Urban rail transit medium-voltage looped network reactive power compensation system and control method thereof

Abstract

The present application relates to the field of urban rail transit power supply control, in particular to a medium-voltage looped network reactive power compensation system in urban rail transit and a control method thereof. The system comprises a medium-voltage looped network, which adopts an AC looped network structure and is connected to at least one main substation and a plurality of traction substations; a flexible DC rectifier, which is arranged in the traction substation, has an AC side connected to the medium-voltage looped network and a DC side connected to a DC traction network; a quadrature adjustable reactive power compensation unit, which is arranged at each node of the medium-voltage looped network and is composed of a quadrature controllable reactor and a fixed capacitor in parallel; a coordinated controller, which comprises a data acquisition module, a working condition identification module, a reactive power demand calculation module and an instruction distribution module; and a communication network, which is connected to the coordinated controller, each quadrature adjustable reactive power compensation unit and each flexible DC rectifier. The present application makes full use of the redundant capacity of the flexible DC system, realizes fine adjustment of reactive power and improves the reliability of the system. The present application is suitable for urban rail transit power supply control.

Description

Technical Field

[0001] This invention relates to the field of power supply control for urban rail transit, specifically to a medium-voltage ring network reactive power compensation system and its control method for urban rail transit. Background Technology

[0002] In special power grid supply scenarios such as rail transit, the power grid operates under complex conditions, with highly random and rapidly changing load fluctuations. Simultaneously, the power grid needs to possess the ability to transfer active power across lines and flexibly regulate power flow, placing extremely high demands on the adaptability, stability, and response speed of reactive power compensation devices. The reactive power compensation problem in urban rail transit systems mainly manifests in the following aspects:

[0003] Reactive power backfeeding is severe under light-load conditions at night. Urban rail transit exhibits significant diurnal load differences. During non-operating hours at night (typically from 00:00 to 04:00), medium-voltage ring network lines are essentially unloaded, and the load rate of each traction substation is extremely low. However, medium-voltage ring networks extensively utilize power cable lines, significantly increasing the charging reactive power generated by the ground capacitance of these cables. This leads to capacitive reactive power excess in the medium-voltage ring network, resulting in a low power factor and even reactive power backfeeding to the grid. Traditional reactive power compensation schemes primarily rely on centralized installation of Static Var Generators (SVG) in main substations for centralized compensation. This causes the SVG equipment to operate under heavy load for extended periods under light-load conditions at night, leading to decreased equipment reliability and increased operating losses.

[0004] Existing dynamic compensation equipment is costly and slow to respond. Currently, the mainstream dynamic reactive power compensation solutions include SVG (Static Var Compensator) and Thyristor-Controlled Reactor (TCR) type Static Var Compensators (SVCs). While SVGs offer four-quadrant operation and fast response, their high cost, especially in large-capacity applications, makes large-scale distributed deployment economically impractical. TCR-type SVCs rely on thyristor-controlled reactors for reactive power regulation. Their core problem lies in the significant harmonics generated during thyristor triggering, requiring additional filtering devices, increasing system complexity and investment costs.

[0005] The reactive power regulation capability of flexible DC converters is not fully utilized. In recent years, flexible DC traction power supply technology has been gradually promoted and applied in urban rail transit. Bidirectional converters (flexible DC rectifiers) have four-quadrant operation capability, allowing for flexible adjustment of active and reactive power. Urban rail flexible DC traction power supply systems are typically designed with a certain amount of redundant capacity. When the traction load level is low, this redundant capacity is idle and not fully utilized. Existing technologies include research on using flexible DC converters for grid reactive power compensation, but most of these studies involve independent operation or simple parallel operation with SVG (Static Var Generator), lacking organic coordination with low-cost basic reactive power compensation devices, making it difficult to achieve system-level overall optimization.

[0006] In summary, existing reactive power compensation schemes for medium-voltage ring networks in urban rail transit suffer from problems such as high investment in compensation equipment, low equipment utilization under light load conditions at night, ineffective utilization of redundant capacity of flexible DC converters, poor overall system economy, and insufficient power supply reliability. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a medium-voltage ring network reactive power compensation system and its control method for urban rail transit. This invention solves the problems of limited power converter capacity and inability to simultaneously address active power flow regulation and global reactive power compensation in traditional single SVG compensation schemes. Through a hierarchical collaborative compensation mechanism of orthogonal controllable reactors and flexible DC rectifiers, it simultaneously meets the requirements of long-term steady-state reactive power compensation and instantaneous dynamic reactive power regulation of the power grid, thereby improving the power quality management effect and economy of the rail transit power supply system.

