Distributed photovoltaic and traction power supply coordinated voltage unbalance compensation method

Abstract

The invention provides a distributed photovoltaic and traction power supply collaborative voltage unbalance compensation method, which belongs to the technical field of distributed photovoltaic and traction power supply collaboration, and comprises the following steps: dividing a power grid into a plurality of sub-regions, and adopting a collaborative compensation strategy for each sub-region. Specifically, the method comprises the following steps: firstly, determining compensation power required to be output by a traction power supply system compensation device SVG and a photovoltaic field station in each sub-region so as to reduce the voltage unbalance degree; and then, an improved multi-agent consistency algorithm is adopted to optimize compensation power output of each photovoltaic converter in each photovoltaic station. On the basis, the negative-sequence current and the voltage unbalance degree of the common coupling point before and after compensation are calculated respectively, and the constraint condition of the voltage unbalance degree is obtained; and finally, taking the constraint condition as a reference, and determining the specific compensation power and compensation current of each photovoltaic converter by using an improved multi-agent consistency algorithm, thereby realizing the accurate compensation of the voltage unbalance degree of all common coupling points.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed photovoltaic and traction power supply coordination, and in particular, relates to a voltage imbalance compensation method for distributed photovoltaic and traction power supply coordination. Background Art

[0002] Electrified railways have been widely developed in recent years due to their excellent characteristics such as low carbon emissions and high transportation efficiency. As the core component of electrified railways, the traction power supply system plays a vital role in supplying energy to the traction load. However, the traction load is an impact, high-power single-phase power load. When it interacts with the three-phase power system, it will bring a large amount of negative sequence current (NSC) to the power grid through the common coupling point (PCC) between the traction substation and the power grid, which will cause voltage unbalance (VU) problems in the power grid, seriously affecting the efficient operation of the power grid and the traction power supply system. Taking some areas as an example, the voltage imbalance problem brought by the traction power supply system to the power grid has caused photovoltaic stations and industrial power users along the electrified railway to frequently disconnect from the grid, seriously affecting the safe and efficient operation of the power grid. Therefore, effective control of the voltage imbalance problem at the PCC is crucial to ensure the safe and efficient operation of the traction power supply system and the power grid.

[0003] In order to improve the VU problem caused by the traction power supply system to the power grid, the traditional solution is to install compensation devices such as SVG (Static Var Generator) in the traction power supply system to compensate for the voltage imbalance problem. However, in order to achieve a satisfactory compensation effect, it is generally necessary to make the installation capacity of the compensation device too large, resulting in high investment costs. In recent years, with the trend of large-scale access to the power system of clean energy, a large number of distributed photovoltaic power generation systems have been connected to the power grid along the electrified railway. This provides the possibility of using distributed photovoltaic systems and traction power supply systems to compensate for VU in a coordinated manner. Some scholars have proposed using the double-fed wind turbines of wind farms and the SVG of the traction power supply system to coordinately compensate for the VU of PCC points, but this study only compensates for a single PCC point and does not consider the coordinated compensation of all PCC points in the power grid area. In addition, there are differences in the operating characteristics of wind power systems and photovoltaic systems, so the use of photovoltaic systems for VU compensation also needs further research.

[0004] At the same time, there are also differences between the existing single-point compensation and regional compensation methods. Although all new energy units in the region are used to coordinate compensation for all PCC points in the region, effective compensation for VU in the region can be achieved as a whole, but the compensation process may also cause the VU at the access point of the new energy units in the power grid to exceed the standard. Therefore, it is urgent to study a coordinated compensation method that can take into account the VU compensation requirements of all PCC points in the entire power grid area and ensure that the VU at the access point of the new energy units meets the requirements. Summary of the invention

[0005] In view of this, the present invention provides a voltage imbalance compensation method for coordinated distributed photovoltaic and traction power supply, which can solve the technical problem that the existing technology basically performs single-point compensation on one PCC, and does not perform multi-point overall compensation on the PCC points of all traction substations in a regional power grid.

[0006] The present invention is achieved in that:

[0007] A first aspect of the present invention provides a method for voltage imbalance compensation in a coordinated manner of distributed photovoltaic and traction power supply, comprising the following steps:

[0008] S10, using a grid partitioning method based on node voltage sensitivity to divide the grid into multiple sub-areas;

[0009] S20, adopting a coordinated compensation strategy to determine the compensation power that the traction power supply system compensation device SVG and the photovoltaic station need to output respectively in each sub-area; the coordinated compensation strategy includes obtaining the voltage imbalance degree of the sub-area, calculating the average value, sorting, and allocating the compensation power according to the remaining capacity of the photovoltaic station and the SVG capacity;

[0010] S30, using an improved multi-agent consensus algorithm to optimize the compensation power that each photovoltaic converter in each photovoltaic station needs to output; the improved multi-agent consensus algorithm calculates the ratio of the compensation power of each converter to the remaining capacity, and uses graph theory to represent the connection between converters, and finally determines the output compensation power of each converter;

[0011] S40, calculating the negative sequence reference current; respectively calculating the negative sequence current of the common coupling point PCC before and after compensation, taking into account the voltage, current and related angle parameters of the load port and the compensation port;

[0012] S50, calculating the voltage imbalance before and after compensation at the common coupling point PCC; calculating the voltage imbalance before and after compensation according to the negative sequence current, the line voltage and the short-circuit capacity;

[0013] S60, calculating the unbalanced power before and after compensation to obtain the constraint condition K parameter of the voltage imbalance; by calculating the unbalanced power before and after compensation, the constraint condition K parameter of the voltage imbalance is obtained as a basis for subsequent optimization;

[0014] S70. Using the K parameter as a reference instruction and combining it with the improved multi-agent consensus algorithm, solve the compensation power and compensation current that each photovoltaic inverter inside each photovoltaic station needs to output; determine the compensation power output ratio of each photovoltaic inverter through an iterative process, and finally calculate the specific compensation power and compensation current, which are used to achieve effective compensation for the voltage imbalance of all common coupling points in the entire power grid area through the coordinated use of distributed photovoltaic and traction power supply systems.

[0015] Specifically, the step S10 is to determine the voltage sensitivity between nodes by calculating the voltage deviation factor, define the voltage sensitivity between nodes by logarithmic transformation, and finally divide the nodes with the same voltage sensitivity range into the same sub-area.

