  This paper proposes a reactive power planning tool for sub-transmission
systems to mitigate voltage violations and fluctuations caused by high
photovoltaic (PV) penetration and intermittency with a minimum investment cost.
The tool considers all existing volt-ampere reactive (var) assets in both
sub-transmission and distribution systems to reduce the need of new equipment.
The planning tool coordinates with an operational volt-var optimization tool to
determine all scenarios with voltage violations and verify the planning
results. The planning result of each scenario is the solution of a proposed
optimal power-flow framework with efficient techniques to handle a high number
of discrete variables. The final planning decision is obtained from the
planning results of all selected violated scenarios by using two different
approaches - direct combination of all single-step solutions and final
investment decision based only on the scenarios that are representative for the
power-flow voltage violations at most time steps. The final planning decision
is verified using a realistic large-scale sub-transmission system and 5-minute
PV and load data. The results show a significant voltage violation reduction
with a less investment cost for additional var equipment compared to
conventional approaches.
