The five general types of high-voltage circuit breakers are as follows.
1 Oil circuit breakers use standard transformer oil, an effective medium for quenching the arc and providing an open break after current has dropped to zero. There are two general types of oil circuit breakers: dead-tank for the higher voltage ranges and live-tank for lower voltages.
Oil circuit breakers have been improved by adding such features as oil-tight joints, vents, and separate chambers to prevent the escape of oil.
Also, improved operating mechanisms prevent gas pressure from reclosing the contacts, making them reliable for system voltages up to 362 kV. However, above 230 kV, oil-less breakers are more economical.
2 Air-blast circuit breakers were developed as alternatives to oil circuit breakers as voltages increased. They depend on the good insulating and arc-quenching properties of dry and clean compressed air injected into the contact region.
3 Magnetic-air circuit breakers use a combination of strong magnetic field with a special arc chute to lengthen the arc until the system voltage is unable to maintain the arc any longer. They are used principally in power distribution systems.
4 Gas circuit breakers take advantage of the excellent arc-quenching and insulating properties of sulfur hexafluoride (SF6) gas. These outdoor breakers can interrupt system voltages up to 800 kV.
These circuit breakers are typically included in gasinsulated substations (GISs) that offer space-saving and environmental advantages over conventional outdoor substations. Gas (SF6) circuit breakers are made with ratings up to 800 kV and continuous cur rent up to 4000 A.
They are alternatives to oil and vacuum breakers for metal-clad and metal-enclosed switchgear up to 38 kV.
5 Vacuum circuit breakers, more accurately termed vacuum-bottle interrupters, are generally used for voltages up to 38 kV and continuous current ratings to 3000 A.
They are used for higher system voltage, current, and interrupting ratings, and are typically specified for metal-clad and metal-enclosed switchgear in distribution systems.
TRANSFORMER TERMS GLOSSARY BASIC INFORMATION AND TUTORIALS
The following technical terms apply to transformers.
BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.
The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.
Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.
Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.
Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.
Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers.
Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.
Isolating transformer: See insulating transformer.
Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.
Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.
Temperature rise: The incremental temperature rise of the windings and insulation above the ambient
temperature.
Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.
Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.
BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.
The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.
Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.
Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.
Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.
Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers.
Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.
Isolating transformer: See insulating transformer.
Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.
Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.
Temperature rise: The incremental temperature rise of the windings and insulation above the ambient
temperature.
Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.
Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.
BUCK BOOST AUTOTRANSFORMERS BASIC INFORMATION AND TUTORIALS
The buck-boost transformer is a simple and economical means for raising a voltage that is too low or decreasing a voltage that is too high. This transformer can raise or lower voltage being supplied to the load more than ±5 percent, to improve the efficiency of the device or system.
Buck-boost transformers are small single-phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 to 20 percent. A common application is boosting 208 V to 230 or 240 V AC.
For example, there might be a requirement to power the motor in an air conditioner with a 230- or 240-V AC motor from the 208-V AC supply line. This can be done with a buckboost transformer.
Buck-boost transformers are standard distribution transformers with ratings ranging from 50 VA to 10 kVA. Commercial units are made with primary voltages of 120, 240, or 480 V AC.
They can also power low-voltage circuits for control or lighting applications requiring 12, 16, 24, 32, or 48 V AC. Schematics of buck-boost transformers that can transform 120 and 240 V AC to 12 and 24 V AC are shown in the figure below.
When the primary and secondary lead wires of buck-boost transformers are connected together electrically in a recommended bucking or boosting connection, they become autotransformers. Some typical connection diagrams for these transformers in autotransformer arrangements for single-phase systems are shown below.
Buck-boost transformers have four windings for versatility. Their two primary and two secondary windings can be connected eight different ways to provide many different voltage and kVA outputs.
Because their output voltage is a function of input voltage, they cannot be used as voltage stabilizers. Output voltage will vary by the same percentage as the input voltage.
These transformers can also function in three-phase systems. Two or three units can be used to buck or boost three-phase voltage. The number of units needed in a three-phase installation depends on the number of wires in the supply line.
Buck-boost transformers are small single-phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 to 20 percent. A common application is boosting 208 V to 230 or 240 V AC.
For example, there might be a requirement to power the motor in an air conditioner with a 230- or 240-V AC motor from the 208-V AC supply line. This can be done with a buckboost transformer.
Buck-boost transformers are standard distribution transformers with ratings ranging from 50 VA to 10 kVA. Commercial units are made with primary voltages of 120, 240, or 480 V AC.
