Transcript
Shut out potential disaster by
Providing Reliable Switchgear Control Power By RONALD J. KASPER, Executive Engineer-National, Industrial Risk Insurers, Hartford, CT
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HEN a 350 MCM cable faulted in a manhole of a 13.8 kv distribution system in a cable manufacturing plant, the 15 kv switchgear failed to operate to clear the fault. Passing the fault through the system damaged some oil circuit breakers and a 600 kva voltage-regulating transformer; the cost of repairs exceeded $200,000. The 48v lead-acid storage battery supplying switchgear trip power was found to be discharged. The battery, in a steel cabinet in the switchyard, was out of sight and, apparently, out of mind. A 6000 hp, 13.8 kv synchronous motor powering a log grinder in a paper mill was seriously damaged when battery power to the motor control system and motor power circuit switchgear was lost. When dc control power was lost, the motor reverted to the “start” mode, but this condition would have been sensed, and the motor breaker would have tripped, if trip power to operate the breaker had been available. Continued operation in the start mode caused overheating and motor failure, upon which the plant main breaker-which drew its control power from another battery--tripped to clear the fault imposed on the power system by the failed motor. The cost for motor repairs and lost production was $250,000. The debacle would have been prevented if the switchgear dc control power system for the motor had been properly maintained.
In a cement plant, an electrical fault and ensuing tire damaged three 1500 hp, 4160 v synchronous motors, three cubicles of 5 kv switchgear, and hundreds of feet of 1000 MCM 5 kv power cable. Damage exceeded $300,000. Almost all of this damage would have been prevented if either the 5 kv breakers or switchgear in the 69 kv main substation had operated; the switchgear failed to operate because both the 5 kv switchgear and main substation were without control power. All
switchgear was served from a 12.5 v stationary battery system that had one lug burned off because of a ground fault; no one knows how long the situation had existed. Loss of dc excitation to two 300 hp synchronous motors in a cement plant resulted in destruction of the motors and ensuing fire; damage was $500,000. The motor control circuitry sensed the condition, but breakers failed to trip because of lack of control power. The 20 ampere circuit breaker in the ac supply to the battery
“A battery is the preferred, and by far the most commonly used, method of providing reliable switchgear control power.”
charger was found to be tripped, explaining why the battery was charged to only 30 volts. At least 100 volts were needed to trip the breakers. Electrical fault and ensuing fire and explosion caused $1,000,000 in damage to a paper mill’s main transformers and other substation equipment, cable trays, and raceways. This loss would not have been sustained if the 48 v battery serving the plant’s switchgear had been maintained. The breakers failed to trip because battery voltage was only 9.5 volts; the operating range for the breaker trip coils was 28 to 60 volts. Eleven of the 36 cells of the nickel-cadmium battery were found to be dry. All of these disasters have a common denominator; failure to maintain the dc power supply system to switchgear prevented breakers from operating. Loss of dc control power can render tens of thousands of dollars worth of switchgear useless, and leave millions of dollars worth of electrical equipment without protection. Medium-voltage power switchgear (and low-voltage switchgear employing shunt-trip relaying) requires a source of electric power to operate the breaker trip mechanism when a fault is sensed by the breaker’s associated protective relaying. Two methods are commonly used to provide power to the breaker trip coil: charged capacitor (charged through a rectifier supplied by an ac source), and a stationary battery. Of these two methods, the battery is the preferred, and by far the most commonly used, method. A switchgear battery is composed of a number of individual cells, or a group of multicell bat-
An arcing fault initiated this burndown of a piece of 1200 amp switchgear. Inadequate dc control power prevented the circuit breaker from operating to clear the fault.
teries, connected in series or seriesparallel to provide the proper breaker operating voltage. Some breaker control circuits are designed for 48 v dc operation, but voltages of 125 and 250 volts dc are recommended, especially if the battery serves a number of breakers. In addition to providing breaker trip power, the battery also provides power to the closing coil for electrical closure. Proper battery performance will be difficult to maintain if the battery is not installed in a manner conducive to long battery life and to the performance of maintenance operations. The location of the battery and its charger should be selected carefully, and the battery system should be installed in accordance with recognized standards. Standards for battery installation and maintenance are available from the Institute of Electrical and Electronics Engineers, American National Standards Institute, Underwriters Laboratories, National Fire Protection Association, and the National Electrical Manufacturers Association. The battery should be located in
a protected room that is clean, dry, well ventilated, and relatively cool. Battery room temperature should be maintained at 72 to 77 F. As a general rule, battery life will be cut in half for every 18 deg rise in temperature. On the other hand, battery capacity will be reduced if the operating environment is too cold. The battery room should be well ventilated, because lead-acid batteries liberate hydrogen gas as a normal part of their charging process; five to six air changes per hour is recommended. Smoking must be prohibited in the battery room, and as a precaution against failure of the ventilating system, electrical equipment should be approved for Class I, Group B, Division II locations. The battery should be located in a sprinklered masonry room that contains no combustibles to minimize exposure to fire. If the battery is in an open area, rather than in a separate room, there should be no combustibles within 25 feet of the battery. Battery cells should be arranged in a manner that will provide easy
access to all cells and that will permit battery heat to be readily dissipated. Three-high battery racks should not be used because the top tier of batteries might be subjected to excessive temperatures that could reduce battery life. Although the battery charger should be located in a clean, dry cool environment comparable to the environment of the battery room, the charger should not be located in the same room as the battery; a number of major fires have been caused by faulty battery chargers. The charger should be selected with care to ensure its compatibility with the battery and should be certified for its intended use by a qualified electrical testing laboraory. Maximum reliability and best battery life are realized if the charger is automatic. The ac supply to the battery charger should be planned for reliability, and precautions should be taken to ensure that it cannot be inadvertently turned off. A means for monitoring the charger ac supply should be provided.
