New research shows cheaper and simpler is better, most of the time, for improving power quality.
On November 7, 2002, torrential
rains moved through the San Joaquin Valley, CA, causing multiple plant shutdowns
due to voltage sags at Del Monte’s fruit packing plants in Modesto, Kingsburg,
and Hanford. Fortunately, the rains occurred after the summer production months
of July, August, and September, when the plants operate 24 hours per day, seven
days a week. The shutdowns led Mark Stephens, manager of industrial power
quality systems at the Electric Power Research Institute (EPRI), to work
extensively with Del Monte and other food processors in California presenting
ways to make food processing systems more robust and to ride through such
storms.
Alex McEachern, owner of Power
Standards Lab and an acknowledged international expert on power quality, says
the facts of nature and man won’t allow the elimination of voltage sags of less
than a second. “We can’t get rid of birds, snakes, squirrels, tree trimmers, and
lightning,” he says.
As a result, he says, there will
always be power quality problems. The solutions advocated by both men are
based on economics—the smaller and cheaper, the better.
McEachern points out that 95% of
electrical power goes to lights, heaters, and motors. The 5% that goes into
other equipment is the economically important stuff. For example, in a Big Box
store, 1% of the electricity goes to operate cash registers. If electricity is
lost for 10 minutes, losing lights and air conditioning is manageable, but doing
without cash registers adds up to the loss of big bucks.
Stephens, who is also the author
of many EPRI power quality reports over the past 10 years, has become an
advocate of smaller solutions at the control systems level, to make systems more
robust and able to ride through power quality problems. McEachern agrees with
Stephens. As the designated US expert in front of the International
Electrotechnical Commission on power quality measures and voltage sag immunity,
McEachern treats power quality as a compatibility problem, while others may see
power as the problem to be fixed.
Developments in Technology
Uninterruptible power supplies
(UPS) have been the traditional method for coping with poor power quality.
Flywheel technology is now replacing lead acid batteries in UPSs. However, as
McEachern says, flywheels, although much less expensive than batteries and
virtually maintenance-free, are still expensive solutions.
EPRI’s research and case study
investigations over the past decade have identified voltage sags and
interruptions, transients (both capacitor-switching and lightning-induced), and
harmonic distortion to be the power quality phenomena that have the highest
importance to end users. The large majority of power quality problems are
related to these three items.
Due to the high occurrence rate
and the general high cost and complexity of typical solutions, says EPRI,
short-term voltage variations are one of the most, if not the most, important
category of power quality phenomena for end users. It reports that 96% of all
voltage sags and outages last 10 seconds or less. Therefore, “protection of
end-use equipment from voltage sag phenomena is the most important power quality
consideration.”
Stephens says earlier generations
used relay-based control systems to power computer-controlled process lines.
With the transition to programmable logic controllers (PLCs), which have become
the backbone of industrial automation, manufacturers have found PLCs are
susceptible to voltage sags, interruptions, and transients. More recently,
manufacturers have made improvements in equipment—especially variable frequency
(adjustable speed) drives—to make systems more robust.
“We are moving away from making
power better and toward making equipment more tolerant of disturbances,” says
McEachern.
On average, once a month there
will be voltage sag. Yet, sensitive equipment may not be tough enough to handle
the available incoming power. All equipment should be designed to handle those
sags, he argues.
To assist equipment manufacturers,
the International Electrotechnical Commission (IEC) has established tolerance
level specifications, IEC 61000-4-34, to identify normal voltage sag, adds
McEachern.
EPRI documented Stephens’ study of
the Del Monte fruit production facilities and the proposed solutions, in
“Assessing Power Quality Impacts and Solutions for the California
Food-Processing Industry.” It was funded in part by the California Energy
Commission’s Public Interest Energy Research Fund (PIER).
