Low-voltage transformer inverter voltage limiting method and system when grid voltage is unbalanced

Abstract

This invention discloses a voltage limiting method and system for inverters in low-voltage distribution areas when there is grid voltage imbalance. It relates to the field of inverter voltage limiting technology. The method involves detecting and decomposing the point of common coupling (PCC) voltage and calculating the angles between phase sequence voltages. The phase voltage amplitude is calculated based on the decomposed PCC voltage and the angles between the phase sequence voltages. A reference current is calculated based on the constraints of the phase voltage amplitude, and this reference current is used as the input to a proportional resonant controller to achieve voltage limiting control of the PCC voltage. This solves the problem of inaccurate voltage limiting calculations in low-voltage distribution area inverters and also provides some support for the large power grid.

Description

Technical Field

[0001] This invention belongs to the field of inverter voltage limiting technology, and particularly relates to a method and system for voltage limiting of inverters in low-voltage distribution areas when the grid voltage is unbalanced. Background Technology

[0002] New energy power generation is the primary power generation method for future power systems, and its integration into the distribution network will profoundly alter the power flow patterns and voltage distribution of the network. Solar, wind, and other new energy sources are typically connected to the distribution network via inverters. However, due to the low inertia and high controllability of inverters, several key issues still exist affecting the safe and stable integration of new energy sources. When the grid voltage is unbalanced, inverters connected to low-voltage distribution areas will operate under unbalanced conditions, leading to low voltage issues at the point of common coupling (PCC). In addition, high voltage may occur on non-faulty phases. The occurrence of low and high voltage problems will cause inverter protection to trip and shut down, resulting in serious problems such as changes in power flow and voltage instability in the low-voltage distribution network.

[0003] Existing voltage limiting solutions for inverters under grid voltage imbalance are still immature and mainly fall into two categories. One category addresses the voltage support problem when the inverter experiences low voltage by distributing the active and reactive power output of the inverter proportionally to the grid impedance to maximize support for the PCC under low voltage, thereby solving the low voltage problem. The other category addresses the voltage limiting problem when the inverter experiences high voltage. Existing technical solutions mainly utilize the actual voltage of the PCC and the target voltage to form a voltage outer loop and a current inner loop to limit the high voltage.

[0004] To address the simultaneous low and high voltage issues in inverters, existing methods for voltage limiting, both domestically and internationally, suffer from inaccurate calculations of the PCC phase voltage amplitude and overly complex control structures. Antonio Camacho et al. calculated the PCC phase voltage amplitude in a three-wire system using the inverse Clarke transform formula; however, this method is only effective for three-phase three-wire systems without zero-sequence current. Masoud M. Shabestary et al. extended the scope of this problem to three-phase four-wire systems, introducing the influence of zero-sequence voltage. While their calculations of the PCC phase voltage amplitude were accurate for minor faults, they became highly inaccurate for more severe grid faults, and the calculation of the flexible reference current was complex. Soumya Ranjan Mohapatra et al. also considered zero-sequence voltage in three-phase four-wire systems, but their solutions simplified many factors, which does not reflect real-world conditions.

[0005] This technology considers a low-voltage distribution area inverter. When a grid fault occurs, a zero-sequence voltage appears at the PCC point, making the calculation of the PCC amplitude more difficult. The aforementioned domestic and international technical solutions for considering the impact of zero-sequence voltage are still immature, suffering from drawbacks such as inaccurate PCC phase voltage amplitude calculation and complex control processes. To address this, this invention proposes a voltage limiting method and system for low-voltage distribution area inverters when the grid voltage is unbalanced, effectively solving the problem of inaccurate voltage limiting calculation for low-voltage distribution area inverters. Summary of the Invention

[0006] The purpose of this invention is to provide a voltage limiting method and system for low-voltage transformer inverters when the grid voltage is unbalanced, thereby overcoming the shortcomings of inaccurate voltage limiting calculation in existing low-voltage transformer inverters.

[0007] To achieve the above objectives, the present invention provides a method for voltage limiting of inverters in low-voltage distribution areas when there is grid voltage imbalance, comprising the following steps:

[0008] The voltage at the common coupling point (PCC) is detected and decomposed, and the angle between each phase sequence voltage is calculated.

