In order to pair control variables following the system properties for designing grid-forming wind turbines in the diode rectifier unit(DRU)- high-voltage direct current(HVDC) connected offshore wind farm (OWF) with controlled hydrogen electrolysis and small grid connection onshore, both steady-state and small-signal relationship between voltage at the OWF and power transmitted to DRU-HVDC have to be clearly understood. However, apart from uncontrolled nature of DRU, interface constituting by passive devices between OWF and the offshore platform impact the voltage-power relationships as well. To including physical properties of DRU to the AC circuit analysis, chain rule is implemented to bridge the voltage at point of common connection (PCC) and the OWF, reflecting the voltage divider role of MVAC cables, DRU filters and reactive power compensation devices.The research work reveals large gain between the voltage magnitude of OWF and power transmitted to HVDC link, while adjusting frequency hardly affect the power, which are different from voltage-power relationship in conventional power systems. The rigorous analytical results are verified by PSCAD/EMT simulation.