Power Quality causes

Power quality is depends upon the rated voltage, frequency(pure sinusoidal). The power delivered to the customer is must be in quality. The causes of power quality in electrical system are listed as follows,

• Sags, dips & swells,
• Transient over voltage,
• Harmonics,
• Flickers,
• Voltage regulation,
• Other disturbances.

Power quality is the set of limits of electrical properties that allows electrical systems to function in their intended manner without significant loss of performance or life. The term is used to describeelectric power that drives an electrical load and the load's ability to function properly with that electric power. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power.

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. TRANSIENT OVER VOLTAGE 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-hundred-hertz 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 electronic systems, especially delicate instrumentation. Low frequency transients are caused when a discharged power-factor-correction 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.) Contact Alex McEachern for more information about: transient standards effects of transients on equipment solutions to transient problems. Example capacitor switching transient. Information 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 countries have established different limits, based on the characteristics of their distribution systems: Australia, Japan, etc. 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.