The concept of low voltage ride through and related specifications
The so-called low voltage ride-through (LVRT) specifically refers to the voltage drop at the grid connection point of the wind farm due to grid failure or disturbance, within a certain range of voltage drop, wind turbines can be connected to the grid uninterruptedly and provide a certain amount of reactive power to the grid to support grid voltage recovery until the grid returns to normal, thereby “traversing” this low voltage time(area). (Influence of low voltage on wind turbines). As shown in Figure 1, the low voltage ride-through requirements of wind turbines are given. It can be seen from this figure that when the voltage at the grid connection point of the power plant is in the area above the contour line in the figure, the wind turbines in the field must ensure uninterrupted grid-connected operation;
When the voltage of the grid connection point is below the contour line, the wind turbines in the field are allowed to be cut out from the grid.
Combining the specifications of low-voltage ride-through of wind turbines in various countries in the world, three common points can be found, namely, continuous operation without off-grid, fast active power recovery, and providing reactive current as much as possible. The relevant standards in this regard are as follows:
(1) The wind turbine has a low voltage ride-through capability that can maintain grid-connected operation for 625ms when the grid-connected point voltage drops to 20% of the rated voltage.
(2) The voltage at the grid-connected point of the wind farm can recover to 90% of the rated voltage within 2s after the drop occurs, and the wind turbine should be capable of uninterrupted grid-connected operation.
(3) For the wind turbines that are not switched out during the grid fault, their active power shall be restored to the state before the fault at a rate of change of at least 10% of the rated power per second after the fault is cleared.
Introduction of low voltage ride through technology of permanent magnet synchronous wind turbine
For permanent magnet synchronous wind turbines, the main problem during the voltage sag is that the energy mismatch causes the DC voltage to rise. The excess energy can be stored or consumed to solve the energy matching problem, which can be achieved by the following steps:
(1) In terms of power converter design, when selecting devices, relax the withstand voltage and overcurrent values of power electronic devices, and increase the rated voltage of DC capacitors. In this way, when the voltage drops, the voltage limit value of the DC bus can be increased to store excess energy and allow the current of the grid-side power converter to increase to output more energy. However, considering the cost of the device, there is a limit to increasing the device rating, and under long-term and severe faults, the power mismatch will be serious and may exceed the device capacity, so this method is more suitable for short-term voltage drop faults.
(2) In terms of wind turbine control, the electromagnetic torque setting value of the permanent magnet synchronous wind turbine can be reduced, which will cause the speed of the generator to increase, so that the temporary increase of the speed can be used to store the input energy of the wind turbine and reduce the output power of the generator. At the same time, pitch control can be adopted to fundamentally reduce the input power of the wind turbine, which is beneficial to the power balance when the voltage drops.
(3) Units with additional circuits can be considered to store or consume excess energy. As shown in Figure 2, two schemes for implementing low voltage ride-through with external circuit units are presented. Among them, the solution given in Figure 2(a) is to use a buck converter to directly consume the excess DC bus energy with a resistor; the scheme given in Figure 2(b) is to connect an energy storage system to the DC bus; when it is detected that the DC voltage is too high, the IGBT of the energy storage system is triggered, the excess DC energy storage is transferred, and the stored energy is fed into the grid after the fault is recovered.