Distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method

Abstract

The invention discloses a distributed photovoltaic low-voltage grid-connected four-layer combined flexible voltage control method. The method comprises the following steps: firstly, acquiring distributed photovoltaic output power prediction data and user load power prediction data of the next day, and predicting a voltage upper limit exceeding time period without flexible control; determining a starting strategy of the water storage type electric water heater based on the upper limit exceeding starting time period predicted in the previous day and the actual voltage upper limit exceeding condition of the current day; if the condition that the voltage of the user side exceeds the upper limit still occurs, the reactive power consumption of the inverter is increased to reduce the voltage to a qualified range; if the reactive power consumption of the photovoltaic inverter reaches the limit value and is still insufficient to realize the control purpose, adjusting the inverter to reduce the active power of photovoltaic power generation so as to reduce the voltage to a qualified range; and if the condition that the voltage exceeds the upper limit still exists, the photovoltaic active power is forcibly reduced through tripping of the asymmetric grouping intelligent switch until the voltage is reduced to be below the allowable upper limit value. The problem that the voltage exceeds the upper limit caused by distributed photovoltaic large-scale grid connection can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed photovoltaic flexible control, and in particular to a distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method. Background Art

[0002] With the continuous construction and improvement of new power systems, large-scale new energy development and grid connection have been carried out. Among them, light energy, as an important component of the new energy structure, has developed rapidly with distributed photovoltaics. Due to the volatility, intermittency and uncertainty of its power generation, the large-scale grid connection of distributed photovoltaics poses a huge challenge to the safe and stable operation of the power grid. Limited by the high cost of current electrochemical energy storage, during the period of abundant sunshine, the excess power generated cannot be stored or consumed on site. If it is not controlled, the power generated during the peak period of photovoltaic power generation will all flow into the power grid, which is easy to cause overvoltage problems and lead to insulation aging of electrical equipment, thereby affecting the safety and power supply quality of the power grid. Therefore, during the period of large-scale photovoltaic reverse transmission, the overvoltage problem can be avoided or alleviated by taking measures to abandon light. However, this measure will directly affect the economic benefits of photovoltaic investors. Therefore, there is an urgent need for a method that can ensure the safe and stable operation of the power grid while taking into account the economic benefits of photovoltaic investors.

[0003] When dealing with large-scale photovoltaic access to distribution networks, existing technologies have some shortcomings and defects, such as voltage over-limit problems, unfair distributed photovoltaic power distribution, single control strategy, high-cost energy storage requirements, insufficient prediction accuracy and lack of flexible control methods.

[0004] At present, when large-scale photovoltaic power generation is connected to the distribution network, the peak period of photovoltaic power generation is prone to cause the grid voltage to exceed the limit, resulting in insulation aging of electrical equipment, affecting the safety of the grid and the quality of power supply. In order to solve this problem, the method of limiting photovoltaic power generation power or installing energy storage equipment is generally adopted to achieve the purpose of grid voltage stability. However, the existing technology often adopts a single day-ahead control or intraday control control scheme for distributed photovoltaics, and performs unfair power reduction. Due to the insufficient accuracy of photovoltaic output and load prediction in the existing technology, there is often a significant deviation between the generated control scheme and the actual operating conditions. Such a control method not only has low control accuracy and wastes a lot of photovoltaic energy, but also has poor control effect on distributed photovoltaics. However, due to the large investment cost of energy storage equipment, the method of adding energy storage equipment is less economical. Therefore, it is necessary to propose new technologies to solve the above problems with good economic efficiency. Summary of the invention

[0005] In view of the overvoltage problem caused by the large-scale access of distributed photovoltaics, while taking into account the economic benefits of photovoltaic investors, the present invention provides a distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method, which adopts a four-layer control strategy to hierarchically regulate photovoltaic output. The use of multiple flexible controls makes the distributed photovoltaic output more stable. At the same time, the storage of the excess electricity in water heaters is considered first, so that this part of the energy can be utilized instead of directly abandoning the light, which increases the economy.

[0006] The purpose of the present invention is achieved through the following technical solutions.

[0007] A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method comprises the following steps: S1. Obtain the planned output value of each distributed photovoltaic power station in the low-voltage (380 / 220V) access distribution area, the historical data of each distributed photovoltaic output and the historical data of each user's load power, and predict the photovoltaic output power, the load power of each user and the voltage characteristic curve without flexible control; S2. Calculate the voltage reference value of each node in the low-voltage distribution station area, compare the predicted voltage characteristic curve with the voltage reference value to determine whether the voltage exceeds the upper limit, obtain the photovoltaic excess power based on the photovoltaic day-ahead planned output value and the predicted photovoltaic output power, and determine the start-up strategy of the photovoltaic user's storage electric water heater (such as the roof-mounted hot water tank / water tank) based on the photovoltaic excess power and the predicted voltage characteristic curve, and calculate the maximum total power consumption of the storage electric water heater With time interval Excess photovoltaic power Compare and execute the first level of control. If , then continue to execute step S3, otherwise end the calculation; S3, calculate and determine the power regulation mode of the photovoltaic power station, and execute the second layer control. If the reactive power regulation mode is entered, execute step S4, otherwise execute step S5; S4, increase the reactive power consumption of the inverter to reduce the voltage to the qualified range, calculate the power generation plan, determine whether the voltage of each node meets the standard, and execute the third layer of control. If it meets the standard, issue instructions according to the current power generation plan and end the calculation. If it does not meet the standard, enter step S6; S5. At this time, the reactive power consumption of the photovoltaic inverter reaches the limit value but is still insufficient to achieve the control purpose. The inverter is adjusted to reduce the active power of photovoltaic power generation to try to reduce the voltage to a qualified range. It is determined whether the voltage of each node meets the standard. The third layer of control is executed. If it meets the standard, the instruction is issued according to the current power generation plan and the calculation is terminated. If it does not meet the standard, it goes to step S6. S6: Execute the fourth level of control to forcibly reduce the PV active power by tripping the asymmetric group intelligent switch until the voltage drops below the allowable upper limit.

[0008] Furthermore, in step S1, the specific steps are as follows: S1.1. Input low voltage access distribution station area The planned output values ​​of each distributed photovoltaic power plant at and before the time, the historical output data of each distributed photovoltaic power plant and the historical data of each user's load power are normalized and divided into training set, test set and validation set; S1.2, perform variational mode decomposition (VMD) on the training set, test set and validation set respectively, and obtain four independent intrinsic mode components (IMFs); S1.3. The four intrinsic mode components (IMFs) corresponding to the training set are used as the input values ​​of the LSTM model. The output of the LSTM model is the modal components corresponding to the photovoltaic output power and the load power of each user without flexible control. Each intrinsic mode component is learned using one LSTM model. S1.4. Use the trained LSTM model to predict the modal components corresponding to the photovoltaic output power and each user's load power in the future without flexible control, and then reconstruct the various modal components of the prediction results back to the original signal form to obtain the photovoltaic output data and each user's load power data without flexible control; S1.5. According to the predicted photovoltaic output data without flexible control and the load power data of each user, set the load and power generation conditions at each time point, select the forward-backward method to calculate the power flow of the radial distribution network, and obtain the first voltage prediction value, current prediction value and power prediction value of each node in the power grid at each time point; use the obtained data to draw the voltage characteristic curve.

[0009] Furthermore, in step S1.1, the details are as follows: S1.1.1. In order to make full use of the existing data, the data is divided into training set, test set and validation set according to the set ratio; the training set is used to train the LSTM model; the validation set is used to verify the generalization ability of the current LSTM model during iterative training, and finally decide whether to stop training; the test set is used to evaluate the generalization ability of the final LSTM model and ensure the reliability of the LSTM model prediction data; S1.1.2. In order to eliminate the dimensional differences between the data and accelerate the convergence speed of the LSTM model, the data is normalized and scaled to the range of [0,1].

