A voltage support method for coordinated distributed power supply and flexible DC converter

Abstract

The present invention discloses a voltage support method for the coordination of distributed power sources and flexible direct current converters. The support method comprises the following steps: establishing a three-sequence network for upstream faults of an active distribution network; compounding a three-sequence network for upstream faults of an active distribution network; simplifying a compound sequence network for upstream faults of an active distribution network; and deriving the optimal output current phase angle of the distributed power source and the flexible direct current converter. The present invention optimizes the characteristics of the output current of each converter in the distribution network under fault conditions, and improves the fault ride-through capability of the DG when a two-phase grounding fault occurs upstream of the distribution network.

Description

Technical Field

[0001] The present invention relates to the field of smart grid technology, and in particular to a voltage support method for coordinated distributed power supply and flexible direct current converter. Background Art

[0002] The distribution network is at the end of the power grid and is responsible for distributing electric energy. Therefore, its power supply reliability and quality are crucial to the development of the national economy. Driven by the "dual carbon" goal, the distribution network is gradually developing in the direction of "active", "multi-source" and "closed-loop". A large number of power electronic converters have begun to be connected to the distribution network, turning the originally passive, passive and open-loop distribution network into an active, active and closed-loop distribution network. The penetration rate of distributed power sources (DG, distribution generator) has increased year by year, and multi-terminal distribution networks have gradually achieved flexible interconnection through flexible DC converters (flexible DC converters).

[0003] In the distribution network, power electronic converters are low-inertia components. If they are disconnected from the grid during a grid fault, it will have many negative impacts on the grid. In response to this problem, governments and power companies have issued converter low-voltage fault ride-through guidelines, requiring converters to remain connected to the grid during grid faults and inject sufficient reactive power into the grid to support the voltage at the converter connection point. However, such guidelines are basically for transmission networks with inductive lines, and have poor adaptability to distribution networks with resistive lines.

[0004] The capacity of DG is usually only 25% of the grid capacity, and its current output capacity is weak. When a two-phase grounding fault occurs in the upstream of the distribution network, DG cannot inject sufficient power into the grid, and it is difficult to significantly support the voltage at its grid connection point, causing it to be disconnected from the grid due to low voltage at the grid connection point. The flexible DC converter usually carries the rated power of the grid, has a large capacity, and has a strong current output capacity. Therefore, the strong output capacity of the flexible DC converter can be used to assist DG in achieving grid connection point voltage support.

[0005] Therefore, it is necessary to study the method of collaboratively supporting the voltage of the DG grid connection point by the DG and flexible DC converter in the distribution network when a fault occurs upstream of the distribution network, so as to improve the fault ride-through capability of the DG. Summary of the invention

[0006] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or be understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the description and other drawings of the description.

[0007] The purpose of the present invention is to overcome the above shortcomings and provide a voltage support method for the coordinated use of distributed power sources and flexible DC converters. The present invention optimizes the characteristics of the output current of each converter in the distribution network under fault conditions and improves the fault ride-through capability of the DG when a two-phase grounding fault occurs upstream of the distribution network.

[0008] The technical solution adopted by the present invention to solve the technical problem is: a voltage support method for a distributed power source and a flexible direct current converter, comprising the following steps:

[0009] Establish a three-sequence network for upstream faults of the active distribution network, and establish positive, negative, and zero-sequence equivalent networks of the active distribution network to form a three-sequence network for upstream faults of the active distribution network;

[0010] The three-sequence network of the upstream fault of the composite active distribution network is equivalent to the distributed power supply and the flexible DC converter as the positive sequence current source, and the three-sequence network of the upstream fault of the active distribution network is connected in parallel at the fault point to obtain the composite sequence network of the active distribution network when a two-phase grounding fault occurs in the upstream of the distribution network;

[0011] Simplifying the composite sequence network of the upstream fault of the active distribution network, simplifying the composite sequence network of the upstream fault of the active distribution network to obtain a simplified composite sequence network of the upstream fault of the active distribution network;

[0012] The optimal output current phase angle of distributed power generation and flexible DC converter is derived. The simplified composite sequence network of upstream fault of active distribution network is used to analyze the influence of distributed power generation and flexible DC converter on the voltage of distributed power generation grid connection point, and the optimal output current phase angle of distributed power generation and flexible DC converter is derived.

