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Power Quality Issues, Disturbance Sources, Financial Impacts, Control, Monitoring & Remedies Brief power quality tutorials for engineers Sags, dips, and swells: Introduction to the most common disturbance on AC mains Transient overvoltages: Introduction to capacitor-switching and high-frequency transients Harmonics: Introduction to voltage and current harmonics issues Flicker: Introduction to voltage flicker Voltage regulation: Introduction to voltage regulation issues Other disturbances: Frequency variations, noise bursts, and other less common problems Industry Standards IEEE power quality standards o o o o o o o o o IEEE SCC-22: Power Quality Standards Coordinating Committee IEEE 1159:Monitoring Electric Power Quality IEEE 1159.1: Guide For Recorder and Data Acquisition Requirements IEEE 1159.2: Power Quality Event Characterization IEEE 1159.3: Data File Format for Power Quality Data Interchange IEEE P1564:Voltage Sag Indices IEEE 1346:Power System Compatibility with Process Equipment IEEE P1100: Power and Grounding Electronic Equipment (Emerald Book) IEEE 1433: Power Quality Definitions IEEE P1453: Voltage flicker IEEE 519: Harmonic Control in Electrical Power Systems IEEE Harmonics Working Group Single-phase Harmonics Task Force IEEE P519A Guide for Applying Harmonic Limits on Power Systems o Interharmonics Task Force Harmonics Modeling and Simulation Task Force Probabilistic Aspects of Harmonics Task Force Surge Protective Devices Committee Seventeen sub-committee links can be found at the "Sub-committee pages" link... o o o IEEE P446: Emergency and standby power IEEE P1409: Distribution Custom Power IEEE P1547: Distributed Resources and Electric Power Systems Interconnection IEC 61000-4-11 - voltage sag immunity - 16 amps or less IEC 61000-4-34 - voltage sag immunity - more than 16 amps IEC 61000-4-30 - Power quality measurement methods General IEC power quality standards o o IEC Power quality standards - numbering system 61000-1-X - Definitions and methodology 61000-2-X - Environment (e.g. 61000-2-4 is compatibility levels in industrial plants) 61000-3-X - Limits (e.g. 61000-3-4 is limits on harmonics emissions) 61000-4-X - Tests and measurements (e.g. 61000-4-30 is power quality measurements) 61000-5-X - Installation and mitigation 61000-6-X - Generic immunity & emmissions standards IEC SC77A: Low frequency EMC Phenomena -- essentially equivalent of "power quality" in American terminology TC 77/WG 1: Terminology (part of the parent Technical Committee) SC 77A/WG 1: Harmonics and other low-frequency disturbances SC 77A/WG 6: Low frequency immunity tests SC 77A/WG 2: Voltage fluctuations and other low-frequency disturbances SC 77A/WG 8: Electromagnetic interference related to the network frequency SC 77A/WG 9: Power Quality measurement methods SC 77A/PT 61000-3-1: Electromagnetic Compatibility (EMC) - Part 3-1: Limits Overview of emission standards and guides. Technical Report Common Power Problems Here are some of the most common power supply problems and their likely effect on sensitive equipment: Power Surges A power surge takes place when the voltage is 110% or more above normal. The most common cause is heavy electrical equipment being turned off. Under these conditions, computer systems and other high tech equipment can experience flickering lights, equipment shutoff, errors or memory loss. Possibile Solutions: Surge Suppressors, Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners High-Voltage Spikes High-voltage spikes occur when there is a sudden voltage peak of up to 6,000 volts. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well. The effects on vulnerable electronic systems can include loss of data and burned circuit boards. Possibile Solutions: Surge Suppressors,Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners Transients Transients are potentially the most damaging type of power quality disturbance that you may encounter. Transients fall into 2 categories. Impulsive Oscillatory For more details about transients view our newsletter archive here. Possibile Solutions: Surge Suppressors, Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners Frequency Variation A frequency variation involves a change in frequency from the normally stable utility frequency of 50 or 60 Hz, depending on your geographic location. This may be caused by erratic operation of emergency generators or unstable frequency power sources. For sensitive equipment, the results can be data loss, program failure, equipment lock-up or complete shut down. Possibile Solutions: Voltage Regulators, Power Conditioners Power Sag A sag is the reduction of AC Voltage at a given frequency for the duration of 0.5 cycles to 1 minute’s time. Sages are usually caused by system faults, and often the result of switching on loads with high demand startup currents. For more details about power sags visit our newsletter archives. Possibile Solutions: Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners Electrical Line Noise Electrical line noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) and causes unwanted effects in the circuits of computer systems. Sources of the problems include motors, relays, motor control devices, broadcast transmissions, microwave