Low-Voltage Ride-Through Control Method for Wind Power Grid Connection with Hybrid Energy Storage and Crowbar Resistance

Abstract

The invention discloses a low-voltage ride-through control method for wind power grid connection with hybrid energy storage and crowbar resistance. First, a hybrid energy storage model is built using SIMULINK, and a lithium battery model and a supercapacitor model are combined to form the hybrid energy storage model. Then, the topological circuit structure of wind power grid connection is designed, the parameters of each device in the model are designed, and a wind power grid connection simulation model is built, where the wind turbine uses a doubly-fed induction generator, and the inverters on the rotor side and grid side of the doubly-fed induction generator are controlled by a PWM control strategy. Finally, the hybrid energy storage model is connected to the DC side of the rotor side converter in the wind power grid connection simulation model, and at the same time, a Crowbar circuit is incorporated to build a low-voltage ride-through model, so as to achieve low-voltage ride-through when the grid connection point voltage drops. The invention solves the problem of power quality caused by low-voltage ride-through in the doubly-fed wind power grid connection system in the prior art.

Description

Technical Field

[0001] The invention belongs to the technical field of low voltage ride through control of wind power generation, and particularly relates to a low voltage ride through control method for wind power grid connection with hybrid energy storage and crowbar resistance. Background Technique

[0002] With the integration of wind power generation systems into traditional power grids, the evolution of traditional power grid systems has become increasingly complex. Since the stator of a doubly-fed induction generator is directly connected to the grid, it is quite sensitive to voltage dips. Problems such as harmonic distortion, imbalance, and voltage dips in doubly-fed wind power grid-connected systems have become increasingly prominent. Using various enhanced control technologies to improve the operating performance of doubly-fed wind turbines or improve the power quality at the grid connection point, when the grid voltage drops, various software and hardware protection technologies are required to ensure that the doubly-fed wind turbines do not trip off the grid and provide reactive power support to the grid, which is a hot topic in current wind power grid connection research.

[0003] Existing low voltage ride through control methods for doubly-fed induction wind turbines mainly include: (1) Using a three-phase discontinuous DVR with all-vanadium redox flow battery energy storage to improve the flexible fault ride through ability of the doubly-fed wind power system. However, when the grid voltage is normal, the energy storage unit is not effective, and the control complexity is increased. (2) Using a hybrid energy storage system with a series grid-side converter to limit the rotor overcurrent and the DC bus overvoltage, which is composed of a parallel combination of lithium batteries and supercapacitors. (3) By adding a Crowbar circuit in front of the rotor converter to short-circuit the rotor converter, consuming the load to achieve the purpose of raising the terminal voltage of the machine and realizing low voltage ride through. To sum up, the existing control methods for low voltage ride through of doubly-fed wind turbines have serious energy waste, are restricted by resistance values, the volume and voltage level of energy storage devices, and cannot solve practical engineering problems, etc. Summary of the Invention

[0004] The purpose of the invention is to provide a low voltage ride through control method for wind power grid connection with hybrid energy storage and crowbar resistance, which solves the problem of power quality caused by low voltage ride through in a doubly-fed wind power grid-connected system in the prior art.

[0005] The technical solution adopted by the invention is a low voltage ride through control method for wind power grid connection with hybrid energy storage and crowbar resistance, which is specifically implemented according to the following steps:

[0006] Step 1: Build a hybrid energy storage model using SIMULINK, combine the lithium battery model and the supercapacitor model. When the voltage drops to more than 0.5 pu, control the input of the lithium battery to absorb the excess electric energy. When the voltage drops to less than 0.5 pu, control the input of the supercapacitor and the Crowbar circuit to absorb the excess electric energy, realizing the rapidity and capacity demand of the hybrid energy storage model to provide or absorb a large amount of energy in a timely manner. A hybrid energy storage model is constituted;

[0007] Step 2: Design the topological circuit structure for wind power generation and grid connection, design the parameters of each device in the model, and build a wind power grid connection simulation model. The wind turbine used is a doubly-fed induction generator, and the inverters on the rotor side and grid side of the doubly-fed induction generator are controlled by the PWM control strategy;

[0008] Step 3: Combine the hybrid energy storage model built in Step 1 with the lithium battery and supercapacitor and connect it to the DC side of the rotor side converter in the wind power grid connection simulation model in Step 2. At the same time, incorporate the Crowbar circuit to build a low voltage ride-through model that combines hybrid energy storage and Crowbar to achieve low voltage ride-through when the grid connection point voltage drops.

