A method and system for dynamic reactive current control of a grid-forming grid-connected converter

Abstract

The application discloses a kind of dynamic reactive current control methods and systems of network type grid-connected converter, this method is in the voltage amplitude control link of network type grid-connected converter on the basis of network type grid-connected converter control, by superimposing voltage compensation signal on voltage reference value, to achieve the goal of limiting network type grid-connected converter reactive current, control is divided into signal acquisition and calculation, low voltage ride through state judgment, reactive power limit calculation under normal state, reactive power limit calculation under low voltage ride through state, voltage compensation control, voltage setting value control.The application is superimposed on voltage control based on reactive current limit calculation obtained voltage compensation term, limit voltage reference value when low voltage ride through, since compensation term is based on reactive current limit calculation, this way is more direct to the limitation of reactive current, protect converter while let converter have stronger reliability in transient process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of grid-forming grid-connected converter control design, and particularly relates to a dynamic reactive current control method and system of a grid-forming grid-connected converter, a storage medium and a computing device. BACKGROUND

[0002] In recent years, grid-forming grid-connected converters have attracted extensive attention from the academic and engineering fields due to their ability to provide voltage support and frequency support for power grids. However, during the operation of the grid-connected converter, it may face many problems. Low voltage ride through (LVRT) is a basic function written into domestic and foreign grid-connected standards. During LVRT, the grid-forming grid-connected converter may generate a large short-circuit current, and the overcurrent capacity of the converter is limited. Therefore, the grid-forming grid-connected converter needs to inject a virtual impedance to limit the current during LVRT. However, since the grid-forming grid-connected converter directly controls the amplitude and phase of the voltage, the injection of the virtual impedance does not directly limit the current, i.e., the amplitude of the current cannot be directly limited during the injection of the virtual impedance. SUMMARY

[0003] The first object of the present application is to overcome the shortcomings and deficiencies of the prior art, and to provide a dynamic reactive current control method of a grid-forming grid-connected converter. The method adds a compensation control to the voltage amplitude control link of the grid-forming grid-connected converter, and superimposes a voltage compensation signal on the voltage reference value to limit the reactive current of the grid-forming grid-connected converter. The method includes the following steps:

[0004] The second object of the present application is to provide a dynamic reactive current control system of a grid-forming grid-connected converter.

[0005] The third object of the present application is to provide a storage medium.

[0006] The fourth object of the present application is to provide a computing device.

[0007] The first object of the present application is achieved by the following technical solution: a dynamic reactive current control method of a grid-forming grid-connected converter. The method adds a compensation control to the voltage amplitude control link of the grid-forming grid-connected converter, and superimposes a voltage compensation signal on the voltage reference value to limit the reactive current of the grid-forming grid-connected converter. The method includes the following steps:

[0008] 1) Collecting signals, including the three-phase voltage instantaneous value, the three-phase current instantaneous value and the voltage phase of the grid-forming grid-connected converter. Based on the collected signals, the voltage and current in the rotating two-phase coordinate system are obtained according to the Park transformation. Based on the obtained voltage and current, the positive sequence voltage amplitude and the reactive power are calculated.

[0009] 2) According to the magnitude of the positive sequence voltage amplitude obtained in step 1), low voltage ride through state is judged, if it is judged as normal state, step 3) is executed, if it is judged as low voltage ride through state, step 4) is executed;

[0010] 3) Reactive power limit calculation in normal state, upper and lower limits of reactive power output are obtained;

[0011] 4) Reactive power limit calculation in low voltage ride through state, upper and lower limits of reactive power output are obtained;

[0012] 5) According to the reactive power obtained in step 1) and the upper and lower limits of reactive power output obtained in step 3) or step 4), voltage compensation control is performed based on the upper and lower limits of reactive power output, over reactive voltage compensation value and under reactive voltage compensation value are obtained;

[0013] 6) According to the over reactive voltage compensation value and under reactive voltage compensation value obtained in step 5), voltage reference value is compensated, control voltage difference is obtained, finally, voltage set value obtained by the controller with limiting amplitude is input into the inner loop voltage / current control link of the grid-forming grid-connected converter, voltage set value control is realized.

