Line Conducted Disturbances — Origins and Control
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LINE CONDUCTED DISTURBANCES — ORIGINS AND CONTROL François D. Martzloff Corporate Research and Development General Electric Company Schenectady, New York 12345 Reprint of handout for the Forum “Power and Grounding for a Computer Facility” National Bureau of Standards, May 1984 Significance Part 6: Tutorials, Textbooks, and Reviews This invited lecture was presented at a forum organized by the National Bureau of Standards (NBS) under the auspices of the same organization that developed the publication FIPS PUB 94 “Guideline on Electrical Power for ADP Installations, ” a widely disseminated and often-cited tutorial document in the eighties and nineties. My subjects were presented within the broad scope of the forum, with emphasis naturally given to surge protection. Delivering this lecture at NBS marked a turning point in my career because it catalyzed the establishment of a program aimed at surge protection and EMI control which I joined in 1985, allowing me to continue my work in this area within the supporting environment of NBS (soon to become NIST) until my retirement in 2003. Such support for 18 years of original research and dissemination of results via contributions to IEEE, IEC, and UIE standards is gratefully acknowledged. Power and Grounding for a Computer Facility National Bureau of Standards May 1984 LINE CONDUCTED DISTURBANCES - ORIGINS AND CONTROL Fraqois D. Martzloff Corporate Research and Development General Electric Company Schenectady, New York 12345 THE ORIGINS OF TRANSIENT OVERVOLTAGES Transient overvoitages in power systems originate from one Abnormal Switching Transients cause. energy being injected into the power system, but from two Several mechanisms are encountered in practical power clrcuits. sources: lightning discharges, or switching within the power sys- These mechan~sms can produce translent overvoltages far in tem. However, in communication and data systems, there is another excess of the theoretical twice-normal limit mentioned above. Two source of transients: the coupling of power system transients into such mechan~srnsoccur frequently: current chopping and restrikes. the data system. the latter being espec~allytroublesome when capacitor switchmg is Lightning discharges may not necessarily mean direct termina- involved. tion of a lightning stroke onto the power system. A lightning stroke Current chopp~ngis the name given to the rapid current reduc- term~nattngnear a power line, e~therby hitting a tree or the bare tion which fuses or which c~rcultbreakers can force when clear~ng earth, will create very fast-changmg magnet~c field that can a a circuit prlor to the natural current zero of the power system. induce voltages and mject energy into the loop formed by the - - When there is inductance In the c~rcuit.this rapid current change conductors of the power system. Lightning can also inject can produce high overvoltages - some 10 times the normal circuit overvoltages in a power system by raising the ground potential on voltage. A classical example is an unloaded transformer where the the surface of the earth where the stroke term~nates,while more magnetizing inductance is high and the energy stored in the mag- distant "ground" points remain at a lower voltage. closer to the netic core can charge only the winding capacitance. potential of "true earth." The literature and other presentations in this conference provide information on the characteristics of light- Capacitor switching can be troublesome if the switch restrikes ning discha~ges.('-~) after current interruption. Restrike occurs followmg the initial inter- ruption: the capacitor voltage remains nearly constant at maximum Surges from power system switching create overvoltages as a system voltage. since the interruption occurred at zero current. result of either trapped energy in loads being switched off. or res- which is 90. apart from the voltage zero. wh~lethe system voltage trikes in the switchgear. These will be examined in greater detail in follows the normal sine wave (Figure 1). At 180' after mterruptlon. the following paragraphs. the sw~tchhas to support twice the amount of system voltage, a SWITCHING TRANSIENTS stress it mrght not be able to withstand with its contacts not com- pletely separated. A restr~kecan occur under these circumstances. A transient is created whenever a sudden change occurs in a In such a case, the capacitance of the clrcuit will tend to drive the power c~rcuit.especially during power switching either the closing - voltage not toward the system voltage but beyond 11 theoretically or openlng of a clrcuit. It is important to recognize the difference - up to twice the difference. Such an overshoot means a poss~ble between the intended switching (the mechanical action of the switch) and the actual happenmg in the circuit. Dur~ngthe closing sequence of a switch. the contacts may bounce, producing open- VOLTAGE ACROSS ings of the clrcuit with reclosing by restrikes and reopening by CONTACT CAPACITOR CONTACTS clearmg at the high-frequency current zero. In medlum-voltage CLEARS