Multilevel Converter Submodule Voltage Ripple Suppression
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Solution Overview
Problem
Modular multilevel converters experience significant challenges with ripple on sub-module voltages due to mains frequency operation, leading to increased hardware requirements and strain on power semiconductors, as existing methods rely on signal filtering that slows down energy control loops and generates harmful current harmonics.
Innovation Solution
Regulating the state of charge of sub-module energy storage using an actuating signal generated by an energy controller, with control error signal evaluation at a rate less than four times the mains frequency, allowing the energy control to operate without signal filtering and reducing ripple influence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If signal filtering is used to smooth sub-module voltage ripple, then voltage stability is improved, but control loop speed decreases and hardware requirements increase
Solution Approach 1:
The patent applies periodic sampling of sub-module voltages at a frequency lower than the ripple frequency (specifically, sampling at a rate that is less than four times the mains frequency). This periodic sampling approach allows the control system to capture essential voltage information while naturally filtering out high-frequency ripple components, thereby maintaining control loop speed without requiring additional signal filtering hardware.
2Stability of the object's composition
If signal filtering is used to smooth sub-module voltage ripple, then voltage stability is improved, but device complexity and hardware requirements increase
Solution Approach 1:
The patent extracts only the necessary voltage information by sampling at a reduced frequency, effectively removing the need for complex signal filtering hardware. By taking out only the essential control-relevant voltage components and discarding high-frequency ripple information through selective sampling, the system achieves voltage stability without additional filtering components.
3Measurement precision
If sub-module voltages are continuously monitored at high frequency, then control precision is improved, but current harmonics and modulation products increase
Solution Approach 1:
The patent changes the sampling frequency parameter to be lower than the ripple frequency (specifically less than four times the mains frequency). This parameter change transforms the measurement approach from continuous high-frequency monitoring to periodic low-frequency sampling, which maintains sufficient control precision while avoiding the generation of harmful current harmonics and modulation products associated with high-frequency sampling.
Data Source
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AI summary
The invention relates, amongst other things, to a method for operating a converter, in particular a multilevel converter (100) that is connected to a power supply network (1), which operates at a prespecified network frequency, and has at least one series circuit comprising at least two submodules (13) which are connected in series, wherein the submodules (13) of the series circuit each comprise at least two switching elements (S1-S4) and an energy store, in particular capacitor (19). According to the invention, the state of charge of the energy stores of the submodules (13) is controlled, specifically using a control signal (i_Stell_Aus), which is generated by an energy controller (20), as part of the method, by evaluating so as to form control error sampling values (e_Abtast) with a rate which is smaller than four times the network frequency, a control error signal (e), which indicates the deviation of the respective state of charge of the energy stores from a setpoint charging value (u_SM_Soll) or at least is dependent on this, and by generating the control signal (i_Stell_Aus) depending on these control error sampling values (e_Abtast).