Double-fed fan fault ride-through method and system based on hybrid energy storage and crowbar protection

Abstract

The invention provides a doubly-fed fan fault ride-through method based on hybrid energy storage cooperating with crowbar circuit protection, and belongs to the technical field of new energy power systems. Comprising the following steps: step 1, obtaining the voltage of a PCC point at an outlet of the doubly-fed fan; 2, whether UPCC is smaller than 0.9 pu or not is judged, and if the UPCC is smaller than 0.9 pu, GSC and RSC are switched to run in the first mode; if not, switching the GSC into the second mode operation, switching the RSC into the second mode operation, switching the storage battery into the second mode operation, and locking crowbar protection; 3, judging whether UPCC is less than 0.5 pu or not, and if so, switching the storage battery into a mode I; if not, switching the storage battery into a mode 2; 4, judging whether the condition that Ir is larger than 2pu or Vdc is larger than 1.15 pu is met currently, if yes, activating crowbar protection, and continuing the step 5; if not, locking crowbar protection, and returning to the step 2; 5, judging whether Ir is less than 1.9 pu and Vdc is less than 1.05 pu at the same time, if so, locking crowbar protection, and returning to the step 2; if not, returning to the step 4.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy power systems, and in particular to a method and system for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection. Background Art

[0002] Since the stator winding of the doubly fed wind turbine is directly connected to the grid, it is more sensitive to the voltage sag on the grid side. When a grid fault occurs, the voltage sag of the system will cause the doubly fed wind turbine rotor overcurrent and the DC bus overvoltage during the fault, which may damage the rotor-side converter of the doubly fed wind turbine and even cause the DFIG to be disconnected from the grid. Therefore, in order to avoid the rotor overcurrent and DC bus overvoltage of the doubly fed wind turbine during the fault, a large number of scholars have proposed many methods to implement and enhance the fault ride-through strategy of the doubly fed wind turbine. These methods are mainly divided into two categories: improving the control method and adding hardware measures:

[0003] In terms of control improvement, the existing technology proposes to control the output power and terminal voltage of the doubly fed wind turbine by adjusting the internal voltage in the dual-loop decoupling control based on the new coordinate reference system design, improve the system damping and the system power angle jump during the fault, so as to improve the fault ride-through of the doubly fed wind turbine; and proposes to use the robust model predictive control method based on Q-learning to suppress the rotor overcurrent of the doubly fed wind turbine, which solves the problems of overshoot and insufficient robustness of traditional demagnetization control. These control improvement methods cannot achieve fault ride-through during severe voltage sags, so some hardware protection is required in the doubly fed wind turbine system.

[0004] In terms of adding hardware measures to assist the doubly fed wind turbine in fault ride-through, the existing technology proposes to use crowbar circuit protection to prevent damage to the rotor-side converter, thereby assisting the doubly fed wind turbine in achieving fault ride-through. The crowbar protection circuit has the advantages of high cost-effectiveness, simple control, and easy implementation. However, during the start-up of the crowbar circuit protection, the doubly fed wind turbine will absorb reactive power from the power grid, thereby affecting the recovery of the fault voltage at the doubly fed wind turbine grid connection point. Summary of the invention

[0005] In view of this, the present invention provides a doubly-fed wind turbine fault ride-through method and system based on hybrid energy storage and crowbar protection, which utilizes the control strategy switching of batteries and superconducting energy storage to improve the triggering process of the crowbar protection, thereby assisting the doubly-fed wind turbine in achieving fault ride-through while reducing the number of times the crowbar protection is started.

[0006] The technical solution adopted by the embodiment of the present invention to solve the technical problem is:

[0007] A method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection is implemented based on a hybrid energy storage coordinated control system, wherein the hybrid energy storage coordinated control system includes a doubly-fed wind turbine, a converter group, an RSC control module, a GSC control module, a battery, a Buck-Boost control module, a superconducting energy storage module, a DC / DC control module, a crowbar protection module, and a demagnetization current input unit:

[0008] The rotor side of the doubly-fed wind turbine is connected to the converter group, then connected to the stator side winding, and then connected to the power grid;

[0009] The converter group includes a rotor-side converter RSC and a grid-side converter GSC connected at the DC side, the DC side of the rotor-side converter RSC is connected to the grid-side converter GSC, the rotor-side converter RSC and the grid-side converter GSC share a capacitor and a DC chopper protection;

[0010] The crowbar protection module is connected to the rotor side of the double-fed wind turbine to provide crowbar protection; the demagnetization current input unit is used to continuously provide demagnetization current to the RSC;

[0011] The RSC control module is connected to the control end of the rotor-side converter RSC;

[0012] The GSC control module is connected to the control end of the grid-side converter GSC;

[0013] The battery is connected to the DC bus between the RSC and the GSC through a Buck-Boost converter; the Buck-Boost control module is connected to the Buck-Boost converter;

[0014] The superconducting energy storage module is connected to the DC bus between the RSC and the GSC through a DC / DC chopper; the DC / DC control module is connected to the DC / DC chopper;

