Understanding Power & Power Quality Engineer’s Measurements

Notebook Typical voltage: phase-to-phase = 208/480V phase-to-neutral = 120/277V The threatened limitations of conventional electrical power sources have focused a great Balanced vs. Unbalanced Loads deal of attention on power, its application, A balanced load is an AC power system using monitoring and correction. Power economics more than two wires, where the current flow is now play a critical role in industry as never equal in each of the current-carrying before. With the high cost of power generation, conductors. Many systems today represent an transmission, and distribution, it is of unbalanced condition due to uneven loading on paramount concern to effectively monitor and a particular phase. This often occurs when control the use of energy. electrical expansion is affected with little regard to even distribution of loads between For economic reasons, electric power is phases or several nonlinear loads on the generated by utility companies at relatively high same system. voltages (4160, 6900, 13,800V are typical). These high voltages are then reduced at the RMS vs. Average Sensing consumption site by step-down transformers to The term RMS (root-mean-square) is used in lower values, which may be safely, and more relation to alternating current waveforms and easily used in commercial, industrial and simply means “equivalent” or “effective,” residential applications. referring to the amount of work done by the equivalent value of direct current (DC). The requirements remained constant. To provide for Personnel and property safety are the most term RMS is necessary to describe the value this time-varying demand, the utility must important factors in the operation of electrical of alternating current, which is constantly invest in the proper size equipment to provide system operation. Reliability is the first changing in amplitude and polarity at regular for these power peaks. Brief high peaks such consideration in providing safety. The reliability intervals. RMS measurements provide a more as those present when large equipment initially of any electrical system depends upon accurate representation of actual current or comes on line are not critical in the overall knowledge, preventive maintenance and voltage values. This is very important for equation because the duration is short with subsequently the test equipment used to nonlinear (distorted) waveforms. respect to the demand averaging interval. monitor that system. Until recently, most loads were “linear”; that is, CONSUMPTION TYPICAL VOLTAGE CONFIGURATIONS the load impedance remained essentially Watts and vars are instantaneous Single-Phase Systems constant regardless of the applied voltage. measurements representing what is happening Single-phase residential loads are almost With expanding markets of computers, uninterruptable power supplies, and variable in a circuit at any given moment. Since these universally supplied through 120/240V, 3-wire, parameters vary so greatly within any period, it single-phase services. Large appliances such speed motor drives, resulting nonlinear waveforms are drastically different. is necessary to integrate (sum) electrical usage as ranges, water heaters, and clothes dryers over time. The fundamental unit for measuring are supplied at 240V. Lighting, small usage is the watthour (Wh), or more typically Measuring nonsinusoidal voltage and current appliances, and outlet receptacles are supplied the kilowatthour (kWh). This value represents waveforms requires a True RMS meter. at 120V. In this system the two “hot” or usage of 1000W for one hour. Typical costs in Conventional meters usually measure the current carrying conductors are 180 degrees the United States for one kilowatthour range average value of the amplitudes of a waveform. out-of-phase with respect to the neutral. from 8 to 15 cents. Some meters are calibrated to read the Three-Phase, 3-Wire Systems equivalent RMS value (.707 x peak); this type calibration is a true representation only when POWER FACTOR In this type of system, commonly known as the the waveform is a pure sine wave (i.e., no Power factor is the ratio of ACTUAL POWER “DELTA” configuration, the voltage between distortion). When distortion occurs, the used in a circuit to the APPARENT POWER each pair of line wires is the actual transformer relationship between average readings and delivered by a utility. Actual power is expressed voltage. This system is frequently used for True RMS values changes drastically. in watts (W) or kilowatts (kW); apparent power power loads in commercial and industrial in voltamperes (VA) or kilovoltamperes (kVA). buildings. In such cases, service to the Only a meter which measures True RMS values Apparent power is calculated simply by premises is made at 208V, three-phase. gives accurate readings for a nonsinusoidal multiplying the current by the voltage. Feeders carry the power