A distributed photovoltaic voltage over-limit control method

Abstract

The present application relates to a kind of distributed photovoltaic voltage over-limit control method, for the voltage lifting problem of distribution network caused by photovoltaic high penetration rate, the voltage difference relationship model of grid-connected point and power grid voltage is established with photovoltaic power, load power and line impedance;Inverter model is used to construct inverter voltage and current double-loop control;Deduce the transfer function of grid-connected point and power grid voltage difference relative to inverter reactive current, based on transfer function design inverter voltage and current double-loop PWM control voltage control loop proportional integral regulator;Focus on whether grid-connected point voltage is over-limit, whether the limit of reference q-axis current regulation output is reached;When grid-connected point voltage is over-limit, according to whether the limit of reference q-axis current regulation output is reached to iterate according to difference adjustment reference q-axis current and reduce bus voltage reference value, until grid-connected point voltage does not exceed voltage over-limit value, finally under the premise of guaranteeing that active power output maximization, effectively solve voltage over-limit problem.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic control technology, and in particular to a method for controlling voltage over-limit in distributed photovoltaic systems. Background Technology

[0002] The statements in this section are merely background information relating to this disclosure and do not necessarily constitute prior art.

[0003] Distribution networks are not ideal zero-impedance conductors, but rather physical entities with resistance and inductance. In traditional models, electrical energy flows unidirectionally from the substation to the load end, with voltage naturally decreasing along the line. However, with the increasing penetration of photovoltaic (PV) systems in distribution networks, when distributed PV systems are connected to the end and generate high-power electricity, the power flow direction reverses. At this point, the PV current is superimposed on the original impedance, and the resulting voltage drop no longer offsets the source voltage but is superimposed on the end voltage. This is equivalent to applying reverse voltage at the end, causing a passive rise in the grid connection voltage, which affects grid stability. If the voltage rise is too high, it can even damage household appliances, leading to the decommissioning of PV systems. In practical applications, reactive power compensation is often added for regulation, but this not only increases the initial investment of the system but also leads to increased maintenance frequency and failure rate over time due to the presence of components. Summary of the Invention

[0004] To solve the above-mentioned technical problems, or at least partially solve them, the present invention provides a method for controlling voltage over-limit in distributed photovoltaic systems.

[0005] This invention provides a method for controlling voltage over-limit in distributed photovoltaic systems, comprising:

[0006] During operation, the grid connection point voltage is collected in real time to detect whether the grid connection point voltage exceeds the preset voltage over-limit and whether the output adjustment according to the reference q-axis current reaches the limit. If the preset voltage over-limit is exceeded, but the output adjustment according to the reference q-axis current does not reach the limit, the proportional-integral regulator of the voltage control loop generates a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit. The inverter adjusts the output according to the reference q-axis current to reduce the grid connection point voltage. If the output adjustment according to the reference q-axis current reaches the inverter limit and the grid connection point voltage still exceeds the limit, the output is adjusted by gradually reducing the reference value of the distributed photovoltaic bus voltage to reduce the grid connection point voltage and provide more adjustment space for the output adjustment according to the reference q-axis current. The two work iteratively until the grid connection point voltage does not exceed the voltage over-limit.

[0007] Furthermore, the established model relating the voltage difference between the grid connection point and the grid to the voltage difference between photovoltaic power, load power, and line impedance is as follows:

[0008] ;

[0009] in, This refers to the active power of distributed photovoltaic systems. The active load of the local load at the grid connection point; The reactive power of distributed photovoltaic power generation; The reactive load of the local load at the grid connection point; For line resistance and For line reactance; The voltage at the grid connection point; This refers to the grid voltage, where the grid is considered an ideal voltage source. It is constant, and electrical energy is delivered to the grid connection point through the line impedance; the grid connection point voltage U is dynamically affected by the power flow flowing through the line.

