A Modulation Error Compensation Method for Modular Multilevel Converter Used in DC Transmission

Abstract

The invention discloses a modulation error compensation method for a modularized multi-level converter of direct-current transmission. The converter uses the modularized multi-level structure, and comprises a plurality of valve sub-modules. Each sub-module is provided with a capacitor, a switch tube and a diode. The modulation uses a method of valve sub-module number rounding and capacitor voltage sorting. The method is capable of feeding and superposing the error generated by the sub-module number rounding in the modulation and the error caused by the deviation of the sub-module capacitor voltage from the average capacitor voltage to the command input sub-module number of the next control period. The method is capable of, in the steady-state operation of the converter, obviously reducing the error between the output voltage and the modulation voltage of the converter, reducing the current harmonic wave, avoiding the vibration, further broadening the difference between the capacitor voltages, reducing the switching frequency, and reducing the loss of the converter.

Description

technical field [0001] The invention relates to the technical field of flexible direct current transmission system control, in particular to a modulation error compensation method for a modular multilevel converter used for direct current transmission. Background technique [0002] Voltage source DC transmission system, also known as flexible DC transmission system, supports independent active / reactive power control of converter stations, does not require huge AC filters, supports grid black start, and supports power supply to weak networks such as islands / wind farms , is an effective method to solve the problem of China's regional new energy grid connection and consumption. At present, the modular multilevel converter is one of the main methods to realize the voltage source DC transmission system, but the control and protection technology of the modular multilevel converter is complicated, especially the pulse modulation technology of its valve sub-module. [0003] Current...

AI Technical Summary

Problems solved by technology

The first one is carrier cascaded modulation. Due to its complex capacitor voltage sorting algorithm and high switching frequency, it is usually not suitable for flexible high-voltage direct current transmission fields with a large number of valve modules.

The second is carrier phase-shift modulation. Its main advantage is that the switching frequency is fixed, but when the capacitor voltage deviation is large, the balance speed is slow; and a balance controller needs to be designed for each sub-module, resulting in a large amount of balance strategy data calculation. and the hardware is difficult to implement

These two errors will lead to a large difference between the output voltage of the bridge arm of the converter and the command voltage, and eventually cause output harmonics or even slight oscillations

On the other hand, in order to reduce the loss of the converter, it is hoped to reduce the switching frequency as much as possible, and it is necessary to further relax the difference between the capacitor voltages of each sub-module, which further exacerbates the deviation between the command voltage and the actual output voltage

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