LLC Resonant PV Inverter With High-Frequency Grid Isolation
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Solution Overview
Problem
Existing grid-connected inverter systems face challenges with high cost, size, weight, and efficiency due to the use of line frequency transformers, and conventional DC-DC converters suffer from narrow zero-voltage switching (ZVS) ranges and complex control algorithms, limiting power density and reliability in high power renewable energy applications.
Innovation Solution
A three-phase power conversion system using LLC resonant converters with high-frequency isolation transformers and unfolding inverters, employing phase-shift and frequency control to generate rectified sine waves, eliminating the need for low-frequency transformers and enhancing ZVS regions for improved efficiency and power density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If line frequency transformers are used in grid-connected voltage source inverters, then voltage matching and isolation requirements are fulfilled, but cost, weight and size increase while efficiency decreases
Solution Approach 1:
The patent transforms the transformer operating frequency from line frequency (50/60 Hz) to high frequency (20-100 kHz), fundamentally changing the operational parameters to achieve both isolation requirements and improved efficiency. The high frequency operation enables smaller transformer size and reduced core losses while maintaining galvanic isolation.
Solution Approach 2:
The patent replaces the conventional line frequency transformer with a high frequency transformer integrated into a DC-DC converter stage, substituting the traditional isolation approach with a high-frequency magnetic coupling system that achieves the same isolation function with superior efficiency and reduced size.
2Reliability
If conventional phase-shift full-bridge converters are used, then galvanic isolation and zero voltage-switching are provided, but narrow ZVS range and higher circulating current limit power density improvements
Solution Approach 1:
The patent introduces dynamic frequency modulation capability to the DC-DC converter, allowing the switching frequency to vary dynamically to maintain optimal ZVS conditions across different load ranges. This dynamic adjustment extends the effective ZVS range beyond what fixed-frequency converters can achieve, thereby improving power density.
Solution Approach 2:
The patent implements preliminary action by ensuring zero-voltage-switching conditions are established before actual power switching occurs through resonant current circulation. The LLC resonant circuit pre-charges or discharges the switch nodes to zero voltage before main power switches operate, eliminating switching losses and enabling higher frequency operation.
3Power
If dual active bridge converters are used, then higher power capability and soft-switching features are achieved, but higher circulating currents and narrower ZVS region result
Solution Approach 1:
The patent converts the potentially harmful circulating current into a beneficial resonant current that enables soft-switching. The LLC resonant circuit deliberately circulates current at resonant frequency, which naturally zeros the voltage across switches before they switch, transforming what would be wasted circulating current into a useful mechanism for achieving lossless switching.
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 system achieves higher power density, efficiency, and reduced size with lower switching losses, transformer core losses, and cost, while providing grid isolation and lower line harmonics, suitable for medium voltage PV systems.
Implementation Method 1
LLC resonant converters have been designed and used for different applications... They provide natural ZVS for primary side switches and zero-current-switching (ZCS) features
Implementation Method 2
high frequency transformers (HFTs) embedded in DC-DC converters are used instead of the LFTs... providing the same advantages as the LFT, it also decreases the size and weight of the converter
Data Source
AI summary
A power conversion system comprising, three full wave LLC resonant converters each of which has an associated high frequency isolation transformer, a full wave rectifier and an unfolding inverter, to provide a direct connection to a Medium Voltage (MV) three-phase grid for a high power photovoltaic system.