[0014] Further, in step 1), three-phase voltage instantaneous values U a , U b , U c and three-phase current instantaneous values I a , I b , I c of the grid-forming grid-connected converter are collected, voltage phase θ is collected by using phase-locked loop, according to Park transformation, voltage U d , U q and current I d , I q in rotating two-phase coordinate system are obtained;

[0015] Wherein, the relationship between U d , U q and U a , U b , U c is:

[0016]

[0017] The relationship between I d , I q and I a , I b , I c is:

[0018]

[0019] The positive sequence voltage amplitude U and reactive power Q are calculated as follows:

[0020]

[0021] Further, in step 2), the Flag LVRT is set as a low voltage ride through flag bit, 0 represents a normal state, 1 represents a low voltage ride through state, and the initial value is 0;

[0022] When Flag LVRT = 0, if U >= U dip , Flag LVRT = 0, and it is judged as a normal state, and step 3) is entered; if U < U dip , Flag LVRT = 1, a low voltage ride through state is entered, and the reactive current value at this time is locked, recorded as I0, and step 4) is entered;

[0023] When Flag LVRT = 1, if U >= U rec , Flag LVRT = 0, and it is converted into a normal state, and step 3) is entered; if Flag LVRT = 1 and U < U rec , Flag LVRT = 1, and it is still in a low voltage ride through state, and step 4) is entered;

[0024] Wherein, U dip is a low voltage ride in threshold value; U rec is a low voltage ride out threshold value, and the value is greater than U dip .

[0025] Further, in step 3), the reactive power limit in the normal state is calculated as follows:

[0026] Q min = -U max I qmax

[0027] Q max = U max I qmax

[0028] In the formula, Q max is an upper limit of reactive power output, Q min is a lower limit of reactive power output, U max is a maximum voltage amplitude, and I qmax is a maximum value of reactive current in a normal state;

[0029] If the grid-connected converter of the network type works in a power factor control mode, then For the set power factor; if the grid-connected converter works in the reactive power control mode, then

[0030] Further, in step 4), the reactive power limit in the low voltage ride through state is calculated as follows:

[0031] Q min = -U max I qmax

[0032] Q max = UI R

[0033] In the formula, the current I R is calculated as: I R = I0+ K1(U Lref -U)I N , K1 is a dynamic reactive current proportion coefficient, U Lref is a voltage reference for dynamic reactive current calculation, and I N is a rated current.

[0034] Further, in step 5), according to the reactive power Q obtained in step 1) and the lower limit Q min and the upper limit Q max of the reactive power output obtained in step 3) or step 4), voltage compensation control is performed with the lower limit Q min and the upper limit Q max of the reactive power output as the reference, and the specific process is as follows:

[0035] 5.1) Calculate the difference AQ max between the upper limit Q oel of the reactive power output and the reactive power Q, and calculate the difference between the reactive power Q and the lower limit Q min of the reactive power output:

[0036] AQ oel = Q max -Q

[0037] AQ uel = Q-Q min

[0038] 5.2) Obtain the over reactive voltage compensation value and the under reactive voltage compensation value through the controller by using the difference AQ oel , AQ uel obtained in step 5.1), and the specific process is as follows:

[0039] First, the voltage compensation is divided into two parts, namely over reactive voltage compensation and under reactive voltage compensation, which are respectively used for compensation when the reactive power exceeds the upper limit and compensation when the reactive power is lower than the lower limit.

[0040] ΔQ oel Over reactive power voltage compensation value U oel ; ΔQ uel Under reactive power voltage compensation value U uel ;

[0041] U oel The upper limit of U oel_max , the lower limit of U oel_min ; In order not to affect the voltage control in steady state operation, U oel will be limited to U oel_max ;

[0042] U uel The upper limit of U uel_max , the lower limit of U uel_min ; In order not to affect the voltage control in steady state operation, U uel will be limited to U oel_min .

[0043] Further, in step 6), according to the over reactive power voltage compensation value U oel and the under reactive power voltage compensation value U uel obtained in step 5), the voltage reference value is compensated, and the voltage setting value control is carried out, as follows:

[0044] 6.1) Calculate the voltage difference value

[0045] The calculated over reactive power voltage compensation value U oel , under reactive power voltage compensation value U uel and positive sequence voltage amplitude U, voltage reference value U ref are superimposed to obtain the control voltage difference ΔU:

[0046] ΔU = U ref - U + U oel + U uel

[0047] In the formula, the voltage reference value U ref is the set voltage reference value;

[0048] 6.2) Obtain the voltage setting value through the controller

[0049] The voltage setting value U set is obtained through the controller with amplitude limit, the upper limit of U set is U set_max , and the lower limit of U se t _m i n ;

[0050] Finally, the obtained U set The input voltage / current control link of the grid-connected converter is taken as the inner loop of the grid-connected converter.