[0010] Furthermore, in step S2, the specific steps are as follows: S2.1. Calculate the voltage reference value of each node in the low-voltage distribution station area as follows: Use sliding window algorithm to pre-process the voltage of each node in the low-voltage distribution station area; calculate the time interval The average voltage of each node in If the voltage of each node does not exceed the upper limit, the time interval is Neidi The average voltage of the nodes is taken as the The first voltage reference value of the node , is the number of nodes in the distribution network, =1,2……, ; If If the voltage of a node exceeds the upper limit, the specified voltage operation upper limit is used as the a first voltage reference value of a node; The first voltage reference value of the node The calculation formula is: ; in, For time interval Internal voltage sampling number; For time interval Neidi The historical measured values ​​of the voltage of each node; is obtained by the power flow calculation in step S1.5 Moment A first voltage prediction value of a node; is the upper limit of the specified voltage operation, which is taken as 1.06pu; S2.2, determine whether the voltage exceeds the upper limit: the first voltage prediction value of each node Compare with the first voltage reference value of each node. If the first voltage prediction value of the node is greater than the first voltage reference value, it means that the voltage of the node exceeds the upper limit. Otherwise, it does not exceed the upper limit. If the voltage does not exceed the upper limit, execute step S2.3. If the voltage exceeds the upper limit, execute step S2.4. S2.3. Set the water heater to start heating to the set temperature at 4:00 p.m. , end the calculation; S2.4. Subtract the photovoltaic output data predicted in step S1 from the photovoltaic output value planned on the previous day to obtain the photovoltaic excess power. ; S2.5. Set the start time of the storage electric water heater as follows: The voltage characteristic curve obtained in step S1 is compared with the first voltage reference value to obtain the time period when the voltage exceeds the upper limit; the water heater is set to start heating during the time period when the voltage exceeds the upper limit, and in principle, heating is not performed during other time periods; S2.6. Calculate the regulating capacity of the storage electric water heater: Set the storage water heater to If the heating is stopped immediately afterwards, the maximum total power consumption of the storage water heater is The calculation formula is as follows: ; in, is the specific heat capacity of water, Indicates the total volume of water in the water heater tank that needs to be heated. represents the initial temperature of the water in the tank, Indicates the number of storage-type electric water heaters in the low-voltage distribution station area; S2.7. Maximum total power consumption of storage electric water heater With time interval Comparison of excess photovoltaic power in: ; like , that is, the excess photovoltaic power is not enough to heat the water in the water heater to the set temperature , then the storage electric water heater will also consume additional electricity Heating; after the voltage exceeds the upper limit, the storage water heater operates at a constant power. Continue heating for a while , The calculation formula is as follows: ; in, The operating power of the storage water heater when the overvoltage ends and the heating continues, and the time required for the heating to continue is set. Then the operation ends; like , that is, the storage electric water heater just completely consumes the surplus active power during the overvoltage period, then the calculation ends and there is no need to carry out the next level of control; like , that is, the storage electric water heater has reached the threshold value but has not yet completely consumed the excess photovoltaic power, then the inverter reactive power optimization adjustment mode of the next level of control needs to be started; at this time, the water heater operates at a constant power Heat to the set temperature ; Photovoltaic excess power after the first level of control The calculation formula is as follows: .

[0011] Further, in step S3, the storage electric water heater cannot completely absorb the excess photovoltaic power, and the inverter active and reactive power are adjusted to reduce the voltage to a qualified range, and the photovoltaic power station power regulation mode is calculated and determined. The specific steps are as follows: S3.1, based on the load power data of each user predicted in step S1 and the photovoltaic excess power calculated in step S2 , recalculate the power flow to obtain the second voltage prediction value of each node in the low-voltage distribution station area , is the number of nodes in the distribution network, =1,2……, ; Recalculate the second voltage reference value of each node according to step S2.1 ; S3.2: According to step S2.5, at this time, a node voltage must have exceeded the upper limit, and the second voltage prediction value calculated in step S3.1 is determined. The highest node is the dominant node, and the dominant node number is ; Pre-adjustment value of voltage at the leading node for: ; S3.3. The power factor of the photovoltaic power station shall be dynamically adjustable within the range of 0.95 leading to 0.95 lagging. Considering the power factor limitation of the photovoltaic power station, Photovoltaic power plants The first maximum output reactive power at the moment for: ; Where: is the power factor of the photovoltaic power station, which is 0.95; For the Photovoltaic power plants The predicted value of the maximum output active power at the moment; is the number of PV power stations in the distribution network, =1,2,……, ; S3.4. Fitting the second voltage prediction value of each node by using nonlinear least squares method The relationship between power and real-time operation data is used to calculate the real-time voltage sensitivity between nodes; S3.5, reactive power can be reduced to the lowest , calculate the maximum adjustable voltage of the dominant node : ; Where: is the voltage-reactive sensitivity matrix The first ( , ) element, indicating the The node pair is injected into The voltage sensitivity of the grid-connected node power change of a photovoltaic power station, The node is The grid-connected nodes of a photovoltaic power station; for Moment The reactive power output of each photovoltaic power station; S3.6. Calculate the photovoltaic reactive power evaluation coefficient of the distribution network : ; When the reactive power of the power station is adjusted, the voltage amplitude of each node will increase, and the reactive power + active power reduction mode needs to be entered; when the reactive power evaluation coefficient is positive, if , enter the reactive power regulation mode, if , then it enters the reactive power + active power reduction mode; If the reactive power + active power reduction mode is entered, step S5 is executed; if the reactive power regulation mode is entered, step S4 is executed.

[0012] Furthermore, the specific steps of step S3.4 are as follows: S3.4.1. Use the historical data of distributed photovoltaic output and the historical data of user load power to calculate the power flow to obtain the historical injection power and historical voltage data of each node, and use the nonlinear least squares method to fit the mapping relationship between the historical injection power and historical voltage data of each node; S3.4.2. Collect the current voltage and power of each node and randomly generate a matrix of small changes in injected power , define a scaling factor α, 0<α<<1, so that ;in, Indicates The absolute value of the current injected power of each node, Indicates The absolute value of the change in the current injected power of each node; keep the reactive power of each node unchanged, and obtain the voltage distribution of each node after the power change through the curve fitted in step S3.4.1, and further obtain the voltage slight change matrix ; S3.4.3. Use the least squares method to solve the current voltage-active power sensitivity matrix ; is the voltage-active power sensitivity matrix The first ( , ) element, indicating the The node pair is injected into Voltage sensitivity of active power changes at each node; S3.4.4, Similarly, the voltage-reactive sensitivity matrix of node voltage to injected reactive power can be calculated: ; is the voltage-reactive sensitivity matrix The first ( , ) element, indicating the The node pair is injected into The voltage sensitivity of each node to the reactive power change.

[0013] Furthermore, in step S4, the reactive power consumption of the inverter is increased to reduce the voltage to a qualified range, and the power generation plan is calculated. The specific steps are as follows: S4.1, subtract the second voltage prediction value of each grid-connected node of each photovoltaic power station calculated in step S3.1 from the second voltage reference value, and combine them to obtain a voltage error matrix , the active power of the power station does not change at this time, so the active power change matrix of the photovoltaic power station is ; Extract the voltage-reactive sensitivity synthesis matrix between the grid-connected nodes of the photovoltaic power station , we can get: ; Where: is the reactive power variation matrix of the photovoltaic power station, and the voltage-reactive power sensitivity matrix The first ( , ) element, indicating the The grid-connected nodes of each photovoltaic power station inject The voltage sensitivity of the grid-connected node power change of a photovoltaic power station, is the voltage error matrix of the grid-connected node; S4.2. After calculating the reactive power change matrix of the photovoltaic power station, Reactive power output of a photovoltaic power station for: ; Where: for Reactive power change matrix of photovoltaic power station at every moment The elements; S4.3, add the limit function, if Exceeding the maximum output reactive power of the photovoltaic power station , then The reactive power output plan is issued; the present invention performs intraday regulation based on a day-ahead power generation plan. At this time, each power station outputs active power according to the day-ahead power generation plan, and only the reactive power output needs to be changed. Photovoltaic power plants The first reactive power output plan at the moment for: ; S4.4. Calculate the power flow of the distribution network to obtain the third voltage prediction value of each node in the distribution network after each power station executes the current power generation plan. , update the third voltage reference value ; Calculate the voltage deviation and : ; S4.5. Determine whether the voltage of each node of the distribution network meets the standard. If And the third voltage prediction of all nodes All less than If the voltage of the distribution network node meets the standard, the command will be issued according to the current power generation plan and the calculation will be ended. Indicates the allowable voltage deviation range, which needs to be set according to the actual situation and requirements of the power system; if the voltage of the distribution network node does not meet the standard, the photovoltaic power generation power is updated to the current power generation plan, the active power output plan remains unchanged, and the reactive power output plan is updated to the first reactive power output plan , fitting the voltage sensitivity matrix between the current nodes and , execute step S6 to enter the reactive power + active power regulation mode.

