A new energy flexible direct transmission system overhead line fault clearing method and system

Abstract

This invention belongs to the technical field of ultra-high voltage flexible direct current (UHVDC) transmission, specifically relating to a method and system for clearing faults on overhead lines in a new energy flexible direct current transmission system. This invention introduces a dual closed-loop control strategy on the DC side at the sending-end converter. Before a fault is detected at the sending end, the submodule voltage changes are tracked, effectively setting up a buffer control within the time difference between fault occurrence and control strategy triggering. This ensures timely adjustment and control as soon as a remote fault affects the sending-end submodule voltage and DC current. Therefore, it solves the problem of AC voltage and module voltage drops at the sending end caused by remote faults on ultra-long-distance overhead lines, and also promptly reduces the overcurrent level of the bridge arm caused by the rapid development of near-end faults. Furthermore, after a fault is detected at the sending end, the DC side control directly switches to current closed-loop control, no longer affected by the submodule voltage, thus enabling rapid suppression of DC current and ensuring rapid fault clearing.

Description

Technical Field

[0001] This invention belongs to the technical field of ultra-high voltage flexible direct current transmission, specifically relating to a method and system for clearing faults in overhead lines of a new energy flexible direct current transmission system. Background Technology

[0002] my country has a vast territory, and its energy resources and energy demand are inversely distributed. More than 80% of its energy resources, including coal and clean energy (hydropower, wind power, and solar energy), are located in the western and northern regions, while more than 70% of its electricity consumption is concentrated in the eastern and central regions. Areas rich in wind and solar energy resources are thousands of kilometers away from load centers. Under the backdrop of building a new power system based on new energy sources, my country must develop hydropower, wind power, and photovoltaic resources in the northwest on a large scale. Large-scale energy development and nationwide resource optimization necessitate the construction of long-distance power transmission channels. Compared to the economic challenges faced by traditional AC transmission technology for long-distance transmission, high-voltage direct current (HVDC), especially ultra-high-voltage direct current (UHVDC), offers significant technological advantages such as long transmission distances, high transmission efficiency, flexible and rapid control methods, no system stability issues, and the ability to limit short-circuit currents. Therefore, developing high-voltage, high-capacity DC transmission technology to achieve large-scale optimal allocation of energy resources is a key direction for the development and construction of my country's power grid.

[0003] Domestically operational ultra-high voltage direct current (UHVDC) transmission projects generally adopt a series connection method based on line commutator converters (LCCs), which has advantages such as large transmission capacity, good operational stability, low system loss, and low construction cost, playing an irreplaceable role in long-distance, large-capacity power transmission. With the development of flexible DC transmission technology, the voltage and capacity of modular multilevel converters (MMCs) have gradually approached the level of LCCs. Due to their flexible controllability and independence from the AC grid, they have unparalleled technical advantages, especially in areas such as clean energy transmission, connection to weak AC systems, or solving commutation failure problems in conventional DC multi-infeed systems. To reduce project costs, half-bridge submodule topologies of MMCs are often used in engineering projects. However, due to their inherent structural characteristics, half-bridge submodule MMCs lack the ability to isolate DC faults and cannot achieve DC fault ride-through. To ensure the safe operation of MMCs, DC circuit breakers need to be installed or some submodule topologies need to be replaced with submodule topologies capable of isolating DC faults.

[0004] Regarding fault clearing methods for UHV flexible DC overhead lines in active power systems, existing literature has researched this approach. This involves configuring a certain number of full-bridge sub-modules in the flexible DC converter valve and controlling the rapid adjustment of DC current to reduce voltage, thereby achieving self-clearing of the fault current. However, for… Figure 1 The large-scale isolated renewable energy transmission system shown here faces several challenges in its control strategy during a fault in an ultra-high-voltage flexible DC overhead line. Firstly, the system must consider the unique characteristics of ultra-long-distance overhead lines. Secondly, it needs to address the AC voltage stability of the isolated renewable energy system during the fault period. Current technologies that use rapid DC current regulation to clear fault current have limitations when faults occur at the far end of the overhead line. This is because the distance between the sending and receiving ends is long, resulting in weak fault detection capability of the sending-end converter and a prolonged fault detection time. During this period before the fault is detected at the sending end, the fault characteristics of the sending-end converter manifest as submodule discharge simultaneously causing overcurrent in the bridge arm. As the submodule continues to discharge, its voltage gradually decreases. When the modulation wave reaches saturation, the voltage level output by the bridge arm is insufficient to maintain a constant AC voltage. Consequently, the AC voltage at the sending end will continue to drop, resulting in the sending end being significantly affected by the fault before it is detected, and the fault cannot be cleared in time. When a fault occurs near the overhead line, the fault develops rapidly, and the DC port voltage of the sending-end converter drops to zero instantaneously. The fault circuit impedance is low, and the discharge of the submodule causes the bridge arm current to rise rapidly. This places extremely stringent requirements on the fault detection time; otherwise, it may trigger overcurrent lockout of the converter valve, which will also affect the normal power transmission of the flexible DC overhead line. Summary of the Invention

[0005] The purpose of this invention is to provide a method and system for clearing faults in overhead lines of a new energy flexible DC transmission system, which solves the problem that faults at the far and near ends of flexible DC overhead lines cannot be cleared in a timely manner, affecting the normal power transmission of the overhead lines.

