A new energy island grid is sent out through a flexible HVDC overhead line

Abstract

The application discloses a new energy island power grid transmission system through flexible DC overhead lines, adopts a bipolar DC structure, improves the flexibility and reliability of system operation, and energy storage devices are used for inhibiting intermittent characteristics and fluctuation characteristics of new energy; the coordinated cooperation of an AC energy consumption device at a sending end, a sending end flexible DC converter and new energy units realizes the consumption of surplus power at the sending end; the same outer loop controller is applied to a double closed loop control system of the sending end bipolar, so that the fighting phenomenon possibly occurring in the voltage frequency control of the sending end bipolar can be avoided; and the receiving end flexible DC converter station adopts a fixed DC voltage control strategy and a fixed active power control strategy, and when a DC line is faulty, the faulty current is controlled to be cut off, so that the DC fault self-clearing of the new energy island power grid transmission system through the flexible DC overhead lines is realized, and the technical problems of the coordinated control difficulty of the new energy and the flexible DC, the influence of power surplus on the stable operation of the system and the incapability of self-clearing of the DC side fault are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current transmission, in particular to a new energy island power grid transmission system through flexible direct current overhead lines. BACKGROUND

[0002] New energy bases are mostly built in remote areas, and have low load levels and weak grid structures, so the stable transmission of new energy islands is obviously required. Flexible direct current transmission has the characteristics of flexibility, controllability and high efficiency, and is one of the important power transmission methods for new energy transmission. For the sending end, the flexible direct current converter can provide stable alternating current voltage for the new energy power plant, can operate in island mode, and can dynamically compensate and reduce the risk of new energy unit disconnection, thereby improving the utilization rate of new energy. For the receiving end, the flexible direct current does not have the problem of commutation failure, and can provide dynamic reactive power compensation, which is of great significance in effectively solving the stability of multi-direct current feeding into alternating current power grids, resisting serious faults of power grids and other problems.

[0003] When the true bipolar flexible direct current transmission system is applied to the transmission of new energy through overhead lines, the following problems still need to be solved: 1) the coordinated control problem of new energy and flexible direct current and the sending end bipolar coordinated control problem; 2) under the disturbance of alternating current serious faults and emergency blocking of the converter valve, the power surplus of the direct current system and the large fluctuation of the direct current voltage and current are easily caused, which seriously affects the stable operation of the system; 3) the self-clearing problem of the direct current overhead line fault, when the direct current fault occurs, the direct current needs to be quickly removed to quickly extinguish the arc generated by the flashover, so as to realize the rapid clearing of the fault. SUMMARY

[0004] The present application provides a new energy island power grid transmission system through flexible direct current overhead lines, which is used to solve the technical problems of the coordinated control difficulty of new energy and flexible direct current, the power surplus affecting the stable operation of the system, and the self-clearing problem of the direct current side fault.

[0005] Therefore, the first aspect of the present application provides a new energy island power grid transmission system through flexible direct current overhead lines, which comprises a new energy power plant, a sending end flexible direct current converter station, a receiving end flexible direct current converter station, a bipolar direct current overhead line, an energy storage device and an alternating current energy consumption device.

[0006] The sending end flexible direct current converter station comprises a sending end step-up converter transformer and a sending end flexible direct current converter, and the new energy power plant is connected to the sending end step-up converter transformer through a three-phase alternating current bus.

[0007] The sending end flexible direct current converter station has multiple sending end flexible direct current converter stations, and the multiple sending end flexible direct current converter stations are connected in parallel.

[0008] The receiving end flexible DC converter station comprises a receiving end converter transformer and a receiving end flexible DC converter, and the receiving end flexible DC converter is connected with the receiving end converter transformer;

[0009] The sending end flexible DC converter is connected with the receiving end flexible DC converter through bipolar DC overhead lines.

[0010] The AC energy consumption device and the energy storage device are both connected between the new energy power plant and the sending end converter station.

[0011] All the sending end flexible DC converter stations adopt double-loop constant AC voltage and frequency control strategy, only one of the receiving end flexible DC converter stations adopts constant DC voltage control strategy, and the rest of the receiving end flexible DC converter stations adopt constant active power control strategy, and the double-loop constant AC voltage and frequency control strategy of the sending end flexible DC converter station adopts the same outer loop controller.

