3-Level Full Power Converter Control for Fault Ride-Through
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Solution Overview
Problem
Conventional FPC systems face challenges in continuing operation during power system faults due to overcurrent issues and voltage imbalances, particularly in the event of single line-to-ground faults, which require large capacitor capacities and complex control methods, limiting their applicability and efficiency.
Innovation Solution
A 3-level converter system with independent current control for three phases, utilizing a second current controller with proportional gain and a dead time compensation circuit, along with mode command switching to suppress overcurrent and voltage imbalance, allowing for reduced capacitor capacities and stable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional FPC systems use standard 3-level converters during power system faults, then the system structure remains simple, but overcurrent exceeds the arc-extinguishing current capacity of electronic switches
Solution Approach 1:
The converter is divided into multiple independent phase units (three-phase structure), each capable of independent control. During faults, individual phases can be controlled separately to prevent overcurrent propagation, with each phase equipped with its own electronic switches and control circuitry for localized current management
Solution Approach 2:
The converter employs dynamic control of electronic switches with variable switching frequencies and modes. During normal operation, standard switching is used, but during faults the control dynamically adjusts switching patterns to limit current, transitioning between different operational modes to maintain reliability while preventing overcurrent conditions
2Reliability
If capacitor capacities are increased to withstand overcurrent during faults, then operational continuity is improved, but system size and cost increase
Solution Approach 1:
The invention changes the operational parameters of the converter during faults, including switching frequency, voltage levels, and current distribution patterns. By dynamically adjusting these parameters, the system can withstand fault conditions with smaller capacitor capacities than would be required with static operation, reducing both size and cost while maintaining fault tolerance
Solution Approach 2:
Dynamic control strategies adjust the converter's operational state in real-time during faults, optimizing the use of available capacitor energy. The system transitions between different control modes to maximize fault ride-through capability with minimal capacitor sizing, avoiding the need for oversized capacitors that would be required with static control approaches
3Device complexity
If conventional control methods are used during faults, then control simplicity is maintained, but voltage imbalance occurs between positive and negative DC capacitors
Solution Approach 1:
The control system incorporates feedback mechanisms that continuously monitor DC voltage levels and adjust switching patterns accordingly. During faults, the controller uses voltage balance feedback to detect imbalances between positive and negative DC capacitors and automatically adjusts the switching of electronic switches to restore and maintain voltage equilibrium, preventing instability while keeping control complexity manageable
Solution Approach 2:
The control method dynamically changes operational parameters including switching frequency, pulse width modulation duty cycles, and current reference values based on real-time voltage balance conditions. These parameter adjustments are made in response to fault conditions to maintain voltage stability without requiring overly complex control algorithms
4Reliability
If bypass switch circuits are used to withstand overcurrent, then arc-extinguishing capacity is improved, but AC system stability deteriorates due to significant shaking
Solution Approach 1:
The invention extracts the overcurrent protection function from passive bypass circuits and implements it through active control of electronic switches within the converter. By removing the need for external bypass switch circuits and integrating protection capabilities directly into the converter's switching devices, the system achieves overcurrent withstand capability without introducing the harmful AC system disturbances associated with bypass circuit operation
Data Source
AI summary
There is provided an adjustable speed generator motor system with full power converter (FPC system) in which two 3-level converters are connected back-to-back on the DC side, and by providing a mode in which a fly-wheeling diode is forced to conduct a current independently for three phases according to the current polarity and a mode in which the fly-wheeling diode is forced to conduct a current independently for three phases according to magnitude relation of the absolute value of a 3-phase AC current and the current polarity of each phase the FPC system is capable of continuing stable operation.