[0006] To achieve the above objectives, the present invention provides a method for clearing faults in overhead lines of a new energy flexible direct transmission system. The method involves real-time detection of faults in the overhead lines at both the sending and receiving ends of the flexible direct transmission system. Based on the detection results, different voltage modulation waves are determined at the sending and receiving converters according to different control strategies. Trigger pulses are generated based on the voltage modulation waves and output to the corresponding sub-modules of the sending and receiving converters to clear the fault current.

[0007] The control strategy at the sending-end converter includes AC-side control and DC-side control. If no overhead line fault is detected at the sending end, the DC-side control is a dual-loop control. The outer loop control is a submodule voltage control, and the output control quantity is determined based on the difference between the submodule voltage reference value and the feedback value of the sending-end converter. The inner loop control is a DC current control, and the output control quantity is determined based on the difference between the DC current reference value and the feedback value of the DC side of the sending-end converter. The control quantity output by the inner loop control is superimposed with the first DC voltage command value and used as the DC component of the voltage modulation wave corresponding to the sending-end converter. The first DC voltage command value is set by the DC side voltage of the sending-end converter during steady-state operation via a flexible DC transmission line.

[0008] The reference value of DC current on the DC side of the sending-end converter in the DC-side inner loop control is determined based on the control quantity output by the DC-side outer loop control.

[0009] The sending-end converter refers to the MMC converter connected to the power generation system in the flexible DC transmission system, and the receiving-end converter refers to the MMC converter connected to the AC power grid; both the sending-end converter and the receiving-end converter are MMC converters that include a full-bridge submodule.

[0010] The beneficial effects of the above technical solution are as follows: By introducing a DC-side dual closed-loop control strategy at the sending-end converter, the voltage change of the sub-module is tracked before the sending end detects the fault when a remote fault occurs. This is equivalent to setting up a buffer control within the time difference between the fault occurrence and the triggering of the control strategy. This ensures that the voltage and DC current of the sending-end sub-module are adjusted and controlled in a timely manner as soon as the remote fault affects them. Therefore, it can solve the problem of the AC voltage drop and module voltage drop at the sending end caused by the remote fault of the ultra-long-distance overhead line, and can also reduce the overcurrent level of the bridge arm caused by the rapid development of the near-end fault in a timely manner.

[0011] Furthermore, if a fault is detected in the overhead line of the system at the sending end, the outer loop control of the DC side control is disabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop control is switched to zero; after the fault is detected to have been cleared, the outer loop controller is re-enabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop controller is switched to the reference value determined according to the control quantity output by the outer loop control of the DC side.

[0012] The beneficial effects of the above technical solution are as follows: the DC-side control strategy related to the submodule voltage change is continuously implemented only when no fault is detected at the sending-end converter. After a fault is detected at the sending end, the DC-side control at the sending-end converter is directly switched to current closed-loop control, which is no longer affected by the submodule voltage. This avoids the fact that the suppression of fault current in the later stage of the fault basically depends on the control result of the outer loop of the submodule voltage, which leads to a longer fault current clearing time. Therefore, it can quickly suppress DC current to ensure the speed of fault clearing.

[0013] Furthermore, the control strategy at the receiving-end converter includes AC-side control: if no overhead line fault is detected at the receiving end, the AC-side control is a dual closed-loop control, wherein the outer loop control is DC voltage control and the inner loop control is AC current control. The control quantity output by the inner loop control is used as the AC component of the voltage modulation wave corresponding to the receiving-end converter; the control quantity output by the outer loop control is determined based on the difference between the reference value and the feedback value of the reactive component of the DC voltage on the AC side of the receiving-end converter; the control quantity output by the inner loop control is determined based on the difference between the reference value and the feedback value of the AC current on the AC side of the receiving-end converter.

[0014] The second DC voltage command value is used as the DC component of the voltage modulation wave corresponding to the receiving-end converter; the second DC voltage command value is set by the DC side voltage of the receiving-end converter during steady-state operation of the flexible DC transmission system.

[0015] If a DC overhead line fault is detected at the receiving end, the AC side outer loop control is switched from DC voltage control to submodule voltage outer loop control. The control quantity output by the submodule voltage outer loop control is determined based on the difference between the receiving end converter submodule voltage reference value and the feedback value. The DC component of the voltage modulation wave is switched to the control quantity output by the DC current closed loop control. The control quantity output by the DC current closed loop control is determined based on the difference between the receiving end converter DC side DC current reference value and the feedback value.

[0016] Furthermore, an AC energy dissipation device is configured at the sending end of the flexible direct transmission system, and the AC energy dissipation device is activated after a fault in the overhead line of the system is detected at the sending end, and the energy dissipation device is deactivated after the fault is cleared.

