Network construction type multi-terminal flexible interconnection equipment and non-power-cut load transfer method based on network construction type multi-terminal flexible interconnection equipment

Abstract

The invention belongs to the field of power systems, and particularly discloses a network construction type multi-terminal flexible interconnection device and a non-power-outage load transfer method based on the network construction type multi-terminal flexible interconnection device, and the method comprises the steps: judging whether the voltage fluctuation of a DC bus is smaller than a set threshold value or not when a feeder line connected with the network construction type multi-terminal flexible interconnection device is subjected to non-planned power outage, and if yes, executing the step 2; if yes, adopting a VSC converter and an energy storage unit in grid-connected operation to spontaneously respond to the change of the voltage of the direct-current bus, and providing power support for the power-losing feeder line; otherwise, based on the DC bus voltage and the rated value of the DC bus voltage, calculating the unbalanced power of the networking type multi-terminal flexible interconnection equipment; power instructions are distributed to the remaining VSC converters or the bidirectional DC / DC converters of the VSC converters except the VSC converters connected with the power-losing feeder lines through the coordination control terminal, and power supporting is carried out on the power-losing feeder lines. According to the invention, stable voltage and frequency can be spontaneously provided for the load on the power-losing feeder line, and load transfer without power failure is realized.

Description

Technical Field

[0001] The present application belongs to the field of power systems, and more specifically, to a network-building multi-terminal flexible interconnection equipment and a non-stop load transfer method based thereon. Background Art

[0002] As modern society becomes increasingly dependent on electricity, the impact of power shortages and feeder power outages on social production and life is becoming increasingly serious. Flexible interconnection devices can connect multiple feeders to provide backup power supply, and are an effective means to achieve load transfer in distribution networks. However, existing flexible interconnection devices still face two difficulties when applied to load transfer: (1) Most existing flexible interconnection devices use grid-following control and do not have the ability to independently build a microgrid. When an unplanned power outage occurs in the connected feeder, it is necessary to switch to grid-building control to complete the load transfer. However, the control switching requires instructions from the distribution dispatcher, which will inevitably cause a short-term power outage in the distribution network, making it impossible to achieve uninterrupted load transfer. (2) During normal operation, one end of the flexible interconnection device acts as the main control unit and operates in constant DC voltage control ( U dc Q The DC voltage of the equipment is maintained stable by the use of flexible interconnection devices (control). When the feeder connected to the main control unit loses power unexpectedly, the entire distribution network will become unstable or even collapse. Therefore, the traditional flexible interconnection device can only realize the load transfer of specific feeders, which has strong limitations in actual operation. Summary of the invention

[0003] In view of the defects of the prior art, the purpose of this application is to provide a meshed multi-terminal flexible interconnection equipment and a method for non-stop load transfer based thereon, aiming to solve the problem that the existing flexible interconnection devices cannot achieve non-stop load transfer when applied to load transfer.

[0004] To achieve the above-mentioned purpose, in the first aspect, the present application provides a non-stop load transfer system of a meshed multi-terminal flexible interconnected equipment, including a VSC converter, an energy storage unit, a bidirectional DC / DC converter, an electrical sensor and a coordinated control terminal; The VSC converter and the bidirectional DC / DC converter are cascaded on the same DC bus; each VSC converter provides three low-voltage AC ports; each VSC converter and bidirectional DC / DC converter output end is connected to an electrical sensor, and the energy storage unit is connected to the DC bus through the bidirectional DC / DC converter; each electrical sensor is connected to the coordination control terminal; The energy storage unit and the VSC converter are both used to spontaneously respond to changes in the DC bus voltage when the DC bus voltage fluctuation is less than a set threshold; and respond to power commands issued by the coordination control terminal when the DC bus voltage fluctuation is not less than a preset threshold; The VSC converter is a grid-type control, which is used to make the VSC converter grid-connected and when the feeder connected to the VSC converter is unplanned power outage, the VSC converter has voltage source characteristics; Electrical sensors are used to collect operating information of VSC converters and bidirectional DC / DC converters; The coordinated control terminal is used to collect operating information sent by electrical sensors. When the DC bus voltage fluctuation is not less than the preset threshold, it spontaneously coordinates the operating status of the VSC converter and the bidirectional DC / DC converter, issues power instructions, and realizes uninterrupted load transfer based on the grid-based control of the VSC converter.

