Temporary blocking method for removing direct current fault of new energy island power grid sending-out system

Abstract

The application discloses a method for removing DC fault by temporary blocking of a new energy island power grid sending-out system. When the receiving end detects that the DC current reaches zero for the first time, the fault current is removed by temporary blocking, and the sending end island energy balance is realized by cooperating with an AC energy consumption device, so that the system DC overhead line fault is self-cleared. Compared with the prior art, the proportion of full-bridge sub-modules is reduced, the engineering investment is saved, and the DC current can be prevented from repeatedly fluctuating around zero, and the DC current arc extinction time is shortened. When the DC fault is detected, the outer ring controller of the fault pole of the sending end is switched to the stator module capacitor voltage control. When single-pole operation is performed, the sending end also selects whether to input the AC energy consumption device according to the size of the sub-module voltage, so as to reduce the risk of overvoltage of the fault pole sub-module capacitor and improve the technical problems that the output negative pressure strategy of the prior art needs a high proportion of full-bridge sub-modules, the cost is high, the sub-module overvoltage tripping risk is high, and the DC current arc extinction time is long.

Description

Technical Field

[0001] This application relates to the field of power grid technology, and in particular to a method for temporarily blocking and clearing DC faults in a renewable energy islanded power grid transmission system. Background Technology

[0002] Most renewable energy bases are located in remote areas with low local load levels and weak grid structures, highlighting the significant need for stable power transmission from isolated renewable energy sources. Flexible DC transmission, characterized by its flexibility, controllability, and high efficiency, serves as a crucial means of power transmission for renewable energy. For the sending end, flexible DC converters can provide stable AC voltage to renewable energy fields, enabling islanded operation and sufficient rapid dynamic reactive power compensation, reducing the risk of renewable energy units disconnecting from the grid, and improving renewable energy utilization. For the receiving end, flexible DC transmission eliminates commutation failure issues and provides dynamic reactive power compensation, effectively addressing the stability problems of multiple DC feeds into the AC grid and mitigating severe grid faults.

[0003] However, considering the new scenario of applying ultra-high voltage flexible DC transmission to large-scale onshore renewable energy development and long-distance transmission, the transmission capacity is large, the distance is long, and the operation mode is complex. It should be noted that as the transmission distance and transmission capacity are further increased, it will have a qualitative change on the operating characteristics of the system and equipment. At present, there is no mature DC engineering design experience. Among them, DC fault ride-through is a key technology, which faces great technical difficulties and challenges. One of the more prominent problems is that longer-distance and larger-capacity overhead lines will reduce the arc-extinguishing speed, and may even lead to the inability to complete the arc-extinguishing within the set deionization time, affecting the stability of the system. In the existing technology of DC fault self-clearing technology for the system of transmitting renewable energy islanded grids through flexible DC overhead lines, both the sending and receiving ends adopt the output negative voltage strategy, and the converter valve is not locked throughout the process to achieve DC line fault self-clearing. However, it should be noted that this method requires a high proportion of full-bridge sub-modules, which is costly, and the capacitor voltage of the sub-module in the sending-end converter valve is close to the device limit, which poses a risk of sub-module overvoltage tripping. Summary of the Invention

[0004] This application provides a method for temporarily blocking DC fault clearing in a new energy islanded power grid transmission system. This method is used to improve the technical problems of existing technologies, such as the need for a high proportion of full-bridge sub-modules for output negative voltage strategies, high cost, risk of sub-module overvoltage tripping, and long DC current arc extinction time.

[0005] In view of this, the first aspect of this application provides a method for temporarily blocking and clearing DC faults in a renewable energy islanded power grid transmission system, comprising:

[0006] S1. Construct a new energy islanded power grid transmission system. The new energy islanded power grid transmission system includes a new energy power plant, a sending-end converter station, a receiving-end converter station, a DC overhead line, and an AC energy dissipation device. The new energy power plant is connected to the sending-end converter station through a three-phase AC line. The AC energy dissipation device is set between the new energy power plant and the sending-end converter station. The sending-end converter station is connected to the receiving-end converter station through a DC overhead line. The sending-end converter station, the DC overhead line, and the receiving-end converter station constitute a DC system.

[0007] S2. Configure the control strategies for the sending-end converter station and the receiving-end converter station. All sending-end converter stations adopt a dual closed-loop constant AC voltage and frequency control strategy. Only one receiving-end converter station adopts a constant DC voltage control strategy, while the rest of the receiving-end converter stations adopt a constant active power control strategy.

[0008] S3. Real-time detection of the DC current flowing out of each pole of the sending-end converter station and real-time detection of the DC current flowing into each pole of the receiving-end converter station.

