1 Micro-network mechanism characteristics
In the traditional power grid, whether the power angle is stable, the frequency is stable or the voltage is stable, the synchronous generator is in the center of its research. The stability problem of the large traditional power grid is closely related to the dynamic behavior of the synchronous generator. However, the primary source of electrical energy in the microgrid is the microsource. The characteristics of the micro-source of the inverter interface lead to a large difference between the transient characteristics of the micro-grid and the traditional grid. Its characteristics mainly include the following:
1 piconet can run in multiple operating modes such as grid connection, islanding and outage;
2 The composition of the microgrid is diverse;
3 The control strategy has a significant impact on the voltage and frequency changes of the inverter interface micro-source;
4 The time scale of the microgrid is wider;
5 microgrid inertia is small;
6 microgrid power, voltage and frequency adjustments are diversified.
2 Research status of microgrid transient stability
The transient stability of the microgrid mainly refers to whether the microgrid can reach a new stable running state or return to the original state after a sudden large disturbance in normal operation. Among them, the large disturbance mainly refers to short-circuit fault, load shedding and disconnection, etc. The transient time scale is mainly electromagnetic transient and electromechanical transient process. The research on transient stability of microgrid mainly focuses on the analysis of different types of micro-source operation characteristics in grid faults, the study of micro-network transient processes in different load types, and the transient stability analysis of micro-grids in different types of faults. The methods of microgrid transient stability research mainly include digital simulation analysis and Lyapunov stability judgment method. The most widely used method is digital simulation analysis.
The micro-sources in the micro-grid mainly include fuel cells, photovoltaic power generation systems, micro gas turbines, wind turbines, batteries, flywheel energy storage, and super capacitors. Different types of micro-source fault response characteristics are different when the grid is faulty, which makes the micro-sources with different characteristics have different effects on the transient stability of the micro-grid. At present, the fault response characteristics of micro-sources and the transient process of micro-networks are studied by modeling and simulating different types of micro-sources. The existing research results indicate that the temperature dynamics of the fuel cell, the level of PV penetration, the inertia constant of the micro gas turbine, and the fluctuation and intermittentity of the wind turbine generator are the main factors affecting the transient stability of the micro-source.
Different types of load operation characteristics are very different, and their influence on the transient stability of the microgrid at the time of failure is also different. This paper usually discusses the transient stability of microgrids for load types such as RLC load, active load, motor load and three-phase asymmetric load. By modeling different types of loads and analyzing the operational characteristics, it can be concluded that the impact of different types of loads and loads on the transient stability of the microgrid is different. Among them, the impact of motor load and three-phase unbalanced load on the transient stability of the microgrid is more complicated.
Different types of faults such as short circuit and disconnection will cause a series of transient stability problems for the microgrid. In addition to considering the faults from the inside of the microgrid, it is also necessary to consider the impact of the fault of the grid where the microgrid is located on the transient stability of the microgrid. When the fault occurs, the microgrid considers its own and the safety factor of the grid, and automatically disconnects the island from the grid. However, if the microgrid has a high proportion of power supply in the grid, if it is operated on an island in case of failure, it may cause instability or even collapse of the grid. . Whether it is for the safe operation of the microgrid itself or for the reliability of the power supply of the grid, the microgrid should have a certain fault ride-through capability.
Different micro-sources, different characteristic loads and different types of faults in the micro-grid will have certain influence on the transient stability of the micro-grid. A certain research result has been obtained by studying the influence of a certain factor on the transient stability of the micro-grid. However, there are few studies on the transient stability of microgrids that take into account multiple factors at the same time. In addition, most of the current research mainly uses digital simulation, and fails to form a perfect and reasonable theoretical analysis method; the simulation process concerned rarely involves the transient process of different micro-source inverters after short circuit.
3 Microgrid transient stability control measures
Some breakthroughs have been made in the optimization control strategy for the microgrid. Using wireless communication technology to obtain the overall active and reactive information of the system, the power sharing control can effectively improve the transient stability of the microgrid; the active damping method based on the virtual resistor can effectively eliminate the microgrid during load shedding and increase the constant power load. Transient instability, but may increase system energy loss; using the virtual inertia droop correction control can effectively improve the frequency response of the microgrid when the frequency offset is large, and improve the transient stability of the microgrid.
In addition, some peripherals such as energy storage and reactive power compensation devices can also be used to improve the transient stability of the microgrid. Taking into account the energy storage device's service life, cost performance, efficiency, charge and discharge speed and environmental friendliness, the flywheel energy storage has a relatively high performance, which can inject megawatts of energy into the system in a quarter cycle. The static var generator (SVG) can perform reactive compensation on the microgrid to control the fluctuation of the microgrid voltage, so the SVG can be connected next to the important load of the microgrid. SVG is mainly used to compensate the voltage harmonics of the line to improve the power quality. When the microgrid is running, the line voltage will drop sharply, and SVG will maintain the voltage stability by injecting reactive power.
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