What is LVRT? What is LVRT?

Low voltage ride through capability refers to the ability of a wind turbine to maintain operation without leaving the grid when the terminal voltage of a wind turbine drops to a certain value, and can even provide a certain amount of reactive power to the system to help the system recover the voltage. Wind turbines with low voltage ride-through capability can avoid protection action time and resume normal operation after the fault is removed. This can greatly reduce the number of times wind turbines are repeatedly connected to the grid in the event of a failure, reducing the impact on the power grid.

It has low voltage ride-through capability to ensure that the wind turbine can be connected to the grid as much as possible in the case of reduced grid fault voltage, maintain power generation operation capacity, and reduce grid fluctuations. Generally, the fault of the line of 230 kV or higher voltage level is cut off within 6 cycles (120 ms), and it takes 100 ms for the voltage to return to 15% of the normal level, and 1 s to restore to 75% of the normal level or higher.

How LVRT capability is implemented.

At present, there are generally three schemes to achieve LVRT capability: 1).Rotor short-circuit protection technology is adopted, 2).Introduction of a new topology, 3).Adopt a reasonable excitation control algorithm.

1. Rotor short circuit protection technology (Crowbar circuit).

This is the method adopted by some wind power manufacturers at present, which is equipped with a crowbar circuit on the rotor side of the generator to provide a bypass for the rotor side circuit, and when the voltage drop is detected in the fault of the power grid system, the doubly-fed induction generator excitation converter is locked, and the bypass (energy release resistance) protection device of the rotor circuit is put into place at the same time to limit the current through the excitation converter and the overvoltage of the rotor winding. In this way, the generator is maintained to run without going off the grid (at this time, the doubly-fed induction generator operates in the mode of an induction motor).

2. The new topology includes the following: 1).New bypass system 2).Parallel connection grid-side converter 3).The converter on the grid side is connected in series.

3. Adopt a new excitation control strategy.

From the point of view of manufacturing costs, the best approach is not to change the hardware structure of the system, but to modify the control strategy to achieve the same LVRT effect: in the event of a grid failure, the generator can be safely faulted, while the converter continues to operate in a safe state.

1. Wind turbine low voltage ride-through technology.

1. Question raising.

For variable frequency constant-speed doubly-fed wind turbines, in the case of grid voltage drops, because the supporting power electronic converter equipment belongs to the AC DC AC type, it is easy to generate peak inrush on its rotor side, damage the converter equipment, and cause the wind turbine to be disconnected from the power grid. In the past, when the capacity of wind turbines was small, in order to protect the excitation device on the rotor side, the way of disconnection from the power grid was adopted, but at present, the capacity of wind power generation is very large, and the stability of the entire power grid will be affected after being disconnected from the power grid, and even chain failures will occur. Therefore, according to this situation, foreign experts raised the problem of low voltage ride-through for wind power generation.

2. Explanation of the concept of LVRT.

When the power grid fails, the wind farm needs to maintain a period of time to connect with the grid without disconnecting, and even requires the wind farm to provide reactive power in the process to support the recovery of the grid voltage, i.e., low voltage ride-through.

At present, the low-voltage operation standard for wind power generation is mainly based on the reference proposed by the German company Netz.

Due to the characteristics of doubly-fed wind turbines, there are the following difficulties in realizing LVRT:

a. Ensure that the inrush current on the rotor side and the overvoltage of the DC bus are within the acceptable range of the system during the fault;

b. The countermeasures taken should be effective under various types of faults;

c. The control strategy must meet the adaptability of different units and different parameters;

d. In engineering applications, the cost must be increased as little as possible under the premise of achieving the goal.

Second, the specific implementation of low voltage ride-through technology.

At present, there are generally three schemes for LVRT: one is to adopt rotor short-circuit protection technology, the other is to introduce a new topology, and the third is to adopt a reasonable excitation control algorithm.