[0008] The present invention achieves the above-mentioned objectives by adopting the following technical solution: Firstly, the present invention provides a reactive power compensation system for a medium-voltage ring network in urban rail transit, comprising:

[0009] The medium-voltage ring network adopts an AC ring network structure, connecting at least one main substation and multiple traction substations;

[0010] The flexible DC rectifier is installed in the traction substation. Its AC side is connected to the medium-voltage ring network and its DC side is connected to the DC traction network. The flexible DC rectifier is a voltage source type bidirectional converter with four-quadrant operation capability.

[0011] An orthogonal adjustable reactive power compensation unit is installed at each node of the medium-voltage ring network and consists of an orthogonal controllable reactor and a fixed capacitor connected in parallel.

[0012] The coordination controller includes a data acquisition module, a working condition identification module, a reactive power demand calculation module, and a command distribution module;

[0013] The communication network connects the coordination controller, each orthogonal adjustable reactive power compensation unit, and each flexible DC rectifier.

[0014] Furthermore, the coordination controller is configured to: identify the current operating condition of the system, calculate the reactive power demand of each node, control the orthogonal adjustable reactive power compensation unit to undertake basic reactive power compensation, control the flexible DC rectifier to use the surplus to supplement the remaining reactive power gap, and schedule the flexible DC rectifiers of other nodes to provide cross-node reactive power support when the node capacity is insufficient.

[0015] Furthermore, the orthogonal controllable reactor operates based on the principle of orthogonal magnetization and includes a working winding and a control winding. The working winding is connected to a medium-voltage ring network, and the control winding receives external control signals to adjust the reactance value. By adjusting the excitation current of the control winding, the inductance value of the orthogonal controllable reactor can be continuously adjusted between the maximum and minimum values.

[0016] Furthermore, the core of the orthogonal controllable reactor is composed of an AC core and a DC core. The AC core has a U-shaped structure with air gaps on its left and right core columns. The DC core consists of two C-shaped cores, which are respectively arranged on both sides of the AC core. The DC magnetic circuit direction is perpendicular to the AC magnetic circuit direction. AC coils are wound on the left and right sides of the AC core and connected in series to form the working winding. DC coils are wound on the left and right sides of the DC core and connected in series to form the excitation winding.

[0017] Furthermore, the fixed capacitor is a capacitor bank with group switching, and the communication network adopts fiber optic communication or industrial Ethernet.

[0018] Secondly, the present invention provides a control method based on the above system, comprising the following steps:

[0019] S1. The coordination controller acquires electrical quantity data of each node of the medium-voltage ring network and the operating status and available capacity of the flexible DC rectifier in real time, and identifies the current operating condition based on system time and load data;

[0020] S2. Based on the current operating conditions and electrical quantity data of each node, calculate the reactive power demand of each node in the medium-voltage ring network;

[0021] S3. Based on the reactive power demand of each node, generate compensation instructions for each orthogonal adjustable reactive power compensation unit, so that it can undertake basic reactive power compensation.

[0022] S4. Determine the remaining reactive power gap of each node after basic reactive power compensation, and generate dynamic compensation instructions for the flexible DC rectifier based on the available capacity of the flexible DC rectifier.

[0023] S5. When the available capacity of the flexible DC rectifier of a certain node is insufficient to compensate for the remaining reactive power deficit of its node, the coordination controller schedules the redundant capacity of the flexible DC rectifiers of other nodes to provide cross-node reactive power support.

[0024] Furthermore, the operating conditions described in step S1 include: nighttime light load operating conditions, daytime normal operation operating conditions, and peak load operating conditions;

[0025] Among them, when the system is in the non-operation period at night and the load rate of each traction substation is lower than the preset threshold, it is judged as a light-load condition at night.

[0026] Under light load conditions at night, all or part of the fixed capacitors are disconnected, and the quadrature controllable reactor is adjusted to a reactance value greater than the preset threshold to absorb capacitive reactive power. The remaining capacitive reactive power is then absorbed by the flexible DC rectifier operating in inverter mode.

[0027] Furthermore, in step S3, the objective of the basic reactive power compensation unit is to compensate the power factor of each node to a first preset value, or to compensate the reactive power of each node to a first preset range.

[0028] Furthermore, in step S4, the maximum reactive power compensation of the flexible DC rectifier is determined based on its available capacity and the currently transmitted active power; if the remaining reactive power deficit does not exceed the maximum reactive power compensation, the flexible DC rectifier outputs reactive power according to the remaining reactive power deficit; if the remaining reactive power deficit exceeds the maximum reactive power compensation, the flexible DC rectifier outputs reactive power according to the maximum reactive power compensation, and the remaining deficit is compensated across nodes by the flexible DC rectifiers of other nodes.

