1. Characteristics of the Microgrid Mechanism
In traditional power grids, stability is typically analyzed in terms of power angle, frequency, and voltage, with synchronous generators at the core of the research. The stability of large-scale conventional power systems is closely tied to the dynamic behavior of these generators. However, in a microgrid, the primary energy source is the microsource, and the characteristics of inverter-interfaced microsources lead to significant differences in transient behavior compared to traditional grids. These differences are mainly reflected in the following aspects:
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, incorporating various types of distributed energy resources;
3. Control strategies significantly affect the voltage and frequency variations of inverter-based microsources;
4. The time scale of microgrid dynamics spans a broader range;
5. The inertia of a microgrid is relatively low;
6. Power, voltage, and frequency adjustments in a microgrid are more varied.
2. Current Research on Microgrid Transient Stability
Transient stability in a microgrid refers to its ability to return to a stable state or reach a new equilibrium after experiencing a sudden major disturbance during normal operation. Major disturbances include short-circuit faults, load shedding, and disconnection events. The transient process typically involves electromagnetic and electromechanical time scales. Research focuses on analyzing the operational characteristics of different microsources during grid faults, studying transient processes under various load conditions, and evaluating the stability of microgrids under different fault scenarios.
Common methods for analyzing microgrid transient stability include digital simulation and Lyapunov stability analysis, with digital simulation being the most widely used. Microsources in a microgrid include fuel cells, photovoltaic systems, micro gas turbines, wind turbines, batteries, flywheels, and supercapacitors. Each type exhibits distinct fault response behaviors, which influence the transient stability of the microgrid. Factors such as fuel cell temperature dynamics, PV penetration levels, micro gas turbine inertia, and wind turbine variability play critical roles in determining stability.
Load types also vary significantly in their operational characteristics and impact on transient stability. Commonly studied loads include RLC, active, motor, and three-phase unbalanced loads. Motor and unbalanced loads tend to have more complex effects on stability. Faults like short circuits and disconnections can trigger transient instability, and it's essential to consider both internal and external grid faults. When a fault occurs, the microgrid may automatically isolate from the main grid for safety, but if it provides a high proportion of power, islanding could cause instability or even collapse. Therefore, microgrids must have fault ride-through capability to ensure safe and reliable operation.
Although studies have explored the impact of individual factors on microgrid stability, few have considered multiple factors simultaneously. Most current research relies on digital simulations without developing comprehensive theoretical frameworks. Additionally, the transient behavior of inverter-based microsources after faults is often overlooked in simulations.
3. Control Measures for Microgrid Transient Stability
Significant progress has been made in optimizing control strategies for microgrids. Wireless communication allows for real-time monitoring of active and reactive power, enabling better power sharing and improving transient stability. Active damping techniques using virtual resistors can reduce instability caused by load changes or constant power loads, though they may increase energy loss. Virtual inertia droop control enhances frequency response during large deviations, improving overall stability.
Additionally, peripheral devices such as energy storage and reactive power compensation systems can enhance transient stability. Flywheel energy storage, for example, offers high performance in terms of lifespan, cost, efficiency, and fast charge/discharge capabilities, making it suitable for rapid energy injection. Static Var Generators (SVGs) help stabilize voltage by compensating reactive power, especially during sudden voltage drops. They are commonly placed near critical loads to maintain power quality and system stability during microgrid operation.
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