Substations - McDonald, John D.pdf (Electric Power Generation)

McDonald, John D. "Substations" The Electric Power Engineering Handbook Ed. L.L. Grigsby Boca Raton: CRC Press LLC, 2001 5 Substations John D. McDonald KEMA Consulting 5.1 Gas Insulated Substations Philip Bolin 5.2 Air Insulated Substations — Bus/Switching Conﬁgurations Michael J. Bio 5.3 High-Voltage Switching Equipment David L. Harris 5.4 High-Voltage Power Electronics Substations Gerhard Juette 5.5 Considerations in Applying Automation Systems to Electric Utility Substations James W. Evans 5.6 Substation Automation John D. McDonald 5.7 Oil Containment Anne-Marie Sahazizian and Tibor Kertesz 5.8 Community Considerations James H. Sosinski 5.9 Animal Deterrents/Security C.M. Mike Stine and Sheila Frasier 5.10 Substation Grounding Richard P. Keil 5.11 Grounding and Lightning Robert S. Nowell 5.12 Seismic Considerations R.P. Stewart, Rulon Fronk, and Tonia Jurbin 5.13 Substation Fire Protection © 2001 CRC Press LLC Al Bolger and Don Delcourt 5 Philip Bolin Substations Mitsubishi Electric Power Products, Inc. Michael J. Bio Power Resources, Inc. David L. Harris 5.1 Gas Insulated Substations 5.2 Air Insulated Substations — Bus/Switching Conﬁgurations SF6 • Construction and Service Life • Economics of GIS Waukesha Electric Systems Gerhard Juette Siemens Single Bus • Double Bus, Double Breaker • Main and Transfer Bus • Double Bus, Single Breaker • Ring Bus • Breaker-and-a-Half • Comparison of Conﬁgurations James W. Evans Detroit Edison Company John D. McDonald 5.3 Ambient Conditions • Disconnect Switches • Load Break Switches • High-Speed Grounding Switchers • Power Fuses • Circuit Switchers • Circuit Breakers • GIS Substations • Environmental Concerns KEMA Consulting Anne-Marie Sahazizian Hydro One Networks, Inc. 5.4 High-Voltage Power Electronics Substations Types • Control • Losses and Cooling • Buildings • Interference • Reliability • Speciﬁcations • Training and Commissioning • The Future Tibor Kertesz Hydro One Networks, Inc. James H. Sosinski High-Voltage Switching Equipment Consumers Energy Considerations in Applying Automation Systems to Electric Utility Substations C. M. Mike Stine Physical Considerations • Analog Data Acquisition • Status Monitoring • Control Functions Raychem Corporation 5.5 5.6 Southern Engineering Richard P. Keil Dayton Power & Light Company 5.7 Georgia Power Company 5.8 Fronk Consulting Tonia Jurbin BC Hydro Al Bolger BC Hydro Don Delcourt BC Hydro © 2001 CRC Press LLC Community Considerations Community Acceptance • Planning Strategies and Design • Permitting Process • Construction • Operations BC Hydro Rulon Fronk Oil Containment Oil-Filled Equipment in Substation • Spill Risk Assessment • Containment Selection Consideration • Oil Spill Prevention Techniques Robert S. Nowell Robert P. Stewart Substation Automation Deﬁnitions and Terminology • Open Systems • Substation Automation Technical Issues • IEEE Power Engineering Society Substations Committee • EPRI-Sponsored Utility Substation Communication Initiative Sheila Frasier 5.9 Animal Deterrents/Security Animal Types • Mitigation Methods 5.10 Substation Grounding Accidental Ground Circuit • Permissible Body Current Limits • Tolerable Voltages • Design Criteria 5.11 Grounding and Lightning Lightning Stroke Protection • Lightning Parameters • Empirical Design Methods • The Electromagnetic Model • Calculation of Failure Probability • Active Lightning Terminals 5.12 Seismic Considerations A Historical Perspective • Relationship Between Earthquakes and Substations • Applicable Documents • Decision Process for Seismic Design Consideration • Performance Levels and Desired Spectra • Qualiﬁcation Process 5.13 Substation Fire Protection Fire Hazards • Fire Protection Measures • Hazard Assessment • Risk Analysis • Conclusion 5.1 Gas Insulated Substations Philip Bolin A gas insulated substation (GIS) uses a superior dielectric gas, SF6, at moderate pressure for phase-tophase and phase-to-ground insulation. The high voltage conductors, circuit breaker interrupters, switches, current transformers, and voltage transformers are in SF6 gas inside grounded metal enclosures. The atmospheric air insulation used in a conventional, air insulated substation (AIS) requires meters of air insulation to do what SF6 can do in centimeters. GIS can therefore be smaller than AIS by up to a factor of ten. A GIS is mostly used where space is expensive or not available. In a GIS the active parts are protected from the deterioration from exposure to atmospheric air, moisture, contamination, etc. As a result, GIS is more reliable and requires less maintenance than AIS. GIS was ﬁrst developed in various countries between 1968 and 1972. After about 5 years of experience, the use rate increased to about 20% of new substations in countries where space is limited. In other countries with space easily available, the higher cost of GIS relative to AIS has limited use to special cases. For example, in the U.S., only about 2% of new substations are GIS. International experience with GIS is described in a series of CIGRE papers (CIGRE, 1992; 1994; 1982). The IEEE (IEEE Std. C37. 