[0010] Furthermore, the process of establishing an inverter model in a synchronous rotating coordinate system includes:

[0011] Inverter model constructed in a three-phase stationary coordinate system:

[0012] ;

[0013] in, and These are the components coupled between the inverter and the power grid. Inductors on the inverter side and grid side of the filter; , and This refers to the three-phase current on the output side of the inverter bridge of the inverter; , and This refers to the three-phase voltage on the output side of the inverter bridge of the inverter. , and The three-phase voltage at the grid connection point; This is the DC bus voltage of the inverter bridge in the inverter. This is the voltage between the neutral point and ground in the power grid. , These are the capacitors on the upper and lower sides of the inverter bus. For the output current of the Boost converter; The inverter bridge input current of the inverter;

[0014] The inverter model in the three-phase stationary coordinate system is transformed by Park to obtain the inverter model in the two-phase synchronous rotating dq coordinate system:

[0015] ;

[0016] in, The angular frequency of the grid voltage; and These are the three-phase voltages on the output side of the inverter bridge. Axial components; and These are the three-phase currents on the output side of the inverter bridge. Axial components; and These are the grid connection point voltages. Axial components; These are the inverter bridge switching functions. Axial components.

[0017] Furthermore, based on the mathematical model of the photovoltaic inverter, a dual-loop control system of voltage and current is adopted. The voltage control loop stabilizes the DC bus voltage and outputs the inner loop setpoint of the inverter's d-axis current. The current control loop controls the inverter bridge output current to track the setpoint, incorporating the grid connection point voltage. Shaft component and coupled inverter bridge output current The axial components are decoupled by feedforward, and finally the switching signals of each switch of the inverter bridge are generated by space vector pulse width modulation.

[0018] Furthermore, the transfer function of the voltage difference between the grid connection point and the grid voltage relative to the inverter reactive current is derived. Based on this transfer function, a voltage control loop proportional-integral regulator for dual-loop PWM control of inverter voltage and current is designed, including:

[0019] Under grid voltage orientation, the d-axis voltage output by the distributed photovoltaic inverter is oriented towards the grid connection point voltage vector. The reactive power injected into the grid by the distributed photovoltaic inverter is:

[0020] ;

[0021] in, and These are the three-phase voltages at the grid connection point. Shaft component and three-phase current on the output side of the inverter bridge Axial components;

[0022] Substituting reactive power into the voltage difference relationship model yields the transfer function of voltage difference and inverter q-axis current, as well as the open-loop transfer function of the voltage control loop.

[0023] Based on the open-loop transfer function, the proportional coefficient of the voltage control loop proportional-integral controller is tuned by setting the desired stability index. and integral coefficient .

[0024] Furthermore, by substituting reactive power into the voltage difference relationship model, and applying a set signal disturbance near the steady-state operating point, the relationship between the voltage difference change and the q-axis current change is obtained to determine the transfer function gain of the voltage difference change relative to the inverter q-axis current.

[0025] ;

[0026] Based on the transfer function gain and the influence of the current control loop, the open-loop transfer function of the voltage control loop is:

[0027] ;

[0028] in, This refers to the proportional coefficient of the proportional-integral controller in the voltage control loop. Let be the integral coefficient of the proportional-integral controller in the voltage control loop, and s be the Laplace operator. The first-order inertia introduced by the current control loop; among which... is the equivalent time constant of the current control loop.

[0029] Furthermore, the reference q-axis current command is calculated by the proportional-integral regulator of the voltage control loop and is expressed as:

[0030] ;

[0031] in, This is the voltage exceeding the limit.

[0032] Furthermore, when the cumulative decrease in the reference value of the distributed photovoltaic bus voltage reaches a set value, the proportional-integral regulator of the voltage control loop is cyclically activated to generate a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit value, and the inverter adjusts its output according to the reference q-axis current.

[0033] Furthermore, when the grid connection point voltage meets the voltage over-limit requirements, priority is given to increasing the active power output of distributed photovoltaics and reducing the reactive power output of distributed photovoltaics, thereby maximizing the active power of distributed photovoltaics while meeting the inverter capacity constraints.

[0034] Secondly, the present invention provides a distributed photovoltaic voltage over-limit control device, comprising: at least one processing unit, the processing unit being connected to a storage unit via a bus unit, the storage unit storing a computer program that can run on a processor, and the processing unit implementing the distributed photovoltaic voltage over-limit control method by running the computer program stored in the storage unit.

