Studies on the Effects of High Renewable Penetrations on Driving Point Impedance and Voltage Regulator Performance: National Renewable Energy Laboratory/Sacramento Municipal Utility District Load Tap Changer Driving Point Impedance Project

Abstract

This project explored the role of controllers-load tap changers (LTCs) and line regulators-on distribution feeders with increased distributed energy resources (DERs). Distribution feeders are facing a steep increase in DER penetration from specific classes of devices, including photovoltaic (PV) generators, energy storage systems, and demand response. Line regulators and LTCs represent a subset of control devices on distribution feeders used to regulate voltage delivered to customer locations. These devices operate in discrete steps, with each step changing the tap rate by 0.625% (hence a 10% change requires 16 steps). Before DERs were introduced, power flow in a distribution feeder was unidirectional, i.e., from the substation to loads. Additionally, a predictable loading pattern that peaks in the afternoons or evenings and dips during nighttime made the operation of voltage regulators easier and understandable. However, with increased penetration of renewable generation on distribution feeders, including homeowners installing solar panels on rooftop and additional utility-scale or commercial PV installations, a paradigm shift is occurring in the operational expectations of voltage regulators and LTCs. It has become very common to expect reverse power flows to the substation as a result of increased PV penetration. Voltage regulators perform as desired when regulating from the source to the load and when regulating from a strong source (utility) to a weak source (distributed generation). (See the glossary for definitions of a strong source and weak source.) Even when the control is provisioned for reverse operation, it has been observed that tap-changing voltage regulators do not perform as desired in reverse when attempting regulation from the weak source to the strong source. The region of performance that is not as well understood is the regulation between sources that are approaching equal strength. As part of this study, we explored all three scenarios: regulator control from a strong source to a weak source (classic case), control from a weak source to a strong source (during reverse power flow), and control between equivalent sources. In this report, we reassess the topic of existing LTC and line-regulator controls and on a realworld distribution feeder with renewable penetration, and we explore their benefits and disadvantages. This study makes use of metrics such as driving point impedance (DPI), the effectiveness of tap change (tap-delta), reactive power flow, and transformer efficiency to evaluate the performance of LTCs/line regulators on distribution feeders with substantial DER penetrations. These metrics were carefully chosen because they unmask the shortcomings of existing LTC/line regulator control practices on feeders with high DER penetrations. The first chapter takes a deeper dive into the description of these metrics and the reasons we choose them. This project was executed in three sequential tasks to help the Sacramento Municipal Utility District (SMUD) understand the role of existing LTC/...