Alternating current fault ride-through control method of direct current sending-out system and storage device thereof

Abstract

The invention provides an alternating-current fault ride-through control method of a direct-current sending-out system and storage equipment thereof. The method comprises the steps that the operation condition of a wind power flexible direct-current system is monitored in real time; when it is detected that the direct current side voltage of the wind power flexible direct current system is larger than a set threshold value at the receiving end converter station, a direct current side energy consumption device of the flexible direct current system is started to dissipate surplus power; when an alternating current fault is detected at the sending end converter station based on the direct current side voltage, the sending end converter station is switched to a current limiting mode, a fan and a power transmission system of the wind power flexible direct current system are controlled, and surplus power is dissipated again through an unloading circuit; when the AC voltage recovers, the sending-end converter station is adjusted back to a normal mode; and power transmission of the wind power flexible direct current system is recovered. According to the invention, on the basis of the operation condition of the receiving-end and sending-end converter station direct-current voltage monitoring system, the energy consumption device, the fan, the power transmission system and other links are used for cooperative control, power can be balanced, equipment can be protected, and the system stability can be enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric power systems, and in particular relates to a method for controlling AC fault ride-through of a DC transmission system and a storage device thereof. Background Art

[0002] With the growth of large-scale renewable energy demand, the development of large-scale new energy bases with a capacity of tens of millions of kilowatts has become an inevitable trend. Modular multilevel converters (MMC) have self-commutation capabilities and can provide AC grid-connected voltage for wind farms and photovoltaic power generation. They also have modular design, which makes it easy to achieve high voltage and large capacity. They also have the ability of independent power control. Therefore, MMC-based high voltage direct current (MMC-HVDC) is one of the important ways to reliably and efficiently transmit large-scale new energy bases.

[0003] The isolated flexible DC transmission system of large-scale renewable energy bases such as wind farms faces many challenges in actual operation. When renewable energy power generation is at a high output or full power state, if the AC power grid fails, the power transmission capacity of the receiving-end MMC will be greatly reduced. Since the sending-end MMC mostly adopts fixed AC voltage control, relying solely on the AC voltage provided by the renewable energy power plant, it is impossible to effectively limit the flow of wind power into the DC system, resulting in a serious imbalance of AC and DC power of the receiving-end MMC. At this time, the surplus power will increase the capacitor voltage of the receiving-end MMC submodule, which may cause overvoltage of the submodule and DC line, which poses a serious threat to the safe and stable operation of the entire system.

[0004] To solve the above problems, the prior art has proposed a variety of solutions. One is to use only full-capacity centralized energy-consuming devices, and configure DC energy-consuming devices on the DC side of the receiving-end converter to dissipate surplus power. However, this solution has the defects of large dv / dt of centralized DC energy-consuming devices, high cost of energy-consuming resistors of distributed DC energy-consuming devices, complex heat dissipation design, and the capacity of energy-consuming devices needs to be selected according to the rated capacity of the wind farm, resulting in a significant increase in cost and floor space. Another is to combine communication to reduce the output power of wind farms with centralized energy-consuming devices. Although it can reduce the cost of centralized energy-consuming devices, it depends on the reliability of the communication system and still needs to cooperate with the energy-consuming devices. There is also a frequency-up method to quickly reduce the output power of wind farms. Due to the limitations of frequency response speed and variation margin, the load reduction effect may not be ideal and the wind turbine needs to change the original operation control strategy; and the voltage reduction method to quickly reduce the output power of wind farms may cause the electromechanical equipment of wind turbines to bear excessive stress and trigger protection action, and the impact of voltage reduction and load reduction margin needs to be considered. Therefore, it is urgent to study new technical solutions that can effectively solve the problem of surplus power in wind power flexible direct transmission systems. Summary of the invention

[0005] In view of this, the present invention provides a DC transmission system AC fault ride-through control method and a storage device thereof, so as to solve the problems of the existing wind power flexible DC transmission system when an AC power grid fails, such as severe AC / DC power imbalance, surplus power causing overvoltage of submodules and DC lines, threatening the safe and stable operation of the system, etc.

[0006] In order to achieve the above object, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for controlling AC fault ride-through of a DC transmission system, comprising the following steps:

[0008] Monitor the operation status of the wind power flexible direct current system in real time according to the DC voltage conditions at the receiving end converter station and the sending end converter station;

[0009] When the DC side voltage of the wind power flexible DC system is detected to be greater than the set threshold at the receiving end converter station, the DC side energy consumption device of the flexible DC system is started to dissipate the surplus power; the set threshold is the DC side voltage value at which the wind power flexible DC system needs to dissipate the surplus power;

[0010] When an AC fault is detected at the sending-end converter station based on the DC side voltage, the sending-end converter station is switched to the current limiting mode, and the wind turbine and transmission system of the wind power flexible DC system are controlled, and the surplus power is dissipated again through the unloading circuit, thereby matching the current limiting mode;

[0011] Determine whether the AC voltage has recovered, and if so, adjust the sending-end converter station back to normal mode;

[0012] Restore power transmission of the wind power flexible DC system and complete AC fault ride-through control.

[0013] Furthermore, at the receiving-end converter station, the operating status of the wind power flexible DC system is monitored based on the DC voltage measured by the DC side unloading device of the receiving-end converter station.

[0014] Furthermore, after starting the DC side energy consumption device of the flexible DC system to dissipate the surplus power, the method further includes:

[0015] Determine whether the DC side voltage meets the stability requirement. If not, continue to start the DC side energy consumption device of the flexible DC system to dissipate the surplus power until the DC side voltage stabilizes:

[0016] Wait for AC failure to be restored;

[0017] Determine whether the grid voltage is restored. If not, continue to wait for the AC fault to be restored until the AC fault is restored.