[0015] The Buck-Boost control module is used to output a control signal to the Buck-Boost converter to control the working mode of the battery. There are two working modes of the battery: Mode 1 is that during the fault period when the PCC voltage of the power grid is 0-0.5pu, the reference value i of the battery output current is b * Set to the actual value of the DC bus voltage V dc With reference value V dc * The difference is used to stabilize the DC bus voltage; Mode 2 is that when the power grid is operating normally or a shallow fault is 0.5 to 0.9 pu, the power P generated by the double-fed wind turbine is tur With the power scheduling instruction P dp-refThe power difference between them is transmitted to the DC bus side of DFIG through GSC, and the reference value of battery output current i b * The value of is the actual value of the battery active output power P battery With reference value P b * The difference between the two, the battery is used to process the low frequency band power;

[0016] The RSC control module is used to output a control signal to the RSC for controlling the RSC, wherein the RSC has two control modes:

[0017] Mode 1 is to use the reverse current tracking control of the stator current to control the demagnetization of the rotor winding; Mode 2 is to obtain the active power reference P through MPPT control MPPT , specifically by controlling the d-axis component i of the rotor current rd , so that the stator output power P s Follow active power reference P MPPT , the reactive output power of the stator Q s The q-axis component i of the rotor current rq to regulate;

[0018] The GSC control module is used to output a control signal to the GSC to control the operation mode of the GSC. There are two operation modes of the GSC: mode 1 is to provide reactive power to the grid when the grid fails; mode 2 is to provide active power to the grid when the system is in a normal state and manage the system output power P out ;

[0019] The steps of the method include:

[0020] Step S1, obtaining the PCC voltage U at the outlet of the doubly fed wind turbine PCC ;

[0021] Step S2, determine whether U PCC <0.9pu, if the judgment result is yes, the GSC and RSC of the doubly fed wind turbine are switched to mode 1 operation, the DC voltage of the doubly fed wind turbine is stabilized by coordinating superconducting energy storage, the demagnetization current control of the rotor converter is maintained to suppress the rotor fault current in real time, and step S3 is continued; if the judgment result is no, the GSC is switched to mode 2 operation, the RSC is switched to mode 2 operation, the battery is switched to mode 2 operation, the crowbar protection is locked, and the current working state is maintained;

[0022] Step S3, determine whether U PCC<0.5pu, if the judgment result is yes, the battery is switched to mode 1 operation to stabilize the DC voltage of the doubly-fed wind turbine in cooperation with the superconducting energy storage, and step S4 is continued; if the judgment result is no, the battery is switched to mode 2 operation;

[0023] Step S4, determine whether the current r >2pu or V dc >1.15pu, where I r is the rotor overcurrent, V dc If the DC bus overvoltage is detected, the crowbar protection is activated and step S5 is continued; if the judgment result is negative, the crowbar protection is locked and the process returns to step S2;

[0024] Step S5, continue to determine whether the current conditions I and r <1.9pu and V dc <1.05pu, if the judgment result is yes, the crowbar protection is locked and the process returns to step S2; if the judgment result is no, the process returns to step S4 to continuously activate the crowbar protection.

[0025] Preferably, the operation mode 1 of the GSC is as follows: assisting the DFIG to achieve low voltage ride-through when a grid fault occurs. The GSC output current vector is divided into two components i in the synchronously rotating dq axis reference frame. gd 、i gq ; Among them, the d-axis component of the GSC output current is used to control the DFIG DC bus voltage, and the q-axis component provides reactive power to the grid according to the voltage drop at the DFIG grid connection point;

[0026] The second operation mode of GSC is as follows: GSC is used to manage the output power of the system and reduce the power fluctuation of wind power caused by random wind speed; in the control strategy, the d-axis component i of the GSC output current gd Used to dispatch the power according to the instruction P dp-ref Control system output power P out ; Reactive power Q of the system out The q-axis component i of the output current through the grid gq To adjust, refer to the reactive power Q dp-ref Set to zero.

[0027] Preferably, the magnitude of the demagnetization current is K times the stator current.

[0028] Preferably, the switch state of the crowbar protection module is controlled by an IGBT, and the control signal is the rotor side current.

[0029] Preferably, the DC / DC control module outputs a control signal to the DC / DC chopper to control the working mode of the superconducting energy storage module, wherein, under normal operation of the power grid, the superconducting energy storage module processes the high-frequency power demand of the system by stabilizing the DC bus voltage; when the power grid fails, the actual value V of the DFIG DC bus voltage is used to control the working mode of the superconducting energy storage module. dc With reference value V dc * The difference between the two values ​​is used to obtain the reference value of the SMES output current i s * , assisting the doubly-fed wind turbine to stabilize the DC voltage and enhance the fault ride-through capability of the doubly-fed wind turbine.

[0030] Preferably, when the power grid is in a normal state, RSC, GSC and HESS are all controlled in mode 2, and the crowbar circuit fails; the battery is used to process the low-frequency power component of the power difference, and the power difference is the difference between the power emitted by the doubly fed wind turbine and the power dispatch instruction; SEMS is used to process the high-frequency power component of the power difference; RSC and GSC are used to manage the output power of the wind storage system.

