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 grids, and in particular to a flexible DC loop closing control device for power grids and a control method thereof. [Background technology]

[0002] Traditional power distribution systems face problems such as insufficient power supply capacity, difficulty in ensuring power quality, and low reliability of power supply. In order to achieve the optimization of the grid structure, traditional power distribution networks use disconnection switches and bus tie switches in closed loops, but flexible mutual support of power cannot be realized and cannot be realized, and the voltage difference and phase difference at the loop closing point are likely to cause large inrush currents, which affect the safe and stable operation of the power grid. At present, in order to improve the reliability of power supply, there are related research and demonstration projects, such as petal-type power distribution networks, honeycomb-type power distribution networks, and multi-port flexible DC loop closing, among other distribution technologies. Taking the petal-type power network that supplies power without crossing regions as an example, this power supply mode has the characteristics of simple structure, flexible load transfer, short time required for fault isolation, high reliability, and strong scalability. However, there are also the following problems. Basically, it uses a single-power loop closing, which is more reliable than open-loop operation, but when a failure occurs in the upper power grid, it cannot guarantee a reliable power supply. In the event of a fault, the load of the entire loop will be transferred to a single feeder, so feeder line redundancy design must be considered.

[0003] The introduction of large amounts of renewable energy, the diversification of load equipment, and the increase in the proportion of DC loads have brought great challenges to the configuration and operation methods of traditional power distribution networks. With the development of power electronics devices, scholars at home and abroad have proposed the concept of flexible interconnection devices such as smart soft open points (flexible multi-state switches) using power, unified power flow controllers, and loop balance controllers, and use the fast and efficient control capabilities of power electronics devices to realize flexible interconnection of power distribution networks, thereby realizing flexible control and power mutual support for multiple power distribution networks with different voltage frequencies, amplitudes, and phases, promoting the consumption of new energy, and meeting the demand for high-quality power supply, thereby improving the reliability, flexibility, and controllability of the power distribution network. Flexible interconnection devices are roughly divided into two types: one is the AC loop closure device based on power electronics devices, and its typical feature is the use of back-to-back bidirectional converters, which have strong power flow control capabilities and can realize flexible interconnection of cross-regional power distribution networks. However, it has the disadvantages of being too large in device capacity and volume, high cost, and the need to consider the phase angle loop closure conditions compared with DC loop closure, and the time required for operation. The other is the DC loop closure method, where the transfer power grid is usually designed as an AC-DC hybrid power supply method and uses the DC bus loop closure solution in different regions. Compared with AC loop closure, loop closure does not need phase angle detection, and the converter can control the DC voltage to reach the loop closure condition quickly, so the loop closure efficiency is higher and it is more suitable for loop closure in areas with a large DC load and new energy concentration. The adoption of multi-port DC loop closure is not only conducive to the local introduction of energy storage and DC loads, flexibly changing the power supply method of the power grid, and improving the new energy consumption level, but also has more benefits in terms of adjustment ability and operation reliability because multiple feeders support each other. When a fault occurs, the operation modes of multiple sets of converters can be switched quickly and seamlessly, ensuring that important loads can be transferred quickly.However, the current flexible interconnection device is composed of back-to-back / multi-port power electronic converters, which also have problems such as equipment capacity / volume, operating loss, 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 popularization.

[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 closure device and a control method thereof that are simple in structure, more efficient, and less costly are needed. Summary of the Invention [Means for solving the problem]

[0005] In order to solve the above problems, the present invention provides a flexible DC loop closing control device for power grid, the device is a series-parallel device, which has two AC ports and one DC port, and the device further includes a three-port series coupling unit, a two-port parallel coupling unit, a bypass switch and a DC loop closing main controller; The two AC ports of the three-port series combination unit are connected in series to the line, and the DC port is connected to the DC side of the parallel combination 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 combination unit, and the loop closure controller is used for logic control of the loop closure and for receiving remote control.

[0006] The device enables loop closure on the DC side, which not only enables flexible transfer supply 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 more than 99.99%, realizes 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. The only monitoring quantity for loop closure is the DC voltage amplitude, and compared with AC loop closure, the monitoring of the voltage phase angle for loop closure can be ignored, thereby improving the economy, safety and efficiency of power supply in the distribution network in the supply area per transformer.

