IEEE Power & Energy Magazine - July/August 2021 - 55

From Fixed Import/Export Limits
to Operating Envelopes
Existing aggregators typically control DERs without considering
the underlying constraints in distribution networks,
such as voltage and thermal limits. This is justified because
this is a systemic network issue that needs to be resolved in
the presence of multiple competing parties. Also, aggregators
do not have access to the necessary data to model the
network. Aggregators simply need to comply with the fixed
export/import limits that are associated with the connection
point of each DER customer in their portfolio.
For instance, in Australia, an aggregator can inject up
to 5 kW per phase (a residential export limit used by many
distribution companies today). However, given the variable
nature of customer demand and local generation, such a
fixed export limit can be too constraining at times when the
network can deal with larger power flows (i.e., there is still
available voltage headroom and thermal capacity). Moreover,
as DER penetration grows, fixed export limits might
prove insufficient in ensuring network integrity since there
will be more customers injecting power simultaneously.
Therefore, the use of operating envelopes-time-varying
export/import limits-at the connection point of active customers
can ensure network integrity while unlocking further
flexibility, thus addressing the drawbacks of fixed limits.
Operating Envelope Fundamental Concepts
The concept of operating envelopes for active power and how
they differ from the fixed export limits already employed
by many distribution companies (in Australia and around
the world) is illustrated by Figure 2. The low-voltage (LV)
feeder shown in the figure has four single-phase customers,
of which two are active customers, as denoted by 1
and 2
.
For each passive customer (i.e., nonaggregator-managed
customer), the maximum permitted power exports/imports
are fixed at all times, as per connection agreements with the
distribution company and/or the local electricity regulations.
These fixed limits for power imports/exports are shown as
flat red lines.
On the other hand, for each active customer, the maximum
permitted power exports/imports are periodically (e.g.,
every 5 min) issued by the distribution company. Overall,
this results in the time-varying red curves shown in Figure 2,
which ultimately define the operating envelope for the corresponding
active customer. The fixed limits of a passive customer
are shown as gray lines to illustrate that the permitted
range under the operating envelope can be larger or smaller
than the fixed limits (as it depends on the available voltage
headroom and thermal capacity throughout the day).
Given that the operating envelopes will be used by aggregators
to determine the most profitable way of exploiting
their DER portfolio, the values need to be made available in
advance. This also means that certain information must be
forecasted by the distribution company when calculating the
operating envelopes for the horizon of interest.
july/august 2021
Lastly, the operating envelope is defined by two limits,
one for power imports and one for power exports. For illustration
purposes, the focus of this article is on active power
exports as it is expected to be a common service provided by
aggregators to help the system balance demand and supply.
The same principle also applies to power imports.
Calculating the Operating Envelopes
The operating envelopes of active customers at a given time
interval are calculated by assessing their maximum feasible
exports. This can be achieved in different ways. For
instance, the optimal power flow used at the transmission
level could be adapted for three-phase LV feeders to determine
the maximum exports of each customer while still satisfying
network constraints. Another, perhaps more practical,
approach is to run a series of power flow calculations
that explore different combinations of the power exports
from active customers.
In either case, the net demand of passive customers as well
as three-phase voltage magnitudes at the head of the feeder
(Figure 2) are required; these can come from recent measurements
or forecasted values. This, in turn, means that advanced
metering infrastructure (also known as smart meters), headof-the-feeder
monitoring, and an adequate three-phase model
of the circuit are needed to make such calculations possible.
A simplified flowchart of the basic process to calculate operating
envelopes is shown in Figure 3.
Given that a set of operating envelopes is to be determined
for each of the intervals (e.g., every 15 min) in the
horizon of interest (e.g., the next 6 h), the process in Figure 3
is repeated for each of the intervals using the corresponding
forecasted values. Since operating envelopes are used in an
operational setting, their calculation also must be routinely
carried out (e.g., every hour) to remain up to date with the
changes of the network state.
Setting the length of the intervals, the length of the
horizon, and how often operating envelopes are calculated
have associated tradeoffs between accuracy and complexity.
Head of Feeder
kW
1
kW
Import
Operating Envelope
for Active Customer
kW Export
2
kW
Import
figure 2. Operating envelopes and fixed export limits.
ieee power & energy magazine
55
t
Fixed Limits for
Passive Customer
Export
t
IEEE Power & Energy Magazine - July/August 2021

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