IEEE Electrification Magazine - June 2018 - 90

transactive gates may have to recognize individual
phases while accounting for respecting predetermined
hosting capabilities of distribution system nodes. We delineate three types of transactive nodes based on their
topological location.
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transactive top nodes (ttns), at the interface between
transmission and distribution networks.
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transactive end nodes (tens), wherein hosting capacities are defined by the utility or limitations are
imposed on injection into or withdrawal from the distribution grid (e.g., point of common coupling).
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simple transactive nodes (stn), i.e., distribution substations, or other nodes in the distribution system,
that are neither a ttn nor a ten.
the t-derMs uses an optimization engine (shown
schematically in Figure 3) to process schedules and, where
relevant, submit bids and offers subject to resource and
network constraints, based on regional transmission organizations/independent system operators (isO/rtO) and
dsO processes and governing regulatory framework. the
optimization engine accommodates nonpriced schedules,
priced or nonpriced bilateral transactions, as well as voluntary bids and offers, while respecting resource and distribution grid constraints. the optimization engine
accommodates various scenarios.
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distribution planning studies are conducted to identify the substations with hosting capacities and establish their respective ten default import/export limits.
the ten default import/export limits from planning
studies are often conservative since they usually cover
a range of operating conditions; the dsO can adjust
them on an hourly basis based on prevailing operating conditions.
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the ttn import/export limits may change hourly due
to bulk power wheeling, parallel flows, and so on.
these limits have default values that the dsO can
adjust for each hour in the scheduling time horizon.
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in the permitting process for ders, the utility may
establish not-to-exceed injection or withdrawal limits
at the respective service points (points of common
coupling) of the prosumer premises. the dsO can use
t-derMs to either enforce these limits or simply
check for any violation and monitor the corresponding schedules, actual values, and limits.
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schedules may represent forecasts provided to the
dsO regarding consumption (withdrawal from) of
generation (injection into) the distribution grid by der
operators, aggregators, microgrid operators, community choice aggregation (ccA), and building energy
management system operators, among others. Generally, the points of injection must be specified at
agreed-upon topological granularity (e.g., point of
common coupling, distribution feeder, distribution
zone, transactive node, and so on).
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A der resource can be an aggregate of der assets
with different nodes (substations) for injection/with-

I EEE E l e c t r i f i c a t i on M a gaz ine / j un e 2018

drawal (ttns and tens). the participant submitting
each of the source and sink schedules provides the
schedules for various sources and sinks separately. Alternatively, the participants can provide distribution
factors for tdMss to allocate the total schedules submitted by the participant among the relevant ttns
and/or tens, respectively.
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Bilateral transactions represent balanced schedules
(equal injection and withdrawal) with designated
sources and sinks. Bilateral transactions are based on
contractual arrangements among stakeholders. Multisource, multisink bilateral transaction schedules are
accommodated.
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Offers for injection or bids for withdrawals are voluntary. Offers may be submitted from an aggregate der
resource (VPP), and a microgrid, among others. each
offer may include multiple price-quantity pairs.
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imports from the bulk power system may be tied to
prices at the corresponding ttn. the ttn prices may
be obtained from the locational marginal prices (lMP)
data published by the isO/rtO or obtained from an
lMP forecast subfunction in tdMss.
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the dsO will have dispatchable assets with associated costs at its disposal to accommodate submitted
single-sided and bilateral schedules. this may establish different prices in various tens in case any
transactive node or transactive gate limits become
binding.
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More generally, the interplay of distribution transport
losses and constraints can lead to price differences at
dlMPs.
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Bilateral transactions may submit a ttcc to limit
their exposure to price differences between the transaction sources and sink.
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When the t-derMs runs out of dispatchable assets,
the schedules will be adjusted (curtailed) using a minimum curtailment objective function to accommodate
all of them simultaneously.
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rules for schedule curtailment (when the t-derMs
runs out of dispatchable assets).
1) t-derMs will reduce schedules to minimize the
total curtailment (i.e., the schedules that are most
effective in relieving ttn/ten limit violations will
be minimized).
2) For schedules with the same effectiveness, the
t-derMs will accommodate priority orders specified by the participants.
3) in curtailing bilateral and multisource/sink schedules, t-derMs will respect dependent curtailment,
i.e., if either the source or the sink is reduced, so
will the other leg.
the dsO may issue reference prices based on submitted
bids, offers and schedules, and the hard limits of the distribution constraints through an iterative advisory execution
of the t-derMs. this leads to a price discovery mechanism
in which mathematical iterations within t-derMs are

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2018