POWER SYSTEMS EE 499
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This 9 page Class Notes was uploaded by Amara Morar on Monday October 12, 2015. The Class Notes belongs to EE 499 at Idaho State University taught by Staff in Fall. Since its upload, it has received 22 views. For similar materials see /class/222195/ee-499-idaho-state-university in Electrical Engineering at Idaho State University.
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Date Created: 10/12/15
Harmonic voltages The effect of harmonic voltages and currents is basically the same as with normal 60 Hz voltages and currents They all obey Ohm s law which is the following E X Z voltage current X impedance As customer load current ows through the impedance resistance of the circuit a voltage drop is developed Linear nonharmonic currents cause a linear or non distorted voltage drop The result to the customer is a voltage that is still linear However when the load current is nonlinear contains harmonics a nonlinear or distorted voltage results The result is that other customers may then experience a distorted voltage at their service location Nonlinear or distorted voltage can cause electrical equipment to missoperate Electrical equipment is designed to operate on a normal 60 Hz nondistorted voltage The impact ofthe harmonic voltage depends on the magnitude ofthe distortion and the sensitivity of the equipment Examples oftypical concerns are that sensitive electronics may not properly function and motors can overheat Utility and customer wiring and equipment may also need to be oversized The magnitude ofthe voltage distortion depends on the magnitude of the customer load current and the impedance resistance of the circuit The impedance is different at different locations on the electric circuit 80 equipment will be impacted to a greater degree at different locations on the electric system IEEE 519 is a standard for limiting harmonics The basic goal is forthe customer to limit the harmonic current and for the utility to limit the harmonic voltage Back to Ohm s law E x Z The customer strives to limit the harmonic current and the utility strives to design a system which will limit the harmonic voltage Working together the customer and utility can both operate effectively The following article gives a more detailed expanafion of harmonics The How39s and Why39s of Harmonic Distortion Jun 1 2006 1200 PM By John DeDad Senior Director Editorial and ECampM Development Harmonics problems counter many of the conventional rules of power system design and operation At one time almost all electrical loads were linear those that weren39t made up such a small portion of the total that they had little effect on electrical system operation That all changed however with the coming ofthe solidstate electronic revolution Today we have an environment rich in nonlinear loads such as UPS equipment computers variablespeed drives and electronic fluorescent lighting ballasts Operation of these devices represents a doubleedged sword Although they provide greater efficiency they can also cause serious consequences to power distribution systems in the form of harmonic distortion Let39s take a closer look at linear and nonlinear loads to get a better understanding of the how39s and why39s of this distortion R Linear loads and current waveforms Pure resistance inductance and capacitance are all linear What that means is if we place a sine wave voltage of a certain magnitude across a circuit containing pure resistance the current in the circuit follows Ohm39s Law E R So for a specific value of ohms the relationship of volts and amperes is a straight line Figure 1 Current For example let39s say we apply 100V across a 10 ohm resistor Per Ohm39s Law the current would then be 10A If we double the voltage to 200V the resulting current is 20A For 400V the current would be 40A etc WEE Fig 1 With a linear load the relationship between voltage and current is linear and proportional The 45 diagonal line represents a fixed resistance lnductances Ohm39s Law as it applies to these types of loads is expressed as E XL where XL is the inductive reactance which is equal to 21TfL which is equal to ohms Here current is directly proportional to the voltage drop and inversely proportional to both the selfinductance and the frequency f If the frequency is constant say 60 Hz the relationship of voltage and current is a straight line as we39ve seen with resistance Note that this relationship involves magnitudes only It does notgive the phase relationship between voltage E and current I Here voltage leads current by 90 Also the ohmic value of XL increases in direct proportion to frequency So if frequency is doubled from say 60 Hz to 120 Hz XL will also double etc thus maintaining a linear relationship Capacitances A similar situation holds for these types of loads Here the Ohm39s Law equivalent is expressed as E Xc where Xc represents capacitive reactance which is equal to 1 21TfC which is equal to ohms As with inductance if the frequency is constant the voltage and current relationship is a straight line If however the frequency is doubled from 60 Hz to 120 Hz the Xc ohms will be half of what it was at 60 Hz Nevertheless the relationship is still a straight line Nonlinear loads and current waveforms Solid E state