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Low Voltage Dry-type Transformer

“Low-voltage” means that it has an input voltage of 600 volts or less.

  1. Low-voltage dry-type transformers

    2. "Dry-type" is a reference to the type of insulation medium, which means that the core and coil is cooled and insulated by air, as opposed to "liquid immersed" transformers that use oil as the coolant/insulant.

  2. Application

    3. Low-voltage dry-type transformers are generally used inside buildings and industrial facilities to reduce voltage to the values necessary to power lighting and loads with input voltages ranges from 480V and below.

Transformer Types
Low Voltage, Greater than 600V
Step Down

Changes a higher voltage to a lower voltage, such as 480V to 208V, and provides isolation.

Step Up

Changes a lower voltage to a higher voltage, such as 240V to 480V, and provides isolation. Note: In most cases the transformers (describe above) that provide isolation also incorporate a grounded “Faraday Shield”, between the primary and secondary windings, to reduce the transmittance of high frequency “noise”. Although shielding is generally included, it should be specified

Isolation Transformer

Does not change voltage. It does, however, provide isolation from the facilities electrical system, which eliminates much of the buildings electrical “noise” on the output..

Auto-Transformer
  1. Transformers which have one winding are called auto transformers

    2. An autotransformer has the usual magnetic core but only one winding, which is common to both the primary and secondary circuits.

  2. Auto transformers usually consist of a single coil with a tap connection at a designated location on the winding.
Buck/Boost:

For example, a machine has an electric motor which requires 208 V, but the electrical supply is 240 V. If ordering the machine with a 240-V motor is expensive, a Jess costly solution to the problem may be to buck the voltage from 240 V down to 208 V with a buck and boost transformer.

Transformer Nameplate Data
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KVA Ratings:

The nameplate kVA rating of a transformer represents the amount of kVA loading that will result in the rated temperature rise when the unit is operated under normal service conditions. When operating under these conditions (including the accepted hot-spot temperature with the correct class of insulation materials), you should achieve a "normal" life expectancy for the transformer.

Standard KVA Ratings
3KVA 6KVA 9KVA 15KVA
25KVA 30KVA 45KVA 50KVA
75KVA 30KVA 112.5KVA 225KVA
300KVA 500KVA 750KVA 1000KVA
Standard Voltage ratings for single Phase and three phase transformers:
  1. 120V, 208, 240V
  2. 480V, 600V
FAQ: Can Transformers be Operated at Voltages other than Nameplate Voltages?

In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated at a voltage in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly. For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts and if the transformer were originally rated 10 KVA, the reduced rating

Current Ratings:

Single Phase Current

\(I_{FLA}=\frac{KVA \cdot 1000}{V_{Phase}} \)

Where:

Phase voltage can be either Line-to-line or line-to-neutral

Three Phase Current

\(I_{FLA}=\frac{KVA \cdot 1000}{\sqrt{3}\cdot V_{Phase}} \)

Frequency (Hz):

  1. Hertz(Hz) is also termed “Cycles per Second” or “Frequency”. With relation to sine waves, it’s the time it takes for one complete cycle.
  2. The three common frequencies available are 50Hz, 60Hz and 400Hz.

    3. European power is typically 50Hz while North American power is usually 60hz.
    The 400 Hz is reserved for high-powered applications such as aerospace and some special-purpose computer power supplies and hand-held machine tools. Such high frequencies cannot be economically transmitted long distances, so 400 Hz power systems are usually confined to the building or vehicle.

  3. Transformer frequency is fixed

    4. Transformers cannot change the frequency of the source kva (voltage & current). If the source frequency is 60 Hz, the output will also be 60 Hz. In most parts of the Americas, it is typically 60Hz, and in the rest of the world it is typically 50Hz. Places that use the 50 Hz frequency tend to use 230 V RMS, and those that use 60Hz tend to use 117 V RMS.

Winding Configurations
Delta-Wye
Wye-Delta
Delta-Delta
Wye-Wye
Open Delta
Scott (T-T)
Interconnected-Star or Zig-Zag
What is Meant by "Impedance" in Transformers?

Impedance is the current limiting characteristic of a transformer and is expressed in percentage.

