EE 221 CIRCUITS II. Chapter 12 Three-Phase Circuit

Transcription

1 EE 221 CRCUTS Chapter 12 Three-Phase Circuit 1

2 THREE-PHASE CRCUTS CHAPTER What is a Three-Phase Circuit? 12.2 Balanced Three-Phase oltages 12.3 Balanced Three-Phase Connection 12.4 Power in a Balanced System 12.5 Unbalanced Three-Phase Systems 12.6 Application Residential Wiring 2

3 12.1 What is a Three-Phase Circuit? t is a system produced by a generator consisting of three sources having the same amplitude and frequency but out of phase with each other by 120. Three sources with 120 out of phase Four wired system 3

4 12.1 What is a Three-Phase Circuit? Advantages: 1. Most of the electric power is generated and distributed in three-phase. 2. The instantaneous power in a three-phase system can be constant. 3. The amount of power in a three-phase system is more economical that the single-phase. 4. n fact, the amount of wire required for a three-phase system is less than that required for an equivalent single-phase system. 4

5 12.2 Balanced Three-Phase oltages A three-phase generator consists of a rotating magnet (in the rotor) surrounded by stationary windings (in the stator). A three-phase generator The generated voltages 5

6 12.2 Balanced Three-Phase oltages Two possible configurations: Three-phase voltage sources: (a) Y-connected ; (b) Δ-connected 6

7 12.2 Balance Three-Phase oltages Balanced phase voltages are equal in magnitude and are out of phase with each other by 120. The phase sequence is the time order in which the voltages pass through their respective maximum values. A balanced load is one in which the phase impedances are equal in magnitude and in phase 7

8 12.2 Balance Three-Phase oltages Example 1 Determine the phase sequence of the set of voltages. v v v an bn cn 200 cos( ωt + 10 ) 200 cos( ωt 230 ) 200 cos( ωt 110 ) 8

9 12.2 Balanced Three-Phase oltages Solution: The voltages can be expressed in phasor form as an bn cn We notice that an leads cn by 120 and cn in turn leads bn by 120. Hence, we have an a-c-b sequence. 9

10 12.3 Balanced Three-Phase Connection Four possible connections 1. Y-Y connection (Y-connected source with a Y-connected load) 2. Y-Δ connection (Y-connected source with a Δ-connected load) 3. Δ-Δ connection 4. Δ-Y connection 10

11 12.3 Balance Three-Phase Connection A balanced Y-Y system is a three-phase system with a balanced y-connected source and a balanced y-connected load. L p L 3 p, where an ab bn bc cn ca n a b c ( + + ) 0 11

12 12.3 Balanced Three-Phase Connection Example 2 Calculate the line currents in the three-wire Y-Y system shown below: Ans a b c A A A *Refer to in-class illustration, textbook 12

13 12.3 Balanced Three-Phase Connection A balanced Y-Δ system is a three-phase system with a balanced y-connected source and a balanced Δ-connected load. L 3 p, where L a b c p AB BC CA 13

14 12.3 Balanced Three-Phase Connection Example 3 A balanced abc-sequence Y-connected source with ( an ) is connected to a Δ-connected load (8+j4)Ω per phase. Calculate the phase and line currents. Solution Using single-phase analysis, an Z / a Δ A Other line currents are obtained using the abc phase sequence *Refer to in-class illustration, textbook 14

15 12.3 Balanced Three-Phase Connection A balanced Δ-Δ system is a three-phase system with a balanced Δ -connected source and a balanced Δ -connected load. 15

16 12.3 Balance Three-Phase Connection Example 4 A balanced Δ-connected load having an impedance 20-j15 Ω is connected to a Δ-connected positive-sequence generator having ( ab ). Calculate the phase currents of the load and the line currents. Ans: The phase currents A; A; A AB BC AB The line currents A; A; A a b c *Refer to in-class illustration, textbook 16

17 12.3 Balanced Three-Phase Connection A balanced Δ-Y system is a three-phase system with a balanced y-connected source and a balanced y-connected load. 17

18 12.3 Balanced Three-Phase Connection Example 5 A balanced Y-connected load with a phase impedance 40+j25 Ω is supplied by a balanced, positive-sequence Δ-connected source with a line voltage of 210. Calculate the phase currents. Use ab as reference. Answer The phase currents AN BN CN A; A; A; *Refer to in-class illustration, textbook 18

19 12.4 Power in a Balanced System P Q 3 cos( θ ) 3 cos( θ ) p 3 sin( θ ) 3 sin( θ ) p L L S 3 3 p L 19

20 12.3 Balanced Three-Phase Systems Example 6 Determine the total average power, reactive power, and complex power at the source and at the load Ans At the source: S s ( j834.6) A P s 2087W Q s 834.6AR At the load: S L ( j1113) A P L 1392W Q L 1113AR *Refer to in-class illustration, textbook 20

21 12.5 Unbalanced Three-Phase Systems An unbalanced system is due to unbalanced voltage sources or an unbalanced load. a Z AN A, b Z BN B, c Z CN C, n ( a + b + c ) To calculate power in an unbalanced three-phase system requires that we find the power in each phase. The total power is not simply three times the power in one phase but the sum of the powers in the three phases. 21

22 12.4 Power in a unbalanced System Three Watt-Meter Method: P T P 1 +P 2 +P 3 Two Watt-Meter Method: P T P 1 +P 2 Special case: Balanced Load using two-wattmeter method: P T P 1 + P 2 Q T 3 1 ( P 2 P ) 22

23 12.6 Application Residential Wiring A 120/240 household power system 23

24 12.6 Application Residential Wiring Single-phase three-wire residential wiring 24

25 12.6 Application Residential Wiring A typical wiring diagram of a room 25