radiation, and distant electrical storms. RFI, EMI and other frequency problems can cause equipment to lock-up, and data error or loss. Possibile Solutions: Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners Brownouts A brownout is a steady lower voltage state. An example of a brownout is what happens during peak electrical demand in the summer, when utilities can't always meet the requirements and must lower the voltage to limit maximum power. When this happens, systems can experience glitches, data loss and equipment failure. Possibile Solutions: Voltage Regulators, Uninterruptable Power Supplies, Power Conditioners Blackouts A power failure or blackout is a zero-voltage condition that lasts for more than two cycles. It may be caused by tripping a circuit breaker, power distribution failure or utility power failure. A blackout can cause data loss or corruption and equipment damage. Voltage sags (dips) and swells Voltage sags -- or dips which are the same thing -- are brief reductions in voltage, typically lasting from a cycle to a second or so, or tens of milliseconds to hundreds of milliseconds. Voltage swells are brief increases in voltage over the same time range. (Longer periods of low or high voltage are referred to as "undervoltage" or "overvoltage".) Voltage sags are caused by abrupt increases in loads such as short circuits or faults, motors starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically caused by a loose connection. Voltage swells are almost always caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged or loose neutral connection. A typical voltage sag. Voltage sags are the most common power disturbance. At a typical industrial site, it is not unusual to see several sags per year at the service entrance, and far more at equipment terminals. Voltage sags can arrive from the utility; however, in most cases, the majority of sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors. Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags. 1996 Version of the IT Industry Tolerance Curves (update from original CBEMA curve). The vertical axis is percent of nominal voltage. "Well-designed" equipment should be able to tolerate any power event that lies in the shaded area. Note that the curve includes sags, swells, and transient overvoltages. The semiconductor industry developed a more recent specification (SEMI F47) for tools used in the semiconductor industry in an effort to achieve better ride through of equipment for commonly occurring voltage dips and therefore improving the overall process performance. It is basically the same as the ITI Curve but specifies an improved ride through requirement down to 50% retained voltage for the first 200 msec. Many short voltage dips are covered by this additional requirement. IEC 61000-4-11 and IEC 61000-4-34 provide similar voltage dip immunity standards. Many utilities have benchmarked performance of the supply system for voltage dips but it has not been the general practice to specify any required performance levels for the system. Performance is often specified using the SARFI index that provides a count of all events with magnitudes and durations outside of some specifications. For instance, SARFI-70 would provide a count of all voltage dips with a retained voltage less than 70% (regardless of duration). SARFI-ITIC would provide a count of all voltage dips that exceeded the ride through specifications of the ITI Curve. The table below provides a summary of voltage dip performance levels from a few major benchmarking efforts. Note that these are average performance levels and it would not be reasonable to develop limits based on an average expected performance (although these are the correct values to use when evaluating the economics of investments in ride through solutions). Example of average voltage dip performance from major benchmarking projects. These values represent voltage dip performance on medium voltage systems. The voltage dip performance can vary dramatically for different kinds of systems (rural vs urban, overhead vs underground). It may be important to include some of these important factors in the specification of the power quality grades. It will also be important to specify the performance for momentary interruptions. These events can be a particular problem for customers and are not included in most assessments of reliability. A previous CEA Technologies report prepared by Electrotek Concepts recommended that the SARFI indices be calculated for the following magnitude and duration categories: Recommended magnitude and duration categories for calculating voltage dip performance. The reasons for these categories were explained as follows: The 90% level provides an indication of performance for the most sensitive equipment. The 80% level corresponds to an important break point on the ITI curve and some sensitive equipment may be susceptible to even short sags at this level. The 70% level corresponds to the sensitivity level of a wide group of industrial and commercial equipment and is probably the most important performance level to specify. The 50% level is important, especially for the semiconductor industry, since they have adopted a standard that specifies ride through at this level. Interruptions affect