What are K-Rated Transformers?
UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.
The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:
a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.
b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.
c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.
For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to
As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.
Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.
STANDBY GENERATOR SYSTEM FOR ELECTRONIC EQUIPMENT BASIC INFORMATION AND TUTORIALS
Incompatibility issues regarding emergency standby generator systems and downstream electronic load equipment are gaining more recognition in modern power systems. UPS systems and electronic load equipment can be very susceptible to voltage waveform distortion and frequency variations.
The distortion of the voltage waveform is primarily a function of the magnitude and harmonic content of the load current and the impedance of the upstream electrical distribution system. Standby generator systems generally have a much higher impedance than the utility system.
Therefore, the voltage waveform distortion typically increases when loads are fed by standby generator power. One of the most common incompatibility situation is with generator systems and downstream UPS systems.
These situations can range from problems with the UPS inverter trying to synchronize to the static bypass circuit to the UPS input failing to accept the input voltage and thus causing the UPS system to go to battery power.
In this latter condition, the voltage distortion typically improves when the load is fed from battery power (the load is now on battery and not acting on the impedance of the generator system) and the UPS input accepts the line voltage. Once again, voltage distortion can increase when the loads are powered via the UPS system, and the UPS
a) Isochronous electronic governor to regulate frequency. These governors typically maintain frequency regulation within 0.25% of the setting, as opposed to approximately 3% for mechanical governors.
b) Permanent magnet excitation system or filtering means to isolate the voltage regulator
power circuit from the distorted waveform.
c) Generators with a 2/3 pitch stator winding design to minimize third harmonic waveform distortion.
d) Low subtransient reactance to minimize voltage waveform distortion. cycles back and forth on battery power. Recommended practice is to provide the standby generator manufacturer with information on the type, rating, and characteristics of the electronic load equipment.
Many generator manufacturers and UPS manufacturers have guidelines for sizing emergency generators when supplying UPS systems. This rating will typically depend on the type and size of the UPS system.
In general, the standby generators should have the following characteristics to minimize adverse interactions when supplying nonlinear loads.
The distortion of the voltage waveform is primarily a function of the magnitude and harmonic content of the load current and the impedance of the upstream electrical distribution system. Standby generator systems generally have a much higher impedance than the utility system.
Therefore, the voltage waveform distortion typically increases when loads are fed by standby generator power. One of the most common incompatibility situation is with generator systems and downstream UPS systems.
These situations can range from problems with the UPS inverter trying to synchronize to the static bypass circuit to the UPS input failing to accept the input voltage and thus causing the UPS system to go to battery power.
In this latter condition, the voltage distortion typically improves when the load is fed from battery power (the load is now on battery and not acting on the impedance of the generator system) and the UPS input accepts the line voltage. Once again, voltage distortion can increase when the loads are powered via the UPS system, and the UPS
a) Isochronous electronic governor to regulate frequency. These governors typically maintain frequency regulation within 0.25% of the setting, as opposed to approximately 3% for mechanical governors.
b) Permanent magnet excitation system or filtering means to isolate the voltage regulator
power circuit from the distorted waveform.
c) Generators with a 2/3 pitch stator winding design to minimize third harmonic waveform distortion.
d) Low subtransient reactance to minimize voltage waveform distortion. cycles back and forth on battery power. Recommended practice is to provide the standby generator manufacturer with information on the type, rating, and characteristics of the electronic load equipment.
Many generator manufacturers and UPS manufacturers have guidelines for sizing emergency generators when supplying UPS systems. This rating will typically depend on the type and size of the UPS system.
In general, the standby generators should have the following characteristics to minimize adverse interactions when supplying nonlinear loads.
ANALYSES OF HARMONIC CURRENTS AND VOLTAGE OF ELECTRONIC EQUIPMENT BASIC INFORMATION AND TUTORIALS
Refer to IEEE Std 519-1992 for a general discussion of harmonic currents. Recommended practice is for all power distribution systems intended for use with electronic load equipment comply with IEEE Std 519-1992 and IEEE Std 399-1997 guidelines. Calculation or estimation of load harmonic profiles is a necessary requirement when installing power factor correction equipment, selecting K-factor rated transformers or derating existing conventional transformers.
Improvements in power factor may be desired for financial reasons (to lower utility costs associated with power factor penalties) or operational reasons (to lower system losses, increase system reserve capacity, or improve voltage conditions). Extreme caution should be used when applying capacitors.