[0005] Further preferably, the active power control equation in the grid-type control of the VSC converter is:

[0006] in, P ref It is the power command value sent by the coordinated control terminal to the VSC converter; K 1 is the primary voltage regulation control coefficient; J is the fictitious inertia coefficient; D 1 is the damping coefficient; ω is the angular frequency of the VSC converter output voltage; ω n is the rated angular frequency; δ is the output VSC converter modulation voltage phase; is the DC bus voltage; is the rated value of the DC bus voltage; is the actual value of the active power of the VSC converter; t For time; The reactive power control equation in the grid-type control of the VSC converter is:

[0007] in, K 2 is the integral coefficient of reactive power control; Q ref It is the reactive power instruction issued by the coordinated control terminal; Q vsc is the reactive power output by the VSC converter; D 2 is the voltage deviation coefficient; U n and U m They are the rated value and actual value of the VSC converter output voltage respectively; is the Laplace operator; The three-phase voltage reference value of the VSC converter is: .

[0008] Further preferably, the output power reference value of the energy storage unit is:

[0009] in, P * baref Output power reference value of energy storage unit; P baref It is the power command value issued by the coordination control terminal to the bidirectional DC / DC converter; K dc is the DC bus voltage control coefficient.

[0010] Further preferably, the bidirectional DC / DC converter adopts power outer loop and current inner loop control, and the control equation is:

[0011]

[0012] in, P ba Output power to the energy storage battery; k p1 and k i1 are the proportional coefficient and integral coefficient of the power outer loop; I Lref It is the inner loop current reference value output by the power outer loop; I L is the actual value of the current flowing through the inductor of the DC / DC converter; d is the duty cycle of the DC / DC converter; k p2 and k i2 are the proportional coefficient and integral coefficient of the inner current loop.

[0013] Further preferably, three VSC converters are provided.

[0014] In the second aspect, based on the above-mentioned meshed multi-terminal flexible interconnection equipment, the present application provides a corresponding non-stop load transfer method, which specifically includes the following steps: Step 1: When an unplanned power outage occurs in the feeder connected to the grid-type multi-terminal flexible interconnection equipment, determine whether the DC bus voltage fluctuation is less than the set threshold. If so, the grid-connected VSC converter and energy storage unit are used to spontaneously respond to the change in DC bus voltage to provide power support for the power-lost feeder; otherwise, go to step 2; Step 2: Based on the DC bus voltage and the rated value of the DC bus voltage, calculate the unbalanced power of the meshing multi-terminal flexible interconnection equipment; Step 3: Based on the unbalanced power of the meshed multi-terminal flexible interconnection equipment, a coordinated control terminal is used to distribute power instructions to the remaining VSC converters or bidirectional DC / DC converters except the VSC converter connected to the power-lost feeder to provide power support for the power-lost feeder.

[0015] Further preferably, the unbalanced power of the networking type multi-terminal flexible interconnection equipment in step 3 is:

[0016] in, P ub_ref Unbalanced power reference value calculated for the coordinated control terminal; k p3 and k i3 It is the proportional coefficient and integral coefficient of the unbalanced power calculation link; is the DC bus voltage; is the rated value of the DC bus voltage.

[0017] Further preferably, when there are three VSC converters, step 4 is specifically as follows: If an unplanned power outage of the first feeder is detected, the second feeder and the third feeder jointly provide power support to the first feeder under the power instruction of the coordination control terminal, where the power instruction is:

[0018] in, P ub_ref Unbalanced power reference value calculated for the coordinated control terminal; P ref2 and P ref3 are power command values ​​issued by the coordination control terminal to the second VSC converter and the third VSC converter respectively; m 2 and m 3 is an unbalanced power distribution coefficient, which is proportional to the port capacities of the second VSC converter and the third VSC converter; wherein the second VSC converter is connected to the second feeder, and the third VSC converter is connected to the third feeder; If an unplanned outage is detected on two feeders, the only feeder that is operating normally is used. j As a power source, it supports the power of other power-lost feeders; among them, the coordination control terminal sends power to the grid-connected operation terminal. j The power command issued by each VSC converter is:

[0019] in, P refj It is the coordination control terminal to the jThe power command value issued by the VSC converter; j A VSC converter is connected to the j on feeder lines; If all feeders are detected to have an unplanned power outage, the energy storage unit is used as a power source to provide power support for the power-lost feeders; the power command sent by the coordination control terminal to the DC / DC converter P baref for: .

