Embedded dc flexible interconnected power distribution network structure and control method thereof

Abstract

The application provides an embedded direct-current flexible interconnected power distribution network structure and a control method thereof. The embedded direct-current flexible interconnected power distribution network structure comprises a plurality of feeders and a plurality of converters. Adjacent feeders are connected with each other through tie switches. The AC end of each converter is connected with each feeder through sectionalizing switches. The DC end of each converter is connected with a circuit breaker and then connected with each other through a medium-voltage DC line. The application realizes power supply and mutual aid among multiple feeders, realizes flexible adjustment of power flow and dynamic reactive power compensation of a power distribution feeder when a high proportion of distributed energy is connected to the feeder, improves the carrying capacity of distributed power in the power distribution network, and realizes high-reliability power supply of the power distribution network.

Description

Technical Field

[0001] This invention relates to an embedded DC flexible interconnected distribution network structure and its control method. Background Technology

[0002] In the context of new power systems, distribution networks are gradually transforming from traditional passive single-radial power supply to active distribution networks with a high proportion of distributed power sources such as wind and solar power. However, the volatility and randomness of the output of distributed power sources such as wind and solar power can cause power quality problems such as voltage fluctuations. At the same time, the large-scale integration of distributed power sources increases the uncertainty of the distribution network's source and load, leading to load imbalance and reduced power supply reliability. In addition, the insufficient capacity of single feedback lines can also limit the capacity of distributed generation (DG) integration, resulting in a decrease in the utilization rate of new energy sources such as wind and solar power. When a high proportion of distributed energy is integrated into the feeder, flexible adjustment of power flow and dynamic reactive power compensation between feeders become particularly important. On the other hand, since tie switches cannot be frequently switched, using tie switches to interconnect multiple feedback lines is obviously not suitable for meeting the real-time power support requirements posed by the randomness of distributed power sources. Although using soft open points (SOPs) to partially replace tie switches can achieve continuous real-time control of power flow, it still suffers from the drawbacks of large capacity, high losses, and a large number of converters in the flexible interconnection equipment at the feeder end of back-to-back converters. This makes it difficult for the existing distribution network structure to meet the requirements for safe operation of the distribution network under the background of new power systems, especially in terms of high penetration rate of distributed energy, reliability improvement, and adjustment and control of power flow.

[0003] For example, CN108964149A discloses a coordinated control method for AC / DC flexible interconnected distribution networks considering fault conditions. This method connects the AC side of a multi-terminal flexible interconnection device to multiple AC distribution networks and the DC side to a DC distribution network as a tie switch, forming an AC / DC flexible interconnected distribution network. However, it connects the DC and AC networks through the terminal flexible interconnection device; the AC networks are not actually connected to each other. Power exchange between feeders can only be achieved through the DC network, and direct power exchange between AC feeders is not possible, thus failing to balance power. Furthermore, the converter can only be connected from the end of the feeder, making it impossible to flexibly select based on parameters such as the load capacity of the feeder, the capacity of the distributed power source, and the locations of sectionalizing switches and tie switches. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides an embedded DC flexible interconnected distribution network structure and its control method.

[0005] The present invention is achieved through the following technical solutions.

[0006] This invention provides an embedded DC flexible interconnected distribution network structure and its control method; it includes several feeders and several converters, with adjacent feeders interconnected by tie switches, the AC terminals of the converters and each AC feeder connected by sectionalizing switches, and the DC terminals of the converters interconnected by a DC switch and a medium-voltage DC line.

[0007] The converters connected to each AC feeder are interconnected through DC switches and medium-voltage DC lines to form an embedded DC network. They are also connected in parallel with the original AC distribution network through the sectionalizing switches on the AC side of the converters. The interconnection switches between each feeder and other feeders are retained, without changing the topology of the original AC distribution network.

[0008] The connection location of the converter is not fixed and can be flexibly selected based on parameters such as the load capacity of the feeder, the capacity of the distributed power supply, and the location of the sectionalizing switch and tie switch.

[0009] A control method for an embedded DC flexible interconnected distribution network, the method being as follows:

[0010] Mutual support and supply: When the output of distributed power sources in a certain feeder is insufficient or the load is heavy, while the output power of distributed power sources in other feeders is sufficient or the load is light, the power supply between multiple feeder lines can be mutually supported and supplied by adjusting the active power and reactive power commands of the converters connected to the feeders, and dynamic voltage compensation can be performed on the feeders.

[0011] Independent reactive power compensation: When a converter's DC switch is disconnected, disconnecting it from the DC side of the other converters, it can operate in independent reactive power compensation mode. Independent reactive power compensation can directly adjust the compensation reactive power command according to the dispatching regulations, or it can detect the AC voltage at the AC side sectionalizing switch of the converter and dynamically adjust the reactive power output of the converter according to the voltage curve specified by the dispatching regulations.

[0012] Load power restoration: When an AC feeder fails, the sectionalizing switches at both ends of the faulty section will operate to isolate the faulty area. Loads connected to the original feeder will be powered by the original feeder, while other loads will be powered by adjacent feeders or medium-voltage DC lines according to the current topology.

