A control method for a multi-terminal flexible interconnection device with networking capability

Abstract

The present application discloses a control method for a multi-terminal flexible interconnection device with networking capability, wherein the multi-terminal flexible interconnection device comprises n VSCs, wherein the AC sides of the n VSCs are connected to a plurality of distribution network feeders, and the DC sides of the n VSCs are connected to the DC bus through a DC capacitor, and the control method comprises the following steps: S10, performing synchronous networking control of the DC bus voltage on any VSC1 among the n VSCs; S20, obtaining the DC bus voltage and the operating status of VSC1, and when VSC1 is connected to the grid, the remaining VSCi are controlled by fixed active power and fixed reactive power, i=2,…,n, to achieve flexible interconnection between distribution networks; when VSC1 is off-grid and the DC bus voltage drops to a set value, the VSC connected to the maximum power supply margin feeder is controlled by fixed DC bus voltage to stabilize the DC bus voltage. The present application can independently build a microgrid on the basis of achieving flexible interconnection.

Description

Technical Field

[0001] The present application belongs to the field of power systems, and more specifically, to a control method for a multi-terminal flexible interconnection device with networking capability. Background Art

[0002] In recent years, under the national "dual carbon" goal, distributed photovoltaic and energy storage have been connected to the distribution system on a large scale, becoming an important part of the new distribution system, making the distribution network increasingly show the characteristics of a weak power grid. In addition, the large-scale access of distributed energy has caused the distribution system to change from passive to active, and there has been greater uncertainty at both the load and power ends of the distribution network, and the difficulty of source-load power and energy balance has increased. At the same time, with the development of power electronics and its related control technology and information technology, scholars have proposed flexible interconnection devices represented by flexible multi-state switches (soft open point, SOP), energy routers, and intelligent power / information exchange base stations. Through the flexible interconnection between distribution networks, a channel for power mutual assistance has been opened, providing a new solution to the problems caused by the access of distributed energy.

[0003] Traditional flexible interconnection devices are mostly two-terminal flexible interconnection devices, that is, two back-to-back voltage source converters (VSCs) are used. During normal operation, fixed active power and fixed reactive power (PQ) and fixed DC bus voltage (U dc Q) Grid-following control, which realizes flexible interconnection between two regional distribution networks by controlling the output current of the control device. However, this type of grid-following control converter exhibits current source characteristics and can only passively track the frequency of the grid. It cannot independently build a microgrid (establish the voltage and frequency of the grid), and it is difficult to ensure the stable operation of important loads. Summary of the invention

[0004] In view of the defects of the prior art, the purpose of this application is to provide a control method for a multi-terminal flexible interconnection device with networking capability, which can independently build a microgrid on the basis of realizing flexible interconnection between distribution networks to ensure the stable operation of important loads.

[0005] To achieve the above object, the present application provides a control method for a multi-terminal flexible interconnection device with networking capability, wherein the multi-terminal flexible interconnection device includes n VSCs, n>2, the AC sides of the n VSCs are correspondingly connected to multiple feeders, and the DC sides of the n VSCs are connected to the DC bus through a DC capacitor. The control method includes the following steps:

[0006] Perform DC bus voltage synchronization network control on any VSC1 among n VSCs;

[0007] The DC bus voltage and the operating status of VSC1 are obtained. When VSC1 is connected to the grid, fixed active power and fixed reactive power control are adopted for the remaining VSCi, i=2,…,n, so as to realize the tracking of the power command by the output power of VSCi, thereby realizing flexible interconnection between distribution networks; when VSC1 is off-grid and the DC bus voltage drops to the set value, fixed DC bus voltage control is adopted for the VSC connected to the feeder with the maximum power supply margin, so as to stabilize the DC bus voltage;

[0008] Among them, the VSC1 performs DC bus voltage synchronous grid control, which is used to adjust the active power output of VSC1 according to the change of DC bus voltage when VSC1 is connected to the grid, so as to achieve power balance of the device; when VSC1 is off-grid, the phase and amplitude of VSC1 modulation voltage are calculated according to the change of DC bus voltage, so as to establish a stable frequency and voltage, and form a local microgrid to ensure the power supply of important loads in the area.