[0051] Further, U oel_max = 0, U oel_min =-1; U oel_max = 1, U oel_min = 0.

[0052] Further, the controller is P control, PI control or PID control.

[0053] The second object of the application is achieved by the following technical scheme: a dynamic reactive current control system of a grid-connected converter, used for implementing the dynamic reactive current control method of the grid-connected converter, comprising:

[0054] A signal acquisition and calculation module is configured to acquire signals, including three-phase voltage instantaneous values, three-phase current instantaneous values and voltage phases of the grid-connected converter, and based on the acquired signals, to obtain voltage and current in a rotating two-phase coordinate system according to a Park transformation, and to calculate a positive sequence voltage amplitude and a reactive power based on the obtained voltage and current;

[0055] A judgment module is configured to judge a low voltage ride through state according to the size of the positive sequence voltage amplitude obtained by the signal acquisition and calculation module, and if the judgment is normal state, to execute a first reactive power limitation calculation module, and if the judgment is low voltage ride through state, to execute a second reactive power limitation calculation module;

[0056] The first reactive power limitation calculation module is configured to calculate the reactive power limitation in the normal state to obtain upper and lower limits of the reactive power output;

[0057] The second reactive power limitation calculation module is configured to calculate the reactive power limitation in the low voltage ride through state to obtain upper and lower limits of the reactive power output;

[0058] A voltage compensation control module is configured to obtain over-reactive voltage compensation values and under-reactive voltage compensation values based on the reactive power obtained by the signal acquisition and calculation module and the upper and lower limits of the reactive power output obtained by the first reactive power limitation calculation module or the second reactive power limitation calculation module, and to perform voltage compensation control based on the upper and lower limits of the reactive power output to obtain the over-reactive voltage compensation values and the under-reactive voltage compensation values;

[0059] A voltage set value control module is configured to compensate a voltage reference value based on the over-reactive voltage compensation values and the under-reactive voltage compensation values obtained by the voltage compensation control module to obtain a control voltage difference, and finally to input the voltage set value obtained by the controller with limiting amplitude to the inner loop voltage / current control link of the grid-connected converter to implement voltage set value control.

[0060] The third object of the present application is achieved by the following technical solution: a storage medium storing a program, which, when executed by a processor, implements the dynamic reactive current control method of the grid-forming grid-connected converter.

[0061] The fourth object of the present application is achieved by the following technical solution: a computing device comprising a processor and a memory for storing a program executable by the processor, which, when executed by the processor, implements the dynamic reactive current control method of the grid-forming grid-connected converter.

[0062] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0063] The present application limits the reactive current by adding compensation control in the control of the grid-forming grid-connected converter, only needs to improve the control algorithm, and does not need to increase additional hardware cost.

[0064] The present application has a more direct limitation on the reactive current after the grid-forming grid-connected converter enters the low-penetration state, and improves the reliability of the converter during the transient state.

[0065] The limiting of the present application limits the interval in which the compensation control acts, does not interfere with the normal operation of the grid-forming grid-connected converter, and only plays a role in limiting the reactive current during the low-penetration period. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 is a control schematic diagram of the method of the present application; in the figure, P is the active power, P ref is the active power reference, U ref0 is the original voltage reference value, T J is the virtual rotor inertia, 1 / s is the integral, D is the virtual rotor damping, ω is the virtual synchronous speed, θ vs is the virtual synchronous phase, ω b is the speed reference value, and ω0 is the speed reference value.

[0067] Figure 2 is a control flowchart of the method of the present application.

[0068] Figure 3 is a signal acquisition schematic diagram of the grid-forming grid-connected converter.

[0069] Figure 4 is an architecture diagram of the system of the present application. DETAILED DESCRIPTION

[0070] The present application will be further described in detail below in combination with embodiments and drawings, but the implementation manner of the present application is not limited thereto.