electronics is based on the use of semiconductors These materials are totally different in that their response to voltage is not a straight line In general the relationship of voltage to current is represented by a curve as shown in Figure 2 Even this is misleading because each solidstate device will have a unique response curve that is different from that of other types of semiconductorbased devices Current What this means is that with a nonlinear load you cannot easily predict the relationship between voltage and current unless you have an exact curve for each device With equipment containing 0 many solidstate devices such an approach is impossible Voltage Fig 2 With a nonlinear load the line is curved not straight as with a linear load The amount of curvature is unique to each type of nonlinear semiconductor or device The only logical way is to use test instruments to plot the individual voltagecurrent relationships The test results are often baffling With an incoming source having a near perfect 60 Hz sine wave the current will be significantly distorted However mathematical Fourier analysis ofthese distorted waves shows that they are made up of the fundamental sine wave plus one or more harmonic current waves having a frequency that is a whole integer multiple ofthe fundamental frequency For example a 60 Hz fundamental combined with 180 Hz and 300 Hz waves will result in a specific type of distorted wave Any wave shape can be reproduced exactly by adding together a series of sine waves of particular frequency amplitude and timing although it may require an infinite number of them Therefore nonsinusoidal waveforms consist of and can be broken down into some finite number of pure sine waves The chart click here to see chart shows how harmonic current waveforms combine with the fundamental to form distorted waveforms Voltage waveform distortion Ohm39s Law also helps explain another phenomenon distorted voltage waveforms caused by distorted current waveforms Each harmonic current in a facility39s electrical distribution system will cause a voltage at the same harmonic to exist when the harmonic current flows into an impedance This results in volta e harmonics appearin at the load bus For example a 5 h harmonic current will produce a 5 harmonic voltage a 7 harmonic current will produce a 7 h harmonic voltage etc The equation takes the form of E l X Z where Z is the impedance of an electrical load in the circuit which in the case of motors transformers and similar devices is mostly inductive Because a distorted current waveform is made up ofthe fundamental plus one or more harmonics currents each of these currents flowing through the impedance will cause a voltage drop across the impedance The magnitude of the current and voltage waveform distortion will depend upon the relative size of the nonlinear loads with respect to that system the type of equipment and the source impedance The amount of voltage distortion increases as the percentage of nonlinear loads increases If you assume the load bus distortion stays within reasonable limits say less than 5 the amount of harmonic current generated by the load is generally constant Let39s talk about system impedance According to the book Electrical Power Systems Quality Dugan McGranaghan Santoso and Beaty ISBN 007138622 X while the nonlinear load39s harmonic currents cause the voltage distortion the load itself has no control over the amount of distortion If we put the same load in two different facilities with two different system impedances we would have two different distortion values lEEE 5191992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems recognizes this by basically saying The control over the amount of harmonic current injected into the system takes place at the enduse application Assuming the harmonic current injection is within reasonable limits the control over the voltage distortion is exercised by the entity having control over the system impedance which is often the electric utility While electric utilities everywhere generate very good sine wave voltages we must remember that the distortion increases closer to the load and that some loads chop the current into seemingly arbitrary waveforms When passed through a system impedance this current can actually cause voltage distortion Sidebar All Loads are Not Created Equal Motors incandescent lighting and heating loads are linear in nature That is the load impedance is essentially constant regardless of the applied voltage As seen in Fig A the current in AC circuits increases proportionately as the voltage increases and decreases proportionately as the voltage decreases The current in these circuits is in phase with the voltage for a resistive circuit with a power factor PF of unity lt lagsthe voltage by some phase angle for the more typical partialy inductive circuit with a PF commonly between 080 and 095 And it leads the voltage by some phase angle in a capacitive circuit Nevertheless this current is always proportional to the voltage For a sinusoidal voltage the current is also sinusoidal Voltage E lnphase current lle Leading current Ic Lagging current IL Fig A shown are plots of different linear load currents ID is a pure resistive circuit current IL is a partially inductive lagging circuit current IC is a