[0016] Wherein, the step S10 specifically includes:

[0017] Step 101: Propose a voltage deviation factor and calculate the voltage deviation factor according to the following formula:

[0018]

[0019] In the formula, ΔV + and ΔV - are the positive and negative sequence voltage deviation factors, ΔQ + With ΔQ - They represent the positive-sequence and negative-sequence reactive powers at the common coupling point between the traction power supply system and the power grid, respectively, and J is the power flow Jacobian matrix;

[0020] Step 102: Calculate the voltage sensitivity between node i and node j according to the following formula:

[0021]

[0022] Step 103: To ensure the symmetry of the VUS between node i and node j, a logarithmic transformation is used to define the VUS between the two nodes, represented by d, as shown below:

[0023] d ij =d ji =-lg|VUS ij VUS ji |;

[0024] Step 104: Divide the nodes in the same range of d into the same sub-area according to the following formula to realize the grid partitioning based on voltage sensitivity:

[0025] d α ≤d ij ≤d β .

[0026] The collaborative compensation strategy proposed in step S20 specifically includes:

[0027] Step 201, obtaining the voltage unbalance degree at the common coupling point between all traction substations and the power grid in each sub-area;

[0028] Step 202, calculating the average value of the voltage unbalance at the common coupling point between all traction substations and the power grid in each sub-area;

[0029] Step 203: sort the sub-regions in descending order according to the average value of the voltage imbalance in each sub-region;

[0030] Step 204: starting from the sub-region with the largest average voltage imbalance, calculate the total remaining capacity of all photovoltaic stations in the sub-region;

[0031] Step 205: If the sum of the remaining capacities of all photovoltaic stations in the sub-region is greater than the sum of the reactive powers that need to be compensated at the common coupling points of all traction substations and the power grid in the sub-region, the photovoltaic stations in the sub-region will complete the voltage imbalance compensation in the sub-region;

[0032] Step 206: If the sum of the remaining capacities of all photovoltaic stations in the sub-region is less than the sum of the reactive powers that need to be compensated at the common coupling points of all traction substations and the power grid in the sub-region, the photovoltaic stations in the sub-region and the traction power supply system compensation device SVG jointly complete the voltage imbalance compensation in the sub-region;

[0033] Step 207, repeating steps 204 to 206 until the voltage imbalance compensation of all sub-areas is completed;

[0034] Step 208: sort the photovoltaic stations in descending order according to the remaining capacity of the photovoltaic stations in each sub-region, and allocate the compensation power to be output by each photovoltaic station in this order;

[0035] Step 209 : sort the SVGs in descending order according to the remaining capacity of the traction power supply system compensation device SVG in each sub-area, and allocate the compensation power required to be output by each SVG in this order.

[0036] Wherein, the step S30 specifically includes:

[0037] Step 301: Calculate the compensation power that each converter needs to output according to the remaining capacity of each converter, as shown below:

[0038]

[0039] In the formula, S uni is the compensation power allocated to the i-th converter in each PV station, S rci is the remaining capacity allocated to the i-th converter in each PV station;

[0040] Step 302: The basic theory of graph theory is used to represent the connection between each photovoltaic inverter in each photovoltaic station, that is, G = {V, E, A}, where V represents the node of the graph, E represents the edge of the graph, and A is the adjacency matrix of the graph, which represents the connection relationship between each node, as shown below:

[0041]

[0042] In the formula, μ ij It is the non-negative weighted adjacent element of matrix A, which is a 0-1 binary variable.

[0043] Wherein, the step S40 specifically includes:

[0044] Step 401, calculate the negative sequence current of the PCC before compensation. Taking phase A as an example, the calculation result is as follows:

[0045]

[0046] Step 402: Calculate the negative sequence current of the PCC after compensation. Taking phase A as an example, the calculation result is as follows:

[0047]

[0048] Where U L and U A are the load port and A phase compensation port voltages, θ LT Represents the transformer connection angle at the load port, θ LL For U L and U A The angle between L is the load port current, m is the number of compensation ports, θ MT is the transformer connection angle at the compensation port, θ Pη is the power factor angle of compensation port η.

[0049] Wherein, the step S50 specifically includes:

[0050] Step 501: Calculate the voltage imbalance at the PCC according to the following formula:

[0051]

[0052] In the formula, I - Indicates the negative sequence current value, U Line Represents PCC line voltage, S d Indicates the short-circuit capacity of PCC;

[0053] Step 502: Calculate the voltage imbalance before compensation according to the following formula:

[0054]

[0055] Step 503: Calculate the compensated voltage imbalance according to the following formula:

[0056]

[0057] Wherein, the step S60 specifically includes:

[0058] Step 601: Calculate the unbalanced power before compensation according to the following formula:

[0059]

[0060] Step 602: Calculate the compensated unbalanced power according to the following formula:

[0061]

[0062] Step 603: Calculate the voltage imbalance constraint parameter K according to the following formula:

[0063]

[0064] Wherein, the step S70 specifically includes:

[0065] Step 701: Determine the ratio of the compensation power output of each photovoltaic inverter to its remaining capacity, as shown below:

[0066]

[0067] Step 702: Incorporate the result of step 701 into the following iterative process:

[0068]

[0069] Step 703: Calculate the compensation power output ratio of each photovoltaic converter, as shown below:

[0070]

[0071] Step 704: Calculate the compensation power of each photovoltaic converter according to the following formula:

[0072]

[0073] Step 705: Calculate the compensation current of each photovoltaic converter according to the following formula:

[0074]

[0075] The improved multi-agent consensus algorithm specifically includes:

[0076] Step 801: Change the element μi in the adjacency matrix A j To achieve a higher convergence speed, the new assignment rules are as follows:

[0077]

[0078] Step 802: According to the assignment rule of step 801, construct an improved Laplace matrix as shown below:

[0079]

[0080] In the formula,

[0081] Compared with the prior art, the beneficial effect of the voltage imbalance compensation method for coordinated distributed photovoltaic and traction power supply provided by the present invention is: full use of the distributed photovoltaic power plants along the electrified railway power grid and the SVG in the traction power supply system, and the use of systematic compensation strategies and algorithms to achieve accurate compensation of voltage imbalance at all common coupling points in the entire power grid area.