[0031] Preferably, after step S5, the method further includes:

[0032] Step S6, when the fault disappears or the voltage recovers, the battery and the superconducting hybrid energy storage module remain connected to the grid to smooth the power fluctuation caused by the wind speed fluctuation.

[0033] The present invention also provides a hybrid energy storage coordinated control system, comprising: a double-fed wind turbine, a converter group, an RSC control module, a GSC control module, a battery, a Buck-Boost control module, a superconducting energy storage module, a DC / DC control module, a crowbar protection module and a demagnetization current input unit, wherein:

[0034] The rotor side of the doubly-fed wind turbine is connected to the converter group, then connected to the stator side winding, and then connected to the power grid;

[0035] The converter group includes a rotor-side converter RSC and a grid-side converter GSC connected at the DC side, the DC side of the rotor-side converter RSC is connected to the grid-side converter GSC, the rotor-side converter RSC and the grid-side converter GSC share a capacitor and a DC chopper protection;

[0036] The crowbar protection module is connected to the rotor side of the double-fed wind turbine to provide crowbar protection; the demagnetization current input unit is used to continuously provide demagnetization current to the RSC;

[0037] The RSC control module is connected to the control end of the rotor-side converter RSC;

[0038] The GSC control module is connected to the control end of the grid-side converter GSC;

[0039] The battery is connected to the DC bus between the RSC and the GSC through a Buck-Boost converter; the Buck-Boost control module is connected to the Buck-Boost converter;

[0040] The superconducting energy storage module is connected to the DC bus between the RSC and the GSC through a DC / DC chopper; and the DC / DC control module is connected to the DC / DC chopper.

[0041] It can be seen from the above technical solution that the embodiment of the present invention provides a method and system for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection. The method is implemented based on a hybrid energy storage collaborative control system, which includes a doubly-fed wind turbine, a converter group, an RSC control module, a GSC control module, a battery, a Buck-Boost control module, a superconducting energy storage module, a DC / DC control module, a crowbar protection module, and a demagnetization current input unit: the rotor side of the doubly-fed wind turbine is connected to the converter group and then connected to the stator side winding and then to the power grid; the converter group includes a rotor-side converter RSC and a grid-side converter GSC connected to the DC side, the DC side of the rotor-side converter RSC is connected to the grid-side converter GSC, the rotor-side converter RSC and the grid-side converter GSC share a capacitor, and a DC chopper Protection; the crowbar protection module is connected to the rotor side of the doubly-fed wind turbine to provide crowbar protection; the demagnetization current input unit is used to continuously provide demagnetization current to the RSC; the RSC control module is connected to the control end of the rotor-side converter RSC; the GSC control module is connected to the control end of the grid-side converter GSC; the battery is connected to the DC bus between the RSC and the GSC through the Buck-Boost converter; the Buck-Boost control module is connected to the Buck-Boost converter; the superconducting energy storage module is connected to the DC bus between the RSC and the GSC through the DC / DC chopper; the DC / DC control module is connected to the DC / DC chopper. The present invention utilizes the control strategy switching of the battery and the superconducting energy storage to improve the triggering process of the crowbar protection, assist the doubly-fed wind turbine in achieving fault ride-through, and reduce the number of startups of the crowbar protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic diagram of the strategy framework of the doubly-fed wind turbine fault ride-through method based on hybrid energy storage and crowbar protection.

[0043] Figure 2 This is a fault ride-through flow chart of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection.

[0044] Figure 3 This is the crowbar circuit protection topology diagram of the doubly fed wind turbine.

[0045] Figure 4 Schematic diagram of the DC / DC chopper control mode of the superconducting energy storage system.

[0046] Figure 5 This is a control diagram of the DC / DC converter of the battery system.

[0047] Figure 6 Schematic diagram of the control mode of the grid-side converter of the doubly fed wind turbine.

[0048] Figure 7 Schematic diagram of the control mode of the doubly fed wind turbine rotor-side converter. DETAILED DESCRIPTION

[0049] The technical scheme and technical effects of the present invention are further elaborated in detail below in conjunction with the accompanying drawings of the present invention.

[0050] For the doubly-fed wind turbine and energy storage grid-connected system, in order to prevent the doubly-fed wind turbine from being disconnected from the grid due to DC bus overvoltage or rotor overcurrent during a grid fault, the present invention proposes a doubly-fed wind turbine fault ride-through method and system based on hybrid energy storage and crowbar protection. When a grid fault occurs, the method uses a hybrid energy storage device to stabilize the DC bus side voltage of the doubly-fed wind turbine, and uses demagnetization current control and crowbar protection to coordinate the hybrid energy storage device to avoid rotor overcurrent from damaging the rotor side converter, so as to improve the low voltage ride-through capability of the DFIG wind power generation system. Figure 1 The hybrid energy storage coordinated control system shown in the figure adopts the traditional doubly fed wind turbine architecture, including doubly fed wind turbine, converter group, RSC control module, GSC control module, battery, Buck-Boost control module, superconducting energy storage module, DC / DC control module, crowbar protection module and demagnetization current input unit:

[0051] The rotor side of the doubly-fed wind turbine is connected to the converter group, then to the stator winding, and then to the grid;

