Distributed photovoltaic cooperative voltage support control method based on direct current voltage detection

Abstract

This invention discloses a distributed photovoltaic (PV) coordinated voltage support control method based on DC voltage detection. First, the PV DC-side voltage is detected in real time, and the positive-sequence active / reactive current reference values ​​output from the PV AC side are dynamically corrected based on the PV DC-side voltage. Next, the negative-sequence reactive current output from the PV AC side under overvoltage constraints, the negative-sequence reactive current of each phase output from the PV AC side under active power oscillation constraints, and the maximum negative-sequence reactive current of each phase output from the PV AC side under overcurrent constraints are calculated. When the maximum negative-sequence reactive current output from the PV AC side is less than or equal to the phase current limit, there is no need to reduce the positive-sequence reactive current output from the PV DC side; otherwise, the positive-sequence reactive current output from the PV DC side needs to be reduced. The negative-sequence reactive current reference value is the minimum value of the negative-sequence reactive current under overvoltage, active power oscillation, and overcurrent constraints. By judging the PV operating status, the inverter capacity is fully utilized to maximize the voltage support effect.

Description

Technical Field

[0001] This invention belongs to the field of power distribution network control technology, and in particular to a distributed photovoltaic coordinated voltage support control method based on DC voltage detection. Background Technology

[0002] The integration of distributed renewable energy sources significantly weakens the voltage support capability of distribution networks during faults, posing a major challenge to fault ride-through. During a fault, when both distributed renewable energy sources and the distribution network struggle to provide effective voltage support, a cascading disconnection of distributed renewable energy sources from the grid may occur. Therefore, utilizing inverter-interfaced distributed power sources to provide voltage support during faults, rather than solely relying on the distribution network, has become a crucial measure to improve the fault ride-through capability of distribution networks.

[0003] Existing voltage support control methods are mainly divided into two categories: single-unit fault ride-through control and multi-unit coordinated control. Single-unit fault ride-through control methods, including constant current method, linear current method, and droop voltage regulation method, have advantages such as low computational load and fast response speed. However, they lack flexible adjustment capabilities in different fault scenarios, and the capacity of a single renewable energy device is limited; therefore, coordinating multiple devices is necessary to meet the voltage support requirements during faults. During a fault, the available power of each renewable energy device varies due to its different operating conditions, directly affecting the voltage support effect. Therefore, accurately estimating the real-time support capacity of each renewable energy device and setting its output current accordingly is crucial to maximizing the voltage support effect during distribution network faults.

[0004] Multi-machine coordinated voltage support control considers the coordinated control of multiple converters, but it does not take into account the upper limit constraint of node voltage, and this type of method is mainly applicable to strong grid conditions. In the field of distribution networks, existing coordinated control is mainly divided into two categories: centralized control and distributed control. Centralized control methods usually rely on communication networks to adjust the output of each inverter interface-type distributed power source, but real-time communication and rapid calculation during faults are difficult to guarantee; distributed control methods achieve rapid voltage support through local measurement, but this type of method often requires additional hardware / communication equipment, and mainly addresses steady-state voltage imbalance problems, rarely considering the impact of the real-time available capacity of the inverter and the active current output of distributed renewable energy on fault ride-through performance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to propose a distributed photovoltaic coordinated voltage support control method based on DC voltage detection.

