153 Questions
What is the preferred construction for low-voltage switchgear?
Metal-enclosed, using air-break or vacuum, drawout-type low-voltage power circuit breakers
What is the interrupting classification range for medium-voltage switchgear?
250 MVA to 1,500 MVA
What are the applications of switchgear primarily used for?
Generation, transmission, distribution, and conversion of electric power
What is the typical voltage class for medium-voltage switchgear?
5-kV to 38-kV
What type of transformers are used for indoor locations?
Ventilated dry-type transformers
What type of fluid is generally not used above 35 kV in transformers?
Silicon insulating fluid
What are transformers containing PCBs no longer manufactured due to?
Federal environmental and health regulations
What are the substitutes for PCB-type transformers?
Less-flammable hydrocarbon fluid and silicon insulating fluid-filled transformers
What is generally required for transformers above 35 kV?
Surge capacitors and arresters
What is the purpose of surge capacitors and arresters in transformers?
To compensate for the lower basic impulse level
What type of transformers have a lower standard basic impulse level compared to liquid-filled transformers?
Ventilated dry-type transformers
What is the purpose of forced-air cooling equipment in transformers?
To increase transformer capacity
What are cast-coil transformers fabricated with?
Solid dielectric
What should transformers be provided with for tap changing under no-load conditions?
Fully rated kVA taps
What is used to adjust voltage under full-load conditions in large power transformers?
Automatic, fully rated kilovolt-ampere taps
What is the determining factor for the kilovolt-ampere rating of the transformer?
Temperature rating of the winding insulation
What is the maximum continuous current rating for switchgear mentioned in the text?
6,000 amps
What are integral current limiting fuse devices used for in switchgear?
To achieve higher interrupting duties
Why is medium-voltage switchgear transitioning to vacuum and SF6 circuit breakers?
Superior characteristics and lower maintenance requirements
Where should switchgear be located for use in a classified location?
Adjacent to the classified location or installed in a pressurized room
What are unit substations frequently used for in processing plants?
Supplying utilization voltage to feeder circuit breakers
What is the recommended interval for preventive maintenance of indoor switchgear?
1 to 5 years
What is a drawback of indoor switchgear compared to outdoor switchgear?
Requires indoor space, affecting the overall cost
How is outdoor switchgear different from indoor switchgear?
Essentially indoor switchgear mounted in a weatherproof enclosure
What can be shipped as a unit or in modular sections with switchgear and other equipment already installed?
Prefabricated electrical power centers
What are distribution and power transformers used for?
Isolate different voltage systems and reduce or increase voltages
Besides stepping down plant distribution voltage, what are power transformers frequently used for?
Desalting and precipitation processes
What are instrument transformers used for?
Metering and relaying, offering a high degree of accuracy and limited capacity
What is the typical secondary voltage for voltage transformers?
120V
What is the typical secondary current for current transformers?
5A
What are autotransformers used for?
Tying systems of different voltage levels economically
What do zigzag grounding, constant voltage, and low-noise isolation transformers have in common?
They are all specialty transformers
What do voltage tap ratings provide flexibility for?
Matching transformers to system voltages
What should the recommended kilovolt-ampere (kVA) ratings of transformers be based on?
ANSI or NEMA standards
What does the rated kVA of a transformer indicate?
The load that can be carried continuously at rated voltage and frequency without exceeding the specified temperature rise
What are the temperature rise limitations for oil-filled transformers?
65°C or 55°C/65°C
How does insulation life change with operating temperature?
It halves for every 10°C rise
What is permissible for oil-filled transformers as long as the hottest spot copper temperature does not exceed 150°C?
Short-time overloads of 1 hour or less
What correction factors are oil-filled transformers subject to when operated at altitudes greater than 1,000 m above sea level?
Rating correction factors
What does the basic impulse level (BIL) indicate?
Transformer's ability to withstand transient over-voltages
What is the purpose of insulation-class correction factors for altitudes over 1,000 m?