[0029] Furthermore, the allocation principle for cross-node reactive power support in step S5 is: to prioritize scheduling the flexible DC rectifier that is closest to the node where the remaining reactive power gap is located and / or has the largest available capacity.

[0030] Compared with the prior art, the present invention has the following significant innovative advantages and beneficial effects:

[0031] Significantly reducing equipment investment costs. This invention employs orthogonal controllable reactors and fixed capacitors to form a basic compensation unit, which is distributed and installed in various traction substations to undertake most of the basic reactive power compensation needs. The orthogonal controllable reactor operates based on the principle of orthogonal magnetization, requiring only low-power DC control to achieve a wide range of reactance value adjustment. It has a simple structure, low manufacturing cost, and low operating losses. At the same time, this invention fully utilizes the redundant capacity of existing flexible DC rectifiers for dynamic fine-tuning, eliminating the need for additional large-capacity SVG, thus significantly reducing the overall equipment investment and operation and maintenance costs of the system.

[0032] This invention achieves precise, layered, and coordinated control of reactive power compensation. It utilizes a multi-layered architecture to precisely allocate reactive power compensation tasks: the first layer provides fixed capacitive base compensation for fixed capacitors; the second layer uses orthogonal controllable reactors to continuously adjust inductive reactive power, compensating for remaining base demand; and the third layer uses flexible DC rectifiers to quickly and accurately compensate for remaining deficits. Each layer performs its specific function and works in concert, ensuring both continuous and smooth compensation (stepless adjustment by the orthogonal controllable reactors) and rapid and accurate compensation (millisecond-level response by the flexible DC rectifiers), achieving a technological leap from coarse compensation to fine adjustment.

[0033] This invention significantly improves the operational reliability of the power supply system. In existing centralized compensation schemes, the overall compensation capacity drops drastically once the SVG (Static Var Generator) fails. This invention adopts a distributed architecture, with each traction substation equipped with a basic compensation unit and a flexible DC rectifier. When a node device fails or its compensation capacity is insufficient, the flexible DC rectifiers of adjacent nodes can provide cross-node reactive power support through the medium-voltage ring network. This design significantly improves the operational reliability and fault tolerance of the power supply system.

[0034] This invention fully utilizes the redundant capacity of flexible DC rectifiers. Urban rail transit flexible DC traction power supply systems are typically designed with a certain amount of redundant capacity to cope with peak loads, which remains idle during off-peak hours. By coordinating the unified scheduling of the controller, this invention fully utilizes the redundant capacity of flexible DC rectifiers to participate in reactive power compensation across the entire network without affecting the traction power supply function, thereby improving equipment utilization and overall system efficiency.

[0035] This invention effectively improves power quality under light-load conditions at night. Addressing the problem of excess capacitive reactive power in cables during light-load nighttime operation of urban rail transit, this invention proposes a specific control strategy: disconnecting fixed capacitor banks, adjusting orthogonal controllable reactors to their maximum reactance value to absorb capacitive reactive power, and activating the flexible DC rectifier in inverter mode to absorb the remaining capacitive reactive power. This effectively avoids reactive power backfeeding and ensures that the power factor of the medium-voltage ring network meets grid requirements under all operating conditions. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the overall architecture of the medium-voltage ring network reactive power compensation system for urban rail transit according to the present invention;

[0037] Figure 2 This is a circuit topology diagram of the orthogonal adjustable reactive power compensation unit of the present invention;

[0038] Figure 3 This is a block diagram showing the modular composition of the coordination controller of the present invention;

[0039] Figure 4 This is a schematic diagram of the orthogonal core structure of the orthogonal controllable reactor of the present invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0041] Example 1: System Architecture

[0042] like Figure 1 As shown, the reactive power compensation system for the medium-voltage ring network of urban rail transit in this embodiment includes:

[0043] A 110kV main substation, after being stepped down by two main transformers, forms a 35kV double-busbar segmented structure, supplying power to a 35kV medium-voltage ring network. The medium-voltage ring network adopts a double-ring network connection method, with several traction substations and step-down substations set up along the line, and each traction substation connected by a ring network cable.