This work with Del Monte led the
Northwest Food Processors Association to ask EPRI to develop food-processing
guidelines for power quality. More recently, Stephens and an engineering team
has been working on a new IEEE (Institute of Electrical and Electronics
Engineers Inc.) standard, P1668, “Recommended Practice for Voltage Sag and
Interruption Ride-through Testing for End-use Electrical Equipment less than
1,000 Volts.”
This standard is aimed at
providing a general voltage sag standard that can be used throughout different
industries. “We hope this standard will be more cross-cutting throughout
industry and provide a means for more complete testing of equipment to voltage
sags,” says Stephens.
EPRI was also instrumental in the
earlier development of industry-specific standards, such as SEMI F47, which lays
out specifications for semiconductor processing equipment voltage sag immunity.
It defines the voltage sag tolerance expected from semiconductor tooling
equipment. SEMI (Semiconductor Equipment and Materials International) is the
global industry association serving the advanced manufacturing supply chain
through development of international standards and other activities.
Small Is Beautiful ...and
Cheap
In an industrial process facility,
components within the control panel “should be able to survive minor voltage
sags without causing the system to trip offline,” using robust relays,
contactors, and power supplies, according to EPRI. If the control for the
process machinery consists of mainly PLCs, relays, contactors, and drives, the
use of a control level power conditioner or 24-V-direct current (DC) power
scheme for the control can be very effective, says Stephens.
Stephens and his colleagues at
EPRI have become proponents of utilizing 24-V DC to power PLCs or automated
controller power supply and input/output (I/O) control power on new designs to
make them more robust. Robustness is tied back to the DC signal.
“Most of the problems I see [can
be fixed] within control systems,” he says.
As he explains, a 24-V DC power is
installed as a main source for cabinet controls instead of a control power
transformer. If the I/O control voltage is already DC, only the alternating
current (AC) input power supply module needs to be replaced with the comparable
DC input power supply module.
“You don’t need power conditioning
on a motor for short events,” continues Stephens. “Up to 85% of events have a
15% drop in voltage for one cycle and this will shut down a PLC. The further you
go away from the control cabinet the more expensive it becomes to solve power
quality issues; it’s a matter of economics.”
Energy savings might also be
available, depending on the efficiency of the DC power supply, notes Stephens.
By lowering the amount of current, demand could be reduced and power factor
improved. If AC control circuits are used, highly efficient power conditioner
options are available, such as units like a dynamic sag corrector that use
series injection technology. When an event occurs, the power conditioner will
draw more current and inject the missing voltage back into the line.
Where a control level solution is
not feasible because sensitive three-phase loads cannot be easily mitigated at
that level, he recommends the use of large three-phase power conditioners such
as a flywheel or series injection technology.
Stephens also identified another
issue—the AC ice cube relay. It is often used to interface between PLCs and
variable frequency drives or motor starters. It can also be used in the “machine
on” or “emergency off” circuits. With sensitivities typically in the range of
70% of nominal voltage sag, these relays present a challenge to a control system
that is trying to ride through voltage sags. For this reason, circuits with AC
“ice cube” relays need power conditioning to keep them from causing the controls
to be affected during voltage sag events.
Other Solutions
The EPRI food processing study
contained additional troubleshooting recommendations of interest to a wider
audience.
- Avoid mismatched control power
voltages. If the actual control system nominal voltage is lower than the
expected nominal input voltage, for example, if a 230-V AC input power supply is
connected to a 208-V AC source, the entire control system will be more
susceptible to voltage sags.
- Most electronic devices come
with power supplies rated at 100 V to 240 V, to cover standard voltages
worldwide. In the US, if the current is at 120 V, equipment will continue to
operate with backup unless the voltage sag drops 60 V. If equipment is connected
to 208 V or 240 V, it will have more margin to be protected from sags.
- Use a robust DC power supply
scheme to ensure that the 24-V DC power source remains steady during typical
voltage sag events. Unregulated DC power supplies have the worst ride-through,
while universal input switch-mode DC power supplies are the most immune over the
range of possible voltage sag scenarios.
