Series Resonant LC Circuit for Transformer Current Limiting
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Solution Overview
Problem
Conventional 12-step three-phase transformer rectifier power supplies lack current limiting protection, fail to reduce AC harmonic currents effectively, and do not provide unity power factor, requiring separate circuits for protection against failures and short circuits, and experience high EMI due to abrupt current changes.
Innovation Solution
Incorporating a series resonant LC circuit with a nonlinear inductor and capacitor at each transformer phase input to create an adjustable impedance that reduces harmonic currents to below 2% and enhances EMI filtering, achieving near unity power factor and inrush current limiting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional 12-step three-phase transformer rectifier power supply is used, then the power supply can provide DC output voltage, but it lacks current limiting protection and requires separate circuits for protection against failures and short circuits
Solution Approach 1:
The patent combines multiple functions (current limiting, EMI filtering, and power factor correction) into a single series resonant LC circuit connected in series with each phase input. This eliminates the need for separate protection circuits, fuses, and interrupt switches, thereby improving reliability while reducing device complexity.
Solution Approach 2:
The series resonant LC circuit serves multiple functions simultaneously: it limits inrush current, provides overcurrent protection, filters EMI, and corrects power factor. This multi-functionality resolves the contradiction by achieving current limiting protection without adding separate circuits.
2Object-generated harmful factors
If a conventional 12-step transformer rectifier power supply is used, then the power supply can operate, but it produces high AC harmonic currents exceeding 10% which is undesirable
Solution Approach 1:
The patent integrates EMI filtering and harmonic current reduction functions into the series resonant LC circuit. The resonant circuit is tuned to attenuate harmonic frequencies while maintaining low impedance at the fundamental frequency, thereby reducing AC harmonic currents to below 2% without requiring separate filtering circuits.
3Object-affected harmful factors
If a conventional power supply is used, then the power supply can deliver power, but it experiences high EMI due to abrupt current changes
Solution Approach 1:
The series resonant LC circuit combines EMI filtering with current limiting and power factor correction. The resonant circuit attenuates high-frequency EMI noise while allowing fundamental frequency current to pass, thereby reducing EMI without requiring separate filtering circuits.
4Loss of energy
If a conventional power supply is used, then the power supply can operate, but it does not provide unity power factor
Solution Approach 1:
The series resonant LC circuit provides power factor correction by compensating for the reactive power drawn by the rectifier circuit. The capacitor component of the resonant circuit supplies leading reactive power that counteracts the lagging reactive power of the rectifier, achieving near unity power factor without requiring separate correction circuits.
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
The solution effectively reduces harmonic currents to less than 2%, achieves a power factor greater than 0.98, limits inrush currents, and provides short circuit protection, while improving EMI filtering and efficiency, thus enhancing the reliability and performance of the power supply.
Implementation Method 1
a series resonant LC circuit including a series connected passive nonlinear inductor (L) and capacitor element (C)
Implementation Method 2
a series connected passive nonlinear inductor (L) and capacitor element (C) at each transformer phase input
Data Source
AI summary
Circuit and apparatus for improving operating features characteristics of DC power supplies implementing three Phase transformer devices of the type such as 12-step and 24-step transformers. The circuit and apparatus reduces harmonic AC input current while providing almost unity power factor for DC power supply outputs intended for aircraft or marine applications where size and weight are concerns. The circuit includes a passive series connected nonlinear resonant LC circuit connected at each phase of the input to the three phase transformer With the three phase transformer having the added series nonlinear resonant LC circuit, the power supply is enhanced with current limiting for the entire transformer, rectifier and load, due to load shorting, input voltage transients, transformer winding short circuit or rectifier failure. Further, such apparatus provides limiting of power inrush currents during voltage application or turn on, while also providing EMI filtering.