[0017] The beneficial effects of the above technical solution are: it can consume the fault current in a timely manner, improve the fault clearing efficiency, and also ensure the safety of the fault clearing process.

[0018] Furthermore, the AC side control at the sending-end converter is an AC voltage dual closed-loop control; wherein the outer loop control of the AC voltage dual closed-loop control is an AC voltage active / reactive amplitude closed-loop control, and the control quantity output of the AC voltage active / reactive amplitude closed-loop control is determined based on the difference between the reference value and feedback value of the AC voltage active component on the sending-end converter's AC side, and the difference between the reference value and feedback value of the AC voltage reactive component on the sending-end converter's AC side; the inner loop control of the AC voltage dual closed-loop control is an AC current active / reactive closed-loop control, and the control quantity output of the AC current active / reactive amplitude closed-loop control is determined based on the difference between the reference value and feedback value of the AC current active component on the sending-end converter's AC side, and the difference between the reference value and feedback value of the AC current reactive component on the AC side, and the final output control quantity of the AC voltage dual closed-loop control is used as the AC component of the voltage modulation wave corresponding to the sending-end converter; the final output control quantity of the AC voltage dual closed-loop control is determined based on the control quantity output of the AC current active / reactive amplitude closed-loop control.

[0019] The reference values ​​for the active component of the AC current on the sending-end converter and the reference values ​​for the reactive component of the AC current on the sending-end converter are determined based on the control quantity output by the closed-loop control of the AC voltage active / reactive amplitude.

[0020] The present invention also provides a fault clearing system for overhead lines of a new energy flexible direct transmission system, including a data collector and a processor; the data collector is used to collect the current and / or voltage at the sending end and receiving end of the flexible direct transmission system in real time, and is used to detect faults in the overhead lines of the system.

[0021] The processor is used to detect overhead line faults in the flexible DC transmission system in real time at the sending end and receiving end. Based on the detection results, it determines the corresponding voltage modulation wave at the sending end converter and the receiving end converter according to different control strategies. It generates trigger pulses based on the voltage modulation wave and outputs them to the corresponding sub-modules of the sending end converter and the receiving end converter respectively to clear the fault current.

[0022] The control strategy at the sending-end converter includes AC-side control and DC-side control. If no overhead line fault is detected at the sending end, the DC-side control is a dual-loop control. The outer loop control is a submodule voltage control, and the output control quantity is determined based on the difference between the submodule voltage reference value and the feedback value of the sending-end converter. The inner loop control is a DC current control, and the output control quantity is determined based on the difference between the DC current reference value and the feedback value of the DC side of the sending-end converter. The control quantity output by the inner loop control is superimposed with the first DC voltage command value and used as the DC component of the voltage modulation wave corresponding to the sending-end converter. The first DC voltage command value is set by the DC side voltage of the sending-end converter during steady-state operation via a flexible DC transmission line.

[0023] The reference value of DC current on the DC side of the sending-end converter in the DC-side inner loop control is determined based on the control quantity output by the DC-side outer loop control.

[0024] The sending-end converter refers to the MMC converter connected to the power generation system in the flexible DC transmission system, and the receiving-end converter refers to the MMC converter connected to the AC power grid; both the sending-end converter and the receiving-end converter are MMC converters that include a full-bridge submodule.

[0025] The beneficial effects of the above technical solution are as follows: By introducing a DC-side dual closed-loop control strategy at the sending-end converter, the voltage change of the sub-module is tracked before the sending end detects the fault when a remote fault occurs. This is equivalent to setting up a buffer control within the time difference between the fault occurrence and the triggering of the control strategy. This ensures that the voltage and DC current of the sending-end sub-module are adjusted and controlled in a timely manner as soon as the remote fault affects them. Therefore, it can solve the problem of the AC voltage drop and module voltage drop at the sending end caused by the remote fault of the ultra-long-distance overhead line, and can also reduce the overcurrent level of the bridge arm caused by the rapid development of the near-end fault in a timely manner.

[0026] Furthermore, if a fault is detected in the overhead line of the system at the sending end, the outer loop control of the DC side control is disabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop control is switched to zero; after the fault is detected to have been cleared, the outer loop controller is re-enabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop controller is switched to the reference value determined according to the control quantity output by the outer loop control of the DC side.

[0027] The beneficial effects of the above technical solution are as follows: the DC-side control strategy related to the submodule voltage change is continuously implemented only when no fault is detected at the sending-end converter. After a fault is detected at the sending end, the DC-side control at the sending-end converter is directly switched to current closed-loop control, which is no longer affected by the submodule voltage. This avoids the fact that the suppression of fault current in the later stage of the fault basically depends on the control result of the outer loop of the submodule voltage, which leads to a longer fault current clearing time. Therefore, it can quickly suppress DC current to ensure the speed of fault clearing.

