# Modular Combined DC-DC Autotransformer for Offshore Wind Power Integration with DC Collection
## 1 Introduction  
Offshore wind energy has drawn wide interest due to its distinctive advantages, such as rich wind resources, high utilization hours, and no need to occupy land resources [1]. Recently, China has proposed to build new power systems dominated by renewable energy, which facilitates the rapid development of offshore wind farms (OWFs). The high voltage direct current (HVDC) transmission system is preferred to the traditional AC transmission system for the grid integration of large-scale OWFs. In the conventional HVDC transmission system for OWFs, wind turbines (WTs) are integrated together with an AC collection grid and then connected to an offshore substation containing a step-up transformer and a rectifier [2]. However, OWFs based on AC collection grids need lots of AC cables, bulky step-up transformers, and large expensive offshore platforms. To address this problem, OWFs based on DC collection grids are researched broadly [3, 4]. In DC-collection-grids OWFs, WTs are interconnected with DC cables and connected to DC-DC converters, and the low DC voltage is raised to the HVDC transmission voltage. Compared with AC-collection-grids OWFs, DC-collection-grids OWFs could reduce the footprint requirement and get the higher power transmission efficiency [5, 6]. According to interconnection schemes of WTs, DC-collection-grids OWFs are classified into DC parallel-connection and DC series-connection schemes [7], as shown in Figure 1. In the series-connection scheme, WTs at two ends have to withstand a high DC voltage. In addition, lots of WTs will be bypassed when unexpected faults happen. If the DC voltage of the OWF falls below the threshold limit, the entire OWF will be shut down undesirably [8]. These defects definitely limit the application of DC series-connection scheme. Therefore, the DC parallel-connection scheme is adopted in this paper. The DC parallel-connection scheme is as shown in Figure 1b. An internal DC-DC converter inside the WT boosts the low DC output voltage of AC-DC converters to the medium voltage (MV) [9]. Several WTs are connected in parallel forming an MVDC grid. Then the MV is stepped up to the high voltage (HV) by a central offshore DC-DC converter. In other words, the DC output voltage of a WT needs two-stage step-up. This two-stage step-up method allows the ratio of DC-DC converters not too large to design, and also enhances the fault-ride-through ability of the wind farm [10]. Conclusively, the DC parallel-connection scheme with the DC voltage two-stage step-up is more suitable for the OWF integration currently. According to the classification criteria of galvanic isolation, DC-DC converters for HVDC are divided into isolated topologies and non-isolated topologies. With the help of the AC transformers, isolated topologies could achieve the requirement of the high stepping ratio and galvanic isolation, but they show characteristics of high cost and large operation losses. Non-isolated topologies require less power semiconductor devices and no need for a transformer or the transformer which does not have to be rated for the full power of the DC-DC converter [11]. Compared with isolated topologies, non-isolated topologies have a smaller footprint and less operation losses and reduce the footprint of an offshore platform which is the major part of the total cost in an OWF [12]. So non-isolated DC-DC converters are preferred to isolated DC-DC converters for OWFs based on HVDC transmission. The primary classification of non-isolated topologies is made based on the presence of transformers, and they are divided into DC autotransformer converters and transformeless converters. [13] proposed a transformeless converter with resonant topologies. Its principle is to use the resonance as a step-up mechanism while achieving the soft switching of semiconductors at the same time. The main disadvantage of these topologies is that it is difficult to balance the sharing of currents or voltages in semiconductor devices and achieve large power transmission.