The percentage impedance of a transformer (Z%) is the voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage.
Electrical impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit.

In general, impedance has a complex value, which means that loads generally have a resistance to the source that is in phase with a sinusoidal source signal and reactance that is out of phase with a sinusoidal source signal.
The total impedance is the vector sum of the resistance and the reactance. The impedance is measured by shorting the low voltage terminals.

Why is Impedance Important?

It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer. Example: Determine a minimum circuit breaker trip rating and nterrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt  50 Hz source.

Maximum Short Circuit Secondary Current

The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by multiplying the reciprocal of the impedance times the full load current.

\(I_{FLA}=\frac{KVA \cdot 1000}{V_{phase} \cdot Z} \)

Where:

\( {V_{phase}: V_{Line-Line} } \)

Z : %Z (decimal form)

Example:

Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt  50 Hz source.

Calculate as follows:

Normal Full Load Current:

\(= \frac{Nameplate Volt Amps}{Line Volts} \)

\(= \frac{10,000 VA}{480V} \)

Example:

Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt  50 Hz source.

Calculate as follows:

Maximum Short Circuit Amps:

\(= \frac{Full Load Amps}{Z} \)

\(= \frac{20.8 Amps}{0.04} =520 Amps\)

Voltage Taps:

Voltage supplied to a Transformer may vary from nominal voltage due to the distance from the substation or source. Higher or lower input voltage will result in higher or lower output voltage if there are no voltage adjustment taps present.

In order to compensate for this voltage difference, transformers secondary voltage can be adjusted to nominal levels by adjusting the transformer's primary winding's voltage Tap.

Transformer voltage taps change the voltage ratio of a transformer so that its secondary voltage stays at nominal. On large power transformers, taps on the primary are used to offset any higher or lower input voltages. These tap connections are usually set at the factory for nominal line voltage. If the voltage at the site is different, the taps are changed accordingly.

Changing Voltage Taps
  1. decreasing the primary voltage Tap will increase secondary voltage
  2. Increasing the primary voltage Tap will decrease secondary voltage

    3. Standard tap arrangements are at two and one-half and five percent of the rated primary voltage for both high and low voltage conditions. For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the pr'imary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating. The standard ASA and NEMA designation for taps are "ANFC" (above normal full capacity) and "BNFC" (below normal full capacity).

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Temperature Rise:

Transformer temperature rise is defined as the average temperature rise of the windings above the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating.

Dry-type transformers are available in three standard temperature rises:

  • 80C
  • 115C
  • 150C

Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life.

Are Temperature Rise and Actual Surface Temperature Related?

No. This can be compared with an ordinary light bulb. The filament temperature of a light bulb can exceed ■ 2000 degrees, yet the surface temperature of the bulb is low enough to perm-it touching with bare hands.

Sound Level:

Unless otherwise specified, transformer sound levels shall conform to ANSI/NEMA as follows:

10 to 50 kVA 45 dB
51 to 150 kVA 50 dB
351 to 500 kVA 60 dB
51 to 150 kVA 50 dB
151 to 300 kVA 55 dB
K Factor:

Per the NEC, transformers feeding non-linear loads shall be K-factor rated.
K-factor is the ratio between the additional losses due to harmonics and eddy current losses at 60Hz.

What is BIL and How Does it Apply to Transformers Listed

BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown. All transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV. This assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar surge protection devices.

