Homework_4

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University Of Connecticut *

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Mechanical Engineering

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Dec 6, 2023

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ECE 3231 Introduction to Modern Power Systems Homework 4 Due 11/30/2023 by 11:59 pm Q1) A 90 MVA transformer is rated 345kV/115 kV and is connected in wye/wye configuration. The transformer impedance (purely inductive) is 7.5% on its own base. The transformer is tied to a 115 kV line whose impedance is 0.375+j1.5 ohms. Using 100MVA and rated voltage as the base values, find the total p.u. impedance of the transformer and line. (20 points) Q2) A single-phase 100-kVA, 1200/120-volt, 60-Hz distribution transformer has been showed in below. The load, which is connected to the 120-volt secondary winding, absorbs 60 kVA at 0.85 power factor lagging and is at 115 volts. Assuming an ideal transformer, calculate the following: (20 points) (a) primary voltage (b) load impedance (c) load impedance referred to the primary (d)the real and reactive power supplied to the primary winding Q3) A single phase transformer rated 1.2kV/120V, 7.2kVA yields the following test results at 60Hz: (20 points) Open circuit test (primary open) Voltage V 2 = 125V; current I 2 = 1.5A; power consumption W 2 = 60W Short circuit test (secondary shorted) Voltage V 1 = 25V; current I 1 = 8.0A; power consumption W 1 = 54W The diagram below already converts all variables to the primary side. Determine the Z P and Z m in per Z = 0.375 + j 1.5 X = 7.5% 115 kV 345 kV E 2 = 115 V < 0 ° 1200:120
unit value; (hint: Z P can be neglected in calculating Z m ; Z m can be neglected in calculating Z P Q4) A single-phase 50-kVA, 1200/120-volt, 60-Hz distribution transformer is used as a step-down transformer at the load end of a 1200-volt feeder whose series impedance is given below. (20 points) The load at a 0.90 power factor lagging and at a rated secondary voltage. Neglecting the transformer exciting current, determine: (a) Determine the voltage I 2 , I 1 , E 1 (b) Determine the voltage V s (c) Calculate the P and Q power delivered to the sending end of the feeder Q5) A 2400/240 V, 60 Hz XFMR below. (20 points) R 1 = 1 , X l 1 = 2.5 R 2 = 0.012 Ω, X l 2 = 0.015 X M = 2000 , R he = negligible 1200:120 120<0°
Determine input voltage if output voltage is 240 V (rms) and feeding a load 1.5 ohm and PF = 0.9 (lagging).
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