Low voltage ride-through (LVRT) refers to the fact that when the voltage of the wind turbine grid drops, the wind turbine can remain connected to the grid, and even provide a certain amount of reactive power to the grid to support the recovery of the grid until the grid returns to normal, so as to "cross" this low voltage time (area). LVRT is a specific functional requirement for grid-connected wind turbines to remain connected to the grid when the voltage drops in the grid.

LVRT requirements vary from country to country.

At present, in some countries where wind power generation is dominant, such as Denmark and Germany, new grid operation guidelines have been formulated, quantitatively giving the conditions for the wind power system to go off the grid (such as the minimum voltage drop depth and drop duration), only when the grid voltage drop is lower than the specified curve before the wind turbine is allowed to go off the grid, when the voltage is in the sunken part, the generator should provide reactive power.

This requires the wind power system to have a strong low voltage ride-through (LVRT) capability, and at the same time can easily provide reactive power support for the power grid, but whether the current doubly-fed wind power generation technology can cope with it freely, there is still a debate in the academic community, and the permanent magnet direct drive key qiao variable speed constant frequency wind power generation system has been proven to have excellent performance in this regard.

Low voltage ride-through (LVRT) refers to the fact that when the voltage of the wind turbine grid drops, the wind turbine can remain connected to the grid, and even provide a certain amount of reactive power to the grid to support the recovery of the grid until the grid returns to normal, so as to "cross" this low voltage time (area). LVRT is a specific functional requirement for grid-connected wind turbines to remain connected to the grid when voltage drops in the grid.

LVRT requirements vary from country to country.

At present, in some countries where wind power generation is dominant, such as Denmark and Germany, new grid operation guidelines have been formulated, quantitatively giving the conditions for the wind power system to go off the grid (such as the minimum voltage drop depth and drop duration), only when the grid voltage drop is lower than the specified curve before the wind turbine is allowed to go off the grid, when the voltage is in the sunken part, the generator should provide reactive power.

This requires the wind power system to have a strong low voltage ride-through (LVRT) capability, and at the same time can easily provide reactive power support for the grid, but whether the current doubly-fed wind power technology can cope with it is still debated in the academic community, and the permanent magnet direct drive variable speed constant frequency wind power generation system has been proven to have excellent performance in this regard.

Low voltage ride-through capability means that when the installed capacity of wind power in the power system is large, after the voltage drop caused by the power system failure, the removal of the wind farm will seriously affect the stability of the system operation, which requires the wind turbine to have low voltage ride through (LVRT) capability to ensure the uninterrupted grid-connected operation of the wind turbine after the system failure.

Wind turbines should have LVRT capability:

a) The wind farm must have a low voltage ride-through capability that can maintain grid-connected operation for 620ms when the voltage drops to 20% of the rated voltage; b) When the wind farm voltage can recover to 90% of the rated voltage within 3s after the drop, the wind farm must maintain grid-connected operation; c) When the voltage of the step-up and high-voltage side of the wind farm is not less than 90% of the rated voltage, the wind farm must be connected to the grid for uninterrupted operation.

LVRT means that when the voltage of the wind turbine is connected to the grid, the wind turbine can remain connected to the grid, and even provide a certain amount of reactive power to the grid to support the recovery of the power grid until the power grid returns to normal, so as to "cross" this low voltage time (area).

The so-called low voltage ride-through refers to a capability of wind turbines.

With the increase of wind turbine assembly capacity, when the power grid fails and the voltage drops, the wind turbines that do not have the ability of low voltage ride-through, or the ability of low voltage ride-through is not enough, will withdraw from the grid in order to protect themselves, and if a large number of wind turbines withdraw from the power grid, the grid voltage will continue to fall, resulting in the paralysis of the power supply grid.

Wind turbines with low voltage ride-through capability are different, and when the voltage drops, they increase their efforts to deliver reactive power to the grid and try to maintain the grid voltage. When the grid voltage is restored, the normal active output is restored.

Super resistance ride-through is when electricity opens another door to the two-dimensional voltage ride-through is like this.