AU2020101819A4

AU2020101819A4 - A kind of fault handling system and unit structure of doubly-fed wind turbine generator

Info

Publication number: AU2020101819A4
Application number: AU2020101819A
Authority: AU
Inventors: Ruju Fang; Yu Ge; Guanzhong Hu; Geli Zhang; Junli Zhang
Original assignee: Xuchang University
Current assignee: Xuchang University
Priority date: 2020-08-14
Filing date: 2020-08-14
Publication date: 2020-09-24
Anticipated expiration: 2028-08-14

Abstract

The invention provides a fault handling system and a unit structure of a doubly-fed wind turbine generator set, wherein the fault handling system of the doubly-fed wind turbine generator set controls three-phase electronic switches K1, K2, K3 and K4 to be sequentially switched on and off according to the situation that the voltage in point of common coupling drops, and selects a mild fault handling controller or a deep fault handling controller to work according to a fault handling logic controller. Adopting the above scheme, by setting hardware circuit and improving software control strategy, the doubly-fed wind turbine can realize voltage drop fault ride-through of different degrees, increase the reactive power generated by the fault handling system of doubly-fed wind turbines and effectively suppress the dynamic overvoltage and overcurrent of the machine-side converter. There is no need to add complex hardware devices, and the control method is simple and convenient for engineering practice. The harmonic pollution on the AC side of the back-to-back PWM converter 102 is reduced, and the DFIG startup and shutdown times of the doubly-fed generator are reduced, which provides a certain technical support for a large number of applications of the doubly-fed wind turbine. 1/6 109 101 106 105 110 111 112 K2 K /PCC T /QF/ K3 - 107 108 104 LCL1 C 103 102 Fig, 1 Fig, 2

Description

1/6

109

101 106 105 110 111 112

K2 K /PCC T /QF/

K3 - 107 108

104 LCL1 C 103

102

Fig, 1

Fig, 2

A kind of fault handling system and unit structure of doubly-fed wind turbine generator

TECHNICAL FIELD

[01] The invention relates to a kind of structure of a doubly-fed wind turbine, in particular to a fault handling system of the doubly-fed wind turbine generator.

BACKGROUND

[02] The penetration rate of wind turbine connected to power system is increasing, so it is necessary to improve its fault ride-through capability and meet the requirements of Technical Regulations for Wind Farm Connected to Power System GB/T19963-2011 issued by State Grid Corporation of China. As one of the main types of variable speed constant frequency wind power generation technology, the operation capability (i.e., fault ride-through capability) of doubly-fed wind turbine with DFIG in voltage fault of point of common coupling PCC needs to be improved.

[03] The fault ride-through capability of doubly-fed wind turbine is limited by the capacity of back-to-back PWM converter, which is sensitive to the voltage fault of point of common coupling. Therefore, it is necessary to take protective measures to restrain the current of DFIG rotor side of doubly-fed generator and protect the back-to back PWM converter. The improved control technologies of back-to-back PWM converter, such as proportional resonance control technology of converter, double dq PI rotor current control technology, voltage-based power compensation method and active damper control method, can restrain rotor overcurrent of back-to-back PWM converter and improve the operation capability of doubly-fed wind turbine when there is a grid voltage drop fault.

[04] Rotor Crowbar and stator Crowbar protection technology are adopted as one of the effective means to realize fault ride-through of doubly-fed wind turbine. During the voltage drop fault of point of common coupling, the trigger pulse of back to-back PWM converter is blocked, and crowbar resistance is connected on the rotor side of doubly-fed generator, which can effectively limit the overcurrent of DFIG rotor of doubly-fed generator.

[05] Using DC side unloading Chopper protection circuit to suppress DC side overvoltage of back-to-back PWM converter or using super capacitor to stabilize DC side voltage can improve the performance of back-to-back PWM converter, and then improve the fault ride-through capability of doubly-fed wind turbine.

[06] Adding auxiliary devices such as dynamic voltage restorer to stator side of doubly-fed generator DFIG can improve the fault ride-through capability of wind turbine.

[07] Some documents combine the above-mentioned fault ride-through methods to realize the fault ride-through of doubly-fed wind turbines, and have achieved certain results.

