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witchgear covers switching and interrupting devices and their combination with control, metering, pro- tection, and regulating devices and also covers the assembly of these devices with associated interconnections, accesso- ries, enclosures, and supporting structures. Switchgear is used primarily in...

witchgear covers switching and interrupting devices and their combination with control, metering, pro- tection, and regulating devices and also covers the assembly of these devices with associated interconnections, accesso- ries, enclosures, and supporting structures. Switchgear is used primarily in connection with the generation, transmis- sion, distribution, and conversion of electric power. Applica- tions include controlling circuits serving generators, large motors, transformers, power circuit feeders, and other large electrical equipment. 4.1 0.2 Medium-Voltage Switchgear In general, the 5-kv to 15-kv medum-voltage switchgear used in facilities is the metal-clad type with drawout circuit breakers and all pertinent auxiliaries contained within their own individual enclosures. Switchgear above the 15-kV class may be the metal-clad or stationary type. Other equipment installed in the switchgear is necessary buses, disconnecting devices, current and voltage transformers, control power transformers, interIocks, meters, relays, and control devices. The switchgear is generally in the 5-kv to 38-kv class with current ratings up to 3,000 amperes, and interrupting classifi- cations ranging from 250 MVA to 1,500 MVA. 4.10.3 Low-Voltage Switchgear Switchgear rated at 600 V is available for small loads that cannot be served economically at 5 kv and above. The pre- ferred construction is metal-enclosed, using air-break or vac- uum, drawout-type low-voltage power circuit breakers. Continuous current ratings are available to 6,000 amps, and interrupting current ratings range from 30,000 amps to 200,000 amps. Integral current limiting fuse devices are used to achieve higher interrupting duties. 4.10.4 Interrupting Medium There are several options to consider when selecting the interrupting medium for medium-voltage switchgear. Air- break and oil-immersed circuit breakers are rapidly being phased out by vacuum and SF6 circuit breakers. An assess- ment of the circuit breaker installed cost, operating charac- teristics, and maintenance requirements is required and must be evaluated to determine which type should be applied. Low-voltage switchgear is most commonly either air-break or vacuum-break. Where an interrupting medium is considered for an appli- cation for the first time, interrupting characteristics should be reviewed at rated .conditions. Maintenance procedure details should be fully understood prior to the selection. 4.10.5 Location Manufacturers do not list switchgear as suitable for use in a classified location. In practice, switchgear that serves process units must be either located adjacent to the classified location or installed in a pressurized room. Processing plant switchgear frequently takes the form of a unit substation, which consists of a transformer (single- ended) or transformers (double-ended) that supply utilization voltage to a group of feeder circuit breakers. Unit substations may be purchased as neat, compact units well adapted for either indoor or outdoor plant use. Transformers and switch- gear may be purchased separately and installed indoors or outdoors as desired. 4.10.6 Installation Types 4.10.6.1 Indoor Switchgear with the time interval between inspections varying with environment and service. The range of intervals is usually 1 to 5 years, with experience with the particular installations dictating any changes to the schedule. Preventive mainte- nance should include all tasks necessary to assure the reli- able operation of the switchgear during the maintenance interval. This maintenance should include inspecting the overload unit settings and other breaker parts, such as con- tacts and arc chutes for air circuit breakers; or vacuum inter- rupter, vacuum integrity, and contact erosion indicators on vacuum circuit breakers. Maintenance also includes check- ing the trip devices by a test set available from the equip- ment manufacturer. 4.1 1 TRANSFORMERS 4.1 1.1 General Indoor switchgear (NEMA 1 or NEMA 12 enclosure) is This information is confined primarily to distribution and not as expensive as outdoor switchgear (NEMA 3R Or 4x power transformers. Other types of transformers which are enclosure); however, the former requires indoor space which applied within the petroleum industry are mentioned briefly, affects the overall cost. The cost of providing the indoor but these other types usually operate as of an electrical location may be offset by the reduced maintenance and equipmentpackage. equipment costs and by the increased reliability resulting from a more benign equipment environment. A common 4-11 .2 Transformer Types location is often used to house both switchgear and motor control equipment. 4.1 1.2.1 Distribution and Power Transformers 4.10.6.2 Outdoor Switchgear Outdoor switchgear is basically indoor switchgear mounted in a weatherproof enclosure. The following types of enclosures are available: a. Enclosure without an aisle. b. Enclosure with an aisle in front of the switchgear. c. Enclosure with a common aisle between two switchgear lineups. An aisle facilitates the maintenance and operation of the switchgear. 4.10.6.3 Electrical Power Centers The use of prefabricated electrical power centers is appro- priate for some applications where work at the site is to be minimized. These prefabricated electrical power centers are modular structures equipped with lighting, heating, and venti- lating equipment. Requiring only assembly and internal con- nections on site, they can be shipped as a unit or in modular sections with switchgear, motor control centers, and other equipment already installed. 