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The electrical system of a typical oil field consists of power generation, power distribution, electric motors, system protection, and electrical grounding. The power is either generated on site or purchased from a local utility company. To ensure continuous production from an oil field, it is of utmost importance that the associated electrical systems be designed adequately.
Electrical codes and standards
Various organizations in the U.S. and other countries have developed many electrical codes and standards that are accepted by industry and governmental bodies throughout the world. These codes and standards provide guidelines or rules for design and installation of electrical systems. Table 1  lists some of the major local and international codes and standards used in the oil field.
In addition to the voltage drop caused by load current, a voltage drop during the starting of a large induction motor also must be calculated. Large induction motors and industrial synchronous motors draw several times full-load current from their power supply under full voltage across the line starting. The starting power factor ranges from 0.15 to 0.50 lagging, which causes an inrush current as high as 6 to 7 times the full-load current of the motor. This large current flowing through motor impedance, cable impedance, and all other impedances between the supply and the motor causes a significant voltage drop. Undesirable effects of this voltage drop include dimming lights or lamp flicker, control relay or contactor dropout (de-energizing), and inability to start motor.
Motor starting voltage drop (off a transformer)
Determining the percent voltage drop (ΔE) on a motor fed by a transformer bank, which is fed by an infinite utility bus, requires knowing the transformer impedance (Z), the three-phase impedance of the cable between the transformer and the motor (Zc), and the motor-starting impedance (Zm). The approximate formula to determine the percent voltage drop is:
where Zt = total impedance, in Ω, given as:
where Ztr = transformer impedance, %; Pt = transformer-rated kVA; and Et = transformer voltage, kV. For Eq. 4, the cable impedence is calculated as:
where R = cable resistance, Ω; X = cable three-phase reactance, Ω; θ = the power factor angle; and cos θ = power factor. (See [[Power factor and use of capacitors for a discussion of power factor.)
For Eq. 4, the motor-starting impedance is calculated as:
where Em = motor voltage, kV, and Pm = motor-starting kVA.
Motor starting voltage drop (off a generator)
The voltage drop while starting a motor off a limited-capacity generator is an important factor in sizing the generator and determining the starting method for the motor. The generator cannot supply the large motor inrush current without a momentary voltage falloff while the voltage regulator works to increase excitation and to re-establish the voltage level.
The magnitude and duration of voltage drop depends on the size of the motor and its inrush current, the kVA capacity of the generator, the performance characteristics of the voltage regulator, and the amount of initial load on the generator before starting the motor. Most new installations use fast-response solid-state voltage regulators, which considerably reduce the amount and duration of voltage drop.
Along with large voltage drop, another problem encountered during motor starting is possible excessive kilowatt loading of the generator prime mover. The motor input horsepower during its acceleration period creates a large load, which reflects to the prime mover of the generator. If large enough, this load will stall the prime mover in the worst case, or cause it to shut down because of overload and/or temperature rise.
In determining the voltage drop when starting a motor off a generator that has limited capacity, the motor-feeder-cable impedance generally is disregarded because its impact on the calculation is negligible. Also, the resistance component of the generator impedance and motor impedance is neglected because reactance values are far greater than the resistance values. The voltage drop therefore is a simple ratio of the reactances in the circuit.
The approximated formula to determine the percent voltage drop when starting a generator is:
where Xm =motor reactance during starting, Ω, and Xg = generator reactance, Ω. In Eq. 8, Xm is calculated as:
In Eq. 8, Xg is calculated as:
where X'′’d' = the transient reactance of the generator, %; Pg = generator kVA; and Eg = generator voltage, V.
The presence of initial load on the generator before starting a motor could have substantial effect on the voltage drop, depending on the amount and nature of the load. A constant impedance load (e.g., resistors or lights) might increase the voltage drop only slightly, but might cause a longer time to recover voltage to normal value. Many generator manufacturers provide graphs, personal computer (PC)-based programs, and data to determine voltage drop during motor starting on their generators, with and without an initial load.
|A||=||cross-sectional area of conductor, circular mil|
|Eg||=||generator voltage, V|
|Em||=||transformer voltage, kV|
|Et||=||motor voltage, kV|
|Fp||=||power factor, cos θ|
|L||=||length of conductor, ft|
|N||=||rotor speed, rev/min|
|Nm||=||motor speed, rev/min|
|Ns||=||synchronous speed, rev/min|
|P||=||number of poles|
|Pr||=||reactive power, kVAR|
|X||=||cable three-phase reactance, Ω|
|Xg||=||generator reactance, Ω|
|Xm||=||motor reactance during starting, Ω|
|X′d||=||transient reactance of the generator, %|
|Z||=||transformer impedance, Ω|
|Zc||=||the three-phase impedance of the cable between the transformer and the motor, Ω|
|Zm||=||motor-starting impedance, Ω|
|Zt||=||total impedance, Ω|
|Ztr||=||transformer impedance, %|
|ΔE||=||voltage drop, V|
|θ||=||power factor angle|
|ρ||=||resistivity of conductor, Ω-circular mil/ft|
- API RP4F, Recommended Practice for Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Division 1 and Division 2 Locations, fourth edition. 1999. Washington, DC: API.
- API RP500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2, second edition. 1998. Washington, DC: API.
- API RP505, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2, first edition. 1998. Washington, DC: API.
- API RP540, Recommended Practice for Electrical Installation in Petroleum Processing Plants, fourth edition. 1999. Washington, DC: API.
- API RP2003, Recommended Practice for Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents, sixth edition. 1998. Washington, DC: API.
- ANSI C37.12, For AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis—Specification Guide—1991. 1991. New York City: American Natl. Standards Inst.
