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1、IEEE Std 1050-2004 (Revision of IEEE Std 1050-1996) 1050 TM IEEE Guide for Instrumentation and Control Equipment Grounding in Generating Stations 3 Park Avenue, New York, NY10016-5997, USA IEEE Power Engineering Society Sponsored by the Energy Development and Power Generation Committee 14 September
2、2005 Print: SH95270 PDF: SS95270 Recognized as an American National Standard (ANSI) The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright 2005 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 14
3、September 2005. Printed 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 educational classroom use can also be obtained through the Copyright Clearance Center. NOTEAttention is called t
4、o the possibility that implementation of this standard may require use of subject matter 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 ide
5、ntifying patents for which 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. iii Introduction The original version of IEEE Std 1050 was publi
6、shed in 1989 after a five year development cycle. Specific recommendations for the grounding of distributed control systems (DCS) were intentionally omitted from the 1989 edition since at the time the document was being written (19841987) there was not a large base of installed systems and user expe
7、rience on which to write a guide. Experience since 1989 has shown that DCS grounding is essentially no different from the concepts presented in the 1989 version, and would not require a specialized treatment in the guide. The 1996 revision consisted of three major changes to the document. The first
8、was the incorporation of comments, corrections, and clarifications that have been brought to the attention of the working group. The second change was a significant rearrangement of the document for enhanced user-friendliness. This included a complete redrawing of the significant figures in Clause 5
9、 to more clearly depict the concepts being illustrated. 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 the various concepts that ar
10、e introduced. Additional enhancements have been made to Clause 5 and its figures for clarity, including new subclauses on power source grounding and surge protection. Clause 6 on cable shields receives another reformatting to make the topics directly relate to the various types of I but their effect
11、s can be controlled. b) Incidental sources Those caused by human activity; but they are not intentional. c) Intentional sources These are emissions of potentially interfering energy produced for specific purposes unrelated to the equipment or systems under consideration. 4.1.1 Natural sources Probab
12、ly the most severe noise source to which any control system will be exposed is lightning. While most electronic control systems will probably fail under a direct lightning strike, even a remote power line strike can cause interference as the lightning-induced surge travels along power lines and is d
13、issipated through leakage, radiation, and power loss in the distribution system. In addition to the currents created in the power system s conductors by a direct strike, lightning can also cre- ate similarly rapidly changing and high current flows through the earth and through numerous grounded meta
14、llic systems and items such as cable shields, equipment grounding conductors, building steel, metallic piping systems, conduits, raceways, and metallic equipment enclosures. Single-point grounding of the above metallic items does not prevent the indicated lightning current from flowing because of th
15、e distributed capacitance of the involved items, which completes the current path via stray 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 typica
16、l lightning strike is composed of a downward-stepped leader stroke, usually negatively charged, a first upward positive return stroke, then two or more downward leader strokes, each followed by a positive return stroke. On average, subsequent strokes contain about 40% of the first stroke s amplitude
17、. IEEE CONTROL EQUIPMENT GROUNDING IN GENERATING STATIONSStd 1050-2004 Copyright 2005 IEEE. All rights reserved. 7 A continuing current is usually present between stroke sequences. There may be as many as twenty stroke sequences in a typical lightning flash. Characteristics of a typical lightning fl
18、ash are as follows: Potential30 000 000 V Peak current 34 000 A Maximum di/dt40 000 A/ s Time interval between strokes30 ms Continuing current140 A Continuing current duration150 ms Analysis of the continuing current component of the lightning flash striking a power line indicates that it initially
19、behaves as a traveling wave and subsequently as a dc source. In cases where the lighting 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- tem
20、s, building wiring, and telecommunications system cables may be obtained by reference to IEEE Std 1100-1999, 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 sources Since one of the largest potential sources of elect
21、rical noise in an electrical generating station is the adjacent high-voltage substation, some 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 the power distribution sy
22、stem may reach the generating station I therefore, when the ESD strikes the external surface, its wavefront also travels through the thickness of the door or panel and is re- radiated from the inside surface into the enclosure s volume containing the ESD susceptible circuits. IEEE CONTROL EQUIPMENT
23、GROUNDING IN GENERATING STATIONSStd 1050-2004 Copyright 2005 IEEE. All rights reserved.15 An example of ESD would be a 5000 V, 5 A current pulse of 200 ns duration. While the energy contained in this pulse is only about 1.25 mJ, this is sufficient to interfere with computer logic levels. An arc disc
24、harge does not have to occur for an electrostatic field to interfere with a control circuit. Any object that has picked up a large electrostatic charge can create a voltage shift of several volts when brought in close proximity to a control circuit or cable. 4.1.2.16 Cable resonance Avoiding unwante
25、d resonances in signal and grounding cables from environmental EMI occurring at radio frequencies has become increasingly important. Without proper preventive design measures being taken, signal and grounding cables may become resonant to some frequency of radiated (far-field), coupled (near field),
26、 or conducted (galvanic) EMI, and thereby subject the circuits connected to them to unintentionally high currents and/or voltages. Resonance is related to the LC ratio of the involved conductor and its associated electrical length expressed in terms of wavelength. In general, it is recommended that
