Dynamic reactive current control method and system of network construction type grid-connected converter

Abstract

The invention discloses a dynamic reactive current control method and a dynamic reactive current control system for a grid-forming type grid-connected converter, and the method comprises the steps: adding compensation control in a voltage amplitude control link of the grid-forming type grid-connected converter on the basis of the control of the grid-forming type grid-connected converter, and superposing a voltage compensation signal on a voltage reference value so as to realize the dynamic reactive current control of the grid-forming type grid-connected converter. The control method comprises the steps of signal acquisition and calculation, low voltage ride through state judgment, reactive power limitation calculation in a normal state, reactive power limitation calculation in a low voltage ride through state, voltage compensation control and voltage set value control. According to the method, the voltage compensation item calculated based on the reactive current limitation is superposed on voltage control, the voltage reference value is limited during low-voltage ride through, and the compensation item is calculated based on the reactive current limitation, so that the limitation on the reactive current is more direct, and the accuracy of the low-voltage ride through is improved. The converter is protected, and meanwhile the converter has higher reliability in the transient process.

Description

Technical Field

[0001] The present invention relates to the technical field of control design of a grid-connected converter, and in particular to a method, system, storage medium and computing device for dynamic reactive current control of a grid-connected converter. Background Art

[0002] In recent years, grid-connected converters have attracted extensive attention from academia and engineering circles because they can provide voltage and frequency support for the power grid. However, the converter will face many problems during operation after being connected to the grid. Low voltage ride-through is written into domestic and international grid standards as a basic function. After entering the low voltage ride-through, the grid-connected converter may generate a large short-circuit current, and the converter's overcurrent capacity is limited. The grid-connected converter will invest in virtual impedance to limit the current during the low voltage ride-through process. However, since the grid-connected converter directly controls the amplitude and phase of the voltage, the investment of virtual resistance does not directly limit the current, that is, the amplitude of the current cannot be directly limited during the investment of virtual impedance. Summary of the invention

[0003] The first purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and to provide a dynamic reactive current control method for a grid-connected grid-connected converter. By superimposing a voltage compensation term calculated based on reactive current limitation on the voltage control, the voltage reference value is limited during low voltage crossing. Since the compensation term is calculated based on reactive current limitation, this method is more direct in limiting reactive current, thereby protecting the converter while making the converter more reliable during transient processes.

[0004] A second object of the present invention is to provide a dynamic reactive current control system for a grid-connected converter.

[0005] A third object of the present invention is to provide a storage medium.

[0006] A fourth object of the present invention is to provide a computing device.

[0007] The first object of the present invention is achieved by the following technical solution: a dynamic reactive current control method of a grid-connected converter, which is to add compensation control in the voltage amplitude control link of the grid-connected converter, and to achieve the goal of limiting the reactive current of the grid-connected converter by superimposing a voltage compensation signal on a voltage reference value, and comprising the following steps:

[0008] 1) Collecting signals, including the instantaneous value of the three-phase voltage, the instantaneous value of the three-phase current and the voltage phase of the grid side of the grid-connected converter, obtaining the voltage and current in the rotating two-phase coordinate system according to the Park transformation based on the collected signals, and calculating the positive sequence voltage amplitude and reactive power based on the obtained voltage and current;

[0009] 2) According to the magnitude of the positive sequence voltage amplitude obtained in step 1), a low voltage ride-through state is determined. If it is determined to be a normal state, step 3) is executed; if it is determined to be a low voltage ride-through state, step 4) is executed;

[0010] 3) Calculate reactive power limit under normal conditions to obtain the upper and lower limits of reactive power output;

[0011] 4) Calculate the reactive power limit under low voltage ride-through state to obtain the upper and lower limits of reactive power output;

[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 to obtain an over-reactive voltage compensation value and an under-reactive voltage compensation value;

[0013] 6) According to 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 limiting is input into the inner loop voltage / current control link of the grid-connected converter to realize voltage setting value control.

