MMC sub-module capacitor voltage balance control method suitable for carrier phase shift modulation

Abstract

The invention provides an MMC slave module capacitance voltage balance control method suitable for carrier phase shifting modulation. According to the invention, pulses in a carrier period are divided into first half period pulses and second half period pulses based on traditional carrier phase shifting modulation so that a ranking rule of pulse duty ratio in half a carrier period can be obtained. Dynamic ranking is performed on all slave module capacitance voltage in each half carrier period. Pulses in the half carrier periods are reallocated according to the magnitude of the capacitance voltage, so that dynamic balance of slave module capacitance voltage is realized. According to the invention, slave module capacitance voltage balance can be realized without changing prior modulation waves and the modulation effect totally equals to that carrier phase shifting modulation. No external loop current is generated at the same time of realizing the slave module capacitance voltage balance, so that output current quality is ensured.

Description

technical field [0001] The invention relates to the technical fields of flexible direct current transmission and power electronics in a power system, and in particular relates to a method for controlling voltage balance of capacitors of MMC sub-modules. Background technique [0002] Modular multilevel converter (MMC) is widely used in the field of flexible DC transmission. It is a power electronic converter suitable for high-voltage and high-power applications. It has strong scalability and the voltage stress of power switches in sub-modules Small, with more levels, and higher output power quality. Sub-module capacitor voltage balance control is one of the key control technologies of MMC. MMC needs to adopt an appropriate sub-module capacitor voltage balance control method to keep the capacitor voltages of each sub-module consistent and avoid sub-module capacitor voltage divergence. [0003] MMC with fewer modules often adopts carrier phase-shift modulation method, which ca...

AI Technical Summary

Problems solved by technology

The defect of this method is: the correction component distorts the original modulation wave, and produces non-ideal components in the output voltage and current, which affects the quality of the output power of the MMC; higher hardware resource requirements

The defect of this method is that the unequal phase-shifted carrier forms a circulating current in the bridge arm, which will be reflected in the DC side current or AC side current of the MMC, affecting the quality of the MMC output current

[0009] After searching, "Modular multilevel converter capacitor voltage balance control using carrier phase shift technology" (Proceedings of the Chinese Society for Electrical Engineering, Vol. 31, No. 21, 2011), in this document, in order to achieve capacitor voltage balance control, Multiple controllers are designed to modify the modulation waves of the sub-modules; the distortion generated by the modified modulation waves will affect the output of the MMC DC side and AC side, making the current harmonics larger and the circulation increased; each sub-module needs With multiple independent controllers, the algorithm is relatively complex. When the number of MMC sub-modules increases, a large number of real-time calculations are required, which is difficult to implement

"Modular multilevel converter capacitor voltage balance control using carrier phase-shift modulation" (Proceedings of the Chinese Society for Electrical Engineering, Vol. 32, No. 9, 2012), this document introduces a bridge arm The additional reference voltage of the current phase essentially achieves the purpose of equalizing the capacitor voltage by modifying the modulation wave. The distortion generated by the modified modulation wave will affect the output of the MMC DC side and AC side, making the current harmonics larger, increased circulation, which is an inherent problem with this type of approach

"A ControlMethod for Voltage Balancing in Modular Multilevel Converters" (IEEEtransactions on power electronics, vol.29, no.1, 2014), the phase-shift angle of each carrier in the carrier phase-shift modulation used in this document is different, and the A circulating current with a frequency higher than the carrier wave is used to charge and discharge the capacitor to balance the capacitor voltage. The defect of this method is that the high-frequency circulating current in the bridge arm will be reflected in the DC current or AC current of the MMC, making the current harmonic change. large, is an unavoidable problem with this method

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