Power distribution network flexible direct current interconnection system based on double feeder lines and energy storage cooperative control and method thereof

Abstract

The invention discloses a power distribution network flexible direct current interconnection system and method based on double feeder lines and energy storage cooperative control. The system comprises at least two double-end voltage source type converters, at least two power distribution network access units, at least two local load units and a distributed energy storage module. The direct current sides of the double-end voltage source type converters are connected to form a back-to-back structure, and an energy router topology is formed through a direct current bus. The power distribution network access unit is respectively connected with the alternating current side of the double-end voltage source type converter through a grid-connected switch; the local load units and alternating current outlets of the double-end voltage source type converters form parallel loops respectively and then are connected to the corresponding power distribution network access units, so that a double-end redundant power supply structure is formed; the distributed energy storage module and the double-end voltage source converter work cooperatively, and the method corresponding to the system can realize adaptive switching of control modes, optimize cooperative control of the energy storage system and improve the reliability of the system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of flexible DC interconnection control, and particularly relates to a power distribution network flexible DC interconnection system based on double-fed line and energy storage collaborative control and a method thereof. BACKGROUND

[0002] With the development of society and the progress of science and technology, the demand and structure of power systems have changed significantly. On the one hand, due to the characteristics of one-way power supply, the traditional power distribution network is difficult to meet the high requirements of modern power systems on power supply reliability and power quality. On the other hand, the large-scale access of distributed power sources (such as solar energy, wind energy, etc.) makes the operation and control of the traditional power distribution network more complex. In order to cope with these challenges, flexible DC interconnection technology has emerged. It realizes flexible control and energy management of the power distribution network by using advanced power electronic devices and control strategies, and improves the reliability and power quality of the power distribution network.

[0003] In the existing flexible DC interconnection technology, there are some technical solutions similar to the present application. For example, Chinese patent CN109834593A discloses a flexible DC interconnection system based on voltage source converter and a control method. The system realizes the interconnection of the power distribution network through double-ended voltage source converter, and can switch between different control modes according to the voltage condition of the power distribution network to improve the stability and power quality of the power distribution network. In addition, some researches have proposed multi-feed line fault ride-through technology, voltage source converter control technology, energy storage system control technology, etc. These technologies have improved the reliability and power quality of the power distribution network to some extent, but compared with the present application, there are still some shortcomings.

[0004] Specifically, in the prior art, the switching of the voltage source converter (VSC) between different control modes is not flexible and timely enough, and it is difficult to adapt to the rapid changes of the power distribution network operation state, resulting in that the system cannot respond quickly and effectively when facing voltage sag or drop and other sudden situations.

[0005] The collaborative control of the energy storage system and the VSC is not close enough, and the role of the energy storage system in the power distribution network cannot be fully played, which cannot effectively optimize the load curve and reduce the load peak-valley difference, affecting the overall performance of the system.

[0006] The connection mode of the VSC AC outlet and the local load unit is relatively single, and there is a lack of redundant power supply structure. Once a link fails, it may affect the power supply reliability of the entire system.

[0007] Therefore, in view of the technical problems and defects of the above control mode switching not being flexible enough, the energy storage system collaborative control being insufficient, and the system reliability being to be improved, it is urgent to design and develop a power distribution network flexible DC interconnection method and circuit based on double-fed line and energy storage collaborative control. SUMMARY

[0008] In order to overcome the deficiencies and difficulties existing in the prior art, the purpose of the present application is to provide a power distribution network flexible DC interconnection system based on double-fed line and energy storage collaborative control and a method thereof, so as to realize adaptive switching of control mode, optimize energy storage system collaborative control and improve system reliability.

[0009] The first purpose of the present application is to provide a power distribution network flexible DC interconnection system based on double-fed line and energy storage collaborative control; and the second purpose of the present application is to provide a power distribution network flexible DC interconnection method based on double-fed line and energy storage collaborative control.

[0010] The first purpose of the present application is achieved in that the system comprises at least two double-ended voltage source type converters, and at least two power distribution network access units, at least two local load units and distributed energy storage modules;

[0011] The DC sides of the double-ended voltage source type converters are connected in series to form a back-to-back structure, and an energy router topology is formed through a DC bus; the power distribution network access units are respectively connected with the AC sides of the double-ended voltage source type converters through grid-connected switches; the local load units are respectively connected with the AC outlets of the double-ended voltage source type converters to form a parallel loop and then access the corresponding power distribution network access units, forming a double-ended redundant power supply structure; the distributed energy storage modules work cooperatively with the double-ended voltage source type converters;

[0012] The output end of the first power distribution network access unit in the power distribution network access units is connected with the AC end of the first double-ended voltage source type converter in the double-ended voltage source type converters; the output end of the second power distribution network access unit in the power distribution network access units is connected with the AC end of the second double-ended voltage source type converter in the double-ended voltage source type converters, and the double-ended voltage source type converters are configured with an adaptive mode switching mechanism to realize seamless conversion between power stability control mode and voltage support control mode based on real-time voltage parameters on the power distribution network side.