[0009] The features of the present invention also lie in that,

[0010] Step 1 is specifically as follows:

[0011] The hybrid energy storage model includes a lithium battery part and a supercapacitor part. The lithium battery is connected to a Boost circuit to make the output voltage of the lithium battery higher than the input voltage;

[0012] The supercapacitor is connected to a Buck-Boost circuit to make the output voltage of the supercapacitor can be lower or higher than the input voltage;

[0013] The two are combined together for energy output and input. When the voltage drops to more than 0.5 pu, control is exerted to put the lithium battery into operation to absorb the excess electric energy. When the voltage drops to less than 0.5 pu, control is exerted to put the supercapacitor and the Crowbar circuit into operation to absorb the excess electric energy.

[0014] Step 2 is specifically as follows:

[0015] The wind turbine is a doubly-fed induction generator model, and the converter is composed of a rotor side converter and a grid side converter;

[0016] The control strategy of the rotor side converter is as follows:

[0017] Equation (9) is the rotational speed relationship formula between the stator and the rotor, and Equation (10) is the frequency relationship formula between the stator and the rotor. When the doubly-fed induction motor is in a stable operating state, the rotating magnetic fields between the stator and the rotor exhibit a relatively stationary characteristic, which is expressed by the following formula:

[0018] n1 = n2 + n r (9)

[0019] Because There is:

[0020]

[0021] n1 is the stator speed, n2 is the speed relative to the stator speed, and n r is the rotor magnetic potential speed, f1 is the stator frequency, f2 is the rotor frequency, and p is the number of pole pairs;

[0022] Grid-side converter control strategy:

[0023] For the three-phase winding voltage equation, for the three-phase stator windings, their voltages can be calculated using the following equation:

[0024]

[0025] The corresponding three-phase rotor winding voltage equation is:

[0026]

[0027] In the above equations, the parameter u sA u sB u sC u ra u rb u rc means the instantaneous phase voltages of the A, B, and C phases of the stator and rotor; the parameter i sA i sB i sC i ra i rb i rc means the instantaneous phase currents of the A, B, and C phases of the stator and rotor; the parameter ψ sA ψ sB ψ sC is the magnetic flux linkage of the A, B, and C phase windings of the stator and rotor; the parameters R s R r respectively represent the stator winding resistance and the rotor winding resistance. Similarly, all the above parameters will be referred to the stator side;

[0028] Equation (13) is the DFIG stator voltage equation in the synchronous rotating coordinate system, and Equation (14) is the DFIG rotor voltage equation in the synchronous rotating coordinate system:

[0029] DFIG stator voltage equation in the synchronous rotating coordinate system:

[0030]

[0031] DFIG rotor voltage equation in the synchronous rotating coordinate system:

[0032]

[0033] In the above formula, u d1 is the voltage of the d-axis of the stator in the dq coordinate system; u q1is the voltage of the stator on the q-axis; u d2 is the voltage of the rotor on the d-axis in the dq coordinate system; u q2 is the voltage of the rotor on the q-axis; i d1 is the current of the stator on the d-axis in the dq coordinate system; i q1 is the current of the stator on the q-axis; i d2 is the current of the rotor on the d-axis in the dq coordinate system; i d2 is the current of the rotor on the q-axis.

[0034] Step 3 is specifically as follows:

[0035] The hybrid energy storage model combining lithium batteries and supercapacitors is juxtaposed on the DC side of the rotor converter of the wind power model, and a Crowbar circuit is added. It is controlled by a DC / DC converter, and at the same time, a PI double-loop control strategy is used. According to the level of voltage dip, the lithium battery, supercapacitor or Crowbar circuit is selectively put into use to absorb or release energy, achieving lower energy consumption for low voltage ride-through. Among them, when there is a large voltage dip, the Crowbar protection circuit is used to improve the low voltage ride-through ability of the fan.

