Distribution of Electricity 1

1
Distribution of Electricity (1)
Household Systems
Single-phase Two-wire
Single-phase Three-wire
wires
Transmission Models
Single-phase Three-wire system is more flexible
as it can offer two working rms voltages (twice
as large) that suit the two standards of most
electrical appliances, such as 110 Vrms and 220
Vrms standards, BUT THIS MEANS MORE WIRES!
2
Distribution of Electricity (2)
Long-distance Power Distribution
Poly-phase Systems
The most commonly used is Three-Phase Systems!
cables

• Overall instantaneous power is
  CONSTANT!
• Thus, less power fluctuation during transmission
• More economical in terms of the amount of
  wires in volume (for the same distributed power
  level)

generator
load
Transmission Model for Three-Phase Four-Wire
systems
3
Distribution of Electricity (3)
Long-distance Power Distribution
Household Systems
4
Voltage Generator Basics (1)
Michael Faraday and his Law Faradays Law
Time-varying magnetic field produces
electromotive force

Magnetic flux (Tesla)
Electromotive force (VOLT???)
SIGNIFICANCE of the minus sign!
Equivalent integral form


Electric Field (Volt/m)
Magnetic Flux Density (Tesla/m2)
the total area enclosed by the integral path
!
5
Voltage Generator Basics (2)
A physical interpretation of Faradays Law
stationary loop (coil), time varying field
area enclosed by the contour
Volt meter measures the produced emf
Contour determined by a wire (assumed
a perfect conductor)
Practical Generator
is produced by the rotating magnet
Perpendicular fluxes that cut through area
is the projection of the circular movement of
the rotating magnet Hence we get a SINUSOIDAL
output!
6
Voltage Generator Basics (3)
THREE-phase generator
A magnet rotated inside three coils placed 120o
apart
Stator stationary windings (multi-turn
coils)
Rotor rotating magnet (producing the
fluxes)
Windings a-a/, b-b/ and c-c/ are placed 120o
apart around stator
a-c-b sequence
clockwise
3-phase generated voltage when the rotor rotates
counter clockwise
a-b-c sequence
7
Three-Phase Generator PHYSICAL Models
Y-Connected
-Connected
Windings
8
Three-Phase Generator Electrical Models (1)
Y-Connected Model (a-c-b sequence)
For balanced phase voltages


with the same frequency but 120o out-phase
Rotating Phasor Equivalence
Observation!
0
9
Three-Phase Generator Electrical Models (2)
-Connected Model (a-c-b sequence)
NO NEUTRAL (n) line!!! Each phase voltage
references to each other
For balanced phase voltages


Rotating Phasor Equivalence
Observation!
0
10
Three-Phase Generator Electrical Models (3)
Relationship between the Y- and -Connected
Generator Models
Y-Connected
-Connected

• Neutral line has been ignored!
• In practice, this help save wire
• cost!

11
Three-Phase Generator Electrical Models (4)
Phasor Equivalence
Observation
For balanced phase voltages

• Conversion between Y- and
• -connected voltages

12
Three-Phase Load Arrangements
Y-Connected Load
-Connected Load
For balanced loads


same magnitude and phase!
Conversion between Y- and -connected loads

Based on equivalent impedance seen at any pairs
of the terminals a-b-c!
13
Three-Phase Circuit Classifications (1)
Four possible connections between the source and
the load (assuming a-c-b sequence)
1.
For balanced loads and balanced phase
voltages, 0 and hence the
neutral line may not be used.
Y-Y Connection
Definitions
Phase voltage voltage (mag) appearing at
Line voltage voltage difference (mag)
between any pair of the transmission cable
Phase current current (mag) passing
through
Line current current (mag) in one
cable
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Three-Phase Circuit Classifications (2)
2.




Y- Connection (most practical connection)
- connected load does not facilitate the
use of the neutral line
Phasor diagram of Line and Phase Currents
15
Three-Phase Circuit Classifications (3)
3.




- Y Connection
4.




- Connection
16
Three