Three-phase three-wire system asymmetric high-low penetrating reactive power control method

Abstract

The invention provides a three-phase three-wire system asymmetric high-low penetrating reactive power control method. The method comprises the following steps that S1, a three-phase three-wire system asymmetric high-low penetrating reactive power control system is established; obtaining the current three-phase voltage of the power grid, and converting the current three-phase voltage into a per-unit value; calculating the average value of the per-unit values of the three-phase voltages in t periods before the high-low voltage ride-through occurs, and taking the average value as a reference voltage; the method effectively solves the problems of insufficient reactive power output, weak supporting capability and the like caused by zero-sequence interference and frequency doubling fluctuation of a traditional control strategy under the asymmetric high-low voltage ride-through working condition, and remarkably improves the dynamic reactive power regulation performance of the inverter under the complex power grid fault. It is ensured that positive-sequence reactive current and negative-sequence reactive current can be accurately and sufficiently sent out at the same time in the fault period, rapid voltage recovery of a common connection point is powerfully supported, system stability is enhanced, and related requirements of existing grid connection technical specifications are comprehensively met.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy grid-connected control technology, more particularly to a control method for three-phase three-wire asymmetric high-low penetration reactive power. BACKGROUND

[0002] In the grid-connected application of new energy generation systems (such as photovoltaic and wind power), three-phase voltage source inverters, as the key electric energy conversion interface, need to have the ability of fault ride-through (FRT), especially during high voltage ride-through (HVRT) or low voltage ride-through (LVRT) of the power grid, need to inject reactive current to the system according to the grid specification to support the recovery of the grid voltage. To achieve this function, a reactive current instruction generation strategy based on voltage deviation is usually adopted.

[0003] A common engineering method is: according to the difference between the current grid phase voltage and the rated voltage (or the set reference voltage), the required reactive current effective value of each phase is calculated by multiplying the proportional coefficient k and the rated current of the inverter, Then the three-phase reactive current is synthesized and coordinate transformation (such as Clark and Park transformation) is performed to obtain the reactive current component i in the synchronous rotating dq coordinate system, which is used for subsequent current loop control.

[0004] This method has simple structure and is easy to implement. In the working condition that the grid voltage is relatively stable and has small fluctuation, it can effectively generate reasonable reactive support instructions and meet the basic reactive power demand during high-low penetration. However, this strategy has several inherent defects: The above calculation is based on three-phase independent processing, and the zero sequence component under asymmetric fault or non-ideal grid conditions is not excluded. When the three-phase voltage is unbalanced or contains significant zero sequence component, coordinate transformation of three-phase reactive current directly will introduce error, resulting in distortion of dq-axis reactive instruction.

[0005] In the process of rapid change or large drop / rise of the grid voltage, the difference between the fixed reference voltage and the instantaneous voltage cannot accurately reflect the dynamic reactive support amount required by the system, and the generated reactive current may deviate from the theoretical optimal value, or even be in the wrong direction.

[0006] Due to the interference of zero sequence component and the error of coordinate transformation, in the case that the voltage is seemingly "stable" but there is a small asymmetry, the actual injected reactive current of the inverter is much smaller than the theoretical calculation value, which is difficult to effectively support the recovery of the grid voltage and weakens the FRT performance. SUMMARY

[0007] The technical problem solved by the present application is to provide a three-phase three-wire asymmetric high-low penetration reactive power control method to avoid zero sequence interference when voltage is unstable, thereby preventing incorrect three-phase reactive power calculation and less reactive power output.