To adjust for the decrease in insulation capability at higher altitudes
What determines the impedance of a transformer?
Internal characteristics like core design and winding geometry
What is the recommended location for transformers and associated secondary switchgear?
Near load centers
What is the purpose of neutral grounding of transformer secondaries?
To ensure continuity of service
What is required for proper operation of parallel transformers?
Tolerances in internal impedance, transformation ratio, and phase relationship
What components are part of the standard accessories for oil-filled transformers?
Ground pad, nameplate, top filter valve
When should consideration be given to a hottest-spot temperature detector?
When the transformer is exposed to emergency loading conditions
What is the purpose of providing alarm contacts for transformer gauges?
To allow remote annunciation of transformer problems
What is the purpose of transformer oil?
To insulate and cool the windings
What should be established for transformers to ensure proper maintenance?
A systematic testing and maintenance program
What is the purpose of standard accessories for oil-filled transformers?
To ensure safe and efficient operation
What is the determining factor for the impedance of a transformer?
Internal characteristics like core design and winding geometry
What is the purpose of forced-air cooling in transformers?
To increase the effective cooling of the transformer and allow for increased capacity
What method of cooling reduces oil temperature and is common in the design of larger power transformers?
Forced-oil cooling
When might emergency loading of transformers be necessary?
During system growth
What is the purpose of providing fans for forced-air cooling or the brackets, temperature switch, and wiring necessary to accommodate the future addition of fans?
To anticipate load growth and increase effective cooling of the transformer
What is the purpose of system grounding in electrical systems?
To protect electrical equipment and ensure the reliability of electrical systems.
Which type of grounding ensures the protection of personnel against electric shock?
Equipment grounding
What is the primary purpose of lightning protection in electrical systems?
To prevent fire, explosion, and equipment damage caused by lightning.
Which standards discuss the protection against static electricity and stray currents?
API RP 2003 and NFPA 77
What does electric power distribution system grounding involve?
Intentional conductive connection between the neutral of the system and the ground.
Which classification of grounding methods provides the greatest control of overvoltages?
Solidly grounded
What significantly affects line-to-ground voltages under steady-state and transient conditions?
System grounding
According to NFPA 70, which systems are required to be solidly grounded?
Systems rated at less than or equal to 480 V supplying phase-to-neutral loads.
What characterizes ungrounded systems?
They have no intentional connection to ground.
What is the primary function of resistance grounding?
To limit ground-fault current and eliminate high transient overvoltages.
What is the purpose of low-resistance grounded systems?
To limit ground-fault current and eliminate high transient overvoltages.
What is the characteristic of solidly grounded systems?
They provide the greatest control of overvoltages.
What type of cable provides an alternative to open conductor distribution for overhead electrical distribution systems?
Three-conductor aerial cable
What is the recommended routing for overhead lines for electrical distribution systems?
Along facility roads or streets
What should be installed to isolate live parts from accidental contact for employee safety in overhead electrical distribution systems?
Adequate clearance to ground or guards
What should overhead lines be located far enough from to avoid fouling insulation and corrosion?
Water spray sources
What should be used to withstand the effects of exposure to petrochemicals for overhead lines?
Specially approved silicon-type grease
Where should overhead lines installed adjacent to or traversing Class I locations be placed?
Outside the space that may contain flammable gases or vapors
What type of cable may be used as an alternative to aerial cable for overhead electrical distribution systems?
Metal-clad (Type MC) cable
What should be readily accessible for examination or adjustment in overhead distribution systems?
All parts of the system
What should be installed to ensure employee safety in overhead electrical distribution systems?
Adequate clearance to ground or guards
What are most pole-line materials designed to conform to?
EEI standards
What is the primary purpose of equipment grounding in industrial power systems?
To prevent equipment damage and shock hazards
What should be the connection between motor and generator enclosures and the overall plant grounding system?
Mechanically and electrically continuous equipment-grounding conductor
What is the purpose of grounding the metallic sheath and shield of power cables?
