1、IEEE Std 1050-2004(Revision of IEEE Std 1050-1996)1050TMIEEE Guide for Instrumentation andControl Equipment Grounding inGenerating Stations3 Park Avenue, New York, NY10016-5997, USAIEEE Power Engineering SocietySponsored by theEnergy Development and Power Generation Committee14 September 2005Print:
2、SH95270PDF: SS95270Recognized as anAmerican National Standard (ANSI)The Institute of Electrical and Electronics Engineers, Inc.3 Park Avenue, New York, NY 10016-5997, USACopyright 2005 by the Institute of Electrical and Electronics Engineers, Inc.All rights reserved. Published 14 September 2005. Pri
3、nted in the United States of America.IEEE is a registered trademark in the U.S. Patent +1 978 750 8400. Permission to photocopy portions of any individual standard for educationalclassroom use can also be obtained through the Copyright Clearance Center.NOTEAttention is called to the possibility that
4、 implementation of this standard may require use of subjectmatter covered by patent rights. By publication of this standard, no position is taken with respect to the exist-ence or validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifyingpatents for whic
5、h a license may be required by an IEEE standard or for conducting inquiries into the legal valid-ity or scope of those patents that are brought to its attention.Copyright 2005 IEEE. All rights reserved.iiiIntroductionThe original version of IEEE Std 1050 was published in 1989 after a five year devel
6、opment cycle. Specificrecommendations for the grounding of distributed control systems (DCS) were intentionally omitted fromthe 1989 edition since at the time the document was being written (19841987) there was not a large base ofinstalled systems and user experience on which to write a guide. Exper
7、ience since 1989 has shown that DCSgrounding is essentially no different from the concepts presented in the 1989 version, and would not requirea specialized treatment in the guide.The 1996 revision consisted of three major changes to the document. The first was the incorporation ofcomments, correcti
8、ons, and clarifications that have been brought to the attention of the working group. Thesecond change was a significant rearrangement of the document for enhanced user-friendliness. Thisincluded a complete redrawing of the significant figures in Clause 5 to more clearly depict the conceptsbeing ill
9、ustrated. The third change was the reformatting of the document to conform to the latest style man-ual for IEEE standards.The revision includes major improvements in terminology consistency along with further elaboration of thevarious concepts that are introduced. Additional enhancements have been m
10、ade to Clause 5 and its figuresfor clarity, including new subclauses on power source grounding and surge protection. Clause 6 on cableshields receives another reformatting to make the topics directly relate to the various types of I but their effects can becontrolled.b)Incidental sourcesThose caused
11、 by human activity; but they are not intentional.c)Intentional sourcesThese are emissions of potentially interfering energy produced for specificpurposes unrelated to the equipment or systems under consideration. 4.1.1 Natural sourcesProbably the most severe noise source to which any control system
12、will be exposed is lightning. While mostelectronic control systems will probably fail under a direct lightning strike, even a remote power line strikecan cause interference as the lightning-induced surge travels along power lines and is dissipated throughleakage, radiation, and power loss in the dis
13、tribution system.In addition to the currents created in the power systems conductors by a direct strike, lightning can also cre-ate similarly rapidly changing and high current flows through the earth and through numerous groundedmetallic systems and items such as cable shields, equipment grounding c
14、onductors, building steel, metallicpiping systems, conduits, raceways, and metallic equipment enclosures.Single-point grounding of the above metallic items does not prevent the indicated lightning current fromflowing because of the distributed capacitance of the involved items, which completes the c
15、urrent path viastray reactive coupling. In addition, insulation of these items is not always a reliable protection for this prob-lem since the large lightning induced voltages can often arc-over through six-feet of air.A typical lightning strike is composed of a downward-stepped leader stroke, usual
16、ly negatively charged, afirst upward positive return stroke, then two or more downward leader strokes, each followed by a positivereturn stroke. On average, subsequent strokes contain about 40% of the first strokes amplitude.IEEECONTROL EQUIPMENT GROUNDING IN GENERATING STATIONS Std 1050-2004Copyrig
17、ht 2005 IEEE. All rights reserved.7A continuing current is usually present between stroke sequences. There may be as many as twenty strokesequences in a typical lightning flash. Characteristics of a typical lightning flash are as follows:Potential 30 000 000 VPeak current 34 000 AMaximum di/dt 40 00
