Transmission-distribution long-term volt-var planning considering reactive power support capability of distributed PV

Abstract

High penetration of grid-edge, inverter-based photovoltaic (PV) can cause significant voltage fluctuations not only at the distribution but also at the sub-transmission levels due to PV output intermittency. This paper proposes a reactive power planning tool for sub-transmission systems to mitigate voltage violations and fluctuations caused by high PV penetration and intermittency with a minimum investment cost. The planning tool considers all existing var assets in both sub-transmission and distribution systems to reduce the need of new equipment. It also closely coordinates with an optimization-based volt-var operational tool for selecting a set of scenarios with voltage violations due to PV intermittency and testing the final investment decision. The planning tool obtains an investment need for each scenario based on a proposed optimal power-flow framework. Because of a high number of discrete variables in this optimization problem, an efficient technique to recover the feasibility for the solution of the relaxed optimization problem is included. After obtaining separated solutions for all scenarios, two options are provided for making final planning decision: i) a conservative direct combination of investment need solutions and ii) a machine learning-based selection of representative investment needs at most time steps. The final investment decision options are verified using a realistic large-scale sub-transmission system and 5-min PV and load data. The results show a significant voltage performance improvement with a lower investment cost for additional var equipment compared to conventional approaches.