[0006] The present invention solves the aforementioned technical problem by adopting the following technical solution: A distributed photovoltaic coordinated voltage support control method based on DC voltage detection, characterized by the following steps: Step 1: Obtain the grid connection point voltage. If the grid connection point voltage is less than 0.9pu, then detect the photovoltaic DC side voltage. If the photovoltaic DC side voltage satisfies equation (4), then obtain the positive sequence active / reactive current reference value of the photovoltaic AC side output according to equation (5). If the photovoltaic DC side voltage does not satisfy equation (4), then obtain the positive sequence active / reactive current reference value of the photovoltaic AC side output according to equation (10). (4) (5) (10) In the formula, Indicates the DC side voltage of the photovoltaic system. Indicates the DC-side current of the photovoltaic system. , , Indicates the AC side voltage of the photovoltaic system. Indicates the inverter's rated current. Indicates the equivalent resistance on the grid side. Indicates the equivalent reactance on the grid side. This indicates the DC voltage at which the photovoltaic system is operating at its rated capacity. Represents the logarithmic function of the product. Indicates time, , This represents the reference values ​​for the positive-sequence active and reactive current output from the photovoltaic AC side. This indicates the maximum active current on the DC side of the photovoltaic system. Step 2: Under asymmetrical fault conditions, considering overvoltage, active power oscillation, and overcurrent constraints, the reference values ​​of positive / negative sequence reactive current output from the photovoltaic AC side are corrected. Negative sequence reactive current output from the photovoltaic AC side under overvoltage constraint for: (14) In the formula, This represents the equivalent negative sequence voltage on the grid side. Indicates the positive sequence voltage at the grid connection point. Indicates the voltage at the grid connection point. This represents the initial phase difference between the positive and negative sequence voltages at the grid connection point; The range of values ​​for the negative sequence reactive current of each phase output from the photovoltaic AC side under active power oscillation constraint is as follows: (16) In the formula, The first output of the photovoltaic AC side under active power oscillation constraint Negative sequence reactive current, This represents the maximum oscillation amplitude of the active power output from the AC side of the photovoltaic system. , The output of the photovoltaic AC side is the first Positive-sequence voltage and negative-sequence voltage, , The output of the photovoltaic AC side is the first Phase-sequence active and reactive currents; The maximum negative sequence reactive current of each phase output from the photovoltaic AC side under overcurrent constraint is expressed as: (18) In the formula, The first output of the photovoltaic AC side Maximum negative sequence reactive current of phase This refers to the phase current limit output from the photovoltaic AC side. When the maximum negative sequence reactive current output by the photovoltaic AC side is less than or equal to the phase current limit, the reference value of the positive / negative sequence reactive current output by the photovoltaic AC side is still obtained according to equation (10); otherwise, the reference value of the positive / negative sequence reactive current output by the photovoltaic AC side is obtained according to equation (19). (19) (20) In the formula, , These represent the first output of the photovoltaic AC side. Phase positive / negative sequence reactive current reference values The first output of the photovoltaic AC side Negative sequence reactive current, This is the equivalent positive sequence voltage on the grid side. This represents the overvoltage amplitude at the grid connection point.

[0007] Compared with the prior art, the beneficial effects of the present invention are: This invention determines the photovoltaic (PV) operating status solely by detecting the DC-side voltage. It dynamically corrects the positive-sequence active and reactive currents based on the PV operating status and simultaneously determines the available inverter capacity, maximizing voltage support by fully utilizing the inverter's capacity. Furthermore, considering overvoltage, active power oscillations, and overcurrent constraints during PV operation, it dynamically corrects the positive / negative-sequence reactive current output from the PV AC side, ensuring stable system operation and solving the problem of poor transient voltage support capability during fault ride-through in distributed PV distribution networks. Under asymmetrical fault conditions, by suppressing overvoltage and active power oscillations in non-faulty phases, it ensures that the voltage of each node does not exceed limits in a multi-machine grid-connected environment, achieving coordination within the system's safe operating boundary. This invention can be extended to any number of distributed PV systems connected to the distribution network, solving multi-machine coordination problems under various fault scenarios to improve the overall fault ride-through capability of the system. Attached Figure Description

[0008] Figure 1 This is the overall control flowchart of the present invention; Figure 2 This is a power distribution system topology diagram for an example. Figure 3 The controllable boundary curves of positive-sequence reactive current and negative-sequence reactive current output from the photovoltaic AC side in this embodiment; Figure 4 The grid connection point voltage waveforms are shown for different methods when a two-phase short-circuit ground fault occurs in the distribution line in the embodiment; where (a) is the traditional method and (b) is the method of the present invention. Figure 5 The grid connection point voltage waveforms are shown for different methods when a three-phase (ABC) ground fault occurs in the distribution line in the embodiment; where (a) is the traditional method and (b) is the method of the present invention. Detailed Implementation

[0009] Specific embodiments are given below with reference to the accompanying drawings. These specific embodiments are only used to describe the technical solution of the present invention in detail and are not intended to limit the scope of protection of this application.

