Decoupled AC/AC MMC Modulation for Asynchronous Networks
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Solution Overview
Problem
In medium voltage direct AC/AC Modular Multilevel Converters (MMC), harmonic coupling occurs between three-phase and single-phase networks due to frequency ratio asymmetry, making it complex to design sophisticated modulators like optimized pulse patterns (OPPs), especially in asynchronous operations where phase-shifts are involved.
Innovation Solution
A decoupled modulation concept is applied, where each converter side is modulated independently, with results mapped into branch-level command signals using a sorting and selection algorithm for capacitor voltage balancing, and optionally incorporating circulating current control for stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a modulator is used on the level of each converter branch to control the AC/AC MMC, then the converter can operate with low cell numbers and low switching frequencies, but an inherent harmonic coupling occurs between the two AC networks due to lack of symmetry in the modulation patterns
Solution Approach 1:
The patent segments the modulation process into two independent parts: one for the three-phase AC network and another for the single-phase AC network. Each side is modulated separately using its own modulation references, and the results are combined through a mapping function at the branch level. This segmentation eliminates the harmonic coupling that occurs when a single modulator tries to handle both networks simultaneously, as each independent modulation process maintains its own symmetry properties.
Solution Approach 2:
The patent introduces a mapping function as an intermediary between the independent modulations of the two AC networks and the branch-level command signals. This mapping function takes the modulation results from both three-phase and single-phase sides and combines them appropriately to generate the final switching commands for each converter branch, ensuring that the benefits of independent modulation are realized while maintaining proper coordination between the two networks.
2Object-generated harmful factors
If sophisticated modulators such as optimized pulse patterns (OPPs) are applied to handle frequency ratios, then harmonic coupling can be reduced, but the design complexity becomes unrealistically high especially in asynchronous network operation
Solution Approach 1:
Instead of designing a single complex modulator that handles both frequency ratios and phase shifts, the patent segments the problem into two simpler independent modulation tasks. Each AC network side has its own modulation process that can use standard techniques, avoiding the need for sophisticated OPPs that would be required to handle the combined complexity of asynchronous operation with frequency ratios.
Solution Approach 2:
The decoupled modulation approach creates a universal framework that works for both synchronous and asynchronous network operations. The same independent modulation structure can handle different frequency ratios and phase shifts without requiring redesign, making the system universally applicable to various operating conditions without the need for complex adaptive modulators.
3Adaptability or versatility
If a single-phase network is connected to a three-phase utility grid with different frequencies, then railway supply requirements can be met, but the frequency ratio asymmetry causes harmonic coupling between the networks
Solution Approach 1:
The patent segments the frequency handling into independent modulation processes for each network side. The three-phase side operates at its utility frequency while the single-phase side operates at the railway frequency, with each having its own modulation references. This segmentation allows the system to connect networks with different frequencies without the harmonic coupling that would result from a unified modulation approach trying to accommodate both frequencies simultaneously.
Data Source
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AI summary
The present disclosure relates to a method of decoupled modulation of a direct AC/AC MMC 1 between a first AC network L having a first waveform and a second AC network R having a second waveform, the MMC having a double-star topology with a plurality of phase legs 11, each phase leg having a first branch 12a and a second branch 12b, each of the first and second branches comprising a plurality of series connected bipolar cells 13. The method comprises performing a first modulation based on a reference signal of the first AC network, independently of a reference signal of the second AC network, to generate, for each phase leg, a first integer command signal corresponding to a first combination of cell states in the first and second branches of the phase leg needed for generating the first waveform. The method also comprises performing a second modulation based on the reference signal of the second AC network, independently of the reference signal of the first AC network to generate,for each phase leg, a second integer command signal corresponding to a second combination of cell states in the first and second branches of the phase leg needed for generating the second waveform. The method also comprises, based on the first and second integer command signals, mapping to each branch a number of cell states to be used for concurrently generating both the first and second waveforms, generating branch-level command signals to a capacitor voltage balancing algorithm. The method also comprises, based on the mapping and the balancing algorithm, sending firing signals to the plurality of cells of each branch.