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(EE328)
ELECTRIC POWER and MACHINES
Prepared by: Dr. Nahla Zakzouk
LECTURE 1
Electric Power SystemA 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
Voltage Ratings
Low Voltage
Medium Voltage
High Voltage
For a phase-to-
phase voltage
between 100 V
and 1000 V.
The standard
ratings are: 400
V -690 V -1000
V (at 50 Hz)
For a phase-to-
phase 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-to-
phase voltage
between 35 kV
and 230 kV. The
standard ratings
are: 45 kV -66 kV-
110 kV -132 kV -
150 kV -220 kV
System voltage classes according to IEC (International Electro technical Committee) Standard no. 38
II. Voltage Ratings
III. Egypt Network
System voltage ratings are inspired by IEC Standard
IV. Power System Components
Components
Transformers
Cables
Protective devices
Loads
Capacitors and
reactors
Supplies
Coal-fired power stations Renewable energy sources
11/71 kV, 33MVA step-upsubstation 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 highvoltages from the generating centers tothe load centers, or the distributionsystem, which feeds smaller amounts ofpower at lower voltages from the loadcenters to nearby homes and industry.
• Choice of conductors is based uponconsiderations such as cost,transmission losses and other desirablecharacteristics. Copper has lowerresistivity than aluminum, while thelatter aluminum has lower cost for thesame current carrying capacity and isthe primary metal used for transmissionline conductors.
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
Cables
Oil circuit breaker
Load majority is inductive; thusthe current lags behind thevoltage. This leads to theemergence of reactive powerdecreasing power factor and in turnsystem efficiency. Hence, powerfactor correction is mandatory.Capacitors are often placed nearinductive loads to reduce currentdemand on the power system (i.e.,increase the power factor). Somereactors are partnered with the p.f.capacitors to prevent resonance
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
R3R2 Req
R1
R2
-+
+
-
+
-
v
i
v1
v2
Voltage division:
Parallel connection
Current division:
R1 R2
+
-
v
i
i1 i2
Impedance: RLC Circuits
Three-Phase Circuit
•Three sinusoidal voltages form a set of balanced voltages when they have the same amplitudes and
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