IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 1, JANUARY/FEBRUARY 2001 223 Low-Voltage Power System Surge Overvoltage Protection Dev Paul, Senior Member, IEEE Abstract—The unpredictable threat of transient overvoltages is ever increasing in today’s low-voltage power supplies for every aspect of industry. To calculate the magnitude, duration, and energy of such transient overvoltages is not an easy task. Some loads are becoming very sensitive to such overvoltages, thereby creating a challenge for the application engineer to design a reliable power supply system. To apply surge overvoltage mitigation devices requires technical knowledge to understand their application limitations and configuration within a power system. Technical overview of fundamentals of transients and associated noise is presented. The importance of understanding applicable UL, IEEE, and IEC Standards, and thorough review of manufacturers’ data on transient voltage surge suppressors (TVSSs) is included. TVSS testing requirements per the second edition of UL 1449 is presented. An overview of how to design a low-voltage power supply system to suppress transient overvoltages is included. Index Terms—Electromagnetic compatibility, electromagnetic interference, metal-oxide varistor, noise, spikes, surges, transient voltage surge suppressor. switching frequencies and switched-mode power supplies, used for their greater efficiencies contribute to noise. The ever increasing application of silicon-controlled-rectifier (SCR)-based power circuits and capacitors do contribute to the noise problem for their own control systems. Transient overvoltages are harmful to sensitive control circuits and should be suppressed by application of transient voltage surge suppressors (TVSSs). However, the problem is to quantify the magnitude, duration, and energy parameters of the surge for proper evaluation to select TVSS devices. This paper provides a fundamental overview of transient surge and associated noise in low-voltage power distribution systems. The TVSS application approach is included to help design engineers in the selection of proper transient surge-suppression devices. II. ELECTRICAL SURGES AND NOISE I. INTRODUCTION V OLTAGE transients are brief and unpredictable. These two characteristics make it difficult to detect and measure them. Considerable work is in progress and much data is now available from the power quality group to better understand these transients . Low-voltage power supply systems are getting more disturbed in terms of power quality issues. The switch-mode power supplies used for equipment such as fax machines, printers, variable-frequency drives (VFDs) for energy-efficient heating, ventilation, and air-conditioning (HVAC) systems, elevators, escalators, electronic ballasts, etc., and a great percentage of other loads are becoming nonlinear. Such loads generate current harmonics, leading to distorted voltage. In addition to this distorted voltage, the switching and lightning surges propagating through the distribution system lead to transient overvoltages. Both analog and digital circuits use complex solid-state components in today’s control systems and are inherently susceptible to damage or malfunction from the electrical surges. The current trend toward more performance in smaller size has itself contributed to the noise problem. Digital circuits with high Paper ICPSD 00–03, presented at the 2000 IEEE/IAS Industrial and Commercial Power Systems Technical Conference, Clearwater Beach, FL, May 7–11, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Power Systems Protection Committee of the IEEE Industry Applications Society. Manuscript submitted for review May 12, 2000 and released for publication July 26, 2000. The author is with EARTH TECH, Oakland, CA 94612-3060 USA (e-mail: Dev_Paul@earthtech.com). Publisher Item Identifier S 0093-9994(01)00283-3. Electrical noise generated within the facility by transient surges corrupts low-voltage power supplies. In discussing the characteristics of transient surges and associated noise, it is helpful to understand other related terms. A. Definitions—Noise and Noise-Related Terms 1) Noise can be defined as any form of electromagnetic energy other than the desired signal and its harmonic components. 2) Transients are the momentary amplitude changes in voltage or current or both; 3) Surge is a term for either high-voltage noise or long-duration transients. 