[0008] The present application provides a three-phase three-wire asymmetric high-low penetration reactive power control method, which is applied to a battery energy storage system, an electric vehicle charger or a direct current microgrid interface; the method comprises the following steps: S1: obtaining the current three-phase voltage of the power grid and converting it into a per-unit value; calculating the average value of the three-phase voltage per-unit value in the t periods before high-low voltage penetration occurs, and taking the average value as the reference voltage; S2: for each phase, multiplying the difference between the reference voltage and the current phase voltage per-unit value by a preset proportionality coefficient k and the inverter rated current to obtain the initial reactive current instruction of the phase, thereby forming the three-phase initial reactive current instruction; S3: performing amplitude limiting processing on the three-phase initial reactive current instruction: if the current amplitude of any phase exceeds the set maximum allowable current, calculating a scaling coefficient and scaling the three-phase current instruction by the same proportion so that the current of the maximum phase is equal to the maximum allowable current; S4: based on the constraint condition that the sum of the three-phase current instantaneous values is zero, selecting the two-phase currents with larger amplitudes and recalculating the current of the minimum one-phase according to the two-phase currents with larger amplitudes to eliminate the zero sequence component introduced by asymmetric calculation, thereby obtaining the three-phase reactive current instruction that satisfies Kirchhoff's current law and does not contain zero sequence components; S5: performing twice sampling and coordinate transformation on the corrected three-phase current instruction: a) the first sampling is performed at the power grid fundamental phase angle θ=0, Clark transformation and Park transformation are performed to obtain the dq-axis current components 、 ; b) the second sampling is performed at the power grid fundamental phase angle θ=π / 2, Clark transformation and Park transformation are performed again to obtain the dq-axis current components 、 ; S6: taking the arithmetic mean of the two transformation results to obtain the final dq-axis current given value , , to eliminate the two-frequency alternating disturbance generated by the Park transformation of the negative sequence component and retain the stable direct current component; S7: based on the maximum output current capacity of the inverter and the upper limit of the q-axis current, dynamically limiting the 、 to generate positive sequence reactive power, negative sequence reactive power and positive sequence active current instructions that satisfy the power constraint. S8; The and The current loop setpoint is input to the d-axis and q-axis PI controllers, which output modulated voltage signals to drive the inverter. Simultaneously, the actual three-phase current output by the inverter is fed back to the coordinate transformation module. After Clark-Park transformation, it is compared with the setpoint to form a closed-loop control until the output current accurately tracks the command.

[0009] In the three-phase three-wire unbalanced high-low current control method of the present invention; in step S2, the first Initial reactive current command of phase Calculate using the following formula: , ; in The reference voltage, This represents the per-unit value of the current i-th phase voltage. This is the rated current of the inverter.

[0010] In the three-phase three-wire asymmetrical high-low power transmission control method of the present invention, the proportional coefficient k in step S2 is dynamically adjusted according to the requirements of the grid connection technical specifications or the reactive power support capability of the inverter under the current operating conditions, and the value range is 0.1 to 6.0.

[0011] In the three-phase three-wire unbalanced high-low current reactive power control method of the present invention; in step S2, when the reactive current of a certain phase exceeds the set maximum current, the three-phase current is reduced proportionally, specifically by the following formula: ; ; ; ; in It is the largest current value among the three phases.

[0012] In the three-phase three-wire unbalanced high-low current control method of the present invention; in step S4, if the three-phase initial reactive current command satisfies Then let This forces the sum of the instantaneous values ​​of the three-phase currents to be zero, thus eliminating the zero-sequence component.

[0013] In the three-phase three-wire asymmetrical high-low current control method of the present invention, the Clark transformation in step S5 adopts the standard α and β transformation matrix, specifically:

[0014] Where a, b, and c represent the three-phase coordinate system

[0015] In the three-phase three-wire asymmetric high-low penetration reactive power control method, the Park transformation in the step S5 is based on the fundamental phase angle θ output by the grid voltage phase-locked loop to realize the synchronous rotating coordinate system conversion, and the following formula is adopted: , ; θ is the fundamental phase angle of the grid voltage.

[0016] In the three-phase three-wire asymmetric high-low penetration reactive power control method, the two samplings in the step S5 correspond to the time points with phase angles of 0 and π / 2 in the same grid fundamental period, and the interval between the two samplings is one-fourth of the fundamental period, which is used to capture the symmetric extreme points of the two-frequency disturbance caused by the negative sequence component.

[0017] In the three-phase three-wire asymmetric high-low penetration reactive power control method, the average of the two Park transformation results in the step S5 is obtained as and satisfy: ,

[0018] Among them, the two-frequency alternating component generated by the negative sequence component is completely offset, and only the direct current component corresponding to the positive sequence and negative sequence current is reserved.