To provide a path for fault currents
What is the aim of the equipment grounding system in industrial power systems?
Maintain the resistance to earth at the lowest practicable value
What is the requirement for grounding exposed metallic noncurrent-carrying enclosures of electrical devices according to NFPA 70?
They must be grounded
What is the recommended method for grounding steel building frameworks and similar installations?
At multiple points with substantial connections to the grounding system grid
What should equipment grounding conductors be connected to minimize under fault conditions?
As directly as possible to the electrical system ground
What is the purpose of grounding interconnected equipment enclosures and structure ground grids in large plants?
To provide a common reference point for fault currents
What should cable shields be sized to carry without exceeding thermal limitations?
Ground-fault current
Why are reactance-grounded systems not typically used in industrial power systems?
To avoid the potential for sustained arcing
What is the purpose of grounding the metal framework of buildings and noncurrent-carrying metal parts of electrical equipment?
To ensure structures and enclosures are not at a dangerous voltage
What is the requirement for grounding systems of large plants involving interconnected equipment enclosures and structure ground grids?
To maintain the resistance to earth at the lowest practicable value
What is the recommended grounding connection for enclosures of electrical equipment according to the text?
Ground bus
What is mandatory for the enclosures of portable electrical equipment operating above 50 V?
Double insulation
What is the purpose of ground-fault circuit interrupters according to the text?
Personnel safety
What is the primary aim of the equipment grounding system in industrial power systems?
Personnel safety
What does NFPA 70 require for all 125 V single-phase 15 amp and 20 amp receptacle outlets on temporary wiring used for maintenance or construction?
Ground-fault circuit interrupters
What is the purpose of surge capacitors and arresters in transformers according to the text?
Overvoltage protection
What does IEEE Std 142 provide information on?
Portable electrical equipment operating at higher voltages
What is the recommended alternative to ground-fault circuit interrupters according to NFPA 70?
Assured grounding program
What is the purpose of connecting power supply, equipment, and cable shields to a single point on the overall plant grounding system?
Safe troubleshooting
What is the recommended method for grounding steel building frameworks and similar installations according to the text?
Tying to the overall plant grounding system
What is the purpose of neutral grounding of transformer secondaries?
Limiting overvoltages
What is the aim of connecting metal fences and gates enclosing electrical equipment or substations to the grounding system grid?
Ensuring continuity of the enclosure
What is the maximum resistance allowed for the grounding grid at large substations and generating stations?
1 ohm
What is the maximum resistance allowed for a single-made electrode according to NFPA 70?
25 ohms
What is the resistance to ground of a single 3/4-in. x 10-ft ground rod even in soil of low resistivity (2,000 ohm-cm)?
Over 6 ohms
What type of grounding electrodes are the most common type but not adequate when relatively low resistance is required?
Driven ground rods
What is the primary advantage of using small three-phase motors from a maintenance and safety standpoint?
They contain no contact-making device
When should an engineering analysis be made to determine if small three-phase motors can be used?
For each application when the answer is not obvious
For which type of applications are synchronous motors generally considered?
Large-horsepower and slow-speed applications
What are low-speed engine-type synchronous motors well suited for?
Use as drives for slow-speed equipment
In what situations are synchronous motors preferred over squirrel-cage induction motors?
When the resulting improvement in power factor or efficiency justifies the additional investment
What is usually the most efficient selection for synchronous motors?
A 1.0 power factor synchronous motor
What type of motors are now used extensively in petroleum facilities for excitation?
Brushless synchronous motors
What conditions are considered when determining the type of motor design to be used?
Exposed to chemical fumes, operated in damp places, driven at speeds in excess of rated speed, and other unusual conditions
What should be used for adjustable speed applications according to the text?
A chemical-type motor
What is recommended for motors exposed to abnormal shock or vibration from external sources?
Oversize the motor
What is generally required for transformers above 35 kV according to the text?