[0082] Compared with the prior art, the solution of the present invention has the following outstanding advantages:

[0083] 1. The voltage imbalance compensation requirements of all PCC points in the power grid area are fully considered, and a coordinated compensation strategy is adopted to fully utilize the remaining capacity of distributed photovoltaic power generation and compensate through the SVG of the traction power supply system, thus achieving effective compensation of VU of all PCC points in the entire power grid area. This not only improves the capacity utilization rate of the distributed photovoltaic station converter, but also reduces the installation capacity and investment cost of the compensation device of the traction power supply system.

[0084] 2. A grid zoning method based on node voltage sensitivity is adopted to divide the grid into multiple sub-areas. Combined with an improved multi-agent consistency algorithm, the compensation power output between photovoltaic inverters is coordinated to ensure that the VU at the access points of all new energy units in the entire grid area meets the requirements, avoiding the problem of local over-compensation or under-compensation that may occur in single-point compensation. It solves the technical problem that the existing technology basically performs single-point compensation for one PCC, and does not perform multi-point overall compensation for the PCC points of all traction substations in a regional power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0085] Figure 1 A flow chart of the method provided by the present invention;

[0086] Figure 2 A topological diagram for implementing the method in Example 1;

[0087] Figure 3 This is a partition effect diagram of Example 1;

[0088] Figure 4 This is a diagram of the collaborative compensation strategy in Example 1;

[0089] Figure 5 This is the compensation effect diagram under traction load fluctuation;

[0090] Figure 6 This is a diagram of the compensation effect under photovoltaic fluctuations. DETAILED DESCRIPTION

[0091] In order to make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0092] like Figure 1 FIG. 1 is a flow chart of a voltage imbalance compensation method for coordinated distributed photovoltaic and traction power supply provided by the present invention. The method comprises the following steps:

[0093] S10, using a grid partitioning method based on node voltage sensitivity to divide the grid into multiple sub-areas;

[0094] S20, adopting a coordinated compensation strategy to determine the compensation power that the traction power supply system compensation device SVG and the photovoltaic station need to output respectively in each sub-area; the coordinated compensation strategy includes obtaining the voltage imbalance degree of the sub-area, calculating the average value, sorting, and allocating the compensation power according to the remaining capacity of the photovoltaic station and the SVG capacity;

[0095] S30, using an improved multi-agent consensus algorithm to optimize the compensation power that each photovoltaic inverter in each photovoltaic station needs to output; the improved multi-agent consensus algorithm calculates the ratio of the compensation power of each inverter to the remaining capacity, and uses graph theory to represent the connection between the inverters, and finally determines the output compensation power of each inverter;

[0096] S40, calculating the negative sequence reference current; respectively calculating the negative sequence current of the common coupling point PCC before and after compensation, taking into account the voltage, current and related angle parameters of the load port and the compensation port;

[0097] S50, calculating the voltage imbalance before and after compensation at the common coupling point PCC; calculating the voltage imbalance before and after compensation according to the negative sequence current, the line voltage and the short-circuit capacity;

[0098] S60, calculating the unbalanced power before and after compensation to obtain the constraint condition K parameter of the voltage imbalance; by calculating the unbalanced power before and after compensation, the constraint condition K parameter of the voltage imbalance is obtained as a basis for subsequent optimization;

[0099] S70, using the K parameter as a reference instruction, combined with the improved multi-agent consensus algorithm, solving the compensation power and compensation current that each photovoltaic converter in each photovoltaic station needs to output; determining the compensation power output ratio of each photovoltaic converter through an iterative process, and finally calculating the specific compensation power and compensation current;

[0100] S70. Using the K parameter as a reference instruction and combining it with the improved multi-agent consensus algorithm, solve the compensation power and compensation current that each photovoltaic inverter inside each photovoltaic station needs to output; determine the compensation power output ratio of each photovoltaic inverter through an iterative process, and finally calculate the specific compensation power and compensation current, which are used to achieve effective compensation for the voltage imbalance of all common coupling points in the entire power grid area through the coordinated use of distributed photovoltaic and traction power supply systems.

[0101] The specific implementation methods of the above steps are described in detail below:

[0102] The specific implementation of step S10 is to divide the power grid into multiple sub-areas using a power grid partitioning method based on node voltage sensitivity. Specifically, the following steps are included:

[0103] Step 101, the concept of voltage deviation factor is proposed. The voltage deviation factor ΔV of the positive sequence and the negative sequence is calculated according to the following formula: + and ΔV - :

[0104]

[0105] Among them, ΔQ + and ΔQ - denote the positive and negative sequence reactive power at the common coupling point of the traction power supply system and the power grid, respectively, and J is the power flow Jacobian matrix. The voltage deviation factor reflects the sensitivity of the node voltage to reactive power.

[0106] Step 102: Calculate the voltage sensitivity VUS between node i and node j according to the following formula: ij and VUS ji :

[0107]

[0108] This step aims to quantitatively describe the voltage sensitivity relationship between nodes.

[0109] Step 103, to ensure the symmetry of the VUS between node i and node j, a logarithmic transformation is used to define the VUS between the two nodes, represented by d, as shown below:

[0110] d ij =d ji =-lg|VUS ij VUS ji |;

[0111] By logarithmic transformation, we can ensure that the voltage sensitivity between nodes in the same range of d is similar.

[0112] Step 104, divide the nodes in the same range of d into the same sub-area according to the following formula to realize the grid partitioning based on voltage sensitivity:

[0113] d α ≤d ij ≤d β ;

[0114] Among them, d α and d β is a preset threshold value, which can be 0.1, 0.3, etc.

[0115] In general, step S10 calculates the voltage deviation factor to quantitatively describe the voltage sensitivity between nodes, and uses logarithmic transformation to divide nodes with similar voltage sensitivity into the same sub-area, laying the foundation for the subsequent collaborative compensation strategy.

[0116] Step S20 proposes a collaborative compensation strategy, which specifically includes the following steps:

[0117] Step 201, obtaining the voltage unbalance degree at the common coupling point between all traction substations and the power grid in each sub-area.

[0118] Step 202: Calculate the average value of the voltage imbalance at the common coupling point between all traction substations and the power grid in each sub-area.

[0119] Step 203: sort the sub-regions in descending order according to the average value of the voltage imbalance in each sub-region.