[08] A few literatures consider installing three-phase electronic switches between stator side, rotor side and AC side of machine-side converter and grid-side converter of DFIG, and controlling the on and off of electronic switches according to the voltage in point of common coupling , which can control back-to-back PWM converter to be used as static var generator in a short time and help the voltage in point of common coupling recovery, which has certain theoretical significance.

[09] However, the prior art mainly has the following shortcomings:

[010] 1. Improving the control technology of the back-to-back PWM converter can only achieve the fault ride-through of the doubly-fed wind turbine in a small range, but cannot achieve the fault ride-through when the voltage in point of common coupling drops deeply.

[011] 2. For the fault ride-through technology of doubly-fed wind turbine adopting Crowbar protection circuit, when Crowbar protection circuit is put into operation, the back-to-back PWM converter on rotor side of doubly-fed generator DFIG stops working, and the back-to-back PWM converter loses its control function on doubly-fed generator DFIG, which cannot provide reactive power to support the voltage in point of common coupling recovery.

[012] 3. The method of adding unloading Chopper protection circuit or super capacitor to DC side of back-to-back PWM converter can not effectively restrain dynamic overvoltage and overcurrent of machine side converter.

[013] 4. The method of installing auxiliary devices on stator side of doubly-fed generator DFIG increases hardware cost, and coordination control must be considered.

[014] 5. Only installing three-phase electronic switches on stator side, rotor side and AC side between machine side converter and grid side converter of doubly-fed induction generator DFIG will cause frequent start and stop of DFIG in case of the voltage in point of common coupling failure, which is not conducive to the control of doubly-fed generator DFIG. At the same time, back-to-back PWM converter will generate a certain amount of harmonic pollution while generating reactive power.

SUMMARY

[015] The technical problem to be solved by the present invention is to provide a fault handling system and a unit structure of a doubly-fed wind turbine generator set, which can not only realize fault ride-through when the voltage in point of common coupling drops slightly, but also realize fault ride-through when the voltage in point of common coupling drops deeply. The fault handling system of doubly-fed wind turbine can also provide reactive power support to point of common coupling to a greater extent when the voltage in point of common coupling drops deeply. It can avoid overvoltage of DC capacitor of back-to-back PWM converter and effectively restrain dynamic overvoltage and overcurrent of machine-side converter. There is no need to add complex hardware devices, and the control method is simple and convenient for engineering practice. The reactive power generated by the doubly-fed wind turbine generator fault handling system is increased, the harmonic pollution of the AC side of the back-to-back PWM converter is reduced, the start-stop times of the doubly-fed generator DFIG are reduced, and the control performance of the doubly-fed generator is improved when the voltage in point of common coupling fails.

[016] In order to solve the above technical problems, the present invention is realized by the following technical scheme. A fault handling system for doubly-fed wind turbine generator includes doubly-fed induction generator DFIG, back-to-back PWM converter, filter circuit LCL1, filter circuit LCL2, three-phase electronic switch K1, three-phase electronic switch K2, three-phase electronic switch K3, three-phase electronic switch K4, three-phase crowbar circuit RL, point of common coupling PCC, three-phase transformer t, three-phase circuit breaker QF, fault handling logic controller, light fault handling controller and deep fault handling controller.

[017] The turns ratio of rotor winding to stator winding of DFIG is 1.5: 1.

[018] The three-phase electronic switches K1, K2, K3 and K4 are circuits composed of power electronic switching devices GTO, IGBT or IGCT.

[019] One side of the three-phase breaker QF is connected with the other side of the three-phase transformer T, and the other side of the three-phase breaker QF is connected with the main power grid.

[020] And one side of the three-phase transformer t is connected to a point of common coupling PCC.

[021] Preferably, one side of the three-phase electronic switch K1 is connected with the stator three-phase port of the doubly-fed induction generator DFIG and the other side of the three-phase electronic switch K2. The other side of the three-phase electronic switch K1 is connected to the point of common coupling PCC. One side of the three-phase electronic switch K2 is connected to the three-phase crowbar circuit RL. One side of the three-phase electronic switch K3 is connected to a rotor three-phase port of the doubly-fed induction generator DFIG, and the other side of the three-phase electronic switch K3 is connected to a connection point between one side of the filter circuit LCL2 and one side of the three-phase electronic switch K4. And that other side of the three-phase electronic switch K4 is connected to the point of common coupling PCC.

