Flexible DC loop closure control device for power grid and control method thereof

Abstract

The present invention provides a flexible DC loop closing control device for a power grid and a control method thereof. The present invention discloses a flexible DC loop closing control device for a power grid and a control method thereof. The device is a series-parallel device including a series coupling unit, a parallel coupling unit, another bypass switch circuit, and a loop closing main controller. Each set of series-parallel devices is configured to be connected in series between the distribution line and the load of each section. After receiving a loop closing command, the loop closing control device is set to operate in a voltage control mode, and after the DC bus voltages of the two sets of loop closing devices are consistent, the system enters a loop closing state. Before the loop closing is terminated, the device is set to a PQ control mode, which gradually reduces P and Q to zero, sequentially turns off the introduction switches, and the loop closing flow is terminated. The device has the advantages of simple structure, low cost, less loop closing monitoring amount than AC loop closing, and high efficiency. It can achieve flexible power mutual support and rapid shutdown in the event of a fault, and not only can it optimally adjust the power factor of the supply area per loop-closed transformer, but it can also flexibly change the power supply method of the power distribution grid to improve the new energy consumption level and DC load acceptance capacity.

Description

[Technical Field]

[0001] The present invention relates to the technical field of optimized operation control of power distribution networks, and in particular to a flexible DC loop closing control device for power networks and a control method thereof. [Background technology]

[0002] Traditional power distribution systems face challenges such as insufficient power supply capacity, difficulty in ensuring power quality, and low power supply reliability. To optimize grid structure, traditional distribution networks use disconnect switches and bus tie switches in closed loops. However, these switches do not provide flexible power support and are prone to large voltage and phase differences at the loop closure points, resulting in inrush currents that affect the safe and stable operation of the power grid. To improve power supply reliability, related research and demonstration projects are currently underway, including petal-type distribution networks, honeycomb-type distribution networks, and multi-port flexible DC loop closure. Taking the petal-type power grid, which supplies power without crossing geographical areas, as an example, this power supply mode features simple structure, flexible load transfer, short fault isolation time, high reliability, and strong scalability. However, it also suffers from the following problems: While it primarily uses a single-source loop closure, this method offers improved reliability compared to open-loop operation, it cannot guarantee reliable power supply in the event of a failure in the upper-level power grid. In the event of a fault, the load of the entire loop will be transferred to a single feeder, so redundant design of the feeder lines must be considered.

[0003] The large-scale introduction of renewable energy, the diversification of load equipment, and the increasing proportion of DC loads pose significant challenges to traditional power distribution grid configurations and operation methods. With the development of power electronics devices, domestic and international scholars have proposed flexible interconnection device concepts, such as smart soft open points (flexible multi-state switches), unified power flow controllers, and loop balance controllers. These devices utilize the fast and efficient control capabilities of power electronics devices to achieve flexible interconnection of power distribution grids. This enables flexible control and power mutual support across multiple power distribution grids with different voltage frequencies, amplitudes, and phases, promoting the consumption of new energy sources and meeting the demand for high-quality power supply, thereby improving the reliability, flexibility, and controllability of power distribution grids. Flexible interconnection devices can be broadly divided into two types: power electronics-based AC loop closure devices, which typically feature powerful power flow control capabilities and the use of back-to-back bidirectional converters to achieve flexible interconnection of cross-regional power distribution grids. However, these devices have drawbacks: large device volume, high cost, and, compared to DC loop closure, the need to consider phase angle loop closure conditions and slow operation. The other is the DC loop closure method, where transfer power systems are usually designed as AC-DC hybrid power supply systems and use DC bus loop closure solutions in different regions. Compared to AC loop closure, loop closure eliminates the need for phase angle detection, and converters can quickly control the DC voltage to reach the loop closure conditions. This results in higher loop closure efficiency and is more suitable for loop closure in areas with high DC loads and new energy concentrations. The adoption of multi-port DC loop closure not only helps with the local introduction of energy storage and DC loads, flexibly changing the power supply mode of the distribution grid, and improving new energy consumption levels, but also offers more benefits in terms of coordination ability and operational reliability, as multiple feeders support each other. In the event of a fault, the operating modes of multiple converters can be quickly and seamlessly switched, ensuring the rapid transfer of critical loads.However, current flexible interconnection devices are composed of back-to-back / multi-port power electronic converters, which also have problems such as large equipment capacity and volume, large operating losses, and high operation and maintenance costs. Therefore, the equipment utilization rate of this type of flexible loop closure device is low and the overall cost is high, which greatly limits its application and widespread use.