[0071] Example 1

[0072] This embodiment discloses a method for controlling dynamic reactive current of a grid-connected converter. The method is based on the control of the grid-connected converter. Figure 1 As shown in Figure 1, compensation control is added to the voltage amplitude control link of the grid-connected converter. By superimposing the voltage compensation signal on the voltage reference value, the reactive current of the grid-connected converter can be limited. The control is divided into six steps: signal acquisition and calculation, low voltage ride-through state judgment, reactive power limit calculation under normal state, reactive power limit calculation under low voltage ride-through state, voltage compensation control, and voltage set value control. Figure 2 The specific situation is as follows:

[0073] 1) Signal acquisition and calculation

[0074] See Figure 3 As shown, the instantaneous value of the three-phase voltage on the grid side of the grid-connected converter is collected. a 、U b 、U c And the instantaneous value of three-phase current I a , I b , I c , use the phase-locked loop to collect the voltage phase θ, and obtain the voltage U in the rotating two-phase coordinate system according to the Park transformation d 、U q and current I d , I q ;

[0075] Among them, U d 、U q with U a 、U b 、U c The relationship is:

[0076]

[0077] I d , I q with I a , I b , I c The relationship is:

[0078]

[0079] The positive sequence voltage amplitude U and reactive power Q are calculated as follows:

[0080]

[0081] 2) Based on the positive sequence voltage amplitude obtained in step 1), the low voltage ride-through state is determined as follows:

[0082] Set Flag LVRT Low voltage ride through flag bit, 0 represents normal state, 1 represents low voltage ride through state, initial value is 0;

[0083] When Flag LVRT = 0, if U >= U dip , Flag LVRT = 0, judge as normal state, enter step 3); if U < U dip , Flag LVRT = 1, enter low voltage ride through state, and lock the reactive current value at this time, recorded as I0, enter step 4);

[0084] When Flag LVRT = 1, if U >= U rec , Flag LVRT = 0, convert to normal state, enter step 3); if Flag LVRT = 1, and U < U rec , Flag LVRT = 1, still in low voltage ride through state, enter step 4);

[0085] Wherein, U dip is low voltage ride through threshold, generally less than 0.9, which can be adjusted according to actual situation; U rec is low voltage ride through exit threshold, generally slightly greater than U dip .

[0086] 3) Reactive power limit calculation in normal state, get upper and lower limits of reactive power output;

[0087] Q min = -U max I qmax

[0088] Q max = U max I qmax

[0089] In the formula, Q max is upper limit of reactive power output, Q min is lower limit of reactive power output, U max is maximum voltage amplitude, I qmax is maximum value of reactive current in normal state;

[0090] If grid-connected converter working in power factor control mode, then is set power factor; if grid-connected converter working in reactive power control mode, then

[0091] 4) Calculate reactive power limits in low voltage ride-through state to obtain upper and lower limits of reactive power output;

[0092] Q min =-U max I qmax

[0093] Q max =UI R

[0094] Where, current I R The calculation method is: I R =I0+K1(U Lref -U)I N , K1 is the dynamic reactive current proportional coefficient, U Lref The voltage reference for dynamic reactive current calculation (usually 0.9), I N is the rated current.

[0095] 5) Voltage compensation control

[0096] According to the reactive power Q obtained in step 1) and the reactive power output lower limit Q obtained in step 3) or step 4) min and reactive power output upper limit Q max , with the reactive power output lower limit Q min and reactive power output upper limit Q max The voltage compensation control is performed based on the reference, as follows:

[0097] 5.1) Calculate the upper limit of reactive power output Q max The difference ΔQ from the reactive power Q oel And calculate the reactive power Q and the reactive power output lower limit Q min The difference:

[0098] ΔQ oel =Q max -Q

[0099] ΔQ uel =QQ min

[0100] 5.2) The difference ΔQ obtained in step 5.1) is oel , ΔQ uel The over-reactive voltage compensation value and under-reactive voltage compensation value are obtained through the controller, as follows:

[0101] First, voltage compensation is divided into two parts: over-reactive voltage compensation and under-reactive voltage compensation, which are used to compensate when the reactive power exceeds the upper limit and when the reactive power falls below the lower limit respectively.