A higher standard basic impulse level
What is the primary purpose of providing fans for forced-air cooling or the brackets, temperature switch, and wiring necessary to accommodate the future addition of fans?
To maintain proper operating temperature
What is the purpose of lightning protection systems?
To provide a pathway for lightning current to reach the ground
Why is copper used for grounding systems?
For its resistance to corrosion and high conductivity
What is the purpose of surge arresters in electrical systems?
To protect against direct lightning strokes or voltage surges
What is the zone of protection of an air terminal in a lightning protection system?
A circular arc passing through the tip and tangent to the ground plane
What is the potential risk of lightning to electric power distribution systems?
Damage to equipment and personnel shock hazards
Why might surge arresters be installed close to the equipment to be protected?
To minimize the voltage drop
What is the purpose of galvanic coupling in grounding systems?
To accelerate corrosion
What is the characteristic of lightning that can pose a hazard to personnel and structures?
Its potential gradients in the soil
Why do lightning protection systems use air terminals?
To intercept and divert lightning current to ground
What is the purpose of cathodic protection for buried steel in grounding systems?
To reduce resistance to corrosion
What is the primary aim of lightning protection systems?
To provide a pathway for lightning current to reach the ground
What factors are considered for deciding on lightning protection?
Frequency of thunderstorms and personnel hazards
What is the purpose of surge capacitors in transformers?
To reduce the rate-of-rise of voltage surges to protect AC rotating machines and other equipment
What is the recommended insulation class for motors in petroleum facility atmospheres?
Class F insulation
What is the preferred standard frequency for all AC systems and equipment in North America?
60 Hz
What is the result of the last rerate program for motors?
NEMA T-frame series (143T through 445T)
What is the minimum insulation used for motors?
Class B insulation
What type of motors are well suited for constant speed applications in petroleum facilities?
Three-phase AC motors
What is the purpose of lightning protection for process instruments and control systems?
To bypass voltage surges to ground
What is the purpose of series resistors with Zener diodes or other devices in lightning protection?
To bypass voltage surges to ground
What type of insulation is normally specified for motors?
Class F insulation
What are surge arresters used for in petroleum facilities?
Protection from voltage surges at transformer terminals
What is the recognized standard frequency for operation of standard motors in North America?
60 Hz
What type of motors are not common in petroleum facilities due to installation requirements and higher equipment and maintenance costs?
DC motors
What is the typical voltage rating for three-phase motors in 60 Hz low-voltage service?
575 V
For induction motors above 600 V, what are the preferred rated voltages for motors up to 5,000 HP?
2,300 V and 4,000 V
At what HP range is the economic breakpoint between low-voltage and medium-voltage motors typically found?
250 HP-300 HP
At what ambient temperature should motors be considered as special and have design ambient temperatures typically nameplated higher than 40°C (104°F)?
45°C (113°F)
What is the typical nameplate voltage rating for synchronous motors?
Depends on the service voltage of the system
What is the preferred rated voltage for induction motors above 600 V for larger motors?
6,600 V and 13,200 V
What is the typical voltage rating for portable motors, except for equipment designed for lower voltage?
115 V
What is the typical voltage rating for motors with ratings of 200 V, 208 V, and 230 V?
460 V
For motors above 5,000 HP, what may dictate the use of 4,000 V, 6,600 V, or 13,200 V?
Economic breakpoint
What should supply voltage and frequency at the motor terminals be maintained within the limits of?
NEMA MG 1
What ambient temperature should standard design motors be suitable for operation at, provided it does not exceed?
40°C (104°F)
Study Notes
Transformer Types and Ratings
- Voltage transformers step down primary voltage to a secondary voltage, usually 120V, while current transformers transform primary current to a secondary current, typically 5 amps.
- Autotransformers are single-winding transformers used to economically tie systems of different voltage levels and for motor control in reduced voltage starter packages.
- Other specialty transformers include zigzag grounding, constant voltage, and low-noise isolation transformers, each serving specific purposes.
