برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

What is the difference between K-13 (Type NLP) and K-4 (Type NL) rated transformers?

K-Factor is a measure of current distortion in the load fed by a transformer. The higher the number, the greater the capacity of the transformer to handle high levels of continuous single-phase, line-to-neutral non-linear loads. Square D recommends K-4 rated transformers for computer and other similar loads found in commercial and industrial systems for duty cycles (24 hour average loading) of 80-95%. If the duty cycle is expected to exceed 95%, then K-13 would be needed. For duty cycles up to 50%, use Type EE energy efficient transformers. For duty cycles of 50-80%, use Watchdog Low Temperature Rise transformers.
The need for K-Factor rated transformers is currently in question because standard, general purpose transformers can supply typical commercial and industrial `non-linear` loads up to more than 80% of nameplate rating without overheating. Studies have shown that typical loading of low voltage dry type transformers is less than 35% as a daily average.

Is there a more technical way to calculate K-Factor? 
 Answer:

1. List the kVA value for each load category to be supplied. Next, assign an ILK value that corresponds to the relative level of harmonics drawn by each type of load. See Table 1.
2. Multiply the kVA of each load times the ILK rating that corresponds to the assigned K-factor rating. This result is an indexed kVA-ILK value:

kVA x ILK = kVA_ILK

3. Tabulate the total connected load kVA for all load categories to be supplied.
4. Next, add-up the kVA_ILK values for all loads or load categories to be supplied by the transformer.
5. Divide the grand total kVA_ILK value by the total kVA load to be supplied. This will give an average ILK for that combination of loads.

(Total kVA_ILK) ÷ (Total kVA) = average ILK

6. From Table 2, find the K-factor rating whose ILK is equal to or greater than the calculated ILK. Corresponding to this ILK is the K-factor of the transformer required.


Problem 1
Calculate the overall K-factor for several non-linear loads.

Load Category kVA Load x ILK = kVA_ILK Value

                         Welders (4.0 x 25.82) = 103.28
        UPS no input filter (5.0 x 57.74) = 288.70
Main frame computers (3.0 x 80.94) = 242.82
               Motor w/drive (0.75 x 80.94) = 60.71
                 Motor w/o drive (3.0 x 0.00) = 0.00
                                       Totals (15.75) = 695.51

            Total kVA_ILK / Total kVA = average ILK
                  695.51/15.75 = 44.16 = average ILK

From Table 2, the nearest K-factor greater than or equal to the average ILK of 44.16 is K-9 with an ILK of 44.72.

Problem 2
Calculate the amount of additional K-20 load that can be handled by a 25 kVA, K-13 transformer with 9 kVA of spare capacity.

1. Determine the available spare K-13 kVA_ILK, using the ILK that corresponds to the transformer’s K-factor rating.

spare kVA x ILK = spare kVA_ILK
9 x 57.74 = 519.66 spare kVA – ILK

2. Divide the spare kVA_ILK by the Index of Load K-rating for the load to be supplied. The ILK for a K-20 load is 80.94

spare kVA_ILK / new load ILK@K-20 = maximum additional kVA
519.66 / 80.94 = 6.4 kVA maximum additional kVA

3. Therefore, an additional 6.4 kVA of K-20 load could be added to this transformer. This additional loading represents the absolute maximum non-linear loading for that transformer.

For a transformer already partially loaded, any additional kVA loading must take into consideration the K-factor of each of the new loads to be added.


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