  High solar Photovoltaic (PV) penetration on distribution systems can cause
over-voltage problems. To this end, an Optimal Tap Control (OTC) method is
proposed to regulate On-Load Tap Changers (OLTCs) by minimizing the maximum
deviation of the voltage profile from 1~p.u. on the entire feeder. A secondary
objective is to reduce the number of tap operations (TOs), which is implemented
for the optimization horizon based on voltage forecasts derived from high
resolution PV generation forecasts. A linearization technique is applied to
make the optimization problem convex and able to be solved at operational
timescales. Simulations on a PC show the solution time for one time step is
only 1.1~s for a large feeder with 4 OLTCs and 1623 buses. OTC results are
compared against existing methods through simulations on two feeders in the
Californian network. OTC is firstly compared against an advanced rule-based
Voltage Level Control (VLC) method. OTC and VLC achieve the same reduction of
voltage violations, but unlike VLC, OTC is capable of coordinating multiple
OLTCs. Scalability to multiple OLTCs is therefore demonstrated against a basic
conventional rule-based control method called Autonomous Tap Control (ATC).
Comparing to ATC, the test feeder under control of OTC can accommodate around
67\% more PV without over-voltage issues. Though a side effect of OTC is an
increase in tap operations, the secondary objective functionally balances
operations between all OLTCs such that impacts on their lifetime and
maintenance are minimized.