[0007] Preferably, the series coupling unit comprises a series coupling transformer and a bidirectionally regulating inverter, the primary side of the series coupling transformer is connected in series with the AC line and the secondary side is connected with the AC side of the inverter, the parallel coupling unit is composed of a bidirectionally regulating inverter, the AC side of the inverter is connected in parallel with the AC grid line and the DC side is connected with the DC side of the series coupling unit, thus forming the basic topology of a series-parallel loop closing device.

[0008] Preferably, the DC loop closing method comprises connecting flexible loop closing devices in series between the lines of the supply areas per transformer requiring loop closure and the loads, respectively, connecting the DC ports of the flexible loop closure devices of the supply areas per transformer requiring loop closure, respectively, and transmitting active power flow between the supply areas per transformer requiring loop closure via the DC ports.

[0009] Preferably, in order to simplify the topology structure of the above-mentioned series-parallel loop closing device and further reduce the manufacturing cost, flexible loop closing devices are respectively connected in series between the line and the load where the supply area per transformer of loop closure is required, and only the series coupling unit is reserved in the flexible loop closing device of the supply area per transformer of one section, and the DC port of the series coupling unit is connected to the DC busbar of the series-parallel loop closing device of the supply area per transformer of the loop closure of the other section, so that flexible loop closure between the supply areas per transformer of two sections can be realized in the same way. Since the parallel coupling unit of the supply area per transformer of one section is omitted, it is possible to save costs while meeting the flexible loop closure.

[0010] Preferably, an energy storage device with a DC / DC converter connected to the DC bus of the device can be provided to improve the flexibility of power supply and the support ability of active power flow, and relieve the pressure on the power supply capacity, and the charging and discharging ability of the energy storage device can further improve the flexibility of the power supply control of the supply area per loop-closed transformer. At the same time, the above topology structure is conducive to improving the acceptance capacity of new energy, and can realize the continuous co-supply of new energy and energy storage, and can also supply power to DC loads.

[0011] In order 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: 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 to be closed over a long period of time, and preparing for loop closure in real time; Step 2, in which the bus tie switches and the introduction switches before the loop closure are all in an open state, and both of the two sets of loop closure control devices operate in a voltage control mode; Step 3: after receiving the loop closing command, judge whether the DC bus voltages of the two sets of loop closing devices are consistent; if they are not consistent, use the constant DC voltage control mode of the parallel coupling unit to adjust the DC bus voltage; after the consistency is confirmed, 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 to realize flexible power mutual support of the supply area per transformer of the two sections of loop closing; Step 4: Before the loop closure is completed, the two sets of flexible loop closure controllers are set to operate in PQ control mode. 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. The present invention is characterized by comprising:

[0012] Preferably, in the step 1, the system model of the flexible DC loop closing device of the power grid is constructed as follows: the device of the method includes two sets of series-parallel devices and one set of introduction switches, each set of series-parallel devices is connected in series between the distribution line and the load of each section, and the DC loop closure is realized by connecting the DC ports of the two sets of flexible loop closing devices, 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, two AC ports of the three-port series coupling unit are connected in series to the line, 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 the DC output port of the series-parallel device; The bypass switch is characterized by being connected in parallel with the two AC ports of the series-coupled unit.

[0013] Preferably, an energy storage device can be connected to the DC bus of the device to improve the flexibility of power supply and the support capacity of 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, so as to increase the new energy consumption level of the grid and flexibly change the power supply mode of the power grid.

[0014] Advantageously, in order to meet the economical construction needs, the system model of the DC loop closing 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, in which the series coupling device only reserves the series coupling module part of the series-parallel device to reduce the manufacturing cost, 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 that needs to be loop closed, and the DC bus of the series-parallel devices and the DC port of the series coupling device are connected to realize the DC loop closure.

[0015] Preferably, in step 2, the two sets of loop closure control devices both operate in a voltage control mode, which is realized by the two sets of the above-mentioned devices, and constant DC voltage control is performed through the parallel coupling unit of the devices, and the DC bus voltages of the devices in the supply areas per transformer in the two sections are consistent, thereby realizing DC loop closure.

[0016] Preferably, in step 3, adjusting the DC bus voltage using the constant DC voltage control mode of the parallel coupling unit is characterized in that: the DC side voltage of the parallel device is detected and compared with a given value of the DC voltage; and the obtained difference value is subjected to current inner loop control by a PI controller to output the parallel converter PWM voltage control signal, wherein the controller can use PI control or other types of controllers.