[0009] The present application provides a control method for a multi-terminal flexible interconnected device with networking capability. When VSC1 is connected to the grid and operates, it can automatically adjust the output active power in response to changes in the DC bus voltage of the device, thereby achieving power balance and stable operation of the device. When VSC1 is off-grid, the DC bus voltage synchronous networking control can calculate the phase and amplitude signals of the converter modulation voltage according to the changes in the DC bus voltage, and then obtain the pulse signal through the voltage and current dual-loop control and PWM generator, thereby establishing a stable frequency and voltage. Without following the external power grid, a local microgrid can be formed to ensure the power supply of important loads in the area.

[0010] As further preferred, the DC bus voltage synchronous networking control comprises the following steps:

[0011] (1) Collecting DC bus voltage U dc , the DC bus voltage U dc After PI control, the phase δ of the VSC1 modulation voltage is obtained;

[0012] (2) The terminal voltage E of VSC1 output is obtained through reactive power control, and then the terminal voltage E is transformed by abc / dq to obtain the dq axis component reference value u of the VSC1 output voltage tdref and u tqref Then, the dq axis component u of the VSC1 modulation voltage is obtained through the voltage and current dual loop control. sd and u sq ; Among them, the d-axis component reference value u of the VSC1 output voltage tdref Equal to the terminal voltage E, the q-axis component reference value u of the VSC1 output voltage tqref is equal to zero;

[0013] (3) According to the phase δ, the dq axis component u of the VSC1 modulation voltage is sd and u sq The PWM modulation wave is obtained through abc / dq conversion, and then the PWM modulation wave is input into the PWM generator to generate a pulse signal to control the on and off of the power switch tube in VSC1.

[0014] As a further preferred embodiment, in step (1), the calculation formula of the phase δ of the VSC1 modulation voltage is:

[0015]

[0016] Where U dcref is the reference value of DC bus voltage; U dc0 is the steady-state value of the DC bus voltage; ω 0 is the rated angular frequency of VSC1; K D is the damping coefficient; s is the complex variable in the Laplace transform.

[0017] As further preferred, in step (2), the mathematical model of reactive power control is:

[0018]

[0019] In the formula, Q e is the reactive power output by VSC1; Q eref K is the reactive power reference value output by VSC1; Q is the integral coefficient in reactive power control; E init is the initial value of the internal potential of VSC1; s is the complex variable in the Laplace transform.

[0020] As further preferred, in step (2), the voltage-current dual-loop control includes voltage outer-loop control and current inner-loop control;

[0021] The mathematical model of the voltage outer loop control in the voltage and current dual loop control is:

[0022]

[0023] The mathematical model of the current inner loop control in the voltage and current dual loop control is:

[0024]

[0025] In the formula, i Ldref and i Lqref It represents the reference value of the dq component of the output current of VSC1, which is the output result of the voltage outer loop control; u td and u tq Represents the dq axis component of the VSC1 grid connection point voltage; Kp1 and K i1 They represent the proportional coefficient and integral coefficient of PI control in the voltage outer loop control respectively; i Ld and i Lq Represents the dq axis component of the VSC1 output current; K p2 and K i2 They represent the proportional coefficient and integral coefficient of PI control in the current inner loop control of the voltage and current dual loop control respectively; L c represents the equivalent reactance of VSC1; ω represents the angular frequency of the output voltage of VSC1; s is the complex variable in Laplace transform.

[0026] As a further preferred embodiment, the active power P output by the VSC1 is e The calculation formula is:

[0027]

[0028] Where, E represents the terminal voltage output by VSC1; U g Indicates the voltage of the distribution network connected to VSC1; X c represents the equivalent impedance of VSC1; δ represents the phase of the modulation voltage of VSC1.

[0029] As a further preferred embodiment, the fixed active power and fixed reactive power control and the fixed DC bus voltage control all adopt a dual-loop structure, and the fixed active power and fixed reactive power control and the fixed DC bus voltage control adopt the same current inner loop control.

[0030] As a further preferred embodiment, the mathematical model of the outer loop control in the fixed active power and fixed reactive power control is:

[0031]

[0032] In the formula, i' Ldref and i' qref represents the dq component reference value of the VSCi output current input by the current inner loop control, which is the output result of the outer loop control in the fixed active power and fixed reactive power control; K p3 and K i3 Respectively represent the proportional coefficient and integral coefficient of the PI control in the outer loop control in the fixed active power and fixed reactive power control; P ref Indicates the active reference power output by VSCi; Q ref represents the reactive reference power output by VSCi; P represents the active power output by VSCi; Q represents the reactive power output by VSCi; s is a complex variable in Laplace transform.