[022] Preferably, the back-to-back PWM converter comprises a machine-side converter MC, a grid-side converter GC and a DC capacitor C, wherein the machine side converter MC and the grid-side converter GC have the same structure. A DC side of the machine-side converter MC is connected in parallel with the DC capacitor C, and an AC side of the machine-side converter MC is connected with the other side of the filter circuit LCL2. A DC side of the grid-side converter GC is connected with the DC capacitor C in parallel, and an AC side of the grid-side converter GC is connected with the other side of the filter circuit LCL1.

[023] Preferably, the filter circuit LCL1 and the filter circuit LCL2 have the same structure. The filter circuit LCL1 has one side connected to the point of common coupling PCC, and the other side connected to the AC side of the grid-side converter GC. The filter circuit LCL2 is connected at the connection point between the other side of the three-phase electronic switch K3 and the three-phase electronic switch K4. The filter circuits LCL1 and LCL2 filter out harmonic content of AC side current of the grid side converter GC and the machine side converter MC and reduce harmonic pollution.

[024] Preferably, the three-phase crowbar circuit RL is an energy consumption resistance circuit, which can adopt a star connection or a triangle connection, and the three-phase end of the three-phase crowbar circuit RL is connected to one side of the three-phase electronic switch K2. The three-phase crowbar circuit RL maintains the energy balance of the doubly-fed induction generator DFIG, which is helpful for the doubly-fed induction generator DFIG to operate in grid connection again.

[025] Preferably, the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time, and when the rotor current of the doubly-fed induction generator DFIG is less than the cut-in current, the three-phase electronic switch K1 and K3 are closed, the three-phase electronic switch K2 and the three-phase electronic switch K4 are opened, and the back-to-back PWM converter works with the mild fault handling controller.

[026] The mild fault handling controller adopts a variable damper control method to switch the machine-side converter MC. Adjust the size of the virtual variable resistor according to the voltage drop amplitude of point of common coupling PCC, and control the doubly-fed generator DFIG to output active power and reactive power.

[027] Preferably, the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time. When the rotor current of the doubly-fed induction generator DFIG is greater than the cut-in current, the three-phase electronic switch K1 and K3 are opened, the three-phase electronic switch K2 and the three-phase electronic switch K4 are closed, and the back-to-back PWM converter works by the deep fault handling controller.

[028] The deep fault handling controller switches the machine-side converter MC and the grid-side converter GC by adopting a similar control method and generates reactive power to the maximum extent.

[029] Preferably, the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time, and when the rotor current of the doubly-fed induction generator DFIG is less than the cut-out current, the three-phase electronic switch K1 and K3 are closed, the three-phase electronic switch K2 and the three-phase electronic switch K4 are opened, and the back-to-back PWM converter works with the mild fault handling controller.

[030] Yet another technical scheme of that invention is as follow: a doubly-fed wind turbine structure is provided with any one of the fault handling system of the doubly-fed wind turbine describe above, and it comprises a vertical axis wind turbine WT, a tower PY, a gear box GB and a doubly-fed induction generator DFIG.

[031] The tower PY is located at the tuyere.

[032] The wind turbine WT is connected with one side of the gear box GB.

[033] And the other side of the gear box GB is connected with the rotating shaft of the doubly-fed induction generator DFIG.

[034] The vertical axis wind turbine WT, the gear box GB, the doubly-fed induction generator DFIG, and the doubly-fed wind turbine fault handling system are located on the tower PY.

[035] Preferably, the rotating shaft of the doubly-fed generator DFIG is connected to the other side of the gear box GB, the three-phase port of the DFIG stator is connected to the junction of one side of the three-phase electronic switch K1 and one side of the three-phase electronic switch K2, and the three-phase port of rotor of the doubly-fed generator DFIG is connected to one side of the three-phase electronic switch K3.

[036] The unique technical characteristics of the fault handling system and the unit structure of the doubly-fed wind turbine are as follows: the AC side of the machine side converter and the AC side of the grid side converter are respectively connected in series with LCL filter circuits; a three-phase crowbar energy consumption circuit is connected in parallel to the stator side of doubly-fed generator DFIG; a mild fault handling controller adopts variable damper.

