A low voltage flexible interconnection equipment set and a low voltage flexible interconnection system

Abstract

The present invention relates to a low-voltage flexible interconnection complete set of equipment and a low-voltage flexible interconnection system, wherein the low-voltage flexible interconnection complete set of equipment includes: a low-voltage flexible interconnection device, with a built-in bidirectional AC / DC conversion unit, the AC side of which is connected to a transformer in an area; a DC protection unit, located on the DC side of the low-voltage flexible interconnection device, for shutting off the flow path between the low-voltage flexible interconnection device and the fault point when the short-circuit current on the DC side exceeds a design threshold, thereby completing the blocking of the fault point. The present invention can improve the reliability of the flexible interconnection device and the AC / DC hybrid distribution network based on the flexible interconnection of multiple areas in the case of single-stage faults on the DC side or inter-pole short-circuit faults.

Description

Technical Field

[0001] The present invention relates to the technical field of low-voltage AC / DC hybrid power distribution, and in particular to a low-voltage flexible interconnection complete set of equipment and a low-voltage flexible interconnection system. Background Art

[0002] In the flexible interconnection system of distribution substations, multiple low-voltage flexible interconnection devices are interconnected through a common DC bus (centralized) or a DC bus segment (distributed) to achieve interconnection and mutual supply of loads with complementary time and space characteristics. Most flexible interconnection devices are voltage source converters composed of fully controlled power electronic devices. When the DC side of the interconnected system is connected to multiple types of DC elements such as distributed photovoltaics, energy storage, and DC loads, the number of possible fault points on the DC side will increase accordingly, and the risk of failure will also increase. If a single-pole grounding fault or an inter-pole short circuit fault occurs on the DC side of the system, the fully controlled device in the low-voltage flexible interconnection device will be quickly locked, while the low-voltage circuit breaker on the DC side is a mechanical circuit breaker with a reaction time and action time of ≥30ms. During the period when the fully controlled device is locked but the DC circuit breaker is not reliably disconnected, the diode in the device that is anti-parallel to the fully controlled device may burn out due to overcurrent. For scenarios with DC distributed power supplies, energy storage, charging piles, and even 220V AC and DC loads, when a short circuit occurs in the DC line, the short-circuit current will be directly cut off due to the rapid locking of the fully controlled devices. The fault point cannot be identified and removed through hierarchical protection, which expands the scope of the fault.

[0003] The existing patent document CN219833762U discloses a centralized protection device applied to a DC multi-branch power grid. The protection device is only used in conjunction with a DC switch cabinet with one input and multiple outputs or N input and N outputs. It still relies on a mechanical circuit breaker to achieve fault current interruption, and cannot solve the problem of diode overcurrent damage and the coordination of upper and lower level protection of the DC interconnection system.

[0004] The existing patent document CN117424194A discloses a fault-blocking type flexible interconnection device without isolation transformer and its control method. The device only considers the flexible interconnection device without isolation transformer, and the midpoint of the DC side of the device is grounded through a resistor. When a single-stage grounding or inter-pole short circuit fault occurs on the DC side, there will be a large short-circuit current. In addition, the device configures the blocking type DC protection unit in the DC distribution cabinet, which cannot be linked with the equipment. When the blocking type distribution unit detects overcurrent, it will be quickly shut down, causing the entire interconnection system to lose fault current and thus unable to achieve fault location and hierarchical protection, and does not have the ability to ride through DC faults. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a low-voltage flexible interconnection complete set of equipment and a low-voltage flexible interconnection system to improve the reliability of the flexible interconnection device and the AC / DC hybrid distribution network based on the flexible interconnection of multiple substations in the event of a single-stage fault on the DC side or an inter-pole short-circuit fault.

[0006] The technical solution adopted by the present invention to solve the technical problem is: to provide a low-voltage flexible interconnection complete set of equipment, including:

[0007] Low-voltage flexible interconnection device, with built-in bidirectional AC / DC conversion unit, the AC side is connected to the substation transformer;

[0008] The DC protection unit is located on the DC side of the low-voltage flexible interconnection device and is used to shut off the current path between the low-voltage flexible interconnection device and the fault point when the short-circuit current on the DC side exceeds the designed threshold, thereby completing the blocking of the fault point.

[0009] The DC protection unit is a current-limiting DC protection unit, which is implemented using a primary circuit. When a fault occurs on the DC side, the fault current is maintained by a current-limiting reactance, and the fault point is isolated after a set time is reached, thereby completing the blocking of the fault point.