all customers so it is important to specify this level separately. These will usually have longer durations than the voltage sags. The first range of durations is up to 0.2 seconds (12 cycles at 60 Hz). This is the range specified by the semiconductor industry that equipment should be able to ride through sags as long as the minimum voltage is above 50%. The second range is up to 0.5 seconds. This corresponds to the specification in the ITIC standard for equipment ride through as long as the minimum voltage is above 70%. It is also an important break point in the definition of sag durations in IEEE 1159 (instantaneous vs. momentary). The third duration range is up to 3 seconds. This is an important break point in IEEE 1159 and in IEC standards (momentary to temporary). The final duration is up to one minute. Events longer than one minute are characterized as long duration events and are part of the system voltage regulation performance, rather than voltage sags. As a final note, remember that voltage sags are voltages, and therefore always occur between two conductors - there is no such thing as a "sag on phase A" -- it must be a sag between phase A and phase B, or a sag between phase A and Neutral. Sources of Voltage Sags Voltage sags are brief reductions in the voltage on ac power systems. (The American "sag" and the British "dip" have exactly the same meaning, and may be used interchangeably.) How brief? Between 1/2 cycle and a few seconds. Disturbances that last less than 1/2 cycle are commonly called "low frequency transients"; voltage reductions that last longer than a few seconds are commonly called "undervoltage." A typical voltage sag, graphed as an RMS voltage vs. time. This sag affected a three-phase system, dropping the voltage to 22.5% of nominal for 0.236 seconds. (Captured and displayed by a PSL PQube) Voltage sags have two main characteristics: depth and duration. Do a scatter plot of depth vs. duration, and you get a CBEMA or ITIC graph. Power systems have non-zero impedances, so every increase in current causes a corresponding reduction in voltage. Usually, these reductions are small enough that the voltage remains within normal tolerances. But when there is a large increase in current, or when the system impedance is high, the voltage can drop significantly. So conceptually, there are two sources of voltage sags: Large increases in current Increases in system impedance As a practical matter, most voltage sags are caused by increases in current. It is convenient to think of the power system as a tree, with your sensitive load connected to one of the twigs. Any voltage sag on the trunk of the tree, or on a branch leading out to your twig, will cause a voltage sag at your load. But a short circuit out on a distant branch can cause the trunk voltage to diminish, so even faults in a distant part of the tree can cause a sag at your load. Most voltage sags originate within your facility. The three most common causes of facility-sourced voltage sags are: Starting a large load, such as a motor or resistive heater. Electric motors typically draw 150% to 500% of their operating current as they come up to speed. Resisitive heaters typically draw 150% of their rated current until they warm up. Loose or defective wiring, such as insufficiently tightened box screws on power conductors. This effective increases your system impedance, and exaggerates the effect of current increases. Faults or short circuits elsewhere in your facility. Although the fault will be quickly removed by a fuse or a circuit breaker, they will drag the voltage down until the protective device operates, which can take anywhere from a few cycles to a few seconds. The most common cause of voltage sags: starting a large load, such as a motor or a computer room. The motor might be a pump, a compressor, a fan, part of an HVAC system... Experts can identify the specific source of a voltage sag with an advanced power quality monitor, such as those found at PQMonitoring.com. Voltage sags can also originate on your utility's electric power system. The most common types of utility-sourced voltage sags are: Faults on distant circuits, which cause a corresponding reduction in voltage on your circuit. Typically, these faults are removed by "reclosers", or self-resetting circuit breakers. These reclosers typically delay 1 to 5 seconds before self-resetting. Occasionally, a voltage sag will If the fault is still present when the recloser resets, originate on the utility grid. But inyou may see a series of voltage sags, spaced 1 to 5 plant causes are far more common. seconds apart. Faults on utility systems may be phase-to-phase, or phase-to-earth; depending on the transformers between you and the fault, you will see different levels of voltage reduction. Voltage regulator failures are far less common. Utilities have automated systems to adjust voltage (typically using power factor correction capacitors, or tap switching transformers), and these systems do occasionally fail. It is important to understand the source of the voltage sags before trying to eliminate them, because the wrong solution can actually make the problem worse. For example, if you install a ferro-resonant transformer as a voltage regulator, or a battery-operated UPS (a