122-1993; IEEE Std C37. 122.1-1993) and the IEC (IEC, 1990) have standards covering all aspects of the design, testing, and use of GIS. For the new user, there is a CIGRE application guide (Katchinski et al., 1998). IEEE has a guide for speciﬁcations for GIS (IEEE Std. C37.123-1996). SF6 Sulfur hexaﬂouride is an inert, non-toxic, colorless, odorless, tasteless, and non-ﬂammable gas consisting of a sulfur atom surrounded by and tightly bonded to six ﬂourine atoms. It is about ﬁve times as dense as air. SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences. SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about one hundred times better than air for interrupting arcs. It is the universally used interrupting medium for high voltage circuit breakers, replacing the older mediums of oil and air. SF6 decomposes in the high temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in GIS. There are some reactive decomposition byproducts formed because of the trace presence of moisture, air, and other contaminants. The quantities formed are very small. Molecular sieve absorbants inside the GIS enclosure eliminate these reactive byproducts. SF6 is supplied in 50-kg gas cylinders in a liquid state at a pressure of about 6000 kPa for convenient storage and transport. Gas handling systems with ﬁlters, compressors, and vacuum pumps are commercially available. Best practices and the personnel safety aspects of SF6 gas handling are covered in international standards (IEC, 1995). The SF6 in the equipment must be dry enough to avoid condensation of moisture as a liquid on the surfaces of the solid epoxy support insulators because liquid water on the surface can cause a dielectric breakdown. However, if the moisture condenses as ice, the breakdown voltage is not affected. So dew points in the gas in the equipment need to be below about –10°C. For additional margin, levels of less than 1000 ppmv of moisture are usually speciﬁed and easy to obtain with careful gas handling. Absorbants © 2001 CRC Press LLC inside the GIS enclosure help keep the moisture level in the gas low, even though over time, moisture will evolve from the internal surfaces and out of the solid dielectric materials (IEEE Std. 1125-1993). Small conducting particles of mm size signiﬁcantly reduce the dielectric strength of SF6 gas. This effect becomes greater as the pressure is raised past about 600 kPa absolute (Cookson and Farish, 1973). The particles are moved by the electric ﬁeld, possibly to the higher ﬁeld regions inside the equipment or deposited along the surface of the solid epoxy support insulators, leading to dielectric breakdown at operating voltage levels. Cleanliness in assembly is therefore very important for GIS. Fortunately, during the factory and ﬁeld power frequency high voltage tests, contaminating particles can be detected as they move and cause small electric discharges (partial discharge) and acoustic signals, so they can be removed by opening the equipment. Some GIS equipment is provided with internal “particle traps” that capture the particles before they move to a location where they might cause breakdown. Most GIS assemblies are of a shape that provides some “natural” low electric ﬁeld regions where particles can rest without causing problems. SF6 is a strong greenhouse gas that could contribute to global warming. At an international treaty conference in Kyoto in 1997, SF6 was listed as one of the six greenhouse gases whose emissions should be reduced. SF6 is a very minor contributor to the total amount of greenhouse gases due to human activity, but it has a very long life in the atmosphere (half-life is estimated at 3200 years), so the effect of SF6 released to the atmosphere is effectively cumulative and permanent. The major use of SF6 is in electrical power equipment. Fortunately, in GIS the SF6 is contained and can be recycled. By following the present international guidelines for use of SF6 in electrical equipment (Mauthe et al., 1997), the contribution of SF6 to global warming can be kept to less than 0.1% over a 100-year horizon. The emission rate from use in electrical equipment has been reduced over the last three years. Most of this effect has been due to simply adopting better handling and recycling practices. Standards now require GIS to leak less than 1% per year. The leakage rate is normally much lower. Field checks of GIS in service for many years indicate that the leak rate objective can be as low as 0.1% per year when GIS standards are revised. Construction