[0102] ΔQ oelThe over-reactive voltage compensation value U is obtained through the controller with limiting oel ;ΔQ uel The under-reactive voltage compensation value U is obtained through the controller with amplitude limiting uel The controller here can be P control, PI control or PID control, and the manufacturer can choose according to the actual situation;

[0103] U oel The upper limit is U oel_max , the lower limit is U oel_min ; In general, U oel_max =0, U oel_min =-1, the manufacturer can adjust it according to the actual situation; in order not to affect the voltage control during steady-state operation, under normal conditions, U oel Will be restricted to U oel_max ;

[0104] U uel The upper limit is U uel_max , the lower limit is U uel_min ;Usually oel_max =1, U oel_min =0, the manufacturer can adjust it according to the actual situation; in order not to affect the voltage control during steady-state operation, under normal conditions, U uel Will be restricted to U oel_min .

[0105] 6) Voltage set value control

[0106] According to step 5) the over-reactive voltage compensation value U oel And the reactive voltage compensation value U uel , compensate the voltage reference value and control the voltage set value, as follows:

[0107] 6.1) Calculate the voltage difference

[0108] The calculated over-reactive voltage compensation value U oel , under-reactive voltage compensation value U uel Compared with the actual voltage value U and the voltage reference value U ref Superposition is performed to obtain the control voltage difference ΔU:

[0109] ΔU=U ref -U+U oel +U uel

[0110] Where, voltage reference value U ref is the set voltage reference value;

[0111] 6.2) Get the voltage setting value through the controller

[0112] ΔU is obtained by the controller with band-limited amplitude set The controller herein can be P control, PI control or PID control, which can be selected by the manufacturer according to actual conditions set The upper limit of U is U set_max The lower limit of U is U set_min ;

[0113] Finally, U is obtained set which will be input to the inner loop voltage (current) control link of the grid-forming grid-connected converter.

[0114] Embodiment 2

[0115] The embodiment discloses a dynamic reactive current control system of a grid-forming grid-connected converter, which is used to realize the dynamic reactive current control method of the grid-forming grid-connected converter described in Embodiment 1, as shown in the figure, comprising the following functional modules: Figure 4

[0116] The signal acquisition and calculation module is used to acquire signals, including the three-phase voltage instantaneous value, the three-phase current instantaneous value and the voltage phase of the grid-forming grid-connected converter grid-side, based on the acquired signals, the voltage and current in the rotating two-phase coordinate system are obtained according to the Park transformation, and the positive sequence voltage amplitude and the reactive power are calculated based on the obtained voltage and current;

[0117] The judgment module judges the low voltage ride through state according to the size of the positive sequence voltage amplitude obtained by the signal acquisition and calculation module, if it is judged as a normal state, the first reactive power limitation calculation module is executed, if it is judged as a low voltage ride through state, the second reactive power limitation calculation module is executed;

[0118] The first reactive power limitation calculation module is used for reactive power limitation calculation in the normal state, and the upper and lower limits of the reactive power output are obtained;

[0119] The second reactive power limitation calculation module is used for reactive power limitation calculation in the low voltage ride through state, and the upper and lower limits of the reactive power output are obtained;

[0120] The voltage compensation control module obtains the over-reactive voltage compensation value and the under-reactive voltage compensation value according to the reactive power obtained by the signal acquisition and calculation module and the upper and lower limits of the reactive power output obtained by the first reactive power limitation calculation module or the second reactive power limitation calculation module, and performs voltage compensation control with the upper and lower limits of the reactive power output as the reference;

[0121] ​The voltage setting value control module compensates the voltage reference value according to the over reactive voltage compensation value and the under reactive voltage compensation value obtained by the voltage compensation control module, obtains a control voltage difference, and finally inputs the voltage setting value obtained by the controller with limiting amplitude into the inner loop voltage / current control link of the grid-forming grid-connected converter to realize voltage setting value control.

[0122] Embodiment 3

[0123] The embodiment discloses a storage medium, which stores a program, and the program is executed by a processor to realize the dynamic reactive current control method of the grid-forming grid-connected converter.

[0124] The storage medium in the embodiment can be a disk, an optical disk, a computer memory, a read-only memory (ROM), a random access memory (RAM), a U disk, a mobile hard disk and the like.

[0125] Embodiment 4

[0126] The embodiment discloses a computing device, which comprises a processor and a memory for storing a program executable by the processor, and the processor realizes the dynamic reactive current control method of the grid-forming grid-connected converter when executing the program stored in the memory.

[0127] The computing device in the embodiment can be a desktop computer, a notebook computer, a smart phone, a PDA handheld terminal, a tablet computer, a programmable logic controller (PLC) or other terminal devices with a processor function.