[0009] S4. If each pole of the sending-end converter station detects an increase in the DC current flowing out of its own pole, the pole will automatically reduce the DC voltage bias through the DC current controller, reduce the number of sub-modules connected on the DC side, directly control the DC voltage, output a negative DC voltage, and actively clear the DC fault.

[0010] S5. If each pole of the receiving-end converter station detects a decrease in the DC current flowing into its own pole and a reverse increase, the pole will automatically reduce the DC voltage bias through the DC current controller, reduce the number of sub-modules connected on the DC side, directly control the DC voltage, output a negative DC voltage, and actively clear the DC fault. If the receiving-end converter station detects that the DC current reaches zero for the first time, a temporary blocking signal will be generated. During the effective period of the temporary blocking signal, the receiving-end converter station will be temporarily blocked and the inner loop controller and DC current margin controller will be reset.

[0011] S6. Each sending-end converter station and receiving-end converter station monitors DC voltage and current in real time, and detects whether a DC fault has occurred based on the DC voltage and current. If so, steps S7 and S8 are executed simultaneously.

[0012] S7. The control systems of each sending-end converter station and receiving-end converter station enter the de-ionization logic. Each sending-end converter station and receiving-end converter station sets the DC current reference value input by the DC current controller of the faulty pole to zero and performs zero DC current control. In addition, both sending-end converter stations and receiving-end converter stations set the active current command output by the outer loop controller of the faulty pole to zero.

[0013] S8. Determine whether the DC system is operating in bipolar mode. If yes, proceed to step S9; otherwise, proceed to step S10.

[0014] S9. All sending-end converter stations will switch the outer loop controller of the faulty pole to stator module capacitor voltage control and transfer the power of the faulty pole to the non-faulty pole. If there is still a power surplus in the sending-end converter stations, the first target number of AC energy consumption devices will be put into operation to balance the energy of the sending-end converter stations.

[0015] S10 and the sending-end converter station both switch the outer loop controller of the faulty pole to stator module capacitor voltage control, and put the second target number of AC energy consumption devices into operation to balance the energy of the sending-end converter station. At the same time, the sending-end converter station monitors the sub-module capacitor voltage in real time and selects whether to put the AC energy consumption device into operation based on the sub-module capacitor voltage.

[0016] S11. When the deionization time is reached, each sending-end converter station and receiving-end converter station will restore the slope of the DC current reference value input by the DC current controller of the faulty pole, and each receiving-end converter station will switch the faulty pole from stator module voltage control back to outer loop controller control to restore DC voltage.

[0017] S12. When each sending-end converter station and receiving-end converter station detects that the DC voltage has recovered to the preset value, the sending-end converter station disconnects the AC energy-consuming devices one group at a time to restore the DC current and realize the restart in the event of a DC fault.

[0018] Optionally, the duration of the temporary blocking signal is 100ms.

[0019] Optionally, the calculation process for the first target quantity group is as follows:

[0020] P 送端正常极剩余容量 =P 额定功率 -P 送端正常极 ;

[0021] N 交流耗能组数 =(P 送端双极功率 -P 送端故障极 -P 送端正常极剩余容量 ) / P 每组交流耗能容量 ;

[0022] In the formula, P 送端正常极剩余容量 For the normal remaining capacity of the sending-end converter station, P 额定功率 P is the rated power of the sending-end converter station. 送端正常极 N represents the normal active power of the sending-end converter station. 交流耗能装置组数 For the first target quantity group that needs to be equipped with AC power consumption devices, P 送端双极功率 P represents the bipolar active power of the sending-end converter station. 送端故障极 For the fault-based active power of the sending-end converter station, P 每组交流耗能装置容量 The capacity of each group of AC power-consuming devices.

[0023] Optionally, step S10 specifically includes:

[0024] All sending-end converter stations switched the outer loop controller of the faulty pole to stator module capacitor voltage control.

[0025] Upon detecting a DC fault, the AC energy-consuming devices of the second target number group are immediately activated to balance the energy of the sending-end converter station. At the same time, the sending-end converter station monitors the submodule capacitor voltage in real time and controls the AC energy-consuming devices of other groups in each sending-end converter station to switch on and off according to the submodule capacitor voltage.

[0026] Optionally, the calculation process for the second target quantity group is as follows:

[0027] N 交流耗能组数 =P 送端故障极 / P 每组交流耗能容量 ;

[0028] In the formula, N 交流耗能组数 For the second target quantity group that needs to be equipped with AC power consumption devices, P 送端故障极 For the fault-based active power of the sending-end converter station, P 每组交流耗能装置容量 The capacity of each group of AC power-consuming devices.

[0029] Optionally, the AC energy consumption device controlling other groups in each sending-end converter station is switched on and off according to the submodule capacitor voltage, including:

[0030] Calculate the average voltage of the faulty submodule in the sending-end converter station based on the submodule capacitor voltage.