[0033] As a further preferred embodiment, the mathematical model of the outer loop control in the constant DC bus voltage control is:

[0034]

[0035] In the formula, i' Ldref and i' qref K represents the dq component reference value of the VSCi output current input by the current inner loop control, which is the output result of the outer loop control in the fixed DC bus voltage control; p4 and K i4 They respectively represent the proportional coefficient and integral coefficient of the PI control in the outer loop control in the constant DC bus voltage control; Q ref represents the reactive reference power output by VSCi; Q represents the reactive power output by VSCi; U dcref is the reference value of DC bus voltage; U dc is the DC bus voltage; s is the complex variable in Laplace transform.

[0036] As a further preferred embodiment, the mathematical model of the current inner loop control in the fixed active power and fixed reactive power control and the fixed DC bus voltage control is:

[0037]

[0038] In the formula, u' sd and u' sq Respectively represent the dq axis components of the VSCi modulation voltage; u' td and u' tq Represents the dq axis component of the VSCi grid-connected point voltage; L' c represents the equivalent reactance of VSCi; ω' represents the angular frequency of the output voltage of VSCi; i' Ld and i' Lq Represents the dq axis component of VSCi output current; K p5 and K i5 Respectively represent the proportional coefficient and integral coefficient of the PI control in the current inner loop control; i' Ldref and i' qref represents the dq component reference value of the VSCi output current; s is a complex variable in the Laplace transform. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a flow chart of a control method of a multi-terminal flexible interconnection device with networking capability provided by the present application;

[0040] Figure 2 is a structural diagram of a multi-terminal flexible interconnection device provided by the present application;

[0041] Figure 3 This is a control strategy diagram for DC bus voltage synchronous networking control provided by this application;

[0042] Figure 4 It is a control strategy diagram of voltage and current dual-loop control provided by the present application;

[0043] Figure 5 It is a control strategy diagram of fixed active power and fixed reactive power control provided by this application;

[0044] Figure 6 is a structural diagram of a multi-terminal flexible interconnection device provided in an embodiment of the present application;

[0045] Figure 7 is a grid voltage waveform diagram established by VSC1 provided in an embodiment of the present application;

[0046] Figure 8 It is a grid frequency waveform diagram established by VSC1 provided in an embodiment of the present application. DETAILED DESCRIPTION

[0047] 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.

[0048] It should be understood that, in the description of the present application, the term "several" means at least one, such as one, two, etc., unless otherwise clearly and specifically defined; the term "plurality" means two or more, unless otherwise clearly and specifically defined; the terms "first" and "second" etc. are used to distinguish different objects rather than to describe a specific order of objects; the term "and / or" includes any and all combinations of one or more related listed items.

[0049] In order to solve the problem that most traditional flexible interconnected devices are based on grid-following control, cannot actively build grid frequency and voltage, and can only operate in grid-connected mode, the present application provides a control method for a multi-terminal flexible interconnected device with networking capability, which can enable the multi-terminal flexible interconnected device to have grid-connected and off-grid operation capabilities.

[0050] like Figure 2 As shown, the multi-terminal flexible interconnection device provided by the present application includes n VSCs, n>2, the AC sides of the n VSCs are respectively connected to multiple 380V feeders of the distribution network, and the DC sides of the n VSCs are connected to a DC bus through a DC capacitor. By controlling the VSCs accordingly, power conversion in the form of DC / AC, AC / DC, etc. can be achieved.

[0051] like Figure 1 As shown, the multi-terminal flexible interconnection device control method provided in the present application includes steps S10 and S20, which are described in detail as follows:

[0052] S10, performing DC bus voltage synchronous networking control on any VSC1 among the n VSCs.

[0053] S20, obtain the DC bus voltage and the operating status of VSC1. When VSC1 is connected to the grid, the remaining VSCi are controlled by fixed active power and fixed reactive power (PQ control), i = 2, ..., n, to achieve the tracking of the power command by the output power of VSCi, thereby realizing the flexible interconnection between distribution networks. When VSC1 is off-grid (islanded) and the DC bus voltage drops to a set value, which can be specifically set to drop to 0.95pu, according to the power supply margin of the feeders connected to each VSCi, the VSC connected to the feeder with the maximum power supply margin is controlled by fixed DC bus voltage to stabilize the DC bus voltage.