[0014] Further, in step S5, at this time, the reactive power consumption of the photovoltaic inverter reaches the limit value but is still insufficient to achieve the control purpose, and the inverter is adjusted to reduce the active power of photovoltaic power generation in an effort to reduce the voltage of each node in the low-voltage distribution station area to a qualified range. The specific steps are as follows: S5.1. If the photovoltaic active power is limited, the maximum adjustable reactive power will change accordingly; calculate the Photovoltaic power plants The second maximum output reactive power at the moment : ; S5.2, press Output reactive power to maximize the use of photovoltaic power station capacity, then the output reactive power change for: ; S5.3. Define fairness as allocating active power reduction to each PV power station in proportion to the maximum power generation. Therefore, each PV power station is set with the same active power reduction ratio. , conduct active and reactive power joint control; Active power reduction ratio The details are as follows: ; In the formula, For the Photovoltaic power plants The predicted value of the maximum output active power at the moment, For the The active power reduction of each photovoltaic power station is as follows: ； In the formula, is the first Photovoltaic power plants Active power output plan value at the moment; The specific values ​​of the dominant node voltage pre-adjustment are as follows: ; In the formula, is an unknown variable that varies between [0,1), and the rest are constants. After simplification, we can get The quadratic equation of one variable is There is a unique solution within the range of variation; S5.4, find After that, the active power output plan of each photovoltaic power station and the second reactive power output plan The details are as follows: ； S5.5. Calculate the power flow of the distribution network to obtain the fourth voltage prediction value of each node of the distribution network after each photovoltaic power station is executed according to the current power generation plan. ; S5.6, determine whether the voltage of each node meets the standard; in the active power reduction mode, if the fourth voltage prediction value of each node All less than If the voltage of the distribution network node meets the standard, the command is issued according to the current power generation plan and the calculation ends; if the voltage of the distribution network node does not meet the standard, the photovoltaic power generation power is updated to the current power generation plan, and the active power output plan is updated to , the reactive power output plan is updated to the second reactive power output plan , fitting the voltage sensitivity matrix between the current nodes and , execute step S6 and enter the next layer of asymmetric grouping intelligent switch control.

[0015] Further, in step S6, at this time, the first three layers of control have not achieved the purpose, and a strategy of partial photovoltaic power cut-off is adopted, and the voltage rise caused by the photovoltaic active power reverse transmission is forcibly reduced by tripping the external asymmetric group intelligent switch, as follows: Most traditional intelligent switches are only equipped with one switch, and can only use 100% abandonment to reduce voltage when overvoltage occurs, resulting in a large waste of light energy. The present invention proposes to configure a flexible control switch terminal in front of each photovoltaic user meter, and configure 2 groups of switches, one group with a breaking capacity of 1 / 3 and the other group with a breaking capacity of 2 / 3, and start abandonment when the control effects of the water storage electric water heater and photovoltaic inverter in the first three levels of control have not reduced the area voltage to below the allowable upper limit.

[0016] Furthermore, the specific steps of step S6 are as follows: S6.1, first, the edge intelligent terminal issues a command to the flexible control switch terminal of each user end, uniformly tripping the first flexible control switch with a disconnection capacity of 1 / 3 for each user, and fairly requiring all photovoltaic users to uniformly abandon 1 / 3 of the light. If the voltage of each node returns to normal, it will be maintained for a period of time until the voltage of each node continues to drop by a set amount, and then the first flexible control switch will be closed again to avoid unnecessary abandonment of light, and the calculation is ended. According to the calculation in step S5.6, and Issue the instruction, otherwise execute step S6.2; S6.2, at this time, the first flexible control switch with a breaking capacity of 1 / 3 fails to solve the problem of voltage exceeding the upper limit, and then the second flexible control switch with a breaking capacity of 2 / 3 is switched to uniformly trip all users, and all photovoltaic users are required to uniformly abandon 2 / 3 of the light; if the voltage of each node returns to normal, it will be maintained for a period of time until the voltage of each node continues to drop by the set amount, and then the second flexible control switch will be closed again to avoid unnecessary abandonment of light, and the calculation is ended. According to the calculation in step S5.6 and Issue the instruction, otherwise execute step S6.3; S6.3, if the node voltage still exceeds the upper limit, the first flexible control switch and the second flexible control switch are both tripped, and the calculated value in step S5.6 is and Send the command and end the calculation. Of course, this scenario rarely occurs.

[0017] Compared with existing traditional switches, this method adds a set of switches, but when the voltage exceeds the upper limit, it can reduce a lot of abandoned light compared to the configuration of a single set of switches. It is a more user-acceptable "flexible" control method.

[0018] Compared with the prior art, the advantages of the present invention are: The present invention solves the shortcomings of the prior art by introducing a four-layer joint flexible voltage control strategy. First, a distributed photovoltaic power generation plan is obtained, and a collaborative control strategy integrating "day-ahead control" and "intraday real-time regulation" is further proposed. Such a control strategy can effectively respond to emergencies encountered during photovoltaic operation and modify the regulation plan in time, thereby improving the flexibility and accuracy of regulation. Flexible loads and asymmetric grouping intelligent switches are introduced to make full use of existing energy storage resources and inverters, minimize abandoned light, and achieve fair distribution of active power with the relative capacity and power generation power of each distributed photovoltaic, and avoid a large increase in investment in hardware equipment, thereby having good economy.

[0019] The present invention is optimized from the perspective of ensuring safe and stable operation of the power grid. It can effectively solve the problem of voltage exceeding the upper limit caused by large-scale grid-connected distributed photovoltaics, improve the power supply quality of new distribution station areas, and at the same time minimize the amount of abandoned light and ensure the economic benefits of photovoltaic investors. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flowchart of the steps of a distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method in an embodiment; Figure 2 It is an execution flow chart of a distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method in an embodiment; Figure 3 Schematic diagram of the low voltage power distribution network system used in the embodiment. DETAILED DESCRIPTION

[0021] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and examples, but the implementation and protection of the present invention are not limited thereto.

[0022] Example: A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method, such as Figure 1 and Figure 2 As shown, the following steps are included: S1. In one embodiment, the planned output value of each distributed photovoltaic power station in the 380 / 220V low-voltage access distribution station area, the historical data of each distributed photovoltaic output and the historical data of each user's load power are obtained, and the photovoltaic output power, the load power of each user and the voltage characteristic curve without flexible control are predicted. The specific steps are as follows: S1.1, input 380 / 220V low voltage to access the distribution station area The planned output values ​​of each distributed photovoltaic power generation unit at and before the time, the historical output data of each distributed photovoltaic power generation unit, and the historical load power data of each user are normalized and divided into training set, test set, and validation set, as follows: S1.1.1. In order to make full use of the existing data, in one embodiment, the data is divided into a training set, a test set, and a validation set in a ratio of 20:4:1; wherein the training set is used to train the LSTM model; the validation set is used to verify the generalization ability of the current LSTM model during iterative training, and ultimately decide whether to stop training; the test set is used to evaluate the generalization ability of the final LSTM model and ensure the reliability of the LSTM model prediction data; S1.1.2. In order to eliminate the dimensional differences between the data and accelerate the convergence speed of the LSTM model, the data is normalized and scaled to the range of [0,1].

