The Application and Selection of Lightning Arresters

By Larry Pryor, P.E., GE, Sr. Specification Engineer

Abstract arrester to circuits and systems rated 1000 V and greater. Each piece of electrical equipment in an electrical system needs to be protected Nature of the Problem from voltage surges. To prevent damage to electrical equipment, surge protection The states around the Gulf of Mexico considerations are paramount to a well- have the highest annual average number designed electrical system. Modern of lightning storms in the United States. metal oxide arresters provide exceptional This is called the isokeraunic level for an overvoltage protection of equipment area. On average, between 40 and 80 connected to the power system. The thunderstorms hit the areas along the proper selection and application of the Gulf Coast each year, while California arrester, however, involves decisions in and some states along the Canadian several areas, which will be discussed in border have an average of less than 5 per the paper. year. So, it is easy to understand that surge protection of electrical equipment Introduction is a very important part of the electrical system design. The original lightning arrester was nothing more than a spark air gap with Lightning strikes are not the only one side connected to a line conductor sources of voltage surges in the electrical and the other side connected to earth system. The following are a few of the ground. When the line-to-ground voltage more frequently encountered causes of reached the spark-over level, the voltage transient voltage surges: surge would be discharged to earth 1. Surge voltages associated ground. with switching capacitors 2. Surge voltages due to a The modern metal oxide arrester failure in equipment provides both excellent protective insulation resulting in a short characteristics and temporary circuit on the distribution overvoltage capability. The metal oxide system disks maintain a stable characteristic and 3. Surge voltages associated sufficient non-linearity and do not with the discharge of require series gaps. lightning arresters at other locations within the facility Due to the broad nature of this subject, this paper will concentrate on the When capacitors are switched in and out application of the gapless metal oxide of the circuit, it is possible to get a restrike when interrupting the capacitor

1 circuit current. A steep-front voltage machine may be only 25 ft. per excursion may be created from each microsecond. restrike. These voltage excursions may be high enough to damage rotating The current resulting from a traveling machines applied at the same voltage. A wave is equal to the voltage divided by surge capacitor applied at the motor the impedance (E/Z). Wave current is terminals can change the steepness of the approximately two to four amps per wave front enough to protect the motor. kilovolt of surge voltage. Lightning waves on overhead lines gradually A short circuit can cause a voltage surge attenuate with travel. in excess of 3 times the normal line to neutral crest value. The magnitude and When the wave runs into a change in steepness of the wave front is not as impedance (transformer, another line, severe as that of a lightning strike, but etc.), the wave continues in the same can cause damage or weaken motor direction at a different magnitude. It will windings that do not have the higher also reflect back in the direction from Basic Impulse Insulation Level (BIL) which it came. ratings of other equipment. When a wave E1 traveling on surge When lightning discharges through an impedance Z1 encounters another surge arrester, surge currents are discharged impedance Z2, the voltage on the new into the grounding terminal. It is very wave Z2 becomes: important that substations and overhead lines be protected with well-grounded E2 = E1( 2 * Z2 ) shield wires. It is also equally important E1 + E2 that the ground system between pieces of equipment be bonded together with Note: as the new surge impedance Z2 interconnected ground wires dedicated to approaches infinity, representative of an the grounding system. open line, E2 = 2E1.

When a surge is released on a line by The reflected wave will actually double direct strokes or induced strokes, the in magnitude in its return in the opposite stroke travels in both directions from the direction. Unless the wave is discharged point where the stroke originated. Wave to ground (lightning arrester connected velocity is an inverse function of the to ground), the reflected wave can surge impedance. Waves travel on an severely damage electrical equipment. overhead line at approximately 1000 ft. per microsecond, in cables about 300 – Surges produced by lightning have high 600 ft. per microsecond and in a buried magnitudes, but their durations are very conductor about 300 ft. per microsecond. short. See figure below: The velocity internal to a rotating

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PSI Letter #3

Protective Margin The lightning discharge may reach its crest value in approximately 1 to 20 Most electrical equipment is rated for microseconds and produce conduction traveling wave voltage surge capability flashover voltages of 5 to 20 times by the Impulse Test. The Impulse Test normal in 1 microsecond or less. is most common and consists of applying a full-wave voltage surge of a The wave shape is customarily specified crest value to the insulation of expressed by two intervals associated the equipment involved. The crest value with the wave geometry. The first time of the wave is called the Basic Impulse interval is between a virtual zero and Insulation Level (BIL) of the equipment. crest; the second time interval is between Each type of electrical equipment has a the virtual zero and the half crest value standard BIL rating. Lightning arresters on the wave tail. The wave is defined if are coordinated with standard electrical the crest value is added to the two time- equipment insulation levels so that they interval designations. For example, a will protect the insulation against 20000 amp 10 x 20 microsecond current lightning over voltages. This wave rises to a crest of 20000 amperes in coordination is obtained by having an 10 microseconds after virtual zero and arrester that will discharge at a lower decays to 10000 amperes in 20 voltage level than the voltage required to microseconds after virtual zero. break down the electrical equipment insulation. Equipment has certain applicable impulse levels or BIL as defined in industry standards.

