A. Basic System
Emergency/stand by systems is categorized by the sources of electric power.
The design of the system depends on the duration of operation, the power required, the reliability and the time to transfer from normal source to the alternative source.
1. Storage battery
It is used primarily for emergency lighting, fire alarms and emergency communication system. The system consists of an automatic charger, battery and emergency dc bus.
2. Generator set
The most common stand by power is an engine-generator set. The set consist of a gasoline or diesel engine or a gas turbine, an ac synchronous generator, and the controls. The set is always used with a transfer switch arrangement.
3. Uninterruptible power supply (UPS)
UPS: Power conversion, energy storage, power quality
a. Size for present and future equipment loads based on square footage.
b. If existing equipment, get actual load measurement and add at least 25% for growth.
c. If new equipment add nameplate rating and take half off for demand/diversity, then add 25% for growth.
d. If future unknown equipment, size based on square footage.
UPS batteries and other energy storage options:
a. “Sealed, Maintenance Free” batteries.
b. Flooded “Wet” cell batteries
d. Super conducting Magnet storage
e. Super Capacitors, Ultra capacitors
f. Fuel Cells
PDU: Modular power distribution Unit, Transformer, grounding, monitor
a.S tep down voltage(480-208/120V)
b. Shielded isolation transformer provides high frequency noise attenuation, grounding reference close to IT equipment.
c. Modular power distribution close to IT equipment.
d. Typically bottom cable feed, using flex cable under raised floor.
e.Top feed option for non-raised floor applications.
f. 2-5 pane; boards built in with external or remote distribution options.
g. Redundancy options: transformers, panelboards, branch breakers, duel feed to input or to remote panels to eliminate single points of failure.
h. PDU/STS combinations
STS: stand-alone or PDU Static Transfer Switch, for high-speed transfer between two sourced.
c. Both future capacity increase and redundancy
2. Expansion for capacity
a.Parallel, up to 6 units, N+1 redundancy
b.Distributes, up to 3 distribution sections
c. Expandable/upgradeable unit-get upgrade cost up front.
3. Expansion for redundancy
a. Distributed / dual bus – add 2nd or 3rd unit, using dual or three corded loads and static switches for single corded loads.
b. Parallel redundant N+1; dual bus parallel 2N+1
c.Isolated or series redundant N+ 1 ; dual bus 2N+1
ATS: Automatic and manual transfer of power between normal utility and standby generators.
a. Separate ATS for HVAC and UPS
b. If more than one UPS, separate ATS for each UPS
c. If only one ATS, include bypass isolation
d. Open transition when mostly UPS and small VAC units
e. Close transition, delay in neutral, or phase transfer if larger motor loads are fed from the ATS
f. Try to keep the ATS design 3 phase 3 wire plus ground. UPS, HVAC and transformer inputs do not need neutrals. Add 480:277V transformer for lighting; 480:208/120V transformer for general purpose loads.
High Reliable Critical power design:
a. Design in the ability to power the critical load directly from the generator while testing the UPS from the utility.
b. Use more small, rather than few large ATS units.400-1200A range.
c. Use more small, rather than few large PDU/STS units. 100-400A range.
d. Use fewer, larger UPS units. Try not to parallel more than 3 units.
e. Reduce or eliminate single points of failure as the budget allows.
f. Design in flexibility and expandability as the budget allows.
g. Plan for load bank testing with IR scan, for switchgear, UPS, batteries, generators.
4. Separate service
The load is normally supplied by normal supply line from the utility. If the normal supply fails the transfer switch automatically transfer the emergency load to the alternative supply line. To be effective the alternative line should come by a different rout, and even from different substation, than the normal line.