4.10.7 Preventive Maintenance Preventive maintenance, including inspection and testing of switchgear, should be carried out on a regular schedule, Distribution and power transformers are used to isolate dif- ferent voltage systems from each other and to reduce or increase voltages to their optimum utilization levels. These transformers may be integral parts of unit substations and motor control centers, or they may be located at a remote site. Unit substation transformers are mechanically and electri- cally connected to unit substation equipment or motor control centers. Aside from the physical size and certain features of construction, unit substation transformers are applied in the same manner and for the same purposes as distribution and power transformers. Power transformers are frequently used to step-down plant distribution voltage to motor utilization levels (e.g., 13.8 kv to 4,160 V or 6,600 V). Often, a captive transformer is used to supply a single large motor, usually greater than or equal to 2,500 HP. The added impedance of the captive transformer in the motor supply circuit lowers voltage and starting in-rush current. The captive transformer should be designed for the required motor starting and operating duty. The captive trans- former-motor combination may be selected over the direct- connected motor for reason of design, system stability, or motor economics. Step-up power transformer or transformedrectifier sets are often used for desalting and precipitation processes where the plant voltage must be increased to the level required at the desalter or precipitator electrodes. 4.1 1.2.2 Instrument Transformers Instrument transformers are used for metering and relay- ing, have a high degree of accuracy, and have limited capac- ity. The accuracy of transformation depends on the application because meteling and relaying require different accuracies. The degree of accuracy is also subject to the effects of load and fault current. Voltage transformers are employed to step down primary voltage to a secondary voltage, normally 120 V, at the rated primary voltage. Current transformers are employed to trans- form primary current to a secondary current, normally 5 amps, at the rated primary current. For some applications, current transformers with multiratio p’imary taps are used. 4.1 1.2.3 Autotransformers The autotransformer is a single-winding transformer in which the lower voltage is obtained by a tap position between the line terminals. Unlike a two-winding distribution or power transformer, a single-winding transformer does not isolate the high-voltage and low-voltage windings. Autotransformers are frequently used to provide an eco- nomical tie between two systems of different voltage levels (e.g., a 4,160-V to a 2400-V system and a 138-kv to a 69-kv system). They are also used for motor control in some types of reduced voltage starter packages. 4.1 1.2.4 Other Transformers Other specialty transformers are zigzag grounding, con- stant voltage, and low-noise isolation transformers. Zigzag grounding transformers are used to derive a neutral for sys- tem grounding purposes and can be used to provide a ground connection for delta-connected transformer second- aries. They permit ground-fault relaying and eliminate high transient voltages that can ,occur on ungrounded systems. Constant-voltage transformers provide a stable power sup- ply for instrumentation and other loads requiring a constant voltage. Low-noise isolation transformers are used to supply power to digital-based systems, such as computers, that are highly susceptible to voltage transients. Transformers are frequently applied to provide isolation for the input to adjustable speed drives. A three-winding transformer (a sin- gle primary with a wye- and a delta-connected secondary) can be used to reduce power system harmonics through har- monic current cancellation. 4.1 1.3 Ratings 4.1 1.3.1 Voltage and Frequency The voltage rating for transformers is determined primarily by the system voltage available and the utilization voltage required. For 60-Hz electric power systems, it is recom- mended that the voltage rating conform to one of the voltage ratings given in ANSI (34.1. Consideration in the selection of these voltage ratings could result in procurement and maintenance economies due to the ability to parallel and interchange transformers. Atten- tion to voltage tap ratings will permit added flexibility in matching transformers to system voltages. Due to a worldwide lack of standardization of AC system frequency, the transformer frequency should always be specified. 4.1 1.3.2 Capacity and Duty The recommended kilovolt-ampere (kVA) ratings of trans- formers are given in ANSI or NEMA standards. These ratings should be on a continuous basis without exceeding the tem- perature limitations for continuous-rated transformers. Cap- tive transformer design should take into account the magnitude of starting current, the duration of motor accelera- tion, and the permissible starting frequency of the motor. 