[0023] S1.2, perform variational mode decomposition (VMD) on the training set, test set and validation set respectively, and obtain four independent intrinsic mode components (IMFs); S1.3. The four intrinsic mode components (IMFs) corresponding to the training set are used as the input values ​​of the LSTM model. The output of the LSTM model is the modal components corresponding to the photovoltaic output power and the load power of each user without flexible control. Each intrinsic mode component is learned using one LSTM model. S1.4. Use the trained LSTM model to predict the modal components corresponding to the photovoltaic output power and each user's load power in the future without flexible control, and then reconstruct the various modal components of the prediction results back to the original signal form to obtain the photovoltaic output data and each user's load power data without flexible control; S1.5. According to the predicted photovoltaic output data without flexible control and the load power data of each user, set the load and power generation conditions at each time point, select the forward-backward method to calculate the power flow of the radial distribution network, and obtain the first voltage prediction value, current prediction value and power prediction value of each node in the power grid at each time point; use the obtained data to draw the voltage characteristic curve.

[0024] S2. Calculate the voltage reference value of each node in the low-voltage distribution station area, compare the predicted voltage characteristic curve with the voltage reference value to determine whether the voltage exceeds the upper limit, obtain the photovoltaic excess power based on the photovoltaic day-ahead planned output value and the predicted photovoltaic output power, and determine the start-up strategy of the photovoltaic user's storage electric water heater based on the photovoltaic excess power and the predicted voltage characteristic curve, and calculate the maximum total power consumption of the storage electric water heater. With time interval Excess photovoltaic power Compare and execute the first level of control. If , then continue to execute step S3, otherwise end the calculation. The specific steps are as follows: S2.1. Calculate the voltage reference value of each node in the low-voltage distribution station area as follows: Use sliding window algorithm to pre-process the voltage of each node in the low-voltage distribution station area; calculate the time interval The average voltage of each node in If the voltage of each node does not exceed the upper limit, the time interval is Neidi The average voltage of the nodes is taken as the The first voltage reference value of the node , is the number of nodes in the distribution network, =1,2……, ; If If the voltage of a node exceeds the upper limit, the specified voltage operation upper limit is used as the a first voltage reference value of a node; The first voltage reference value of the node The calculation formula is: ; in, For time interval Internal voltage sampling number; For time interval Neidi The historical measured values ​​of the voltage of each node; is obtained by the power flow calculation in step S1.5 Moment A first voltage prediction value of a node; is the specified upper voltage operating limit, in one embodiment, it is taken as 1.06 pu; S2.2, determine whether the voltage exceeds the upper limit: the first voltage prediction value of each node Compare with the first voltage reference value of each node. If the first voltage prediction value of the node is greater than the first voltage reference value, it means that the voltage of the node exceeds the upper limit. Otherwise, it does not exceed the upper limit. If the voltage does not exceed the upper limit, execute step S2.3. If the voltage exceeds the upper limit, execute step S2.4. S2.3. Set the water heater to start heating to the set temperature at 4:00 p.m. , end the calculation; In one embodiment, the set temperature Set to 75°C; S2.4. Subtract the photovoltaic output data predicted in step S1 from the photovoltaic output value planned on the previous day to obtain the photovoltaic excess power. ; S2.5. Set the start time of the storage electric water heater as follows: The voltage characteristic curve obtained in step S1 is compared with the first voltage reference value to obtain the time period when the voltage exceeds the upper limit; the water heater is set to start heating during the time period when the voltage exceeds the upper limit, and in principle, heating is not performed during other time periods; S2.6. Calculate the regulating capacity of the storage electric water heater: If the storage water heater is set to stop heating immediately after the hot water temperature reaches 75°C, the maximum total power consumption of the storage water heater is The calculation formula is as follows: ; in, is the specific heat capacity of water, Indicates the total volume of water in the water heater tank that needs to be heated. represents the initial temperature of the water in the tank, Indicates the number of storage-type electric water heaters in the low-voltage distribution station area; S2.7. Maximum total power consumption of storage electric water heater With time interval Comparison of excess photovoltaic power in: ; like , that is, if the excess photovoltaic power is not enough to heat the water in the water heater to 75°C, the storage electric water heater will need to consume additional power Heating; after the voltage exceeds the upper limit, the storage water heater operates at a constant power. Continue heating for a while , The calculation formula is as follows: ; in, The operating power of the storage water heater when the overvoltage ends and the heating continues, and the time required for the heating to continue is set. Then the operation ends; like , that is, the storage electric water heater just completely consumes the surplus active power during the overvoltage period, then the calculation ends and there is no need to carry out the next level of control; like , that is, the storage electric water heater has reached the threshold value but has not yet completely consumed the excess photovoltaic power, then the inverter reactive power optimization adjustment mode of the next level of control needs to be started; at this time, the water heater operates at a constant power Heating to 75℃; excess photovoltaic power after first-layer control The calculation formula is as follows: . S3, calculate and determine the power regulation mode of the photovoltaic power station, and execute the second layer control. If the reactive power regulation mode is entered, execute step S4, otherwise execute step S5; The storage electric water heater cannot completely absorb the excess photovoltaic power. The inverter active and reactive power are adjusted to reduce the voltage to a qualified range. The power regulation mode of the photovoltaic power station is calculated and determined. The specific steps are as follows: S3.1, based on the load power data of each user predicted in step S1 and the photovoltaic excess power calculated in step S2 , recalculate the power flow to obtain the second voltage prediction value of each node in the low-voltage distribution station area , is the number of nodes in the distribution network, =1,2……, ; Recalculate the second voltage reference value of each node according to step S2.1 ; S3.2: According to step S2.5, at this time, a node voltage must have exceeded the upper limit, and the second voltage prediction value calculated in step S3.1 is determined. The highest node is the dominant node, and the dominant node number is ; Pre-adjustment value of voltage at the leading node for: ; S3.3. The power factor of the photovoltaic power station shall be dynamically adjustable within the range of 0.95 leading to 0.95 lagging. Considering the power factor limitation of the photovoltaic power station, Photovoltaic power plants The first maximum output reactive power at the moment for: ; Where: is the power factor of the photovoltaic power station, which is 0.95; For the Photovoltaic power plants The predicted value of the maximum output active power at the moment; is the number of PV power stations in the distribution network, =1,2,……, ; S3.4. Fitting the second voltage prediction value of each node by using nonlinear least squares method The relationship between power and real-time operation data is combined to calculate the real-time voltage sensitivity between nodes. The specific steps are as follows: S3.4.1. Use the historical data of distributed photovoltaic output and the historical data of user load power to calculate the power flow to obtain the historical injection power and historical voltage data of each node, and use the nonlinear least squares method to fit the mapping relationship between the historical injection power and historical voltage data of each node; S3.4.2. Collect the current voltage and power of each node and randomly generate a matrix of small changes in injected power , define a scaling factor α, 0<α<<1, so that ;in, Indicates The absolute value of the current injected power of each node, Indicates The absolute value of the change in the current injected power of each node; keep the reactive power of each node unchanged, and obtain the voltage distribution of each node after the power change through the curve fitted in step S3.4.1, and further obtain the voltage slight change matrix ; S3.4.3. Use the least squares method to solve the current voltage-active power sensitivity matrix ; is the voltage-active power sensitivity matrix The first ( , ) element, indicating the The node pair is injected into Voltage sensitivity of active power changes at each node; S3.4.4, Similarly, the voltage-reactive sensitivity matrix of node voltage to injected reactive power can be calculated: ; is the voltage-reactive sensitivity matrix The first ( , ) element, indicating the The node pair is injected into The voltage sensitivity of each node to the reactive power change.

[0025] S3.5, reactive power can be reduced to the lowest , calculate the maximum adjustable voltage of the dominant node : ; Where: is the voltage-reactive sensitivity matrix The first ( , ) element, indicating the The node pair is injected into The voltage sensitivity of the grid-connected node power change of a photovoltaic power station, The node is The grid-connected nodes of a photovoltaic power station; for Moment The reactive power output of each photovoltaic power station; S3.6. Calculate the photovoltaic reactive power evaluation coefficient of the distribution network : ; When the reactive power of the power station is adjusted, the voltage amplitude of each node will increase, and the reactive power + active power reduction mode needs to be entered; when the reactive power evaluation coefficient is positive, if , enter the reactive power regulation mode, if , then it enters the reactive power + active power reduction mode; If the reactive power + active power reduction mode is entered, step S5 is executed; if the reactive power regulation mode is entered, step S4 is executed.