[0054] In step S20, the DC bus voltage is used to synchronize the grid control of VSC1. When VSC1 is connected to the grid, the DC bus voltage U dc The active power output of VSC1 is adjusted according to the changes in the DC bus voltage to achieve power balance of the device; when VSC1 is off-grid (islanded), the phase and amplitude of the VSC1 modulation voltage can be calculated according to the changes in the DC bus voltage, thereby establishing a stable frequency and voltage, forming a local microgrid to ensure the power supply of important loads in the area.

[0055] Specifically, the operating state of VSC1 is determined by the output voltage and output current of VSC1. When the output voltage of VSC1 is lower than the set minimum limit U min , the output current exceeds the set maximum limit i max When the feeder is judged to have a serious fault and the port has lost the ability to control the DC voltage, VSC1 is in an off-grid state; when the output voltage and output current of VSC1 are both within the set range, VSC1 is in a grid-connected state.

[0056] Figure 3 This is a strategy diagram for DC bus voltage synchronous network control provided by this application, such as Figure 3 As shown, the present application is based on the structural and dynamic similarities between the DC capacitor and the synchronous generator rotor. VSC1 establishes a DC bus voltage U dc The matching relationship between the output voltage angular frequency ω of VSC1 relies on the inertia of the DC capacitor to achieve the autonomous synchronization function of the power grid, that is, the DC bus voltage U dc The measured value is controlled by PI to obtain the phase δ of the VSC1 modulation voltage, thereby realizing the real-time linkage between the DC bus voltage and the output voltage angular frequency.

[0057] Specifically, the method for controlling the DC bus voltage synchronous networking provided by the present application is:

[0058] Step 1: Collect DC bus voltage U dc , the DC bus voltage U dc After PI control, the phase δ of the VSC1 modulation voltage is obtained.

[0059] In step 1, the calculation formula of the phase δ of the VSC1 modulation voltage is:

[0060]

[0061] In formula (1), U dcref is the reference value of DC bus voltage; U dc0 is the steady-state value of the DC bus voltage, generally 1.0pu; ω 0 is the rated angular frequency of VSC1, which is 100pi; K D is the damping coefficient; s is the complex variable in the Laplace transform.

[0062] When VSC1 is connected to the grid, the active power P output by VSC1 is e It is related to the phase δ and the calculation formula is:

[0063]

[0064] In formula (2), E represents the terminal voltage output by VSC1; U g Indicates the voltage of the distribution network connected to VSC1; X c Represents the equivalent impedance of VSC1.

[0065] It can be seen from equations (1) and (2) that the multi-terminal flexible interconnection device provided in the present application can respond to the device DC bus voltage U when VSC1 is connected to the grid. dc The output active power P is adjusted automatically by the change of e , achieving power balance and stable operation of the device. For example, when the load on the feeder side to which it is connected suddenly increases, the output power of the device is unbalanced with the input power, the DC capacitor discharges, and the DC bus voltage u dc Decrease; From equations (1) and (2), we can see that u dc Less than u dcref This will reduce the phase angle δ of the VSC1 modulation voltage, further reducing the output power P of VSC1. e Reduce, and finally achieve power balance inside the device.

[0066] Step 2: Obtain the terminal voltage E output by VSC1 through reactive power control, and then transform the terminal voltage E through abc / dq to obtain the dq axis component reference value u of the VSC1 output voltage tdref and u tqref Then, the dq axis component u of the VSC1 modulation voltage is obtained through the voltage and current dual loop control. sd and usq ; Among them, the d-axis component reference value u of the VSC1 output voltage tdref Equal to the terminal voltage E, the q-axis component reference value u of the VSC1 output voltage tqref is equal to zero.

[0067] In step 2, the mathematical model of reactive power control is:

[0068]

[0069] In formula (3), Q e is the reactive power output by VSC1; Q eref K is the reactive power reference value output by VSC1; Q is the integral coefficient in reactive power control; E init is the initial value of the internal potential of VSC1, which is 1.0.