[0010] The current-limiting DC protection unit includes an upper IGBT tube, a lower IGBT tube and a DC reactor. The upper IGBT tube and the lower IGBT tube are connected in series. One end of the DC reactor is connected to the connecting end of the upper IGBT tube and the lower IGBT tube, and the other end serves as the first input end of the current-limiting DC protection unit. The connecting end of the lower IGBT tube that is not connected to the upper IGBT tube serves as the second input end of the current-limiting DC protection unit. A capacitor is also connected in parallel at both ends of the upper IGBT tube and the lower IGBT tube connected in series.

[0011] The DC protection unit is a blocking type DC protection unit, which is implemented by a primary IGBT switch. When a fault occurs on the DC side, the IGBT switch quickly closes to isolate the current path between the fault point and the low-voltage flexible interconnection device.

[0012] The blocking type DC protection unit comprises an upper IGBT tube, a lower IGBT tube and a diode, wherein the upper IGBT tube, the diode and the lower IGBT tube are sequentially connected in series, and the two ends of the diode serve as the first input end and the second input end of the blocking type DC protection unit respectively, and a capacitor is also connected in parallel at both ends of the series-connected upper IGBT tube, the diode and the lower IGBT tube.

[0013] The low voltage flexible interconnection equipment set also includes:

[0014] The leakage protection monitoring unit is arranged on the DC side of the low-voltage flexible interconnection device and is used for early warning of DC single-stage faults.

[0015] When there are multiple outgoing lines on the DC side of the low-voltage flexible interconnection device, and the number of outgoing lines connected to DC loads among the multiple outgoing lines exceeds a preset percentage, the DC protection unit is configured on the total outgoing line on the DC side of the low-voltage flexible interconnection device, and the circuit breaker on each outgoing line is configured with a DC line microcomputer protection device, and the DC line microcomputer protection device is used to monitor current signals and / or voltage signals, and when the current signal or voltage signal is abnormal, cooperate with the DC protection unit to realize fault line selection.

[0016] When there are multiple outgoing lines on the DC side of the low-voltage flexible interconnection device and the number of adjustable power supplies and loads connected in the multiple outgoing lines exceeds a preset percentage, the DC protection unit is configured on each outgoing line on the DC side of the low-voltage flexible interconnection device.

[0017] The technical solution adopted by the present invention to solve its technical problem is: to provide a low-voltage flexible interconnection system, characterized in that it includes multiple low-voltage flexible interconnection complete sets of equipment as mentioned above, the AC sides of the multiple low-voltage flexible interconnection complete sets of equipment are connected to the corresponding substations, and the DC sides are connected to the DC bus, and the DC bus is connected to the DC source, DC load and / or DC energy storage through multiple output lines.

[0018] Beneficial Effects

[0019] Due to the adoption of the above-mentioned technical scheme, the present invention has the following advantages and positive effects compared with the prior art: the present invention uses a DC protection unit to quickly block the short-circuit current or limit the short-circuit current to a certain range when a fault occurs on the DC side, thereby protecting the power electronic components in the low-voltage flexible interconnection device from damage. At the same time, in conjunction with the line protection and equipment protection of the grid, accurate fault positioning can be achieved, the scope of fault power outages can be effectively reduced, and the reliability of system operation can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of a typical Santai District flexible interconnection system;

[0021] Figure 2 is a schematic diagram of a current limiting DC protection unit in an embodiment of the present invention;

[0022] Figure 3 is a schematic diagram of a blocking type DC protection unit in an embodiment of the present invention;

[0023] Figure 4 It is a schematic diagram of a non-isolated flexible interconnection complete set of equipment connected to a station area according to an embodiment of the present invention;

[0024] Figure 5 It is a schematic diagram of an isolated flexible interconnection complete set of equipment connected to a station area according to an embodiment of the present invention;

[0025] Figure 6 is a schematic diagram of the configuration of a DC protection unit in a DC load power supply scenario in an implementation manner of the present invention;

[0026] Figure 7 is a schematic diagram of the configuration of a DC protection unit in a multi-resource access scenario in an implementation manner of the present invention;

[0027] Figure 8 It is a waveform diagram of DC inter-pole short circuit fault of isolated flexible interconnection device;