[0026] S4, increase the reactive power consumption of the inverter to reduce the voltage to the qualified range, calculate the power generation plan, judge whether the voltage of each node meets the standard, and execute the third layer of control. If it meets the standard, issue instructions according to the current power generation plan and end the calculation. If it does not meet the standard, enter step S6. The specific steps are as follows: S4.1, subtract the second voltage prediction value of each grid-connected node of the photovoltaic power station calculated in step S3.1 from the second voltage reference value to obtain a voltage error matrix , the active power of the power station does not change at this time, so the active power change matrix of the photovoltaic power station is ; Extract the voltage-reactive sensitivity synthesis matrix between the grid-connected nodes of the photovoltaic power station , we can get: ; Where: is the reactive power variation matrix of the photovoltaic power station, and the voltage-reactive power sensitivity matrix The first ( , ) element, indicating the The grid-connected nodes of each photovoltaic power station inject The voltage sensitivity of the grid-connected node power change of a photovoltaic power station, is the voltage error matrix of the grid-connected node; S4.2. After calculating the reactive power change matrix of the photovoltaic power station, Reactive power output of a photovoltaic power station for: ; Where: for Reactive power change matrix of photovoltaic power station at every moment The elements; S4.3, add the limit function, if Exceeding the maximum output reactive power of the photovoltaic power station , then The reactive power output plan is issued; at this time, each power station outputs active power according to the day-ahead power generation plan, and only the reactive power output needs to be changed. Photovoltaic power plants The first reactive power output plan at the moment for: ; S4.4. Calculate the power flow of the distribution network to obtain the third voltage prediction value of each node in the distribution network after each power station executes the current power generation plan. , update the third voltage reference value ; Calculate the voltage deviation and : ; S4.5. Determine whether the voltage of each node of the distribution network meets the standard. If And the third voltage prediction value of all nodes All less than If the voltage of the distribution network node meets the standard, the command will be issued according to the current power generation plan and the calculation will be ended. Indicates the allowable voltage deviation range, which needs to be set according to the actual situation and requirements of the power system; if the voltage of the distribution network node does not meet the standard, the photovoltaic power generation power is updated to the current power generation plan, the active power output plan remains unchanged, and the reactive power output plan is updated to the first reactive power output plan , fitting the voltage sensitivity matrix between the current nodes and , execute step S6 to enter the reactive power + active power regulation mode.

[0027] S5. At this time, the reactive power consumption of the photovoltaic inverter reaches the limit value but is still insufficient to achieve the control purpose. The inverter is adjusted to reduce the active power of photovoltaic power generation to try to reduce the voltage to a qualified range. It is determined whether the voltage of each node meets the standard. The third layer of control is executed. If it meets the standard, the instruction is issued according to the current power generation plan and the calculation is terminated. If it does not meet the standard, it goes to step S6. At this time, the reactive power consumption of the photovoltaic inverter reaches the limit value, which is still insufficient to achieve the control purpose. The inverter is adjusted to reduce the active power of photovoltaic power generation in an effort to reduce the voltage of each node in the low-voltage distribution station area to a qualified range. The specific steps are as follows: S5.1. If the photovoltaic active power is limited, the maximum adjustable reactive power will change accordingly; calculate the Photovoltaic power plants The second maximum output reactive power at the moment : ; S5.2, press Output reactive power to maximize the use of photovoltaic power station capacity, then the output reactive power change for:

[0028] S5.3. Define fairness as allocating active power reduction to each PV power station in proportion to the maximum power generation. Therefore, each PV power station is set with the same active power reduction ratio. , conduct active and reactive power joint control; Active power reduction ratio The details are as follows: ; In the formula, For the Photovoltaic power plants The predicted value of the maximum output active power at the moment, For the The active power reduction of each photovoltaic power station is as follows: ； In the formula, is the first Photovoltaic power plants Active power output plan value at the moment; The specific values ​​of the dominant node voltage pre-adjustment are as follows: ; In the formula, is an unknown variable that varies between [0,1), and the rest are constants. After simplification, we can get The quadratic equation of one variable is There is a unique solution within the range of variation; S5.4, find After that, the active power output plan of each photovoltaic power station and the second reactive power output plan The details are as follows: ； S5.5. Calculate the power flow of the distribution network to obtain the fourth voltage prediction value of each node of the distribution network after each photovoltaic power station is executed according to the current power generation plan. ; S5.6, determine whether the voltage of each node meets the standard; in the active power reduction mode, if the fourth voltage prediction value of each node All less than If the voltage of the distribution network node meets the standard, the command is issued according to the current power generation plan and the calculation ends; if the voltage of the distribution network node does not meet the standard, the photovoltaic power generation power is updated to the current power generation plan, and the active power output plan is updated to , the reactive power output plan is updated to the second reactive power output plan , fitting the voltage sensitivity matrix between the current nodes and , execute step S6 and enter the next layer of asymmetric grouping intelligent switch control.

[0029] S6, execute the fourth layer control, and forcibly reduce the photovoltaic active power by tripping the asymmetric group intelligent switch until the voltage drops below the allowable upper limit; At this time, the first three levels of control have not achieved their goals, so a strategy of partial photovoltaic power cut-off is adopted, and the voltage rise caused by the photovoltaic active power reverse transmission is forcibly reduced by tripping the external asymmetric grouping intelligent switch. The details are as follows: Most traditional intelligent switches are only equipped with one switch, and can only use 100% abandonment to reduce voltage when overvoltage occurs, resulting in a large waste of light energy. The present invention proposes to configure a flexible control switch terminal in front of each photovoltaic user meter, and configure 2 groups of switches, one group with a breaking capacity of 1 / 3 and the other group with a breaking capacity of 2 / 3, and start abandonment when the control effects of the water storage electric water heater and photovoltaic inverter in the first three levels of control have not reduced the area voltage to below the allowable upper limit.

[0030] The specific steps of step S6 are as follows: S6.1, first, the edge intelligent terminal issues a command to the flexible control switch terminal of each user end, uniformly tripping the first flexible control switch with a disconnection capacity of 1 / 3 for each user, and fairly requiring all photovoltaic users to uniformly abandon 1 / 3 of the light. If the voltage of each node returns to normal, it will be maintained for a period of time until the voltage of each node continues to drop by a set amount, and then the first flexible control switch will be closed again to avoid unnecessary abandonment of light, and the calculation is ended. According to the calculation in step S5.6, and Issue the instruction, otherwise execute step S6.2; S6.2, at this time, the first flexible control switch with a breaking capacity of 1 / 3 fails to solve the problem of voltage exceeding the upper limit, and then the second flexible control switch with a breaking capacity of 2 / 3 is switched to uniformly trip all users, and all photovoltaic users are required to uniformly abandon 2 / 3 of the light; if the voltage of each node returns to normal, it will be maintained for a period of time until the voltage of each node continues to drop by the set amount, and then the second flexible control switch will be closed again to avoid unnecessary abandonment of light, and the calculation is ended. According to the calculation in step S5.6 and Issue the instruction, otherwise execute step S6.3; S6.3, if the node voltage still exceeds the upper limit, the first flexible control switch and the second flexible control switch are both tripped, and the calculated value in step S5.6 is and Send the command and end the calculation. Of course, this scenario rarely occurs.

[0031] Compared with existing traditional switches, this method adds a set of switches, but when the voltage exceeds the upper limit, it can reduce a lot of abandoned light compared to the configuration of a single set of switches. It is a more user-acceptable "flexible" control method.

[0032] In one embodiment, using Figure 3 The power system network architecture shown in the figure is simulated and analyzed. The low-voltage distribution network includes 14 nodes (9 load nodes and 5 power supply nodes), 20 branches and 3 transformers. Among them, the first node 1, the second node 2, and the third node 3 are connected to the photovoltaic power generation group, corresponding to the first photovoltaic power station 1, the second photovoltaic power station 2, and the third photovoltaic power station 3, respectively. At the same time, in order to simulate the impact of other power generation methods on the power system, the sixth node 6 and the eighth node 8 are connected to the thermal power generation group and the hydropower generation group, respectively, and the remaining nodes are all load nodes. The system baseline capacity is 150MVA, and the node types include PV nodes and PQ nodes.

