IEEE Power & Energy Magazine - July/August 2021 - 58

facilitate maximum market participation of aggregators. In
this regard and in the context of a distributed energy marketplace,
it becomes important to understand the potential economic
value of reactive power, especially as reactive power
support from inverter-based DERs could limit their active
power market participation while enabling the participation
of other DERs.
Modeling of the network constraints in the operating
envelope concept is essential for their application to distributed
energy markets explicitly considering reactive power.
Detailed distribution network modeling can demonstrate
how additional network capacity can be accessed through
proper management and control of reactive power. A DSO
can expand its reactive power capability either
through
investing in reactive power infrastructure (e.g., capacitor
banks) or acquiring reactive power from aggregators. In both
cases, the key question is how to value reactive power from a
network and then market perspectives. From an aggregator's
point of view, the provision of reactive power could potentially
maximize their market revenues, for example, by providing
local network capacity support. However, this could
also restrict active power participation from some DERs in
other energy and ancillary service markets. Developing an
understanding of these issues is pivotal to inform decision
making, drive investment, establish relevant business cases,
and bring different actors (DSO, aggregators, and active customers)
to the negotiating table.
The underlying challenge is to determine methodologies
suitable for modeling distribution networks considering ac
power flow and all possible combinations of active/reactive
power export/import from all of the DERs, as discussed
previously. In this respect, a useful concept to demonstrate
the idea is that of a nodal operating envelope (NOE), which
characterizes the aggregate network limits as seen upstream
of a given reference node, in contrast to operating envelopes
that specify import/export limits at each DER connection
point. An NOE thus represents the aggregated behavior (i.e.,
minimum/maximum real/reactive power injection/absorption)
of downstream resources at a reference node that can
be accommodated without violating the operational limits
of the DERs and the network. Alternatively, an NOE can
be interpreted as the active power import/export capacity of
downstream resources at the reference node, represented as
a function of reactive power.
" Capability " and " Feasibility "
Regions of DER Aggregates
To highlight the impact of reactive power on DER market
participation via a graphical representation of an NOE, consider
four DERs with diverse active-reactive power (P-Q)
characteristics [shown on a pu scale in Figure 7(a)] that are
to be aggregated at a generic upstream node. Although each
DER can operate at any point within their P-Q characteristics,
their convex hull can be established by proper selection
of a few boundary points, as shown by the dots in Figure 7.
The aggregated flexibility of the DER, called capability,
is also presented in Figure 7(a), representing a much larger
NOE as compared to the summation of individual DER
envelopes, which demonstrates the benefits of aggregation.
Note that the power import is taken with a positive and the
power export with a negative sign.
As this capability NOE does not account for network
constraints, this effectively represents the potential
active-reactive power region ( " capability " ) of DERs for
commercial purposes as would ideally be deployed by
one or more aggregators. The NOE of the downstream
DERs can also be mathematically derived through a geometric
computation approach known as Minkowski summation.
The starting point is the description of an active
and reactive power dispatch point for each DER as a position
vector in the P-Q space. The Minkowski summation
then performs additions of such vectors over all possible
2
1
2
1
-1
-2
-3
-2
Active Power (pu)
(a)
-1
-2
-1 01 2-3-2-10 12
Active Power (pu)
(b)
Capability/Feasibility
DER 1
DER 2
figure 7. (a) A capability NOE and (b) a feasibility NOE from DER aggregation.
58
ieee power & energy magazine
july/august 2021
DER 3
DER 4
Reactive Power (pu)
Reactive Power (pu)
IEEE Power & Energy Magazine - July/August 2021

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