[0018] Furthermore, after the sending-end converter station is switched to the current limiting mode, the method further includes:

[0019] The output DC power of the sending-end converter station is reduced to increase the capacitor voltage and frequency of the converter submodule;

[0020] The frequency is transmitted to the wind farm so that the wind farm can coordinate to reduce the frequency through wind turbine control.

[0021] Furthermore, in the control of the wind turbine and the transmission system of the wind power flexible direct current system, the wind turbine control includes: machine-side converter control and grid-side converter control, and the machine-side converter control and the grid-side converter control are used to coordinately control the output power of the wind turbine to match the current limiting mode.

[0022] Furthermore, in the control of the wind turbines and transmission system of the wind power flexible direct current system, the transmission system control includes: sending-end converter station control and receiving-end converter station control, and the sending-end converter station control and the receiving-end converter station control are used to coordinately control the transmission power of the transmission system to match the current limiting mode.

[0023] Furthermore, the control of the sending-end converter station and the receiving-end converter station both include: an AC controller, a DC controller, a circulating current controller and a voltage linear conversion.

[0024] Furthermore, the threshold is set to 1.05 pu.

[0025] In a second aspect, the present invention provides a computer device, the device comprising a processor and a memory:

[0026] The memory is used to store the computer program and send the instructions of the computer program to the processor;

[0027] The processor executes a method for controlling AC fault ride-through at a receiving end of a DC transmission system according to the instructions of the computer program as described in the first aspect.

[0028] In a third aspect, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements a method for controlling AC fault ride-through of a DC transmission system as in the first aspect.

[0029] In summary, the present invention provides a method for controlling AC fault ride-through of a DC transmission system and a storage device thereof, including real-time monitoring of the operating status of a wind power flexible DC system according to the DC voltage conditions at a receiving-end converter station and a sending-end converter station; when the DC side voltage of the wind power flexible DC system is detected to be greater than a set threshold at the receiving-end converter station, starting the DC side energy consumption device of the flexible DC system to dissipate surplus power; the threshold is set as the DC side voltage value at which the wind power flexible DC system needs to dissipate surplus power; when an AC fault is detected at the sending-end converter station based on the DC side voltage, the sending-end converter station is switched to a current limiting mode, and the wind turbine and the transmission system of the wind power flexible DC system are controlled, and the surplus power is dissipated again through a load unloading circuit, thereby matching the current limiting mode; judging whether the grid voltage is restored, and if so, adjusting the sending-end converter station back to normal mode; restoring power transmission of the wind power flexible DC system and completing AC fault ride-through control. The present invention effectively responds to AC faults, balances power, protects equipment and enhances system stability by monitoring the operating conditions of the DC voltage monitoring system at the receiving and sending converter stations, utilizing energy consumption devices, current limiting modes and coordinated control of various links. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0031] Figure 1 A flow chart of a method for controlling AC fault ride-through in a DC transmission system provided by an embodiment of the present invention;

[0032] Figure 2 The wind power-flexible direct current grid-connected system structure diagram provided by the present invention;

[0033] Figure 3 A topological structure diagram of a DC energy consumption device provided by the present invention;

[0034] Figure 4 A schematic diagram of wind turbine load reduction using the communication method provided by the present invention;

[0035] Figure 5 A flowchart of an AC fault ride-through control method provided by an embodiment of the present invention;

[0036] Figure 6 A fan control block diagram provided by an embodiment of the present invention;

[0037] Figure 7 A control block diagram of a flexible direct current transmission system provided by an embodiment of the present invention;

[0038] Figure 8 A block diagram of a DC transmission system AC fault ride-through control device provided by an embodiment of the present invention;

[0039] Fig. 9 A block diagram of a computer device provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0040] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] See also Figure 1-4 , Figure 1 The present invention shows a flow chart of a method for controlling AC fault ride-through of a DC transmission system; Figure 2 The structure of the wind power-flexible direct current grid-connected system is shown; Figure 3 The topology of the DC energy consumption device is shown; Figure 4 The communication method is shown to reduce the load of the wind turbine. Figure 2-4 First, the existing technology for solving the surplus power problem is further introduced.

[0042] like Figure 2 As shown, for the isolated flexible DC transmission system of large-scale renewable energy bases such as wind farms, when the renewable energy power generation is at a high output or full power, a fault in the AC power grid will cause the receiving-end MMC to significantly reduce its power transmission capacity. The sending-end MMCs all use fixed AC voltage control, and only the renewable energy power plant can provide reliable AC voltage, which cannot limit the flow of wind power into the DC system, resulting in a serious imbalance in the AC and DC power of the receiving-end MMC. The surplus power will increase the capacitor voltage of the receiving-end MMC submodule, which may cause overvoltage in the submodule and DC line, threatening the safe and stable operation of the system. Solutions (1)-(4) are existing solutions for transferring or dissipating the surplus power of wind power flexible DC transmission systems, as follows:

[0043] (1) Only full-capacity centralized energy consumption devices are used

[0044] Configuring energy dissipation devices is one of the most direct and effective solutions to solve the problem of power surplus. In order to solve the problem of AC fault in the isolated DC transmission system, DC energy dissipation devices are usually configured on the DC side of the receiving converter to dissipate the surplus power. DC energy dissipation devices are mainly based on energy dissipation circuits of fully controlled devices, including centralized energy dissipation devices, distributed energy dissipation devices and hybrid energy dissipation devices, such as Figure 3By controlling the switching of the energy-consuming device, the voltage across the energy-consuming resistor can be changed, and the power of the system can be quickly balanced.

