Two-Stage Solid-State Transformer for Fault-Tolerant Isolation
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Solution Overview
Problem
Conventional solid-state transformers face challenges in achieving high power density for higher voltage classes due to the coupling of AC/DC and isolated DC/DC stages, which require large creepage and insulation distances, and are vulnerable to AC grid faults.
Innovation Solution
A two-stage solid-state transformer design with a first stage operating at a modulation index higher than 1, decoupling the AC/DC and isolated DC/DC stages, reducing insulation requirements and using a medium-frequency transformer for isolation, along with modular multilevel converters and zero-sequence current suppression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If the AC/DC stage and isolated DC/DC stage are coupled together to handle high voltage, then the voltage handling capability is improved, but the creepage distance and insulation distance increase, reducing power density
Solution Approach 1:
The patent divides the high voltage handling function into two separate stages: AC/DC conversion stage and isolated DC/DC conversion stage. Each stage handles a portion of the voltage conversion independently, allowing the insulation requirements to be distributed rather than concentrated in a single transformer, thereby improving power density while maintaining voltage handling capability.
Solution Approach 2:
The patent introduces a DC bus as an intermediary between the AC/DC stage and the isolated DC/DC stage. This DC bus serves as a voltage transfer medium that decouples the two stages, allowing independent optimization of each stage's insulation design and enabling higher power density without compromising voltage handling capability.
2Reliability
If the MFT insulation voltage is increased to handle AC grid faults, then the fault ride-through capability is improved, but the insulation distance increases, reducing power density
Solution Approach 1:
The patent segments the fault handling capability across two stages. The AC/DC stage with modulation index greater than 1 handles AC grid faults by blocking zero-sequence currents, while the isolated DC/DC stage maintains stable DC output. This segmentation allows each stage to be optimized for its specific fault handling requirements without requiring excessive insulation voltage in a single component.
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 design enhances power density and reliability by reducing insulation voltage and handling AC grid faults effectively, while maintaining efficient operation under fault conditions.
Implementation Method 1
The first stage is configured to operate at a modulation index higher than 1 so that the insulation requirement for the second stage is reduced
Implementation Method 2
using a medium-frequency transformer for isolation
Data Source
AI summary
A solid-state transformer is provided. The solid-state transformer includes a first stage, a first DC bus and a second stage. The first stage is configured to receive an AC power and convert the AC power into a first DC power. The first DC bus is electrically connected to the first DC power. The second stage is electrically connected to the first stage through the first DC bus, and is configured to perform isolated step-down DC/DC conversion on the first DC power to generate a second DC power. The first stage is configured to operate at a modulation index higher than 1, and the modulation index is defined by a ratio of twice AC voltage provided by the AC power to voltage across the first DC bus.