[0013] Further, when the double-ended voltage source type converter is in power stability control, the internal control structure comprises a first sampling module, a phase-locked loop module, a power outer loop control module, a first current inner loop control module and a first PWM module;

[0014] When the double-ended voltage source type converter is in voltage support control, the internal control structure comprises a second sampling module, a virtual synchronous machine outer loop control module, a voltage outer loop control module, a second current inner loop control module and a second PWM module.

[0015] Further, the first sampling module is used to synchronously sample three-phase power, line voltage, phase current and synchronous phase parameters of the double-ended voltage source type converter at a switching period.

[0016] The phase-locked loop module is used to obtain the AC side voltage amplitude and phase angle by tracking the AC bus voltage of the double-ended voltage source type converter;

[0017] The power outer loop control module generates a current reference value based on instantaneous power theory and outputs a current reference signal;

[0018] The first current inner loop control module is used to subtract the current reference signal from the actual current value, and the difference is adjusted by a PI controller to generate a voltage reference signal, which is converted by coordinate transformation to generate a modulation signal of the VSC in the three-phase stationary coordinate system;

[0019] The first PWM module is used to generate a trigger signal from the modulation signal to control the on-off time of the insulated gate bipolar transistor inside the double-ended voltage source type converter, so that the output voltage of the double-ended voltage source type converter is controlled within the preset threshold range.

[0020] Further, the second sampling module is used to synchronously sample the three-phase power, line voltage, phase current and synchronous phase parameters of the double-ended voltage source type converter at a switching period;

[0021] The virtual synchronous machine outer loop control module is used to adjust the three-phase power of the double-ended voltage source type converter and output a voltage amplitude reference signal and a voltage phase reference signal;

[0022] The voltage outer loop control module is used to adjust the voltage amplitude reference signal and the voltage phase reference signal by a PI controller to obtain the current reference signal of the inner loop;

[0023] The second current inner loop control module is used to subtract the current reference signal from the actual current value, and the difference is adjusted by a PI controller to generate a voltage reference signal, which is converted by coordinate transformation to generate a modulation signal of the double-ended voltage source type converter in the three-phase stationary coordinate system;

[0024] The second PWM module is used to generate a trigger signal from the modulation signal to control the on-off time of the insulated gate bipolar transistor inside the double-ended voltage source type converter, so that the output voltage of the double-ended voltage source type converter is controlled within the preset threshold range.

[0025] Further,

[0026] The local load unit includes a first local load unit and a second local load unit.

[0027] The second object of the application is achieved in that the method is applied to the flexible DC interconnection system, and the method comprises the following steps:

[0028] Real-time monitoring of the first distribution network access unit and the second distribution network access unit side first AC bus voltage and second AC bus voltage, judging the stability of the distribution network, and adaptively selecting the control mode;

[0029] When the AC side bus voltage of the first and second dual-ended voltage source type converters is higher than the target threshold value, the first and second distribution network access units are in normal operation state, and the first and second dual-ended voltage source type converters are operated in power stabilization control mode.

[0030] Further, when voltage sag or voltage drop occurs on the first distribution network access unit side, that is, the AC side bus voltage of the first dual-ended voltage source type converter is lower than the target threshold value, the first dual-ended voltage source type converter automatically switches to voltage support control mode, the distributed energy storage module cooperates with the second dual-ended voltage source type converter to provide stable DC voltage support for the first dual-ended voltage source type converter, the first dual-ended voltage source type converter provides stable voltage and frequency support for the first distribution network access unit, and provides virtual inertia and damping to enhance the adaptability of the first distribution network access unit, at this time, the first distribution network access unit, the second dual-ended voltage source type converter and the distributed energy storage module jointly supply power to the first load, when the AC side bus voltage of the first dual-ended voltage source type converter recovers to above the target threshold value, the first dual-ended voltage source type converter automatically switches back to power stabilization control mode, and continues to comprehensively manage voltage fluctuation and harmonics in the distribution network.

[0031] When voltage sag or voltage drop occurs on the second distribution network access unit side, that is, the AC side bus voltage of the second dual-ended voltage source type converter is lower than the target threshold value, the second dual-ended voltage source type converter automatically switches to voltage support control mode, the distributed energy storage module cooperates with the first dual-ended voltage source type converter to provide stable DC voltage support for the second dual-ended voltage source type converter, the second dual-ended voltage source type converter provides stable voltage and frequency support for the second distribution network access unit, and provides virtual inertia and damping to enhance the adaptability of the second distribution network access unit, at this time, the second distribution network access unit, the first dual-ended voltage source type converter and the distributed energy storage module jointly supply power to the second load, when the AC side bus voltage of the second dual-ended voltage source type converter recovers to above the target threshold value, the second dual-ended voltage source type converter automatically switches back to power stabilization control mode, and continues to comprehensively manage voltage fluctuation and harmonics in the distribution network.