[0045] Its disadvantages are: 1) The dv / dt of the centralized DC energy dissipation device is large, which may threaten the safety of system operation; 2) The energy dissipation resistor of the distributed DC energy dissipation device is expensive and the heat dissipation design is complex. 3) If only the energy dissipation device is used to solve the surplus power during the fault ride-through process, the capacity of the energy dissipation resistor needs to be large enough to cope with various fault conditions, so it needs to be selected according to the rated capacity of the wind farm, which will greatly increase the cost and floor space of the system.

[0046] (2) Communications reduce wind farm output power + centralized energy consumption devices

[0047] In order to reduce the cost of DC energy consumption devices, the output power of the wind farm can also be quickly reduced after a fault occurs to reduce the requirements of the centralized energy consumption device for the dissipation power capacity. This will reduce the installation cost of the centralized energy consumption device. Figure 4 As shown in the figure, the communication method means that after the MMC at the sending end detects a fault, it transmits the load reduction command to the wind farm after a period of signal delay. The wind farm reduces the output power after receiving the load reduction signal. The disadvantages of this method are: 1) It depends on the reliability of the communication system; 2) Considering the influence of signal delay, it still needs to cooperate with the energy consumption device.

[0048] (3) Frequency increase method to quickly reduce wind farm output power

[0049] In order to avoid the delay problem caused by communication, the load reduction of the wind farm can also be achieved by changing the output voltage electrical quantity of the MMC at the sending end. The frequency increase method is to increase the output voltage frequency of the MMC at the sending end (that is, the frequency of the wind farm) when a fault occurs, and at the same time, the wind farm machine-side converter has an additional frequency response control link, which can reduce the output power accordingly when the frequency increases. The disadvantages of this method are: 1) The frequency increase method is limited by the frequency response speed and frequency change margin, and the load reduction effect may not be good; 2) The wind turbine needs to change the original operation control strategy.

[0050] (4) Voltage reduction method to quickly reduce wind farm output power

[0051] The voltage reduction rule is to directly reduce the power output from the wind farm to the sending-end MMC by reducing the output AC voltage amplitude of the sending-end MMC. The disadvantages of this method are: 1) it may cause excessive stress on the electromechanical equipment of the wind turbine, thereby triggering protection action and affecting the stable operation of the wind farm; 2) the impact of voltage reduction and load reduction margin needs to be considered.

[0052] Based on this, the present invention provides a DC transmission system AC fault ride-through control method and related devices to solve the problems of severe AC / DC power imbalance and surplus power causing overvoltage of submodules and DC lines threatening the safe and stable operation of the system in the existing wind power flexible DC transmission system when an AC power grid fault occurs.

[0053] See also Figure 1 The embodiment of the present invention provides a method for controlling AC fault ride-through of a DC transmission system, comprising the following steps:

[0054] S11: Monitor the operation status of the wind power flexible DC system in real time according to the DC voltage conditions at the receiving-end converter station and the sending-end converter station.

[0055] It should be noted that a reasonable range of DC voltage under normal operating conditions and related judgment logic are preset at the receiving-end converter station and the sending-end converter station, and then the DC side voltage is monitored. By comparing the real-time collected data with the preset range, it can be determined whether the system is in normal operating condition.

[0056] S12: When the DC side voltage of the wind power flexible DC system is detected to be greater than the set threshold at the receiving end converter station, the DC side energy consumption device of the flexible DC system is started to dissipate the surplus power; the set threshold is the DC side voltage value at which the wind power flexible DC system needs to dissipate the surplus power.

[0057] It should be noted that the DC side energy dissipation device is a device installed on the DC side of the flexible DC transmission system. It is usually composed of components such as energy dissipation resistors. Its purpose is to maintain the stability of parameters such as the DC side voltage by dissipating excess electrical energy in the form of heat when abnormal conditions such as power surplus occur in the system, thereby avoiding damage to system equipment due to excess power.

[0058] The threshold is determined based on comprehensive consideration of multiple factors such as the withstand voltage of the system electrical equipment, the normal operating voltage fluctuation range, and the power balance characteristics (for example, through a large amount of system modeling, simulation, and statistical analysis of actual operating data). Once the receiving end converter station detects that the DC side voltage is greater than the set threshold, the DC side energy dissipation device of the flexible DC system is started, so that the energy dissipation resistor and other components in the energy dissipation device are connected to the circuit to form a current path, so that the surplus power is consumed in the energy dissipation resistor in the form of heat energy, thereby reducing the energy accumulation on the DC side and stabilizing the DC voltage.

[0059] When the voltage on the DC side of the receiving end exceeds the normal range and reaches the set threshold, the energy-consuming device is started in time to dissipate the surplus power. This can effectively deal with the power imbalance in the initial stage of the AC fault at the receiving end and prevent the DC side voltage from further abnormally increasing. It can effectively protect the electrical equipment in the DC system from the impact of excessive voltage.

[0060] S13: When an AC fault is detected at the sending-end converter station based on the DC side voltage, the sending-end converter station is switched to the current limiting mode, and the wind turbine and the transmission system of the wind power flexible DC system are controlled, and the surplus power is dissipated again through the unloading circuit, thereby matching the current limiting mode.

[0061] It should be noted that the unloading circuit is located inside equipment such as fans. It is a circuit used to quickly consume excess power under specific working conditions (such as system failure or power imbalance). Through reasonable circuit design and control, the surplus power inside the equipment is converted into other forms such as heat energy and released, thereby ensuring the stable operation of the equipment itself and the entire system.