They can also power low-voltage circuits for control or lighting applications requiring 12, 16, 24, 32, or 48 V AC. Schematics of buck-boost transformers that can transform 120 and 240 V AC to 12 and 24 V AC are shown in the figure below.
When the primary and secondary lead wires of buck-boost transformers are connected together electrically in a recommended bucking or boosting connection, they become autotransformers. Some typical connection diagrams for these transformers in autotransformer arrangements for single-phase systems are shown below.
Buck-boost transformers have four windings for versatility. Their two primary and two secondary windings can be connected eight different ways to provide many different voltage and kVA outputs.
Because their output voltage is a function of input voltage, they cannot be used as voltage stabilizers. Output voltage will vary by the same percentage as the input voltage.
These transformers can also function in three-phase systems. Two or three units can be used to buck or boost three-phase voltage. The number of units needed in a three-phase installation depends on the number of wires in the supply line.
OVER CURRENT PROTECTIVE DEVICES FOR GENERATORS BASIC INFORMATION AND TUTORIALS
What are the basic overcurrent protection devices for generators?
As with other motors, NEC 445.11 requires a generator to have a nameplate giving the manufacturer’s name, the rated frequency, power factor, number of AC phases, the subtransient and transient impedances, the rating in kilowatts or kilovolt amperes, a rating for the normal volts and amps, rated revolutions per minute, insulation system class, any rated ambient temperature or temperature rise, and a time rating.
The size and type of OCPD will be based on this critical data. NEC 445.12 defines the basic overcurrent protection standards for various types of generators. A constant-voltage generator must be protected from overloads by either the generator’s inherent design or circuit breakers, fuses, or other forms of overcurrent protection that are considered suitable for the conditions of use.
This is true except for AC generator exciters.
Two-wire, DC generators are allowed to have overcurrent protection in only one conductor if the overcurrent device is triggered by the entire current that is generated other than the current in the shunt field. For this reason, the overcurrent device cannot open the shunt field.
If the two-wire generator operates at 65 V or less and is driven by an individual motor then the overcurrent device protection device needs to kick-in if the generator is delivering up to 150% of its full-load rated current.
When a two-wire DC generator is used in conjunction with balancer sets it accomplishes the neutral points for the three-wire system. This means it requires an overcurrent device that is sized to disconnect the three-wire system if an extreme unbalance occurs in the voltage or current.
For three-wire DC generators, regardless of whether they are compound or shunt wound, one overcurrent device must be installed in each armature lead, and must be connected so that it is activated by the entire current from the armature.
These overcurrent devices need to have either a double-pole, double-coil circuit breaker or a four-pole circuit breaker connected in both the main and equalizer leads, plus two more overcurrent devices, one in each armature lead.
The OCPD must be interlocked so that no single pole can be opened without simultaneously disconnecting both leads of the armature from the system.
The ampacity of the conductors that run from the generator terminals to the first distribution device that contains overcurrent protection cannot be less than 115% of the nameplate current rating for the generator per NEC 445.13.
All generators must be equipped with at least one disconnect that is lockable in the open position that will allow the generator and all of its associated protective devices and controls to be disconnected entirely from the circuits that are supplied by the generator.
As with other motors, NEC 445.11 requires a generator to have a nameplate giving the manufacturer’s name, the rated frequency, power factor, number of AC phases, the subtransient and transient impedances, the rating in kilowatts or kilovolt amperes, a rating for the normal volts and amps, rated revolutions per minute, insulation system class, any rated ambient temperature or temperature rise, and a time rating.
The size and type of OCPD will be based on this critical data. NEC 445.12 defines the basic overcurrent protection standards for various types of generators. A constant-voltage generator must be protected from overloads by either the generator’s inherent design or circuit breakers, fuses, or other forms of overcurrent protection that are considered suitable for the conditions of use.
This is true except for AC generator exciters.
Two-wire, DC generators are allowed to have overcurrent protection in only one conductor if the overcurrent device is triggered by the entire current that is generated other than the current in the shunt field. For this reason, the overcurrent device cannot open the shunt field.
If the two-wire generator operates at 65 V or less and is driven by an individual motor then the overcurrent device protection device needs to kick-in if the generator is delivering up to 150% of its full-load rated current.
When a two-wire DC generator is used in conjunction with balancer sets it accomplishes the neutral points for the three-wire system. This means it requires an overcurrent device that is sized to disconnect the three-wire system if an extreme unbalance occurs in the voltage or current.
