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Publication Date: 10/1/2011
Archive >  October 2011 Issue >  Special Features: PCB and Production > 

Protecting Mission Critical Electronics in the Industrial Plant

Today's engineers are designing more sophisticated control systems that bring higher productivity, while keeping system costs under control. In order to achieve these results, they are employing more electronic equipment, often adopted from non-industrial applications, and almost always more sensitive to electrical disturbances than the equipment being replaced.

These new systems, when mixed with the inherently poor power environment of an industrial facility and the aging power generation and distribution facilities — both inside and outside of the plant — lead to a wide variety of power and electrical noise problems.

Understanding these problems, along with their causes and solutions, can help ensure reliable, cost-effective design of mission-critical electronic systems.

Power Problem Sources
Power problems, which can cause destruction, degradation, or disruption of mission critical equipment, may originate either "inside" or "outside" the industrial facility. While "outside" events are typically the most obvious and spectacular, up to 80 percent of power problems in industrial facilities originate on the customer's side of the meter.

"Inside" problems are caused by a variety of factors including stopping and starting motors, welding equipment, electronic motor speed controls, poor grounding, as well as some of the same problems facing the utility company — fault clearing, and capacitor switching.

Power Interruptions
The most noticeable power quality problem is a power interruption. While interruptions are relatively infrequent, their effect can be dramatic and obvious as operations grind to a halt. Solutions to combat interruptions include alternate power feeds, local backup power generation (diesel or gas powered generators) and UPSs on selected equipment.

Voltage Sags
Voltage sags are the most measured power line problem. The typical approach to protect against sags is a voltage control device in the power path supplying the equipment. Choices include a transformer that stores energy (Constant Voltage Transformer,) transformers with boost windings to raise voltages during sags (tap switching transformer), and Uninterruptible Power Supplies that supply energy from batteries during sags.

In the past, the Constant Voltage Transformer (CVT) was a common method for sag control; however, today's control systems have changed. Typically, equipment now has a Switch Mode Power Supply (SMPS) and systems are no longer based on proprietary software that "crashes" well, but on commercially available operating systems that need to be properly shut down to retain status. Today, load requirements also change more often as control schemes are frequently updated with the latest technology.

While changes have been made in CVTs to adapt to new technology, the best solution is one specifically designed to support SMPS and has energy to ride through severe sags, such as a UPS with integral isolation transformer to provide regulation, isolation, and backup. If a local isolation transformer already supplies the load, a UPS with double conversion topology also serves effectively.

Voltage Transients
Transients on power lines usually occur randomly below a level that causes destruction and are usually difficult to measure. Among the most difficult to find are the high-speed transients that cause disruption of electronic equipment. Special power quality monitors are required to capture these high-speed impulse and oscillatory events that disrupt sensitive electronics. This "least measured" problem can be a major contributor to random errors and "lock-ups" that occur in a system.

Fortunately, most transient events are ignored by electronic equipment. If they were not, it would be impossible to run a computer. In mission critical applications, however, the goal is to push disruptions as close to zero as is possible. Thus, reducing the amplitude and edge speed of transients becomes paramount to achieve the goal of system reliability. To understand the methods that are used to control the amplitude and edge speed of transient voltages, it is useful to review how transient noise appears to electronic equipment.

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