Capacitor Degradation Detection in Modular Multilevel Converter Cells
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Solution Overview
Problem
In power converters, especially modular multilevel converters, the aging of electrolytic capacitors leads to capacitance drop and increased equivalent series resistance, making health monitoring crucial for maintaining high reliability, but existing methods require offline measurements and are sensitive to noise.
Innovation Solution
A method and arrangement that determine cell capacitor degradation by measuring the time difference between a starting and ending voltage during online operation, using a bypassed converter cell, which allows for simple and reliable capacitance assessment without needing current or voltage integration, thereby reducing noise sensitivity and allowing operation during measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitance measurement is performed using integration of detected branch currents and capacitor voltage, then capacitance can be determined, but the method is sensitive to noise and requires high measurement precision
Solution Approach 1:
The patent changes the measurement parameter from small voltage variations on top of high dc-link voltage to large voltage differences (from high dc-link voltage to near-zero voltage). This parameter transformation fundamentally improves measurement resolution and reduces noise sensitivity, as A/D converter resolution is no longer limited by the small ripple magnitude relative to the high operating voltage.
Solution Approach 2:
The patent employs periodic switching of the cell capacitor through the converter cell's switching modules, creating a controlled discharge cycle. The capacitor is charged during normal operation and then discharged through a defined path for measurement purposes. This periodic action enables capacitance determination during online operation while isolating the measurement from continuous noise sources.
2Measurement precision
If offline capacitance measurement is performed, then measurement accuracy can be improved, but the power converter must be stopped
Solution Approach 1:
The patent enables capacitance measurement to be performed during online operation by utilizing the converter cell's existing switching modules and capacitor discharge paths. The measurement is integrated into the normal operation cycle, allowing the power converter to remain productive while health monitoring is performed. The cell capacitor is periodically discharged through controlled switching actions without requiring system shutdown.
3Measurement precision
If small voltage variations are measured on top of high dc-link voltage, then capacitance can be determined, but measurement resolution is limited by A/D converter precision
Solution Approach 1:
The patent transforms the measurement parameter from small voltage variations (ripples) on top of high dc-link voltage to large voltage differences (from high dc-link voltage level down to near-zero voltage during discharge). This parameter transformation fundamentally improves measurement resolution, as the voltage swing used for measurement is relatively large (in the order of 100 V), greatly enhancing sensitivity and reducing the impact of A/D converter resolution limitations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables reliable online determination of capacitor degradation, reducing measurement resolution issues and allowing continuous operation of power converters by using large voltage differences, enhancing sensitivity and reducing noise impact.
Implementation Method 1
determining a value of a capacitor degradation parameter based on a measurement parameter value associated with discharging of the cell capacitor, forming a dc-link, from a measurement start voltage level at a measurement start time to a measurement end voltage level at a measurement end time
Data Source
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AI summary
A method and arrangement for determining a capacitor degradation parameter associated with the degree of degradation of a cell capacitor in a converter cell of a power converter comprising a plurality of identical converter cells, including redundant converter cells, which converter cell, prior to determining the capacitor degradation parameter, has been bypassed. The method comprises a) obtaining a measurement parameter value associated with discharging of the cell capacitor, forming a dc-link, from a measurement start voltage level at a measurement start time to a measurement end voltage level at a measurement end time, and b) determining a value of the capacitor degradation parameter based on the measurement parameter value.