The manner in which they are applied can cause resonance conditions that can magnify harmonic levels and cause excessive voltage distortion. Power factor correction equipment may be applied directly at or close to the facility service entrance, or as close as practicable to the load equipment.
The location of the power factor equipment will depend on economic reasons, as well as operational and design considerations.
Thorough analysis of distribution system characteristics and load characteristics should be made prior to applying power factor correction capacitors to determine what effect harmonic currents will have on the system, and to determine proper harmonic mitigation techniques. Refer to IEEE Std 141-1993 for further discussion on application of power factor correction capacitors.
It is recommended practice to measure and record the harmonic profile of load currents at the transformers serving the load. When the harmonic profiles of individual loads at downstream locations are measured, there is a tendency to calculate a higher than necessary K-factor.
This is also the case in new installations where the current harmonic profile is estimated from typical
individual pieces of electronic load equipment based upon experience or data supplied by the OEM. Due to cancellation, the combined contribution to K-factor of several loads is always less than the sum of individual loads.
This reduction may be substantial when there is a large number and a diversity of nonlinear load types. Figure below shows an example of how harmonic levels vary in a typical electrical distribution system. Note that the level of harmonic current distortion decreases from the individual electronic load equipment to the branch circuit panelboards, through delta-wye stepdown transformers, and upstream to the power source.
However, when loads are removed from the electrical distribution system, the cancellation benefit produced by these loads is also removed. In many cases, this will not be a problem for a transformer that is conservatively loaded or is K-factor rated. It may be a problem if the load or K-factor rating is marginal.
Cancellation results when harmonics produced by different loads are phase-shifted relative to each other. Impedance in branch circuit wiring, as well as isolation transformers or series inductors and shunt capacitors that may be incorporated in the loads, shift harmonic currents.
A delta-wye transformer serving single-phase nonlinear loads randomly distributed among the three phases will trap the balanced triplen load harmonics in the primary winding. This may substantially reduce the triplen harmonic currents and the related current and voltage distortion that would otherwise appear on the primary side.
It is difficult to predict a harmonic diversity factor without modeling the nonlinear loads and the electrical distribution system. Computer programs and methods that allow modeling and simulation are becoming available.
With more experience, these computer analysis tools are expected to provide diversity factors for typical loads in industrial and commercial power systems. For new installations, where such diversity factors are not available, recommended practice is to monitor the load current distortion and diversity relative to the load mix in a comparable facility.
Improvements in power factor may be desired for financial reasons (to lower utility costs associated with power factor penalties) or operational reasons (to lower system losses, increase system reserve capacity, or improve voltage conditions). Extreme caution should be used when applying capacitors.
The manner in which they are applied can cause resonance conditions that can magnify harmonic levels and cause excessive voltage distortion. Power factor correction equipment may be applied directly at or close to the facility service entrance, or as close as practicable to the load equipment.
The location of the power factor equipment will depend on economic reasons, as well as operational and design considerations.
Thorough analysis of distribution system characteristics and load characteristics should be made prior to applying power factor correction capacitors to determine what effect harmonic currents will have on the system, and to determine proper harmonic mitigation techniques. Refer to IEEE Std 141-1993 for further discussion on application of power factor correction capacitors.
It is recommended practice to measure and record the harmonic profile of load currents at the transformers serving the load. When the harmonic profiles of individual loads at downstream locations are measured, there is a tendency to calculate a higher than necessary K-factor.
This is also the case in new installations where the current harmonic profile is estimated from typical
individual pieces of electronic load equipment based upon experience or data supplied by the OEM. Due to cancellation, the combined contribution to K-factor of several loads is always less than the sum of individual loads.
This reduction may be substantial when there is a large number and a diversity of nonlinear load types. Figure below shows an example of how harmonic levels vary in a typical electrical distribution system. Note that the level of harmonic current distortion decreases from the individual electronic load equipment to the branch circuit panelboards, through delta-wye stepdown transformers, and upstream to the power source.
However, when loads are removed from the electrical distribution system, the cancellation benefit produced by these loads is also removed. In many cases, this will not be a problem for a transformer that is conservatively loaded or is K-factor rated. It may be a problem if the load or K-factor rating is marginal.