[0036] In Step 3:

[0037] After the Crowbar is put into use, the voltage on the rotor side should satisfy:

[0038]

[0039] To prevent overvoltage in the rotor circuit and overvoltage in the DC bus during low voltage ride-through, the rotor voltage of the doubly-fed fan is smaller than the minimum voltage of the maximum voltage that the rotor converter can withstand and the voltage limit of the DC side bus:

[0040] U r <min{U rmax ,U dc,lim}

[0041] From the above two equations, it can be obtained that the resistance value of the Crowbar must satisfy:

[0042]

[0043] At the same time, in order to limit the overcurrent in the short-circuit loop, the maximum value that the rotor side can reach during a fault should be less than the safety current threshold set by the rotor converter, so:

[0044]

[0045] A value range of the resistance of the Crowbar circuit is obtained. The larger the Crowbar resistance, the better the effect of suppressing the overcurrent on the rotor side.

[0046] The beneficial effects of the present invention are as follows. For the low-voltage ride-through control method of a wind power grid connection with hybrid energy storage and a crowbar resistor, in view of the advantages of high battery energy density and rapid charge and discharge of supercapacitors, a hybrid energy storage model is adopted to give full play to the advantages of the two energy storage elements and maintain the stable operation of the motor. The Crowbar circuit is used to protect the converter on the rotor side and can also be used as an energy-dissipating load for the energy storage element, solving the power quality problems caused by low-voltage ride-through in the existing doubly-fed wind power generation system. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a comparison diagram of the power demand (larger amplitude) and power output (smaller amplitude) of the hybrid energy storage in a low-voltage ride-through model combining hybrid energy storage and a Crowbar in a doubly-fed wind power system of the present invention;

[0048] Figure 2 It is a low-voltage ride-through simulation data diagram of a low-voltage ride-through model combining hybrid energy storage and a Crowbar in a doubly-fed wind power system of the present invention;

[0049] Figure 3 It is a waveform diagram of the grid-connected point voltage and current without a low-voltage ride-through control strategy when the grid voltage drops to 0.3 pu of the terminal voltage of the machine;

[0050] Figure 4 It is a waveform diagram of the grid-connected point voltage and current after the hybrid energy storage is put into use when the grid voltage drops to 0.6 pu of the terminal voltage of the machine;

[0051] Figure 5 It is a waveform diagram of the grid-connected point voltage and current after the hybrid energy storage is put into use when the grid voltage drops to 0.3 pu of the terminal voltage of the machine;

[0052] Figure 6 It is a physical model diagram of the DFIG under the dq axis;

[0053] Figure 7 It is a schematic diagram of the converter in the doubly-fed asynchronous wind generator model;

[0054] Figure 8 It is a PWM control block diagram of the rotor-side converter of the converter in the doubly-fed asynchronous wind generator model; DETAILED DESCRIPTION OF THE INVENTION

[0055] The present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0056] The low-voltage ride-through control method of the wind power grid connection with hybrid energy storage and a crowbar resistor of the present invention is specifically implemented according to the following steps:

[0057] Step 1: Use SIMULINK to build a hybrid energy storage model by combining a lithium battery model and a supercapacitor model. When the voltage drops to more than 0.5 pu, control the lithium battery to be put into operation to absorb the excess electrical energy. When the voltage drops to less than 0.5 pu, control the supercapacitor and the Crowbar circuit to be put into operation to absorb the excess electrical energy, achieving the rapidity and capacity requirements for the hybrid energy storage model to promptly provide or absorb a large amount of energy, thus forming the hybrid energy storage model;

[0058] Combine Figure 1 , and Step 1 is specifically as follows:

[0059] The hybrid energy storage model includes a lithium battery part and a supercapacitor part. Among them, the lithium battery is connected to a Boost circuit to make the output voltage of the lithium battery higher than the input voltage; the supercapacitor is connected to a Buck - Boost circuit to make the output voltage of the supercapacitor can be lower or higher than the input voltage; the two are combined together for energy output and input. When the voltage drops to more than 0.5 pu, control the lithium battery to be put into operation to absorb the excess electrical energy. When the voltage drops to less than 0.5 pu, control the supercapacitor and the Crowbar circuit to be put into operation to absorb the excess electrical energy, achieving the rapidity and capacity requirements for the hybrid energy storage model to promptly provide or absorb a large amount of energy.