[0052] The converter group includes a rotor-side converter RSC and a grid-side converter GSC connected at the DC side, the DC side of the rotor-side converter RSC is connected to the grid-side converter GSC, the rotor-side converter RSC and the grid-side converter GSC share a capacitor and a DC chopper protection;

[0053] The crowbar protection module is connected to the rotor side of the doubly-fed wind turbine to provide crowbar protection;

[0054] The de-excitation current input unit is used to continuously provide the de-excitation current to the RSC;

[0055] The RSC control module is connected to the control end of the rotor-side converter RSC;

[0056] The GSC control module is connected to the control terminal of the grid-side converter GSC;

[0057] The battery is connected to the DC bus between the RSC and the GSC through a Buck-Boost converter; the Buck-Boost control module is connected to the Buck-Boost converter;

[0058] The superconducting energy storage module is connected to the DC bus between the RSC and the GSC through a DC / DC chopper; and the DC / DC control module is connected to the DC / DC chopper.

[0059] Topology and specific activation methods of crowbar protection in the system:

[0060] The circuit topology of the crowbar protection is as follows Figure 3 As shown, it is mainly composed of a diode bridge, a self-turn-off device IGBT and a DC crowbar resistor.

[0061] The crowbar circuit protection controls the opening and closing of the IGBT through a binary signal, thereby realizing the input and removal of the crowbar protection. When subjected to severe voltage sag, the rotor overcurrent of the doubly fed wind turbine may endanger the rotor converter within a few milliseconds. In order to prevent the rotor overcurrent from damaging the rotor converter, the control strategy of the crowbar protection will send a turn-on signal to the IGBT and activate the crowbar circuit protection. During the activation period, the RSC converter will be locked. However, when the crowbar circuit is activated, the RSC loses its control ability, and the DFIG must draw reactive power from the stator side for excitation; therefore, after the crowbar circuit is disconnected, the PCC fault voltage may be further deteriorated to produce a secondary voltage drop due to the high reactive power absorbed by the DFIG stator from the grid.

[0062] Therefore, the crowbar protection in this paper will adopt the minimum threshold method to assist HESS in achieving low voltage drop of DFIG, that is, the rotor current is i r ≥2pu or V dc When ≥1.15pu, the crowbar circuit is put into the DFIG system and the rotor current is i r ≤1.9pu and V dc When ≤1.05pu, the crowbar circuit is cut off from the DFIG system. This method can reduce the activation time of the crowbar circuit protection and prevent more reactive power from being absorbed from the grid during a grid fault.

[0063] Control strategy of hybrid energy storage:

[0064] (a) Control mode of superconducting energy storage

[0065] The DC / DC control module outputs a control signal to the DC / DC chopper to control the working mode of the superconducting energy storage module. In the wind storage system based on hybrid energy storage, the control target of the DC-DC chopper of SEMS is to stabilize the DC bus voltage of DFIG. Therefore, the DC-DC converter strategy of SMES is as follows: Figure 4 shown.

[0066] The actual value of the DFIG DC bus voltage V dc With reference value V dc * The difference between the two can be used to obtain the reference value of the SMES output current i s * SMES system has a faster response to the fluctuation of DC bus voltage when the power grid fails, which helps to stabilize the DC voltage of the doubly fed wind turbine and enhance the fault ride-through capability of the doubly fed wind turbine. When the power grid is operating normally, the SMES system handles the high-frequency power demand of the system by stabilizing the DC bus voltage.

[0067] When the voltage drops, the superconducting energy storage module performs positive compensation for the voltage at the outlet of the doubly-fed wind turbine; active power compensation support is used to provide the voltage deficit for the voltage drop, so that the voltage amplitude at the outlet of the doubly-fed wind turbine is restored to the voltage level during normal operation;

[0068] When the voltage rises, the voltage at the outlet of the doubly-fed wind turbine is reversely compensated with the help of batteries and superconducting energy storage devices, so that the voltage at the outlet of the doubly-fed wind turbine returns to normal.

[0069] (b) Battery control mode

[0070] The Buck-Boost control module is used to output a control signal to the Buck-Boost converter to control the working mode of the battery. The battery DC / DC buck / boost converter control mode is divided into two types: Mode 1 is used to assist the SMES system in stabilizing the DC bus voltage. According to the DC reference voltage command, the voltage stability of the DC bus is controlled by processing the power on the DC bus side, thereby enhancing the low voltage ride-through capability of the DFIG in the event of a grid fault. Mode 2 is used to process the low-frequency power in the normal state. According to the low-frequency power command, the battery is controlled to process the power on the DC bus side. Figure 5 The converter strategy is described in detail as follows:

[0071] Mode 1: During a deep fault in the power grid with a PCC voltage of 0 to 0.5 pu, the reference value of the battery output current i b * Set to the actual value of the DC bus voltage V dc With reference value V dc *This mode can help the SMES system stabilize the DFIG DC bus voltage and improve the DFIG's low voltage ride-through capability during grid faults.

