Zhejiang University

Zheren Zhang

Zheng Xu

Ying Huang

Wentao Liu

Yuzhe Xu

Yiyan Dong

Yingjie Tang

Yanqiu Jin

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. Compared with the existing sub-module hybrid MMC, the MMC of the present disclosure can also deal with DC faults under the condition of using fewer full-bridge sub-modules, which can reduce the number of required power electronic devices, greatly reduces the cost of building an overhead line high-voltage flexible DC transmission system, and has very strong reference significance and use value in engineering.

Submodule hybrid MMC with low full-bridge ratio and DC fault processing method thereof

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.

Distributed voltage clamping method for 100% renewable-energy sending-end grid

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.

Resistive sub-module hybrid MMC and direct current fault processing strategy thereof

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.

Offshore wind farm low-frequency alternating-current uncontrolled rectification electric power transmission system

A backup voltage and frequency support method for a 100%-renewable energy sending-end grid, including: (S) selecting a plurality of support nodes in the 100%-renewable energy sending-end grid; (S) mounting a backup voltage and frequency support device at each support node; and (S) 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.

Backup voltage and frequency support method for 100%-renewable energy sending-end grid

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.

High-frequency uncontrolled rectifier-based DC transmission system for offshore wind farm

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.

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Hangzhou, China

Zheren Zhang