[0032] Further, the double-ended voltage source type converter adjusts power quality, comprehensively manages voltage fluctuation and harmonic in the power distribution network, and supplies power to the load from the power distribution network, while comparing the first AC bus voltage and the second AC bus voltage of the two converter AC side bus voltages, and the distributed energy storage module supplies energy to the power distribution network with lower AC side bus voltage.

[0033] The application comprises at least two double-ended voltage source type converters, at least two power distribution network access units, at least two local load units and a distributed energy storage module;

[0034] The DC sides of the double-ended voltage source type converters are connected to form a back-to-back structure, and the DC bus constitutes an energy router topology; the power distribution network access units are connected with the AC sides of the double-ended voltage source type converters through grid-connected switches; the local load units are connected with the AC outlets of the double-ended voltage source type converters to form a parallel loop and then access the corresponding power distribution network access units, forming a double-ended redundant power supply structure; and the distributed energy storage module works cooperatively with the double-ended voltage source type converters;

[0035] The output end of the first power distribution network access unit in the power distribution network access units is connected with the AC end of the first double-ended voltage source type converter in the double-ended voltage source type converters; the output end of the second power distribution network access unit in the power distribution network access units is connected with the AC end of the second double-ended voltage source type converter in the double-ended voltage source type converters, and the double-ended voltage source type converters are configured with an adaptive mode switching mechanism to realize seamless conversion between the power stability control mode and the voltage support control mode based on real-time voltage parameters on the power distribution network side; and the method corresponding to the system can realize adaptive switching of the control mode, optimized cooperative control of the energy storage system and improved system reliability.

[0036] That is, the adaptive switching of the control mode can be realized by the present solution: by configuring the adaptive mode switching mechanism, the VSC can realize seamless conversion between the power stability control mode and the voltage support control mode based on real-time voltage parameters on the power distribution network side, improving the adaptability and response speed of the system to the operation state changes of the power distribution network. Optimized cooperative control of the energy storage system: through the close cooperation of the ESS and the VSC, the ESS can draw power from the power distribution network during the load valley and release power during the load peak, reducing the load peak-valley difference between the flexible interconnected power distribution networks, optimizing the load curve and improving the overall performance of the system. Improved system reliability: the double-ended redundant power supply structure is adopted, the VSC AC outlet and the local load unit form a parallel loop and then access the power distribution network, enhancing the redundancy and reliability of the system and reducing the risk of power supply interruption caused by failure of a single link. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative effort.

[0038] Figure 1 A schematic diagram of a power distribution network flexible DC interconnection system framework based on double-fed line and energy storage collaborative control is provided.

[0039] Figure 2 A schematic diagram of a power distribution network flexible DC interconnection method flow framework based on double-fed line and energy storage collaborative control is provided.

[0040] In the figure: VSC1-first double-ended voltage source converter; VSC2-second double-ended voltage source converter; G1-first power distribution network access unit; G2-second power distribution network access unit; L1-first local load unit (load 1); L2-second local load unit (load 2); ESS-distributed energy storage module; Uabc1-first AC bus voltage; Uabc2-second AC bus voltage. DETAILED DESCRIPTION

[0041] In order to better understand the purpose, technical solutions and advantages of the present application, the present application will be further described below in combination with the drawings and specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in the present description.

[0042] The present application can also be implemented or applied through other different specific examples, and each detail in the present description can be modified and changed in various ways based on different viewpoints and applications without departing from the spirit of the present application.

[0043] It should be noted that if the embodiments of the present application involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative positional relationship, movement condition, etc. between the components in a certain specific posture (as shown in the drawings), and if the specific posture changes, the directional indications will also change accordingly.

[0044] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Secondly, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0045] The present invention will be further described below with reference to the accompanying drawings.

[0046] like Figure 1 As shown, the present invention provides a flexible DC interconnection system for distribution networks based on the coordinated control of dual feeders and energy storage. The system includes at least two dual-terminal voltage source converters, at least two distribution network access units, at least two local load units, and a distributed energy storage module.

[0047] The DC sides of the dual-ended voltage source converters are connected to form a back-to-back structure and form an energy router topology through a DC bus. The distribution network access units are connected to the AC sides of the dual-ended voltage source converters via grid-connected switches. The local load units are connected to the corresponding distribution network access units after forming parallel circuits with the AC outputs of the dual-ended voltage source converters, forming a dual-ended redundant power supply structure. The distributed energy storage module works in conjunction with the dual-ended voltage source converters.