[0013] In some embodiments, the active distribution network includes a power grid 1, a power grid 2, a flexible DC converter T1, and a flexible DC converter T2. The power grid 1 and the power grid 2 are interconnected through the flexible DC converter T1 and the flexible DC converter T2. The neutral point of the power grid 1 is grounded through a resistor, and the neutral point of the power grid 2 is grounded through an arc suppression coil. The flexible DC converter T1 and the flexible DC converter T2 are both connected to a 20kV DC bus.

[0014] In some embodiments, in the power grid 1, the length of the AB segment is L 1. Line impedance is Z 1. The length of BC segment is L 2. Line impedance is Z 2. The length of the CD segment is L 3. Line impedance is Z 3. Failure point f 1 The fault point is located at the junction of section AB and section BC f 1. The fault is an upstream fault of the distributed generation DG1. In the power grid 2, the line length of the EF section is L 4. Line impedance isZ 4. The length of the FG segment is L 5. Line impedance is Z 5.

[0015] In some embodiments, both the flexible DC converter T1 and the flexible DC converter T2 adopt a modular multi-level converter structure.

[0016] In some embodiments, in a three-sequence network with an upstream fault in the active distribution network, the grid voltage phasor before the fault is , the current phasor of distributed power source DG1 And the current phasor of the flexible DC converter T1 The expression is:

[0017]

[0018] In formula (1), U s|0| and θ s They are The magnitude and phase angle of and α DG1 They are The magnitude and phase angle of and β T1 They are Amplitude and phase angle.

[0019] In some embodiments, simplifying the composite sequence network of the upstream fault of the active power distribution network includes primary simplification, secondary simplification and tertiary simplification.

[0020] In some embodiments, the first simplification is specifically: simplifying the circuit on the right side of the fault point f1 in the composite sequence network to form an impedance Z A , whose expression is:

[0021]

[0022] In formula (2), yes Z 1 negative sequence impedance, 3 Z f is the zero-sequence fault impedance, is the zero-sequence impedance of Z1, 3 Z N is the zero-sequence impedance of the neutral point impedance of grid 1.

[0023] In some embodiments, the impedance Z is simplified again as follows: A Further simplification gives the impedance Z B , whose expression is:

[0024]

[0025] In formula (3), express , Z eq Expressed in amplitude and phase angle form, | Z eq | and δ eq are their amplitude and phase angle respectively.

[0026] In some embodiments, the third simplification is as follows: for the impedance Z B Further simplification gives the impedance Z C , whose expression is:

[0027]

[0028] In formula (4), |Z C | and φ C Z C The magnitude and phase angle of the voltage phasor at the grid connection point of DG1 can be , and Add them together and you get, and are expressed in the form of amplitude and phase angle, and" α DG1 + φ C ” are The magnitude and phase angle of and" β T1 + φ C ” are Amplitude and phase angle.

[0029] In some embodiments, if and only if , , and When parallel, The magnitude of " is the largest, then the optimal phase angle of the output current of DG1 and flexible DC converter T1 is expressed as:

[0030]

[0031] In formula (5), α DG1_opt and β T1_optare the optimal phase angles of the output currents of DG1 and flexible DC converter T1, that is, α DG1 and β T1 The best phase angle.

[0032] By adopting the above technical solution, the beneficial effects of the present invention are:

[0033] The present invention first establishes the positive, negative, and zero-sequence equivalent networks of the active distribution network and equates the DG and the flexible DC converter to positive-sequence current sources, so that the three-sequence networks are connected in parallel at the fault point to obtain the composite sequence network of the active distribution network when a two-phase grounding fault occurs upstream of the distribution network, simplifies the composite sequence network, and uses the network to analyze the influence of each power source of the distribution network on the voltage of the DG grid connection point. And derive the optimal phase angle of the output current of the DG and the flexible DC converter required to maximize the support of the voltage at the DG grid connection point when a fault occurs upstream of the distribution network. The present invention optimizes the characteristics of the output current of each converter in the distribution network under fault conditions, and improves the fault ride-through capability of the DG when a two-phase grounding fault occurs upstream of the distribution network.

[0034] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present disclosure.

[0035] Undoubtedly, these and other objects of the present invention will become more apparent after the following detailed description of the preferred embodiment described with reference to various figures and drawings.

[0036] In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, one or several preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the present invention and do not constitute a limitation of the present invention.

[0038] In the drawings, the same reference numerals are used for the same components and the drawings are schematic and not necessarily drawn to scale.

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only one or several embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on such drawings without paying creative work.