- IEEE C37.20.1-2002, Standard for Metal-Enclosed Low Voltage Power Circuit Breaker Switchgear, revision of ANSI/IEEE C37.20.1-1993 (R1998). 1998. New York City: American Natl. Standards Inst.
- ANSI C37.20.2, Standard for Metal-Clad and Station-Type Cubicle Switchgear. 1999. New York City: American Natl. Standards Inst.
- ANSI C37.12.70, Terminal Markings and Connections for Distribution and Power Transformers. 2000. New York City: American Natl. Standards Inst.
- ANSI C84.1, Voltage Rating for Electrical Wiring and Equipment (60Hz). 1989. New York City: American Natl. Standards Inst.
- CSA C22.1, Canadian Electrical Code, Part I, 19th edition. 2002. Rexdale, Ontario, Canada: Canadian Standards Assn.
- IEC 60050-426 (1990-10), International Electrotechnical Vocabulary, Chapter 426: Electrical Apparatus for Explosive Atmospheres, ed. 1.0 bilingual. 1990. Geneva: Intl. Electrotechnical Commission (IEC).
- IEC 331, Fire-Resisting Characteristics of Electrical Cables. 1999. Geneva: IEC.
- IEC 529, Degrees of Protection Provided by Enclosures (IP Code). 2001. Geneva: IEC.
- IEEE Std. 100, Standard Dictionary of Electrical and Electronics Terms, sixth edition. 1996. New York City: IEEE.
- IEEE Std. 141, Electrical Power Distribution for Industrial Plants. 1993. New York City: IEEE.
- IEEE Std. 142, Grounding of Industrial and Commercial Power Systems. 1991. New York City: IEEE.
- IEEE Std. 242, Protection and Coordination of Industrial and Commercial Power Systems. 2001. New York City: IEEE.
- IEEE Std. 315, Graphic Symbols for Electrical and Electronics Diagrams (R1993). 1975. New York City: IEEE.
- IEEE Std. 446, Emergency and Standby Power Systems for Industrial and Commercial Applications. 1995. New York City: IEEE.
- IEEE Std. 485, Sizing Large Lead Storage Batteries for Generating Stations and Substations (R2003). 1997. New York City: IEEE.
- TIESNA RP1 American Natl. Standard Practice for Office Lighting. 2004. New York City: The Illuminating Engineering Soc. of North America.
- TIESNA RP7 American Natl. Standard Practice for Industrial Lighting. 2001. New York City: The Illuminating Engineering Soc. of North America.
- ISA-5.1, Instrumentation Symbols and Identification (R1992). 1984. Research Triangle Park, North Carolina: ISA.
- ANSI/ISA-12.00.01, Electrical Apparatus for Use in Class I, Zones 0, 1, and 2, Hazardous (Classified) Locations: General Requirements. 2002. Research Triangle Park, North Carolina: ISA.
- ISA-RP 12.1 Recommended Practice for Electrical Instruments in Hazardous Atmospheres. 1999. Research Triangle Park, North Carolina: ISA.
- NEMA MG 1 Motors and Generators (revised 2004). 2003. Rosslyn, Virginia: NEMA.
- NEMA MG 2 Safety Standard for Construction and Guide for Selection, Installation, and Use of Electrical Motors and Generators. 2001. Rosslyn, Virginia: NEMA.
- NEMA MG 10 Energy Management Guide for Selection and Use of Polyphase Motors. 2001. Rosslyn, Virginia: NEMA.
- NFPA 30, Flammable and Combustible Liquids Code. 2003. Quincy, Massachusetts: Natl. Fire Protection Assn. (NFPA).
- NFPA 70, Natl. Electrical Code (NEC). 2005. Quincy, Massachusetts: NFPA.
- NFPA 78, Lightning Protection Code. 1989. Quincy, Massachusetts: NEFPA.
- NFPA 496, Standard for Purged and Pressurized Enclosures for Electrical Equipment. 2003. Quincy, Massachusetts: NFPA.
- NFPA 497, Recommended Practices for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Area. 2004. Quincy, Massachusetts: NFPA.
- US DOI 30 CFR Part 250, Oil and Gas and Sulfur Operation in the Outer Continental Shelf. 2004. Washington, DC: US Dept. of the Interior.
- US DOT 49 CFR Part 190, Pipeline Safety Programs and Rulemaking Procedures. 2004. Washington, DC: US Dept. of Transportation.
- US DOT 49 CFR Part 191, Transportation of Natural and Other Gas by Pipeline; annual reports, incident reports, and safety-related condition reports. 2004. Washington, DC: US Dept. of Transportation.
- US DOT 49 CFR Part 192, Transportation of Natural and Other Gas by Pipeline: Minimum Federal Safety Standards. 2004. Washington, DC: US Dept. of Transportation.
- US DOT 49 CFR Part 195, Transportation of Hazardous Liquids by Pipeline. 2004. Washington, DC: US Dept. of Transportation.
- OSHA 29 CFR Part 1910, Subpart H, Hazardous Materials. 2004. Washington, DC: Occupational Safety and Health Administration (OSHA).
- SHA 29 CFR Part 1910, Subpart S Electrical. 2004. Washington, DC: OSHA.
- OSHA 29 CFR Part 1926, Subpart K Electrical Construction. 2004. Washington, DC: OSHA.
- USCG 33 CFR Part 67, Subchapter C Aids to Navigation. 2004. Washington, DC: US Coast Guard.
- SCG 46 CFR Part 110-113, Shipping Subchapter J Electrical Engineering. 2004. Washington, DC: US Coast Guard.
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