It is important to recognize that rotating machines do not have BIL ratings.

3 However, it is generally accepted that a distance between the arrester and the rotating machine can withstand a voltage transformer. wave at the motor terminals, which rises to a maximum level at a rate not exceeding 1.25 time the crest value of Switching Surge Withstand > = 1.15 the one-minute hi-pot test voltage in 10 Switching Surge Protective Level microseconds. Steepness of the wave front should also be considered when Full Wave Withstand (BIL) > = 1.20 applying surge protection to rotating Impulse Protective Level machines. A fast rising voltage at the motor terminals lifts the potential of the Chopped Wave Withstand > = 1.25 terminal turn, but the turns deeper in the Front-of-Wave Protective Level windings are delayed in their response to the arriving voltage wave due to the large capacitance coupling between conductors and the grounded core iron. The protective ratios typically exceed The result is a high voltage gradient the minimum protective ratios across the end-turns of the terminal coil, recommended by ANSI by a resulting in severe voltage stress on the considerable amount in actual power turn-to-turn insulation. system applications.

Surge arresters are applied to limit the Overwhelming evidence points to the magnitude of impulse waves. Surge fact that surge voltage is a significant capacitors reduce the steepness of the contributor to cable failure. When wave fronts so that the equipment being transformers are subjected to surge protected is not subjected to as severe a voltages, if the insulation strength of the voltage wave. Surge capacitors should transformer is sufficient to withstand the be considered only for rotating machine surge, there is typically no damage to the protection and must be applied at the transformer. However, cable has motor terminals. memory. Each impulse on a cable contributes to the deterioration of cable For insulation coordination, protective insulation strength, such that the cable ratios are calculated at three separate may ultimately fail by overvoltage levels points within the volt-time regions. below the BIL. These are: the switching surge withstand, the full wave withstand and The important thing to note is that each the chopped wave withstand. The apparatus has a design rating. Many following protective ratios must be met lightning and switching surges are of a or exceeded if satisfactory insulation magnitude much higher than the design coordination is to be achieved, according rating. It is of vital importance, to the minimum recommendations given therefore, to properly select, locate, and in ANSI C62.22. apply surge arresters in order to avoid damaging equipment. Properly applied These protective ratios assume surge protection can reduce the negligible arrester lead length and magnitude of the traveling wave to a level within the rating of the equipment.

4 Arrester Voltage Rating

Arrester Selection The lower the arrester voltage rating, the lower the discharge voltage, and the The objective of arrester application is to better the protection of the insulation select the lowest rated surge arrester system. The lower rated arresters are which will provide adequate overall also more economical. The challenge of protection of the equipment insulation selecting and arrester voltage rating is and have a satisfactory service life when primarily one of determining the connected to the power system. The maximum sustained line-to-ground arrester with the minimum rating is voltage that can occur at a given system preferred because it provides the greatest location and then choosing the closest margin of protection for the insulation. rating that is not exceeded by it. This A higher rated arrester increases the maximum sustained voltage to ground is ability of the arrester to survive on the usually considered to be the maximum power system, but reduces the protective voltage on the unfaulted phases during a margin it provides for a specific single line-to-ground fault. Hence, the insulation level. Both arrester survival appropriate arrester ratings are and equipment protection must be dependent upon the manner of system considered in arrester selection. grounding.

The proper selection and application of Table 1 lists arrester ratings, for one lightning arresters in a system involve manufacturer, that would normally be decisions in three areas: applied on systems of various line-to- line voltages. The rating of the arrester is 1. Selecting the arrester voltage defined as the RMS voltage at which the rating. This decision is based arrester passes the duty-cycle test as on whether or not the system defined by the reference standard. Metal is grounded and the method oxide arresters are designed and tested in of system grounding. accordance with ANSI/IEEE C62.11. 2. Selecting the class of arrester. In general there are three For surge arrester applications the classes of arresters. In order “solidly grounded” classification is of protection, capability and usually found in Electric Utility cost, the classes are: distribution systems where the system is Station class usually only grounded at the point of Intermediate class supply. These systems can exhibit a Distribution class wide range of grounding coefficients The station class arrester has depending upon the system or location in the best protection capability the system. Accordingly, these systems and is the most expensive. may require a study to ensure the most 3. Determine where the arrester economical, secure, arrester rating should be physically located. selection. If this information is not known or available, the ungrounded classification should be used.

