A three-phase substation bus supplies one wye and one delta connected load. The constant impedance loads are connected in parallel. A three-phase feeder with an impedance per phase of Z = 0.6+j1.8 Ω connects the loads to a source, as seen in the one-line diagram of Figure 4.33
Three Phase circuits
Problem 1
A three-phase substation bus supplies one wye and one delta connected load. The constant impedance loads are connected in parallel. A three-phase feeder with an impedance per phase of Z = 0.6+j1.8 Ω connects the loads to a source, as seen in the one-line diagram of Figure 4.33
.
The loads draw the following three-phase power:
P1 = 35 kW pf1 = 0.90 (leading) wye connected
S2 = 30 kVA pf2 = 0.85 (lagging) delta connected
The line-to-line load voltage is 480 V. Determine:
(a) the impedance of each load per phase,
(b) the total line current flowing through the feeder,
(c) the line-to-line voltage at the substation bus, and (d) the total three-phase real and reactive powers supplied by the bus.
Problem 2
A delta-connected load and a wye-connected capacitor bank are supplied through a feeder. The delta-connected load consists of three identical resistive-inductive impedances of Z =50 Ω at an angle of +40 degrees per phase.
The source is wye connected and consists of a three-phase voltage source of 240 V (line-to-line). The feeder has a conductor impedance of Z = 2+j2 Ω per phase.
Draw the equivalent circuit, without and with the capacitor bank
Determine:
(a) The current and line-to-line voltage at the load terminals without the capacitor bank.
(b) The resulting line-to-line voltage at the load terminals if the wye-connected capacitor bank is connected in parallel with the load. The capacitor bank has a reactance of X = – 35 Ω per phase.
(c) Discuss the effects of the capacitor bank on the voltage at the load terminals.
Problem 3
A balanced three-phase source supplies power to three wye-connected loads, as depicted in the single-phase diagram of Figure 4.34. The loads are
Load 1: S1 = 38 kVA pf1 = 0.7 lagging
Load 2: S2 = 30 kVA pf2 = 0.75 leading
Load 3: Unknown
The line voltage at the loads and the line current at the source are 240 V and 150 A, respectively. If the combined power factor at the loads is unity, determine:
(a) the real, apparent, and reactive powers of the unknown load; and
(b) thepf of the unknown load.
(c) Plot the total pf vs. impedance of load 3, assuming that only the resistance value is variable (take values from 0 to 10 ohms, with steps of 0.1).
Determine the resistance value that belongs to a power factor of pfR3 = 0.96
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