reasonable and common approach), but inadvertently install it upstream from the motor that is causing your voltage sags, the voltage sags will get worse, not better. In most cases, the correct solution is to adjust the equipment so that it is less sensitive to voltage sags. Information about voltage sag sensitivity Why does equipment fail when there are voltage sags on ac power systems? There is one obvious way, and four not-so-obvious ways. 1. Equipment fails because there isn't enough voltage. This is the obvious way -- if there is not enough voltage on the ac power system to provide the energy that the equipment needs, it is going to fail. Actually, the problem is slightly more subtle. In a typical sensitive load, the ac voltage is rectified and coverted to pulsed dc. With a bridge rectifier, the pulsing will typically be either twice the power line frequency (for single-phase loads) or six times the power line frequency (for three-phase loads). This pulsing DC is stored in a filter capacitor, which in turn supplies smooth DC as raw material for the rest of the power supply: regulators, etc. Filter capacitors store voltage in power supplies (yellow trace). If their voltage drops below a critical level (typically several cycles after the sag begins), there will not be enough voltage for the rest of the power supply to operate properly. If the DC supplied by the filter capacitor drops below some critical level, the regulators will not be able to deliver their designed voltage, and the system will fail. Note that the filter capacitor always stores energy, so there is always an ability to ride through some sags -- after all, the ac power system delivers zero voltage 100 or 120 times each second! But with a deep enough sag that lasts long enough, the filter capacitor voltage will drop below a critical level. 2. Equipment fails because an undervoltage circuit trips. Careful system designers may include a circuit that monitors the ac power system for adequate voltage. But "adequate voltage" may not be well defined, or understood. For example, if the sensitive system is running at half load, it may be able to operate at only 70% ac voltage, even though it may be specified to operate with 90% - 110% Quick-operating relays, such as this "ice-cube" relay, can ac voltage. So the voltage sags to 70%; inadvertently shut down sensitive systems during voltage the equipment can operate without a sags, especially in EMO circuits. problem; but the undervoltage monitor may decide to shut the system down. 3. Equipment fails because an unbalance relay trips.On three-phase systems, voltage sags are often asymmetrical (they affect one or two phases more than the remaining phases). Three-phase motors and transformers can be damaged by sustained voltage unbalance; it can cause the transformer or motor to overheat. So it makes sense to put in an unbalance relay, which is a device that shuts down the system if the voltage unbalance exceeds some threshold, typically a few percent. Unbalance relays, if their trip time is set too short, can shut a system down during a hamrless voltage sag. Typically, you can adjust both the trip delay and the re-start delay. Some relays combine unbalance, undervoltage tripping. But a voltage sag that causes 20-50% unbalance for a second or two is never going to cause a motor or transformer to overheat. It just doesn't last long enough. Still, unbalance relays with inadequate delays can cause the sensitive system to shut down, even for a brief voltage sag. 4. A quick-acting relay shuts the system down, typically in the EMO circuit. The EMO (emergency off) circuit in an industrial load typically consists of a normally-closed switch that can disconnect power to a latched relay coil. If the relay operates Emergency Off circuits can inadvertently shut equipment quickly enough, it may interpret a down during brief, harmless voltage sags. brief voltage sag as an operator hitting the EMO switch. The whole system will shut down unnecessarily. 5. A reset circuit may incorrectly trip at the end of the voltage sag. This is the most subtle problem caused by voltage sags. Many electronic reset circuits are designed to operate at "power up" -- when you first turn on the equipment, these circuits will ensure that the microprocessors all start up properly, the latches are all Reset circuits deserve close attention; they can improperly properly initialized, the displays are in reset a system, or part of a system, at the end of a voltage their correct mode, etc. These circuits sag. are difficult to design, because they must operate correctly when power is uncertain. One common design detects a sudden increase in voltage, which always happens when you turn the equipment on. Unfortunately, it also happens at the end of a voltage sag. If the reset circuit misinterprets the end of a voltage sag, the equipment will operate perfectly during the voltage sag, but will abruptly reset itself when the voltage returns to normal. To make this problem even more difficult, it is quite common for different parts of a system to have different reset circuits, so it is possible for one part of the system to be reset even when the rest of the system is not. Without a sag generator with a good data acquisition system, this problem is