[0009] Calculate the phase voltage amplitude based on the angle between the decomposed PCC voltage and each phase sequence voltage;

[0010] The reference current is calculated based on the constraint of the phase voltage amplitude, and the reference current is used as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

[0011] Preferably, the common coupling point (PCC) voltage is detected and decomposed, specifically including:

[0012] Acquire instantaneous values ​​of PCC voltage and current;

[0013] The instantaneous values ​​of PCC voltage and current are transformed from three-phase instantaneous values ​​to two-phase stationary coordinates using the Clarke transform. Then, the positive and negative sequence two-phase stationary voltages and the included angles of each vector are obtained through a dual second-order generalized integrator phase-locked loop. Finally, the amplitudes of the positive and negative sequence voltages from the PCC to the low-voltage grid are obtained by combining the case of the low-voltage grid.

[0014] Preferably, the phase voltage amplitude is calculated based on the angle between the decomposed PCC voltage and each phase sequence voltage, specifically including:

[0015] By adding the positive and negative sequence voltages and combining them with the zero sequence voltage and the rotating vector, a right-angled triangle relationship is obtained.

[0016] Based on the lag-lead relationship between phase B and phase C voltages and phase A voltage, and combined with the right-angled triangle relationship, the phase voltage amplitude of PCC voltage is calculated.

[0017] Preferably, the expression for the phase voltage amplitude of the PCC voltage is:

[0018]

[0019]

[0020] In the above formula, V + V - This represents the magnitude of the positive and negative sequence voltages of the PCC, V. g + and V g - This represents the magnitude of the positive and negative sequence voltages of the power grid, where L and R are the inductance and resistance of the power grid, respectively. ω represents the magnitude of the positive-sequence active current and the magnitude of the negative-sequence reactive current, and ω represents the angular frequency of the grid voltage.

[0021] Preferably, calculating the reference current based on the constraint of the phase voltage amplitude specifically includes:

[0022] Calculate the positive and negative sequence reference voltages based on the constraints of the phase voltage amplitudes;

[0023] The positive and negative sequence reference voltages are solved to obtain the reference values ​​of the positive and negative sequence voltage amplitudes;

[0024] Substituting the magnitudes of the positive and negative sequence voltages obtained by decomposing the PCC voltage at the common coupling point into the reference value of the positive and negative sequence voltage magnitudes yields two inequalities;

[0025] The positive and negative sequence reactive currents are obtained by solving the two inequalities based on impedance matching current.

[0026] Substituting the positive and negative sequence reactive currents into the reference current in the two-phase stationary coordinate system, we obtain the final reference current, which is the reference value of the current control loop.

[0027] Preferably, the positive and negative sequence reference voltages are calculated based on the constraints of the phase voltage amplitudes, specifically including:

[0028] When the grid voltage is unbalanced, the upper and lower voltage limits required for the inverter's PCC point in the low-voltage distribution area;

[0029] Extract the maximum and minimum values ​​of the phase voltage amplitude from the PCC voltage amplitude;

[0030] The upper and lower limit amplitude requirements of the voltage at the PCC point in the low-voltage distribution area are calculated, and the maximum and minimum values ​​of the phase voltage amplitude are extracted from the PCC voltage amplitude to calculate the positive and negative sequence reference voltages.

[0031] Preferably, when the grid voltage is unbalanced, the inverter's control loop is set to a single current loop.

[0032] This invention also provides a voltage limiting system for low-voltage transformer inverters when there is grid voltage imbalance. The low-voltage transformer inverter voltage limiting system applies the aforementioned method for voltage limiting of low-voltage transformer inverters when there is grid voltage imbalance, and includes:

[0033] The PCC voltage detection module is used to detect and decompose the PCC voltage at the common coupling point, and to calculate the angle between each phase sequence voltage.

