Transitioning PV Inverters from Unbalanced to Balanced Power Flow Models

As higher penetrations of distributed PV and storage technologies are installed on the distribution feeders, utility software tools must model the thousands of single phase and poly-phase inverters accurately to determine the effects on voltage, frequency, fault current, transients, reactive control, and other dimensions under steady state and contingency conditions.  The unbalanced distribution system must integrate with transmission power flow models to study the system wide effect of high penetrations of all types and sizes of renewable technologies.

BEW has been working with HECO and SMUD on the detailed modeling of single phase and poly-phase inverters on their respective grids.  For HECO, BEW is modeling the entire distribution grid which contains more than 4,000 inverters.  Some HECO feeders have PV penetrations over 30 percent.

Utilities are already having operational problems with high penetrations but lack the capability to simulate the penetrations with any kind of accuracy.  BEW developed a methodology for modeling and studying inverters in the unbalanced distribution model for the entire distribution grid or selected areas under steady state conditions.  Dynamic studies must be completed using transmission models but lack the capability to model single phase inverters and high numbers of inverters.  Under WECC definitions, an inverter requires three modules to simulate inverter functions.  For HECO, the 4,000 inverters require 12,000 modules.

BEW, HECO and SMUD participated in the development of a methodology and application to transition the inverter modules from the unbalanced distribution model to the balanced model.  As utilities are requiring inverters to have more flexibility for VAR generation, voltage control, frequency control, and other characteristics, the inverter modules must have the same capability.  HECO and SMUD are providing their system data for the development of the methodology. This paper will highlight the progress on the transition of inverters and the results to date.