Modular Multilevel Converter Insertion Index Calculation
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Solution Overview
Problem
Modular multilevel converters experience circulation currents between DC terminals due to unbalanced voltage distribution across submodules, leading to increased peak currents and losses.
Innovation Solution
A method for calculating insertion indices for a DC to AC modular multilevel converter that adjusts the number of submodules in voltage insert mode based on real-time calculations of desired and actual arm voltages, using feedback control and open-loop approaches to control total energy and energy balance between arms, thereby reducing harmonic currents and stabilizing capacitor voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the number of inserted submodules is changed to equalize capacitor voltages, then voltage balance is improved, but circulation currents increase between DC terminals
Solution Approach 1:
The patent implements a feedback control mechanism where the modulator continuously monitors the actual arm voltages and compares them with desired values. Based on this feedback, the modulator dynamically adjusts the insertion indices to equalize capacitor voltages while simultaneously controlling circulation currents. The feedback loop ensures that voltage balance is maintained without allowing circulation currents to increase excessively.
Solution Approach 2:
The patent changes the control parameters by introducing a new control variable that directly regulates circulation currents in addition to the traditional capacitor voltage balancing control. By adjusting this parameter, the system can modify the insertion strategy to reduce circulation currents while still achieving voltage equalization across submodules.
2Loss of energy
If circulation currents are reduced through control mechanisms, then losses are minimized, but control complexity increases
Solution Approach 1:
The patent merges the circulation current control function with the existing capacitor voltage balancing control into a unified modulator. Instead of adding a separate complex control system, the patent integrates both control objectives into a single modulation algorithm that calculates insertion indices considering both voltage balance and circulation current reduction simultaneously.
Solution Approach 2:
The control mechanism uses the existing arm voltage measurements and submodule state information to automatically adjust insertion indices for reducing circulation currents. The system serves itself by utilizing already-available data from the converter operation without requiring additional sensors or external control inputs.
Data Source
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AI summary
It is presented a method for calculating insertion indices for a phase leg of a DC to AC modular multilevel converter. Each phase leg comprises two serially connected arms, wherein each arm comprises a number of submodules, wherein each submodule can be in a bypass state or a voltage insert mode. The insertion index comprises data representing the portion of available submodules that should be in the voltage insert mode. The method comprises the steps of : calculating a desired arm voltage for an upper arm connected to the upper DC source common bar and a lower arm connected to the lower DC source common bar, obtaining values representing actual total arm voltages in the upper arm and lower arm, respectively, and calculating modulation indices for the upper and lower arm, respectively, using the respective desired arm voltage and the respective value representing the total actual arm voltage. A corresponding apparatus is also presented.