[037] The fault handling system and the unit structure of the doubly-fed wind turbine provided by the invention have the beneficial effects that the fault handling system and the unit structure of the doubly-fed wind turbine can not only realize the fault ride-through when the voltage in point of common coupling drops slightly, but also realize the fault ride-through when the voltage in point of common coupling drops deeply. The doubly-fed wind turbine fault handling system can also provide reactive power support to point of common coupling to a greater extent when the voltage in point of common coupling drops deeply. It can avoid overvoltage of DC capacitor of back-to-back PWM converter and effectively restrain dynamic overvoltage and overcurrent of machine-side converter. There is no need to add complex hardware devices, and the control method is simple and convenient for engineering practice. The reactive power generated by the doubly-fed wind turbine generator fault handling system is increased, the harmonic pollution of the AC side of the back-to-back PWM converter is reduced, the start-stop times of the doubly-fed generator DFIG are reduced, and the doubly-fed generator control performance is improved when the voltage in point of common coupling fails.

BRIEF DESCRIPTION OF THE FIGURES

[038] Fig. 1 is a schematic structural diagram of a fault handling system for doubly-fed wind turbines.

[039] Fig. 2 is a structural diagram of a three-phase electronic switch.

[040] Fig. 3 is a structural diagram of a three-phase full-bridge PWM converter.

[041] Fig. 4 is a structural diagram of a three-phase half-bridge PWM converter.

[042] Fig. 5 is a structural diagram of LCL filter circuit.

[043] Fig. 6 is a schematic diagram of star connection of energy consumption resistors.

[044] Fig. 7 is a schematic diagram of triangle connection of energy consumption resistors.

[045] Fig. 8 is a flow chart of control logic software.

[046] Fig. 9 is a control block diagram of single-phase rotor current with DFIG introducing variable damper.

[047] Fig. 10 is the equivalent circuit of DFIG single-phase rotor with variable damper.

[048] Fig. 11 is a structural diagram of doubly-fed wind turbine.

DESCRIPTION OF THE INVENTION

[049] The following detailed description of the present invention will be made with reference to the drawings and specific embodiments. The following embodiments can be used in combination, and the present invention can be realized in various forms, which are not limited to the specific embodiments described in this specification. These embodiments are provided for a more thorough and comprehensive explanation of the disclosure of the present invention, which is convenient for understanding.

[050] As an example of the invention, a fault handling system for doubly-fed wind turbines is shown in Figurel, including DFIG 101, back-to-back PWM converter 102, filter circuit LCL1 103, filter circuit LCL2 104, three-phase electronic switch K1 105, three-phase electronic switch K2 106, three-phase electronic switch K3 107, three phase electronic switch K4 108, three-phase crowbar circuit RL 109, point of common coupling PCC 110, three-phase transformer T 111 and three-phase circuit breaker QF 112, fault handling controller, mild fault handling controller and deep fault handling controller.

[051] The turns ratio of the rotor winding to the stator winding of the doubly-fed induction generator DFIG 101 is 1.5:1, and the pole number p of the doubly-fed induction generator DFIG 101 can be 1, 2, 3, 4, etc.

[052] The three-phase electronic switch K1 105, K2 106, K3 107 and K4 108 are circuits composed of power electronic switching devices GTO, IGBT or IGCT. The three-phase electronic switches 105, 106, 107 and 108 have similar structures, and the one-phase circuit of the electronic switch is shown in Figure 2. The current and voltage levels of the three-phase electronic switches 105, 106 are basically equivalent, and the current and voltage levels of the three-phase electronic switches 107 , 108 are basically equivalent , and the current and voltage ratings of the three-phase electronic switches 107 and 108 can be selected to be the same as those of the three-phase electronic switches 105 and 106, or one third of the values can be taken.

[053] One side of the three-phase breaker QF 112 is connected to the other side of the three-phase transformer T 111, and the other side of the three-phase breaker QF

112 is connected to the main power grid. One side of the three-phase transformer T 111 is connected to the point of common coupling PCC 110.

