POWER QUALITY

 

 

What is power quality?

Power Quality events
What causes power quality problems?

Power quality is simply the interaction of electrical power with electrical equipment. If electrical equipment operates correctly and reliably without being damaged or stressed, we would say that the electrical power is of good quality. On the other hand, if the electrical equipment malfunctions, is unreliable, or is damaged during normal usage, we would suspect that the power quality is poor.

As a general statement, any deviation from normal of a voltage source (either DC or AC) can be classified as a power quality issue. Power quality issues can be very high-speed events such as voltage impulses / transients, high frequency noise, waveshape faults, voltage swells and sags and total power loss. (See Glossary for definitions.)  Each type of electrical equipment will be affected differently by power quality issues. By analyzing the electrical power and evaluating the equipment or load, we can determine if a power quality problem exists. See Power Quality events for a more detailed description of power quality problems.

We can verify the power quality by installing a special type of high-speed recording test equipment to monitor the electrical power. This type of test equipment will provide information used in evaluating if the electrical power is of sufficient quality to reliably operate the equipment. The process is similar to a doctor using a heart monitor to record the electrical signals for your heart. Monitoring will provide us with valuable data, however the data needs to be interpreted and applied to the type of equipment being powered. Lets look at two examples of interpreting data for a USA location (other countries use different voltages but the same principal applies).

Example No. 1.Example No. 1. A standard 100-watt light bulb requires 120 volts to produce the designed light output (measured in lumens). If the voltage drops to 108 volts (-10%), the light bulb still works but puts out less lumens and is dimmer. If the voltage is removed as during a power outage, the light goes out. Either a low voltage or complete power outage does not damage the light bulb. If however the voltage rises to 130 volts (+10%), the light bulb will produce more lumens than it was intended to, causing overheating and stress to the filament wire. The bulb will fail much sooner than its expected design life; therefore, we could conclude that as far as a standard light bulb is concerned, a power quality issue that shortens bulb life is high voltage. We could also conclude that low voltage or a power outage would cause the lumen output to vary, which effects the intended use of the bulb.

Example No. 2.Example No. 2. A CRT or monitor for a personal computer uses a 120 volt AC power supply to convert the incoming voltage to specific DC voltages required to run the monitor, these voltages include 5 VDC for logic circuits and high voltage DC to operate the cathode ray tube (CRT). If the incoming voltage drops to 108 volts (-10%), the power supply is designed to draw more current or amps to maintain the proper internal voltages needed to operate the monitor. As a result of the higher current draw, the power supply runs hotter and internal components are stressed more. Although the operator of the monitor does not notice a problem, the long term effect of running on low voltage is reduced reliability and increased failures of the monitor. If the power drops below the operating range of the power supply, the monitor will shut down. If the voltage goes above 132 volts AC (+10%), the power supply will not be able to regulate the internal voltages and internal components will be damaged from high voltage; therefore, we conclude that the power quality requirements for the PC monitor are much higher than for a light bulb. Both high and low voltage can cause premature failures. The economic issues are much greater for the PC monitor in both replacement cost and utilization purposes.

The above examples can be applied to any electrical or electronic systems.   It is the task of the power quality consultant to determine if the power, grounding, and infrastructure of a facility is inadequate to operate the technological equipment.   Once this assessment is made steps can be taken to remediate the problems.  To use the physician example, the diagnosis has to be made before the medicine is prescribed.   Many clients are buying power quality medicine without a proper diagnosis.   This is both costly and many times ineffective.

 

Power Quality Events

Power quality problems have many names and descriptions. Surges, spikes, transients, blackouts, noise, are some common descriptions given, but what do they mean? This section delves into defining power quality issues and terminology.

Power quality issues can be divided into short duration, long duration, and continuous categories.  The computer industry has developed a qualification standard to categorize power quality events.  The most common standard is the CBEMA curve (Computer Business Equipment Manufacturing Association).  Other standards include ANSI and ITIC.   Figure 1 is an example of the CBEMA curve for site. The various power quality events are plotted on the curve based on time and magnitude.   Any event outside the curve would be a suspect power problem.

 

 

 

 

 

 




Figure 1

 

Figure 2 is a table of the power quality data divided into time categories.

 

 

 

 

 

 

 

Figure 2

By qualifying the events, we can determine what type of power protection equipment is required to protect the technology.

See Glossary for descriptions of power terms.

What can cause power quality problems?

We have found that the majority of power quality problems are related to issues within a facility as opposed to the utility.  Based on over 20 years of field experience, we have found that 90% of power quality problems are caused within the site.  Typical problems include grounding and bonding problems, code violations and internally generated power disturbances.

Other internal issues include powering different equipment from the same power source.  Lets take an example of a laser printer and a personal computer.  Most of us would not think twice about plugging the laser printer into the same power strip that runs the PC.  We are more concerned about the software and communication compatibility than the power capability; however, some laser printers can generate neutral-ground voltage swells and line-neutral voltage sags every minute or so.  The long term effect to the PC may be power supply failure.  We have to be careful in how technology is installed and wired.

The case studies give examples of how we found and solved power quality problems for our clients.  Please go to this section for more information.   

 

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