18、0 A/sTime interval between strokes 30 msContinuing current 140 AContinuing current duration 150 msAnalysis of the continuing current component of the lightning flash striking a power line indicates that itinitially behaves as a traveling wave and subsequently as a dc source. In cases where the light
19、ing stroke ter-minates on a tower or lightning terminal, it may be analyzed through circuit analysis.More information about the magnitudes and effects of lightning surge currents on structures, electrical sys-tems, building wiring, and telecommunications system cables may be obtained by reference to
20、 IEEE Std1100-1999, IEEE Std C62.23-1995, IEEE Std C62.41-1991 (R1995), IEEE Std C62.43-1999, NFPA-780-1997, and IEC/TR 61312-1:1995.4.1.2 Incidental sourcesSince one of the largest potential sources of electrical noise in an electrical generating station is the adjacenthigh-voltage substation, some
21、 of the incidental sources mentioned in the following subclauses originate pre-dominantly in the substation environment. Experience has shown that the electrical noise generated in thepower distribution system may reach the generating station I therefore, when the ESDstrikes the external surface, it
22、s wavefront also travels through the thickness of the door or panel and is re-radiated from the inside surface into the enclosures volume containing the ESD susceptible circuits.IEEECONTROL EQUIPMENT GROUNDING IN GENERATING STATIONS Std 1050-2004Copyright 2005 IEEE. All rights reserved. 15An example
23、 of ESD would be a 5000 V, 5 A current pulse of 200 ns duration. While the energy contained inthis pulse is only about 1.25 mJ, this is sufficient to interfere with computer logic levels. An arc dischargedoes not have to occur for an electrostatic field to interfere with a control circuit. Any objec
24、t that has pickedup a large electrostatic charge can create a voltage shift of several volts when brought in close proximity to acontrol circuit or cable. 4.1.2.16 Cable resonanceAvoiding unwanted resonances in signal and grounding cables from environmental EMI occurring at radiofrequencies has beco
25、me increasingly important. Without proper preventive design measures being taken,signal and grounding cables may become resonant to some frequency of radiated (far-field), coupled (nearfield), or conducted (galvanic) EMI, and thereby subject the circuits connected to them to unintentionallyhigh curr
26、ents and/or voltages.Resonance is related to the LC ratio of the involved conductor and its associated electrical length expressedin terms of wavelength. In general, it is recommended that no conductor be allowed to have an electricallength that exceeds approximately 1/20 at the highest frequency of
27、 the EMI environment into which it isintended to be operated. This minimizes the effects of EMI on the conductor since it cannot becomeresonant.The worst conditions of resonance occur at the first quarter-wave point and succeeding odd-multiplesthereof (0.25 , 0.75 , 1.25 , .). At these points of res
28、onance, the voltage will be maximum at one end ofthe conductor (with a current minimum), and the current will be maximum at the opposite end of the con-ductor (with a voltage minimum). As a result, the electrical components and insulation systems are stressedat one end of the path where the voltage
29、is high, and at the opposite end where the peak or rms current carry-ing ability of the components or conductors is stressed because the current is high. With EMI currentsextending into half, full, or multi-cycle durations, the true-rms value of the current is what is of concern, ascompared to trans
30、ients such as lightning and faults where the concern is for the paths current carrying abil-ity and is expressed in terms of I2t.Resonances of the first half-wave point and succeeding even-multiples thereof (0.5, 1.0, 1.5, .) produceessentially identical EMI current and voltage distribution conditio
31、ns at each end of the affected conductor.The voltage at one end is essentially the same as that at the other end, and the same holds true for the current.Figure 2Electrostatic discharge noise generationIEEEStd1050-2004 IEEE GUIDE FOR INSTRUMENTATION AND 16 Copyright 2005 IEEE. All rights reserved.Co
32、nductors installed in free-space will have self-resonant points that will normally be somewhat higher infrequency than those installed in close proximity to the earth, or in particular ferrous metal items. This is theresult of mutual coupling that exists between the conductor and the earth or ferrou
33、s items, and the result isgenerally that the self-resonant frequency of the conductor is lowered. In addition, depending upon theamount of stray coupling involved, the velocity factor of the path is also generally reduced to values that areless then that of a conductor in free space.The full-wave, s
