###### Abstract  
A diode-based rectifier (DR) is an attractive transmission technology for offshore wind farms, which reduces the volume of large bulk platforms. A novel parallel-series DC wind farm based on a distributed DR is proposed, which meets the requirements of high voltage and high power with an isolation capability from other units. The coupling mechanism between a modular multilevel converter (MMC) and a DR has been built, and the coordinate control strategy for the whole system has been proposed based on the MMC triple control targets with intermediate variables. Under the proposed control strategy, the system automatically operates at maximum power point tracking (MPPT). The feasibility of topology and the effectiveness of the control strategy are verified under start-up, power fluctuation, onshore alternating current (AC) fault, and direct current (DC) fault based on the power systems computer-aided design (PSCAD)/electromagnetic transients including direct current (EMTDC) simulation. parallel-series; diode rectifier; coordinate control strategy +
Footnote †: journal: Applied sciences  
0000-0001-5000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-0000-00000-0000-0000-00000-0000-00000-00000-00000-00000-00000-00000-  
Several methods exist to build direct current (DC) transmission voltage, including the series-parallel topology, series topology, and parallel topology. An offshore wind power medium-voltage direct-current transmission system based on paralleled DR is proposed in [10] to solve the issues. The parallel ones reduce the offshore platforms, and it is suitable for a small-scale wind farm as the cable current is limited. A series-connected distributed DR is proposed for a higher voltage transmission system in [11; 12], while the alternating current (AC) collection lines still exist. The series-connected topology can transmit larger power but shows less reliability. Once one cable is broken, the whole string will be out. An enhanced series-connected system with fault-resilient capability was proposed previously [13], in which a grouping scheme or extra auxiliary cables will be added. In addition, the top wind turbine unit in the series string will withstand a high DC voltage, thus meeting the insulation challenge and limiting the transmission voltage level. Considering the reliability and insulation, the parallel topology is more suitable for an offshore wind farm. To improve the transmission capacity, an offshore station with a DC/DC converter is needed to increase the voltage level. Several DC/DC converter topologies have been proposed [14; 15]. Refs. [16; 17; 18; 19; 20] propose a DC/DC converter based on a modular multilevel converter (MMC) with galvanic isolation. Refs. [21; 22] propose a non-isolated high-power DC/DC converter with a lower current stress on the medium voltage (MV) side and lower voltage stress for the high voltage (HV) side. Compared with traditional voltage source converter (VSC) platforms, the DC/DC platforms are smaller and reduce the voltage stress of wind turbines, making the parallel scheme more attractive. A higher transformation ratio with a smaller volume is more urgent. The fault isolation and multilevel coordinate control strategy should be analyzed further. This paper aimed to propose a wind farm topology based on parallel-series distributed DRs with higher reliability and a higher transformation ratio. Firstly, the requirements for topology are pointed out, a parallel-series structure with DC fault isolation capability is proposed, and its balancing mechanism is presented in Section 2. Secondly, the mathematical model of DR is built, and a control strategy for converters is proposed to achieve the decoupling control strategy for parallel and series strings in Section 3. Under the proposed control strategy, the wind turbines automatically operate at maximum power point tracking (MPPT) without overvoltage and overcurrent.