(EE328)
ELECTRIC POWER and MACHINES
Prepared by: Dr. Nahla Zakzouk
LECTURE 1
Electric Power System
A network that involves the generation, transmission, distribution and utilization of electric power as well as dealing with electrical
components including generators, transformers, motors, ……… etc—and is known as electric power system (grid).
It is be broadly divided into the generators that supply the power, the transmission system that carries the power from the
generating centers to the load centers and the distribution system that feeds the power to nearby homes and industries
I. Power System Layout
13.8 kV
II. Voltage Ratings
System voltage classes according to IEC (International Electro technical Committee) Standard no. 38
Voltage Ratings
Low
Voltage
Medium
Voltage
High
Voltage
For a phase-tophase voltage
between 100 V
and 1000 V.
The standard
ratings are: 400
V -690 V -1000
V (at 50 Hz)
For a phase-tophase voltage
between 1000 V
and 35 kV. The
standard ratings
are: 3.3 kV -6.6
kV -11 kV -22 kV
-33 kV
For a phase-tophase voltage
between 35 kV
and 230 kV. The
standard ratings
are: 45 kV -66 kV110 kV -132 kV 150 kV -220 kV
III. Egypt Network
System voltage ratings are inspired by IEC Standard
IV. Power System Components
Coal-fired power stations Renewable energy sources
Transformers
Supplies
Components
Load majority is inductive; thus
the current lags behind the
voltage. This leads to the
emergence of reactive power
decreasing power factor and in turn
system efficiency. Hence, power
factor correction is mandatory.
Capacitors are often placed near
inductive loads to reduce current
demand on the power system (i.e.,
increase the power factor). Some
reactors are partnered with the p.f.
capacitors to prevent resonance
Capacitors
and
reactors
Loads
11/71 kV, 33MVA step-up
substation in generating plant Substation power transformer Three 11 kV/380-V, 125kVA
Transformer
• Carry power from the generators to the
load. (In the transmission system,
carrying large amounts of power at high
Cables
voltages from the generating centers to
the load centers, or the distribution
system, which feeds smaller amounts of
power at lower voltages from the load
centers to nearby homes and industry.
• Choice of conductors is based upon
considerations
such
as
cost,
transmission losses and other desirable
characteristics. Copper has lower
Protective
resistivity than aluminum, while the
Cables
devices
latter aluminum has lower cost for the
same current carrying capacity and is
the primary metal used for transmission
line conductors.
Oil circuit breaker
During faults the current flow is
interrupted by opening the circuit to
prevent equipment failure. This can be
done by circuit breakers which can be
reset after faults and switches the
circuits in and out the substation
Questions
• Discuss what is a power system, showing its layout and stating its components.
• What are different voltage ratings according to IEC standards
Review on electric circuits
Kirchhoff’s Voltage Law
The sum of the voltages around any loop of a circuit at any instant is zero.
Kirchhoff’s Current Law
At every instant of time the sum of the currents flowing into any node of a
circuit must equal the sum of the currents leaving the node,
Series connection
R1
R3
R2
Req
i
+
R1
v
R2
-
+
v1
+
v2
-
Voltage division:
Parallel connection
i
+
v
-
i1
R1
i2
R2
Current division:
Impedance: RLC Circuits
Three-Phase Circuit
•Three sinusoidal voltages form a set of balanced voltages when they have the same amplitudes an
frequency and are shifted in phase by 120o from each other
•The standard practice is to name those phases by a, b and c and use phase a as reference.
•These voltages represent phase a voltage, phase b voltage and phase voltage.
Three-Phase Circuit