[0010] like Figure 1 As shown, this invention proposes a distributed photovoltaic coordinated voltage support control method based on DC voltage detection, comprising the following steps: Step 1: Obtain the grid connection point voltage. If the grid connection point voltage is less than 0.9 pu, then detect the DC side voltage of the photovoltaic system. Based on the DC side voltage of the photovoltaic system, dynamically correct the positive sequence active / reactive current reference values ​​output by the AC side of the photovoltaic system. A detection step is added to the inverter control process. By detecting the DC-side voltage of the photovoltaic system, the active current output from the AC-side of the photovoltaic system is determined in real time. The positive-sequence active / reactive current is then corrected based on the DC-side voltage to fully utilize the inverter capacity. This embodiment uses a single photovoltaic unit as an example. The maximum active power output from the AC-side of the photovoltaic system is: (1) In the formula, This indicates the maximum active power output from the AC side of the photovoltaic system. This represents the active power output from the DC side of the photovoltaic system. This represents the DC bus capacitance. Indicates the DC-side voltage of the photovoltaic system; From equation (1), we can further solve for the maximum available active current that the photovoltaic AC side can provide. for: (2) In the formula, Indicates the AC side voltage of the photovoltaic system. Indicates the DC-side current of the photovoltaic system; When the maximum available active current that the photovoltaic AC side can provide is less than the reference value of the active current output by the photovoltaic AC side, i.e., equation (3), the photovoltaic DC side voltage range of equation (4) can be obtained. (3) (4) In the formula, This represents the reference value of the active current output from the AC side of the photovoltaic system. Indicates the equivalent resistance on the grid side. Indicates the equivalent reactance on the grid side. Indicates the inverter's rated current. , , This indicates the DC voltage at which the photovoltaic system is operating at its rated capacity. Represents the logarithmic function of the product. Indicates time; When the DC voltage of the photovoltaic system satisfies equation (4), it is considered that the active power output of the DC side of the photovoltaic system is insufficient. In order to make full use of the inverter capacity, all the active power output of the AC side of the photovoltaic system should be injected into the system, and the remaining capacity of the inverter should be used to generate reactive power. The reference value of the positive sequence active / reactive current output of the AC side of the photovoltaic system satisfies equation (5). When the positive sequence active / reactive current of the AC side of the photovoltaic system is output in this way, the overcurrent capability of the inverter can be used to maximize the voltage support effect. (5) In the formula, , This represents the reference values ​​for the positive-sequence active and reactive current output from the photovoltaic AC side. This indicates the maximum active current on the DC side of the photovoltaic system. When the DC voltage of the photovoltaic system does not satisfy equation (4), it is assumed that the active power output of the DC side of the photovoltaic system is sufficient, that is, the available capacity of the inverter is insufficient. Then, according to equation (10), the positive sequence active / reactive current reference value of the AC side output of the photovoltaic system is obtained, and the ratio of the active current to the reactive current reference value of the AC side output of the photovoltaic system is equal to the impedance ratio of the distribution line, so that the fault transient voltage support effect is optimal. For a single photovoltaic unit, the grid connection voltage is expressed as: (6) In the formula, Indicates the voltage at the grid connection point. Indicates the equivalent voltage on the grid side. This represents the reactive current output from the AC side of the photovoltaic system. For transmission lines, the line resistance is much smaller than the reactance. The current national standard control scheme supports the grid connection point voltage by increasing the reactive current output of photovoltaic power and reducing the active current. For distribution lines, the line resistance is often close to the reactance. In this case, the active current injected by photovoltaic power is also conducive to supporting the grid connection point voltage. Considering inverter capacity limitations, the active / reactive current of the photovoltaic injection system satisfies the following relationship: (7) Based on equation (6), construct the following Lagrange function: (8) In the formula, Represents the Lagrange multipliers; For the Lagrange function , Taking the first-order partial derivative and setting it to 0, we can further obtain the reference values ​​for the reactive / active current output from the photovoltaic AC side: (9) In the formula, This indicates the reference value of the reactive current output from the AC side of the photovoltaic system. As can be seen from equation (9), the grid connection point voltage support effect is best when the output current is equal to the inverter's rated current, that is, when the ratio of the active current reference value to the reactive current reference value output by the photovoltaic AC side is equal to the impedance ratio of the distribution line. In contrast, when using the existing fault ride-through strategy, if a serious fault occurs in the system, the active current output by the photovoltaic AC side may be reduced to zero. This control strategy will not only reduce the photovoltaic's ability to support the grid connection point voltage in the distribution network, but may also cause an active power deficit in the system, affecting the system's frequency stability. Therefore, after a fault occurs in the system, the photovoltaic system injects positive sequence active / reactive current into the system according to the impedance ratio of the distribution line to support the grid connection point voltage. Currently, the mainstream control strategy used during asymmetrical faults is balanced positive sequence control. Even if the negative sequence current output by the photovoltaic AC side is zero, only the positive sequence active / reactive current reference value is output. The settings are as follows: (10) In the formula, , These are the reference values ​​for the positive sequence active current and reactive current output from the photovoltaic AC side, respectively.