Rated Voltage of windings Windings Bushings
Insulation Class Bil Insulation Class Bil
480V 1.2KV 10KV 1.2KV 10KV
4160V 5KV 30KV 5KV 30KV
13.8KV 15KV 95KV 15KV 95KV
34.5KV 34,500V 150KV 34,500V 150KV
AA Ventilated, self-cooled transformers. These transformers have ventilation ports located in outside walls of the enclosure. There are no fans to force air into and out of the enclosure with typically no external fins or radiators. Cooler air enters the lower ports, is heated as it rises past windings, and exits the upper ventilation ports.
AFA Self-cooled (A) and additionally cooled by forced circulation of air (FA). These transformers have ventilation ports for fan inlets and outlets only. Normally, there are no additional ventilation ports for natural air circulation.
AA/FA Ventilated, self-cooled (same as Class AA). These transformers have a fan or fans providing additional forced-air cooling. Fans may be wired to start automatically when the temperature reaches a pre-set value. These transformers generally have a dual load rating, one for AA (self-cooling natural air flow) and a larger load rating for FA (forced air flow).
ANV Self-cooled (A), non-ventilated (NV). These transformers have no ventilation ports or fans on the enclosure and is not sealed to exclude migration of outside air, but there are no provisions to intentionally allow outside air to enter and exit. Cooling is by natural circulation of air around the enclosure. This transformer may have some type of fins attached outside the enclosure to increase surface area for additional cooling.
GA Sealed with internal gas (G) and self-cooled (A). These transformers typically have a gas, such as nitrogen, SF6, or freon, to provide high dielectric and good heat removal. Cooling occurs by natural circulation of air around the outside of the enclosure. There are no fans to circulate cooling air; however, there may be fins attached to the outside to aid in cooling. The enclosure is hermetically sealed to prevent leakage.
Transformer Test Procedures
Dry Type, Low Voltage 600 volts or less
  1. Visual and Mechanical Inspection
  2. Transformer Turns Ratio (TTR)
  3. Winding Resistance
  4. Insulation Resistance
Transformer Test Procedure
NETA Standard

NETA ATS

7.2.1.1 Transformers, Dry Type, Air-Cooled, Low-Voltage, Small

NOTE: This category consists of power transformers with windings rated 600 volts or less and sizes equal to or
less than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition..
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed.
  5. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.2.1.1.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.
  6. Verify that as-left tap connections are as specified.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.2.1.1.A.6.1.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data or in the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. *Perform turns-ratio tests at all tap positions.
  4. Verify correct secondary voltage phase-to-phase and phase-to-neutral after energization and prior to loading.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.2.1.1.A.6.1)
  2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.2.1.1.A.6.2)
  3. Results of the thermographic survey shall be in accordance with Section 9. (7.2.1.1.1.6.3)
  4. Tap connections are left as found unless otherwise specified. (7.2.1.1.A.7)
D. Test Values – Electrical
  1. Compare bolted electrical connection resistances to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. Turns-ratio test results shall not deviate by more than one-half percent from either the adjacent coils or the calculated ratio.
  4. Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement with nameplate data.

NETA ATS

7.2.1.2 Transformers, Dry Type, Air-Cooled, Large

NOTE: This category consists of power transformers with windings rated higher than 600 volts and low-voltage transformers larger than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Compare equipment nameplate data with drawings and specifications.
  2. Inspect physical and mechanical condition..
  3. Inspect anchorage, alignment, and grounding.
  4. Verify that resilient mounts are free and that any shipping brackets have been removed..
  5. Verify the unit is clean.
  6. *Verify that control and alarm settings on temperature indicators are as specified.
  7. Verify that cooling fans operate and that fan motors have correct overcurrent protection.
  8. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.2.1.2.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.
  9. Perform specific inspections and mechanical tests as recommended by the manufacturer.
  10. Verify that as-left tap connections are as specified.
  11. Verify the presence of surge arresters.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.2.1.2.A.8.1.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform power-factor or dissipation-factor tests on all windings in accordance with the test equipment manufacturer’s published data.
  4. *Perform a power-factor or dissipation-factor tip-up test on windings greater than 2.5 kV.
  5. Perform turns-ratio tests at all tap positions.
  6. *Perform an excitation-current test on each phase.
  7. *Measure the resistance of each winding at each tap connection.
  8. Measure core insulation resistance at 500 volts dc if the core is insulated and the core ground strap is removable.
  9. *Perform an applied voltage test on all high- and low-voltage windings-to-ground. See ANSI/IEEE C57.12.91, Sections 10.2 and 10.9.
  10. Verify correct secondary voltage, phase-to-phase and phase-to-neutral, after energization and prior to loading.
  11. Test surge arresters in accordance with Section 7.19.
C. Test Values – Visual and Mechanical
  1. Control and alarm settings on temperature indicators shall operate within manufacturer’s recommendations for specified settings. (7.2.1.2.A.6)
  2. Cooling fans shall operate. (7.2.1.2.A.7)
  3. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.2.1.2.A.8.1)
  4. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.2.1.2.A.8.2)
  5. Results of the thermographic survey shall be in accordance with Section 9. (7.2.1.2.A.8.3)
  6. Tap connections shall be left as found unless otherwise specified. (7.2.1.2.A.10)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. The following values are typical for insulation power factor tests:
    1. CHL Power transformers: 2.0 percent or less
    2. CHL Distribution transformers: 5.0 percent or less
    3. CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. Consult transformer manufacturer’s or test equipment manufacturer’s data for additional information.
  4. Power-factor or dissipation-factor tip-up exceeding 1.0 percent shall be investigated.
  5. Turns-ratio test results shall not deviate more than one-half percent from either the adjacent coils or the calculated ratio.
  6. The typical excitation current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading.
  7. Temperature-corrected winding-resistance values shall compare within one percent of previously-obtained results.
  8. Core insulation-resistance values shall not be less than one megohm at 500 volts dc.
  9. AC dielectric withstand test voltage shall not exceed 75 percent of factory test voltage for one minute duration. DC dielectric withstand test voltage shall not exceed 100 percent of the ac rms test voltage specified in ANSI C57.12.91, Section 10.2 for one minute duration. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand test, the test specimen is considered to have passed the test.
  10. Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement with nameplate data.
  11. Test results for surge arresters shall be in accordance with Section 7.19.

