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Ch 8. Three-phase systems, Study notes of Electrical and Electronics Engineering

Three-phase systems can produce rotating field without special control. • Three phase generator produce more power than single phase one with the same volume. • ...

Typology: Study notes

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Anouar Belahcen
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Ch 8. Three-phase systems
Lecture outcomes (what you are supposed to learn):
Generation of three-phase voltages
Connection of three-phase circuits
Wye-Delta transformation
Power of three-phase connected loads
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Download Ch 8. Three-phase systems and more Study notes Electrical and Electronics Engineering in PDF only on Docsity!

Anouar Belahcen

Ch 8. Three-phase systems

• Lecture outcomes (what you are supposed to learn):

  • Generation of three-phase voltages
  • Connection of three-phase circuits
  • Wye-Delta transformation
  • Power of three-phase connected loads

Anouar Belahcen

• High power equipments are built as three-phase systems.

• Three-phase systems can produce rotating field without special control.

• Three phase generator produce more power than single phase one with the

same volume.

• Three-phase systems are more reliable. They can deliver power even if one

phase fails.

Introduction

Anouar Belahcen

a

a -400 0 0.01 0.02 0.03 0.04 0.05 0.

0

100

200

300

400

Time (s)

induced voltage (V)

ea

-400 0 0.01 0.02 0.03 0.04 0.05 0.

0

100

200

300

400

Time (s)

induced voltage (V)

e ea b

-400 0 0.01 0.02 0.03 0.04 0.05 0.

0

100

200

300

400

Time (s)

induced voltage (V)

e ea ebc

Generation of three-phase voltages

(kolmivaihejärjestelmä)

S N

• Simple three-phase generator e^ < nBl^ ϖ

Anouar Belahcen

Generation of three-phase voltages

N S ݁ ௖

Z

Z

Z

vaa (^) ´ < ea <Vmax cos( ϖ t)

vbb (^) ´ < eb < Vmax cos( ϖ t, 120 )↓

´ max max

cos( 240 ) cc c cos( 120 ) v e V t V t

ϖ ϖ

• 3 single-phase circuits at different phase angle!

Anouar Belahcen

Connecting the 3-phase voltages

݁ ௖

• The potential difference is known but not the potentials!

Anouar Belahcen

Connecting the 3-phase voltages

Line-to-line voltage= Pääjännitte

Anouar Belahcen

• Three similar terminal of each coil connected to the same point called

neutral or N

Wye connection (Y- tai tähtikytkentä)

Anouar Belahcen

• For balanced symmetrical three phase system:

Wye connection (Y- tai tähtikytkentä)

V an < Vbn < Vcn <Vph

Van < Vph∉ ↓ 0 Vbn < Vph∉ , 120 ↓ Vcn < Vph∉ 120 ↓

Anouar Belahcen

Line-to-line voltages in Wye connection

V bc < Vbn ,Vcn 120 120 3 90

bc ph ph ph

V V V

V

V ca < Vcn ,Van 120 0 3 150

ca ph ph ph

V V V

V

V ab < Van ,Vbn 0 120 3 30

ab ph ph ph

V V V

V

Vll < 3 Vph

Anouar Belahcen

• The entrance terminal of one coil is connected to the end terminal of the

next coil

Delta connection (kolmiokytkentä)

Anouar Belahcen

• Residential loads are usually single phase loads (230 V)

• Industrial and commercial loads are mostly three phases loads (400 V)

• Clustered residential areas are powered by three phases

• Single houses might be powered by single phase

• Neutral point is grounded to ensure that all loads are powered regardless of

fluctuations in current

Single and three phases loads

Anouar Belahcen

• Load impedances connected to a common neutral point from one terminal

• The load is powered by a three phases source

Wye connected load

Connection

Z an < Zbn < Zcn <Z

Anouar Belahcen

• Neutral current

• If source and loads are balanced

the neutral current is zero

• In transmission this means no

need for a neutral line

Wye connected load

I n < Ia ∗ I b ∗Ic

I n < I a ∗ I a ∉ , 120 ∗ Ia∉ 120 < 0

Anouar Belahcen

• Loads connected between two transmission lines

• Voltage across the single load is the line-to-line voltage

• Balanced load and source balanced currents

Delta-connected load

I ab < I a ∗Ica

I bc < I b ∗Iab

I ca < I c ∗Ibc

I ab^ V^ ab

< Z

I bc^ V^ bc

< Z

I ca^ V^ ca

< Z