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Understanding & Power Quality Measurements The threatened limitations of conven- Typical : tional electrical power sources have -to-phase = 208/480V focused a great deal of attention on phase-to-neutral = 120/277V power, its application, monitoring and correction. Power economics now play a critical role in industry as never before. With the high cost of power generation, transmission, and distribution, it is of paramount concern to effectively moni- tor and control the use of energy. The electric utility’s primary goal is to meet the power demand of its customers Figure 1: 1Ø System at all times and under all conditions. But as the electrical demand grows in Three-Phase, 3-Wire Systems size and complexity, modifications and Figure 3: 3Ø, 4-Wire System In this type of system, commonly known additions to existing as the “DELTA” configuration, the volt- networks have become increasingly Balanced vs. Unbalanced Loads age between each pair of line wires is the expensive. The measuring and monitoring actual voltage. This system A balanced load is an AC power system of electric power have become even is frequently used for power loads in using more than two wires, where the more critical because of down time commercial and industrial buildings. In current flow is equal in each of the associated with equipment breakdown such cases, service to the premises is c u r r e n t - c a r rying conductors. Many and material failures. made at 208V, three-phase. Feeders systems today represent an unbalanced For economic reasons, electric power is carry the power to panel boards supply- condition due to uneven loading on a generated by utility companies at rela- ing branch circuits for motor loads. particular phase. This often occurs when tively high (4160, 6900, 13,800 Lighting loads are usually handled by a electrical expansion is affected with little volts are typical). These high voltages separate single-phase service. The 480V regard to even distribution of loads are then reduced at the consumption site distribution is often used in industrial between phases or several nonlinear by step-down to lower val- buildings with substantial motor loads. loads on the same system. ues which may be safely and more easi- RMS vs. Sensing ly used in commercial, industrial and residential applications. The term RMS (root--) is used in relation to Personnel and property safety are the and simply “equiva- most important factors in the operation lent” or “effective,” referring to the of electrical system operation. Reliability amount of work done by the equivalent is the first consideration in providing value of (DC). The term safety. The reliability of any electrical RMS is necessary to describe the value system depends upon knowledge, pre- of alternating current, which is ventive maintenance and subsequently constantly changing in and the test equipment used to monitor that polarity at regular intervals. RMS system. Figure 2: 3Ø, 3-Wire System measurements provide a more accurate Three-Phase, 4-Wire Systems representation of actual current or TYPICAL VOLTAGE voltage values. This is very important for CONFIGURATIONS Known as the “WYE” type connection, nonlinear (distorted) waveforms. Single-Phase Systems this is the system most commonly used in commercial and industrial buildings. Until recently, most loads were “linear”; Single-phase residential loads are almost In office or other commercial buildings, that is, the load impedance remained universally supplied through 120/240V, 3- the 480V three-phase, 4-wire feeders essentially constant regardless of the wire, single-phase services. Large appli- are carried to each floor, where 480V applied voltage. With expanding markets ances such as ranges, water heaters, and three-phase is tapped to a power panel of computers, uninterruptable power clothes dryers are supplied at 240V. or motors. General area fluorescent supplies, and variable speed motor Lighting, small appliances, and outlet lighting that uses 277V ballasts is drives, resulting nonlinear waveforms receptacles are supplied at 120V. In this connected between each leg and are drastically different. system the two “hot” or current carryi n g neutral; 208Y/120 three-phase, 4-wire Measuring nonsinusoidal voltage and conductors are 180 degrees out-of-phase c i rcuits are derived from step-down current waveforms requires a True RMS with respect to the neutral. transformers for local lighting and m e t e r. Conventional meters usually receptacle outlets. measure the average value of the

TRANSCAT.COM Go To TRANSCAT.COM of a . Some meters The fundamental unit for measuring In the case of a single-phase motor, the are calibrated to read the equivalent RMS usage is the watthour (Wh), or more typ- actual power is the sum of several value (.707 x peak); this type calibration ically the kilowatthour (kWh). This value components: is a true representation only when the represents usage of 1000W for one hour. a. the work performed by the system; waveform is a pure sine (i.e., no Typical costs in the United States for one that is, lifting with a crane, moving air ). When distortion occurs, the kilowatthour range from 8 to 15 cents. with a fan, or moving material, as with relationship between average readings a conveyer. and True RMS values changes drastically. b. heat developed by the power lost in Only a meter which measures True RMS Power factor is the ratio of ACTUAL the motor winding resistance values gives accurate readings for a non- POWER used in a circuit to the APPAR- sinusoidal waveform. RMS measuring ENT POWER delivered by a utility. Actual c. heat developed in the iron through circuits sample the input signal at a high power is expressed in (W) or kilo- eddy currents and hysteresis losses rate of speed. The meter’s internal cir- watts (kW); apparent power in