A smooth switching method for flexible direct current transmission system from parallel to island operation

Abstract

The invention discloses a smooth switching method for switching a flexible direct current transmission system from parallel operation into isolated island operation. According to the smooth switching method, a self-adaptive isolated island detection method which comprehensively takes phase position and frequency of the system into consideration is disclosed by extracting electrical parameter information such as the phase position and frequency of the AC system, and a convertor station control system is automatically switched to an isolated island control strategy when detecting that the system is in an isolated island operation state. The smooth switching method avoids the frequency collapse caused by switching the flexible direct current transmission system into isolated island operation, can adopt different detection methods according to the different transmission powers of an AC transmission line, can accurately and quickly identify which operation mode the system is in, ensures the stable operation of the two end systems during the entire switching process, cannot cause wide fluctuations of the electrical parameters, can avoid the large-scale generator shedding of wind turbine generators caused by the switching of control strategies, and can be widely applied in many occasions such as new energy grid connection and isolated island power supply.

Description

technical field [0001] The present invention relates to the field of power transmission and distribution of electric power systems, and more specifically, relates to a smooth control switching method for converting a flexible direct current transmission system from parallel operation of alternating current and direct current to island operation. Background technique [0002] With the vigorous development of renewable energy such as wind energy and solar energy, the grid-connection of new energy has attracted increasing attention, and the flexible DC transmission technology based on voltage source converters can solve the problem of grid-connection of new energy well, and power electronic devices The rapid development of HVDC also provides a good foundation for the development of flexible HVDC technology, which has been widely used in recent years. [0003] There are usually three operating modes of the flexible DC transmission system: (1) The wind farm is connected to the re...

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Problems solved by technology

[0004] At present, there have been extensive studies on the control strategies adopted by the flexible DC transmission system in the above-mentioned various operation modes, but there are no literature reports on the control switching methods between various operation modes, and there is no research on the control strategy between different operation modes. Relevant research on the switching basis adopted at the time of switching

The DC system operates in parallel with AC and DC during normal operation, but the AC line may trip due to an accident during operation. , the output power of the system will be redistributed on the AC line, and the power flow will be transferred from the faulty line to the sound line; but when all the AC lines are tripped, the system is in the island operation mode at this time, and the sending end system can only use flexible DC transmission The line injects power into the receiving end system, and because the flexible DC transmission system adopts constant active power control, most of the electric energy cannot be sent out, resulting in the phenomenon of "storage power". If the system is in the island operation mode for a long time, the frequency of the system will continue It will eventually lead to frequency collapse, which will have a very serious impact on the system and equipment, so how to accurately and timely judge that the system is in an island operation state, and then perform smooth switching of the corresponding control mode to ensure that the power generated by the system passes through The flexible direct current transmission line continuously transmits outwards, which is crucial to the safe and stable operation of the entire power grid system

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