An unbalanced current compensation control method and system with limiting function

Abstract

The present application relates to the field of power system protection and control, in particular to a kind of unbalanced current compensation control method and system with limiting amplitude function, the method includes: considering a section of rural area in low-voltage distribution network with three-phase grid-connected inverter, measure the voltage of current three-phase grid-connected inverter and grid common connection point and the output current of inverter, and separate the positive sequence, negative sequence and zero sequence components in the voltage and current;According to the positive sequence, negative sequence and zero sequence components in the measured voltage and current, design load unbalanced current compensation strategy, so as to calculate the negative sequence, zero sequence reference current value that needs to be compensated;According to the maximum amplitude of the output current of three-phase grid-connected inverter, the negative sequence, zero sequence reference current value that needs to be compensated is limited amplitude processing.The present application designs the distribution method of negative sequence and zero sequence current compensation amount under the condition of limited inverter capacity, and builds simulation model through Matlab / Simulink platform, demonstrates its performance and effectiveness.

Description

Technical Field

[0001] This invention relates to the field of power system protection and control, specifically to an unbalanced current compensation control method and system with amplitude limiting function. Background Technology

[0002] In rural power grids, a large number of single-phase loads are connected. Due to the uneven distribution of single-phase loads in the three-phase system and the randomness of single-phase load electricity consumption, three-phase power imbalance is common in rural power grids. This power imbalance leads to unbalanced current in the grid, causing greater energy loss and three-phase voltage imbalance, reducing the power quality of the distribution network and affecting the normal operation of electrical equipment connected to the grid. To improve power quality, it is necessary to compensate for the unbalanced current caused by unbalanced loads in the grid.

[0003] Existing technologies employ series power quality regulators to compensate for unbalanced voltage in the system, or parallel power quality regulators to compensate for unbalanced current introduced by unbalanced loads, both of which effectively solve the problem of unbalanced current caused by unbalanced loads. However, the additional power quality management equipment increases investment costs and system size. With the large-scale penetration of distributed renewable energy into the power grid, a large number of grid-connected inverters are being connected to the grid. Therefore, many studies have proposed control methods to compensate for unbalanced current, starting from the control of the grid-connected inverters themselves.

[0004] For grid-connected inverters, their core function is grid-connected control. Therefore, their compensation function should not affect their normal grid-connected operation. This makes the compensation current that the inverter can output limited. Furthermore, the compensation of unbalanced current makes the three-phase current of the grid-connected inverter unbalanced. It is not possible to simply limit the amplitude of each phase current. It is necessary to allocate and limit the zero-sequence current and negative-sequence current that it compensates.

[0005] However, current solutions for unbalanced current compensation caused by unbalanced loads typically only add the required unbalanced current to the grid-connected reference current, without considering the limitations of the inverter's output capacity. Therefore, a three-phase unbalanced load compensation control strategy is needed to compensate for the unbalanced load current without affecting the normal positive-sequence operation of the grid-connected inverter.

[0006] The invention patent with publication number CN114884118A discloses an electrified railway traction power supply system and its photovoltaic power generation phase current control method. It can control positive and negative sequence currents without phase sequence separation. At the same time, under the condition of ensuring the priority of photovoltaic active power, it can use the converter margin to compensate for the negative sequence current generated by single-phase traction load, realizing the unified control of photovoltaic grid-connected power generation and negative sequence compensation. It can reduce the negative sequence current on the high-voltage side and improve power quality. However, firstly, this application is for a railway power supply system, which is a three-phase three-wire system, unlike the three-phase four-wire system of rural power grids, and there is no zero sequence current. Secondly, the connection method of the railway power supply system is fixed, and they are all phase-separated connections. The negative sequence current that needs to be compensated can be directly calculated from the measured positive sequence current, which is not very accurate.