to panel boards waveform. RMS measuring circuits sample the supplying branch circuits for motor loads. Power Factor = Actual Power = kW input signal at a high rate of speed. The Lighting loads are usually handled by a Apparent Power kVA meter’s internal circuitry digitizes and squares separate single-phase service. The 480V Certain loads (e.g., inductive type motors) each sample, adds it to the previous samples distribution is often used in industrial buildings create a phase shift or delay between the Power & squared, and takes the square root of the with substantial motor loads. current and voltage waveforms. An inductive total. This is the True RMS value. type load causes the current to lag the voltage Three-Phase, 4-Wire Systems by some angle, known as the phase angle. Known as the “WYE” type connection, this is DEMAND the system most commonly used in The amount of electrical energy consumed over On purely resistive loads, there is no phase commercial and industrial buildings. In office or time is known as demand. Demand is the difference between the two waveforms; other commercial buildings, the 480V average load placed on the utility to provide therefore the power factor on such a load three-phase, 4-wire feeders are carried to each power (kilowatts) to a customer over a will be 0 degrees, or unity. Quality Measurements floor, where 480V three-phase is tapped to a utility-specified time interval (typically 15 or 30 power panel or motors. General area minutes). If demand requirements are irregular, fluorescent lighting that uses 277V ballasts is the utility must have more capability available connected between each leg and neutral; than would be required if the customer load 208Y/120 three-phase, 4-wire circuits are derived from step-down transformers for local lighting and receptacle outlets.

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ELECTRICAL HARMONICS largest harmonic amplitude Until fairly recently, power quality referred to numbers that adjustable speed Notebook the ability of the electric utilities to supply drives create. When this electric power without interruption. Today, the situation arises, capacitor phrase encompasses any deviation from a banks should be resized to shift perfect sinusoidal waveform. Power quality now the resonant point to relates to short-term transients as well as another frequency. continuous state distortions. Power system harmonics are a continuous state problem with Detection and Measurement dangerous results. harmonics can be present In harmonic analysis, field measurements are in current, voltage, or both. It is estimated that performed to identify frequency and magnitude current without abnormal as many as 60% of all electrical devices of harmonic currents generated by susceptible heating. Some distribution operate with non-linear current draw. equipment (e.g., electronic equipment, variable transformers are now being designed speed motors, etc.). Remember that most with magnetic cores and windings to Utility companies invest millions of dollars each distribution systems are designed specifically accommodate harmonic content. A K-rated year to ensure that voltage supplied to their to carry 60 Hz. transformer is specifically designed to handle customers is as close as possible to a nonlinear loads. The higher the K factor value, the better the transformer’s ability to handle sinusoidal waveform. If the power user Most nonlinear harmonic problems can be nonlinear loads. connects loads to the system which are detected at the electrical panel. Excessive resistive, such as incandescent light bulb, the current flow on the neutral can be detected resulting current waveform will also be with a True RMS current meter, but may be Meter Readings Quality Measurements sinusoidal. However, if the loads are nonlinear, indicated by a resonant buzzing sound or by Harmonic problems can be analyzed more which is typically the case, the current is drawn discolored connections on the neutral buss. easily when the proper test equipment is used. in short pulses and the current waveform will be distorted. Total current that is then drawn Beginning at the service entrance panel, The term “True RMS”, or Root-Mean-Square, by the nonlinear load would be the fundamental measure and record the True RMS current in relates to the equivalent DC heating value of as well as all the harmonics. each phase, as well as the neutral of the the current or voltage waveform. If a pure sine distribution transformer secondary. Compare wave and a distorted sine wave were both Harmonic distortion can cause serious this measured neutral current to the applied to a resistive load, the point where they Power & problems for the users of electric power, from anticipated current due to phase imbalance. If both create the same heating value is the point inadvertent tripping of circuit breakers to the phase currents are equal, the vector sum where they both have the same