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What are the key differences between grid-forming and grid-following inverters, and how do they influence microgrid operation?



Grid-forming and grid-following inverters are two fundamental types of inverters used to interface distributed energy resources (DERs) with a microgrid, each with distinct characteristics and control strategies impacting microgrid operation. A grid-forming inverter, also known as a voltage-source inverter (VSI) operating in voltage control mode, is capable of independently establishing and maintaining the voltage and frequency of an AC microgrid. It acts as an ideal voltage source, providing the voltage and frequency reference for other devices within the microgrid to synchronize with. Grid-forming inverters are essential for islanded operation, where the microgrid is disconnected from the main grid, as they are responsible for creating a stable and reliable AC power supply. Control strategies like droop control or virtual synchronous generator (VSG) control are typically employed to enable grid-forming inverters to regulate voltage and frequency and share load proportionally among multiple inverters. A grid-following inverter, also known as a current-source inverter (CSI) or a VSI operating in current control mode, relies on an external voltage and frequency reference provided by the main grid or a grid-forming inverter. It operates by injecting current into the grid or microgrid, with the magnitude and phase angle of the current controlled to deliver active and reactive power. Grid-following inverters synchronize their output with the existing voltage and frequency using a phase-locked loop (PLL) and cannot function independently without a stable voltage reference. In grid-connected mode, grid-following inverters inject power from DERs into the grid, following the grid's voltage and frequency. How these inverters influence microgrid operation: Islanded Operation: Grid-forming inverters are crucial for islanded operation, providing the necessary voltage and frequency reference. Grid-following inverters cannot operate in islanded mode without a grid-forming inverter to provide a reference. Black Start Capability: Grid-forming inverters possess black start capability, meaning they can energize a de-energized microgrid without relying on an external power source. Grid-following inverters lack this capability. Short-Circuit Current Contribution: Grid-forming inverters contribute to the short-circuit current during fault conditions, aiding in fault detection and clearing. Grid-following inverters typically have limited short-circuit current contribution due to their current-limiting control. Stability: Grid-forming inverters enhance microgrid stability by actively regulating voltage and frequency. Grid-following inverters, while contributing power, do not directly contribute to voltage and frequency regulation. Control Complexity: Grid-forming inverter control is generally more complex than grid-following inverter control, requiring sophisticated algorithms to regulate voltage and frequency and maintain stability. An example scenario illustrates the difference: If a microgrid is operating in islanded mode with a photovoltaic (PV) system and a battery energy storage system (BESS), the BESS inverter needs to be a grid-forming inverter to establish and maintain the microgrid's voltage and frequency. The PV inverter can be a grid-following inverter, injecting power into the microgrid under the control of the grid-forming BESS inverter. Understanding the distinct characteristics and roles of grid-forming and grid-following inverters is crucial for designing and operating stable and reliable microgrids.