Directional Earth-Fault Protection (Compensated Network)
A protection relay under IEC 60255-121 that, unlike a simple (non-directional) earth-fault protection which only measures the magnitude of the zero-sequence current, also determines the phase angle between the zero-sequence (residual) voltage and the zero-sequence current in order to establish in which direction — upstream or downstream of the measuring point — an earth fault is located. This distinction is essential in medium-voltage networks with a Petersen coil (resonant earthed neutral), because the compensation coil largely cancels out the capacitive earth-fault current of the healthy network, causing the remaining, active residual current during an earth fault to become so small that a conventional, non-directional overcurrent protection cannot reliably distinguish the fault from normal operating currents or determine which outgoing feeder contains the fault. By analysing the phase-angle relationship between the zero-sequence voltage and the active component of the zero-sequence current, the directional relay can accurately determine which specific feeder contains the fault, even though the measured current amplitude is very low, thereby enabling selective disconnection of only the faulty feeder without unnecessarily de-energising the entire network.
In een middenspanningsring met Petersen-compensatie lokaliseert de directionele aardfoutbeveiliging de fout op de juiste uitgaande kabel ondanks een reststroom van slechts enkele ampères, waar een conventionele relais dit signaal niet van ruis had kunnen onderscheiden.
In een gecompenseerd (Petersen-geaard) net een standaard niet-directionele aardfoutbeveiliging toepassen omdat die 'goedkoper en eenvoudiger' is — door de sterk gereduceerde reststroom kan deze de fout niet selectief lokaliseren en resulteert dit vaak in het onnodig uitschakelen van het gehele net of het missen van de fout.