[0007] Chinese patent application CN118646034A discloses a three-phase power decoupling control method and related apparatus based on a three-phase four-arm photovoltaic grid-connected inverter. Applied to a topology based on a three-phase four-arm photovoltaic grid-connected inverter, the method includes: power compensation for the three phases of the low-voltage distribution network based on the difference between the transmission power of the photovoltaic array and the output power of the three phases of the low-voltage distribution network; current compensation for the three phases of the low-voltage distribution network based on the power compensation value and the voltage value at the common connection point between the three-phase four-arm photovoltaic inverter and the low-voltage distribution network; and current compensation... The components are decomposed into positive and negative zero-sequence components to obtain three-phase positive and negative zero-sequence current components. The positive and negative zero-sequence current components are then transformed using the abc / dq0 coordinate system to obtain three-phase DC current components. For the three-phase four-arm photovoltaic inverter, a feedforward compensation is introduced into the inner-loop current controller based on a PI link in the synchronous rotating coordinate system to eliminate coupling terms in the synchronous rotating coordinate coefficient mathematical model, resulting in the output current of independent decoupled control. The output current is then transformed using the dq0 / abc system and summed, and 3D-SVPWM modulation is used to obtain the drive signal for the switching transistors of the three-phase four-arm inverter. Although this application also proposes the concept of current compensation, firstly, it requires sampling of the grid current, and secondly, it assumes that the compensation power is sufficiently large without power limiting, which is not realistic. Summary of the Invention

[0008] Purpose of the invention: In order to solve the technical problem of three-phase unbalanced load in rural power grids, the present invention provides an unbalanced current compensation control method and system with amplitude limiting function.

[0009] Technical solution: To achieve the above objectives, the present invention is implemented through the following technical solution:

[0010] An unbalanced current compensation control method with limiting function, the method comprising:

[0011] Consider a low-voltage distribution network in a rural area connected to a three-phase grid-connected inverter. Measure the voltage at the common connection point between the three-phase grid-connected inverter and the grid, as well as the output current of the inverter, and separate the positive-sequence, negative-sequence, and zero-sequence components of the voltage and current.

[0012] Based on the positive-sequence, negative-sequence, and zero-sequence components in the measured voltage and current, a load unbalanced current compensation strategy is designed, thereby calculating the negative-sequence and zero-sequence reference current values ​​that need to be compensated.

[0013] The negative sequence and zero sequence reference current values ​​that need to be compensated are limited based on the maximum amplitude of the output current of the three-phase grid-connected inverter. Then, the corresponding switching signals are obtained after being modulated by the positive, negative and zero sequence controllers and the sinusoidal pulse width modulation (SPWM).

[0014] Furthermore, including:

[0015] The separation of the positive-sequence, negative-sequence, and zero-sequence components in the voltage and current includes:

[0016] Under unbalanced load conditions, based on the symmetrical component method, the system components are decomposed into positive-sequence, negative-sequence, and zero-sequence components, which are independent of each other and controlled independently. The expression for this is:

[0017]

[0018] Where L1 is the inverter-side filter inductor, L2 is the grid-side filter inductor, and i d i q i0 and u are the inverter output currents in the dq0 rotating coordinate system, respectively. d u q The inverter output voltages u and u0 are respectively located in the dq0 rotating coordinate system. pccd ,u pccq ,u pcc0 Let ω be the voltage at the point of common connection in the rotating coordinate system dq0, and let ω be the fundamental frequency. The superscripts + and - in the above formula represent the positive and negative sequence components, respectively.

[0019] Furthermore, including:

[0020] The design incorporates a load imbalance current compensation strategy, which calculates the negative-sequence and zero-sequence reference current values ​​that need to be compensated, including:

[0021] For a three-phase grid-connected inverter, if the negative-sequence and zero-sequence components of its output current can completely compensate for the negative-sequence and zero-sequence components of the load current, then the grid-side current contains only the positive-sequence component, and the three-phase current on the grid side is balanced; otherwise...

[0022] Since the grid-connected inverter on the user side cannot directly use the negative sequence and zero sequence components of the load current as reference values ​​for the output negative sequence and zero sequence current of the three-phase grid-connected inverter, a compensation strategy is adopted to obtain the negative sequence and zero sequence reference current values ​​that need to be compensated.

[0023] Furthermore, including:

[0024] The method of obtaining the negative-sequence and zero-sequence reference current values ​​that need to be compensated using the compensation strategy includes:

[0025] Considering that the load dq-axis current is a DC component under steady state, the inverter output negative sequence reference current value is calculated by the following formula:

[0026]

[0027] The superscript * indicates the reference current, L line The grid-side impedance is determined by measuring the voltage. With current Generate inverter output reference current value This allows the negative sequence component of the inverter output current to follow the negative sequence component of the unbalanced load, thus achieving compensation for the unbalanced load.

[0028] Zero-sequence current is an alternating current, controlling u pcc0 When the value is 0, the output zero-sequence current i0 of the three-phase grid-connected inverter is used to fully compensate for the zero-sequence current of the unbalanced load. Therefore, the output of the voltage outer loop controller is directly used as the reference value of the zero-sequence current inner loop controller.