Cancellation results when harmonics produced by different loads are phase-shifted relative to each other. Impedance in branch circuit wiring, as well as isolation transformers or series inductors and shunt capacitors that may be incorporated in the loads, shift harmonic currents.
A delta-wye transformer serving single-phase nonlinear loads randomly distributed among the three phases will trap the balanced triplen load harmonics in the primary winding. This may substantially reduce the triplen harmonic currents and the related current and voltage distortion that would otherwise appear on the primary side.
It is difficult to predict a harmonic diversity factor without modeling the nonlinear loads and the electrical distribution system. Computer programs and methods that allow modeling and simulation are becoming available.
With more experience, these computer analysis tools are expected to provide diversity factors for typical loads in industrial and commercial power systems. For new installations, where such diversity factors are not available, recommended practice is to monitor the load current distortion and diversity relative to the load mix in a comparable facility.
THREE PHASE VS SINGLE PHASE SYSTEM ELECTRONIC EQUIPMENT- WHAT'S THE DIFFERENCE, WHICH IS BETTER? BASIC INFORMATION
When to use single phase, and three phase system?
Some power conditioning and electronic load equipment are operable only from a three phase power source. Often single-phase equipment can be operated directly from a single phase component of a three-phase system. However, these alternatives should be carefully determined before selecting an electrical system design.
The acceptable voltage limits of all equipment must be determined and carefully evaluated to ensure proper operation on the electrical system into which it is installed. Some equipment may have features such as internal taps or other adjustments that will allow it to accept common utilization voltages.
When evaluating the choice between three-phase and single-phase systems, consideration should always be given to the fact that three-phase systems may generally support larger loads with greater efficiency. In addition, the source impedance of three-phase systems is generally lower than single-phase systems, which is important to minimize voltage waveform distortion due to nonlinear load currents. Three-phase power may also be derived from single- phase systems.
However, the derivation of three-phase power from a single-phase system is not always practical and is not recommended. Certain methods of converting a single-phase circuit to supply three-phase loads such as capacitor phase shifters are considered inappropriate for electronic load equipment and may damage these loads per IEEE Std 141-1993.
Still other methods, such as utilizing single-phase motors to drive three-phase generators, may be used to convert single-phase to three-phase. Even so, special precautions should be observed such as balancing the load among the three phases.
Most three-phase electronic load equipment cannot tolerate the application of single-phase power to its input. The resulting downtime and equipment damage can be extensive.
Because fuses and circuit breakers generally cannot prevent all types of single-phasing conditions, recommended practice is that electronic phase-failure or voltage-unbalance relays be installed where necessary to mitigate single-phasing events.
Some power conditioning and electronic load equipment are operable only from a three phase power source. Often single-phase equipment can be operated directly from a single phase component of a three-phase system. However, these alternatives should be carefully determined before selecting an electrical system design.
The acceptable voltage limits of all equipment must be determined and carefully evaluated to ensure proper operation on the electrical system into which it is installed. Some equipment may have features such as internal taps or other adjustments that will allow it to accept common utilization voltages.
When evaluating the choice between three-phase and single-phase systems, consideration should always be given to the fact that three-phase systems may generally support larger loads with greater efficiency. In addition, the source impedance of three-phase systems is generally lower than single-phase systems, which is important to minimize voltage waveform distortion due to nonlinear load currents. Three-phase power may also be derived from single- phase systems.
However, the derivation of three-phase power from a single-phase system is not always practical and is not recommended. Certain methods of converting a single-phase circuit to supply three-phase loads such as capacitor phase shifters are considered inappropriate for electronic load equipment and may damage these loads per IEEE Std 141-1993.
Still other methods, such as utilizing single-phase motors to drive three-phase generators, may be used to convert single-phase to three-phase. Even so, special precautions should be observed such as balancing the load among the three phases.
Most three-phase electronic load equipment cannot tolerate the application of single-phase power to its input. The resulting downtime and equipment damage can be extensive.
Because fuses and circuit breakers generally cannot prevent all types of single-phasing conditions, recommended practice is that electronic phase-failure or voltage-unbalance relays be installed where necessary to mitigate single-phasing events.
SELECTION OF SYSTEM VOLTAGE FOR ELECTRONIC EQUIPMENT SAFETY BASIC INFORMATION AND TUTORIALS
The selection of the ac supply system voltage typically begins at the service entrance of the facility. In most commercial environments in the U. S., the utility supplies three-phase power at 480 Y/277 V (or 600 Y/347 V) or 208 Y/120 V.
