Improved Power Quality Can Boost Productivity, Safeguard Equipment.
Nearly all types of electronic equipment are sensitive to power fluctuations. According to the Computer Business Equipment Manufacturers Association (CBEMA), electronic equipment should not be operated out of a range starting at 20% lower than the rated voltage and going up to 15% higher for a period of longer than one-half second. Maintaining the power supply within these limits is not difficult within the secure confines of data processing centers that are designed to isolate the computer's load from power fluctuations. These data processing centers are usually operated from day one with elaborate power conditioning systems.
But in recent years there has been a major trend in which computer controls have moved from the secure environment of a data processing center to the plant floor. The plant floor is traditionally an environment where little attention is paid to power conditioning issues. This is because most manufacturing processes were nearly all controlled by electromechanical systems that were more robust and could handle a wide range of voltages without any impact on operations. While the new generation of computer-based controls offers increased throughput, flexibility and ease of operation, in many cases they are placed in environments that are electrically unstable. The result is that they sometimes operate less efficiently than the older controls they replaced.
The most common power-quality problem is voltage sags. A multi-year study conducted by the Electric Power Research Institute (EPRI) shows that power users have more than a 90% probability that voltage will sag below 80% of rated voltage every month. Such sags are below the tolerance range of sensitive electronic controls. Most sags are caused by faults in the utility distribution system. A particular type of sag in which voltage falls by a relatively small amount for an extended period of time is called a brownout. Transients are conditions in which the voltage is outside of acceptable levels for two to five cycles, with one cycle being equal to 16.66 milliseconds.
Consider a piece of equipment that can tolerate a 15% power disturbance. If an 8% transient sag and an 8% transient surge occur consecutively, the total power disturbance is 16%, more than the control can tolerate. Combined effects such as these are common. The reason for the brownout is that the power company is under so much load stress that it may have to reduce its voltage to lower the load demand. In this stress condition, transients become particularly common.
A voltage surge may be caused by a lightning strike that induces a large but short high-voltage condition. Additionally, when switching events occur on utility distribution systems, longer duration transients occur, often consisting of a combination of under- and over-voltage. These surges often damage computer equipment.
A rapid drop of utility voltage to near zero followed by a recovery to nominal is a short-term outage. An example of such an outage is the rapid opening and reclosing of a three-phase circuit breaker immediately up line of the equipment. This type of disturbance usually is caused by a temporary fault on the utility line. The most common cause is that a protective switchgear briefly opens the line and tries to reconnect the utility power. Utility experience demonstrates that more than 80% of these faults have disappeared by the time that power is reconnected. The majority of interruptions caused by the action of protective switchgear are five cycles long or above 80 milliseconds.
To maintain the performance of computer-based control systems, plants need to operate within the same tight limits as data-processing centers. Early attempts to apply static solid-state systems to improve power quality have not delivered satisfactory results. Facility engineers have found that solid-state technology is not robust enough to handle the dynamic mixture of motor and electronic loads found in today's typical manufacturing environment.
Power-quality problems can have a big impact on CNC machining. Any variation in voltage can affect the quality of the part being machined. Some machine tools are intentionally made sensitive to voltage variations of only 10% below nominal for safety reasons. The machine tool controls shutdown and some lose their program when a voltage sag occurs. A typical plant experiences about 30 events like this per year. The time needed to restart the machine and the value of scrap materials that are produced can be very large. If the CNC controller does not continue to operate in a minor sag event, the entire machine will shut down. Newer generations of CNC controllers can operate down to almost 80% of nominal voltage for short cycle sags, but will still fail at the CBEMA limit.
DC drives are used in many industrial processes, including printing presses and plastics manufacturing. Plastic extrusion is a typical application where voltage sags can be particularly disruptive. During a voltage sag, the controls to DC drives and winders may trip. These extrusion operations are usually completely automated and an interruption may involve expensive cleanup and restarting requirements. Some extruders begin to have problems when the voltage sags to only 88% of nominal. A typical plant may experience 25 such events per year.
PLCs' sensitivity to sags vary greatly, but certain types, such as remote IO units, have been found to trip for voltages as low as 90% of nominal. Newer types are sensitive at 50% to 60% of nominal voltage at less than one cycle, while older PLCs can ride through zero voltage for up to 15 cycles. Another issue to be considered is that noise from variable-frequency drives can disrupt PLC communications.
Delivering a power-conditioning solution to the plant floor is more complicated than to the data processing center for several reasons. First, most plant electrical systems have evolved over time to the point that it is cost prohibitive or even impossible to isolate the controls from the rest of the plant. Second, the power load in a plant is usually unpredictable. In most data centers, computers are never turned off and the load will vary only slightly. In a plant, turning on and off a large motor on one production line can create enough of a power sag or surge that controls on another line are disrupted.
Several types of equipment are available that will protect sensitive equipment from power-quality problems.
Shielded isolation transformers, for example, are equipped with a Faraday electrostatic shield between primary and secondary windings. Because high-frequency transients are capacitatively coupled through the windings as opposed to being coupled by transformer action, the shield is an effective barrier against some transients. But these devices have no voltage-regulation capability so they provide no protection against brownouts.
Ferroresonant and automatic tap-changing dynamic transformers can protect equipment from brownout and transient conditions, but do not protect equipment from extreme high-voltage transients. They also have little or no stored energy and therefore cannot protect equipment from outages of more than four to eight milliseconds. While tap changers are generally efficient throughout their load range, ferroresonant units have low part-load efficiency because the core has to be saturated to cause oscillation, so losses are the same at full and part load.
Motor generator sets can give 100% protection against brownout conditions, all transients and outages up to 200 milliseconds. Most generator sets have their motor and generator in separate frames which gives inherent protection from lightning and electromagnetic interference because input and output windings are separated by a thick, magnetic shield and by at least 1 ft. of air gap. A motor generator set that is correctly sized will provide uninterruptible power supply to a plant's critical load for up to 200 milliseconds. This will cover 96.5% of a plant's power-quality problems at substantially less than the cost of battery backup.
In the event that more ride-through is needed, an extended ride--through motor generator set with a flywheel will provide a minimum of 15 seconds of ridethrough. Even more ride-through capability can be provided by adding an engine generator to the above configuration, resulting in a battery-less uninterruptible power supply than can sustain a plant through any type of power outage. The motor-generator set can also be used with batteries.
The migration of sensitive computer equipment to the factory floor presents a major power-quality challenge. Delicate controls need to be protected from voltages sags and surges that can adversely affect quality and productivity. A variety of solutions are available, the most complete being the motor-generator set which can take raw power from a utility source and clean it up so that power variations will not affect plant operation.
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|Author:||Butts, Boota de|
|Publication:||Industrial Maintenance & Plant Operation|
|Date:||Sep 1, 2000|
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