A Fault Current Suppression Method of MMC AC Side Based on Virtual Impedance

Abstract

The invention relates to a virtual impedance-based AC side fault current suppression method for an MMC, and belongs to the technical field of power transmission and distribution. An AC side fault of the MMC causes an over-current of a switching device. On the basis of an existing control strategy, a virtual impedance shunt circuit in a primary system is mapped into a controller through a feedback function; and an impedance of a secondary side valve outlet circuit of a transformer is equivalently increased, so that the transient increase rate of a fault current when an AC side fault of the MMC appears is effectively suppressed; and the increase rates of other types of fault currents are not increased. According to the core technical scheme, the method comprises the following steps: firstly, measuring three-phase voltage and current, a reactive power and a DC side current of a PCC point of an MMC system in real time; secondly, supposing that a virtual impedance circuit is connected to the secondary side valve outlet circuit of the transformer in series, carrying out reactive power deviation calculation on D and Q-axis currents through the virtual impedance shunt circuit; and finally adding related deviation correction to a measurement value of reactive control quantity of a DQ decoupling controller for correction.

Description

technical field [0001] The invention belongs to the technical field of power transmission and distribution, and in particular relates to a virtual impedance-based MMC AC side fault current suppression method. Background technique [0002] Modular Multilevel Converter (MMC) has the advantages of high power transmission capacity, no need for a large number of IGBTs to be crimped and connected in series, low voltage change rate of devices, and good harmonic characteristics, and has gradually become the preferred choice for flexible DC transmission. development trend. [0003] Faults on the AC side of the MMC (AC system side, secondary side of the transformer) will lead to an increase in the AC current of the converter, charging and discharging of the sub-modules, and then cause overcurrent of the switching device. Under fault conditions, the instantaneous current flowing through the device before the latching action of the switching device directly affects the selection of the...

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Problems solved by technology

[0004] Aiming at the fault current suppression problem of the AC side fault of the MMC mentioned in the above-mentioned background technology, the present invention adopts a virtual impedance control strategy, based on the existing DQ decoupling control strategy, by adding a reactive power feedback link on the Q axis, the system transfer function Make corrections, map the feedback control to the primary system to form a virtual impedance parallel circuit, and equivalently increase the impedance of the valve outlet circuit on the secondary side of the transformer, thereby effectively suppressing the instantaneous rise rate of the AC current and the bridge arm current when the MMC AC side fails

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