TECHNICAL | SAFETY/VENTILATION
DRIVES IN TUNNELS
Variable speed drives (VSDs) offer benefits to control ventilation fans and drainage pumps in tunnels, explains Josh Paikada, Strategic Business Development Manager, ABB
Tunnel ventilation and drainage systems provide safe, clean air for people within the tunnel, limit water intrusion, remove harmful vehicle emissions, and even play a vital role in fire safety. For these critical applications, it is essential to choose the right equipment and that can include variable speed drives (VSDs) to control fans and pumps. Historically, tunnel ventilation systems used electric
motors to power fans which ran continuously at full speed. The fan output was controlled by mechanical throttling. Drives, meanwhile, can adjust the frequency
and voltage provided to an AC motor to match the requirements of the application. This enables the control systems to automatically change the speed of the fan to meet ventilation demands, depending on the traffic within the tunnel. In tunnel ventilation systems, this enables precise,
real-time control of airflow through the system. This avoids unnecessarily high fan speeds and reduces wear on the motors, lengthening the economic lifespan of equipment. When combined with sensors throughout the tunnel,
drives in ‘comfort mode’ can automatically adjust fan speeds to maintain air quality at a specified level. In ‘fire mode’, drives run the fans at full speed – including in the opposite direction – to establish fire control zones. Fire safety use cases will remain essential even as
electric vehicles become more common and the need for exhaust ventilation is reduced.
There are also significant energy efficiency benefits to
using drives. Running a motor continuously at full speed and throttling the output is an inefficient approach, similar to using the brakes to control a car’s speed while leaving the other foot on the accelerator pedal. With a drive, any time a motor is not running at full
speed, it is saving power. This is particularly significant considering that the relationship between speed and energy use is nonlinear – slowing a motor by just 20% cuts energy use by half. As a result, adding a drive to an existing fan or pump’s motor typically reduces total power consumption by 20% to 60%.
Harmonics and active front end technology Any non-linear electrical equipment on a network though, like VSDs, disrupts the waveform and introduces ‘electrical pollution’ in the form of harmonics. This can result in the overheating of cables, transformers, generators and motors, reducing their lifespan and wasting electricity. To eliminate harmonics, operators can specify ultra-
low harmonic (ULH) drives with an active front end. This eliminates the need to oversize other components on the network, reducing both the system’s installation footprint and the capital expenditure required. Drives with an active front end enable active braking.
This can save power through regenerative braking and has significant safety benefits. In a tunnel with a unidirectional draft, where a fan may be spinning even when unpowered, reversing a fan’s direction in an emergency depends on quickly ramping down its speed.
size down Motor, cabling, fan size down size down Cabling Transformer size down Above: Downsizing and cutting capital costs are benefits with active front end (ULH) drives PHOTO CREDIT: ABB 18 | August 2023 20% 10% ~30%
50%
Generator space savings Up to 85%
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