voltam- d.frictional losses in the moor bearings cuitry digitizes and squares each sample, peres (VA) or kilovoltamperes (kVA ) . adds it to the previous samples squared, Apparent power is calculated simply by e. air friction losses in turning the motor and takes the of the total. multiplying the current by the voltage. r o t o r, more commonly known as This is the True RMS value. windage losses. Power Factor = Actual Power = k W Apparent Power kVA We now observe that with a single-phase motor, the apparent power obtained is Certain loads (e.g., inductive type greater than the actual power. This dif- motors) create a phase shift or delay ference is the power factor. between the current and voltage wave- forms. An inductive type load causes the Power factor reflects the difference current to lag the voltage by some angle, which exists between loads. The solder- known as the phase angle. ing iron is a purely resistive load which absorbs the current, which is then On purely resistive loads, there is no absorbed directly into heat. The current phase difference between the two wave- is called actual current because it directly Figure 4: Nonlinear Current Waveform forms; therefore the power factor on contributes to the production of actual such a load will be 0 degrees, or unity. power. DEMAND The following examples of a soldering On the other hand, the single-phase elec- iron and a single-phase motor illustrate The amount of electrical energy con- tric motor represents a partially inductive how power factor is consumed in sumed over time is known as demand. load consisting of actual current which different types of loads. Demand is the average load placed on will be converted into actual power, and the utility to provide power (kilowatts) to In a soldering iron, the apparent power magnetizing current which generates the a customer over a utility-specified time supplied by the utility is directly convert- required to operate the interval (typically 15 or 30 minutes). If ed into heat, or actual power. In this . This magnetizing current, demand requirements are irregular, the case, the actual power is equal to the called the reactive current, corresponds utility must have more capability avail- apparent power, so that the power factor to an exchange of energy between the able than would be required if the cus- is equal to “1” or 100% (unity). generator and the motor, but it is not tomer load requirements remained con- converted into actual power. stant. To provide for this time-varying demand, the utility must invest in the REACTIVE COMPENSATION proper size equipment to provide for POWER these power peaks. Brief high peaks such as those present when large equip- Reactive compensation power refers to ment initially comes on line are not criti- the capacitive values required to correct cal in the overall equation because the low power factor to as close to unity duration is short with respect to the (1.0) as possible. Most industrial loads demand averaging interval. are inductive, so the load current lags the line voltage by some degree. CONSUMPTION In order to bring the value closer to unity, Watts and vars are instantaneous meas- something must be added to the load to urements representing what is happen- draw a leading current. This is done by ing in a circuit at any given moment. connecting a in parallel with Since these parameters vary so greatly the load. Since a capacitor will not dissi- pate any real power, the charge for real within any period, it is necessary to inte- Figure 5: Power Factor on Nondistorted Sine Wav e grate (sum) electrical usage over time. power will be the same.

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Several AEMC power monitoring that is then drawn by the nonlinear load • Saturated transformers instruments will display the actual power would be the fundamental as well as all • Solid state elevator controls factor correction capacitor values directly. the harmonics. AEMC recommends consulting a power • Medical equipment Harmonic distortion can cause serious factor correction capacitor manu- problems for the users of electric power, facturer prior to any installation to This is by no means an exhaustive list of from inadvertent tripping of circ u i t reduce the possible effects of harmonics, equipment which generates harmonics. breakers to dangerous overheating of resonance, etc. Any electronic-based equipment should transformers and neutral conductors, as be suspected of producing harmonics. well as heating in motors and capacitor ELECTRICAL HARMONICS failure. Harmonics can cause problems Due to the ever increasing use of , the percentage of equipment Until fairly recently, power quality that are easy to recognize but tough to which generates harmonic current has referred to the ability of the electric diagnose. increased significantly. The harmonic utilities to supply electric power without It is becoming increasingly important to problem manifests itself with pro- interruption. Today, the phrase encom- understand the fundamentals of liferation of equipment using diode- passes any deviation from a perf e c t harmonics, and to be able to recognize capacitor input power supplies. This type sinusoidal waveform. Power quality now and monitor the presence of damaging of equipment draws current in a short relates to short-term transients as well harmonics. Harmonics within an electri- pulse only during the peak of the sine as continuous state . Power cal system vary greatly within different wave. The result of this action, aside system harmonics are a continuous state parts of the same distribution system from improved efficiency, is that high problem with dangerous results. and are not limited simply to the supply harmonics are superimposed harmonics can be present in current, of the harmonic producing device. onto the fundamental 60Hz frequency. voltage, or both. It