[0019] In the three-phase three-wire asymmetric high-low penetration reactive power control method, the d-axis current instruction is subjected to amplitude limiting processing in the step S7 to meet the maximum current output capability constraint of the inverter, and the specific constraint is: ; At the same time, the q-axis current instruction satisfies: , wherein, is the maximum output current amplitude allowed by the inverter, and is the upper limit value of the q-axis reactive current set according to the grid connection specification during high-low voltage penetration.

[0020] The three-phase three-wire asymmetric high-low penetration reactive power control method of the application effectively avoids the distortion of the reactive power command caused by voltage mutation or transient fluctuation by using the average value of the voltage standard value in the t fundamental period before the fault as the reference voltage; at the same time, the constraint mechanism based on the instantaneous sum of three-phase current being zero is introduced, and the minimum one-phase current is reconstructed by using the two-phase current with larger amplitude, so that the zero sequence component generated in the calculation process is completely eliminated. On this basis, further Clark-Park coordinate transformation is carried out at two moments of fundamental phase angle θ=0 and θ=π / 2, and the arithmetic average of the obtained d-axis and q-axis currents is taken to eliminate the twice frequency disturbance caused by the Park transformation of the negative sequence component.

[0021] The method effectively solves the problems of insufficient reactive power output and weak support capacity caused by zero sequence interference and twice frequency fluctuation under the asymmetric high-low voltage penetration working condition of the traditional control strategy, significantly improves the dynamic reactive power regulation performance of the inverter under complex grid fault, ensures that the inverter can accurately and fully output positive sequence reactive current and negative sequence reactive current at the same time during the fault, effectively supports the rapid recovery of the point of common coupling voltage, enhances the system stability, and fully meets the relevant requirements of the current grid connection technical specification. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a flowchart of an embodiment of the three-phase three-wire asymmetric high-low penetration reactive power control method of the application. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme and advantages of the application more clear and explicit, the application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application, and do not limit the application.

[0024] It should be noted that the terms "first", "second" and the like in the specification and claims of the application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to only those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to the process, method, product or device.

[0025] As Figure 1The diagram shown is a flowchart illustrating an embodiment of a three-phase three-wire asymmetrical high-low current control method according to the present invention. The method provides a three-phase three-wire asymmetrical high-low current control method applicable to battery energy storage systems, electric vehicle chargers, or DC microgrid interfaces; characterized in that the method includes the following steps: In step S1: Obtain the current three-phase voltage of the power grid and convert it to per-unit value; calculate the average value of the per-unit value of the three-phase voltage within t cycles before the high-low voltage crossover occurs, and use this average value as the reference voltage; In step S2; for each phase, the difference between the reference voltage and the current phase voltage per unit value is multiplied by a preset proportional coefficient k and the inverter rated current to obtain the initial reactive current command for that phase, thereby forming a three-phase initial reactive current command. In step S3, the initial reactive current command of the three phases is subjected to amplitude limiting: if the current amplitude of any phase exceeds the set maximum allowable current, the scaling factor is calculated, and the three phase current commands are scaled by the same proportion so that the current of the largest phase is equal to the maximum allowable current. In step S4; based on the constraint that the sum of the instantaneous values ​​of the three-phase currents is zero, the two-phase currents with larger amplitudes are selected, and the current of the smallest phase is recalculated based on the two-phase currents with larger amplitudes to eliminate the zero-sequence component introduced by the asymmetrical calculation, so as to obtain a three-phase reactive current command that satisfies Kirchhoff's current law and does not contain zero-sequence components. In step S5, the corrected three-phase current command is sampled and transformed twice: a) The first sampling is performed when the fundamental phase angle of the power grid is θ=0. Clark and Park transforms are then performed to obtain the dq-axis current components. , ; b) The second sampling is performed when the fundamental phase angle of the power grid is θ = π / 2. Clark and Park transformations are performed again to obtain the dq-axis current components. , ; In step S6, the arithmetic mean of the two transformation results is taken to obtain the final dq-axis current setpoint. , This is to eliminate the second harmonic AC disturbance generated by the negative sequence component after Park transformation, and retain the stable DC component. In step S7; based on the inverter's maximum output current capability and q-axis current limit, the... , Dynamic limiting is performed to generate positive-sequence reactive, negative-sequence reactive, and positive-sequence active current commands that meet power constraints. In step S8; the... and The current loop given value is input to the d, q axis PI controller, and the PI controller outputs a modulation voltage signal to drive the inverter; at the same time, the three-phase current actually output by the inverter is fed back to the coordinate transformation module, compared with the given value after Clark-Park transformation, to form a closed-loop control, until the output current accurately tracks the instruction

