To address the issue of active power transfer limits for new energy units in weak grids, current research lacks analytical methods that consider multiple electrical constraints of new energy systems, including constraints such as maximum steady-state operating current and the voltage variation range at the grid connection point. To this end, the static voltage feasible region of new energy converters under multiple electrical constraints based on electrical isoclines is characterized, and analytical expressions for the power transfer limit under these constraints are derived accordingly. Then, the mechanism of active power transfer limitation for new energy units in weak power grids is analyzed, and the impact of various parameters and the static characteristics of Q-V control on active power transfer is studied. Next, to satisfy the requirement of operate delivering power in weak girds within the rated range, the minimum parameter requirements to ensure normal operation of converters are investigated, and the boundary range of the Q-V droop coefficient that satisfies multiple electrical constraints is derived. Finally, the proposed theoretical method is comprehensively validated through PSCAD/EMTDC simulations under an ultra-weak power grid with a short-circuit ratio of 1.05.