[0062] After the sending-end converter station determines that an AC fault occurs in the receiving-end power grid based on abnormal changes in the DC side voltage (such as voltage surges, abnormal fluctuations, etc., which are identified through a preset fault judgment algorithm), it adjusts the control parameters of the DC controller, such as changing the current reference value or adjusting the gain of the current control loop, so that the DC controller switches to the current limiting mode, thereby limiting the DC power output of the sending-end converter station. At the same time, the sending-end converter station will send a control signal to the control system of the wind turbine. The machine-side converter and the grid-side converter of the wind turbine coordinate to reduce the output power of the wind turbine according to the corresponding control strategy (such as the machine-side converter changes the electromagnetic torque of the wind turbine by adjusting the current and voltage reference values ​​to control the power, and the grid-side converter controls the reactive power and current parameters). In addition, after receiving the control signal, the unloading circuit guides the excess power to the energy-consuming elements for dissipation through operations such as turning on the internal switching devices, so as to match the power output in the current limiting mode and jointly maintain the power balance of the system.

[0063] After the AC fault of the receiving power grid is detected at the sending end, the system power is further adjusted from the overall perspective of the power generation end and the transmission link by switching to the current limiting mode and coordinating the control of the wind turbine and the transmission system, and using the unloading circuit to dissipate the surplus power again. It matches the current limiting mode and ensures the power balance and stable operation of the system under fault conditions.

[0064] S14: Determine whether the AC voltage is restored, and if so, adjust the sending-end converter station back to normal mode.

[0065] It should be noted that the voltage condition of the receiving-end power grid is monitored in real time, and the voltage value collected in real time is compared with the voltage standard value during normal operation (such as the rated voltage of the power grid). When it is detected that the voltage value has returned to the normal range and remains stable for a certain period of time (for example, the voltage fluctuation is within the allowable error range for several consecutive seconds), it is determined that the voltage of the receiving-end power grid has been restored and the conditions for adjusting the sending-end converter station back to normal mode are met.

[0066] By continuously judging the voltage of the receiving grid, determining whether the fault has been restored and providing a decision basis for whether the subsequent system can switch from fault mode to normal mode is a key judgment link in achieving the transition of the system from fault state to normal operation.

[0067] S15: Restore power transmission of the wind power flexible DC system and complete AC fault ride-through control.

[0068] It should be noted that when it is determined that the voltage of the receiving-end grid has been restored, the control system will adjust the parameters of the DC controller and other related control modules of the sending-end converter station back to the set values ​​in the normal mode (for example, restore the normal current reference value, control loop gain, etc.), so that the sending-end converter station can restore its normal power output capacity. At the same time, the control system of the wind turbine will also adjust the operating status of the wind turbine accordingly, gradually restore it to the normal power output level before the fault, or re-determine the appropriate power output value according to the actual situation such as the current wind speed, so as to realize the power transmission recovery of the entire wind power flexible DC system and complete the fault ride-through control.

[0069] This embodiment provides a method for controlling AC fault ride-through in a DC transmission system. Based on the close connection between the DC voltage and the system power balance in a wind power flexible DC system, the occurrence of AC faults can be keenly captured by real-time monitoring of the DC voltage at the receiving and sending converter stations. Once there is a sign of a fault, the receiving end first uses an energy-consuming device to preliminarily dissipate the surplus power, and then the sending end adopts a current limiting mode and coordinates the wind turbine and the transmission system control and the unloading circuit to further adjust the power balance. During the duration of the fault, the voltage recovery of the receiving-end grid is continuously monitored. After it is restored, the sending-end converter station and the entire system are orderly adjusted back to normal mode to restore power transmission, forming a complete closed-loop control mechanism from fault monitoring, response to recovery, to ensure that the wind power flexible DC system can still operate stably and continue to supply power to the grid in the event of an AC fault at the receiving end.

[0070] The embodiment of the present application simultaneously utilizes the DC voltage monitoring information of the receiving-end converter station and the sending-end converter station, and realizes comprehensive control and coordinated control of the entire wind power flexible direct current system from power generation to power transmission and power receiving, so that the system can jointly adjust the power balance from multiple key nodes when facing AC faults, and enhance the overall ability of the system to cope with faults. At the same time, a fault crossing timing coordination method for wind turbines, DC energy consumption devices and flexible direct current transmission systems is proposed. At different stages of the fault, each part can automatically and flexibly adjust the power output and energy consumption according to the system status, so that the entire wind power flexible direct current system can always maintain a relatively stable power balance under complex fault conditions, ensuring the stable operation of the system and the continuous and reliable delivery of wind power, showing good adaptability and robustness.

[0071] See also Figure 5 , Figure 5The process of the AC fault ride-through control method proposed in the above embodiment includes the following steps:

[0072] 1) Real-time monitoring of the operating status of the wind power flexible direct current system. If the receiving-end MMC DC side unloading device detects that the voltage is greater than the set threshold of 1.05pu, the energy dissipation device is started and the energy dissipation resistor is put into use to dissipate the surplus power. If the DC voltage can remain stable, continue to wait for the fault to recover; if the voltage continues to rise and cannot remain stable, continue to start the energy dissipation device.

[0073] 2) At the same time, the MMC at the sending end detects the DC voltage to determine whether a fault has occurred. If a fault occurs, the DC controller is switched to current limiting mode to reduce the output DC power. This causes the submodule capacitor voltage to rise, the frequency to rise, the fan to coordinate to reduce power, and dissipate power through the internal unloading circuit.

[0074] 3) After the above process, the AC and DC power at both ends of the receiving MMC and the sending MMC are balanced, and then wait for the fault to be restored.

[0075] 4) After detecting that the grid voltage has recovered, the sending-end MMC DC controller returns to normal mode.

[0076] 5) Restore the delivery of active power.

[0077] The following combination Figure 1 and 5 Other embodiments of the present invention are introduced in detail with reference to the process.

[0078] In one embodiment, at the receiving-end converter station, the operating status of the wind power flexible DC system is monitored based on the DC voltage measured by the DC side unloading device of the receiving-end converter station.