4) Electromagnetic interference (EMI) is the impairment of a desired electromagnetic signal by an electromagnetic disturbance such as noise. Since noise-related EMI often occurs in the radio frequency range of 10 kHz to 30 MHZ, the term radio frequency interference (RFI) is often used instead of the general term EMI; 5) Electromagnetic compatibility (EMC) is a device’s capability to perform its intended function within a given electromagnetic environment without adversely affecting or being adversely affected by other devices sharing that environment; 6) Electromagnetic environment is a set of conditions characterized by: 1) the presence of one or more kinds of disturbance, energy, rise time, frequency, duration, and amplitude of disturbance and 2) the effect that these disturbances have on equipment within the environment. 0093–9994/01$10.00 © 2001 IEEE 224 Fig. 1. Fig. 2. IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 1, JANUARY/FEBRUARY 2001 Fig. 4. Voltage spikes. Fig. 5. Oscillatory decaying disturbances. Common-node (CM) noise. Normal-mode (NM) noise. Fig. 3. Normal-mode (NM) noise on primary winding transformed into common-mode (CM) through distribution transformer. B. Modes of Noise Various modes of noise and their characteristics are defined as follows. 1) Common-Mode Noise: The term “common” indicates that the noise signals on each of the current-carrying conductors are in phase and equal in magnitude. Thus, a voltage signal is not generated between these conductors. The common mode is also known as the longitudinal mode (see Fig. 1). 2) Normal-Mode Noise: This is defined as the noise appearing between the current-carrying conductors. It is also known as transverse-mode, differential-mode, metallic-mode, or symmetrical RFI (see Fig. 2). 3) Normal-to-Common-Mode Transformation: Common-mode noise is more troublesome than normal-mode noise. Noise is always transmitted through a distribution transformer as common-mode noise, regardless of the mode in which it was generated (see Fig. 3). 4) Low-Voltage Noise: Noise with peak voltage less than 2000 V is considered low voltage. 5) Voltage or Current Spikes (Impulses): They are called fast transients due to their fast rise times in the order of 1 ns–10 s. They occupy a broad frequency spectrum from 4 kHz to 5 MHz, occasionally reaching 30 MHz. A typical duration is within 100 ns–150 s. Voltage reaches 150% or more of the peak nominal line voltage. They may occur in bursts lasting for as long as one cycle of line voltage (see Fig. 4). 6) Oscillatory Decaying Disturbances: These disturbances have a frequency range of 400 Hz–5 kHz or more (see Fig. 5). 7) High-Voltage Surges: These range upwards from 2000 V. A typical amplitude of surges at a 120-V receptacle is between 100–500 V. In very rare circumstances, the magnitude reaches 4000–6000 V. The upper limit results because sparkover (arcing between the conductors) will occur at about 6000 V, at the distance between the terminal blocks of the indoor power system supply rated at 240 V. 8) Broad-Band Noise: The spectral characteristics of a noise waveform dictate the frequency attenuation requirements of a noise protection device. The shorter the rise time of a noise pulse, the broader is the band of energies that must be attenuated. Voltage spikes contain a power spectrum of broad frequency range and are considered broad-band noise. 9) Narrow-Band Noise: Such noise has much lower frequency content than broad-band noise, in the less than 1-MHz frequency range. 10) Attenuation of Common-Mode Noise: Power transformers do not transform high-frequency signals in the same way as they do 60-Hz power. To high-frequency transients, the transformer is nothing more than a network of capacitance and iron-core reactance. The iron core of the transformer cannot respond to the high frequency and, thus, becomes a negligible factor. In some applications, the use of an isolation transformer or a shielded transformer is effective to attenuate the common-mode noise, which otherwise could disturb the performance of sensitive equipment. In the case of an isolation transformer, the degree PAUL: LOW-VOLTAGE POWER SYSTEM SURGE OVERVOLTAGE PROTECTION Fig. 6. Attenuation of common-mode noise through isolation transformer. Fig. 7. Attenuation of common-mode noise through shielded transformer. of attenuation depends upon the relative magnitudes and of transformer inter-winding capacitance (see Fig. 6). winding-to— ground capacitance In the case of a shielded transformer, the introduction of a grounded shield between the windings results in much better attenuation to common-mode transients. The electrostatic charge around the primary winding is conducted to ground by the shield before it can induce voltage into the secondary winding. The small amount of coupling of the electrostatic field around the shield and is is generally called “effective capacitance” much smaller than the inter-winding capacitance of an unshielded transformer (see Fig. 7). 