very difficult to detect and solve. Information about Transient Overvoltages Transient overvoltages are brief, high-frequency increases in voltage on AC mains. Broadly speaking, there are two different types of transient overvoltages: low frequency transients with frequency components in the few-hundredhertz region typically caused by capacitor switching, and high-frequency transients with frequency components in the few-hundred-kilohertz region typically caused by lighting and inductive loads. Low frequency transients are often called "capacitor switching transients". High frequency transients are often called "impulses", "spikes", or "surges". Surge suppressors are devices that conduct across the power line when some voltage threshold is exceeded. Typically, they are used to absorb the energy in high frequency transients. However, the resulting high frequency current pulses (often in the hundreds of amps) can still create problems for sensitive instrumentation. electronic systems, especially delicate Low frequency transients are caused when a discharged power-factorcorrection capacitor is switched on across the line. The capacitor then resonates with the inductance of the distribution system, typically at 400 600 Hz, and produce and exponentially damped decaying waveform. The peak of this waveform, in theory, cannot exceed twice the peak voltage of the sine wave, and is more typically 120% - 140% of the sine peak. However, in some specific cicumstances, there can be "multiplication" of this transient by resonance with other power factor correction capacitors. High frequency transients are caused by lightning, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of a tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 kHz, which corresponds to the frequency of equivalent inductor/capacitor model of low voltage power lines. Typical peak voltages for end-use applications are hundreds of volts to a few thousand volts; several thousand amps of current may be available. (Extremely fast transients, or EFT's, have rise and fall times in the nanosecond region. They are caused by arcing faults, such as bad brushes in motors, and are rapidly damped out by even a few meters of distribution wiring. Standard line filters, included on almost all electronic equipment, remove EFT's.) About Harmonics The electric power distribution system is designed to operate with sinusoidal voltages and currents. But not all waveforms are sine waves. Electronic loads, for example, often draw current only at the peak of the voltage waveform, which always means that the current is distorted, and may distort the voltage as well. One convenient way to describe these waveforms is to make a list of sine waves that, when added together, reproduce the distorted waveform. The sine waves in this list are always multiples, or harmonics, of the fundamental frequency (50 Hz or 60 Hz). A typical input circuit of a single-phase supply. All of the graphs below are automatically produced by the Industrial Power Corruptor's Power Flow Option. A typical distorted current waveform, drawn by the supply above. It only draws current at the peak of the voltage waveform, because the diodes in BR1 only conduct when the AC voltage is higher than the voltage on C1. This is the same waveform, expressed as a frequency spectrum. Note that the frequency content of the waveform consists of odd multiples (3,5,7,9, etc.) of the fundamental. This is typical for electronic loads. Again, the same waveform, expressed as a frequency spectrum. This time the values are listed. Sometimes, the phase angles of the harmonics can be important, too, but they are not shown here. THD, or Total Harmonic Distortion, is one measure of the total distortion. It is the RMS sum of the harmonics, divided by one of two values: either the fundamental value, or the RMS value of the total waveform. Both are legitimate definitions of THD. For small values of distortion, they both produce roughly the same number. For the waveform above, using the fundamental as the reference produces a THD value of 93.2%, and using the RMS as the reference produces a THD value of 67.8%. Both values are correct. For this and other reasons, most experts in power system harmonics frown on using THD as a measure of harmonics. Other measures such as TDD (IEEE 519) or volts and amps make more sense. For example, the waveform above consists of 32.4 amps at 60 Hz, plus 25.4 amps at 180 Hz, plus 14.8 amps at 300 Hz, etc. Many devices on the power system respond poorly to non-sinusoidal waveforms. Transformers, for example, become less efficient. Many revenue meters become less accurate. Protection devices such as circuit breakers may trip too soon, or too late. Balanced harmonics at multiples-of-3-of-the-fundamental, or triplen harmonics (3rd, 9th, 15th, etc.), fail to rotate on three-phase systems. As a result, neutral conductors may overheat, and transformers and motors become less efficient. Information about Flicker Flicker is a very specific problem related to human perception and incandescent light bulbs. It is not a general term for voltage variations. Humans can be very sensitive to light flicker that is caused by voltage fluctuations. Human perception of light flicker is almost always the limiting criteria for