[0028] Fig. 9 It is a single-stage ground fault waveform of an isolated flexible interconnection device;

[0029] Fig.10 It is a waveform diagram of DC inter-pole short circuit fault of non-isolated flexible interconnection device;

[0030] Fig.11 It is a single-stage grounding fault waveform diagram of a non-isolated flexible interconnection device;

[0031] Fig.12 This is the DC inter-pole fault waveform after adding the blocking protection unit;

[0032] Fig.13 This is the waveform of DC single-stage grounding fault after adding blocking protection unit;

[0033] Fig.14 This is the measured diagram of the blocking protection unit;

[0034] Fig.15 This is the actual measurement diagram of the current limiting protection unit;

[0035] Fig.16 This is a schematic diagram of an AC / DC hybrid distribution network with two substations based on flexible interconnection. DETAILED DESCRIPTION

[0036] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0037] The embodiment of the present invention relates to a low-voltage flexible interconnection complete set of equipment, which is suitable for lines and electronic systems with an AC rated voltage of no more than 1000V and a DC rated voltage of no more than 1500V. When a fault occurs on the DC side, the short-circuit current can be quickly blocked or the short-circuit current can be limited to a certain range, thereby protecting the power electronic components in the device from damage. At the same time, in conjunction with the line protection and equipment protection of the grid, accurate fault positioning can be achieved, the scope of fault power outage can be effectively reduced, and the reliability of system operation can be improved.

[0038] The low-voltage flexible interconnection equipment of this embodiment can be applied to a low-voltage flexible interconnection system in which multiple flexible interconnection devices are connected to the grid, or to an AC / DC hybrid distribution network based on a grid structure of flexible interconnection between stations. Figure 1 Taking the typical topological structure of three substations flexible interconnection as an example, the figure includes a typical low-voltage distribution substation, a low-voltage flexible interconnection complete set of equipment A, a 750V DC bus, and source, load, and storage functional units such as photovoltaics on the DC side. Among them, the low-voltage flexible interconnection complete set of equipment includes: a low-voltage flexible interconnection device and a DC protection unit. The low-voltage flexible interconnection device has a built-in bidirectional AC / DC conversion unit, and its AC side is connected to the substation transformer; the DC protection unit is located on the DC side of the low-voltage flexible interconnection device, which is used to shut off the flow path between the low-voltage flexible interconnection device and the fault point when the short-circuit current on the DC side exceeds the design threshold, thereby completing the blocking of the fault point.

[0039] DC protection units can be divided into current limiting DC protection units and blocking DC protection units according to topology design.

[0040] like Figure 2 As shown, the current limiting DC protection unit is implemented by a primary circuit, for example, a primary Buck / Boost circuit is adopted, and its main circuit components are an upper IGBT tube, a lower IGBT tube, a diode and a DC reactor. The upper IGBT tube and the lower IGBT tube are connected in series with each other, one end of the DC reactor is connected to the connection end of the upper IGBT tube and the lower IGBT tube, and the other end serves as a first input end of the current limiting DC protection unit, and the connection end of the lower IGBT tube that is not connected to the upper IGBT tube serves as a second input end of the current limiting DC protection unit. A capacitor is also connected in parallel at both ends of the upper IGBT tube and the lower IGBT tube connected in series. When a fault occurs on the DC side, the fault current is maintained by the current limiting reactor, and the fault point is isolated after the set time is reached, thereby completing the blocking of the fault point.

[0041] like Figure 3As shown, the blocking type DC protection unit is implemented by a primary IGBT switch, and its main circuit components are an upper IGBT tube, a lower IGBT tube and a diode, wherein the upper IGBT tube, the diode and the lower IGBT tube are connected in series in sequence, and the two ends of the diode serve as the first input end and the second input end of the blocking type DC protection unit respectively, and a capacitor is also connected in parallel at both ends of the series-connected upper IGBT tube, the diode and the lower IGBT tube. When a fault occurs on the DC side, the IGBT switch is quickly locked to isolate the current path between the fault point and the low-voltage flexible interconnection device.