11) One or more of the following parameters can characterize noise distortion: • single impulses or oscillatory signals; • rise and/or fall times; • duration; • rate of repetition; • amplitude; • frequency. The attenuation is generally described in decibel (dB) units. The logarithmic relationship of dB to attenuation in volts or amperes is shown by dB or dB Thus, 60 dB could be stated as resulting in: (1) 225 *In some locations, sparkover of clearances may limit the overvoltages. Fig. 8. Rate of surge occurrences versus voltage level at unprotected locations for different exposures. III. CAUSE OF SURGES AND NOISE The various causes of surge overvoltage and associated noise are well documented. 1) Lightning: Lightning can produce various modes of noise and transient disturbances as follows: • lightning and common-mode noise; • lightning-induced electrostatic coupling; • lightning-induced magnetic coupling; • lightning-induced conductive coupling; • lightning and normal-mode noise. 2) Power-Factor-Correction Capacitors: Oscillation frequencies are in the range of 1–20 kHz. 3) Power System Switching: Included are fault-clearing devices and load switching and electronic switching devices. 4) Lighting Exposure and Surge Intensity: The probability of surge voltage exceeding specific peak values is related to the levels of lightning exposure, as shown in Fig. 8 . IV. WAVE SHAPE OF SURGES Working groups of IEEE and IEC Standards have developed different standard surge waves for testing TVSS devices meant for outdoor and indoor application locations to the low-voltage power distribution system . 1) Outdoor: Combo Wave—1.2/50 s voltage wave and 8/20 s current wave are predominant at the service entrance outdoor location. However, lightning discharges 226 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 1, JANUARY/FEBRUARY 2001 TABLE I LOCATION CATEGORIES Fig. 9. Test waveforms as described by ANSI/IEEE C62.41-1991 . induce oscillations, reflections that ultimately appear as decaying oscillations in a low-voltage power system. 2) Indoor: Ring Wave—A surge impinging on the system excites the natural resonant frequencies of the conductor system. As a result, not only are the surges typically oscillatory, but also may have different amplitudes and wave shapes at different places in the low-voltage power system. These oscillatory frequencies of surges range from 5 to more than 500 kHz. Based upon such conclusions, a Ring Wave, 0.5 s with 100 kHz which rises in 0.5 s, then decays while oscillating at 100 kHz, each peak being 60% of the preceding peak. Such waves are depicted in Fig. 9. V. SURGE ENERGY AND SOURCE IMPEDANCE There is a lack of definite data on the duration, waveform, and source impedance of transient overvoltages in ac power circuits. These three parameters are important for estimating the energy that a transient can deliver to a suppressor. The impedance presented by a source of energy to the input terminals of a device or network is defined as the source impedance. Because of the wide range of possible source impedance and the difficulty of selecting a specific value, three broad categories of building locations as defined in Table I  have been considered for the application of surge suppressors in the low-voltage power system. The degree to which the source impedance is important depends largely on the type of surge suppressors that are used. The surge suppressors must be able to withstand the current passed through them by the surge source. Source impedance should not be confused with the surge impedance. Surge impedance is the concept relating to the parameters of a line to the propagation of the traveling wave, for the low voltage wiring system it may be in the range of 150–300 . Clamping surge protective devices work only if there is a finite source impedance . Demarcation between location Categories B and C is arbitrary, taken to be at the main meter or at the main disconnect or at the secondary of the service transformer if the service is provided to the user at a higher voltage. Fig. 10. Power quality pyramid-relative cost. VI. SURGE SUPPRESSOR DEVICES The majority of the power quality problems in the low-voltage distribution system are being solved by the application of TVSSs and their combinations with other noise-filtering devices. TVSSs go by a variety of names provided by individual manufacturers, creating a challenge for the power system designer for proper selection and application. In the low-voltage power distribution system, series-connected TVSSs are available, however, such devices carry normal current continuously and need to have short-circuit withstand capability without creating a dangerous situation. Increased application demand of TVSSs to solve power quality problems in general is dedicated by the relative cost of such devices as compared to other devices shown in the power quality pyramid (see Fig. 10). Broad categories of TVSS devices are defined as follows. 