partially capacitive leading circuit current which is uncommon Nonlinear loads on the other hand are loads in which the load current is not proportional to the instantaneous voltage This is because the load current is often not continuous as shown in Fig B below This is the result of the nonlinear load being switched on for only part of the cycle as in a thyristorcontrolled circuit or pulsed as in a controlledrectifier circuit Sinusoidal voltage Fig B Shown is a nonlinear load current which is drawn only at the peak ofthe voltage waveform This is the result of the nonlinear load being switched on for only part ofthe cycle as in a thyristorcontrolled or pulsed piece of equipment which incorporates a controlledrectifier circuit Curre nit pulses Sidebar Speak the Same Language Depending on what industry you39re in the word harmonics can mean two different things to two different people In their book Electrical Power Systems Quality ISBN 007138622 X authors Dugan McGranaghan Santoso and Beaty point out that by popular convention in the power industry the majority of times the term harmonics is used by itselfto refer to the load apparatus therefore the speaker is referring to harmonic currents On the electric utility side however many use the term to mean harmonic voltages So when talking about harmonics with manufacturers utility people technicians etc make sure everyone is on the same page to avoid any confusion This document describes various technical specifications pertaining to voltage commonly provided by the electric utility to customers I t is a compilation of brief excerpts from a number of documents that have been prepared over the years for power quality reference Three areas are addressed that frequently prompt customer questions They are 1 voltage level 2 voltage reliability and 3 voltage waveshape quality 1 Voltage levels A frequent concern expressed by customers electricians and equipment repair personnel is the level of voltage supplied by the electric utility Many times the voltage level is viewed as the culprit when equipment malfunctions orfails to properly operate Life expectancy of electric equipment can vary to some degree in proportion to the applied voltage Efficiency of equipment can also be affected For these and other reasons it is important to understand what constitutes a normal voltage range and how different equipment might respond as the applied steady state voltage varies Idaho Power Company strives to maintain the steady state voltage at its point of delivery within a maximum deviation of or 5 from the nominal supply voltage This is consistent with typical utility standards based on ANSI 08411989 and IEEE Std 1411993 Chapter 3 Section 32 Italics bold and underlining added in following quote for emphasis 322 System Voltage tolerance limits ANSI 08411989 specifies the preferred nominal voltages and operating voltage ranges for utilization and distribution equipment operating from 12034 500 V in the United States It sgeci es voltages for two critical goints on the distribution szstem the goint of delive b1 the sugglzing utilitz and the point of connection to utilization eguigment The actual voltage measured at any point on the system will vary depending on the location of the point of measurement and the system load at the time the measurement is made Fixed voltage changes take place in transformers in accordance with the transformer ratio Voltage variations occur from the operation of voltage control equipment changes in voltage drop due to changes in load current and other reasons It should be reco nized that because of conditions beyond the control of the supplier or userl or bothl there will be infreguent and limited geriods when sustained voltages outside range B limits will occur Table 32 Standard voltage profile for lowvoltage regulated power distribution system 120 V base For utilization voltage of 120 600 V For building wiring circuits supplying lighting equipment The tolerance limits for the service voltage provide guidance to the supplying utility for the design and operation of its distribution system The service voltage is the voltage at the point where the utl y conductors connect to the user conductors It is generally measured at the service switch for services of 600 V and below and at the billing meter voltage potential transformers of services over 600 V The tolerance limits for the voltage at the point of connection of utilization equipment provide guidance to the user for the design and operation ofthe user distribution system and to utilization equipment manufacturers for the design of utilization equipment Electric supply systems are to be designed and operated so that most service voltages fall within the range A limits User systems are to be designed and operated so that when the service voltages are within range A the utilization voltages are within range A Utilization eguigment is to be designed and rated to give fully satisfactom gerformance within the range A limits for utilization voltages Range B allows limited excursions of voltage outside the range A limits that necessarily result from gractical design and ogerating conditions When voltages are outside range A and inside range B the corrective action should be taken within a reasonable time to restore service voltages to range