[0044] Each traction substation is equipped with:

[0045] (1) A flexible DC rectifier with a rated capacity of 8MVA is connected to the 35kV bus via an isolation transformer on the AC side and outputs DC 750V on the DC side to supply power to the DC traction network and provide traction power to the contact network or contact rail. This flexible DC rectifier is a voltage source type two-level or three-level PWM converter, using IGBT power devices, and has four-quadrant operation capability. It can flexibly switch between rectification mode and inverter mode, and also has independent active and reactive power regulation functions. Under traction conditions, the flexible DC rectifier converts AC power into DC power to supply the train; under regenerative braking conditions, it can invert the DC side regenerative energy into AC power to feed back to the medium-voltage ring network, realizing bidirectional energy flow.

[0046] (2) A set of orthogonal adjustable reactive power compensation units consists of an orthogonal controllable reactor (rated capacity 2Mvar) and two sets of fixed capacitors (each set 1Mvar, which can be switched in groups) connected in parallel, and connected to the 35kV bus via a circuit breaker. The orthogonal controllable reactor operates based on the orthogonal magnetization principle, and its inductance value can be continuously adjusted within the range of 50% to 100% of the rated value, with a response time of no more than 100ms. The two sets of fixed capacitors can be switched separately, and the combination provides 0Mvar, 1Mvar or 2Mvar of fixed capacitive reactive power.

[0047] The orthogonal adjustable reactive power compensation units and flexible DC rectifiers of each traction substation are connected to the coordination controller located in the main substation via a fiber optic communication network. The fiber optic communication network adopts a dual-ring network redundancy structure to ensure high reliability and real-time performance, with a communication rate of no less than 100Mbps and a data transmission delay of no more than 10ms. The coordination controller is an intelligent control device based on an industrial-grade embedded platform (such as ARM or PowerPC architecture), with built-in data acquisition, operating condition identification, reactive power demand calculation, and command allocation modules, and has an interface for communication with the upper-level energy management system (EMS / PSCADA).

[0048] like Figure 2As shown, in the circuit topology of the quadrature adjustable reactive power compensation unit, the quadrature controllable reactor and the fixed capacitor bank are connected in parallel and then connected to the medium-voltage bus. The working winding of the quadrature controllable reactor is connected in series in the circuit branch, and the control winding receives the DC control current signal from the coordinating controller. Each branch of the fixed capacitor bank is equipped with a switching switch (circuit breaker or contactor), which can connect or disconnect the corresponding capacitor bank according to the command.

[0049] like Figure 3 As shown, the coordinated controller includes a data acquisition module, a working condition identification module, a reactive power demand calculation module, and a command distribution module. The data acquisition module collects electrical quantities such as voltage, current, and power at each node, as well as the converter's operating status, via a communication network. The working condition identification module identifies the current working condition category based on the collected data and time information. The reactive power demand calculation module calculates the reactive power demand of each node based on the working condition and electrical quantities. The command distribution module generates and issues compensation commands to each execution unit according to a preset strategy. The modules interact with each other via an internal data bus, forming a closed-loop control system.

[0050] Example 2: Control Strategy under Light Load Conditions at Night

[0051] When the system enters the non-operational period at night (00:00-04:00), the load rate of each traction substation is less than 2%, and the operating condition identification module of the coordination controller determines that the system is in a light-load condition at night.

[0052] Under this condition, the train is stopped and the traction load is close to zero, but the charging reactive power of the medium-voltage ring network cable line increases significantly, and the system exhibits characteristics of excess capacitive reactive power. The data acquisition module collects voltage data (which may be higher due to capacitive effects), reactive power data (negative values), and power factor data (lagging or leading) for each node. The reactive power demand calculation module calculates negative reactive power demand for each node (i.e., the system needs to absorb inductive reactive power to offset the excess capacitive reactive power).

[0053] The instruction allocation module allocates compensation tasks according to the following strategy:

[0054] (1) Send a tripping command to the capacitor switching switches of each node through the communication network to disconnect all the fixed capacitors in the quadrature adjustable reactive power compensation unit, so as to avoid the fixed capacitors providing additional capacitive reactive power and aggravating the problem of excess capacitive reactive power. At the same time, send a DC control signal to the control winding of the quadrature controllable reactor to adjust its excitation current to the maximum value, so that the quadrature region of the core of the quadrature controllable reactor is deeply saturated, the inductance value is reduced to the minimum value, and the inductive reactive power output reaches the maximum, thereby absorbing the capacitive reactive power in the medium voltage ring network to the maximum extent.

