Cost-Effective Power Supply Scheme for Tunnel Booster Fans in Long Tunnels

An increased voltage supply for tunnel booster fans is considered as a cost-saving solution for some tunnel projects in Singapore and Hong Kong.

Background – The Technical Challenge 

For long vehicular tunnels or tunnels with slip roads (ramps), tunnel booster fans may assist the ventilation of the tunnel. For metro systems, tunnel booster fans may be installed near the crossover to direct the air flow in the desired direction. 

In general, tunnel booster fans are powered from a motor control center (MCC) and operated at a low voltage of about 400V. However, if the fans are not located near the power supply, it becomes necessary to run long cables to the tunnel booster fans, which leads to: 

• An excessive voltage drop in the cable due to the long cables from the power supply to the fans; 
• Increased cable sizes and possibly additional cable sets to reduce the expected voltage drop (which is expensive); 
• Reinforcing the feeder cable mounting system due to the additional weight of the upgraded cables; 
• Space congestion along the entire tunnel services mounting system, in particular due to the upgraded cabling systems; 
• Provision of a large cable termination box to connect the upgraded cables to the tunnel booster fans; and 
• Risk of fan operational problems due to substandard voltage regulation, especially when starting “across the line” with electro-mechanical contractors. 

Since it can be shown that an increase in a feeder nominal voltage results in a significantly higher power capacity (proportional to the square of the percentage voltage increase for the same voltage drop), boosting the system nominal voltage was considered in order not to increase feeder sizes in long fan booster feeder applications; these solutions can be economical, especially if the low voltage equipment insulation classification does not have to be upgraded. Therefore, an increase in the nominal voltage of the serving power distribution system for big motors and fans, such as tunnel ventilation fans or smoke exhaust fans, has been adopted as a feasible solution for some tunnel projects, including a road tunnel project in Hong Kong (forty 660V tunnel ventilation fans, rated at 200 to 600kW). 

For smaller fans (e.g., tunnel booster fans), increased voltage is also becoming popular as the cost savings for the feeder cabling system is very attractive, especially when tunnel space is at a premium. For example, there are more than 70 tunnel booster fans (1250mm diameter, 60kW for each fan) in another road tunnel in Hong Kong. 

Figure 1– Schematic of Scheme 1

Examples of Solutions 

Some projects in Hong Kong, India, and Singapore have considered or adopted this approach of using tunnel booster fans of increased voltages of 660V (i.e., √3 x 380V), or 690V (i.e., √3 x 400V) as appropriate¹ (using step-up transformers inside the MCC rooms). This nominal voltage boost approach will reduce the nominal operating currents of the tunnel booster fans proportionally; however, the percentage voltage drop along the feeder will be reduced with the square of the nominal voltage boost, which in turn results in significantly reduced feeder cable sizes for the same voltage drop requirements, and therefore results in significant savings for the overall feeder cabling costs. 

To illustrate this approach/solution, it is assumed a tunnel project in Singapore has tunnel booster fans (45kW each) with an average cabling distance² of over 200 metres (656 feet) from the MCC room. For this situation, two schemes were considered: 

Scheme 1 – Low Voltage Supply for Tunnel Booster Fans 

The tunnel booster fans’ power supply is connected from the MCC which is operating at 400V. The MCC is dual-fed from two low-voltage service switchboards (see Figure 1). 

Figure 2 – Schematic of Scheme 2

Scheme 2 – Higher Voltage Supply for Tunnel Booster Fans 

Transformers for step-up of the voltage from 400V to 690V are proposed to be connected to the MCC for the power supply of the tunnel booster fans. With the increased operating voltage, the tunnel booster fans’ operation current would be reduced by 1/√3 proportionally. This would also reduce the cable size required and lower the cable costs. 

However, the size of the MCC room would have to be enlarged to accommodate the two step-up transformers and the additional switchboard that supplies the tunnel booster fans (see Figure 2). 

Cost Considerations 

An example of a cost comparison is shown in Figure 3: twenty (20) 45kW tunnel booster fans spaced along three parallel tunnels at an average cabling distance of 255 metres (836 feet) from the MCC. 

It shall be noted that the cost of a MCC (690V) may be sufficiently high in some countries, as additional effort is required to design and construct the upgraded equipment, especially if the local low voltage classifications and certifications cannot be met by off-the-shelf units. Also, additional tests may be required to obtain the authorization and required equipment certifications, especially when a variable voltage variable frequency (VVVF) drive system is used. 

Figure 3– Cost comparison between Scheme 1 and Scheme 2

Other Considerations 

• Cabling System
  The cost saving as illustrated in Figure 3 is 7.6 percent, based on the adoption of low smoke zero halogen (LSOH³) type fire resistant cables. This is generally a cabling requirement for emergency/life safety systems of all underground tunnel installations in Singapore. In Singapore and Hong Kong, where the low voltage (LV) power distribution system is rated at 230V/400V and 220V/380V respectively, the standard LV cable (U_o/U) is rated at 600V/1000V. 
• Energy Savings/Efficiency 
  From a viewpoint of energy savings, VVVF drives can be considered for many fan/motor installations. VVVF drives can achieve a very steady current ramping for different fan speeds to suit different air flow requirements. As the energy consumption for a fan with VVVF drive is in direct proportion to the speed, the energy savings will be generally as shown in Figure 4. 

Figure 4 – Energy consumption for a fan with VVVF drive is in direct proportion to the speed

• Location of VVVF Drive 
  For this project, the VVVF drive for each fan will be located inside the MCC room so maintenance does not have to be performed in a tunnel environment. 

Recommendation 

Technically, the use of a step-up boost transformer or autotransformer for tunnel booster fans requires more distribution equipment and more space to accommodate the plant. If the tunnel booster fans are too close to the MCC (e.g., within 200 metres) and the fan quantity is small (e.g., four fans), it will not be worthwhile to consider the use of a higher supply voltage cable distribution system. 

WSP | Parsons Brinckerhoff has successfully applied the above-mentioned cost saving scheme to various road tunnels and metro projects. 

¹Note: In the US – the “nominal” voltage of a fan motor is relatively standardized (3-phase 208V, 480V, 575V, 2400V, etc.) and normally cannot be customized as stated in this paper.

²Average cabling distance = Total cable length / Number of tunnel booster fans.

³In the US, the acronym for low smoke zero halogen is LSZH.