[0034] The PCC phase voltage amplitude calculation module is used to calculate the phase voltage amplitude based on the decomposed PCC voltage; and

[0035] The current control loop module calculates a reference current based on the constraint of the phase voltage amplitude, and uses the reference current as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

[0036] Compared with existing technologies, the present invention has the following advantages:

[0037] The present invention provides a method and system for voltage limiting of inverters in low-voltage distribution areas when there is grid voltage imbalance. This method detects and decomposes the voltage at the point of common coupling (PCC) and calculates the angle between each phase sequence voltage. It then calculates the phase voltage amplitude based on the decomposed PCC voltage and the angle between each phase sequence voltage. Finally, it calculates a reference current based on the constraint of the phase voltage amplitude and uses this reference current as the input to a proportional resonant controller to achieve voltage limiting control of the PCC voltage. This solves the problem of inaccurate voltage limiting calculation for inverters in low-voltage distribution areas and also provides some support for the large power grid. Attached Figure Description

[0038] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a flowchart of a method for limiting the voltage of a low-voltage transformer inverter when the grid voltage is unbalanced, according to the present invention.

[0040] Figure 2 This is a topology diagram of the low-voltage distribution area inverter;

[0041] Figure 3 This is a structural diagram of the PCC voltage detection module of the present invention;

[0042] Figure 4This is the phasor diagram of the positive-sequence, negative-sequence, and zero-sequence voltages of the PCC at the initial moment of this invention;

[0043] Figure 5 This is a structural diagram of the current control loop of the present invention;

[0044] Figure 6 This is a schematic diagram of the simulation results of the PCC voltage amplitude calculation method of the present invention.

[0045] Figure 7 This is a schematic diagram of the simulation results of the present invention. Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. 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.

[0047] like Figure 1 As shown, the voltage limiting method for low-voltage transformer inverters in the event of grid voltage imbalance provided by this invention includes the following steps:

[0048] S1. Detect and decompose the voltage at the common coupling point (PCC), and calculate the angle between each phase sequence voltage;

[0049] S2. Calculate the phase voltage amplitude based on the angle between the decomposed PCC voltage and each phase sequence voltage;

[0050] S3. Calculate the reference current based on the constraint of the phase voltage amplitude, and use the reference current as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

[0051] The aforementioned method for voltage limiting of inverters in low-voltage distribution areas when there is grid voltage imbalance detects and decomposes the voltage at the point of common coupling (PCC) and calculates the angle between each phase sequence voltage. It then calculates the phase voltage amplitude based on the decomposed PCC voltage and the angle between each phase sequence voltage. Finally, it calculates a reference current based on the constraint of the phase voltage amplitude and uses this reference current as the input to a proportional resonant controller to achieve voltage limiting control of the PCC voltage. This solves the voltage limiting problem of the inverter bridge in low-voltage distribution areas and also provides some support to the large power grid.

[0052] In one embodiment, step S1, detecting and decomposing the common coupling point (PCC) voltage, specifically includes:

[0053] Acquire instantaneous values ​​of PCC voltage and current;

[0054] The instantaneous values ​​of PCC voltage and current are transformed from three-phase instantaneous values ​​to two-phase stationary αβ coordinates using the Clarke transform. Then, the positive and negative sequence two-phase stationary voltages and the included angles of each vector are obtained through a dual second-order generalized integrator phase-locked loop. Finally, the amplitudes of the positive and negative sequence voltages from the PCC to the low-voltage grid are obtained by combining the case of the low-voltage grid.

[0055] In one embodiment, step S2, calculating the phase voltage amplitude based on the angle between the decomposed PCC voltage and each phase sequence voltage, specifically includes:

[0056] By adding the positive and negative sequence voltages and combining them with the zero sequence voltage and the rotating vector, a right-angled triangle relationship is obtained.

[0057] Based on the lag-lead relationship between phase B and phase C voltages and phase A voltage, and combined with the right-angled triangle relationship, the phase voltage amplitude of PCC voltage is calculated.

[0058] The above-mentioned PCC phase voltage amplitude calculation method has two advantages: first, it can still ensure high accuracy in voltage amplitude calculation even when the grid fault is deep, thus ensuring the precision of control; second, in the current control module, the process of solving the current inner loop reference value is relatively simple, avoiding a large amount of calculation and facilitating the construction of the control system.