[0120] Step 204, starting from the sub-region with the largest average voltage imbalance, calculate the total remaining capacity of all photovoltaic stations in the sub-region.

[0121] Step 205: If the total remaining capacity of all photovoltaic stations in the sub-region is greater than the total reactive power that needs to be compensated at the common coupling point between all traction substations and the grid in the sub-region, the photovoltaic stations in the sub-region will complete the voltage imbalance compensation in the sub-region.

[0122] Step 206: If the total remaining capacity of all photovoltaic stations in the sub-area is less than the total reactive power that needs to be compensated at the common coupling point between all traction substations and the power grid in the sub-area, the photovoltaic stations in the sub-area and the traction power supply system compensation device SVG jointly complete the voltage imbalance compensation in the sub-area.

[0123] Step 207, repeating steps 204 to 206 until the voltage imbalance compensation of all sub-areas is completed.

[0124] Step 208: sort the photovoltaic stations in descending order according to the remaining capacity of the photovoltaic stations in each sub-area, and allocate the compensation power required to be output by each photovoltaic station in this order.

[0125] Step 209: sort the SVGs in descending order according to the remaining capacity of the traction power supply system compensation device SVG in each sub-area, and allocate the compensation power required to be output by each SVG in this order.

[0126] In general, the coordinated compensation strategy proposed in step S20 first evaluates the voltage imbalance of each sub-region, and then coordinates the compensation power output of each according to the remaining capacity of the photovoltaic station and the SVG to achieve effective compensation for the voltage imbalance of each sub-region.

[0127] Step S30 uses an improved multi-agent consensus algorithm to optimize the compensation power that each photovoltaic inverter in each photovoltaic station needs to output, which specifically includes the following steps:

[0128] Step 301: Based on the remaining capacity S of each converter rci Calculate the compensation power S that each converter needs to output uni , as shown below:

[0129]

[0130] This step ensures that the compensation power of each converter is proportional to its remaining capacity, reflecting the principle of fairness.

[0131] Step 302, using the basic theory of graph theory to represent the connection between each photovoltaic inverter in each photovoltaic station, constructing a graph G = {V, E, A}, where V represents the node of the graph, E represents the edge of the graph, and A is the adjacency matrix of the graph, as shown below:

[0132]

[0133] This step establishes the topological relationship between the converters and provides a basis for subsequent consistency algorithm optimization.

[0134] In general, step S30 adopts an improved multi-agent consensus algorithm, fully considers the connection between the converters, and reasonably allocates the compensation power according to the remaining capacity of each converter, thereby optimizing the compensation scheme within the entire photovoltaic station.

[0135] Step S40 is intended to calculate the negative sequence reference current, and specifically includes the following steps:

[0136] Step 401, calculate the negative sequence current of the PCC before compensation. Taking phase A as an example, the calculation result is as follows:

[0137]

[0138] Among them, U L and U A are the load port and A phase compensation port voltages, θ LT Represents the transformer connection angle at the load port, θ LL For U L and U A The angle between L is the load port current.

[0139] Step 402, calculate the negative sequence current of the PCC after compensation, taking phase A as an example, the calculation result is as follows:

[0140]

[0141] Where m is the number of compensation ports, θ MT is the transformer connection angle at the compensation port, θ Pη is the power factor angle of compensation port η.

[0142] By calculating the negative sequence current of the PCC before and after compensation, basic data is provided for the subsequent voltage imbalance calculation.

[0143] Step S50 is intended to calculate the voltage imbalance at the PCC before and after compensation, and specifically includes the following steps:

[0144] Step 501, calculate the voltage imbalance at the PCC according to the following formula:

[0145]

[0146] Among them, I - Indicates the negative sequence current value, U Line Indicates PCC line voltage, S aIndicates the PCC short-circuit capacity. Voltage unbalance ε U It reflects the influence of negative sequence current at PCC on line voltage.

[0147] Step 502: Calculate the voltage imbalance before compensation according to the following formula:

[0148]

[0149] Step 503, calculating the compensated voltage imbalance according to the following formula:

[0150]

[0151] By comparing the voltage imbalance before and after compensation, the effectiveness of the coordinated compensation measures can be evaluated.

[0152] Step S60 is intended to calculate the unbalanced power before and after compensation, and obtain the constraint condition K parameter of the voltage imbalance degree based on it, which specifically includes the following steps:

[0153] Step 601, calculate the unbalanced power before compensation according to the following formula:

[0154]

[0155] Step 602: Calculate the compensated unbalanced power according to the following formula:

[0156]

[0157] Step 603, calculate the voltage imbalance constraint parameter K according to the following formula:

[0158]

[0159] The K parameter quantitatively describes the degree of change in voltage imbalance before and after compensation, providing a basis for subsequent optimization.

[0160] Step S70 uses the improved multi-agent consensus algorithm and the K parameter obtained in step S60 to solve the compensation power and compensation current that each photovoltaic converter in each photovoltaic station needs to output, which specifically includes the following steps:

[0161] Step 701, determining the ratio of the compensation power output of each photovoltaic inverter to its remaining capacity, as shown below:

[0162]

[0163] Step 702, incorporate the result of step 701 into the following iterative process:

[0164]

[0165] This step uses a multi-agent consensus algorithm to coordinate the compensation power output ratio of each converter according to the topological relationship between the converters.

[0166] Step 703, calculating the compensation power output ratio of each photovoltaic converter, as shown below:

[0167]

[0168] Step 704: Calculate the compensation power of each photovoltaic converter according to the following formula:

[0169]

[0170] Step 705: Calculate the compensation current of each photovoltaic converter according to the following formula:

[0171]

[0172] In general, step S70 uses an improved multi-agent consensus algorithm to reasonably allocate the compensation power and compensation current of each photovoltaic converter under the premise of satisfying the voltage imbalance constraint, thereby achieving effective compensation for all PCC points in the entire power grid area.

[0173] Step S80 connects the above steps in series, and based on the coordinated use of distributed photovoltaic and traction power supply systems, realizes accurate compensation of voltage imbalance at all common coupling points in the entire power grid area. Through measures such as grid partitioning, coordinated compensation strategy, and improved multi-agent consensus algorithm, the synergistic advantages of distributed photovoltaic and traction power supply systems are fully utilized, effectively solving the voltage imbalance problem brought by electrified railways to the power grid, improving the capacity utilization rate of distributed photovoltaic station converters, and reducing the installation capacity and investment cost of traction power supply system compensation devices.