[0042] Figure 4 The diagram shows a schematic diagram of a non-isolated flexible interconnection equipment set connected to a distribution area. For a non-isolated flexible interconnection equipment set connected to a distribution area system, if a single-pole grounding fault occurs on the DC side, the grounding bus of the transformer in the area will form a current loop, thereby forming a leakage current or a short-circuit current. Therefore, in order to ensure the reliability of system operation and personal safety, this embodiment configures a leakage protection monitoring unit on the DC side of the flexible interconnection device. The leakage protection monitoring unit is used to warn of a single-pole DC fault. At the same time, a DC protection unit is configured on the DC side to prevent the DC inter-pole short circuit from causing an impact on the interconnection device, thereby affecting the operational stability of the DC microgrid.

[0043] Figure 5 The diagram shows the schematic diagram of the isolated interconnection equipment connected to the substation. For the isolated flexible interconnection device connected to the distribution substation system, if a single-stage grounding fault occurs on the DC side, the grounding bar of the substation transformer cannot be directly connected due to the existence of the isolation transformer, so there is no current loop. When the single-stage grounding occurs, the voltage to the ground of the fault level drops to 0, but it does not affect the inter-pole voltage. The system can still operate for a short time, and the leakage protection monitoring unit is optional. However, in order to prevent the DC inter-pole short circuit from impacting the low-voltage flexible interconnection device, thereby affecting the reliable power supply of the DC side access equipment and the operating stability of the microgrid, a DC protection unit still needs to be configured on the DC side.

[0044] Taking the case where a single interconnected device has multiple outgoing lines on the DC side as an example, from the perspective of power supply reliability, in order to prevent a single DC outgoing line from failing and causing overall protection of the device, this implementation method considers protection measures for each DC outgoing line to facilitate hierarchical protection.

[0045] like Figure 6As shown, for a power supply scenario where there are multiple outgoing lines on the DC side and they are mainly loads (i.e., there are multiple outgoing lines on the DC side of the low-voltage flexible interconnection device, and the number of outgoing lines connected to DC loads among the multiple outgoing lines exceeds a preset percentage), a current-limiting DC protection unit can be configured on the DC main outgoing line of the low-voltage flexible interconnection device, and the circuit breaker on each outgoing line is configured with a DC line microcomputer protection device. The DC line microcomputer protection device includes functions such as current quick-break protection, composite voltage overcurrent protection, overvoltage protection, undervoltage protection, leakage current protection, and DC line insulation monitoring. It can monitor the current signal and / or voltage signal on each outgoing line, and through the fault current limiting threshold, time threshold, and microcomputer protection overcurrent threshold and delay coordination, it can realize fault line selection, accurately isolate line faults, and improve system power supply reliability.

[0046] like Figure 7 As shown, for the scenario where there are multiple outgoing lines on the DC side and a large number of distributed photovoltaic, energy storage, charging piles and other adjustable power sources and loads are connected (that is, there are multiple outgoing lines on the DC side of the low-voltage flexible interconnection device, and the number of adjustable point sources and loads connected in the multiple outgoing lines exceeds the preset percentage), a blocking type DC protection unit can be configured on each outgoing line; when a single-stage DC fault or an inter-pole fault occurs on a certain line, the blocking type DC protection unit of the corresponding outgoing line will quickly lock out and block the short-circuit current, directly isolating the fault area without affecting the normal operation of other branches and the low-voltage flexible interconnection device.

[0047] In order to verify the effectiveness of this implementation method, a low-voltage flexible interconnection device model with isolation in which the N line is not led out from the DC midpoint, a low-voltage flexible interconnection device model with the DC midpoint led out but not isolated, and an AC / DC hybrid distribution network model with distributed power sources and loads connected on the DC side of the flexible interconnection-based grid were built.

[0048] For the isolated low-voltage flexible interconnection device without N-line lead-out at the midpoint of the capacitor, the corner side of the isolation transformer is connected to the AC / DC power module, and the star side is connected to the grid. When a short circuit occurs between the positive and negative poles of the DC bus, the transient capacitor discharges to the fault point, and the steady-state AC voltage discharges to the fault point through the uncontrolled rectification of the bridge arm diode. Typical fault currents are as follows: Figure 8 As shown in the figure, the DC current rises rapidly at the fault point at 0.002s, reaches a peak value and then decays. In steady state, there is continuous uncontrolled rectified current. When a single-pole (positive or negative pole) short circuits to ground, there is no short-circuit current, and only a small leakage current appears at the fault pole. The typical fault waveform is as follows Fig. 9 shown.