[0035] The technical solutions provided in the embodiments of the present invention have the following advantages compared with the prior art:

[0036] This invention establishes a transfer function between the grid connection point voltage and the inverter's q-axis current, and designs a voltage control loop for the distributed photovoltaic (PV) inverter based on this transfer function. It employs a hierarchical alternating control strategy. When the grid connection point voltage exceeds the limit, it iteratively adjusts the reference q-axis current and lowers the bus voltage reference value based on the voltage difference, depending on whether the output adjustment according to the reference q-axis current reaches its limit, until the grid connection point voltage does not exceed the voltage over-limit value. The invention utilizes the inverter's q-axis current to change the reactive power of the distributed PV inverter to regulate the grid connection point voltage, and gradually lowers the distributed PV bus voltage reference value to adjust the output, thereby reducing the grid connection point voltage and providing more adjustment space for adjusting the output according to the reference q-axis current. These two approaches iteratively cooperate until the grid connection point voltage does not exceed the voltage over-limit value. When the grid connection point voltage meets the voltage over-limit requirement, it prioritizes increasing the distributed PV active power output and reducing the distributed PV reactive power output, maximizing the distributed PV active power while meeting inverter capacity constraints. This achieves a balance between voltage over-limit management and maximizing active power output. Attached Figure Description

[0037] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0038] 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, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 A flowchart of a distributed photovoltaic voltage over-limit control method provided in an embodiment of the present invention;

[0040] Figure 2 A schematic diagram of the equivalent model of distributed photovoltaic grid connection provided in an embodiment of the present invention;

[0041] Figure 3 A schematic diagram of voltage and current dual-loop control of an inverter provided in an embodiment of the present invention;

[0042] Figure 4 The voltage reduction effect diagram is shown for the distributed photovoltaic voltage over-limit control method provided in the embodiment of the present invention.

[0043] Figure 5 This is a schematic diagram of a distributed photovoltaic voltage over-limit control device provided in an embodiment of the present invention. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, 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.

[0045] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0046] Example 1

[0047] like Figure 1 As shown, a distributed photovoltaic voltage over-limit control method of the present invention includes:

[0048] S100, construct an equivalent model of distributed photovoltaic grid connection that can reflect the power flow of the power grid, such as Figure 2 As shown, the components of the equivalent model for distributed photovoltaic grid connection include: grid voltage. The equivalent resistance R and reactance X of the line, and the active power output of the distributed photovoltaic system. and reactive power Active load of local load at the grid connection point and reactive load And the grid connection point voltage U.

[0049] An equivalent model of distributed photovoltaic (PV) grid connection is used to model the relationship between the voltage difference between the grid connection point and the grid, and the PV power, load power, and line impedance. Specifically, the voltage difference relationship model models the relationship between the voltage difference between the grid connection point and the grid voltage, the active and reactive loads at the grid connection point, the active and reactive power of the distributed PV, and the equivalent line impedance.

[0050] The voltage difference relationship model is as follows:

[0051] ;

[0052] The power grid is considered an ideal voltage source. It is constant, and electrical energy is delivered to the grid connection point through the line impedance; the grid connection point voltage U is dynamically affected by the power flow flowing through the line.

[0053] According to the voltage difference relationship model, the voltage difference mainly consists of two parts. The first part depends on the difference between the photovoltaic active power and the active load of the local load at the grid connection point, and the equivalent resistance of the line. The second part depends on the difference between the photovoltaic reactive power and the reactive load of the local load at the grid connection point, and the equivalent reactance of the line. When the distributed photovoltaic power is large and the local load at the grid connection point is low to a certain extent, the power flow reverses, which will cause the line voltage drop to change from negative to positive, directly raising the voltage at the grid connection point.

[0054] S200: Establish an inverter model of the photovoltaic inverter in a synchronous rotating coordinate system, and use the inverter model to construct a dual-loop PWM control for inverter voltage and current.

[0055] In the specific implementation process, an inverter model is constructed in a three-phase stationary coordinate system:

[0056] ;

[0057] in, and These are the components coupled between the inverter and the power grid. Inductors on the inverter side and grid side of the filter; , and This refers to the three-phase current on the output side of the inverter bridge of the inverter; , and This refers to the three-phase voltage on the output side of the inverter bridge of the inverter. , and The three-phase voltage at the grid connection point; This is the DC bus voltage of the inverter bridge in the inverter. This is the voltage between the neutral point and ground in the power grid. , These are the capacitors on the upper and lower sides of the inverter bus. For the output current of the Boost converter; The inverter bridge input current of the inverter;

[0058] The inverter model in the three-phase stationary coordinate system is transformed by Park to obtain the inverter model in the two-phase synchronous rotating dq coordinate system:

[0059] ;

[0060] in, The angular frequency of the grid voltage; and These are the three-phase voltages on the output side of the inverter bridge. Axial components; and These are the three-phase currents on the output side of the inverter bridge. Axial components; and These are the grid connection point voltages. Axial components; These are the inverter bridge switching functions. Axial components.