[0079] This embodiment relies on the unloading device equipped on the DC side of the receiving-end converter station to detect the DC side voltage, and uses this as a basis to grasp the operation status of the wind power flexible direct current system. The reason for adopting this method is that in the wind power flexible direct current system, the numerical change of the DC side voltage can intuitively reflect the operation status of the system. Under normal circumstances, the DC side voltage will be in a relatively stable range, and once the receiving-end AC power grid fails or other abnormal conditions occur, the DC side voltage will change accordingly. The DC side unloading device of the receiving-end converter station has the function of detecting voltage, and can continuously obtain DC side voltage data at a certain detection frequency. Then, by comparing the judgment logic such as the preset normal voltage range, it can be known in real time whether the wind power flexible direct current system is currently in a normal operating state, or an abnormal state such as power imbalance that requires further countermeasures has occurred, thereby providing a key judgment basis for subsequent possible fault handling and other operations.

[0080] In one embodiment, after starting the DC side energy consumption device of the flexible DC system to dissipate surplus power, the method further includes:

[0081] S21: Determine whether the DC side voltage meets the stability requirement. If not, continue to start the DC side energy consumption device of the flexible DC system to dissipate the surplus power until the DC side voltage stabilizes:

[0082] S22: Waiting for the AC fault of the receiving-end power grid to be restored;

[0083] S23: Determine whether the AC voltage is restored, if not, continue to wait for the AC fault to be restored until the AC fault is restored.

[0084] This embodiment is based on the operating principle of the wind power flexible direct current system. When the AC power grid fails and causes the DC side voltage to fluctuate due to power surplus, the DC side unloading device of the receiving end converter station detects the voltage to monitor the system operation status. Once the voltage exceeds the set threshold, the DC side energy consumption device is started to dissipate the surplus power. After that, it is continuously judged whether the DC side voltage is stable. If it is not stable, it continues to consume energy. After it is stable, it waits for the AC fault of the power grid to recover and continuously detects its voltage. If it is not recovered, it continues to wait. Its function is to timely and effectively handle power surplus through accurate monitoring of the DC side voltage and power grid faults and corresponding control actions, ensure the safety of system equipment, maintain stable operation of the system, and make an orderly transition during the fault recovery process, and finally realize the smooth transition of the wind power flexible direct current system from the fault state to the normal operation, ensuring the continuous and reliable access of wind power to the power grid.

[0085] In one embodiment, after the sending-end converter station is switched to the current limiting mode, the method further includes:

[0086] S31: reducing the output DC power of the sending-end converter station to increase the capacitor voltage and frequency of the converter submodule;

[0087] S32: transmitting the frequency to the wind farm so that the wind farm can coordinate and reduce the frequency through wind turbine control.

[0088] After the sending-end converter station is switched to the current limiting mode, this embodiment reduces its output DC power through a specific control strategy. According to the system energy conservation and electrical characteristics, this will cause the electric energy to accumulate in the capacitor of the converter submodule, thereby increasing the capacitor voltage, and at the same time, the frequency will increase due to the coupling relationship between power and frequency. The frequency information generated by this change is then transmitted to the wind farm. The wind farm uses wind turbine control means, such as the coordinated adjustment of the machine-side converter and the grid-side converter, to coordinate and change the operating state of the wind turbine according to the received frequency signal, reduce the output power of the wind turbine, and then adjust the power balance of the entire wind power flexible direct current system. Its function is to effectively respond to the power imbalance caused by AC faults by regulating the capacitor voltage and frequency of the converter submodule and linking with the wind turbine control in the current limiting mode of the sending-end converter station, avoid system failures due to excess power, ensure the stable operation of the wind power flexible direct current system during the fault, and form a close collaborative working mechanism between the various components, improve the overall reliability and adaptability of the system, and promote the system to smoothly transition from a fault state to a normal operating state.

[0089] In a further embodiment, in the control of the wind turbine and the transmission system of the wind power flexible direct current system, the wind turbine control includes: machine-side converter control and grid-side converter control, and the machine-side converter control and the grid-side converter control are used to coordinately control the output power of the wind turbine to match the current limiting mode.

[0090] In this embodiment, the wind turbine control is mainly composed of the machine-side converter control and the grid-side converter control. The machine-side converter is located on the connection path between the wind turbine motor and the grid, close to the wind turbine motor. It is mainly responsible for controlling the electrical parameters of the wind turbine motor itself, such as current and voltage. The grid-side converter is more focused on controlling the quality of power output from the wind turbine to the grid side, including parameters such as reactive power and current.

[0091] See also Figure 6 , Figure 6 The fan control process is shown in Figure 2. Figure 6 As shown in the figure, the machine-side converter control is the same as the conventional method, the d-axis current reference value is 0, and the d-axis current reference value is i * sd With the actual value i sd Subtract the feedforward amount (ω e L s i sq , where ω e is the rotor electrical angular velocity, L s is the stator and rotor inductance, i sq is the q-axis component of the rotor current) to obtain the d-axis voltage reference value u * d . Actual speed value ω m After MPPT control, the fan power reference value P is obtained.* WT . Actual value of fan power P WT With power reference value P * WT Subtract, and then pass through the proportional integral controller to get the q-axis current reference value. * sq With the actual value i sq Subtract and pass through the proportional integral controller to get the deviation. This deviation plus the feedforward (ω e ψ f + ω e L s i sd , where ψ f is the magnetic link, i sd is the d-axis component of the rotor current) to obtain the q-axis voltage reference value u * q Finally, u * d and u * q The three-phase voltage reference value of the converter port is obtained through park transformation, and then the switching signal is obtained through PWM.

