Long-distance power supply method based on low-voltage flexible DC technology

Abstract

The present invention relates to a long-distance power supply method based on low-voltage flexible DC technology. A flexible DC system is installed in parallel on the power supply AC line. The flexible DC system includes a rectifier host with an AC / DC module, an inverter slave with a DC / AC module, and a DC transmission line. The rectifier host and the inverter slave are respectively installed at the front end and the end of the AC line, and the rectifier host is connected to the inverter slave through the DC transmission line. This method is easy to construct, has a low implementation cost and a short cycle, and can quickly and effectively solve the low-voltage problem of long-distance power supply.

Description

Technical Field

[0001] The present invention belongs to the technical field of power supply, and particularly relates to a long-distance power supply method based on low-voltage flexible DC technology. Background Art

[0002] The problem of low voltage in distribution areas widely exists in the current power distribution system, showing characteristics such as universality, periodicity, load followability, and coexistence of high and low voltages.

[0003] Currently, the following main measures are taken to address the low voltage problem:

[0004] 1. Expansion and transformation of distribution areas

[0005] Adopt the method of using small-capacity and multiple distribution points to reduce the power supply radius. To a certain extent, this method "radically cures" the low voltage problem, but there are problems such as large renovation investment, long construction period, seasonal load reducing the utilization rate of transformer assets, large no-load loss, and increased line loss.

[0006] 2. Reactive power and three-phase unbalance compensation

[0007] This scheme can be adopted for some distribution areas with three-phase unbalance and excessive reactive power. However, the voltage boosting ability of reactive power compensation and three-phase unbalance control methods is limited, and the application scenarios are few, and it cannot be applied to most distribution areas.

[0008] 3. Use of series voltage regulators

[0009] This scheme is applied to scenarios where there are few end users, generally less than 10 households, and the wire diameter is above 50mm 2 In the above scenarios. If the end power is large or the wire diameter is too thin, this "pumping" energy supplement method will affect the voltage at the front end of the equipment, and the voltage boosting ability is generally only 20 - 30V, and the application scenarios are also limited. Summary of the Invention

[0010] The purpose of the present invention is to provide a long-distance power supply method based on low-voltage flexible DC technology, which is easy to construct, has low implementation cost, short cycle, and can quickly and effectively solve the low voltage problem of long-distance power supply.

[0011] To achieve the above object, the technical solution adopted by the present invention is: a long-distance power supply method based on low-voltage flexible DC technology, in which a flexible DC system is installed in parallel on the power supply AC line. The flexible DC system includes a rectifier host with an AC / DC module, an inverter slave with a DC / AC module, and a DC transmission line. The rectifier host and the inverter slave are respectively installed at the front end and the end of the AC line, and the rectifier host is connected to the inverter slave through the DC transmission line.

[0012] Further, the rectifier main unit rectifies and boosts 380V alternating current into ±375V to ±400V direct current and outputs it. The DC transmission line conducts ±375V to ±400V DC power supply, and the inverter slave unit inversely outputs 380V or 220V alternating current and connects it to the user side. When the flexible DC system operates, the transmission power on the AC line is shunted to the flexible DC system. The rectifier main unit dynamically adjusts the DC bus voltage according to the end load to meet the long-distance power supply requirements.

[0013] Further, the flexible DC system has an energy-saving operation mode, and its implementation method is as follows:

[0014] When the inverter slave unit detects that the user voltage is higher than the set target voltage threshold, the system automatically enters the energy-saving operation mode, and the inverter slave unit stops working. The non-operation of the inverter slave unit causes the DC bus voltage and current to decrease. When the rectifier main unit detects that the DC bus current is lower than the set value, the rectifier main unit also stops working.

[0015] When the inverter slave unit detects that the user voltage is not higher than the set target voltage threshold, the system automatically exits the energy-saving operation mode, the inverter slave unit starts to resume work, and raises the DC bus voltage to the set value for a certain period of time. When the rectifier main unit detects that the DC bus voltage reaches the set value, the rectifier main unit starts to work normally and raises the user voltage.

[0016] Further, the flexible DC system has a reactive power compensation function, and its implementation method is as follows:

[0017] When the voltage of the end user is normal and there is no need to generate active current for compensation, the inverter slave unit performs reactive power local compensation. The inverter slave unit externally connects a current transformer CT to continuously monitor the system current, and processes and analyzes the system current information through an internal controller to judge the reactive power load state of the system, calculates the current generated by the system reactive power load, and then sends a control signal to the internal IGBT of the inverter slave unit and drives it to act for current compensation, so as to achieve the purpose of reactive power compensation.