NETA MTS

7.2.1.1 Transformers, Dry Type, Air-Cooled, Low-Voltage, Small

NOTE: This category consists of power transformers with windings rated 600 volts or less and sizes equal to or less than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition.
  2. Inspect anchorage, alignment, and grounding.
  3. Prior to cleaning the unit, perform as-found tests, if required.
  4. VClean the unit.
  5. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.2.1.1.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.
  6. Perform as-left tests..
  7. Verify that as-left tap connections are as specified.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.2.1.1.A.5.1.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data or in the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. *Perform turns-ratio tests at all tap positions.
C. Test Values – Visual and Mechanical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.2.1.1.A.5.1)
  2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.2.1.1.A.5.2)
  3. Results of the thermographic survey shall be in accordance with Section 9. (7.2.1.1.A.5.3)
  4. Tap connections are left as found unless otherwise specified. (7.2.1.1.A.7)
D. Test Values – Electrical
  1. Compare bolted electrical connection resistances to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. Turns-ratio test results shall not deviate by more than one-half percent from either the adjacent coils or the calculated ratio.

NETA MTS

7.2.1.2 Transformers, Dry Type, Air-Cooled, Large

NOTE: This category consists of power transformers with windings rated higher than 600 volts and low-voltage transformers larger than 167 kVA single-phase or 500 kVA three-phase.
A. Visual and Mechanical Inspection:
  1. Inspect physical and mechanical condition..
  2. Inspect anchorage, alignment, and grounding.
  3. Prior to cleaning the unit, perform as-found tests, if required.
  4. Clean the unit.
  5. *Verify that control and alarm settings on temperature indicators are as specified.
  6. Verify that cooling fans operate correctly.
  7. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.2.1.2.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform thermographic survey in accordance with Section 9.
  8. Perform specific inspections and mechanical tests as recommended by the manufacturer.
  9. Perform as-left tests.
  10. Verify that as-left tap connections are as specified.
  11. Verify the presence of surge arresters.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.2.1.2.A.7.1.
  2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.
  3. Perform insulation power-factor or dissipation-factor tests on all windings in accordance with the test equipment manufacturer’s published data.
  4. *Perform a power-factor or dissipation-factor tip-up test on windings rated greater than 2.5 kV.
  5. Perform turns-ratio tests at the designated tap position.
  6. *Perform an excitation-current test on each phase.
  7. *Measure the resistance of each winding at the designated tap position.
  8. Measure core insulation resistance at 500 volts dc if the core is insulated and if the core ground strap is removable.
  9. *Perform an applied voltage test on all high- and low-voltage windings-to-ground. See IEEE C57.12.91-2001, Sections 103.
  10. Verify correct secondary voltage phase-to-phase and phase-to-neutral after energization and prior to loading.
  11. Test surge arresters in accordance with Section 7.19.
  12. *Perform online partial-discharge survey on winding rated higher than 600 volts in accordance with Section 11.
C. Test Values – Visual and Mechanical
  1. Control and alarm settings on temperature indicators shall operate within manufacturer’s recommendations for specified settings. (7.2.1.2.A.5)
  2. Cooling fans shall operate. (7.2.1.2.A.6)
  3. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value. (7.2.1.2.A.7.1)
  4. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.2.1.2.A.7.2)
  5. Results of the thermographic survey shall be in accordance with Section 9. (7.2.1.2.A.7.3)
