Recently, the wind generator capacity is growing bigger, and the wind generator affects the grid seriously. The LVRT during voltage drop is the most important and it is one of the biggest problem of affecting the power grid. When there is voltage drop, the LVRT of the fans can’t be satisfied, the accidents of generator switching will happen which causes the system unstable, even electricity short cut. So, the State Grid has published related regulation to instruct the grid. Low voltage ride through (LVRT), is what an electric device, especially wind generator, may be required to be capable of when the voltage in the grid is temporarily reduced due to a fault or load change in the grid. The voltage may be reduced in one, two or all the three phases of the AC grid. The severity of the voltage dip is defined by the voltage level during the dip (may go down to zero) and the duration of the dip

Now, the man fans are required to show the authorized test report on LVRT before grid. In previous period, the fans below 850kW can’t get the LVRT certificate. In order to realize the LVRT function, the auxiliary equipment on LVRT is required to install.

The principle on improving the LVRT ability is that the voltage drop affects the wind generator minimally to suppress the impact between voltage and current in the voltage drop of the fans and transmit the energy in time. How to provide an effective way for the wind generators becomes key and basic.

If the wind capacity accounts for more than 5% of other power source capacity, the wind farm running the area should have the ability of LVRT showing in the requirements.

  • The wind generators in wind farm should own the ability that when the voltage drops to 20% of rated voltage, it ensures not off-grid and continues to run 625ms.
  • When the voltage drops in the wind farm, it can recover to 90% of the rated voltage within 2s, and the wind generators can continue to work without off-grid.

Asynchronous fan LVRT auxiliary equipment
According to above requirements, PONOVO promotes the LVRT improvement equipment in absorbing LVRT. When there is voltage drop in the grid sides, it isolates the fan and grid quickly and absorbs the energy sent by fans. Meanwhile, it controls the phase differential between the fan and grid by circuit, and provides the conditions for the re-grid connection.

The simulation result of energy absorption LVRT equipment is showing in the figure 1.

Pon Voltage change

Figure 1 Voltage change curve during voltage drop

 
From figure 1, when the voltage drops to 20% and lasts 625ms, the fan’s voltage will rise to the rated value in fan side and keep stable. The equipment exists the working until the voltage is recovered. The fan begins re-grid and injects active power into the grid. When the equipment exists, the impact on the grid will be reduced to the lowest.