[0017] Preferably, in step 3, flexible power mutual support in the supply areas per transformer in the two-section closed loop connection is realized by dynamically adjusting the amplitude and phase of the additional voltage of the series-coupled transformer, which is achieved by adjusting the output voltage of the series-coupled transformer in the supply area per transformer that needs to be powered, 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 transmission lines.

[0018] 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 transform using the phase of the collected AC bus voltage to obtain U d , U q and obtaining an output current reference value by comparing the difference value obtained through a PI controller (the controller may be a PI controller or another type of controller); (3) outputting a voltage control signal to the PWM series-side converter through a current inner-loop control based on the obtained current reference value to control the amplitude and phase angle of the additional voltage ΔV of the series-coupled transformer; The present invention is characterized by comprising:

[0019] 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 current of the AC system connected to the inverter of the parallel coupling device, converting it into a current command after dq transformation, and outputting a PWM voltage control signal through a PI controller based on the difference with the given value, so that the parallel converter outputs the reactive power required by the system and realizes reactive compensation (the controller can be a PI controller or other types of controller); The present invention is characterized by comprising: Effect of the Invention

[0020] The present invention has the following advantages:

[0021] (1) The present invention provides a flexible DC loop closing device and a control method thereof, which can realize loop closure on a line, avoid reverse load operation after a power outage, and improve the reliability of power supply.

[0022] (2) The flexible DC loop closure device provided by the present invention has a simple structure, a low design capacity of the converter in the series-parallel device, and a lower cost than the traditional back-to-back loop closure device.

[0023] (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 for DC loads.

[0024] (4) The present invention receives loop closing commands in real time by monitoring the real-time power of the supply area per transformer in loop closing, and realizes flexible loop closing or loop closing termination by controlling the amplitude of the DC bus voltage of the two sections of the line, thereby ensuring high efficiency and stability of loop closing operation.

[0025] (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 fast response speed and high control accuracy. Compared to conventional closed-loop operation methods that use bus tie switches, this results in less mechanical wear and a longer equipment life.

[0026] (6) The present invention has a high reliability because, if one device fails, the bypass switch can be used to switch to a bypass state through the DC loop closure of the two devices, without affecting the operation of the original power supply line.

[0027] (7) The present invention uses a DC loop closing method, and can satisfy the loop closing conditions by simply considering that the amplitude of the DC side voltage is equal. In contrast, the AC loop closing 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 closing is higher. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a block diagram of an embodiment of an apparatus for realizing flexible DC loop closure in a power grid according to the present invention. [Diagram 2] FIG. 1 is a block diagram showing the introduction of the flexible DC loop closure device of the present invention into a DC power distribution network. [Diagram 3] FIG. 1 is a block diagram of an embodiment of an improved apparatus for achieving flexible DC loop closure in a power grid according to the present invention. [Figure 4] 2 is a control flow chart for implementing flexible DC loop closing control of a power grid according to the present invention; [Diagram 5] FIG. 2 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. 2 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. 2 is a diagram showing vectors of a voltage outer loop control strategy for implementing the DC voltage control of the present invention. [Figure 8]FIG. 2 is a diagram showing vectors of a current inner loop control strategy for implementing the 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 closing control of a power grid; [Figure 10] FIG. 2 is a block diagram of a vector control strategy for implementing flexible loop closure control of the power grid according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The technical means of the present invention will be described in more detail below with reference to the drawings.

[0030] Step 1: As shown in FIG. 1, a system model of a flexible DC loop closing device of a power grid is constructed. The device of this embodiment includes two sets of series-parallel devices and one set of introduction switches. Each set of series-parallel devices is connected in series between the distribution line and the load of each section, and DC loop closing is realized by connecting the DC ports of the flexible loop closing devices of the supply area per transformer of the loop closing. Each set of series-parallel devices includes two AC output ports and one DC output port. The inside is composed of a series coupling unit, a parallel coupling unit, a bypass switch and a DC loop closing main controller. The two AC ports of the three-port series coupling unit are connected in series to the line, 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 distribution 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 also connected in parallel to the two AC ports of the series coupling unit. The system model is shown in FIG. 2. Advantageously, in order to meet the economical construction needs, the system model of the DC loop closing 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, the series coupling device only reserves the series coupling module part of the series-parallel device to reduce the manufacturing cost, 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 that needs loop closure, and the DC bus of the series-parallel devices and the DC port of the series coupling device are connected to realize the DC loop closure. The model is shown in Figure 3.