[0048] Specifically, the output terminal of the first distribution network access unit in the distribution network access unit is connected to the AC terminal of the first dual-terminal voltage source converter in the dual-terminal voltage source converter; the output terminal of the second distribution network access unit in the distribution network access unit is connected to the AC terminal of the second dual-terminal voltage source converter in the dual-terminal voltage source converter, and the dual-terminal voltage source converter is equipped with an adaptive mode switching mechanism to achieve seamless switching between power stability control mode and voltage support control mode based on real-time voltage parameters on the distribution network side.

[0049] When the dual-terminal voltage source converter is under power stabilization control, its internal control structure includes a first sampling module, a phase-locked loop module, a power outer loop control module, a first current inner loop control module, and a first PWM module.

[0050] When the dual-terminal voltage source converter is under voltage support control, its internal control structure includes a second sampling module, a virtual synchronous machine outer loop control module, a voltage outer loop control module, a second current inner loop control module, and a second PWM module.

[0051] The first sampling module is configured to synchronously sample three-phase power, line voltage, phase current and synchronous phase parameters of the double-ended voltage source converter in a switching cycle.

[0052] The phase-locked loop module is configured to obtain AC side voltage amplitude and phase angle by tracking AC bus voltage of the double-ended voltage source converter.

[0053] The power outer loop control module is configured to generate a current reference value based on instantaneous power theory and output a current reference signal.

[0054] The first current inner loop control module is configured to subtract the current reference signal from an actual current value, and generate a voltage reference signal after PI controller adjustment, and the voltage reference signal is converted into a modulation signal of the VSC in a three-phase stationary coordinate system.

[0055] The first PWM module is configured to generate a trigger signal from the modulation signal to control the on-off time of the insulated gate bipolar transistor in the double-ended voltage source converter, so that the output voltage of the double-ended voltage source converter is controlled within a preset threshold range.

[0056] The second sampling module is configured to synchronously sample three-phase power, line voltage, phase current and synchronous phase parameters of the double-ended voltage source converter in a switching cycle.

[0057] The virtual synchronous machine outer loop control module is configured to adjust three-phase power of the double-ended voltage source converter and output a voltage amplitude reference signal and a voltage phase reference signal.

[0058] The voltage outer loop control module is configured to adjust the voltage amplitude reference signal and the voltage phase reference signal through a PI controller to obtain a current reference signal of the inner loop.

[0059] The second current inner loop control module is configured to subtract the current reference signal from an actual current value, and generate a voltage reference signal after PI controller adjustment, and the voltage reference signal is converted into a modulation signal of the double-ended voltage source converter in a three-phase stationary coordinate system.

[0060] The second PWM module is configured to generate a trigger signal from the modulation signal to control the on-off time of the insulated gate bipolar transistor in the double-ended voltage source converter, so that the output voltage of the double-ended voltage source converter is controlled within a preset threshold range.

[0061] The local load unit includes a first local load unit and a second local load unit.

[0062] Specifically, in the embodiment of the present application, a power distribution network flexible DC interconnection system based on double-fed line and energy storage collaborative control is provided, and the topological structure thereof comprises: double-ended voltage source converters (VSC1, VSC2) connected on the DC side to form a back-to-back structure and constitute an energy router topology through a DC bus; power distribution network access units (G1, G2) connected with the AC sides of the VSC1 and VSC2 through grid-connected switches respectively; local load units (L1, L2) (i.e. load 1 and load 2) connected with the AC outlets of the VSC1 and VSC2 to form a parallel loop and then connected with the corresponding power distribution network access units, forming a double-ended redundant power supply structure; and a distributed energy storage module (ESS) working collaboratively with the double-ended voltage source converters; wherein the output end of the first power distribution network access unit is connected with the AC end of the VSC1, the output end of the second power distribution network access unit is connected with the AC end of the VSC2, and the double-ended voltage source converters are configured with an adaptive mode switching mechanism to realize seamless conversion between power stability control mode and voltage support control mode based on real-time voltage parameters on the power distribution network side.

[0063] When the VSC is in power stability control, the internal control structure thereof comprises a sampling module, a phase-locked loop module, a power outer loop control module, a current inner loop control module and a PWM module;

[0064] The sampling module is used for synchronously sampling the three-phase power, line voltage, phase current and synchronous phase parameters of the VSC at a switching period; the phase-locked loop module is used for obtaining the voltage amplitude and phase angle on the AC side by tracking the AC bus voltage of the VSC; the power outer loop control module generates a current reference value and outputs a current reference signal based on the instantaneous power theory; the current inner loop control module is used for subtracting the current reference signal from the actual current value, and the difference is adjusted by a PI controller to generate a voltage reference signal, which is converted into a modulation signal of the VSC in a three-phase stationary coordinate system; and the PWM module is used for generating a trigger signal from the modulation signal to control the on-off time of the internal insulated gate bipolar transistor of the VSC, so that the output voltage of the VSC is controlled within a preset threshold range.