1) Clamps: A metal-oxide varistor (MOV) has high-energy absorption capability and response time of nanoseconds 2) Crowbar: Gas tubes do not response quickly; transient may occur faster than the device can respond. 3) Hybrid: These devices may be considered as a combination of an MOV and a Crowbar. PAUL: LOW-VOLTAGE POWER SYSTEM SURGE OVERVOLTAGE PROTECTION 4) Sine-Wave Tracking: New technology tracks the ac sine wave; this enables response to minor spikes and transients that may pass through other categories of protection devices. 5) Others: These include data/signal/telephone TVSSs and noise-filtering devices . VII. SURGE SUPPRESSOR TEST REQUIREMENTS The second edition of UL Std. 1449  has important changes affecting the test requirements for safety and performance of permanently connected TVSS products. This edition includes the "end of life" mode testing, which could result in damage to products from a large surge event, a sustained overvoltage such as a loss of neutral, or installation error such as improper bonding or misoperation. MOV element of TVSS can cause a short circuit condition when its goes into a thermal runaway condition. If not properly contained, equipment damage may result. The TVSS must pass the following tests without the evidence of risk of fire or electric shock . 1) Overvoltage: The TVSS must withstand 110% of rated voltage for seven hours. 2) Abnormal Overvoltage, Full Phase Voltage—High Current: The TVSS must withstand 25 kA at full phase voltage of 208 V for 120 V and 480 V for 277 V. The overvoltage is applied for seven hours. 3) Abnormal Overvoltage, Full Phase Voltage—Limited Current: The TVSS must withstand5 A (also 2.5, 0.5, and0.125 A) at full phase voltage of208 V for 120 V and 480 Vfor277 V. The overvoltage is applied for seven hours. 4) Voltage-Limiting Test and Duty Cycle (Pulse Life) Test: The test is performed for L-L, L-N, L-G, and N-G connections of TVSS subjecting an impulse surge of 6 kV and 0.5 kA, limiting voltage is measured and recorded. The device is then subjected to ten consecutive 6-kV and 3-kA positive impulses at 60-s intervals, and ten consecutive 6-kV and 3-kA negative impulses at 60-s intervals. Following the duty cycle test, the TVSS is subjected to another 6 kV and 0.5-kA impulse and the limiting voltage is measured and recorded. The average of the limiting voltage test is to fall within the minimum suppressed voltage ratings and is not to exceed the suppressed voltage rating by greater than 10%. 5) Surge Current Test: This test is designed to qualify each device to withstand a standard Category C3 transient. The device must withstand two consecutive (one positive and one negative) 6-kV and 10-kA surge impulses (C3). After testing, the device must stay connected to the service at rated voltage for seven hours or until thermal equilibrium. VIII. TVSS APPLICATION APPROACH It is important that the application engineer has the basic understanding of the published literature on transient surge overvoltage environment and the surge wave shapes used in testing the TVSS , . To avoid guesswork and misapplication of the TVSS, the application engineer, perhaps, should develop his/her own list of evaluation factors. These evaluation factors, in general, will es- 227 tablish the roadmap for analysis and solution to the problem of voltage transients and associated high-frequency noise. 1) Develop a complete one-line diagram of the low-voltage power distribution system showing main transformer, main switchboard, subpanels, feeders distribution transformers, type of loads, and metering devices at each panel. Show control, data communication, and other sensitive loads at appropriate locations throughout the facility in one— line representation (see Fig. 11). 2) Review the utility company incoming primary power source grounding method and configuration in order to select proper primary surge arrester to be located at the primary side of the main power transformer. 3) Establish an equipment layout sketch indicating raceway and wiring types, configuration, and approximate lengths. 