[0020] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the prior art: Traditional flexible interconnection devices usually adopt a grid-following control method. When the connected feeder has an unplanned power outage, it is necessary to switch the converter on the power outage side to a grid-building control under the instruction of the power distribution dispatcher, which inevitably brings about the problem of short-term power outages. All AC ports of the grid-building multi-terminal flexible interconnection equipment proposed in this application adopt grid-building control. When any connected feeder loses power, there is no need to switch the control method, and a microgrid can be seamlessly established on the power-lost side; compared with the transfer method of traditional devices, it avoids waiting for the distribution dispatcher and the control switching process, and can spontaneously provide stable voltage and frequency for the load on the power-lost feeder, realizing non-stop load transfer.

[0021] Traditional flexible interconnection devices can only realize load transfer of specific feeders. When the feeder connected to the main control unit side or all connected feeders suffer an unplanned power outage, the entire equipment will become unstable or even collapse, and stable operation and load transfer of the equipment cannot be achieved. The load transfer method proposed in this application can coordinate the output power of multiple converters through a coordinated control terminal to handle load transfer under any feeder power outage condition. Even when all feeders suffer an unplanned power outage, the feeder load can still be transferred without power outage, thereby improving the practicability of the equipment and the power supply reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural diagram of a networking type multi-terminal flexible interconnection equipment provided in an embodiment of the present application; Figure 2 is a control structure diagram of a VSC converter provided in an embodiment of the present application; Figure 3 is a control structure diagram of a DC / DC converter provided in an embodiment of the present application; Figure 4 It is a flow chart of a method for transferring power from a load to a grid-type multi-terminal energy conversion device provided in an embodiment of the present application; Figure 5 The voltage waveform output by the equipment before and after the unplanned power failure of all connected feeders using the method of the present application provided in the embodiment of the present application; Figure 6 The embodiment of the present application provides that the power output of the equipment fluctuates before and after the unplanned power failure of all connected feeders using the method of the present application. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0024] The term "and / or" in this article is a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. The symbol " / " in this article indicates that the associated objects are in an or relationship, for example, A / B means A or B.

[0025] The terms “first”, “second” and the like in the specification and claims herein are used to distinguish different objects rather than to describe a specific order of the objects.

[0026] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0027] In the description of the embodiments of the present application, unless otherwise specified, “plurality” means two or more than two.