[0040] Figure 1A flow chart of a voltage support method for a distributed power source and a flexible DC converter in cooperation with each other according to the present invention;

[0041] Figure 2 The active power distribution network system topology diagram of the present invention;

[0042] Figure 3 It is a positive sequence network diagram of an upstream fault of an active power distribution network of the present invention;

[0043] Figure 4 A negative sequence network diagram of an upstream fault of an active power distribution network of the present invention;

[0044] Figure 5 It is a zero-sequence network diagram of an upstream fault of an active power distribution network of the present invention;

[0045] Figure 6 A composite sequence network diagram of an upstream fault of an active power distribution network of the present invention;

[0046] Figure 7 The composite sequence network simplification of the upstream fault of the active distribution network of the present invention Figure 1 ;

[0047] Figure 8 The composite sequence network simplification of the upstream fault of the active distribution network of the present invention Figure 2 ;

[0048] Fig. 9 The composite sequence network simplification of the upstream fault of the active distribution network of the present invention Figure 3 ;

[0049] Fig.10 For the present invention , , and Relationship diagram between them;

[0050] Fig.11 For the present invention In different α DG1 and β The change diagram under T1;

[0051] Fig.12 For the present invention in 8 groups α DG1 The instantaneous voltage and current changes of T1 and DG1 under the action;

[0052] Fig.13 For the present invention in 8 groups α DG1 Under the effect , α DG1 and β T1 Graph of changes in

[0053] Fig.14 For the present invention in 8 groups β T1 The instantaneous voltage and current changes of T1 and DG1 under the action;

[0054] Fig.15 For the present invention in 8 groups β T1 Under the effect , α DG1 and β T1 Graph of changes in . DETAILED DESCRIPTION

[0055] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but are not used to limit the present invention.

[0056] In the present invention, unless otherwise clearly specified and limited, the first feature "on" or "under" the second feature may be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in an appropriate manner in any one or more embodiments or examples.

[0057] Reference Figure 1 , Figure 1 The present invention is a flow chart of a voltage support method for a distributed power source and a flexible DC converter in coordination.

[0058] According to some embodiments of the present invention, the present invention provides a voltage support method for a distributed power source and a flexible direct current converter, comprising the following steps:

[0059] Establish a three-sequence network for upstream faults of the active distribution network, and establish positive, negative, and zero-sequence equivalent networks of the active distribution network to form a three-sequence network for upstream faults of the active distribution network;

[0060] The three-sequence network of the upstream fault of the composite active distribution network is equivalent to the distributed power supply and the flexible DC converter as the positive sequence current source, and the three-sequence network of the upstream fault of the active distribution network is connected in parallel at the fault point to obtain the composite sequence network of the active distribution network when a two-phase grounding fault occurs in the upstream of the distribution network;

[0061] Simplifying the composite sequence network of the upstream fault of the active distribution network, simplifying the composite sequence network of the upstream fault of the active distribution network to obtain a simplified composite sequence network of the upstream fault of the active distribution network;

[0062] The optimal output current phase angle of distributed power generation and flexible DC converter is derived. The simplified composite sequence network of upstream fault of active distribution network is used to analyze the influence of distributed power generation and flexible DC converter on the voltage of distributed power generation grid connection point, and the optimal output current phase angle of distributed power generation and flexible DC converter is derived.

[0063] Reference Figure 2 , Figure 2 This is a topological diagram of the active power distribution network system of the present invention.

[0064] According to some embodiments of the present invention, optionally, the active distribution network includes a power grid 1, a power grid 2, a flexible direct current converter T1, and a flexible direct current converter T2. The power grid 1 and the power grid 2 are interconnected through the flexible direct current converter T1 and the flexible direct current converter T2. The neutral point of the power grid 1 is grounded through a resistor, and the neutral point of the power grid 2 is grounded through an arc suppression coil. The flexible direct current converter T1 and the flexible direct current converter T2 are both connected to a 20kV DC bus.

[0065] According to some embodiments of the present invention, optionally, the AC voltage levels of the power grid 1 and the power grid 2 are both 10 kV.

[0066] According to some embodiments of the present invention, optionally, the grid connection points of the flexible DC converter T1 and the flexible DC converter T2 are PCC1 and PCC2 respectively.

[0067] According to some embodiments of the present invention, optionally, in the power grid 1, the length of the AB segment is L 1. Line impedance is Z 1. The length of BC segment is L 2. Line impedance is Z 2. The length of the CD segment is L 3. Line impedance is Z 3. Failure point f 1 The fault point is located at the junction of section AB and section BC f 1. The fault is an upstream fault of the distributed generation DG1. In the power grid 2, the line length of the EF section is L 4. Line impedance is Z 4. The length of the FG segment is L 5. Line impedance is Z 5.

