SELECTING THE CORRECT UPS FOR THE JOB
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When most people think about poor power quality, blackouts or
brownouts come to mind. In fact, these are some of the most uncommon
power quality issues facing the industry today. The most typical
power quality problems are under voltage (sags), spikes and surges,
over voltage, noise, and lastly, blackouts. While outages are
the most visible of power problems, equipment damage, data loss,
and spurious errors are usually caused by these other more common
power quality problems.
The best way to safeguard from power quality issues is to set
up an Uninterruptible Power Supply (UPS).
There are several considerations when deciding on what type
of UPS to use. The major options include using a large centralized
three-phase UPS or a smaller-application specific single-phase
UPS. To decide what type of UPS is best for your application,
consider the following:
How important are the loads (non-essential vs critical loads)?
Which UPS topology best supports the applications (online, line
interactive, off-line)?
What are the power requirements (is three-phase power in use)?
UPS Capability and battery run time?
How difficult is UPS installation?
When maintenance is required and how extensive is that maintenance?
What is the life-cycle cost of the system?
Non-Essential Vs Critical Load
The consequence of equipment failure is the single most important
factor in deciding on the type of UPS. For small office PC loads
where single station or small network data protection is required,
a small single-phase UPS is often an adequate solution. Most
single-phase UPSs use off-line or line-interactive topologies,
resulting in a small (but acceptable for standard PCs and servers)
interruption when transferring to and from battery power. Because
PCs, workstations and peripherals are often located in a decentralized
manner throughout offices, a dedicated off-line UPS or inexpensive
line-interactive UPS often priced under $200 provides an ideal
solution.
If the equipment to be protected is critical, an online UPS
is the best choice. This topology transfers to and from battery
and internal bypass without any power interruption to the load,
providing a truly seamless transfer to battery power.
For loads above 16kVA, the most practical solution is a three-phase
UPS. Three-phase online UPSs offer the advantage of providing
centralized protection using a single UPS. This simplifies maintenance
and battery replacement while supplying high quality uninterruptible
power to critical loads.
UPS Topology Choices
The power protection technology supporting lighter load equipment
such as desktop computers and workstations differs significantly
from the advanced power management systems that protect mission-critical
equipment. This creates a lot of confusion in trying to understand
the three basic types of UPSs:
Off-Line UPS
For a UPS with offline topology the inverter is connected in
parallel and acts simply to backup utility power. The first
component in an off-line UPS is input surge protection to protect
the load from high-voltage surges/spikes. The second element
is the battery to supply the inverter with power, and the third
element is the inverter. The inverter takes the dc battery voltage
and creates the ac voltage required to power your equipment.
The fourth component is the battery charger. Under normal conditions,
the inverter is sitting idle until the input voltage goes above
or below a usable level. At that point, the inverter will turn
on and supply the load with ac power.
Line - Interactive UPS
With the line-interactive topology, the inverter is connected
in parallel and acts to backup utility power. It also charges
the battery. Through its reversible operation, it interacts
with utility power to stabilize the voltage.
A line-interactive UPS is similar to an off-line UPS because
when sensing an under voltage or over voltage situation, it
also requires a transfer time for the inverter to turn on and
to supply power to the load. The main difference between an
off-line and a line-interactive UPS is that a line-interactive
UPS in the stand-by mode has active voltage regulation. The
primary advantage of a line-interactive over an off-line UPS
is that it doesn't use its batteries nearly as often, which
extends battery life
Online UPS
With online topology, the inverter is connected in series between
the ac input and the load. Power for the load flows continuously
through the inverter. An online UPS has some of the same components
as an off-line UPS with a few differences — most notably,
a rectifier. An online UPS has input surge protection, batteries,
inverter components and a rectifier. The rectifier takes the
input voltage and changes it from ac voltage (alternating current
coming from the utility power) to dc voltage to charge the battery
and provide dc power to the inverter. In most online UPSs, the
rectifier or filtering is used to make sure that the load, as
well as the UPS, does not allow unnecessary noise and harmonics
to be fed back into a building's power. Because an online UPS
is always creating its own power from the rectifier and inverter,
it never has transfer interruptions and can provide a much higher
level of power quality.
All of the topologies offered differ in price and performance,
the key to successful power protection is choosing the UPS that
is right for your needs, no less and no more.
Facility Power requirements
In larger facilities the electrical distribution network, as
a power source, is generally a three-phase power configuration.
This configuration provides multiple voltages: three-phase power,
single-phase (phase-to-phase), and single-phase voltage (phase-to-neutral).
Smaller facilities often have to contend with the availability
of only 208Vac Single Phase.
For three-phase power, during plant evaluation and UPS installation,
the engineering team distributes the electrical loads equally
between the three phases of the electrical network system by
ensuring that all are supplying the same amount of power to
within 30% of each other.
