Comparative Analysis of Low Voltage Ride Through Technology of Doubly-Fed Variable Speed Constant Frequency Wind Turbine
Grid-connected wind power is the fastest growing renewable energy technology in the world in the past decade. The biggest difference between grid-connected wind turbines and traditional grid-connected power generation equipment is that they cannot maintain the voltage and frequency of the grid during grid faults, which is very detrimental to the stability of the power system. Grid fault is a kind of abnormal operation mode of the power grid. It mainly has short circuit or open circuit of transmission line, such as three relative ground, single ground and short circuit or open circuit between lines, which will cause drastic changes in grid voltage amplitude.
The doubly-fed variable-speed constant-frequency wind turbine is the mainstream model of wind turbines at home and abroad. Its power generation equipment is a doubly-fed induction generator. When a grid fault occurs, the existing protection principle is to immediately send the doubly-fed induction generator from the grid. The network is disconnected to ensure the safety of the unit. With the continuous increase of the single unit capacity of wind turbines and the continuous expansion of the scale of wind farms, the mutual influence between wind turbines and power grids has become increasingly serious. There is growing concern that once a grid failure forces a large-area wind turbine to be disconnected due to its own protection, it will seriously affect the operational stability of the power system. Therefore, with the increasing capacity of the doubly-fed induction generators connected to the grid, the grid is increasingly demanding. Generally, the generator set is required to operate without faults in the event of a voltage drop in the grid fault (faultride-through). ), and can help the power system to resume stable operation as soon as possible after the fault is removed, that is, the wind turbine has a certain low voltage ride-through capability. To this end, some new grid operating rules have been proposed internationally. For example, the power company in northern Germany (e.onnetz) requires that the wind farm can operate without the off-grid in the voltage range shown in Figure 1 (ie, the shaded area in the figure) [1][33], and the grid voltage drops to 15%. In the future, the wind turbine should not run off the grid for 300ms. When the grid voltage drops below the curve, the wind turbine is allowed to be disconnected. The voltage here refers to the voltage at the point where the wind farm is connected. The nationalgrid power company, which supplies electricity to parts of the UK, requires that when a transmission line higher than 200kv fails, all power plants or wind farms that are connected to the grid must remain off-grid within 140ms [2]. In addition, the Scottish Power Company (scottishhydro-electric) has similar requirements for non-off-grid operation of power stations or wind farms in the event of grid failure [3].
