8/13/2019 Tutorial Transformer

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TUTORIAL

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TRANSFORMER TUTORIAL QUESTIONS

1. A 20-kVA 8000/480-V distribution transformer has the following resistances andreactances:

RP= 32 RS= 0.05

XP= 45 RS= 0.06 RC= 250 k XM= 30 k

The excitation branch impedances are given referred to the high-voltage side ofthe transformer.(a) Find the equivalent circuit of this transformer referred to the high-voltage

side.(b) Assume that this transformer is supplying rated load at 480 V and 0.8 PF

lagging. What is this transformers input voltage?(c) What is the transformers efficiency under the conditions of part (b)?

2. A 20-kVA 20,000/480-V 60-Hz distribution transformer is tested with thefollowing results:

Open-circuit test Short-circuit test(measured from secondary side) (measured from primary side)------------------------------------------------------------------------------------------------------------

VOC= 480 V VSC= 1130 VIOC= 1.60 A ISC= 1.00 APOC= 305 W PSC= 260 W

(a) Find the equivalent circuit for this transformer referred to the low-voltageside.

(b) Find the transformers efficiency if the transformer is supplying a load atrated voltage and half-rated current and 0.8 lagging power factor. Sketchthe phasor diagram.

3. A 13.2-kV single-phase generator supplies power to a load through atransmission line. The loads impedance is Zload=500 36.87 , and thetransmission lines impedance is Zline= 60 53.1.

(a) If the generator is directly connected to the load (Figure below), what is

the ratio of the load voltage to the generated voltage? What are thetransmission losses of the system?(b) If a 1:10 step-up transformer is placed at the output of the generator

and a 10:1 transformer is placed at the load end of the transmissionline, what is the new ratio of the load voltage to the generated voltage?What are the transmission losses of the system now? (Note: Thetransformers may be assumed to be ideal.)

8/13/2019 Tutorial Transformer

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TUTORIAL

EEEB413

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4. Three 25-kVA 24,000/277-V distribution transformers are connected in -Y. Theopen-circuit test was performed on the low-voltage side of this transformer bank,and the following data were recorded:

Vline,OC = 480 V Iline,OC= 4.10 A P3phase, OC = 945 W

The short-circuit test was performed on the high-voltage side of thistransformer bank, and the following data were recorded:

Vline,SC = 1600 V Iline,SC= 2.00 A P3phase, SC = 1150 W

(a) Find the per-unit equivalent circuit of this transformer bank.(b) Find the voltage regulation of this transformer bank at the rated load

and 0.90 PF lagging.(c) What is the transformer banks efficiency under these conditions?