[0012] Optionally, the DC energy consumption device is further included.

[0013] The DC energy consumption device is connected at the DC outlet of the receiving end flexible DC converter.

[0014] Optionally, the DC energy consumption device is composed of a resistance in series with an IGBT device.

[0015] Optionally, the configuration group number of the AC energy consumption device is calculated according to a preset formula, and the preset formula is:

[0016]

[0017] Wherein, δ is 1 or 2, N 交流耗能装置组数 is the configuration group number of the AC energy consumption device, P 送端双极功率 is the active power of the sending end flexible DC converter, P 每组交流耗能装置容量 is the capacity of each group of AC energy consumption devices.

[0018] Optionally, each group of AC energy consumption devices is composed of a resistance in series with an IGBT device connected in star.

[0019] Optionally, the sending end booster converter transformer and the receiving end converter transformer are △ / Y0 double-winding transformers or Y / Y0 double-winding transformers or three-winding transformers.

[0020] Optionally, the bipolar DC overhead lines are connected with the sending end flexible DC converter and the receiving end flexible DC converter through smoothing reactors.

[0021] Optionally, the number of the receiving end flexible DC converter stations is one or more than two.

[0022] Optionally, the sending end flexible DC converter and the receiving end flexible DC converter are in the form of single valve group or double valve group series connection, each valve group is in three-phase six-bridge-arm structure, each bridge arm is composed of a plurality of power modules and a bridge arm reactor in series connection, and the power module is in full-bridge sub-module or hybrid module of full-bridge sub-module and half-bridge sub-module.

[0023] Optionally, the sending end flexible DC converter and the receiving end flexible DC converter of each pole are composed of two or more flexible DC converters in series connection.

[0024] From the above technical solutions, the new energy island power grid through the flexible DC overhead line transmission system has the following advantages:

[0025] The new energy island power grid through the flexible DC overhead line transmission system adopts a bipolar DC structure, improves the flexibility and reliability of system operation, and can maintain the stable operation of the sending end island system by the normal pole after single-pole fault even leads to final locking. The energy storage device is used for inhibiting the intermittent and fluctuation characteristics of new energy, and when the new energy station generates more power, part of the energy is stored, and when the new energy station generates less power, the stored energy is released, so as to maintain the DC power transmission as much as possible, avoid frequent start and stop of DC, facilitate system dispatching and operation and maintenance, the coordinated cooperation of the AC energy consumption device of the sending end, the sending end flexible DC converter and the new energy unit realizes the consumption of the sending end power surplus power, the same outer ring controller is applied in the double closed loop control system of the sending end bipolar, the possible fighting phenomenon of the sending end bipolar voltage frequency control can be avoided, only one of the receiving end flexible DC converter station adopts the fixed DC voltage control strategy, and the rest of the receiving end flexible DC converter station adopts the fixed active power control strategy, and the new energy island power grid through the flexible DC overhead line transmission system realizes the DC fault self-clearing, solves the technical problems of the coordination control difficulty of new energy and flexible DC, the influence of power surplus on system stable operation and the failure of DC side fault self-clearing.

[0026] Meanwhile, the DC energy consumption device is arranged at the DC outlet of the receiving end flexible DC converter station, and the DC energy consumption device is coordinated with the receiving end flexible DC converter, and is used for dissipating the DC power that cannot be sent out on the DC side when the receiving end AC fails, so as to avoid the overcurrent and overvoltage phenomenon caused by the continuous charging of the DC surplus power to the bridge arm sub-module of the receiving end flexible DC converter station. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings described below are only some embodiments of the present application, and for those skilled in the art, other related drawings can also be obtained without creative labor.

[0028] Figure 1 The structural diagram of a new energy island grid through a flexible DC overhead line sending-out system is provided in the embodiments of the present application.

[0029] Figure 2 The schematic diagram of a single-converter overhead line sending-out system of the new energy island grid through a flexible DC overhead line sending-out system is provided in the embodiments of the present application.

[0030] Figure 3 The schematic diagram of a double-converter overhead line sending-out system of the new energy island grid through a flexible DC overhead line sending-out system is provided in the embodiments of the present application.