[0061] Based on the mathematical model of the photovoltaic inverter, a dual-loop control system of voltage and current is adopted. The voltage control loop stabilizes the DC bus voltage and outputs the inner loop setpoint of the inverter's d-axis current. The current control loop controls the inverter bridge output current to track the setpoint, and the grid connection point voltage is added. Shaft component and coupled inverter bridge output current The axial components are decoupled via feedforward, and finally, the switching signals for each switch in the inverter bridge are generated using space vector pulse width modulation. The voltage control loop is designed to accommodate over-limit control.

[0062] S300, derive the transfer function of the voltage difference between the grid connection point and the grid with respect to the inverter reactive current, and design a voltage control loop proportional-integral regulator for dual-loop PWM control of inverter voltage and current based on the transfer function.

[0063] In the specific implementation process, under grid voltage orientation, the d-axis of the synchronous rotating coordinate system is oriented to the grid connection point voltage vector. At this time, the reactive power injected into the grid by the distributed photovoltaic inverter is:

[0064] ;

[0065] Substituting reactive power into the voltage difference relationship model yields the relationship between voltage difference and inverter q-axis current.

[0066] Near the steady-state operating point, by applying a set signal disturbance, the relationship between the voltage difference change and the q-axis current change is obtained to determine the transfer function gain of the voltage difference change relative to the inverter q-axis current; where the voltage difference change originates from the grid connection point voltage change, and the transfer function gain of the voltage difference change relative to the inverter q-axis current is:

[0067] ;

[0068] Based on the transfer function gain and the influence of the current control loop, the open-loop transfer function of the voltage control loop is:

[0069] ;

[0070] in, This refers to the proportional coefficient of the proportional-integral controller in the voltage control loop. The integral coefficient of the proportional-integral controller in the voltage control loop is set according to the system stability requirements.

[0071] In practical applications, the inner current loop after being tuned by the proportional-integral regulator of the current control loop is equivalent to a first-order inertial element. The voltage control loop introduces the first-order inertia brought about by the current control loop. ;in, is the equivalent time constant of the current control loop.

[0072] Based on the open-loop transfer function, the proportional coefficient of the voltage control loop proportional-integral controller is tuned by setting the desired stability index. and integral coefficient Stability metrics include phase margin and cutoff frequency, thereby ensuring the dynamic response and steady-state performance of the voltage control loop.

[0073] The voltage control loop proportional-integral regulator generates a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit value. The inverter adjusts its output according to the reference q-axis current to reduce the grid connection point voltage.

[0074] During operation, the S400 collects the grid connection point voltage in real time, detects whether the grid connection point voltage exceeds the preset voltage limit, and analyzes whether the output adjustment according to the reference q-axis current reaches the limit. In specific implementation, the grid connection point voltage is filtered using a filtering algorithm, and the filtered grid connection point voltage is detected to see if it exceeds the voltage limit.

[0075] If the voltage limit is not exceeded, then S500 is executed.

[0076] If the voltage exceeds the limit, and the output is adjusted according to the reference q-axis current but does not reach the limit, then S600 is executed.

[0077] If the output reaches the inverter limit after adjusting the reference q-axis current and the grid connection point voltage still exceeds the limit, then execute S700.

[0078] The S500 inverter optimizes the allocation of reactive and active power. Under the premise of not exceeding voltage limits and inverter capacity constraints, it gradually reduces reactive power output while simultaneously increasing active power output according to maximum power point tracking (MPPT) until it reaches the maximum active power point within the voltage tolerance range, thus maximizing the active power of distributed photovoltaic (PV) power. While not exceeding voltage limits, it optimizes the allocation of active and reactive power according to inverter capacity constraints to maximize the economic benefits of PV power generation.

[0079] S600, the voltage control loop proportional-integral regulator generates a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit value, and the inverter adjusts the output according to the reference q-axis current to reduce the grid connection point voltage.