[0068] According to some embodiments of the present invention, optionally, the current phasor of the flexible DC converter T1 is , the current phasor of the flexible DC converter T2 is .

[0069] According to some embodiments of the present invention, optionally, both the flexible DC converter T1 and the flexible DC converter T2 adopt a modular multi-level converter structure.

[0070] Reference Figure 3-Figure 5 , Figure 3 It is a positive sequence network diagram of an upstream fault of an active power distribution network of the present invention; Figure 4 A negative sequence network diagram of an upstream fault of an active power distribution network of the present invention; Figure 5 It is a zero-sequence network diagram of an upstream fault of the active distribution network of the present invention.

[0071] exist Figure 3 middle, is the grid voltage phasor before the fault, is the current phasor of DG1, is the current phasor of T1. yes Z 1 positive sequence impedance, and They are Z 2 and Z 3's positive sequence impedance. Is the inflow fault point f 1 positive sequence fault current, It is the failure point f 1 positive sequence voltage. Figure 4 middle, yes Z 1 negative sequence impedance, Is the inflow fault point f 1 negative sequence fault current, It is the failure point f 1 negative sequence voltage. Figure 5 middle, yes Z 1 zero sequence impedance, Is the inflow fault point f 1 zero sequence fault current, It is the failure point f 1 zero sequence voltage, 3 Z N is the zero-sequence impedance of the neutral point impedance of grid 1.

[0072] According to some embodiments of the present invention, optionally, in a three-sequence network with an upstream fault in an active distribution network, the grid voltage phasor before the fault , the current phasor of distributed power source DG1 And the current phasor of the flexible DC converter T1 The expression is:

[0073]

[0074] In formula (1), U s|0| and θ s They are The magnitude and phase angle of and α DG1 They are The magnitude and phase angle of and β T1 They are Amplitude and phase angle.

[0075] Reference Figure 6 , Figure 6 It is a composite sequence network diagram of the upstream fault of the active distribution network of the present invention.

[0076] According to some embodiments of the present invention, optionally, simplifying the composite sequence network of the upstream fault of the active distribution network includes primary simplification, secondary simplification and tertiary simplification.

[0077] Reference Figure 7 , Figure 7 The composite sequence network simplification of the upstream fault of the active distribution network of the present invention Figure 1 .

[0078] According to some embodiments of the present invention, optionally, the first simplification is specifically: simplifying the circuit on the right side of the fault point f1 in the composite sequence network to form an impedance Z A , whose expression is:

[0079]

[0080] In formula (2), yes Z 1 negative sequence impedance, 3 Z f is the zero-sequence fault impedance, is the zero-sequence impedance of Z1, 3 Z N is the zero-sequence impedance of the neutral point impedance of grid 1.

[0081] Reference Figure 8 , Figure 8 The composite sequence network simplification of the upstream fault of the active distribution network of the present invention Figure 2 .

[0082] According to some embodiments of the present invention, optionally, it can be simplified again as follows: A Further simplification gives the impedance Z B , whose expression is:

[0083]

[0084] In formula (3), express , Z eq Expressed in amplitude and phase angle form, | Z eq | and δ eq are their amplitude and phase angle respectively.

[0085] Reference Fig. 9 , Fig. 9 The composite sequence network simplification of the upstream fault of the active distribution network of the present invention Figure 3 .

[0086] According to some embodiments of the present invention, optionally, the third simplification is specifically: for the impedance Z B Further simplification gives the impedance Z C , whose expression is:

[0087]

[0088] In formula (4), |Z C | and φ C Z C The magnitude and phase angle of the voltage phasor at the grid connection point of DG1 can be , and Add them together and you get, and are expressed in the form of amplitude and phase angle, and" α DG1 + φ C ” are The magnitude and phase angle of and" β T1 + φ C ” are Amplitude and phase angle.

[0089] Reference Fig.10 , Fig.10 For the present invention , , and Relationship diagram between them.

[0090] According to some embodiments of the present invention, optionally, if and only if , , and When parallel, The amplitude The optimal phase angle of the output current of DG1 and flexible DC converter T1 is expressed as:

[0091]

[0092] In formula (5), α DG1_opt and β T1_opt are the optimal phase angles of the output currents of DG1 and flexible DC converter T1, that is, α DG1 and β T1 The best phase angle.