[0028] Furthermore, the control strategy at the receiving-end converter includes AC-side control: if no overhead line fault is detected at the receiving end, the AC-side control is a dual closed-loop control, wherein the outer loop control is DC voltage control and the inner loop control is AC current control. The control quantity output by the inner loop control is used as the AC component of the voltage modulation wave corresponding to the receiving-end converter; the control quantity output by the outer loop control is determined based on the difference between the reference value and the feedback value of the reactive component of the DC voltage on the AC side of the receiving-end converter; the control quantity output by the inner loop control is determined based on the difference between the reference value and the feedback value of the AC current on the AC side of the receiving-end converter.

[0029] The second DC voltage command value is used as the DC component of the voltage modulation wave corresponding to the receiving-end converter; the second DC voltage command value is set by the DC side voltage of the receiving-end converter during steady-state operation of the flexible DC transmission system.

[0030] If a DC overhead line fault is detected at the receiving end, the AC side outer loop control is switched from DC voltage control to submodule voltage outer loop control. The control quantity output by the submodule voltage outer loop control is determined based on the difference between the receiving end converter submodule voltage reference value and the feedback value. The DC component of the voltage modulation wave is switched to the control quantity output by the DC current closed loop control. The control quantity output by the DC current closed loop control is determined based on the difference between the receiving end converter DC side DC current reference value and the feedback value.

[0031] Furthermore, an AC energy dissipation device is configured at the sending end of the flexible direct transmission system, and the AC energy dissipation device is activated after a fault in the overhead line of the system is detected at the sending end, and the energy dissipation device is deactivated after the fault is cleared.

[0032] The beneficial effects of the above technical solution are: it can suppress fault current in a timely manner, improve fault clearing efficiency, and also ensure the stability and safety of system operation during the fault clearing process.

[0033] Furthermore, the AC side control at the sending-end converter is an AC voltage dual closed-loop control; wherein the outer loop control of the AC voltage dual closed-loop control is an AC voltage active / reactive amplitude closed-loop control, and the control quantity output of the AC voltage active / reactive amplitude closed-loop control is determined based on the difference between the reference value and feedback value of the AC voltage active component on the sending-end converter's AC side, and the difference between the reference value and feedback value of the AC voltage reactive component on the sending-end converter's AC side; the inner loop control of the AC voltage dual closed-loop control is an AC current active / reactive closed-loop control, and the control quantity output of the AC current active / reactive amplitude closed-loop control is determined based on the difference between the reference value and feedback value of the AC current active component on the sending-end converter's AC side, and the difference between the reference value and feedback value of the AC current reactive component on the AC side, and the final output control quantity of the AC voltage dual closed-loop control is used as the AC component of the voltage modulation wave corresponding to the sending-end converter; the final output control quantity of the AC voltage dual closed-loop control is determined based on the control quantity output of the AC current active / reactive amplitude closed-loop control.

[0034] The reference values ​​for the active component of the AC current on the sending-end converter and the reference values ​​for the reactive component of the AC current on the sending-end converter are determined based on the control quantity output by the closed-loop control of the AC voltage active / reactive amplitude. Attached Figure Description

[0035] Figure 1This is a schematic diagram of the topology of a large-scale new energy ultra-high voltage flexible direct transmission system in the background technology of this invention;

[0036] Figure 2 This is a block diagram of the fault clearing control strategy at the sending-end converter in an embodiment of the fault clearing method for the overhead line of the new energy flexible direct transmission system of the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] Example of a method for clearing faults in overhead lines of new energy flexible direct transmission systems:

[0039] This embodiment provides a technical solution for clearing faults in overhead lines of a new energy flexible direct transmission system. The fault clearing method detects overhead line faults in real time at both the sending and receiving ends of the flexible direct transmission system. Based on the detection results, it determines the corresponding voltage modulation wave at the sending-end converter and the receiving-end converter according to different control strategies. It generates trigger pulses based on the voltage modulation wave and outputs them to the corresponding sub-modules of the sending-end converter and the receiving-end converter to clear the fault voltage and current.

[0040] In this embodiment, the sending-end converter refers to the MMC converter connected to the power generation system in the flexible DC transmission system, and the receiving-end converter refers to the MMC converter connected to the AC power grid. Both the sending-end converter and the receiving-end converter are MMC converters that include a full-bridge submodule.

[0041] When a fault occurs at the far end of an overhead line, the sending-end converter has a weaker ability to detect the fault due to the long distance between the sending and receiving ends, resulting in a longer fault detection time. To avoid the control method triggered after fault detection causing submodules to be significantly affected by the fault and unable to clear the fault in a timely and reliable manner, refer to... Figure 2 In this embodiment, the control strategy at the sending-end converter includes two types: AC-side control and DC-side control. The specific application methods are as follows:

[0042] ① If no overhead line fault is detected at the sending end, the DC side control adopts dual closed-loop control. Figure 2 The Trip_DCF value is 0; the DC-side outer loop control is a submodule voltage control, and the output control quantity is determined based on the difference between the sending-end converter submodule voltage reference value and the feedback value; for example... Figure 2 As shown, V sm_rated v sm_avg These are the reference and feedback values ​​for the submodule average voltage (submodule voltage), respectively. The submodule average voltage is the average voltage across all bridge arm submodules of the sending-end converter; V sm_rated and v sm_avgAfter PI control, the output control quantity is used as the reference value of DC current on the DC side of the sending-end converter in the subsequent DC side inner loop control.

