CEFISA installs, in record time, the system for parallel-substation interlocking between seven substations of line six of the Madrid Metro.

The overhead power line, or catenary (overhead contact line), of a metro or railway is divided into sectors which are supplied from substations which supply electromotive power. Each sector can be connected to adjacent sectors by way of switchgear normally known as catenary switches.

When a catenary switch is closed between two sectors supplied by two different substations, these then supply both sectors in parallel.

This situation is normally applied when the power available from one substation is insufficient to supply the number of trains thay may be circulating though the sector supplied by that substation, or when, due to voltage drop in the overhead power line, the available voltage at locations most distant from the substation is not sufficient for correct supply of trains.

The power supply in parallel from two or more substations to a sector of overhead line carries two implications:

1. In the case of a fault in the overhead contact line of a sector, in order for this to cleared it is necessary to effect simultaneous disconnection of the sector from all substations supplying the said sector. The time necessary for clearing the fault is critical, in order to minimise secondary knock-on effects of the fault.
2. In the case of disconnection of a single substation from the public power supply network, the substation will remain energised by the voltage present in the overhead line. This reversal of power flow is extremely dangerous for maintenance personnel because of the risk of direct or indirect contact with a system that remains energised in this way.
   

The system for parallel-substation interlocking between substations alleviates both of the foregoing situations by means of the installation of an extremely fast and reliable communications system that permits power cut-off of adjacent substations to be made from a single substation.

These power cut-off manoeuvres must be subject to the topology of the electrical network.