Transformers Question
1.
What are Power Transformers?
Hammond offers a wide range of dry type power transformers, rated from 750kVA up to 25 MVA. They are suitable for commercial, industrial, manufacturing or production process applications. With three phase ratings to 46 kV class and up to 25 MVA, Hammond offers the latest technology and manufacturing processes available today.
2. Explain the Banking of Transformers?
Two or three, single-phase transformers can be connected to make a three-phase bank. The primary windings of the single-phase transformers can be connected in delta or Wye and the secondary windings can be connected in either a delta or Wye configuration. The equivalent capacity of the bank will be equal to three times the nameplate rating of each single-phase transformer. Usually this type of installation is more expensive than using a single three phase transformer.
3. What are Primary Voltage Taps?
In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments. Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.
4. Can you operate a 60Hz Transformer at 50 Hz?
Transformers rated at 60Hz should not be used on a 50Hz supply due to higher losses and core saturation, and the resultant higher temperature rise. Transformers rated for 50Hz, however, can be operated on a 60Hz supply.
5. Explain Balance Loading on Single and Three Phase Transformers?
A single-phase transformer with 120/240V secondary has two separate 120V secondary windings and is usually connected into a 3-wire system. Care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total load.
Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded.
6. When is Sound Level an issue in the design?
Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission as discussed in the Hammond "Field Service Guide". Hammond Transformers are built to meet the latest ANSI, CSA and UL standards. These standards are outlined in the accompanying table.
7. When can you Reverse Connect a transformer?
In general, distribution transformers can be reverse connected without de-rating the nameplate
KVA capacity. However some precautions need to be taken for reverse connection of some smaller transformers. On Hammond transformers under 6kVA three phase and 3kVA single phase, there is a "turns ratio compensation" on the low voltage winding.
When the input voltage, equal to the nameplate rated voltage, is connected to the low voltage winding, the output voltage will be slightly lower than the nameplate rating.
When a three-phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. Further, the reverse connected transformer may draw a higher inrush current during energization. Hence the sizing of the line fusing or circuit breaker may be affected.
8. Under what circumstance does need D.C. Resistance Measurement?
Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of I2R for use in the winding temperature test, and as base data for future assessment in the field.
9. What is a Polarity and Phase-Relation test for?
Polarity and phase-relation tests are made to determine angular displacement and relative phase sequence to facilitate connections in a transformer. Determining polarity is also essential when paralleling or banking two or more transformers.
10. Explain Voltage Ratio (turns ratio)?
To confirm the voltage ratio of a transformer, the ratio of the number of turns in the high-voltage winding with respect to the number of turns in the low-voltage winding, is measured.
11. What are No-Load and Excitation Current tests?
No-load losses (excitation losses) are the core losses of a transformer that are "excited" at rated voltage and frequency, but which do not supply load. No-load losses include core loss, dielectric loss, and losses in the windings due to exciting current. The transformer is excited at rated voltage with all other windings open circuited. The exciting current and no load loss is then measured.
(Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.)
12. What are Dielectric tests?
The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the voltages associated with specified insulation levels.
13. What is an Applied Voltage test?
A normal power frequency such as 60hz is applied to each winding for one minute. These tests are in accordance with table (3) in ANSI C57-12-01.
14. What is an Induced Voltage test?
The induced voltage test is applied for 7200 cycles or 60 seconds whichever is shorter. The voltage applied is twice the operating voltage, and confines the integrity of the insulation
15. What are Impedance Voltage and Load Loss tests?
The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20°C reference temperature. (Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.)
16. What are Type Tests?
Type Tests are required either to qualify a new product or to further certify a production product. The following is a list of type tests performed on Hammond Transformers.
Temperature Rise Test
Sound Level Test
Partial Discharge (corona)
Basic Impulse Insulation Level (BIL)
Short-Circuit Test
17. What is a NEMA 1 enclosure?
This is a general-purpose ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.
18. What is a NEMA 1-N enclosure?
This is a general-purpose non-ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.
19. What is a NEMA 2 enclosure?
This is a general-purpose enclosure for indoor use primarily to provide a further degree of protection against limited amounts of falling water (drip proof) and dirt.
20. What is a NEMA 2-S enclosure?
This enclosure is the sprinkler proof version of the NEMA 2.