[0029] Furthermore, including:

[0030] The step of limiting the zero-sequence reference current value to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter includes:

[0031] The maximum amplitude of the output current is obtained based on the three-phase output current of the three-phase grid-connected inverter.

[0032] The maximum compensation value of the zero-sequence current is obtained based on the maximum amplitude of the output current, and then the result after limiting the zero-sequence reference current value is obtained.

[0033] Furthermore, including:

[0034] The maximum amplitude of the output current obtained from the three-phase output current of the three-phase grid-connected inverter is expressed as follows:

[0035] I max =C max +I0=A + +A - +I0;

[0036] Among them, A+ A - I0 and I0 are the amplitudes of the positive, negative, and zero-sequence currents, respectively;

[0037] C max for The maximum value of , where x = a, b, c;

[0038] If the rated output current amplitude of the inverter is I N To ensure the inverter functions properly, we have I max ≤I N .

[0039] Furthermore, including:

[0040] The process of obtaining the maximum compensation value of the zero-sequence current based on the maximum amplitude of the output current, and then obtaining the result after limiting the zero-sequence reference current value, includes:

[0041] The maximum compensation value of the zero-sequence current is expressed as:

[0042] I 0max =I max -A + ;

[0043] At this time, A - If the value is zero, the result after limiting the zero-sequence reference current value is expressed as:

[0044]

[0045] in, This is the reference amplitude of the zero-sequence current. This is the zero-sequence reference current value after limiting. This is the reference value for the output current at the 0-axis.

[0046] Furthermore, including:

[0047] The negative sequence reference current value requiring compensation is limited based on the maximum amplitude of the output current of the three-phase grid-connected inverter, including:

[0048] If I0 < I 0max If the inverter still has current margin to compensate for negative sequence current, then the maximum compensation value for negative sequence current is:

[0049] A -max =I max -A + -I0.

[0050] Furthermore, including:

[0051] The limitation processing of the negative sequence reference current value to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter is expressed as follows:

[0052]

[0053] in, This is the reference amplitude for the negative sequence current. This is the d-axis negative sequence reference current value after limiting. This is the negative sequence reference current value for the q-axis after limiting.

[0054] On the other hand, the present invention also provides an unbalanced current compensation control system with limiting function, the system comprising:

[0055] The component separation module is used to first consider a low-voltage distribution network in a rural area with a three-phase grid-connected inverter, measure the voltage at the common connection point between the current three-phase grid-connected inverter and the grid, as well as the output current of the inverter, and separate the positive sequence, negative sequence, and zero sequence components in the voltage and current.

[0056] The compensation module is used to design a load unbalanced current compensation strategy based on the positive sequence, negative sequence, and zero sequence components in the measured voltage and current, thereby calculating the negative sequence and zero sequence reference current values ​​that need to be compensated.

[0057] The limiting module is used to limit the negative sequence and zero sequence reference current values ​​that need to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter. After being modulated by the positive, negative and zero sequence controllers and the sinusoidal pulse width modulation (SPWM), the corresponding switching signals are obtained.

[0058] Compared with existing technologies, it has the following advantages:

[0059] This invention focuses on the mathematical model of grid-connected inverters, unbalanced current compensation methods, reference current calculation, compensation current allocation, and limiting. It proposes a current compensation method considering the amplitude limitation of inverter output current, designs a method for allocating negative-sequence and zero-sequence current compensation amounts under limited inverter capacity, and demonstrates its performance and effectiveness through a simulation model built on the Matlab / Simulink platform. This method achieves balanced control of the grid-side current using current margin without affecting the normal grid connection of the inverter. The current margin is the current remaining after subtracting the positive-sequence grid-connected output from the rated output current; it is the margin available for compensating the negative-sequence and zero-sequence components of the load current, thereby balancing the three-phase current on the grid side and improving grid-side power quality. Specifically, because the inverter outputs negative-sequence and zero-sequence compensation currents, the unbalanced portion of the load current is borne by the inverter, thus balancing the grid-side current; the imbalance on the grid side is essentially transferred to the inverter side. Meanwhile, this method does not require measuring the load current and generates negative-sequence and zero-sequence reference currents using the voltage at the point of common coupling, making it highly practical and feasible for distributed generation equipment in rural power grids. Attached Figure Description