is estimated that as Harmonics can interact within the sys- many as 60% of all electrical devices tem through direct system connections The harmonics are produced by the operate with non-linear current draw. or even through capacitive or inductive diode-capacitor input section which rectifies the AC signal into DC. The Utility companies invest millions of coupling. circuit draws current from the line only dollars each year to ensure that voltage A harmonic may be defined as an integer during the peaks of the voltage wave- supplied to their customers is as close multiple of a fundamental frequency. form, thereby charging a capacitor to the as possible to a sinusoidal waveform. If Harmonics are designated by the peak of line voltage. The equipment DC the power user connects loads to the harmonic number. For our discussion, requirements are fed from this capacitor system which are resistive, such as we will focus on the 60Hz power and as a result the current waveform , the resulting frequency. The second harmonic would becomes distorted. current waveform will also be sinusoidal. be two times the fundamental or 120Hz. H o w e v e r, if the loads are nonlinear, The third would be three times the which is typically the case, the current is fundamental or 180Hz, and so on. drawn in short pulses and the current waveform will be distorted. Total current Nonlinear equipment generates harmonic . The nonlinear nature of a device draws current waveforms that do not follow the voltage waveform. Electronic equipment is a good example. While this broad category encompasses many different types of equipment, most Figure 7: Nonlinear Current Draw of these devices have one characteristic in common. They rely on an internal DC Harmonics in the power source for their operation. combine with the fundamental frequency to create distortion. The level of distor- Loads which produce harmonic currents tion is directly related to the frequencies include: and amplitudes of the harmonic current. • Electronic lighting ballasts The contribution of all harmonic frequency currents to the fundamental current is • Adjustable speed drives known as “Total Harmonic Distortion” or • Electric arc furnaces THD. This THD value is expressed as a percentage of the fundamental current. • Personal computers THD values of over 10% are reason for • Electric welding equipment concern. • Solid state THD is calculated as the square root of the sum of the squares of all the • Industrial process controls harmonics divided by the fundamental Figure 6: Composite Wav e f o r m • UPS systems signal (50 or 60Hz). This calculation

TRANSCAT.COM Go To TRANSCAT.COM arrives at the value of distortion as a Wherever there are large numbers of percentage of the fundamental. nonlinear loads, there are sure to be harmonics in the distribution system. Mathematically, %THD is the ratio of the Harmonic-producing equipment is found sum of the root-mean-square (RMS) of in varied locations from administrative the harmonic content to the root-mean- offices to manufacturing facilities. In the square (RMS) value of the fundamental factory environment, electronic power 50 or 60Hz signal, and expressed as a converters such as variable speed percentage. drives, SCR drives, etc., are the largest contribu- Figure 9: Delta Primary, Circulating Current tors to harmonic distor- tion. It is not uncommon on the zero crossing of the sinusoidal to have THD levels as high waveform, such as clock timing devices, as 25% within some heavy harmonic content can cause a zero industrial settings. crossing point offset. Most single-phase office Odd number harmonics (third, fifth and equipment draws nonlin- seventh) cause the greatest concern in ear current. While fluores- the electrical distribution system. cent lighting with electronic ballasts and Because the harmonic waveform usually Another useful parameter is the many types of office equipment con- swings equally in both the positive and Distortion Factor, or %DF. Distortion tribute to creating harmonics, personal negative direction, the even number Factor is the Total Harmonic Distortion computer power supplies are the largest harmonics are mitigated. referenced to the total RMS signal. The contributor within the office environ- Heating effect causes the greatest TH D is expressed as a percentage and ment. Although THD levels will be lower problem in electric equipment. Many may not be greater than the fundamental. than in an industrial setting, the suscep- times, electrical distribution equipment The %DF never exceeds 100%. We pro- tibility of office equipment to variations has overheated and failed even when vide this term because of the market in power quality is extremely high. need and the requirement of this value operating well below the suggested under the international standard IEC- In the industrial environment, there can rating requirements. Te m p e r a t u r e 555. Mathematically, it is the ratio of the be many three-phase, nonlinear loads increase is directly related to the sum of the root-mean-square (RMS) of drawing high levels of load current. The increase in RMS current. the harmonic content to the root-mean- most prevalent harmonic frequencies are Because harmonic frequencies are square (RMS) value of the total signal, the odd integer multiples of the 60Hz fre- always higher than the 60Hz fundamen- and expressed as a percentage. quency. The third harmonic (180Hz) is tal frequency, “skin effect” also becomes always the most prevalent a factor. Skin effect is a phenomenon and troublesome. where the higher frequency causes the Large commercial build- electrons to flow toward the outer sides ings have many different of the conductor, effectively reducing the sizes and types of loads. In cross-sectional diameter of the conduc- most installations the tor and thereby reducing the ampacity power is distributed with