5 The “ungrounded” classification ungrounded system. This simply means includes resistance grounded systems, that the maximum continuous operating ungrounded systems and temporarily voltage, as discussed below, must be at ungrounded systems. Both high least 100 % of the maximum operating resistance and low resistance systems are voltage of the system. considered ungrounded for the selection of the proper surge arrester since during a line-to-ground fault the unfaulted phases and their arresters experience essentially line-to-line voltage. The same is true for the infrequently used

FETA – 100A

Continuous System Voltage This is referred to as the Maximum Continuous Operating Voltage (MCOV). When arresters are connected to the The MCOV values for each arrester power system they continually monitor rating are shown, for one manufacturer, the system operating voltage. For each in Table 2a below. These values meet or arrester rating, there is a limit to the exceed those values contained in the magnitude of voltage that may be referenced standard. Note that the continuously applied to the arrester. MCOV is less that the arrester voltage

6 rating. The arrester selected must have a line-to-line, then the phase-to-phase MCOV rating greater than or equal to voltage must be considered. There are the maximum continuous system also special applications that require voltage. Attention must be given to the additional consideration such as the use circuit configuration (single phase, wye, of a delta tertiary winding of a or delta) as well as the arrester transformer where one corner of the connection (line-to-ground or line-to- delta is permanently grounded. In this line). Most arresters are connected line- case the normal voltage applied to the to-ground, in which case, the grounding arrester will be full phase-to-phase methods discussed above must be voltage even though the arresters are considered. If the arrester is connected connected line-to-ground.

FETA – 100A

7 that the overvoltages may be applied before the arrester voltage must be Temporary Overvoltages reduced to the arrester’s continuous operating voltage capability. These Temporary overvoltages (TOV) may be capabilities are independent of system caused by a number of events such as impedance and are, therefore, valid for line-to-ground faults, circuit back voltages applied at the arrester location. feeding, load rejection, and ferro- If detailed transient system studies or resonance. The system configuration and calculations are not available, the operating practices will identify the most overvoltages due to a single line-to- probable forms of temporary overvoltage ground fault should be considered as a that may occur at the arrester location. minimum. The arrester application The temporary overvoltage capability standard ANSI C62.22 gives some must meet or exceed the expected guidance in determining the magnitude temporary overvoltages. of single line-to-ground fault overvoltages. The effects of TOV on Table 3 identifies the temporary metal oxide arresters are increased overvoltage capability of one current and power dissipation and a manufacture's arresters in per unit of rising arrester temperature. MCOV. It also defines the time duration

FETA – 100A

8 over a one-minute period. Arresters Switching Surges have more capability if the discharges take place over a longer period of time. The energy capability of an arrester After a one-minute rest period, the determines the ability of the arrester to discharges, as defined, may be repeated. dissipated switching surges. These energy ratings assume that The units used in defining the energy switching surges occur in a system capability of metal oxide arresters are having surge impedances of several kilojoules per kilovolt. hundred ohms, which is typical for overhead transmission circuits. In The maximum amount of energy that circuits having low surge impedance can be dissipated in one manufacture’s involving cables or shunt capacitors, the arresters is given in Table 4. These energy capability of metal oxide capabilities are based on the assumption arresters may be reduced because that multiple discharges are distributed currents can exceed the values stated.

FETA – 100A

Arrester Failure and Pressure Relief Should a complete failure of the arrester occur, a line-to-ground arc will develop Metal oxide disks may crack or puncture and pressure will build up inside the if the capability of the arrester is housing. This pressure will be safely exceeded reducing the arrester internal vented to the outside and an external arc electrical resistance. This will limit the will be established, provided the fault arrester’s ability to survive future system current is within the pressure relief fault conditions but does not jeopardize the current capability of the arrester. insulation protection provided by the arrester. Once an arrester has safely vented, it no longer possesses its pressure relief/fault

9 current capability and should be intended arrester location taking in immediately replaced. For a given allowances for future growth. application, the arrester selected should Table 5 shows relief/fault current have a pressure relief/fault current capability for one manufacture’s capability greater than the maximum arresters. short-circuit current available at the