[054] Preferably, one side of the three-phase electronic switch K1 105 is connected with the stator three-phase port of the DFIG 101 and the other side of the three-phase electronic switch K2 106, and the other side of the three-phase electronic switch K1 105 is connected with the point of common coupling PCC 110. One side of the three-phase electronic switch K2 106 is connected with the three-phase crowbar circuit RL109. One side of the three-phase electronic switch K3 107 is connected to the rotor three-phase port of the doubly-fed induction generator DFIG 101, and the other side of the three-phase electronic switch K3 107 is connected to the connection point between one side of the filter circuit LCL2 104 and one side of the three-phase electronic switch K4 108.The other side of the three-phase electronic switch K4 108 is connected to the point of common coupling PCC 110.

[055] Preferably, the back-to-back PWM converter 102 includes a machine-side converter MC, a grid-side converter GC and a DC capacitor C. A DC side of the machine-side converter MC is connected in parallel with the DC capacitor C, and an AC side of the machine-side converter MC is connected with the other side of the filter circuit LCL2 104. A DC side of the grid-side converter GC is connected with the DC capacitor C in parallel, and an AC side of the grid-side converter GC is connected with the other side of the filter circuit LCL1 103. The grid-side converter GC and the machine-side converter MC have the same structure and can adopt a three-phase full bridge structure as shown in Figure 3 or a three-phase half-bridge structure as shown in Figure 4. GTO, IGBT or IGCT can be used as the switching device.

[056] Preferably, one side of the filter circuit LCL1 103 is connected to the point of common coupling PCC 110, and the other side of the filter circuit LCL1 103 is connected to the AC side of the grid-side converter GC. The filter circuit LCL2 104 is connected to the connection point between the other side of the three-phase electronic switch K3 107 and one side of the three-phase electronic switch K4 108.The filter circuit LCL1 103 and the filter circuit LCL2 104 have the same structure, as shown in Figure 5, in which the values of six inductors can be the same and the values of three capacitors can be the same.

[057] Preferably, the three-phase crowbar circuit RL 109 is an energy consumption resistance circuit, and the three-phase end of the three-phase crowbar circuit RL 109 is connected to one side of the three-phase electronic switch K2 106. The three-phase crowbar circuit RL 109 can adopt a star connection of three-phase resistors as shown in Figure 6, or a triangle connection of three-phase resistors as shown in Figure 7. Each resistor branch in Figure 6 and Figure 7 can also be composed of series connection, parallel connection and hybrid connection of resistors, in which the resistor must be a high power consumption resistor.

[058] When the voltage in point of common coupling does not fall down, the three-phase electronic switches K1 105 and K3 107 of the doubly-fed wind turbine generator fault handling system are closed, and K2 106 and K4 108 are opened. The grid-side converter of the back-to-back PWM converter 102 adopts voltage and current double closed-loop control, and both the voltage and the current loop adopt proportional-integral resonance control method, which can meet the control requirements of the grid-side converter under the conditions of three-phase balance and imbalance of the voltage in point of common coupling. The speed and current double closed-loop control method is adopted in the machine-side converter, and the control goal is to stabilize the speed of the doubly-fed generator and generate rotor current whose amplitude and frequency meet the requirements. The two LCL filters are mainly used to filter out the harmonic current near the switching frequency during the operation of the switching device, to reduce the harmonic pollution and the jitter of the doubly fed generator DFIG 101, and to reduce the loss and heat generation.

[059] Preferably, the control logic of the fault handling logic controller is as shown in fig.8. The fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG 101 in real time. When the rotor current of the doubly-fed induction generator DFIG 101 is less than the cut-in current, the three-phase electronic switches K1 and K3 are closed, and the three-phase electronic switches K2 and K4 are opened. The back-to-back PWM converter 102 uses the mild fault handling controller to work. The mild fault handling controller switches the machine-side converter to adopt a variable damper control method, adjusts the size of the virtual variable resistor according to the voltage drop amplitude of the point of common coupling PCC 110, and controls the doubly-fed induction generator DFIG 101 to output active power and reactive power.

[060] Figure 9 is a single-phase rotor current control block diagram of DFIG with variable damper. In Figure 9, is2a is the reference value of DFIG a-phase rotor current,

u' is the DFIG a-phase rotor voltage after introducing variable damper, U2. is the

open-circuit voltage of DFIG a-phase rotor, and R(s) is the transfer function of current

regulator introduced into variable damper control. The traditional PI regulator is used

here. G(s) is the rotor voltage-current relationship inside DFIG. In G(s)=

, R 2 +-L 22 s

F(s) is introduced in variable damper control as the negative feedback function, and it

will be designed as a proportional differential function F(s)= R+Lfs

.