[0013] DC section fault in the distribution network: A blocking signal is issued to block the converter, all AC-side sectionalizing switches and DC-side switches of the converters are turned off, the embedded DC network is taken out of operation, and the AC distribution network remains in its original operating state. If it is a converter fault, the AC-side sectionalizing switches and DC-side switches of the faulty converter remain open, the AC-side sectionalizing switches and DC switches of the remaining converters are closed, and the embedded DC network is restarted; if it is a permanent DC line fault, all DC switches of the embedded DC network remain open, all AC-side sectionalizing switches of the converters are closed, the converters are restarted, and they operate in dynamic reactive power compensation mode.

[0014] The beneficial effects of this invention are: it realizes mutual power supply and support among multiple feeders, enables flexible power flow adjustment and dynamic reactive power compensation of distribution feeders when a high proportion of distributed energy is connected to the feeders, improves the carrying capacity of distributed power sources in the distribution network, and achieves highly reliable power supply to the distribution network. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the power grid structure of the present invention;

[0016] Figure 2 This is a schematic diagram of the power grid mutual assistance and supply principle of the present invention;

[0017] Figure 3 This is a schematic diagram of the fault handling principle of the DC side of the distribution network according to the present invention;

[0018] Figure 4 This is a schematic diagram of the AC feeder fault handling principle of the present invention;

[0019] Figure 5 These are example diagrams illustrating several feeder fault handling methods of the present invention. Detailed Implementation

[0020] The technical solution of the present invention is further described below, but the scope of protection is not limited to what is described.

[0021] An embedded DC flexible interconnected distribution network structure and its control method are disclosed. The existing single-radial power supply mode of the distribution network is transformed into a multi-feeder closed-loop power supply mode, which can realize power mutual supply and support between multiple feeders, and effectively reduce the equipment capacity and number of converters of the flexible interconnection equipment of feeders, so as to achieve high reliability power supply of the distribution network.

[0022] Multiple bidirectional AC / DC converters are connected to the feeder, and each AC / DC converter is interconnected via a medium-voltage DC cable line or overhead line. The AC / DC converters can be modular converters (MMC), two-level converters, or three-level converters.

[0023] Without altering the existing feeder topology, a flexible interconnection method for multiple AC feeders consisting of an embedded multi-terminal DC distribution network is provided. Converters are connected via medium-voltage DC lines, including cable lines or overhead lines, which can be newly built or modified along existing AC cable or overhead line corridors.

[0024] Each feeder is connected to an AC / DC converter. The location of the AC / DC converter can be flexibly selected based on parameters such as the load capacity of the feeder, the capacity of the distributed power supply, and the location of the sectionalizing switch and tie switch.

[0025] The AC / DC converters connected to each feeder can continuously adjust the active / reactive power transmitted between the feeders, enabling real-time power support for mutual assistance and supply among multiple feeder lines.

[0026] When the distributed power supply and load of each feedback line are unbalanced, the load rate of the multiple feedback lines can be balanced by adjusting the active power / reactive power command of the AC / DC converter.

[0027] An embedded multi-terminal DC network is used to form a flexible interconnected AC / DC hybrid power distribution configuration among multiple AC feeders. This can be achieved by constructing new feeders or retrofitting existing AC cable or overhead line corridors without altering the original feeder topology. The existing single-radial power supply mode of the distribution network is transformed into a multi-feeder closed-loop power supply model, enabling power exchange and mutual support among multiple feeders. This allows for flexible power flow adjustment and dynamic reactive power compensation when a high proportion of distributed energy is integrated into the feeders, increasing the carrying capacity of distributed power sources in the distribution network and achieving high-reliability power supply. A sophisticated converter control strategy enables continuous adjustment of the AC / DC converters in each feeder and the transfer of active / reactive power between feeders, achieving real-time power support for mutual support and mutual supply among multiple feeders and resolving the problem of uneven distribution power and load among feeders. The flexible interconnection method effectively reduces the equipment capacity and number of converters required for flexible feeder interconnection, while overcoming the shortcomings of large equipment capacity and high losses associated with existing back-to-back converter-based flexible interconnection at the feeder ends.

[0028] In existing feeder networks, AC / DC converters are connected via medium-voltage DC cables or overhead lines. The AC / DC converters can be modular, two-level, or three-level converters.

[0029] Each feeder is connected to an AC / DC converter. The location of the AC / DC converter can be flexibly selected based on parameters such as the load capacity of the feeder, the capacity of the distributed power supply, and the location of the sectionalizing switch and tie switch.

[0030] The AC / DC converters connected to each feeder can continuously adjust the active / reactive power transmitted between the feeders, enabling real-time power support for mutual assistance and supply among multiple feeder lines.

[0031] When the distributed power supply and load of each feedback line are unbalanced, the load rate of the multiple feedback lines can be balanced by adjusting the active power / reactive power command of the AC / DC converter.

[0032] When uneven load occurs between power supply areas, the embedded flexible interconnected DC distribution network will control the active power flow from lightly loaded areas to heavily loaded areas in real time according to the load level of different zones. This will balance the load of each power supply zone and optimize the power flow distribution of the distribution network.

