A Low-order Circulating Current Suppression Method for Modular Multilevel Converter

Abstract

The invention discloses a method for suppressing low-order circulating currents of a modular multilevel converter. The steps are: according to the capacitor voltage value u of the upper and lower bridge arm sub-modules of a certain phase collected at the k-1 and k-2 sampling times dcui , u dcli , to obtain the estimated value of the sub-module capacitor voltage for calculation at the kth moment; calculate the estimated value of the common-mode component and differential-mode component of the sub-module capacitor voltage of the phase, and calculate the common-mode component with the total DC voltage control capability of the phase Modulation ratio m' cm ; The phase common-mode modulation ratio m′ with low secondary circulation suppression ability cm_C , the compensated common-mode modulation ratio m′ cm_C and differential mode modulation ratio m dm The difference is taken as the actual modulation signal of the upper bridge arm of the phase; the compensated common-mode modulation ratio m' cm_C and differential mode modulation ratio m dm The sum value of is used as the actual modulation signal of the lower bridge arm of this phase. The invention realizes the suppression of the wide-band circulating current ripple, is easy to realize, has clear physical concepts, and has high engineering application value.

Description

technical field [0001] The invention belongs to the technical field of application of high-voltage and high-power power electronic technology in power systems, and relates to a method for suppressing low-order circulating currents of modular multilevel converters. Background technique [0002] Modular multilevel converter (MMC) is gradually becoming a popular topology for higher power applications due to its special design of cascaded half-bridges that do not require separate half-bridge DC power supplies. They can therefore operate at high voltage levels without losing properties such as modularity, redundancy, and switching frequency harmonic cancellation. Therefore, although MMC was first proposed to be applied to high-voltage direct current transmission systems, it is not suitable for these insulated gate bipolar transistors such as static synchronous compensators (STATCOM), active filters (APF), high-voltage frequency converters, and micro-grid energy storage systems. ...

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Problems solved by technology

However, the design of the resonant filter circuit increases the cost and power consumption; CN104333032A superimposes the double-frequency circulating current component on the basis of the fundamental wave current and the DC component of the converter valve, and the device loss is relatively large; CN104393745A estimates the sub Module voltage, using the modulation method to obtain the number of sub-modules expected to be input in each bridge arm; although it can ensure that the capacitor voltage of the sub-modules in the same bridge arm remains balanced, it does not consider the energy balance between the upper and lower bridge arms

If the uneven distribution of energy occurs between the upper and lower bridge arms of the MMC, it will also have an adverse effect on the stable operation of the MMC and reduce its work performance; the unbalanced voltage reference value used for circulating current suppression by this method in CN105119509A is obtained by direct calculation , which can simultaneously suppress the positive sequence, negative sequence and zero sequence circulating current components under the operating state of the asymmetrical grid. Different negative-sequence control targets, this method has fast dynamic response and a small amount of control communication data, but requires accurate system component parameters, and the real-time solution to the dynamic equation of the converter is relatively cumbersome

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