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Understanding Power & Power Quality Measurements

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Understanding Power & Power Quality Measurements Go To TRANSCAT.COM Understanding Power & Power Quality Measurements The threatened limitations of conven- Typical voltage: tional electrical power sources have phase-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 electric power 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 transformer 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 voltages (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 transformers to lower val- buildings with substantial motor loads. loads on the same system. ues which may be safely and more easi- RMS vs. Average Sensing ly used in commercial, industrial and residential applications. The term RMS (root-mean-square) is used in relation to alternating current Personnel and property safety are the waveforms and simply means “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 direct current (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 amplitude 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 amplitudes of a waveform. 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 wave (i.e., no Typical costs in the United States for one that is, lifting with a crane, moving air distortion). 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. POWER FACTOR 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 watts (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 square root 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- magnetic field required to operate the interval (typically 15 or 30 minutes). If ed into heat, or actual power. In this electric motor. 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 capacitor 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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