Common power quality problems are grouped into two broad areas: voltage anomalies and harmonic distortion issues. Voltage anomalies can cause several problems, many easily corrected. The key is to spot the symptoms.
Voltage dips or sags
Voltage dips or sags are responsible for up to 80% of all power quality issues. A dip or sag occurs when the system voltage drops to 90% or less of nominal system voltage for a half-cycle to one minute. Common symptoms of dips include incandescent lights dimming if the dip lasts more than three cycles, computer lockup, spurious shutdown of sensitive electronic equipment, data (memory) loss on programmable controls, and relay control problems.
Detecting sags can be challenging because it’s difficult to predict when they will occur. To troubleshoot potential dip problems, the load where the dip symptoms first occurred is monitored. Generally, an upstream event will be indicated by a drop in both voltage and current. A downstream or load dip event would be indicated by an increase in current and a drop in voltage. The time of the equipment’s operational failure is compared to the time at which the voltage dip occurred and if there is no correlation, the problem is most likely not caused by a voltage dip. Troubleshooting is then continued by monitoring further upstream until the source is located.
Voltage swells or surges
Voltage swells or surges occur about only half as often as dips, and ares caused by increases in system voltage for short periods up to a cycle or more. Symptoms of swells often include immediate failure of equipment, typically the power supply section. However, some equipment failures may not occur immediately, because voltage swells can occur over a prolonged period and prematurely break down components. If analysis of electronic equipment reveals faulty power supplies, then the feeders and branch circuits feeding this equipment should be monitored.
When analysing power quality survey results, any sudden line-to-ground faults on a single-phase line should be identified. This type of fault causes the voltage to suddenly swell on the two non-faulted phases. Large plant loads suddenly dropping offline, and power factor correction capacitor switching, can also cause voltage swells.
Voltage transients
Voltage transients can cause symptoms ranging from computer lockups and damaged electronic equipment to flashover, and damaged insulation on distribution equipment and motors. Sometimes referred to as spikes, transients are substantial increases in voltage, but for only a matter of microseconds.
Common causes of transients are lightning strikes and mechanical switching. Equipment failure during a storm is often rightfully attributed to transients, even though no power quality monitoring was performed.
Other causes of transients include switching of capacitors or capacitor banks, reenergising systems after a power failure, the switching of motor loads, turning off or on fluorescent and HID lighting loads, switching transformers, and sudden stoppage of certain equipment. Normal arcing across contacts by interrupting large loads can also be a cause of transients.
Voltage interruptions
Voltage interruptions can last anywhere from two to five seconds or more. The symptom is usually quite simple: the equipment stops operating. Interruptions for longer than five seconds are typically referred to as sustained interruptions. Most motor control circuits and process control systems are not designed to restart, even after a brief interruption of power.
If a voltage interruption occurs when equipment is unattended, the cause of the equipment shutdown might not be properly identified. Only monitoring equipment and correlating the time of any power interruptions to the time of equipment issues will help identify a voltage interruption.
Voltage unbalance
Voltage unbalance is one of the most common problems on three-phase systems, and can result in severe equipment damage, yet is often overlooked. For example, a voltage unbalance of 2,3% on a 230 V motor results in a current unbalance of almost 18%, causing a temperature rise of 30°C. While a digital multimeter (DMM) and some quick calculations can be used for averaging voltage readings, a power quality analyser provides the most accurate information about unbalance.
Unbalance can occur at any point throughout the distribution system. Loads should be divided equally across each phase feeding a panel. Should one phase become too heavily loaded in comparison to others, voltage will be lower on that phase. Transformers and three-phase motors fed from that panel may run hotter, be unusually noisy, vibrate excessively, and even suffer premature failure.
Monitoring over time is the key to capturing unbalanced phases. Accurate, real-time unbalance measurements need a three-phase power quality analyser to identify the problems. In a three-phase system, the maximum variation in voltage between phases should be no more than 2%, otherwise significant equipment damage can occur.
Harmonics
Harmonics are voltages and currents at a frequency which is an integer multiple of the fundamental frequency. For example, the third harmonic is the voltage or current that is occurring at 180 Hz in a 60 Hz system. These unwanted frequencies can cause numerous symptoms, including overheating in neutral conductors and the transformers supplying these circuits. Reverse torque creates heat and efficiency losses in motors.
The most severe symptoms created by harmonics are typically the result of the harmonics distorting the fundamental 60 Hz sine wave. This sine wave distortion results in improper operation of electronic equipment, spurious alarms and data losses. These are often reported as ‘mysterious’ problems.
When symptoms of harmonics occur, troubleshooting can be performed by observing total harmonic distortion (THD). Harmonics at the point of common coupling can be measured using a power quality analyser.
Power quality problems
Voltage problems and the creation of harmonic currents are the two broad areas under which power quality problems occur. Dips and swells, voltage transients, power interruptions, and voltage unbalance can all be monitored, analysed, and compared to equipment operation histories to determine the cause and severity of the power quality problem. The same can be done with the various harmonic currents in a system.
Power quality issues are frequently interrelated. Looking at the big picture by using a three-phase power quality analyser enables the correction of the causes of power quality issues, and not just the doctoring of the symptoms!
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