[0055] (2) The data acquisition module continuously monitors the reactive power and power factor of each node. If, after all fixed capacitors are disconnected and the quadrature controllable reactor is adjusted to its maximum inductive output, there is still excess capacitive reactive power that has not been fully absorbed (power factor is still below 0.95), the coordinating controller sends an inverter mode control command to the flexible DC rectifier, causing it to operate in inverter mode, absorb the remaining capacitive reactive power, and simultaneously feed the absorbed energy back to the medium-voltage ring network. The flexible DC rectifier achieves rapid and precise adjustment of reactive power by adjusting the reactive component of its AC side current, with a response time in the millisecond range.

[0056] Through the above-mentioned hierarchical and coordinated compensation strategy, the power factor of each node of the medium-voltage ring network is maintained above 0.95 (leading or lagging) under light load conditions at night, effectively avoiding reactive power backfeeding to the grid and meeting the grid access requirements.

[0057] Example 3: Control Strategy under Normal Daytime Operating Conditions

[0058] Under normal daytime operating conditions (during operation, the load rate of each traction substation is between 5% and 80%), the traction load and the station power and lighting load generate inductive reactive power demand, while the medium-voltage ring network cable lines simultaneously generate capacitive charging reactive power. Under certain conditions, the two cancel each other out. However, due to the uneven load status and line distribution of each traction substation, the reactive power distribution at each node varies greatly, requiring fine-tuning.

[0059] The coordination controller aims to achieve a power factor of 0.95 for each node and allocates compensation tasks according to the following priority:

[0060] First priority: Adjusting the quadrature adjustable reactive power compensation unit. The instruction allocation module calculates the required excitation current instruction for the quadrature controllable reactor and the fixed capacitor switching instruction based on the reactive power demand of each node. First, based on the magnitude and direction of the node's reactive power demand, the number of fixed capacitor banks to be engaged is determined, ensuring that the capacitive reactive power provided by the fixed capacitors closely approximates, but does not completely compensate, the node's reactive power demand. Then, by continuously adjusting the excitation current of the quadrature controllable reactor, its inductive reactive power output is precisely adjusted to compensate for the difference between the fixed capacitive compensation and the target compensation value, achieving fine adjustment of the basic reactive power compensation. Because the quadrature controllable reactor has no thyristor switching devices, its output waveform has good sinusoidal characteristic and low harmonic content.

[0061] Second priority: When the adjustment range of the orthogonal adjustable reactive power compensation unit is insufficient to meet the compensation demand (e.g., the reactive power demand of a node exceeds the rated capacity of the compensation unit), the command allocation module activates the reactive power compensation function of the flexible DC rectifier. The coordination controller reads the current active power and available capacity of the flexible DC rectifier in the traction substation where the node is located, and calculates the maximum reactive power compensation it can provide. If the remaining reactive power gap is within this range, a corresponding reactive current command is generated and sent to the control system of the flexible DC rectifier, causing it to output the corresponding reactive power to accurately fill the remaining gap.

[0062] Third Priority: When the spare capacity of a flexible DC rectifier at a certain node is insufficient, the cross-node reactive power mutual assistance function is activated. The coordination controller reads the operating status and available capacity of the flexible DC rectifiers of other traction substations on the medium-voltage ring network, and selects the optimal support node according to the allocation principle of "nearest distance and largest available capacity". After determining the support node, the coordination controller issues instructions to the supported node and the flexible DC rectifier of the support node respectively: the flexible DC rectifier of the supported node outputs reactive power at its maximum capacity, and the flexible DC rectifier of the support node outputs reactive power at its available capacity. The sum of the two is exactly equal to the total residual shortfall of the supported node, and the compensation current flows from the support node to the supported node through the medium-voltage ring network line.

[0063] Example 4: Cross-node reactive power support

[0064] Assuming there are 5 traction substations (substation 1 to substation 5) on a medium-voltage ring network of a certain urban rail transit system, under normal daytime operating conditions, the reactive power demand and compensation of each node calculated by the coordinating controller are shown in Table 1 below:

[0065] Table 1 Reactive power demand and compensation at each node

[0066]

[0067] Institute 1 and Institute 3 have remaining gaps (0.5 Mvar gap for Institute 1 and 1.0 Mvar gap for Institute 3), and the flexible DC rectifiers of Institute 4 and Institute 5 have redundant capacity.

[0068] The coordination controller performs scheduling according to a preset cross-node support strategy:

[0069] For the 0.5 Mvar shortfall in Institute 1, the coordinating controller selects Institute 2 (which already meets its own needs and has some redundancy) and Institute 4, which are closest to Institute 1, for support. Calculations show that Institute 2 has 0.2 Mvar of remaining available capacity, and Institute 4 has 0.8 Mvar. The controller prioritizes scheduling Institute 2 to provide 0.2 Mvar, and then schedules Institute 4 to provide 0.3 Mvar, for a total of 0.5 Mvar, thus meeting the shortfall in Institute 1.