[0059] In one embodiment, step S3, calculating the reference current based on the constraint of the phase voltage amplitude, specifically includes:

[0060] S31. Calculate the positive and negative sequence reference voltages based on the constraints of the phase voltage amplitudes;

[0061] S32. Solve for the positive and negative sequence reference voltages to obtain the reference values ​​of the positive and negative sequence voltage amplitudes;

[0062] S33. Substitute the magnitudes of the positive and negative sequence voltages obtained by decomposing the PCC voltage at the common coupling point into the reference value of the positive and negative sequence voltage magnitudes to obtain two inequalities;

[0063] S34. Solve the two inequalities based on impedance matching to obtain the positive and negative sequence reactive currents;

[0064] S35. Substitute the positive and negative sequence reactive currents into the reference current in the two-phase stationary coordinate system to obtain the final reference current, which is the reference value of the current control loop.

[0065] Specifically, step S31, calculating the positive and negative sequence reference voltages based on the constraints of the phase voltage amplitude, includes:

[0066] When the grid voltage is unbalanced, the upper and lower voltage limits required for the inverter's PCC point in the low-voltage distribution area are obtained. At the same time, the inverter's control loop is set to a single current loop.

[0067] Extract the maximum and minimum values ​​of the phase voltage amplitude from the PCC voltage amplitude;

[0068] The upper and lower limit amplitude requirements of the voltage at the PCC point in the low-voltage distribution area are calculated, and the maximum and minimum values ​​of the phase voltage amplitude are extracted from the PCC voltage amplitude to calculate the positive and negative sequence reference voltages.

[0069] This invention provides a voltage limiting system for low-voltage transformer inverters when the grid voltage is unbalanced. The voltage limiting system for low-voltage transformer inverters when the grid voltage is unbalanced, as described below, can be referred to in correspondence with the voltage limiting method for low-voltage transformer inverters when the grid voltage is unbalanced, as described above.

[0070] A voltage limiting system for low-voltage transformer inverters in the event of grid voltage imbalance includes: a PCC voltage detection module, a PCC phase voltage amplitude calculation module, and a current control loop module.

[0071] The PCC voltage detection module is used to detect and decompose the PCC voltage at the common coupling point, and to calculate the angle between each phase sequence voltage.

[0072] The PCC phase voltage amplitude calculation module is used to calculate the phase voltage amplitude based on the angle between the decomposed PCC voltage and each phase sequence voltage.

[0073] The current control loop module calculates a reference current based on the constraint of the phase voltage amplitude, and uses the reference current as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

[0074] The aforementioned voltage limiting system for low-voltage transformer substation inverters under grid voltage imbalance utilizes a PCC voltage detection module to detect and decompose the PCC voltage at the common coupling point, and to calculate the angle between each phase sequence voltage. A PCC phase voltage amplitude module calculates the phase voltage amplitude based on the decomposed PCC voltage and the angle between each phase sequence voltage. A current control loop module calculates a reference current based on the constraint of the phase voltage amplitude, and uses this reference current as the input to a proportional resonant controller to complete the voltage limiting control of the PCC voltage. This solves the voltage limiting problem of the inverter bridge in low-voltage transformer substations and also provides some support to the large power grid.

[0075] The present invention provides a detailed description of the voltage limiting system for low-voltage transformer area inverters under grid voltage imbalance, in order to enable those skilled in the art to better understand the present invention:

[0076] like Figure 2 The diagram shows the inverter topology for the low-voltage distribution area. Figure 2 In the middle, V dc It is the DC-side voltage of the micro-source, L cf and L gfIt is a filter inductor, C f It is a filter capacitor, R d It is a series passive damper, where L and R are the grid inductance and resistance. When an asymmetrical fault occurs in the grid, the PCC voltage contains a zero-sequence component, while the PCC current does not contain a zero-sequence component because there is no path for zero-sequence current.

[0077] First, the PCC voltage at the common coupling point is detected and decomposed, and the angles between the phase sequence voltages are calculated. After acquiring the instantaneous PCC voltage and current signals, the three-phase instantaneous values ​​are first transformed into a two-phase stationary αβ coordinate system using the Clarke transform. Then, the positive and negative sequence two-phase stationary voltages, i.e., the positive and negative zero sequence voltage signals in the two-phase stationary coordinate system, are obtained using a double second-order generalized integrator-phase-locked loop (DSOGI-PLL). The structure of the PCC voltage detection module is as follows: Figure 3 As shown.

