Part 3 - UPS System Specifications

Electrical Phases

When specifying Standby Power and UPS System Specifications, make it clear if it is to be for either single or three phase electrical supply.

Low power UPS equipment will be one phase input and output. Larger equipment will have three phases for both the input and output. In the mid range, three phase input with a single phase output is used.

UPS System Specifications

Cooling Systems

Not all critical loads need to be protected using a UPS system when it comes to specifications. Only systems that cannot withstand short breaks in the power, such as servers, need to be protected by a UPS. Other critical systems that can withstand a short break in power, such as server room cooling systems, can be protected using a generator. Server room cooling systems are regarded as critical loads because, if the cooling system fails, a thermal shutdown of the servers will occur, to protect them from overheating.

Power Conditioning

Much of the function of an on-line UPS is to clean up irregularities in the mains power supply. These aberrations typically last for very short periods of time, five seconds being regarded as a long time. However, there are occasions when there is a complete loss of power. If such a situation occurs the UPS will take up the load using battery power, and if you have a generator connected the UPS will act as a power bridge between the loss of power and the connected generator or generators taking up the load.

UPS System Specifications

On-line UPS will give a break-free electrical supply in the event of mains power failure and also a break-free connection when the system reverts back to mains power once the problem has passed. It will also provide you with an automatic system bypass to mains if an overload or fault condition occurs.

On-Line / Double Conversion Technology

One of the types of UPS systems is called, On-line/double conversion technology for specifications, provides the highest levels of power protection from all mains-borne power problems and is often regarded as the most dependable type of technology.

The UPS converts the incoming mains AC power to DC power. The DC current provides power to the battery and the inverter, which converts the DC back to AC and provides a clean, regulated and conditioned AC output to the protected load. The battery is always in circuit and therefore when a mains failure occurs the battery carries on supplying power to the inverter which means there is no-break in the supply to the protected load and no switching is involved. A static bypass circuit is incorporated into the UPS which provides a failsafe mechanism, by providing AC mains to the load should the UPS be overloaded or in the unlikely event of an internal fault.

Online UPS systems are normally recommended when used with generators due to the UPS tolerance to voltage and frequency variations that can be associated with generators. An RS232 communications port for use with SNMP/server close down software along with Emergency Power Off (EPO) capability are normally a standard item on these systems. Power ratings are typically from 700VA to > 600kVA in single module format, with parallel systems and N+1 capability available from typically 5kVA upwards. Battery extension packs are available to extend the back-up time from a few minutes to a few hours.

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Offline / Standby Technology

An offline / standby UPS provides very basic protection against mains-borne problems. AC mains power passes straight through the UPS which has RFI filtering and limited spike protection provided.

In the event of a mains failure, or the AC mains voltage is outside the UPS operating window, the inverter starts up, a relay closes and allows the inverter to provide power to the load. Upon AC mains returning the mains output is switched back to mains and the inverter then shuts down. There are a few milli-seconds break during transfer to/from the inverter. The output when on inverter is invariably a square wave or stepped sine wave output.

Offline units typically have power ratings from 250VA to 2kVA. They typically have limited communications capability and whilst relatively cheap are normally recommended for only a single PC / workstation type application.

Line-Interactive Technology

A Line-Interactive UPS provides more superior protection than an offline UPS. However, it still does not provide protection against all mains-borne problems unlike an on-line double conversion UPS.

These UPS's work in a similar way to an off-line UPS but have superior spike protection than an off-line system. They have a wider tolerance to voltage variations due to a voltage trimming transformer or similar. This increases the incoming AC mains voltage when it is low and decreases the incoming AC mains voltage when it is high and prevents the UPS switching to battery power and draining the batteries needlessly. During a mains failure, or if the incoming mains is outside operating limited the inverter starts up and provides power as described in an offline system. The output waveform on a line interactive UPS is normally a sine wave output.

Some manufacturers offer a range of battery packs which extend the back-up time from a few minutes to several hours. However, the battery charger in these UPS systems are limited and depending on how many battery packs are used; it may not be large enough to re-charge the batteries or the re-charge time could take many days. An RS232 communications port for use with SNMP / server close down software along with Emergency Power Off (EPO) capability are normally a standard item on these systems. Power ratings are typically from 400VA to 5kVA and can be offered in either a tower type or rack-mounted format and whilst not the recommended option, still provide cost-effective protection for server type applications.

You reached the end of our guide of UPS System Specifications

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