[0031] Figure 4 The schematic diagram of another double-converter overhead line sending-out system of the new energy island grid through a flexible DC overhead line sending-out system is provided in the embodiments of the present application.

[0032] Figure 5 The structural schematic diagram of a flexible DC converter (modular multilevel converter) of the new energy island grid through a flexible DC overhead line sending-out system is provided in the embodiments of the present application.

[0033] Figure 6 The circuit structural diagram of a full-bridge sub-module and a half-bridge sub-module is provided in the embodiments of the present application.

[0034] Figure 7 The schematic diagram of a sending-end converter station double-loop VF control is provided in the embodiments of the present application.

[0035] Figure 8 The schematic diagram of a receiving-end converter station double-loop control is provided in the embodiments of the present application. DETAILED DESCRIPTION

[0036] In order to make the technical personnel in the art better understand the present application scheme, the following will combine the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0037] For the convenience of understanding, please refer toFigure 1 The embodiment of the new energy island power grid through the flexible DC overhead line sending-out system provided in the present application comprises a new energy power plant, a sending-end flexible DC converter station, a receiving-end flexible DC converter station, a bipolar DC overhead line, an energy storage device and an AC energy consumption device;

[0038] The sending-end flexible DC converter station comprises a sending-end step-up converter transformer and a sending-end flexible DC converter, and the new energy power plant is connected to the sending-end step-up converter transformer through a three-phase AC bus, and the sending-end step-up converter transformer is connected to the sending-end flexible DC converter.

[0039] There are two sending-end flexible DC converter stations, and the two sending-end flexible DC converter stations are connected in parallel.

[0040] The receiving-end flexible DC converter station comprises a receiving-end converter transformer and a receiving-end flexible DC converter, and the receiving-end flexible DC converter is connected to the receiving-end converter transformer.

[0041] The sending-end flexible DC converter is connected to the receiving-end flexible DC converter through the bipolar DC overhead line.

[0042] The AC energy consumption device and the energy storage device are both connected between the new energy power plant and the sending-end converter station.

[0043] All the sending-end flexible DC converter stations adopt a double-loop constant AC voltage and frequency control strategy, only one of the receiving-end flexible DC converter stations adopts a constant DC voltage control strategy, and the rest of the receiving-end flexible DC converter stations adopt a constant active power control strategy, and the double-loop constant AC voltage and frequency control strategy of the sending-end flexible DC converter station adopts a same outer loop controller for bipolar control.

[0044] It should be noted that the new energy island power grid through the flexible DC overhead line sending-out system built in the embodiment of the present application is as shown in Figure 1 The new energy power plant comprises a wind power plant and a photovoltaic power plant and the like renewable energy power plant. The DC overhead line has simple structure, low line cost, high corridor utilization rate, small operation loss and convenient maintenance, and improves the distance and capacity of new energy sending-out. The sending-end flexible DC converter station comprises a sending-end step-up converter transformer and a sending-end flexible DC converter, the sending-end flexible DC converter is grounded, the new energy power plant is connected to the sending-end step-up converter transformer through a three-phase AC bus, and the sending-end step-up converter transformer is connected to the sending-end flexible DC converter. The sending-end flexible DC converter can adopt a single valve group form or a double valve group series form, each valve group has a three-phase six-bridge-arm structure, and each bridge arm is composed of a plurality of power modules and a bridge arm reactor in series (as shown in Figure 5 The power module adopts a full-bridge sub-module (as shown in Figure 6The full-bridge circuit structure shown in the diagram consists of four fully controllable switching devices that can be turned off, four anti-parallel diodes, and an energy storage capacitor. It can output positive, negative, and zero levels. Alternatively, it can use full-bridge submodules and half-bridge submodules (such as...). Figure 6 The half-bridge circuit structure shown in the diagram consists of two fully controllable switching devices that can be turned off, two anti-parallel diodes, and an energy storage capacitor. It is a module capable of outputting both positive and zero-level voltages. When the flexible DC converter adopts a hybrid topology of full-bridge and half-bridge submodules—that is, a topology containing both a certain number of full-bridge and half-bridge submodules within a single bridge arm—the cost of the flexible DC converter can be further reduced while still meeting the requirements for DC voltage regulation. The sending-end flexible DC converter mainly connects to the sending-end step-up converter transformer and to the receiving-end flexible DC substation via a bipolar DC overhead line. The sending-end step-up converter transformer collects and boosts the electrical energy from the connected new energy power field. The AC power after transformation is rectified by the sending-end flexible DC converter to obtain DC power, which is then transmitted to the receiving-end flexible DC converter station. The receiving-end flexible DC converter station includes a receiving-end converter transformer and a receiving-end flexible DC converter. The receiving-end flexible DC converter is grounded. The receiving-end flexible DC converter can be in the form of a single valve group or a double valve group in series. Each valve group is a three-phase six-arm structure, and each arm consists of several power modules and an arm reactor connected in series (e.g., Figure 5 As shown). The power module uses a full-bridge submodule (such as...). Figure 6 The full-bridge circuit structure shown in the figure) or using full-bridge submodules and half-bridge submodules (such as...) Figure 6 The half-bridge circuit structure shown in the diagram is a hybrid module. The receiving-end flexible DC converter converts the DC power supplied by the sending-end flexible DC converter into AC power and supplies it to the receiving-end converter transformer. The receiving-end converter transformer then transforms the received AC power before outputting it.

