How To Balance Three Phase Power In A Balanced System

Transcription

1 Three-Phase Circuits

2 Three-Phase Circuit Three-Phase Circuits What is a Three-Phase Circuit? Balance Three-Phase oltages Balance Three-Phase Connection Power in a Balanced System Unbalanced Three-Phase Systems Application Residential Wiring

3 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

4 What is a Three-Phase Circuit?(2) 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, the three-phase system is more economical that the single-phase. 4. n fact, the amount of wire required for a threephase system is less than that required for an equivalent single-phase system.

5 Three-Phase Circuit A three-phase generator consists of a rotating magnet (rotor) surrounded by a stationary winding (stator). A three-phase generator The generated voltages

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

7 Terminology 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

8 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

9 Y-Y 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 P 3 ph L L 3 p p ab a p b 3 bc L c L, where ca

10 Example Three - Phase Circuit Calculate the line currents in the three-wire Y-Y system shown below: Ans a b c A A A

11 Y-Δ connection A balanced Y-Δ system is a three-phase system with a balanced y-connected source and a balanced Δ-connected load. L L p 3 p, a where AB b BC c CA P 3 3 3ph p p L L

12 Example A balanced abc-sequence Y-connected source with an is connected to a Δ-connected load (8+j4) per phase. Calculate the phase and line currents. Solution Using single-phase analysis, an Z / a A Other line currents are obtained using the abc phase sequence

13 Δ -Δ connection A balanced Δ-Δ system is a three-phase system with a balanced Δ -connected source and a balanced Δ -connected load.

14 Example A balanced Δ-connected load having an impedance 20- j15 is connected to a Δ-connected positive-sequence generator having ( ab 3300 ). Calculate the phase currents of the load and the line currents. Ans: The phase currents AB A; BC A; AB A The line currents a A; b A; c A

15 Δ -Y connection A balanced Δ-Y system is a three-phase system with a balanced y-connected source and a balanced y-connected load.

16 Example A balanced Y-connected load with a phase impedance 40+j25 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;

17 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.