[0011] Step 2: Under asymmetrical fault conditions, considering overvoltage, active power oscillation, and overcurrent constraints, the reference values ​​of positive / negative sequence reactive current output from the photovoltaic AC side are corrected to ensure the safe and reliable operation of the system. (1) Overvoltage and active power oscillation constraints Under asymmetrical fault conditions, the grid connection point voltage can also be expressed as: (11) In the formula, , Indicates the positive and negative sequence voltages at the grid connection point. This represents the initial phase difference between the positive and negative sequence voltages at the grid connection point; As shown in equation (11), the voltage amplitude at the grid connection point is related to the positive-sequence voltage, the negative-sequence voltage, and the initial phase difference. For non-faulty phases, the overvoltage problem can be solved by reducing the positive-sequence voltage and the negative-sequence voltage at the grid connection point. The positive-sequence voltage and the negative-sequence voltage at the grid connection point can be written as: (12) In the formula, , These represent the equivalent positive-sequence and negative-sequence voltages on the grid side, respectively. , These are the negative-sequence active current and negative-sequence reactive current output from the photovoltaic AC side, respectively. As can be seen from equation (12), reducing the magnitude of the negative sequence voltage can reduce the overvoltage problem of the non-faulty phase, while reducing the negative sequence current can reduce the negative sequence voltage at the grid connection point.

