Abstract: The present disclosure provides a control method for suppressing an overvoltage of the receiving-end F/HMMC under the fault at the receiving-end alternating-current (AC) system of the hybrid high-voltage direct-current (HVDC) transmission system. After a short-circuit fault occurs in a receiving-end alternating-current (AC) system of the hybrid HVDC transmission system, an AC voltage drop is generated, and the effective voltage of the receiving-end AC bus at the current time is measured. Because the hybrid modular multilevel converter (MMC) has a capability of actively decreasing the DC voltage for operation, on a premise of ensuring that its output current is always within a rated current range, a control manner of setting the reference DC voltage and making the reference DC voltage not greater than the effective voltage of the AC bus enables receiving-end power to be sent out, thereby suppressing an overvoltage of the receiving-end F/HMMC.

Abstract: The present disclosure provides a method for controlling the receiving-end alternating-current (ac) fault ride-through of the hybrid cascaded high-voltage direct-current (hvdc) transmission system. According to the method, a line commutated converter (lcc) on the rectifier side determines the occurrence of a receiving-end ac fault based on a change in an electrical quantity of its dc port, and quickly reduces the dc voltage on the rectifier side by increasing the firing angle, so as to quickly suppress an overcurrent. The mmcs on the inverter side, which use the constant active power control, correct the outer-loop active power reference values to transmit as much active power as possible, thereby reducing the surplus power of the receiving-end system and suppressing an overvoltage for submodule capacitors of the mmcs.

Abstract: The present disclosure discloses a sub-module hybrid MMC with low full-bridge ratio and a DC fault processing method thereof. The hybrid MMC prevents the energy at the AC side from entering a DC system by artificially creating three-phase interphase short circuit at the AC side of a converter in the DC fault processing process, which greatly reduces the ratio of full-bridge sub-modules required for DC fault processing. Moreover, the hybrid MMC has the same DC fault processing speed as the sub-module hybrid MMC with high full-bridge. The duration of the artificially created three-phase interphase short circuit fault in the fault processing process does not exceed 30 ms, which will not have a great impact on the AC system.

Abstract: The present disclosure discloses a sub-module hybrid MMC with low full-bridge ratio and a DC fault processing method thereof. The hybrid MMC prevents the energy at the AC side from entering a DC system by artificially creating three-phase interphase short circuit at the AC side of a converter in the DC fault processing process, which greatly reduces the ratio of full-bridge sub-modules required for DC fault processing. Moreover, the hybrid MMC has the same DC fault processing speed as the sub-module hybrid MMC with high full-bridge. The duration of the artificially created three-phase interphase short circuit fault in the fault processing process does not exceed 30 ms, which will not have a great impact on the AC system.

Abstract: A distributed voltage clamping method for a 100% renewable-energy sending-end grid, including: selecting key nodes from the 100% renewable-energy sending-end grid, including their voltage levels and positions; installing a dynamic reactive power compensation device on each key node, where the dynamic reactive power compensation device is controlled by a constant alternating-current (AC) voltage effective value, and the instruction value of the constant AC voltage effective value is adjustable according to an operation mode; according to AC voltage variation of the sending-end grid under a typical working condition, judging whether the key nodes meet the checking requirements; if not, selecting more key nodes.

Abstract: The present disclosure discloses a resistive sub-module hybrid MMC and a direct current fault processing strategy thereof. The hybrid MMC prevents fault current from entering a direct current system from the alternating current side by artificially creating three-phase earthing short circuit on the alternating current side of a converter during the direct current fault processing process, and can reduce the number of required power electronic devices compared with the module hybrid MMC. At the same time, the direct current fault processing speed of the hybrid MMC is fast, and the duration of the artificially creating three-phase short circuit fault in the fault processing process is less than 60 ms, which will not have a great impact on the alternating current system. The present disclosure greatly reduces the cost of building an overhead line high-voltage flexible direct current transmission system, and has very strong reference significance and use value in engineering.

