A topological structure and control method of a meshed flexible direct current system

Abstract

The present application provides a grid-type flexible direct current system topology structure and control method, including: at least one grid-type MMC converter station, a DC-side centralized energy storage device and an open DC bus; at least one grid-type MMC converter station and a DC-side centralized energy storage device are respectively connected to the open DC bus to form a grid-type flexible direct current single-port system topology structure or a grid-type flexible direct current multi-port system topology structure; the open DC bus of the grid-type flexible direct current single-port system topology structure or the grid-type flexible direct current multi-port system topology structure is connected to a DC grid-connected device, and each grid-type MMC converter station is connected to an AC system; the grid-type flexible direct current single-port system topology structure and the grid-type flexible direct current multi-port system topology structure of the present application can improve the regulation capability and flexibility of the power system, smooth out fluctuations in new energy power generation, improve system inertia, voltage and frequency support capabilities, control short-circuit capacity and improve grid damping characteristics, and improve the stability of new energy grid connection.

Description

Technical Field

[0001] The present application belongs to the field of power systems, and specifically relates to a grid-type flexible direct current system topology structure and control method. Background Art

[0002] The foundation of the new power system is still the AC synchronous operation mechanism. In order to ensure energy and power security and meet the power needs of economic and social development, the new power system requires a deep integration of new digital technologies and traditional technologies, supported by the interaction of sources, grids, loads and storage and multi-energy complementarity, to improve the stability of the power system.

[0003] In the past decade, large-scale access of new energy sources such as wind and solar power to the power grid through power electronic converters has become one of the main technical features of the new generation of power systems. The cluster development of new energy sources based on resource distribution has become an important means of large-scale development and utilization of new energy sources. However, multiple clusters are distributed over a wide area, the power grid is relatively weak, and the stability of the weak power grid interconnection system has become an important factor restricting the large-scale development and utilization of new energy sources.

[0004] At present, the development of new energy clusters adopts AC system collection and interconnection or DC system collection and interconnection. The AC system is used to collect power in a wide area, which has large power fluctuations, and its voltage stability and the grid connection stability of wind and solar power sources make this technology face great challenges. The solution of using flexible DC system to collect new energy power and send it out to realize the interconnection of asynchronous power grids where new energy bases are located has received widespread attention.

[0005] In typical regional interconnected flexible DC transmission systems, grid-following control technology is often used. The receiving-end converter ensures the transmission of active power through DC voltage control, and adjusts the reactive power exchanged with the receiving-end grid through reactive power control. This traditional control strategy mainly relies on vector control of phase-locked loop tracking. After a disturbance occurs in the receiving-end grid, this control method cannot provide effective inertia support, resulting in an increase in the frequency deviation of the receiving-end grid during the transient period, which seriously affects the stable operation of the system. In a weak grid interconnection system, the dynamic process of the phase-locked loop affects the stability of the control system; under certain extreme conditions, the phase-locked loop will introduce negative damping to the system, causing subsynchronous oscillation of the converter. Summary of the invention

[0006] In order to overcome the above-mentioned deficiencies of the prior art, the present application proposes a grid-type flexible direct current system topology structure, comprising: at least one grid-type MMC converter station, a DC-side centralized energy storage device and an open DC bus;

[0007] The at least one grid-type MMC converter station is connected to the open DC bus, and the at least one grid-type MMC converter station is connected to an AC system outside the grid-type flexible DC system topology structure;

[0008] The DC side centralized energy storage device is connected to the open DC bus;

[0009] The open DC bus is connected to a DC grid-connected device outside the grid-forming flexible DC system topology structure.

[0010] Preferably, the meshed flexible direct current system topology structure includes: a meshed flexible direct current single-port system topology structure and a meshed flexible direct current multi-port system topology structure;

[0011] The grid-type flexible DC single-port system topology structure includes: a DC side of a grid-type MMC converter station and a DC side centralized energy storage device are respectively connected to the open DC bus;

[0012] The grid-type flexible DC multi-port system topology structure includes: the DC sides of at least two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to an open DC bus.

[0013] Preferably, the meshed flexible direct current multi-port system topology structure includes: a meshed flexible direct current dual-port system topology structure and a meshed flexible direct current multi-port system topology structure;

[0014] The grid-type flexible DC dual-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to an open DC bus; the AC side of each grid-type MMC converter station is respectively connected to a corresponding AC system, and the open DC bus is connected to a DC grid-connected device;

[0015] The grid-type flexible DC multi-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to the open DC bus, and other grid-type MMC converter stations are connected in parallel to the open DC bus through other open DC bus; the AC side of each grid-type MMC converter station is respectively connected to the corresponding AC system, and the open DC bus is connected to the DC grid-connected equipment.

