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A Design and Control Method for Reducing the Capacitance of a Full-Bridge MMC Sub-module
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A control method and sub-module technology are applied in electrical components, output power conversion devices, and AC power input is converted into DC power output, etc. It can reduce the fluctuation rate of capacitor voltage, reduce the fluctuation rate of capacitor voltage, and reduce the capacitor value.
Active Publication Date: 2019-06-07
HUAZHONG UNIV OF SCI & TECH
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[0006] In view of the above defects or improvement needs of the prior art, the present invention provides a design and control method for reducing the capacitance of the full-bridge MMC sub-module, thus solving the problems of high construction cost and operating loss in the existing full-bridge MMC application technology Large, and it is difficult to realize the technical problem of reducing the capacitor voltage fluctuation without increasing the current flow of the switching device
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Embodiment 1
[0032] Embodiment 1 of the present invention is used to illustrate that while keeping the transmission capacity of the converter and the voltage of the DC side unchanged, the phase voltage of the AC side is increased so that the modulation coefficient is m 2 =1.414, the superiority in reducing the voltage fluctuation rate of the sub-module capacitor and reducing the demand for the sub-module capacitor.
[0033] For a clearer description, the following analysis is carried out:
[0034] Assuming that the converter works in the rectification state, and the harmonic circulating current is effectively suppressed, taking the upper bridge arm of phase a as an example, the voltage and current of the bridge arm can be obtained as:
[0035]
[0036] Among them, v pa , i pa is the voltage and current of the upper bridge arm of phase a, U dc is the rated voltage of DC side, i dc , i a are the instantaneous values of the DC side current and the AC side phase current respectively,...
Embodiment 2
[0058] This example is used to illustrate how to make m 2 =1.414 The scheme combined with double-frequency circulating current optimization control has advantages in reducing the voltage fluctuation rate of sub-module capacitors and reducing the demand for sub-module capacitors.
[0059] For a clearer description, the following analysis is carried out:
[0060] Step 1: Calculate the minimum capacitor voltage fluctuation rate in different circulating current injection schemes under each power factor angle, and select the maximum value of the fluctuation rate as the final control target ε target ;Double frequency voltage 0≤U in different circulation injection schemes h ≤0.5U dc , the phase angle 0≤α≤2π.
[0061] Step 2: According to the control target ε in the first step target , calculate all the volatility satisfying 0.98ε target ≤ε≤1.02ε target The double frequency voltage U h and phase angle α;
[0062] Step 3: Compute each group obtained in the second step (U h , α...
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Abstract
The invention discloses a design and control method for reducing full-bridge MMC submodule capacitance. The AC side phase voltage peak is increased by using the negative level output capability of thefull-bridge submodule under the steady state under the condition of maintaining the current converter transmission capacity and the DC side voltage to be constant so that the modulation coefficient m2 is enabled to be greater than 1, AC voltage boost operation can be realized and the fluctuation of submodule capacitance voltage can be reduced to a certain extent. The fluctuation ratio of the submodule capacitance voltage in the whole operation area can be further reduced through combination of frequency doubled circulation optimization control on the basis of voltage boost operation so that the submodule capacitance requirement can be effectively reduced and the cost of the submodule can be reduced.
Description
technical field [0001] The invention belongs to the technical field of multi-level power electronic converters, and more specifically relates to a design and control method for reducing the capacitance of a full-bridge MMC sub-module. Background technique [0002] Modular Multilevel Converter (MMC) has become the preferred choice of converters in flexible DC transmission due to its advantages of no dynamic and static voltage equalization problems for switching devices, easy expansion, high system efficiency and good output waveform quality. Preferred topology. [0003] In the existing MMC-based flexible direct current transmission projects, converters based on the half-bridge sub-module topology are used. The half-bridge MMC does not have the ability to defend against DC short-circuit faults, and needs to rely on the AC-side circuit breaker or DC-side circuit breaker to clear the DC-side fault. However, if the AC side circuit breaker is used, the circuit breaker can only c...
Claims
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