There are three new tunnels on the new high-speed link that runs from Ebbsfleet in North Kent to the newly refurbished St Pancras station, passing under central London and the River Thames. The tunnels are twin-bore, one for each direction of travel, with regular cross passages that facilitate escape to safety in the event of an incident.
Ventilation and escape/access are important aspects of tunnel safety and Rail Link Engineering entrusted the control system for this to Johnson Controls who, in turn, chose Mitsubishi Electric as its key technology supplier. Several different architectures were considered for the project, the one finally selected being a unified dual redundant network running all the way from the Ashford control centre to St Pancras and back again. Johnson Controls’ Ian Spencer takes up the story:
“The whole system is based on Mitsubishi’s MX4SCADA and Q series programmable automation controllers (PACs). These are high-performance, state-of-the-art PLCs, designed for use in large complex systems and the most demanding applications. One of their great advantages for this job is that that they are easy to set up in a redundant or standby configuration. Previously such belt-and-braces systems design was very expensive to implement, but with the Q series, costs are barely more than for a conventional system.”
A pair of PACs, configured in warm standby mode, are assigned to control the main tunnel ventilation fans at each of the eleven portal/shaft locations throughout the length of the tunnels. These fans are used to move smoke away from an incident train in one tunnel bore and pressurise the other tunnel bore to create ‘a place of relative safety’ to which passengers and train crew can escape.
The fans are generally used for cooling the trains during periods of congestion and to provide ventilation for maintenance teams in the tunnels. However, it must be possible to configure them in one of many hundreds of possible ventilation modes, dependent upon the ‘incident’ train’s position at a moment’s notice. This ability is tested regularly through the PACs’ pre-programming.
Along with the bespoke interface to the signalling system, which enables the position of trains within the tunnels to be monitored, is a bespoke interface to the EMIS (Electrical and Mechanical Information System). The status and position of cross passage doors and other essential equipment within the cross passages is sent to EMIS from the MX4SCADA system.
Critical to the tunnel’s safety regime are the cross passages. If an incident occurs in one tunnel, they allow the passengers rapid egress to the other and ultimately escape from danger. There are frequent cross passages so that evacuation can be rapid and safe, no matter where in the tunnel the incident occurs.
The Mitsubishi control system is designed to maintain a positive pressure in the cross passages, so that they can be guaranteed clear of smoke in all events. The fans for this safety-critical duty are also controlled by warm standby pairs of Q series PACs. The cross passage door positions are monitored 24/7 by Q series PACs on a dual redundant Melsec 10 network that connects back to the warm standby PACs.
It is an inescapable reality that all major tunnels are likely to have low lying points where water tends to accumulate, causing at best, a nuisance or hygiene risk, at worse, local flooding. To overcome this potential problem, Rail Link Engineering has installed banks of large pumps in several susceptible locations. These are all controlled by Mitsubishi variable speed drives integrated into the pump control network.
“We considered putting the pumps on a dedicated network, but the advantages of integration are significant,” explains Mr Spencer. “Maintenance of a single system is far easier, and condition monitoring of the environment and technical systems is straightforward.”