Renewable energy grid integration and resilience: 1) simulation of microgrids and 2) smart inverter testing and communications
Date
2022
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
The electric grid is considered the biggest and most complex human-built system in the world. It was originally designed for unidirectional power flow with centralized power plants generating electricity to be transported for long distances and then delivered to final customers. This old paradigm of producing and consuming electricity has changed during the last decades due to the development and adoption of Distributed Energy Resources (DERs), such as Photovoltaic (PV) arrays, wind energy, Battery Energy Storage (BES) systems, and other technologies that allow the customer to become a prosumer (producer and consumer) of electricity. ☐ The aging infrastructure and the still dominant centralized power generation in the electric grid bring on challenges for the grid operator. The main problems of managing and maintaining the grid include: its vulnerability to natural disasters (e.g. hurricanes and wildfires), cyber and physical attacks, difficulty to integrate large amounts of variable renewable energy (PV and wind), CO2 emissions from burning fossil fuels (coal and natural gas), significant transmission and distribution losses, and the use of expensive and polluting peak generators during periods of very high demand. ☐ Many of these challenges can be alleviated by localized microgrids which integrate multiple renewable and fossil fuel based energy sources, storage, and local energy management to provide reliable electric energy and can operate either connected to the main grid or in islanded mode. A key component of microgrids and renewable energy systems is the inverter which interfaces the renewable or battery source with the grid. IEEE standard 1547 defines grid supporting functions for smart inverters such as voltvar, volt-watt, and voltage or frequency ride-through to help in the stabilization of the macro grid and to allow a high penetration of renewable energy. ☐ The main goal of this dissertation is to identify and quantify different ways in which microgrids can facilitate a high penetration of renewable energy into the electric grid while increasing resiliency. To accomplish this objective, the dissertation is divided into two separate, but related efforts. First, I focus on modeling, simulation, and data analysis of microgrids using HOMER Grid software. The second part involves experiments in a newly installed Power Hardware-in-the-Loop (P-HIL) lab that includes smart inverters, PV simulators, a grid emulator, and a Lithium-ion (Li-ion) battery system. ☐ In the modeling and simulation section, I used high-resolution real electric load data from three buildings at the University of Delaware and two electric tariff structures (flat rate and time-of-use rate) to quantify the value of Li-ion BES coupled with PV. The peak shaving and energy arbitrage functions of BES were analyzed for different combinations of PV and battery sizes. I also evaluated the effect of the battery degradation limit and the battery capital cost on the Net Present Cost (NPC) of the system. From the results, I found that delaying the replacement of the battery has a substantial economic benefit for the system owner. Letting the battery degrade to 50% of its initial capacity is comparable to a 30% reduction in the battery capital cost during the lifetime of the project because the battery will be replaced only once instead of twice lowering the Net Present Cost. The cost-effectiveness of PV+BES for a given building depends on the degradation limit and tariff structure, but it does not depend strongly on the load pattern and size. I conclude that Time-of-Use (TOU) tariffs would promote more rapid cost-effective adoption of PV systems with batteries in commercial buildings in the upcoming years. ☐ In a related effort, I conducted an environmental, economic and resilience analysis of a microgrid consisting of PV, battery, natural gas generator, and the electric load of an office building that consumes an average of 2 MWh per day. Different component sizes were used to determine the configuration with the lowest generator size to power a two-day outage during the summer peak load. Environmental and economic analysis were performed to show the tradeoffs between different system design goals. The results indicate that installing a microgrid in an office building with a 600 kW PV array and 2.8 MWh lithium-ion battery can avoid the release of up to 287 tons of CO2 per year which come from the electric utility generation. The same microgrid configuration can endure a two-day blackout during the highest electric demand in the hurricane season without the need of a polluting backup generator. Based on current technology costs, large PV systems with small batteries are economically more attractive than the base case configuration, which refers to the system with a generator only (no PV and no BES). ☐ A more in-depth resilience analysis was performed considering commercial buildings with different load patterns in three cities across the U.S. with contrasting weather conditions. Here I propose a new resilience metric called “area under the curve of the surviving probability”. Using this metric, I concluded that load patterns have a higher impact on resilience of renewable-only microgrids (no back-up generator) than weather conditions in the cases under study. I also defined and analyzed worst- and best-case 3-day outages and run simulations for several scenarios. I found that adding a backup generator to renewable-only microgrids can decrease the net present cost of 100% PV configurations, but it does not affect the net present cost of 50% PV with 50% wind systems because they do not incur in the penalty of not meeting the load during the outage. ☐ For the second part of my dissertation, I investigated how PV inverters can help to stabilize grid voltage. For that, I developed a Python library to communicate to the HIL equipment (grid emulator, PV simulators, and power meter), set up and configured a server hosting a customized RESTful API to allow remote control of the components, similar to how the utilities will control inverters in a distribution network. I conducted efficiency tests on the inverters and characterized the grid supporting functions for grid voltage stabilization, specifically constant power factor, volt-var, and volt-watt. I identified some abnormalities in the operation of the volt-var-watt control in one of the inverters and presented a method to overcome the limitation in remote control of another inverter using Modbus communication. Identifying, understanding, and overcoming shortcomings on the operation of PV smart inverters that provide grid supporting functions is key for the quick adoption of this technology and can help regulatory agencies to determine what is the appropriate control mode that will facilitate higher PV capacity.
Description
Keywords
Battery energy storage, Hardware in the loop, HOMER grid, Microgrid, PV, Smart inverters