4.1 1.3.3 Temperature Rise The rated kVA of a transformer is the load which can be carried continuously at rated voltage and frequency without exceeding the specified temperature rise. A transformer . . should have a normal life at its rated kVA if the specified tem- perature rise is not exceeded, the ambient temperature does not peak above 40°C (104“F), and the ambient temperature does not rise above 30°C (86°F) for a 24-hour average. Oil-filled transformers are limited to a winding tempera- ture rise, as measured by resistance, of 65°C (1 17°F) or 55”C/ 65°C (99“F/117”F) and a hottest-spot winding temperature rise of 80°C (144°F). Dry-type transformers are divided into the following temperature rise specifications: a. Class 150 C has Class B winding insulation and is lim- ited to an average rise of 80°C (144°F) with a hot spot of 110°C (198°F). b. Class 185 C has Class F winding insulation and is limited to an average rise of 115°C (207°F) with a hot spot of 145°C (261°F). c. Class 220 C has Class H winding insulation and is limited to an average rise of 150°C (270°F) with a hot spot of 180°C (324°F). More detailed information on temperature rise specifica- tions is contained in IEEE C57.91 and IEEE C57.96. If operated outside of these temperature limitations, the transformer must be rerated on the basis of the actual load cycle and ambient temperature to attain its normal life expect- ancy. The transformer manufacturer should be consulted for these figures. As a rule of thumb, insulation life is cut in half for each 10°C rise in operating temperature. When load is applied to a transformer, the heating and cooling curves vary exponentially. The time constant for the windings is 5 to 10 minutes and for the oil is 2 to 4 hours. A time equal to approximately five time constants is required for the items to reach their ultimate temperature. Short-time overloads of 1 hour or less are permissible, however, as long as the hottest spot copper temperature does not exceed 150°C (302°F) for an oil-filled transformer. Overloads of more than one-hour in duration should be avoided. Transformers operated to altitudes greater than 1,000 m (3,300 ft) above sea level are subject to special rating correc- tion factors which may be obtained from the manufacturer. 4.1 1.3.4 Insulation The basic impulse level @IL), which indicates a trans- former’s ability to withstand transient over-voltages, and the applicable manufacturer’s test voltages are given in IEEE C57.12.00 for liquid-filled transformers, and IEEE C57.12.01 for dry-type transformers. The dielectric strength of transform- ers that depend on air for insulation decreases as altitude increases. Insulation-class correction factors for altitudes greater than 1,000 m (3,300 feet) are covered in IEEE C57.12.01. 4.11.3.5 Efficiency and Regulation Efficiency and regulation are fixed by the manufacturer’s design, although more efficient designs are available at higher cost, if the loss evaluation warrants them. 4.1 1.3.6 Impedance The impedance is expressed as a percentage of the trans- former base kilovolt-ampere rating and is determined by the internal characteristics of the transformer, which include its core design, resistance, and geometry of windings. The manu- facturer’s standard impedance, in accordance with ANSI stan- dards, is normally acceptable to facilitate parallel operation and minimize cost. In some instances, it may be desirable to install a transformer with greater-than-standard impedance to limit the short-circuit duty on secondary switchgear. In other instances, a transformer with lower-than-standard impedance is used to facilitate motor starting by reducing the voltage drop. 4.1 1.4 Applications 4.1 1.4.1 Location Transformers and associated secondary switchgear should be located as near to their load centers as practical while mini- mizing exposure to fire and mechanical damage. The location should preferably be unclassified. In cases where the trans- former must be in a classified location, all auxiliary devices associated with the transformer must be suitable for the classi- fication. For Class I, Division 2, or Zone 2 locations, it is some- times practical to locate the transformer outside a pressurized switchgear room with a secondary throat connection for a busway supply through the wall of the room to the switchgear. 4.1 1.4.2 Grounding Neutral grounding of transformer secondaries should be considered. The type of grounding chosen is based on factors such as voltage levels, ground-fault levels, and continuity of service. The neutral ground is obtained by bringing out the neutral connection on a wye-connected secondary or by using a zigzag transformer on a delta-connected secondary. The neutral is either solidly grounded or grounded through resis- tance or reactance. 4.1 1.4.3 Parallel Operation Proper operation of parallel transformers requires that the transfo1mers be connected properly and that their characteris- tics be within certain tolerances-refer to IEEE C57.12.00 and IEEE C57.12.01 for acceptable tolerances for parallel operation. To divide the connected load according to the rat- ing of the parallel banks, the following must be the same: the internal impedance, the transformation ratio, and the phase relationship. It is not possible to parallel delta-wye or wye- delta banks with a delta-delta bank because of the 30” phase shift that is present in the secondary. 4.1 1.4.4 Testing and Maintenance A systematic testing and maintenance program should be established for transformers. It should include the inspection and cleaning of bushings, the testing and gas analysis of trans- former oil, and the vacuum cleaning of dry-type transformers. The manufacturer’s test report for each transformer should be kept on record. This report contains results of dielectric tests and measurements of resistance, excitation current, impedance, ratio, temperature rise, polarity, and phase relation. The dielec- tric strength of new transformer oil should not be less than 30 kv when measured in accordance with ASTM D877. Refer to IEEE C57.106 for mineral oil testing, to BEE C57.111 for silicone fluid testing, and to IEW C57.121 for Less Flammable Hydrocarbon Huid testing. When dielectric testing of transformer windings is per- formed, the test parameter limitations set forth in the stan- dards should not be exceeded (see 4.1 1.3.4). Dielectric testing of bushings is also covered by standards. High-voltage DC test equipment is available to provide non- destructive and accurate testing of insulation. This method of testing is preferable to high-voltage AC test procedures. Power factor tests are also used to indicate the condition of transformer insulation. When a program of power factor test- ing is planned, the transformer factory testing should include a power factor test so that the results will be available for comparison with later field testing. 