41 / 108
www. e l e c t r i c a l c o n n e c t i o n . c om . a u
41
unless lower than 0.7 when the
automatic voltage regulator (AVR) runs
out of excitation current control range.
Leading power factor is a problem, in
that lower and lower excitation current
is necessary to control voltage as more
power is drawn. The AVR therefore runs
out of range much more quickly and
voltage can rise to dangerous levels.
HARMONICS
The export of current harmonics
is mainly a problem for static UPS
systems.
In an unfiltered state the commonly
used six-pulse converter generates more
than 30% of total harmonic current
distortion (THID). This can be reduced to
about 7% by a second transformer and
parallel connected six-pulse converter,
making a 12-pulse converter.
The latter makes sense for higher
rating UPS systems. For a six-pulse
system the principal harmonics are
the fifth and seventh; for a 12-pulse
converter, the eleventh and thirteenth.
Passive filters for mitigating
harmonics employ capacitors as
parallel elements and inductors as
series elements.
Under light IT load, the input to
the UPS converter appears mainly
capacitive, and that can be a problem
for the standby generator. This is
sometimes taken care of by switching
out capacitors under light load
conditions. The use of active harmonic
filters removes the problem but adds
substantially to the overall cost.
OPERATIONWITHSTANDBY
GENERATORS
Frequency stability is of importance
– there are no golden rules, but it is an
issue if the generator full load capacity
is closely matched to the static UPS.
With a prolonged outage, say, in
excess of 20 seconds, the generator
will start up and supply the AC to DC
converter. Frequency and voltage
stability at the site should be such that
the DC link battery bank is not being
needlessly exercised.
The static UPS double conversion
topology (Figure 1) is better suited to
stable synchronisation than the delta
conversion topology. This is because
the former is entirely isolated from the
mains (yet is mains synchronised under
normal operation).
When a power outage occurs, the
inverter supplying the critical load is
self-synchronised, maintaining this
state until power restoration when
synchronisation is derived from the
static transfer switch.
In the delta scheme, frequency
stability is important. The inverter
has to be in synch with the generator,
because in essence it is regulating the
voltage to the mission-critical load.
Frequency stability is the main issue
for DRUPS systems. The employment of
synchronous motor-generator-flywheel
combinations provides a stable basis.
DIESEL ENGINES
Diesel generators, particularly when
not fully loaded for extensive periods,
are more prone to ‘wet stacking’.
Most standby generator systems up
to 5MW use the reciprocating internal
combustion engine as the power source
to drive the generator.
Diesel is a convenient fuel source,
and the compression ignition systems
of diesel engines have a much higher
thermal efficiency.
When a diesel engine is operated on
light loads, it will not attain its correct
operating temperature. When operated
for extended periods, unburnt fuel is
exhausted and noticed as wetness in
the exhaust system, hence the term
wet stacking.
The obvious solution is to run the
generator set with an electrical load
that reaches the designed operational
temperature of the diesel, or
approximately 75% of full load.
Prolonged operation at low load
will eventually mean an expensive
engine overhaul.
The use of resistor load banks to be
switched in when light load conditions
pertain is recommended. This can be an
automatic switching-in, or as part of a
regular maintenance procedure.
BATTERYANDSYSTEMMONITORING
The monitoring of batteries is
essential for UPS systems.
On-line monitoring requires
string current monitoring, and also
monitoring of individual cell voltages –
that being necessary to prevent a
cell of lower capacity being driven
into reverse condition.
For preventive maintenance, an off-line
‘state of health’ test is recommended.
Capacity tests (discharge tests) should
be done at the installation (acceptance
test) within the first two years of
service, and at intervals no greater than
25% of the expected service life.
For on-line battery monitoring, a
radio-based method is preferable, as it
obviates extensive cable harnesses.
USB and RS-232 (serial port)
connectivity may be suitable for small
installations. However, data centre
UPS systems should include internet
protocol connectivity to allow remote
management across an Ethernet local
area network – the same mechanism
that supports remote server, storage
and other systems management in the
data centre.
Selection of static or rotary UPS should take account
of power quality of incoming supply.