[0012] The maximum phase voltage at the grid connection point is controlled within 1.1 pu, and the overvoltage amplitude at the grid connection point is... Represented as: (13) Combining equations (12) and (13), the negative sequence reactive current output from the photovoltaic AC side under overvoltage constraint is obtained. for: (14) Maximum oscillation amplitude of active power output from the photovoltaic AC side Represented as: (15) To limit active power oscillation, the range of values ​​for the negative sequence reactive current output from the photovoltaic AC side can be obtained as follows: (16) In the formula, The first output of the photovoltaic AC side under active power oscillation constraint Negative sequence reactive current, The first output of the photovoltaic AC side Phase-sequence reactive current, The first output of the photovoltaic AC side Positive-sequence active current , The output of the photovoltaic AC side is the first Positive-sequence voltage and negative-sequence voltage; In summary, under asymmetrical fault conditions, considering overvoltage and active power oscillation constraints, the reference value of the negative sequence reactive current output from the photovoltaic AC side is taken as the smaller value of equations (14) and (16); since the negative sequence active current will reduce the system voltage and weaken the voltage support effect, it is set to zero. Further considering overcurrent constraints, under asymmetrical fault conditions, the three-phase current output from the photovoltaic AC side is expressed as: (17) In the formula, , , , , , The first output of the photovoltaic AC side Phase-negative sequence active and reactive currents; Set the phase current limit of the photovoltaic AC side output to Under the premise of keeping the positive-sequence active current of photovoltaic output constant and the negative-sequence active current zero, the maximum negative-sequence reactive current of each phase output by the photovoltaic AC side can be expressed as: (18) In the formula, The first output of the photovoltaic AC side Maximum negative sequence reactive current of phase; If the maximum negative sequence reactive current output by the photovoltaic AC side is less than or equal to the phase current limit, then there is no need to reduce the positive sequence reactive current, and the positive / negative sequence reactive current reference value is still output according to formula (10); otherwise, the positive sequence reactive current needs to be reduced to solve the overvoltage problem of the non-faulty phase, and the positive / negative sequence reactive current reference value output by the photovoltaic AC side is obtained according to formula (19). (19) (20) In the formula, , These represent the first output of the photovoltaic AC side. Reference values ​​for positive / negative sequence reactive current.

[0013] Example This embodiment uses a 10kV IEEE 69-bus distribution system as an example, in which 20 distributed photovoltaic units are connected. The locations of the fault points and measurement points are as follows: Figure 2 As shown. The capacity of each photovoltaic unit is set to 0.1MW, the system short-circuit ratio is set to 2, and the power distribution line impedance is 0.0351+j0.04082Ω / km; the fault is set to occur at 0s and the fault duration is 0.2s.

[0014] Considering overvoltage, overcurrent, and active power oscillation constraints simultaneously, and combining equations (14), (16), and (17), controllable boundary curves for the positive-sequence reactive current and negative-sequence reactive current output from the photovoltaic AC side are constructed, as follows: Figure 3 As shown.

[0015] Figure 4 The voltage waveforms at the grid connection point under the control of the traditional method (linearly increasing the reactive current output by a fixed coefficient according to the voltage drop amplitude) and the method of this invention are compared when a two-phase short-circuit ground fault (fault resistance 15Ω) occurs in a power distribution line. Figure 4 (a) is the traditional method. Figure 4 (b) is the method of the present invention. As can be seen from the figure, when a two-phase short-circuit ground fault occurs in the distribution line under the control of the conventional method, the maximum phase voltage amplitude at the grid connection point is 0.45 pu, while under the control of the method of the present invention, the maximum phase voltage amplitude at the grid connection point is 0.62 pu. Compared with the conventional method, the grid connection point voltage increases by 0.17 pu, which is an improvement of 37.8%.

[0016] Figure 5 The voltage waveforms at the grid connection point under the control of the traditional method and the method of this invention are shown when a three-phase ABC ground fault (fault resistance 15Ω) occurs in a power distribution line. Figure 5 (a) is the traditional method. Figure 5 (b) is the method of the present invention. As can be seen from the figure, when a three-phase ground fault occurs in the distribution line under the control of the conventional method, the maximum phase voltage amplitude at the grid connection point is 0.11 pu, while under the control of the method of the present invention, the maximum phase voltage amplitude at the grid connection point is 0.23 pu. Compared with the conventional method, the grid connection point voltage increases by 0.12 pu, which is an improvement of 109.1%.