[0060] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0061] Figure 1 This is a flowchart of the unbalanced current compensation control method with amplitude limiting function according to an embodiment of the present invention;

[0062] Figure 2 This is a schematic diagram of the grid-connected inverter topology according to an embodiment of the present invention;

[0063] Figure 3 This is a control flowchart of the unbalanced current compensation control method with limiting function according to an embodiment of the present invention;

[0064] Figure 4 This is a block diagram of the unbalanced current compensation control according to an embodiment of the present invention;

[0065] Figure 5 These are the negative-sequence part control structure diagram and the zero-sequence part control structure diagram with amplitude limiting function as described in the embodiments of the present invention;

[0066] Figure 6A schematic diagram of the inverter's output current, grid-side current, grid-side current sequence component, and grid-side current imbalance during the full compensation described in this embodiment of the invention.

[0067] Figure 7 This is a schematic diagram showing the inverter's output current, grid-side current, grid-side current sequence component, and grid-side current imbalance when the negative sequence is not fully compensated as described in this embodiment of the invention.

[0068] Figure 8 This is a schematic diagram of the inverter's output current, grid-side current, grid-side current sequence component, and grid-side current imbalance when only zero-sequence incomplete compensation for the load current is possible, as described in an embodiment of the present invention. Detailed Implementation

[0069] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0070] This invention addresses three-phase unbalanced loads in rural power grids. Utilizing the spare capacity of existing three-phase four-wire grid-connected inverters, it estimates the unbalanced current based solely on the inverter's sampled point of common coupling voltage and designs a limited-power three-phase unbalanced load compensation control strategy. Grid-connected inverters are generally used for renewable energy grid-connected power generation, but they typically do not operate at full power. Their spare capacity can be used to compensate for three-phase unbalanced current, but this spare capacity is usually small and uncertain, generally insufficient to fully compensate for the unbalanced current. Therefore, this invention proposes an unbalanced current compensation method with limited inverter output current. This method allows the inverter to compensate for the negative-sequence and zero-sequence currents caused by the unbalanced load as much as possible within the rated output current amplitude range, thereby balancing the three-phase current on the grid side.

[0071] Specifically, such as Figure 1 As shown, the present invention provides an unbalanced current compensation control method with limiting function, the method comprising the following steps:

[0072] S1 considers a low-voltage distribution network in a rural area connected to a three-phase grid-connected inverter. Measure the voltage at the common connection point between the three-phase grid-connected inverter and the grid, as well as the inverter's output current, and separate the positive-sequence, negative-sequence, and zero-sequence components of the voltage and current.

[0073] In this embodiment, as Figure 2As shown, it is a topology diagram of a grid-connected inverter, showing the voltage at the common connection point between the grid-connected inverter and the power grid, and the output current of the inverter, and separating the positive and negative zero-sequence components in the voltage and current.

[0074] The point of common connection (PCC) refers to the connection point where a user system (power generation or power consumption) connects to the public power grid. It is usually a shared node for more than one user load in the power system, and its definition is clearly defined in national standards and power quality specifications.

[0075] Rural power grids are mostly low-voltage distribution networks, and the three-phase grid-connected inverters typically use a three-phase four-wire system. At the fundamental frequency, the inverter output voltage equation can be transformed to the dq0 rotating coordinate system using the Park transformation. Its expression is:

[0076]

[0077] Under unbalanced load conditions, based on the symmetrical component method, the system components are decomposed into positive-sequence, negative-sequence, and zero-sequence components. These components are independent of each other and can be controlled independently. The expression is:

[0078]

[0079] In the formula, L1 is the inverter-side filter inductor, and C f L1 is the filter capacitor, L2 is the grid-side filter inductor, and i d i q i0 is the inverter output current, u pccd u pccq u pcc0 ω is the voltage at the point of common coupling, ω is the fundamental frequency, and the superscripts + and - represent the positive and negative sequence components, respectively.

[0080] This invention first measures the voltage at the point of common coupling and the inverter output current, and separates the positive and negative zero-sequence components for the sequential control of the inverter.

[0081] S2 designs a load unbalanced current compensation strategy based on the positive-sequence, negative-sequence, and zero-sequence components in the measured voltage and current, thereby calculating the negative-sequence and zero-sequence reference current values ​​that need to be compensated.

