1. Characteristics of the Microgrid Mechanism
In traditional power grids, the stability of the system—whether in terms of power angle, frequency, or voltage—is primarily determined by synchronous generators. The dynamic behavior of these generators plays a central role in the stability of large-scale power systems. However, in microgrids, the primary energy source is the microsource, typically connected through an inverter. This difference leads to significant variations in transient characteristics compared to traditional grids. The main features of microgrids include:1. A piconet can operate in multiple modes, such as grid-connected, islanded, or during outages;
2. The composition of the microgrid is highly diverse, including various types of distributed energy resources;
3. Control strategies have a major impact on the voltage and frequency changes of inverter-interfaced microsources;
4. The time scale of microgrid operations is broader, covering both fast electromagnetic transients and slower electromechanical processes;
5. Microgrids have low inertia, making them more sensitive to disturbances;
6. Power, voltage, and frequency adjustments in microgrids are more varied and complex.
2. Current Research on Microgrid Transient Stability
Transient stability in microgrids refers to the ability of the system to return to a stable state after experiencing a sudden large disturbance, such as a short circuit, load shedding, or disconnection. These events occur over electromagnetic and electromechanical time scales. Current research focuses on analyzing the operational characteristics of different microsources during faults, studying transient processes under various load conditions, and evaluating stability across different fault types.Common methods for analyzing transient stability include digital simulation and Lyapunov-based stability analysis, with digital simulation being the most widely used. Microsources in microgrids include fuel cells, photovoltaic systems, microturbines, wind turbines, batteries, flywheels, and supercapacitors. Each type has distinct fault response characteristics, which affect the overall transient stability of the system. Factors such as fuel cell temperature dynamics, PV penetration levels, microturbine inertia, and wind turbine variability significantly influence stability.
Load types also vary greatly, and their impact on transient stability differs. Commonly studied loads include RLC, active, motor, and three-phase unbalanced loads. Motor and unbalanced loads tend to have more complex effects on stability. Additionally, different fault types—such as short circuits and disconnections—can trigger instability. When a fault occurs, microgrids may automatically disconnect from the main grid for safety. However, if the microgrid supplies a large share of the grid’s power, islanding could lead to instability or even collapse.
While many studies have explored individual factors affecting transient stability, there is limited research that considers multiple factors simultaneously. Most current approaches rely on digital simulations, lacking a comprehensive theoretical framework. Furthermore, few studies examine the transient behavior of inverters in detail after a short-circuit event.
3. Transient Stability Control Measures for Microgrids
Recent advances in optimization control strategies have improved microgrid transient stability. Wireless communication allows for real-time monitoring of active and reactive power, enabling better power sharing and enhancing system resilience. Active damping techniques using virtual resistors help reduce instability caused by load changes or constant power loads, though they may increase energy losses. Virtual inertia droop control improves frequency response during large deviations, thus improving overall stability.Energy storage systems and reactive compensation devices also play a key role in maintaining stability. Flywheel energy storage, for example, offers high performance in terms of lifespan, efficiency, and rapid charge/discharge capabilities. It can inject megawatts of energy into the system within a quarter cycle. Static Var Generators (SVGs) provide reactive power compensation, helping to stabilize voltage fluctuations. They are particularly useful near critical loads and can maintain voltage stability during sudden drops by injecting reactive power.
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