Modular Multilevel Converter Input Voltage Control for Low Frequency Operation
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Solution Overview
Problem
Modular multilevel power converters face challenges in reducing capacitor and semiconductor demands, associated costs, and simplifying control and regulation at low frequencies, leading to restricted operating ranges and increased losses, especially in applications requiring continuous operation or constant torque over a wide speed range.
Innovation Solution
The method involves controlling the input voltage as a function of angular frequency to reduce capacitor expenses and influence energy input into sub-module capacitors, allowing for variable DC voltage and optional modulation of common mode voltage, using techniques like Pulse Width Modulation and adjustable transformers to optimize energy distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the operating frequency of the converter is limited to a minimum frequency, then the energy input into sub-module capacitors is restricted, but continuous operation at low frequencies and soft start are not possible
Solution Approach 1:
The patent applies dynamics by making the converter capable of operating at variable frequencies including zero frequency, transitioning from a static frequency limitation to a dynamic adaptive system. The control system dynamically adjusts switching patterns to maintain stable operation across the entire frequency range from 0 Hz to nominal frequency, enabling soft start and continuous low-frequency operation while managing capacitor energy input through adaptive switching strategies
Solution Approach 2:
The patent changes the operating parameters by allowing frequency to vary continuously from 0 Hz rather than being constrained to a minimum frequency. This parameter change enables the converter to adapt to different operating conditions,实现 soft start by gradually increasing frequency from zero, and maintains stable operation at low frequencies through modified switching patterns that account for the changed frequency parameter
2Use of energy by moving object
If the amplitude of load current is matched as a function of frequency to restrict energy input, then the power of the M2C is reduced
Solution Approach 1:
The patent applies segmentation by separating the control of load current amplitude from the frequency matching constraint. Instead of reducing power by limiting current amplitude based on frequency, the system segments the power delivery into controlled intervals where full power can be delivered when needed while using adjusted switching patterns to manage capacitor energy input during low-frequency operation, maintaining both power capability and energy control
Solution Approach 2:
The patent uses dynamics to allow the load current amplitude to vary independently based on actual power demands rather than being statically matched to frequency. The system dynamically adjusts switching patterns and duty cycles to deliver required power while managing capacitor energy input through time-varying control strategies that adapt to instantaneous operating conditions rather than imposing fixed current-frequency relationships
3Use of energy by moving object
If additional current components are introduced to influence energy input into capacitors, then switching and conduction losses increase
Solution Approach 1:
The patent extracts and eliminates the harmful additional current components (circulating currents) that cause increased switching and conduction losses. By redesigning the switching patterns and control strategy to operate without these parasitic current components, the system achieves stable low-frequency operation and soft start capability while minimizing energy losses in switches and conductors
Solution Approach 2:
The patent converts the challenge of managing capacitor energy input at low frequencies into a benefit by developing switching patterns that naturally avoid generating harmful circulating currents. The control strategy transforms what would be a loss-inducing situation into an efficient operating mode where energy is delivered effectively without the penalty of additional switching and conduction losses
Data Source
AI summary
In a method for generating with a modular multilevel power converter (M2C) having a plurality of sub-modules a frequency-variable output voltage, the sub-modules are switched on and off to generate from an input voltage discrete voltage steps approximating an approximately sinusoidal alternating output voltage having a first angular frequency located between a zero frequency and a second angular frequency, and the input voltage is controlled or regulated as a function of the first angular frequency so as to be located between a lower angular frequency, which is equal to or greater than the zero frequency, and a third angular frequency, such that the input voltage increases with increasing first angular frequency, thereby reducing capacitor complexity in the power converter.