[0065] When the VSC is in voltage support control, the internal control structure thereof comprises a sampling module, a virtual synchronous machine outer loop control module, a voltage outer loop control module, a current inner loop control module and a PWM module;

[0066] The sampling module is used for synchronously sampling the three-phase power, line voltage, phase current and synchronous phase parameters of the VSC at a switching period; the virtual synchronous machine outer loop control module is used for adjusting the three-phase power of the VSC and outputting a voltage amplitude reference signal and a voltage phase reference signal;

[0067] The voltage outer loop control module is configured to adjust the voltage amplitude reference signal and the voltage phase reference signal through a PI controller to obtain a current reference signal of an inner loop;

[0068] The current inner loop control module is configured to subtract the current reference signal from an actual current value, and generate a voltage reference signal after adjusting the difference through a PI controller, and generate a modulation signal of the VSC in a three-phase static coordinate system through coordinate transformation;

[0069] The PWM module is configured to generate a trigger signal from the modulation signal to control the on-off time of the internal insulated gate bipolar transistor of the VSC, so that the output voltage of the VSC is controlled within a preset threshold range.

[0070] In addition, the core protection key point of the present application is a dual-mode switching mechanism: VSC1 and VSC2 are configured with an adaptive mode switching mechanism, which realizes seamless conversion between power stability control mode and voltage support control mode based on real-time voltage parameters on the G1 / G2 side. This key point can make the system respond more quickly and accurately to the changes in the operation state of the distribution network, and improve the adaptability and response speed of the system to the changes in the operation state of the distribution network.

[0071] Energy storage collaborative control: ESS and VSC work closely together to draw power from the distribution network during load valleys and release power during load peaks, reducing the load peak-valley difference between flexible interconnected distribution networks and optimizing the load curve. At the same time, when voltage sag or drop occurs in the distribution network, the VSC is provided with stable DC voltage support. This key point can fully utilize the role of the energy storage system in the distribution network and improve the overall performance of the system.

[0072] Dual-end redundant power supply structure: a dual-end redundant power supply structure is adopted, and the VSC AC outlet and the local load unit form a parallel circuit and are connected to the distribution network. This key point can enhance the redundancy of the system, improve the power supply reliability of the system, and reduce the risk of power supply interruption caused by failure of a single link.

[0073] Power quality regulation: in the power stability control mode, the VSC can regulate power quality and comprehensively manage voltage fluctuations, harmonics, etc. in the distribution network. This key point can improve the power quality of the distribution network and meet the high requirements of modern power systems on power quality.

[0074] To achieve the purpose of the present application, a flexible DC interconnected distribution network based on dual-fed line and energy storage collaborative control is also provided, as shown in Figure 2 The method is applied to the flexible DC interconnected system, and the method comprises the following steps:

[0075] Real-time monitoring of the first distribution network access unit and the second distribution network access unit side first AC bus voltage and second AC bus voltage, judging the stability of the distribution network, and adaptively selecting the control mode;

[0076] When the AC side bus voltage of the first and second double-ended voltage source type converters is higher than the target threshold value (the target threshold value is set to 0.9p.u), the first and second distribution network access units are in normal operation state, and the first and second double-ended voltage source type converters are operated in power stabilization control mode. At this time, the double-ended voltage source type converter adjusts power quality, comprehensively manages voltage fluctuation and harmonic in the distribution network, and supplies power to the load by the distribution network. Meanwhile, the first and second AC bus voltages are compared, and the distributed energy storage module supplies energy to the distribution network with lower AC bus voltage.

[0077] When voltage sag or voltage drop occurs on the first distribution network access unit side, that is, the AC side bus voltage of the first double-ended voltage source type converter is lower than the target threshold value, the first double-ended voltage source type converter automatically switches to voltage support control mode. The distributed energy storage module cooperates with the second double-ended voltage source type converter to provide stable DC voltage support for the first double-ended voltage source type converter. The first double-ended voltage source type converter provides stable voltage and frequency support for the first distribution network access unit, and provides virtual inertia and damping to enhance the adaptability of the first distribution network access unit. At this time, the first distribution network access unit, the second double-ended voltage source type converter and the distributed energy storage module jointly supply power to the first load. When the AC side bus voltage of the first double-ended voltage source type converter recovers to above the target threshold value, the first double-ended voltage source type converter automatically switches back to power stabilization control mode, and continues to comprehensively manage voltage fluctuation and harmonic in the distribution network.

