IEEE Electrification - September 2020 - 83

coordinated along the distribution system to keep the
feeder voltage profile within a permissible range. However,
PV integration has complicated distribution voltage control as there are multiple generation feeds in addition to
those at the substation. By their very nature, distribution
networks are resistance-dominated circuits in which their
voltage magnitude is strongly coupled with active power,
more so than reactive power. Accordingly, solar PV penetration directly affects the distribution voltage profile by
altering active power flow magnitudes and direction. In
such conditions, the voltage profile may rise along the
feeder, which has been exacerbated by lowering the load
in 2020.
Figure 5 depicts the voltage profile of a typical distribution network feeder. In case A, with a normal loading and
no PV integration, the voltage monotonically declines to
within the desired level. In case B, the voltage magnitude
drops further as the load increases, which exceeds the
permissible limit. In case C, the transformer tap changer
or the capacitor banks equipped with automatic voltage
regulators are applied to adjust the voltage profile to within the permissible range. Case D illustrates the voltage
profile in the heavily loaded distribution circuit with an
abundant level of PV. In case E, the voltage profile, which
is reversed to within the permissible range, becomes
excessive at terminal nodes when the load drops. In case
F, the situation is exacerbated when the minute power
intake from the main grid causes an overvoltage on the
partially loaded transmission lines, which is due to dominant capacitive charges. In case G, transformer tap changing might not be effective enough to lower the voltage
profile to within the acceptable range. In this case,
switching out lightly loaded transmission lines can mitigate the overvoltage in both the transmission and distribution levels. However, the N-1 security criterion might be
violated when the lines are switched out, which may not
be an acceptable grid-operation practice. Finally, in a
largely PV-integrated distribution network, the system
could undergo nearly all of the listed loading and solar-generation
conditions in a day. Therefore, a
Voltage Magnitude
suite of strategies and tools that
feature autonomous distribution
Upper Limit
system operation will be necessary
1 (p.u.)
for such circumstances to fulfill the
voltage-regulation requirements.

centers are mostly equipped with a load forecasting module, which applies statistical techniques, artificial neural
networks, or a combination of such algorithms for daily
load forecasts. Accordingly, generating units are committed
and dispatched based on forecasted daily load profiles.
Although load forecasting is inevitably an error-prone process, the error grade should be essentially very limited,
otherwise the balancing power would have to be supplied
by the (expensive) real-time market. In addition, solar PV
generation is intimately dependent on daily solar irradiation and weather conditions, which are critical factors in
net thermal load forecasts. Hence, given that the net thermal load is affected by two random variables, i.e., load
demand and renewable energy generation, net thermal
load forecasting loses its cyclical pattern and repetitive
behavior. This issue makes forecasting a challenging task.

Distribution System Protection
Conventional radial and passive distribution networks are
subjected to unidirectional power flows and fault currents.
These networks are traditionally protected by cost-effective current-based protection schemes, such as overcurrent, Earth fault, and recloser relays. Current magnitude
with a fixed direction is a decision index in these protection devices. These assumptions are no longer true when
sizeable solar PV quantities, integrated into the distribution network, contribute to fault currents and affect current magnitudes and directions. During the normal
operation of a distribution system, there may be certain
daily hours (e.g., midday hours with substantial solar PV
power generation and small residential loads) when
reversed power might flow from the customer side to the
main substation. This reversed flow would require protection schemes and devices to be reengineered to satisfy the
specifications of the designated PV-integrated active distribution network. Accordingly, a wide spectrum of
advanced and novel protection schemes and devices,
based on not only the line current magnitude but also on

Lower Limit

Daily Forecast of Net
Thermal Load
Daily load profiles have cyclic patterns associated with similar days
of the week or the same days in
consecutive weeks. This feature has
made forecasting models sufficiently accurate for power system operations. Power system dispatching

Main Substation
Case A
Case B

End Feeder Length
Case C
Case D

Case E
Case F

Case G

Figure 5. The voltage profile of a distribution network feeder. p.u.: per unit.

IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 2 0

IEEE Electrification - September 2020

Table of Contents for the Digital Edition of IEEE Electrification - September 2020