Validating the grid integration of state-of-the-art photovoltaic technologies: smart inverter grid controls and bifacial modules performance
Date
2024
Authors
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Publisher
University of Delaware
Abstract
The penetration of renewable energy systems into the centralized electric grid has increased significantly during the last decade. Photovoltaic (PV) is now the fastest-growing of all types of electrical generation. A variety of innovative approaches have been introduced to promote higher deployment of distributed energy resources (DERs) such as residential-scale PVs. A decrease in technology costs, strong government incentives, and technological advancements have resulted in exponential growth in worldwide PV installed capacity. Although solar electricity can deliver clean and cost-effective energy, the intermittent nature of solar energy can lead to challenges with electric grid stability as the percentage of solar increases. High penetration of PV generation can cause over-voltages due to active power injection when load demand is low. Other problems associated with large fractions of PV in distribution networks include negative impacts on protection devices, reverse power flow, and frequency stability. ☐ Smart inverter-based resources (IBRs) and battery energy storage systems (BESS) can be used to mitigate the impact of such high penetration of renewable energy, as well as to support grid reliability by improving voltage and frequency stability and serving residential loads during grid failures. Nevertheless, control protocols for smart inverters are vulnerable to infiltration and cyberattacks. To deploy smart PV inverters to the field, it is crucial to evaluate the primary performance and grid-supporting control functionalities with the IEEE Standard for Interconnection and Interoperability of DERs with Associated Electric Power Systems Interfaces (IEEE 1547-2018). Additionally, to ensure the security of the electric grid, the ability to withstand cyberattacks on the smart inverters needs to be assessed to have a better long-term preparation for more DER integration. ☐ In the first half of this dissertation, I addressed the challenges of electric grid operation due to high PV penetration and presented possible solutions to overcome the electric grid's reliability and stability issues in order to support more solar energy integration. To evaluate the impacts of PV integration on the grid and smart inverter's performance, I conducted experiments on two commercially available smart PV inverters under different grid conditions and inverter grid-supporting functionalities, including abnormal responses caused by cyberattacks, using the Power Hardware-in-the-Loop (P-HIL) test facility. I designed and applied twenty possible cyberattack scenarios based on their high efficacy in causing severe disturbance to the grid and damage to the inverters and compared their responses to the cyber threats. In this work, I developed an abnormality detection strategy using power flow measurements compared to the forecasted PV power obtained from the field as well as developed Python codes to measure and control the lab inverters via Modbus communication. Additionally, to observe possible interactions between the unstable grid and the inverters under different reactive power controls, I designed and fabricated two impedance circuits to be installed between P-HIL equipment to represent the impedance in an electric distribution line. Grid voltage and frequency were varied between extreme values outside of the normal range to test the response of the two inverters operating under different controls. ☐ The key findings highlighted that different inverters that have met the same requirements of IEEE 1547-2018 can have different abilities, limitations, and levels of vulnerability. In addition to the grid control, the residential PV installed capacity and physical distances between PV homes and the substation, which impact the distribution wiring impedance which we characterize by the ratio of reactive to real impedance (X/R), should be considered when assigning the grid-supporting control setpoints to smart inverters. I found that high X/R allowed for a more effective control for both voltage and frequency stability. This dissertation also highlights the significance of validating the cyberattack detection method and analyzes the inverters' responses to unstable grid conditions. ☐ The results further show that most cyberattacks can be noticed immediately after the attacks are launched, and they can be detected from the active power analysis. Conversely, some attacks are detectable by the reactive power measurements. However, some types of attacks could be invisible when high power levels are assigned to the inverters and need to be addressed with higher measured sampling rates. ☐ The second part of this dissertation presents another challenge of high PV integration in the residential sector. As PV technologies become more advanced, the performance of PV has increased with a reduction in PV module costs. Bifacial PV is one of the cutting-edge technologies that is predicted to overwhelm the market in the next decade due to its ability to generate additional power from the rear side of the modules. In a practical aspect, the reduction in PV module cost compared to other components such as inverter and BESS has led to oversized PV systems, leading to inverter clipping where the DC power exceeds the inverter AC rating. With the bifacial PV technology, the inverter clipping could conceal the benefit of high reflective ground increasing the rear side energy gain thus minimizing the energy generation potential of the bifacial PV. ☐ The goals of this part are to explore the seasonal impact of inverter clipping loss on a bifacial PV system in different ground conditions and to introduce the BESS management strategy to utilize the clipped PV power from the oversized system. I collected and analyzed the measured data using a 5-kW bifacial PV testbed connected to a 3.8-kW residential grid-tied PV inverter, and a 10-kWh Li-ion battery. The key findings indicate that the inverter clipping potentially occurs in summer from 10 am to 3 pm when the solar irradiance is the highest. The daily PV generation with the white ground experienced higher clipping loss than the gravel, consequently resulting in negligible bifacial gain. To solve the issue of clipped energy, I implemented a concept of residential BESS management where the clipped power is utilized in the system to lower the wasted energy and help peak load shaving for residential-scale systems. ☐ In a related effort, a modeling study was done to identify the impacts of technical and financial variables on PV+BESS systems using modeling software. A BESS with the proposed dispatch strategy from the hardware experiment was also evaluated using a software simulation. The results show that geographic location and tariff structures play an important role in the cost-effectiveness of PV systems. The optimal DC/AC ratio is found to lie between 1.1 to 1.3, and locations with higher solar irradiance would have a higher optimal DC/AC ratio. While a bifacial PV system with a flat rate tariff is more cost-effective than PV+BESS due to a substantially lower payback period, the PV+BESS system does add more value to the system owners with a time-of-use structure. To identify the optimal system design for the PV+BESS system, sensitivity analyses of battery size and battery replacement threshold were done using different financial criteria. The results validate that each economic measure offers a different optimal configuration so they should be evaluated together for the most cost-effective system design.
Description
Keywords
Battery energy storage, Grid stability, Photovoltaic, Smart inverter, Solar energy