- The voltage rating for transformers should conform to ANSI standards, and attention to voltage tap ratings allows flexibility in matching transformers to system voltages.
- The recommended kilovolt-ampere (kVA) ratings of transformers are given in ANSI or NEMA standards, and should be on a continuous basis without exceeding temperature limitations.
- The rated kVA of a transformer is the load that can be carried continuously at rated voltage and frequency without exceeding the specified temperature rise.
- Oil-filled transformers are limited to a winding temperature rise of 65°C or 55°C/65°C and a hottest-spot winding temperature rise of 80°C, while dry-type transformers have different temperature rise specifications based on insulation class.
- Insulation life is halved for each 10°C rise in operating temperature, and short-time overloads of 1 hour or less are permissible as long as the hottest spot copper temperature does not exceed 150°C for oil-filled transformers.
- Transformers operated at altitudes greater than 1,000 m above sea level are subject to special rating correction factors.
- The basic impulse level (BIL) indicates a transformer’s ability to withstand transient over-voltages, and the applicable manufacturer’s test voltages are given in IEEE standards for liquid-filled and dry-type transformers.
- The dielectric strength of transformers that depend on air for insulation decreases as altitude increases.
- The accuracy of transformation depends on the application, and the degree of accuracy is subject to the effects of load and fault current.
IEEE C57.12.01 Covers Insulation-Class Correction Factors for Altitudes over 1,000 m (3,300 feet)
- Insulation-class correction factors for altitudes over 1,000 m are addressed in IEEE C57.12.01.
- Efficiency and Regulation are Fixed by Manufacturer's Design
- Efficiency and regulation are fixed by the manufacturer's design, and more efficient designs are available at a higher cost if loss evaluation warrants them.
- Impedance is Expressed as a Percentage of Transformer Base Kilovolt-Ampere Rating
- Impedance is expressed as a percentage of the transformer base kilovolt-ampere rating and is determined by internal characteristics like core design, resistance, and winding geometry.
- Location of Transformers Should Be Near Load Centers
- Transformers and associated secondary switchgear should be located as near to their load centers as practical while minimizing exposure to fire and mechanical damage.
- Neutral Grounding of Transformer Secondaries Should Be Considered
- Neutral grounding of transformer secondaries should be considered based on factors such as voltage levels, ground-fault levels, and continuity of service.
- Proper Operation of Parallel Transformers Requires Certain Tolerances
- Proper operation of parallel transformers requires certain tolerances in characteristics like internal impedance, transformation ratio, and phase relationship.
- Systematic Testing and Maintenance Program Should Be Established for Transformers
- A systematic testing and maintenance program should be established for transformers, including inspection and cleaning of bushings, testing and gas analysis of transformer oil, and vacuum cleaning of dry-type transformers.
- Transformer Oil is Used to Insulate and Cool the Windings
- Transformer oil is used to insulate and cool the windings and protect the core and windings from corrosive and hazardous vapors.
- Standard Accessories for Oil-Filled 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.
- Consideration Should Be Given to Hottest-Spot Temperature Detector
- Consideration should be given to a hottest-spot temperature detector where the system operation may subject the transformer to emergency loading conditions.
- Transformer Gauges Can Be Provided with Alarm Contacts
- Transformer gauges can be provided with alarm contacts to allow remote annunciation of transformer problems.
Guidelines for Overhead Electrical Distribution Systems
- Overhead electrical distribution systems should be designed and installed in accordance with NFPA 70, ANSUEEE C2, and applicable state and local codes.
- Most pole-line materials conform to the standards of the Edison Electric Institute (EEI) or similar specifications and designs.
- Aerial cable provides an alternative to open conductor distribution and is available in single- and three-conductor types, shielded or nonshielded.
- Metal-clad (Type MC) cable supported by a messenger may be used as an alternative to aerial cable, with necessary insulation and shielding.
- All parts of the overhead distribution system that need examination or adjustment should be readily accessible, with established electrical clearances.