[061] After the introduction of proportional differential negative feedback, for the rotor side of doubly-fed generator DFIG, the transfer function of part of rotor voltage which is closely related to the rotor current changes, that is, the transfer function of the inner closed loop in Figure 9 becomes

G'(s)= G(s) 1+F(s)G(s) (R 2 a+Rf)+(-L2 2 s±Lfs)1)

[062] Introducing the proportional differential negative feedback is equivalent to introducing an additional resistor-inductor circuit on the rotor side, which is used to suppress the rotor overcurrent. However, this resistor-inductor circuit does not exist on the actual DFIG rotor side, thus avoiding some drawbacks of the actual resistance, which is called variable damper. The equivalent circuit of DFIG single-phase rotor with variable damper is shown in Figure 10.

[063] If the proportional coefficient value of the proportional differential function F(s) in the variable damper is too small, it is beneficial to adjust the internal parameters

of the AC excitation power supply. However, if the proportional coefficient is too small, the ability to suppress the rotor overcurrent in the case of grid voltage failure is too poor. The larger the proportional coefficient value of the proportional differential function F(s) in the variable damper is, the more beneficial it is for the doubly-fed wind power system to suppress the rotor overcurrent under fault condition, and the fault ride through can be realized under the deep drop range of the grid voltage. However, its value cannot be increased indefinitely, and it cannot be increased when it reaches the maximum value Rf , that is, the limiting value is Rf 1ax. In order to improve the fault ride-through capability of doubly-fed wind power system with variable damper, the value Rf of negative feedback function F(s) should be optimized in real time according to the severity of voltage drop fault.

[064] In the negative feedback function, Rf should be related to the severity of

grid voltage drop. When the grid voltage drops slightly, Rf of the negative feedback

function F(s) is inversely proportional to X. When the grid voltage has a deep drop fault, Rf should be increased, but a too large value of the proportional function will increase

the burden of the AC excitation power supply on the one hand, so that it cannot provide the reactive power required by the doubly-fed generator DFIG fully and effectively. On the other hand, it may cause overvoltage on the rotor side, which needs to limit R, , and

cannot continue to increase with the decrease of X. At this time, the fault handling controller of doubly-fed wind turbine should consider to work with the deep fault handling controller. In order to make the introduced variable damper meet the requirement of restraining current without increasing the burden of AC excitation power supply, the value of the variable damper is determined by Formula (2).

2 a -U - L2 Ra ! R: w'A42vd - R2)a(2) 2 a vdc - u2a

[065] Comparing the control method with variable damper and Crowbar circuit, it can be seen that after Crowbar protection circuit is introduced into the rotor side, the excitation power supply on the rotor side will exit, and the DFIG will become an induction asynchronous generator without excitation power supply on the rotor side. Crowbar protection circuit can restrain the rotor overcurrent amplitude and DFIG can output active power when the AC excitation power supply of doubly-fed wind power generation system stops working, but it is equivalent to asynchronous generator at this time, which requires the grid to provide reactive power. DFIG, which is equivalent to asynchronous generator working mode, needs to connect other reactive generators in parallel to provide reactive power support for grid voltage recovery. Adopting a negative feedback control method with variable damper, a variable resistor is introduced at the DFIG rotor side to suppress rotor overcurrent.

[066] Preferably, the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time, and when the rotor current of the doubly-fed induction generator DFIG is greater than the cut-in current, the back-to back PWM converter uses the deep fault handling controller to work;

[067] The deep fault handling controller firstly turns off the three-phase electronic switches K1 and K3, secondly closes the three-phase electronic switches K2 and K4, and thirdly switches the control strategies of the machine-side converter and the grid side converter to make them work in the state of static reactive power generator and send out reactive power to help the voltage in point of common coupling recovery.

[068] Compared with the traditional method that blocking the machine side converter to trigger pulse and only letting the grid-side converter work to send out reactive power, this method needs to send out more reactive power, which is more conducive to the voltage recovery in point of common coupling . Using LCL filter circuit can reduce the harmonic content of current output by doubly-fed wind turbine fault handling system and reduce its pollution to point of common coupling.

