There are several categories of static UPS systems available. Broadly
speaking, UPS Modules fall within one of three operational design
architectures, namely off-line, line interactive and on-line. However,
irrespective of their individual design criteria certain features are
common to all forms of static UPS systems - i.e., they all contain
batteries which store energy when the mains supply is available and a
means of converting the battery charge into an alternating current (ac)
supply in times of mains failure. All systems must therefore include a
battery charger and a power inverter circuit. As described above, the
battery provides a power source for the inverter when the mains
supply fails, whereupon it discharges at a rate determined by the
critical load connected to the UPS output. The inverter automatically
shuts down when its dc supply falls below a certain voltage, therefore
the duration for which the critical load can be supported in times of
mains failure depends upon the battery capacity and the percentage
applied load. A typical UPS system, which may be referred to as,
Uninteruptible power supplies, Uninteruptable power supplies,
Uninterruptable power supplies, Uninterruptible power supplies, Energy
Systems, or simply UPS, will contain sufficient battery capacity to
support its fully rated output load for 5 to 15 minutes. However, in
most cases this can be extended by adding further battery cabinets or
selecting batteries of a higher capacity. The battery backup time is
often referred to as the autonomy time. Virtually all systems contain a
`bypass` system which, in conjunction with some form of output
switching circuit, provides a means of connection the critical load
directly to the mains supply. In most cases the output switching
circuit is implemented using solid-state switching devices. The rules
governing the static switch control depend on the UPS operating mode.
Line-Interactive System This type of UPS covers a range of hybrid
devices that attempt to offer a higher level of performance than
conventional off-line designs by adding voltage regulation features in
the bypass line. The two most popular types of system in this
category employ either a buck/boot transformer or a ferroresonant
transformer. Like off-line models, line-interactive UPS normally
supply the critical load through the bypass line and transfer it to
the inverter in the event of a bypass supply failure. The battery,
charger and inverter power blocks are utilised in the same manner as in
an off-line system but due to the added `regulation` circuits in the
bypass line the load is transferred to the battery-fed inverter supply
less often, making this type of system slightly more efficient in
terms of running cots and batter `wear` compared with an off-line
system. Buck/Boost Transformer Design One of the drawbacks of the
straightforward off-line design is that the load must be transferred to
the inverter immediately the bypass supply voltage reaches voltage
limits acceptable to the load. This means that the UPS might transfer
between bypass and inverter quite frequently if it is set up to operate
with a critical load having a tight voltage tolerance. Apart from the
power break each time this occurs, this method of operation incurs
frequent battery usage which reduces battery life and might perhaps
result in a battery that is inadequately charged when it is called upon
to support a prolonged mains blackout. A buck-boost transformer
connected in the bypass line helps overcome this problem. The
transformer has tapped secondary windings which are selected by relays
to either step-up or step-down the bypass voltage as appropriate to
maintain the UPS output voltage within the required output voltage
limits. This means of controlling the output voltage permits a wider
variation of bypass voltage to exist before the output voltage reaches
its limits and initiates a load transfer to inverter. A typical UPS in
this category will sustain the load voltage over a bypass voltage
range of +20. Note that although the output voltage is maintained
within its preferred window using this method, buck/boot switching
unavoidably leads to a degree of step voltage changes as tap changes
take place. On-Line System An immediate difference between this design
and the previously described off-line system is that the battery
charger is replaced by a `rectifier/charger` black. The
rectifier/charger may be two separate units or a combined power black.
When the mains supply is present this black float charges the battery
and supplies the inverter with a stable de voltage. In the absence of
the mains supply the charger shuts down and the inverter dc supply is
provided by the battery, which begins to discharge. The connection
between the rectifier/battery and inverter is often known as the dc
busbar , or dc bus. As part of its control function the
rectifier/charger generally includes an input current limit feature to
provide overload protection an a dc overvoltage shutdown mechanism to
protect the batter/inverter and dc filter components This UPS design,
which is sometimes also referred to as a double conversion UPS, offers
the greatest degree of critical supply integrity in that the load is
supplied with processed power at all times. That is, when the UPS
input mains supply is resent the rectifier, charger and inverter power
blocks are all active and the load is connected to the inverter
output via the static switch. As the load is powered from the inverter
under normal circumstances it is well protected from input supply
aberrations because the rectifier and inverter act as a barrier to
mains borne noise and transient voltage excursions, in addition to
providing a well regulated output voltage. If the input supply goes
outside a preset voltage range (typically +10), or suffers a total
failure, the inverter continues operating from battery power and the
event is totally transparent to the load as there is no transfer
operation involved. When operating from battery power the inverter
supplies the same degree of supply regulation as when the main is
present. If the mains is not restored before the battery reaches its
end-of-discharge voltage the inverter shuts down and, in some models,
the static switch may attempt to transfer the load to the bypass line.
