Accelerating Development of Advanced Inverters.

Abstract

Increasing the penetration of distributed renewable sources, including photovoltaic (PV) generators, poses technical challenges for grid management. The grid has been optimized over decades to rely on large centralized power plants with well-established feedback controls. Conventional generators provide relatively constant dispatchable power and help to regulate both voltage and frequency. In contrast, photovoltaic (PV) power is variable, is only as predictable as the weather, and provides no control action. Thus, as conventional generation is displaced by PV power, utility operation stake holders are concerned about managing fluctuations in grid voltage and frequency. Furthermore, since the operation of these distributed resources are bound by certain rules that require they stop delivering power when measured voltage or frequency deviate from the nominal operating point, there are also concerns that a single grid event may cause a large fraction of generation to turn off, triggering a black out or break-up of an electric power system. To mitigate effects on grid voltage and frequency due to increased penetration of distributed energy resources (DERs), new grid support functions (GSFs) have been investigated to allow DERs to participate in voltage and frequency regulation. In addition, new voltage and frequency ride-through (V/FRT) functions are being developed to enable these inverters to remain connected even when voltage and frequency deviate considerably. Unfortunately, the implementation of these functions brings with it a new set of engineering concerns including an increased propensity to unintentionally island, the need to ensure standards of performance, as well as interoperability of multiple inverters on one bus, dynamic response of inverters to load rejection and ground fault, and finally how to address the cyber security threat. To accelerate advanced inverter adoption, this project addresses these potential obstacles, thus enabling increased PV penetration. This project was successful in developing both general/generic and manufacturerspecific transient inverter models and adapting them to include GSFs and V/FRT capability to aid in extensive simulation studies of anti-islanding efficacy. Simulation and experimental studies were done to investigate efficacy of existing anti-islanding schemes and quantify performance. In addition, two new anti-islanding methods were developed and tested in the lab that are robust against the effects of GSFs and V/FRT and are expected to be cost-effective. Furthermore, an extensive testing protocol was developed in collaboration with industry, documented, and applied to commercial and pre-commercial inverters. This test protocol is a potential means by which to certify gridconnected equipment for GSF and V/FRT implementation and ensure a prescribed standard of performance. Finally, a Sandia team began the evaluation of cybersecurity implications of remote commands to DERs implementing GSFs and V/FRT functions.