[0018] Further, the flexible DC system has a leakage current detection function, and its implementation method is as follows:

[0019] The rectifier main unit is internally provided with a leakage current detection device and a 4G module. The leakage current detection device detects the total current of the AC line connected to the front end of the flexible DC system. When the vector sum of the currents of the A, B, C, and N phases is not 0, it is judged that there is a leakage current situation. If the leakage current magnitude exceeds the set threshold, the leakage current detection device sends information to the maintenance personnel through the internal 4G module.

[0020] Further, a concentrator is installed at the back end of the flexible DC system to read the voltage data of the user's electricity meter. The concentrator communicates with the slave inverter in real time through a 485 communication line. A 4G module is provided on the slave inverter. If the user voltage is lower than the set threshold, that is, there is a low voltage situation, the slave inverter sends an alarm message to the set user through the 4G module.

[0021] Further, the rectifier host and the slave inverter adopt a fanless structure. Heat dissipation aluminum sheets are arranged on the device shell and the internal array, and no ventilation holes are arranged on the device shell to improve the sealing performance and dust and waterproof performance of the device.

[0022] Compared with the prior art, the present invention has the following beneficial effects: A long-distance power supply method based on low-voltage flexible DC technology is provided. By installing a flexible DC system in parallel on the original power supply AC line, without affecting the reliability of the line itself, the line loss is effectively reduced, thereby solving the low voltage problem in the long-distance power supply area. In addition, this method is convenient for construction, easy to transform and implement on the existing power supply system, has a low implementation cost, a short construction period, and has strong practicability and broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic structural diagram of the flexible DC system in the embodiment of the present invention.

[0024] Figure 2 It is a schematic diagram of the energy-saving working mode of the flexible DC system in the embodiment of the present invention.

[0025] Figure 3 It is a schematic diagram of the reactive power compensation function of the flexible DC system in the embodiment of the present invention.

[0026] Figure 4 It is a schematic diagram of the leakage current detection function of the flexible DC system in the embodiment of the present invention.

[0027] Figure 5 It is a schematic diagram of installing a concentrator in the flexible DC system in the embodiment of the present invention.

[0028] Figure 6 It is a schematic diagram of the external structure of the rectifier host in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0031] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0032] As Figure 1 shown, this embodiment provides a long-distance power supply method based on low-voltage flexible DC technology. A flexible DC system is installed in parallel on the power supply AC line. The flexible DC system includes a rectifier host with an AC / DC module, an inverter slave with a DC / AC module, and a DC transmission line. The rectifier host and the inverter slave are respectively installed at the front end and the end of the AC line, and the rectifier host is connected to the inverter slave through the DC transmission line. Since the flexible DC system is connected to the grid in parallel, it does not affect the power supply reliability.

[0033] When the flexible DC system operates, the transmission power on the AC line is shunted to the flexible DC system. Under the same power, voltage and current are inversely proportional, while voltage drop and line loss are proportional to current. Therefore, the flexible DC system can reduce voltage drop and line loss, thereby achieving the purpose of low voltage governance. The parallel connection method can govern the low voltage of multiple users, and the voltage governance range is wide.

[0034] The DC bus voltage can be dynamically adjusted according to the end load. Because to enable the inverter slave to stably output 220V alternating current, the DC bus voltage on the input side of the inverter slave should be maintained above 680V. In this embodiment, the rectifier host rectifies and boosts 380V alternating current into ±375V~±400V direct current and outputs it. The DC transmission line conducts ±375V~±400V DC power supply, and the inverter slave inversely outputs 380V or 220V alternating current and connects it to the user side. According to the length of the power supply radius and the different diameters of the power supply lines, the rectifier host dynamically adjusts the DC bus voltage, which can meet the maximum power supply distance exceeding 3km.

[0035] Preferably, the rectifier host and the inverter slave are connected by ±375V DC transmission. Compared with single-phase AC 220V transmission, under the same power, the transmission current is reduced to 1 / 3.4 of the original, and the loss in the line will be 1 / 10 of the original loss, effectively reducing the line loss.