The result of the transfer action depends on whether or not the
module`s bypass line is connected to the same mains supply as the
modules rectifier, and if the bypass supply is live. (Known as a split
bypass system.) What Happens if the UPS Fails? A UPS Fault is
generally seen as the inability of the inverter to provide the correct
voltage or requency at the UPS output terminals and the resulting
actions that take place may vary between models. Usually, the UPS
control logic will detect the failing output voltage/frequency as the
fault occurs and immediately signal the static switch control system
to transfer the load to the bypass line in a make-before-break
fashions. However, if the inverter is not synchronised to the bypass
supply when the transfer is called fro it will be impossible to
perform a break-free transfer operation, consequently there will be a
brief supply break while the transfer takes place. These are the only
circumstances under which the load is subjected to a (brief) supply
break in a true on-line ups system. Note that although the break-free
transfer to bypass is transparent to the load it is no longer supplied
with processed power once it is transferred to the bypass supply;
also, if the bypass supply is unavailable when the `fault` transfer is
necessary a total loss of power to the critical load is unavoidable.
The static switch usually transfers the critical load back to the
inverter automatically once the inverter fault clears. This feature is
occasionally described as auto-retransfer. The response of an on-line
system to an output overload is usually similar to that of the UPS
failure described above in that the load is transferred to bypass
until the cause of the overload clears whereupon it automatically
re-transfers back to the inverter. If the bypass supply is unavailable
this will lead to a total loss of load supply, therefore some systems
allow an overload condition to continue to be supplied from the
inverter for a finite time ? that is the UPS equipment is able to
supply enough current to a faulty piece of load equipment to ensure
that the load protection fuse or circuit breaker will automatically
disconnect if from the UPS. While feeding the overload under these
circumstances the inverter operates in a current-limit mode and its
output voltage may be reduced deliberately, but in most cases this is
preferable to total power loss and of course conditions will return to
normal if the overload is cleared during the allotted time. Parallel
Systems This type of system comprises two or more UPS modules
sometimes referred to operating in parallel to feed a common critical
load bus, and is generally applicable to medium/high rated modules of
on-line design. Units forming part of such a multi-module system are
almost identical in operation to that of their corresponding single
module counterparts. In fact, some manufacturers deign their UPS
modules such that they can be used in either configuration without the
need for complex modification. Each module contains a static switch
to provide a means of transferring the load between inverter and
bypass. However, a certain amount of inter-module electronic control
logic is added to ensure that all the module's static switches operate
simultaneously when transferring from one power source to the other.
Damage would result if one module attempted to transfer its output to
the bypass line which the others remained on inverter. Additional
inverter control functionality is also required to facilitate
inter-module load sharing and frequency synchronisation. These control
signals, and others, are passed between the modules over low voltage
control cables which are normally connected in a ring configuration to
allow each module to communicate with every other module in the
system. One advantage of providing external input and output isolators
for each of the modules is that it allows modules to be fully
isolated and `hot-swapped` if necessary without disrupting the
remainder of the system. There are two major reasons for installing a
parallel system. The first is to increase the effective UPS capacity to
enable the 'system' to power a larger load that is otherwise possible
with single module. The second is to introduce a measure of module
redundancy to improve the anticipated system reliability. Parallel UPS
systems are therefore commonly categorised as either `capacity` or
`redundancy` systems, although some are intelligent enough to operate
as either, depending on the prevailing circumstances. Irrespective of
the intended mode, all the modules forming part of a parallel system
must be of the same type and rating - i.e. it is not possible to
parallel a 30kVA unit with one of 120kVA. Installing the UPS Delivery
and Positioning The importance of planning the installation and
delivery of the UPS system cannot be overstated. Having chosen a
particular system and topology it is important to decide:1. Will the
system fit into the space reserved for it? 2. Is the proposed location
suitable? 3. Can and how will the system be transported to the
location Size & Weight Improvements in UPS technology and design
have provided much higher power densities which when combined with the
flexible installation options for modern parallel systems make it
much easier to find space for UPS systems. Also, because the most
modern designs no longer need bulky and heavy input transformers,
installation of very powerful UPS systems is no longer limited to the
ground floor or basement plant room. The manufacturer or supplier will
provide details of space requirements and details of module weights
in the UPS system specification. Be sure to consider possible future
expansion when choosing a UPS location and if you can allow extra
space over and above the manufacturers recommended minimum,
maintenance and service will be easier. A UPS system is not just a big
battery box. It contains electronic components similar to those found
in computers and therefore requires careful handling when being
transported. Additionally, large UPS equipment will be heavy and
unwieldy and will require specialist contractors using `air-ride`
suspension vehicles and specialised lifting equipment to unload and
position it. The UPS supplier should be able to recommend handling
procedures and suitable contractors with experience in this field.