[0014] Further, in step 1), the instantaneous value U 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 ;

[0015] Among them, U d , U q with U a , U b , U c The relationship is:

[0016]

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

[0018]

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

[0020]

[0021] Further, in step 2), set Flag LVRT It is the low voltage ride-through flag. 0 indicates normal state, 1 indicates low voltage ride-through state, and the initial value is 0.

[0022] In Flag LVRT =0, if U>=U dip , then Flag LVRT =0, it is judged as 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 enter step 4);

[0023] 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 rec , then Flag LVRT =1, still in low voltage ride-through state, go to step 4);

[0024] 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 .

[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 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;

[0029] If the grid-connected converter works in the power factor control mode, then ​​is 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 limitation in the low voltage ride-through state is calculated as follows:

[0031] Q min =-U max I qmax

[0032] Q max =UI R

[0033] Where, the 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.

[0034] Further, in step 5), 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 as a reference, as follows:

[0035] 5.1) Calculate the upper limit of reactive power output Q max The difference ΔQ with the reactive power Q oel And calculate the reactive power Q and the reactive power output lower limit Q min The difference:

[0036] ΔQ oel =Q max -Q

[0037] ΔQ uel =QQ min

[0038] 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:

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

[0040] ΔQ oel The over-reactive voltage compensation value U is obtained through the controller with limiting oel ; ΔQ uel The voltage compensation value U is obtained by the controller with limiting uel ;

[0041] 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 ;

[0042] 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 .

[0043] Further, in step 6), the over-reactive voltage compensation value U obtained in step 5) is oel And the reactive voltage compensation value U uel , compensate the voltage reference value and control the voltage setting value, as follows:

[0044] 6.1) Calculate the voltage difference

[0045] The calculated over-reactive voltage compensation value U oel , under-reactive voltage compensation value U uel With positive sequence voltage amplitude U, voltage reference value U ref Superposition is performed 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) Get the voltage setting value through the controller

[0049] ΔU passes through a controller with a limiting value to obtain the voltage setting value U set , U set The upper limit is U set_max , the lower limit is U se t _m i n ;

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

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

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

[0053] The second object of the present invention is achieved by the following technical solution: a dynamic reactive current control system of a grid-connected converter, used to implement the above-mentioned dynamic reactive current control method of the grid-connected converter, comprising:

[0054] The signal acquisition and calculation module is used to acquire signals, including the instantaneous value of the three-phase voltage, the instantaneous value of the three-phase current and the voltage phase of the grid-connected converter on the 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. Based on the obtained voltage and current, the positive sequence voltage amplitude and reactive power are calculated.

[0055] A judgment module, which judges the low voltage ride-through state according to 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;

[0056] 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;

[0057] The second reactive power limit calculation module is used for reactive power limit calculation in the low voltage ride-through state to obtain the upper and lower limits of reactive power output;

[0058] 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 takes 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;

[0059] 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 limiting is input into the inner loop voltage / current control link of the grid-connected converter to realize voltage set value control.

[0060] The third object of the present invention is achieved through the following technical solution: a storage medium stores a program, and when the program is executed by a processor, the dynamic reactive current control method of the above-mentioned grid-connected converter is implemented.

[0061] The fourth purpose of the present invention is achieved through the following technical solution: a computing device, comprising a processor and a memory for storing processor executable programs, when the processor executes the program stored in the memory, the dynamic reactive current control method of the above-mentioned grid-connected converter is implemented.

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

[0063] The present invention limits reactive current by adding compensation control in the control of the grid-connected converter, and only requires improvement of the control algorithm without increasing additional hardware costs.

[0064] The present invention has a more direct limitation on reactive current after the grid-connected converter enters the low-breakdown state, thereby improving the reliability of the converter during the transient period.

[0065] The amplitude limiting of the present invention limits the interval in which the compensation control takes effect, will not interfere with the normal operation of the grid-connected converter, and will only play a role in limiting the reactive current during the low-voltage run-through period. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 is a control schematic diagram of the method of the present invention; in the figure, P is 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 synchronization phase, ω b is the speed reference value, ω0 is the speed reference value.

[0067] Figure 2 It is a control flow chart of the method of the present invention.