[0036] In this embodiment, the flexible DC system has a bypass operation mode and can automatically disconnect from the grid during a fault without affecting the normal power supply of the original line.

[0037] In this embodiment, the flexible DC system has an energy-saving operation mode. As Figure 2 shown, its implementation method is:

[0038] Enter the energy-saving working mode: When the slave inverter detects that the user voltage (the voltage on the load side of the slave unit) is higher than the set target voltage threshold, the system automatically enters the energy-saving working mode and the slave inverter stops working; when the slave inverter does not work, the DC bus voltage and current decrease. When the master rectifier detects that the DC bus current is lower than the set value, the master rectifier also stops working;

[0039] Exit the energy-saving working mode: When the slave inverter detects that the user voltage is not higher than the set target voltage threshold, the system automatically exits the energy-saving working mode, the slave inverter starts to resume work, and raises the DC bus voltage to the set value for a certain period of time; when the master rectifier detects that the DC bus voltage reaches the set value, the master rectifier starts to work normally and raises the user voltage.

[0040] In this embodiment, the flexible DC system not only has the function of voltage correction (managing high and low voltages), but also has the function of reactive power compensation. As Figure 3 shown, the implementation method is:

[0041] When the voltage of the end user is normal and there is no need to generate active current for compensation, the slave inverter performs local reactive power compensation; the slave inverter is externally connected with a current transformer CT to monitor the system current in real time, and processes and analyzes the system current information through an internal controller to judge the reactive power load state of the system, calculates the current generated by the system reactive power load, and then sends a control signal to the IGBT inside the slave inverter and drives it to act for current compensation, so as to achieve the purpose of reactive power compensation.

[0042] In this embodiment, the flexible DC system has a leakage current detection function, as Figure 4 shown, the implementation method is:

[0043] The master rectifier is built-in with a leakage current detection device and a 4G module. The leakage current detection device detects the total current of the AC line connected to the front end of the flexible DC system, and the leakage current detection accuracy is 10 mA. When the vector sum of the currents of the four phases A, B, C, and N is not 0, it is judged that there is a leakage current situation; if the magnitude of the leakage current exceeds the set threshold, the leakage current detection device sends information to the maintenance personnel through the internal 4G module.

[0044] As Figure 5 shown, in this embodiment, a concentrator is installed at the back end of the flexible DC system to read the voltage data of the user electricity meter, and the concentrator communicates with the slave inverter in real time through a 485 communication line; a 4G module is set on the slave inverter. If the user voltage is lower than the set threshold, that is, there is a low voltage situation, the slave inverter sends an alarm message to the set user through the 4G module.

[0045] As Figure 6As shown, the rectifier host and inverter slave adopt a fanless structure, heat dissipation aluminum sheets are arranged on the device housing and the internal array, and the device housing is not provided with ventilation channels, so as to improve the sealing and dustproof and waterproof performance of the device.

[0046] In this embodiment, the DC power transmission interface of the flexible DC system has a lightning protection function and lightning protection failure warning. The equipment is designed according to the surge interference capability of level 4 (open circuit test voltage 4.0±10%kV) specified in GB / T 17626.5-2006, the DC lightning surge protection flow is 120kA, and it has a failure warning mechanism. When the surge protection device is damaged, it will automatically report to the 4G main station to prompt replacement, ensuring stable operation during the thunderstorm season.

[0047] Application scenario examples

[0048] 1. Basic situation of the area

[0049] Basic information of Yanzike substation: Yanzike substation is located in Xingcun Town, Wuyishan City, and was put into operation on August 9, 2000. On April 21, 2021, the capacity was increased from the original 100kVA to 160kVA, with a total of 18 users connected, 2 low-voltage outgoing lines, a low-voltage power supply radius of about 1.3km, and a daily power supply of 130-210 kWh. The maximum load rate in 2021 is 45.19%. Due to the long low-voltage power supply line, during the seasonal load period (tea making, returning home), the voltage of the end users in the substation area is in a low voltage state for a long time.

[0050] 2. Project implementation

[0051] Before treatment: the back-end user voltage was as low as 165V and as high as 197V, with an average of about 180V.

[0052] Construction process: Install a rectifier host at the voltage stabilization point (A4 pole) at the front end of the substation to take power, convert the three-phase AC power into ±375V DC power, and set up two DC busbars on the same pole from A4 to A11 for DC power transmission. At the end of the line, install an inverter slave on the A11 pole in the low-voltage area to convert DC power into 220V AC power for users.

