resistance are transferred to the secondary side The transferred primary and the second impedances are combined together In the next slide, the simplified circuit of Fig. 7.26 is presented 52
Short Circuit Test Figure 7.26 Simplified circuit for calculation of series impedance Primary and secondary impedances are combined The combined leakage impedances and coil resistances are calculated by repeated use of the basic power equation and Ohm’s law 53 s p s e R T R R 2 _ s p s e X T X X 2 _
Short Circuit Test The short circuit test evaluation results are evaluated using 54 2 e_s 2 sh_s e_s 2 2 sh_s sh_s sh_s 2 sh_s sh_s e_s ) (R ) (Z X I V Z ) (I P R X R Z
Equivalent Circuit Equivalent circuit obtained by measurement is Figure 7.29 Equivalent circuit for a real transformer resulting from the open and short circuit tests 55
Equivalent Circuit Figure 7.30 Modified equivalent circuit The circuit can be further modified to a form similar to the traditional equivalent circuit. The calculated series impedance is transferred from the secondary to the primary side and divided by two. 56
Transformers Example for Single-Phase Transformer Operation Analysis 57
Operation Analysis Example Single-phase 150 kVA transformer. Primary voltage is 7.2 kV and secondary voltage is 240 V. Calculate the primary and secondary current, required supply voltage and voltage regulation when the load is 80% of the transformer capacity at rated secondary voltage and the power factor is 0.8 lagging The equivalent circuit parameters are R p = 0.12 Ω R s = 0.03 Ω R m = 2500 Ω X p = 0.3 Ω X s = 0.09 Ω X m = 800 Ω 58
Operation Analysis Example Equivalent circuit used for the calculation S Tr = 150 kVA E p = 7.2 kV E s = 240 V pf L = 0.8 59
Operation Analysis Example With the load at 80% of the transformer capacity and a 0.8 lagging power factor, the load complex power is The corresponding load current is 60
Operation Analysis Example The transformer secondary voltage is calculated by the loop equation Transformer primary voltage 61
Operation Analysis Example Transformer primary current is Transformer magnetizing current is 62
Operation Analysis Example The supply current is the sum of the primary current and magnetizing current Supply voltage is the sum of the primary voltage and voltage drop on primary impedance 63
Operation Analysis Example Finalized supply voltage is The voltage regulation is Unacceptably high; it should be less than 10% 64
Operation Analysis Example The supply complex power and real power are Supply power factor is calculated Transformer rating is 150 kVA, serious overload 65
Three-Phase Transformers Descriptions and Basic Features 66
Three-Phase Transformers A three-phase transformer is described by its total apparent power capability (| S |) and voltage ratings ( V rate ).
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- Fall '08
- Volt, SPX Transformer Solutions, Diagram, Bushings, Fig., ThreePhase Transformer