[0078] exist Figure 3 In the middle, v abc This represents the instantaneous value of the three-phase voltage collected from the PCC, v αβ V represents the instantaneous value of the two-phase voltage generated by the Clarke transform. α + v β + v α - v β - This represents the positive and negative sequence two-phase static voltages obtained by decomposition using the DSOGI-PLL module.

[0079] Specifically, the specific information of the PCC voltage is obtained, and the PCC voltage is obtained from the PCC voltage detection module as shown in equation (1).

[0080]

[0081] In equation (1), v α v β v0 represents the instantaneous voltage value in the two-phase stationary coordinate system, ω represents the angular frequency of the grid voltage, t represents time, and δ + δ - δ 0 V represents the positive-sequence voltage phase, negative-sequence voltage phase, and zero-sequence voltage phase. + V - V 0 This represents the magnitude of the positive and negative zero-sequence voltages of the PCC, V. + V - V 0It can be calculated using equation (2).

[0082]

[0083] The initial phase difference between the positive and negative zero-sequence voltages of PCC is:

[0084]

[0085] in,

[0086]

[0087] In equation (4), δ represents the initial phase difference between positive and negative sequence voltages, δ' represents the initial phase difference between zero and positive sequence voltages, and δ” represents the initial phase difference between negative and zero sequence voltages.

[0088] After obtaining the specific information about the PCC voltage, the circuit equation from the PCC to the low-voltage grid can be written as shown in equation (5).

[0089]

[0090] In equation (5), V g + and V g - V represents the magnitude of the positive and negative sequence voltages of the power grid. + V - This represents the magnitude of the positive and negative sequence voltages of the PCC, where L and R are the mains inductance and resistance. ω represents the magnitude of the positive-sequence active current and the magnitude of the negative-sequence reactive current, and ω represents the angular frequency of the grid voltage.

[0091] The PCC phase voltage amplitude calculation module is used to calculate the phase voltage amplitude based on the angle between the decomposed PCC voltage and each phase sequence voltage.

[0092] like Figure 4 The diagram shows the phase relationship of the positive and zero-sequence voltages of the PCC at the initial moment. At the initial moment, the rotating voltage vector rotates to the A-axis (α-axis), and the positive-sequence voltage vector coincides with the rotating voltage vector. The negative-sequence voltage vector, the zero-sequence voltage vector, and the angles between each vector can be obtained through the PCC voltage detection module. For simplicity of analysis, the small phase and amplitude deviations of the positive and negative-sequence voltages caused by current injection are ignored here.

[0093] according to Figure 4 We can first calculate the voltage obtained by adding the positive and negative sequence phasors. Its relationship with zero-sequence voltage and rotation vector They form a right triangle relationship, as shown in equation (6).

[0094] [V0 sin(π-δ')] 2 +[V+V 0 cos(π-δ')] 2 =(V +- ) 2 (6)

[0095] in,

[0096]

[0097] Based on equations (6) and (7), and utilizing the lag-lead relationship between phase B and phase C voltages and phase A voltage, the amplitude V of the PCC voltage can be obtained completely. a V b V c As shown in equation (8).

[0098]

[0099] Equation (8) contains very few coupling terms related to positive and negative zero-sequence voltages, so the inner current loop can be easily designed in the current control loop module according to the high voltage and low voltage limitation requirements.

[0100] The current control loop module calculates the reference current based on the constraint of the phase voltage amplitude, and uses the reference current as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

[0101] Specifically, by utilizing the upper and lower limits of the PCC voltage, the inverter's reference current is calculated, and a current control loop is designed. When the grid voltage is unbalanced, the inverter control loop is designed as a single current loop, such as... Figure 5 As shown.

[0102] The reference current in the two-phase stationary coordinate system is:

[0103]

[0104] In equation (9), i α * and i β * This is the reference value for the current control loop. and These are the positive and negative sequence active and reactive reference current components. Different reference current components can be designed according to different control objectives.

[0105] When the grid voltage is unbalanced, the upper and lower limit values ​​of the inverter PCC voltage in the low-voltage distribution area are required as follows:

[0106]

[0107] In equation (10), V is the upper limit of the PCC voltage amplitude, and V is the lower limit of the PCC voltage amplitude. Therefore, the current control loop module first needs to extract the maximum and minimum values ​​of the phase voltage amplitude from the PCC voltage amplitude calculation results, as shown in the formula.