[069] After the three-phase electronic switch K2 is closed, the three-phase crowbar circuit RL is connected to the stator side of the doubly-fed induction generator DFIG, which consumes excess energy at the stator side, maintains the energy balance of the doubly-fed induction generator DFIG and is beneficial to the re-grid operation of the doubly-fed wind turbine generator.

[070] Preferably, the fault handling logic controller detects the current of the rotor of the doubly-fed generator DFIG in real time. When the current of the rotor of the doubly-fed generator DFIG is less than the cut-out current, the three-phase electronic switches K2 and K4 of the doubly-fed wind turbine fault handling system are turned off, and K1 and K3 are closed, and the back-to-back PWM converter uses the mild fault handling controller to work. Generally, the cutting current should be less than the cutting current.

[071] After the voltage in point of common coupling is restored, the grid-side converter adopts voltage and current double closed-loop control again, and the voltage loop and current loop adopt proportional-integral resonance control method. The speed and current double closed-loop control method is adopted in the machine-side converter, and the control goal is to stabilize the speed of the doubly-fed generator and generate rotor current whose amplitude and frequency meet the requirements.

[072] Yet another embodiment of the present invention is as follows: a doubly fed wind turbine structure, as shown in Figure11, has any one of the doubly-fed wind turbine fault handling system 1104 described above, and includes a vertical axis wind turbine WT 1101, a tower PY 1102, a gear box GB 1103, and a doubly-fed induction generator DFIG.

[073] The tower PY 1102 is located at the tuyere.

[074] The wind turbine WT 1101 is connected to one side of the gear box GB 1103.

[075] And the other side of the gear box GB 1103 is connected with the rotating shaft of the doubly-fed induction generator DFIG.

[076] The vertical axis wind turbine WT 1101, the gear box GB 1103, the doubly fed induction generator DFIG, and the doubly-fed wind turbine fault handling system are located on the tower PY 1102.

[077] Preferably, the rotating shaft of the doubly-fed generator DFIG is connected to the other side of the gear box GB 1103, the three-phase port of the DFIG stator is connected to one side of the three-phase electronic switch K1 and one side of the three phase electronic switch K2, and the three-phase port of the rotor of the doubly-fed generator DFIG is connected to one side of the three-phase electronic switch K3.

[078] It should be noted that the above technical features continue to be combined with each other to form various embodiments not listed above, which are regarded as the scope recorded in the specification of the present invention. Furthermore, for those of ordinary skill in the art, improvements or transformations can be made according to the above description, and all these improvements and transformations should belong to the protection scope of the appended claims of the present invention.

[079] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms, in keeping with the broad principles and the spirit of the invention described herein.

[080] The present invention and the described embodiments specifically include the best method known to the applicant of performing the invention. The present invention and the described preferred embodiments specifically include at least one feature that is industrially applicable

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A fault handling system for doubly-fed wind turbines is characterized by comprising doubly-fed induction generator DFIG, back-to-back PWM converter, filter circuit LCL1, filter circuit LCL2, three-phase electronic switch K1, three-phase electronic switch K2, three-phase electronic switch K3, three-phase electronic switch K4, three-phase crowbar circuit RL, a point of common coupling PCC, three-phase transformer T, three-phase circuit breaker QF, fault handling logic controller, mild fault handling controller and deep fault handling controller.

The turns ratio of rotor winding to stator winding of DFIG is 1.5: 1.

The three-phase electronic switches K1, K2, K3 and K4 are circuits composed of power electronic switching devices GTO, IGBT or IGCT.

One side of the three-phase circuit breaker QF is connected with the other side of the three-phase transformer T, and the other side of the three-phase circuit breaker QF is connected with the main power grid.

And one side of the three-phase transformer T is connected with a point of common coupling PCC.

2. The fault handling system for doubly-fed wind turbines according to claim 1 is characterized in that one side of the three-phase electronic switch K1 is connected with the stator three-phase port of the doubly-fed generator DFIG and the other side of three-phase electronic switch K2, and the other side of the three-phase electronic switch K1 is connected with the point of common coupling PCC; One side of the three-phase electronic switch K2 is connected with the three-phase crowbar circuit RL; One side of the three-phase electronic switch K3 is connected to a rotor three-phase port of the doubly-fed generator DFIG, and the other side of the three-phase electronic switch K3 is connected to a connection point between one side of the filter circuit LCL2 and one side of the three-phase electronic switch K4. And that other side of the three-phase electronic switch K4 is connected to the point of common coupling PCC.