[0070] Specifically, Figure 4 As shown, the voltage and current dual-loop control provided by the present application can adopt a dual closed-loop control architecture of voltage outer loop control and current inner loop control, which can automatically implement the current limiting function in the event of a fault. In the abc / dq transformation, the d-axis component reference value u of the output voltage of VSC1 is selected tdref The q-axis component u of the VSC1 output voltage is in the same direction as the terminal voltage vector of the VSC1 output. tqref is 0, the formula is as follows:

[0071]

[0072] The mathematical model of the voltage outer loop control in the dual closed-loop control architecture is:

[0073]

[0074] The mathematical model of the current inner loop control in the dual closed-loop control architecture is:

[0075]

[0076] In the formula, u td and u tq represents the dq axis component of the PCC voltage at the VSC1 grid connection point; i Ldref and i Lqref Represents the dq component reference value of the VSC1 output current, which is the output result of the voltage outer loop control; K p1 and K i1 They represent the proportional coefficient and integral coefficient of PI control in the voltage outer loop control respectively; i Ld and i Lq Represents the dq axis component of the VSC1 output current; K p2 and Ki2 They represent the proportional coefficient and integral coefficient of PI control in the current inner loop control of the voltage and current dual loop control respectively; L c represents the equivalent reactance of VSC1; ω represents the angular frequency of the output voltage of VSC1.

[0077] Step 3: According to the phase δ, the dq axis component u of the VSC1 modulation voltage is modulated sd and u sq The PWM modulation wave is obtained through abc / dq conversion, and then the PWM modulation wave is input into the PWM generator to generate a pulse signal to control the on and off of the power switch tube in VSC1.

[0078] It can be seen from the above formulas (1) and (3) that when the VSC1 island is independently operating with load, the multi-terminal flexible interconnection device provided by the present application can obtain the phase and amplitude (terminal voltage) signals of the VSC1 modulation voltage through the DC bus voltage synchronization network control, and then obtain the pulse signal for controlling VSC1 through the voltage and current dual-loop control and PWM generator, thereby establishing a stable frequency and voltage. Without following the external power grid, a local microgrid can be formed to ensure the power supply of important loads in the area.

[0079] In this application, when VSC1 is connected to the grid, VSC2~VSCn adopt PQ control, i=2,…,n, and the voltage and frequency are given by the grid, which is used to realize the tracking of the power command of the VSCi output power, thereby realizing the flexible interconnection between the distribution networks; when VSC1 is off the grid and the DC bus voltage of the device is detected to drop to 0.95pu, according to the power supply margin of the feeder connected to VSC2~VSCn, the VSC connected to the selected power supply margin feeder is switched to U dc Q control is used to stabilize the DC bus voltage.

[0080] Specifically, Figure 5 As shown, the PQ control and U dc Both PQ and Q control adopt a dual-loop structure, and both use the same current inner loop control. The control strategy is: when PQ control is adopted, the dq axis component reference value of the VSCi output current is first calculated according to the VSCi active power and reactive power instructions; when U dc When Q is controlled, according to the DC bus voltage U dc The dq axis component reference value of the VSCi output current is calculated based on the VSCi reactive power instruction; then the VSCi output current after abc / dq conversion is compared with the dq axis component reference value of the VSCi output current and PI control is performed to obtain the dq axis component u' of the VSCi modulation voltage sd and u' sq ; Then the dq axis component u' sd and u' sqIt is sent to the PWM modulator and modulated by a triangular carrier to generate a trigger pulse sequence u cA *、u cB * and u cC * to VSCi, completing the entire control process.

[0081] In this application, the outer loop control in the PQ control dual-loop structure adopts power outer loop control, and its mathematical model is:

[0082]

[0083] U dc The outer loop control in the Q control dual-loop structure adopts voltage outer loop control, and its mathematical model is:

[0084]

[0085] PQ control and U dc The mathematical models of the current inner loop control in Q control are:

[0086]

[0087] In the formula, i' Ldref and i' qref Represents the dq component reference value of the VSCi output current, which is the output result of the outer loop control; K p3 and K i3 They represent the proportional coefficient and integral coefficient of PI control in the power outer loop control in the dual-loop structure respectively; P ref Indicates the active reference power output by VSCi; Q ref represents the reactive reference power output by VSCi; P represents the active power output by VSCi; Q represents the reactive power output by VSCi; K p4 and K i4 They represent the proportional coefficient and integral coefficient of PI control in UdcQ control respectively; U dcref is the reference value of DC bus voltage; U dc is the actual value of the DC bus voltage; u' sd and u' sq represents the dq axis component of the VSCi modulation voltage; i' Ld and i' Lq represents the dq axis component of the VSCi output current; u' td and u' tq K represents the dq axis component of the PCC voltage at the VSCi grid connection point; p5 and K i5 Respectively represent the proportional coefficient and integral coefficient of PI control in the current inner loop control in the dual-loop structure; L' c represents the equivalent reactance of VSCi; ω' represents the angular frequency of the output voltage of VSCi.