[0031] Step 1.1: A model of the series-coupled unit is constructed, and a specific embodiment is shown in FIG. 5. The series-coupled unit has a total of three ports, including 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, and the DC end of the inverter constitutes 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 topology of the inverter can be a two-level or cascaded multilevel structure. The series inverter can quickly control the switching operation through PWM to realize voltage stabilization regulation and harmonic suppression, and isolate DC bus voltage disturbance.

[0032] Step 1.2: A model of the parallel coupling unit is constructed, and a specific embodiment is shown in FIG. 6. The parallel coupling 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 coupling unit, and the DC output port of the inverter is the DC port of the parallel coupling unit. The inverter is composed of a three-phase full-bridge arm power unit with bidirectional power flow, and the topology of the inverter can be a two-level or cascade multilevel structure. The parallel inverter can realize functions such as reactive compensation, harmonic suppression, and stabilization of DC bus voltage by controlling the power electronics device in real time.

[0033] Step 2: The control logic of the series-parallel combination 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:

[0034] Step 2.1: The power of each of the two sections of transmission lines that need to be closed is monitored over a long period of time, and the system prepares for the loop closure 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.

[0035] Step 2.2: After receiving the loop closure command, it is judged whether the DC bus voltages of the two sets of loop closure devices are consistent; if they are not consistent, the constant DC voltage control mode of the parallel coupling unit is used to adjust the DC bus voltage; after the consistency is confirmed, the DC introduction switch K is closed, so that the system enters the loop closure state; after the loop closure, the amplitude and phase of the additional voltage of the series coupling transformer are dynamically adjusted, thereby realizing flexible power mutual support in the supply area per loop-closed transformer.

[0036] 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 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.

[0037] 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 by detecting the DC side voltage of the parallel device and comparing it with the given value of the DC voltage, and 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, and the controller can use PI control or other types of controllers.

[0038] 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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[0039]

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[0040] 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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[0041] 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 sections of the busbars can be adjusted, thereby realizing the requirement of flexible loop closure control of the power grid.

[0042] In step 2.2, the control strategy block diagram of the flexible DC loop closing control is shown in Fig. 10. The power of the supply area per transformer of the two-section loop closing is measured and calculated, and the given value of the power is obtained, and the difference value with the real-time measured power is calculated by PID control. ΔP is obtained, and a given value of a series compensation voltage ΔV proportional to ΔP is calculated. The sum of the 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 the current inner loop, and the output value is the voltage control signal of the series side inverter.

[0043] Both series-parallel combined units can select a vector control strategy including control modules such as a phase-locked loop (PLL) for detecting the AC bus voltage phase, a PI controller (other controllers can be used depending on the required control effect), a coordinate conversion module (abc / dq converter and dq / abc converter), and a comparator. By using vector control, multiple electrical quantities can be controlled to reach given values, such as DC bus voltage, grid side power, grid side voltage d-axis and q-axis components, and load side current d-axis and q-axis components.

[0044] 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 problem of active power flow control, specifically including the following steps: (1) Bus voltage phase θ of a power supply connected to a 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 transform using the phase of the collected AC bus voltage to obtain U d , U q and obtaining an output current reference value by comparing the difference value obtained through a PI controller (the controller may be a PI controller or another type of controller); (3) The obtained current reference value is used to output a voltage control signal to the PWM series-side converter through 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.

[0045] 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) AC bus voltage phase θ connected to a parallel device via a phase-locked loop (PLL) s and collecting (2) Collecting reactive current of the AC system connected to the inverter of the parallel coupling device, converting it into a current command after dq transformation, and outputting a PWM voltage control signal through a PI controller based on the difference with the given value, so that the parallel converter outputs the reactive power required by the system and realizes reactive compensation (the controller can be a PI controller or other types of controller); The present invention is characterized by comprising:

[0046] 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. For those skilled in the art, some equivalent replacements or obvious modifications can be made without departing from the concept of the present invention, but the same performance or application 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 with 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 two AC ports of the three-port series coupling unit are connected in series to a line, 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 combination unit, and the loop closure controller is used for logic control of loop closure and receiving remote control. 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 bidirectionally regulating inverter, the primary side of the series coupling transformer is connected in series with the AC line and the secondary side is connected with the AC side of the inverter; and the parallel coupling unit is composed of a bidirectionally regulating inverter, the AC side of the inverter is connected in parallel with the AC grid line and the DC side is 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 power grid according to claim 1 or 2, characterized in that the DC loop closing method comprises: connecting flexible loop closure devices in series between a line in a supply area per transformer requiring loop closure and a load; connecting DC ports of the flexible loop closure devices in the supply areas per transformer for loop closure; and transmitting active power flows between the supply areas per transformer for loop closure through the DC ports.