[0093] Reference Figure 11-15 , Fig.11 For the present invention In different α DG1 and β The change diagram under T1; Fig.12 For the present invention in 8 groups α The instantaneous voltage and current changes of T1 and DG1 under the action of DG1; Fig.13 For the present invention in 8 groups α Under the action of DG1 , α DG1 and β T1 change diagram; Fig.14 For the present invention in 8 groups β The instantaneous voltage and current changes of T1 and DG1 under the action of T1; Fig.15 For the present invention in 8 groups β Under the action of T1 , α DG1 and β Graph showing changes in T1.

[0094] In order to verify the correctness of the above conclusions, according to the parameters in Table 1 and the above formula, the code is written in Matlab software to draw The amplitude Follow α DG1 and β T1 A three-dimensional graph of changes, such as Fig.11 As shown. Fig.11 The values ​​indicated on the grid plane represent the grid f When a two-phase ground fault occurs at 1, but DG1 and flexible DC converter T1 do not inject current The value is 3928 V. Fig.11It can be seen that if and only if α DG1 and β T1 When both are 309°, reaches a maximum value of 7581 V. If α DG1 and β T1 The value of is not chosen properly, for example α DG1 and β T1 The average value is 100°. The value is even lower than that before current injection ( The curve is below the plane of the grid), in other words, α DG1 and β T1 An unreasonable value will lower the voltage amplitude of the DG1 grid connection point . Fig.11 The correctness of the above theory was verified.

[0095] Table 1 Parameters of the two-terminal distribution network studied

[0096]

[0097] To further verify the correctness of the present invention, according to Figure 1 According to the parameters in Table 1, a specific electromagnetic transient simulation model was built in Matlab / Simulink software for simulation verification.

[0098] According to the superposition theorem, the effects of the output currents of DG1 and T1 can be superimposed. Therefore, the present invention selects to fix the output of one converter and change the phase angle of the output current of another converter to verify the proposed method. The verification is divided into two steps. First, the output of T1 is kept unchanged to verify the optimal phase angle of the output current of DG1. Secondly, the output of DG1 is kept unchanged to verify the optimal phase angle of the output current of T1. The simulation results are shown in Table 2, Table 3, Fig.12 , Fig.13 , Fig.14 and Fig.15 shown.

[0099] Table 2 and Fig.12 , Fig.13 It shows that the output current amplitude of T1 remains unchanged, and β T1 Maintain optimal ( β T1 =309°), change the DG1 output current phase angle 8 times, that is, in 8 groups of different α DG1 Under the effect, From Table 2 and Fig.12 , Fig.13 It can be seen that at the beginning of the fault, before T1 and DG1 inject current, is only 3.953 kV. α DG1 =309° (-51°), Reaching a maximum value of 7.585 kV, Fig.11 middle The maximum value is 7.581 kV, and the error is only about 0.05%. α DG1 The value is inappropriate, for example α DG1 =100°, It is only 5.985 kV, which is far below the maximum value.

[0100] Table 2 In 8 different α DG1 Changes under the action of β T1 constant)

[0101]

[0102] Table 3 and Fig.14 , Fig.15 It shows that the output current amplitude of DG1 remains unchanged, and α DG1 Maintain optimal ( α DG1 =309°), change the output current phase angle of T1 8 times, that is, in 8 groups of different β T1 Under the effect, From Table 3 and Fig.14 , Fig.15 It can be seen that at the beginning of the fault, before T1 and DG1 inject current, is only 3.953 kV. β T1 =309° (-51°), Reaching a maximum value of 7.585 kV, Fig.10 middle The maximum value is 7.581 kV, and the error is only about 0.05%. β T1 The value is inappropriate, for example β T1 =100°, It is only 2.669 kV, which is far below the maximum value.

[0103] Table 3 In 8 different β T1 Changes under the action of α DG1 constant)

[0104]

[0105] It should be understood that the embodiments disclosed in the present invention are not limited to the specific processing steps or materials disclosed herein, but should be extended to equivalent substitutions of such features understood by ordinary technicians in the relevant field. It should also be understood that the terms used herein are only used for the purpose of describing specific embodiments and are not meant to be limiting.

[0106] The "embodiment" mentioned in the specification means that the specific features or characteristics described in conjunction with the embodiment are included in at least one embodiment of the present invention. Therefore, the phrases or "embodiment" appearing in various places throughout the specification do not necessarily refer to the same embodiment.

