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39
BY
PHIL
KREVELD
duplication, triplication, etc, aspects
to achieve redundancy – also implies
special engineering measures, some of
which are highlighted here.
UPS SYSTEMS
The systems dealt with include:
>
Battery-backed static inverters
supported by diesel generators;
>
flywheel-backed (RUPS – or rotary
UPS) static inverters (the energy
stored in the flywheel takes the place
of batteries, and the flywheel is driven
by a motor generator) with long
(more than 5-30 seconds) interruption
support from a diesel generator; and
>
DRUPS without batteries.
We can’t do justice to the ‘economy’
argument when choosing a particular
system, as this depends on particular
site requirements.
However, in the case of DRUPS, the
idea is to eliminate batteries altogether.
A battery is a limited-life storage
medium, and in Tier IV there must be
no risk regarding availability, including
batteries. Long power interruptions,
outstripping battery capacity, must be
taken into account.
Valve-regulated lead acid (VRLA)
batteries are commonly used, although
there are nickel-cadmium batteries in use
for some major energy storage systems.
VRLAs are considered to be reliable, with
relatively low maintenance.
Notwithstanding the marketing
arguments from suppliers, battery-
backed UPS systems are usually
employed for ratings below 500kVA. The
energy storage density of flywheels is
lower than that of VRLAs, and by a factor
of 10 compared with Li-ion batteries.
BATTERY-BACKEDSTATICUPS
The two important topologies are the
double conversion and delta schemes, as
shown in Figure 1.
The double conversion topology has
the advantage of isolating the load but is
less efficient than the delta circuit.
The delta inverter provides
incremental energy to the load (to make
up for system losses). The primary of
the delta transformer is, in effect, a
variable impedance controlled by the
delta inverter.
When the mains static switch operates
to the off position, the load receives
power from the parallel converter.
The series (or delta) converter makes
up for any difference between the
voltage on the output of the UPS and
the voltage from the mains during
normal operation. It also controls the
input power factor to unity as it is it
takes up mains current that is sinusoidal
and in-phase with the mains voltage.
Furthermore the series-inverter
controls the charging of the battery.
The mains static switch protects against
back-feed into the mains.
Double conversion systems can draw
a considerably higher harmonic current
than delta systems unless fitted with
input filtering or harmonic traps. In both
topologies the static bypass switch has
to be synchronised with the inverter/
parallel converter.
FLYWHEEL-BACKEDSTATIC
INVERTERS
The earliest forms of UPS systems
seem to have been rotary, although that
might be historically debatable.
A rectifier circuit maintained a
battery bank driving a DC motor/AC
generator. The inertia of the motor/
generator provided change-over energy.
A supplemental flywheel accommodated
changeover in a few seconds, allowing a
ride-through with sufficient frequency
as well as voltage stabilisation.
The energy stored in a flywheel
is proportional to the square of the
rotational speed, and to its moment
of inertia. In an ideal flywheel, all of
the mass is concentrated at its outer
radius, giving it a moment of inertia
proportional to the square of the
radius. Thus the energy stored is
proportional to:
MR
2
N
2
Where M is the mass, R the radius, and
N the revolutions per second or minute.
The principle of the UPS is shown in
Figure 2.
The AC motor/DC generator
flywheel unit can be powered from
the auto-transfer switch end or the
inverter output.
The flywheel includes a hub that is
formed from aerospace-grade steel, a
high-speed permanent magnet motor
generator, and contact-free magnetic
bearings that levitate and sustain the
rotor during operation.
Typically the DC generator will supply
a 500V DC link. The manufacturer,
Vycon, states that a 300kVA UPS
would have up to seven seconds of
uptime for the diesel generator to
come on line. For additional capacity,
flywheels can be paralleled.
DRUPS
These systems are generally devoid of
battery static USP circuitry.
As in the system described above,
the bridging energy during change over
from mains to UPS is supplied by a
flywheel. The flywheel inertia should be
thought of as the combined inertia of a
motor generator and coupling. The basic
idea is shown in Figure 3.
The synchronous motor/generator
is connected to the second motor/
generator coupling flywheel combination
via a bi-directional power converter.
Under normal mains conditions, the
synchronous machine operates as an
over-excited synchronous condenser,
correcting power factor and drawing
only energy for windage and bearing
losses for the entire ‘rotating system’.
The buffer inductor (tapped inductor)
has a low mutual inductance impedance
but a high impedance providing
isolation for the synchronous motor/
generator, which regulates the critical
load voltage.