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 use multiple conductors for the coils. These properties allow them to endure the additional heat caused by harmonic currents much better than a standard transformer.
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| K-Factor or K-Rated Transformer |
K-Factor
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)2 (1)2] + [(I2/Irms)2 (2)2] + [(I3/Irms)2 (3)2] +…..+ [(In/Irms)2 (n)2]
where:
I1 = fundamental current
I2 = 2nd harmonic current
I3 = 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 derating. Thus, transformers are said to be K-rated when they use the K-factor.
In addition, Underwriters Laboratories (UL) recognized the problems of derating standard transformers with nonlinear loads. New test procedures that matched with ANSI/IEEE C57.110, Recommended Practice for Establishing Capability When Supplying Nonsinusoidal Load Currents, were established. Recently, only those manufacturers that have their transformers evaluated by UL for harmonic loads can apply the label, "Suitable for Non-Sinusoidal Current Load”.
The typical load K-factors, which are derived from ANSI/IEEE and UL 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
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| Common Load K-Factors |
K-Factor Transformers differ in construction from standard transformers as their designs were changed to accommodate the effects of harmonics:
Double sized neutral conductor
Ø This is to protect against triplen harmonics (3rd, 6th, 9th, etc.) that add up together in the neutral
Electrostatic shielding between the primary and secondary windings of each coil
Ø Reduce eddy current losses and heating
Lower induction core
Ø Reduce the core flux density to protect against harmonic voltage distortion
Heavier conductor and transposition of winding conductor
Ø Reduce magnetic losses
Parallel smaller windings on the secondary (multiple winding conductors)
Ø To cancel out skin effect from high frequency currents
K-Rated vs. Derated
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, derating a standard transformer has several disadvantages:
· It is oversized
· Less efficient
· Loads added without reference back to initial derating, resulting to overloading & failure
· Potential maintenance problem – long after installation
· Problematic protection level
Furthermore, derating a standard transformer will not assure optimal performance under nonlinear loads and aluminum wound transformers are prone to premature failure. In the end, a K-factor transformer is usually more economical than using a derated, oversized transformer.
References:
Controlled Power Company. (1998). What is a K-Rated Transformer?
Cooper Development Association. (2000). Harmonics, Transformers and K-Factors
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