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 study. The stability of large-scale traditional power systems is closely related to the dynamic behavior of these generators. However, in microgrids, the primary energy source is the microsource, which is often connected through inverters. This leads to significant differences in transient behavior compared to conventional grids. Key characteristics of microgrids include:
- Piconets can operate in multiple modes such as grid-connected, islanded, or during outages;
- The composition of microgrids is highly diverse;
- Control strategies have a major impact on the voltage and frequency changes of inverter-based microsources;
- The time scale of microgrid operations is broader;
- Microgrids have low inertia;
- Power, voltage, and frequency adjustments are more varied.
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 disturbances typically occur over electromagnetic or electromechanical time scales. Current research focuses on analyzing how different types of microsources behave during grid faults, studying transient processes under various load conditions, and evaluating microgrid stability in different fault scenarios. Common methods include digital simulation and Lyapunov stability analysis, with digital simulation being the most widely used.
Microsources in microgrids include fuel cells, photovoltaic systems, micro gas turbines, wind turbines, batteries, flywheels, and supercapacitors. Each has unique fault response characteristics, affecting the microgrid’s transient stability differently. Studies show that factors like fuel cell temperature dynamics, PV penetration levels, micro gas turbine inertia, and wind turbine variability significantly influence stability.
Load types also play a critical role. RLC loads, active loads, motor loads, and three-phase unbalanced loads all affect transient stability in different ways. Motor and unbalanced loads tend to introduce more complex behaviors, requiring careful modeling and analysis for accurate stability assessment.
Faults such as short circuits or disconnections can cause instability. It's essential to consider both internal microgrid faults and those occurring in the main grid. During a fault, a microgrid may disconnect from the grid to ensure safety. However, if it contains a high proportion of local generation, continued islanded operation could lead to instability. Thus, microgrids must possess fault ride-through capability to support both their own stability and grid reliability.
While individual factors affecting transient stability have been studied, there is limited research on the combined effects of multiple variables. Most studies rely on digital simulations without developing comprehensive theoretical models. Additionally, few investigations focus on the transient behavior of inverter-based microsources following short circuits.
3. Control Measures for Microgrid Transient Stability
Recent advances in 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 can reduce instability caused by load changes or constant power loads, though they may increase energy losses. Virtual inertia droop control helps improve frequency response during large frequency deviations, thus supporting transient stability.
Energy storage systems and reactive power compensation devices also contribute to stability. Flywheel energy storage offers high performance in terms of efficiency, lifespan, and fast charge/discharge capabilities, making it suitable for rapid energy injection. Static Var Generators (SVGs) provide reactive power compensation, stabilizing voltage fluctuations and improving power quality. They are particularly useful near critical loads, helping maintain voltage stability during sudden drops.
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