[0060]

[0061] In the formula, U b is the battery terminal voltage; I b is the battery current; R b is the battery internal resistance; P b is the battery output power.

[0062] From Equation (1), the current when the lithium battery output power is P b can be obtained as

[0063]

[0064] Since in the hybrid energy storage system, the lithium battery provides or absorbs the energy of the load motor via a DC - DC converter, so

[0065]

[0066]

[0067] In the formula, P b is the power released or absorbed by the battery on the DC bus side through the DC - DC converter; η is the working efficiency of the DC - DC converter, and the value is taken as 97% through actual experiments. The supercapacitor model satisfies

[0068]

[0069] In the formula, P sc is the power released or absorbed by the super capacitor on the DC bus side through the DC-DC converter; SOC sc is the SOC of the super capacitor; U scm is the rated voltage of the super capacitor.

[0070] Therefore, the current of the super capacitor is

[0071]

[0072] When the grid voltage of the doubly-fed induction wind power grid-connected system drops, there are problems of overcurrent in the rotor and overvoltage of the DC capacitor, so an energy storage module is needed to absorb energy; when the wind speed drops and the output of the wind turbine is insufficient, the energy storage module is required to release energy to maintain the normal operation of the grid. This model mainly takes into account that the energy density of the super capacitor is small and it cannot discharge for a long time, and the long-term discharge efficiency is even lower than that of the battery; however, the super capacitor has a long life and a large power density, and can almost be charged and discharged instantaneously. Therefore, the lithium battery and the super capacitor are complementary, with the lithium battery providing long-term energy and the super capacitor providing fast response and high power to protect power equipment, infrastructure, and the entire power grid.

[0073] Step 2: Design the topological circuit structure of the wind power grid connection, design the parameters of each device in the model, and build a wind power grid connection simulation model. The wind turbine used is a doubly-fed induction wind generator (DFIG), and the inverters on the rotor side and grid side of the doubly-fed induction wind generator are controlled by the PWM control strategy; both its stator and rotor can exchange power with the grid, and the unit realizes constant-frequency power generation at different speeds; the main control purpose of the rotor side is to control the speed of the wind turbine to capture the maximum wind energy at the current wind speed and obtain the maximum output power, while realizing the P and Q decoupling control of the stator winding of the doubly-fed motor. The control of the grid side is mainly to achieve two goals: maintaining the stability of the DC bus voltage and controlling the power factor of the grid side of the doubly-fed wind turbine.

[0074] Step 2 is specifically as follows:

[0075] The wind turbine is a doubly-fed induction wind generator model. The model requires that the wind turbine unit withstand a long-term voltage unbalance of 2% and a short-term voltage unbalance of 4%, and according to the different grid connection point voltage levels, withstand a harmonic distortion rate of 2% - 5%; at the same time, it is required that when the terminal voltage of the wind turbine drops to 20%, it can maintain grid-connected operation for at least 625 ms and provide reactive power support to the grid.

[0076] When there are harmonics or unbalances in the grid connection point voltage, a control strategy is adopted to improve the problems of stator and rotor current distortion or unbalance, torque fluctuation, output power fluctuation, and DC bus voltage fluctuation that the doubly-fed wind turbine unit will face.

[0077] The converter consists of a rotor-side converter and a grid-side converter, which are independently controlled. The main principle of the power electronic converter is that the rotor-side converter controls the active power and reactive power by controlling the rotor current components, while the grid-side converter controls the DC bus voltage and ensures that the converter operates at a unity power factor (i.e., zero reactive power).

