Active current limiting method applicable to direct current fault of MMC direct current transformer

Abstract

The invention discloses an active current limiting method applicable to a direct current fault of an MMC direct current transformer. Additional bias <delta>u of a direct current component and gain Kuof an alternating current component of a bridge arm voltage are generated based on induction voltage drop <delta>U<dc2> output by a virtual impedance module; when the system is in steady state operation, the output current is constant, wherein <delta>U<dc2> is equal to 0, <delta>u is equal to 0, and Ku is equal to 1; when direct current fault occurs, the output current is increased suddenly, and <delta>U<dc2> is greater than 0; when <delta>U<dc2> exceeds the upper bond of a dead region, <delta>u is increased suddenly from 0 and then is limited to be kept stable until the <delta>u<max> is equalto U<dc2> / 2, so that the direct current bias in the bridge arm voltage is eliminated; meanwhile, by virtue of change of <delta>u, Ku is changed from 1 to 2 / n, so that the alternating current component amplitude value is in the bridge arm voltage is lowered, and the reference value of the bridge arm voltage is adjusted; and by virtue of the effect of a modulation module, the number of sub modulesinput in each phase of MMC in the fault side is reduced from n<sum>=n to n'<sum> belonging to {0, 1}, so that the number of sub modules participated in discharge at the same moment is reduced, the capacitor discharging speed is lowered, and current limiting is achieved.

Description

technical field [0001] The invention relates to an active current limiting method suitable for DC faults of MMC type DC transformers, and belongs to the technical field of modularized multi-level and high-power DC conversion. Background technique [0002] Modular Multilevel Converter (MMC) technology has been successfully commercialized in the field of HVDC transmission in recent years, and MMC-based high-voltage and high-power DC transformers have good performance in future flexible HVDC grids. Application prospects. [0003] Since the flexible high-voltage DC power grid is a "low inertia" system with a small response time constant, the research on the DC fault protection of the MMC DC transformer is one of the technical problems faced by the researchers. Since the bipolar short-circuit fault on the DC side of the MMC is a major problem in the MMC system It is the most serious fault type in the MMC type DC transformer, and the overcurrent level of the sub-module caused by ...

AI Technical Summary

Problems solved by technology

[0004] In order to isolate the bipolar short-circuit fault of the MMC type DC transformer, the MMC converter on the fault side needs to be blocked, and the DC fault needs to be identified before the blocking action is issued. The blocking action generally lags behind the occurrence of the fault, so the rising fault before the blocking action occurs The current increases the current stress of the switching device, which may not meet the requirements of the flexible DC grid for fast current limiting

At present, some scholars have begun to study the active current limiting method that is in advance of fault identification and relies on MMC itself to control automatic current limiting when a fault occurs, such as the document "Active Control of DC Fault Currents in DC Solid-State Transformers During Ride-Through Operation of Multi -Terminal HVDCSystems" realizes DC fault ride-through by setting proportional-integral-differential controller dynamically to adjust the bridge arm voltage, and the control link is cumbersome; the document "MMC AC and DC side fault overcurrent suppression method based on virtual impedance" proposes A current limiting strategy based on virtual impedance is proposed, which equivalently increases the reactance in the discharge circuit and suppresses the overcurrent caused by DC faults, but the current limiting effect of this method is limited

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