The battery system should be installed with a load-test circuit that permits testing the battery to ensure that proper voltage and current are available for breaker trip-coil operation, and an alarm circuit should be installed to signal if the proper voltage for breaker operation is not available. If breakers are equipped with red (closed) and green (open) indicating lights, the lights should be monitored regularly; an extinguished red light that coincides with an extinguished green light should never be ignored. The red light monitors the continuity of, and the dc supply to, the trip circuit, in addition to signifying that the breaker is closed. An extinguished red indicating light with the breaker closed can mean a discontinuous trip circuit because of a burned-out trip coil or other reasons, loss of dc voltage, or simply a burned-out lamp. A backup supply of dc power for switchgear operation should be considered because the plant is exposed to the possibility of severe losses if a fault should occur in the electrical system when the normal
dc supply is not available. The best backup supply is a redundant battery: as an alternative to a redundant battery, an engine-driven generator can be installed to provide switchgear control power when the normal (battery) supply is not available. The most reliable arrangement is to have several batteries interconnected with transfer switching arrangements. With such a system, selective portions of the power system are served from different batteries, rather than a single battery serving all of the plant’s switchgear. Batteries should be sized to permit each battery to carry its normal load plus the load of other batteries whose load it might be called upon to assume. Whether only one battery or several are used, switchgear batteries should be specified explicitly for switchgear service. Batteries designed for switchgear service are built to deliver very high discharge currents (but only for brief periods), because when a fault occurs, several breakers are likely to trip simultaneously. It is not uncommon, however, to use the switchgear battery to provide dc control power to instrumentation and other equipment in the plant. If this is done, care must be taken not to burden the battery to the point at which it will be unable to perform its primary function. Both the battery and the charger should be serviced on a regular basis; service of a switchgear control power supply system should “ever be relegated to “job of convenience” status. Both battery and charger should be cleaned
Failure of a circuit breaker to trip caused extensive damage to this 93,000 kva. 138/69 kv transformer; repairs to the damaged windings, core, and bushings took 4 months to complete. Failure of the breaker to trip was traced to inadequate maintenance of the switchgear dc control power supply system.
“The integrity of the ac supply to the charger should be checked periodically.”
periodically, and battery terminals and intercell connections should be checked for tightness when the battery is cleaned. Battery posts, studs, and cable ends should be cleaned with a solution of bicarbonate of soda and left with a liberal coating of antioxidant grease to retard corrosion. The charger should be kept free of dirt and dust to permit proper heat transfer. The preferred method of cleaning is to use a soft-
bristled brush in conjunction with low-pressure compressed air to loosen dust and dirt deposits and then to remove the dislodged contaminants with a vacuum cleaner. The integrity of the ac supply to the charger should be checked periodically to ensure that fuses are not blown or the breaker is not tripped. If the charger is served through fuses, fuses and fuse clips should be inspected to see if there is any sign of overheating.
Batteries must be inspected regularly for proper electrolyte level, and hydrometer readings should be taken regularly to ensure that the electrolyte specific gravity is at the recommended value for the charged state. The hydrometer readings must be correlated with temperature according to the battery manufacurer’s instruction manual, because specific gravity varies with temperature for a given state of charge. The charge rate should be set at a value that will maintain the battery at full charge without overcharging; the charge rate might require adjustment from time to time. Overcharging will cause liberation of excessive amounts of hydrogen, increasing water consumption, generating high temperatures that reduce battery life, and increasing explosion potential because of increased hydrogen liberation. Undercharging, on the other hand, leads to a condition known as “sulfation,” in which the active plate area of the battery is reduced, thereby reducing battery capacity. A rubber apron, gloves, boots, and eye protection should be used when batteries are serviced. As an added precaution against electrolyte spillage, and to prevent contaminants from entering the battery, vent plugs should be left in place unless hydrometer readings are being taken or water is being added to the cell. Smoking and open flames must not be permitted in the battery room, and precautions must be taken to ensure that no sparks will be produced in the room. Before workers enter the battery room, tool pouches should be removed, and tools should be removed from trouser pockets to prevent metal objects from contacting battery posts and causing sparks. Metal tools should never be rested on batteries.