[0072] Mode 2: When the power grid is operating normally or a shallow fault is 0.5 to 0.9 pu, the power P generated by the double-fed wind turbine tur With the power scheduling instruction P dp-ref The power difference between the two is transmitted to the DC bus side of the DFIG through the GSC. In this mode, the actual value P of the active power output of the battery is battery With reference value P b * The difference between the battery output current and the reference value i can be obtained. b * The power difference is filtered through a low-pass filter to obtain the active power reference value P of the battery. b * Through this control strategy, the battery can realize the function of processing low-frequency power.

[0073] Control mode of double-fed wind turbine GSC-RSC

[0074] (a) Control mode of GSC

[0075] The GSC control module is used to output control signals to the GSC to control the operation mode of the GSC. The GSC operation mode is divided into two modes: mode 1 is to provide reactive power to the grid when the grid fails, and mode 2 is to provide active power to the grid when the system is in a normal state and manage the system output power P out , reducing the power fluctuation of wind power caused by random wind speed. Figure 6 The specific control strategy is described as follows:

[0076] Mode 1: Applied during grid fault, the d-axis control of GSC stabilizes the DC voltage by following the DC voltage reference command. During the fault, GSC is used to assist DFIG to achieve low voltage ride-through during grid fault. First, the GSC output current vector is divided into two components i in the synchronously rotating dq axis reference frame. gd 、i gq ; Secondly, the d-axis component of the GSC output current is used to control the DFIG DC bus voltage, and its q-axis component provides reactive power to the grid according to the voltage drop at the DFIG grid connection point. This mode can not only stabilize the DFIG DC bus voltage, but also provide reactive power to the grid, thereby alleviating the drop in fault voltage.

[0077] Mode 2: Applied during normal operation of the power grid, the d-axis control of the GSC controls the power output at the AC side grid connection according to the power dispatch instruction. During normal operation of the power grid, the GSC is used to manage the output power of the system and reduce the power fluctuation of wind power caused by random wind speed. In the control strategy, the d-axis component i of the GSC output current gd Used to dispatch the power according to the instruction P dp-ref Control system output power P out ; Reactive power Q of the system out The q-axis component i of the output current through the grid gq To adjust, under normal circumstances, refer to the reactive power Q dp-ref Set to zero so that no reactive power is supplied to or absorbed from the grid under normal conditions.

[0078] (b) RSC control mode

[0079] The RSC control module is used to output control signals to the RSC for controlling the RSC. The RSC control strategy of the doubly fed wind turbine is as follows: Figure 7 As shown, the control strategy of RSC is divided into two modes: Mode 1 is to use the reverse current tracking control of the stator current to perform demagnetization control of the rotor winding; Mode 2 is maximum power tracking control.

[0080] Mode 1: During the fault period, the RSC converter control adopts the demagnetization current control. When the transmission line detects the voltage drop, the RSC control switches to mode 1, and uses the reverse current tracking control of the stator current to perform the demagnetization control of the rotor winding, suppressing the sudden increase of the rotor current during the fault period, preventing the repeated triggering of the crowbar protection, and improving the operation stability of the system; specifically, the rotor converter of the doubly fed wind turbine receives the demagnetization current i d-stator , demagnetization current i q-stator , the demagnetization current used here is K times the stator current;

[0081] Mode 2: In order to make the power P generated by DFIG under random wind speed tur Maximize and obtain active power reference P through MPPT control MPPT Specifically, by controlling the d-axis component i of the rotor current rd , so that the stator output power P s Follow active power reference P MPPT , the reactive output power of the stator Q s The q-axis component i of the rotor current rq To adjust, that is, maximum power tracking control.

[0082] In summary, when the power grid is in normal state, RSC, GSC and HESS are all controlled in mode 2, and the crowbar circuit fails; the battery is used to process the low-frequency power component of the power difference, and the power difference is the difference between the power issued by the doubly fed wind turbine and the power dispatch instruction; SEMS is used to process the high-frequency power component of the power difference; RSC and GSC are used to manage the output power of the wind storage system.

[0083] Please refer to Figure 2 , the implementation steps of the method of the present invention include:

[0084] Step S1, obtaining the PCC voltage U at the outlet of the doubly fed wind turbine PCC ;

[0085] Step S2, determine whether U PCC <0.9pu, if the judgment result is yes, the GSC and RSC of the doubly fed wind turbine are switched to mode 1 operation, the DC voltage of the doubly fed wind turbine is stabilized by coordinating superconducting energy storage, the demagnetization current control of the rotor converter is maintained to suppress the rotor fault current in real time, and step S3 is continued; if the judgment result is no, the GSC is switched to mode 2 operation, the RSC is switched to mode 2 operation, the battery is switched to mode 2 operation, the crowbar protection is locked, and the current working state is maintained;

[0086] Step S3, determine whether U PCC <0.5pu, if the judgment result is yes, the battery is switched to mode 1 operation to stabilize the DC voltage of the doubly-fed wind turbine in cooperation with the superconducting energy storage, and step S4 is continued; if the judgment result is no, the battery is switched to mode 2 operation;

[0087] Step S4, determine whether the current r >2pu or V dc >1.15pu, where I r is the rotor overcurrent, V dc If the DC bus overvoltage is detected, the crowbar protection is activated and step S5 is continued; if the judgment result is negative, the crowbar protection is locked and the process returns to step S2;

[0088] Step S5, continue to determine whether the current conditions I and r <1.9pu and V dc <1.05pu, if the judgment result is yes, the crowbar protection is locked and the process returns to step S2; if the judgment result is no, the process returns to step S4 to continuously activate the crowbar protection.

