As energy production is tending toward being less centralized, it is becoming of high importance to study the implications of introducing intermittent renewable energy sources in the conventional power grid. It is well known that microgrids provide a reliable way of improving sustainability, resiliency and penetration of distributed energy resources (DERs). However, some important technical challenges need to be addressed to provide a realistic large-scale integration of these microgrids. These challenges include harmonic mitigation, power quality control, stability, energy storage coordination and mitigation of voltage and power fluctuations. With the use of energy storage systems, some of the challenges related to the intermittence of the DERs’ power availability and demand variability can be addressed. In addition, standards such as IEEE 1547-2018 provide important guidelines for grid-following inverter-based renewable (IBR) generators to support the power quality of the network.
An important phenomenon that the industry is still in the process of understanding is the inter-area oscillations when there is a high penetration of IBRs, which are also known as inter-IBR oscillations. These oscillations can cause power, frequency and/or voltage fluctuations, which can provoke unplanned disconnection of DERs and a possible microgrid collapse. This phenomenon has been analyzed using historical data and also modeled to understand the mathematical root causes of it. It has been found that some of the oscillatory events are related to the common interaction of the IBRs with the grid. This typically occurs when the grid is considered weak, and it is directly related to the power level being injected or absorbed (e.g., short circuit ratio). Other oscillatory events occur when multiple IBRs oscillate against each other, which can be caused by many factors such as the electrical components, the internal control parameters and/or the intermittence in the direct current (DC)-link bus.
Vendors, integrators and asset owners may face challenges analyzing the interaction of commercial IBRs with other microgrid components. However, this analysis could provide them a clearer perspective about the operational ranges to avoid conditions such as inter-IBR oscillations. Digital simulation can be employed for this purpose, but it lacks fidelity because it does not consider all the processes that occur inside of the commercial devices and other elements in the microgrid. Power hardware-in-the-loop (PHIL) is a test technique where real equipment is interfaced to the power system simulation using a power amplifier (PA) in which its output is regulated by a real-time simulator. The purpose of this technique is to mimic the behavior at the point of connection (POC) to the real IBR with a virtual microgrid or other complex power systems. This technique allows us to simulate complex systems and vary their parameters to assess the interaction with real DERs. For example, parameters such as the voltage in the DC link, the value of the output filters, line impedances, grid strength or control parameters in the virtual microgrid could be adjusted in different scenarios to observe which of these factors mainly affect the inter-IBR oscillations or other phenomena. As shown in Figure 1, the commercial IBR, also known as the device under test (DUT), is directly connected to the PA, which reads currents at the POC and uses them to feed a virtual current source connected to one of the nodes in the virtual microgrid.
With PHIL, many different virtual scenarios could be tested with reduced testing time and costs. To analyze weak grid oscillations, the microgrid could be tested in grid-following mode modifying the line impedances or in islanded mode modifying the control parameters of the grid-forming DERs. Also, to analyze inter-IBR oscillations, the virtual DC-link voltage can be modified changing the availability of the primary energy source (solar, wind, etc.). For both cases, users can change control parameters on-the-fly to determine acceptable ranges for each scenario.
Since the dynamics of the IBRs are directly associated with the control parameters, it is also necessary to use standardized control models in the virtual IBRs. A comprehensive repository of open-source models for IBRs has been presented by NREL for non-real-time simulation. Also, Natural Resources Canada has developed a real-time inverter modeling toolbox based on the IEEE 1547-2018 standard for grid-following IBRs that includes grid-support functions and protection elements. These will allow grid operators, researchers, system planners and associated stakeholders to model the IBRs with appropriate grid forming and grid support requirements for different power system studies. Thus, inter-IBR oscillations can be thoroughly investigated with different scenarios of control as well as generation mix using these inverter models both in offline simulation and real-time simulation incorporating actual controller hardware. This can also facilitate the real-time assessment of actual devices or microgrid controllers for such inter-IBR oscillations, significantly reducing their development and testing time.