rating of the cable. This effect increases 208/120 volt transformers as the frequency and the amplitude in a Delta-Wye configura- increase. As a result, higher harmonic tion. When multiple loads frequencies cause a greater degree of Please note that our %DF is not the same are supplied, each generates triplen har- heating in conductors. value as the Distortion Factor as monic currents on the neutral conductor On balanced three-phase systems with expressed the the IEEE standard 519- which are sent onto the transformer sec- no harmonic content, the line currents 1992 (in which Distortion Factor is the ondary and reflected into the delta pri- are 120 degrees out-of-phase, canceling same as THD). mary. These currents circulate within the each other and resulting in very little delta primary causing neutral current. However, when there is overheating and short- distortion in any one of the phase cur- ened service life. rents, the harmonic currents increase Harmonics can cause and the cancellation effect is lessened. a variety of problems The result is typically a neutral current to any user of electric that is significantly higher than power. For large users, planned.The triplen harmonics (odd mul- the problems can be tiples of three) are additive in the neutral intense. For electronic and can quickly cause dangerous over- equipment that relies heating. Figure 8: Computer Current Wav e f o r m

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neutral current to the anticipated current due to phase imbalance. If the phase currents are equal, the vector sum of the neutral currents will add to zero. If there are excessive amounts of triplen harmonics in the neutral, neutral current may exceed phase current. Consult the NEC® for the maximum ampacity for each of the tested conductors. Measure each feeder for harmonic content. A high degree at this location can often be heard as a buzzing sound. A voltage THD reading is also useful at this location. IEEE standard 519-1992 specifies both maximum distortion levels and recom- Figure 10: Power Factor at Resonant Frequency mended correction levels. A harmonic distortion limit of 5% is the point where In theory, the maximum current that the Detection and Measurement harmonics begin to have a detrimental neutral will carry is 1.73 times the phase In harmonic analysis, field measure- effect on electrical distribution systems. current. If not sized correctly, over- ments are performed to identify heating will result. Higher than normal Harmonic current measurements define frequency and magnitude of harmonic neutral current will cause voltage drops the harmonic generating characteristics currents generated by susceptible between neutral and ground which are of the load, so measurements should be equipment (e.g., electronic equipment, well above normal. Readings above taken at the load when possible. variable speed motors, etc.). Remember 4 volts indicate high neutral current. Vo l t a g e measurements define the that most distribution systems are system response and are usually taken at False tripping of circuit breakers is also a designed specifically to carry 60Hz. the individual busses. problem encountered with the higher Most nonlinear harmonic problems can frequencies that harmonics produce. Effects on the System be detected at the electrical panel. peak sensing circuit breakers often will Excessive current flow on the neutral can To compound the problems that trip even though the amperage value has be detected with a True RMS current harmonic currents present to the not been exceeded. Harmonic current m e t e r, but may be indicated by a system, nonlinear harmonic load also peak values can be many times higher resonant buzzing sound or by discolored have an Ohm’s law relationship with the than sinusoidal waveforms. connections on the neutral buss. s o u rce impedance of the system to Power factor correction capacitor failure produce voltage harmonics. Consider a Beginning at the service entrance panel, in many cases can be directly attributed heavily loaded transformer that is measure and record the True RMS to harmonic content. Capacitors appear affected by one branch circuit feeding a current in each phase, as well as the as extremely low impedance values and non-linear load. The creation of voltage neutral of the distribution transformer are more susceptible to harmonics. harmonics can then be passed down to s e c o n d a ry. Compare this measured Inductive reactance varies directly with all the remaining circuits being fed by frequency (XL = 2πfl). Parallel resonance that transformer. between the capacitor bank and the Voltage harmonics may cause havoc s o u rce impedance can cause system within the electrical system. Motors are resonance resulting in higher than typically considered to be linear loads; normal currents and voltages. High however, when the source voltage supply harmonic currents have been known to is rich in harmonics, the motor will draw overheat correction capacitors, causing harmonic current. The typical result is a premature failure and sometimes higher than normal operating tempera- resulting in explosion. ture and shortened service life. Most harmonic problems result when Different frequency harmonic currents the resonant frequency is close to the can cause additional rotating fields in the fifth or seventh harmonic. These happen motor. Depending on the frequency, the to be the largest harmonic amplitude motor will rotate in the opposite numbers that adjustable speed drives direction (counter-torque). The fifth create. When this situation arises, harmonic, which is very prevalent, is a capacitor banks should be resized to negative sequence harmonic causing the shift the resonant point to another Figure 11: Measuring at the Service Entrance motor to have a backward rotation, frequency. Panel shortening the service life.