For three-wire DC generators, regardless of whether they are compound or shunt wound, one overcurrent device must be installed in each armature lead, and must be connected so that it is activated by the entire current from the armature.
These overcurrent devices need to have either a double-pole, double-coil circuit breaker or a four-pole circuit breaker connected in both the main and equalizer leads, plus two more overcurrent devices, one in each armature lead.
The OCPD must be interlocked so that no single pole can be opened without simultaneously disconnecting both leads of the armature from the system.
The ampacity of the conductors that run from the generator terminals to the first distribution device that contains overcurrent protection cannot be less than 115% of the nameplate current rating for the generator per NEC 445.13.
All generators must be equipped with at least one disconnect that is lockable in the open position that will allow the generator and all of its associated protective devices and controls to be disconnected entirely from the circuits that are supplied by the generator.
ARC FLASH BOUNDARY SAFE DISTANCE BASIC INFORMATION AND TUTORIALS
Arc-flash boundaries need to be established around electrical equipment such as switchboards, panelboards, industrial control panels, motor control centers, and similar equipment if you plan to work on or in the proximity of exposed energized components.
Parts are considered exposed if they are energized and not enclosed, shielded, covered, or otherwise protected from contact. Work on these parts includes activities such as examinations, adjustment, servicing, maintenance, or troubleshooting.
Equipment energized below 240 V does not require arc-flash boundary calculation unless it is powered by a 112.5 KVA transformer or larger.
The arc-flash boundary is the limit at which a person working on energized parts can be standing at the time of an arc-flash without risking permanent injury unless they are wearing flame-resistant clothing. Permanent injury results from an arc-flash that causes an incident energy of 1.2 calories/centimeter2 (cal/cm2) or greater and causes a minimum of second-degree burns.
This distance can only be effectively determined by calculating the destructive potential of an arc.
First you must determine the magnitude of the arc based on the available short circuit current, then estimate how long the arc will last based on the interrupting time of the fuse or circuit breaker.
Finally, you will need to calculate how far away an individual must be to avoid being exposed to an incident energy of 1.2 cal/cm2. It may sound like a lot of math and factoring in of potentials, but believe me the extra time you take to determine the arc flash boundary is well worth your safety and well-being.
Calculating flash protection boundaries for systems over 600 V requires performing a flash hazard analysis coupled with either the NFPA 70E Hazard Risk Category/PPE tables or the Incident Energy Formula.
Additionally, Section 4 of IEEE 1584 Guide for Arc Flash Hazard Calculations states that the results of the arc flash hazard analysis are used to identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical system.
The purpose is to establish safe work distances and the PPE required to protect workers from injury. A flash-hazard analysis is comprised of the following three different electrical system studies:
1. A short circuit study
2. A protective device time-current coordination study
3. The flash-hazard analysis and application of the data
Parts are considered exposed if they are energized and not enclosed, shielded, covered, or otherwise protected from contact. Work on these parts includes activities such as examinations, adjustment, servicing, maintenance, or troubleshooting.
Equipment energized below 240 V does not require arc-flash boundary calculation unless it is powered by a 112.5 KVA transformer or larger.
The arc-flash boundary is the limit at which a person working on energized parts can be standing at the time of an arc-flash without risking permanent injury unless they are wearing flame-resistant clothing. Permanent injury results from an arc-flash that causes an incident energy of 1.2 calories/centimeter2 (cal/cm2) or greater and causes a minimum of second-degree burns.
This distance can only be effectively determined by calculating the destructive potential of an arc.
First you must determine the magnitude of the arc based on the available short circuit current, then estimate how long the arc will last based on the interrupting time of the fuse or circuit breaker.
Finally, you will need to calculate how far away an individual must be to avoid being exposed to an incident energy of 1.2 cal/cm2. It may sound like a lot of math and factoring in of potentials, but believe me the extra time you take to determine the arc flash boundary is well worth your safety and well-being.
Calculating flash protection boundaries for systems over 600 V requires performing a flash hazard analysis coupled with either the NFPA 70E Hazard Risk Category/PPE tables or the Incident Energy Formula.
Additionally, Section 4 of IEEE 1584 Guide for Arc Flash Hazard Calculations states that the results of the arc flash hazard analysis are used to identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical system.
The purpose is to establish safe work distances and the PPE required to protect workers from injury. A flash-hazard analysis is comprised of the following three different electrical system studies:
1. A short circuit study
2. A protective device time-current coordination study
3. The flash-hazard analysis and application of the data
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