Establishing maintenance-free wireless sensor networks for online monitoring of power transmission lines is crucial for realizing smart grids, exhibiting necessitating energy harvesters with compact volume, excellent robustness, and efficient, well-regulated output. This study introduces a hybrid magnetic energy harvester (HMEH), seamlessly integrating a magneto-mechanical energy conversion module, a non-contact rotational triboelectric nanogenerator (TENG) module, and an electro-magnetic generator (EMG) module for magnetic energy harvesting and self-powered sensing in power transmission lines. The HMEH converts magnetic energy into synchronized mechanical rotation using the magnetic phase difference principle and smoothly transforms it into a regulated electrical energy output. With a compact size of 5 cm × 5 cm × 3 cm and a light weight of 56 g, the HMEH showcases unprecedented volume and weight output power densities of 48.9 μW cm−3 and 65.5 μW g−1, respectively, outperforming common current transformer (CT) energy harvesters in power transmission lines while avoiding issues like magnetic core saturation, leading to self-overheating or vulnerability to load breakdown due to current fluctuations. Furthermore, the non-contact rotational design ensures minimal performance degradation in both EMG and TENG modules, even after continuous operation for 1200 000 and 600 000 cycles, respectively. Environmental robustness testing verifies the IPX7-rated waterproof capabilities of the HMEH, along with its resilience against high temperatures, humidity, and vibration. All these demonstrate the exceptional robustness of the HMEH. Finally, a self-powered wireless power transmission line's temperature sensing system is demonstrated as a proof-of-concept. This work offers an effective strategy for self-powered wireless sensor networks within power grid transmission lines.