[0080] The reference q-axis current command is calculated by the proportional-integral regulator of the voltage control loop and is expressed as:

[0081] ;

[0082] in, This is the voltage exceeding the limit.

[0083] The S700 adjusts its output to reduce the grid connection point voltage until it does not exceed the limit by gradually lowering the reference value of the distributed photovoltaic bus voltage.

[0084] In the control of grid-connected photovoltaic (PV) inverters, lowering the reference value of the distributed PV bus voltage essentially deviates the PV array from the maximum power point tracking (MPPT) mode, entering a proactive power-limiting operation mode. Normally, the bus voltage reference value is provided by the MPPT algorithm, aiming to draw maximum power. Lowering the bus voltage reference value causes the voltage control loop to detect a positive deviation. To eliminate this deviation, the voltage control loop reduces its output, which is then sent to the current control loop. The current control loop, through its proportional-integral (PI) regulator, adjusts and decouples the current, forcing a reduction in the actual active current output of the inverter, thereby reducing the active power injected into the grid. According to the principle of line voltage drop, the reduction in active power directly leads to a decrease in the grid connection point voltage, ultimately suppressing voltage over-limiting.

[0085] The current control loop reduces the active current, directly decreasing the active power. According to the voltage difference relationship model, when the active power decreases, the active voltage drop on the line decreases, thus lowering the grid connection point voltage. Moreover, due to the reduction in active current, the inverter's total apparent power capacity is released, increasing the scope for adjusting the output according to the reference q-axis current to reduce the grid connection point voltage.

[0086] Therefore, when the cumulative decrease in the reference value of the distributed photovoltaic bus voltage reaches the set value, the proportional-integral regulator of the voltage control loop is activated to generate a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit value, and the inverter adjusts the output according to the reference q-axis current.

[0087] Example 2

[0088] like Figure 5 As shown, this embodiment of the invention provides a distributed photovoltaic voltage over-limit control device, including: at least one processing unit, the processing unit being connected to a storage unit via a bus unit, the storage unit serving as a computer-readable storage medium, which can be used to store software programs, computer-executable programs, and modules, such as the software program, computer-executable program, and module corresponding to a distributed photovoltaic voltage over-limit control method in this embodiment of the invention. The processing unit implements the aforementioned distributed photovoltaic voltage over-limit control method by running the software program, computer-executable program, and module stored in the storage unit, including:

[0089] Establish a model relating the voltage difference between the grid connection point and the grid to the voltage difference between photovoltaic power, load power, and line impedance;

[0090] An inverter model is established in a synchronous rotating coordinate system, and a dual-loop control of inverter voltage and current is constructed using the inverter model.

[0091] The transfer function of the voltage difference between the grid connection point and the grid with respect to the inverter reactive current is derived, and a voltage control loop proportional-integral regulator for dual-loop PWM control of inverter voltage and current is designed based on the transfer function.

[0092] During operation, the grid-connected point voltage is collected in real time to detect whether the grid-connected point voltage exceeds a preset voltage over-limit value and whether the output adjustment according to the reference q-axis current reaches its limit. If it exceeds the preset voltage over-limit value, but the output adjustment according to the reference q-axis current does not reach its limit, the proportional-integral regulator of the voltage control loop generates a reference q-axis current based on the difference between the grid-connected point voltage and the voltage over-limit value. The inverter adjusts its output according to the reference q-axis current to reduce the grid-connected point voltage. If the output adjustment according to the reference q-axis current reaches the inverter's limit and the grid-connected point voltage still exceeds the limit, the output is adjusted by gradually reducing the reference value of the distributed photovoltaic bus voltage to reduce the grid-connected point voltage and provide more adjustment space for the output adjustment according to the reference q-axis current. The two work iteratively until the grid-connected point voltage does not exceed the voltage over-limit value. Of course, the computer program stored in the memory of the distributed photovoltaic voltage over-limit control device provided in this embodiment is not limited to the operation of the method described above, and can also execute related operations in the distributed photovoltaic voltage over-limit control method provided in any embodiment of this invention.

[0093] Example 3

[0094] This invention provides a computer-readable storage medium storing a computer program, characterized in that, when executed, the computer program implements the distributed photovoltaic voltage over-limit control method, comprising:

[0095] Establish a model relating the voltage difference between the grid connection point and the grid to the voltage difference between photovoltaic power, load power, and line impedance;

[0096] An inverter model is established in a synchronous rotating coordinate system, and a dual-loop control of inverter voltage and current is constructed using the inverter model.

