Understading Distribution Transformer Nameplate
Transformer becomes the most expensive equipment in the electrical distribution system. Understanding transformer is begun by understanding transformer namelate. As reference in this post, I use IEC 60076.
Capacity means the ability of transformer to deliver certain amount of electric power. In that nameplate, the capacity of transformer is 10 MVA.
ONAN refers method of cooling. ONAN means oil natural air natural. ON means the circulation of primary coolant (in this example is oil) while the AN means the circulation of secondary coolant (in this example is air). The other common method of cooling is ONAF (oil natural air force) which equipped fan at transformer radiator. By changing the method of cooling from ONAN to ONAF, we can increase the capacity of transformer, typically 125%.
Temp rise is the allowable increase of temperature inside transformer. The temp rise is 55C from the nameplate.
Vector group is the phase arrangement of primary and secondary winding. From the nameplate, the primary is delta winding and the secondary is wye winding. The phase shift between primary and secondary is 300. From the nameplate, vector group is Dyn11. D means delta (the upper case means primary side), y means star (the lower case means secondary side), n means neutral terminal, while 11 means the vector of secondary terminal refers to primary.
HV insulation level means the maximum limit of primary side to resist the effect of overvoltage. The overvoltage can be lightning (Li), switching impulse (SI), and power frequency (AC). From the nameplate, the HV insulation level is 75/28 kV. 75 kV shows lightning impulse while 28 kV shows power frequency (50 Hz). For transformer with Um < 72,5 kV the insulation level is just lightning impulse and power frequency. The other rules also applies to low voltage insulation level. The value of insulation level varies and depends on the voltage rating of primary and secondary. The complete guidance to determine the insulation level is presented in IEC 60076.
Primary voltage is the input voltage to the transformer while the secondary voltage is the output of the transformer. From the nameplate of transformer, the primary voltage is 11 kV with +2.5%, +5%, -2.5%, -5%. The primary side equips off-load tap changer. The purpose of tap changer is to raise of lower the output voltage. The off-circuit is usually placed at the side with lower current. In the step down transformer, the tap changer is placed at the primary side.
Impedance means the resistance of transformer itself. As you know, transformer consist of copper winding which will raise certain impedance. The impedance parameter depends on the geometry of transformer. The impedance parameter is very important in short circuit calculation.
Transformer oil is specially designed based on IEC 60298. The main character of transformer oil is the ability to resist against the oxidation.
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