[0049] For non-isolated flexible interconnection devices, the N line of the transformer in the substation is directly connected to the midpoint of the capacitor. When a short circuit occurs between the positive and negative poles of the DC bus, the transient capacitor discharges to the fault point, and the steady-state AC voltage discharges to the fault point through the uncontrolled rectification of the bridge arm diode. The typical fault current is basically the same as that of the isolated device, such as Fig.10 When a single-pole (positive or negative) short circuit occurs to the ground, there is no overcurrent at the non-fault pole. The overcurrent state of the fault pole in the transient state is basically the same as the short circuit between poles. After entering the steady state, there is a continuous fault current at the fault pole. The fault current path is mainly the half-cycle of the AC voltage through the corresponding diode and the AC inductor to form a loop. The typical fault waveform is as follows Fig.11 shown.

[0050] When a blocking protection unit is configured on the DC side of the non-isolated flexible interconnection device, the inductance of the positive and negative DC bus lines in the short-circuit fault loop is 1μH and the resistance is 1mΩ. The cut-off frequency of the controller low-pass filter is 300kHz, and the overcurrent threshold is set to 1500A. After the fault occurs 0.002s, the current rises rapidly to 1700A, and then the IGBT is locked, and the current is continued through the freewheeling diode. The waveform is as follows Fig.12 If a single-pole ground fault occurs, the current rises rapidly to 1600A, and then the IGBT is locked, with a response time of 5us. Fig.13 shown.

[0051] contrast Figure 10-11 and Figure 12-13 It can be seen that a blocking protection unit is configured on the DC side of the device. When the short-circuit current exceeds the overcurrent threshold of the protection unit, the IGBT can be turned off within 10us, thereby blocking the short-circuit current and reducing the impact on the interconnected devices and systems.

[0052] When a blocking protection unit is configured on the DC side of the flexible interconnection device and the DC side is directly short-circuited, the DC side current and voltage waveforms are measured as follows: Fig.14 When a short-circuit fault occurs, the DC voltage drops rapidly, and the capacitor discharges to the fault point. The short-circuit current rises to 440A and reaches the IGBT setting threshold. At this time, the IGBT is locked and the short-circuit current is cut off.

[0053] A current limiting protection unit is connected to the DC side of the flexible interconnection device, and the current limiting reactor is configured to be 150A, and the current limiting threshold is set to 120A. Under the condition of DC voltage of 100V, the DC side is short-circuited. After the fault occurs, the current rises rapidly to 140A. At this time, the Hall sensor detects the current and triggers the current protection value, outputs an overcurrent alarm signal (valid at low level), and sends it to the current limiting module controller. The controller quickly blocks the upper tube IGBT, and the inductor current slowly drops to the protection value. After the overcurrent alarm signal returns to normal (high level), the upper tube IGBT continues to be turned on in the next switching cycle. At this time, the current climbs to the protection value again, triggers the protection again to block the upper tube IGBT, and the inductor current drops to the protection value again. The upper tube IGBT is turned on again in the next switching cycle, and this is repeated. The inductor current always fluctuates near a protection value to achieve the current limiting effect. See Fig.15 .

[0054] by Fig.16 The system topology shown in the figure is used as an example to illustrate the feasibility of combining the current limiting protection unit with the microcomputer protection method in the actual operation of the system. The grid shown in the figure is an interconnected system of two substations, and both interconnected devices are equipped with current limiting DC protection units on the DC side.

[0055] Substation 1 is a DC substation. The low-voltage side of the transformer is connected to the DC distribution cabinet after AC / DC conversion through a low-voltage flexible interconnection device. The DC distribution cabinet is equipped with a microcomputer protection device, which is equipped with 4 outgoing lines, two of which are used to power household inverters, one is connected to the energy storage device, and the other is connected to the next-level DC distribution box to provide an interface for charging piles and DC loads. The current limiting protection unit is designed with a current limiting threshold of 320A and a duration of 200ms. The microcomputer protection device adopts composite voltage overcurrent protection. Among them, the parameters are set as follows:

[0056] Compound voltage setting: Compound voltage low voltage setting = 90% rated voltage = 675V setting;

[0057] Time setting value: Fault duration of the fault current limiter of the low-voltage flexible interconnection device is 200ms / reliability coefficient-circuit breaker breaking time, which is tentatively set at 170ms;

[0058] Breaking time: fault current limiter fault duration 200ms / reliability factor = 190ms;

[0059] AC / DC feeder (DC side main incoming line) overcurrent threshold: short-circuit current / reliability factor = 305A;