  6. Tap connections shall be left as found unless otherwise specified. (7.2.1.2.A.10)
D. Test Values – Electrical
  1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
  2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.
  3. The following values are typical for insulation power factor tests:
    1. CHL Power transformers: 2.0 percent or less
    2. CHL Distribution transformers: 5.0 percent or less
    3. CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. Consult transformer manufacturer’s or test equipment manufacturer’s data for additional information.
  4. Power-factor or dissipation-factor tip-up exceeding 1.0 percent shall be investigated.
  5. Turns-ratio test results shall not deviate more than one-half percent from either the adjacent coils or the calculated ratio.
  6. The typical excitation current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading.
  7. Temperature-corrected winding-resistance values shall compare within one percent of previously-obtained results.
  8. Core insulation-resistance values shall not be less than one megohm at 500 volts dc.
  9. AC dielectric withstand test voltage shall not exceed 65 percent of factory test voltage for one minute duration. DC dielectric withstand test voltage shall not exceed 100 percent of the ac rms test voltage specified in IEEE C57.12.91, Section 10.2 for one minute duration. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectric withstand voltage test, the test specimen is considered to have passed the test.
  10. Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement with nameplate data.
  11. Test results for surge arresters shall be in accordance with Section 7.19.
  12. Results of online partial-discharge survey should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, refer to Table 100.23.
NETA ATS / MTS
TABLE 100.5
Neta Table 100.5
NETA ATS / MTS
Neta Table 100.5
Neta Table 100.5
Transformer Turns Ratio
DESCRIPTION:

The transformer turns ratio test, or TTR test, confirms that the transformer has the correct ratio of primary turns to secondary turns

PURPOSE:

Verifies the transformer's input and output voltage ratio.

PROCEDURE:

Transformer Turns Ratio

Single-Phase Models:
Single-Phase Models:

  1. Connect the exciting leads (X1 and X2) to the lower-voltage winding.

  2. Connect the H2 lead to the other high voltage terminal.

  3. Where both windings are grounded on one side, connect X1 and H1 to the grounded sides.

  4. Always excite the entire low-voltage winding. For polyphase transformers, repeat procedure on each set of windings to be measured.

  5. Match transformer polarity by connecting the H1 secondary lead to the higher-voltage terminal which corresponds to the X1 connection. See Figure.

TTR-test-diagram

Transformer Turns Ratio

Three-Phase Models:
  1. Connect the H0, H1, H2, and H3 test cables to the corresponding high-voltage winding terminals of the transformer under test.
  2. Note: With delta connected windings, H0 is not used.
  3. Connect the X0, X1, X2, and X3 test cables to the corresponding low-voltage winding of the transformer under test.
  4. Note: With delta connected windings, X0 is not used.
  5. It is possible to test single phase two-winding transformers with a three-phase TTR test set by only utilizing the H1, H2 and X1, X2 leads.
TTR-test-diagram
Winding Resistance Test:
PURPOSE:

Winding resistance in transformers will change due to shorted turns, loose connections, or deteriorating contacts in tap changers. Regardless of the configuration, the resistance measurements are normally made phase-to-phase and the readings are compared with each other to determine if they are acceptable.

DESCRIPTION:

This test measures the DC resistance of the transformer leads and windings and is made with a low-resistance ohmmeter or a Kelvin bridge. The test procedure for measuring DC winding resistance requires the transformer to be de-energized and isolated. Both the primary and secondary terminals should be isolated from external connections, and measurements made on each phase of all windings.