21. What is a NEMA 3 enclosure?
This is a general purpose ventilated enclosure for outdoor use designed primarily to provide a degree of protection against rain, sleet, wind blown snow or dust and damage from external ice formation. It is considered ideal for construction sites, subways etc.
22. What is a Non-ventilated enclosure?
A non-ventilated enclosure is constructed to restrict unintentional circulation of external air through the enclosure.
23. What is a Ventilated enclosure?
A ventilated enclosure is constructed to provide circulation of external air through the enclosure to remove excess heat.
24. What is a NEMA 3R enclosure?
Similar to the NEMA 3, it is also intended for outdoor use. It provides a greater degree of protection against rain, sleet, falling snow or dirt and damage from external ice formation. It is ideal for any outdoor installation where no blowing snow or blowing conductive dust exists.
25. What is a NEMA 3R-E enclosure?
Although similar to the NEMA 3R it also provides added protection against blowing snow and dirt. It is suitable for out-door installations where blowing snow or blowing conductive dust are present.
26. What is a NEMA 4 enclosure?
NEMA 4 is a non-ventilated indoor or outdoor enclosure designed primarily to provide a degree of protection against windblown dust and rain, splashing water, hose-directed water, and damage from external ice formation.
It is suitable in areas where exposure to large amounts of water from any direction. (Note: not submersible)
27. What is a NEMA 4X enclosure?
This enclosure is the same as the NEMA 4, and is also corrosion resistant. It is ideal for environments such as food processing plants and refineries.
28. What is a NEMA 12 enclosure?
This is a non-ventilated indoor enclosure designed primarily for providing a degree of protection against circulating dust, falling dirt, and dripping non-corrosive liquids. This enclosure is both oil and rust resistant suitable for applications such as oil refineries where oil or other chemical liquids may be prevalent. (Note: not watertight)
29. Why is Clean Power so critical?
Your computer is a delicate electronic instrument. When you use the keyboard, you're sending a series of tiny electronic impulses through the computers circuits.
The computer 'reads' these electronic impulses and makes calculations or performs tasks according to your programmed instructions. If the electrical power feeding your computer is smooth and clean, your computer will behave normally.
However, if the power fed into your computer is "dirty", you could be in for many unpleasant surprises.
30. What is Dirty Power?
Dirty power is caused by a number of things. Simply put, dirty power is what causes your radio or telephone to 'crackle' during an electrical storm; or what causes 'snow' on your TV when someone is using a power tool, sewing machine or other appliances in your house. This dirty power, or electrical noise, is a nuisance when it appears on your radio, TV or telephone. When it gets into your computer, it can cause serious errors; improper readouts, printing problems, or even damage your computers circuit.
11. How does Dirty Power affect my electronic equipment?
Your computer operates by reading electronic impulses. Dirty power contains a great number of random pulses riding on the normally smooth surface of a power wave. As these random pulses enter the circuits, your computer 'reads' them as data. This can cause a whole range of problems. You may suddenly get garbled numbers or letters in a readout or printout.
You could loose files, skip program steps, have trouble loading programs or have connection problems while on the Internet.
32. How bad can the Dirty Power problem get?
One form of dirty power usually called a surge can burn out computer, audio, video or nay other electronic circuitry in seconds. A surge is a high voltage pulse riding the normal power wave. Surges will commonly measure 600 to 2500 volts. Even though they occur for only mille-seconds, this is enough time to melt down circuits.
33. What are the most common power problems?
Research conducted by both IBM and Bell indicates that most line disturbances to sensitive equipment are line noise and voltage fluctuations.
34. What are Voltage Fluctuations?
Under voltages and over voltages are caused by faults on power lines, and the subsequent actions of fault clearing devices. Also, by heavy loads, such as machinery start-up and by the slow reaction of power company regulating equipment. Since computer equipment is designed to operate close to nominal voltages, the effects of these voltage variations can cause serious problems. Voltages can drop as much as 20% of nominal. This can result in expensive and time consuming errors, loss of information, downtime, recovery and rerun costs and possible equipment damage.