In industrial environments, the utility may supply three-phase power at even higher voltages such as 4160 V, 13 800 V and higher. The magnitude of the voltage will typically depend on the size of the facility, the load conditions, and the voltage ratings of the utilization equipment in the facility.
In some cases, the facility owners may design, install, and maintain their own medium-voltage electrical distribution system.
Recommended practice is to provide distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V) rather than at the actual utilization equipment level of most electronic load equipment (208 Y/120 V). Electrical distribution systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following benefits over 208 Y/120 V systems:
a) The source impedance of 480 Y/277 V systems are typically less than 208 Y/120 V systems. This characteristic provides a more stable source with better voltage regulation, and minimizes voltage distortion due to the nonlinear load currents.
b) 480 Y/277 V systems are less susceptible to on-premises generated disturbances. Step-down transformers (and other power enhancement devices) for 208 Y/120 V utilization equipment help attenuate disturbances originating on the 480 V system.
c) 480 Y/277 V systems distribute power at lower currents, which result in lower heat losses in feeders. 480 Y/277 V systems may also decrease material and labor costs associated with installing long feeder circuits.
Step-down transformers (and other power enhancement devices) may be located physically close to the electronic load equipment to minimize the buildup of common-mode voltage.
Delta-connected transformer primaries trap balanced triplen harmonic currents generated on the secondary side by nonlinear electronic load equipment. This action serves to reduce distortion of the voltage waveform at the 480 Y/277 V level.
It is not recommended practice to step-up the voDue to the generally lower impedance of 480 Y/277 V distribution systems, higher short-circuit currents may be available throughout the system. Overcurrent protective devices with higher interrupting capabilities and equipment with higher withstand ratings may be required.
In some situations, electrical distribution at 208 Y/120 V is unavoidable. This may be due to limitations of the utility or facility to provide higher voltages. As previously noted, nonlinear electronic load equipment may cause undesirable voltage distortion that can adversely affect the entire premises.
In these situations, a system analysis may be performed to determine proper mitigation techniques such as the installation of isolation transformers, and other power conditioning or filtering equipment located close to the electronic load equipment tage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment.
Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used.
In industrial environments, the utility may supply three-phase power at even higher voltages such as 4160 V, 13 800 V and higher. The magnitude of the voltage will typically depend on the size of the facility, the load conditions, and the voltage ratings of the utilization equipment in the facility.
In some cases, the facility owners may design, install, and maintain their own medium-voltage electrical distribution system.
Recommended practice is to provide distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V) rather than at the actual utilization equipment level of most electronic load equipment (208 Y/120 V). Electrical distribution systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following benefits over 208 Y/120 V systems:
a) The source impedance of 480 Y/277 V systems are typically less than 208 Y/120 V systems. This characteristic provides a more stable source with better voltage regulation, and minimizes voltage distortion due to the nonlinear load currents.
b) 480 Y/277 V systems are less susceptible to on-premises generated disturbances. Step-down transformers (and other power enhancement devices) for 208 Y/120 V utilization equipment help attenuate disturbances originating on the 480 V system.
c) 480 Y/277 V systems distribute power at lower currents, which result in lower heat losses in feeders. 480 Y/277 V systems may also decrease material and labor costs associated with installing long feeder circuits.
Step-down transformers (and other power enhancement devices) may be located physically close to the electronic load equipment to minimize the buildup of common-mode voltage.
Delta-connected transformer primaries trap balanced triplen harmonic currents generated on the secondary side by nonlinear electronic load equipment. This action serves to reduce distortion of the voltage waveform at the 480 Y/277 V level.
It is not recommended practice to step-up the voDue to the generally lower impedance of 480 Y/277 V distribution systems, higher short-circuit currents may be available throughout the system. Overcurrent protective devices with higher interrupting capabilities and equipment with higher withstand ratings may be required.
In some situations, electrical distribution at 208 Y/120 V is unavoidable. This may be due to limitations of the utility or facility to provide higher voltages. As previously noted, nonlinear electronic load equipment may cause undesirable voltage distortion that can adversely affect the entire premises.
In these situations, a system analysis may be performed to determine proper mitigation techniques such as the installation of isolation transformers, and other power conditioning or filtering equipment located close to the electronic load equipment tage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment.
Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used.
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