The main purpose of this test is to check for gross differences between windings and for opens in the connections. Measuring the resistance of transformer windings assures that each circuit is wired properly and that all connections are tight.

PROCEDURE:

Transformer winding resistance measurements are obtained by passing a known DC current through the winding under test and measuring the voltage drop across each terminal (Ohm's Law). Modern test equipment for this purposes utilizes a Kelvin bridge to achieve results; you might think of a winding resistance test set as a very large low-resistance ohmmeter (DLRO).

TEST RESULTS:

The IR test is of value for future comparative purposes and also for determining the suitability of the transformer of energizing or application of the high-potential (hi-pot) test. Insulation-resistance tests should be performed on winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index.

Test Equipment:
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Insulation Resistance
PURPOSE:

  1. Insulation-resistance tests determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration.

  2. Test Values should be recorded for future comparative purposes and also for determining the suitability of the transformer of energizing or application of the high-potential (hi-pot) test. This test should be conducted before and after repair or when maintenance is performed.

DESCRIPTION:

  1. Insulation-resistance tests are performed at or above rated voltage. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5.

  2. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer.

  3. The test values should be normalized to 20°C for comparison purposes The test data should be recorded for future comparative purposes. Calculate polarization index

PROCEDURE:
Insulation Resistance Test:
Test Duration : 1 minute

Megohmmeter reading should be maintained for a period of 1 min.
Make the following readings for two-winding transformers:

Test Connections
  1. High-voltage winding to low-voltage winding and to ground
  2. High-voltage winding to ground
  3. Low-voltage winding to high-voltage winding and to ground
  4. Low-voltage winding to ground
  5. High-voltage winding to low-voltage winding
TEST RESULTS:
Polarization Index (PI) test:
Test Duration : 10 minute
A PI below 2 is indicative of insulation deterioration and cause for further investigation.

This is an extension of the IR test. In this test, the two IR measurements are taken, the fi rst reading at 1 min and the second reading at 10 min.

Then the ratio of the 10 min reading to 1 min reading is calculated to give the PI dielectric absorption value. A PI of winding-to-winding and winding-to-ground should be determined.

NETA ATS TABLE 100.5
Insulation Resistance Test Values
Electrical Apparatus and Systems Other han Rotating Machinery
Neta Table 100.5
NETA MTS TABLE 100.5
Insulation Resistance Test Values
Electrical Apparatus and Systems Other Than Rotating Machinery
Neta Table 100.5
NETA ATS TABLE 100.14
Insulation Resistance Conversion Factors (20°C)
Neta Table 100.5
Circuit Breakers
Transformer Protection
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NEC Article 450:
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450.4 Autotransformers 1000 Volts, Nominal, or Less

(A) Overcurrent Protection. Each autotransformer 1000 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. Where this calculation does not correspond to a standard

  1. 3KVA, 6KVA, 9KVA
  2. 15KVA, 30KVA, 45KVA
  3. 25KVA, 37.5KVA, 50KVA, 75KVA
  4. 112.5KVA, 225KVA, 300KVA, 500KVA
  5. 750KVA, 1000KVA

Primary and Secondary Protection Chart

KVA Primary Voltage: 480V Secondary Voltage: 208V
Pri.FLA 250% Max CB Rating Sec. FLA 125% Max CB Rating
30 36.08 A 90.21 A 90 83.27 A 104.09 A 100 A
45 36.08 A 90.21 A 90 83.27 A 104.09 A 100 A
50 60.14 A 105.35 A 150 A 138.79 A 173.48 A 200 A
75 90.21 A 225.53 A 225 A 208.18 A 260.22 A 300 A
112.5 135.32 A 150 A 312.27 A 300 A 300 A 300 A
150 180.42 A 451.06 A 450 A 416.36 A 520.45 A 600 A
225 270.63 A 676.58 A 700 A 624.54 A 780.67 A 680 A
300 360.84 A 902.11 A 900 A 832.72 A 1040.90 A 1000 A
500 601.41 A 1503.52 A 1500 A 1387.86 A 1734.83 A 2000 A
750 902.11 A 2255.28 A 2500 A 2081.79 A 2602.24 A 3000 A
1000 1202.81 A 3007.03 A 3000 A 2775.72 A 3469.65 A 4000 A
Standard Voltage ratings for single Phase and three phase transformers:
  1. 120V, 208, 240V
  2. 480V, 600V
FAQ: Can Transformers be Operated at Voltages other than Nameplate Voltages?

