THREE-PHASE SYSTEMS, Study notes of Electrical and Electronics Engineering

Download THREE-PHASE SYSTEMS and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 THREE-PHASE SYSTEMS The three-phase systems are used for - generation - transmission - distribution of electrical power because - The three-phase generators are less bulky and have a lower weight than the other electrical systems single-phase and d.c. - The three-phase electrical lines have a lower weight than the other match all electrical parameters. From the electrical point of view, a three-phase generator is composed of three single-phase generators with voltages of equal amplitudes and phase differences of 120°, according to the following relationships: The phase voltage is the voltage of each of the single phase generator 2 These generator can be connected in different ways: - Wye or Star connection (Y): the three voltage sources are connected to a common point with or without neutral wire; - delta (Δ) connection The three-phase systems are classified according to the sets of three electromotive forces (emf) and currents. A "Symmetric 3-phase set" is a three phase system where the set of three sinusoidal voltages satisfy these requirements:  All three voltages have the same amplitude  All three voltages have the same frequency  All three voltages are 120 o in phase So is called symmetric a system where the emfs A "balanced 3-phase set" is a three phase system where the set of three sinusoidal currents satisfy these requirements:  All three currents have the same amplitude  All three currents have the same frequency  All three currentes are 120 o in phase So is called balanced a three phase system where the currents If the load (Wye or Delta) has equal impedances, the system is Balanced If the load (Wye or Delta) hasn’t equal impedances, the system is Unbalanced 5 Symmetric and balanced three-phase systems ( DELTA-DELTA CONNECTION) The Line currents are different than the load currents Line currents ≠ load currents I1 = I12 – I31 I2 = I23 – I12 I3 = I31 – I23 Z12 = Z23 = Z31 = Z inductive resistive load relationship between the line currents and phase: 20" 3 Ty = Tyg — Igy = yg — ype? ta (s405-)S) 3 3 ly ( -r2) = V3h, (2-5) = V3I,6 7° lt] = VB ylZt, = 21,2 — 30° Wdq| = V3q3121, = 2Iz3 — 30° [fg] = V3UIgq14Ig = ZI, — 30° 7 Symmetric and balanced three-phase systems ( WYE-DELTA CONNECTION) We can use the Wye-Delta transformation to get the case of wye load already studied Symmetric and balanced three-phase systems (DELTA-WYE CONNECTION) We can use the Delta-Wye transformation to get the case of wye sources already studied: Z12 = Z23 = Z31 =Z = 3ZY 10 11 SYMMETRIC AND UNBALANCED THREE-PHASE SYSTEMS (DELTA-DELTA CONNECTION) E1, E2, E3 the set of three symmetric voltages Z1 ≠ Z2 ≠ Z3: the set of three unbalanced impedances 12 NOT SYMMETRIC AND UNBALANCED THREE-PHASE SYSTEMS E1, E2, E3 the set of three symmetric voltages Z1 ≠ Z2 ≠ Z3: the set of three unbalanced impedances (Delta load) DELTA LOAD WYE LOAD as above after having transformed the wye load into delta load