[0092] The q-axis current control of the grid-side converter is the same as the conventional method, and the q-axis current reference value i * gq It is 2 / 3 of the reactive power reference value Q * g Divided by the grid-side AC voltage d-axis component u gd . q-axis current reference value i * gq With the actual value i gq Subtract and pass through the proportional integral controller to get the deviation. The deviation plus the feedforward (u gq + ω WF L g i gd , where u gq is the q-axis component of the grid-side AC voltage, ω WF is the grid voltage frequency, L g is the grid-side filter inductor, i gd is the actual value of the d-axis component of the grid-side current) to obtain the q-axis voltage reference value u * q . d-axis current reference value i * gd There are two sources. One is the reference value i generated by the conventional constant DC voltage control. * gd1 , and the other part is based on the reference value i generated by the additional frequency control * gd2 . Actual value of fan DC voltage udcW With the rated value u dcWN Subtract, and after the proportional integral controller, we get i * gd1 . Wind turbine grid-side frequency rating ω WFN With the actual value ω WF Subtract, and after the proportional integral controller, we get i * gd2 . d-axis current reference value i * gd With the actual value i gd Subtract and pass through the proportional integral controller to get the deviation. The deviation plus the feedforward (u gd – ω WF L g i gq ) to obtain the d-axis voltage reference value u * d Finally, u * d and u * q The three-phase voltage reference value of the converter port is obtained through park transformation, and then the switching signal is obtained through PWM.

[0093] According to the above control method, the machine-side converter and the grid-side converter cooperate with each other through their respective control signals. When the sending-end converter station switches to the current limiting mode, the machine-side converter changes the output power of the wind turbine by adjusting the electromagnetic torque of the motor according to the power command of the system (such as the command to reduce power). The grid-side converter synchronously adjusts the reactive power and current injected into the grid according to the action of the machine-side converter and the changes in parameters such as voltage and frequency on the grid side, so that the power output of the wind turbine can match the current limiting mode of the sending-end converter station. This coordinated control can ensure that the wind turbine can quickly and stably adjust the output power when the system fails (such as AC failure), avoiding problems such as aggravated system power imbalance caused by the mismatch between the wind turbine power output and system demand, thereby maintaining the stable operation of the wind power flexible direct current system.

[0094] In one embodiment, in controlling the wind turbines and the transmission system of the wind power flexible direct current system, the transmission system control includes: sending-end converter station control and receiving-end converter station control, and the sending-end converter station control and the receiving-end converter station control are used to coordinately control the transmission power of the transmission system to match the current limiting mode.

[0095] In this embodiment, when the wind power flexible direct current system enters the current limiting mode in response to a fault, the control of the transmission system is refined into two parts: the control of the sending-end converter station and the control of the receiving-end converter station. The two parts work together through relevant control means such as the AC controller, the DC controller, and the circulating current controller based on their respective functions in the system and their mutual relationship. The sending-end converter station controls and adjusts the output power on the wind farm side, and the receiving-end converter station controls and manages the power conversion on the AC power grid side, so as to jointly accurately regulate the transmission power of the transmission system to make it compatible with the current limiting mode. Its function is to ensure that the transmission power can be reasonably adjusted according to the overall needs of the system when a system fault occurs, through this method of coordinated control of the converter stations at both ends, to avoid system instability caused by power mismatch, to maintain the power balance of the wind power flexible direct current system during the fault period and the subsequent recovery process, to ensure the stable operation of the entire system, and to help smoothly achieve fault crossing and restore normal wind power transmission functions.

[0096] In a further embodiment, the sending-end converter station control and the receiving-end converter station control both include: an AC controller, a DC controller, a circulating current controller and a voltage linear conversion.

[0097] See also Figure 7 , Figure 7 The control process of the flexible DC transmission system is shown in Figure 2. Figure 7 As shown in the figure, the control of the flexible DC transmission system includes the control of the sending-end converter station, i.e., the wind farm-side MMC (WFMMC) converter station and the receiving-end converter station, i.e., the onshore converter station (Grid-side MMC, GSMMC) control.

[0098] The sending-end converter station controller is divided into four parts: AC controller, DC controller, circulating current controller, and voltage linear conversion.

[0099] ①AC controller: actual value square of capacitor voltage of MMC submodule at the sending end u 2 C2 Subtract the square of the submodule capacitor voltage rating u 2 CN , and then through the proportional integral controller, the frequency addition Δω is obtained. This frequency addition Δω plus the rated frequency ω N Get the AC voltage frequency ω WF , and then pass through the integrator to get the phase angle θ. The AC voltage amplitude control adopts the conventional voltage and current double closed-loop vector control. The output of the AC controller is the AC internal potential e a2 , e b2 , e c2 .

[0100] ② DC controller: The DC controller includes normal mode and fault mode. In normal mode, the rated voltage of the MMC submodule capacitor at the sending end is u CNSubtract the actual value u C2 , and then the DC current reference value i is obtained through the proportional integral controller * dc2 . DC current reference value i * dc2 Subtract the actual value i dc2 , and then the deviation is obtained through the proportional integral controller. The deviation plus the feedforward DC voltage u dc2 , we get the DC internal potential e dc2 In fault mode, the DC current reference value is limited to a given value i* dc2limit , the subsequent DC current control is consistent with the normal mode, and the DC internal potential e is obtained dc2 .

[0101] ③Circulation controller: This part is the same as the circulation controller of the existing MMC, which suppresses the circulation component to 0, so it is not expanded in detail in the figure. The output is the potential e in the circulation cira2 and e cirb2 .

[0102] ④ Linear voltage transformation: convert the AC, DC, and circulating current potential e a2 , e b2 , e c2 , e dc2 , e cira2 , e cirb2 After linear transformation, it becomes the output voltage reference value u of the six bridge arms * ap2 , u * an2 , u * bp2 , u * bn2 , u * cp2 , u * cn2 Wherein, the subscripts p and n represent the upper bridge arm and the lower bridge arm respectively.

[0103] The receiving-end converter station controller is divided into four parts: AC controller, DC controller, circulating current controller, and voltage linear conversion.