[0026] In an embodiment, in the step S2, the initial reactive current instruction of the i-th phase is calculated according to the following formula: The initial reactive current instruction of the i-th phase is calculated according to the following formula:

[0027] In an embodiment, in the step S2, the proportional coefficient k is dynamically adjusted according to the requirements of the grid interconnection technical specification or the reactive power support capability of the inverter under the current operating condition, and the value range is 0.1 to 6.0.

[0028] In an embodiment, in the step S2, when the reactive current of a certain phase exceeds the set maximum current, the three-phase currents are proportionally reduced, and the specific formula is as follows:

[0029] In an embodiment, in the step S4, if the sum of the initial reactive current instructions of the three phases meets the condition , then , so as to force the sum of the instantaneous values of the three-phase currents to be zero and eliminate the zero sequence component.

[0030] In an embodiment, in the step S5, the Clark transformation adopts a standard α, β transformation matrix, and the specific formula is as follows:

[0031] Wherein, a, b, and c represent the three-phase coordinate system

[0032] In an embodiment, in the step S5, the Park transformation is based on the fundamental phase angle θ output by the grid voltage phase-locked loop to realize synchronous rotating coordinate system conversion, and the following formula is adopted: ​​​​​​​​​​​​​​​Wherein, θ is the fundamental phase angle of grid voltage.

[0033] In an embodiment, the two sampling times in step S5 correspond to the time points with phase angle 0 and π / 2 respectively in the same fundamental period of grid, and the interval between the two sampling times is one quarter of the fundamental period, which is used to capture the symmetric extreme point of the twice-frequency disturbance caused by negative sequence component.

[0034] In an embodiment, the average of the two Park transformation results in step S5 is and satisfies: ,

[0035] wherein the twice-frequency AC component caused by negative sequence component is completely offset, and only the DC component corresponding to positive sequence and negative sequence current is reserved.

[0036] In an embodiment, the d-axis current instruction in step S7 is limited to satisfy the maximum current output capability constraint of inverter, specifically: ; Meanwhile, the q-axis current instruction satisfies: , wherein, is the maximum output current amplitude allowed by inverter, wherein is the upper limit value of q-axis reactive current set according to grid connection specification during high-low voltage ride-through.

[0037] This embodiment takes a three-phase grid-connected inverter with rated power of 1MW as an example, whose rated line voltage is 690V, rated phase current , maximum allowed output current

[0038] , and current loop given value . When the grid occurs asymmetric low voltage ride-through (LVRT) fault, the A-phase voltage drops to 0.2pu, the B-phase and C-phase voltages are 0.85pu and 0.9pu respectively, and the duration is 300ms.

[0039] The three-phase grid voltage , , is collected in real time, and is converted into per-unit value , , after phase-locked loop (PLL) synchronization.Simultaneously, calculate the moving average of the per-unit values ​​of the three-phase voltages within t=5 fundamental cycles (i.e., 100ms, corresponding to a 50Hz system) before the high-low voltage ride-through occurs: Where N is the number of sampling points per cycle. In this application, the power grid was normal before the crossing, therefore... .

[0040] Set the scaling factor 1.5 (Dynamically set according to GB / T19964-2012 "Technical Regulations for Photovoltaic Power Stations Connected to Power Systems"), calculate the initial reactive current command for each phase using the following formula: ; ; .

[0041] because ; After scaling , , .

[0042] Under asymmetrical faults, the sum of the instantaneous values ​​of the three-phase currents calculated directly is not zero (a zero-sequence component exists), and Kirchhoff's current law must be enforced. Sort the instantaneous values ​​of the three-phase currents by absolute value. According to Kirchhoff's laws, the instantaneous sum of the three-phase currents is 0, that is... ,like Minimum, can be obtained .