- Adequate clearance to ground or guards should be installed to isolate live parts from accidental contact for employee safety.
- Grounding of circuits and equipment should conform to ANSUEEE C2 and any applicable state and local codes.
- The clearances specified for conductors in ANSVIEEE C2, Section 23, are a minimum recommendation, and local and state requirements must also be considered.
- Recommended routing for overhead lines is along facility roads or streets, complying with applicable fire codes and minimizing exposure to damage.
- Overhead lines should be located far enough from water spray sources to avoid fouling insulation and corrosion, or designed to withstand such exposure.
- When exposed to petrochemicals, lines should be designed to withstand the effects, with specially approved silicon-type grease providing a deterrent to current leakage and insulator flashover.
- Overhead lines installed adjacent to or traversing Class I locations should be placed outside the space that may contain flammable gases or vapors.
Equipment Grounding in Industrial Power Systems
- High currents may cause equipment damage and shock hazards if equipment grounding is inadequate
- Cable shields must be sized to carry ground-fault current without exceeding thermal limitations
- Reactance-grounded systems are not typically used in industrial power systems
- Equipment grounding ensures that structures and enclosures are not at a dangerous voltage, and provides a path for fault currents to flow without sparking or overheating
- The metal framework of buildings and noncurrent-carrying metal parts of electrical equipment should be grounded
- Equipment grounding conductors should be connected as directly as possible to the electrical system ground to minimize voltage drop under fault conditions
- The equipment grounding system aims to maintain the resistance to earth of structures and equipment enclosures at the lowest practicable value
- Grounding systems for large plants may involve interconnected equipment enclosures and structure ground grids
- Steel building frameworks and similar installations should be grounded at multiple points with substantial connections to the grounding system grid
- Motor and generator enclosures should be connected to the overall plant grounding system with a mechanically and electrically continuous equipment-grounding conductor
- Metallic sheath and shield of power cables should be continuous over the entire run and grounded at each end
- NFPA 70 requires that exposed metallic noncurrent-carrying enclosures of electrical devices be grounded
Motor Voltage and Temperature Considerations in Industrial Applications
- For portable motors, 115 V is generally preferred, except for equipment designed for lower voltage, such as 32 V.
- Single-phase motors with potentially sparking mechanisms should be used cautiously in classified areas.
- Three-phase motors typically have ratings of 460 V or 575 V for 60 Hz low-voltage service.
- Motors with ratings of 200 V, 208 V, and 230 V are generally not used except where power is readily available at those voltages.
- Induction motors for service above 600 V have preferred rated voltages of 2,300 V and 4,000 V for motors up to 5,000 HP, and 4,000 V, 6,600 V, and 13,200 V for larger motors.
- Synchronous motors usually have nameplate voltage ratings identical to the service voltage of the system they are connected to.
- The economic breakpoint between low-voltage and medium-voltage motors is typically in the range of 250 HP-300 HP.
- Motors with ratings of 2,300 V and 4,000 V are used for sizes up to 5,000 HP, with the choice depending on the economics of the individual plant.
- For motors above 5,000 HP, the economic breakpoint may dictate the use of 4,000 V, 6,600 V, or 13,200 V, with consideration for local conditions and the number of large and small motors to be served.
- Supply voltage and frequency at the motor terminals should be maintained within the limits of NEMA MG 1.
- Motors of standard design are suitable for operation at their standard ratings, provided the ambient temperature does not exceed 40°C (104°F).
- Motors to be installed where the ambient temperature will normally exceed 40°C (104°F) should be considered as special, with design ambient temperatures typically nameplated higher than 40°C (104°F), usually 45°C (113°F) and 50°C (122°F).
Test your knowledge of transformer types and ratings with this quiz. Explore topics such as voltage and current transformers, autotransformers, specialty transformers, ANSI and NEMA standards, kVA ratings, temperature rise specifications, altitude correction factors, and more. This quiz covers essential information for engineers and professionals working with transformers.
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