[0033] Three low-voltage distribution network operation scenarios were set. In scenario 1, the actual photovoltaic power generation value was 1.1 times the day-ahead forecast value, and the actual load value was 0.9 times the day-ahead forecast value; in scenario 2, the actual photovoltaic power generation value was 1.3 times the day-ahead forecast value, and the actual load value was 0.8 times the day-ahead forecast value; the actual photovoltaic power generation value was 1.4 times the day-ahead forecast value, and the actual load value was 0.7 times the day-ahead forecast value. All three scenarios were simulated and verified at 13:00 noon.

[0034] For scenario 1, since the actual values ​​of photovoltaic output and load do not exceed the predicted values ​​by much, the water storage electric water heater timing adjustment mode can be used to control the voltage to an appropriate range. Therefore, there is no need to consider the reactive power adjustment mode and the active power reduction mode. Two solutions are set for simulation comparison, namely, the photovoltaic power station is shut down when the voltage exceeds the upper limit and only the water storage electric water heater is adjusted on a timing basis. The specific implementation of the two solutions is as follows: Solution 1: Take extreme measures. During the peak power generation period (10:00 am to 3:00 pm), the overvoltage protection is activated and the power station is shut down to ensure that the voltage at the grid connection point is within a safe range. The power station attempts to restart after 1 minute. If the voltage still exceeds the upper limit, it will continue to be shut down.

[0035] Solution 2 adopts the storage electric water heater timing adjustment mode. In the distribution network PRMC, a storage electric water heater timing adjustment plan is formulated according to the overall operation data and instructions are issued to the storage electric water heater control center for execution.

[0036] The voltage quality evaluation indicators of each control scheme are shown in the following table, where r Q Indicates the voltage qualification rate, D D represents the voltage discrete degree, Watt% and Var% represent the proportion of the total active and reactive power injected by the photovoltaic power station in the distribution network to the maximum active and reactive power that can be generated. The evaluation indicators under the two control schemes are compared with the results without control. The voltage qualification rate reaches 100%, and the voltage discreteness decreases. Compared with Scheme 1, the actual total output of the photovoltaic power station is 100%, the total reactive power output of the power station is 25.42%, and the voltage discreteness of the distribution network is reduced by 11.31%, which significantly improves the voltage quality. The calculation results show that when only a small part of the photovoltaic output is in excess, the use of the storage electric water heater timing adjustment mode can better solve the voltage quality problem and ensure the safe and stable operation of the system.

[0037] Table 1 Scenario 1 Voltage quality evaluation table

[0038] In scenario 2, the photovoltaic output is in excess to a certain extent, and the voltage exceeding the upper limit is more serious than in scenario 1. Three solutions are set for simulation comparison, namely, the photovoltaic power station is shut down when the voltage exceeds the upper limit, only the storage electric water heater is adjusted on time, and the storage electric water heater is adjusted on time + reactive power adjustment mode. The specific implementation of the three solutions is as follows: Solution 3: The power station will be shut down when the voltage of the grid-connected node of the photovoltaic power station exceeds the upper limit. The voltage of the grid-connected node of the first photovoltaic power station 1, the grid-connected node of the second photovoltaic power station 2, and the grid-connected node of the third photovoltaic power station 3 did not exceed the upper limit during the simulation period, and all of them generated electricity at maximum power.

[0039] Solution 4: Only implement the timed regulation mode of the storage electric water heater. Due to the limited regulation capacity, the storage electric water heater performs heating work at a certain power at each moment and cannot absorb all the remaining photovoltaic capacity. Calculate the voltage distribution after the water heater absorbs the power and compare it with other solutions.

[0040] Solution 5: Implement an operation mode that combines timed regulation of storage electric water heaters with reactive power regulation. According to the previous method, when the water heater's ability to absorb photovoltaic power reaches its limit, start the reactive power regulation mode, re-formulate the photovoltaic power generation plan, and issue a command to the photovoltaic inverter control center to execute, thereby adjusting the system reactive power.

[0041] The following table lists the voltage quality and power distribution evaluation indicators of the distribution network under different control schemes in scenario 2. Under scheme 3, if the voltage exceeds the upper limit, the photovoltaic power station will be shut down, and the photovoltaic power station abandonment rate is as high as 43.87%. The economic benefits are too low and are not considered. Compared with scheme 4, scheme 5 not only achieves 100% photovoltaic active power output, but also has no voltage exceeding the upper limit at all times, the reactive power output exceeds 50%, and the voltage deviation is reduced by 18.68%, which meets the basic requirements of voltage quality and realizes full absorption of photovoltaic output. The calculation results show that when the photovoltaic output is in excess to a certain extent, it cannot be fully absorbed by only using the timed adjustment of the water storage electric water heater. At this time, the reactive power adjustment mode can be added to achieve full absorption of photovoltaic output within the reactive power adjustment range of the inverter and ensure voltage quality.

[0042] Table 2 Voltage quality evaluation table for scenario 2

[0043] In scenario 3, the photovoltaic output is relatively surplus, and the voltage exceeds the upper limit. Three solutions are set for simulation comparison, namely, the photovoltaic power station is shut down when the voltage exceeds the upper limit, the water storage electric water heater timing adjustment + reactive power adjustment mode, and the water storage electric water heater timing adjustment + reactive power adjustment mode + active power reduction mode. The three solutions are implemented as follows: Solution 6: The grid-connected nodes of the photovoltaic power station will be shut down when the upper limit is exceeded. The grid-connected nodes of the first photovoltaic power station 1, the second photovoltaic power station 2, and the third photovoltaic power station 3 did not exceed the upper limit during the simulation period, and all generated electricity at the maximum power.

[0044] Scheme 7: Implement an operation mode that combines timed regulation and reactive power regulation for storage-type electric water heaters. According to the previous method, when the water heater's ability to absorb photovoltaic power reaches its limit, start the reactive power regulation mode, re-formulate the photovoltaic power generation plan, and issue a command to the photovoltaic inverter control center to execute, thereby adjusting the system reactive power. Due to the limited reactive power regulation capability of the inverter, calculate the voltage distribution after the reactive power regulation capability reaches the maximum regulation margin and compare it with other schemes.

[0045] Plan 8: Implement an operation mode that combines timed regulation of storage electric water heaters with reactive power regulation and active power reduction. When the reactive power regulation capacity of the inverter reaches its limit, start the active power reduction mode, re-formulate the photovoltaic power generation plan, and issue a command to the photovoltaic inverter control center for execution, so as to meet the voltage quality requirements by reducing part of the active power.

[0046] The following table lists the voltage quality and power allocation evaluation indicators of the distribution network under different control schemes in scenario 3. Under scheme 6, if the voltage exceeds the upper limit, the photovoltaic power station will be shut down, and the photovoltaic power station abandonment rate is as high as 49.89%. The economic benefits are too low and are not considered. Compared with scheme 7, although scheme 8 fails to fully absorb photovoltaic power, it guarantees the voltage quality, reduces the voltage deviation by 26.42%, and effectively reduces the risk of voltage fluctuation. The calculation results show that the proposed power optimization strategy makes full use of the reactive power regulation function of the power station. After the reactive power regulation of the photovoltaic power station reaches the threshold, the active and reactive power of the power station are coordinated and controlled, which improves the voltage quality of the photovoltaic grid-connected point.

