Keeping current: Strike back against common power distortions and disturbances
Written by Harshad Singh
Downtime due to problems with the quality of electric power can be
costly. With rising levels of automation and computerization in
manufacturing facilities, voltage sag lasting just a few milliseconds
can bring an automated factory floor to a halt. Power disturbances are
bound to occur, and while some may only briefly interfere with sensitive
equipment, others could result in the total loss of power for days.
Most international standards define power quality as the physical
characteristics of the electrical supply provided under normal operating
conditions that do not disrupt or disturb the customer’s processes.
(The quality of power supply implies voltage quality and supply
reliability.) Therefore, a power quality problem exists if any voltage,
current or frequency deviation results in a failure or in a bad
operation of equipment. Voltage quality problems relate any failure due
to deviations of the line voltage from its normal characteristics, and
the supply reliability is characterized by its adequacy (ability to
supply the load), security (ability to withstand sudden disturbances)
and availability (focusing especially on long interruptions).
When power is generated, it has very predictable characteristics. It
energizes all electrical equipment equally and satisfactorily. However,
as the power travels through the wires and energizes the equipment, the
various pieces of equipment can change its quality, making it less
suitable for the next application. These changes are especially common
in large industrial complexes and include increases and decreases in
voltage, momentary power outages and noise on the electrical system; at
its most extreme, poor power quality can even cause equipment to
malfunction. This can cause business problems such as lost productivity,
idle people and equipment and lost data.
Voltage sags
Voltage sag, or “undervoltage,” is a momentary decrease in voltage
outside the normal tolerance, typically caused by the starting of heavy
loads, lightning and power system faults. A good example is the starting
of a motor. Motors draw more current when they are starting than when
they are running at their rated speed. Consequently, the starting of a
motor causes voltage sag while it accelerates to its rated speed.
Excessive load changes may also cause sags. Computer equipment and
controllers may power down depending on the duration and magnitude of
the voltage sag. In addition, voltage sags cause loss of data.
One way of reducing damage would be to connect sensitive electronic
devices to circuits other than the ones large motor-driven appliances
are on, try to lighten the load on the affected circuit. If a sag lasts
longer, it is classified as undervoltage and is usually caused by
circuit overloads, poor voltage regulation and intentional reduction by
the utility. The impacts of undervoltages include equipment shutdowns,
and the overheating of motors.
Voltage swells
A voltage swell, or “overvoltage,” is a momentary increase in voltage
outside the normal tolerance. Voltage swells are caused by sudden
decreases or the turning off of heavy loads and can damage equipment by
breaking down insulation. If voltage swells last longer, they are
classified as overvoltage, frequently caused by poor voltage regulation.
For instance, shunt capacitors designed to improve upon the voltage
profile might supply excessive reactive power and cause overvoltages.
Periodic loads, such as pumps, cause periodic increases and decreases of
amplitude (sags and swells); this is referred to as voltage modulation.
Outages
Outages, or blackouts, simply occur when power completely drops off line
for anywhere between brief moments to several hours. Power outages can
be caused by many circumstances, such as storms, which can be
accompanied by heavy wind, ice, precipitation and lightning.
Momentary outages, which are seen as a dimming or flickering of lights
or even a brief loss of power, are caused by short circuits. Short
circuits happen when something, such as a tree limb, comes into contact
with power lines or when the lines touch each other. When a short
circuit occurs, a breaker automatically de-energizes the circuit and
interrupts the flow of power. Electrical equipment is designed to
quickly open and close the breaker two or three times automatically
attempting to clear the problem. An uninterruptible power supply or
generator could help mitigate the drop in power in both instances.
Harmonics
For most of the 20th century, the predominant use of electricity for
business and industry was to power motors, lights and heating devices.
These uses have little effect on the 60 Hz (cycles per second) sine
waveform of the electricity delivered to them from their utility. They
are referred to as linear loads, because the current rises and falls in
proportion to the voltage wave. A few industries, such as steel mills
and aluminum smelters, used electricity to power arc furnaces, which
distorted the sine waveform because the current flow was not
proportional to the voltage. These loads are referred to as non-linear
loads.
Non-linear loads cause waveforms that are multiples of the normal 60
Hertz sine wave to be superimposed on the base waveform. These multiples
are called harmonics. In the last 20 years, there has been an explosion
of microprocessor-based equipment, which are also non-linear loads.
Equipment widely used in offices and manufacturing not only creates
harmonic issues but are also susceptible to harmonic disorders. These
include are computers, monitors, adjustable-speed drives, welding
equipment, transformers, etc. The list goes on and on. The invention of
all these sophisticated electronic gadgets and circuitry has increased
the problems associated with harmonics immensely.
Transients
Transients can be defined as sudden, brief increases in current or
voltage in a circuit that can damage sensitive components and
instruments. These disturbances are shorter than sags or swells, and are
caused by sudden changes in the power system. Two main categories of
transients based on their duration are switching surge and impulse
spike.
Switching surges are caused as a result of resonating circuits with
switching devices. Large capacitor bank switching can cause resonant
oscillations, leading to surges causing tripping or even damaging
protective devices. Electronically controlled industrial motors are
particularly susceptible to these transients.
Impulses are very short spikes, in the order of a few microseconds. They
are mainly caused by lightning strikes, arcing and insulation
breakdowns. Most power systems are protected from surges and spikes
using a transient voltage surge suppressors (TVSS) device and by surge
dividers and arc-gaps at high voltages and avalanche diodes at low
voltages. If these disturbances are encountered by monitoring systems,
depending on the magnitude of the spike, it may saturate or damage the
monitoring sensors.
Furthermore, TVSS systems are an effective first step toward power
quality management. They’re generally specified to protect the incoming
AC power at the service entrance, the branch panels and individual
sensitive loads. Additionally, signal and control circuit conductors are
also protected to eliminate any activity from coming in the “back
door.” It should be considered mandatory where there are safety-to-live
and public safety considerations involved. Applications include water
and wastewater treatment plants, manufacturing systems, industrial
robots, power system controllers, and controllers in food-processing and
petrochemical plants.
Power-quality monitoring is extremely important: it provides a
continuous health check of a facility’s power system to see, diagnose
and avert looming problems.
Harshad Singh is a technical sales specialist with Gescan Ontario in Milton, a division of Sonepar Canada.