4. 4. The flexible DC loop closure control device for power grid according to claim 3, characterized in that, in order to simplify the topology structure of the series-parallel loop closure device and further reduce the manufacturing cost, the flexible loop closure devices are connected in series between the lines and loads requiring a supply area per transformer for loop closure, and only the series coupling unit is reserved in the flexible loop closure device for the 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 the supply area per transformer for loop closure in the other section, thereby enabling flexible loop closure between the supply areas per transformer in two sections to be realized in the same way.

5. The flexible DC loop-closed control device for power grid as claimed in 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, which is connected to the DC bus of the device; the charging and discharging ability 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 ability of new energy, realize the continuous co-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 have a need for loop closure over a long period of time, and preparing for loop closure in real time; Step 2, in which the bus tie switches and the introduction switches before the loop closure are all in an open state and both of the two sets of loop closure control devices operate in a voltage control mode; Step 3: after receiving the loop closing command, judge whether the DC bus voltages of the two sets of loop closing devices are consistent; if they are not consistent, use the constant DC voltage control mode of the parallel coupling unit to adjust the DC bus voltage; after the consistency is confirmed, 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 to realize flexible power mutual support of the supply area per transformer of the two sections of loop closing; Step 4: before the loop closing is completed, the two sets of flexible loop closing control devices are set to operate in a PQ control mode, and the two sets of flexible loop closing control devices respectively reduce P and Q to zero, and then turn off the introduction switch K, thus completing the flexible DC loop closing process; A method comprising:

7. In the step 1, constructing a system model of the flexible DC loop closing device of the power grid includes the following steps: the device of the method includes two sets of series-parallel devices and one set of introduction switches, each set of the series-parallel devices is connected in series between the distribution line and the load of each section, and 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 includes a three-port series coupling unit, a two-port parallel coupling unit, a bypass switch and a DC loop closure main controller, two AC ports of the three-port series coupling unit are connected in series to the line, 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 with the two AC ports of the series combination 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; at the same time, the DC bus of the device can be connected to a DC power distribution 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 distribution grid.

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 as follows: the system model comprises a set of series-parallel devices, a set of series coupling devices, and a set of introduction switches; the series coupling device only reserves a series coupling module part of the series-parallel devices to reduce manufacturing costs; similarly, the series-parallel devices and the series coupling devices are respectively connected in series between the distribution lines and the loads of each section that needs loop closure; and the DC bus of the series-parallel devices and the DC port of the series coupling device are connected to realize the DC loop closure.

10. The method according to any one of claims 6 to 9, characterized in that in step 2, the two sets of loop closure control devices both operate in voltage control mode, which is realized by two sets of the devices, and constant DC voltage control is performed through the 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. The method according to any one of claims 6 to 9, characterized in that in step 3, regulating 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 outputting the parallel converter PWM voltage control signal through a current inner loop control by a PI controller, wherein the controller can use PI control or can use other types of controller.

12. 12. The method according to claim 11, 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-section closed loop connection, comprises adjusting the output voltage of the series-coupled transformer in the supply area per transformer that needs to be powered, 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 lines.

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) Collect the AC voltage U on the power supply side of the inverter terminal on the series side, and perform a dq transformation using the phase of the collected AC bus voltage to obtain U ｄ , U ｑ and obtaining an output current reference value by comparing the difference value obtained through a PI controller (the controller may be a PI controller or another type of controller); (3) outputting a voltage control signal to the PWM series-side converter through a current inner-loop control based on 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 current of the AC system connected to the inverter of the parallel coupling device, converting it into a current command after dq transformation, and outputting a PWM voltage control signal through a PI controller based on the difference with the given value, so that the parallel converter outputs the reactive power required by the system 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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