Generators and UPSs must join forces
in the war on power outages
The
days when generators and UPS (Uninterruptible Power Supply) systems were
specified, bought and installed separately are over. Today, standby power plants,
particularly in large enterprises, have to be designed holistically – as a
unified solution.
The
key is compatibility. Because UPSs
and generators must work together, they need to be specified accordingly. For
example, UPS’ that use IGBT (insulated gate bipolar transistor) technology to create AC waveforms will
provide a nonlinear waveform. This nonlinear waveform contains harmonics which
can cause voltage distortion across the reactor of the generator, which results
in unacceptable transient performance, adversely impacting on the performance
of the entire system.
Harmonic distortion can cause excessive heating at the generator, resulting in excessive voltage distortion across the system.
Harmonic distortion can cause excessive heating at the generator, resulting in excessive voltage distortion across the system.
The
UPS selected must be able to damp down this distortion. To this end, UPS
systems that include a 12-pulse rectifier will be more generator-friendly than
those with a six-pulse rectifier.
A
six-pulse rectifier will, typically, have about 30% total harmonic distortion
(THD), rich in fifth and seventh harmonics. A 12-pulse rectifier will have less
than one-half of the THD of a six-pulse rectifier and will be rich in eleventh
and thirteenth harmonics.
In
addition, some UPS manufacturers provide a passive filter to reduce the THD
seen by the generator even further. UPS harmonics can cause voltage waveform
distortion at the generator, but permanent magnet generators that derive their
own excitation can help to mitigate this problem. Digital voltage regulators and
digital excitation controls can further provide immunity to the harmonic effect
of nonlinear loads.
Proper
specification of both the generator and UPS components is essential and will
ensure the desired level of power quality and uptime.
In
my next blog we’ll discuss the challenge of ‘step loading’.
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