- Properly maintain the
controller’s battery. Many PLCs utilize lithium-ion batteries to maintain its
control programs and non-volatile memory data in the event of a power loss or
voltage sag-induced shutdown. Battery death can cause loss of the PLC program
and extended downtime due to the need to locate the latest back up, reload, and
restart the process. Alternatively, utilize a state-machine programming method
or non-volatile PLC memory.
- Consider the power source for
analog input signals to ensure that the source is stable throughout normal
voltage sag events.
- Utilize only compatible power
conditioners. For control level power conditioning, use battery-less power
conditioners if possible rather than the standard small battery-based UPS.
There are many alternatives
available now for providing power conditioning without the use of a battery,
including constant voltage transformers, dip proofing inverters, dynamic sag
corrector (a series injection device), or voltage dip compensator. Stephens has
observed many small battery-based systems abandoned in place in control
cabinetry. Once the battery dies, the manufacturer will bypass the UPS and get
the line up-and-running again. Therefore, battery-less power conditioners offer
the freedom from battery replacement.
If a UPS is used, avoid those with
square-wave outputs, unless the controller manufacturer can assure that the
system power supply and I/O cards are compatible. A line-interactive UPS that
produces a true sine-wave output has effectively mitigated voltage sags in
tests, says EPRI.
 |
Photo: EPRI |
| EPRI's Mark Stephens conducts a power quality audit on a typical control cabinet in a glass bottle manufacturing facility, while Baskar Vairamohan (behind him) takes notes. |
In the case of the Del Monte fruit
production facilities, EPRI found that adjustable speed drives with sizes
ranging from a few horsepower to 75 horsepower were not susceptible to
single-phase voltage sags, and could withstand a zero-voltage event on a
single-phase lasting up to one second or more and continue operation. The other
two remaining phases are able to provide peak-charge voltage needed to keep the
drive above the trip level. However, two-phase voltage sags caused adjustable
speed drives to drop offline during the event.
EPRI says adjustable speed drives
have a variety of setup parameters that can be changed, that will influence the
ability of the unit to ride through voltage sags. These parameters include
flying restart, kinetic buffering, and adjustment of the DC bus trip level.
Transients
Transient over-voltages are
usually caused by switching operations or lightning strikes to electric
facilities and can have significant potential to damage electric power equipment
or disrupt operation. Off-the-shelf and inexpensive transient voltage surge
suppressor products are available to the user or original equipment
manufacturer.
Low-frequency oscillatory
transients are more difficult to treat. They are associated with the energizing
of shunt capacitor banks on a utility’s distribution system. Because they
contain substantial energy, their effects can be felt quite far electrically
from the point of origin.
Harmonics
Harmonics, says EPRI, have
increased significantly over the past two decades due to the increased use of
non-linear loads, such as adjustable speed motor drives and switch-mode power
supplies. McEachern went further: Over the past 70 years, loads like light
dimmers and industrial heaters that draw wave forms have been added to the
distribution system that was built for sinusoidal loads like heaters, lights,
and motors.
And because harmonic distortion
levels will begin to cause more problems for utilities, harmonics have received
continuous attention from standards-making bodies and technical groups. IEEE has
developed rules-of-thump to estimate limits on the percentage of total load
represented by adjustable speed drives. Problems related to harmonics are
usually confined to locations with inordinate amounts of nonlinear, harmonic
current-producing loads, such as wastewater treatment plants where the entire
load may be adjustable speed motor drives powering pumps. Or, they may occur
where power factor correction capacitors on the end-user system, or at the
utility distribution level create resonances that amplify the effects of
nonlinear loads.
McEachern’s solution, again using
the philosophy of finding cheaper solutions, is to install a larger transformer,
rather than a harmonics filter. Usually the effects of harmonics at the customer
level are seen in telephone interference and burned out transformers, he
says.
