IEEE Power & Energy Magazine - January/February 2020 - 67

As the widespread adoption of DERs continues and more consumers
become active participants, managing the effects of the corresponding
reverse power flows is becoming an increasingly difficult task.
(with and without DERs) and network elements across mul-
tiple voltage levels, i.e., performing a network-level optimi-
zation. However, in a realistic distribution system, the sheer
number of controllable devices is prohibitively cumbersome
for rule-based approaches, which may potentially become
inefficient or even inadequate. Model-based ADMS appli-
cations using an ac OPF as the decision-making engine
show great potential to determine set points for controllable
devices in a network. OPF is a mathematical optimization
problem that embeds the well-known power flow equations,
i.e., a set of nonlinear equations used to calculate the steady-
state behavior of a given network (e.g., voltages and currents).
If controllable devices are incorporated as variables, such as
some DERs and OLTCs, the power flow equations (and con-
sequently, the set points of these devices) can be optimized
according to a given objective.
The concept of performing network-level optimiza-
tion is certainly not new. Transmission network operators
have long benefited from this concept during their day-
to-day operations. For instance, in Australia, OPF (albeit
a simplified version of the full ac OPF formulation) is
integrated in the Australian Energy Market Operator's
dispatch process, where the operating schedules of gen-
eration plants and ancillary service providers are deter-
mined once every 5 min to ensure that demand is always
matched by supply.

Transmission

MV

In the context of distribution systems, having the OPF
as the decision-making engine allows for myriad ADMS
applications because its formulation can be adapted
to the corresponding objective. From an environmental
perspective, it can be used to maximize the energy har-
vesting of DERs installed in constrained networks, which
also increases the financial viability of the corresponding
investments. However, because the OPF would produce
DER settings that maximize overall energy harvesting,
some locations (particularly the remote ends of feeders)
might be heavily penalized. To this end, different fair-
ness schemes can be incorporated into the formulation,
such as using last-in, first-out rules to manage utility-scale
DERs or enforcing the same export limit for all LV-con-
nected PV systems.
From the customers' perspective, particularly for residential
customers, the OPF can be applied to reduce their electricity
bills. A well-known technique is conservation voltage reduc-
tion (CVR), which reduces energy consumption by exploit-
ing the voltage/power relationship of loads. By performing a
network-level optimization, it is possible to fully exploit the
flexibility of voltage-regulation devices to lower the network's
voltages to acceptable limits so as to minimize energy con-
sumption, resulting in lower electricity bills. From the distri-
bution company's perspective, OPF can also be used to meet/
improve performance targets, such as loss minimization across

Passive
Solution

Transmission

Install capacity is limited
to mitigate issues.

MV

DER settings are
centrally optimized.

Volt-watt settings unfairly penalize
LV customers at the end of feeders.
LV

Network-Level
Optimization

LV

figure 1. A passive solution versus network-level optimization.
january/february 2020

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IEEE Power & Energy Magazine - January/February 2020

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