Voltage Source Converter Commutation Control
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Solution Overview
Problem
Voltage transients occur during current commutation in voltage source converters due to differences in magnitude and direction between AC and DC side currents, leading to potential damage and electromagnetic interference, and existing solutions require large commutation capacitors, increasing cost, size, and weight.
Innovation Solution
A voltage source converter with a controller that synthesizes a driving commutation voltage to minimize differences between AC and DC side currents, using sub-converters in series or parallel with the H-bridge to manage current switching, and forming AC or DC crowbars to prevent voltage transients, thereby eliminating the need for large capacitors and enhancing reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If large commutation capacitors are used to prevent voltage transients during current commutation, then reliability is improved, but cost, size, and weight increase
Solution Approach 1:
The patent extracts the commutation function from the traditional capacitor-based approach and implements it through a controller that synthesizes commutation voltage using switching elements. This removes the need for large commutation capacitors while maintaining commutation reliability through active voltage control during the commutation process.
Solution Approach 2:
The patent replaces the passive capacitor-based commutation system with an active electronic control system using switching elements (IGBTs, MOSFETs, or IGCTs) and a controller. This substitution allows for precise control of commutation voltage and current, eliminating the need for bulky capacitors while improving reliability through adaptive control.
2Reliability
If large commutation capacitors are used to prevent voltage transients during current commutation, then reliability is improved, but device complexity increases
Solution Approach 1:
The switching elements in the H-bridge configuration serve multiple functions: power conversion during normal operation and commutation control during switching transitions. This multi-functionality eliminates the need for separate commutation capacitors and associated circuitry, reducing device complexity while maintaining reliability.
Solution Approach 2:
The controller uses the existing switching elements and circuit topology to generate the commutation voltage needed for safe current switching. The system serves itself by utilizing its own components (switching elements, H-bridge structure) to perform the commutation function that would traditionally require external capacitors, thereby simplifying the overall device architecture.
3Speed
If current commutation is performed without minimizing differences between AC and DC side currents, then switching speed is maintained, but voltage transients occur causing electromagnetic interference
Solution Approach 1:
The controller synthesizes the commutation voltage in advance and applies it during the commutation process to minimize current differences between AC and DC sides before switching occurs. This preliminary action prevents voltage transients and electromagnetic interference while maintaining fast commutation speed through proactive control.
Solution Approach 2:
The controller monitors the currents on both AC and DC sides and dynamically adjusts the commutation voltage synthesis to minimize differences between them. This feedback control ensures that commutation occurs with minimal current mismatch, preventing voltage transients and electromagnetic interference while maintaining high switching speed.
Data Source
Figure 1a
Figure 1b~1c
Figure 2~3
AI summary
A voltage source converter (30) comprises first and second DC terminals (32,34) for connection to a DC network (58). The voltage source converter (30) further includes at least one limb connected between the first and second DC terminals (32,34). The or each limb includes: a phase element (36) including two parallel-connected sets of series-connected switching elements (40) connected in an H-bridge to define first and second diagonal switching pairs, a respective junction between each set of series-connected switching elements (36) defining an AC terminal (40) for connection to an AC network (50); and a sub-converter (38,39) configured to be controllable to act as a voltage waveform synthesiser; wherein the voltage source converter (30) further includes a controller (60) to operate the sub-converter (38,39) to selectively synthesise a driving commutation voltage to modify a DC side current at a DC side of the H-bridge so as to minimise any differences in magnitude and direction between the DC side current and an AC side current at an AC side of the H-bridge and thereby carry out commutation of current from one of the first and second diagonal switching pairs to the other of the first and second diagonal switching pairs.