[0017] In summary, the method of this invention significantly improves the system's fault transient voltage support capability while ensuring equipment and system safety. This method can be triggered in real time based on voltage dips at the grid connection point. During the control process, only the grid connection point voltage and the DC-side voltage of the photovoltaic system are needed to accurately distinguish and coordinate the distributed photovoltaic support capability under different connection locations and operating conditions. Furthermore, this method does not rely on communication links, is computationally simple, and has clear physical meaning. While ensuring that the voltage of each node does not exceed limits, it maximizes the fault ride-through reliability and voltage support effect of the distribution network. It can be extended to multi-unit grid-connected distribution network systems, solving power coordination problems in various fault ride-through scenarios and improving the overall fault ride-through capability of the system.

[0018] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A distributed photovoltaic coordinated voltage support control method based on DC voltage detection, characterized in that, Includes the following steps: Step 1: Obtain the grid connection point voltage. If the grid connection point voltage is less than 0.9pu, then detect the photovoltaic DC side voltage. If the photovoltaic DC side voltage satisfies equation (4), then obtain the positive sequence active / reactive current reference value of the photovoltaic AC side output according to equation (5). If the photovoltaic DC side voltage does not satisfy equation (4), then obtain the positive sequence active / reactive current reference value of the photovoltaic AC side output according to equation (10). (4) (5) (10) In the formula, Indicates the DC side voltage of the photovoltaic system. Indicates the DC-side current of the photovoltaic system. , , Indicates the AC side voltage of the photovoltaic system. Indicates the inverter's rated current. Indicates the equivalent resistance on the grid side. Indicates the equivalent reactance on the grid side. This indicates the DC voltage at which the photovoltaic system is operating at its rated capacity. Represents the logarithmic function of the product. Indicates time, , This represents the reference values ​​for the positive-sequence active and reactive current output from the photovoltaic AC side. This indicates the maximum active current on the DC side of the photovoltaic system. Step 2: Under asymmetrical fault conditions, considering overvoltage, active power oscillation, and overcurrent constraints, the reference values ​​of positive / negative sequence reactive current output from the photovoltaic AC side are corrected. Negative sequence reactive current output from the photovoltaic AC side under overvoltage constraint for: (14) In the formula, This represents the equivalent negative sequence voltage on the grid side. Indicates the positive sequence voltage at the grid connection point. Indicates the voltage at the grid connection point. This represents the initial phase difference between the positive and negative sequence voltages at the grid connection point; The range of values ​​for the negative sequence reactive current of each phase output from the photovoltaic AC side under active power oscillation constraint is as follows: (16) In the formula, The first output of the photovoltaic AC side under active power oscillation constraint Negative sequence reactive current, This represents the maximum oscillation amplitude of the active power output from the AC side of the photovoltaic system. , The output of the photovoltaic AC side is the first Positive-sequence voltage and negative-sequence voltage, , The output of the photovoltaic AC side is the first Phase-sequence active and reactive currents; The maximum negative sequence reactive current of each phase output from the photovoltaic AC side under overcurrent constraint is expressed as: (18) In the formula, The first output of the photovoltaic AC side Maximum negative sequence reactive current of phase This refers to the phase current limit output from the photovoltaic AC side. When the maximum negative sequence reactive current output by the photovoltaic AC side is less than or equal to the phase current limit, the reference value of the positive / negative sequence reactive current output by the photovoltaic AC side is still obtained according to equation (10); otherwise, the reference value of the positive / negative sequence reactive current output by the photovoltaic AC side is obtained according to equation (19). (19) (20) In the formula, , These represent the first output of the photovoltaic AC side. Phase positive / negative sequence reactive current reference values The first output of the photovoltaic AC side Negative sequence reactive current, This is the equivalent positive sequence voltage on the grid side. This represents the overvoltage amplitude at the grid connection point.

2. The distributed photovoltaic coordinated voltage support control method based on DC voltage detection according to claim 1, characterized in that, The formula for calculating the maximum oscillation amplitude of the active power output from the photovoltaic AC side is: (15) In the formula, , These are the negative-sequence active current and negative-sequence reactive current output from the photovoltaic AC side, respectively.

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