35. What is electrical Noise?
Noise is a very broad term that can be applied to a number of AC power line disturbances. Lightening surges or any other sudden changes in load, such as switching motor loads or power factor correcting capacitors can produce voltage spikes and ringing. Phase controlled rectifier loads and arcing devices produce continuous noise unless adequately filtered. Noise sources are either common mode, which appears between both sides of a power line and ground or of transverse mode, which appears from line to line. Hammond Clean Power products, such as our Computer Regulators remove these noise sources.
36. What are Harmonics?
Harmonics, in an electrical system, are currents created by non-linear loads that generate non-sinusoidal (non-linear) current waveforms. These current and voltage wave forms operate on frequencies that are in multiples of the fundamental 60hz frequency. That is, the fundamental frequency is at 60 hertz, the 2nd harmonic is at 120hz frequency (60 x 2), the 3rd at 180 hertz, and so forth. Harmonics are principally the by-product of switch-mode power supply technology where AC is rectified to DC, and back again. In the process, a capacitor is charged in the first half-cycle, and then discharged in the next half-cycle, in supplying current to the load. This cycle is repeated. This action of recharging causes AC current to flow only during a portion of the AC voltage wave, in abrupt pulses. These abrupt pulses distort the fundamental wave shape causing distortion to the various harmonic frequencies.
37. What are Non-Linear Loads?
Today, non-linear loads make up a large percentage of all electrical demand. Rectified input, switching power supplies and electronic lighting ballasts are the most common single-phase non-linear loads. Harmonic currents and voltages produced by single phase, non-linear loads which are connected phase-to-neutral in a three phase four wire system, are third order, zero sequence harmonics (the third harmonic and its odd multiples - 3rd, 9th, 15th, 21st, etc., phasors displaced by zero degrees). These third order, zero sequence harmonic currents, do not cancel but add up arithmetically on the neutral bus, creating a primary source of excessive neutral current.
38. Explain the K-Factor rating?
K factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses at 60Hz. It is used to specify transformers for non-linear loads. Transformers with a rated K factor of 4, 7, 13, 20 and 30 are available. For balanced loading, a transformer with a K factor of 4 should be specified when no more than 50% of the total load is non-linear. A transformer with K factor 13 should be specified when 100% of the load is non-linear.
Hammond offers a wide range of dry type power transformers, rated from 750kVA up to 25 MVA. They are suitable for commercial, industrial, manufacturing or production process applications. With three phase ratings to 46 kV class and up to 25 MVA, Hammond offers the latest technology and manufacturing processes available today.
2. Explain the Banking of Transformers?
Two or three, single-phase transformers can be connected to make a three-phase bank. The primary windings of the single-phase transformers can be connected in delta or Wye and the secondary windings can be connected in either a delta or Wye configuration. The equivalent capacity of the bank will be equal to three times the nameplate rating of each single-phase transformer. Usually this type of installation is more expensive than using a single three phase transformer.
3. What are Primary Voltage Taps?
In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments. Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.
4. Can you operate a 60Hz Transformer at 50 Hz?
Transformers rated at 60Hz should not be used on a 50Hz supply due to higher losses and core saturation, and the resultant higher temperature rise. Transformers rated for 50Hz, however, can be operated on a 60Hz supply.
5. Explain Balance Loading on Single and Three Phase Transformers?
A single-phase transformer with 120/240V secondary has two separate 120V secondary windings and is usually connected into a 3-wire system. Care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total load.
Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded.
6. When is Sound Level an issue in the design?
Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission as discussed in the Hammond "Field Service Guide". Hammond Transformers are built to meet the latest ANSI, CSA and UL standards. These standards are outlined in the accompanying table.
7. When can you Reverse Connect a transformer?
In general, distribution transformers can be reverse connected without de-rating the nameplate
KVA capacity. However some precautions need to be taken for reverse connection of some smaller transformers. On Hammond transformers under 6kVA three phase and 3kVA single phase, there is a "turns ratio compensation" on the low voltage winding.
When the input voltage, equal to the nameplate rated voltage, is connected to the low voltage winding, the output voltage will be slightly lower than the nameplate rating.
When a three-phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. Further, the reverse connected transformer may draw a higher inrush current during energization. Hence the sizing of the line fusing or circuit breaker may be affected.
8. Under what circumstance does need D.C. Resistance Measurement?
Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of I2R for use in the winding temperature test, and as base data for future assessment in the field.