27、no conductor be allowed to have an electrical length that exceeds approximately 1/20 at the highest frequency of the EMI environment into which it is intended to be operated. This minimizes the effects of EMI on the conductor since it cannot become resonant. The worst conditions of resonance occur a
28、t the first quarter-wave point and succeeding odd-multiples thereof (0.25 , 0.75 , 1.25 , .). At these points of resonance, the voltage will be maximum at one end of the conductor (with a current minimum), and the current will be maximum at the opposite end of the con- ductor (with a voltage minimum
29、). As a result, the electrical components and insulation systems are stressed at one end of the path where the voltage 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 currents extendin
30、g into half, full, or multi-cycle durations, the true-rms value of the current is what is of concern, as compared to transients such as lightning and faults where the concern is for the path s current carrying abil- ity and is expressed in terms of I2t. Resonances of the first half-wave point and su
31、cceeding even-multiples thereof (0.5, 1.0, 1.5, .) produce essentially identical EMI current and voltage distribution conditions 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 2Electrosta
32、tic discharge noise generation -,-,- IEEE Std1050-2004IEEE GUIDE FOR INSTRUMENTATION AND 16Copyright 2005 IEEE. All rights reserved. Conductors installed in free-space will have self-resonant points that will normally be somewhat higher in frequency than those installed in close proximity to the ear
33、th, or in particular ferrous metal items. This is the result of mutual coupling that exists between the conductor and the earth or ferrous items, and the result is generally that the self-resonant frequency of the conductor is lowered. In addition, depending upon the amount of stray coupling involve
34、d, the velocity factor of the path is also generally reduced to values that are less then that of a conductor in free space. The full-wave, self-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, approximate
35、ly 300 meters per microsecond. Measuring time in microseconds yields a result for f in megahertz. Equation (2) may be used to approximate the self-resonant conditions of a cable or grounding conductor s path. If the result is divided by 20, the 1/20 point may be estimated and used as a recommended l
36、imit. Sim- ilarly, dividing the result by 4.0 or by 2.0 respectively gives the quarter-wave point and half-wave point estimates. Rearranging the equation allows the estimated length of the conductor to be determined in view of a given amount of EMI frequency. 4.1.2.17 Reflections and traveling waves
37、 Excessively high currents and voltages on EMI affected cables, or grounding conductors may also occur from traveling waves on the path, which encounter a severe impedance mismatch such as an open or shorted-end. In this type of situation, the traveling wave is partially or fully reflected by the im
38、pedance mis- match and the reflected portion is instantaneously added to the original wave at the point of reflection. As a result, the current or voltage 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, the i
39、mpedance is very high (open circuit), so the reflection occurs on the voltage waveform and not the current waveform. There is no current flow in the open circuit, but a very high potential may be created. In the case of a short-circuit termination such as where a surge-arrester is applied on the end
40、 of an overhead radial dis- tribution feeder, the impedance is very low (it approaches a short-circuit condition in respect to the traveling wave), so the reflection occurs on the current waveform and not on the voltage waveform. There is little voltage developed across a “ short circuit.” EMI condi
41、tions at or near the shorted or open-end termination for a traveling wave can be very severe. For example, near-field conditions are worst for H-fields nearest the shorted-termination (highest current, lowest voltage) while E-field conditions are similarly serious nearest the open-termination (highe
42、st voltage, lowest current). Radiation of far-field EMI can occur all along the conductor s path once it is subjected to EMI, which forms a traveling wave on it. Hence, unwanted EMI effects are unavoidable under these kinds of con- ditions if there are any victim power, signal, grounding, or other c
43、onductors located near the conductor car- rying the traveling wave. 4.1.2.17.1 Velocity factor Traveling waves move through a conductive medium (such as a wire) at a velocity that may be considerably less than that for the radiated wave in free space or air. The free space velocity factor of 1.0x is
44、 approxi- mately 299 m/s for a radiated wave. Velocity factors less than 1.0x always occur when a wave travels through a physical medium such as a wire, and this affects calculations regarding how long a conductor may be in relation to conditions of actual self-resonance vs. equivalent free space el
45、ectrical length. For example, the leading edge (first transition) of a radiated wave will travel 30 m in free space during one cycle of a 10 MHz clock signal in a microprocessor. However, within an insulated conductor in a cable, it -,-,- IEEE CONTROL EQUIPMENT GROUNDING IN GENERATING STATIONSStd 10
46、50-2004 Copyright 2005 IEEE. All rights reserved.17 may 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 from the cable termination where the cable has been terminated “ open” or at least in a very high impedance in c
47、omparison to the signal cable s characteristic impedance, and is in phase with a new wave, resonance will occur and line oscillations will be greatly magnified. Also, if one end of the circuit is grounded, the first resonance at 10 MHz occurs when the conductor is only 5.25 m or 1/4 wave- length lon
48、g. At this frequency, the 5.25 m long cable appears to be virtually an open circuit between ends or at least a very high impedance. It is incapable of equalizing the voltages appearing between its ends. A cable or grounding conductor, longer than 1/20 cannot be counted upon to adequately equalize vo
49、ltages between its ends. This amounts to only 1.5 m of length at 10 MHz, so it should become apparent that the use of long grounding/bonding conductors in a facility that is a part of a “ single-point” or similar grounding system will not be effective for high-frequency EMI. At high frequencies, signal transmission lines are often terminated in their characteristic surge impedance to eliminate most of the reflection and resona