[0031] If the average voltage of the faulty pole module of the sending-end converter station is greater than the first preset threshold, an additional set of AC energy-consuming devices will be put into operation, and the average voltage of the faulty pole module of the sending-end converter station will be recalculated. If the newly calculated average voltage of the faulty pole module of the sending-end converter station is less than the second preset threshold, the additional set of AC energy-consuming devices will be disconnected.

[0032] Optionally, the sending-end converter station and the receiving-end converter station can adopt a single valve group or a dual valve group in series, and the converter valve can adopt a full-bridge sub-module or a combination of full-bridge sub-module and half-bridge module.

[0033] Optionally, the number of sending-end converter stations can be one or more, and the number of receiving-end converter stations can be one or more.

[0034] Optionally, the new energy power plant includes wind farms and photovoltaic power plants.

[0035] Optionally, the deionization time is 300ms to 400ms.

[0036] As can be seen from the above technical solutions, this application has the following advantages:

[0037] This application provides a method for temporarily blocking and clearing DC faults in a renewable energy islanded power grid transmission system, comprising: S1, constructing a renewable energy islanded power grid transmission system, the renewable energy islanded power grid transmission system including a renewable energy field, a sending-end converter station, a receiving-end converter station, a DC overhead line, and an AC energy dissipation device, the renewable energy field being connected to the sending-end converter station via a three-phase AC line, the AC energy dissipation device being installed between the renewable energy field and the sending-end converter station, and the sending-end converter station being connected to the receiving-end converter station via a DC overhead line, the sending-end… The converter station, the overhead DC line, and the receiving-end converter station constitute a DC system; S2, configure the control strategies for the sending-end and receiving-end converter stations, wherein all sending-end converter stations adopt a double closed-loop constant AC voltage and frequency control strategy, and only one receiving-end converter station adopts a constant DC voltage control strategy, while the remaining receiving-end converter stations adopt a constant active power control strategy; S3, each pole of the sending-end converter station detects the DC current flowing out of its pole in real time, and each pole of the receiving-end converter station detects the DC current flowing into its pole in real time; S4, sending... If each pole of the sending-end converter station detects an increase in the DC current flowing out of its own pole, the pole automatically reduces the DC voltage bias through the DC current controller, reduces the number of sub-modules connected on the DC side, directly controls the DC voltage, outputs a negative DC voltage, and actively clears the DC fault; S5, if each pole of the receiving-end converter station detects a decrease in the DC current flowing into its own pole and then an increase in the reverse direction, the pole automatically reduces the DC voltage bias through the DC current controller, reduces the number of sub-modules connected on the DC side, directly controls the DC voltage, outputs a negative DC voltage, and actively clears the DC fault; if the receiving-end converter station detects that the DC current has reached zero for the first time, a temporary blocking signal is generated. During the effective period of the temporary blocking signal, the receiving-end converter station is temporarily blocked and the inner loop controller and DC current margin controller are reset; S6, each sending-end converter station and receiving-end converter station monitors the DC voltage and current in real time, and detects whether a DC fault has occurred based on the DC voltage and current. If so, steps S7 and S8 are executed simultaneously;

[0038] S7. The control systems of each sending-end converter station and receiving-end converter station enter de-ionization logic. Each sending-end and receiving-end converter station sets the DC current reference value input to the DC current controller of the faulty pole to zero, performing zero DC current control. Furthermore, both sending-end and receiving-end converter stations set the active current command output from the outer loop controller of the faulty pole to zero. S8. Determine if the DC system is operating in bipolar mode. If yes, proceed to step S9; otherwise, proceed to S10. S9. Each sending-end converter station switches the outer loop controller of the faulty pole to stator module capacitor voltage control and transfers the power from the faulty pole to the non-faulty pole. If any sending-end converter station still has power surplus, the first target number of AC energy dissipation devices are activated to balance the energy of the sending-end converter stations. S10. Each sending-end converter station... The outer loop controller of the faulty pole is switched to stator module capacitor voltage control, and the second target number of AC energy dissipation devices are put into operation to balance the energy of the sending-end converter station. At the same time, the sending-end converter station monitors the sub-module capacitor voltage in real time and selects whether to over-activate the AC energy dissipation devices based on the sub-module capacitor voltage; S11, when the deionization time is reached, each sending-end converter station and receiving-end converter station restores the slope of the DC current reference value input by the DC current controller of the faulty pole, and each receiving-end converter station switches the faulty pole from stator module voltage control back to outer loop controller control to restore the DC voltage; S12, when each sending-end converter station and receiving-end converter station detects that the DC voltage has recovered to the preset value, the sending-end converter station disconnects the AC energy dissipation devices group by group to restore the DC current, realizing the restart in the case of DC fault.

