Flying Capacitor Inverter Control for FRT Grid Disturbances
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Solution Overview
Problem
Flying capacitor multilevel power converters may suspend operation due to excess voltage or insufficient voltage of the flying capacitor, even with excess current protection circuits, leading to potential grid disturbance issues during FRT requirements.
Innovation Solution
The power converter includes an inverter circuit with a multilevel output unit and polarity switching unit, along with an excess current protection circuit that restricts voltages other than positive, zero, and negative voltages from being output during abnormal conditions, ensuring continuous operation during grid disturbances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a switching element with low withstand voltage is used, then conduction loss and switching loss are reduced, but the element is vulnerable to voltage in excess of its withstand voltage
Solution Approach 1:
Flying capacitors are introduced as intermediary energy storage elements between the DC power supply and the switching elements. These capacitors buffer voltage fluctuations and prevent excess voltage from reaching the low-withstand-voltage switching elements, enabling the use of more efficient low-voltage devices while maintaining reliability
Solution Approach 2:
The patent transforms the voltage parameter distribution in the circuit by using flying capacitors to create multiple voltage levels. This parameter transformation allows switching elements to operate at lower voltage differences, reducing conduction and switching losses while the capacitor voltage monitoring ensures voltage parameters remain within safe limits
2Reliability
If excess current protection circuit is activated during grid disturbance, then excess current is suppressed, but flying capacitor voltage imbalance occurs due to unequal charge/discharge amounts
Solution Approach 1:
The patent implements a feedback mechanism by continuously monitoring the voltage across flying capacitors. When voltage imbalance is detected, the control unit adjusts the switching patterns to restore voltage balance, preventing operation suspension while maintaining current protection capabilities
Solution Approach 2:
The control strategy dynamically adjusts switching patterns based on real-time flying capacitor voltage states. During grid disturbances, the system flexibly modifies switching sequences to prevent both excess current and voltage imbalance, adapting protection mechanisms to current operating conditions
3Reliability
If inverter operation is suspended due to flying capacitor voltage abnormality, then switching element protection is ensured, but FRT requirement cannot be met
Solution Approach 1:
Flying capacitors serve as intermediary energy buffers that decouple the switching elements from direct exposure to voltage abnormalities. This intermediary structure allows the system to maintain operation during grid disturbances by absorbing voltage fluctuations, protecting switching elements without requiring operation suspension
Solution Approach 2:
The patent changes the voltage parameter monitoring approach from fixed threshold suspension to dynamic adjustment of switching patterns. By continuously adapting switching strategies based on flying capacitor voltage levels, the system maintains both element protection and operation continuity during grid disturbances
Data Source
AI summary
An inverter circuit includes a plurality of flying capacitors and converts a DC voltage supplied from a DC power supply into an AC voltage. A filter circuit approximates a waveform of an output voltage of the inverter circuit to a sinusoidal wave. An excess current protection circuit supplies a block signal for turning off a plurality of switching elements to the driving circuit when an excess current is detected. When at least one of an abnormal voltage in any of the plurality of flying capacitors and an abrupt change in an output voltage of the power converter occurs, voltages other than a positive voltage of the DC power supply, a negative voltage of the DC power supply, and a zero voltage are restricted from being output from the inverter circuit.