Solid-State Transformer AC/DC Fault Handling
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Solution Overview
Problem
Traditional distribution grids face challenges in handling AC/DC faults and ensuring uninterrupted operation, particularly in multi-voltage-grade and multi-form AC/DC hybrid systems, due to inadequate relay protection and power supply reliability, especially when faults occur in networks connected to any port of a solid-state transformer.
Innovation Solution
A hybrid modular multilevel solid-state transformer with isolated dual-active-bridge converters and a three-phase full-bridge inverter, utilizing half-bridge and full-bridge submodules interconnected via DC capacitors, allows for uninterrupted operation by locking fault ports and maintaining stable voltage and power flow across non-fault ports through advanced control strategies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a traditional distribution grid architecture is used, then the system structure is simple and easy to operate, but the regulation and control ability is insufficient to meet high-precision real-time operation optimization demands
Solution Approach 1:
The distribution grid is segmented into multiple voltage grades (AC and DC) with independent control zones. The solid-state transformer divides the system into primary and secondary sides, allowing localized regulation and control without requiring system-wide complexity. Each segment can be optimized independently while maintaining overall system functionality.
2Reliability
If the number of distributed power sources and connection ports is increased, then the energy balance control and power supply reliability improve, but the AC/DC fault handling ability and system complexity worsen
Solution Approach 1:
The solid-state transformer is equipped with pre-configured fault detection and isolation mechanisms. Protection relays and control circuits are预先 set up to automatically detect faults and isolate affected ports before they can propagate through the system. This preliminary preparation enables rapid fault response without requiring complex real-time decision-making.
Solution Approach 2:
The solid-state transformer acts as an intermediary device between different voltage grades and network types. It provides galvanic isolation and controlled coupling, allowing faults in one network to be contained while maintaining stability in other networks. The modular architecture with isolated ports serves as a buffer that prevents fault propagation.
3Adaptability or versatility
If a solid-state transformer with multiple ports is used to connect AC/DC networks, then the energy balance control capability improves, but the impact of device outage on power supply reliability increases
Solution Approach 1:
The solid-state transformer is divided into independent modular units, each handling specific voltage grades and network connections. When a fault occurs in one module or port, only that specific segment is affected while other modules continue to operate normally. This segmentation ensures that multi-voltage-grade interconnection capabilities are maintained through healthy modules.
Solution Approach 2:
The system is designed to automatically discard (isolate) faulted ports or modules and recover normal operation through the remaining healthy components. The control system redistributes power flow and adjusts operating parameters to maintain energy balance using available functional ports, ensuring continuous power supply reliability.
4Reliability
If traditional relay protection technology is applied to DC distribution grids, then the protection difficulty is high and fault handling ability is insufficient, but adopting converter topologies with DC fault blocking ability increases device complexity
Solution Approach 1:
The solid-state transformer employs a universal converter topology that can handle both AC and DC voltage grades with integrated fault blocking capabilities. The same basic converter structure serves multiple functions: voltage transformation, power conversion, and fault protection across different network types. This multi-functionality reduces the need for separate specialized equipment for AC and DC protection.
Data Source
AI summary
Disclosed is a solid-state transformer having an uninterrupted operation ability under an AC/DC fault, in which bridge arms of a hybrid modular multilevel converter include half-bridge submodules and full-bridge submodules. The half-bridge submodules and the full-bridge submodules are connected with input ends of isolated dual-active-bridge converters via DC capacitors of the half-bridge submodules and full-bridge submodules; output ends of the isolated dual-active-bridge converters are connected in parallel to form a low-voltage DC bus; and a three-phase full-bridge inverter is connected to the low-voltage DC bus. The solid-state transformer may be provided with four ports including a medium-voltage AC port, a medium-voltage DC port, a low-voltage DC port and a low-voltage AC port which are beneficial to the interconnection of multi-voltage-level and multi-form AC/DC hybrid distribution grids. The solid-state transformer has the uninterrupted operation ability under the AC/DC fault.