Abstract : The power frequency change distance relay is an important component of the "LFP-901 Ultra-High Voltage Line Complete Fast Protection Device". Its principle is novel. To facilitate proper commissioning, this paper provides a detailed theoretical analysis and field application discussion of this line protection component. Keywords: Power frequency change, setting value, memory, floating. The "LFP-901 Ultra-High Voltage Line Complete Fast Protection Device" produced by our company has become one of the main protection devices for 220kV lines and is widely used in North China. However, due to the novel principle of the power frequency change distance relay in this device and the lack of specific test methods in the commissioning outline, many testers do not fully understand it and cannot conduct correct tests. In July 1999, the power frequency change distance relay of the 220kV Han-Gao double-circuit line protection at the Liulihe Substation in Beijing tripped beyond its designated level. Fortunately, one of the reclosing circuits operated correctly; otherwise, it would have caused the breakdown of the Hebei Nantong and Beijing-Tianjin-Tangshan power grids, with serious consequences. The investigation revealed that the main reason was that the commissioning personnel did not understand the principle of the power frequency variation distance relay in the protection device. When the setting value was incorrect (10 times larger than the specified value), they were unable to correctly verify the device's operation due to their inability to calculate the test values, resulting in malfunction of the upstream protection system when a downstream line fault occurred. To improve the commissioning process, the power frequency variation distance relay was studied, and discussions were held with the manufacturer's designers. My understanding is presented in two parts. [b][align=center]For details, please click: Field Test Methods for Power Frequency Variation Distance Relays[/align][/b]