[0016] Preferably, when the grid-type flexible DC single-port system topology is a true bipolar topology, the grid-type MMC converter station includes a first grid-type MMC unit, a neutral line, a second grid-type MMC module unit, a first transformer, a second transformer and a first grid connection point; the DC side centralized energy storage device includes a first energy storage branch and a second energy storage branch; the open DC bus includes: a first open DC bus and a second open DC bus; one end of the first transformer is connected to the first grid-type MMC unit, and the other end of the first transformer is connected to the first grid connection point; one end of the second transformer is connected to the second grid-type MMC unit, and the other end of the second transformer is connected to the first grid connection point;

[0017] One end of the first meshed MMC unit is connected to the first open DC bus, and the other end of the first meshed MMC unit is connected to the neutral line; one end of the second meshed MMC unit is connected to the neutral line, and the other end of the second meshed MMC unit is connected to the second open DC bus;

[0018] One end of the first energy storage branch is connected to the first open-type DC bus, and the other end of the first energy storage branch is connected to the neutral line; one end of the second energy storage branch is connected to the neutral line, and the other end of the second energy storage branch is connected to the second open-type DC bus;

[0019] The first open DC bus and the second open DC bus are connected to a DC grid-connected device as required, and the first grid-connected point is connected to an AC system.

[0020] Preferably, when the topology structure of the grid-type flexible DC single-port system is a pseudo-bipolar topology, the grid-type MMC converter station includes a third grid-type MMC unit, a third transformer and a second grid connection point; the DC side centralized energy storage device includes a third energy storage branch; the open DC bus includes: a third open DC bus and a fourth open DC bus; one end of the third transformer is connected to the third grid-type MMC unit, and the other end of the third transformer is connected to the second grid connection point;

[0021] One end of the third meshed MMC unit is connected to the third open-type DC bus, and the other end of the third meshed MMC unit is connected to the fourth open-type DC bus;

[0022] One end of the third energy storage branch is connected to the third open-type DC bus, and the other end of the third energy storage branch is connected to the fourth open-type DC bus;

[0023] The third open DC bus and the fourth open DC bus are connected to a DC grid-connected device as required, and the second grid-connected point is connected to an AC system.

[0024] Preferably, the third network-type MMC unit comprises: three phase units;

[0025] The three phase units are connected in parallel, one end of each of the phase units connected in parallel is connected to the third open-type DC bus, and the other end of each of the phase units is connected to the fourth open-type DC bus;

[0026] The third transformer is connected to each of the phase units;

[0027] Wherein, the phase unit includes: an upper bridge arm and a lower bridge arm, and the upper bridge arm or the lower bridge arm is formed by a reactor and a plurality of MMC submodules connected in series. Preferably, the third energy storage branch includes: a main branch reactor L, a cascade energy storage submodule and a soft start loop CB1+R;

[0028] The main branch reactance L, the cascade energy storage submodule and the soft start loop CB1+R are connected in series in sequence;

[0029] The other end of the main branch reactor L and the other end of the soft-start loop CB1+R are connected to an open DC bus.

[0030] Based on the same inventive concept, the present application also provides a control method for the topological structure of a meshed flexible direct current system, including:

[0031] Based on the obtained network control parameters, a network control strategy is adopted through the network MMC converter station of the network flexible direct current system topology structure to control the network MMC converter station to exchange energy with the corresponding connected AC system; based on the obtained voltage parameters, a DC voltage control strategy is adopted through the DC side centralized energy storage device of the network flexible direct current system topology structure to control the voltage of the open DC bus;

[0032] The meshed flexible DC system topology structure is constructed based on the meshed flexible DC system topology structure described above.

[0033] Preferably, the method of adopting a networking control strategy based on the acquired networking control parameters through the networking type MMC converter station of the networking type flexible direct current system topology structure to control the networking type MMC converter station to exchange energy with the corresponding connected AC system includes:

[0034] Based on the obtained network control parameters, the network control strategy is adopted through the network type MMC converter station to calculate the MMC k-phase upper bridge arm reference voltage and the MMC k-phase lower bridge arm reference voltage;

[0035] Based on the MMC k-phase upper bridge arm reference voltage and the MMC k-phase lower bridge arm reference voltage, the meshing MMC submodule is triggered through modulation and capacitor voltage balancing control to control the active power, reactive power, AC side system frequency and AC side system voltage exchanged between the meshing MMC converter station and the corresponding connected AC system;

[0036] Among them, the network control parameters include: output active power command value, grid-connected side frequency reference value, output reactive power command value, grid-connected side voltage reference value, grid-connected side power grid frequency, grid-connected point electromagnetic active power, grid-connected point electromagnetic reactive power and grid-connected point voltage.

