As the use of distributed generation with power electronics-based interfaces increases, the separation between DC and AC parts of the grid is reduced. In such inverter-dominated AC grids, impedance-based analysis methods are proving to be more powerful than traditional state-space-based analysis methods. Even the conventional parameters and standards used to estimate the stability of generators and stronger grids cannot fully capture the dynamics of weaker, inverter-dominated grids. It then stands to reason that system impedances that are commonly used to analyze DC systems will be useful in the analysis of grid-forming inverters in these hybrid systems. To understand the value of studying the impedances of inverters and other elements in weak AC grids, this article reviews and describes the various ways in which impedance-based analyses can be used to define, assess, and improve the performance of grid-forming inverter controllers. An exemplary case using the conventional P-f/Q-V droop control demonstrates the application of impedance-based analyses to determine the impact of the controller on the input and output stability of the inverter.

A grid‐forming inverter in an inverter‐dominated grid should operate as a dispatchable voltage source, which is difficult to achieve when the inverter is interfaced with nonlinear dc sources such as photovoltaic (PV) arrays. This paper presents a new grid‐forming controller which considers the PV source dynamics and limitations and maintains dc‐link stability under transient and overload conditions. A single‐loop voltage controller without ac current feedback is used, and the synchronization is achieved using dc voltage‐frequency droop. Additional dc current‐virtual impedance feedforward compensation is included to improve the ac output passivity. The controller limits the operation of the PV source inverter in the linear portion of its characteristic by regulating its modulation index, thus preventing dc voltage collapse. The proposed controller is implemented and tested on a controller‐in‐the‐loop simulation platform. The simulation results show that the controller shares power in proportion to the dc source capacities of parallel inverters, effectively limits the output current during faults, and limits dc‐link voltage drop when the inverter is overloaded. The controller is also validated on a hardware testbed.

In contrast with conventional microgrids (MGs) with fixed boundaries, a smart and flexible MG with dynamic boundary is introduced in this paper. Such a MG can dynamically change its boundary by picking up or shedding load sections of a distribution feeder depending on its available power, leading to more flexible operation, better utilization of renewables, smaller size of energy storage system, higher reliability, and lower cost. To achieve a flexible MG, the main challenges in MG design are addressed, including recloser placement, MG asset sizing considering resilience, system grounding design, and protection system design. Meanwhile, a hierarchical structure is employed to design and implement the MG controller. On top of the functions defined in IEEE 2030.7-2018, a few new functions, e.g., online topology identification and PQ balance, are added, while the planned/unplanned islanding and reconnection functions are enhanced. The controller is implemented on a CompactRIO, a general-purpose hardware platform provided by National Instruments (NI), and tested on a controller hardware-in-the-loop setup based on an OPAL-RT real-time simulator and a reconfigurable power electronic converter-based hardware testbed. The test results have validated the performance of the developed controllers. Such a flexible MG and its controller have been deployed at a municipal utility, and part of the controller’s functions have been tested on-site.

The dynamic boundary concept enables more flexible and efficient operation of microgrids with distributed energy resources (DER) that are intermittent in nature. As the integration of renewables continues to accelerate, an adaptive power management module that enables dynamic boundary operations in microgrids with an increasing number of source locations is essential for the fast and low-cost deployment of microgrid controllers. The power management module introduced in this paper is capable of handling the increased complexity in topological variations and transitions stemming from dynamic boundaries and multiple source locations. This includes real-time operation of multiple islands with dynamic boundaries, initiation of topological transitions (merging and separation of islands), and automatic source coordination for power sharing and frequency regulation. All functions in the power management module are designed to be automatically adaptable to arbitrary microgrids with non-meshed topologies so that the deployment of the controller at new microgrid sites can be expedited with a reduced cost. The module has been implemented on NI’s CompactRIO system as an essential part of an MG controller and tested on a converter-based hardware testbed (HTB). Testing results validated the effectiveness of the algorithms under various operating conditions.

With increasing deployment of inverter-based sources in microgrids, inverter control methods are constantly being modified and improved. What remains constant, however, is the phase-locked loop (PLL). Regardless of the control technique adopted and the type of microgrid, a PLL is used to synchronize the output PWM signal of the inverter with the main grid or other inverters. But as penetration of inverter-based sources increases and grids become weaker, the impact of PLLs on controller behavior becomes more pronounced. The issues caused by these influences are described in this paper to make a case for finding a solution for inverter synchronization that better fits the needs of inverter-dominated microgrids than PLLs. Simulation results from a study of a low-voltage microgrid supported by parallel inverters are also presented to demonstrate some of these characteristics.