[0039] In this application, when the DC current is detected to reach zero for the first time at the receiving end, the fault current is temporarily blocked off, and the energy balance of the sending-end island is achieved in conjunction with the AC energy dissipation device. This realizes the self-clearing of DC overhead line faults in the new energy island grid transmission system. Compared with the prior art, it can further reduce the proportion of full-bridge submodules, save engineering investment, and prevent the DC current from fluctuating repeatedly near zero, thereby shortening the DC current arc extinction time. When a DC fault is detected in the system, the sending-end converter station switches the outer loop controller of the fault pole to stator module capacitor voltage control. When operating on a single pole, the sending-end converter station also selects whether to over-connect the AC energy dissipation device according to the submodule voltage to reduce the risk of overvoltage of the fault pole submodule capacitor when clearing the DC fault. This improves the technical problems of the prior art, which requires a high proportion of full-bridge submodules for the output negative voltage strategy, resulting in high cost, risk of submodule overvoltage tripping, and long DC current arc extinction time. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 A flowchart illustrating a method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system, provided as an embodiment of this application.

[0042] Figure 2 A schematic diagram of a new energy islanded power grid transmission system provided in this application embodiment;

[0043] Figure 3 This is a control schematic diagram of the receiving-end converter station provided in an embodiment of this application;

[0044] Figure 4 This is a control diagram of the sending-end converter station provided in an embodiment of this application. Detailed Implementation

[0045] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0046] For easier understanding, please refer to Figure 1 This application provides a method for temporarily blocking and clearing DC faults in a renewable energy islanded power grid transmission system, including:

[0047] S1. Construct a new energy islanded power grid transmission system. The new energy islanded power grid transmission system includes a new energy power plant, a sending-end converter station, a receiving-end converter station, a DC overhead line, and an AC energy dissipation device. The new energy power plant is connected to the sending-end converter station through a three-phase AC line. The AC energy dissipation device is set between the new energy power plant and the sending-end converter station. The sending-end converter station is connected to the receiving-end converter station through a DC overhead line. The sending-end converter station, the DC overhead line, and the receiving-end converter station constitute a DC system.

[0048] The new energy islanded power grid transmission system implemented in this application includes a new energy power plant, a sending-end converter station, a receiving-end converter station, a DC overhead line, and AC energy dissipation devices. For details, please refer to [reference needed]. Figure 2All renewable energy power plants are connected to the sending-end converter station via three-phase AC lines. These renewable energy power plants include wind farms, photovoltaic power plants, and other renewable energy sources. AC energy dissipation devices are mandatory equipment, installed between the renewable energy power plant and the sending-end converter. These devices dissipate the continuous active power output of the renewable energy power plant when the DC line's active power transmission path is obstructed, preventing overvoltage on the sending-end AC feeder due to energy accumulation.

[0049] Both the sending-end and receiving-end converter stations can adopt a single-valve-group configuration or a dual-valve-group configuration in series. The converter valves can be full-bridge submodules or a combination of full-bridge and half-bridge submodules. The number of sending-end converter stations can be one or more, enabling single-power-source or multi-power-source supply. The number of receiving-end converter stations can also be one or more, enabling single-point or multi-point power reception.

[0050] The sending-end converter station is connected to the receiving-end converter station via an overhead DC line, specifically via a bipolar overhead DC line. If there are multiple sending-end converter stations or multiple receiving-end converter stations, the overhead DC line can connect the DC sides of each sending-end converter station and each receiving-end converter station to each other via a star connection or a delta connection.

[0051] The sending-end converter station, the overhead DC line, and the receiving-end converter station constitute a DC system. The DC system is a true bipolar DC system, with pole 1 and pole 2 being completely symmetrical. Bipolar operation greatly improves the stable operation capability of the DC transmission system. The grounding method is simple to design, and even after a single pole fault or even the final blockage, the normal pole can still maintain the stable operation of the sending-end islanded system.

[0052] S2. Configure the control strategies for the sending-end converter station and the receiving-end converter station. All sending-end converter stations adopt a dual closed-loop constant AC voltage and frequency control strategy. Only one receiving-end converter station adopts a constant DC voltage control strategy, while the remaining receiving-end converter stations adopt a constant active power control strategy.

[0053] In this embodiment of the application, when configuring the control strategies for the sending-end converter station and the receiving-end converter station, the sending-end converter station adopts a dual closed-loop constant AC voltage and frequency (VF) control strategy to provide the AC voltage and frequency required for the operation of its respective new energy power plant. Only one of the receiving-end converter stations adopts a constant DC voltage control strategy to provide a stable DC voltage for the DC system, while the other receiving-end converter stations adopt a constant active power control strategy.

[0054] S3. Real-time detection of the DC current flowing out of each pole of the sending-end converter station, and real-time detection of the DC current flowing into each pole of the receiving-end converter station.