[0047] Table 3 Voltage quality evaluation table for scenario 3

[0048] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method, characterized in that: The following steps are involved: S1. Obtain the planned output value of each distributed photovoltaic power station in the low-voltage access distribution station area, the historical data of each distributed photovoltaic output and the historical data of each user's load power, and predict the photovoltaic output power, each user's load power and voltage characteristic curve without flexible control; S2. Calculate the voltage reference value of each node in the low-voltage distribution station area, compare the predicted voltage characteristic curve with the voltage reference value to determine whether the voltage exceeds the upper limit, obtain the photovoltaic excess power based on the photovoltaic day-ahead planned output value and the predicted photovoltaic output power, and determine the start-up strategy of the photovoltaic user's storage electric water heater based on the photovoltaic excess power and the predicted voltage characteristic curve, and calculate the maximum total power consumption of the storage electric water heater. With time interval Excess photovoltaic power Compare and execute the first level of control: If , then continue to execute step S3, otherwise end the calculation; S3, calculate and determine the power regulation mode of the photovoltaic power station, and execute the second layer of control: if the reactive power regulation mode is entered, execute step S4, otherwise execute step S5; S4, increase the reactive power consumption of the inverter to reduce the voltage to the qualified range, calculate the power generation plan, determine whether the voltage of each node meets the standard, and execute the third layer of control: if it meets the standard, issue instructions according to the current power generation plan and end the calculation; if it does not meet the standard, enter step S6; S5. At this time, the reactive power consumption of the photovoltaic inverter reaches the limit value but is still insufficient to achieve the control purpose. The inverter is adjusted to reduce the active power of photovoltaic power generation to try to reduce the voltage to a qualified range. It is determined whether the voltage of each node meets the standard and the third level of control is performed: if it meets the standard, the instruction is issued according to the current power generation plan and the calculation is terminated. If it does not meet the standard, it goes to step S6. S6. Execute the fourth level of control: forcibly reduce the PV active power by tripping the asymmetric group intelligent switch until the voltage drops below the allowable upper limit.

2. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 1, characterized in that: In step S1, the specific steps are as follows: S1.

1. Input low voltage access distribution station area The planned output values ​​of each distributed photovoltaic power plant at and before the time, the historical output data of each distributed photovoltaic power plant and the historical data of each user's load power are normalized and divided into training set, test set and validation set; S1.2, perform variational mode decomposition (VMD) on the training set, test set and validation set respectively, and obtain four independent intrinsic mode components; S1.

3. The four intrinsic mode components corresponding to the training set are used as the input values ​​of the LSTM model. The output of the LSTM model is the modal components corresponding to the photovoltaic output power and the load power of each user without flexible control. Each intrinsic mode component is learned using one LSTM model. S1.

4. Use the trained LSTM model to predict the modal components corresponding to the photovoltaic output power and each user's load power in the future without flexible control, and then reconstruct the various modal components of the prediction results back to the original signal form to obtain the photovoltaic output data and each user's load power data without flexible control; S1.

5. According to the predicted photovoltaic output data without flexible control and the load power data of each user, set the load and power generation conditions at each time point, select the forward-backward method to calculate the power flow of the radial distribution network, and obtain the first voltage prediction value, current prediction value and power prediction value of each node in the power grid at each time point; use the obtained data to draw the voltage characteristic curve.

3. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 2, characterized in that: In step S1.1, the details are as follows: S1.1.

1. Divide the data into training set, test set and validation set according to the set ratio; the training set is used to train the LSTM model; the validation set is used to verify the generalization ability of the current LSTM model during iterative training and ultimately decide whether to stop training; the test set is used to evaluate the generalization ability of the final LSTM model and ensure the reliability of the LSTM model prediction data; S1.1.

2. Normalize the data and scale it to the range of [0,1].

4. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 1, characterized in that: In step S2, the specific steps are as follows: S2.

1. Calculate the voltage reference value of each node in the low-voltage distribution station area as follows: Use sliding window algorithm to pre-process the voltage of each node in the low-voltage distribution station area; calculate the time interval The average voltage of each node in If the voltage of each node does not exceed the upper limit, the time interval is Neidi The average voltage of the nodes is taken as the The first voltage reference value of the node , is the number of nodes in the distribution network, =1,2……, ; If If the voltage of a node exceeds the upper limit, the specified voltage operation upper limit is used as the a first voltage reference value of a node; The first voltage reference value of the node The calculation formula is: in, For time interval Internal voltage sampling number; For time interval Neidi The historical measured values ​​of the voltage of each node; It is obtained by the power flow calculation in step S1.5 Moment A first voltage prediction value of a node; is the specified upper voltage operating limit; S2.2, determine whether the voltage exceeds the upper limit: the first voltage prediction value of each node Compare with the first voltage reference value of each node. If the first voltage prediction value of the node is greater than the first voltage reference value, it means that the voltage of the node exceeds the upper limit. Otherwise, it does not exceed the upper limit. If the voltage does not exceed the upper limit, execute step S2.

3. If the voltage exceeds the upper limit, execute step S2.

4. S2.

3. Set the water heater to start heating to the set temperature at 4:00 p.m. , end the calculation; S2.

4. Subtract the photovoltaic output data predicted in step S1 from the photovoltaic output value planned on the previous day to obtain the photovoltaic excess power. ; S2.

5. Set the start time of the storage electric water heater as follows: The voltage characteristic curve obtained in step S1 is compared with the first voltage reference value to obtain the time period when the voltage exceeds the upper limit; the water heater is set to start heating during the time period when the voltage exceeds the upper limit, and in principle, heating is not performed in other time periods; S2.

6. Calculate the regulating capacity of the storage electric water heater: Set the storage water heater to If the heating is stopped immediately afterwards, the maximum total power consumption of the storage water heater is The calculation formula is as follows: in, is the specific heat capacity of water, Indicates the total volume of water in the water heater tank that needs to be heated. represents the initial temperature of the water in the tank, Indicates the number of storage-type electric water heaters in the low-voltage distribution station area; S2.

7. Maximum total power consumption of storage electric water heater With time interval Comparison of excess photovoltaic power within: ； like , that is, the excess photovoltaic power is not enough to heat the water in the water heater to the set temperature , then the storage electric water heater will also consume additional electricity Heating; after the voltage exceeds the upper limit, the storage water heater operates at a constant power. Continue heating for a while , The calculation formula is as follows: in, The operating power of the storage water heater when the overvoltage ends and the heating continues, and the time required for the heating to continue is set. Then the operation ends; like , that is, the storage electric water heater just completely consumes the surplus active power during the overvoltage period, then the calculation ends and there is no need to carry out the next level of control; like , that is, the storage electric water heater has reached the threshold value but has not yet completely consumed the excess photovoltaic power, then the inverter reactive power optimization adjustment mode of the next level of control needs to be started; at this time, the water heater operates at a constant power Heat to the set temperature ; Photovoltaic excess power after the first level of control The calculation formula is as follows: 。 5. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 4, characterized in that: In step S3, the storage electric water heater cannot completely absorb the excess photovoltaic power, and the inverter active and reactive power are adjusted to reduce the voltage to a qualified range, and the photovoltaic power station power regulation mode is calculated and determined. The specific steps are as follows: S3.1, based on the load power data of each user predicted in step S1 and the photovoltaic excess power calculated in step S2 , recalculate the power flow to obtain the second voltage prediction value of each node in the low-voltage distribution station area , is the number of nodes in the distribution network, =1,2……, ; Recalculate the second voltage reference value of each node according to step S2.1 ; S3.

2. Determine the second voltage prediction value calculated in step S3.1 The highest node is the dominant node, and the dominant node number is ; Pre-adjustment value of voltage at the leading node for: ； S3.

3. The power factor of the photovoltaic power station is dynamically adjustable within the range of 0.95 leading to 0.95 lagging. Considering the power factor limitation of the photovoltaic power station, Photovoltaic power plants The first maximum output reactive power at the moment for: Where: is the power factor of the photovoltaic power station, which is 0.95; For the Photovoltaic power plants The predicted value of the maximum output active power at the moment; is the number of PV power stations in the distribution network, =1,2,……, ; S3.

4. Fitting the second voltage prediction value of each node by using nonlinear least squares method The relationship between power and real-time operation data is used to calculate the real-time voltage sensitivity between nodes; S3.5, reactive power can be reduced to the lowest , calculate the maximum adjustable voltage of the dominant node : Where: is the voltage-reactive sensitivity matrix The first ( , ) element, indicating the The node pair is injected into The voltage sensitivity of the grid-connected node power change of a photovoltaic power station, The node is The grid-connected nodes of a photovoltaic power station; for Moment The reactive power output of each photovoltaic power station; S3.