Abstract: The present disclosure discloses an offshore wind farm low-frequency alternating-current uncontrolled rectification electric power transmission system, comprising an onshore converter station and an offshore alternating-current system. The offshore alternating-current system comprises wind turbine generators, alternating-current submarine cables, a confluence bus, and offshore booster stations; the onshore converter station comprises a wind field side alternating-current bus, an alternating-current system side alternating-current bus, an alternating-current filter, an energy dissipation device, a rectifier, and a converter; the rectifier is composed of a three-phase six-pulse uncontrolled rectifier bridge, and the converter may be an MMC or an LCC; the rated frequency of the offshore alternating-current system is selected to be close to 10 Hz.

Abstract: A backup voltage and frequency support method for a 100%-renewable energy sending-end grid, including: (S1) selecting a plurality of support nodes in the 100%-renewable energy sending-end grid; (S2) mounting a backup voltage and frequency support device at each support node; and (S3) dynamically adjusting an active power output of a renewable energy station of the 100%-renewable energy sending-end grid according to a frequency of a grid-connection point.

Abstract: The present disclosure discloses a sub-module hybrid MMC with low full-bridge ratio and a DC fault processing method thereof. The hybrid MMC prevents the energy at the AC side from entering a DC system by artificially creating three-phase interphase short circuit at the AC side of a converter in the DC fault processing process, which greatly reduces the ratio of full-bridge sub-modules required for DC fault processing. Moreover, the hybrid MMC has the same DC fault processing speed as the sub-module hybrid MMC with high full-bridge. The duration of the artificially created three-phase interphase short circuit fault in the fault processing process does not exceed 30 ms, which will not have a great impact on the AC system.

Abstract: The present disclosure provides a high-frequency uncontrolled rectifier-based DC transmission system for an offshore wind farm, including a DC system and an offshore AC system. The offshore AC system mainly includes wind turbines based on permanent magnet synchronous generators with full-scale power converters, AC submarine cables, and offshore step-up stations. The DC system includes an offshore station and an onshore station that are connected by DC submarine cables, where a converter of the offshore is a three-phase six-pulse uncontrolled rectifier bridge, while a converter of the onshore station is MMC. Each of the offshore AC system and the offshore station has a rated frequency far above 50 Hz, which can usually be chosen to be in a range of about 100 Hz to 400 Hz. The disclosed transmission system allows for a great reduction in construction costs and demonstrating great application potentials in actual engineering.

Abstract: A distributed voltage clamping method for a 100% renewable-energy sending-end grid, including: selecting key nodes from the 100% renewable-energy sending-end grid, including their voltage levels and positions; installing a dynamic reactive power compensation device on each key node, where the dynamic reactive power compensation device is controlled by a constant alternating-current (AC) voltage effective value, and the instruction value of the constant AC voltage effective value is adjustable according to an operation mode; according to AC voltage variation of the sending-end grid under a typical working condition, judging whether the key nodes meet the checking requirements; if not, selecting more key nodes.

Abstract: A backup voltage and frequency support method for a 100%-renewable energy sending-end grid, including: (S1) selecting a plurality of support nodes in the 100%-renewable energy sending-end grid; (S2) mounting a backup voltage and frequency support device at each support node; and (S3) dynamically adjusting an active power output of a renewable energy station of the 100%-renewable energy sending-end grid according to a frequency of a grid-connection point.

Abstract: A grid-forming wind turbine control method for a diode rectifier unit-based offshore wind power transmission system. A control system for controlling a grid-side converter has a three-layered structure, where a first layer is a combination of an active power controller and a reactive power controller; a second layer is a voltage controller; and a third layer is a current controller. The actual reactive power is represented by a per-unit value of a capacity of a corresponding wind turbine unit. The wind turbine units have the same reactive-power reference value, which is constant and does not change with time. The reactive power controllers of all wind turbine units have the same structure and parameters.

Abstract: Provided is a control method for a parallel MMC unit of a LCC-MMC hybrid cascade converter station. The control strategy includes: 1) numbering all MMC units connected in parallel in a MMC valve manifold; (2) for a MMC unit using a constant direct-current voltage control manner, calculating a direct-current instruction value of the MMC unit according to a direct-current measurement value; (3) for a MMC unit using a constant active power control manner, calculating an active power instruction value of the MMC unit according to the rated capacity of the MMC unit and a direct-current instruction value of a system rectifier station; (4) for the MMC unit using the constant direct-current voltage control manner, correcting a direct-current voltage instruction value of the MMC unit by using the direct-current instruction value and the direct-current measurement value, and controlling the MMC unit according to the corrected direct-current voltage instruction value.