[0055] The DC current flowing out of each pole of the sending-end converter station is detected in real time, and the DC current flowing into each pole of the receiving-end converter station is detected in real time.

[0056] S4. If each pole of the sending-end converter station detects an increase in the DC current flowing out of its own pole, the pole will automatically reduce the DC voltage bias through the DC current controller, reduce the number of sub-modules connected on the DC side, directly control the DC voltage, output a negative DC voltage, and actively clear the DC fault.

[0057] If an increase in DC current flowing out of each pole is detected at the sending-end converter station, the DC current controller at that pole automatically reduces the DC voltage bias, decreases the number of sub-modules connected on the DC side, and directly controls the DC voltage, outputting a negative DC voltage to actively clear DC faults. This process is fully automatic, does not rely on protection for fault detection, can quickly clear DC faults, and effectively avoids the impact of protection malfunctions or failures to operate.

[0058] S5. If each pole of the receiving-end converter station detects a decrease in the DC current flowing into its own pole and a reverse increase, the pole will automatically reduce the DC voltage bias through the DC current controller, reduce the number of sub-modules connected on the DC side, directly control the DC voltage, output a negative DC voltage, and actively clear the DC fault. If the receiving-end converter station detects that the DC current reaches zero for the first time, a temporary blocking signal will be generated. During the effective period of the temporary blocking signal, the receiving-end converter station will be temporarily blocked and the inner loop controller and DC current margin controller will be reset.

[0059] If each pole of the receiving-end converter station detects a decrease in the DC current flowing into its own pole and a subsequent increase in the opposite direction, the receiving-end converter station will automatically reduce the DC voltage bias of that pole through the DC current controller, thereby reducing the number of sub-modules connected on the DC side. This allows for direct control of the DC voltage, outputting a negative DC voltage and actively clearing the DC fault. This process is fully automatic, does not rely on protection for fault detection, can quickly clear DC faults, and effectively avoids the impact of protection malfunctions or failures to operate.

[0060] When the receiving-end converter station detects that the DC current has reached zero for the first time, a temporary blocking signal is generated. This signal lasts for approximately 100ms. During the effective period of this temporary blocking signal, the receiving-end converter station is temporarily blocked to prevent the DC current from fluctuating repeatedly near zero and to shorten the DC current arc extinction time. It should be noted that for ultra-long-distance DC lines, the temporary blocking strategy at the receiving-end converter station can significantly shorten the DC current arc extinction time, enabling the DC fault arc to extinguish quickly. This reduces the DC power interruption time and avoids secondary impacts on the AC system caused by prolonged DC power interruptions, which could lead to large-scale power flow shifts within the AC system and, in severe cases, cause weakly damped oscillations, voltage instability, and transient power angle instability in the AC grid area. Furthermore, during the effective period of this temporary blocking signal, the inner loop controller and the DC current margin controller need to be reset to prevent electrical overstress impacts at the moment the system is unlocked.

[0061] In this embodiment, when the DC current is detected to reach zero for the first time at the receiving end, the fault current is cut off by temporary blocking, and the energy balance of the sending-end island is achieved in conjunction with AC energy consumption. This realizes the self-clearing of DC overhead line faults in the new energy island grid transmitted through the true bipolar flexible DC overhead line system. Compared with the prior art, the temporary blocking strategy can further reduce the proportion of full-bridge submodules and save engineering investment. When the receiving-end converter station adopts a hybrid topology of full and half bridge, the proportion of full bridge can be reduced to 50%.

[0062] S6. Each sending-end converter station and receiving-end converter station monitors DC voltage and current in real time, and detects whether a DC fault has occurred based on the DC voltage and current. If so, steps S7 and S8 are executed simultaneously.

[0063] At the sending-end converter station, the DC current flowing out of each pole is monitored in real time. At the receiving-end converter station, the DC current flowing into each pole is monitored in real time. Simultaneously, both the sending-end and receiving-end converter stations monitor the DC voltage and current in real time. In DC engineering, line protection such as traveling wave protection or sudden change protection is often used to detect whether a DC fault has occurred. If so, steps S7 and S8 are executed simultaneously; otherwise, the detection continues. The DC fault detection process is existing technology and will not be described in detail here.

[0064] S7. The control systems of each sending-end converter station and receiving-end converter station enter the de-ionization logic. Each sending-end converter station and receiving-end converter station sets the DC current reference value input to the DC current controller of the faulty pole to zero and performs zero DC current control. In addition, both sending-end converter stations set the active current command output by the outer loop controller of the faulty pole to zero.

[0065] Upon detecting a DC fault, the control systems of both the sending-end and receiving-end converter stations enter deionization logic. Each station sets the DC current reference value input to the DC current controller of the faulty pole to zero, implementing zero DC current control to assist in rapid arc extinguishing and deionization of the DC current. Simultaneously, both the sending-end and receiving-end converter stations set the active current command output from the outer loop controller of the faulty pole to zero, preventing continuous charging of the faulty pole submodule capacitor by the AC side during a DC fault.