[0045] There are multiple sending-end flexible DC converter stations, which can be connected in parallel, i.e. co-station construction, or constructed in different places. The sending-end flexible DC converter stations all adopt a fixed AC voltage and frequency (VF) control strategy to provide the required AC voltage and frequency for the respective new energy power plant. The VF adopts a vector control architecture, i.e. active component and reactive component control of the flexible DC unit in the synchronous rotating coordinate system, which is structurally divided into outer loop and inner loop control. The d-axis voltage outer loop control realizes control of the d-axis component of the AC voltage, and the q-axis voltage outer loop control realizes control of the q-axis component of the AC voltage. The inner loop control includes active current control and reactive current control, which mainly accept the active and reactive current reference values from the outer loop control and quickly track the reference current to realize direct control of the amplitude and phase of the valve group AC side voltage. Since the entire system is a bipolar system, to avoid the possible fighting phenomenon of the sending-end fixed AC voltage and frequency (VF) control, the double closed-loop control system of the sending-end bipolar should adopt the same outer loop controller, such as the d-axis voltage controller and the q-axis voltage controller shown in Figure 7 The output of the same d-axis voltage controller is used as the input of the active current controller of the four poles of the two sending-end converter stations, and the output of the same q-axis voltage controller is used as the input of the reactive current controller of the four poles of the two sending-end converter stations, to realize common coordinated control of the sending-end AC voltage by the two converter stations. The sending-end converter station detects the DC current flowing out of the pole in real time. If the detected DC current flowing out of the pole increases, the sending-end converter station automatically reduces the DC voltage bias of the pole through the DC current controller, reduces the number of sub-modules put into the DC side, realizes direct control of the DC voltage, outputs a negative DC voltage, and actively clears the DC fault. This process is completely automatic control, which does not depend on the detection of the fault by the protection, can quickly clear the DC fault, and can effectively avoid the influence of protection misoperation or refusal to operate.

[0046] The number of receiving-end flexible DC converter stations can be one or more than two, realizing single landing point or multi-landing point power reception. There is only one receiving-end flexible DC converter station that adopts a fixed DC voltage control strategy to provide stable DC voltage for the DC system, and the rest of the receiving-end flexible DC converter stations all adopt a fixed active power control strategy, and the control architecture is as shown in Figure 8The receiving end basic control is a vector control architecture, which controls the active and reactive power of the flexible DC unit in the synchronous rotating coordinate system. The structure is divided into outer loop and inner loop control. The outer loop control includes active control (active power control and DC voltage control) and reactive control (reactive power control and AC voltage control). The inner loop control includes active current control and reactive current control. The inner loop control mainly receives the reference value of active and reactive current from the outer loop control, and quickly tracks the reference current to achieve direct control of the valve group AC side voltage amplitude and phase. The receiving end converter station detects the DC current flowing into the pole in real time. If the receiving end converter station detects that the DC current flowing into the pole decreases and reverses, the receiving end converter station automatically reduces the DC voltage bias of the pole through the DC current controller, reduces the number of sub-modules put into the DC side, directly controls the DC voltage, and outputs a negative DC voltage to actively clear the DC fault. This process is completely automatic control and does not depend on the detection of the protection fault, which can quickly clear the DC fault.

