Using the telegrapher's equations to verify sample alignment and increase transmission line differential protection security

Differential protection is widely used for protecting transmission lines, providing secure and dependable solution. However, its security relies on the sampling devices sharing a common time reference, or means to compensate clock differences. Two solutions for meeting this requirement are available on many modern substations: external synchronisation (e.g. GPS or Galileo) and “ping-pong”. Both these solutions can provide wrong time information due to malfunction or malicious tampering, furthermore, no clear method was found in the literature to identify time synchronisation problems that could lead to incorrect trips. This paper presents a strategy to identify when the time references are drifting from each other based on the power system measurements and block differential protection algorithms before they maloperate. This strategy has the potential to be used to identify time drifts caused by communication channel asymmetry, drifting synchronisation reference and even cyber-attacks.