[0066] S8. Determine whether the DC system is operating in bipolar mode. If yes, proceed to step S9; otherwise, proceed to step S10.

[0067] After a DC fault is detected, determine whether the DC system is operating in bipolar mode. If yes, proceed to step S9; otherwise, proceed to step S10.

[0068] S9. The outer loop controller of the faulty pole of each sending-end converter station will be switched to stator module capacitor voltage control, and the power of the faulty pole will be transferred to the non-faulty pole. If there is still a power surplus in the sending-end converter station, the first target number of AC energy consumption devices will be put into operation to balance the energy of the sending-end converter station.

[0069] If the DC system operates in bipolar mode, all sending-end converter stations need to switch the outer loop controller of the faulty pole to stator module capacitor voltage control. This avoids continuous charging of the faulty pole's submodule capacitor on the AC side during a DC fault, reducing the risk of overvoltage in the faulty pole's submodule capacitor when the DC fault is cleared. Simultaneously, power is transferred from the faulty pole to the non-faulty pole. If any sending-end converter station still has power surplus, the first target number of AC power units needs to be allocated to balance the sending-end energy and maintain the stable operation of the sending-end islanded system.

[0070] The calculation process for the first target quantity group is as follows:

[0071] P 送端正常极剩余容量 =P 额定功率 -P 送端正常极 ;

[0072] N 交流耗能组数 =(P 送端双极功率 -P 送端故障极 -P 送端正常极剩余容量 ) / P 每组交流耗能容量 ;

[0073] In the formula, P 送端正常极剩余容量 For the normal remaining capacity of the sending-end converter station, P 额定功率 P is the rated power of the sending-end converter station. 送端正常极 N represents the normal active power of the sending-end converter station. 交流耗能装置组数 For the first target quantity group that needs to be equipped with AC power consumption devices, P 送端双极功率 P represents the bipolar active power of the sending-end converter station. 送端故障极 For the fault-based active power of the sending-end converter station, P 每组交流耗能装置容量 The capacity of each group of AC power-consuming devices.

[0074] S10 and the sending-end converter station both switch the outer loop controller of the faulty pole to stator module capacitor voltage control, and put into operation the second target number of AC energy consumption devices to balance the energy of the sending-end converter station. At the same time, the sending-end converter station monitors the sub-module capacitor voltage in real time and selects whether to put into operation the AC energy consumption device according to the sub-module capacitor voltage.

[0075] If the DC system operates in a single-pole configuration, the sending-end converter station needs to switch the outer loop controller of the faulty pole to stator module capacitor voltage control. Furthermore, the sending-end converter station must constantly monitor the sub-module capacitor voltage and determine whether to over-activate AC energy dissipation devices to avoid continuous charging of the faulty pole sub-module capacitor on the AC side during a DC fault, further reducing the risk of overvoltage in the faulty pole sub-module capacitor when the DC fault is cleared. The specific implementation method is as follows: First, upon detecting a DC fault, immediately activate a second target number of AC energy dissipation devices to balance the energy of the sending-end converter station; the calculation process for the second target number is as follows:

[0076] N 交流耗能组数 =P 送端故障极 / P 每组交流耗能容量 ;

[0077] In the formula, N 交流耗能组数 For the second target quantity group that needs to be equipped with AC power consumption devices, P 送端故障极 For the fault-based active power of the sending-end converter station, P 每组交流耗能装置容量 The capacity of each group of AC power-consuming devices.

[0078] Secondly, in addition to the AC energy-consuming devices in the second target group calculated from the data, the AC energy-consuming devices in other groups of the sending-end converter are simultaneously switched on and off based on the capacitor voltage of the sending-end terminal module. Specifically, the average voltage U of the faulty sub-module of the sending-end converter station is calculated based on the capacitor voltage of the sub-module. 送端故障极子模块电压平均值 If the average voltage U of the faulty pole module in the sending-end converter station 送端故障极子模块电压平均值 If the voltage exceeds the first preset threshold (e.g., 2.3kV), an additional AC power dissipation device will be activated (at which point a total of N will be activated). 交流耗能组数 (+1 group of AC energy dissipation devices), after adding an additional group of AC energy dissipation devices, obtain the new submodule capacitor voltage and recalculate the average voltage U' of the faulty pole submodule at the sending-end converter station. 送端故障极子模块电压平均值 If the newly calculated average voltage U' of the faulty pole module of the sending-end converter station 送端故障极子模块电压平均值 If the voltage is less than the second preset threshold (e.g., 1.9kV), then the additional AC power-consuming device will be disconnected.