Abstract: A grid-forming wind turbine control method for a diode rectifier unit-based offshore wind power transmission system. A control system for controlling a grid-side converter has a three-layered structure, where a first layer is a combination of an active power controller and a reactive power controller; a second layer is a voltage controller; and a third layer is a current controller. The actual reactive power is represented by a per-unit value of a capacity of a corresponding wind turbine unit. The wind turbine units have the same reactive-power reference value, which is constant and does not change with time. The reactive power controllers of all wind turbine units have the same structure and parameters.

Abstract: The present disclosure discloses an offshore wind farm low-frequency alternating-current uncontrolled rectification electric power transmission system, comprising an onshore converter station and an offshore alternating-current system. The offshore alternating-current system comprises wind turbine generators, alternating-current submarine cables, a confluence bus, and offshore booster stations; the onshore converter station comprises a wind field side alternating-current bus, an alternating-current system side alternating-current bus, an alternating-current filter, an energy dissipation device, a rectifier, and a converter; the rectifier is composed of a three-phase six-pulse uncontrolled rectifier bridge, and the converter may be an MMC or an LCC; the rated frequency of the offshore alternating-current system is selected to be close to 10 Hz.

Abstract: The present disclosure provides a high-frequency uncontrolled rectifier-based DC transmission system for an offshore wind farm, including a DC system and an offshore AC system. The offshore AC system mainly includes wind turbines based on permanent magnet synchronous generators with full-scale power converters, AC submarine cables, and offshore step-up stations. The DC system includes an offshore station and an onshore station that are connected by DC submarine cables, where a converter of the offshore is a three-phase six-pulse uncontrolled rectifier bridge, while a converter of the onshore station is MMC. Each of the offshore AC system and the offshore station has a rated frequency far above 50 Hz, which can usually be chosen to be in a range of about 100 Hz to 400 Hz. The disclosed transmission system allows for a great reduction in construction costs and demonstrating great application potentials in actual engineering.

Abstract: The present disclosure discloses a resistive sub-module hybrid MMC and a direct current fault processing strategy thereof. The hybrid MMC prevents fault current from entering a direct current system from the alternating current side by artificially creating three-phase earthing short circuit on the alternating current side of a converter during the direct current fault processing process, and can reduce the number of required power electronic devices compared with the module hybrid MMC. At the same time, the direct current fault processing speed of the hybrid MMC is fast, and the duration of the artificially creating three-phase short circuit fault in the fault processing process is less than 60 ms, which will not have a great impact on the alternating current system. The present disclosure greatly reduces the cost of building an overhead line high-voltage flexible direct current transmission system, and has very strong reference significance and use value in engineering.

Abstract: Provided is a control method for a parallel MMC unit of a LCC-MMC hybrid cascade converter station. The control strategy includes: 1) numbering all MMC units connected in parallel in a MMC valve manifold; (2) for a MMC unit using a constant direct-current voltage control manner, calculating a direct-current instruction value of the MMC unit according to a direct-current measurement value; (3) for a MMC unit using a constant active power control manner, calculating an active power instruction value of the MMC unit according to the rated capacity of the MMC unit and a direct-current instruction value of a system rectifier station; (4) for the MMC unit using the constant direct-current voltage control manner, correcting a direct-current voltage instruction value of the MMC unit by using the direct-current instruction value and the direct-current measurement value, and controlling the MMC unit according to the corrected direct-current voltage instruction value.

Abstract: Flash memory on a flash memory device is virtualized using compression that is native to the flash memory device. Through compression, the flash memory device is used to logically store more data in a virtual address space that is larger than the physical address space of the flash memory device. Physical storage capacity of a flash memory device may prevent further storage of data even when the virtual address space is not fully populated. Because compressibility may vary, the extent to which the virtual address space may be populated before physical storage capacity is reached varies. The approaches for virtual memory described herein rely on the memory device client to monitor when this point is reached. In addition, the memory device client is responsible for freeing space as needed to accommodate subsequent requests to store data in the flash memory.