A limits Insofar as gracticablex utilization eguigment mav be exgected to give accegtable gerformance at voltage outside range A but within range B When voltages occur outside the limits or range B prompt corrective action should be taken Resgonsibilitv for corrective action degends ugon where the voltage is out of range A comgared to the limits sgeci ed for each location identi ed in ANSI C8411989 There are many types of electric equipment supplied from today s electric utility system Equipment ranges from large robust motors to sensitive electronic devices Customers can also purchase many different grades of quality in their utilization equipment Customer equipment can be connected at any point on the utility distribution system and in various types of buildings and customer wiring systems Obviously it is impossible for the utility to tailor the electric supply to meet each individual customers perceived needs Therefore utilities strive to maintain voltage levels as per the previously mentioned standards to provide adequate service to all classes of customers This points out the need for customers to be aware of specific individual requirements and to design their electric supply system accordingly 2 Voltage Reliability Idaho Power strives to deliver a steady state 60 Hz voltage to customers However electric service is inherently subject to occasional interruption suspension curtailment and fluctuation Events that cause voltage disturbances andor interruptions include wind blowing lines together lightning strikes tree limbs and animal contact customer large load switching and electric utility switching operations Since most of these events are either beyond the control ofthe utility or necessary for safe reliable power delivery no guarantee of uninterruptible service is made Also customers should be aware that many ofthese events can cause momentary transient voltage changes The frequency of occurrence and the magnitude of a voltage disturbance are dependent on the cause of the disturbance Some events may be planned and predictable such as line switching Other events such as line faults are more probable during certain adverse weather conditions or animal activity Therefore Idaho Power strongly advises customers to protect their equipment particularly that which is sensitive andor critical The majority of problems that cause short circuits or faults on the power lines are temporary in nature A tree limb blowing or animals getting in contact with the wire are examples Power companies use automatic circuit breakers that sense these faults and open the circuit for a few seconds in the hope that the problem will clear itself lfthe problem persists the circuit breaker will remain open after several attempts and a company workman will be sent to clear the problem and manually close the circuit breaker to restore power If a customer requires uninterruptible power a UPS or standby generator should be considered Tr rt te tvottage cnange swttcntng and more neg swttcntng Evert a rentg or on can cause a subst used by uttttty st s can be ca ert ty due to cu omer toad rator or smatt motor swttcntng on rttt t s nt Forthts u t e a a vottage tran trtng be tn an adequate round patn tne TVSS cannot property functton Atso tnere are v r g guattttes ofTVS or devtces tnat catt tnemsetves Tyss sotd on tne martltet Tne T s snoutd meet rope t spectncattons to adeguatety perform tne destred 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constant tmpedance drawtng current tn proportton to tne stnusotdat vot g tetectront devtces cnange tnetr tmpedance y swttcntng on and on near tne peak of tne vottage Waveform swttcntng toads on and on durtng art ofthewav o m sutts tn s om abruptt nonstnusotdat current putses nese cur ent putses tntroduce narmontc currents tnto tne power dtstrtbutton system Tnese narmontc curr nts o erate at neguenctes otner tnan tne fundamentat 60 Hz Currvnl put Tne magnttude ortne vottage dtstortton depends on tne maonttude m tne customer toad current and tne tmpedance reststance of the ctrcu The tmpedance ts dtfferertt at dtnerent tocattons on tne etectrtc ctrcutt So edutoment wttt be tmpacted to greater degree at dtrrerent tocattons on etectrtc system t tEEE sta ts a standard ror ttmtttng narmontcs The best goat ts rortne customerto ttmtt tne narmontc current and ne p r and tne uttttty strtves to destgn a Wstem wntcn wttt ttmtttne narmontc vottage a m z 5 i a 3 lt 9 m a c c 2 0 was dummy muma mm harnuulu um Harmonics affect us all from the secretary operating a computer the electrician trouble shooting equipment failure the electrical contractor having to absorb the cost of equipment replacement the inspector who must investigate the cause of electric fires to the facilities management interested in effective and efficient equipment operation and the avoidance of down time The scope of harmonics impacts architects engineers electricians building maintenance personnel equipment manufactures and private industry The subject of harmonics its causes and methods to mitigate concerns can be a complicated subject If harmonics are an issue the assistance of a qualified engineering consultant is recommended Idaho Power can also provide assistance and additional reference information
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