Abstract: In the field of high voltage direct current (HVDC) power transmission networks there is a need for an improved sensor failure detector. A sensor failure detector, for use in HVDC power transmission networks, includes an intelligent electronic device. The intelligent electronic device has a first input to receive in-use a number of first sample values from a first sensor measuring a given operational characteristic, and a second input to receive in-use a number of second sample values from a second sensor measuring the same given operational characteristic. The intelligent electronic device also includes a comparator module that is programmed to compare the first and second sample values with one another, and to establish a sensor failure when the first and second sample values differ from one another by more than a primary threshold for a predetermined first number of samples.