[0128] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above embodiments, and any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principle of the present application should be equivalent replacement methods, which are all included in the protection scope of the present application.

Claims

1. A method for controlling dynamic reactive current of a grid-connected converter, characterized in that: The method is to add compensation control to the voltage amplitude control link of the grid-connected converter of the grid-connected type, and to achieve the goal of limiting the reactive current of the grid-connected converter of the grid-connected type by superimposing a voltage compensation signal on a voltage reference value. The method includes the following steps: 1) Acquire signals, including the instantaneous three-phase voltage and current values ​​and voltage phase of the grid-side three-phase grid-connected converter. Based on the acquired signals, use the Park transform to obtain the voltage and current in a rotating two-phase coordinate system. Based on the obtained voltage and current, calculate the positive sequence voltage amplitude and reactive power. 2) Based on the magnitude of the positive sequence voltage amplitude obtained in step 1), determine the low voltage ride-through state. If it is determined to be normal, proceed to step 3. If it is determined to be a low voltage ride-through state, proceed to step 4. 3) Calculate reactive power limit under normal conditions to obtain the upper and lower limits of reactive power output; The reactive power limit under the normal state is calculated as follows: Q min =-U max I qmax ; Q max =U max I qmax ; Where Q max is the upper limit of reactive power output, Q min is the lower limit of reactive power output, U max is the maximum voltage amplitude, I qmax The maximum value of reactive current in normal state; If the grid-connected converter operates in the power factor control mode, then , cosφ is the set power factor; if the grid-connected converter works in the reactive power control mode, then ; 4) Calculate reactive power limits in low voltage ride-through state to obtain upper and lower limits of reactive power output; The reactive power limit in the low voltage ride-through state is calculated as follows: Q min =-U max I qmax ; Q max =UI R ; Where, current I R The calculation method is: I R =I0+K1(U Lref -U)I N , K1 is the dynamic reactive current proportional coefficient, U Lref The voltage reference for dynamic reactive current calculation, I N is the rated current; 5) Based on the reactive power obtained in step 1) and the upper and lower limits of reactive power output obtained in step 3) or step 4), voltage compensation control is performed based on the upper and lower limits of reactive power output to obtain an over-reactive voltage compensation value and an under-reactive voltage compensation value; 6) Based on the over-reactive voltage compensation value and under-reactive voltage compensation value obtained in step 5), the voltage reference value is compensated to obtain a control voltage difference. Finally, the voltage setting value obtained by passing the control voltage difference through a controller with a limiter is input into the inner loop voltage and current control link of the grid-connected converter to achieve voltage setting value control.

2. The method for dynamic reactive current control of a grid-connected converter according to claim 1, characterized in that: In step 1), the instantaneous value of the three-phase voltage on the grid side of the grid-connected converter is collected. a 、U b 、U c And the instantaneous value of three-phase current I a , I b , I c , use the phase-locked loop to collect the voltage phase θ, and obtain the voltage U in the rotating two-phase coordinate system according to the Park transformation d 、U q and current I d , I q ; Among them, U d 、U q with U a 、U b 、U c The relationship is: ； I d , I q with I a , I b , I c The relationship is: ； The positive sequence voltage amplitude U and reactive power Q are calculated as follows: ； 。 3. The method for dynamic reactive current control of a grid-connected converter according to claim 2, wherein: In step 2), set Flag LVRT Low voltage ride-through flag, 0 indicates normal state, 1 indicates low voltage ride-through state, and the initial value is 0; In Flag LVRT =0, if U>=U dip , then Flag LVRT =0, it is judged to be normal state and goes to step 3); if U dip , then Flag LVRT =1, enter the low voltage ride-through state, and lock the reactive current value at this time, recorded as I0, and go to step 4);​ In Flag LVRT =1, if U>=U rec , then Flag LVRT =0, convert to normal state and go to step 3); if Flag LVRT =1, and U< U rec , then Flag LVRT =1, still in low voltage ride-through state, go to step 4); Among them, U dip is the low voltage threshold; U rec is the low wear-out exit voltage threshold, which is greater than U dip .