and Service Life GIS is assembled of standard equipment modules (circuit breaker, current transformers, voltage transformers, disconnect and ground switches, interconnecting bus, surge arresters, and connections to the rest of the electric power system) to match the electrical one-line diagram of the substation. A crosssection view of a 242-kV GIS shows the construction and typical dimensions (Fig. 5.1). The modules are joined using bolted ﬂanges with an “O” ring seal system for the enclosure and a sliding plug-in contact for the conductor. Internal parts of the GIS are supported by cast epoxy insulators. These support insulators provide a gas barrier between parts of the GIS, or are cast with holes in the epoxy to allow gas to pass from one side to the other. Up to about 170 kV system voltage, all three phases are often in one enclosure (Fig. 5.2). Above 170 kV, the size of the enclosure for “three-phase enclosure,” GIS becomes too large to be practical. So a “singlephase enclosure” design (Fig. 5.1) is used. There are no established performance differences between three-phase enclosure and single-phase enclosure GIS. Some manufacturers use the single-phase enclosure type for all voltage levels. Enclosures today are mostly cast or welded aluminum, but steel is also used. Steel enclosures are painted inside and outside to prevent rusting. Aluminum enclosures do not need to be painted, but may be painted for ease of cleaning and a better appearance. The pressure vessel requirements for GIS enclosures are set by GIS standards (IEEE Std. C37.122-1993; IEC, 1990), with the actual design, manufacture, and test following an established pressure vessel standard of the country of manufacture. Because of the moderate pressures involved, and the classiﬁcation of GIS as electrical equipment, third-party inspection and code stamping of the GIS enclosures are not required. Conductors today are mostly aluminum. Copper is sometimes used. It is usual to silver plate surfaces that transfer current. Bolted joints and sliding electrical contacts are used to join conductor sections. There are many designs for the sliding contact element. In general, sliding contacts have many individually © 2001 CRC Press LLC © 2001 CRC Press LLC FIGURE 5.1 Single-phase eclosure GIS. FIGURE 5.2 Three-phase enclosure GIS. sprung copper contact ﬁngers working in parallel. Usually the contact ﬁngers are silver plated. A contact lubricant is used to ensure that the sliding contact surfaces do not generate particles or wear out over time. The sliding conductor contacts make assembly of the modules easy and also allow for conductor movement to accommodate the differential thermal expansion of the conductor relative to the enclosure. Sliding contact assemblies are also used in circuit breakers and switches to transfer current from the moving contact to the stationary contacts. Support insulators are made of a highly ﬁlled epoxy resin cast very carefully to prevent formation of voids and/or cracks during curing. Each GIS manufacturer’s material formulation and insulator shape has been developed to optimize the support insulator in terms of electric ﬁeld distribution, mechanical strength, resistance to surface electric discharges, and convenience of manufacture and assembly. Post, disc, and cone type support insulators are used. Quality assurance programs for support insulators include a high voltage power frequency withstand test with sensitive partial discharge monitoring. Experience has shown that the electric ﬁeld stress inside the cast epoxy insulator should be below a certain level to avoid aging of the solid dielectric material. The electrical stress limit for the cast epoxy support insulator is not a severe design constraint because the dimensions of the GIS are mainly set by the lightning impulse withstand level and the need for the conductor to have a fairly large diameter to carry to load current of several thousand amperes. The result is space between the conductor and enclosure for support insulators having low electrical stress. Service life of GIS using the construction described above has been shown by experience to be more than 30 years. The condition of GIS examined after many years in service does not indicate any approaching limit in service life. Experience also shows no need for periodic internal inspection or maintenance. Inside the enclosure is a dry, inert gas that is itself not subject to aging. There is no exposure of any of the internal materials to sunlight. Even the “O” ring seals are found to be in excellent condition because there is almost always a “double seal” system — Fig. 5.3 shows one approach. The lack of aging has been found for GIS, whether installed indoors or outdoors. Circuit Breaker GIS uses essentially the same dead tank SF6 puffer circuit breakers used in AIS. Instead