[0043] The DC-side inner loop control is a DC current control, and the output control quantity is determined based on the difference between the reference value and the feedback value of the DC current on the sending-end converter's DC side; for example... Figure 2 As shown, I dcref i dc These are the reference and feedback values ​​of the DC current on the DC side of the sending-end converter, respectively, where I... dcref The value is V in the aforementioned DC-side outer loop control. sm_rated and v sm_avg After PI control, the output control quantity is: I dcref i dc After PI control, the output control quantity is then superimposed with the first DC voltage command value U. dcref0 Subsequently, the DC component U of the voltage modulation wave corresponding to the sending-end converter dcref The first DC voltage command value here is set by the DC-side voltage of the sending-end converter during steady-state operation of the flexible DC transmission system. Typically, the desired range for controlling the DC-side voltage of the sending-end converter is determined by setting the first DC voltage command value U. dcref0 Which values ​​to set.

[0044] This embodiment introduces a DC-side dual closed-loop control strategy at the sending-end converter that is automatically triggered based on the status of the sending-end converter submodules. Before a remote fault occurs but is detected at the sending end, the submodule voltage change can be automatically tracked. This is equivalent to setting up a buffer control within the time difference between the fault occurrence and the triggering of the control strategy. This ensures that the remote fault affects the sending-end submodule voltage and DC current in a timely manner, thus solving the problem of the drop in AC voltage and module voltage at the sending end caused by a remote fault in an ultra-long-distance overhead line. It can also reduce the overcurrent level of the bridge arm caused by the rapid development of a near-end fault in a timely manner.

[0045] The AC side control at the sending-end converter is a dual closed-loop AC voltage control; refer to Figure 2 The outer loop control of the AC voltage dual closed-loop control is the AC voltage active / reactive amplitude closed-loop control, based on the AC voltage active component reference value U. dpref With feedback value u dp The difference between them and the reference value U of the reactive component of AC voltage qpref With feedback value u qp The difference between them determines the control quantity i of the AC voltage active / reactive amplitude closed-loop control output. dpref and i qpref The inner loop of the AC voltage dual closed-loop control is an AC current active / reactive closed-loop control, based on the AC current active component reference value i.dpref (That is, the control quantity output by the AC voltage active amplitude closed-loop control mentioned above) and the feedback value i dp The difference between them and the reference value i of the reactive component of the AC current qpref (That is, the control quantity output by the AC voltage reactive amplitude closed-loop control mentioned above) and the feedback value i qp The difference between them determines the control quantity of the AC current active / reactive amplitude closed-loop control output, and the final output control quantity v is determined by the AC voltage dual closed-loop control. abc The AC component of the voltage modulation wave corresponding to the sending-end converter; the control quantity v of the final output of the AC voltage dual closed-loop control. abc The control quantity is determined based on the output of the above AC current active / reactive amplitude closed-loop control; such as... Figure 2 As shown, i dpref with i dp i qpref with i qp Each component is controlled by a PI controller, and the negative values ​​are then superimposed on each other as the AC voltage reactive component feedback value u, which serves as the feedforward quantity. dp And the feedback value of reactive component of alternating current i qp The product of ωL and the feedback value of the reactive component of AC voltage, u qp And the feedback value of the active component of alternating current i dp The product of ωL and v is obtained. d and v q Then v d and v q Converting to three-phase voltage yields the control quantity v of the final output of the AC voltage dual closed-loop control. abc The reference values ​​for the active and reactive components of the AC current are determined based on the control output of the AC voltage active / reactive amplitude closed-loop control. Therefore, the control strategy at the sending-end converter ultimately yields the voltage modulation wave v. arm It consists of two parts, namely the control quantity v of the final output of the AC voltage dual closed-loop control. abc (The AC component of the voltage modulation wave corresponding to the sending-end converter) and the DC component U of the voltage modulation wave corresponding to the sending-end converter. dcref Then, the voltage modulation wave is applied to the bridge arm of each submodule to eliminate the impact of overhead line faults in the system.

[0046] ② If a fault is detected in the overhead line at the sending end, the outer loop control of the DC side of the sending-end converter station will be disabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop control will be switched to zero. Figure 2 The Trip_DCF value in the input switches from "0" to "1" (the pulse switches from low to high, corresponding to the input I). dcrefThe value is 0); after the fault is detected and cleared, the outer loop controller is re-enabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop controller is switched to the reference value determined according to the control quantity output by the DC side outer loop controller, that is, Figure 2 The Trip_DCF value in the converter is switched back from "1" to "0"; while the AC side control strategy at the sending-end converter remains unchanged.

