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Electric Power
Lecture 2
The Electrical Power System
• Power is one of the main subdisciplines of
• The power generation, transmission and
distribution system is something we all use
all the time.
• It is an excellent example of a case where
electrical networks are used to model the
flow of energy.
Lecture 2
Consider what we do here.
• We usually generate a huge amount of
power in a centralized location.
• We ship it to you when you flip the switch
• You then decide that you will light a desk
lamp located 18 inches above your desk,
about 2 feet in on the right.
• You can easily change your mind and put
the lamp on the left!
• This is magic.
Lecture 2
Electric Load
• The load changes continuously
– Daily
– Seasonal
• The daily maximum occurs
around 4-6 PM , the minimum
at night.
• The load or demand is defined
as the average load (MW) for
15 minutes
• Seasonal changes: Summer load
is higher than the winter load at
Base load (large thermal and
nuclear plants)
Intermediate loads (medium
steam and hydro)
Peak load (gas turbine and
combined cycle plants)
Intermediate load
Base load
Lecture 2
What to notice about what
• We do NOT create energy out of nowhere.
• Instead, we convert some form of energy. In
a fossil power plant, chemical energy is
converted to a mechanical motion of a
rotating turbine and generator. The result is
electric energy.
Lecture 2
Electrical Power Transmission
1) The generating station converts the energy of gas, oil, coal or nuclear
fuel to electric energy. The generator voltage is around 15-25 kV
2) The main transformer increases the voltage to 380 kV. This reduces the
current and losses.
3) The high voltage transmission line transports the energy from the
generating station to the large loads, like towns. Example: Energy
generated at Elazığ is transported by 380 kV lines to Ankara, İstanbul.
Lecture 2
Electrical Power Transmission
4) The high voltage substation reduces the voltage to 110 kV. The
substation serves as a node point for several lines.
5) The sub-transmission lines (110 kV) connect the high voltage
substation with the local distribution station.
6) Distribution lines (10 kV) distribute the energy along streets or
underground. Each line supplies several step down transformers
distributed along the line.
7) The distribution transformer reduces the voltage to 220/380 V, which
supplies the houses, shopping centers etc..
Lecture 2
Generating Station
Type of Generation stations
• Thermal Power Plant. The large (more than 500 MVA) plants carry
constant load (base load plant), Smaller plants loads are regulated but
they operate continuously. Minimum down time is 20-35 hours.
Nuclear Power Plant. These plants carry constant load and are used as
base loads plants.
Hydroelectric Plants. It is economically desireable to load these plants
to the maximum capacity, because of the low energy cost. (Water is
free). Other factors: flood control, irrigation and salmon migration.
Combined steam and Gas-Turbine Power Plants. High efficiency
plants for variable load.
Gas-turbine. Peak load plants, high operating and low investment cost
• Solar, Wind. Loaded to the maximum capacity, when sun or wind
power available.
Lecture 2
Steam Turbine
Lecture 2
Steam Turbine
The moving blades are attached to the shaft.
The stationary blades are attached to the casing.
The control valves regulate the steam flow.
The turbine often has three stages: high, medium
and low pressure (right to left)
• The high pressure steam drives the turbine (3000
• The generator is connected directly to the turbine
Lecture 2
Generating Station
Lecture 2
Hydro Power Plant
• The water is stored in the lake, which is at higher
• A canal and pipe system transfers the water to the
power house.
• The potential energy of the water is transformed to
mechanical energy in the turbine.
• The turbine drives the generator and converts the
water mechanical energy to electrical energy.
Lecture 2
Electrical Power Transmission
Lecture 2
Transmission Lines
Type of transmission lines:
• Extra high voltage lines
– 380 kV
– Interconnection between
systems. (National Grid)
• High Voltage lines
– 110 kV, 220 kV
– Inter connection between
• Sub-transmission lines
– 45 kV, 69 kV, 120 kV
– Substation and large customer
• Distribution Line
– 2.4 kV- 45 kV , 15 kV
– Supplies houses
• High Voltage DC lines
– 120 kV- 600 kV
– Interconnection between
Lecture 2
The substation forms a node point in the electric network
Substation equipment :
• Transformer to change the voltage and current level
• Circuit breaker(CB) to interrupt the load and fault current. The fault
current automatically triggers the CB
• Disconnect switch to provide visible circuit separation. Permit CB
maintenance. No load operation.
• Voltage and current transformers to reduce the current to 5 A, the
voltage to 220 V and insulate the measuring circuit from the high
• Surge arresters are used for protection against lightning and
switching overvoltages. They are voltage dependent, non linear
Lecture 2
AC versus DC
• AC is alternating
• AC quantities always
vary sinusoidally in
• Usually, we will know
the frequency and
solve for the
amplitude and phase.
• DC is direct current
• DC quantities are
always constant in
• DC can not be directly
transformed to lower
or higher voltages.
Lecture 2
Why do we use AC for power
• Power levels correspond to v2. Therefore,
to get a huge power out of Atatürk
Hidroelectrical station, we need huge v’s.
We need to step the voltage down before it
reaches our classroom. The power
company wants to do this in a lossless
fashion. AC allows them to do this by
using transformers.
Lecture 2
Why Are There Three
Conductors in Power
Transmissions Lines?
• Most AC power transmission systems have
three conductors.
• The voltage on each phase (referenced with
respect to earth ground) is a sinusoid with a
phase difference of 120 from the voltages
in the other two phases.
Lecture 2
Three Phase
• A three conductor transmission system is
called a three-phase system.
• The power delivered by a three phase
system (assuming a balanced load) is
constant, even though the voltages in each
phase vary sinusoidally.
• Three phase systems are more efficient than
single systems due to reduced power losses.
Lecture 2
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