[0107] In addition, the described features or characteristics may be combined in one or more embodiments in any other suitable manner. In the above description, some specific details, such as thickness, quantity, etc., are provided to provide a comprehensive understanding of the embodiments of the present invention. However, those skilled in the relevant art will understand that the present invention can be implemented without one or more of the above specific details or can also be implemented using other methods, components, materials, etc.

Claims

1. A voltage support method for a distributed power source and a flexible direct current converter, characterized in that: The following steps are involved: Establish a three-sequence network for upstream faults of the active distribution network, and establish positive, negative, and zero-sequence equivalent networks of the active distribution network to form a three-sequence network for upstream faults of the active distribution network; The three-sequence network of the upstream fault of the composite active distribution network is equivalent to the distributed power supply and the flexible DC converter as the positive sequence current source, and the three-sequence network of the upstream fault of the active distribution network is connected in parallel at the fault point to obtain the composite sequence network of the active distribution network when a two-phase grounding fault occurs in the upstream of the distribution network; Simplifying the composite sequence network of the upstream fault of the active distribution network, simplifying the composite sequence network of the upstream fault of the active distribution network to obtain a simplified composite sequence network of the upstream fault of the active distribution network; The optimal output current phase angle of the distributed power source and the flexible DC converter is derived. The simplified composite sequence network of the upstream fault of the active distribution network is used to analyze the influence of the distributed power source and the flexible DC converter on the voltage of the distributed power source grid connection point, and the optimal output current phase angle of the distributed power source and the flexible DC converter is derived. Among them, simplifying the composite sequence network of the upstream fault of the active distribution network includes one simplification, two simplifications and three simplifications; The first simplification is as follows: simplify the circuit to the right of the fault point f1 in the composite sequence network to form impedance Z A , whose expression is: In formula (2), is the negative sequence impedance of Z1, 3Z f is the zero-sequence fault impedance, is the zero-sequence impedance of Z1, 3Z N is the zero-sequence impedance of the neutral point impedance of grid 1; Simplified again: for impedance Z A Further simplification gives the impedance Z B , whose expression is: In formula (3), express Z eq Expressed in the form of magnitude and phase angle, |Z eq | and δ eq are their amplitude and phase angle respectively; The third simplification is as follows: for impedance Z B Further simplification gives the impedance Z C , whose expression is: In formula (4), |Z C | and φ C Z C The magnitude and phase angle of the voltage phasor at the grid connection point of DG1 can be and Add them together and you get, and are expressed in the form of amplitude and phase angle, and "α DG1 +φ C ” are The magnitude and phase angle of and "β T1 +φ C ” are The magnitude and phase angle of If and only if and When parallel, The amplitude The optimal phase angle of the output current of DG1 and flexible DC converter T1 is expressed as: In formula (5), α DG1_opt and β T1_opt are the optimal phase angles of the output currents of DG1 and flexible DC converter T1, that is, α DG1 and β T1 The best phase angle.

2. The voltage support method for the coordinated use of distributed power sources and flexible direct current converters according to claim 1 is characterized in that: The active distribution network includes grid 1, grid 2, flexible DC converter T1 and flexible DC converter T2. Grid 1 and grid 2 are interconnected through flexible DC converter T1 and flexible DC converter T2. The neutral point of grid 1 is grounded through a resistor, and the neutral point of grid 2 is grounded through an arc suppression coil. Flexible DC converter T1 and flexible DC converter T2 are both connected to a 20kV DC bus.

3. The voltage support method for coordinated distributed power supply and flexible direct current converter according to claim 2 is characterized in that: In power grid 1, the length of section AB is L1, the line impedance is Z1, the length of section BC is L2, the line impedance is Z2, the length of section CD is L3, the line impedance is Z3, and the fault point f1 is located at the junction of section AB and section BC. This fault is an upstream fault of distributed power source DG1; in power grid 2, the line length of section EF is L4, the line impedance is Z4, the length of section FG is L5, and the line impedance is Z5.

4. The voltage support method for coordinated distributed power supply and flexible direct current converter according to claim 2 is characterized in that: Both the flexible DC converter T1 and the flexible DC converter T2 adopt a modular multi-level converter structure.

5. The voltage support method for coordinated distributed power supply and flexible direct current converter according to claim 1, characterized in that: In the three-sequence network with an upstream fault in the active distribution network, the grid voltage phasor before the fault Current phasor of distributed generation DG1 And the current phasor of the flexible DC converter T1 The expression is: In formula (1), U s|0| and θ s They are The magnitude and phase angle of and α DG1 They are The magnitude and phase angle of and β T1 They are Amplitude and phase angle.

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