[0097] The transfer function of the voltage difference between the grid connection point and the grid with respect to the inverter reactive current is derived, and a voltage control loop proportional-integral regulator for dual-loop PWM control of inverter voltage and current is designed based on the transfer function.

[0098] During operation, the grid connection point voltage is collected in real time to detect whether the grid connection point voltage exceeds the preset voltage over-limit and whether the output adjustment according to the reference q-axis current reaches the limit. If the preset voltage over-limit is exceeded, but the output adjustment according to the reference q-axis current does not reach the limit, the proportional-integral regulator of the voltage control loop generates a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit. The inverter adjusts the output according to the reference q-axis current to reduce the grid connection point voltage. If the output adjustment according to the reference q-axis current reaches the inverter limit and the grid connection point voltage still exceeds the limit, the output is adjusted by gradually reducing the reference value of the distributed photovoltaic bus voltage to reduce the grid connection point voltage and provide more adjustment space for the output adjustment according to the reference q-axis current. The two work iteratively until the grid connection point voltage does not exceed the voltage over-limit.

[0099] In the embodiments provided by this invention, it should be understood that the disclosed structures and methods can be implemented in other ways. For example, the structural embodiments described above are merely illustrative. For instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another assembly system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection through some interfaces, structures, or units, and may be electrical, mechanical, or other forms.

[0100] 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 units can be selected to achieve the purpose of this embodiment according to actual needs.

[0101] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0102] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for controlling voltage over-limit in distributed photovoltaic systems, characterized in that, include: Establish a model relating the voltage difference between the grid connection point and the grid to the voltage difference between photovoltaic power, load power, and line impedance; An inverter model is established in a synchronous rotating coordinate system, and a dual-loop control of inverter voltage and current is constructed using the inverter model. The transfer function of the voltage difference between the grid connection point and the grid with respect to the inverter reactive current is derived, and a voltage control loop proportional-integral regulator for dual-loop PWM control of inverter voltage and current is designed based on the transfer function. During operation, the grid connection point voltage is collected in real time to detect whether the grid connection point voltage exceeds the preset voltage limit and to analyze whether the output is adjusted according to the reference q-axis current to reach the limit. If the preset voltage limit is exceeded and the output is adjusted according to the reference q-axis current to reach the limit, the voltage control loop proportional-integral regulator generates a reference q-axis current based on the difference between the grid connection point voltage and the voltage limit. The inverter adjusts the output according to the reference q-axis current to reduce the grid connection point voltage. If the output reaches the inverter limit after adjusting according to the reference q-axis current and the grid connection point voltage still exceeds the limit, the output is adjusted by gradually reducing the reference value of the distributed photovoltaic bus voltage to reduce the grid connection point voltage and provide more adjustment space for adjusting the output according to the reference q-axis current; the two are iteratively coordinated until the grid connection point voltage does not exceed the voltage limit value.

2. The distributed photovoltaic voltage over-limit control method according to claim 1, characterized in that, The established model relating the voltage difference between the grid connection point and the grid to the voltage difference between photovoltaic power, load power, and line impedance is as follows: ； in, This refers to the active power of distributed photovoltaic systems. The active load of the local load at the grid connection point; The reactive power of distributed photovoltaic power generation; The reactive load of the local load at the grid connection point; For line resistance and For line reactance; The voltage at the grid connection point; This refers to the grid voltage, where the grid is considered an ideal voltage source. It is constant, and electrical energy is delivered to the grid connection point through the line impedance; the grid connection point voltage U is dynamically affected by the power flow flowing through the line.

3. The distributed photovoltaic voltage over-limit control method according to claim 1, characterized in that, The process of establishing an inverter model in a synchronous rotating coordinate system includes: Inverter model constructed in a three-phase stationary coordinate system: ； in, and These are the components coupled between the inverter and the power grid. Inductors on the inverter side and grid side of the filter; , and This refers to the three-phase current on the output side of the inverter bridge of the inverter; , and This refers to the three-phase voltage on the output side of the inverter bridge of the inverter. , and The three-phase voltage at the grid connection point; This is the DC bus voltage of the inverter bridge in the inverter; This is the voltage between the neutral point and ground in the power grid. , These are the capacitors on the upper and lower sides of the inverter bus. For the output current of the Boost converter; The inverter bridge input current of the inverter; The inverter model in the three-phase stationary coordinate system is transformed by Park to obtain the inverter model in the two-phase synchronous rotating dq coordinate system: ； in, The angular frequency of the grid voltage; and These are the three-phase voltages on the output side of the inverter bridge. Axial components; and These are the three-phase currents on the output side of the inverter bridge. Axial components; and These are the grid connection point voltages. Axial components; These are the inverter bridge switching functions. Axial components.

