Flexible DC Transmission System Based on Combined Modular Multilevel Converter

Abstract

The invention proposes a flexible direct current transmission system based on a combined modular multilevel converter, belonging to the field of power electronics technology and power transmission and distribution. A combined modular multilevel converter composed of a multi-level converter and a three-phase converter transformer group composed of a plurality of three-phase converter units; wherein each combined converter is composed of a plurality of MMC units Each MMC unit group is composed of multiple MMC units in series; the secondary side port of the three-phase converter transformer group is connected to the corresponding AC side port of the combined modular multilevel converter, and the primary side is connected to The AC power grid at the originating or receiving end of the flexible DC power transmission system; the system of the present invention has the characteristics of fault suppression, reactive power compensation capability during faults, low cost, high voltage level and power level scalability, and is suitable for high-capacity and long-distance overhead lines. An economical solution for long-distance flexible DC transmission applications with high reliability.

Description

technical field [0001] The invention belongs to the field of power electronics technology and power transmission and distribution, in particular to a flexible direct current transmission system based on a combined modular multilevel converter, capable of blocking direct current short-circuit current under the condition of direct current short-circuit fault, and providing power to the alternating current system at the same time Capability of dynamic reactive power support. Background technique [0002] Compared with the conventional current source HVDC transmission technology, the flexible HVDC transmission technology (VSC-HVDC) uses a fully controlled power electronic switch to realize the control of the converter, which has the advantages of flexible control, small harmonic distortion of the AC side system current, Reactive power can be freely compensated and does not rely on the AC system to achieve conversion and other advantages. Most of the flexible HVDC transmission s...

AI Technical Summary

Problems solved by technology

The main problems of this method are: 1) The reliability is low, and it cannot be compared with the bipolar system of conventional DC transmission. Generally, the system using pseudo-bipolar connection only needs a failure of the unipolar converter unit or a unipolar If the DC line fails, the entire bipolar system will stop running, and one pole cannot be kept in operation like conventional DC transmission; 2) The capacity of the flexible DC system is small, and only one VSC (Voltage Source Converter) can be used Or MMC (Modular Multilevel Converter) unit, unable to realize multi-unit series-parallel combination expansion

However, the problem with this solution is that only one MMC converter per pole is not conducive to the expansion of voltage level and power capacity; secondly, the number and cost of components used by FBSM are high, even if it is mixed with the same number of HBSM, the use of The fully controlled power electronic switch is still higher

But the problem is that although the number of fully controlled power electronic switches used by each CDSM is 1 less than the sum of a FBSM and HBSM, but there are 2 more diodes, the overall economy has no advantage; it is even more unfavorable Unfortunately, due to the inconsistency between CDSM's blocking ability and voltage output ability in positive and negative directions under pulse blocking conditions, it cannot operate in STATCOM mode during DC faults, which greatly limits its application range.

[0009] To sum up, the current flexible DC transmission scheme using true bipolar system wiring has a relatively simple configuration of power sub-modules, and the power capacity level and voltage level, the overall cost of using devices, and the maximum power that can be provided to the AC system during a DC fault It is difficult to take into account and achieve the optimum in multiple key indicators of reactive power capacity at the same time

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