4) Review grounding, bonding, and shielding provisions of the distribution system as needed for equipment protection, personnel safety, and high-frequency shielding of data and control systems , . 5) Establish maximum continuous operating voltage at each main power distribution panel based upon the utility voltage regulation and the effect of the transformer taps, if any. 6) Perform three-phase and single-phase short-circuit analysis to establish maximum expected short-circuit current at all critical locations within the power distribution system where TVSSs are to be applied. 7) Review facility site in association with configuration of power distribution system and loads to determine the degree of outside transient surge exposure as well as the internal switching surge propagation , . 8) Analyze power distribution system design for possible ferroresonance condition which could lead to failure of surge arresters . 9) For each location, establish the TVSS category type, protection modes, high-frequency noise-filtering requirements, and the means of connection to the equipment that needs surge protection. 10) Establish TVSS local or remote monitoring requirements. 11) At Category A locations, make analysis of the choice between the series-connected or the parallel-connected TVSS devices. This may require reviewing the problem with the equipment vendor to assure proper application. 12) Review design of grounding, bonding, and shielding, a key element to the success of TVSS application . 13) Carefully review the TVSS vendor literature for each category for specific requirements and safety test requirements . If possible, contact the vendor to address questions and concerns to obtain clarification of TVSS characteristics and performance specifications. TVSS performance criteria and specifications shall include at least the following: a) surge current capability: performance, safety; b) fail-safe design; 228 Fig. 11. IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 1, JANUARY/FEBRUARY 2001 Category classification defined by IEEE Std. C62.41 and IEC Std 664 for TVSS application. c) d) e) f) g) h) UL listing, testing, and suppression voltage; modes of protection; EMI/RFI filtering capability; life-cycle testing; monitoring features; short-circuit withstand capability and fusing rating; i) response time. IX. CONCLUSION To avoid guesswork and misapplication of TVSSs, an engineering analysis of transient surge overvoltages and high-frequency noise is required. Application engineers must learn to evaluate the transient environment and should have technical knowledge to fully understand the limitations of commercially available TVSSs for each specific location in the power system. Some manufacturers promote their TVSSs merely by their surge-chopping and energy-absorption capability without proper application guidance and criteria. A new UL standard  requires their test withstand capability and not all TVSSs could meet such requirements. With a thorough understanding of the applicable standards, and knowledge of surge propagation through the power distribution system, the design engineer should be able to make a proper selection of TVSS for the application. Proper application of TVSS devices in a low-voltage power distribution system may require all or a combination of the following. 1) Apply both primary and secondary surge-suppressor devices at the main transformer . 2) Implement integrated surge-suppressor device at the main panel . 3) Apply “cascaded network” approach , . 4) Apply power quality pyramid for design of distribution system . PAUL: LOW-VOLTAGE POWER SYSTEM SURGE OVERVOLTAGE PROTECTION 5) Use integrated surge protection devices approach, especially for a new facility, as it provides better margin of protection. In retrofit projects, use as short as possible the specially designed low-impedance coaxial cable to install a surge-suppression device outside the panel. 6) Use twisted shielded pair wires and ground shield at either both ends or at one end, depending upon the overall configuration of equipment ground and shield ground. 7) Provide reference ground grid for sensitive electronic equipment , . 8) Use of ferro-resonant line conditioners can provide better attenuation than isolating power transformers for fast line-to-line transients . 9) Fuses applied to the TVSS shall have surge current withstand capability and shall isolate the device in case of its failure by surge current higher than its let through capability. However, in the case of thermal runaway of the TVSS device, the current is still too low to operate the fuse; only a thermal cut-out in close proximity to the surge device can clear this fault. Development of such proper disconnectors is a challenge to the manufacturers of surge devices. 