[0078] Control strategy of the rotor-side converter: Equation (9) is the speed relationship formula between the stator and the rotor, and Equation (10) is the frequency relationship formula between the stator and the rotor. When the doubly-fed induction motor is in a stable operating state, the relative static characteristic presents between the rotating magnetic fields of the stator and the rotor, which is expressed by the following formula:

[0079] n1 = n2 + n r (9)

[0080] Because There is:

[0081]

[0082] n1 is the stator speed, n2 is the speed relative to the stator speed, n r is the rotor magnetic potential speed, f1 is the stator frequency, f2 is the rotor frequency, and p is the number of pole pairs;

[0083] Control strategy of the grid-side converter:

[0084] For the three-phase winding voltage equation of the three-phase stator windings, its voltage can be calculated by the following equation:

[0085]

[0086] The corresponding three-phase rotor winding voltage equation is:

[0087]

[0088] In the above equations, the parameter u sA u sB u sC u ra u rb u rc means the instantaneous phase voltages of the A, B, and C phases of the stator and the rotor; the parameter i sA i sB i sC i ra i rb i rc means the instantaneous phase currents of the A, B, and C phases of the stator and the rotor; the parameter ψ sA ψ sB ψ sC is the magnetic flux linkage of the A, B, and C phase windings of the stator and the rotor; the parameter Rs R r respectively represent the stator winding resistance and the rotor winding resistance. Similarly, the above parameters will be converted to the stator side;

[0089] Such as Figure 6 The figure shows the physical model diagram of the DFIG under the dq axis. Equation (13) is the stator voltage equation of the DFIG in the synchronous rotating coordinate system, and Equation (14) is the rotor voltage equation of the DFIG in the synchronous rotating coordinate system:

[0090] Stator voltage equation of the DFIG in the synchronous rotating coordinate system:

[0091]

[0092] Rotor voltage equation of the DFIG in the synchronous rotating coordinate system:

[0093]

[0094] In the above equations, u d1 is the voltage of the stator on the d-axis in the dq coordinate system; u q1 is the voltage of the stator on the q-axis; u d2 is the voltage of the rotor on the d-axis in the dq coordinate system; u q2 is the voltage of the rotor on the q-axis; i d1 is the current of the stator on the d-axis in the dq coordinate system; i q1 is the current of the stator on the q-axis; i d2 is the current of the rotor on the d-axis in the dq coordinate system; i d2 is the current of the rotor on the q-axis;

[0095] Control strategy of the converter part:

[0096] Such as Figure 7 The figure shows the converter. The converter includes two independent control parts and is interconnected through a DC-side capacitor. They are independently controlled. The main principle of the power electronic converter is that the rotor-side converter realizes independent decoupling control of active and reactive power by controlling the rotor current components, which is convenient for the stable operation of the fan. Usually, the vector control method is used to control the back-to-back converter. The commonly used vector control methods are as follows: rotor flux-oriented vector control; slip frequency vector control; air-gap magnetic field-oriented vector control; voltage-oriented vector control.

[0097] The inverters on the rotor side and grid side of the doubly-fed induction generator (DFIG) are controlled by the PWM control strategy. The control objective of the rotor side is mainly to control the speed of the wind turbine to capture the maximum wind energy at the current wind speed, obtain the maximum output power, and at the same time achieve the P and Q decoupling control of the stator winding of the doubly-fed motor. The control of the grid side is mainly to achieve two objectives: maintaining the stability of the DC bus voltage and controlling the power factor of the grid side of the doubly-fed wind turbine.

[0098] In the two-phase rotating coordinate system, the instantaneous active and reactive powers of the doubly-fed wind turbine are i rd , i rq and can be expressed as:

[0099]

[0100] It can be seen from the above formula that they are linearly related to the q-axis component i rq and the d-axis component i rd of the rotor current in the dq coordinate system. Since there is no coupling relationship between the active component i rq and the reactive component i rd of the rotor current, by independently adjusting i rq and i rd , the active power P s and reactive power Q s output by the doubly-fed wind turbine can be decoupled and controlled to achieve the goal of PWM control of the rotor-side converter. Then we can get Figure 8 Control block diagram:

[0101] Because it is different from the control objective of the rotor-side converter, the control strategy of the grid-side converter is also different from that of the rotor-side converter.