4.1 1.5 Construction and Accessories 4.1 1.5.1 Oil-Filled Transformers Transformer oil is used to insulate and cool the windings and to protect the core and windings from corrosive and hazardous vapors. Transformer oil should include a suitable oxygen inhibitor to prevent deterioration of the dielectric. The sealed-tank system is standard for transformers rated 2,500 kVA, 200-kv basic impulse level and less, and is often used on larger sizes as well. Inert-gas-pressurized sealed tanks are sometimes provided on larger or critical- service transformers. Standard accessories for oil-filled transformers include a no-load tap changer, a ground pad, a nameplate, a liquid- level gauge, an oil temperature indicator, a drain valve, a top filter valve, a pressure-vacuum gauge, and jack bosses. Optional features pertain to the type of bushings, fan con- trols, winding temperature indication, sudden pressure relay, terminal blocks, junction boxes, disconnect switches, and throat connections. Where applicable, terminal cham- bers must allow adequate space for stress-relief terminations on shielded cable. Current and voltage transformers, to serve metering and relaying, are often provided in special, separate termination chambers. Multi-ratio current trans- formers are often located within the transformer tank. Con- sideration should be given to a hottest-spot temperature detector where the system operation may subject the trans- former to emergency loading conditions (e.g., where auto- matic bus transfer between two transformers is provided). Transformer gauges can be provided with alarm contacts to allow remote annunciation of transformer problems. All transformer accessories must be suitable for the area classi- fication where the transformer is to be installed. 4.1 1.5.2 Transformer Fluid 4.1 1.5.2.1 Mineral Oil-Filled Transformers Regulations require users of transformers containing poly- chlorinated biphenyls (PCBs) to maintain specific records, to comply with specified procedures in case of leakage and for disposal, and to fulfill other requirements. Users of PCB-filled or PCB-contaminated oil-filled transformers should consult applicable federal and state regulations. Transformers contain- ing PCBs are no longer manufactured because of federal envi- ronmental and health regulations (see 40 CFR Part 761.). 4.1 1.5.2.2 Less-Flammable Hydrocarbon Fluid and Silicon Insulating Fluid-Filled Transformers Transformers with less-flammable hydrocarbon fluid and silicon insulating fluid insulation media, are available for use where mineral oil-filled transformers would constitute a fire hazard, and are substitutes for the PCB-type transform- ers. The less-flammable, hydrocarbon type has several spe- cific restrictions on indoor use; while the silicon insulating fluid-type may be used indoors with only the same vault requirement for ratings over 35 kV that applies for PCB- filled transformers (see NFPA 70). Silicon insulating fluid is generally not used above 35 kv. 4.1 1.5.3 Dry-TypeTransformers Ventilated dry-type transformers, as distinguished from sealed dry-type transformers, are used for indoor locations. Weatherproof units are available for outdoor use, primarily for lighting services where the primary voltage is usually 480 V. They are lightweight compared to oil-filled trans- formers, making them more economical to install. Only minimum maintenance, including a periodic cleaning of the windings, is required. Ventilated dry-type transformers have several disadvan- tages, compared with liquid- or fluid-filled transformers: a. They have a lower standard basic impulse level. b. They lack an overload rating. c. Their use may result in a higher noise level. d. Their windings are more exposed to the environment. I. Surge capacitors and arresters can be installed to compen- , sate for the lower basic impulse level, and forced-air cooling equipment can be used to increase transformer capacity. Where the environment presents corrosive vapors, the trans- formers can be obtained as completely sealed, nitrogen-pres- surized units. 4.1 1.5.4 Cast-Coil Transformers Cast-coil transformers are fabricated with a solid dielectric completely encapsulating the primary and secondary coils, which are mounted in a suitable, ventilated enclosure. This results in a transformer that is contaminant and moisture resistant, has low maintenance, and has impulse ratings com- parable to other dry-type transformers. 4.1 1.5.5 Transformer Taps Transformers should be provided with fully rated kVA taps suitable for tap changing under no-load conditions. Tap changers are designed in uniform 2.5% or 5% steps above and below rated voltage. The number of taps above and below rated voltage and their magnitude will depend on individual requirements. Two 2.5% taps above and below rated voltage are often specified. Large power transformers may be equipped with auto- matic, fully rated kilovolt-ampere taps suitable for adjusting voltage under full-load conditions. 4.1 1.5.6 Forced-Air Cooling The kilovolt-ampere rating of the transformer is deter- mined by the temperature rating of the winding insulation. One means of increasing transformer capacity is to keep the winding ins