4. The distributed photovoltaic voltage over-limit control method according to claim 3, characterized in that, Based on the mathematical model of the photovoltaic inverter, a dual-loop control system of voltage and current is adopted. The voltage control loop stabilizes the DC bus voltage and outputs the inner loop setpoint of the inverter's d-axis current. The current control loop controls the inverter bridge output current to track the setpoint, and the grid connection point voltage is added. Shaft component and coupled inverter bridge output current The axial components are decoupled by feedforward, and finally the switching signals of each switch of the inverter bridge are generated by space vector pulse width modulation.

5. The distributed photovoltaic voltage over-limit control method according to claim 1, characterized in that, The transfer function of the voltage difference between the grid connection point and the grid relative to the inverter reactive current is derived. Based on this transfer function, a voltage control loop proportional-integral regulator for dual-loop PWM control of inverter voltage and current is designed, including: Under grid voltage orientation, the d-axis voltage output by the distributed photovoltaic inverter is oriented towards the grid connection point voltage vector. The reactive power injected into the grid by the distributed photovoltaic inverter is: ； in, and These are the three-phase voltages at the grid connection point. Shaft component and three-phase current on the output side of the inverter bridge Axial components; Substituting reactive power into the voltage difference relationship model yields the transfer function of voltage difference and inverter q-axis current, as well as the open-loop transfer function of the voltage control loop. Based on the open-loop transfer function, the proportional coefficient of the voltage control loop proportional-integral controller is tuned by setting the desired stability index. and integral coefficient .

6. The distributed photovoltaic voltage over-limit control method according to claim 5, characterized in that, By substituting reactive power into the voltage difference relationship model, and applying a set signal disturbance near the steady-state operating point, the relationship between the voltage difference change and the q-axis current change is obtained to determine the transfer function gain of the voltage difference change relative to the inverter q-axis current. ； Based on the transfer function gain and the influence of the current control loop, the open-loop transfer function of the voltage control loop is: ； in, This is the grid voltage. The angular frequency of the grid voltage. and These are the components coupled between the inverter and the power grid. Inverter-side and grid-side inductors of the filter, This refers to the proportional coefficient of the proportional-integral controller in the voltage control loop. Let be the integral coefficient of the proportional-integral controller in the voltage control loop, and s be the Laplace operator. The first-order inertia introduced by the current control loop; among which... is the equivalent time constant of the current control loop.

7. The distributed photovoltaic voltage over-limit control method according to claim 5, characterized in that, The reference q-axis current command is calculated by the proportional-integral regulator of the voltage control loop and is expressed as: ； Where U is the grid connection point voltage, Let be the voltage limit value, and s be the Laplace operator.

8. The distributed photovoltaic voltage over-limit control method according to claim 1, characterized in that, When the cumulative decrease in the reference value of the distributed photovoltaic bus voltage reaches the set value, the proportional-integral regulator of the voltage control loop is cyclically activated to generate a reference q-axis current based on the difference between the grid connection point voltage and the voltage over-limit value, and the inverter adjusts the output according to the reference q-axis current.

9. The distributed photovoltaic voltage over-limit control method according to claim 1, characterized in that, When the grid connection point voltage meets the voltage over-limit requirements, priority should be given to increasing the active power output of distributed photovoltaic (PV) and reducing the reactive power output of distributed PV. Under the condition of meeting the inverter capacity constraints, the active power of distributed PV should be maximized.

10. A distributed photovoltaic voltage over-limit control device, comprising: At least one processing unit, the processing unit being connected to a storage unit via a bus unit, the storage unit storing a computer program that can run on a processor, characterized in that the processing unit implements the distributed photovoltaic voltage over-limit control method as described in any one of claims 1-9 by running the computer program stored in the storage unit.

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