[0079] S11. When the deionization time is reached, each sending-end converter station and receiving-end converter station will restore the slope of the DC current reference value input by the DC current controller of the faulty pole, and each receiving-end converter station will switch the faulty pole from stator module voltage control back to outer loop controller control to restore DC voltage.

[0080] Once the deionization time is reached, both the sending-end and receiving-end converter stations restore the slope of the DC current reference value input to the DC current controller of the faulty pole (the rate is adjustable). Each receiving-end converter station switches the control of the faulty pole from stator module voltage control back to control by the outer loop controller, restoring the DC voltage. The control process of the receiving-end converter station can be found in [reference needed]. Figure 3 The deionization time in this embodiment is 300ms to 400ms.

[0081] S12. When each sending-end converter station and receiving-end converter station detects that the DC voltage has recovered to the preset value, the sending-end converter station disconnects the AC energy-consuming devices one group at a time to restore the DC current and realize the restart in the event of a DC fault.

[0082] Once each sending-end and receiving-end converter station detects that the DC voltage has returned to the preset value, the sending-end converter station sequentially disconnects AC power-consuming devices to restore DC current, thus achieving restart after a DC fault. The control process for the sending-end converter station can be found in [reference needed]. Figure 4 .

[0083] In this embodiment, when the DC current is detected to reach zero for the first time at the receiving end, the fault current is temporarily blocked off, and the energy balance of the sending-end island is achieved in conjunction with the AC energy dissipation device. This realizes the self-clearing of DC overhead line faults in the new energy island grid transmission system. Compared with the prior art, it can further reduce the proportion of full-bridge submodules, save engineering investment, and prevent the DC current from fluctuating repeatedly near zero, thereby shortening the DC current arc extinction time. When a DC fault is detected in the system, the sending-end converter station switches the outer loop controller of the fault pole to stator module capacitor voltage control. When operating on a single pole, the sending-end converter station also selects whether to over-connect the AC energy dissipation device according to the submodule voltage to reduce the risk of overvoltage of the fault pole submodule capacitor when clearing the DC fault. This improves the technical problems of the prior art, which requires a high proportion of full-bridge submodules for the output negative voltage strategy, resulting in high cost, risk of submodule overvoltage tripping, and long DC current arc extinction time.

[0084] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0085] It should be understood that in this application, "at least one (item)" means one or more, and "more than" means two or more. "And / or" is used to describe the relationship between related objects, indicating that three relationships can exist. For example, "A and / or B" can represent three cases: only A exists, only B exists, and both A and B exist simultaneously, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one (item) of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one (item) of a, b, or c can represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, and c can be single or multiple.

[0086] In the several embodiments provided in this application, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0087] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0088] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0089] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the methods described in the various embodiments of this application through a computer device (which may be a personal computer, server, or network device, etc.). The aforementioned storage medium includes: USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, optical disks, and other media capable of storing program code.

[0090] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for temporarily blocking and clearing DC faults in a renewable energy islanded power grid transmission system, characterized in that, include: S1. Construct a new energy islanded power grid transmission system. The new energy islanded power grid transmission system includes a new energy power plant, a sending-end converter station, a receiving-end converter station, a DC overhead line, and an AC energy dissipation device. The new energy power plant is connected to the sending-end converter station through a three-phase AC line. The AC energy dissipation device is set between the new energy power plant and the sending-end converter station. The sending-end converter station is connected to the receiving-end converter station through a DC overhead line. The sending-end converter station, the DC overhead line, and the receiving-end converter station constitute a DC system. S2. Configure the control strategies for the sending-end converter station and the receiving-end converter station. All sending-end converter stations adopt a dual closed-loop constant AC voltage and frequency control strategy. Only one receiving-end converter station adopts a constant DC voltage control strategy, while the rest of the receiving-end converter stations adopt a constant active power control strategy. S3. Real-time detection of the DC current flowing out of each pole of the sending-end converter station and real-time detection of the DC current flowing into each pole of the receiving-end converter station. S4. If each pole of the sending-end converter station detects an increase in the DC current flowing out of its own pole, the pole will automatically reduce the DC voltage bias through the DC current controller, reduce the number of sub-modules connected on the DC side, directly control the DC voltage, output a negative DC voltage, and actively clear the DC fault. S5. If each pole of the receiving-end converter station detects a decrease in the DC current flowing into its own pole and a reverse increase, the pole will automatically reduce the DC voltage bias through the DC current controller, reduce the number of sub-modules connected on the DC side, directly control the DC voltage, output a negative DC voltage, and actively clear the DC fault. If the receiving-end converter station detects that the DC current reaches zero for the first time, a temporary blocking signal will be generated. During the effective period of the temporary blocking signal, the receiving-end converter station will be temporarily blocked and the inner loop controller and DC current margin controller will be reset. S6. Each sending-end converter station and receiving-end converter station monitors DC voltage and current in real time, and detects whether a DC fault has occurred based on the DC voltage and current. If so, steps S7 and S8 are executed simultaneously. S7. The control systems of each sending-end converter station and receiving-end converter station enter the de-ionization logic. Each sending-end converter station and receiving-end converter station sets the DC current reference value input by the DC current controller of the faulty pole to zero and performs zero DC current control. In addition, both sending-end converter stations and receiving-end converter stations set the active current command output by the outer loop controller of the faulty pole to zero. S8. Determine whether the DC system is operating in bipolar mode. If yes, proceed to step S9; otherwise, proceed to step S10. S9. All sending-end converter stations will switch the outer loop controller of the faulty pole to stator module capacitor voltage control and transfer the power of the faulty pole to the non-faulty pole. If there is still a power surplus in the sending-end converter stations, the first target number of AC energy consumption devices will be put into operation to balance the energy of the sending-end converter stations. S10 and the sending-end converter station both switch the outer loop controller of the faulty pole to stator module capacitor voltage control, and put the second target number of AC energy consumption devices into operation to balance the energy of the sending-end converter station. At the same time, the sending-end converter station monitors the sub-module capacitor voltage in real time and selects whether to put the AC energy consumption device into operation based on the sub-module capacitor voltage. S11. When the deionization time is reached, each sending-end converter station and receiving-end converter station will restore the slope of the DC current reference value input by the DC current controller of the faulty pole, and each receiving-end converter station will switch the faulty pole from stator module voltage control back to outer loop controller control to restore DC voltage. S12. When each sending-end converter station and receiving-end converter station detects that the DC voltage has recovered to the preset value, the sending-end converter station disconnects the AC energy-consuming devices one group at a time to restore the DC current and realize the restart in the event of a DC fault.

2. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, The duration of the temporary blocking signal is 100ms.

3. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, The calculation process for the first target quantity group is as follows: P 送端正常极剩余容量 =P 额定功率 -P 送端正常极 ; N 交流耗能组数 =(P 送端双极功率 -P 送端故障极 -P 送端正常极剩余容量 ) / P 每组交流耗能容量 ; In the formula, P 送端正常极剩余容量 For the normal remaining capacity of the sending-end converter station, P 额定功率 P is the rated power of the sending-end converter station. 送端正常极 N represents the normal active power of the sending-end converter station. 交流耗能装置组数 For the first target quantity group that needs to be equipped with AC power consumption devices, P 送端双极功率 P represents the bipolar active power of the sending-end converter station. 送端故障极 For the fault-based active power of the sending-end converter station, P 每组交流耗能装置容量 The capacity of each group of AC power-consuming devices.

4. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, Step S10 specifically includes: All sending-end converter stations switched the outer loop controller of the faulty pole to stator module capacitor voltage control. Upon detecting a DC fault, the AC energy-consuming devices of the second target number group are immediately activated to balance the energy of the sending-end converter station. At the same time, the sending-end converter station monitors the submodule capacitor voltage in real time and controls the AC energy-consuming devices of other groups in each sending-end converter station to switch on and off according to the submodule capacitor voltage.

5. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 4, characterized in that, The calculation process for the second target quantity group is as follows: N 交流耗能组数 =P 送端故障极 / P 每组交流耗能容量 ; In the formula, N 交流耗能组数 For the second target quantity group that needs to be equipped with AC power consumption devices, P 送端故障极 For the fault-based active power of the sending-end converter station, P 每组交流耗能装置容量 The capacity of each group of AC power-consuming devices.

6. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 4, characterized in that, The AC energy consumption device controlling other groups in each sending-end converter station is switched on and off according to the submodule capacitor voltage, including: Calculate the average voltage of the faulty submodule in the sending-end converter station based on the submodule capacitor voltage. If the average voltage of the faulty pole module of the sending-end converter station is greater than the first preset threshold, an additional set of AC energy-consuming devices will be put into operation, and the average voltage of the faulty pole module of the sending-end converter station will be recalculated. If the newly calculated average voltage of the faulty pole module of the sending-end converter station is less than the second preset threshold, the additional set of AC energy-consuming devices will be disconnected.

7. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, The sending-end converter station and the receiving-end converter station adopt a single valve group or a double valve group in series configuration, and the converter valve adopts a full-bridge sub-module or a combination of full-bridge and half-bridge modules.

8. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, The number of sending-end converter stations is one or more, and the number of receiving-end converter stations is one or more.

9. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, The new energy power plants include wind farms and photovoltaic power plants.

10. The method for temporarily blocking and clearing DC faults in a new energy islanded power grid transmission system according to claim 1, characterized in that, The de-ionization time is 300ms to 400ms.

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