34、elf-resonant frequency of a conductor in free space may be estimated by Equation (2): f = c/l (2)Where c is the speed of propagation in free space, approximately 300 meters per microsecond. Measuringtime in microseconds yields a result for f in megahertz.Equation (2) may be used to approximate the s
35、elf-resonant conditions of a cable or grounding conductorspath. If the result is divided by 20, the 1/20 point may be estimated and used as a recommended limit. Sim-ilarly, dividing the result by 4.0 or by 2.0 respectively gives the quarter-wave point and half-wave pointestimates. Rearranging the eq
36、uation allows the estimated length of the conductor to be determined in view ofa given amount of EMI frequency.4.1.2.17 Reflections and traveling wavesExcessively high currents and voltages on EMI affected cables, or grounding conductors may also occurfrom traveling waves on the path, which encounte
37、r a severe impedance mismatch such as an open orshorted-end. In this type of situation, the traveling wave is partially or fully reflected by the impedance mis-match and the reflected portion is instantaneously added to the original wave at the point of reflection. As aresult, the current or voltage
38、 at such a point may easily be doubled.In the case of an open-end termination such as at the end of an overhead radial distribution feeder, theimpedance is very high (open circuit), so the reflection occurs on the voltage waveform and not the currentwaveform. There is no current flow in the open cir
39、cuit, but a very high potential may be created. In the caseof a short-circuit termination such as where a surge-arrester is applied on the end of an overhead radial dis-tribution feeder, the impedance is very low (it approaches a short-circuit condition in respect to the travelingwave), so the refle
40、ction occurs on the current waveform and not on the voltage waveform. There is littlevoltage developed across a “short circuit.”EMI conditions at or near the shorted or open-end termination for a traveling wave can be very severe. Forexample, near-field conditions are worst for H-fields nearest the
41、shorted-termination (highest current, lowestvoltage) while E-field conditions are similarly serious nearest the open-termination (highest voltage, lowestcurrent). Radiation of far-field EMI can occur all along the conductors path once it is subjected to EMI,which forms a traveling wave on it. Hence,
42、 unwanted EMI effects are unavoidable under these kinds of con-ditions if there are any victim power, signal, grounding, or other conductors located near the conductor car-rying the traveling wave.4.1.2.17.1 Velocity factorTraveling waves move through a conductive medium (such as a wire) at a veloci
43、ty that may be considerablyless than that for the radiated wave in free space or air. The free space velocity factor of 1.0x is approxi-mately 299 m/s for a radiated wave. Velocity factors less than 1.0x always occur when a wave travelsthrough a physical medium such as a wire, and this affects calcu
44、lations regarding how long a conductor maybe in relation to conditions of actual self-resonance vs. equivalent free space electrical length.For example, the leading edge (first transition) of a radiated wave will travel 30 m in free space during onecycle of a 10 MHz clock signal in a microprocessor.
45、 However, within an insulated conductor in a cable, itIEEECONTROL EQUIPMENT GROUNDING IN GENERATING STATIONS Std 1050-2004Copyright 2005 IEEE. All rights reserved. 17may travel only 21 m due to a reduced velocity factor, which, in this case, would be 0.7x (21m/30m = 0.7).If the voltage wave reflects
46、 from the cable termination where the cable has been terminated “open” or atleast in a very high impedance in comparison to the signal cables characteristic impedance, and is in phasewith a new wave, resonance will occur and line oscillations will be greatly magnified. Also, if one end of thecircuit
47、 is grounded, the first resonance at 10 MHz occurs when the conductor is only 5.25 m or 1/4 wave-length long.At this frequency, the 5.25 m long cable appears to be virtually an open circuit between ends or at least avery high impedance. It is incapable of equalizing the voltages appearing between it
48、s ends. A cable orgrounding conductor, longer than 1/20 cannot be counted upon to adequately equalize voltages between itsends. This amounts to only 1.5 m of length at 10 MHz, so it should become apparent that the use of longgrounding/bonding conductors in a facility that is a part of a “single-poin
49、t” or similar grounding system willnot be effective for high-frequency EMI.At high frequencies, signal transmission lines are often terminated in their characteristic surge impedance toeliminate most of the reflection and resonance. However, no single-grounded conductor within a cable canprovide a virtual short circuit between one end and the other over a very useful portion of a broad frequencyrange, and not at all once 1/4 conditions and odd-multiples thereof, are approached.4.1.2.18 Power circuit inrush currentSmall conductors on higher impedance circ
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1