VOLTAGE 1 sw~tchgear, pre-strlkes can occur just before the contacts close. with a succession of clearings at the high-frequency current zero. t t followed by restrike. Similarly, during an opening sequence of a switch, restrikes can cause electrical closing(s) of the circuit Simple Switching Transients Simple sw~tching transient^'^) include circuit closing transients. transients initiated by clearing a short circuit, and transients pro- duced when the two clrcuits on either side of the- switch being opened oscillate at different frequencies. In circuits having induc- tance and capacitance (all phys~calcircuits have at least some in VOLTAGE the form of stray capacitance and inductance) with little damping. these smple switching transients are inherently limited to twice the peak amplitude of the steady-state sinusoidal voltage. Another limit to remember when analyzing transients associated with current interruption (circu~topening) is that the circuit inductance tends to maintam the current constant. At most, then, a surge protective device provided to divert the current will be exposed to that initial current. Without a surge protective device the current is available CAPACITOR to charge the circuit capacitances at whatever voltage IS requlred VOLTAGE 13E to store the inductive energy from the current by converting it into capacitive energy. Figure 1. Restrike Mechanism on Capacitor Switching Origins voltage of three tlmes the system voltage Wh~le th~s h~gh- The specified waveshape of the voltage is osc~llatorywave- frequency osclllatlon takes place, the sw~tchmay clear at a h~gh- shape, not the h~stor~caluntdirect~onal waveshape. frequency current zero, only to restr~keagaln later w~than even A Source impedance, a characterlstlc undefined in many greater difference of voltage and then escalating to an even hlgher other documents. IS defined. overshoot The outcome w~llbe e~thera breakdown ~n one of the components. or. lf the swltch eventually recovers enough d~electrlc The concept that all lines to the device under test must be w~thstandIn ~tsopenlng gap, no further restr~kes But severe over- subjected to the test is spelled out voltages w~llhave been impressed on the system. Because this useful document was released at a time when little A similar scenarlo can unfold when an ungrounded power sys- other guidance was available. users attempted to apply the recom- tem experiences an arcing ground fault. The switching actlon is mendations of this document to situations where the environment of then not the result of a deliberate partlng of contacts but the inter- a high-voltage substation did not exist. Thus. an important con- mittent connection produced by the arc. sideration in the writing and publishing of documents dealing with transients is a clear definition of the scope. and limitations of the These sw~tchingovervoltages. h~ghas they may be. are some- application. what predictable and can be estimated with reasonable accuracy from the circuit parameters. once the mechanism involved has been The IEC 664 Report identified. There IS still some uncertainty as to where and when The Insulation Coordination Committee of IEC, following a com- they occur because the worst offenders result from some abnormal prehens~vestudy of breakdown characteristics ~n alr gaps, included behawor of a clrcult element. L~ghtning-induced translents are ~n its repon a table ~ndlcatingthe voltages that equipment must be much less predictable because there is a w~derange of coupling capable of withstanding in varlous system voltages and lnstallation possibilities. Moreover. one user, assuming that his system will not categories (Table 1). The table specifies that it is applicable to a be the target of a direct hit may take a casual view of protection. controlled VOltag9 situation, which implies that some surgelim~ting while another, fearlng his system will experience a "worst case." device will have been provided presumably a typical surge may demand the utmost protection. - arrester with charactaristlcs matching the system voltage in each In response to these Concerns. various comrnlttees and working case. The waveshape specified for these voltages is the 1.2150 ps groups have anempted to describe ranges of transtent occurrences wave. a specification constotent with the lnsulation w~thstandcon- or maximum values occurring in power circuits. Three such cerns of the group that prepared thct document No source attempts will be discussed now. impedance is indicated but four 'installation categories" are speci- fied. each with decreasing voltage magnitude as the installation is further removed from the outdoor environment. Thus, this document STANDARDS ON TRANSIENT OVERVOLTAGES addresses primarily the concerns of insulation coordlnatlon. and the specification it implles for the envlronment is more the result of Several standards or gutdes have been issued or proposed in efforts toward coordinattng levels than efforts to describe the Europe by VDE, IEC. CECC. Pro-Electron. CClfT, and in the USA by The latter approach IEEE NEMA. UL, REA. and the military, specifying a surge withstand environment and the occurrence of translents.