FETA – 100A

Arrester Class lower and the pressure relief is greater. The value of the protected equipment The class of lightning arrester to be and the importance of uninterrupted applied depends upon the importance service generally warrant the use of and value of the protected equipment, its station class arresters throughout their impulse insulation level and the voltage range. Industry standards dictate expected discharge currents the arrester the use of both station class and must withstand. intermediate class arresters for equipment protection in the 5-to 20- Station class arresters are designed for mVA size ranges. Above 20 mVA, protection of equipment that may be station class arresters are predominately exposed to significant energy due to line used. switching surges and at locations where significant fault current is available. Intermediate class arresters are They have superior electrical designed to provide economic and performance because their energy reliable protection of medium voltage absorption capabilities are greater, the class power equipment. Intermediate discharge voltages (protective levels) are arresters are an excellent choice for the

10 protection of dry-type transformers, for BIL apparatus (certain dry-type use in switching and sectionalizing transformers and rotating machines) will equipment and for the protection of often require surge protective devices be URD cables. Traditional applications connected directly at the terminals of the include equipment protection in the equipment being protected. range of 1 to 20 mVA for substations and rotating machines. In many switchgear installations, the only exposure to lightning will be Distribution class arresters are through a transformer located on its up frequently used for smaller liquid-filled stream side. When the transformer has and dry-type transformers 1000 kVA adequate lightning protection on its and less. These arresters can also be primary, experience has shown that the used, if available in the proper voltage surge transferred through the transformer rating, for application at the terminals of is usually not of a magnitude that would rotating machines below be harmful to the switchgear. Hence, it is 1000 kVA. The distribution arrester is generally not necessary to provide often used out on exposed lines that are arresters in the switchgear. directly connected to rotating machines. When arresters are located away from All of the system parameters need to be the terminals of the protected equipment, considered while choosing an arrester the voltage wave will reflect positively classification. If the actual arrester on the equipment terminals and the energy duties are not known and a voltage magnitude at the terminal point transient study cannot be performed, will always be higher than the discharge then it is suggested that Station class voltage of the arrester. This, as arresters be applied. This is a discussed earlier, is due to the fact that conservative approach that reduces the the protected equipment usually has a chances of misapplication. higher surge impedance than the line or cable serving it. If the circuit is open at Location of Arresters the protected equipment (infinite surge impedance), the voltage will be double The ideal location for lightning arresters, the arrester discharge voltage. from the standpoint of protection, is directly at the terminals of the equipment The actual surge voltage appearing at the to be protected. At this location, with the protected equipment depends, in part, on arrester grounded directly at the tank, the incoming wave magnitude at the frame or other metallic structure which instant of arrester discharge. If a positive supports the insulated parts, the surge reflected surge from the protected voltage applied to the insulation will be equipment arrives back at the arrester limited to the discharge voltage of the before arrester discharge, it will add to arrester. Practical system circumstance the incoming wave to produce discharge and sound economics often dictate that at a lower incoming wave magnitude. arresters be mounted remotely from the The reflected wave, in this case, results equipment to be protected. Often, one set in improved protection. The closer the of arresters can be applied to protect arrester is to the protected equipment, more than one piece of equipment. Low

11 the greater the effect of the reflected surge on arrester discharge and the better the protection.

Conclusion

All electrical equipment in an electrical absorption capability, compared to system needs to be protected from previous generation arresters. voltage surges. The rating of the arrester, the class of arrester and the location of The application and selection of metal the arrester all play a part in the surge oxide arresters requires a thorough protection. Modern metal oxide arresters review of the power system, including provide markedly superior protective voltage, system stresses, switching characteristics and energy surges, grounding method and MCOV.

References:

1. ANSI/IEEE C62.11 Standard for 4. TRANQUELL Surge Arresters, Metal-Oxide Surge Arresters for Product Selection & Application Alternating Current Systems. Guide – GE Publication no. FETA – 100A 2. ANSI/IEEE C62.22 Guide for Application of Metal-Oxide 5. Bob M. Turner, GE, Power Surge Arresters for AC Systems. Systems Information Letter No. 3, May 1986 3. George W. Walsh, “A Review of Lightning Protection and 6. Bob M. Turner, GE, Power Grounding Practice”, IEEE Systems Information Letter No. Transactions on Industry 4, August 1986 Applications, Vol.IA-9, No. 2, March/April 1973 (reprint/Ref GE Publication GER-2951)

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