[0088] The present application provides a control method for a multi-terminal flexible interconnected device with networking capability. When VSC1 is connected to the grid, it can automatically adjust the output active power in response to changes in the DC bus voltage, thereby achieving power balance and stable operation of the device. When VSC1 is off-grid, the DC bus voltage synchronous networking control can calculate the phase and amplitude signals of the converter modulation voltage according to the changes in the DC bus voltage, and then obtain the pulse signal through the voltage and current dual-loop control and PWM generator, thereby establishing a stable frequency and voltage. Without following the external power grid, a local microgrid can be formed to ensure the power supply of important loads in the area.

[0089] The present application is described below in conjunction with specific embodiments.

[0090] This embodiment provides a multi-terminal flexible interconnection device topology structure, such as Figure 6 As shown, the device connects three 380V feeders of the distribution network through VSC1, VSC2, and VSC3 to achieve flexible interconnection of the distribution network. The control method provided in this embodiment is: VSC1 adopts network control based on DC bus voltage synchronization; VSC2 and VSC3 adopt fixed active power and fixed reactive power control to adjust the transmission power; when the feeder on the VSC1 side loses power and causes the DC voltage to drop to 0.95pu, VSC2 automatically switches to fixed DC voltage control. The multi-terminal flexible interconnection device provided in this embodiment can realize flexible interconnection between distribution networks, has the ability to independently construct voltage and frequency, and forms a local microgrid to ensure the power supply of regional loads.

[0091] In this embodiment, Figure 6 The proposed method is verified in the shown device: when feeder 1 loses power, VSC1 forms a local microgrid. Figure 7 It represents the grid voltage waveform established by VSC1, and the microgrid voltage is 1.0pu; Figure 8 It represents the grid frequency waveform established by VSC1, and the microgrid frequency is 50.1 Hz.

[0092] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A control method for a multi-terminal flexible interconnection device with networking capability, the multi-terminal flexible interconnection device comprising n VSCs, n>2, n The AC side of each VSC is connected to multiple feeders. n The DC side of each VSC is connected to the DC bus through a DC capacitor, which is characterized by: The control method comprises the following steps: right n Any VSC1 among the VSCs performs DC bus voltage synchronous networking control; Get the DC bus voltage and the operating status of VSC1. When VSC1 is connected to the grid, the remaining VSC i Adopting fixed active power and fixed reactive power control, i =2,…, n , used to implement VSC i The output power tracks the power command, thereby realizing flexible interconnection between distribution networks; when VSC1 is off-grid and the DC bus voltage drops to the set value, a fixed DC bus voltage control is adopted for the VSC connected to the maximum power supply margin feeder to stabilize the DC bus voltage; The VSC1 performs DC bus voltage synchronous grid control, which is used to adjust the active power output by the VSC1 according to the change of the DC bus voltage when the VSC1 is connected to the grid, so as to achieve device power balance; when the VSC1 is off-grid, the phase and amplitude of the VSC1 modulation voltage are calculated according to the change of the DC bus voltage, so as to establish a stable frequency and voltage, and form a local microgrid to ensure the power supply of important loads in the area; The DC bus voltage synchronous networking control comprises the following steps: (1) Collecting DC bus voltage U dc , the DC bus voltage U dc After PI control, the phase of VSC1 modulation voltage is obtained. δ ; (2) Obtain the terminal voltage of VSC1 output through reactive power control E , then the terminal voltage E The dq axis component reference value of the VSC1 output voltage is obtained through abc / dq transformation u tdref and u tqref Then, the dq axis components of the VSC1 modulation voltage are obtained through voltage and current dual loop control. u sd and u sq ; Among them, the d-axis component reference value of the VSC1 output voltage u tdref Equal to terminal voltage E , q-axis component reference value of VSC1 output voltage u tqref is equal to zero; (3) According to the phase δ The dq axis components of the VSC1 modulation voltage u sd and u sq The PWM modulation wave is obtained through abc / dq conversion, and then the PWM modulation wave is input into the PWM generator to generate a pulse signal to control the on and off of the power switch tube in VSC1.

2. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 1, characterized in that: In step (1), the phase of the VSC1 modulation voltage δ The calculation formula is: In the formula, U dcref is the reference value of DC bus voltage; U dc0 is the steady-state value of the DC bus voltage; ω 0 is the rated angular frequency of VSC1; K D is the damping coefficient; s is a complex variable in the Laplace transform.

3. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 1, characterized in that: In step (2), the mathematical model of reactive power control is: In the formula, Q e is the reactive power output by VSC1; Q eref is the reactive power reference value output by VSC1; K Q is the integral coefficient in reactive power control; E init is the initial value of the internal potential of VSC1; s is a complex variable in the Laplace transform.

4. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 1, characterized in that: In step (2), the voltage-current dual-loop control includes voltage outer-loop control and current inner-loop control; The mathematical model of the voltage outer loop control in the voltage and current dual loop control is: The mathematical model of the current inner loop control in the voltage and current dual loop control is: In the formula, i Ldref and i Lqref It represents the reference value of the dq component of the output current of VSC1, which is the output result of the voltage outer loop control; u td and u tq Represents the dq axis component of the VSC1 grid connection point voltage; K p1 and K i1 They represent the proportional coefficient and integral coefficient of PI control in the voltage outer loop control respectively; i Ld and i Lq Represents the dq-axis component of the output current of VSC1; K p2 and K i2 They represent the proportional coefficient and integral coefficient of PI control in the current inner loop control of the voltage and current dual loop control respectively; L c represents the equivalent reactance of VSC1; ω Indicates the angular frequency of the VSC1 output voltage; s is a complex variable in the Laplace transform.

5. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 1, characterized in that: The VSC1 outputs active power P e The calculation formula is: In the formula, E Indicates the terminal voltage output by VSC1; U g Indicates the voltage of the distribution network connected to VSC1; X c represents the equivalent impedance of VSC1; δ Indicates the phase of the VSC1 modulation voltage.

6. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 1, characterized in that: The fixed active power and reactive power control and the fixed DC bus voltage control all adopt a double-loop structure, and the fixed active power and reactive power control and the fixed DC bus voltage control adopt the same current inner loop control.

7. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 6, characterized in that: The mathematical model of the outer loop control in the fixed active power and fixed reactive power control is: In the formula, i' Ldref and i' Lqref Represents the current inner loop control input of the VSC i The dq component reference value of the output current is the output result of the outer loop control in the fixed active power and fixed reactive power control; K p3 and K i3 Respectively represent the proportional coefficient and integral coefficient of the PI control in the outer loop control in the fixed active power and fixed reactive power control; P ref Represents VSC i Output active reference power; Q ref Represents VSC i Output reactive reference power; P Represents VSC i Output active power; Q Represents VSC i Output reactive power; s is a complex variable in the Laplace transform.

8. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 6, characterized in that: The mathematical model of the outer loop control in the constant DC bus voltage control is: In the formula, i' Ldref and i' Lqref Represents the current inner loop control input of the VSC i The dq component reference value of the output current is the output result of the outer loop control in the fixed DC bus voltage control; K p4 and K i4 Respectively represent the proportional coefficient and integral coefficient of the PI control in the outer loop control in the constant DC bus voltage control; Q ref Represents VSC i Output reactive reference power; Q Represents VSC i Output reactive power; U dcref is the reference value of DC bus voltage; U dc is the DC bus voltage; s is a complex variable in the Laplace transform.

9. The control method of a multi-terminal flexible interconnection device with networking capability as claimed in claim 6, characterized in that: The mathematical model of the current inner loop control in the fixed active power and fixed reactive power control and the fixed DC bus voltage control is: In the formula, u' sd and u' sq Respectively represent VSC i The dq axis components of the modulation voltage; u' td and u' tq Represents VSC i The dq axis components of the grid connection point voltage; L' c Represents VSC i The equivalent reactance of ω' Represents VSC i Angular frequency of the output voltage; i' Ld and i' Lq Represents VSC i The dq-axis components of the output current; K p5 and K i5 Respectively represent the proportional coefficient and integral coefficient of the PI control in the current inner loop control; i' Ldref and i' Lqref Represents VSC i Reference value of dq component of output current; s is a complex variable in the Laplace transform.

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