[0037] Preferably, the method of controlling the open DC bus voltage by using a DC voltage control method through the DC side centralized energy storage device of the grid-type flexible DC system topology structure based on the acquired voltage parameter includes:

[0038] Based on the voltage parameters of the open DC bus, the DC voltage control strategy is adopted through the DC side centralized energy storage device in the grid-type flexible DC system topology to calculate the reference voltage of the cascaded energy storage submodule;

[0039] Based on the reference voltage of the cascaded energy storage submodule, the cascaded energy storage submodule is triggered through SOC balancing control and modulation to perform open DC bus voltage control;

[0040] Wherein, the control voltage parameters include: an open DC bus voltage value, an open DC bus voltage reference high value and an open DC bus voltage reference low value.

[0041] Compared with the closest prior art, the present application has the following beneficial effects:

[0042] The present application provides a grid-type flexible direct current system topology structure and control method, including: at least one grid-type MMC converter station, a DC-side centralized energy storage device and an open DC bus; the at least one grid-type MMC converter station and the DC-side centralized energy storage device are respectively connected to the open DC bus to form a grid-type flexible direct current single-port system topology structure or a grid-type flexible direct current multi-port system topology structure; the open DC bus of the grid-type flexible direct current single-port system topology structure or the grid-type flexible direct current multi-port system topology structure is connected to the DC grid-connected equipment outside the grid-type flexible direct current system topology structure as needed, and each grid-type MMC converter station is connected to the AC system outside the grid-type flexible direct current system topology structure; the grid-type flexible direct current single-port system topology structure and the grid-type flexible direct current multi-port system topology structure of the present application can improve the regulation capability and flexibility of the power system, can smooth out the fluctuation of new energy power generation, improve the system inertia, voltage and frequency support capabilities, control the short-circuit capacity and improve the damping characteristics of the power grid, and improve the stability of new energy grid connection. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A schematic diagram of the topological structure of a meshed flexible direct current system provided in this application;

[0044] Figure 2 Schematic diagram of the grid-type flexible direct current single-port topology (a) based on centralized energy storage on the DC side provided in this application;

[0045] Figure 3 Schematic diagram of the grid-type flexible DC single-port topology (b) based on DC-side centralized energy storage provided in this application;

[0046] Figure 4 A schematic diagram of the networking MMC submodule topology (a) provided in this application;

[0047] Figure 5 A schematic diagram of the networking type MMC submodule topology (b) provided in this application;

[0048] Figure 6 A schematic diagram of the energy storage branch topology provided for this application;

[0049] Figure 7 A schematic diagram of the energy storage submodule provided for this application;

[0050] Figure 8 A schematic diagram of the topological structure of a grid-type flexible DC dual-port system based on centralized energy storage on the DC side provided in this application;

[0051] Fig. 9 A schematic diagram of three ports in the topology of a grid-type flexible DC multi-port system based on DC-side centralized energy storage provided in this application;

[0052] Fig.10 A schematic diagram of an application scenario of a grid-type flexible DC single-port system topology structure with centralized energy storage on the DC side provided in this application;

[0053] Fig.11 A schematic diagram of an application scenario of a grid-type flexible DC dual-port system topology structure with centralized energy storage on the DC side provided in this application;

[0054] Fig.12 A schematic diagram of an application scenario of a grid-type flexible DC multi-port system topology structure with centralized energy storage on the DC side provided in this application;

[0055] Fig.13 A flow chart of a control method for a topological structure of a meshed flexible direct current system provided in this application;

[0056] Fig.14 This is a block diagram of the joint control of the grid-type MMC and DC-side centralized energy storage provided in this application. DETAILED DESCRIPTION

[0057] The specific implementation methods of the present application are further described in detail below in conjunction with the accompanying drawings.

[0058] Embodiment 1:

[0059] The present application provides a topological structure of a meshed flexible direct current system. Figure 1 As shown, it includes: at least one grid-type MMC converter station, a DC-side centralized energy storage device and an open DC bus;

[0060] The at least one grid-type MMC converter station is connected to the open DC bus, and the at least one grid-type MMC converter station is connected to an AC system outside the grid-type flexible DC system topology structure;

[0061] The DC side centralized energy storage device is connected to the open DC bus;

[0062] The open DC bus is connected to a DC grid-connected device outside the grid-forming flexible DC system topology structure.

[0063] In one implementation, the meshed flexible direct current system topology structure includes: a meshed flexible direct current single-port system topology structure and a meshed flexible direct current multi-port system topology structure;

[0064] The grid-type flexible DC single-port system topology structure includes: a DC side of a grid-type MMC converter station and a DC side centralized energy storage device are respectively connected to the open DC bus;

[0065] The grid-type flexible DC multi-port system topology structure includes: the DC sides of at least two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to an open DC bus.

[0066] For example, the meshed flexible direct current system topology is a single-port system topology and a multi-port system topology, and the number of ports corresponds to the number of meshed MMC converter stations included.