Choosing a Suitable Location The choice of a particular installation
location for the UPS depends on many things: 1. How much space is
available? 2. Can the floor safely support the weight of the equipment?
3. Will the installation cause continued inconvenience to the
existing personnel and business? 4. Are the environmental conditions
at the chosen location suitable? 5. Can access to the UPS equipment be
made secure yet convenient? 6. Does the UPS comprise one module or
several in parallel? 7. What is the effect of the installation on
existing air flow and air conditioning equipment? 8. Will the
switchgear controlling the UPS be in the same area? 9. Can the chose
area safely accommodate the battery installation? In general the
location chosen for modern UPS can be summarised as follows: Small UPS
- less that about 20kVA, can be installed in a normal office
environment although care should be taken to ensure that the
additional noise and heat does not adversely effect the office
environment. Medium UPS - between 20 and 100kVA are designed to be
installed in computer rooms.Large UPS - greater that 100kVA, will
usually be located either in a separate UPS room or in an existing
plant room Transporting the System Having chosen a suitable location to
suit the UPS system it is vital to survey the proposed transportation
route. If a specialist delivery contractor has been employed for the
task they will usually undertake a site access survey before
attempting to deliver any equipment. Even if the location chosen for
the installation could in fact accommodate an addional three or four
UPS, access to the area may prove problematic. Check the access route:
1. Is the site easily accessible by road? Bear in mind the size of
the delivery vehicle and the equipment required to off-load the UPS.
1. Are all doorways large enough for the UPS equipment and any
transportation equipment to pass through? 2. Ensure the equipment can
be moved along the entire route especially around corners. 3. Will the
UPS need to be carried across soft or uneven surfaces? 4. Are there
any stairs between the off-loading point and the final location? 5. If
the equipment must be transported using a goods lift, chick that the
lift has the required capacity. 6. Ensure that site staff are aware
the equipment is being delivered and have made every effort to ensure
that access along the route is unhindered on the day of delivery
Environmental Considerations Heat All UPS manufacturers will quote a
maximum operating temperature for their equipment (typically +40C).
The air conditioning plant must have sufficient capacity to maintain
the conditions stated. Obviously the overall efficiency of the UPS
will have a significant effect on both the size and the operating cost
of the air conditioning plant. The high efficiency figures (up to
97). Whilst most UPS equipment is well designed, high relative
humidity levels may promote corrosion of cabinets and internal parts.
Simple dehumidification equipment is available for sites where this
may be a problem. Audible Noise The unit of sound intensity is the
decibel (dB) and it represents the ratio between the sound level
measured with a microphone and a reference sound level, Odb, which is
defined to be approximately equal to the threshold of human hearing.
However as the human ear is less sensitive to very low and very high
frequencies, an additional `A` Filter is applied when measuring
background or other intrusive noises, hence the dBA unit used by all
UPS manufacturers. Typical audible noise figures for fully loaded UPS
equipment range from 50dBA for 5dVA to 60dBA at 60kVA. Electrical
Installation Installation Contractors Electrically installing a UPS,
sometimes refer to as or protected power system is a specialised task
and should only be performed by a qualified and experienced electrical
contractor. The supplier of the UPS equipment should be able to
undertake the installation work or supply a list of suitable
contractors who can provide references of previous installations. Take
the time to: 1. Check the credentials of the staff who will be
installing the equipment 2. Contact and investigate previous
installations and discuss their work with the staff on the other
sites. It is important to ensure that the installation is carried out
in strict accordance with the supplier's instructions and it complies
with local and national electrical installation regulations.