[0004] Therefore, in order to realize flexible interconnection and power mutual support between power distribution grid areas and improve the reliability of power supply, a flexible loop closing device and a control method thereof that are simple in structure, more efficient, and less costly are needed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-251765 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-071889 [Patent Document 3] Chinese Patent Application Publication No. 109638829 [Patent Document 4] Chinese Patent Application Publication No. 110854882 [Patent Document 5] Chinese Patent Application Publication No. 113054658 [Patent Document 6] Chinese Patent Application Publication No. 113270864 [Patent Document 7] US Patent Application Publication No. 2008 / 0303489 [Patent Document 8] US Patent Application Publication No. 2017 / 0199502 Summary of the Invention [Means for solving the problem]

[0006] To solve the above problems, the present invention provides a flexible DC loop closure control device for a power grid, which is a series-parallel device with two AC ports and one DC port, and further includes a three-port series coupling unit, a two-port parallel coupling unit, a bypass switch, and a DC loop closure main controller; 3-port serially coupled unit teeth, Two AC ports Therefore, line Between the load and to Load and Connected in series, the DC port connects to the DC side of the parallel-coupled unit, The AC side of the two-port parallel coupling unit is connected in parallel to the power grid line, and the DC port is connected to the DC side of the series coupling unit to form a DC output port of the series-parallel device; The bypass switch is connected in parallel to the two AC ports of the series-coupled unit, and the loop closure controller is used for logic control of the loop closure and for receiving remote control.

[0007] The device is capable of loop closure on the DC side, which not only enables flexible transfer of power between distribution networks and rapid shutdown in the event of a fault, but also improves the reliability of power supply in the distribution network to over 99.99%, achieves load balancing between the lines in the supply area per heavy-load transformer and the lines in the supply area per light-load transformer, and ensures the quality of power supply. Since loop closure only monitors the DC voltage amplitude, and compared with AC loop closure, the monitoring of the voltage phase angle in loop closure can be ignored, the economy, safety, and efficiency of power supply in the distribution network in the supply area per transformer are improved.

[0008] Preferably, the series coupling unit can regulate power bidirectionally with the series coupling transformer. Series side an inverter, the primary side of the series coupled transformer being connected to an AC line; Between the load and to Load and They are connected in series, and the secondary side is Series side Connected to the AC side of the inverter, the parallel coupling unit can adjust the power in both directions. Parallel side It consists of an inverter, Parallel sideThe AC side of the inverter is connected in parallel to the AC grid line, and the DC side is connected to the DC side of the series coupling unit, thereby forming the basic topology of a series-parallel loop closing device.

[0009] Preferably, the DC loop closing method includes providing a flexible loop closing device between the line and the load in the supply area for each transformer requiring loop closure. Load and The DC ports of the flexible loop closing devices of the supply areas of each loop-closed transformer are connected in series, and the active power flow between the supply areas of each loop-closed transformer is transmitted mutually through the DC ports.

[0010] Preferably, in order to simplify the topology structure of the series-parallel loop closing device and further reduce the manufacturing cost, a flexible loop closing device is provided between the line and the load, each of which is required for the supply area per transformer of the loop closing. Load and Similarly, flexible loop closure between supply areas per transformer in two sections can be achieved by connecting them in series, reserving only the series coupling unit in the flexible loop closure device of the supply area per transformer in one section, and connecting the DC port of the series coupling unit to the DC bus of the series-parallel loop closure device of the supply area per transformer of the loop closure in the other section.Since the parallel coupling unit of the supply area per transformer in one section is omitted, costs can be saved while meeting flexible loop closure requirements.

[0011] Preferably, an energy storage device with a DC / DC converter connected to the DC bus of the device can be installed to improve the flexibility of power supply and the ability to support active power flow, and to relieve the pressure on the power supply capacity. The charging and discharging capability of the energy storage device further improves the flexibility of power supply control in the supply area per loop-closed transformer. At the same time, the above topology structure helps improve the acceptance capacity of new energy, realizes continuous combined supply of new energy and energy storage, and can also supply power to DC loads.