[0067] In one implementation, the meshed flexible DC multi-port system topology structure includes: a meshed flexible DC dual-port system topology structure (for example, including two meshed MMC converter stations) and a meshed flexible DC multi-port system topology structure (for example, including three or more meshed MMC converter stations);

[0068] The grid-type flexible DC dual-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to an open DC bus; the AC side of each grid-type MMC converter station is respectively connected to a corresponding AC system, and the open DC bus is connected to a DC grid-connected device;

[0069] The grid-type flexible DC multi-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to the open DC bus, and other grid-type MMC converter stations are connected in parallel to the open DC bus through other open DC bus; the AC side of each grid-type MMC converter station is respectively connected to the corresponding AC system, and the open DC bus is connected to the DC grid-connected equipment.

[0070] In this implementation method, the meshing-type flexible DC multi-port system topology in the meshing-type flexible DC system topology has three or more ports. This application designs a meshing-type flexible DC single-port system topology, a meshing-type flexible DC dual-port system topology and a meshing-type flexible DC multi-port system topology to meet a variety of different power grid application scenarios.

[0071] The above-mentioned meshing type MMC converter station can adopt a true bipolar topology or a pseudo bipolar topology, and correspondingly, the meshing type flexible direct current system topology can also be a true bipolar topology or a pseudo bipolar topology. The following mainly takes the meshing type flexible direct current single-port system topology as a true bipolar topology or a pseudo bipolar topology as an example for explanation. The meshing type flexible direct current two-port system topology and the meshing type flexible direct current multi-port system topology as a true bipolar topology or a pseudo bipolar topology are similar, and will not be explained one by one here.

[0072] In one example, when the grid-type flexible direct current single-port system topology is a true bipolar topology, the grid-type MMC converter station includes a first grid-type MMC unit, a second grid-type MMC unit, a neutral line, a first transformer, a second transformer and a first grid connection point; the DC side centralized energy storage device includes a first energy storage branch and a second energy storage branch; the open DC bus includes: a first open DC bus and a second open DC bus; one end of the first transformer is connected to the first grid-type MMC unit, and the other end of the first transformer is connected to the first grid connection point;

[0073] One end of the second transformer is connected to the second grid-type MMC unit, and the other end of the second transformer is connected to the first grid-connected point;

[0074] One end of the first meshed MMC unit is connected to the first open DC bus, and the other end of the first meshed MMC unit is connected to the neutral line; one end of the second meshed MMC unit is connected to the neutral line, and the other end of the second meshed MMC unit is connected to the second open DC bus;

[0075] One end of the first energy storage branch is connected to the first open-type DC bus, and the other end of the first energy storage branch is connected to the neutral line; one end of the second energy storage branch is connected to the neutral line, and the other end of the second energy storage branch is connected to the second open-type DC bus;

[0076] The first open DC bus and the second open DC bus are connected to a DC grid-connected device as required, and the first grid-connected point is connected to an AC system.

[0077] In another example, when the topology of the grid-type flexible DC single-port system is a pseudo-bipolar topology, the grid-type MMC converter station includes a third grid-type MMC unit, a third transformer and a second grid connection point; the DC side centralized energy storage device includes a third energy storage branch; the open DC bus includes: a third open DC bus and a fourth open DC bus;

[0078] One end of the third transformer is connected to the third grid-type MMC unit, and the other end of the third transformer is connected to the second grid-connected point;

[0079] One end of the third meshed MMC unit is connected to the third open-type DC bus, and the other end of the third meshed MMC unit is connected to the fourth open-type DC bus;

[0080] One end of the third energy storage branch is connected to the third open-type DC bus, and the other end of the third energy storage branch is connected to the fourth open-type DC bus;

[0081] The third open DC bus and the fourth open DC bus are connected to a DC grid-connected device as required, and the second grid-connected point is connected to an AC system.

[0082] In the example, the third meshed MMC unit includes: three phase units;

[0083] The three phase units are connected in parallel, one end of each of the phase units connected in parallel is connected to the third open-type DC bus, and the other end of each of the phase units is connected to the fourth open-type DC bus;

[0084] The third transformer is connected to each of the phase units;

[0085] The phase unit comprises an upper bridge arm and a lower bridge arm, and the upper bridge arm or the lower bridge arm is formed by a reactor and a plurality of MMC submodules connected in series.

[0086] In this implementation, the third transformer is connected to the connection point of the upper bridge arm and the lower bridge arm of each phase unit.

[0087] In the example, the third energy storage branch includes: a main branch reactance L, a cascade energy storage submodule and a soft start loop CB1+R;

[0088] The main branch reactance L, the cascade energy storage submodule and the soft start loop CB1+R are connected in series in sequence;

[0089] The other end of the main branch reactor L and the other end of the soft-start loop CB1+R are connected to an open DC bus.

