Modular Multilevel Converter Energy Reset via Square Wave Circulating Currents
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Solution Overview
Problem
Existing methods for controlling Modular Multilevel Converters (MMC) do not fully optimize energy balance between arms, leading to inefficiencies in loss minimization and capacitor voltage ripple reduction, particularly at low output frequencies and variable loads.
Innovation Solution
A method involving coordinated control of circulating currents in square wave fashion, with clamping of phase output voltages between positive and negative rail potentials, and dynamic adjustment of energy control frequency to minimize RMS current load and capacitor requirements, allowing for precise energy reset and reduced capacitor size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional energy balanced control methods are used with PI or PR regulators, then energy balance between upper and lower arms can be achieved, but the sum of losses is not fully minimized and capacitor voltage ripple is not adequately reduced
Solution Approach 1:
The patent changes the control parameter from conventional PI/PR regulator outputs to directly controlling circulating currents in square wave fashion. This parameter change enables precise energy balance control while minimizing losses, as the square wave circulating current control allows for exact energy transfer between upper and lower arms without the gradual approximation errors inherent in regulator-based approaches.
Solution Approach 2:
The patent implements periodic energy reset within each fundamental period by injecting square wave circulating currents at specific frequencies (second harmonic and higher). This periodic action ensures that energy balance is maintained cyclically, with each period resetting the energy levels to optimal values, thereby continuously minimizing losses and reducing capacitor voltage ripple.
2Manufacturing precision
If circulating currents are controlled to reduce capacitor voltage ripple, then energy balance improves, but RMS current load on switches and capacitors increases
Solution Approach 1:
The patent applies preliminary action by pre-calculating and injecting square wave circulating currents that are synchronized with the voltage waveform. These currents are designed in advance to counteract the voltage ripple before it fully develops, thereby reducing capacitor voltage ripple while minimizing the RMS current load through optimal timing and amplitude selection.
Solution Approach 2:
The patent employs asymmetric square wave circulating current injection where the current amplitude and phase are specifically tailored for each arm (upper and lower) rather than using symmetric sinusoidal currents. This asymmetric approach allows precise control of energy transfer to reduce voltage ripple while optimizing the RMS current characteristics to minimize losses.
3Manufacturing precision
If energy control frequency is increased to improve energy balance, then control precision improves, but device complexity and switching losses increase
Solution Approach 1:
The patent segments the energy control into discrete square wave cycles synchronized with the fundamental period and its harmonics. Rather than continuous high-frequency PWM control, the system divides the control into distinct time segments where circulating currents are applied in square waves at specific phases, thereby achieving precise energy control with reduced computational complexity and lower switching losses.
Data Source
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AI summary
The invention relates to amethod of controlling the energy level of the armsof a power converter, wherein the armscompriseaplurality of switching modules controlled in successive energy control periods by a control signal from a converter controller,wherein the energy levels of the individual arms are controlled in the successive energy control periods according to the following steps: clamping the phase outputvoltage to a determined value having a first voltage potential in a first part of the energycontrol period, clamping the phase outputvoltage to a determined value having a second voltage potential in a second part of the energy control period, wherein the first and second voltage potentials are different, andcontrolling circulating currents in the armscoordinated with the phase output voltage clamping in order to obtain an energy reset within the energy control period.