10) Install series-hybrid-type filters as close to the critical loads as possible, such as in front of fire alarm systems, control devices, programmable logic controllers, cash registers, etc. 11) Install surge protection equipment on all data and communication lines, especially noise-suppression devices. 12) At noncritical loads which generate disturbance such as harmonics and noise, installation of less expensive surge strips may be adequate. 13) If possible, the grounding system of the surge arrester shall have minimum resistance to ground in order to minimize ground potential rise above remote earth which acts as reference ground to other sensitive equipment in the facility. REFERENCES  Recommended Practice for Powering and Grounding Sensitive Electronic Equipment, (Emerald Book), 1992.  The New IEEE Standard Dictionary of Electrical and Electronics Terms, 5th ed., IEEE Std. 100, 1992.  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UL Standard for Safety for Transient Voltage Surge Suppressors, 2nd ed., UL 1449, 1998. D. O. Koval, “Rural power quality,” IEEE Trans. Ind. Applicat., vol. 28, pp. 761–766, July/Aug. 1992. T. Muller and D. Graff, “The use of surge protection devices in the petroleum/petrochemical industry,” IEEE Trans. Ind. Applicat., vol. 34, pp. 1351–1358, Nov./Dec. 1998. P. G. Slade, “Vacuum interrupters: The new technology for switching and protecting distribution circuits,” IEEE Trans. Ind. Applicat., vol. 33, pp. 1298–1304, Nov./Dec. 1997. M. B. Marz and S. R. Mendis, “Protecting load devices from the effects of low-side surges,” IEEE Trans. Ind. Applicat., vol. 29, pp. 1196–1203, Nov./Dec. 1993. L. J. Bohmann, J. McDaniel, and E. K. Stanek, “Lightning arrester failure and ferroresonance on a distribution system,” IEEE Trans. Ind. Applicat., vol. 29, pp. 1189–1195, Nov./Dec. 1993. Transient Voltage Suppression Devices Ddata Book, Harris Semiconductor Corp., 1992. IEEE Guide for the Application of Metal Oxide Surge Arresters for AC Systems, Draft ANSI/IEEE Std. C62.22, 1991. IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, ANSI/IEEE Std. C62.41, 1991. IEEE Guide on Surge Testing for Equipment Connected to Low-Voltage AC Power Circuits, ANSI/IEEE Std. C62.45, 1987. IEEE Guide for the Application of Gas Tube Arrester Low-Voltage Surge Protective Devices, ANSI/IEEE Std. C62.42, 1987. IEEE Standard Test Methods for Surge Protectors Used in Low-Voltage Data, Communications, and Signaling Circuits, ANSI/IEEE Std. C62.36, 1991. IEEE Standard Test Specifications for Low-Voltage Air Gap Surge-Protective Devices (Excluding Valve and Expulsion Type Devices), ANSI/IEEE Std. C62.32, 1981. IEEE Standard Test Specifications for Avalanche Junction Semiconductor Surge Protective Devices, ANSI/IEEE Std. C62.35, 1987. IEEE Standard Test Specifications for Gas Tube Surge Protective Devices, ANSI/IEEE Std. C62.31, 1987. IEEE Standard for Metal Oxide Surge Arresters for AC Power Circuits, ANSI/IEEE Std. C62.11, 1987. Insulation Coordination Within Low-Voltage Systems Including Clearances and Creepage Distances for Equipment, IEC 664, 1980. IEEE Standard Test Specifications for Varistor Surge-Protective Devices, ANSI/IEEE Std. C62.33, 1982. Dev Paul (M’73–SM’90) received the B.Sc. degree with honors in mathematics and the B.E. (Honors) and M.S.E.E. degrees in electrical engineering from Punjab University, Chandigarh, India, in 1965, 1969, and 1971, respectively. He completed further studies in power systems at the University of Santa Clara, Santa Clara, CA, in 1975. In 1972, he joined EARTH TECH (formerly Kaiser Engineers), Oakland, CA, as a Design Engineer and has worked on a variety of heavy industrial, cogeneration, commercial, DOD and DOE facilities, and rapid transit rail projects. In his present position as a Principal Electrical Engineer, he is responsible for the overall design, analysis, studies, specification, installation, project engineering, startup work, and system integration. He has authored papers published in IEEE Industry Applications Society (IAS) and APTA conference proceedings. His main fields of interests are power system analysis, protection, grounding, and harmonics. Mr. Paul is an active member of several IAS Committees. He received the Award of Distinction for his M.S.E.E. thesis work on power system stability. He is a Registered Professional Engineer in the States of California, Nevada, and Oregon.