[0102]

[0103] Among them, u ga , u gb , u gc are three-phase symmetrical grid voltages, U1a, U1b, U1c are the voltages of the grid-side converter in the three-phase coordinate system respectively, and I1A1B1C are the currents of the grid-side PWM converter in the three-phase coordinate system respectively. After the 3s / 2r coordinate transformation, we can get:

[0104]

[0105] Among them, ugd and ugq are the components of the grid voltage in the two-phase synchronous rotating coordinate system, and i1d, i1q, u1d, and u1q are the current and voltage of the grid-side PWM converter in the two-phase synchronous rotating coordinate system respectively. The power exchanged between the grid-side PWM converter and the grid, and the rotor power can be expressed as:

[0106]

[0107] It can be seen from the above formula that the expressions of the rotor active and reactive powers are relatively complex. Therefore, we can use the vector control strategy based on grid voltage orientation to simplify the control of the grid-side PWM converter. If the direction of the d-axis in the dq coordinate system is specified as the direction of the grid voltage Ug, and the angle between the d-axis and the α-axis is 90°, then there is

[0108]

[0109] Substituting the above formula into the rotor power formula, we can get:

[0110]

[0111] It can be seen from the above formula that if the grid voltage remains constant, then the active and reactive power exchanges between the grid-side PWM converter and the grid are only affected by i 1d , i 1q 's control. Then, only by changing i 1d , i 1q can we control the active and reactive power exchanges of the grid-side PWM converter respectively.

[0112] Step 3: Combine the hybrid energy storage model of the lithium battery and the super capacitor built in Step 1 and place it on the DC side of the rotor-side converter in the wind power grid-connected simulation model. At the same time, incorporate the Crowbar circuit to build a low-voltage ride-through model of the hybrid energy storage and Crowbar combination to achieve low-voltage ride-through when the grid-connected point voltage drops.

[0113] Combined with Figure 2 , Step 3 is specifically as follows:

[0114] Place the hybrid energy storage model of the lithium battery and the super capacitor in parallel on the DC side of the rotor converter of the wind power model, and add the Crowbar circuit. Control it using a DC / DC converter. At the same time, use the PI double-loop control strategy. According to the level of voltage drop, selectively input the lithium battery, super capacitor or Crowbar circuit to absorb or release energy, achieving a lower-energy-consuming low-voltage ride-through. Among them, when there is a large voltage drop, the Crowbar protection circuit is used to improve the low-voltage ride-through ability of the wind turbine.

[0115] The main purpose of the Crowbar circuit being put into operation is to prevent the overcurrent on the rotor side from being too large and burning out the rotor-side converter. However, if the value of the crowbar resistance is too large, there will be a risk of overvoltage. Therefore, it is necessary to adjust the value of the crowbar resistance.

[0116] After the Crowbar is put in, the voltage on the rotor side should satisfy:

[0117]

[0118] To prevent overvoltage in the rotor circuit and DC bus overvoltage during low-voltage ride-through, the rotor voltage of the doubly-fed wind turbine must be smaller than the minimum of the maximum voltage that the rotor converter can withstand and the DC bus voltage limit:

[0119] U r <min{U rmax ,U dc,lim}

[0120] From the above two equations, it can be obtained that the resistance value of the Crowbar must satisfy:

[0121]

[0122] At the same time, in order to limit the overcurrent in the short-circuit circuit, the maximum value that the rotor side can reach during a fault must be less than the safety current threshold set by the rotor converter, so it can be obtained:

[0123]

[0124] A value range of the resistance of the Crowbar circuit can be obtained. Generally, within this value range, the larger the Crowbar resistance, the better the effect of suppressing the overcurrent on the rotor side.

[0125] For different degrees of voltage dips, hierarchical control (module input, control after input)

[0126] (1) When the grid connection point voltage drops to no less than 50% of the grid voltage, only the lithium battery is connected through detection and switch control strategies.

[0127] (2) When the grid connection point voltage drops to less than 50% and higher than 20% of the grid voltage, the supercapacitor and Crowbar circuit are connected through switch control strategies for energy discharge.