[0012] To solve the above problems, the present invention provides a flexible DC loop closure control method for a power grid, the method comprising: Step 1: Build a system model of the flexible DC loop closure device of the power grid, monitor the power of each of the two sections of transmission lines that need to be closed over a long period of time, and prepare for loop closure in real time. Step 2: before the loop is closed, the bus tie switches and the introduction switches are all in an open state, and both sets of loop closure control devices are operating in voltage control mode; Step 3: after receiving the loop closing command, determine whether the DC bus voltages of the two sets of loop closing devices are consistent; if they are not consistent, adjust the DC bus voltage using the constant DC voltage control mode of the parallel coupling unit; after confirming that they are consistent, close the DC introduction switch K, so that the system enters the loop closing state; after the loop is closed, dynamically adjust the amplitude and phase of the additional voltage of the series coupling transformer, thereby realizing flexible power mutual support in the supply area per transformer of the two sections of loop closing; Before completing the loop closure, the two sets of flexible loop closure controllers are set to operate in PQ control mode, and the two sets of flexible loop closure controllers respectively reduce P and Q to zero, and then turn off the introduction switch K, and the flexible DC loop closure process is completed (step 4). The present invention is characterized by comprising:

[0013] Preferably, in the step 1, the system model of the flexible DC loop closing device of the power grid is constructed by the method comprising two sets of series-parallel devices and one set of introduction switches, each set of series-parallel devices being connected between the distribution line and the load in each section. Load and The DC loop closure is achieved by connecting the DC ports of two sets of flexible loop closure devices connected in series. Each set of series-parallel devices is composed of a three-port series coupling unit, a two-port parallel coupling unit, a bypass switch and a DC loop closure main controller. The three-port series coupling unit teeth, Two AC ports Therefore, line Between the load and to Load andConnected in series, the DC port connects to the DC side of the parallel-coupled unit, The AC side of the two-port parallel coupling unit is connected in parallel to the power grid line, and the DC port is connected to the DC side of the series coupling unit to form the DC output port of the series-parallel device; The bypass switch is characterized by connecting in parallel with the two AC ports of the series-coupled unit.

[0014] Preferably, an energy storage device can be installed on the DC bus of the device to improve the flexibility of power supply and the ability to support active power flow, and the charging and discharging capability of the energy storage device can further improve the flexibility of power supply control in the supply area per loop-closed transformer. At the same time, the DC bus of the device can be connected to a DC power grid equipped with photovoltaic power generation, wind power generation, and DC loads, which will increase the new energy consumption level of the grid and allow the power supply mode of the grid to be flexibly changed.

[0015] Preferably, in order to meet the economical construction needs, the system model of the DC loop closure device can be improved as follows: it comprises one set of series-parallel devices, one set of series coupling devices, and one set of introduction switches, and the series coupling device only reserves the series coupling module part of the series-parallel device to reduce manufacturing costs, and similarly, the series-parallel devices and the series coupling devices are respectively connected in series between the distribution lines and the loads in each section where loop closure is required, and the DC bus of the series-parallel devices and the DC port of the series coupling device are connected to realize DC loop closure.

[0016] Preferably, in step 2, the two sets of loop closure control devices both operate in voltage control mode, which is realized by the two sets of devices, and constant DC voltage control is performed through the parallel coupling unit of the devices, so that the DC bus voltages of the devices in the supply areas per transformer in the two sections are consistent, thereby realizing DC loop closure.

[0017] Preferably, in step 3, adjusting the DC bus voltage using the constant DC voltage control mode of the parallel coupling unit comprises detecting the DC side voltage of the parallel device and comparing it with a given value of the DC voltage, and using a PI controller to control the current inner loop and output the parallel converter PWM voltage control signal, where the controller can be PI control or other types of controllers.

[0018] Preferably, in step 3, the amplitude and phase of the additional voltage of the series-coupled transformer are dynamically adjusted to realize flexible power mutual support in the supply areas per transformer of the two-section loop-closed connection, by adjusting the output voltage of the series-coupled transformer in the supply area per transformer that needs power supply, and adjusting the amplitude and phase angle within a certain adjustment range, thereby changing the active current and reactive current of the standby line, and flexibly adjusting the active power and reactive power transferred between the two sections of the transmission line.

