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Existing conventional isolation transformers designed for linear loads if required to be used on non-linear harmonic loads with different K-factors then they have to be de-rated as per UL-1561 / ANSI 57.110 guideline curve. This is called as Derating.


A typical 120 KVA conventional transformer (LINEAR K1) when used with different k loads

If transformers manufactured for harmonic loads with different k factors like K7,K13, K20 are designed with considerations of


·    Minimizing skin-effect by Multiple parallel conductor combination, 


·    Closely coupled primary and secondary Specific arrangement of layers and properly overlapping primary       and secondary, resulting in minimum stray field and stray losses.


·    High quality low loss magnetic core material design at moderate flux density for reduced iron losses with      extra margin for harmonic losses.


.    Effective cooling by focusing cool air draft effectively thru winding ducts.


They still do require some % of over-sizing in the design to provide for the uncovered harmonic losses mainly due to the higher stray losses at harmonic frequencies.

If  the stray losses @ fundamental frequency 50 Hz , namely Wstray-50Hz, (k1)then Stray losses on Krated loads K(n) become  Wstray-K(n)= n X Wstray-50Hz.


A typical example of the impact of K-factor on stray losses can be illustrated as below.


A, K 13 rated ,120 KVA transformer (LINEAR K1) when used with different k loads-

DERATING vs. OVERSIZING


K1


K-13K-20

NO LOAD LOSS

480W480W480W

I²R LOSS AT 100% LOAD W

1260W1260W1260W

STRAY LOSS W ( W STRAY 50Hz XK (N)

30W390W600W

TOTAL LOSS W

1770W2130W2340W


DERATING REQD


NIL30%40%


SCOPE AND METHODS OF TESTING K-RATED TRANSFORMER


  ​   Routine Test


    1.Visual Inspection

    2.Verifications of BOQ and OGA

    3.Measurement of Insulation Resistance

    4.High voltage withstand test

    5.Measurement of no load loss and current

    6.Measurement of No load voltage ratio and check of voltage vector relationship

    7.Measurement of Impedance voltage / Short circuit Impedance and load loss

    8.Operational test- As applicable to the project requirement


     Heat-run Test – Temp. Rise –losses and Efficiency


    1. We will be offering heat run test by back to back method as per “IS 11171-2001 clause 17-TEMPERATURE  RISE                 TEST-(17.2.2)” which is preferred method by IS.
    2. Loading % will be as per over sizing specified in the offer.

    3. Heat run will be type test as per “IS 11171-2001 clause 13-CLASSIFICATION OF TESTS-13.1.h” thus will be                         conducted on one unit in each rating)

    4.  As  per “IS 11171-2001 clause 17-TEMPERATURE RISE TEST-17.2.4-DETERMINATION OF CONSTANT TEMPERATURE                CONDITIONS” the ultimate temperature rise is reached when the temperature rise becomes constant; this is                    considered to have been achieved when the temperature rise does not vary by more than 2% of the permissible              temperature rise per hour or 2 Deg C per hour, whichever is the smaller. Heat run will be conducted for period              till the differential in winding temperature is < 1Deg C or consecutive  readings.  **
    5.  Measurement of winding temperature T2 can be done by contact thermometer as well as “As per IS 2026-clause 4-            Test Of Temperature-Rise-4.3” by formula 
Where and are measured in 

  





























.





K1


K-13K-20

NO LOAD LOSS

480W480W480W

I²R LOSS AT 100% LOAD W

1260W1260W1260W

STRAY LOSS W ( W STRAY 50Hz XK (N)

100W1300W2000W

TOTAL LOSS W

1820W2940W3840W


DERATING REQD


NIL30%40%