[0128] (3) When the voltage drops to less than 20% of the grid voltage, the wind turbine directly trips off the grid.

[0129] Use the low-voltage ride-through control model combining hybrid energy storage and Crowbar built with SIMULINK for simulation. Among them, the wind turbine output voltage is 690V, the power is 150kW, the main grid voltage is 110kV, the frequency is 50Hz, the simulation duration is 4.5s, the fault type is a three-phase short-circuit fault, the occurrence time is at 2.3s, and the grid-connected system controls the voltage and current in the dq decoupled coordinate system through a PI double closed-loop.

[0130] Figure 3When the corresponding voltage drops to 0.3 pu of the terminal voltage, the waveforms of the grid connection point voltage and current without the low voltage ride-through control strategy are shown in the figure. It can be seen that a fault occurs in the main grid at 2.3 s, the voltage drops sharply, and the current increase doubles, which may cause the converter to burn out, the winding wire insulation to break down, and a short circuit to occur. Figure 4 When the corresponding voltage drops to 0.6 pu of the terminal voltage, the waveforms of the grid connection point voltage and current after the hybrid energy storage is put into use Figure 5 When the corresponding voltage drops to 0.3 pu of the terminal voltage, the waveforms of the grid connection point voltage and current after the hybrid energy storage is put into use are shown in the figure. It can be seen that after the fault occurs, the change amplitudes of the terminal voltage and current are small, and the voltage and current waveforms are stable.

[0131] The low voltage ride-through model combining hybrid energy storage and Crowbar built with SIMULINK makes full use of the advantages of high energy density of lithium batteries and rapid charge and discharge of supercapacitors to form a hybrid energy storage module, and a crowbar resistor is added to the circuit. It can be used as an independent Crowbar circuit to protect the converter on the rotor side, and can also be used as an energy storage element to discharge the load. To a certain extent, it provides an improvement method to solve the problems existing in the wind power grid connection system. Secondly, it can also make full use of emerging energy storage technologies. At the same time, such control ideas, control methods and strategies can provide certain methods and make contributions to the smooth grid connection of other new energy sources such as photovoltaic power generation to a certain extent.

Claims

1. A low-voltage ride-through control method for wind power grid connection with hybrid energy storage and crowbar resistance, characterized in that, The implementation is specifically carried out according to the following steps: Step 1: Build a hybrid energy storage model using SIMULINK, combine the lithium battery model and the supercapacitor model. When the voltage drops to more than 0.5 pu, control the input of the lithium battery to absorb the excess electrical energy. When the voltage drops to less than 0.5 pu, control the input of the supercapacitor and the Crowbar circuit to absorb the excess electrical energy, achieving the rapidity and capacity requirements for the hybrid energy storage model to provide or absorb energy in a timely manner, thus constituting the hybrid energy storage model; Step 2: Design the topological circuit structure of the wind power grid connection, design the parameters of each device in the model, and build a wind power grid connection simulation model. Among them, the wind turbine uses a doubly-fed induction generator, and the inverters on the rotor side and grid side of the doubly-fed induction generator are controlled by the PWM control strategy; Step 3: Incorporate the hybrid energy storage model of the lithium battery combined with the supercapacitor built in Step 1 into the DC side of the rotor side converter in the wind power grid connection simulation model in Step 2, and at the same time incorporate the Crowbar circuit to build a low voltage ride-through model of the combination of the hybrid energy storage and the Crowbar to achieve low voltage ride-through when the grid connection point voltage drops; In Step 3: The voltage on the rotor side after the Crowbar is input should satisfy: To prevent overvoltage in the rotor circuit and overvoltage in the DC bus during the low voltage ride-through process, the rotor voltage of the doubly-fed wind turbine is smaller than the minimum voltage of the maximum voltage that the rotor converter can withstand and the DC bus voltage limit: U r <min{U rmax ,U dc,lim} From the above two formulas, it can be obtained that the resistance value of the Crowbar must satisfy: At the same time, in order to limit the overcurrent in the short-circuit loop, the maximum value that can be reached on the rotor side during a fault should be less than the safety current threshold set by the rotor converter, so: A value range of the resistance of the Crowbar circuit is obtained. The larger the Crowbar resistance, the better the effect of suppressing the overcurrent on the rotor side.

