The protection relays in MV transmission networks protect the MV transmission cables and the MV/MV substations. Splitting it up in the 3 previously mentioned ways of malfunctioning of a protection relay:
1) In case of a spontaneous trip by a relay in the MV transmission network normally only the transport cable will be switched off. Due to the N-1 redundancy no interruption of supply will occur. Therefore there will be no interruption of supply or damage in this case
2) In a case where incorrectly no trip is given either the whole MV transmission network or the HV/MV transformer will be switched off, depending on the location of the relay that fails. Disconnection of a MV transmission network will on average result in 500.000 Customer Minutes Lost (CML), disconnection of a HV/MV transformer in 2 million CML. The effect of these kind of disturbances therefore is in the effect category ‘Considerable’. The switch-off time of the backup protection can be up to several seconds more than that of the main protection. For a failure in a MV-cable this is expected to have little effect, but for a failure in a MV/MV substation this can result in severe damage. The damage is expected to be in the range of € 100,000 to € 200,000 (‘Considerable’)
3) In case of a-selective switch-off of a relay in the MV transmission network normally only a transmission cable will be switched off. Next the main protection of the distribution cable in which the fault occurred can still switch-off the fault correctly. Due to the N-1 redundancy no interruption of supply will occur. It is also possible however that the fault is in another cable of the same transmission network, resulting in switching off two cables instead of one. This can result in interruption in supply, but the chance that this will occur is small however