[0047] The DC-side dual closed-loop control strategy mentioned in ①, which is automatically triggered based on the status of the sending-end converter submodule, can quickly respond to DC current and submodule voltage after a DC fault, without relying on detection information and results transmitted via communication. However, it has the following problems: if the output of the submodule voltage outer loop control is always used as the reference value of the DC current on the sending-end converter, i.e., the command value of the DC current, then the fault current suppression in the later stages of the fault will basically depend on the control result of the submodule voltage outer loop. This will make the fault current clearing time longer, and there may be a situation where the fault current has not dropped to zero after the fault disappears, affecting the rapid recovery of the DC fault. Therefore, in this embodiment, the DC-side control strategy at the sending-end converter is only continuously implemented when no fault is detected. After the fault is detected at the sending end, the DC-side control at the sending-end converter is directly switched to ordinary current closed-loop control (reference value is 0), no longer affected by the submodule voltage. This avoids the lengthy fault current clearing time while ensuring that the clearing strategy still works, thus enabling rapid suppression of DC current and ensuring the speed of fault clearing.

[0048] The control strategy at the receiving-end converter only includes AC-side control: if no overhead line fault is detected at the receiving end, the AC-side control is a dual closed-loop control, wherein the outer loop control is DC voltage control and the inner loop control is AC current control. The control quantity output by the inner loop control is used as the AC component of the voltage modulation wave corresponding to the receiving-end converter; the control quantity output by the outer loop control is determined based on the difference between the reference value and the feedback value of the reactive component of the DC voltage on the AC side of the receiving-end converter; the control quantity output by the inner loop control is determined based on the difference between the reference value and the feedback value of the AC current on the AC side of the receiving-end converter.

[0049] The second DC voltage command value is directly used as the DC component of the voltage modulation wave corresponding to the receiving-end converter; here, the second DC voltage command value is set by the DC side voltage of the receiving-end converter during steady-state operation of the flexible DC transmission system.

[0050] If a DC overhead line fault is detected at the receiving end, the AC side outer loop control is switched from DC voltage control to submodule voltage outer loop control. The control quantity output by the submodule voltage outer loop control is determined based on the difference between the receiving end converter submodule voltage reference value and the feedback value. The DC component of the voltage modulation wave is switched to the control quantity output by the DC current closed loop control. The control quantity output by the DC current closed loop control is determined based on the difference between the receiving end converter DC side DC current reference value and the feedback value.

[0051] In this embodiment, an AC energy dissipation device is also configured at the sending end of the flexible direct transmission system. The AC energy dissipation device is activated after a fault in the overhead line of the system is detected at the sending end, and is deactivated after the fault is cleared. This can consume the fault current in a timely manner, improve the fault clearing efficiency, and also ensure the safety of the fault clearing process.

[0052] Example of an overhead line fault clearing system for new energy flexible direct transmission systems:

[0053] This embodiment provides a technical solution for a fault clearing system for overhead lines in a new energy flexible direct transmission system. The fault clearing system includes a data collector and a processor.

[0054] Among them, the data acquisition device is used to collect the current and / or voltage at the sending and receiving ends of the flexible DC transmission system in real time, and is used to detect faults in the overhead lines of the system.

[0055] The processor is used to detect overhead line faults in the flexible DC transmission system in real time at the sending end and receiving end. Based on the detection results, it determines the corresponding voltage modulation wave at the sending end converter and the receiving end converter according to different control strategies. It generates trigger pulses based on the voltage modulation wave and outputs them to the corresponding sub-modules of the sending end converter and the receiving end converter respectively to clear the fault voltage and current.

[0056] In this embodiment, the sending-end converter refers to the MMC converter connected to the power generation system in the flexible DC transmission system, and the receiving-end converter refers to the MMC converter connected to the AC power grid. Both the sending-end converter and the receiving-end converter are MMC converters that include a full-bridge submodule.

[0057] Since the specific working method and working principle of the fault clearing system in this embodiment have been described in detail in the above embodiment of the fault clearing method for overhead lines of new energy flexible direct transmission system, they will not be repeated here.

[0058] This invention has the following characteristics:

[0059] 1) A DC-side dual closed-loop control strategy is introduced at the sending-end converter. Before the sending end detects the fault when the remote fault occurs, the voltage change of the sub-module is tracked. This is equivalent to setting up a buffer control within the time difference between the fault occurrence and the triggering of the control strategy. This can ensure that the remote fault affects the voltage and DC current of the sending-end sub-module in a timely manner. Therefore, it can solve the problem of the AC voltage drop and module voltage drop caused by the remote fault of the ultra-long-distance overhead line. It can also reduce the overcurrent level of the bridge arm caused by the rapid development of the near-end fault in a timely manner.

[0060] 2) The DC-side control strategy related to the submodule voltage change is only continuously implemented at the sending-end converter when no fault is detected. After a fault is detected at the sending end, the DC-side control at the sending-end converter directly switches to ordinary current closed-loop control (control quantity is 0), and is no longer affected by the submodule voltage. This avoids the fact that the suppression of fault current in the later stage of the fault basically depends on the control result of the outer loop of the submodule voltage, which leads to a longer fault current clearing time. Therefore, it can quickly suppress DC current to ensure the speed of fault clearing.

