DC collecting and transmission systems will be the development tendency for future offshore wind farms because of the advantages of high efficiency, small size and no reactive power compensation. However, the distinct distribution capacitance of submarine cables, in contrast to overhead lines, can lead to significant differences in line currents during external faults, potentially causing false operation in traditional transient pilot protection. Addressing this problem, this paper proposes a pilot protection method using time-frequency mutations in fault components. This paper analyzes transient fault current frequency characteristics in DC transmission lines and converters taking into consideration the differences of frequency band of the transient fault current on both sides when internal and external faults occur, thereby selecting an appropriate frequency band for protection. Using wavelet transformation for time-frequency analysis of the fault current and employing Wasserstein distance, it develops criteria to distinguish internal and external faults. This removes the transient time-domain quantity-based pilot protection false action during external metallic or low-resistance fault. The offshore wind farms collecting and transmission system model is built on a real-time digital simulation (RTDS) platform to verify the proposed protection method. The results show that the proposed method effectively identifies fault locations and operates within 5 ms, with tolerance to transition resistances up to 300 Ω and noise resistance up to 20 dB.