9. What is a Polarity and Phase-Relation test for?
Polarity and phase-relation tests are made to determine angular displacement and relative phase sequence to facilitate connections in a transformer. Determining polarity is also essential when paralleling or banking two or more transformers.
10. Explain Voltage Ratio (turns ratio)?
To confirm the voltage ratio of a transformer, the ratio of the number of turns in the high-voltage winding with respect to the number of turns in the low-voltage winding, is measured.
11. What are No-Load and Excitation Current tests?
No-load losses (excitation losses) are the core losses of a transformer that are "excited" at rated voltage and frequency, but which do not supply load. No-load losses include core loss, dielectric loss, and losses in the windings due to exciting current. The transformer is excited at rated voltage with all other windings open circuited. The exciting current and no load loss is then measured.
(Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.)
12. What are Dielectric tests?
The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the voltages associated with specified insulation levels.
13. What is an Applied Voltage test?
A normal power frequency such as 60hz is applied to each winding for one minute. These tests are in accordance with table (3) in ANSI C57-12-01.
14. What is an Induced Voltage test?
The induced voltage test is applied for 7200 cycles or 60 seconds whichever is shorter. The voltage applied is twice the operating voltage, and confines the integrity of the insulation
15. What are Impedance Voltage and Load Loss tests?
The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20°C reference temperature. (Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.)
16. What are Type Tests?
Type Tests are required either to qualify a new product or to further certify a production product. The following is a list of type tests performed on Hammond Transformers.
Temperature Rise Test
Sound Level Test
Partial Discharge (corona)
Basic Impulse Insulation Level (BIL)
Short-Circuit Test
17. What is a NEMA 1 enclosure?
This is a general-purpose ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.
18. What is a NEMA 1-N enclosure?
This is a general-purpose non-ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.
19. What is a NEMA 2 enclosure?
This is a general-purpose enclosure for indoor use primarily to provide a further degree of protection against limited amounts of falling water (drip proof) and dirt.
20. What is a NEMA 2-S enclosure?
This enclosure is the sprinkler proof version of the NEMA 2.
21. What is a NEMA 3 enclosure?
This is a general purpose ventilated enclosure for outdoor use designed primarily to provide a degree of protection against rain, sleet, wind blown snow or dust and damage from external ice formation. It is considered ideal for construction sites, subways etc.
22. What is a Non-ventilated enclosure?
A non-ventilated enclosure is constructed to restrict unintentional circulation of external air through the enclosure.
23. What is a Ventilated enclosure?
A ventilated enclosure is constructed to provide circulation of external air through the enclosure to remove excess heat.
24. What is a NEMA 3R enclosure?
Similar to the NEMA 3, it is also intended for outdoor use. It provides a greater degree of protection against rain, sleet, falling snow or dirt and damage from external ice formation. It is ideal for any outdoor installation where no blowing snow or blowing conductive dust exists.
25. What is a NEMA 3R-E enclosure?
Although similar to the NEMA 3R it also provides added protection against blowing snow and dirt. It is suitable for out-door installations where blowing snow or blowing conductive dust are present.
26. What is a NEMA 4 enclosure?
NEMA 4 is a non-ventilated indoor or outdoor enclosure designed primarily to provide a degree of protection against windblown dust and rain, splashing water, hose-directed water, and damage from external ice formation.
It is suitable in areas where exposure to large amounts of water from any direction. (Note: not submersible)
27. What is a NEMA 4X enclosure?
This enclosure is the same as the NEMA 4, and is also corrosion resistant. It is ideal for environments such as food processing plants and refineries.
28. What is a NEMA 12 enclosure?
This is a non-ventilated indoor enclosure designed primarily for providing a degree of protection against circulating dust, falling dirt, and dripping non-corrosive liquids. This enclosure is both oil and rust resistant suitable for applications such as oil refineries where oil or other chemical liquids may be prevalent. (Note: not watertight)
29. Why is Clean Power so critical?
Your computer is a delicate electronic instrument. When you use the keyboard, you're sending a series of tiny electronic impulses through the computers circuits.
The computer 'reads' these electronic impulses and makes calculations or performs tasks according to your programmed instructions. If the electrical power feeding your computer is smooth and clean, your computer will behave normally.
