How to ensure efficient and stable DC current for green hydrogen
A multi-pulse rectifier is a common choice (Figure 4). Thyristor-based, this rectifier design has high efficiency, is reliable, supports high current densities, and uses low- cost semiconductors. Multi-pulse, thyristor-based converters are an established and well-known technology. The 12-pulse thyristor rectifier shown in Figure 4 consists of a wye-delta- wye power frequency transformer with two low-voltage secondary windings. The secondary windings drive two six-pulse thyristor rectifiers with their outputs connected in parallel. If this rectifier drives an electrolyzer directly, the thyristor firing angle
controls the output voltage and the current flowing into it. The firing angle can also be used to maintain the current in the system as the electrolyzer cell ages, and the voltage required for the cell stack increases. The transformer may also include an on-load tap changer (OLTC). The OLTC changes the transformer turns ratio by switching among multiple access points or taps on one of the windings to raise or lower the voltage supplied to the rectifier. Infineon Technologies offers a broad range of semiconductor component choices to PCS designers. Thyristor rectifiers are commonly used for these AC-
source applications. For example, the T3800N18TOFVTXPSA1 is a discrete thyristor in a chassis mount TO-200AE disc package that is rated to handle 1800 V at 5970 amperes root mean square (A rms ) on-state current. The disc package offers increased power density due to its double-sided cooling design. The basic rectifier design can be improved by adding buck converters as post-rectification choppers at the rectifier output. Adding the chopper stage enhances control of the process by adjusting the chopper's duty cycle rather than the thyristor's firing angle (Figure 5). This reduces the dynamic range required for the thyristor, allowing optimization of the process.
Figure 4: A multi-pulse rectifier based on thyristors has high efficiency, is reliable, supports high current densities, and uses low- cost semiconductors. Shown is a 12-pulse implementation. (Image source: Infineon Technologies)
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