the P-Q
potential of DERs considering network
constraints; yet, it lacks the
crucial information regarding the
temporal aspects of flexibility
provision, which is particularly
relevant for the representation
of specific grid services. For
instance, several markets, e.g.,
frequency control ancillary services
(FCASs), demand response,
and others, require participants
to respond within a predefined
response time and expect them to
deliver the response for at least a
predefined duration of the service.
Given the diverse nature of
DERs, physical constraints, such
as ramp-rate restrictions of slower
DERs (e.g., diesel generators) and
energy-content limitations of the
distributed storage (e.g., household
batteries), must be considered to
establish the actual DER aggregate
potential to participate in a
july/august 2021
-0.5
-1
-1.5
-2
-2.5
-3 -2.5 -2 -1.5 -1 -0.5
Capability NOE
0 0.5 1 1.5 2
Active Power (pu)
Feasibility NOE
figure 8. A comparison of the capability and feasibility NOEs shown in Figure 9.
2
1.5
1
0.5
-0.5
-1
-1.5
-2
Raise FCAS Potential Lower FCAS Potential
-2.5
-3 -2.5 -2 -1.5 -1 -0.5
Feasibility
Slow Lower
Fast Raise
Delayed Raise
Active Power (pu)
0 0.5 1 1.5 2
Fast Lower
Delayed Lower
Slow Raise
Operating Point
figure 9. An example illustrating " flexibility " NOEs used to describe different FCASs
against the " feasibility " NOE.
ieee power & energy magazine
59
Capacity Unlocked
Through Reactive
Power Management
Maximum Active Power
Export Without Reactive
Power Control
Maximum Active Power Export Potential
With Reactive Power Control
Reactive Power
Required to
Achieve Maximum
Hosting Capacity
particular market. A new class of " flexibility " NOEs can be
defined, whose boundaries in the P-Q space delimit the possible
regions of operation for the DER aggregation while
providing a certain service, subject to the relevant temporal
constraints and starting from a given operating point.
For example, in the Australian National Electricity Market
(NEM), " raise " and " lower " FCASs are used to provide
" upward " and " downward " reserves, respectively. Figure 9
illustrates a generic DER aggregate where the fast, slow, and
delayed FCASs must be delivered within 6 s, 60 s, and 5 min
and last 60 s, 5 min, and 10 min, respectively, again according
to the NEM FCAS market. The size of the NOE monotonically
increases with the response time as more resources
Reactive Power (pu)
Reactive Power (pu)
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