[0070] For the 1.0 Mvar shortfall in Institute 3, the coordination controller selects the nearest Institutes 4 and 5 to provide support. After supporting Institute 1, Institute 4 has 0.5 Mvar of remaining available capacity, and Institute 5 has 0.5 Mvar of available capacity, totaling 1.0 Mvar, which exactly meets the shortfall in Institute 3.

[0071] After the support command was issued, each flexible DC rectifier adjusted its reactive power output according to the command, and the reactive power gap of all nodes in the entire system was completely made up. The power factor of the entire network reached the preset target (above 0.95), and the reactive power was not transmitted in a long-distance detour in the ring network, resulting in low network loss.

[0072] Example 5: Cooperative Control under Peak Load Conditions

[0073] When the system is in operation and the load rate of a traction substation is higher than 80%, it is considered a peak load condition. Under this condition, the flexible DC rectifier of the traction substation undertakes a large active power transmission task, and its available reactive power capacity is reduced accordingly, even approaching zero.

[0074] At this point, the four-quadrant operation capability of the flexible DC rectifier is limited by active power transmission, and the reactive power compensation it can provide decreases significantly. The coordination controller, based on the formula... , This represents the available capacity of the i-th flexible DC rectifier. This represents the active power currently transmitted by the i-th flexible DC rectifier.

[0075] The available reactive power capacity of each flexible DC rectifier is calculated in real time. Under peak load conditions, the reactive power compensation task is mainly undertaken by the orthogonal adjustable reactive power compensation unit.

[0076] Due to the high inductive reactive power demand under peak load, the coordinating controller first activates all fixed capacitor banks and adjusts the excitation current of the quadrature controllable reactor to a lower level (i.e., a higher reactance value) to absorb less inductive reactive power, thereby outputting as much net capacitive reactive power as possible. If there is still a residual inductive reactive power deficit after basic compensation, and the flexible DC rectifier has a small amount of redundant capacity available for adjustment, then the flexible DC rectifier will provide supplementary compensation. If the deficit still cannot be completely compensated, the coordinating controller can appropriately relax the power factor target for this node (e.g., from 0.95 to 0.92) to ensure that the active power transmission capacity of the flexible DC rectifier is prioritized and to avoid affecting the train traction power supply.

[0077] Example 6: Core Structure of an Orthogonal Controllable Reactor

[0078] like Figure 4As shown, the orthogonal controllable reactor of the present invention adopts an orthogonal core structure. The core consists of two parts: an AC core and a DC core, and its overall structure is symmetrically distributed in the vertical, horizontal, and front-back directions.

[0079] The AC core has a "U"-shaped structure, made of stacked silicon steel sheets. Two air gaps are located in the middle of the core columns on both sides, filled with insulating support material to prevent AC flux saturation and improve the linearity of the reactor. An AC coil is wound on each side of the AC core. To ensure that the magnetic flux generated by the two coils is in the same direction, their corresponding terminals are connected end-to-end in series to form the AC working winding. The two ends of the working winding are led out and connected in series to the medium-voltage circuit.

[0080] The DC core consists of two C-shaped cores, positioned on either side of the AC core. The magnetic circuit direction of the DC core is perpendicular (orthogonal) to that of the AC core, which is crucial for achieving orthogonal magnetization control. A set of DC coils is wound on each side of the DC core, connected end-to-end through corresponding terminals to form a series structure, constituting the DC excitation winding. The two ends of the DC excitation winding are led out and connected to a DC control power supply, receiving DC current control signals from the coordinating controller.

[0081] When a DC control current is applied to the DC excitation winding, the DC core generates a DC bias flux, which passes through the orthogonal regions in the core columns on both sides of the AC core. Since the direction of the DC flux is perpendicular to the direction of the AC flux, the DC bias flux changes the magnetic operating point of the core in the orthogonal region, causing the core in this region to enter different degrees of saturation. The higher the degree of saturation, the smaller the permeability μ_a2 of the orthogonal region, and the lower the inductance L of the AC working winding. By continuously adjusting the magnitude of the DC excitation current, the degree of saturation of the core in the orthogonal region can be continuously changed, thereby achieving smooth and stepless adjustment of the AC reactance.

