By Dan Gregorec
Poor voltage quality is a hidden expense that often goes undetected until costly failures occur. Voltage events interrupt sensitive processes, shorten the useful life of equipment, cause computer and communications problems, and trigger needless expense in implementing solutions to misdiagnosed problems. Unless the source of the problem is correctly identified and its cause diagnosed and repaired, the costs and time required to solve the problem quickly add up.
Voltage events are defined in terms of magnitude and duration. Magnitude means amplitude of the event; duration is the length of the event. The events that can affect equipment performance are sags, swells, impulses and total harmonic distortion.
• Voltage sags: A sag is a period of low voltage. Minor sags occur frequently, sometimes without disturbing equipment performance. Major sags will always disturb equipment performance.
• Voltage swells: A swell is a period of high voltage. Swells have serious impact on equipment function and are not as common as sags. Both minor and major swells will affect equipment performance.
• Impulses: An impulse is a short burst of energy that lasts for less than a cycle. Impulses range in magnitude from twice the nominal voltage to several thousand volts. Not every impulse impacts equipment performance. When impulses occur repeatedly over time or when the energy level is very high, however, an impulse will cause equipment degradation or even immediate failure.
• Harmonics: Ac voltage is a sine wave that repeats at 50 or 60 times per second (Hertz = cycles/second). This is the fundamental frequency. Harmonics are alternate frequencies that distort the sinusoidal waveform. Total harmonic distortion (THD) is measured as a percentage of the fundamental frequency, either 50 or 60 Hertz. Equipment will run well on voltage that is a clean or a slightly distorted sine wave. High levels of distortion may cause equipment problems. On single-phase branch circuits, levels of total harmonic distortion greater than 5 percent to 8 percent need to be investigated.
Voltage events are the cause of many intermittent problems that go un-diagnosed in facilities. Problems that exist for years often are traced to undetected voltage events. The ultimate impact of voltage events is determined by the sensitivity of the equipment on the branch circuit.
How do Voltage Events Occur?
The source of 85 percent of all voltage events originates within a facility. Voltage events can be caused by inadequate supply, undersized wiring, an overloaded circuit, and frequently by other equipment turning on or off on the same circuit. Voltage quality changes every time a new piece of equipment is installed in a facility or in a neighboring facility.
Voltage events can also be caused by environmental changes, such as lightning storms, high winds, flooding, physical damage, utility operations or neighboring facilities.
Some perceived power quality problems have nothing to do with the supply of voltage. Many times power quality problems can be traced to human error. Custodial crews unplugging equipment to clean around them and painting crews disconnecting the ground rod are examples of routine activity that will impact voltage supplied to equipment.
|Chart 1: Typical Tolerance Standards|
How Do Events Affect Equipment?
Electrical equipment is designed to work at nominal voltage +/-10 percent. Equipment may not fail the first time an event occurs, but, over time, failure will occur because of excessive stress from repeated voltage events. When voltage is outside of equipment design specifications, equipment has to work harder, run hotter, or insulation may have to withstand extreme voltage levels.
A refrigerator, for example, is designed to operate between 108 and 132 volts ac, a typical range for a nominal 120 Volts (+/-10 percent of nominal voltage). If voltage runs consistently below 108V on the circuit powering the refrigerator, the compressor motor will run hotter, reducing its operation and service life.
Sags are the most prevalent power quality issue for equipment. Momentary sags may not affect the referenced refrigerator-but they will cause problems for more sensitive equipment, such as computers. The greater the voltage sag, the greater the likelihood of damage. Similarly, the greater the number of sags occurring, the greater the chance of failure or damage. Sags occur for many reasons, including voltage drop caused by long runs of wire, switching loads, poor wiring and overloaded branch circuits.
Voltage swells occur less frequently than sags, but even relatively minor swells can damage equipment, so they require immediate attention. The longer a swell's duration, the more extensive the damage will be. An example would be a large motor creating voltage sags by drawing high inrush currents. When the motor is abruptly stopped, voltage swells are generated. Left uncorrected, these sags and swells will lead to computer disruptions and frequent hardware replacement in the office facility of an industrial plant.
|Chart 2: Who Needs to Test Voltage Quality?|
THD can produce excessive heat, generate electro-magnetic interference in communications circuits, and cause electronic controls to fail. Non-linear loads such as personal computers (PCs), copying machines and variable frequency drives create harmonic currents that distort the voltage sine wave. The more electronic devices on a circuit, the greater the likelihood of severe voltage distortion.
A real-life example of such a problem involved a hospital technician who tested a circuit for two days before installing patient monitoring equipment. One instance of voltage harmonics amounting to 5.2 percent THD was noted. Recognizing this low level THD wouldn't cause a problem, the technician installed the patient monitoring device. Within hours, the device failed. The technician reviewed new data to find a THD event reaching 10.2 percent. Further investigation using a circuit analyzer and long term recorders found there were several non-linear loads on the branch circuit that the patient monitoring device was also being plugged into. When certain combinations of these loads were on simultaneously along with the new equipment, excessive harmonics flowed, causing a distorted voltage waveform and sporadic shutdown of the device.
How to Test Voltage Quality
There are a variety of voltage monitoring devices for the electrician to choose from, ranging from high-end power quality analyzers that can be used at the service entrance to simple monitors that connect at the branch level.
Some testers come with pre-loaded monitoring standards based on the most widely used international voltage standards, such as Computer and Business Equipment Manufacturers' Association (CBEMA), International Telecommunications and IT Consultants Group (ITIC), and Institute of Electrical and Electronics Engineers (IEEE) 519 or 1159.1. Different applications require different voltage quality standards. Some are very stringent and will record many events, while others are less stringent, applying tolerances designed for modern, more robust loads, resulting in fewer out-of-tolerance events to be recorded.
In voltage quality there is no such thing as a coincidence, so, while you are testing, carefully examine the simultaneous operation of equipment as a likely cause of voltage events. Dates and times of nuisance events should be noted and the event records kept as part of normal operations. And always consult your local utility for resources to help diagnose power problems, improve efficiency and avoid usage penalties.
About the author: Jim Gregorec is the Business Unit Manager for Test & Measurement, IDEAL Industries Inc.