[0061] It should be understood that the above-described specific embodiments of the present invention are merely illustrative of or explanation of the principles of the present invention, and do not constitute a limitation thereof; any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for clearing overhead line faults in a new energy flexible direct transmission system, characterized in that, The overhead line faults of the flexible DC transmission system are detected in real time at the sending end and receiving end. Based on the detection results, the corresponding voltage modulation wave is determined at the sending end converter and the receiving end converter according to different control strategies. The trigger pulse is generated based on the voltage modulation wave and output to the corresponding sub-modules of the sending end converter and the receiving end converter respectively to clear the fault current. The control strategy at the sending-end converter includes AC-side control and DC-side control. If no overhead line fault is detected at the sending end, the DC-side control is a dual closed-loop control. The outer loop control is a sub-module voltage control, and the output control quantity is determined based on the difference between the sub-module voltage reference value and the feedback value of the sending-end converter. The inner loop control is a DC current control, and the output control quantity is determined based on the difference between the DC current reference value of the sending-end converter DC side determined by the control quantity output by the outer loop control and the feedback value. The control quantity output by the inner loop control is superimposed with the first DC voltage command value set by the DC side voltage of the sending-end converter during steady-state operation of the flexible DC transmission system, and this is used as the DC component of the voltage modulation wave corresponding to the sending-end converter. The control strategy at the receiving-end converter includes AC-side control: if no overhead line fault is detected at the receiving end, the AC-side control is a dual closed-loop control, with the inner and outer loops controlling AC current and DC voltage, respectively. The control quantity output by the inner loop control, determined based on the difference between the AC current reference value and the feedback value of the AC current at the receiving-end converter, is used as the AC component of the voltage modulation wave corresponding to the receiving-end converter. The control quantity output by the outer loop control is determined based on the difference between the reactive component reference value and the feedback value of the DC voltage at the AC side of the receiving-end converter. The second DC voltage command value set by the DC side voltage of the receiving-end converter during steady-state operation of the flexible DC transmission system is used as the DC component of the voltage modulation wave corresponding to the receiving-end converter. The sending-end converter refers to the MMC converter connected to the power generation system in the flexible DC transmission system, while the receiving-end converter refers to the MMC converter connected to the AC power grid; both the sending-end and receiving-end converters are MMC converters that include full-bridge submodules.

2. The method according to claim 1, wherein, If a fault is detected in the overhead line of the system at the sending end, the outer loop control of the DC side control is disabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop control is switched to zero. After the fault is detected to have been cleared, the outer loop controller is re-enabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop controller is switched to the reference value determined according to the control quantity output by the outer loop control of the DC side.

3. The method for clearing the fault of overhead line of new energy flexible direct transmission and delivery system according to claim 1, characterized in that, If a DC overhead line fault is detected at the receiving end, the AC side outer loop control is switched from DC voltage control to submodule voltage outer loop control. The control quantity output by the submodule voltage outer loop control is determined based on the difference between the receiving end converter submodule voltage reference value and the feedback value. The DC component of the voltage modulation wave is switched to the control quantity output by the DC current closed loop control. The control quantity output by the DC current closed-loop control is determined based on the difference between the reference value and the feedback value of the DC current on the DC side of the receiving-end converter.

4. The method according to any one of claims 1-3, characterized in that, An AC energy dissipation device is configured at the sending end of the flexible direct transmission system. The AC energy dissipation device is activated after a fault is detected in the overhead line of the system at the sending end, and is deactivated after the fault is cleared.

5. The method according to any one of claims 1-3, characterized in that, The AC side control at the sending-end converter is an AC voltage dual closed-loop control; wherein the outer loop control of the AC voltage dual closed-loop control is an AC voltage active / reactive amplitude closed-loop control, and the control quantity output of the AC voltage active / reactive amplitude closed-loop control is determined based on the difference between the reference value and the feedback value of the AC voltage active component on the sending-end converter AC side and the difference between the reference value and the feedback value of the AC voltage reactive component on the sending-end converter AC side. The inner loop control of the AC voltage dual closed-loop control is the AC current active / reactive closed-loop control. The control quantity output by the AC current active / reactive amplitude closed-loop control is determined based on the difference between the reference value and the feedback value of the AC current active component on the AC side of the sending-end converter and the difference between the reference value and the feedback value of the AC current reactive component on the AC side. The control quantity finally output by the AC voltage dual closed-loop control is used as the AC component of the voltage modulation wave corresponding to the sending-end converter. The control quantity of the final output of the AC voltage dual closed-loop control is determined based on the control quantity of the output of the AC current active / reactive amplitude closed-loop control. The reference values ​​for the active component of the AC current on the sending-end converter and the reference values ​​for the reactive component of the AC current on the sending-end converter are determined based on the control quantity output by the closed-loop control of the AC voltage active / reactive amplitude.