RMS value. dangerous overheating of transformers and of the neutral currents will add to zero. If there neutral conductors, as well as heating in are excessive amounts of triplen harmonics in True RMS capability is required to accurately motors and capacitor failure. Harmonics can the neutral, neutral current may exceed phase measure systems where harmonic current is cause problems that are easy to recognize but current. Consult the NEC® for the maximum present. Average responding instruments will tough to diagnose. ampacity for each of the tested conductors. yield erroneous measurement results from 25 to 40% below the actual value when harmonic Loads which produce harmonic Effects on the System distortion is present. currents include: To compound the problems that harmonic • Electronic lighting ballasts currents present to the system, nonlinear Crest Factor • Adjustable speed drives harmonic load also have an Ohm’s law Crest factor is the ratio of the Peak value of a • Electric arc furnaces relationship with the source impedance of the sinusoidal waveform to its RMS value. system to produce voltage harmonics. Consider • Personal computers a heavily loaded transformer that is affected by Crest Factor (CF) = Peak Value • Electric welding equipment one branch circuit feeding a non-linear load. RMS Value • Solid state rectifiers The creation of voltage harmonics can then be • Industrial process controls passed down to all the remaining circuits being Crest factor indicates the level of peaking fed by that transformer. • UPS systems which an instrument can handle without measurement errors. For a perfect sine wave • Saturated transformers Voltage harmonics may cause havoc within the the crest factor would be 1.414. This relates to • Solid state elevator controls electrical system. Motors are typically the Peak amplitude that an instrument can • Medical equipment considered to be linear loads; however, when measure accurately. Typical crest factor ratings the source voltage supply is rich in harmonics, are from 2.0 to 6.0. The higher the factor, the Harmonics can cause a variety of problems to the motor will draw harmonic current. The more capable the instrument of measuring a any user of electric power. For large users, the typical result is a higher than normal operating complex waveform correctly. When harmonics problems can be intense. For electronic temperature and shortened service life. are present crest factors may be less than equipment that relies on the zero crossing of (CF of a square wave = 1) or greater the sinusoidal waveform, such as clock timing Different frequency harmonic currents can than 1.414. devices, heavy harmonic content can cause a cause additional rotating fields in the motor. Depending on the frequency, the motor will zero crossing point offset. Limiting the Effects of Harmonics rotate in the opposite direction Derating certain types of electrical equipment Odd number harmonics (third, fifth and (counter-torque). The fifth harmonic, which is is the easiest way to limit the effects that seventh) cause the greatest concern in the very prevalent, is a negative sequence increased heating has on the equipment. A electrical distribution system. Because the harmonic causing the motor to have a 25% derating for transformers and generators harmonic waveform usually swings equally in backward rotation, shortening the service life. is commonly employed in industry. both the positive and negative direction, the even number harmonics are mitigated. Transformer Derating Most generators and transformers base their Filtering is currently the most common method False tripping of circuit breakers is also a operating characteristics on undisturbed 60Hz used to limit the effects that harmonics pres- problem encountered with the higher waveforms. When the waveforms are rich in ent to the rest of the system. Filters typically frequencies that harmonics produce. peak harmonics, shortened service or complete consist of tuned series L – C circuits. Filter sensing circuit breakers often will trip even failure often follows. impedance is negligible with respect to the rest though the amperage value has not been of the system, limiting its interaction effects for exceeded. Harmonic current peak values harmonic control. Filters are sized to withstand The derating K factor can be applied can be many times higher than the RMS current as well as the value of current specifically to transformers to ensure that sinusoidal waveforms. for the harmonics. dangerous heating will not result due to the transformer supplying load currents rich in Most harmonic problems result when the harmonic content. Information for this article contributed by resonant frequency is close to the fifth or AEMC® Instruments. seventh harmonic. These happen to be the The K factor is basically an index of the transformer’s ability to handle nonlinear load

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