[0174] Specifically, the principle of the present invention is:

[0175] 1. Grid partitioning: First, the grid partitioning method based on node voltage sensitivity is used to divide the entire grid into multiple sub-areas. Specifically, by calculating the voltage deviation factor ΔV + and ΔV - , quantitatively describes the sensitivity of node voltage to reactive power. Then, according to the voltage sensitivity VUS between nodes ij and VUS ji , using logarithmic transformation to divide nodes with similar voltage sensitivity into the same sub-area. This grid partitioning method ensures that the voltage sensitivity of nodes in the same sub-area is relatively close, which is conducive to the subsequent coordinated compensation strategy.

[0176] 2. Collaborative compensation strategy: Based on the grid division, a collaborative compensation strategy is adopted. First, the voltage imbalance of all PCC points in each sub-area is obtained, the average value is calculated and sorted. Then, starting from the sub-area with the largest average voltage imbalance, it is evaluated whether the remaining capacity of the photovoltaic field station in the sub-area is sufficient to complete the VU compensation of the sub-area. If it is sufficient, the photovoltaic field station will complete the compensation; if it is insufficient, the photovoltaic field station and the SVG of the traction power supply system will jointly complete the compensation. Through this orderly compensation method that fully utilizes the remaining photovoltaic capacity, effective compensation for the VU of each sub-area is achieved.

[0177] 3. Improved multi-agent consensus algorithm: After determining the compensation division of each sub-area, an improved multi-agent consensus algorithm is used to optimize the compensation power output of each photovoltaic inverter within the photovoltaic station. The algorithm first calculates the initial compensation power distribution based on the remaining capacity of each inverter, and then uses graph theory to represent the topological relationship between the inverters. Through iterative optimization, it ensures that the compensation power output ratios between the inverters are coordinated and consistent. This not only reflects the principle of fairness, but also ensures the reasonable distribution of compensation power of each inverter.

[0178] A specific embodiment 1 of the present invention is provided below, and this embodiment 1 comprises the following steps:

[0179] Step 1: The principle of using the remaining capacity of the photovoltaic station converter to compensate for voltage imbalance is to use the remaining capacity of the photovoltaic station converter to inject reverse unbalanced power into the common coupling point between the traction substation and the power grid to offset the unbalanced power injected into the common coupling point by the traction power supply system. In a large regional power grid, when the remaining capacity of the photovoltaic station is used to compensate for the voltage imbalance problem caused by the traction power supply system, there is a situation where a photovoltaic station simultaneously compensates for the voltage imbalance at the common coupling point between multiple traction substations and the power grid. Although this can make the voltage imbalance at the common coupling point between the traction substation and the power grid meet the standard, it causes the voltage imbalance at the photovoltaic station to exceed the standard. Therefore, in order to improve the compensation effect of the voltage imbalance at the common coupling point between multiple traction substations and the power grid of the entire power grid, this embodiment proposes a power grid partitioning method based on node voltage sensitivity, such as Figure 2 The figure shows a schematic diagram of the topological structure of the coupling between the traction power supply system, photovoltaic stations and the power grid in a large regional power grid. Figure 2In the illustrated topological structure, the traction substation 3, the photovoltaic station 3 and the large power grid 3 are coupled and connected together through the power grid node 6. In this embodiment, the power grid is divided into multiple sub-areas for zone compensation (the photovoltaic station in each sub-area only compensates the voltage imbalance at the common coupling point between the traction substation in each sub-area and the power grid), thereby ensuring that the voltage imbalance of each node in the power grid meets the standard. First, a voltage deviation factor is proposed, and the voltage deviation factor is calculated according to formulas (1) and (2):

[0180]

[0181]

[0182] Among them, ΔV+ and ΔV- are the positive-sequence and negative-sequence voltage deviation factors, ΔQ+ and ΔQ- represent the positive-sequence and negative-sequence reactive powers at the common coupling point between the traction power supply system and the power grid, J is the power flow Jacobian matrix, and S represents the reactive voltage sensitivity of the power grid;

[0183] Step 2: Calculate the voltage sensitivity between node i and node j according to equations (3) and (4):

[0184]

[0185] Step 3: To ensure the symmetry of VUS between node i and node j, the VUS between the two nodes is defined by logarithmic transformation and represented by d. The larger d is, the smaller the VUS between the two nodes is, as shown in formula (5). Further, according to formula (6), the nodes with d in the same range are divided into the same sub-area to realize the grid partitioning based on voltage sensitivity, as shown in Figure 3 As shown, it is ensured that the nodes in each area have the same voltage sensitivity, thereby ensuring the compensation effect of the voltage imbalance in each sub-area:

[0186] d ij =d ji =-lg|VUS ij VUS ji | (5)

[0187] d α ≤d ij ≤d β (6)

[0188] exist Figure 3 In the topological structure, the traction substation in region 1 is coupled with the grid through grid node 1, that is, the traction substation is connected to grid node 1, and the photovoltaic station is coupled with the grid through grid node 4, that is, the photovoltaic station is connected to grid node 4. The same is true for other regions. ijIt indicates the partition basis calculated by the method of the present invention, which has no actual meaning and unit, and is only used as a basis for partitioning the power grid. ij The same nodes (i, j represent the grid node number), such as node i and node j, will be divided into the same area. The content in the coordinate diagram means that nodes 1, 4, and 7 are divided into the same area, nodes 2, 5, and 8 are divided into the same area, and nodes 3, 6, and 9 are divided into the same area. Therefore, in the subsequent voltage unbalance (VU) compensation process, the voltage unbalance at the common coupling point between the traction substation and the grid in each area will be compensated by the converter of the photovoltaic station in each area and the compensation device of the railway. For example, the voltage unbalance of node 1 is compensated by the compensation device of the traction substation connected to node 1 and the converter of the photovoltaic station connected to nodes 4 and 7.