4. The method for dynamic reactive current control of a grid-connected converter according to claim 3, wherein: In step 5), the reactive power Q obtained in step 1) and the reactive power output lower limit Q obtained in step 3) or step 4) are calculated. min and reactive power output upper limit Q max , with the reactive power output lower limit Q min and reactive power output upper limit Q max The voltage compensation control is performed based on the reference, as follows: 5.1) Calculate the upper limit of reactive power output Q max The difference ΔQ from the reactive power Q oel And calculate the reactive power Q and the reactive power output lower limit Q min The difference: ΔQ oel =Q max -Q; ΔQ uel =Q-Q min ; 5.2) Substitute the difference ΔQ obtained in step 5.1) oel , ΔQ uel The over-reactive voltage compensation value and under-reactive voltage compensation value are obtained through the controller, as follows: First, voltage compensation is divided into two parts: over-reactive voltage compensation and under-reactive voltage compensation, which are used to compensate when the reactive power exceeds the upper limit and when the reactive power falls below the lower limit respectively. ΔQ oel The over-reactive voltage compensation value U is obtained through the controller with limiting oel ;ΔQ uel The under-reactive voltage compensation value U is obtained through the controller with amplitude limiting uel ; U oel The upper limit is U oel_max , the lower limit is U oel_min ; In order not to affect the voltage control during steady-state operation, under normal conditions, U oel Will be restricted to U oel_max ; U uel The upper limit is U uel_max , the lower limit is U uel_min ; In order not to affect the voltage control during steady-state operation, under normal conditions, U uel Will be restricted to U oel_min .

5. The method for dynamic reactive current control of a grid-connected converter according to claim 4, characterized in that: In step 6), the reactive voltage compensation value U obtained in step 5) is obtained. oel And the reactive voltage compensation value U uel , compensate the voltage reference value and control the voltage set value, as follows: 6.1) Calculate the voltage difference The calculated over-reactive voltage compensation value U oel , under-reactive voltage compensation value U uel With the positive sequence voltage amplitude U, voltage reference value U ref Superposition is performed to obtain the control voltage difference ΔU: ΔU=U ref -U+U oel +U uel ; Where, voltage reference value U ref is the set voltage reference value; 6.2) Get the voltage setting value through the controller ΔU passes through the controller with limiting to obtain the voltage setting value U set , U set The upper limit is U set_max , the lower limit is U set_min ; Finally, the obtained U set The input is sent to the inner loop voltage and current control link of the grid-connected converter.

6. The method for dynamic reactive current control of a grid-connected converter according to claim 5, characterized in that: IN oel_max =0,U oel_min =-1;U oel_max =1,U oel_min =0.

7. The method for dynamic reactive current control of a grid-connected converter according to claim 6, characterized in that: The controller is P control, PI control or PID control.

8. A dynamic reactive current control system for a grid-connected converter, characterized in that: A method for implementing a dynamic reactive current control method of a grid-connected converter according to any one of claims 1 to 7, comprising: The signal acquisition and calculation module is used to collect signals, including the instantaneous values ​​of the three-phase voltage and the three-phase current on the grid side of the grid-connected converter. Based on the collected signals, the voltage and current in the rotating two-phase coordinate system are obtained according to the Park transformation. Based on the obtained voltage and current, the positive sequence voltage amplitude and reactive power are calculated. A judgment module, which judges the low voltage ride-through state based on the magnitude of the positive sequence voltage amplitude obtained by the signal acquisition and calculation module. If it is judged to be a normal state, the first reactive power limitation calculation module is executed; if it is judged to be a low voltage ride-through state, the second reactive power limitation calculation module is executed; The first reactive power limit calculation module is used for reactive power limit calculation under normal conditions to obtain the upper and lower limits of reactive power output; The second reactive power limit calculation module is used to calculate the reactive power limit in the low voltage ride-through state and obtain the upper and lower limits of the reactive power output; A voltage compensation control module performs voltage compensation control based on the reactive power obtained by the signal acquisition and calculation module and the upper and lower limits of reactive power output obtained by the first reactive power limit calculation module or the second reactive power limit calculation module, and uses the upper and lower limits of reactive power output as a reference to obtain an over-reactive voltage compensation value and an under-reactive voltage compensation value; The voltage set value control module compensates the voltage reference value according to the over-reactive voltage compensation value and under-reactive voltage compensation value obtained by the voltage compensation control module to obtain a control voltage difference. Finally, the voltage set value obtained by passing the control voltage difference through a controller with a limiter is input into the inner loop voltage and current control link of the grid-connected converter to realize voltage set value control.

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