[0082] When no DC current is applied, the permeability of the orthogonal region is the same as that of other regions of the AC core, and the inductance is at its maximum. When a DC current is applied, the saturation degree of the orthogonal region increases with the increase of the current, the permeability decreases, and the inductance decreases. When the magnetic induction intensity of the core reaches the saturation point, the permeability reaches its minimum value, and the inductance reaches its minimum value. The above adjustment process does not involve thyristor switching, does not generate harmonics, and has the advantages of smooth response and high waveform quality.

[0083] Example 7: Complete flow of the control method

[0084] The complete flow of the control method of the present invention will be comprehensively described below in conjunction with the above embodiments:

[0085] Step S1: Data Acquisition and Operating Condition Identification

[0086] The coordinating controller collects real-time electrical quantities such as three-phase voltage, three-phase current, active power, reactive power, and power factor from each medium-voltage ring network node via a fiber optic communication network, as well as the operating status (operating / shutdown), current active power, available capacity, and fault information of each flexible DC rectifier. Simultaneously, it reads the system clock to obtain the current time (to determine if it is during non-operating nighttime hours) and, combined with the load data from each node, comprehensively judges the current system operating condition (light load at night, normal daytime operation, or peak load). The data sampling period is no more than 100ms to ensure the real-time and accurate identification of operating conditions.

[0087] Step S2, Reactive power demand calculation:

[0088] The reactive power demand calculation module calculates the measured reactive power value of each node based on the electrical quantity data obtained in step S1, and compares it with the reactive power target value corresponding to the target power factor to calculate the reactive power demand of each node. The calculation formula is: ,in This represents the reactive power demand of the load carried by node i. This represents the reactive power loss of the medium-voltage ring network line at node i. This represents the reactive power generated by the distributed power source connected to node i. For the reactive power loss of a medium-voltage ring network line... It is calculated based on the line current and line parameters (resistance, reactance). The total reactive power demand of the system is also calculated. .

[0089] Step S3: Generate basic compensation instructions:

[0090] The instruction allocation module allocates instructions based on the nodes'... The value is used to generate basic compensation instructions. For each node, first, based on... The magnitude and direction (positive / negative) of the fixed capacitors determine the number of capacitor banks to be connected. If the demand is positive (inductive), then an appropriate number of fixed capacitor banks should be installed so that the capacitive reactive power does not exceed [the required amount]. ;like If the value is negative (capacitive excess), then all or part of the fixed capacitor bank is disconnected. Next, based on the remaining uncompensated reactive power demand, the required inductive reactive power output of the quadrature controllable reactor is calculated and converted into the corresponding excitation current command value. The excitation current command value is then sent to the control winding of the quadrature controllable reactor at the corresponding node via the communication network.

[0091] Step S4: Dynamic compensation instruction generation:

[0092] After step S3 is completed, recalculate the remaining reactive power deficit for each node. (Actual demand minus basic compensation). Read the available capacity of the flexible DC rectifier at each node. and current active power Calculate the maximum reactive power compensation it can provide. ,Compare and .like ≤ Then, an instruction is issued to make the flexible DC rectifier output according to its reactive current output capacity. reactive power; if > Then, a command is issued to make the flexible DC rectifier output... And record the remaining uncompensated gap. The calculation formula is as follows: .

[0093] Step S5, Cross-node reactive power support:

[0094] If it exists For nodes with a reactive power deficit > 0, a cross-node reactive power support procedure is initiated. The coordinating controller scans the flexible DC rectifiers of all nodes in the network and includes nodes with available capacity after fulfilling their own compensation tasks in the support candidate list. Based on the allocation principle of "closest distance, largest available capacity," a support node is matched sequentially for each node with a deficit, generating a cross-node reactive power support instruction. The support instruction includes the support node number and the target value of the output reactive power. The flexible DC rectifier of the supported node outputs at its maximum capacity, and the flexible DC rectifier of the supporting node outputs according to its allocated support amount. All instructions are synchronously issued through the communication network to ensure coordinated action of all execution units.

[0095] Through the complete closed-loop control process described above, this invention achieves fine-tuning of reactive power in the medium-voltage ring network of urban rail transit under all operating conditions and with full coverage.

[0096] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A reactive power compensation system for a medium-voltage ring network in urban rail transit, characterized in that, include: The medium-voltage ring network adopts an AC ring network structure, connecting at least one main substation and multiple traction substations; The flexible DC rectifier is installed in the traction substation. Its AC side is connected to the medium-voltage ring network and its DC side is connected to the DC traction network. The flexible DC rectifier is a voltage source type bidirectional converter with four-quadrant operation capability. An orthogonal adjustable reactive power compensation unit is installed at each node of the medium-voltage ring network and consists of an orthogonal controllable reactor and a fixed capacitor connected in parallel. The coordination controller includes a data acquisition module, a working condition identification module, a reactive power demand calculation module, and a command distribution module; The communication network connects the coordination controller, each orthogonal adjustable reactive power compensation unit, and each flexible DC rectifier.