[0078] When voltage sag or voltage drop occurs on the second distribution network access unit side, that is, the AC side bus voltage of the second double-ended voltage source type converter is lower than the target threshold value, the second double-ended voltage source type converter automatically switches to voltage support control mode. The distributed energy storage module cooperates with the first double-ended voltage source type converter to provide stable DC voltage support for the second double-ended voltage source type converter. The second double-ended voltage source type converter provides stable voltage and frequency support for the second distribution network access unit, and provides virtual inertia and damping to enhance the adaptability of the second distribution network access unit. At this time, the second distribution network access unit, the first double-ended voltage source type converter and the distributed energy storage module jointly supply power to the second load. When the AC side bus voltage of the second double-ended voltage source type converter recovers to above the target threshold value, the second double-ended voltage source type converter automatically switches back to power stabilization control mode, and continues to comprehensively manage voltage fluctuation and harmonic in the distribution network.

[0079] Specifically, in the embodiment of the application, the AC bus voltages Uabc1 and Uabc2 at the access units G1 and G2 of the power distribution network are monitored in real time, the stability of the power distribution network is judged, and the control mode is adaptively selected;

[0080] When the AC bus voltages of VSC1 and VSC2 are both higher than the target threshold, the power distribution network 1 and the power distribution network 2 are both in normal operation, VSC1 and VSC2 are both operated in the power stabilization control mode, at this time, the VSC adjusts the power quality, comprehensively manages the voltage fluctuation and harmonics in the power distribution network, and supplies power to the load, at the same time, the AC bus voltages Uabc1 and Uabc2 of the two converters are compared, and the ESS supplies energy to the power distribution network with lower AC bus voltage;

[0081] When the voltage sag or voltage drop occurs at the power distribution network 1, that is, the AC bus voltage of VSC1 is detected to be lower than the target threshold, VSC1 automatically switches to the voltage support control mode, the ESS cooperates with VSC2 to provide stable DC voltage support for VSC1, VSC1 provides stable voltage and frequency support for the power distribution network 1, and provides virtual inertia and damping to enhance the adaptability of the power distribution network 1, at this time, the power distribution network 1, VSC2 and the ESS jointly supply power to the load 1 (the first load), when the AC bus voltage of VSC1 recovers to above the target threshold, VSC1 automatically switches back to the power stabilization control mode, and continues to comprehensively manage the voltage fluctuation and harmonics in the power distribution network;

[0082] When the voltage sag or voltage drop occurs at the power distribution network 2, that is, the AC bus voltage of VSC2 is detected to be lower than the target threshold, VSC2 automatically switches to the voltage support control mode, the ESS cooperates with VSC1 to provide stable DC voltage support for VSC2, VSC2 provides stable voltage and frequency support for the power distribution network 2, and provides virtual inertia and damping to enhance the adaptability of the power distribution network 2, at this time, the power distribution network 2, VSC1 and the ESS jointly supply power to the load 2 (the second load), when the AC bus voltage of VSC2 recovers to above the target threshold, VSC2 automatically switches back to the power stabilization control mode, and continues to comprehensively manage the voltage fluctuation and harmonics in the power distribution network.

[0083] The energy storage device as a backup power source can draw electric energy from the power distribution network during the load valley and release electric energy during the load peak, so as to reduce the load peak-valley difference between the flexible interconnected power distribution networks and optimize the load curve

[0084] The application comprises at least two double-ended voltage source type converters, at least two power distribution network access units, at least two local load units and distributed energy storage modules; the direct current sides of the double-ended voltage source type converters are connected to form a back-to-back structure and constitute an energy router topology through a direct current bus; the power distribution network access units are respectively connected with the alternating current sides of the double-ended voltage source type converters through grid-connected switches; the local load units are respectively connected with the alternating current outlets of the double-ended voltage source type converters to form a parallel loop and then access the corresponding power distribution network access units, forming a double-ended redundant power supply structure; the distributed energy storage modules work cooperatively with the double-ended voltage source type converters.

[0085] The output end of the first power distribution network access unit is connected with the alternating current end of the first double-ended voltage source type converter, the output end of the second power distribution network access unit is connected with the alternating current end of the second double-ended voltage source type converter, and the double-ended voltage source type converters are provided with an adaptive mode switching mechanism to realize seamless conversion between the power stability control mode and the voltage support control mode based on real-time voltage parameters on the power distribution network side; and the method corresponding to the system can realize adaptive switching of the control mode, optimize cooperative control of the energy storage system and improve the reliability of the system.