In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated at a voltage in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly. For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts and if the transformer were originally rated 10 KVA, the reduced rating

Current Ratings:
Single Phase Current

\(I_{FLA}=\frac{KVA \cdot 1000}{V_{Phase}} \)

Three Phase Current

\(I_{FLA}=\frac{KVA \cdot 1000}{\sqrt{3}\cdot V_{Phase}} \)

Where:
  • \( {V_{Phase}: V_{Line-Line} , V_{Line-neutral} } \) \(\text{Phase voltage can be either Line to line or line to neutral}\)
Frequency (Hz):
  1. Hertz(Hz) is also termed “Cycles per Second” or “Frequency”. With relation to sine waves, it’s the time it takes for one complete cycle.
  2. The three common frequencies available are 50Hz, 60Hz and 400Hz.

    3. European power is typically 50Hz while North American power is usually 60hz.
    The 400 Hz is reserved for high-powered applications such as aerospace and some special-purpose computer power supplies and hand-held machine tools. Such high frequencies cannot be economically transmitted long distances, so 400 Hz power systems are usually confined to the building or vehicle.

  3. Transformer frequency is fixed

    4. Transformers cannot change the frequency of the source kva (voltage & current). If the source frequency is 60 Hz, the output will also be 60 Hz. In most parts of the Americas, it is typically 60Hz, and in the rest of the world it is typically 50Hz. Places that use the 50 Hz frequency tend to use 230 V RMS, and those that use 60Hz tend to use 117 V RMS.

Winding Configurations
Delta-Wye
Wye-Delta
Delta-Delta
Wye-Wye
Open Delta
Scott (T-T)
Interconnected-Star or Zig-Zag
What is Meant by "Impedance" in Transformers?

Impedance is the current limiting characteristic of a transformer and is expressed in percentage.

The percentage impedance of a transformer (Z%) is the voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage.
Electrical impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedance in the circuit.

In general, impedance has a complex value, which means that loads generally have a resistance to the source that is in phase with a sinusoidal source signal and reactance that is out of phase with a sinusoidal source signal.
The total impedance is the vector sum of the resistance and the reactance. The impedance is measured by shorting the low voltage terminals.

Why is Impedance Important?

It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer. Example: Determine a minimum circuit breaker trip rating and nterrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt  50 Hz source.

Maximum Short Circuit Secondary Current

The maximum short circuit current that can be obtained from the output of the transformer is limited by the impedance of the transformer and is determined by multiplying the reciprocal of the impedance times the full load current.

\(I_{FLA}=\frac{KVA \cdot 1000}{V_{phase} \cdot Z} \)

Where:

\( {V_{phase}: V_{Line-Line} } \)

Z : %Z (decimal form)

Example:

Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt  50 Hz source.

Calculate as follows:

Normal Full Load Current:

\(= \frac{Nameplate Volt Amps}{Line Volts} \)

\(= \frac{10,000 VA}{480V} \)

Example:

Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt  50 Hz source.

Calculate as follows:

Maximum Short Circuit Amps:

\(= \frac{Full Load Amps}{Z} \)

\(= \frac{20.8 Amps}{0.04} =520 Amps\)

Voltage Taps:

Voltage supplied to a Transformer may vary from nominal voltage due to the distance from the substation or source. Higher or lower input voltage will result in higher or lower output voltage if there are no voltage adjustment taps present.

In order to compensate for this voltage difference, transformers secondary voltage can be adjusted to nominal levels by adjusting the transformer's primary winding's voltage Tap.

Transformer voltage taps change the voltage ratio of a transformer so that its secondary voltage stays at nominal. On large power transformers, taps on the primary are used to offset any higher or lower input voltages. These tap connections are usually set at the factory for nominal line voltage. If the voltage at the site is different, the taps are changed accordingly.

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