There are numerous types of power quality issues and power problems each of which might have varying and diverse causes. To further compound the matter, it is all too common that different power quality problems can occur simultaneously, interchangeably or randomly. The following is a brief summary of typical power problems.
Blackout or Interruption
When the voltage drops below 10% of its nominal value it is called an interruption or a blackout. Interruptions have three classifications: momentary (lasting 30 cycles to 3 seconds), temporary (lasting 3 seconds to 1 minute) and sustained (lasting more than 1 minute).
Although interruptions are the most severe form of power problem, they are also the least likely to occur. Voltage sags are often mistaken for an interruption because equipment shuts down or lighting goes off since the voltage dropped below the point that these devices can operate.
Where sags and undervoltage typically represent more than 92% of power problem events, interruptions represent less than 4% of such problems.
The U.S. electrical grid operates at 60 Hertz or 60 cycles per second. Because of, literally, the “mass” of the U.S. electrical system, it is very unlikely to encounter frequency problems. Virtually all electrical devices are capable of operating properly at frequency variations much larger those that could be seen in the U.S.
The concept of very stable frequency does not apply to closed systems where electricity is generated on-site. Even large diesel-generator sets can have frequency problems. Also see Operating Frequency.
Harmonics are a recurring distortion of the waveform that can be caused by various devices including variable frequency drives, non-linear power supplies and electronic ballasts. Certain types of power conditioners like ferroresonant or constant voltage (CVT) transformers can add significant harmonic distortion to the waveform.
Waveform distortion can also be an issue with uninterruptible power supplies (UPS) and other inverter-based power conditioners. The UPS does not actually add distortion, but because the UPS digitally synthesizes a waveform, that waveform may be square or jagged rather than a smooth sine wave.
Symptoms of harmonic distortion include overheating and equipment operational problems.
Noise is a high frequency distortion of the voltage waveform. Caused by disturbances on the utility system or by equipment such as welders, switchgear and transmitters, noise can frequently go unnoticed.
Frequent or high levels of noise can cause equipment malfunction, overheating and premature wear.
Notching is a disturbance of opposite polarity to the normal voltage waveform (which is subtracted from the normal waveform) lasting for less than one-half cycle. Notching is frequently caused by malfunctioning electronic switches or power conditioners.
While it is generally not a major problem, notching can cause equipment, especially electronics, to operate improperly.
Sag or Dip
The American “sag” and the British “dip” are both names for a decrease in voltage to between 10 and 90% of nominal voltage for one-half cycle to one minute
Sags account for the vast majority of power problems experienced by end users. They can be generated both internally and externally from an end users facility.
External causes of sags primarily come from the utility transmission and distribution network. Sags coming from the utility have a variety of cause including lightning, animal and human activity, and normal and abnormal utility equipment operation. Sags generated on the transmission or distribution system can travel hundreds of miles thereby affecting thousands of customers during a single event.
Sometimes externally caused sags can be generated by other customers nearby. The starting of large electrical loads or switching off shunt capacitor banks can generate a sag large enough to affect a local area. If the end user is already subject to chronic undervoltage, then even a relatively small amplitude sag can have detrimental effects.
Sags caused internally to an end user’s facility are typically generated by the starting of large electrical loads such as motors or magnets. The large inrush of current required to starts these types of loads depresses the voltage level available to other equipment that share the same electrical system. As with externally caused sags, ones generated internally will be magnified by chronic undervoltage.
A short circuit (or “short”) is not normally considered a quality problem as much as it is a dangerous operational malfunction or fault. Short circuit refers to a condition where two “hot” lines are connected directly (or through a small impedance) or one “hot” line is connected directly to ground.
A short circuit causes very high fault currents to flow through the wiring and all devices between the point of the short and the incoming power line. Left unchecked, a short circuit can very quickly lead to catastrophic overheating, melting and burning of wiring and devices.
The opening of a breaker or the operation of a protective fuse is the normal means of guarding against damage from shorts circuits. It is imperative that protective breakers and fuses be of the proper size and characteristics to avoid the dangers of short circuits. Also see Fault Clearing.
Transient or Surge
Transients are very short duration (sub-cycle) events of varying amplitude. Often referred to as “surges”, transients are probably most frequently visualized as the tens of thousands of volts from a lighting strike that destroys any electrical device in its path.
Transients can be caused by equipment operation or failure or by weather phenomena like lightning. Even relatively low voltage transients can cause damage to electrical components if the occur with any frequency.
A properly sized industrial-grade surge suppressor is usually ample protection from the damaging effects of high voltage transients.
Undervoltage is a decrease in voltage below 90% of its nominal value for more than one minute. Undervoltage is sometimes called a “brownout” although this term is not officially defined. Brownout is often used when the utility intentionally reduces system voltage to accommodate high demand or other problems.
The symptoms of undervoltage can range from none to daily equipment malfunction or premature equipment failure. Undervoltage may go unnoticed until new equipment is installed or the electrical system is otherwise changed and the new combined load depresses (see Sags) the voltage to a point where symptoms become apparent. Besides the obvious malfunction of equipment, chronic undervoltage can cause excess wear on certain devices like motors as they will tend to run overly hot if the voltage is low.
Undervoltage is generally a chronic problem aggravated by a number of factors beyond the end user’s control. Electric utilities try to maintain voltage levels delivered to customers at ±5%. However, factors like weather, high demand and others can cause the utility voltage to fall within a ±10% range. Even under ideal conditions, most customers will see a drop in utility voltage levels over the course of the day as demand begins to increase around 8 AM and peaks around 3 or 4 PM.
Distribution system characteristics can also contribute to chronically low voltage situations. For example, customers at the end of a long line may be subject to a permanent voltage drop due to line losses on top of the utility voltage variations.