[0028] The present application provides a method for non-stop power supply transfer based on a networking type multi-terminal flexible interconnection device, which specifically includes the following steps: Step 1: Build a network-based multi-terminal flexible interconnection equipment The overall structure of the networking type multi-terminal flexible interconnection equipment is as follows: Figure 1 As shown, it consists of a main circuit and a coordinated control terminal; the main circuit includes three VSC converters, a storage unit, a bidirectional DC / DC converter, a DC voltage-stabilizing capacitor and several electrical sensors; four converters (three VSC converters and one bidirectional DC / DC converter) are cascaded on the same 750V DC bus; each VSC converter provides three low-voltage AC ports, which can realize flexible interconnection between three feeders; the electrical sensor is responsible for collecting the operating information of the network-type multi-terminal flexible interconnection equipment, including the DC voltage of the equipment, the output voltage and output current of multiple VSC converters, etc., and transmits it to the coordinated control terminal of the equipment; The coordination control terminal is deployed inside the meshed multi-terminal flexible interconnected equipment and is the upper-level control of multiple converters. On the one hand, it can receive instructions from the grid dispatching, so that the meshed multi-terminal flexible interconnected equipment responds to the dispatching and management of the grid. On the other hand, by collecting the operating information sent by the electrical sensors, it can spontaneously coordinate the operating status of multiple converters and realize the uninterrupted load transfer of the equipment. Step 2: Introduce the overall equipment control method: VSC converter grid control method VSC converter grid control method Figure 2 As shown in the figure, on the one hand, the VSC converter can realize grid-connected operation; on the other hand, when the feeder connected to the VSC converter has an unplanned power outage, the VSC converter has the characteristics of a voltage source and can spontaneously establish a local microgrid without controlling the switching process to supply power to important loads on the feeder. The active power control equation of the VSC converter is shown in formula (1): (1) in, P ref It is the power command value sent by the coordinated control terminal to the VSC converter; K 1 is the primary voltage regulation control coefficient; J is the fictitious inertia coefficient; D 1 is the damping coefficient; ω is the angular frequency of the VSC converter output voltage; ω n is the rated angular frequency; δ is the output VSC converter modulation voltage phase; is the DC bus voltage; is the rated value of the DC bus voltage; is the actual value of the active power of the VSC converter; t For time; The reactive power control of VSC converter is achieved by changing the output potential of VSC converter. U E The reactive power is adjusted by the size of the reactive power, and the control equation is shown in (2): (2) in, K 2 is the integral coefficient of reactive power control; Q ref It is the reactive power instruction issued by the coordinated control terminal; Q vsc is the reactive power output by the VSC converter; D 2 is the voltage deviation coefficient; U n and Um They are the rated value and actual value of the VSC converter output voltage respectively; is the Laplace operator; Further preferably, according to the modulation voltage phase δ and internal potential amplitude U E , the three-phase voltage reference value of the VSC converter can be obtained u aref 、u bref 、u cref : (3) DC / DC converter control: In this application, the energy storage unit is an energy storage battery, which is connected to the DC bus of the equipment through a DC / DC converter. The overall control block diagram is as follows: Figure 3 As shown, the output power reference value of the energy storage battery P * baref As shown in formula (4), on the one hand, it can respond locally to the power command issued by the coordinated control terminal, and on the other hand, it can spontaneously respond to the change of the DC bus voltage; (4) in, P * baref It is the reference value of the output power of the energy storage battery; P baref It is the power command value issued by the coordination control terminal to the bidirectional DC / DC converter; K dc is the DC voltage control coefficient; The bidirectional DC / DC converter adopts power outer loop and current inner loop control: (5) (6) in, P ba Output power to the energy storage battery; k p1 and k i1 are the proportional coefficient and integral coefficient of the power outer loop; I Lref It is the reference