[0043] Since the two samples are taken at 0 degrees and 90 degrees, Taking 0 and 90 degrees, the two angles are calculated. The instantaneous current value can be obtained. Spend, . Spend, .

[0044] Will and [1004* , * , - Perform Clark-Park transformations on each.

[0045] Park transformation (for example, θ=0): , Similarly, calculate when θ = π / 2 , Take the average: , The second harmonic disturbance caused by negative sequence component is opposite in polarity at two points, and is completely canceled after averaging, leaving only the positive / negative sequence corresponding DC component.

[0046] Let the second harmonic component caused by negative sequence be =A Then: When , ; When , ; after averaging , the second harmonic component is completely canceled. Finally, the pure DC dq component is obtained, which only contains positive sequence and negative sequence current information.

[0047] Assuming that the preliminary , is obtained, check whether the following conditions are met: ; and ; (set according to the specification), so there is no need for further clipping. The final instruction contains positive sequence reactive power (dominant), negative sequence reactive power and a small amount of positive sequence active power (caused by ).

[0048] Input , to the d, q axis PI controller to generate the modulation voltage , , which is driven by the inverse Park transformation and SVPWM modulation. The actual output current is sampled by the sensor and fed back to the Clark-Park module to form a closed loop, ensuring that the output current accurately tracks the instruction.

[0049] After being controlled by the application, the inverter output current during asymmetric low penetration does not contain zero sequence component, and the positive and negative sequence reactive current reaches more than 98% of the theoretical demand value. The negative sequence reactive current effectively suppresses the point of common coupling (PCC) voltage unbalance degree, supports the rapid recovery of the grid voltage, and meets the requirements of GB / T36963-2018 "Technical Regulation for Wind Power Integration into Power System" on asymmetric fault penetration.

[0050] It should be noted that "t cycles" can be adjusted according to the system response speed, and the typical value is 1-10 base wave periods; the proportional coefficient k can be adjusted online to adapt to different grid strength or fault depth; The application is also applicable to high voltage ride through (HVRT) scenarios, and only the reactive power direction needs to be adjusted; the phase angle θ in the Clark-Park transformation is provided by a high-precision phase-locked loop (such as DSOGI-PLL), ensuring accurate synchronization even under voltage distortion.

[0051] It should be noted that, for the foregoing method embodiments, for the sake of simple description, they are all expressed as a series of action combinations, but those skilled in the art should know that the present application is not limited by the action sequence described, because according to the present application, certain steps can be performed in other sequences or simultaneously. Secondly, those skilled in the art should know that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily necessary for the present application.

[0052] Those skilled in the art can clearly understand the method according to the above-mentioned embodiments can be realized by means of software and the necessary general hardware platform, of course, it can also be realized by hardware, but in many cases the former is a better implementation. Based on such understanding, the technical solutions of the present application can be embodied in the form of software products, and the computer software product is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk), including a plurality of instructions to make a terminal device (which can be a mobile phone, computer, server, or network device, etc.) execute the method described in each embodiment of the present application.