[0082] Traditional load imbalance current compensation methods often require extracting the load current to generate the compensation current component, which limits their application in distributed generation systems and does not take into account the limitations of inverter output capacity. In order to achieve load current compensation using limited inverter capacity, researchers need to allocate and limit the zero-sequence and negative-sequence currents used for compensation.

[0083] This invention designs a control block diagram for load unbalanced current compensation strategy, using a PI controller to achieve current control under the positive and negative sequence dq axes, and a PR controller to control the zero sequence current.

[0084] First, the inverter output current and the voltage at the point of common coupling are measured. After positive, negative, and zero-sequence separation, the positive, negative, and zero-sequence voltages and currents under the dq0 axis are obtained. The positive, negative, and zero-sequence output currents of the inverter are controlled separately in the dq0 coordinate system. The positive-sequence reference current is given by the grid-connected inverter's own control strategy, while the negative and zero-sequence reference currents are obtained through reference current calculation and limiting methods. These are then modulated by the positive, negative, and zero-sequence controllers and sinusoidal pulse width modulation (SPWM) to obtain the switching signals. The control block diagram is shown below. Figure 3 and 4 As shown.

[0085] This invention focuses on the method for generating the inverter compensation reference current value, and the method for allocating negative sequence current and zero sequence current when the compensation capacity is limited.

[0086] For grid-connected inverters, if the negative-sequence and zero-sequence components of their output current can completely compensate for the negative-sequence and zero-sequence components of the load current, then the grid-side current contains only the positive-sequence component, and the three-phase current on the grid side is balanced. However, grid-connected inverters on the user side often have difficulty measuring the load current, and therefore cannot directly use the negative-sequence and zero-sequence components of the load current as the inverter's output negative-sequence and zero-sequence reference current values.

[0087] Considering that the load dq-axis current is a DC component under steady state, the negative sequence reference current value of the inverter output can be calculated by the following formula:

[0088]

[0089] In the formula, the superscript * indicates the reference current, L line This is the grid-side impedance. The voltage is measured... With current The inverter output reference current value is generated through calculation. This allows the negative sequence component of the inverter output current to follow the negative sequence component of the unbalanced load, thus compensating for the unbalanced load.

[0090] For zero-sequence current, it is an alternating current, controlling u pcc0 When the value is 0, the inverter output zero-sequence current i0 can completely compensate for the unbalanced load zero-sequence current i. l0 Therefore, the output of the voltage outer loop controller can be used as the reference value for the current inner loop controller to achieve control of the zero-sequence current component. Since i0 and u... pcc0Since both are AC quantities, direct PI control cannot achieve good control results. However, the proportional resonant (PR) controller can achieve zero steady-state error tracking of AC signals. Therefore, a zero-sequence voltage and current dual-loop control structure is designed using a PR controller.

[0091] S3 performs amplitude limiting processing on the negative sequence and zero sequence reference current values ​​that need to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter.

[0092] In this embodiment, when the output current does not exceed the limit, the reference values ​​for the negative sequence and zero sequence components of the inverter output current are the same as the negative sequence and zero sequence components of the load current. However, the inverter output current value is limited. If the load imbalance is severe, it will cause the inverter to require excessive compensation current, affecting the normal operation of the inverter and possibly even damaging it. Therefore, in this invention, as... Figure 5 As shown, this paper focuses on analyzing the relationship between the peak value of the inverter output phase current and each sequence component, and proposes a limiting strategy to ensure that the inverter output current does not exceed the rated value, thereby avoiding damage to the inverter due to excessive compensation current.

[0093] The three-phase output current of the inverter can be expressed as:

[0094]

[0095] In the formula: (x = a, b, c), where x represents the amplitude of each phase current without considering zero sequence, and θ a =0, θ b = -240°, θ c =240°. A + A - I0 and I0 are the positive, negative, and zero-sequence current amplitudes, respectively, and φ0 is the negative and zero-sequence current amplitudes, respectively. + φ - These represent the positive and negative sequence current phases, φ x The phase of each phase current is taken into account without considering the zero-sequence component.

[0096] From the above formula, we can see that the maximum output current of the grid-connected inverter is:

[0097] I max =C max +I0=A + +A - +I0;

[0098] If the rated output current amplitude of the inverter is I N To ensure the inverter functions properly, we have I max ≤I N As the main function of the inverter, the positive sequence output A +It should not be affected during compensation; the inverter output compensation current A can be seen. - The capability of I0 is limited. Therefore, this invention proposes a limiting strategy to allocate a limited compensation current.