[0033] The aforementioned system embeds the DC distribution network between multiple AC feeders, enabling multi-source power supply and improving power supply reliability. It also allows for dynamic zoning of the distribution network's power supply area. When a distributed power source or load fails, the faulty area can be disconnected via a sectionalizing switch, while the rest of the distribution network continues to supply power, achieving fault-tolerant reconfiguration of the distribution network.

[0034] In the aforementioned system, multiple AC feeders are connected through an embedded multi-terminal DC network. Under normal circumstances, the embedded multi-terminal DC network regulates the power flow of the distribution network through AC / DC converters connected to the AC feeders, thereby optimizing the power flow distribution. When the embedded multi-terminal DC network fails, the sectionalizing switches and DC circuit breakers at both ends of the AC / DC converter operate, and the embedded multi-terminal DC network shuts down. The distribution network structure operates according to the original distribution network frame structure, but the power flow of the distribution network cannot be regulated, resulting in an uncontrollable power flow state.

[0035] This embodiment further explains the technical solution of the present invention through a 10kV embedded flexible DC distribution network:

[0036] Example 1

[0037] This embodiment mainly includes a 10kV embedded flexible DC distribution network (see...). Figure 1 ), 10kV AC feeders 1 and 3 are connected to 10kV AC feeders 2 and 4 respectively via tie switches, and the AC sides of AC / DC converters 1, 2, 3 and 4 are connected to tie switches 1, 2, 3 and 4 respectively; the converters are connected to each other via medium-voltage DC lines.

[0038] The 10kV embedded flexible DC distribution network flexibly interconnects four 10kV AC feeders, which are divided into four power supply zones by tie switches. When the distributed power output in power supply zone III is insufficient or the load is heavy, while the distributed power output in power supply zone II is sufficient or the load is light, the active power commands of converters 2 and 3 will be adjusted through control strategies to achieve mutual power supply and assistance between power supply zones II and III (see [link to relevant documentation]). Figure 2 This will balance the load rate between power supply area II and power supply area III.

[0039] Example 2

[0040] When a fault occurs within the 10kV embedded flexible DC distribution network, the sectionalizing switches and DC circuit breakers at both ends of AC / DC converters 1, 2, 3, and 4 will trip, and the embedded flexible DC distribution network will be taken out of service (see [link]). Figure 3 The four 10kV AC feeders can still operate according to the original topology and will not reduce the power supply reliability of the original topology. However, at this time, the power flow of the distribution network cannot be regulated, and the power flow is in an uncontrollable state.

[0041] Example 3

[0042] When a fault occurs at load point 2 (see...) Figure 4 When the sectionalizing switches on both sides of load 2 operate, the faulty section is disconnected, load 1 is powered by feeder 1, and the continuous power supply to load 3 is ensured by the embedded flexible DC distribution network.

Claims

1. A control method for an embedded DC flexible interconnected distribution network, wherein the embedded DC flexible interconnected distribution network includes several feeders and several converters, adjacent feeders are interconnected by tie switches, the AC end of each converter is connected to each AC feeder by a sectionalizing switch, the DC end of each converter is connected to a DC switch and then interconnected by a medium-voltage DC line; the converters connected to each AC feeder are interconnected by DC switches and medium-voltage DC lines to form an embedded DC network, and are connected in parallel with the original AC distribution network by the sectionalizing switch on the AC side of the converter, retaining the tie switches between each feeder and other feeders, without changing the topology of the original AC distribution network; Its features are: The control method for embedded DC flexible interconnected distribution networks is as follows: Mutual support and supply: When the output of distributed power sources in any feeder is insufficient or the load is heavy, while the output power of distributed power sources in other feeders is sufficient or the load is light, the power mutual support and supply between multiple feeder lines can be achieved by adjusting the active power and reactive power commands of the converter connected to the feeder, and dynamic voltage compensation can be performed on the feeder. Independent reactive power compensation: When any converter DC switch is disconnected and disconnected from the DC side of the other converters, it enters the independent reactive power compensation mode; in the independent reactive power compensation mode, the reactive power compensation command can be directly adjusted according to the operation mode specified by the dispatching, or the AC voltage at the AC side sectionalizing switch of the converter can be detected, and the reactive power output of the converter can be dynamically adjusted according to the voltage curve specified by the dispatching. Load power restoration: When an AC feeder fails, the sectionalizing switches at both ends of the faulty section will operate to isolate the faulty area. Loads connected to the original feeder will be powered by the original feeder, while other loads will be powered by adjacent feeders or medium-voltage DC lines according to the current topology. Fault in the DC section of the distribution network: Issue a blocking signal to block the converter, shut down all AC-side sectionalizing switches and DC-side switches of the converters, the embedded DC network is taken out of operation, and the AC distribution network remains in its original working state. If the fault is a converter, keep the AC side sectionalizing switch and DC side switch of the faulty converter open, close the remaining AC side sectionalizing switches and DC switches of the converters, and restart the embedded DC network. If the fault is a permanent DC line fault, keep all DC switches of the embedded DC network open, close all AC side sectionalizing switches of the converters, restart the converters, and operate in dynamic reactive power compensation mode.

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