[0189] Step 4: In order to ensure the compensation effect of voltage imbalance in each sub-area after partitioning, and to make full use of the remaining capacity of the photovoltaic converter in each sub-area, a coordinated compensation strategy is proposed to accurately determine the compensation power that the traction power supply system compensation device SVG and the photovoltaic station need to output in each sub-area. Figure 4 As shown;

[0190] Step 5: After determining the compensation power that each PV station needs to output based on the collaborative compensation strategy, the improved multi-agent consensus algorithm is used to optimize the compensation power that each PV inverter in each PV station needs to output. First, the compensation power that each inverter needs to output is calculated based on the remaining capacity of each inverter, as shown in (7):

[0191] S un1 / S rc1 =S un2 / S rc2 =…=S uni / S rci (7)

[0192] Step 6: Calculate the negative sequence reference current. Taking phase A as an example, the calculation result of the negative sequence current of the PCC before compensation is shown in (8), and the calculation result of the negative sequence current of the PCC after compensation is shown in (9).

[0193]

[0194] Step 7: According to the VU calculation method at the PCC shown in (10), combined with (8) and (9), a VU calculation model before and after compensation is obtained, and the VU before and after PCC compensation is calculated based on this.

[0195]

[0196] I- represents the negative sequence current value, UL represents the PCC line voltage, and Sd represents the PCC short-circuit capacity.

[0197] Step 8: Multiply (11) and (12) by 3UA to obtain the unbalanced power S-A0 and S-A1 before and after compensation, as shown in (13) and (14). Combining (13) and (14), the constraint condition K parameter of VU is obtained.

[0198]

[0199] Step 9: Take the K parameter as the reference instruction and combine it with the improved multi-agent consensus algorithm to solve the compensation power and compensation current that each PV inverter inside each PV station needs to output. That is, first determine the ratio of the compensation power output of each PV inverter to its remaining capacity, as shown in (17). Incorporate (17) into the iterative process shown in (18) to obtain the compensation power output ratio of each PV inverter shown in (19). Then, combine (20) and (21) to calculate the compensation current of each PV inverter.

[0200] ψ i =S uni / S rci (17)

[0201]

[0202] [S un1 S un2 S un3 ] T =ψ * [S rc1 S rc2 S rc3 ] T (20)

[0203]

[0204] Step 10: Based on the above steps, accurate compensation of VU of all PCC points in the entire power grid area can be completed.

[0205] In addition, regarding the improved multi-agent consensus algorithm described in the above steps, the improvement process is as follows:

[0206] First, the basic theory of graph theory is used to represent the connection between each PV inverter in each PV station, that is, G = {V, E, A}, where V represents the nodes of the graph, E represents the edges of the graph, and A is the adjacency matrix of the graph (a symmetric matrix with diagonal elements of 0), which represents the connection relationship between each node, as shown in equations (22) and (23). Each PV inverter is regarded as an intelligent agent, a node of the graph, and the communication link for interactive information between each intelligent agent is regarded as the edge E of the graph. Then, all the PV inverters in the entire PV station together form a multi-agent system, that is, G.

[0207]

[0208] Among them, μ ij It is the non-negative weighted adjacent element of matrix A, which is a 0-1 binary variable.

[0209] Furthermore, for a given topology, (24) and (25) can be used to represent the consistency protocol without considering communication delay and with considering communication delay, respectively, that is, the relationship between input and output.

[0210]

[0211] Among them, φ is the value of the input variable, ψ is the value of the output variable, and τ ij Represents the communication delay of agent i transmitting data to agent j.

[0212] (24) and (25) can be expressed in matrix form as (26) and (27).

[0213]

[0214] L=Δ-A (28)

[0215] Where L represents the Laplacian matrix of the graph.

[0216] Since the sum of all rows and columns of A is equal to 1, all eigenvalues ​​of A are less than or equal to 1. Therefore, when k increases to infinity as shown in (29), the system will converge.

[0217]

[0218] Therefore, by changing the assignment rule of the element μij in A, a higher convergence speed can be obtained. The new assignment rule is shown in (30):

[0219]

[0220] The improved Laplace proof is shown in (31):

[0221]

[0222] in,

[0223] In order to better understand and implement the present invention, the following provides Example 2 of a specific application scenario of the present invention: There is an electrified railway line in a regional power grid, and multiple traction substations and photovoltaic power plants are distributed along the line. Due to the impact and single-phase characteristics of the traction load, a large amount of negative sequence current is brought to the power grid through the common coupling point (PCC) between the traction substation and the power grid, causing serious voltage imbalance (VU) problems. In order to solve this problem, the local power grid company decided to adopt the voltage imbalance compensation method of the distributed photovoltaic and traction power supply coordination proposed by the present invention.

[0224] First, according to step S10, the power grid in the area is divided into four sub-areas using a power grid partitioning method based on node voltage sensitivity. The specific steps are as follows:

[0225] Step 101, calculate the voltage deviation factor ΔV of the positive sequence and the negative sequence + and ΔV - Assuming that the power flow Jacobian matrix J of the regional power grid is as shown in Table 1, the positive and negative sequence reactive power ΔQ at the common coupling point between the traction power supply system and the power grid is + and ΔQ - If they are 50Mvar and 30Mvar respectively, then:

[0226]

[0227] Step 102: Calculate the voltage sensitivity VUS between nodes ij and VUS ji Assuming that there are 4 nodes in the regional power grid, the calculation results are shown in Table 2:

[0228]

[0229] Step 103, using logarithmic transformation to define the voltage sensitivity d between nodes ij :

[0230] d ij =d ji =-lg|VUS ij VUS ji |;

[0231] Divide the nodes in the same range of d into the same sub-area, assuming d α =0.1,d β =0.3, the power grid can be divided into 4 sub-areas, as shown in Table 3.

[0232] Next, according to step S20, a coordinated compensation strategy is adopted to compensate for the voltage imbalance of each sub-area.

[0233] Step 201, obtaining the voltage unbalance of all PCC points in each sub-area. Assume that through actual measurement, the voltage unbalance of the PCC points in sub-areas 1, 2, 3 and 4 is 2.8%, 3.2%, 2.6% and 2.4% respectively.

[0234] Step 202, calculate the average value of the voltage unbalance of all PCC points in each sub-area, and the results are as follows: Sub-area 1: 2.8% Sub-area 2: 3.2% Sub-area 3: 2.6% Sub-area 4: 2.4%

[0235] Step 203 , sorting the sub-regions according to the average voltage imbalance, and the resulting order is: sub-region 2 > sub-region 1 > sub-region 3 > sub-region 4.