[0019] Preferably, in the flexible adjustment of the active power and reactive power transferred between the two sections of the transmission line, the active power control uses the voltage loop of the series coupling unit to control the amplitude and phase angle of the additional voltage of the series coupling transformer to solve the problem of active power flow control, specifically: (1) acquiring a bus voltage phase of a power supply connected to a series device via a phase-locked loop (PLL); (2) Collect the AC voltage U on the power supply side of the inverter in series, and perform a dq transformation using the phase of the collected AC bus voltage to obtain U d , U q and comparing it with a given voltage reference value, and obtaining an output current reference value by using a difference value obtained through a PI controller (the controller may be a PI controller or other type of controller); (3) The obtained current reference value is outputted as a voltage control signal to the PWM series-side converter through the current inner loop control to control the amplitude and phase angle of the additional voltage ΔV of the series-coupled transformer. The present invention is characterized by comprising:

[0020] Preferably, in the flexible adjustment of the active power and reactive power transferred between the two sections of the transmission line, the reactive power control uses a parallel coupling unit to solve the problem of reactive power flow control through closed-loop control, specifically: (1) acquiring an AC bus voltage phase connected to a parallel device via a phase-locked loop (PLL); (2) Collecting reactive currents from the AC grid connected to the inverter of the parallel coupling device, converting them into current commands after dq transformation, and outputting PWM voltage control signals based on the difference between the current command and the given value through a PI controller, so that the parallel converter outputs the required reactive power for the grid and realizes reactive compensation (the controller can be a PI controller or other types of controller); The present invention is characterized by comprising: [Effects of the Invention]

[0021] The present invention has the following advantages:

[0022] (1) The present invention provides a flexible DC loop closing device and its control method, which can realize loop closing on the line, avoid reverse load operation after a power outage, and improve the reliability of power supply.

[0023] (2) The flexible DC loop closure device provided by the present invention has a simple structure, requires a low design capacity of the converter in the series-parallel device, and is less expensive than the conventional back-to-back loop closure device.

[0024] (3) The loop closure method of the present invention belongs to an AC and DC hybrid power supply mode, which can realize dynamic restructuring of the power grid under extreme operating conditions, continuous co-supply of new energy and energy storage, and provide flexible power supply to DC loads.

[0025] (4) The present invention monitors the real-time power consumption of each loop-closed transformer in the supply area, receives loop-closing commands in real time, and controls the amplitude of the DC bus voltages of the two sections of the line to realize flexible loop-closing or loop-closing termination, ensuring high efficiency and stability of loop-closing operation.

[0026] (5) The loop closing device of the present invention uses a power electronics converter as the main controller for loop closing and controls the high-speed operation of the switching elements, resulting in a fast response speed, high control accuracy, less mechanical wear, and a longer lifespan of the equipment compared to conventional closed-loop operation methods that use bus tie switches.

[0027] (6) The present invention has a high reliability because, if a device fails, it can be switched to a bypass state using a bypass switch through the DC loop closure of the two devices, without affecting the operation of the original power supply line.

[0028] (7) The present invention uses a DC loop closure method, and can satisfy the loop closure conditions by simply considering that the amplitude of the DC side voltage is equal. In contrast, the AC loop closure does not require adjusting the two parameters of the AC voltage amplitude and phase angle to close the loop, so the efficiency of the loop closure is higher. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a block diagram of an embodiment of a device for realizing flexible DC loop closure in a power grid according to the present invention; [Figure 2] 1 is a block diagram showing the installation of the flexible DC loop closure device of the present invention into a DC power distribution network. [Figure 3] 1 is a block diagram of an embodiment of an improved device for realizing flexible DC loop closure in a power grid according to the present invention; FIG. [Figure 4] 2 is a control flowchart for realizing flexible DC loop closing control of a power grid according to the present invention; [Figure 5]FIG. 1 is a block diagram of an embodiment of a series coupling unit in the components of the device of the present invention. [Figure 6] FIG. 10 is a block diagram of an embodiment of a parallel coupling unit in the components of the device of the present invention. [Figure 7] FIG. 1 is a diagram showing vectors of a voltage outer loop control strategy for realizing DC voltage control of the present invention. [Figure 8] FIG. 10 is a diagram showing vectors of a current inner loop control strategy for implementing DC voltage control of the present invention. [Figure 9] 1 is a simplified system diagram and vector diagram of the device of the present invention performing flexible DC loop closure control of a power grid. [Figure 10] FIG. 1 is a block diagram of a vector control strategy for implementing flexible loop closure control of a power grid according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0030] The technical means of the present invention will be described in more detail below with reference to the drawings.