[0068] Figure 3 Schematic diagram of signal acquisition for grid-connected converter.

[0069] Figure 4 Schematic diagram of the system of the present invention. DETAILED DESCRIPTION

[0070] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0071] Example 1

[0072] This embodiment discloses a method for controlling a 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 FIG, a compensation control is added to the voltage amplitude control link of the grid-connected converter of the grid-connected type. By superimposing the voltage compensation signal on the voltage reference value, the reactive current of the grid-connected converter of the grid-connected type is 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. See FIG. Figure 2 The specific situation is as follows:

[0073] 1) Signal acquisition and calculation

[0074] See Figure 3 As shown, the instantaneous value U 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:

[0080]

[0081] 2) According to the magnitude of the positive sequence voltage amplitude obtained in step 1), the low voltage ride-through state is judged as follows:

[0082] Setting Flags LVRT It is the low voltage ride-through flag. 0 indicates normal state, 1 indicates low voltage ride-through state, and the initial value is 0.

[0083] In Flag LVRT =0, if U>=U dip , then Flag LVRT =0, it is judged as 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 enter step 4);

[0084] 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 rec , then Flag LVRT =1, still in low voltage ride-through state, go to step 4);

[0085] Among them, U dip is the low penetration voltage threshold, which is generally less than 0.9 and can be adjusted according to actual conditions; U rec It is the low wear-out exit voltage threshold, which is generally slightly larger than U dip .

[0086] 3) Calculate reactive power limit under normal conditions to obtain the 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 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;

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

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

[0092] Q min =-U max I qmax

[0093] Q max =UI R

[0094] Where, the 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 is 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 as a reference, as follows:

[0097] 5.1) Calculate the upper limit of reactive power output Q max The difference ΔQ with 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, namely, over-reactive voltage compensation and under-reactive voltage compensation, which are used for compensation when reactive power exceeds the upper limit and when reactive power is lower than the lower limit respectively;

[0102] ΔQ oelThe over-reactive voltage compensation value U is obtained through the controller with limiting oel ; ΔQ uel The voltage compensation value U is obtained by the controller with 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 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 ; In general, U 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 setting 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 setting 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 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] In the formula, the voltage reference value U ref is the set voltage reference value;

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

[0112] ΔU passes through a controller with a limiting value to obtain the voltage setting value U set The controller here can be P control, PI control or PID control. The manufacturer can choose according to the actual situation. set The upper limit is U set_max , the lower limit is U set_min ;

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

[0114] Example 2

[0115] This embodiment discloses a dynamic reactive current control system of a grid-connected converter, which is used to implement the dynamic reactive current control method of the grid-connected converter described in Example 1. Figure 4 As shown, it includes the following functional modules:

[0116] The signal acquisition and calculation module is used to acquire signals, including the instantaneous value of the three-phase voltage, the instantaneous value of the three-phase current and the voltage phase of the grid-connected converter on the 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. Based on the obtained voltage and current, the positive sequence voltage amplitude and reactive power are calculated.

[0117] A judgment module, which judges the low voltage ride-through state according to 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;

[0118] 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;

[0119] The second reactive power limit calculation module is used for reactive power limit calculation in the low voltage ride-through state to obtain the upper and lower limits of reactive power output;

[0120] 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 takes 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;

[0121] 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 limiting is input into the inner loop voltage / current control link of the grid-connected converter to realize voltage set value control.

[0122] Example 3

[0123] This embodiment discloses a storage medium storing a program. When the program is executed by a processor, the dynamic reactive current control method of the grid-connected converter of the grid-connected type described in Embodiment 1 is implemented.

[0124] The storage medium in this embodiment can be a disk, an optical disk, a computer memory, a read-only memory (ROM), a random access memory (RAM), a USB flash drive, a mobile hard disk, or other media.

[0125] Example 4

[0126] This embodiment discloses a computing device, including a processor and a memory for storing a program executable by the processor. When the processor executes the program stored in the memory, the dynamic reactive current control method of the grid-connected converter described in Example 1 is implemented.