[0099] The purpose of this application is to compensate for voltage dips and rises by injecting and absorbing three-phase positive-sequence reactive power. This application also compensates for three-phase imbalance by injecting and absorbing negative-sequence and zero-sequence currents. Therefore, the inverter first considers compensating for the load's zero-sequence current. At this point, since the inverter's rated output current amplitude is I... N Output A in ascending order. + It should not be affected during compensation, compensation current A - The capacity of I0 is limited, so zero order is compensated first, and then negative order A is obtained. - Initially, set the zero-sequence current to 0. If the remaining capacity is insufficient to compensate for the zero-sequence current, only partial compensation will be performed. If there is still a surplus after compensating for the zero-sequence current, then the negative-sequence current A will be compensated. The maximum compensation value for the zero-sequence current is:

[0100] I 0max =I max -A + ;

[0101] The output zero-sequence reference current value needs to be limited:

[0102]

[0103] In the formula, This is the zero-sequence reference current amplitude.

[0104] If I0 0max The inverter still has current margin to compensate for negative sequence current, and the maximum compensation value for negative sequence current is:

[0105] A -max =I max -A + -I0;

[0106] Otherwise, if I 0max If I ≤ I0, then only the zero-sequence current is compensated.

[0107] Similarly, the calculated negative-sequence reference current value also needs to be limited:

[0108]

[0109] In the formula, This is the negative sequence reference current amplitude.

[0110] S4 generates the corresponding switching signal after being modulated by a positive, negative, and zero-sequence controller and a sinusoidal pulse width modulation (SPWM). ​

[0111] To more clearly and completely illustrate the implementation scheme of this application, the following specific embodiments are given:

[0112] A 20kW grid-connected inverter simulation model was built in the Matlab / Simulink platform to verify the effectiveness of the proposed compensation strategy.

[0113] 1) When the load current is unmeasurable, the negative sequence and zero sequence reference currents can be generated using the common coupling point voltage, thereby controlling the inverter output current to compensate for the unbalanced load current components.

[0114] 2) Without affecting the normal grid connection of the inverter, the current margin can be used to achieve balanced control of the grid-side current, compensating for the negative sequence and zero sequence components of the load current, thereby balancing the three-phase current on the grid side. This invention focuses on exploring the performance of the proposed method in the following three situations:

[0115] a) The inverter is fully compensated;

[0116] When the inverter current margin is sufficient, it can fully compensate for the load current. The simulation waveform at this time is as follows: Figure 6 As shown, where Figure 6 In the figure, (a) represents the inverter output current, such as Figure 6 In the figure, (b) represents the grid-side current, such as Figure 6 In this context, (c) represents the grid-side current sequence component, such as... Figure 6 In the figure, (d) represents the grid-side current imbalance.

[0117] As shown in the figure above, initially, the inverter output is 0. Under the action of an unbalanced load, the grid-side three-phase current i sabc The waveform exhibits significant asymmetry, containing substantial negative-sequence and zero-sequence current components, with a three-phase current imbalance of 48%. At t = 0.2s, the compensation strategy is activated, and the inverter implements unbalanced current compensation. Since the inverter's positive-sequence output is 0 at this time, there is sufficient margin to compensate for the negative-sequence and zero-sequence components in the load current. The grid-side three-phase current waveform returns to a symmetrical state, the negative-sequence and zero-sequence components in the current are suppressed to 0, the unbalanced current is effectively compensated, and the grid-side current imbalance is reduced to 0.

[0118] b) The inverter performs incomplete negative sequence compensation;

[0119] At this point, the inverter current margin is insufficient. Zero-sequence current can be fully compensated for, but negative-sequence current cannot be fully compensated. The simulation waveform at this time is as follows: Figure 7 As shown, where Figure 7 In the figure, (a) represents the inverter output current, such as Figure 7 In the figure, (b) represents the grid-side current, such as Figure 7In this context, (c) represents the grid-side current sequence component, such as... Figure 7 In the figure, (d) represents the grid-side current imbalance.

[0120] In the diagram above, initially, the inverter operates normally connected to the grid, outputting a three-phase symmetrical current, and the grid-side three-phase current i sabc It bears the negative sequence and zero sequence current components of the unbalanced load current, and the three-phase current imbalance reaches 120%. At t = 0.2s, the compensation strategy is activated, at which time the output current of phase c of the inverter exceeds the rated value I. N With amplitude limiting control engaged, the output current amplitude of phase c is limited to I. N In the diagram, the zero-sequence component of the grid-side current drops to 0, while the negative-sequence component only partially decreases. This indicates that the inverter has achieved complete zero-sequence compensation, while the negative-sequence component is only partially compensated. The grid-side current imbalance decreases from 120% to 20%, indicating that the grid-side current imbalance can still be well suppressed.

