07-12-2012, 01:15 PM
Compensation of harmonic currents generated by computers utilizing an innovative active harmonic conditioner
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Abstract
MGE UPS SYSTEMS MGE0128UKI – 01/2000 3
In little more than ten years, electricity power quality has grown from obscurity
to a major issue.
Electronic converters and power electronics gave birth to numerous new
applications, offering unmatched comfort, flexibility and efficiency to the customers.
However, their proliferation during the last decade is creating a growing concern and
generates more and more problems: not only these electronic loads pollute
the AC distribution system with harmonic currents, but they also appear to be very
sensitive to the voltage distortion.
Then, electricity power quality is becoming a major issue for utilities and
for their customers, and both are quickly adopting the philosophy and the limits
proposed by the new International Standards (519-1992 IEEE, 61000.3-2/4 IEC).
Today, recent advances in power electronic technology are providing
an unprecedented capability for conditioning and compensating harmonic distortion
generated by the non-linear loads.
INTRODUCTION
MGE UPS SYSTEMS MGE0128UKI – 01/2000 4
Today, the situation on low-voltage AC systems has become a serious concern.
The quality of electrical power in commercial and industrial installations is
undeniably decreasing.
In addition to external disturbances, such as outages, sags and spikes due to
switching and atmospheric phenomena, there are inherent, internal causes specific
to each site and resulting from the combined use of linear and non-linear loads.
Untimely tripping of protection devices, harmonic overloads, high levels of voltage
and current distortion, temperature rise in conductors and generators all contribute
to reducing the quality and the reliability of a low-voltage AC system.
The above disturbances are well understood and directly related to the proliferation
of loads consuming non-sinusoidal current, referred to as "non-linear loads".
This type of load is used for the conversion, variation and regulation of electrical
power in commercial, industrial and residential installations.
Topologies of active harmonic conditioners
The idea of active harmonic conditioners, also named active filters, is relatively
old, however the lack of an effective technique at a competitive price slowed
its development for a number of years.
Today, the wide-spread use of IGBT components, mastery of their implementation
and the availability of new digital signal processing (DSP) techniques are paving
the way to a much brighter future for the active harmonic conditioner.
The active harmonic conditioner concept uses power electronics to produce
harmonic components which cancel the harmonic components of the non-linear
loads. A number of different topologies are being proposed, whom some of them are
described here after. Within each topologies there are issues of required
components ratings and method of rating the overall conditioner for the loads to be
compensated.
Parallel active harmonic conditioner: system description
The converter comprises a three phase IGBT current inverter leg that chops at
an average switching frequency of 16 kHz, chemical capacitor C2 and C3 providing
back up power. The conditioner draws from the power source the active power
required for its operation.
The control electronics comprise:
n an harmonic-extraction module which generates a regulation set point proportional
to the harmonic components of the load current;
n a module that regulates inverter currents and the DC voltage;
n a monitoring module which ensures filter protection in the event of overload or
an internal fault;
n a control module which generates the control signals necessary for inverter
operation.
Conclusion
A 30 Amp "shunt topology" active harmonic conditioner was successfully developed,
and is being marketed.
All the installations equipped with the SineWave active harmonic conditioner
demonstrate excellent performances in a wide range of applications. Regarding
computer type loads, the presented case study is a clear demonstration of the high
level of harmonic current compensation that the conditioner can achieve. As a
consequence of the compensation of the 3rd harmonic current, the active
conditioner also reduces the neutral (harmonic) current.
These results gives very good reasons to expect in a very short time
the development of active harmonic conditioner to compensate harmonic distortion
in the commercial applications, but also in the industrial sector.