[0090] The grid-type flexible direct current system topology structure of the present application is particularly suitable for weak power grid interconnection systems. The converter station exhibits voltage source characteristics, has four-quadrant power regulation capabilities and millisecond-level response speeds, and provides active support and reactive power regulation functions, thereby improving the stability of weak power grid interconnection systems.

[0091] This application combines grid-type control with flexible DC transmission and energy storage to form a grid-type flexible DC system topology based on centralized energy storage on the DC side. The flexible DC system topology of this application can provide stable support for DC voltage and build a central DC bus; it uses grid-type control technology to simulate the characteristics of synchronous generators, and uses voltage source technology + modular multi-level control technology + energy storage technology to achieve inertia support, voltage support, short-circuit capacity support, voltage regulation and other functions to improve the stability of weak power grid interconnection systems; it can also be jointly controlled with new energy sources as a black start power source for regional power grids after disasters.

[0092] Example 2

[0093] On the basis of the above embodiments, the topological structure of the grid-type flexible DC single-port system based on DC side centralized energy storage of the present invention is as follows Figure 2 or as shown in 3, including:

[0094] (1) The topology consists of three parts: the grid-connected part, the energy storage part and the open DC bus. The grid-connected part is connected to the AC system through a grid-connected MMC converter station via a transformer; the energy storage part is connected in parallel to the open DC bus by multiple power electronically controlled energy storage branches. The open DC bus is used to connect to new energy sources such as wind power and photovoltaic power or DC power loads that require DC grid connection.

[0095] (2) The grid-connected MMC converter station adopts a true bipolar topology or a pseudo bipolar topology, that is, the grid-connected flexible DC single-port system topology is also a true bipolar topology or a pseudo bipolar topology. The topology of the MMC submodules in the grid-connected MMC converter station (an example of the above-mentioned grid-connected MMC converter station) is SMAp1~SMCnN. Figure 4 As shown in 5, a half-bridge submodule is preferably used. According to design requirements, part of the half-bridge submodule + part of the full-bridge submodule or all of the full-bridge submodules can also be used.

[0096] (3) The DC side centralized energy storage device is composed of multiple energy storage branches connected in parallel. The energy storage branch topology is as follows: Figure 6 As shown, the main equipment includes cascaded energy storage submodule topology as shown in Figure 7 As shown, the soft start circuit CB1+R, the main branch reactor L, etc., each cascade energy storage sub-module is mainly composed of a half-bridge power module connected in parallel with multiple groups of parallel energy storage battery cabinets via a connecting reactor. The cascade energy storage sub-module also includes a conduction bypass switch BS and a shunt thyristor T for protecting the half-bridge power module.

[0097] (4) The PCC point (i.e., grid connection point) of the grid-connected part can be connected to AC power grids of different levels.

[0098] (5) The voltage level of the open DC bus is adapted to the voltage of the DC centralized energy storage or can be flexibly set according to the connected DC grid-connected equipment.

[0099] Example 3

[0100] On the basis of the above embodiments, the present invention is based on the topological structure of the grid-type flexible DC dual-port system with centralized energy storage on the DC side. Figure 8 As shown, including:

[0101] (1) The dual-port system consists of two back-to-back grid-type MMC converter stations, each of which is connected to an AC system via a transformer.

[0102] (2) The back-to-back grid-type MMC converter stations are connected by DC lines in the middle; the DC lines are also open DC buses;

[0103] (3) The DC side centralized energy storage device is directly hung on the DC line and is jointly controlled with its proximal grid-type MMC converter station.

[0104] (4) The grid-type MMC converter station 1 and the grid-type MMC converter station 2 can be connected to AC systems of different voltage levels.

[0105] (5) In addition to being used for energy transfer between AC system 1 and AC system 2, the DC line can also be used to connect new energy sources such as wind power and photovoltaic power or DC power loads that require DC grid connection.

[0106] Example 4

[0107] On the basis of the above embodiments, the present invention is based on the example of three ports in the topological structure of the grid-type flexible DC multi-port system with centralized energy storage on the DC side. Fig. 9 As shown, including:

[0108] (1) The multi-port system contains multiple mesh-type MMC converter stations, each of which is connected to an AC system via a transformer.

[0109] (2) Multiple grid-type MMC converter stations are connected in parallel by DC lines.

[0110] (3) The DC side centralized energy storage device is directly hung on the DC line and is jointly controlled with its proximal grid-type MMC converter station.

[0111] (4) Each grid-type MMC converter station can be connected to AC power grids of different voltage levels.

[0112] (5) In addition to being used for energy transfer between multiple AC systems, DC lines can also be used to connect new energy sources such as wind power and photovoltaic power or DC power loads that require DC grid connection.