[0086] That is, the adaptive switching of the control mode can be realized through the present solution: the adaptive mode switching mechanism is configured to enable the VSC to realize seamless conversion between the power stability control mode and the voltage support control mode based on real-time voltage parameters on the power distribution network side, thereby improving the adaptability and response speed of the system to the changes in the operation state of the power distribution network. The cooperative control of the energy storage system is optimized: the close cooperation between the ESS and the VSC enables the ESS to draw electric energy from the power distribution network during the load valley and release electric energy during the load peak, thereby reducing the load peak-valley difference between the flexible interconnected power distribution networks, optimizing the load curve and improving the overall performance of the system. The reliability of the system is improved: the double-ended redundant power supply structure is adopted, the VSC alternating current outlet and the local load unit form a parallel loop and then access the power distribution network, thereby enhancing the redundancy and reliability of the system and reducing the risk of power supply interruption caused by failure of a single link.

[0087] In other words, the control mode switching of the present solution is more flexible: the adaptive mode switching mechanism is configured to enable seamless conversion between the power stability control mode and the voltage support control mode based on real-time voltage parameters on the power distribution network side. This enables the system to respond more quickly and accurately to changes in the operation state of the power distribution network, and when voltage sag or drop occurs in the power distribution network, the VSC can automatically switch to the voltage support control mode to provide stable voltage and frequency support for the power distribution network, thereby enhancing the adaptability of the power distribution network.

[0088] Moreover, the energy storage system cooperative control is more optimal: the ESS cooperates with the VSC closely, the ESS not only draws electric energy from the power distribution network in the load low valley time and releases electric energy in the load peak time, reduces the load peak valley difference between the flexible interconnected power distribution networks, optimizes the load curve, but also provides stable DC voltage support for the VSC when the voltage of the power distribution network is temporarily lowered or dropped, further improves the stability and reliability of the system. The system reliability is higher: the scheme adopts a double-end redundant power supply structure, the VSC AC outlet and the local load unit form a loop and are connected to the power distribution network, the redundancy of the system is enhanced. Even if a certain link fails, the system can still operate stably, reduces the risk of power supply interruption caused by single link failure, and improves the power supply reliability of the whole system. The power quality regulation is more effective: the VSC matched with the ESS can regulate the power quality, comprehensively manages the voltage fluctuation, harmonic and the like in the power distribution network, improves the power quality of the power distribution network, and meets the high requirements of the modern power system on the power quality.

[0089] The above-mentioned embodiments only express several embodiments of the present application, the description is more specific and detailed, but it cannot be understood as the limitation of the patent scope of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A flexible DC interconnection system for distribution networks based on the coordinated control of dual feeders and energy storage, characterized in that, The system includes at least two dual-terminal voltage source converters, at least two distribution network access units, at least two local load units, and distributed energy storage modules; The DC sides of the dual-ended voltage source converters are connected to form a back-to-back structure and form an energy router topology through a DC bus. The distribution network access units are connected to the AC sides of the dual-ended voltage source converters via grid-connected switches. The local load units are connected to the corresponding distribution network access units after forming parallel circuits with the AC outputs of the dual-ended voltage source converters, forming a dual-ended redundant power supply structure. The distributed energy storage module works in conjunction with the dual-ended voltage source converters. The output terminal of the first distribution network access unit in the distribution network access unit is connected to the AC terminal of the first dual-terminal voltage source converter in the dual-terminal voltage source converter; the output terminal of the second distribution network access unit in the distribution network access unit is connected to the AC terminal of the second dual-terminal voltage source converter in the dual-terminal voltage source converter, and the dual-terminal voltage source converter is equipped with an adaptive mode switching mechanism to achieve seamless switching between power stability control mode and voltage support control mode based on real-time voltage parameters on the distribution network side.

2. A flexible DC interconnection system for distribution networks based on the coordinated control of dual feeders and energy storage, as described in claim 1, is characterized in that... When the dual-terminal voltage source converter is in power stability control mode, its internal control structure includes a first sampling module, a phase-locked loop module, a power outer loop control module, a first current inner loop control module, and a first PWM module. When the dual-ended voltage source converter is in voltage support control mode, its internal control structure includes a second sampling module, a virtual synchronous machine outer loop control module, a voltage outer loop control module, a second current inner loop control module, and a second PWM module.

3. A flexible DC interconnection system for distribution networks based on the coordinated control of dual feeders and energy storage, as described in claim 2, is characterized in that... The first sampling module, the phase-locked loop module, and the power outer loop control module are electrically connected inside the dual-terminal voltage source converter; wherein, the first sampling module is used to synchronously sample the three-phase power, line voltage, phase current, and synchronous phase parameters of the dual-terminal voltage source converter at the switching cycle; The phase-locked loop module is used to obtain the AC side voltage amplitude and phase angle by tracking the AC bus voltage of the dual-terminal voltage source converter. The power outer loop control module is used to generate a current reference value and output a current reference signal based on instantaneous power theory; The first current inner loop control module is used to calculate the difference between the current reference signal and the actual current value. The difference is adjusted by the PI controller to generate a voltage reference signal. The voltage reference signal is transformed by coordinate transformation to generate a VSC modulation signal in a three-phase stationary coordinate system. The first PWM module is used to generate a trigger signal from the modulation signal to control the on / off time of the insulated gate bipolar transistor inside the dual-terminal voltage source converter, so that the output voltage of the dual-terminal voltage source converter is controlled within a preset threshold range.