[0060] Energy storage circuit overcurrent threshold: set according to the energy storage DC / DC rated power 200kW / 0.75kV×1.05 times overload / reliability factor, tentatively set at 266A;

[0061] Charging pile circuit overcurrent threshold: set according to the total power of the charging pile + office 90kW / 0.75kV×1.05 times the overload / reliability factor, tentatively set at 120A;

[0062] Load feeder (currently only household inverters) overcurrent threshold: set at 200kW / 0.75kV×1.05 times overload / reliability factor, tentatively set at 266A;

[0063] Overcurrent threshold of standby circuit: 60kW / 0.75kV*1.05 times overload ÷ reliability coefficient of 1 power household + 1 lighting household, temporarily set to 80A.

[0064] Under the coordinated effect of the current threshold, duration, microcomputer protection branch overcurrent threshold and time setting of the above-mentioned current limiting unit, when a fault occurs in any DC outgoing line of the DC distribution cabinet, only the corresponding outgoing line circuit breaker is disconnected without affecting the normal operation of the interconnected device, thereby effectively reducing the scope of the fault power outage and improving the system power supply reliability.

[0065] It is not difficult to find that the present invention uses a DC protection unit to quickly block the short-circuit current or limit the short-circuit current to a certain range when a fault occurs on the DC side, thereby protecting the power electronic components in the low-voltage flexible interconnection device from damage. At the same time, in conjunction with the line protection and equipment protection of the grid, accurate fault positioning can be achieved, effectively reducing the scope of fault power outages and improving system operation reliability.

Claims

1. A low voltage flexible interconnection complete set of equipment, characterized in that: include: Low-voltage flexible interconnection device, with built-in bidirectional AC / DC conversion unit, the AC side is connected to the substation transformer; A DC protection unit is located on the DC side of the low-voltage flexible interconnection device, and is used to shut off the flow path between the low-voltage flexible interconnection device and the fault point when the short-circuit current on the DC side exceeds the design threshold, thereby completing the blocking of the fault point; the DC protection unit is a current-limiting DC protection unit, which is implemented by a primary circuit. When a fault occurs on the DC side, the fault current is maintained by a current-limiting reactance, and the fault point is isolated after a set time is reached, thereby completing the blocking of the fault point; When the non-isolated low-voltage flexible interconnection device is connected to the distribution substation system, it also includes: a leakage protection monitoring unit, which is arranged on the DC side of the low-voltage flexible interconnection device and is used to warn of a DC single-stage fault; When there are multiple outgoing lines on the DC side of the low-voltage flexible interconnection device, and the number of outgoing lines connected to DC loads among the multiple outgoing lines exceeds a preset percentage, the DC protection unit is configured on the total outgoing line on the DC side of the low-voltage flexible interconnection device, and the circuit breaker on each outgoing line is configured with a DC line microcomputer protection device, and the DC line microcomputer protection device is used to monitor the current signal and / or voltage signal on each outgoing line, and when the current signal or voltage signal is abnormal, cooperate with the DC protection unit to realize fault line selection; When there are multiple outgoing lines on the DC side of the low-voltage flexible interconnection device and the number of adjustable power supplies and loads connected in the multiple outgoing lines exceeds a preset percentage, the DC protection unit is configured on each outgoing line on the DC side of the low-voltage flexible interconnection device.

2. The low voltage flexible interconnection equipment set according to claim 1 is characterized in that: The current-limiting DC protection unit includes an upper IGBT tube, a lower IGBT tube and a DC reactor. The upper IGBT tube and the lower IGBT tube are connected in series. One end of the DC reactor is connected to the connecting end of the upper IGBT tube and the lower IGBT tube, and the other end serves as the first input end of the current-limiting DC protection unit. The connecting end of the lower IGBT tube that is not connected to the upper IGBT tube serves as the second input end of the current-limiting DC protection unit. A capacitor is also connected in parallel at both ends of the upper IGBT tube and the lower IGBT tube connected in series.

3. A low voltage flexible interconnection system, characterized in that: It comprises a plurality of low-voltage flexible interconnection complete equipment sets as described in any one of claims 1-2, wherein the AC sides of the plurality of low-voltage flexible interconnection complete equipment sets are connected to corresponding substations, and the DC sides are connected to a DC bus, and the DC bus is connected to a DC source, a DC load and / or a DC energy storage through multiple output lines.

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