2. The low-voltage ride-through control method for wind power grid connection with hybrid energy storage and crowbar resistance according to claim 1, characterized in that, Step 1 is specifically as follows: The hybrid energy storage model includes a lithium battery part and a supercapacitor part. Among them, the lithium battery is connected with a Boost circuit to make the output voltage of the lithium battery higher than the input voltage; The supercapacitor is connected with a Buck-Boost circuit to make the output voltage of the supercapacitor can be lower or higher than the input voltage; Combine the two together for the output and input of energy. When the voltage drops to more than 0.5 pu, control the input of the lithium battery to absorb the excess electrical energy. When the voltage drops to less than 0.5 pu, control the input of the supercapacitor and the Crowbar circuit to absorb the excess electrical energy.

3. The low-voltage ride-through control method for wind power grid connection with hybrid energy storage and crowbar resistance according to claim 2, characterized in that, Step 2 is specifically as follows: The wind turbine is a doubly-fed induction generator model, and the converter consists of a rotor side converter and a grid side converter; The control strategy of the rotor side converter is as follows: Formula (9) is the speed relationship formula between the stator and the rotor, and formula (10) is the frequency relationship formula between the stator and the rotor. When the doubly-fed induction motor is in a stable operating state, the rotating magnetic fields between the stator and the rotor present a relatively stationary characteristic, which is expressed by the following formula: n1 = n2 + n r (9) Because There is / are: n1 is the stator speed, n2 is the speed relative to the stator speed, n r is the rotor magnetomotive force speed, f1 is the stator frequency, f2 is the rotor frequency, and p is the number of pole pairs; The control strategy of the grid side converter: For the three-phase stator windings, the voltage of the three-phase winding voltage equation is calculated by the following equation: The corresponding three-phase rotor winding voltage equation is: In the above equation, the parameter u sA u sB u sC u ra u rb u rc represents the instantaneous phase voltages of the three-phase stator and rotor of A, B, and C; the parameter i sA i sB i sC i ra i rb i rc represents the instantaneous phase currents of the three-phase stator and rotor of A, B, and C; the parameter ψ sA ψ sB ψ sC is the magnetic flux linkage of the three-phase windings of A, B, and C of the stator and rotor; the parameter R s R r respectively represent the stator winding resistance and the rotor winding resistance. Similarly, the above parameters will be converted to the stator side; Equation (13) is the stator voltage equation of the DFIG in the synchronous rotating coordinate system, and Equation (14) is the rotor voltage equation of the DFIG in the synchronous rotating coordinate system: DFIG stator voltage equation in the synchronous rotating coordinate system: DFIG rotor voltage equation in the synchronous rotating coordinate system: In the above formula, u d1 is the voltage of the stator on the d-axis in the dq coordinate system; u q1 is the voltage of the stator on the q-axis; u d2 is the voltage of the rotor on the d-axis in the dq coordinate system; u q2 is the voltage of the rotor on the q-axis; i d1 is the current of the stator on the d-axis in the dq coordinate system; i q1 is the current of the stator on the q-axis; i d2 is the current of the rotor on the d-axis in the dq coordinate system; i q2 is the current of the rotor on the q-axis.

4. The low-voltage ride-through control method for wind power grid connection of hybrid energy storage and crowbar resistance according to claim 3, wherein, The specific steps of Step 3 are as follows: A hybrid energy storage model combining a lithium battery and a supercapacitor is juxtaposed on the DC side of the rotor converter of the wind power model, and a Crowbar circuit is added. It is controlled by a DC / DC converter and at the same time, a PI double closed-loop control strategy is used. According to the level of voltage dip, the lithium battery, the supercapacitor or the Crowbar circuit is selectively put into operation to absorb or release energy, achieving lower power consumption for low voltage ride-through. Among them, when there is a large voltage dip, the Crowbar protection circuit is used to improve the low voltage ride-through ability of the fan.

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