4. A flexible DC interconnection system for distribution networks based on the coordinated control of dual feeders and energy storage, as described in claim 2, is characterized in that... The second sampling module is used to synchronously sample the three-phase power, line voltage, phase current, and synchronous phase parameters of the dual-terminal voltage source converter at the switching cycle. The virtual synchronous machine outer loop control module is used to adjust the three-phase power of the dual-terminal voltage source converter and output voltage amplitude reference signal and voltage phase reference signal; The outer voltage loop control module is used to adjust the voltage amplitude reference signal and voltage phase reference signal through a PI controller to obtain the inner loop current reference signal; The second current inner loop control module is used to calculate the difference between the current reference signal and the actual current value. The difference is adjusted by the PI controller to generate a voltage reference signal. The voltage reference signal is transformed by coordinate transformation to generate the modulation signal of the dual-terminal voltage source converter in the three-phase stationary coordinate system. The second PWM module is used to generate a trigger signal from the modulation signal to control the on / off time of the insulated gate bipolar transistor inside the dual-terminal voltage source converter, so that the output voltage of the dual-terminal voltage source converter is controlled within a preset threshold range.

5. A flexible DC interconnection system for distribution networks based on the coordinated control of dual feeders and energy storage, as described in claim 1 or 2, characterized in that: The local load unit includes a first local load unit and a second local load unit.

6. A method for flexible DC interconnection of distribution networks based on coordinated control of double feeders and energy storage, characterized in that, The method is applied to any of the flexible DC interconnect systems described in claims 1 to 5, and the method includes the following steps: The system monitors the voltage of the first AC bus and the voltage of the second AC bus on the side of the first and second distribution network access units in real time, judges the stability of the distribution network, and adaptively selects the control mode. When the AC bus voltage of both the first and second dual-terminal voltage source converters is higher than the target threshold, both the first and second distribution network access units are in normal operation, and both the first and second dual-terminal voltage source converters are in power stability control mode.

7. The method for flexible DC interconnection of distribution networks based on the coordinated control of dual feeders and energy storage as described in claim 6, characterized in that, When a voltage dip or drop occurs on the first distribution network access unit side, i.e., when the AC bus voltage of the first dual-ended voltage source converter is detected to be lower than the target threshold, the first dual-ended voltage source converter automatically switches to voltage support control mode. The distributed energy storage module and the second dual-ended voltage source converter cooperate to provide stable DC voltage support for the first dual-ended voltage source converter. The first dual-ended voltage source converter provides stable voltage and frequency support for the first distribution network access unit and provides virtual inertia and damping to enhance the adaptability of the first distribution network access unit. At this time, the first distribution network access unit, the second dual-ended voltage source converter, and the distributed energy storage module jointly supply power to the first load. When the AC bus voltage of the first dual-ended voltage source converter recovers to above the target threshold, the first dual-ended voltage source converter automatically switches back to power stability control mode to continue to comprehensively manage voltage fluctuations and harmonics in the distribution network. When a voltage dip or drop occurs on the second distribution network access unit side, i.e., when the AC bus voltage of the second double-ended voltage source converter is detected to be lower than the target threshold, the second double-ended voltage source converter automatically switches to voltage support control mode. The distributed energy storage module cooperates with the first double-ended voltage source converter to provide stable DC voltage support for the second double-ended voltage source converter. The second double-ended voltage source converter provides stable voltage and frequency support for the second distribution network access unit and provides virtual inertia and damping to enhance the adaptability of the second distribution network access unit. At this time, the second distribution network access unit, the first double-ended voltage source converter, and the distributed energy storage module jointly supply power to the second load. When the AC bus voltage of the second double-ended voltage source converter recovers to above the target threshold, the second double-ended voltage source converter automatically switches back to power stability control mode to continue to comprehensively manage voltage fluctuations and harmonics in the distribution network.

8. A method for flexible DC interconnection of distribution networks based on the coordinated control of dual feeders and energy storage, as described in claim 6, is characterized in that... The dual-terminal voltage source converter regulates power quality, comprehensively manages voltage fluctuations and harmonics in the distribution network, and supplies power to the load through the distribution network. At the same time, it compares the AC bus voltages of the two converters, the first AC bus voltage and the second AC bus voltage, and the distributed energy storage module delivers energy to the distribution network on the side with the lower AC bus voltage.

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