[0104] ① AC controller: The receiving end MMC phase angle θ is obtained by tracking the grid voltage phase through a phase-locked loop. The outer loop submodule capacitor voltage control obtains the d-axis current reference value i * s1d , and then the inner loop current control is used to obtain the three-phase AC internal potential d-axis component e d1 . q-axis current reference value i * s1q It is 2 / 3 of the reactive power reference value Q *1 divided by the grid voltage d-axis component u s1d , and then the inner loop current control is used to obtain the q-axis component e of the three-phase AC internal potential q1 .e d1 and e q1 After park transformation, the output of the AC controller is the three-phase AC internal potential e a1 , e b1 , e c1 .

[0105] ② DC controller: The DC controller directly converts the DC voltage rated value u dc1N As the DC internal potential e dc1 reference value.

[0106] ③Circulation controller: This part is the same as the circulation controller of the MMC at the sending end, and outputs the internal potential e of the circulation cira1 and e cirb1 .

[0107] ④ Voltage linear transformation: consistent with the voltage linear transformation of the MMC at the sending end, e a1 , e b1 , e c1 , e dc1 , e cira1 ,e cirb1 Converted into six bridge arm voltage reference values ​​u * ap1 , u * an1 , u * bp1 , u * bn1 , u * cp1 , u * cn1 .

[0108] In one embodiment, the threshold is set to 1.05 pu, where pu represents per unit value.

[0109] The present invention can ensure the reliability of AC fault riding in the system. The control method proposed in the present invention can ensure that the voltage, current and other electrical quantities of each device in the system can be controlled within the safety boundary during the fault riding period. It can also reduce the cost and floor space of DC energy consuming devices. The existing scheme basically adopts full-capacity DC energy consuming devices. This scheme can reduce the design capacity of DC energy consuming devices, thereby reducing costs. Moreover, there is no need to rely on the communication system, and information transmission is more reliable.

[0110] Based on the same inventive concept, the embodiment of the present application also provides a DC transmission system AC fault ride-through control device for implementing the DC transmission system AC fault ride-through control method involved above. The implementation scheme for solving the problem provided by the device is similar to the implementation scheme recorded in the above method, so the specific limitations in the DC transmission system AC fault ride-through control device embodiment provided below can refer to the limitations of the DC transmission system AC fault ride-through control method above, and will not be repeated here.

[0111] See also Figure 8 The embodiment of the present invention further provides an AC fault ride-through control device for a DC transmission system, comprising:

[0112] The monitoring module monitors the operation status of the wind power flexible direct current system in real time according to the DC voltage conditions at the receiving-end converter station and the sending-end converter station;

[0113] The ride-through control module is used to start the DC side energy consumption device of the flexible DC system to dissipate surplus power when the DC side voltage of the wind power flexible DC system is detected to be greater than the set threshold at the receiving end converter station; the set threshold is the DC side voltage value at which the wind power flexible DC system needs to dissipate surplus power; when the sending end converter station detects an AC fault based on the DC side voltage, the sending end converter station is switched to the current limiting mode, and the wind turbine and transmission system of the wind power flexible DC system are controlled, and the surplus power is dissipated again through the unloading circuit, so as to match the current limiting mode; it is determined whether the AC voltage is restored, and if so, the sending end converter station is adjusted back to normal mode; the power transmission of the wind power flexible DC system is restored and the AC fault ride-through control is completed.

[0114] Furthermore, at the receiving-end converter station, the operating status of the wind power flexible DC system is monitored based on the DC voltage measured by the DC side unloading device of the receiving-end converter station.

[0115] Furthermore, after starting the DC side energy consumption device of the flexible DC system to dissipate the surplus power, the method further includes:

[0116] Determine whether the DC side voltage meets the stability requirement. If not, continue to start the DC side energy consumption device of the flexible DC system to dissipate the surplus power until the DC side voltage stabilizes:

[0117] Wait for the AC fault of the receiving-end power grid to be restored;

[0118] Determine whether the receiving-end power grid voltage is restored. If not, continue to wait for the receiving-end power grid AC fault to be restored until the receiving-end power grid AC fault is restored.

[0119] Furthermore, after the sending-end converter station is switched to the current limiting mode, the method further includes:

[0120] The output DC power of the sending-end converter station is reduced to increase the capacitor voltage and frequency of the converter submodule;

[0121] The frequency is transmitted to the wind farm so that the wind farm can coordinate to reduce the frequency through wind turbine control.

[0122] Furthermore, in the control of the wind turbine and the transmission system of the wind power flexible direct current system, the wind turbine control includes: machine-side converter control and grid-side converter control, and the machine-side converter control and the grid-side converter control are used to coordinately control the output power of the wind turbine to match the current limiting mode.

[0123] Furthermore, in the control of the wind turbines and transmission system of the wind power flexible direct current system, the transmission system control includes: sending-end converter station control and receiving-end converter station control, and the sending-end converter station control and the receiving-end converter station control are used to coordinately control the transmission power of the transmission system to match the current limiting mode.

[0124] Furthermore, the control of the sending-end converter station and the receiving-end converter station both include: an AC controller, a DC controller, a circulating current controller and a voltage linear conversion.

[0125] Those skilled in the art can clearly understand that, for the convenience and simplicity of description, only the division of the above-mentioned functional units and modules is used as an example for illustration. In practical applications, the above-mentioned function allocation can be completed by different functional units and modules as needed, that is, the internal structure of the system can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiment can be integrated in a processing unit, or each unit can exist physically separately, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of software functional units. In addition, the specific names of the functional units and modules are only for the convenience of distinguishing each other, and are not used to limit the scope of protection of this application. The specific working process of the units and modules in the above-mentioned system can refer to the corresponding process in the aforementioned method embodiment, which will not be repeated here.

