Failure of the London Undergroundʹs 22 kV power supply system: An exercise in logic and transport management

The power supply to the north-cast sector of London Underground Ltdʹs Central Linc, and therefore the train service, was out of service for the majority of the time between the 24th and the 29th November 1993. During this period the power system faults were investigatcd, identified and corrected. This incident wiis the most extensive powcr fault on Loiidon Undergroundʹs power supply system in its recent history. Tlic key cause was identified as an earth radt on a neutral cable of an alternator, which was locatcd several kilometres from tlie part of the network that became inoperative. Loiidon Undergroundʹs power 22 kV generation and distribution system is described, and tlie fed arrangements historically made to iiiitiiniise the possibility of such an extensive outage are explained. The events leading to the loss of supply, subsequent identification and correction of the underlying main fault are described, and a model of the 150tIz harmonic earth-return current, which was the immediate cause of tripping owing to a series resonant circuit condition, is included. The effect of the neutral cablc fitult wiis to increase tlic I50Hz current to the point where it caused simultaneous multiple fʹccdcr brealm trips from earth-fault relays Jʹor the feeders supplying the northcast sector of thc Central Linc, resulting in loss of power to that sector. A comparison is made between the model and practically measured results.