[0089] Step S6, when the fault disappears or the voltage recovers, the battery and the superconducting hybrid energy storage module remain connected to the grid to smooth the power fluctuation caused by the wind speed fluctuation.

[0090] In specific implementation:

[0091] When a fault occurs, the control mode of the grid-side converter (GSC) is designed to switch to mode 1 to provide reactive power to the grid and stabilize the DC bus voltage of the doubly fed wind turbine. When the voltage at the AC side PCC is less than 0.5pu, the control mode of the battery is designed to switch to mode 1 to coordinate with the SMES to stabilize the DC bus voltage. To prevent the rotor overcurrent from damaging the RSC, the minimum threshold method is used to activate the crowbar circuit in real time according to the rotor current in the rotor circuit. The control framework is as follows: Figure 1 As shown. Under normal conditions, RSC and GSC are switched to Mode 2 to manage the output power of the wind storage system; the battery is also switched to Mode 2 to process the low-frequency power component of the power difference, which is the difference between the power sent by the wind turbine and the power dispatch instruction; SEMS is designed to process the high-frequency power component of the power difference.

[0092] The proposed DFIG fault ride-through flow chart based on HESS coordinated crowbar protection is shown in Figure 2 As shown. When the grid is in normal state, RSC, GSC and HESS are controlled in mode 2 to disable the crowbar circuit; when the voltage at the grid connection point PCC is detected to be less than 0.9pu, RSC and GSC switch to mode 1, which can provide more reactive power for the grid; when the AC side voltage drops sharply below 0.5pu, the battery control mode is switched to mode 1, and if the AC side voltage drops sharply above 0.5pu, the battery control mode is switched to mode 2. If the DFIG rotor current is greater than 2pu or the DC bus voltage is greater than 1.15pu, the minimum threshold method mentioned in Section 4.3.1 is used to activate the crowbar circuit; if the rotor current is still detected to be greater than 2pu, the crowbar circuit is still in the activated state; when the rotor current is detected to be less than 1.9pu and the DC bus voltage is less than 1.05pu, the crowbar circuit is cut off from the DFIG system. Until the grid fault is cleared and no more grid fault occurs, the control of the GSC and the battery is switched to Mode 2 operation, where the HESS system is used to manage the output power of the DFIG and the crowbar circuit is not activated.

[0093] The present invention has the following advantages:

[0094] It can effectively suppress DC bus overvoltage and rotor overcurrent: a battery + superconducting hybrid energy storage device is connected in parallel on the DC bus side of the doubly fed wind turbine. The battery + superconducting hybrid energy storage device can suppress the voltage fluctuation on the DC bus side of the doubly fed wind turbine during a fault, preventing the doubly fed wind turbine from being disconnected from the grid due to DC bus overvoltage; and crowbar protection + demagnetization current control are used to assist the hybrid energy storage device in suppressing rotor overcurrent, preventing the rotor overcurrent from damaging the rotor-side converter, thereby effectively preventing the doubly fed wind turbine from failing during a fault.

[0095] In the battery + superconducting hybrid energy storage module, superconducting energy storage is a power-type energy storage device, and the battery is an energy-type energy storage device. During deep faults, superconducting energy storage can be used to quickly respond to and compensate for voltage fluctuations at the DC bus of the doubly fed wind turbine, while the battery can be used to assist the superconducting energy storage in stabilizing the DC voltage, improving the fault ride-through capability of the doubly fed wind turbine, and increasing the overall capacity of the hybrid energy storage, thereby ensuring the long-term operation of the hybrid energy storage; and during normal operation, the hybrid energy storage can also be used to assist the doubly fed wind turbine in power smoothing and improve the power quality at the outlet of the doubly fed wind turbine.

[0096] The battery + superconducting hybrid energy storage module suppresses the voltage fluctuation at the DC bus of the doubly fed wind turbine through DC / DC converter control, while the demagnetization current control of the rotor converter suppresses the rotor fault current when it becomes larger to prevent overcurrent. If the suppression fails and the rotor overcurrent still occurs, the crowbar protection is triggered in real time according to the rotor overcurrent during the fault period, thereby avoiding the rotor overcurrent and the rotor converter from forming a loop, thereby preventing the doubly fed wind turbine from having DC overvoltage and rotor overcurrent problems during the fault period.

[0097] When the fault disappears or the voltage is restored, the battery + superconducting hybrid energy storage module is still connected to the grid and can smooth out the power fluctuations caused by wind speed fluctuations.