controlling small voltage fluctuations. The figure illustrates the level of perception of light flicker from a 60 watt incandescent bulb for rectangular variations. The sensitivity is a function of the frequency of the fluctuations and it is also dependent on the voltage level of the lighting. Voltage changes that will result in perceptible light flicker with a 60 watt incandescent light bulb. Limits for flicker levels are not specified in IEEE standards. Curves similar to the one shown above have been used by individual utilities as guidelines for controlling flicker. Flicker levels in IEC standards are characterized by two parameters: Pst is a value measured over 10 minutes that characterizes the likelihood that the voltage fluctuations would result in perceptible light flicker. A value of 1.0 is designed to represent the level that 50% of people would perceive flicker in a 60 watt incandescent bulb. Plt is derived from 2 hours of Pst values (12 values combined in cubic relationship). Note that IEEE is also adopting this method of characterizing flicker (IEEE 1453). IEC 61000-2-2 specifies flicker compatibility levels: Compatibility level for short term flicker (Pst) is 1.0. Compatibility level for long term flicker (Plt) is 0.8. Recognizing that it is not always possible to maintain flicker levels within these compatibility levels, EN 50160 specifies less restrictive requirements for the supply system performance. The EN 50160 limit is that 95% of the long term flicker values (Plt) should be less than 1.0 in one week measurement period. Note that individual step changes in the voltage, such as would be caused by motor starting or switching a capacitor bank, are often limited separately from the continuous flicker limits. IEC 61000-2-2 specifies a compatibility level of 3% for the individual voltage variations. EN 50160 specifies a limit of 5% for these variations but mentions that more significant variations (up to 10%) can occur for some switching events. Specific recommendations are not provided in IEEE but individual utilities usually have their own guidelines in the range 4-7%. Information about Voltage Regulation The term "voltage regulation" is used to discuss long-term variations in voltage. It does not include short term variations, which are generally called sags, dips, or swells. The ability of equipment to handle steady state voltage variations varies from equipment to equipment. The steady state voltage variation limits for equipment is usually part of the equipment specifications. The Information Technology Industry Council (ITIC) specifies equipment withstand recommendations for IT equipment according to the ITI Curve (formerly the CBEMA curve). The 1996 ITI Curve specifies that equipment should be able to withstand voltage variations within ± 10% (variations that last longer than 10 seconds). Voltage regulation standards in North America vary from state to state and utility to utility. The national standard in the U.S.A. is ANSI C84.1. Voltage regulation requirements are defined in two categories: Range A is for normal conditions and the required regulation is ± 5% on a 120 volt base at the service entrance (for services above 600 volts, the required regulation is -2.5% to +5%). Range B is for short durations or unusual conditions. The allowable range for these conditions is -8.3% to +5.8%. A specific definition of these conditions is not provided. Voltage regulation requirements from ANSI C84.1. This is not a universal standard; it is only used in North America. Other countries have different standards. For example, IEC 61000-2-2 mentions that the normal operational tolerances are ± 10% of the declared voltage. This is the basis of requirements for voltage regulation in EN 50160 for the European Community. EN 50160 requires that voltage regulation be within ± 10% for 95% of the 10 minute samples in a one week period, and that all 10 minute samples be within -15% to +10%, excluding voltage dips. Other Disturbances The most common disturbances on AC power systems are voltage sags or dips. Other problems, such as transient overvoltages and brief interruptions, occur almost everywhere. Problems with harmonics, voltage regulation, and flicker occur at a wide range of sites. Some other disturbances that occur at specific locations include: Frequency variations. On utility grids, these are rare events, usually associated with catastrophic collapses on the grid. However, at sites with back-up diesel generators, they are common. High frequency noise. This can be caused by anything from arcing brushes on a motor, to local radio transmitters. Mains signalling Some utilities intentionally place small signals on the mains voltage to act as control signals (for example, they may control a capacitor switch, or they may instruct revenue meters to go to a different rate structure). EFT Extremely Fast Transients are nano-second range transient overvoltages. Due to their high frequency content, they do not travel well over the mains circuits, getting damped out within a few meters. However, they can be caused by nearby contact arcing. Unbalance On three-phase systems, the voltages and currents on each phase should, in theory, match the voltages and currents on the other phases. Sometimes they don't.