[0108]

[0109] In equation (11), the maximum value of the three-phase voltage amplitude is denoted as max{V a V b V c}, where the corresponding cos(δ+γ), sin(π-δ'-γ), and cos(π-δ'-γ) are x, b, and a, respectively. Similarly, the minimum value for calculating the three-phase voltage amplitude is min{V a V b V c}, where cos(δ+γ), sin(π-δ'-γ), and cos(π-δ'-γ) are y, n, and m, respectively. The range of γ is... For any type of fault, the maximum and minimum phase voltages can be quickly calculated using equation (11). Therefore, if the maximum phase voltage exceeds 1.1 pu, or the minimum is less than 0.85 pu, it is set as follows:

[0110]

[0111] Then there is

[0112]

[0113] As can be seen from equation (13), the phase voltage amplitude is mainly related to the amplitude of the positive and negative sequence voltages and the phase difference between the positive and negative sequence voltages and the positive zero sequence voltage. Equation (13) can be regarded as a two-variable quadratic equation to solve for the positive and negative sequence reference voltages, and the result is shown in equation (14).

[0114]

[0115] in,

[0116]

[0117] After obtaining the reference values ​​for the positive and negative sequence voltage amplitudes, according to equation (5), we have:

[0118]

[0119] Equation (16) contains only two inequalities, but four unknowns: positive and negative sequence active and reactive currents. Therefore, the equation has infinitely many solutions. By incorporating the current injection strategy based on impedance matching, the inverter output current can be optimally distributed proportionally to the grid impedance. The positive and negative sequence active and reactive currents can then be solved as follows:

[0120]

[0121] Substituting the positive and negative sequence active and reactive current components of equation (17) into equation (9) yields the reference value for the current control loop.

[0122] The simulation results obtained by simulating the phase voltage amplitude calculation method of step S2 according to the present invention are as follows: Figure 6 As shown, an asymmetrical fault occurs in the power grid at 0.1s. Figure 6 It can be seen that the proposed voltage amplitude calculation method can calculate the PCC voltage amplitude including the zero-sequence component very accurately and quickly.

[0123] The simulation results using the low-voltage transformer inverter voltage limiting method for grid voltage imbalance according to the present invention are as follows: Figure 7 As shown, at 0.2s, an asymmetrical ground fault occurs in the power grid, causing voltage imbalance. At this time, the A-phase voltage at the inverter's PCC point drops to 210V, below the lower limit of 264V (0.85pu), while the B-phase voltage rises to 379V, exceeding the upper limit of 342V (1.1pu). At 0.3s, the system employs the proposed control method to limit the voltage, supporting the low voltage and limiting the high voltage. Figure 7 It can be seen that the amplitude of each phase of the PCC voltage has recovered to within the specified range, which can ensure the safe and stable operation of the low-voltage inverter in the distribution area when the grid voltage is unbalanced.

[0124] In summary, when the grid voltage is unbalanced, the voltage limiting method and system for low-voltage transformer inverters provided by this invention can ensure that the voltage at the PCC point of the inverter connected to the low-voltage transformer is within the specified range.

[0125] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without any creative effort.

[0126] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0127] The above description only discloses specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or modifications that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for limiting the voltage of inverters in low-voltage distribution areas when there is grid voltage imbalance, characterized in that, Includes the following steps: The voltage at the common coupling point (PCC) is detected and decomposed, and the angle between each phase sequence voltage is calculated. The common coupling point (PCC) voltage is detected and decomposed, specifically including: Acquire instantaneous values ​​of PCC voltage and current; The instantaneous values ​​of PCC voltage and current are transformed from three-phase instantaneous values ​​to two-phase stationary coordinates through Clark transformation. Then, the positive and negative sequence two-phase stationary voltages and the included angles of each vector are obtained through a dual second-order generalized integrator phase-locked loop. The amplitude of the positive and negative sequence voltages from PCC to the low-voltage grid is obtained in combination with the low-voltage grid situation. The phase voltage amplitude is calculated based on the angle between the decomposed PCC voltage and each phase sequence voltage, specifically including: By adding the positive and negative sequence voltages and combining them with the zero sequence voltage and the rotating vector, a right-angled triangle relationship is obtained. Based on the lag-lead relationship between phase B voltage and phase C voltage and phase A voltage, and combined with the right triangle relationship, the phase voltage amplitude of PCC voltage is calculated. The reference current is calculated based on the constraint of the phase voltage amplitude, and the reference current is used as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