[0113] Embodiment 5:

[0114] Based on the above embodiments, Fig.10 As shown, an embodiment of the present invention provides a schematic diagram of an application scenario of a grid-type flexible DC single-port system topology structure with centralized energy storage on the DC side. As shown in the figure, the wind farm and photovoltaic are collected through the DC bus, connected to the AC grid through the grid-type MMC station, and transmitted to the load center area. This topology can achieve output balance control of wind and photovoltaic power generation; achieve bidirectional isolation of AC and DC faults; participate in dynamic reactive power support under emergency fault conditions, help new energy to achieve fault ride-through, including continuous fault ride-through; and achieve functions such as black start of grid-connected new energy.

[0115] Example 6

[0116] Based on the above embodiments, Fig.11 As shown, an embodiment of the present invention provides a schematic diagram of an application scenario of a grid-type flexible direct current dual-port system topology structure with centralized energy storage on the DC side. The application scenario is large-scale wind power development and transmission through flexible direct current. The centralized energy storage on the DC side provides a stable DC voltage for the flexible direct current at both ends. The converters at both ends adopt grid-type control, and there is no need to autonomously synchronize with the grid through a phase-locked loop. Both ends reflect the characteristics of a voltage source, which can effectively support the stable operation of a weak grid. When a sudden load change occurs in the grid, the flexible direct current transmission system can automatically respond to the power shortage and adjust the transmitted active power; it can provide the necessary inertia support and auxiliary frequency regulation to the power system; when a short circuit occurs in the grid, the grid-type MMC converter station can actively output reactive power and provide dynamic voltage support while ensuring its own safe and stable crossing.

[0117] Example 7

[0118] Based on the above embodiments, Fig.12 As shown, an embodiment of the present invention provides a schematic diagram of an application scenario of a grid-type flexible DC multi-port system topology structure with centralized energy storage on the DC side. This application scenario is an application scenario of multi-terminal weak system interconnection via flexible DC collection for the development of new energy clusters. The topology of the present invention can improve the stability of the weakly interconnected system.

[0119] Embodiment 8:

[0120] Based on the same inventive concept, the present application also provides a control method for the topological structure of a meshed flexible direct current system. Fig.13 As shown, including:

[0121] Step 1: Based on the obtained networking control parameters, a networking control strategy is adopted through a networking MMC converter station of a networking flexible direct current system topology structure to control the networking MMC converter station to exchange energy with a correspondingly connected AC system;

[0122] Step 2: Based on the acquired voltage parameters, a DC voltage control strategy is adopted through the DC side centralized energy storage device of the grid-type flexible DC system topology structure to control the voltage of the open DC bus;

[0123] Among them, the meshed flexible DC system topology structure is constructed based on the above-mentioned meshed flexible DC system topology structure.

[0124] Step 1 is to control the grid-forming MMC converter station and the corresponding connected AC system to exchange energy based on the obtained grid-forming control parameters and adopt a grid-forming control strategy through the grid-forming MMC converter station of the grid-forming flexible direct current system topology structure, including:

[0125] Based on the obtained network control parameters, the network control strategy is adopted through the network type MMC converter station to calculate the MMC k-phase upper bridge arm reference voltage and the MMC k-phase lower bridge arm reference voltage;

[0126] Based on the MMC k-phase upper bridge arm reference voltage and the MMC k-phase lower bridge arm reference voltage, the meshing MMC submodule is triggered through modulation and capacitor voltage balancing control to control the active power, reactive power, AC side system frequency and AC side system voltage exchanged between the meshing MMC converter station and the corresponding connected AC system;

[0127] Among them, the network control parameters include: output active power command value, grid-connected side frequency reference value, output reactive power command value, grid-connected side voltage reference value, grid-connected side power grid frequency, grid-connected point electromagnetic active power, grid-connected point electromagnetic reactive power and grid-connected point voltage.

[0128] Step 2 is to control the open DC bus voltage based on the acquired voltage parameters by using a DC voltage control method through the DC side centralized energy storage device of the grid-type flexible DC system topology structure, including:

[0129] Based on the voltage parameters of the open DC bus, the DC voltage control strategy is adopted through the DC side centralized energy storage device in the grid-type flexible DC system topology to calculate the reference voltage of the cascaded energy storage submodule;

[0130] Based on the reference voltage of the cascaded energy storage submodule, the cascaded energy storage submodule is triggered through SOC balancing control and modulation to perform open DC bus voltage control;

[0131] Wherein, the control voltage parameters include: an open DC bus voltage value, an open DC bus voltage reference high value and an open DC bus voltage reference low value.