[0031] Step 1: As shown in Figure 1, a system model of a flexible DC loop closure device for a power grid is constructed. The device in this embodiment includes two sets of series-parallel devices and one set of introduction switches. Each set of series-parallel devices is connected between the distribution line and the load in each section. Load and The DC loop closure is achieved by connecting the DC ports of the flexible loop closure devices in the supply area of ​​each loop-closed transformer connected in series. Each set of series-parallel devices has two AC output ports and one DC output port. The internal structure consists of a series coupling unit, a parallel coupling unit, a bypass switch, and a DC loop closure main controller. 3-port series coupling unit teeth, Two AC ports Therefore, line Between the load and to Load andThe series-parallel coupling unit is connected in series, and its DC port is connected to the DC side of the parallel coupling unit. The AC side of the two-port parallel coupling unit is connected in parallel to the power distribution network line, and its DC port is connected to the DC side of the series coupling unit, thereby forming the DC output port of the series-parallel device. A bypass switch is connected in parallel to the two AC ports of the series coupling unit. The system model is shown in Figure 2. Preferably, to meet the economical construction needs, the system model of the DC loop closure device can be improved as follows: it includes one set of series-parallel devices, one set of series coupling devices, and one set of introduction switches, and the series coupling devices only reserve the series coupling module part of the series-parallel device to reduce manufacturing costs, and similarly, the series-parallel device and the series coupling device are respectively connected between the distribution line and the load in each section where loop closure is required. Load and The DC loop is closed by connecting the DC bus of the series-parallel device and the DC port of the series-coupled device in series. The model is shown in Figure 3.

[0032] Step 1.1: A model of the series-coupled unit is constructed, and a specific embodiment is shown in Figure 5. The series-coupled unit has three ports: two AC ports and one DC port. The two AC ports are connected to the primary side of the series-coupled transformer, and the secondary side of the series-coupled transformer is connected to the AC side of the series-side inverter, whose DC end forms the DC port of the series-coupled unit. The series-side inverter is composed of a three-phase full-bridge arm power unit with bidirectional power flow, and the inverter topology can be a two-level or cascaded multilevel structure. The series inverter uses PWM to quickly control switching operation, achieving voltage stabilization and harmonic suppression, and isolating DC bus voltage disturbances.

[0033] Step 1.2: A model of the parallel-coupled unit is constructed, and a specific embodiment is shown in Figure 6. The parallel-coupled unit has two ports and is composed of a parallel inverter. The AC output port of the inverter is the AC port of the parallel-coupled unit, and the DC output port of the inverter is the DC port of the parallel-coupled unit. The inverter is composed of a three-phase full-bridge arm power unit with bidirectional power flow, and the inverter topology can be a two-level or cascaded multilevel structure. The parallel inverter can achieve functions such as reactive compensation, harmonic suppression, and DC bus voltage stabilization by controlling power electronic devices in real time.

[0034] Step 2: The control logic of the series-parallel combined unit of the flexible DC loop closed connection of the power grid is constructed as shown in Figure 4. The specific flow of the control logic is as follows:

[0035] Step 2.1: The power of each of the two sections of transmission line that need to be closed is monitored over a long period of time, and preparations for loop closure are made in real time. Before the loop is closed, the bus tie switches and introduction switches are all open, and both sets of loop closure control devices operate in voltage control mode.

[0036] Step 2.2: After receiving the loop closure command, it determines whether the DC bus voltages of the two sets of loop closure devices are consistent; if they are not, it uses the constant DC voltage control mode of the parallel coupling unit to adjust the DC bus voltage; after it is confirmed that they are consistent, it closes the DC introduction switch K, causing the system to enter the loop closure state; after the loop is closed, it dynamically adjusts the amplitude and phase of the additional voltage of the series coupling transformer, thereby realizing flexible power mutual support in the supply area per loop closure transformer.

