Modular Converter Capacitor Selection for Uniform Loading
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Solution Overview
Problem
Existing converter arrangements face challenges in efficiently managing sub-modules and capacitors, particularly when operating with a modulation index greater than 1, leading to uneven electrical and thermal loading, and unipolarly operated capacitors being energetically limited.
Innovation Solution
A method where unipolarly operated capacitors are preferred over bipolarly operated capacitors during specific time ranges, with a selection loop determining which capacitors to switch on or off to maintain uniform sub-module operation, using a sorting list to prioritize capacitor switching based on polarity and voltage, ensuring balanced loading and efficient voltage regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If unipolarly operated capacitors are used in sub-modules, then the converter arrangement can operate with a modulation index greater than 1, but the unipolar capacitors become energetically limited and can only be charged or discharged in short time periods
Solution Approach 1:
The patent applies dynamics by making the capacitor selection adaptive and time-dependent. The control device dynamically selects which capacitors to switch on or off based on the current time range and the sign of the mean current value. This dynamic adaptation allows the system to operate with modulation index greater than 1 while managing the energy limitations of unipolar capacitors through real-time adjustments.
Solution Approach 2:
The patent changes the operational parameters of the capacitors by switching between unipolar and bipolar operation modes. By controlling the sub-module switches, the system can change the voltage polarity across capacitors, allowing unipolar capacitors to be used during specific time periods when the mean current has a different sign, thereby extending their effective operating duration.
2Reliability
If more sub-modules are available than currently required for switching the required voltages, then redundancy and reliability are improved, but selecting and managing the appropriate sub-modules becomes more complex
Solution Approach 1:
The patent implements feedback through the control device that continuously monitors the state of sub-modules and capacitor voltages. The control device uses this feedback information to automatically select which sub-modules and capacitors to activate, managing the complexity of having more available sub-modules than required by making selection decisions based on real-time system state.
Solution Approach 2:
The system performs self-service by automatically managing the selection and coordination of multiple sub-modules. The control device autonomously determines which capacitors to switch on or off based on the predefined control method, eliminating the need for external manual intervention and simplifying the management of redundant sub-modules.
3Adaptability or versatility
If bipolarly operated capacitors are used alongside unipolar capacitors, then operational flexibility is improved, but uneven electrical and thermal loading occurs among sub-modules
Solution Approach 1:
The patent applies local quality by assigning different operational characteristics to different capacitors based on their capabilities. Unipolar capacitors are used during specific time ranges when the mean current has a different sign, while bipolar capacitors are used during other periods. This localized differentiation in usage patterns balances the electrical and thermal loading across all sub-modules.
Solution Approach 2:
The patent implements periodic action by alternating between using unipolar and bipolar capacitors in different time ranges. The control method creates a periodic pattern where unipolar capacitors are activated during periods when the mean current sign changes, and bipolar capacitors are used during other periods, distributing the operational stress evenly across all capacitors.
Data Source
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AI summary
The invention relates, inter alia, to a method for actuating a modular converter assembly (10), for example a modular multilevel converter (MMC), having at least one AC voltage terminal to which or from which an alternating current can be supplied or drawn and at least two DC voltage terminals to which or from which a direct current can be supplied or drawn. The converter assembly comprises at least one series circuit (R1, R2, R3), the external terminals of which form the DC voltage terminals of the converter assembly. The series circuit comprises two sub-circuits which are connected in series and the electric connection points of which form the AC voltage terminal or one of the AC voltage terminals of the converter assembly, and each of the sub-circuits comprises at least two sub-modules connected in series, each said sub-module having at least two switches and a capacitor. In the method, the actuation of the sub-modules, and thus the activation or deactivation of the respective capacitors of the sub-modules, is carried out at least also on the basis of the capacitor voltage of the respective capacitor. According to the invention, at least one capacitor is operated exclusively in a unipolar manner and at least one capacitor is operated in a bipolar manner in at least one of the sub-circuits. As part of the actuation of the converter assembly, the sub-circuit is operated with a modulation index which is greater than one. In a time domain in which the temporal mean value of the current flowing through the sub-circuit has a different sign than the respective actual current flowing through the sub-circuit, the capacitor(s) operating in a unipolar manner is/are preferred over the capacitor(s) operating in a bipolar manner, i.e. the capacitor(s) operating in a unipolar manner is/are preferably reactivated or remain activated. The selection of the capacitors to be activated or deactivated is carried out in a selection loop which is carried out repeatedly.