Mixed HB FB Converter Energization via Capacitor Charging
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Solution Overview
Problem
In high-voltage direct current (HVDC) power transmission systems, converters based on a mix of half-bridge and full-bridge submodules face challenges in evenly charging capacitors, leading to insufficient energy storage for starting operations, resulting in low charging levels for HB cells and potentially insufficient for switching devices, especially when using AC side charging.
Innovation Solution
A method involving direct electrical connection between DC terminals and AC power source to charge FB submodules to a higher level, followed by controlled charging of HB submodules to their minimum voltage requirement, ensuring sufficient energy storage for both FB and HB cells, thereby enabling the startup of switching devices and maintaining zero DC voltage during the process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If AC side charging is used for MMC with mix of HB and FB cells, then the charging process is simple and general, but the capacitors of HB cells charge to very low value and switching devices cannot start operating
Solution Approach 1:
The patent applies preliminary action by first establishing a direct electrical connection between DC terminals to pre-charge the FB cell capacitors to a sufficient level before attempting to charge HB cells. This preliminary charging of FB cells creates the necessary voltage conditions to subsequently charge HB cells to adequate levels for switching device operation.
Solution Approach 2:
The patent uses FB cells as an intermediary to charge HB cells. By first charging FB cells through direct DC terminal connection, and then using controlled switching of FB cells to charge HB cells through capacitor coupling, the system overcomes the limitation of direct AC charging that leaves HB cells insufficiently charged.
2Reliability
If switching devices of FB submodules are controlled to charge HB submodules, then HB cells reach minimum voltage for switching, but the process requires coordinated control of multiple switching devices
Solution Approach 1:
The patent applies segmentation by dividing the charging process into distinct phases: first charging FB cells through direct DC terminal connection, then using controlled switching of specific FB cells to charge HB cells. This segmented approach simplifies the control complexity by breaking down the overall charging process into manageable stages with different control requirements.
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
This approach significantly increases the charging level of FB cells to 80-90% and HB cells to 0-20%, ensuring the capacitors are adequately charged to start operating the switching devices, meeting AC and DC voltage requirements, and supports reactive power during the charging phase.
Implementation Method 1
charging capacitors of the FB submodules connected between these two electrical phase lines
Implementation Method 2
charging capacitors of the HB submodules to at least a minimum voltage required to start switching the switching devices of the HB submodules
Data Source
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AI summary
Methods of energizing converters, and converter stations thereof, based on a mix of half-bridge (HB) converter cells and full-bridge (FB) converter cells are provided. According to one embodiment, a method for energizing a converter may include establishing a direct electrical connection between DC terminals of a converter, electrically connecting the converter to an AC power source via AC connectors and obtaining, from the AC power source and for a certain period of time, an AC voltage between two electrical phase lines for charging capacitors of the FB cells connected to the two electrical phase lines. Accordingly, the FB cells will be charged to a higher value and they may be controlled for charging capacitors of the HB submodules to at least a minimum voltage required to start switching switching devices of the HB submodules by drawing power from the AC power source.