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1. Generator :
In earlier session, you heard the roll of the boiler, and turbine.
At the boiler the burn the coal or oil, we gain the heat and making hot gas or hot steam,
which rotate the turbine, in this way we gain the mechanical energy-torque,
By converting this rotating mechanical energy, we gain the electrical energy.
The role of generator is convert mechanical energy to electrical energy.
This is the generator and basically all generators can be divided into five major assemblies.
STATOR FRAME ASSEMBLY: These components are heavy steel fabrications whose main function is to support and contain the other four major assemblies and contain the pressurized Hydrogen cooling gas.
ROTATING FIELD ASSEMBLY: This is a large rotating steel forging which create creating a very powerful magnetic flux by DC windings. In here, the generator rotor is coupled with turbine’s rotor so it can rotate. It’s designed to provide the rotating flux necessary to generate an AC voltage and current flow in the armature winding coils.
END SHIELD ASSEMBLIES: Located at each end of the stator frame, the end shields are heavy steel fabricated structures designed to support the rotating Field and to withstand the pressure of the hydrogen gas. It consist of End Shield include the main journal bearings, Hydrogen seals and oil deflectors.
Stator CORE ASSEMBLY supports the armature winding and provides a low reluctance path for the magnetic flux.
ARMATURE WINDING ASSEMBLY : in here that the mechanical energy of the turbine is converted to useable electric power and fed into the electrical grid though the High Voltage Bushings in the terminal box
Each component will be more explained in tomorrow.
Generator is divided by cooling type/ Air cooled-Hydrogen Cooled-Hydrogen Water cooled
As the capacity of the generator increased, the more heat load will be created and more complicated cooling system
is needed.
In low capacity, we apply air cooled generator.
Air cooled generator is divided open ventilation and TEWAC.
The Open Ventilated air cooled generator design is suited in clean air environments, free from harsh chemicals, sand, dust and insects.
And about AIR-COOLED "TEWAC" GENERATORS
The Totally Enclosed, Water to Air Cooled (TEWAC) generator design is with adverse air conditions.
Hydrogen cooled generator used H2 gases for cooling, because hydrogen’s cooling ability is better than the Air.
But hydrogen gas can be explode when it meet the air,
Hydrogen cooled generator must have H2 control system and Seal oil system because we must prevent leaking the hydrogen.
we use a single piece stator frame with 4 vertical hydrogen coolers installed in cooling towers located at the four corners of the frame
Hydrogen water cooled generator has the largest capacity, so we use addition method besides hydrogen gas,
stator winding liquid cooling system, for cooling the stator winding.
As generator electrical ratings grew, the generator core also grew in both length and diameter and hydrogen coolers had to became larger to remove the excess internal heat.
But with the larger diameter cores, there was no room for the required larger coolers between the core outer diameter and the wrapper of the frame.
Also it has TWIN DOME for HORIZONTAL COOLER
So we applied “Twin Dome" design, in which the coolers were placed horizontally in separate domes on the top of the stator frame.
The domes are shipped separately and assembled to the top of the main stator at the customer's power plant.
This design is more expensive that the single piece vertical cooler design due to the additional labor required at the construction site.
This slide shows you Doosan generator product line up.
Doosan has handled 100 to 1500MW generators.
They are classified into three types based on the cooling medium.
Because generator capacity closely related to heat dissipation.
With an increase in the generator capacity, heat load will also increase.
As a result improve the cooling system, capacity of the generator is increased.
The generators below 150MW normally are air cooled, 150 to 350MW hydrogen cooled and above 350MW normally water cooled
Before taking about PF, we discuss about type of load.
In a circuit with an induction load such as transformer or motor. An alternating input voltage produces an alternating current, which is lagging the voltage.
In a circuit with a capacitor load such as a camera flash or long transmission lines in a power grid, the alternating input voltage produces an alternating current which is leading the voltage.
This capacitive or inductive load make power factor angle between voltage and current.
The power factor is cosA of this phase difference.
In this triangle, This means apparent power the product of the voltage and current
And this means the real power W which is power plant supply to the customer and this meas imaginary power vars that
Which is needed to control the power grid.
Depending on the application, generators can operate at a leading PF as well as a lagging PF.
Each customer specifies a Lagging PF requirement based on his application; typically lagging PF ranges from 0.85 to 0.90 and occasionally as high as 0.95.
Many older plants were commonly specified with PF as low as 0.80, while newer plants can meet local voltage support requirements with PF in the 0.85 ~ 0.90 range.
These differences are a function of local load characteristics such as motors, capacitors, transformers, etc.
A lower PF results in higher armature and field currents for a given active power output. Customers will usually demand the generator to
supply more reactive power than required to meet their electrical system load. Such generators must be physically larger.
Larger generators introduce higher losses associated with the higher current. This reduces generator efficiency and increases operating costs.
Terminal Voltage is given as Line-to-Line voltage in a 3-Phase Y connection generator. As like Vab, Vbc, Vca.
For Marafiq #5,6 Generators, terminal voltage is 18kV
Terminal Voltages on larger generators are generally the designer’s choice based on other requirements and limitations,
and generally ranges from 18 – 28 kV on 60Hz machines and slightly lower on 50Hz machines.
Stator winding for Marafiq project
is 72slots and has 3 phase-3circuit
3phase is used in synchronous machine,
and circuit is determined by the current,
number of turns is determined by the generating voltage.
Generator’s other name is synchronous machine. Since the frequency of the grid and the frequency of the rotating part is same.
Electrical Frequency throughout the world is either 50 or 60 Hz.
This varies by country, but a few countries, such as Japan, have both 50 and 60 Hz.
Korea and North America Saudi Arabia use 60 Hz, Most of Asia, Africa and Europe use 50 Hz.
Fossil Fired Boilers can obtain high steam pressures and temperatures which allow for higher speed turbines.
The world standard is either 3600 RPM (60 Hz) or 3000 RPM (50 Hz). This requires the 2-Pole generator rotor design.
Due to the lower steam conditions of Nuclear Reactors,
the steam turbines operate at lower speeds of 1800 RPM or 1500 RPM and require 4-Pole generator rotors.
Generator has several kind of the cooling method.
By cooling medium, it is called as air cooling, hydrogen cooling and hydrogen water cooling
As apply more effective medium, to go hydrogen water cooling, generator capacity will be increased.
In here, there is indirect cooled and direct cooled way.
Direct means the cooling material is contact with hot area and remove the heat,
Indirect means the hot area is surrounded by another material such as insulation layer, the cooling material
can’t directly contact with this hot area.
Short Circuit Ratio (SCR), is another generator characteristic specified by the customer.
It can be determined from the Open Circuit Saturation Curve and the Synchronous Impedance curve
and is defined as the ratio of the field current at no-load rated voltage (AFNL) to the field current required
to produce rated armature current with the generator terminals short-circuited (AFSI).
It is a measure of the stiffness of the magnetic link between the stator and the rotor and
an indication of the steady-state stability inherent in a generator design.
It shows the relation between the no-load field strength and the armature reaction strength.
However, a higher SCR is not free, because it is obtained by designing a larger air gap to increase AFNL.
This then increases the excitation required for normal load, which requires a larger field,
more ventilation gas flow, a more costly and larger generator.
Developments in high-speed solid state voltage regulators and
excitation systems provide more cost effective methods of meeting stability needs and thereby permit operation at a lower SCR.
In the early 1950s. SCR specifications ran as high as 0.90.
State of the Art control technology today permit reliable steady-state stability in the 0.50 SCR range.
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