The Application and Selection of Lightning Arresters
Total Page：16
File Type：pdf, Size：1020Kb
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 2 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.