This paper investigates an offshore wind farm based on modular dual-active-bridge (DAB) isolated DC-DC converter. The DAB converter is adopted due to its compact design and the galvanic isolation between its input and output terminals. The synchronous generator in each wind turbine is operated individually with the generator-side rectifier to maximize the power production. The output terminals of multiple DAB converters are connected in series so that the high voltage for DC transmission is achieved and the expensive offshore step-up substation is eliminated. The DC transmission voltage is excluded from the primary side of the DAB transformer by its galvanic isolation. To balance the power sharing among the DAB converters, their input terminals are connected in parallel. In addition, the droop voltage balancing control is presented for the series offshore wind farm. As a result, the input and output voltages of each DAB converter are governed by the same droop characteristic, and the DAB converters' output voltages are kept balanced. The time-domain simulation results show that the DC voltages of each DAB converter and the high voltage direct current (HVDC) transmission system remain stable during the temporal and spatial variations in the wind power generation.