[0047] In one embodiment, the bipolar DC overhead line is connected between the sending end flexible DC converter station and the receiving end flexible DC converter station through a smoothing reactor, which mainly plays a smoothing role to make the DC current smooth.

[0048] In one embodiment, a DC energy consumption device is arranged at the DC outlet of the receiving end flexible DC converter, which is used to dissipate the DC power sent out by the DC side during the receiving end AC fault, so as to avoid the overcurrent and overvoltage phenomenon caused by the continuous charging of the bridge arm sub-module by the surplus DC power. The DC energy consumption device is composed of a resistance in series with an IGBT device, and the capacity of the DC energy consumption device is the rated capacity of the receiving end converter station.

[0049] In one embodiment, the AC energy consumption device is composed of a star-connected resistance in series with an IGBT device. In order to facilitate operation and maintenance, the AC energy consumption device should be redundantly configured, that is, one or two groups of AC energy consumption devices are configured. The number of groups of AC energy consumption devices is calculated according to a preset formula, and the preset formula is:

[0050]

[0051] Wherein, δ is 1 or 2, N 交流耗能装置组数 is the number of groups of AC energy consumption devices, P 送端双极功率 is the active power of the sending end flexible DC converter, P 每组交流耗能装置容量 is the capacity of each group of AC energy consumption devices.

[0052] In one embodiment, the sending end step-up converter transformer and the receiving end converter transformer are △ / Y0 double-winding transformers or Y / Y0 double-winding transformers (as shown in Figure 2 and Figure 3 ) or three-winding transformers (as shown in Figure 4The sending end and the receiving end of each pole are connected by a flexible DC transmission line, and the sending end and the receiving end of each pole are connected by a flexible DC transmission line. Figure 2 As shown in FIG. 4, the flexible DC converters of the sending end and the receiving end of each pole are composed of a single flexible DC converter. The operation mode is simple and flexible, and the system has high reliability. Figure 3 As shown in FIG. 5, the flexible DC converters of the sending end and the receiving end of each pole are composed of two flexible DC converters in series, which can reduce the requirement for the current-carrying capacity of the flexible DC converter, and reduce the manufacturing difficulty and cost of the flexible DC converter. It can be understood that in other embodiments, the flexible DC converters of the sending end and the receiving end of each pole can also be composed of multiple flexible DC converters in series, further improving the voltage level of the flexible DC.

[0053] The new energy island grid transmission system through the flexible DC overhead line provided in the embodiment of the application adopts a bipolar DC architecture, improves the flexibility and reliability of system operation, and can maintain the stable operation of the sending end island system by the normal pole after a single pole fault even leads to the final lockout. The energy storage device is used to suppress the intermittent and fluctuating characteristics of the new energy, and when the new energy station generates more power, part of the energy is stored, and when the new energy station generates less power, the stored energy is released to maintain the DC power transmission as much as possible, avoid frequent start and stop of DC, facilitate system dispatching and operation and maintenance, the coordination of the sending end AC energy consumption device with the sending end flexible DC converter and the new energy unit realizes the consumption of the sending end power surplus power, the same outer loop controller is applied in the double closed loop control system of the sending end bipolar, which can avoid the fighting phenomenon that may occur in the sending end bipolar voltage frequency control, and only one of the receiving end flexible DC converter stations adopts a fixed DC voltage control strategy to provide a stable DC voltage for the DC system, and the rest of the receiving end flexible DC converter stations adopt a fixed active power control strategy. When the DC line is faulty, the fault current is controlled to be cut off, and the DC fault self-clearing of the new energy island grid transmission system through the flexible DC overhead line is realized, and the technical problems of the coordination control difficulty of the new energy and the flexible DC, the influence of the power surplus on the stable operation of the system, and the inability of the DC side fault to self-clear are solved.