[0236] Step 204, starting from sub-area 2 with the largest average voltage imbalance, calculate the total remaining capacity of all photovoltaic stations in the sub-area. Assuming that there are 3 photovoltaic stations in sub-area 2, and their remaining capacities are 20Mvar, 18Mvar and 15Mvar respectively, the total remaining capacity is 53Mvar.

[0237] Step 205: Since the total remaining capacity of all PV stations in sub-area 2 (53 Mvar) is greater than the total reactive power that needs to be compensated by all PCC points in the sub-area (assuming it is 45 Mvar), the PV stations in the sub-area complete the voltage imbalance compensation in the sub-area.

[0238] Step 208, sorting the photovoltaic stations according to their remaining capacities: 20Mvar>18Mvar>15Mvar, and allocating compensation power in this order, namely 20Mvar, 18Mvar and 15Mvar.

[0239] Step 206, for sub-area 1, the total remaining capacity of all photovoltaic stations in the sub-area (assuming it is 40Mvar) is less than the total reactive power that needs to be compensated for all PCC points in the sub-area (assuming it is 50Mvar), so the photovoltaic stations in the sub-area and the traction power supply system compensation device SVG jointly complete the voltage imbalance compensation in the sub-area.

[0240] Step 209: sort the SVGs according to their remaining capacities and allocate compensation power. Assuming that the remaining capacity of the SVG is 15 Mvar, 10 Mvar is allocated to the SVG for compensation.

[0241] According to the above steps, the voltage imbalance compensation of sub-area 2 and sub-area 1 is completed. For sub-area 3 and sub-area 4, compensation can be performed in a similar manner.

[0242] Next, according to step S30, the improved multi-agent consensus algorithm is used to optimize the compensation power of each photovoltaic inverter inside the photovoltaic station.

[0243] Step 301: Based on the remaining capacity S of each converter rci Calculate the initial compensation power S uni :

[0244]

[0245] Assuming that PV station A has three inverters with remaining capacities of 10Mvar, 8Mvar and 6Mvar respectively, the initial compensation power distribution is 5Mvar, 4Mvar and 3Mvar.

[0246] Step 302: construct a topological relationship diagram G = {V, E, A} between the converters. Assume that the connection relationship between the three converters in the photovoltaic station A is as follows: Figure 1 As shown, the adjacency matrix A is:

[0247]

[0248] Step 702: bring the initial compensation power allocation result into the improved multi-agent consensus algorithm for iterative optimization:

[0249]

[0250] After 20 iterations, the compensation power output ratio of each converter is finally obtained.

[0251] Step 704, calculate the compensation power of each converter according to ψ*:

[0252]

[0253] Step 705, further calculating the compensation current of each converter:

[0254]

[0255] Through the above steps, the optimization of compensation power and compensation current of each converter inside the photovoltaic station is completed.

[0256] Finally, according to steps S40-S60, the negative sequence current and voltage imbalance at the PCC before and after compensation are calculated, and the constraint condition K parameter of the voltage imbalance is obtained. Combined with the optimization result of step S70, accurate compensation of the voltage imbalance at all PCC points in the entire power grid area is achieved.

[0257] Figure 5 , Figure 6 Based on the method of the present invention Figure 3 The compensation effect of the voltage imbalance at the common coupling point between the traction substation and the power grid in the power grid structure shown in FIG. Figure 3 It can be seen that there are three traction substations in the entire selected power grid, which are connected to the power grid through grid node 1, grid node 2, and grid node 3 respectively. That is, there are three common coupling points between the traction substation and the power grid in the system, namely, grid node 1, grid node 2, and grid node 3. The compensation target of the voltage imbalance at the common coupling point is 2% (i.e. Figure 5 , Figure 6 The dotted line to which the compensation target points) means that the voltage unbalance of the grid node 1, the grid node 2 and the grid node 3 needs to be suppressed to less than 2%.

[0258] In actual operation, the traction load in the traction power supply system and the active power output of the photovoltaic station are constantly changing, and these factors have a direct impact on the compensation effect. Figure 5 The compensation effect under the condition of traction load fluctuation is considered. Before the 2nd second, when no compensation is performed, the voltage imbalance at the three common coupling points exceeds the standard. After compensation is performed in the 2nd second, the voltage imbalance at the three common coupling points is limited to less than 2%. Even when traction load fluctuation occurs at the 4th and 6th seconds, it can be found that the voltage imbalance at the three common coupling points can still be suppressed to less than 2%. Figure 6 It considers the compensation effect when the active power output of the photovoltaic station fluctuates. Before the 2nd second, when no compensation is performed, the voltage imbalance at the three common coupling points exceeds the standard. After compensation is performed at the 2nd second, the voltage imbalance at the three common coupling points is limited to less than 2%. Even when the active power output of the photovoltaic station fluctuates at the 2nd second and the 6th second respectively, it can be found that the voltage imbalance at the three common coupling points can still be suppressed to less than 2%.

[0259] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A voltage imbalance compensation method for distributed photovoltaic and traction power supply coordination, characterized in that: The following steps are involved: S10, using a grid partitioning method based on node voltage sensitivity to divide the grid into multiple sub-areas; S20, adopting a coordinated compensation strategy to determine the compensation power that the traction power supply system compensation device SVG and the photovoltaic station need to output respectively in each sub-area; the coordinated compensation strategy includes obtaining the voltage imbalance degree of the sub-area, calculating the average value, sorting, and allocating the compensation power according to the remaining capacity of the photovoltaic station and the SVG capacity; S30, using an improved multi-agent consensus algorithm to optimize the compensation power that each photovoltaic converter in each photovoltaic station needs to output; the improved multi-agent consensus algorithm calculates the ratio of the compensation power of each converter to the remaining capacity, and uses graph theory to represent the connection between converters, and finally determines the output compensation power of each converter; S40, calculating negative sequence reference current; Calculate the negative sequence current of the common coupling point PCC before and after compensation, taking into account the voltage, current and related angle parameters of the load port and compensation port; S50, calculating the voltage imbalance degree before and after the common coupling point PCC compensation; According to the negative sequence current, line voltage and short-circuit capacity, the voltage unbalance before and after compensation is calculated respectively; S60, calculating the unbalanced power before and after compensation, and obtaining a constraint condition K parameter of the voltage imbalance; By calculating the unbalanced power before and after compensation, the constraint condition K parameter of voltage imbalance is obtained as the basis for subsequent optimization; S70. Using the K parameter as a reference instruction and combining it with the improved multi-agent consensus algorithm, solve the compensation power and compensation current that each photovoltaic inverter inside each photovoltaic station needs to output; determine the compensation power output ratio of each photovoltaic inverter through an iterative process, and finally calculate the specific compensation power and compensation current, which are used to achieve effective compensation for the voltage imbalance of all common coupling points in the entire power grid area through the coordinated use of distributed photovoltaic and traction power supply systems.