[0037] Step 2.3: Before completing the loop closure, the two sets of flexible loop closure controllers are set to operate in PQ control mode, and after the two sets of flexible loop closure controllers respectively reduce P and Q to zero, the introduction switch K is turned off, and the flexible DC loop closure process is completed.

[0038] In step 2.2, as shown in Figure 7, the constant DC voltage control mode of the parallel coupling unit is used to adjust the DC bus voltage. The DC side voltage of the parallel device is detected and compared with the given DC voltage value. The obtained difference value is passed through the current inner loop control (Figure 8) by the PI controller to output the parallel converter PWM voltage control signal. The controller can be PI control or other types of controllers can be used.

[0039] In step 2.2, the power mutual support vector diagram during flexible DC loop closed control is shown in Figure 9. The power supply voltage in section I is

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[0040]

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[0041] As can be seen from the above analysis, when Section I transfers power to Section II, the voltage difference of the series compensation voltage of Section I is

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[0042] Therefore, by adjusting the voltage between the two AC ports of the series-coupled unit, i.e., the series compensation voltage of the series-coupled transformer, the output active power between the two bus sections can be adjusted, thereby realizing the requirement of flexible loop closure control of the power grid.

[0043] In step 2.2, the control strategy block diagram for flexible DC loop closure control is shown in Figure 10. The power in the supply area per transformer for the two-section loop closure is measured and calculated to obtain a given power value, and the difference value from the real-time measured power is used to obtain ΔP and ΔQ through PID control. A given value for the series compensation voltage ΔV proportional to ΔP and ΔQ is obtained, and the sum of this given value and the AC side voltage of the original series-coupled unit is input as the voltage reference value of the series-coupled unit. Closed-loop control is realized by the voltage outer loop and current inner loop, and the output value is the voltage control signal of the series-side inverter.

[0044] Both series-parallel combined units can select a vector control strategy that includes control modules such as a phase-locked loop (PLL) that detects the AC bus voltage phase, a PI controller (other controllers can be used depending on the required control effect), a coordinate transformation module (abc / dq converter and dq / qbc converter), and a comparator.Vector control allows multiple electrical quantities to be controlled to reach given values, such as DC bus voltage, grid power, grid voltage d-axis and q-axis components, and load current d-axis and q-axis components.

[0045] The closed-loop control of the series coupling unit is used to adjust the capacity of the active power flow mutual support of the grid, and uses a voltage loop to control the amplitude and phase angle of the additional voltage of the series coupling transformer to solve the active power flow control problem, specifically including the following steps: (1) Bus voltage phase θ of the power supply connected to the series device via a phase-locked loop (PLL) S collecting the (2) Collect the AC voltage U on the power supply side of the inverter in series, and perform a dq transformation using the phase of the collected AC bus voltage to obtain U d , U q and comparing it with a given voltage reference value, and obtaining an output current reference value by the difference value obtained through a PI controller (the controller can be a PI controller or other type of controller); (3) The obtained current reference value is used to output a voltage control signal to the PWM series converter via current inner loop control, so as to adjust the amplitude and phase angle of the series-coupled transformer ΔV1 or ΔV2 in the two sections of the line.

[0046] In the flexible adjustment of the active power and reactive power transferred between the two sections of the transmission line, the reactive power control uses a parallel coupling unit to solve the reactive power flow control problem through closed-loop control, specifically: (1) AC bus voltage phase θ connected to parallel devices via a phase-locked loop (PLL) S and collecting (2) Collecting reactive currents from the AC grid connected to the inverter of the parallel coupling device, converting them into current commands after dq transformation, and outputting PWM voltage control signals based on the difference between the current command and the given value through a PI controller, so that the parallel converter outputs the required reactive power for the grid and realizes reactive compensation (the controller can be a PI controller or other types of controller); The present invention is characterized by comprising:

[0047] The above is a detailed description of the present invention in combination with specific preferred embodiments, and the specific implementation of the present invention is limited to these embodiments, and other adjustment methods based on the device structure of the present invention are essentially consistent with the present invention. Those skilled in the art can make some equivalent substitutions or obvious modifications without departing from the concept of the present invention, but the same performance or use should be considered to be within the scope of the present invention.

