Optimal pressure equalizing control method of modular multilevel converter type direct current transmission system

Abstract

The invention discloses an optimal pressure equalizing control method of a modular multilevel converter type direct current transmission system. By setting an upper limit and a lower limit on voltage, the pressure equalizing control is mainly carried out on submodules with out-of-limit capacitor voltage, and capacitor voltage sequencing for submodules with no out-of-limit capacitor voltage is processed by combing the charging/discharging conditions of bridge arm current so as to increase the probability of maintaining an original switching state for the submodules with no out-of-limit capacitor voltage when the next action is in triggering control and lower the switching frequency of devices. The introduction of the pressure equalizing control can not lower the working frequency of triggering control and the tracking speed of a converter by using submodule capacitor voltage sequencing and triggering control to work at different frequencies. The optimal pressure equalizing control method of the invention can greatly lower the switching frequency of the devices on the premise of having no obvious increment on submodule capacitor voltage fluctuation.

Description

technical field [0001] The invention relates to the technical field of multilevel converter capacitor voltage equalization control, in particular to an optimized voltage equalization control method for a modular multilevel converter type direct current transmission system. Background technique [0002] The modular multilevel converter is a relatively new multilevel voltage source converter, and its structure is as follows figure 1 shown. Based on modular design and manufacture, the converter can reach a very high level number, and is suitable for high-voltage and high-power DC transmission fields. [0003] The sub-module capacitances of the modular multilevel converter are independent of each other. The difference in charge and discharge, loss and capacitance value of each sub-module will make the capacitor voltage unbalanced, which will endanger the normal operation of the converter. When the sub-module capacitor is connected to the bridge arm, it is said that the sub-mo...

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Problems solved by technology

[0005] Secondly, the switching frequency of sub-modules is too high, which makes the switching frequency and switching loss of power electronic devices larger, which reduces the efficiency of the modular multilevel converter type DC transmission system

[0006] Thirdly, due to the large number of bridge arm sub-modules in the modular multi-level converter type DC transmission system, it takes a lot of time to monitor and sort the capacitor voltages of the sub-modules, which will introduce a large delay in the trigger control , to reduce the speed of the converter to track the modulating wave

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