Flywheel UPSs
Flywheel technology has really
matured in the last few years, Stephens says. There are now a variety of
manufacturers that offer flywheel UPS systems. Flywheels use motors to collect
energy. Once utility power is disrupted, the momentum of the spinning flywheels
keeps its internal “saved” power flowing to the equipment being served for 10 to
15 seconds, smoothing out the voltage sag. This gives a backup generator enough
time to immediately take over from the flywheel if the interruption lasts longer
than 15 seconds.
Matt Servis, director of
engineering at Vermont television stations WFFF-TV and WVNY-TV, says when two
solid state digital transmitters were installed, they decided to add a 65-kW
flywheel UPS, manufactured by Active Power, because of the poor power quality in
the area. The digital transmitters are driven by computers, he says, which are
much more susceptible to dirty power than were the analog-driven older
transmitters being replaced.
The transmitters are located on
Vermont’s highest peak, Mount Mansfield, and are powered by a distribution line
shared with a ski lift. When it operates, the line experiences voltage sags and
spikes, and power drops out for two to three seconds. UPSs with battery backup
were impractical, he says, because replacement batteries would have to be hauled
up the mountain in a snow mobile if a battery went out between October and May.
The lifetime of a battery is three years on average. Other solutions at the
control panel level were not practical, says Servis, because the control system
is an integral part of the transmitter, with power being fed through one
transformer.
Since the flywheel was installed
in October 2006, Servis reports it has discharged over 300 times, but the
transmitter has experienced no outages. He is aware of problems now only when
competitors go off the air. After it was installed, he experimented and pulled
the power plug. The flywheel supplied power for several minutes—not seconds, and
Servis reconnected the power before the flywheel ran out of power.
Jim Clishem, president and chief
executive officer of Active Power, says his company’s flywheel UPSs offer
particular advantages to healthcare and broadcast facilities where large amounts
of power are required in short periods of time. In these situations, batteries
deplete very quickly. The pulsation of current lends itself to flywheel
operation, he comments. More flywheels, joined in multiples of units, can fit in
smaller spaces than can batteries, Clishem says, by a factor of four.
Active Power flywheel UPSs cost
between $55,000 and $60,000 for a 130-kV-Amps system, and $640,000 for a 2-MW
system. Clishem reports maintenance costs for an Active Power system are
one-third of a UPS system with battery backup, because of the need to replace
batteries periodically.
It’s the Economics,
Stupid!
To evaluate whether the solution
is a small, inexpensive solution such as installing a 24-V DC power supply in
control panels or a flywheel UPS, McEachern explains that the engineer needs to
look first for free solutions, and then measure the cost of the events against
the cost of other solutions.
He cites several examples: a
company produces widgets at $1 million per hour. A sag shuts down the line for
half an hour once a month. The manager’s solution is to pay six workers $500 at
the end of the day to make up for the lost production. On the other hand, a
large auto manufacturing plant operates by receiving its supplies on a
just-in-time basis and receives bumper shipments four times a day. The bumper
factory has power quality problems and the loss of tens of thousands of dollars
at the bumper factory costs the auto plant millions of dollars.
At a pharmaceutical plant
McEachern recently visited, they were experiencing voltage sags once a month.
After each incident, staff restarted machines in a standard routine. He sees
this as a reasonable solution, as long as the steps to restart motors are
written out to accommodate personnel changes.
In each case, McEachern asks
managers what they are willing to pay to fix their problems. “Far more commonly,
they are looking for common-sense, less expensive solutions,” he says.
The auto plant manager will be
eager to seek a far more expensive solution than the widget company.
McEachern says a manager may be
better off tolerating sags or adding a tiny UPS to serve only the control
system, and not heaters, lights, and motors which are not sensitive to sags and
similar interruptions lasting one or two seconds.
Another example: 1% of the power
that runs a bank of elevators goes to the computer that controls the motor.
Protection of the computer against a power quality problem is much less
expensive than a large UPS to protect all the motors as well, given that motors
are usually able to ride through voltage sags.
McEachern will always ask a client
who has sought his analysis: “Is the solution less than the cost of the
problem?”
His bottom-line message on power
quality issues—Find the cheapest solutions!