Claims

1. A flexible DC loop closure control device for a power grid, the flexible DC loop closure control device being a series-parallel device having two AC ports and one DC port, further comprising a three-port series coupling unit, a two-port parallel coupling unit, a bypass switch, and a DC loop closure main controller; The three-port series coupling unit is connected in series with the load between the line and the load by two AC ports, and the DC port is connected to the DC side of the parallel coupling unit; The AC side of the two-port parallel coupling unit is connected in parallel to a power grid line, and the DC port is connected to the DC side of the series coupling unit to form a DC output port of the series-parallel device; The bypass switch is connected in parallel to the two AC ports of the series-coupled unit, and the loop closure controller is used for logic control of loop closure and for receiving remote control.

1. A flexible DC loop closure control device for a power grid, comprising:

2. 2. The flexible DC loop closure control device for power grid according to claim 1, characterized in that the series coupling unit comprises a series coupling transformer and a series-side inverter with bidirectional power regulation, a primary side of the series coupling transformer being connected in series with the load between the AC line and the load and a secondary side being connected with the AC side of the series-side inverter; and the parallel coupling unit comprises a parallel-side inverter with bidirectional power regulation, an AC side of the parallel-side inverter being connected in parallel with the AC power grid line and a DC side being connected with the DC side of the series coupling unit, thereby forming a basic topology of a series-parallel loop closure device.

3. 3. The flexible DC loop closure control device for a power grid according to claim 1 or 2, characterized in that the DC loop closing method comprises connecting flexible loop closure devices in series with the load between a line in a supply area per transformer requiring loop closure and the load, respectively, connecting DC ports of the flexible loop closure devices in the supply areas per transformer for loop closure, respectively, and transmitting active power flows between the supply areas per transformer for loop closure via the DC ports.

4. 4. The flexible DC loop closure control device for power grids according to claim 3, characterized in that, in order to simplify the topology structure of the series-parallel loop closure device and further reduce manufacturing costs, the flexible loop closure devices are connected in series between the lines and loads that require a supply area per transformer for loop closure, and only the series coupling unit is reserved in the flexible loop closure device for a supply area per transformer in one section, and the DC port of the series coupling unit is connected to the DC busbar of the series-parallel loop closure device for a supply area per transformer for loop closure in another section, thereby similarly realizing flexible loop closure between supply areas per transformer in two sections.

5. 5. The flexible DC loop-closed control device for power grids according to claim 4, characterized in that, in order to improve the flexibility of power supply and the ability to support active power flow and relieve the pressure on power supply capacity, an energy storage device with a DC / DC converter can be provided, connected to the DC bus of the device; the charging and discharging capability of the energy storage device can further improve the flexibility of power supply control in the supply area per loop-closed transformer; at the same time, the topology structure can help improve the acceptance capability of new energy, realize the continuous combined supply of new energy and energy storage, and can also supply power to DC loads.

6. 1. A flexible DC loop closure control method for a power grid, comprising: Step 1: building a system model of a flexible DC loop closure device for a power grid, monitoring the power of each of two sections of transmission lines that need loop closure over a long period of time, and preparing for loop closure in real time; Step 2: before the loop is closed, the bus tie switches and the introduction switches are all in an open state, and both sets of loop closure control devices operate in voltage control mode; Step 3: after receiving the loop closing command, determine whether the DC bus voltages of the two sets of loop closing devices are consistent; if they are not consistent, adjust the DC bus voltage using the constant DC voltage control mode of the parallel coupling unit; after confirming that they are consistent, close the DC introduction switch K, and the system enters a loop closing state; after the loop is closed, dynamically adjust the amplitude and phase of the additional voltage of the series coupling transformer to realize flexible power mutual support in the supply areas per transformer of the two sections of loop closing; Step 4: Before the loop closure is completed, the two sets of flexible loop closure control devices are set to operate in PQ control mode, and the two sets of flexible loop closure control devices respectively reduce P and Q to zero, and then turn off the introduction switch K, thereby completing the flexible DC loop closure process. A method comprising:

7. In step 1, the system model of the flexible DC loop closure device of the power grid is constructed as follows: the device of the method comprises two sets of series-parallel devices and one set of introduction switches, each set of the series-parallel devices is connected in series with the load between the distribution line and the load in each section, and the DC loop closure is realized by connecting the DC ports of the two sets of flexible loop closure devices, each set of the series-parallel devices is composed of a three-port series coupling unit, a two-port parallel coupling unit, a bypass switch and a DC loop closure main controller, and the three-port series coupling unit is connected in series with the load between the line and the load by two AC ports, and the DC port is connected to the DC side of the parallel coupling unit; The AC side of the two-port parallel coupling unit is connected in parallel to the power grid line, and the DC port is connected to the DC side of the series coupling unit to form a DC output port of the series-parallel device; The bypass switch is connected in parallel with the two AC ports of the series-coupled unit.