[0132] Step 2 also includes: using different DC bus voltage reference values ​​when charging and discharging the DC side direct-mounted energy storage, and using a high value U when charging. dcref_ch , low value U is used during discharge dcref_disch ; The present invention adopts a joint control method between the grid-type MMC converter station and the DC-side centralized energy storage in the grid-type flexible DC system topology based on the DC-side centralized energy storage, which can improve the regulation ability and flexibility of the power system, smooth the fluctuation of new energy power generation, improve the system inertia, voltage and frequency support capabilities, control the short-circuit capacity and improve the grid damping characteristics, and improve the stability of new energy grid connection; the present invention adopts grid-type control technology to simulate the characteristics of synchronous generators, and uses voltage source technology + modular multi-level control technology + energy storage technology to realize inertia support, voltage support, short-circuit capacity support, voltage regulation and other functions, thereby improving the stability of weak power grid interconnection systems; it can also be jointly controlled with new energy to serve as a black start power source for regional power grids after disasters.

[0133] Embodiment 9:

[0134] A specific implementation of a control method for a topological structure of a meshed flexible direct current system of the present invention is as follows: Fig.14 As shown:

[0135] (1) The grid-forming MMC converter station adopts a grid-forming control strategy to adjust and stabilize the system frequency of the AC grid-connected point and control the voltage of the grid-connected point, thereby controlling the active / reactive power exchanged between the grid-forming MMC converter station and the connected AC system and the AC side system voltage;

[0136] The DC side directly mounted energy storage controls the DC side voltage of the MMC converter station and controls the SOC balance between energy storage branches and between each level of interconnected energy storage submodules.

[0137] (2) The quantities controlled by the grid-type MMC converter station include: output active power command value P ref , grid-connected frequency reference value ω ref , output reactive power command value Q ref , grid-connected voltage reference value U ref , grid frequency on the grid side ω g , Grid-connected point electromagnetic active power P e , electromagnetic reactive power Q at the grid connection point e And the grid voltage U m , the upper and lower bridge arm voltage reference values ​​are output through network control and Each submodule is triggered after modulation and submodule capacitor voltage balancing control.

[0138] (3) The DC side direct-mounted energy storage control quantity is: DC bus voltage value U dc , the DC voltage is controlled to output the energy storage branch submodule reference voltage u ESM_ref

[0139] u ESM-ref After SOC balancing control and modulation, each sub-module is triggered to perform open DC bus voltage control.

[0140] (4) The DC side direct-mounted energy storage uses different DC bus voltage reference values ​​during charging and discharging. A high value U is used during charging. dcref_ch , low value U is used during discharge dcref_disch .

[0141] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0142] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0143] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0144] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0145] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application rather than to limit its protection scope. Although the present application has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that after reading the present application, those skilled in the art can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the application, but these changes, modifications or equivalent substitutions are all within the protection scope of the claims to be approved.

Claims

1. A topological structure of a meshed flexible direct current system, characterized in that: include: At least one grid-type MMC converter station, a DC-side centralized energy storage device and an open DC bus; The at least one grid-type MMC converter station is connected to the open DC bus, and the at least one grid-type MMC converter station is connected to an AC system outside the grid-type flexible DC system topology structure; The DC side centralized energy storage device is connected to the open DC bus; The open DC bus is connected to a DC grid-connected device outside the grid-forming flexible DC system topology structure; The meshed flexible direct current system topology structure includes: a meshed flexible direct current single-port system topology structure, a meshed flexible direct current dual-port system topology structure and a meshed flexible direct current multi-port system topology structure; The grid-type flexible DC single-port system topology structure includes: a DC side of a grid-type MMC converter station and a DC side centralized energy storage device are respectively connected to the open DC bus; The grid-type flexible DC dual-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to the open DC bus; The grid-type flexible DC multi-port system topology structure comprises: at least two grid-type MMC converter stations, the DC sides of which and a DC side centralized energy storage device are respectively connected to an open DC bus; The grid-type flexible DC dual-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to an open DC bus; the AC side of each grid-type MMC converter station is respectively connected to a corresponding AC system, and the open DC bus is connected to a DC grid-connected device; The grid-type flexible DC multi-port system topology structure includes: the DC sides of two grid-type MMC converter stations and a DC side centralized energy storage device are respectively connected to the open DC bus, and other grid-type MMC converter stations are connected in parallel to the open DC bus through other open DC bus; the AC side of each grid-type MMC converter station is respectively connected to the corresponding AC system, and the open DC bus is connected to the DC grid-connected equipment.

2. The topological structure of the meshed flexible direct current system according to claim 1, characterized in that: When the topology structure of the grid-type flexible DC single-port system is a true bipolar topology, the grid-type MMC converter station includes: a first grid-type MMC unit, a second grid-type MMC unit, a neutral line, a first transformer, a second transformer and a first grid connection point; the DC side centralized energy storage device includes a first energy storage branch and a second energy storage branch; the open DC bus includes: a first open DC bus and a second open DC bus; One end of the first transformer is connected to the first grid-type MMC unit, and the other end of the first transformer is connected to the first grid-connected point; One end of the second transformer is connected to the second grid-type MMC unit, and the other end of the second transformer is connected to the first grid-connected point; One end of the first meshed MMC unit is connected to the first open DC bus, and the other end of the first meshed MMC unit is connected to the neutral line; one end of the second meshed MMC unit is connected to the neutral line, and the other end of the second meshed MMC unit is connected to the second open DC bus; One end of the first energy storage branch is connected to the first open-type DC bus, and the other end of the first energy storage branch is connected to the neutral line; one end of the second energy storage branch is connected to the neutral line, and the other end of the second energy storage branch is connected to the second open-type DC bus; The first open DC bus and the second open DC bus are connected to a DC grid-connected device as required, and the first grid-connected point is connected to an AC system.