2. The reactive power compensation system for medium-voltage ring network in urban rail transit according to claim 1, characterized in that, The coordination controller is configured to: identify the current operating condition of the system, calculate the reactive power demand of each node, control the orthogonal adjustable reactive power compensation unit to undertake basic reactive power compensation, control the flexible DC rectifier to use the surplus to supplement the remaining reactive power gap, and schedule the flexible DC rectifiers of other nodes to provide cross-node reactive power support when the node capacity is insufficient.

3. The reactive power compensation system for medium-voltage ring network in urban rail transit according to claim 1, characterized in that, The orthogonal controllable reactor operates based on the principle of orthogonal magnetization and includes a working winding and a control winding. The working winding is connected to a medium-voltage ring network, and the control winding receives external control signals to adjust the reactance value. By adjusting the excitation current of the control winding, the inductance value of the orthogonal controllable reactor can be continuously adjusted between the maximum and minimum values.

4. The reactive power compensation system for medium-voltage ring network in urban rail transit according to claim 1, characterized in that, The core of the orthogonal controllable reactor consists of an AC core and a DC core. The AC core has a U-shaped structure with air gaps on its left and right core columns. The DC core consists of two C-shaped cores, which are arranged on both sides of the AC core. The DC magnetic circuit direction is perpendicular to the AC magnetic circuit direction. AC coils are wound on the left and right sides of the AC core and connected in series to form the working winding. DC coils are wound on the left and right sides of the DC core and connected in series to form the excitation winding.

5. The reactive power compensation system for medium-voltage ring network in urban rail transit according to claim 1, characterized in that, The fixed capacitors are capacitor banks that are switched in groups, and the communication network adopts fiber optic communication or industrial Ethernet.

6. A control method for a medium-voltage ring network reactive power compensation system for urban rail transit based on any one of claims 1 to 5, characterized in that, Includes the following steps: S1. The coordination controller acquires electrical quantity data of each node of the medium-voltage ring network and the operating status and available capacity of the flexible DC rectifier in real time, and identifies the current operating condition based on system time and load data; S2. Based on the current operating conditions and electrical quantity data of each node, calculate the reactive power demand of each node in the medium-voltage ring network; S3. Based on the reactive power demand of each node, generate compensation instructions for each orthogonal adjustable reactive power compensation unit, so that it can undertake basic reactive power compensation. S4. Determine the remaining reactive power gap of each node after basic reactive power compensation, and generate dynamic compensation instructions for the flexible DC rectifier based on the available capacity of the flexible DC rectifier. S5. When the available capacity of the flexible DC rectifier of a certain node is insufficient to compensate for the remaining reactive power deficit of its node, the coordination controller schedules the redundant capacity of the flexible DC rectifiers of other nodes to provide cross-node reactive power support.

7. The control method according to claim 6, characterized in that, The operating conditions described in step S1 include: nighttime light load operating conditions, daytime normal operation operating conditions, and peak load operating conditions; Among them, when the system is in the non-operation period at night and the load rate of each traction substation is lower than the preset threshold, it is judged as a light-load condition at night. Under light load conditions at night, all or part of the fixed capacitors are disconnected, and the quadrature controllable reactor is adjusted to a reactance value greater than the preset threshold to absorb capacitive reactive power. The remaining capacitive reactive power is then absorbed by the flexible DC rectifier operating in inverter mode.

8. The control method according to claim 6, characterized in that, In step S3, the objective of the basic reactive power compensation unit is to compensate the power factor of each node to a first preset value, or to compensate the reactive power of each node to a first preset range.

9. The control method according to claim 6, characterized in that, In step S4, the maximum reactive power compensation of the flexible DC rectifier is determined based on its available capacity and the current active power transmitted. If the remaining reactive power deficit does not exceed the maximum reactive power compensation, the flexible DC rectifier outputs reactive power according to the remaining reactive power deficit. If the remaining reactive power deficit exceeds the maximum reactive power compensation, the flexible DC rectifier outputs reactive power according to the maximum reactive power compensation, and the remaining deficit is compensated across nodes by the flexible DC rectifiers of other nodes.

10. The control method according to claim 6, characterized in that, The allocation principle for cross-node reactive power support in step S5 is: prioritize scheduling the flexible DC rectifier that is closest to the node with the remaining reactive power deficit and / or has the largest available capacity.

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