[0126] Reference Fig. 9 An embodiment of the present invention further provides a computer device, comprising: a memory and a processor and a computer program stored in the memory. When the computer program is executed on the processor, the method for controlling AC fault ride-through at the receiving end of a DC transmission system as described in any one of the above methods is implemented.

[0127] The computer device may be a desktop computer, a notebook, a PDA, a cloud server or other computing device. The computer device may include, but is not limited to, a processor and a memory. Those skilled in the art will understand that Fig. 9It is only an example of a computer device and does not constitute a limitation of the computer device. It may include more or fewer components than shown in the figure, or a combination of certain components, or different components. For example, it may also include input and output devices, network access devices, etc.

[0128] The processor may be a central processing unit (CPU), other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor may be a microprocessor or any conventional processor, etc.

[0129] In some embodiments, the memory may be an internal storage unit of the computer device, such as a hard disk or memory of the computer device. In other embodiments, the memory may also be an external storage device of the computer device, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, etc. equipped on the computer device. Further, the memory may include both an internal storage unit and an external storage device of the computer device. The memory is used to store an operating system, an application program, a boot loader (BootLoader), data, and other programs, such as the program code of the computer program. The memory may also be used to temporarily store data that has been output or is to be output.

[0130] An embodiment of the present invention further provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for controlling AC fault ride-through of a DC transmission system as described in any one of the above methods is implemented.

[0131] In this embodiment, if the integrated unit is implemented in the form of 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 present application implements all or part of the processes in the above-mentioned embodiment method, which can be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by the processor, the steps of the above-mentioned various method embodiments can be implemented. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium can at least include: any entity or device that can carry the computer program code to the camera / terminal device, recording medium, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electric carrier signal, telecommunication signal and software distribution medium. For example, USB flash drive, mobile hard disk, disk or optical disk. In some jurisdictions, according to legislation and patent practice, computer-readable media cannot be electric carrier signals and telecommunication signals.

[0132] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0133] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0134] In the embodiments disclosed in the present application, it should be understood that the disclosed devices / terminal equipment and methods can be implemented in other ways. For example, the device / terminal equipment embodiments described above are only schematic. For example, the division of the modules or units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0135] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling AC fault ride-through in a DC transmission system, characterized in that: The steps include: Monitor the operation status of the wind power flexible direct current system in real time according to the DC voltage conditions at the receiving end converter station and the sending end converter station; When the receiving-end converter station detects that the DC side voltage of the wind power flexible DC system is greater than a set threshold, the DC side energy consumption device of the flexible DC system is started to dissipate the surplus power; The set threshold is the DC side voltage value at which the wind power flexible DC system needs to dissipate surplus power; When an AC fault is detected at the sending-end converter station based on the DC side voltage, the sending-end converter station is switched to a current limiting mode, and the wind turbine and the transmission system of the wind power flexible DC system are controlled, and the surplus power is dissipated again through a load unloading circuit, thereby matching the current limiting mode; Determine whether the AC voltage is restored, and if so, adjust the sending-end converter station back to normal mode; The power transmission of the wind power flexible DC system is restored and the AC fault ride-through control is completed.

2. The method for controlling AC fault ride-through of a DC transmission system according to claim 1, characterized in that: At the receiving-end converter station, the operating status of the wind power flexible DC system is monitored based on the DC voltage measured by the DC side unloading device of the receiving-end converter station.

3. The method for controlling AC fault ride-through of a DC transmission system according to claim 2, characterized in that: After starting the DC side energy consumption device of the flexible DC system to dissipate the surplus power, it also includes: Determine whether the DC side voltage meets the stability requirement. If not, continue to start the DC side energy consumption device of the flexible DC system to dissipate the surplus power until the DC side voltage is stable: Waiting for the AC fault to be restored; Determine whether the grid voltage is restored, if not, continue to wait for the AC fault to be restored until the AC fault is restored.

4. The method for controlling AC fault ride-through of a DC transmission system according to claim 1, characterized in that: After switching the sending-end converter station to the current limiting mode, it also includes: The output DC power of the sending-end converter station is reduced to increase the capacitor voltage and frequency of the converter submodule; The frequency is transmitted to the wind farm so that the wind farm can coordinately reduce the frequency through wind turbine control.

5. The method for controlling AC fault ride-through of a DC transmission system according to claim 1, characterized in that: In controlling the wind turbine and the power transmission system of the wind power flexible direct current system, the wind turbine control includes: machine-side converter control and grid-side converter control, and the machine-side converter control and the grid-side converter control are used to coordinately control the output power of the wind turbine to match the current limiting mode.

6. The method for controlling AC fault ride-through of a DC transmission system according to claim 5, characterized in that: In controlling the wind turbines and the transmission system of the wind power flexible direct current system, the transmission system control includes: sending-end converter station control and receiving-end converter station control, and the sending-end converter station control and the receiving-end converter station control are used to collaboratively control the transmission power of the transmission system to match the current limiting mode.

7. The method for controlling AC fault ride-through of a DC transmission system according to claim 6, characterized in that: The sending-end converter station control and the receiving-end converter station control both include: an AC controller, a DC controller, a circulating current controller and a voltage linear conversion.

8. The method for controlling AC fault ride-through of a DC transmission system according to claim 1, characterized in that: The set threshold is 1.05 pu.

9. A computer device, characterized in that: The device comprises a processor and a memory: The memory is used to store a computer program and send instructions of the computer program to the processor; The processor executes a method for controlling AC fault ride-through at a receiving end of a DC transmission system according to any one of claims 1 to 7 according to instructions of the computer program.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by the processor, the method for controlling AC fault ride-through of a DC transmission system according to any one of claims 1 to 7 is implemented.

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