[0121] c) Inverter performs incomplete zero-sequence compensation.

[0122] At this point, the inverter's current margin is severely insufficient, allowing only incomplete zero-sequence compensation for the load current; it can no longer compensate for the negative-sequence current. The simulation waveform is as follows. Figure 8 As shown, where Figure 8 In the figure, (a) represents the inverter output current, such as Figure 8 In the figure, (b) represents the grid-side current, such as Figure 8 In this context, (c) represents the grid-side current sequence component, such as... Figure 8 In the figure, (d) represents the grid-side current imbalance.

[0123] In this diagram, the excessive c-phase current on the grid side results in a large zero-sequence current component. After inverter compensation is activated at 0.2s, the inverter can only partially compensate for the zero-sequence current. At this point, the grid side still bears the negative-sequence load component, and the negative-sequence current component on the grid side remains essentially unchanged. The grid-side current imbalance in the diagram decreases from 100% to approximately 60%, indicating that the inverter still provides some compensation without affecting its own positive-sequence output.

[0124] On the other hand, the present invention also provides an unbalanced current compensation control system with limiting function, the system comprising:

[0125] The component separation module is used to first consider a low-voltage distribution network in a rural area with a three-phase grid-connected inverter, measure the voltage at the common connection point between the current three-phase grid-connected inverter and the grid, as well as the output current of the inverter, and separate the positive sequence, negative sequence, and zero sequence components in the voltage and current.

[0126] The compensation module is used to design a load unbalanced current compensation strategy based on the positive sequence, negative sequence, and zero sequence components in the measured voltage and current, thereby calculating the negative sequence and zero sequence reference current values ​​that need to be compensated.

[0127] The limiting module is used to limit the negative sequence and zero sequence reference current values ​​that need to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter. After being modulated by the positive, negative and zero sequence controllers and the sinusoidal pulse width modulation (SPWM), the corresponding switching signals are obtained.

[0128] Other technical features of the unbalanced current compensation control system with limiting function described in this invention are similar to other technical features of the unbalanced current compensation control method with limiting function, and will not be repeated here.

[0129] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. "A plurality of" means two or more, unless otherwise explicitly specified.

[0130] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0131] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0132] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0133] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as will be understood by those skilled in the art to which embodiments of the invention pertain.

[0134] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (such as a computer-based system, a processor-included system, or other system that can fetch and execute instructions from, an instruction execution system, apparatus, or device). For the purposes of this specification, "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transmit programs for use by, or in conjunction with, an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of computer-readable media include: an electrical connection having one or more wires (electronic device), a portable computer disk drive (magnetic device), random access memory (RAM), read-only memory (ROM), erasable and editable read-only memory (EPROM or flash memory), fiber optic devices, and portable optical disc read-only memory (CDROM). Alternatively, the computer-readable medium may be paper or other suitable media on which the program can be printed, since the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpreting, or otherwise processing as necessary, and then stored in a computer memory.

[0135] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0136] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0137] Furthermore, the functional units in the various embodiments of the present invention can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0138] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for unbalanced current compensation control with amplitude limiting function, characterized in that, The method includes: Consider a low-voltage distribution network in a rural area connected to a three-phase grid-connected inverter. Measure the voltage at the common connection point between the three-phase grid-connected inverter and the grid, as well as the output current of the inverter, and separate the positive-sequence, negative-sequence, and zero-sequence components of the voltage and current. Based on the positive-sequence, negative-sequence, and zero-sequence components in the measured voltage and current, a load unbalanced current compensation strategy is designed, thereby calculating the negative-sequence and zero-sequence reference current values ​​that need to be compensated. The negative sequence and zero sequence reference current values ​​that need to be compensated are limited based on the maximum amplitude of the output current of the three-phase grid-connected inverter. After being modulated by the positive, negative and zero sequence controllers and the sinusoidal pulse width modulation (SPWM), the corresponding switching signals are obtained. The step of limiting the zero-sequence reference current value to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter includes: The maximum amplitude of the output current is obtained based on the three-phase output current of the three-phase grid-connected inverter. The maximum compensation value of the zero-sequence current is obtained based on the maximum amplitude of the output current, and then the result after limiting the zero-sequence reference current value is obtained. The maximum amplitude of the output current obtained from the three-phase output current of the three-phase grid-connected inverter is expressed as follows: ； in, These represent the amplitudes of the positive, negative, and zero-sequence currents, respectively. for The maximum value, in the formula, x =a, b, c; If the rated output current amplitude of the inverter is In order for the inverter to work properly, there is: ; The maximum compensation value of the zero-sequence current is expressed as: ； at this time If the value is zero, the result after limiting the zero-sequence reference current value is expressed as: ； in, This is the reference amplitude of the zero-sequence current. This is the zero-sequence reference current value after limiting. This is the reference value for the output current at the 0-axis. like If the inverter still has current margin to compensate for negative sequence current, then the maximum compensation value for negative sequence current is: ； The step of limiting the negative sequence reference current value to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter also includes: ； in, This is the reference amplitude for the negative sequence current. After the amplitude is limited d negative sequence reference current value, After the amplitude is limited q Negative sequence reference current value for the shaft.