7. The method according to claim 6.

8. The method according to claim 7, characterized in that, in order to improve the flexibility of power supply and the ability to support active power flow, an energy storage device can be provided connected to the DC bus of the device, and the charging and discharging capability of the energy storage device can further improve the flexibility of power supply control in the supply area per loop-closed transformer; meanwhile, the DC bus of the device can be connected to a DC power grid equipped with photovoltaic power generation, wind power generation and DC loads, so as to increase the new energy consumption level of the grid and flexibly change the power supply mode of the power grid.

9. 9. The method according to claim 8, characterized in that, in order to meet economical construction needs, the system model of the DC loop closure device can be improved to include one set of series-parallel devices, one set of series coupling devices, and one set of introduction switches, in which the series coupling device reserves only the series coupling module part of the series-parallel device to reduce manufacturing costs, and similarly, the series-parallel device and the series coupling device are respectively connected in series with the load between the distribution line and the load in each section where loop closure is required, and the DC bus of the series-parallel device and the DC port of the series coupling device are connected to realize DC loop closure.

10. 10. The method according to claim 6, wherein in step 2, the two sets of loop-closing control devices both operate in voltage control mode, which is realized by the two sets of devices, and constant DC voltage control is performed through parallel coupling units of the devices, and the DC loop closure is realized when the DC bus voltages of the devices in the supply areas per transformer in the two sections are consistent.

11. 10. The method according to claim 6, wherein in step 3, adjusting the DC bus voltage using the constant DC voltage control mode of the parallel coupling unit comprises detecting the DC side voltage of the parallel device and comparing it with a given value of the DC voltage, and using a PI controller to control the current inner loop and output the parallel converter PWM voltage control signal, wherein the controller can be a PI controller or other types of controllers.

12. 12. The method of claim 11, wherein in step 3, dynamically adjusting the amplitude and phase of the additional voltage of the series-coupled transformer to realize flexible power mutual support in the supply areas per transformer of the two sections of closed-loop connection comprises adjusting the output voltage of the series-coupled transformer in the supply area per transformer that needs power supply, and adjusting the amplitude and phase angle within a certain adjustment range to change the active current and reactive current of the standby line, thereby flexibly adjusting the active power and reactive power transferred between the two sections of the transmission line.

13. In the flexible adjustment of the active power and reactive power transferred between the two sections of the transmission line, the active power control uses the voltage loop of the series coupling unit to control the amplitude and phase angle of the additional voltage of the series coupling transformer to solve the problem of active power flow control, specifically: (1) acquiring a bus voltage phase of a power supply connected to a series device via a phase-locked loop (PLL); (2) collecting AC voltage U on the power supply side of the inverter end on the series side, performing dq transformation using the phase of the collected AC bus voltage to obtain Ud and Uq, comparing them with a given voltage reference value, and obtaining an output current reference value from the difference value obtained via a PI controller (the controller can be a PI controller or other type of controller); (3) outputting a voltage control signal to the PWM series-side converter through current inner-loop control using the obtained current reference value to control the amplitude and phase angle of the additional voltage ΔV of the series-coupled transformer; 13. The method of claim 12, comprising:

14. In the flexible adjustment of the active power and reactive power transferred between the two sections of the transmission line, the reactive power control uses a parallel coupling unit to solve the problem of reactive power flow control through closed-loop control, specifically: (1) acquiring an AC bus voltage phase connected to a parallel device via the phase-locked loop (PLL); (2) collecting reactive currents of the AC grid connected to the inverter of the parallel coupling device, and converting them into current commands after dq transformation; the difference between the current command and the given value is output as a PWM voltage control signal through a PI controller, so that the parallel converter outputs the required reactive power to the grid and realizes reactive compensation (the controller can be a PI controller or other types of controller); 14. The method of claim 13, comprising:

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