[0098] In summary, the present invention utilizes a doubly-fed wind turbine fault ride-through method based on battery + superconducting hybrid energy storage to suppress DC bus voltage. By introducing the AC side voltage to judge the control strategy of the battery, the voltage fluctuation of the DC bus of the doubly-fed wind turbine can be effectively reduced. The overvoltage on the DC bus side of the doubly-fed wind turbine can be suppressed during power grid faults, thereby improving the fault ride-through capability of the wind turbine. In the early stage of a fault, the superconducting energy storage can quickly respond to and suppress the voltage fluctuation on the DC bus side of the doubly-fed wind turbine. At the same time, the battery is used to process low-frequency power to increase the overall capacity of the hybrid energy storage device. In addition, in shallow faults, Under fault conditions, the demagnetization current suppresses the rotor overcurrent in real time during the fault period. Under deep fault conditions, the crowbar protection is used to coordinate with the hybrid energy storage to increase the fault ride-through capability of the doubly fed wind turbine, and prevent the doubly fed wind turbine from being disconnected from the grid due to rotor overcurrent. The voltage at the PCC point on the AC side is used to determine whether the battery is switched to DC voltage control, so as to assist superconducting energy storage in enhancing the fault ride-through capability of the doubly fed wind turbine under deep fault conditions. In addition to increasing the fault ride-through capability of the doubly fed wind turbine during a fault, the present invention can also improve the power quality at the outlet of the doubly fed wind turbine when the wind speed fluctuates, and enhance the stability of the wind turbine grid-connected system.

[0099] What is disclosed above is only a preferred embodiment of the present invention, which certainly cannot be used to limit the scope of rights of the present invention. A person skilled in the art can understand that all or part of the processes of the above embodiments and equivalent changes made according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A doubly-fed wind turbine fault ride-through method based on hybrid energy storage and crowbar protection, characterized in that: It is realized based on a hybrid energy storage coordinated control system, which includes a doubly-fed wind turbine, a converter group, an RSC control module, a GSC control module, a battery, a Buck-Boost control module, a superconducting energy storage module, a DC / DC control module, a crowbar protection module, and a demagnetization current input unit: The rotor side of the doubly-fed wind turbine is connected to the converter group, then connected to the stator side winding, and then connected to the power grid; The converter group includes a rotor-side converter RSC and a grid-side converter GSC connected at the DC side, the DC side of the rotor-side converter RSC is connected to the grid-side converter GSC, the rotor-side converter RSC and the grid-side converter GSC share a capacitor and a DC chopper protection; The crowbar protection module is connected to the rotor side of the double-fed wind turbine to provide crowbar protection; the demagnetization current input unit is used to continuously provide demagnetization current to the RSC; The RSC control module is connected to the control end of the rotor-side converter RSC; The GSC control module is connected to the control end of the grid-side converter GSC; The battery is connected to the DC bus between the RSC and the GSC through a Buck-Boost converter; the Buck-Boost control module is connected to the Buck-Boost converter; The superconducting energy storage module is connected to the DC bus between the RSC and the GSC through a DC / DC chopper; the DC / DC control module is connected to the DC / DC chopper; The Buck-Boost control module is used to output a control signal to the Buck-Boost converter to control the working mode of the battery. There are two working modes of the battery: Mode 1 is that during the fault period when the PCC voltage of the power grid is 0-0.5pu, the reference value i of the battery output current is b * Set to the actual value of the DC bus voltage V dc With reference value V dc * The difference is used to stabilize the DC bus voltage; Mode 2 is that when the power grid is operating normally or a shallow fault is 0.5 to 0.9 pu, the power P generated by the double-fed wind turbine is tur With the power scheduling instruction P dp-ref The power difference between them is transmitted to the DC bus side of DFIG through GSC, and the reference value of battery output current i b * The value of is the actual value of the battery active output power P battery With reference value P b * The difference between the two, the battery is used to process the low frequency band power; The RSC control module is used to output a control signal to the RSC for controlling the RSC, wherein the RSC has two control modes: Mode 1 is to use the reverse current tracking control of the stator current to control the demagnetization of the rotor winding; Mode 2 is to obtain the active power reference P through MPPT control MPPT , specifically by controlling the d-axis component i of the rotor current rd , so that the stator output power P s Follow active power reference P MPPT , the reactive output power of the stator Q s The q-axis component i of the rotor current rq to regulate; The GSC control module is used to output a control signal to the GSC to control the operation mode of the GSC. There are two operation modes of the GSC: mode 1 is to provide reactive power to the grid when the grid fails; mode 2 is to provide active power to the grid when the system is in a normal state and manage the system output power P out ; The steps of the method include: Step S1, obtaining the PCC voltage U at the outlet of the doubly fed wind turbine PCC ; Step S2, determine whether U PCC <0.9pu, if the judgment result is yes, the GSC and RSC of the doubly fed wind turbine are switched to mode 1 operation, the DC voltage of the doubly fed wind turbine is stabilized by coordinating superconducting energy storage, the demagnetization current control of the rotor converter is maintained to suppress the rotor fault current in real time, and step S3 is continued; if the judgment result is no, the GSC is switched to mode 2 operation, the RSC is switched to mode 2 operation, the battery is switched to mode 2 operation, the crowbar protection is locked, and the current working state is maintained; Step S3, determine whether U PCC <0.5pu, if the judgment result is yes, the battery is switched to mode 1 operation to stabilize the DC voltage of the doubly-fed wind turbine in cooperation with the superconducting energy storage, and step S4 is continued; if the judgment result is no, the battery is switched to mode 2 operation; Step S4, determine whether the current r >2pu or V dc >1.15pu, where I r is the rotor overcurrent, V dc If the DC bus overvoltage is detected, the crowbar protection is activated and step S5 is continued; if the judgment result is negative, the crowbar protection is locked and the process returns to step S2; Step S5, continue to determine whether the current conditions I and I are satisfied at the same time. r <1.9pu and V dc <1.05pu, if the judgment result is yes, the crowbar protection is locked and the process returns to step S2; if the judgment result is no, the process returns to step S4 to continuously activate the crowbar protection.