[0053] Therefore, the above description is only a preferred embodiment of the present application, the protection scope of the present application is not limited to this, any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A control method of three-phase three-wire asymmetric high-low reactive power, characterized in that, The method comprises the following steps: S1: obtaining the current three-phase voltage of the power grid and converting it into a unit value; calculating the average value of the three-phase voltage unit value in the t periods before the occurrence of high and low voltage ride-through, and taking the average value as the reference voltage; S2: for each phase, multiply the difference between the reference voltage and the current phase voltage unit value by the preset proportionality coefficient k and the inverter rated current to obtain the initial reactive current instruction of the phase, thereby forming the three-phase initial reactive current instruction; S3: limiting the three-phase initial reactive current instruction: if the current amplitude of any phase exceeds the set maximum allowable current, calculate the scaling factor and scale the three-phase current instruction by the same proportion, so that the current of the maximum phase is equal to the maximum allowable current; S4: based on the constraint condition that the sum of the three-phase current instantaneous values is zero, select the two-phase currents with larger amplitudes, and recalculate the current of the minimum one-phase according to the two-phase currents with larger amplitudes, to eliminate the zero sequence component introduced by asymmetric calculation, and obtain the three-phase reactive current instruction that satisfies Kirchhoff's current law and does not contain zero sequence component; S5: perform twice sampling and coordinate transformation on the corrected three-phase current instruction: a) The first sampling is at the grid fundamental phase angle θ = 0, the Clark transformation and Park transformation are carried out to obtain the dq axis current components 、 ; b) second sampling at grid fundamental phase angle θ = π / 2, Clark and Park transformation again to get dq axis current components 、 ; S6; take the arithmetic mean of the two transformation results to obtain the final dq-axis current given value , , to eliminate the twice frequency AC disturbance generated by the Park transformation of the negative sequence component and retain the stable DC component; S7; Based on the inverter's maximum output current capability and q-axis current limit, for the... , Dynamic limiting is performed to generate positive-sequence reactive, negative-sequence reactive, and positive-sequence active current commands that meet power constraints. S8; said and As the current loop given value input to the d, q axis PI controller, PI controller output modulation voltage signal drive inverter; at the same time, the actual output of the three-phase current feedback to the coordinate transformation module, after Clark-Park transformation and given value comparison, constitute a closed loop control, until the output current accurate tracking instruction.

2. The control method of three-phase three-wire asymmetric high-low reactive power of claim 1, characterized in that, In the step S2 the first The initial reactive current command of the phase is calculated as follows: , ; wherein is the reference voltage, is the current i-th phase voltage norm, is the inverter rated current.

3. The control method of three-phase three-wire asymmetric high-low reactive power of claim 2, characterized in that, In the step S2, the proportionality coefficient k is dynamically adjusted according to the requirements of the grid interconnection technical specification or the reactive power support capability of the inverter under the current operating condition, and the value range is 0.1 to 6.

0.

4. The control method of three-phase three-wire asymmetric high-low reactive power of claim 3, characterized in that, In step S2, when the reactive current of a certain phase exceeds the set maximum current, the three-phase current is reduced by the same proportion, and the specific formula is: ; ； ； ； wherein is the largest current value in the three-phase.

5. The method of claim 1, wherein, In step S4, if the three-phase initial reactive current command satisfies then let so that the sum of the three-phase current instantaneous values is zero, eliminating the zero sequence component.

6. The control method of three-phase three-wire asymmetric high-low reactive power of claim 5, characterized in that, In the step S5, the Clark transformation adopts the standard α, β transformation matrix, which is: Wherein, a, b, c represent the three-phase coordinate system.

7. The control method of three-phase three-wire asymmetric high-low reactive power of claim 6, characterized in that, In the step S5, the Park transformation realizes synchronous rotating coordinate system conversion based on the fundamental phase angle θ output by the grid voltage phase-locked loop, and adopts the following formula: ， ； Wherein, θ is the fundamental phase angle of the grid voltage.

8. The control method of three-phase three-wire asymmetric high-low reactive power of claim 7, characterized in that, In the step S5, the two samplings correspond to the time points of phase angle 0 and π / 2 in the same grid fundamental period, and the interval between the two samplings is one-fourth of the fundamental period, which is used to capture the symmetric extreme points of the two-frequency disturbance caused by the negative sequence component.

9. The control method of three-phase three-wire asymmetric high-low reactive power of claim 8, characterized in that, The result of averaging the two Park transform results in the step S5 is and satisfies: , Wherein, the two-frequency alternating component generated by the negative sequence component is completely offset, and only the direct current component corresponding to the positive sequence and negative sequence current is retained.

10. The control method of three-phase three-wire asymmetric high-low reactive power of claim 1, characterized in that, In step S7, the d-axis current command is given. Limiting is performed to meet the inverter's maximum current output capability constraint, specifically: ； Meanwhile, the q-axis current command satisfies: ， wherein, is the maximum output current amplitude allowed for the inverter, wherein is the upper limit of q-axis reactive current set according to grid code during high-low voltage ride through.

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