H. Gao
Washington State University Vancouver,
United States
Keywords: Current-source rectifier, power-to-hydrogen, water electrolysis
Summary:
Renewable power-to-hydrogen (P2H) technology is one of the most promising solutions for fulfilling the increasing global demand for hydrogen and to buffer large-scale, fluctuating renewable energies. The high-power, high-current rectifier plays a vital role in P2H facilities by converting medium-voltage (MV) ac power to a large dc current to supply hydrogen electrolyzers. Though conventional technologies, such as thyristor or diode-based multi-pulse rectifiers, can maturely fulfill the above task, they suffer from several inherent constraints as follows. First, the semiconductor's switching characteristics compromise power quality at the MV ac side, making a passive trap filter or even an active filter inevitable. Second, it is challenging to maintain a power factor close to unity under varying load conditions. Third, they heavily depend on bulky multi-phase line-frequency transformers to provide galvanic isolation and mitigate low-order harmonics. In this study, a paralleled pulse-width modulated current-source rectifier (PWM-CSR) independent of a line-frequency transformer is developed for water electrolysis to overcome the above mentioned constraints. The CSR has been widely applied in MV electric motor drives and is suitable for the dc current regulation, and also features a simple structure, ac-side friendly waveforms, and inherent short-circuit protection. The contribution of the transformerless paralleled PWM-CSR for water electrolysis can be summarized as fourfold. First, the developed paralleled CSR can operate in interleaving mode with power balancing capability to achieve superb power quality with smaller dependence on passive filters. Second, the developed paralleled CSR can enable a unity power factor through a wide range of load conditions. Third, it can totally eliminate the dependence on a multi-phase transformer and realize transformerless operation. Last, it can maintain a high efficiency throughout a wide operating range. To validate the effectiveness of the developed transformerless paralleled CSR, the simulation is conducted on a high-power MV system (5 MW, 6.6 kV) with the rated specification of water electrolysis of 1 kV and 5 kA. Steady state operations under high, medium, and low load conditions are tested. The harmonic distortions and power factor at the MV ac side and the ripples of the dc current are calculated and recorded. The output performance when the load suddenly jumps and drops is presented as well.