6. Calculate the photovoltaic reactive power evaluation coefficient of the distribution network : When the reactive power of the power station is adjusted, the voltage amplitude of each node will increase, and the reactive power + active power reduction mode needs to be entered; when the reactive power evaluation coefficient is positive, if , enter the reactive power regulation mode, if , then it enters the reactive power + active power reduction mode; If the reactive power + active power reduction mode is entered, step S5 is executed; if the reactive power regulation mode is entered, step S4 is executed.

6. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 5, characterized in that: The specific steps of step S3.4 are as follows: S3.4.

1. Use the historical data of distributed photovoltaic output and the historical data of user load power to calculate the power flow to obtain the historical injection power and historical voltage data of each node, and use the nonlinear least squares method to fit the mapping relationship between the historical injection power and historical voltage data of each node; S3.4.

2. Collect the current voltage and power of each node and randomly generate a matrix of small changes in injected power , define a scaling factor , 0 < << 1, satisfy ;in, Indicates The absolute value of the current injected power of each node, Indicates The absolute value of the change in the current injected power of each node; keep the reactive power of each node unchanged, and obtain the voltage distribution of each node after the power change through the curve fitted in step S3.4.1, and further obtain the voltage slight change matrix ; S3.4.

3. Use the least squares method to solve the current voltage-active power sensitivity matrix ; is the voltage-active power sensitivity matrix The first ( , ) element, indicating the The node pair is injected into Voltage sensitivity of active power changes at each node; S3.4.4, Similarly, the voltage-reactive sensitivity matrix of node voltage to injected reactive power is calculated ; is the voltage-reactive sensitivity matrix The first ( , ) element, indicating the The node pair is injected into The voltage sensitivity of each node to the reactive power change.

7. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 6, characterized in that: In step S4, the reactive power consumption of the inverter is increased to reduce the voltage to a qualified range, and the power generation plan is calculated. The specific steps are as follows: S4.1, subtract the second voltage prediction value of each grid-connected node of each photovoltaic power station calculated in step S3.1 from the second voltage reference value, and combine them to obtain a voltage error matrix , the active power of the power station does not change at this time, so the active power change matrix of the photovoltaic power station is ; Extract the voltage-reactive sensitivity synthesis matrix between the grid-connected nodes of the photovoltaic power station ,get: Where: is the reactive power variation matrix of the photovoltaic power station, and the voltage-reactive power sensitivity matrix The first ( , ) element, indicating the The grid-connected nodes of each photovoltaic power station inject The voltage sensitivity of the grid-connected node power change of a photovoltaic power station, is the voltage error matrix of the grid-connected node; S4.

2. After calculating the reactive power change matrix of the photovoltaic power station, Reactive power output of a photovoltaic power station for: Where: for Reactive power change matrix of photovoltaic power station at every moment The elements; S4.3, add the limit function, if Exceeding the maximum output reactive power of the photovoltaic power station , then The reactive power output plan is issued; at this time, each power station outputs active power according to the day-ahead power generation plan, and only the reactive power output needs to be changed. Photovoltaic power plants The first reactive power output plan at the moment for: ； S4.

4. Calculate the power flow of the distribution network to obtain the third voltage prediction value of each node in the distribution network after each power station executes the current power generation plan. , update the third voltage reference value ; Calculate the voltage deviation and : ； S4.

5. Determine whether the voltage of each node of the distribution network meets the standard. If And the third voltage prediction value of all nodes All less than If the voltage of the distribution network node meets the standard, the command will be issued according to the current power generation plan and the calculation will be ended. Indicates the allowable voltage deviation range; if the voltage of the distribution network node does not meet the standard, the photovoltaic power generation power is updated to the current power generation plan, the active power output plan remains unchanged, and the reactive power output plan is updated to the first reactive power output plan , fitting the voltage sensitivity matrix between the current nodes and , execute step S6 to enter the reactive power + active power regulation mode.

8. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 7, characterized in that: In step S5, at this time, the reactive power consumption of the photovoltaic inverter reaches the limit value but is still insufficient to achieve the control purpose. The inverter is adjusted to reduce the active power of photovoltaic power generation in an effort to reduce the voltage of each node in the low-voltage distribution station area to a qualified range. The specific steps are as follows: S5.

1. If the photovoltaic active power is limited, the maximum adjustable reactive power will change accordingly; calculate the Photovoltaic power plants The second maximum output reactive power at the moment : S5.2, press Output reactive power to maximize the use of photovoltaic power station capacity, then the output reactive power change for: S5.

3. Define fairness as allocating active power reduction to each PV power station in proportion to the maximum power generation. Therefore, each PV power station is set with the same active power reduction ratio. , conduct active and reactive power joint control; Active power reduction ratio The details are as follows: In the formula, For the Photovoltaic power plants The maximum output active power prediction value at time For the The active power reduction of each photovoltaic power station is as follows: In the formula, is the first Photovoltaic power plants Active power output plan value at the moment; The specific values ​​of the dominant node voltage pre-adjustment are as follows: In the formula, is an unknown variable that varies between [0,1), and the rest are constants. After simplification, we can get The quadratic equation of one variable is There is a unique solution within the range of variation; S5.4, find After that, the active power output plan of each photovoltaic power station and the second reactive power output plan The details are as follows: S5.

5. Calculate the power flow of the distribution network to obtain the fourth voltage prediction value of each node of the distribution network after each photovoltaic power station is executed according to the current power generation plan. ; S5.6, determine whether the voltage of each node meets the standard; in the active power reduction mode, if the fourth voltage prediction value of each node All less than If the voltage of the distribution network node meets the standard, the command is issued according to the current power generation plan and the calculation ends; if the voltage of the distribution network node does not meet the standard, the photovoltaic power generation power is updated to the current power generation plan, and the active power output plan is updated to , the reactive power output plan is updated to the second reactive power output plan , fitting the voltage sensitivity matrix between the current nodes and , execute step S6 and enter the next layer of asymmetric grouping intelligent switch control.

9. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 1, characterized in that: In step S6, at this time, the first three layers of control have not achieved the purpose, and the strategy of partial photovoltaic power cut-off is adopted, and the voltage rise caused by the photovoltaic active power reverse transmission is forcibly reduced by tripping the external asymmetric group intelligent switch, as follows: A flexible control switch terminal is configured in front of each photovoltaic user meter, and two groups of switches are configured, one group has a breaking capacity of 1 / 3, and the other group has a breaking capacity of 2 / 3; when the control effects of the storage electric water heater and photovoltaic inverter in the first three levels of control have not yet reduced the station voltage to below the allowable upper limit, the abandonment of light is started.

10. A distributed photovoltaic low-voltage grid-connected four-layer joint flexible voltage control method according to claim 8 or 9, characterized in that: The specific steps of step S6 are as follows: S6.1, first, the edge intelligent terminal issues a command to the flexible control switch terminal of each user end, uniformly tripping the first flexible control switch with a disconnection capacity of 1 / 3 for each user, and fairly requiring all photovoltaic users to uniformly abandon 1 / 3 of the light. If the voltage of each node returns to normal, it will be maintained for a period of time until the voltage of each node continues to drop by a set amount, and then the first flexible control switch will be closed again to avoid unnecessary abandonment of light, and the calculation is ended. According to the calculation in step S5.6, and Issue the instruction, otherwise execute step S6.2; S6.2, at this time, the first flexible control switch with a breaking capacity of 1 / 3 fails to solve the problem of voltage exceeding the upper limit, and then the second flexible control switch with a breaking capacity of 2 / 3 is switched to uniformly trip all users, and all photovoltaic users are required to uniformly abandon 2 / 3 of the light; if the voltage of each node returns to normal, it will be maintained for a period of time until the voltage of each node continues to drop by the set amount, and then the second flexible control switch will be closed again to avoid unnecessary abandonment of light, and the calculation is ended. According to the calculation in step S5.6 and Issue the instruction, otherwise execute step S6.3; S6.3, if the node voltage still exceeds the upper limit, the first flexible control switch and the second flexible control switch are both tripped, and the calculated value in step S5.6 is and Send the command to end the calculation.

Images