[0053] After treatment: After the flexible DC system was installed, the end-user's high-power equipment was turned on for testing, and the voltage could be stably maintained at around 220V. The line loss rate in the substation area was reduced from 5.41% to 3.32%.

[0054] The long-distance power supply method based on low-voltage flexible DC technology of the present invention provides an economical, convenient and effective governance measure to solve the low voltage problem in long-distance power supply areas. After practical application, the long-standing low voltage problem in the Yanzike distribution substation area has been effectively solved. At the same time, the DC transmission characteristics of this method provide a convenient access point for subsequent DC equipment such as photovoltaic, storage and charging, and have good promotion and application value.

[0055] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A long-distance power supply method based on low-voltage flexible DC technology, characterized in that, A flexible DC system is installed in parallel on the power supply AC line. The flexible DC system includes a rectifier main unit with an AC / DC module, an inverter slave unit with a DC / AC module, and a DC transmission line. The rectifier main unit and the inverter slave unit are respectively installed at the front end and the end of the AC line, and the rectifier main unit is connected to the inverter slave unit via the DC transmission line. The flexible DC system has an energy-saving operation mode, and its implementation method is as follows: When the inverter slave unit detects that the user voltage is higher than the set target voltage threshold, the system automatically enters the energy-saving operation mode and the inverter slave unit stops working. The non-operation of the inverter slave unit causes the DC bus voltage and current to decrease. When the rectifier main unit detects that the DC bus current is lower than the set value, the rectifier main unit also stops working. When the inverter slave unit detects that the user voltage is not higher than the set target voltage threshold, the system automatically exits the energy-saving operation mode, the inverter slave unit starts to resume work, and raises the DC bus voltage to the set value for a certain period of time. When the rectifier main unit detects that the DC bus voltage reaches the set value, the rectifier main unit starts to work normally and raises the user voltage. The flexible DC system has a reactive power compensation function, and its implementation method is as follows: When the voltage of the end user is normal and there is no need to generate active current for compensation, the inverter slave unit performs local reactive power compensation. The inverter slave unit is externally connected with a current transformer CT to monitor the system current in real time, and processes and analyzes the system current information through an internal controller to judge the reactive power load state of the system, calculates the current generated by the system reactive power load, and then sends a control signal to the IGBT inside the inverter slave unit to drive its action for current compensation, so as to achieve the purpose of reactive power compensation.

2. The long-distance power supply method based on low-voltage flexible DC technology according to claim 1, characterized in that The rectifier main unit rectifies and boosts 380V alternating current into ±375V~±400V direct current and outputs it. The DC transmission line conducts ±375V~±400V DC power supply. The inverter slave unit inversely outputs 380V or 220V alternating current and connects it to the user side. When the flexible DC system is running, the transmission power on the AC line is shunted to the flexible DC system. The rectifier main unit dynamically adjusts the DC bus voltage according to the end load to meet the long-distance power supply requirements.

3. The long-distance power supply method based on low-voltage flexible DC technology according to claim 1, characterized in that, The flexible DC system has a leakage current detection function, and its implementation method is as follows: The rectifier main unit is internally provided with a leakage current detection device and a 4G module. The leakage current detection device detects the total current of the AC line connected to the front end of the flexible DC system. When the vector sum of the currents of the four phases A, B, C, and N is not 0, it is judged that there is a leakage current situation. If the leakage current magnitude exceeds the set threshold, the leakage current detection device sends information to the maintenance personnel through the internal 4G module.

4. The long-distance power supply method based on low-voltage flexible DC technology according to claim 1, characterized in that A concentrator is installed at the back end of the flexible DC system to read the voltage data of the user electricity meter. The concentrator communicates with the inverter slave unit in real time through a 485 communication line. A 4G module is set on the inverter slave unit. If the user voltage is lower than the set threshold, that is, there is a low voltage situation, the inverter slave unit sends an alarm message to the set user through the 4G module.

5. The long-distance power supply method based on low-voltage flexible DC technology according to claim 1, characterized in that The rectifying main machine and the inverse converting slave machine adopt a fanless structure, heat dissipation aluminum fins are arranged on the equipment housing and the internal array, and ventilation channels are not arranged on the equipment housing so as to improve the sealing performance and the dust and water proof performance of the equipment.

Images