CONSULT | Case Study - UPS Cooling
Client & Application
Scope of Supply
- Advice and recommendations to client
- Support client Engineers throughout
- Participate in multi-corporation, global teleconferences
- Develop suitable power supply strategy
- Control supply harmonics
- Control supply peak AC currents
- Mitigate effects of commmon-mode voltages & currents
- Attend & facilitate system FAT
This project centred around HVAC equipment for ExxonMobil's Hebron platform, under construction at St. Johns, Newfoundland and Labrador. The client had been contracted to provide HVAC systems for each of six rooms on the platform's topside.
AC power to the six HVAC systems is provided by two static inverters, with both inverters being fed from a single common battery. This battery also supplies the platform's essential services, including radio & telecomms equipment.
The six rooms' HVAC heating & cooling capacity requirements were specified by the main contractor. However, the available electrical capacity of the inverter supplies was also separately limited. This meant that some design effort was required, to reconcile the two requirements within the systems.
The systems are intended to provide a hospitable environment for the rooms' personnel & equipment. In the event of an catastrophic emergency situation arising, the systems would run "single shot", to maintain habitable conditons for as long as possible, i.e. until the battery is exhausted.
The systems comprise compressors, Ex condenser fans, evaporator fans & control electronics. Direct online starting of the compressor & fan motors is not possible, as the resulting demand currents would exceed the inverters' overload ratings. Soft starters would help but present limitations as to the number of starts per hour. VSDs present additional harmonic & peak current considerations, along with high frequency common-mode issues.
In addition, the UPS inverters are high impedance sources, compared to mains supplies. Large load current transients or harmonic content would present additional risks of voltage distortion (notching), possible nuisance tripping, loss of frequency control or instabilities.
We were able to propose a suitable supply strategy, in the form of outline schematic & bill of materials, which included recommendations & mitigating measures for:
- Inverter RMS & peak currents
- Inverter RMS & peak transient currents
- Motor starting transients & staging
- Inverter current harmonics
- Inverter voltage harmonics
- Common-mode voltage effects
- Screening & Earthing for EMC
- Use of VSDs/softstarts with Zone1/Zone2 Ex Motors
Once manufacture of the six systems was complete, we attended at client's site, to assist with Factory Acceptance Test. Measurements made during these tests confirmed that the systems were performing as intended and that the relevant design criteria were being met.