(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