This requirement has to be considered from the initial installation
and must be reevaluated every time a new piece of equipment
is added or removed. Ignoring this key step or increasing the
load imbalance between phases could cause problems with the
power source, which results in voltage regulation being lost
or displacing the phase angles. It might also cause overheating
and overloading of the power source.
Both single-phase and three-phase UPSs provide power protection
for critical loads, but there are several important power loading
considerations to evaluate.
A three-phase UPS can supply a 100% unbalanced load on its output
without causing any problems or affecting its performance. With
the inverter generating the power, it can also compensate for
the unbalanced load and regulate the output voltage accordingly.
Single-phase UPSs (phase-to-phase or phase-to-neutral) act
as a load on the main power source. This situation can cause
an unbalanced load on the power source. The loads upstream to
the UPS may have to be rewired to compensate for the unbalanced
load.
Current demand on the input of a three-phase UPS is much less
than the input current on a single-phase. The reduction in the
input current results in a reduction in the input cable size
and the upstream protection devices, such as circuit breakers.
Example:
If the load requirement is 10 kVA @ 280 Vac
Single-phase UPS input current is:
(10 kVAx1000)÷208 Vac = 48 amps
Three-phase UPS input current is:
(10 kVAx1000)÷208 Vac÷1.732 = 28 amps
This calculation indicates that the input current of a single-phase
UPS is nearly twice that of a three-phase. Reduced input current
requires smaller cables and circuit breakers, which shrinks
installation and maintenance costs.
On the other hand, a single-phase UPS may be fine in a highly
distributed cell or light manufacturing environment where there
are a very large number of small UPSs. The higher upfront costs
of networking these cells into one large system (if plant wide
data collection is unimportant) may outweigh the benefit of
a larger systems network approach. Data back up systems also
can be installed to save information on a periodic basis. If
the facility wasn't initially set up for three-phase power,
the additional retrofitting costs also might make a three-phase
UPS prohibitive.
UPS Capability and battery run time
UPS capacity should suit your needs. Include any future expansion
requirement and remember that, typically, networks grow by five
to 10 per cent on average. Also remember that spare capacity
in the early years of your UPS results in longer runtimes when
mains fail.
Consider the amount of runtime required for your application.
Generally five minutes is acceptable if you have a generator
in place however much more time would be required should you
need to gracefully shut down. For longer run time applications,
you’ll need to consider the heat impact on your data center
when air conditioning is not available.
Cost of installation
Single-phase plug and play off-line or line-interactive UPSs
are easy to install. The end user often does it during initial
PC setup. Combining each single-phase UPS into a network may
not be easy, requiring extra cabling hardware as compared to
supporting the entire network with a single three-phase UPS.
Three-phase UPSs protecting larger facilities or more critical
loads require an electrical contractor to install the system.
Upfront costs of using an electrical contractor can be relatively
expensive, but a properly designed electrical power system with
a UPS and backup generator can pay large dividends over the
long-term by reducing electrical power costs and plant downtime.
Daily operation
With one three-phase UPS, all status and alarm functions can
be monitored from a central point. To monitor the status of
each single-phase UPS, they must be connected individually resulting
in costly wiring runs. In addition, the ability to quickly identify
a defective single-phase UPS from a central location may be
difficult. With a three-phase UPS, there is no doubt as to the
location of the UPS.
Maintenance
Battery monitoring and replacement are by far the biggest maintenance
tasks with both single and three-phase UPSs. Single-phase UPSs
are often considered a disposable peripheral; just toss and
replace when the batteries are dead. While this is convenient
in one sense, it gets to be a time-consuming task when a plant
is full of discrete single-phase UPSs in multiple locations
that require constant checking one at a time, especially if
there is no network on-line monitoring capability.
For many smaller-to-midsize facilities, which are experiencing
growth, one of two problems begin to occur after retrofits.
Either isolated (non-networked) UPSs fail during power interruptions
due to undetected dead batteries in inconvenient or forgotten
about locations or battery monitoring software goes off frequently
in multiple locations, causing the plant maintenance staff to
spend too much time on battery or UPS replacement.
With a single three-phase UPS, battery testing and replacement
are more conveniently done at the network level. Testing can
be performed from one central monitoring location with a single
battery set, as opposed to testing the batteries of each single-phase
UPS located throughout the building (which also can be done
in a networked single-phase UPS system). Battery maintenance
and replacement are performed in the plant's computer room.
It is far less expensive to maintain one single set of batteries
for a three-phase UPS than a large quantity of single-phase
UPS module batteries. In some cases, the batteries for a single-phase
UPS are not replaceable and the entire UPS must be replaced.
Three-phase UPS modules can have single batteries replaced if
they fail.
This white paper is written by Guy Lacerte with the
assistance of MGE UPS Systems.
Guy Lacerte can be reached at 800.876.9373 (x282), glacerte@power-solutions.com
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