2. The method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection according to claim 1, characterized in that: The specific operation mode of GSC is: assisting DFIG to achieve low voltage ride through when the grid fails. The GSC output current vector is divided into two components i in the synchronous rotating dq axis reference frame. gd 、i gq ; Among them, the d-axis component of the GSC output current is used to control the DFIG DC bus voltage, and the q-axis component provides reactive power to the grid according to the voltage drop at the DFIG grid connection point; The second operation mode of GSC is as follows: GSC is used to manage the output power of the system and reduce the power fluctuation of wind power caused by random wind speed; in the control strategy, the d-axis component i of the GSC output current gd Used to dispatch the power according to the instruction P dp-ref Control system output power P out ; Reactive power Q of the system out The q-axis component i of the output current through the grid gq To adjust, refer to the reactive power Q dp-ref Set to zero.

3. The method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection as claimed in claim 2, characterized in that: The magnitude of the demagnetization current is K times the stator current.

4. The method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection as claimed in claim 3, characterized in that: The switch state of the crowbar protection module is controlled by the IGBT, and the control signal is the rotor side current.

5. The method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection as claimed in claim 4, characterized in that: The DC / DC control module outputs a control signal to the DC / DC chopper to control the working mode of the superconducting energy storage module. In the normal operation of the power grid, the superconducting energy storage module processes the high-frequency power demand of the system by stabilizing the DC bus voltage. In the event of a power grid failure, the actual value V of the DFIG DC bus voltage is used to control the working mode of the superconducting energy storage module. dc With reference value V dc * The difference between the two values ​​is used to obtain the reference value of the SMES output current i s * , assist the doubly-fed wind turbine to stabilize the DC voltage and enhance the fault ride-through capability of the doubly-fed wind turbine; When the voltage drops, the superconducting energy storage module performs positive compensation for the voltage at the outlet of the doubly-fed wind turbine; active power compensation support is used to provide the voltage deficit of the voltage drop, so that the voltage amplitude at the outlet of the doubly-fed wind turbine is restored to the voltage value during normal operation; When the voltage rises, the voltage at the outlet of the doubly-fed wind turbine is reversely compensated by means of the battery and the superconducting energy storage device, so that the voltage at the outlet of the doubly-fed wind turbine is restored to normal.

6. The method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection according to claim 5, characterized in that: When the power grid is in a normal state, RSC, GSC and HESS are all controlled in mode 2, and the crowbar circuit fails; the battery is used to process the low-frequency power component of the power difference, and the power difference is the difference between the power emitted by the doubly fed wind turbine and the power dispatch instruction; SEMS is used to process the high-frequency power component of the power difference; RSC and GSC are used to manage the output power of the wind storage system.

7. The method for fault ride-through of a doubly-fed wind turbine based on hybrid energy storage and crowbar protection as claimed in claim 6, characterized in that: After step S5, the method further includes: Step S6, when the fault disappears or the voltage recovers, the battery and the superconducting hybrid energy storage module remain connected to the grid to smooth the power fluctuation caused by the wind speed fluctuation.

8. A hybrid energy storage coordinated control system, characterized in that: include: Doubly-fed wind turbine, converter group, RSC control module, GSC control module, battery, Buck-Boost control module, superconducting energy storage module, DC / DC control module, crowbar protection module and demagnetization current input unit, including: The rotor side of the doubly-fed wind turbine is connected to the converter group, then connected to the stator side winding, and then connected to the power grid; The converter group includes a rotor-side converter RSC and a grid-side converter GSC connected at the DC side, the DC side of the rotor-side converter RSC is connected to the grid-side converter GSC, the rotor-side converter RSC and the grid-side converter GSC share a capacitor and a DC chopper protection; The crowbar protection module is connected to the rotor side of the double-fed wind turbine to provide crowbar protection; the demagnetization current input unit is used to continuously provide demagnetization current to the RSC; The RSC control module is connected to the control end of the rotor-side converter RSC; The GSC control module is connected to the control end of the grid-side converter GSC; The battery is connected to the DC bus between the RSC and the GSC through a Buck-Boost converter; the Buck-Boost control module is connected to the Buck-Boost converter; The superconducting energy storage module is connected to the DC bus between the RSC and the GSC through a DC / DC chopper; the DC / DC control module is connected to the DC / DC chopper; The hybrid energy storage collaborative control system is used to implement the method described in any one of claims 1-7.

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