6. A fault clearing system for overhead lines in a new energy flexible direct transmission system, characterized in that, Includes collectors and processors; The data acquisition unit is used to collect the current and / or voltage at the sending and receiving ends of the flexible DC transmission system in real time, and is used for fault detection of the overhead lines of the system. The processor is used to detect overhead line faults in the flexible DC transmission system in real time at the sending end and receiving end. Based on the detection results, it determines the corresponding voltage modulation wave at the sending end converter and the receiving end converter according to different control strategies. It generates trigger pulses based on the voltage modulation wave and outputs them to the corresponding sub-modules of the sending end converter and the receiving end converter respectively to clear the fault current. The control strategy at the sending-end converter includes AC-side control and DC-side control. If no overhead line fault is detected at the sending end, the DC-side control is a dual closed-loop control. The outer loop control is a submodule voltage control, and the output control quantity is determined by the difference between the reference value of the sending-end converter submodule voltage determined by the control quantity output by the outer loop control and the feedback value. The inner loop control is a DC current control, and the output control quantity is determined by the difference between the reference value of the DC current on the sending-end converter DC side and the feedback value. The control quantity output by the inner loop control is superimposed with the first DC voltage command value set by the DC side voltage of the sending-end converter during steady-state operation of the flexible DC transmission system, and this is used as the DC component of the voltage modulation wave corresponding to the sending-end converter. The control strategy at the receiving-end converter includes AC-side control: if no overhead line fault is detected at the receiving end, the AC-side control is a dual closed-loop control, with the inner and outer loops controlling AC current and DC voltage, respectively. The control quantity output by the inner loop control, determined based on the difference between the AC current reference value and the feedback value of the AC current at the receiving-end converter, is used as the AC component of the voltage modulation wave corresponding to the receiving-end converter. The control quantity output by the outer loop control is determined based on the difference between the reactive component reference value and the feedback value of the DC voltage at the AC side of the receiving-end converter. The second DC voltage command value set by the DC side voltage of the receiving-end converter during steady-state operation of the flexible DC transmission system is used as the DC component of the voltage modulation wave corresponding to the receiving-end converter. The sending-end converter refers to the MMC converter connected to the power generation system in the flexible DC transmission system, while the receiving-end converter refers to the MMC converter connected to the AC power grid; both the sending-end and receiving-end converters are MMC converters that include full-bridge submodules.

7. The overhead line fault clearing system for a new energy flexible direct transmission system according to claim 6, characterized in that, If a fault is detected in the overhead line of the system at the sending end, the outer loop control of the DC side control is disabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop control is switched to zero. After the fault is detected to have been cleared, the outer loop controller is re-enabled, and the reference value of the DC current on the DC side of the sending-end converter in the inner loop controller is switched to the reference value determined according to the control quantity output by the outer loop control of the DC side.

8. The overhead line fault clearing system for a new energy flexible direct transmission system according to claim 6, characterized in that, If a DC overhead line fault is detected at the receiving end, the AC side outer loop control is switched from DC voltage control to submodule voltage outer loop control. The control quantity output by the submodule voltage outer loop control is determined based on the difference between the receiving end converter submodule voltage reference value and the feedback value. The DC component of the voltage modulation wave is switched to the control quantity output by the DC current closed loop control. The control quantity output by the DC current closed-loop control is determined based on the difference between the reference value and the feedback value of the DC current on the DC side of the receiving-end converter.

9. The overhead line fault clearing system for a new energy flexible direct transmission system according to any one of claims 6-8, characterized in that, An AC energy dissipation device is configured at the sending end of the flexible direct transmission system. The AC energy dissipation device is activated after a fault is detected in the overhead line of the system at the sending end, and is deactivated after the fault is cleared.

10. The overhead line fault clearing system for a new energy flexible direct transmission system according to any one of claims 6-8, characterized in that, The AC side control at the sending-end converter is an AC voltage dual closed-loop control; wherein the outer loop control of the AC voltage dual closed-loop control is an AC voltage active / reactive amplitude closed-loop control, and the control quantity output of the AC voltage active / reactive amplitude closed-loop control is determined based on the difference between the reference value and the feedback value of the AC voltage active component on the sending-end converter AC side and the difference between the reference value and the feedback value of the AC voltage reactive component on the sending-end converter AC side. The inner loop control of the AC voltage dual closed-loop control is the AC current active / reactive closed-loop control. The control quantity output by the AC current active / reactive amplitude closed-loop control is determined based on the difference between the reference value and the feedback value of the AC current active component on the AC side of the sending-end converter and the difference between the reference value and the feedback value of the AC current reactive component on the AC side. The control quantity finally output by the AC voltage dual closed-loop control is used as the AC component of the voltage modulation wave corresponding to the sending-end converter. The control quantity of the final output of the AC voltage dual closed-loop control is determined based on the control quantity of the output of the AC current active / reactive amplitude closed-loop control. The reference values ​​for the active component of the AC current on the sending-end converter and the reference values ​​for the reactive component of the AC current on the sending-end converter are determined based on the control quantity output by the closed-loop control of the AC voltage active / reactive amplitude.

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