2. The unbalanced current compensation control method with limiting function according to claim 1, characterized in that, The separation of the positive-sequence, negative-sequence, and zero-sequence components in the voltage and current includes: Under unbalanced load conditions, based on the symmetrical component method, the system components are decomposed into positive-sequence, negative-sequence, and zero-sequence components, which are independent of each other and controlled independently. The expression for this is: ； in, L 1 For inverter-side filter inductance, L 2 For grid-side filter inductors, The inverter outputs current in the dq0 rotating coordinate system. The inverter output voltage in the dq0 rotating coordinate system is respectively. Let ω be the voltage at the point of common connection in the rotating coordinate system dq0, and let ω be the fundamental frequency. The superscripts + and - in the above formula represent the positive and negative sequence components, respectively.

3. The unbalanced current compensation control method with limiting function according to claim 2, characterized in that, The design of the load imbalance current compensation strategy calculates the negative-sequence and zero-sequence reference current values ​​that need to be compensated, including: For a three-phase grid-connected inverter, if the negative-sequence and zero-sequence components of its output current can completely compensate for the negative-sequence and zero-sequence components of the load current, then the grid-side current contains only the positive-sequence component, and the three-phase current on the grid side is balanced; otherwise... Since the grid-connected inverter on the user side cannot directly use the negative sequence and zero sequence components of the load current as reference values ​​for the output negative sequence and zero sequence current of the three-phase grid-connected inverter, a compensation strategy is adopted to obtain the negative sequence and zero sequence reference current values ​​that need to be compensated.

4. The unbalanced current compensation control method with limiting function according to claim 3, characterized in that, The method of obtaining the negative-sequence and zero-sequence reference current values ​​that need to be compensated using the compensation strategy includes: Considering that the load dq-axis current is a DC component under steady state, the inverter output negative sequence reference current value is calculated by the following formula: ； The superscript * indicates the reference current. The grid-side impedance is determined by measuring the voltage. With current Generate inverter output reference current value This allows the negative sequence component of the inverter output current to follow the negative sequence component of the unbalanced load, thus achieving compensation for the unbalanced load. Zero-sequence current is an alternating current, used for control. When the output zero-sequence current of the three-phase grid-connected inverter is 0, To fully compensate for the zero-sequence current of an unbalanced load, the output of the outer voltage loop controller is directly used as the reference value for the inner zero-sequence current controller.

5. A control system based on the unbalanced current compensation control method with limiting function as described in claim 1, characterized in that, The system includes: The component separation module is used to first consider a low-voltage distribution network in a rural area with a three-phase grid-connected inverter, measure the voltage at the common connection point between the current three-phase grid-connected inverter and the grid, as well as the output current of the inverter, and separate the positive sequence, negative sequence, and zero sequence components in the voltage and current. The compensation module is used to design a load unbalanced current compensation strategy based on the positive sequence, negative sequence, and zero sequence components in the measured voltage and current, thereby calculating the negative sequence and zero sequence reference current values ​​that need to be compensated. The limiting module is used to limit the negative sequence and zero sequence reference current values ​​that need to be compensated based on the maximum amplitude of the output current of the three-phase grid-connected inverter. After being modulated by the positive, negative, and zero sequence controllers and the sinusoidal pulse width modulation (SPWM), the corresponding switching signals are obtained.

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