## 1 Introduction
A commercial thyristor-based converter can supply 1.5-10 kA dc current with a dc voltage of 1 kV, delivering a maximum of 10 MW power per unit [22]. Nevertheless, the large firing angle operation of thyristors can result in heavy current distortion and low power factor at the ac input. Hence, passive/active power filters and reactive power compensators are mandatory, increasing the system-level costs [16]. Moreover, a significant trend in recent years is to use a buck-type chopper as the rear stage of diode/thyristor rectifier [20; 21; 23; 24; 25; 26; 27; 28]. A commercial chopper rectifier can supply a maximum of 10.4 kA and 1.1 kV at the dc output, delivering 10 MW power to the electrolyzers [28]. This brings better power quality and higher power factor throughout a wide operation range of the converter. Moreover, the emerging active front end (AFE), based on the B6 converter, can also be an attractive option for power-to-hydrogen applications [29]. It brings much fewer input current distortions by fully regulating the input ac current waveform. In this way, the active/passive power filters and the reactive power compensators can be eliminated. Figure 1: The general architecture of renewable hydrogen system, including renewable energy generation, the distribution and conversion of electricity, P2H electrolysis, and the application of renewable hydrogen. In the literature, there are some comparative works available. In [19], four thyristor-based ac/dc high-current rectifier topologies are compared, where the chopper-rectifier and AFE solutions are not covered. Reference [20] reviews the thyristor-phase-controlled rectifiers, IGBT chopper rectifiers, and pulse-width-modulated current source rectifiers for various industry applications requiring kiloampere supplies. It is concluded that the IGBT chopper rectifier has had fulfilled field experience to perform as an alternative for supplying kiloampere currents. Reference [21] overviews the diode- and thyristor-based, multi-pulse rectifiers with an on-load tap changing transformer, as well as the chopper-rectifier topologies. A future research trend on modular converter topologies with medium-frequency transformers (MFT) and high-frequency transformers (HFT) is introduced in [21]. Nevertheless, none of the aforementioned provides an insight into the P2H water electrolysis system. Moreover, the performance matrix of using AFE converters in such P2H water electrolysis systems has not been investigated in prior studies. This paper focuses on the P2H water electrolysis system and serves as the overview and comparison of P2H power electronic converter topologies. This paper first introduces the general requirements of P2H converters from both the electrolyzer side and grid side in Section 2. Then, several power converter topologies used in P2H water electrolysis are introduced and studied from multiple perspectives in Section 3. The comparative conclusions regarding the power quality, efficiency, cost, reliability, and control complexity are drawn in Section 4. The future trend of using the modular multicell converter to eliminate the bulky line frequency transformer (LFT) is discussed in Section 5. Finally, the conclusions are drawn in Section 6.