Low voltage ride through control circuit of doubly-fed variable-speed constant-frequency wind power generator unit

Abstract

The invention relates to a low voltage ride through control circuit of doubly-fed variable-speed constant-frequency wind power generator unit, which is mainly technically characterized in that: first three-phase alternating current switches are connected in series between stators of the power grid and the doubly-fed power generator in the doubly-fed variable-speed constant-frequency wind power generator unit, second three-phase alternating current switches are connected in series between a rotor of the doubly-fed power generator in the doubly-fed variable-speed constant-frequency wind power generator unit and a rotor-side inverter of the doubly-fed frequency converter, and third three-phase alternating current switches are connected in series between the rotor-side inverter of the doubly-fed frequency converter and a grid side inverter of the doubly-fed frequency converter. The low voltage ride through control circuit has the characteristics of reasonable design, great simplicity and stable and reliable performance, guarantees the implementation of low voltage ride through function of doubly-fed power generator unit and safe and stable power grid, reduces the cost thereof, and can be widely applied to the field of wind power generation.

Description

technical field [0001] The invention belongs to the field of wind power generation, in particular to a low-voltage ride-through control circuit of a double-fed variable-speed constant-frequency wind power generating set. Background technique [0002] In recent years, wind power generation has developed rapidly all over the world. With the continuous expansion of installed capacity of wind power generation, the influence of wind power generation system on the power grid is increasing. When the grid voltage drops by a certain value, if a large number of wind turbines cut off the grid, it will cause a large change in the power flow of the system and may even cause a large-scale power outage. Therefore, the traditional method of automatically disconnecting the wind turbines from the grid is no longer suitable for the new grid operation criteria. . In order to keep wind power generators connected to the grid when the grid voltage drops instantaneously, countries have formulated ...

AI Technical Summary

Problems solved by technology

[0003] For doubly-fed variable-speed constant-frequency wind turbines, the doubly-fed generator stator is directly connected to the grid. This special topology enables the frequency converter to only partially control the generator, and the generator stator voltage equation has Under-damping characteristics, in this way, during the instantaneous drop and recovery of the grid voltage, a large electromagnetic transition process will occur inside the doubly-fed motor, and the stator flux linkage cannot quickly follow the sudden change of the stator terminal voltage, resulting in a large transient state Oscillation attenuation component and DC component, due to the decrease of the integral amount, the stator flux linkage speed hardly changes, while the rotor continues to rotate, a large slip will be generated, which will cause overvoltage and overcurrent of the rotor winding, and at the same time The rapid increase of the rotor side current will cause the DC bus voltage of the rotor inverter to increase, and the active and reactive power of the generator will oscillate

[0004] Due to the limited withstand voltage and overcurrent capacity of the power device of the frequency converter in the doubly-fed generator set, it is difficult to withstand the severe transition process of the doubly-fed motor when the grid voltage drops

In order to protect the frequency converter and meet the requirements of the power system for low-voltage ride-through of doubly-fed generators, active or active crowbar protection circuits (crowbar) are mainly used at present. The basic circuit structure is as follows: Image 6 As shown, the generator rotor is equipped with a rotor short-circuit protection circuit controlled by IGBT, GTO and other devices that can be turned off. When the rotor current exceeds a certain set value, the crowbar protection circuit is put into operation, and the rotor inverter is out of operation. The large current on the rotor side provides a bypass to limit the current and protect the frequency converter. This protection circuit can basically realize the LVRT operation of the doubly-fed generator set, but it has the following problems: 1. Since the generator stator is still connected to the grid, Once the crowbar protection circuit operates, the doubly-fed generator becomes an ordinary asynchronous motor and rotates freely, absorbing reactive excitation power from the grid, which is not conducive to the recovery of the grid voltage. According to the reactive power-voltage characteristics of the generator, when the terminal voltage drops by more Within a certain range, the reactive power absorbed by the generator from the grid increases sharply as the voltage decreases. Since the reactive power characteristics of the system reactive power supply decrease with the decrease of the voltage, the grid voltage will further drop due to insufficient reactive power of the grid. , in severe cases, it may lead to voltage collapse; 2. During the oscillation process when the power grid is restored, a larger rotor current may appear, which makes the crowbar protection circuit work again, and absorbs a large amount of reactive power from the power grid, which leads to the aggravation of the power grid oscillation process. It is not conducive to the recovery of the power grid, and even leads to further deterioration of the power grid, resulting in power outages; 3. Since the degree of rotor overcurrent varies greatly under different grid drop conditions, it is difficult to choose a crowbar protection circuit that can prevent DC side overvoltage and protect the power grid. Current limiting resistor that can effectively suppress overcurrent on the rotor side

To sum up, the LVRT control circuit of double-fed variable-speed constant-frequency wind turbines generally has problems such as complex control methods, poor reliability, and high cost.

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