World Grain - September 2024 - 60

MILLING OPERATIONS
Energy cost management:
POWER QUALITY
Poor power quality can damage motors, other equipment
by Jeff Gwirtz
In the article, " Energy cost management - looking at your
energy bill " (World Grain November 2023, pages 50-57),
many hidden charges in the electrical bill were identified that
impact milling energy costs. In " Energy cost management -
power factor " (World Grain March 2024, pages 58-62), the
concept of power factor and its impact on energy cost was presented.
This final installment of the energy cost management
series will examine issues of power quality. The references
identified at the end of this article are encouraged for further
development and understanding of power quality.
Power quality refers to how well the voltage, frequency and
waveform fits expectations of a normal sinusoidal waveform.
Divergence from expectation impacts both motors and sensitive
electronic equipment used in milling operations. Unless
identified and corrected, significant damage can result from
poor power quality.
Minor deviations within specifications often go unnoticed
and have little long-term consequences on electric motors, for
example. However, microprocessor-based equipment may be
more sensitive to minor power quality issues. Disruptions including
unexpected resets, electronic component failures, and
random tripping of process control equipment may be caused
by poor power quality.
Power quality concerns and issues
Power quality is not a line item on the mill electrical bill, but
poor power quality has hidden costs to the milling operation.
Power quality issues and cost are not as easy to identify as
power outages, for example, resulting in lost production affecting
both fixed and variable manufacturing costs. The cost
of poor-quality power or poor power quality events is hidden
as the damage caused by overheating, which shortens equipment
lifespan.
Power quality always has been a concern for electric motors,
the workhorse of the milling industry. The growing
60
presence of electronic sensors, instrumentation and controls
has increased the complexity of electrical systems within the
mill. The presence of highly sensitive electronic equipment in
the mill is challenging and requires careful attention as these
components are interconnected and their interaction must be
managed for optimal efficiency and equipment life.
Major power quality issues are well defined and graphically
illustrated by Accuenergy and Helios Power Solutions
(Hiliosps) websites (page 63). However, harmonic distortions
are more complex in both definition and origin. All these issues
are a cause for concern and require careful evaluation and
consideration for optimal equipment performance and maximum
equipment lifetime.
The following is a summary of power quality issues.
* Voltage Sag/Dip: A sag or dip is a 10% to 90% decrease
in voltage that lasts from a few milliseconds
(half a cycle) up to 60 seconds. It can be caused by
large motors starting up or shutting down, undersized
circuits, short circuits or operations of surge suppressors
in the power system. Microprocessor-based equipment
(process controllers, computers, PLC, Variable
Speed Drives) can malfunction due to voltage sags.
Contactors or relays may trip, and machinery used in
processing operations may lose efficiency or stop functioning.
Dips can also cause momentary dimming of
lights or Human Machine Interface (HMI) display issues.
*
Voltage Swell: A voltage swell is the opposite of a sag/
dip and is a short increase in the voltage above normal
tolerances, typically at 110% or above. They can be
caused by lightning, switching of lines or power factor
correction capacitors, incorrectly sized power sources,
startup or shutdown of large loads, or a fault on one
phase of a three-phase system. Like sags, voltage
swells can cause equipment shutdowns and failures.
September 2024 / World Grain / World-Grain.com
http://www.World-Grain.com
World Grain - September 2024

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