Presentation on theme: "1 Eeng 224 Chapter 12 Three Phase Circuits Huseyin Bilgekul Eeng224 Circuit Theory II Department of Electrical and Electronic Engineering Eastern Mediterranean."— Presentation transcript:
1 Eeng 224 Chapter 12 Three Phase Circuits Huseyin Bilgekul Eeng224 Circuit Theory II Department of Electrical and Electronic Engineering Eastern Mediterranean University Chapter Objectives: Be familiar with different three-phase configurations and how to analyze them. Know the difference between balanced and unbalanced circuits Learn about power in a balanced three-phase system Know how to analyze unbalanced three-phase systems Be able to use PSpice to analyze three-phase circuits Apply what is learnt to three-phase measurement and residential wiring
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3 Three phase Circuits An AC generator designed to develop a single sinusoidal voltage for each rotation of the shaft (rotor) is referred to as a single-phase AC generator. If the number of coils on the rotor is increased in a specified manner, the result is a Polyphase AC generator, which develops more than one AC phase voltage per rotation of the rotor In general, three-phase systems are preferred over single-phase systems for the transmission of power for many reasons. 1. Thinner conductors can be used to transmit the same kVA at the same voltage, which reduces the amount of copper required (typically about 25% less). 2. The lighter lines are easier to install, and the supporting structures can be less massive and farther apart. 3. Three-phase equipment and motors have preferred running and starting characteristics compared to single-phase systems because of a more even flow of power to the transducer than can be delivered with a single-phase supply. 4. In general, most larger motors are three phase because they are essentially self- starting and do not require a special design or additional starting circuitry.
4 Eeng 224 a) Single phase systems two-wire type b) Single phase systems three-wire type. Allows connection to both 120 V and 240 V. Two-phase three-wire system. The AC sources operate at different phases. Single Phase, Three phase Circuits
5 Eeng 224 Three-phase Generator The three-phase generator has three induction coils placed 120° apart on the stator. The three coils have an equal number of turns, the voltage induced across each coil will have the same peak value, shape and frequency.
6 Eeng 224 Balanced Three-phase Voltages Three-phase four-wire system Neutral Wire A Three-phase Generator Voltages having 120 phase difference
7 Eeng 224 Balanced Three phase Voltages a) Wye Connected Sourceb) Delta Connected Source a) abc or positive sequence b) acb or negative sequence Neutral Wire
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9 Balanced Three phase Loads a) Wye-connected load b) Delta-connected load A Balanced load has equal impedances on all the phases
10 Eeng 224 General Delta to Wye conversion Delta to Wye Wye to Delta works the same way for complex impedances
11 Eeng 224 Three phase Connections Both the three phase source and the three phase load can be connected either Wye or DELTA. We have 4 possible connection types. Y-Y connection Y-Δ connection Δ-Δ connection Δ-Y connection Balanced Δ connected load is more common. Y connected sources are more common.
12 Eeng 224 Balanced Wye-wye Connection A balanced Y-Y system, showing the source, line and load impedances. Source Impedance Line Impedance Load Impedance
13 Eeng 224 Balanced Wye-wye Connection Phase voltages are: V an, V bn and V cn. The three conductors connected from a to A, b to B and c to C are called LINES. The voltage from one line to another is called a LINE voltage Line voltages are: V ab, V bc and V ca Magnitude of line voltages is √3 times the magnitude of phase voltages. V L = √3 V p Line current I n add up to zero. Neutral current is zero: I n = -(I a + I b + I c )= 0
14 Eeng 224 Balanced Wye-wye Connection Magnitude of line voltages is √3 times the magnitude of phase voltages. V L = √3 V p Line current I n add up to zero. Neutral current is zero: I n = -(I a + I b + I c )= 0
15 Eeng 224 Balanced Wye-wye Connection Phasor diagram of phase and line voltages
16 Eeng 224 Single Phase Equivalent of Balanced Y-Y Connection Balanced three phase circuits can be analyzed on “per phase “ basis.. We look at one phase, say phase a and analyze the single phase equivalent circuit. Because the circuıit is balanced, we can easily obtain other phase values using their phase relationships.
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18 Eeng 224 Balanced Wye-delta Connection Line currents are obtained from the phase currents I AB, I BC and I CA Three phase sources are usually Wye connected and three phase loads are Delta connected. There is no neutral connection for the Y-∆ system.
19 Eeng 224 Balanced Wye-delta Connection Single phase equivalent circuit of the balanced Wye-delta connection Phasor diagram of phase and line currents
20 Eeng 224 Balanced Delta-delta Connection Both the source and load are Delta connected and balanced.
21 Eeng 224 Balanced Delta-wye Connection Transforming a Delta connected source to an equivalent Wye connection Single phase equivalent of Delta Wye connection
22 Eeng 224 Chapter 12 Three Phase Circuits Huseyin Bilgekul Eeng224 Circuit Theory II Department of Electrical and Electronic Engineering Eastern Mediterranean University Chapter Objectives: Be familiar with different three-phase configurations and how to analyze them. Know the difference between balanced and unbalanced circuits Learn about power in a balanced three-phase system Know how to analyze unbalanced three-phase systems Be able to use PSpice to analyze three-phase circuits Apply what is learnt to three-phase measurement and residential wiring
23 Eeng 224 Power in a Balanced System The total instantaneous power in a balanced three phase system is constant.
24 Eeng 224 Power in a Balanced System The complex power per phase is S p. The total complex power for all phases is S.
25 Eeng 224 Power in a Balanced System Notice the values of V p, V L, I p, I L for different load connections. VLVL VLVL VLVL VpVp VpVp VpVp IpIp IpIp IpIp VLVL VpVp IpIp VLVL VLVL VpVp VpVp IpIp IpIp Y connected load.Δ connected load.
26 Eeng 224 Power in a Balanced System
27 Eeng 224 Single versus Three phase systems Three phase systems uses lesser amount of wire than single phase systems for the same line voltage V L and same power delivered. a) Single phase system b) Three phase system If same power loss is tolerated in both system, three-phase system use only 75% of materials of a single-phase system
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29 Eeng 224 V L =840 V (Rms) Capacitors for pf Correction ILIL
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31 Eeng 224 Unbalanced Three Phase Systems An unbalanced system is due to unbalanced voltage sources or unbalanced load. In a unbalanced system the neutral current is NOT zero. Unbalanced three phase Y connected load. Line currents DO NOT add up to zero. I n = -(I a + I b + I c ) ≠ 0
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33 Eeng 224 Three Phase Power Measurement Two-meter method for measuring three-phase power