3. The fault handling system of doubly-fed wind turbine according to claim 1 is characterized in that the back-to-back PWM converter comprises a machine-side converter MC, a grid-side converter GC and a DC capacitor C. The machine-side converter MC and the grid-side converter GC have the same structure; A DC side of the machine-side converter MC is connected in parallel with the DC capacitor C, and an AC side of the machine-side converter MC is connected with the other side of the filter circuit LCL2. A DC side of the grid-side converter GC is connected with the DC capacitor c in parallel, and an AC side of the grid-side converter GC is connected with the other side of thefilter circuit LCL1.

4. The fault handling system of doubly-fed wind turbine according to claims 1 to 3 is characterized in that the filter circuit LCL1 and the filter circuit LCL2 have the same structure; the filter circuit LCL1 has one side connected to the point of common coupling PCC, and the other side connected to the AC side of the grid-side converter GC. The filter circuit LCL2 is connected at the connection point between the other side of the three-phase electronic switch K3 and the three-phase electronic switch K4.The filter circuit LCL1 and the filter circuit LCL2 filter out harmonic content of AC side current of the grid side converter GC and the machine side converter MC.

5. The fault handling system of doubly-fed wind turbine according to claim 1 is characterized in that the three-phase crowbar circuit RL is an energy consumption resistance circuit, which can adopt a star connection or a triangle connection, and the three-phase end of the three-phase crowbar circuit RL is connected to one side of the three-phase electronic switch K2. The three-phase crowbar circuit RL maintains the energy balance of the doubly-fed induction generator DFIG, which is helpful for the doubly-fed induction generator DFIG to operate in grid connection again.

6. The fault handling system for doubly-fed wind turbine according to claim 1 is characterized in that the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time, and when the rotor current of the doubly-fed induction generator DFIG is less than the cut-in current, the three-phase electronic switch K1 and K3 are closed, the three-phase electronic switch K2 and the three-phase electronic switch K4 are opened, and the back-to-back PWM converter uses the mild fault handling controller to work.

The mild fault handling controller switches the machine-side converter MC using a variable damper control method. It can adjust the size of the virtual variable resistor according to the voltage drop in the common connection point PCC, and control the doubly-fed induction generator DFIG to output active power and reactive power.

7. The fault handling system for doubly-fed wind turbine according to claim lis characterized in that the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time, and when the rotor current of the doubly-fed induction generator DFIG is greater than the cut-in current, the three-phase electronic switch K1 and K3 are opened, the three-phase electronic switch K2 and the three-phase electronic switch K4 are closed, and the back-to-back PWM converter works adopting the deep fault handling controller.

The deep fault handling controller switches the machine-side converter MC and the grid-side converter GC by adopting a similar control method, and generates reactive power to the maximum extent.

8. The fault handling system for doubly-fed wind turbine according to claim 1 is characterized in that the fault handling logic controller detects the rotor current of the doubly-fed induction generator DFIG in real time, and when the rotor current of the doubly-fed induction generator DFIG is less than the cut-out current, the three-phase electronic switch K1 and K3 are closed, the three-phase electronic switch K2 and the three-phase electronic switch K4 are opened, and the back-to-back PWM converter uses the mild fault handling controller to work.

9. A doubly-fed wind turbine structure is characterized by comprising a vertical axis wind turbine WT, a tower PY, a gear box GB, a doubly-fed induction generator DFIG and a fault handling system of the doubly-fed wind turbine.

The tower PY is located at the tuyere.

The wind turbine WT is connected with one side of the gear box GB;

And the other side of the gear box GB is connected with the rotating shaft of the doubly-fed induction generator DFIG.

The vertical axis wind turbine WT, the gear box GB, the doubly-fed induction generator DFIG, and the doubly-fed wind turbine fault handling system are located on the tower PY.

10. The structure of doubly-fed wind turbine according to claim 9 is characterized in that the rotating shaft of DFIG is connected to the other side of the gear box GB, the stator three-phase port of DFIG is connected to the junction of one side of the three phase electronic switch KI and one side of the three-phase electronic switch K2, and the rotor three-phase port of DFIG is connected to one side of the three-phase electronic switch K3.