3. The topological structure of the meshed flexible direct current system according to claim 1, characterized in that: When the topology structure of the grid-type flexible DC single-port system is a pseudo-bipolar topology, the grid-type MMC converter station includes a third grid-type MMC unit, a third transformer and a second grid connection point; the DC-side centralized energy storage device includes a third energy storage branch; the open DC bus includes: a third open DC bus and a fourth open DC bus; one end of the third transformer is connected to the third grid-type MMC unit, and the other end of the third transformer is connected to the second grid connection point; One end of the third meshed MMC unit is connected to the third open-type DC bus, and the other end of the third meshed MMC unit is connected to the fourth open-type DC bus; One end of the third energy storage branch is connected to the third open-type DC bus, and the other end of the third energy storage branch is connected to the fourth open-type DC bus; The third open DC bus and the fourth open DC bus are connected to a DC grid-connected device as required, and the second grid-connected point is connected to an AC system.

4. The topological structure of the meshed flexible direct current system according to claim 3, characterized in that: The third meshed MMC unit comprises: three phase units; The three phase units are connected in parallel, one end of each of the phase units connected in parallel is connected to the third open-type DC bus, and the other end of each of the phase units is connected to the fourth open-type DC bus; The third transformer is connected to each of the phase units; The phase unit comprises an upper bridge arm and a lower bridge arm, and the upper bridge arm or the lower bridge arm is formed by a reactor and a plurality of MMC submodules connected in series.

5. The topological structure of the meshed flexible direct current system according to claim 3, characterized in that: The third energy storage branch includes: a main branch reactance L, a cascade energy storage submodule and a soft start loop CB1+R; The main branch reactance L, the cascade energy storage submodule and the soft start loop CB1+R are connected in series in sequence; The other end of the main branch reactor L and the other end of the soft-start loop CB1+R are connected to an open DC bus.

6. A control method for a topological structure of a meshed flexible direct current system, characterized in that: include: Based on the obtained network control parameters, a network control strategy is adopted through a network-forming MMC converter station of a network-forming flexible direct current system topology structure to control the network-forming MMC converter station to exchange energy with a correspondingly connected AC system; Based on the obtained voltage parameters, a DC voltage control strategy is adopted through the DC side centralized energy storage device of the grid-type flexible DC system topology structure to control the voltage of the open DC bus; The meshed flexible DC system topology structure is constructed based on a meshed flexible DC system topology structure as described in any one of claims 1 to 5.

7. The method according to claim 6, characterized in that The method adopts a networking control strategy based on the obtained networking control parameters through the networking type MMC converter station of the networking type flexible direct current system topology structure to control the networking type MMC converter station to exchange energy with the corresponding connected AC system, including: Based on the obtained network control parameters, the network control strategy is adopted through the network type MMC converter station to calculate the MMCk phase upper bridge arm reference voltage and the MMC k phase lower bridge arm reference voltage; Based on the MMC k-phase upper bridge arm reference voltage and the MMC k-phase lower bridge arm reference voltage, the MMC submodule is triggered through modulation and capacitor voltage balancing control to control the active power, reactive power, AC side system frequency and AC side system voltage exchanged between the network-type MMC converter station and the corresponding connected AC system; Among them, the network control parameters include: output active power command value, grid-connected side frequency reference value, output reactive power command value, grid-connected side voltage reference value, grid-connected side power grid frequency, grid-connected point electromagnetic active power, grid-connected point electromagnetic reactive power and grid-connected point voltage.

8. The method according to claim 6, characterized in that Based on the obtained voltage parameter, the DC side centralized energy storage device of the grid-type flexible DC system topology structure adopts a DC voltage control method to perform open DC bus voltage control, including: Based on the voltage parameters of the open DC bus, the DC voltage control strategy is adopted through the DC side centralized energy storage device in the grid-type flexible DC system topology to calculate the reference voltage of the cascaded energy storage submodule; Based on the reference voltage of the cascaded energy storage submodule, the cascaded energy storage submodule is triggered through SOC balancing control and modulation to perform open DC bus voltage control; The voltage parameters include: an open DC bus voltage value, an open DC bus voltage reference high value, and an open DC bus voltage reference low value.

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