value of the inner loop current output by the power outer loop; I L is the actual value of the current flowing through the inductor of the DC / DC converter; d is the duty cycle of the DC / DC converter; k p2 and ki2 are the proportional coefficient and integral coefficient of the inner current loop; Step 3: Feeder load transfer method based on coordinated control terminal: When an unplanned power outage occurs on the feeder to which the equipment is connected, the power at the equipment DC bus is no longer balanced, further causing fluctuations in the equipment DC voltage. The coordination control terminal divides the load transfer methods of the equipment into the following two categories based on the collected equipment DC bus voltage: (a)|U dc0 -U dc |<U0 When the DC voltage fluctuation is less than the set threshold U When the feeder power failure is 0, the unbalanced power caused by the feeder power failure is small. The VSC converter and energy storage unit in grid operation can provide power support for the power failure feeder by spontaneously responding to the change of DC voltage and changing its output power. Specifically, when the DC voltage of the equipment drops due to the unplanned power failure of the first feeder, the VSC converter will be negative ( U dc - U dc0 ) and reduce its output power, the DC / DC converter will be based on the positive ( U dc0 - U dc ) Increase its output power to maintain the DC power balance of the equipment and supply power to the important loads of the power-lost feeder; (b) |U dc0 -U dc |≥U0 When the DC voltage fluctuation exceeds the set threshold U 0, the unbalanced power caused by the loss of feeder power is large, which may cause the DC voltage of the equipment to become unstable; therefore, the coordinated control terminal needs to collect the DC bus voltage of the equipment. U dc Calculate the unbalanced power inside the equipment: (7) in, P ub_ref Unbalanced power reference value calculated for the coordinated control terminal; k p3 and k i3 It is the proportional coefficient and integral coefficient of the unbalanced power calculation link; Coordinate and control multiple converters to jointly bear the above unbalanced power and provide power support for the power-lost feeder; Unplanned power outage on one feeder: When an unplanned power outage is detected on only one connected feeder, for example, the first feeder, the second and third feeders jointly supply power to the important loads of the first feeder under the power command of the coordinated control terminal; the power will be distributed according to the static capacity of the second VSC converter and the third VSC converter, and the power command satisfies: (8) in, P ref2 and P ref3 are power command values ​​issued by the coordination control terminal to the second VSC converter and the third VSC converter respectively; m 2 and m 3 is the unbalanced power distribution coefficient, which is proportional to the port capacity of the second VSC converter and the third VSC converter; Two feeder unplanned outage: When an unplanned outage of two connected feeders is detected, the only feeder operating normally j It will serve as a power source and provide power support for all power-off feeders. At this time, the coordination control terminal sends a signal to the grid-connected VSC. j The power command issued is: (9) in, P refj It is the coordinated control terminal to VSC j The power command value issued by the converter; All connected feeders have unplanned power outages. When all feeders have unplanned power outages due to a fault in the upper power grid, the energy storage battery becomes the only power source; the power command issued by the coordination control terminal to the DC / DC converter P baref for: (10) In summary, compared with the prior art, the present application has the following advantages: Traditional flexible interconnection devices usually adopt a grid-following control method. When the connected feeder has an unplanned power outage, it is necessary to switch the converter on the power outage side to a grid-building control under the instruction of the power distribution dispatcher, which inevitably brings about the problem of short-term power outages. All AC ports of the grid-building multi-terminal flexible interconnection equipment proposed in this application adopt grid-building control. When any connected feeder loses power, there is no need to switch the control method, and a microgrid can be seamlessly established on the power-lost side; compared with the transfer method of traditional devices, it avoids waiting for the distribution dispatcher and the control switching process, and can spontaneously provide stable voltage and frequency for the load on the power-lost feeder, realizing non-stop load transfer.