[0054] Meanwhile, the new energy island grid transmission system through the flexible DC overhead line provided in the embodiment of the application is provided with a DC energy consumption device at the DC outlet of the receiving end flexible DC converter station, and the DC energy consumption device is coordinated with the receiving end flexible DC converter to dissipate the DC power that cannot be sent out on the DC side when the receiving end AC is faulty, thereby avoiding the overcurrent and overvoltage phenomenon caused by the continuous charging of the bridge arm sub-module of the receiving end flexible DC converter station by the DC surplus power.

[0055] The above-described embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those skilled in the art that the technical solutions recorded in the foregoing embodiments can still be modified, or some technical features can be replaced by equivalent replacements; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A new energy islanded power grid transmission system through flexible DC overhead line, characterized in that, The new energy power plant, the sending end flexible DC converter station, the receiving end flexible DC converter station, the bipolar DC overhead line, the energy storage device and the AC energy consumption device are connected in parallel. The sending end flexible DC converter station includes a sending end booster converter transformer and a sending end flexible DC converter, and the new energy power plant is connected to the sending end booster converter transformer through a three-phase AC bus. The sending end flexible DC converter station includes a sending end booster converter transformer and a sending end flexible DC converter, and the new energy power plant is connected to the sending end booster converter transformer through a three-phase AC bus. The sending end flexible DC converter station includes a sending end booster converter transformer and a sending end flexible DC converter, and the new energy power plant is connected to the sending end booster converter transformer through a three-phase AC bus. The sending end flexible DC converter station includes a sending end booster converter transformer and a sending end flexible DC converter, and the new energy power plant is connected to the sending end booster converter transformer through a three-phase AC bus. The AC energy consumption device and the energy storage device are connected between the new energy power plant and the sending end converter station. All the sending end flexible DC converter stations adopt a double-closed-loop AC voltage and frequency control strategy, and the receiving end flexible DC converter station adopts a DC voltage control strategy, and the rest of the receiving end flexible DC converter stations adopt an active power control strategy.

2. The new energy islanded power grid transmitted by flexible HVDC overhead line according to claim 1, characterized in that, The DC energy consumption device is connected to the DC outlet of the receiving end flexible DC converter. The DC energy consumption device is composed of a resistance in series with an IGBT device. 3.The new energy islanded grid through flexible HVDC overhead line transmission system according to claim 2, characterized in that, The number of groups of the AC energy consumption device is calculated according to a preset formula.

4. The new energy islanded power grid transmitted by flexible HVDC overhead line according to claim 1, characterized in that, Each group of the AC energy consumption device is composed of a star-connected resistance in series with an IGBT device. wherein δ is 1 or 2, N 交流耗能装置组数 is the number of configuration groups of the AC energy consumption device, P 送端双极功率 is the active power of the sending end flexible DC converter, P 每组交流耗能装置容量 is the capacity of each group of AC energy consumption devices.

5. The new energy islanded power grid transmitted by flexible HVDC overhead line according to claim 4, characterized in that, The sending end booster converter transformer and the receiving end converter transformer are △ / Y0 double-winding transformers, Y / Y0 double-winding transformers or three-winding transformers. 6.The new energy islanded grid through flexible HVDC overhead line transmission system according to claim 1, characterized in that, The bipolar DC overhead line is connected to the sending end flexible DC converter through a smoothing reactor, and is connected to the receiving end flexible DC converter through a smoothing reactor. 7.The new energy islanded grid through flexible HVDC overhead line transmission system according to claim 1, characterized in that, The number of the receiving end flexible DC converter stations is one or more than two. 8.The new energy islanded grid system through flexible HVDC overhead line for sending out power according to claim 1, characterized in that, The sending end flexible DC converter and the receiving end flexible DC converter adopt a single valve group form or a double valve group series form, each valve group is a three-phase six-bridge arm structure, each bridge arm is composed of a plurality of power modules and a bridge arm reactor in series, and the power module adopts a full-bridge sub-module or a hybrid module of a half-bridge sub-module. 9.The new energy islanded grid through flexible HVDC overhead line transmission system according to claim 1, characterized in that, Each sending end flexible DC converter and each receiving end flexible DC converter is composed of two or more flexible DC converters in series. 10.The new energy islanded grid system through flexible HVDC overhead line for sending out power according to claim 1, characterized in that, ​

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