of SF6-to-air as connections into the substation as a whole, the nozzles on the circuit breaker enclosure are directly connected to the adjacent GIS module. FIGURE 5.3 © 2001 CRC Press LLC Gas seal for GIS enclosure. FIGURE 5.4 Current transformers for GIS. Current Transformers CTs are inductive ring type installed either inside the GIS enclosure or outside the GIS enclosure (Fig. 5.4). The GIS conductor is the single turn primary for the CT. CTs inside the enclosure must be shielded from the electric ﬁeld produced by the high voltage conductor or high transient voltages can appear on the secondary through capacitive coupling. For CTs outside the enclosure, the enclosure itself must be provided with an insulating joint, and enclosure currents shunted around the CT. Both types of construction are in wide use. Voltage Transformers VTs are inductive type with an iron core. The primary winding is supported on an insulating plastic ﬁlm immersed in SF6. The VT should have an electric ﬁeld shield between the primary and secondary windings to prevent capacitive coupling of transient voltages. The VT is usually a sealed unit with a gas barrier insulator. The VT is either easily removable so the GIS can be high voltage tested without damaging the VT, or the VT is provided with a disconnect switch or removable link (Fig. 5.5). FIGURE 5.5 © 2001 CRC Press LLC Voltage transformers for GIS. FIGURE 5.6 Disconnect switches for GIS. Disconnect Switches Disconnect switches (Fig. 5.6) have a moving contact that opens or closes a gap between stationary contacts when activated by a insulating operating rod that is itself moved by a sealed shaft coming through the enclosure wall. The stationary contacts have shields that provide the appropriate electric ﬁeld distribution to avoid too high a surface stress. The moving contact velocity is relatively low (compared to a circuit breaker moving contact) and the disconnect switch can interrupt only low levels of capacitive current (for example, disconnecting a section of GIS bus) or small inductive currents (for example, transformer magnetizing current). Load break disconnect switches have been furnished in the past, but with improvements and cost reductions of circuit breakers, it is not practical to continue to furnish load break disconnect switches, and a circuit breaker should be used instead. Ground Switches Ground switches (Fig. 5.7) have a moving contact that opens or closes a gap between the high voltage conductor and the enclosure. Sliding contacts with appropriate electric ﬁeld shields are provided at the enclosure and the conductor. A “maintenance” ground switch is operated either manually or by motor drive to close or open in several seconds and when fully closed to carry the rated short-circuit current for the speciﬁed time period (1 or 3 sec) without damage. A “fast acting” ground switch has a high speed drive, usually a spring, and contact materials that withstand arcing so it can be closed twice onto an energized conductor without signiﬁcant damage to itself or adjacent parts. Fast-acting ground switches are frequently used at the connection point of the GIS to the rest of the electric power network, not only in case the connected line is energized, but also because the fast-acting ground switch is better able to handle discharge of trapped charge and breaking of capacitive or inductive coupled currents on the connected line. Ground switches are almost always provided with an insulating mount or an insulating bushing for the ground connection. In normal operation the insulating element is bypassed with a bolted shunt to the GIS enclosure. During installation or maintenance, with the ground switch closed, the shunt can be removed and the ground switch used as a connection from test equipment to the GIS conductor. Voltage © 2001 CRC Press LLC FIGURE 5.7 Ground switches for GIS. and current testing of the internal parts of the GIS can then be done without removing SF6 gas or opening the enclosure. A typical test is measurement of contact resistance using two ground switches (Fig. 5.8). Bus To connect GIS modules that are not directly connected to each other, an SF6 bus consisting of an inner conductor and outer enclosure is used. Support insulators, sliding electrical contacts, and ﬂanged enclosure joints are usually the same as for the GIS modules. Air Connection SF6-to-air bushings (Fig. 5.9) are made by attaching a hollow insulating cylinder to a ﬂange on the end of a GIS enclosure. The insulating cylinder contains pressurized SF6 on the inside and is suitable for exposure to atmospheric air on the outside. The conductor continues up through the center of the insulating cylinder to a metal end plate. The outside of the end plate has provisions for bolting to an air © 2001 CRC Press LLC

Substations - McDonald, John D

Nội dung