[0029] Traditional flexible interconnection devices can only realize load transfer of specific feeders. When the feeder connected to the main control unit side or all connected feeders suffer an unplanned power outage, the entire equipment will become unstable or even collapse, and stable operation and load transfer of the equipment cannot be achieved. The load transfer method proposed in this application can coordinate the output power of multiple converters through a coordinated control terminal to handle load transfer under any feeder power outage condition. Even when all feeders suffer an unplanned power outage, the feeder load can still be transferred without power outage, thereby improving the practicability of the equipment and the power supply reliability of the system.

[0030] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0031] The present application discloses a method for non-stop load transfer based on a meshed multi-terminal flexible interconnection device, such as Figure 1 The above is a topological structure of a meshed multi-terminal energy conversion equipment; the equipment is connected to three 380V feeders through three VSC converters, and the energy storage battery is connected to the DC bus of the device through a DC / DC converter; the coordination control terminal of the equipment serves as the upper-level control of multiple converters, and can receive operating information collected by sensors and issue power instructions to each converter; multiple VSC converters all adopt meshed control, and when the connected feeders are unplanned outages, the VSC converter has voltage source characteristics and can spontaneously establish a local microgrid without controlling the switching process to supply power to important loads on the feeders.

[0032] like Figure 4 As shown, the present application provides a feeder power failure transfer method; first, monitor the operating status of the equipment, when the DC voltage fluctuation exceeds the set threshold U 0, the coordination control terminal calculates the unbalanced power reference value inside the equipment according to formula (7); then, the power failure of the equipment connecting feeder is judged: when one connecting feeder is unplanned, the coordination control terminal sends power instructions to the two VSC converters connected to the grid according to formula (8); when two connecting feeders are unplanned, the coordination control terminal sends power instructions to the two VSC converters connected to the grid according to formula (9); when all connecting feeders are unplanned, the coordination control terminal sends power instructions to the energy storage DC / DC converter according to formula (10); in the above process, all off-grid VSCs can spontaneously establish stable voltage and frequency under grid-type control, without the need to switch control strategies, and can realize load transfer in the power outage area without power outage; This application is in Figure 1 The proposed method was verified in the system shown in the figure. At 4 s, all feeders experienced an unplanned power outage. Figure 5 represents the voltage dynamics of the first VSC converter, the second VSC converter and the third VSC converter output at each port of the equipment; Figure 5It can be seen that the feeder voltage remains stable before and after the feeder power failure. Based on the grid-based control, stable frequency and voltage can be established after the feeder power failure without control switching, realizing non-stop load supply. Figure 6 represents the active power output by the first VSC converter, the second VSC converter and the third VSC converter of each port of the equipment; Figure 6 It can be seen that in the extreme case of power failure on all feeders, the VSC converter can quickly change its output power to provide power support to the important loads connected to the power-failed feeders.

[0033] It should be understood that the various numerical numbers involved in the embodiments of the present application are only used for the convenience of description and are not used to limit the scope of the embodiments of the present application.

[0034] It should be understood that expressions such as "including" and "may include" that may be used in the present application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In the present application, terms such as "including" and / or "having" may be interpreted as indicating specific characteristics, numbers, operations, constituent elements, components, or combinations thereof, but may not be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0035] In addition, in the present application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0036] In the description of the embodiments of the present application, it should be noted that, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium.

[0037] In addition, in the embodiments of the present application, the mathematical concepts mentioned are symmetry, equality, parallelism, verticality, etc. These limitations are all for the current state of the art, rather than being absolutely strict definitions in a mathematical sense, and allow a small amount of deviation, approximation to symmetry, approximation to equality, approximation to parallelism, approximation to verticality, etc. are all possible. For example, A and B are parallel, which means that A and B are parallel or approximately parallel, and the angle between A and B can be between 0 and 10 degrees. A and B are perpendicular, which means that A and B are perpendicular or approximately perpendicular, and the angle between A and B can be between 80 and 100 degrees.

[0038] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A networking type multi-terminal flexible interconnection equipment, characterized in that: It includes VSC converter, energy storage unit, bidirectional DC / DC converter, electrical sensor and coordinated control terminal; The VSC converter and the bidirectional DC / DC converter are cascaded on the same DC bus; each VSC converter provides three low-voltage AC ports, and each VSC converter and bidirectional DC / DC converter output terminal is connected to an electrical sensor, and the energy storage unit is connected to the DC bus via the bidirectional DC / DC converter; each electrical sensor is connected to the coordination control terminal; The energy storage unit and the VSC converter are both used to spontaneously respond to changes in the DC bus voltage when the DC bus voltage fluctuation is less than a set threshold; and respond to power commands issued by the coordination control terminal when the DC bus voltage fluctuation is not less than a preset threshold; The VSC converter is a grid-type control, which is used to make the VSC converter grid-connected and when the feeder connected to the VSC converter is unplanned power outage, the VSC converter has voltage source characteristics; Electrical sensors are used to collect operating information of VSC converters and bidirectional DC / DC converters; The coordinated control terminal is used to collect operating information sent by electrical sensors. When the DC bus voltage fluctuation is not less than the preset threshold, it spontaneously coordinates the operating status of the VSC converter and the bidirectional DC / DC converter, issues power instructions, and realizes uninterrupted load transfer based on the grid-based control of the VSC converter.

