Balanced Three-Phase Circuits and Ideal Transformers ELEC 308 Elements of Electrical Engineering Dr. Ron Hayne Images Courtesy of Allan Hambley and Prentice-Hall
Three-Phase Sources Most common multiple-phase AC voltage source configuration Wye connected (Y connected) ELEC 3082
Three-Phase Sources ELEC 3083
Wye-Wye Connection ELEC 3084
Wye-Wye Connection Lines Wires a-A, b-B, c-C Neutral Wire n-N Balanced Load Three load impedances are equal Phase Voltage or Line-to-Neutral Voltage Peak magnitude of one source, V Y Line Currents Values I aA, I bB, and I cC ELEC 3085
Wye-Wye Connection Neutral current is ZERO in a BALANCED three- phase system Can eliminate the neutral wire ELEC 3086
Cost Advantage Three (or four) wires needed to connect a three-phase source with three loads Six wires needed to connect three single- phase sources with three loads ELEC 3087
Three-Phase Power Total power is CONSTANT with respect to time ELEC 3088
Three-Phase Reactive Power The reactive power delivered to a balanced three-phase load is given by ELEC 3089
Line-to-Line Voltages ELEC 30810
Analysis of a Wye-Wye System Balanced wye-connected 60-Hz three-phase source has line- to-neutral voltages of V Y = 1000 V. Balanced wye-connected load consisting of a 0.1H inductance in series with a 50-Ω resistance. Find the line currents, the line-to-line voltages, the power, and the reactive power delivered to the load. Draw a phasor diagram showing the line-to-neutral voltages, the line-to-line voltages, and the line currents. ELEC 30811
Analysis of a Wye-Wye System ELEC 30812
Delta-Connected Sources ELEC 30813
Delta-Connected Loads ELEC 30814
Delta-Delta Connection ELEC 30815
Power Transmission Losses Power Line Losses Large Voltages and Small Currents Smaller Line Loss ELEC 30816
Ideal Transformers ELEC 30817
Ideal Transformers ELEC 30818
Example Determine the rms values of the currents and voltages. ELEC 30819
Power Transmission Step-Up and Step-Down Transformers 99% Efficiency (vs. 50% with no transformers) ELEC 30820
U.S. Power Grid ELEC 30821
Summary Three-Phase Circuits Wye-Wye Connection Three-Phase Power Ideal Transformers U.S. Power Grid ELEC 30822