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Noise can be picked up in communication This method is established by The equipment and systems when Computer & Business Equipment harmonics at audio or frequencies Manufacturers Association (CBEMA). are inductively or capacitively coupled into communication or data lines. CBENA Derating Factor = 1.414 When induction-disc -hour meters are monitoring nonlinear loads, depend- Meter Readings ing on the content of the harmonics, the Harmonic problems can be analyzed disk may run slower or faster, resulting more easily when the proper test equip- in erroneous readings. Figure 12: RMS — Avg — Peak ment is used. Relationship in Sine Wav e Transformer Derating The term “True RMS”, or Root-Mean- complex waveform correctly. When Most generators and transformers base Square, relates to the equivalent DC harmonics are present crest factors may their operating characteristics on undis- heating value of the current or voltage be less than (CF of a = 1) or turbed 60Hz waveforms. When the waveform. If a pure and a dis- greater than 1.414. waveforms are rich in harmonics, torted sine wave were both applied to a Limiting the Effects of Harmonics shortened service or complete failure resistive load, the point where they both often follows. create the same heating value is the Derating certain types of electrical equip- point where they both have the same ment is the easiest way to limit The derating K factor can be applied RMS value. the effects that increased heating has on specifically to transformers to ensure the equipment. A 25% derating for that dangerous heating will not result True RMS capability is required to accu- transformers and generators is due to the transformer supplying load rately measure systems where harmonic commonly employed in industry. currents rich in harmonic content. current is present. Average responding instruments will yield erroneous Filtering is currently the most common The K factor is determined by measuring measurement results from 25 to 40% method used to limit the effects that the True RMS current of each harmonic, below the actual value when harmonic harmonics present to the rest of the sys- multiplied by the harmonic order and distortion is present. tem. Filters typically consist of tuned squared. The total sum of this is series L – C circuits. Filter impedance is then multiplied by the harmonic order Many instruments on the market negligible with respect to the rest of the and squared. The total sum of this measure average or Peak values of a system, limiting its interaction effects for is then multiplied by the waveform and internally multiply by 1.11 harmonic control. Filters are sized to losses. Transformer parasitic heating or .707 respectively to indicate RMS withstand the RMS current as well as the due to harmonic currents is frequency- values. These devices work well when value of current for the harmonics. dependent, i.e., higher frequency measuring a pure sine wave. harmonic currents cause a higher degree In the future, systems may be available Instruments with True RMS converters of transformer heating and failure. which will offset the harmonics by apply- sample the waveform at many different ing signals that are equal in amplitude The K factor is basically an index of the points and provide accurate readings on but opposite in phase, thereby canceling transformer’s ability to handle nonlinear distorted waveforms. Microprocessor- or severely limiting harmonic effects. load current without abnormal heating. based circuits sample, digitize and Some distribution transformers are now square each sample, add it to the being designed with magnetic cores and previous sample squared, and take the windings to accommodate harmonic square root of the total. This process content. A K-rated transformer is specif- yields a True RMS value regardless of ically designed to handle nonlinear the amount of distortion. loads. The higher the K factor value, the Crest Factor better the transformer’s ability to handle nonlinear loads. Crest factor is the ratio of the Peak value of a sinusoidal waveform to its RMS value. IEEE C57.110-1986 is a prescribed procedure used to derate the trans- Crest Factor (CF) = Peak Value former loading based on the specific RMS Value harmonic content. Each specific electri- cal application is unique in type and Crest factor indicates the level of peaking amount of harmonic interaction. which an instrument can handle without measurement errors. For a perfect sine IEEE C57.1200-1987 has proposed a wave the crest factor would be 1.414. limit of 5% for transformer harmonic This relates to the Peak amplitude that an current factor. instrument can measure accurately. An alternate method for derating trans- Typical crest factor ratings are from 2.0 formers is available for buildings which to 6.0. The higher the factor, the more Figure 13: Single tuned shunt filter supply single phase, 120V receptacles. capable the instrument of measuring a

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