2. The networking type multi-terminal flexible interconnection equipment according to claim 1, characterized in that: The active power control equation in the grid-type control of the VSC converter is: in, P ref It is the power command value sent by the coordinated control terminal to the VSC converter; K 1 is the primary voltage regulation control coefficient; J is the fictitious inertia coefficient; D 1 is the damping coefficient; ω is the angular frequency of the VSC converter output voltage; ω n is the rated angular frequency; δ is the output VSC converter modulation voltage phase; is the DC bus voltage; is the rated value of the DC bus voltage; is the actual value of the active power of the VSC converter; t For time; The reactive power control equation in the grid-type control of the VSC converter is: in, K 2 is the integral coefficient of reactive power control; Q ref It is the reactive power instruction issued by the coordinated control terminal; Q vsc is the reactive power output by the VSC converter; D 2 is the voltage deviation coefficient; U n and U m They are the rated value and actual value of the VSC converter output voltage respectively; is the Laplace operator; The three-phase voltage reference value of the VSC converter is: 。 3. The networking type multi-terminal flexible interconnection equipment according to claim 1 or 2, characterized in that: The reference value of the output power of the energy storage unit is: in, P * baref The output power reference value of the energy storage unit; P baref It is the power command value issued by the coordination control terminal to the bidirectional DC / DC converter; K dc is the DC bus voltage control coefficient.

4. The networking type multi-terminal flexible interconnection equipment according to claim 3, characterized in that: The bidirectional DC / DC converter adopts power outer loop and current inner loop control, and the control equation is: in, P ba Output power to the energy storage battery; k p1 and k i1 are the proportional coefficient and integral coefficient of the power outer loop; I Lref It is the reference value of the inner loop current output by the power outer loop; I L is the actual value of the current flowing through the inductor of the DC / DC converter; d is the duty cycle of the DC / DC converter; k p2 and k i2 are the proportional coefficient and integral coefficient of the inner current loop.

5. The networking type multi-terminal flexible interconnection equipment according to claim 1, characterized in that: There are three VSC converters, and the energy storage unit is an energy storage battery.

6. A method for non-stop load transfer based on the networking type multi-terminal flexible interconnection equipment according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step 1: When an unplanned power outage occurs in the feeder connected to the grid-type multi-terminal flexible interconnection equipment, determine whether the DC bus voltage fluctuation is less than the set threshold. If so, the grid-connected VSC converter and energy storage unit are used to spontaneously respond to the change in DC bus voltage to provide power support for the power-lost feeder; otherwise, go to step 2; Step 2: Based on the DC bus voltage and the rated value of the DC bus voltage, calculate the unbalanced power of the meshing multi-terminal flexible interconnection equipment; Step 3: Based on the unbalanced power of the meshed multi-terminal flexible interconnection equipment, a coordinated control terminal is used to distribute power instructions to the remaining VSC converters or bidirectional DC / DC converters except the VSC converter connected to the power-lost feeder to provide power support for the power-lost feeder.

7. The method for non-stop load transfer according to claim 6, characterized in that: The unbalanced power of the meshed multi-terminal flexible interconnection equipment in step 3 is: in, P ub_ref Unbalanced power reference value calculated for the coordinated control terminal; k p3 and k i3 It is the proportional coefficient and integral coefficient of the unbalanced power calculation link; is the DC bus voltage; is the rated value of the DC bus voltage.

8. The method for non-stop load transfer according to claim 7, characterized in that: When there are three VSC converters, step 4 is as follows: If an unplanned power outage of the first feeder is detected, the second feeder and the third feeder jointly provide power support to the first feeder under the power instruction of the coordination control terminal, where the power instruction is: in, P ub_ref Unbalanced power reference value calculated for the coordinated control terminal; P ref2 and P ref3 are power command values ​​issued by the coordination control terminal to the second VSC converter and the third VSC converter respectively; m 2 and m 3 is an unbalanced power distribution coefficient, which is proportional to the port capacities of the second VSC converter and the third VSC converter; wherein the second VSC converter is connected to the second feeder, and the third VSC converter is connected to the third feeder; If an unplanned outage is detected on two feeders, the only feeder that is operating normally is used. j As a power source, it supports the power of other power-lost feeders; among them, the coordination control terminal sends power to the grid-connected operation terminal. j The power command issued by each VSC converter is: in, P refj It is the coordination control terminal to the j The power command value issued by the VSC converter; j A VSC converter is connected to the j on feeder lines; If all feeders are detected to have an unplanned power outage, the energy storage unit is used as a power source to provide power support for the power-lost feeders; the power command sent by the coordination control terminal to the DC / DC converter P baref for: 。

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