Whilst preparing for the redevelopment of one of London’s busiest Underground Stations, it was realised early in the planning process that the 1 MVA electrical supply available for the Civil Engineering and Construction Works would not be large enough. The deficit was calculated to be in the region of 350kVA. The use of generators to supplement the supply was considered. However, in addition to the cost of hiring generators for the duration of the project, the cost of fuel alone was calculated to be £80,000 per annum. Given that the project was scheduled to run for 5 years, the cost of installing generators was considered prohibitive and, the additional CO2 emissions, which would be considerable, were considered unacceptable.
From previous experience of similar projects and from initial measurements on site, it was anticipated that the power factor for the installation could be as low as 0.64. Correcting this to 0.9 or 0.95 would enable significantly more usable power to be extracted from the 1 MVA supply transformer, thus avoiding the need to supplement the mains supply with generators. In addition, penalties for poor power factor would be avoided.
Working with the Blakley Electrics project team, it was calculated that a 550kVAr rated Power Factor Correction (PFC) assembly would be required to improve the power factor from 0.5 to 0.95, which would be more than sufficient to correct the expected 0.64 power factor. Due to the use of equipment with a high harmonic content, it was also decided that detuned capacitors would be required.
The resultant PFC assembly comprises of a robust, free-standing enclosure fitted with 11 no. capacitor banks each rated at 50kVAr, controlled by a power factor relay. The relay monitors the power factor of the installation and automatically switches banks of capacitors in and out to maintain the target power factor of 0.95. Due to the sensitivity of the capacitors to heat, the assembly incorporates forced ventilation with thermostatically controlled fans installed at diagonally opposite points of the enclosure.
The PFC assembly is installed alongside the 1600A Mains Distribution Assembly (MDA), which supplies the overall site. A CT is fitted to the incoming side of the MDA, which is connected via a 2 core cable to the PFC relay in the PFC assembly and enables the power factor to be determined. An 800A MCCB fitted within the MDA is connected to the PFC assembly via a short, fully rated cable.
Due to the duration of the project, it is recommended that the capacitors are checked periodically (every 2 years) to ensure that they are functioning correctly. It is also recommended that the PFC assembly is installed in a shaded location, due to capacitors being susceptible to high temperatures. If a shaded location cannot be found on site, a sun shade can be incorporated to protect the assembly from intense overhead sunlight.
Although installing PFC equipment does not reduce kilowatt consumption, it can reduce the size of supply required for a project. It can therefore reduce monthly availability charges and eliminate penalties for poor power factor. It therefore has a significant role to play in improving the electrical efficiency of construction site installations.
If you have a requirement for a Power Factor Correction Assembly, the Blakley Electrics Project Team is on hand to work with clients and contractors to develop the optimum solution. Please contact us via our Wakefield or Crayford Customer Service Centres. We look forward to being of service.