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Power Systems I

by: Dr. Reina Hane
Dr. Reina Hane
GPA 3.79

Clifford Grigg

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Clifford Grigg
Class Notes
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This 5 page Class Notes was uploaded by Dr. Reina Hane on Monday October 19, 2015. The Class Notes belongs to ECE 470 at Rose-Hulman Institute of Technology taught by Clifford Grigg in Fall. Since its upload, it has received 18 views. For similar materials see /class/225080/ece-470-rose-hulman-institute-of-technology in Electrical Engineering & Computer Science at Rose-Hulman Institute of Technology.

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Date Created: 10/19/15
51 Power Factor Correction The power factor pf is the ratio of real power to apparent power It is dimensionless and between 0 and 1 i a pure resistance has unity pf ii a pure inductance has zero pf lagging iii a pure capacitance has zero pf leading Almost all industrial loads are lagging Low pf results in unnecessarily high currents requiring larger conductors and resulting in unnecessary losses and in too much voltage being dropped across the feeders the power lines feeding the load lfa power source is required to deliver power average power to a load which has a low pf pf cos e the required current will be higherthan it would be ifthe power factor were larger This excess current causes excessive losses in the power lines and in the entire power system The power company is then required to devote capacity to this excess current that is present only because the load power factor is small that is because the load appears too strongly inductive I RL M 4 VS V load What is physically going on is that energy is being temporarily stored at the load during one part ofthe cycle and then sent back to the source during another part A low pf indicates a significant transfer ofstored energy with respect to energy dissipation the transfer ofstored energy does no useful work Energy is just transferred between the source and load lecture 5 outline 52 Almost all industrial load are inductive because of motors and other electric machines This provides a practical way to correct a low power factor nearer to one By adding a capacitor a power factorcorrecting capacitor in parallel with the inductive load we may improve ef ciency and save a great deal of money Power companies encourage power factor correction so that taking care to maintain a high power factor can easily result in savings in the million of dollars per year for a large plant lt s usual to correct the power factorto 09095 Adding too much capacitance is wasteful and ifwe overcorrect the power factor becomes leading and its magnitude to decrease with additional capacitance load VS 23 jmc s P jQL General Analysis lecture 5 outline 53 Example i Given 132 kV 60 Hz source nd the pfc capacitor required to correct a load of 36 MW at pf 0707 lag to pf 095 lag Compare the line current just magnitude before and after installing the pfc capacitor iii Compare the apparent power before and after installing the pfc capacitor Suppose the power company charges 9 KVAmonth in demand charges How much money do we save by installing pfc capacitors v iv v What is going on physically with power factor correction is that less energy is being swapped between the load and source Much of the stored energy is now being swapped between the inductive elements in the original load and the power factor correction capacitor lecture 5 outline A very common way to measure power is via an ammeter a voltmeter and a wattmeter as shown below 0 0 Id VS 0 oa sIjQL The scheme is this with the voltmeter measuring voltage rms the ammeter measuring current rms and the wattmeter measuring power average power in W We can quickly determine all quantities of interest for power measurement Example Suppose I 20 A V 120 V and P 2 kW Find i apparent power i39 power factor assume lagging iii complex power iv reactive power v vt and it assume 60 Hz with vt taken as reference v v lecture 5 outline 55 More sophisticated power meters like those in the power lab measure phasor current and phasor voltage These meters measure current voltage power factor power apparent power and reactive power The meter assumes current and voltage polarities as shown below that is the polarities that result in power flowing from the source to the load 80 for example ifthe load is inductive the reactive power will be positive wattmeter ammeter Would the readings be affected if the meter voltage polarity were reversed How lecture 5 outline


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