Installation Design Small and medium sized UPS equipment will probable
require very little installation work and minimal changes to the
existing electrical wiring. However, if larger, high-power UPS
equipment is being installed then careful consideration of the
switchgear and cabling arrangements must be made. Considerable time
and therefore cost savings can be made by carefully planning the
electrical installation to allow for possible business growth and the
addition of extra UPS Modules. Using an integrated switchgear and
bushbar solution, makes the installation process for a modern parallel
system much simpler by: 1. Providing a single point of entry for the
incoming mains supply 2. a single point of entry for the bypass mains
supply 3. a fully interlocked maintenance (or wrap-around) bypass
circuit 4. correctly sized bushbars and circuit breakers 5.
co-ordinated protection for the load and UPS equipment 6.
straightforward connection of load distribution panels Connecting the
Critical Loads In order to make best use of the UPS equipment and to
ensue maximum protection of the critical load it is important to
consider carefully how best to connect the load components. Large ring
circuits feeding many critical load devices and unsuitable as a fault
on one device may cause the circuit feeding it to trip or fuse and
consequently disconnect power to other pieces of important equipment.
Radial wiring with individual devices protected by their own circuit
breakers is a far better approach? In this way a fault in one device
will cause that device only to be disconnected and remaining critical
load elements will remain undisturbed. To avoid confusion, particular
attention must be paid to the labelling of circuit breakers and fuses in
the load distribution panels. Earthing In any electrical installation
correct earthing is essential for personnel safety and equipment
protection. A protected power installation in no exception, it is a
essential to ensure that all earthing points within the system are
connected to a properly planned and secure earthing system. As a minimum
a properly planned and secure earthing system for a computer and UPS
installation must provide: 1. Protection against electrical shock 2. A
short, low impedance return path for fault currents 3. A path for
induced currents caused by high voltages such as lightning 4.
Straightforward connection facilities for future expansion. Most
earthing installations are based on star or grid configurations.
Generator Maintenance and Testing A regular generator service program
should include tests and checks of the following: 1. Cooling system
Radiator/heat exchanger, coolant, hoses and connections, fan drive
pulley and fan, fan belts, jacket water heater, water pump, thermostats.
1. Fuel systemFuel tank, water trap/separator, fuel lines and
connections, governor and controls, fuel filters ? primary/secondary,
fuel pressure, air induction and exhaust system, air filter, air filter
service indicator, air inlet system, turbocharger, exhaust manifold,
valves and valve rotators. 1. Lubrication oil system Oil, oil filters,
oil pressure, crankcase breather 1. Starting system Batteries, battery
specific gravity, battery charger, starting motor, alternator, engine
monitor and safety controls, gauges, remote annunciators/alarms 1.
Generator Bearings, slip rings and brushes, space heaters, vibration
isolators 1. Control panel Start controls - manual/auto, voltmeter,
ammeter, frequency meter, circuit breaker, auto transfer switch 1. Gas
engine Gas lines and connections, carburettor and linkage,
magneto/distributor, ignition system, spark plugs 1. Insulation test
Main stator, main rotor, exciter stator, exciter rotor 1. Load testing
With full load, perform a two to four hour load test Overall Energy
Systems Testing As well as performing the periodic Planned Maintenance
of each part of the system, consideration should be given to testing
the entire system on a regular basis. An overall system test will
involve putting the critical load at risk so careful arrangements and
agreements must be made. Mains Failure Test Disconnect the mains
supply to the protected power equipment for an extended period and
check: " With no generator : " the UPS battery supports the critical
load for the expected autonomy time " all alarms and control signals
are correct? " the critical load responds correctly to any signals
received from the UPS equipment e.g. system alarms, orderly shutdown
sequence etc. If a generator is installed: 1. the Automatic Mains Fail
(AMF) equipment operates correctly 2. the generator auto-starts after
the expected time 3. the generator output supply is correct and
within acceptable UPS input limits. 4. the UPS accepts the generator
supply 5. battery recharging takes place while the UPS is being
supplied by the generator. Mains Restoration Test Following the Mains
Failure Test, re-connect the mains supply to the protected power
equipment and check: With no generator: If the UPS is fitted with
auto-restart facilities, 1. The system returns to normal operation
automatically, otherwise all UPS modules can be re-started manually
and the system restored to normal operation 2. All alarms can be
reset. If a generator is installed: " the Automatic Mains Fail (AMF
equipment operates correctly " the UPS signals a mains failure during
the changeover " the generator shuts down after the expected time "
the UPS accepts the restored mains power supply and recharges the
batteries " all alarms can be reset. Power Continuity Means Business
Continuity Please visit our website at or call us on 0845 055 8455 for
further details.