Energy Efficient Distribution Transformers

Energy Efficient Distribution Transformers

Linear Load General Purpose Transformers 

They are generally used for supplying appliance, lighting, heating, motorized machine and power loads from electrical distribution systems.

For non-linear loads, all standard energy efficient general purpose distribution

transformers have to be de rated to allow for additional heat due to harmonic losses


Non-Linear Load K-Factor Transformers 

Energy efficient K-factor transformers are designed to tolerate heating due to harmonics associated with non-linear loads.  They have become a popular means of addressing these related overheating problems where electronic ballasts, drives, personal computers, telecommunications equipment, broadcasting equipment and other similar power electronics are found in high concentrations.

Harmonics can indicate their presence in a number of ways: overheating, device malfunctions, telephone interference, equipment vibration, and breakers tripping.  These non-linear loads generate harmonic currents which can substantially increase transformer losses.

The K-rated transformer has a more rugged design intended to prevent failure due to overheating, a 200% rated neutral and full electrostatic shield.

K-factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses

It is used to specify transformers for non-linear loads.  Standard K-factor ratings are 4, 13, 20 and other ratings are available.


Harmonic Mitigating Transformers

Features of Harmonic Mitigating Transformers:

  • Prevents voltage flat-topping caused by non-linear loads
  • Reduces upstream harmonic currents
  • Eliminates transformer overheating and high operating temperatures
  • Eliminates primary winding circulating current
  • Saves energy by reducing harmonic losses
  • Maintains high energy efficiency even under sever non-linear loading conditions
  • Electrostatic shielding for high frequency noise attenuation


Harmonic Mitigating transformers are superior to K-rated and general purpose

Transformers in that they reduce voltage distortion (flat-topping) and power losses due to current harmonics created by single-phase, non-linear loads such as computer equipment.

Secondary windings are arranged to cancel zero sequence fluxes and eliminate primary winding circulating currents.  They treat zero sequence harmonics (3rd, 9th and 15th) within the secondary windings and 5th and 7th harmonics upstream with appropriate phase shifting.

Dual output, phase shifting Harmonic Mitigating Transformers provide extremely low output voltage distortion and input current distortion even under severe non-linear load conditions (Data Centers, Internet Service Providers,

Telecom Sites, Call Centers, Broadcasting Studios, etc.).  Combining zero sequence flux cancellation with phase shifting treats 3rd, 5th, 7th, 9th, 15th, 17th and 19th harmonics within its secondary windings.

K-Factor Transformer, also known as K-Rated transformer, is designed for nonlinear or harmonic generating loads that a standard transformer could not adequately handle due to overheating. K-factor transformers are specially assembled with a double sized neutral conductor, heavier gauge copper and either change the geometry of their conductors or FOIL WINDING for the coils. These properties allow them to endure the additional heat caused by harmonic currents much better than a standard transformer.



It is an index of the transformer’s ability to carry harmonic currents while operating within the temperature limits of its insulating system. K-Factor values range from 1 to 50. Standard transformers have a K-factor of 1.0 and are assigned for linear loads only. Meanwhile, a K-factor of 50 is utilized for the harshest harmonic condition possible.

K =  [(I1/Irms)(1)2] + [(I2/Irms)(2)2] + [(I3/Irms)(3)2] +…………….+ [(In/Irms)(n)2]


I= fundamental current

I= 2nd harmonic current

I= 3rd harmonic current

In = nth harmonic current

Irms = RMS current

Note: RMS current is the square root of the sum of squares of the individual currents.

The rationale is to first compute the K-factor of the load. Subsequently, specify the transformer to have a K-factor that is equal or higher than the calculated value. As a result, the transformer can be sized to the load without de rating. Thus, transformers are said to be K-rated when they use the K-factor.

The typical load K-factors, which are derived from ANSI/IEEE standards are the following: K-4, K-9, K-20, K-30, K-40 and K-50. In theory, a transformer could be designed for other K-factors in-between those values. Nonetheless, K-40 and K-50 are expensive and rarely used.


Design and Construction

K-Factor Transformers differ in construction from standard transformers as their designs were changed to accommodate the effects of harmonics:


Double sized neutral conductor

  • This is to protect against triplen harmonics (3rd, 6th, 9th) that add up together in the neutral
  • Electrostatic shielding between the primary and secondary windings of each coil
  • Reduce eddy current losses and heating
  • Lower induction core
  • Reduce the core flux density to protect against harmonic voltage distortion
  • Heavier conductor and transposition of winding conductor
  • Reduce magnetic losses
  • Parallel smaller windings on the secondary (multiple winding conductors)
  • To cancel out skin effect from high frequency currents


K-Rated vs. De rated

The K-factor transformer’s main advantage is that it is designed considering harmonic generating loads. A good example is the abovementioned changes made in the construction of the transformer.

On the contrary, de rating a standard transformer has several disadvantages:

  • It is oversized
  • Less efficient
  • Loads added without reference back to initial de rating, resulting to overloading & failure
  • Potential maintenance problem – long after installation
  • Problematic protection level

Furthermore, de rating a standard transformer will not assure optimal performance under nonlinear loads. And are prone to premature failure. In the end, a K-factor transformer is usually more economical than using a de rated, oversized transformer.


A Few of the many features are:

  • Multiple K-rating selection, K-4, K-7, K-13, K-20
  • Designed for linear and nonlinear loads
  • All Copper construction
  • Removes triplen harmonic currents from the line
  • High Efficiency
  • Solves 88% of typical power disturbance problems
  • Heavy-duty  construction
  • Optional lightning surge protection
  • Optional input/output breakers

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