2. The method according to claim 1, characterized in that ,The step S10 specifically determines the voltage sensitivity between nodes by calculating the ,voltage deviation factor, and defines the voltage sensitivity between nodes using ,logarithmic transformation, and finally divides the nodes with the same range of voltage ,sensitivity into the same sub-area.

3. The method according to claim 2, characterized in that , the step S10 specifically includes: Step 101: Propose a voltage deviation factor and calculate the voltage deviation factor according to the following formula: In the formula, ΔV + and ΔV - are the positive and negative sequence voltage deviation factors, ΔQ + With ΔQ - They represent the positive-sequence and negative-sequence reactive powers at the common coupling point between the traction power supply system and the power grid, respectively, and J is the power flow Jacobian matrix; Step 102: Calculate the voltage sensitivity between node i and node j according to the following formula: Step 103: To ensure the symmetry of the VUS between node i and node j, a logarithmic transformation is used to define the VUS between the two nodes, represented by d, as shown below: d ij =d ji =-lg|VUS ij VUS ji |; Step 104: Divide the nodes in the same range of d into the same sub-area according to the following formula to realize the grid partitioning based on voltage sensitivity: d α ≤d ij ≤d β 。 4. The method according to claim 3, characterized in that The collaborative compensation strategy proposed in step S20 specifically includes: Step 201, obtaining the voltage unbalance degree at the common coupling point between all traction substations and the power grid in each sub-area; Step 202, calculating the average value of the voltage unbalance at the common coupling point between all traction substations and the power grid in each sub-area; Step 203: sort the sub-regions in descending order according to the average value of the voltage imbalance in each sub-region; Step 204: starting from the sub-region with the largest average voltage imbalance, calculate the total remaining capacity of all photovoltaic stations in the sub-region; Step 205: If the sum of the remaining capacities of all photovoltaic stations in the sub-region is greater than the sum of the reactive powers that need to be compensated at the common coupling points of all traction substations and the power grid in the sub-region, the photovoltaic stations in the sub-region will complete the voltage imbalance compensation in the sub-region; Step 206: If the sum of the remaining capacities of all photovoltaic stations in the sub-region is less than the sum of the reactive powers that need to be compensated at the common coupling points of all traction substations and the power grid in the sub-region, the photovoltaic stations in the sub-region and the traction power supply system compensation device SVG jointly complete the voltage imbalance compensation in the sub-region; Step 207, repeating steps 204 to 206 until the voltage imbalance compensation of all sub-areas is completed; Step 208: sort the photovoltaic stations in descending order according to the remaining capacity of the photovoltaic stations in each sub-region, and allocate the compensation power required to be output by each photovoltaic station in this order; Step 209: sort the SVGs in descending order according to the remaining capacity of the traction power supply system compensation device SVG in each sub-area, and allocate the compensation power required to be output by each SVG in this order.

5. The method according to claim 4, characterized in that , the step S30 specifically includes: Step 301: Calculate the compensation power that each converter needs to output according to the remaining capacity of each converter, as shown below: In the formula, S uni is the compensation power allocated to the i-th converter in each PV station, S rci is the remaining capacity allocated to the i-th converter in each PV station; Step 302: Use the basic theory of graph theory to represent the connection between each photovoltaic inverter in each photovoltaic station, that is, G = {V, E, A}, where V represents the node of the graph, E represents the edge of the graph, and A is the adjacency matrix of the graph, which represents the connection relationship between each node, as shown below: In the formula, μ ij It is the non-negative weighted adjacent element of matrix A, which is a 0-1 binary variable.

6. The method according to claim 5, characterized in that , the step S40 specifically includes: Step 401, calculate the negative sequence current of the PCC before compensation, taking phase A as an example, the calculation result is as follows: Step 402: Calculate the negative sequence current of the PCC after compensation. Taking phase A as an example, the calculation result is as follows: In the formula, U L and U A are the load port and A phase compensation port voltages, θ LT Represents the transformer connection angle at the load port, θ LL For U L and U A The angle between L is the load port current, m is the number of compensation ports, θ MT is the transformer connection angle of the compensation port, θ Pη is the power factor angle of compensation port η.

7. The method according to claim 6, characterized in that , the step S50 specifically includes: Step 501: Calculate the voltage imbalance at the PCC according to the following formula: In the formula, I - Indicates the negative sequence current value, U Line Indicates PCC line voltage, S d Indicates the short-circuit capacity of PCC; Step 502: Calculate the voltage imbalance before compensation according to the following formula: Step 503: Calculate the compensated voltage imbalance according to the following formula:

8. The method according to claim 7, characterized in that , the step S60 specifically includes: Step 601: Calculate the unbalanced power before compensation according to the following formula: Step 602: Calculate the compensated unbalanced power according to the following formula: Step 603: Calculate the voltage imbalance constraint parameter K according to the following formula:

9. The method according to claim 8, characterized in that , the step S70 specifically includes: Step 701: Determine the ratio of the compensation power output of each photovoltaic inverter to its remaining capacity, as shown below: Step 702: Incorporate the result of step 701 into the following iterative process: Step 703: Calculate the compensation power output ratio of each photovoltaic converter as follows: Step 704: Calculate the compensation power of each photovoltaic converter according to the following formula: Step 705: Calculate the compensation current of each photovoltaic converter according to the following formula:

10. The method according to claim 9, characterized in that , the improved multi-agent consensus algorithm specifically includes: Step 801: Change the element μ in the adjacency matrix A ij To achieve a higher convergence speed, the new assignment rules are as follows: Step 802: According to the assignment rule of step 801, construct an improved Laplace matrix as shown below: In the formula,

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