[0127] The computing device described in this embodiment may be a desktop computer, a laptop 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 preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for controlling dynamic reactive current of a grid-connected converter, characterized in that: The method is to add compensation control in the voltage amplitude control link of the grid-connected converter, and to achieve the goal of limiting the reactive current of the grid-connected converter by superimposing a voltage compensation signal on a voltage reference value, which includes the following steps: 1) Collecting signals, including the instantaneous value of the three-phase voltage, the instantaneous value of the three-phase current and the voltage phase of the grid side of the grid-connected converter, obtaining the voltage and current in the rotating two-phase coordinate system according to the Park transformation based on the collected signals, and calculating the positive sequence voltage amplitude and reactive power based on the obtained voltage and current; 2) According to the magnitude of the positive sequence voltage amplitude obtained in step 1), a low voltage ride-through state is determined. If it is determined to be a normal state, step 3) is executed; if it is determined to be a low voltage ride-through state, step 4) is executed; 3) Calculate reactive power limit under normal conditions to obtain the upper and lower limits of reactive power output; 4) Calculate the reactive power limit under low voltage ride-through state to obtain the upper and lower limits of reactive power output; 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 to obtain an over-reactive voltage compensation value and an under-reactive voltage compensation value; 6) According to 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 limiting is input into the inner loop voltage / current control link of the grid-connected converter to realize 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 U 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:

3. The method for dynamic reactive current control of a grid-connected converter according to claim 2, characterized in that: In step 2), set Flag LVRT It is the 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 as 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 enter 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 rec , then Flag LVRT =1, still in low voltage ride-through state, go to step 4);​ Among them, U dip is the low penetration 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, characterized in that: In step 3), the reactive power limit in the normal state is calculated as follows: Q min =-U max I qmax Q max =U max I qmax In the formula, 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 works in the power factor control mode, then is the set power factor; if the grid-connected converter works in the reactive power control mode, then 5. The method for dynamic reactive current control of a grid-connected converter according to claim 4, characterized in that: In step 4), 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, the current I R The calculation method is: 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.

6. The method for dynamic reactive current control of a grid-connected converter according to claim 5, characterized in that: In step 5), 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 as a reference, as follows: 5.1) Calculate the upper limit of reactive power output Q max The difference ΔQ with 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) 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: First, voltage compensation is divided into two parts, namely, over-reactive voltage compensation and under-reactive voltage compensation, which are used for compensation when reactive power exceeds the upper limit and when reactive power is lower than the lower limit respectively; ΔQ oel The over-reactive voltage compensation value U is obtained through the controller with limiting oel ; ΔQ uel The voltage compensation value U is obtained by the controller with 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 .

7. A method for controlling dynamic reactive current of a grid-connected converter according to claim 6, characterized in that: In step 6), the reactive voltage compensation value U obtained in step 5) is oel And the reactive voltage compensation value U uel , compensate the voltage reference value and control the voltage setting 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 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 In the formula, the voltage reference value U ref is the set voltage reference value; 6.2) Get the voltage setting value through the controller ΔU passes through a controller with a limiting value to obtain the voltage setting value U set , U set The upper limit is U set_max , the lower limit is U se t _m i n ; Finally, the obtained U set The input is sent to the inner loop voltage / current control link of the grid-connected converter.

8. The method for dynamic reactive current control of a grid-connected converter according to claim 7, characterized in that: IN oel_max =0,U oel_min =-1;U oel_max =1,U oel_min =0.

9. A method for controlling dynamic reactive current of a grid-connected converter according to claim 8, characterized in that: The controller is P control, PI control or PID control.

10. A dynamic reactive current control system for a grid-connected converter, characterized in that: A method for dynamic reactive current control of a grid-connected converter for realizing any one of claims 1 to 9, comprising: The signal acquisition and calculation module is used to acquire signals, including the instantaneous value of the three-phase voltage, the instantaneous value of the three-phase current and the voltage phase of the grid-connected converter on the 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. 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 according to 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 for reactive power limit calculation in the low voltage ride-through state to obtain the upper and lower limits of 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 takes 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 limiting is input into the inner loop voltage / current control link of the grid-connected converter to realize voltage set value control.

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