2. The method for limiting the voltage of low-voltage transformer inverters when there is grid voltage imbalance according to claim 1, characterized in that, The expression for the phase voltage amplitude of the PCC voltage is: In the above formula, V + , V - This indicates the magnitude of the positive and negative sequence voltages of the PCC. V g + and V g - Indicates the magnitude of the positive and negative sequence voltages of the power grid. L and R It is the inductance and resistance of the power grid. This represents the magnitude of the positive-sequence active current and the magnitude of the negative-sequence reactive current. This represents the angular frequency of the grid voltage.

3. The method for limiting the voltage of low-voltage transformer inverters when there is grid voltage imbalance according to claim 1, characterized in that, The reference current is calculated based on the constraint of the phase voltage amplitude, specifically including: Calculate the positive and negative sequence reference voltages based on the constraints of the phase voltage amplitudes; The positive and negative sequence reference voltages are solved to obtain the reference values ​​of the positive and negative sequence voltage amplitudes; Substituting the magnitudes of the positive and negative sequence voltages obtained by decomposing the PCC voltage at the common coupling point into the reference value of the positive and negative sequence voltage magnitudes yields two inequalities; The positive and negative sequence reactive currents are obtained by solving the two inequalities based on impedance matching current. Substituting the positive and negative sequence reactive currents into the reference current in the two-phase stationary coordinate system, we obtain the final reference current, which is the reference value of the current control loop.

4. The method for limiting the voltage of inverters in low-voltage distribution areas when there is grid voltage imbalance according to claim 3, characterized in that, The positive and negative sequence reference voltages are calculated based on the constraints of the phase voltage amplitude, specifically including: When the grid voltage is unbalanced, the upper and lower voltage limits required for the inverter's PCC point in the low-voltage distribution area; Extract the maximum and minimum values ​​of the phase voltage amplitude from the PCC voltage amplitude; The upper and lower limit amplitude requirements of the voltage at the PCC point in the low-voltage distribution area are calculated, and the maximum and minimum values ​​of the phase voltage amplitude are extracted from the PCC voltage amplitude to calculate the positive and negative sequence reference voltages.

5. The method for limiting the voltage of a low-voltage transformer inverter when there is grid voltage imbalance according to claim 3, characterized in that, When the grid voltage is unbalanced, the inverter's control loop is set to a single current loop.

6. A voltage limiting system for low-voltage transformer inverters when there is grid voltage imbalance, wherein the voltage limiting system for low-voltage transformer inverters when there is grid voltage imbalance applies the voltage limiting method for low-voltage transformer inverters when there is grid voltage imbalance as described in any one of claims 1-5, characterized in that, include: The PCC voltage detection module is used to detect and decompose the PCC voltage at the common coupling point, and to calculate the angle between each phase sequence voltage. The common coupling point (PCC) voltage is detected and decomposed, specifically including: Acquire instantaneous values ​​of PCC voltage and current; The instantaneous values ​​of PCC voltage and current are transformed from three-phase instantaneous values ​​to two-phase stationary coordinates through Clark transformation. Then, the positive and negative sequence two-phase stationary voltages and the included angles of each vector are obtained through a dual second-order generalized integrator phase-locked loop. The amplitude of the positive and negative sequence voltages from PCC to the low-voltage grid is obtained in combination with the low-voltage grid situation. The PCC phase voltage amplitude calculation module is used to calculate the phase voltage amplitude based on the decomposed PCC voltage. The PCC phase voltage amplitude calculation module is specifically used for: By adding the positive and negative sequence voltages and combining them with the zero sequence voltage and the rotating vector, a right-angled triangle relationship is obtained. Based on the lag-leader relationship between phase B and phase C voltages and phase A voltage, and combined with the aforementioned right-angled triangle relationship, the phase voltage amplitude of the PCC voltage is calculated; and The current control loop module calculates a reference current based on the constraint of the phase voltage amplitude, and uses the reference current as the input of the proportional resonant controller to complete the amplitude limiting control of the PCC voltage.

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