However, if the power fed into your computer is "dirty", you could be in for many unpleasant surprises.
30. What is Dirty Power?
Dirty power is caused by a number of things. Simply put, dirty power is what causes your radio or telephone to 'crackle' during an electrical storm; or what causes 'snow' on your TV when someone is using a power tool, sewing machine or other appliances in your house. This dirty power, or electrical noise, is a nuisance when it appears on your radio, TV or telephone. When it gets into your computer, it can cause serious errors; improper readouts, printing problems, or even damage your computers circuit.
11. How does Dirty Power affect my electronic equipment?
Your computer operates by reading electronic impulses. Dirty power contains a great number of random pulses riding on the normally smooth surface of a power wave. As these random pulses enter the circuits, your computer 'reads' them as data. This can cause a whole range of problems. You may suddenly get garbled numbers or letters in a readout or printout.
You could loose files, skip program steps, have trouble loading programs or have connection problems while on the Internet.
32. How bad can the Dirty Power problem get?
One form of dirty power usually called a surge can burn out computer, audio, video or nay other electronic circuitry in seconds. A surge is a high voltage pulse riding the normal power wave. Surges will commonly measure 600 to 2500 volts. Even though they occur for only mille-seconds, this is enough time to melt down circuits.
33. What are the most common power problems?
Research conducted by both IBM and Bell indicates that most line disturbances to sensitive equipment are line noise and voltage fluctuations.
34. What are Voltage Fluctuations?
Under voltages and over voltages are caused by faults on power lines, and the subsequent actions of fault clearing devices. Also, by heavy loads, such as machinery start-up and by the slow reaction of power company regulating equipment. Since computer equipment is designed to operate close to nominal voltages, the effects of these voltage variations can cause serious problems. Voltages can drop as much as 20% of nominal. This can result in expensive and time consuming errors, loss of information, downtime, recovery and rerun costs and possible equipment damage.
35. What is electrical Noise?
Noise is a very broad term that can be applied to a number of AC power line disturbances. Lightening surges or any other sudden changes in load, such as switching motor loads or power factor correcting capacitors can produce voltage spikes and ringing. Phase controlled rectifier loads and arcing devices produce continuous noise unless adequately filtered. Noise sources are either common mode, which appears between both sides of a power line and ground or of transverse mode, which appears from line to line. Hammond Clean Power products, such as our Computer Regulators remove these noise sources.
36. What are Harmonics?
Harmonics, in an electrical system, are currents created by non-linear loads that generate non-sinusoidal (non-linear) current waveforms. These current and voltage wave forms operate on frequencies that are in multiples of the fundamental 60hz frequency. That is, the fundamental frequency is at 60 hertz, the 2nd harmonic is at 120hz frequency (60 x 2), the 3rd at 180 hertz, and so forth. Harmonics are principally the by-product of switch-mode power supply technology where AC is rectified to DC, and back again. In the process, a capacitor is charged in the first half-cycle, and then discharged in the next half-cycle, in supplying current to the load. This cycle is repeated. This action of recharging causes AC current to flow only during a portion of the AC voltage wave, in abrupt pulses. These abrupt pulses distort the fundamental wave shape causing distortion to the various harmonic frequencies.
37. What are Non-Linear Loads?
Today, non-linear loads make up a large percentage of all electrical demand. Rectified input, switching power supplies and electronic lighting ballasts are the most common single-phase non-linear loads. Harmonic currents and voltages produced by single phase, non-linear loads which are connected phase-to-neutral in a three phase four wire system, are third order, zero sequence harmonics (the third harmonic and its odd multiples - 3rd, 9th, 15th, 21st, etc., phasors displaced by zero degrees). These third order, zero sequence harmonic currents, do not cancel but add up arithmetically on the neutral bus, creating a primary source of excessive neutral current.
38. Explain the K-Factor rating?
K factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses at 60Hz. It is used to specify transformers for non-linear loads. Transformers with a rated K factor of 4, 7, 13, 20 and 30 are available. For balanced loading, a transformer with a K factor of 4 should be specified when no more than 50% of the total load is non-linear. A transformer with K factor 13 should be specified when 100% of the load is non-linear.
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