Flexibility of Natural/Forced Ventilated Tunnel for EHV cable Links Across Urban Environments

摘要

Power cables installed in tunnels are often the preferred method to transmit bulk power in largeurban areas, in order to limit the width of the right of way and the visual impact. This solution wasalso selected by the Dubai Electricity and Water Authority to enhance the 400kV network byconnecting three 400kV substations together through two tunnels of 4 km and 11.5 km long.The challenge was to design the tunnel’s cooling system in order to transmit 2 × 1500MVA throughtwo cable circuits where the outside ambient temperature may reach 50°C during the summer period.Since the high demand of energy and peak ambient temperature may occur together during a limitedperiod in summer, the technical proposal has been oriented around a natural ventilation systemdesigned for operating alone most of the time, and can be enhanced by a forced ventilation systemduring the critical period. The natural ventilation system consists of several inlet/outlet shaftspositioned along the cable tunnel, while the forced ventilation consists of several large fans located intechnical buildings above the tunnel which are controlled by a temperature monitoring system.The ventilation system design was one of the main difficulties to overcome during the execution of theproject, and the requirement of transmitting bulk power was not only challenged by the ambienttemperature, but also by other environmental constraints, such as; the restriction on the size of theventilation shafts and buildings to mitigate the visual impact on the surrounding urban architecture; therestriction on the tunnel slope due to the presence of the road crossing; the restriction on the noiseemitted by electrical fans; the restriction on the temperature inside the tunnel at 62°C to protect theauxiliary equipment, etc …Therefore, a computation model of power cables installed in the ventilated tunnel was developed totake into account all the environmental constraints mentioned above, together with the requirement ofbulk power transmission, and meet the cable circuit rating at 2×1500MVA at 50°C ambienttemperature.This paper presents a computation model of the ventilated tunnel based on the fundamental fluiddynamic physics principles.