In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy.
Figure: Electric generator.
Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. So to get voltage, induction is very necessary. There are three conditions of proper induction.
2. Magnetic fields and
3. Relative motion between them.
Figure: Electromagnetic induction
A metallic wire is warped around a metal bar called rotor. Rotor is connected with an external dc source for excitation. By the excitation, a magnetic field is produced where fluxes are flows from North Pole to South Pole. When the rotor rotates the magnetic field also rotates. As a result the fluxes are cut by the conductors and voltage is induced from there. This is the Faraday’s law of electromagnetic induction.
Faraday's law of electromagnetic induction states that:
“The electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path.”
Where, is the electromotive force (emf) in volts.
ΦB is the magnetic flux in webers.
For the common but special case of a coil of wire, composed of N loops with the same area, Faraday's law of electromagnetic induction states that
is the electromotive force (emf) in volts.
N is the number of turns of wire.
ΦB is the magnetic flux in webers through a single loop.
Here, the negative sign indicates the direction which opposes the direction of motion according to Lenz’s law.
Applications of Faraday’s law:
Ø Electrical generator
Ø Moving coil microphone etc.
Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt. It is denoted by P.
Mathematically, P = VI = I2R
Where, V = voltage in volts (V)
I = current in ampere (A)
R = resistance in ohm (Ω)
In every power plant there are two types of power is generated.
1. Megawatt (MW)
2. MegaVARs (M-VAR)
The megawatt is equal to one million (106) watts. Many events or machines produce or sustain the conversion of energy on this scale. For example: lightning strikes, large electric motors, large warships, such as aircraft carriers, cruisers, and submarines, engineering hardware, and some scientific research equipment, such as supercollider’s, and in the output pulses of very large lasers. A large residential or commercial building may consume several megawatts in electric power and heat.
The productive capacity of electrical generators operated by a utility company is often measured in MW.
Mega VAR stands for Mega Volt*Amps Reactive. Although reactive power is not 'real' (i.e. it is not considered power at the source or destination), it still consists of current flowing in the transmission lines. When current flows in the line, power (real power) is lost due to I2R losses. When you are talking about Mega VARs, this power loss is significant and is a direct loss for the power company. This reactive power is usually caused by a company having a large inductive load (lots of motors). If it is bad enough, sometimes companies will put large capacitor banks in or near their factory to try and balance things out. If they don't, the power company may ask them to pay for the losses.
Here, P = VA
MW = VAcosθ
M VAR = VAsinθ
Here cosθ is known as power factor. It is the ratio of real power to apparent power. It lies between the number 0 and 1.
A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour. Generally it is known as unit.
1 unit electricity = 1 KWH power.
Variation of frequency with load:
The load and frequency is inversely proportionate. Load increases with the decrease of frequency and vise versa. When the load is too much, then the frequency will decrease in such an alarming rate that may cause the turbine trip. At this time turbine rpm should be controlled.
In a power plant, power generation is controlled by the equation –
P = (120*f ) / N.
Where, P = number of magnetic poles.
F = frequency and
N = number of revolution per minute (rpm).
In Ashuganj power station, the total number of magnetic poles is 2.
Standard frequency for Bangladesh is 50 Hz.
Now if we calculate the shaft revolution, we get –
N = (120*f ) / P
N = (120*50) / 2 =3000 rpm.
So for Ashuganj power station, the turbines are designed with 3000 rpm at standard frequency 50 Hz. So the turbine rpm must be satisfied at any conditions.
But when the load is high, then frequency decreases with the decrease of turbine rpm.
For example, at certain load the frequency is 49 Hz which < standard frequency 50 Hz.
Hence the rpm is –
N = (120*49) / 2 = 2940 which < 3000 rpm.
So to get standard rpm we have to increase the turbine revolution. Hence more steam is passed through the turbine blades and increased rpm until it catch the standard rpm. When the load is too much to supply, then power transmission lines are cut off for few times to back up the supply.
But when the load is lower, then frequency increases with the increase of turbine rpm.
For example, at certain load the frequency is 51 Hz which > standard frequency 50 Hz.
Hence the rpm is –
N = (120*51) / 2 = 3060 which > 3000 rpm.
So to get standard rpm we have to decrease the turbine revolution. Hence less steam is passed through the turbine blades and decreased rpm until it catch the standard rpm.
When the demand is too low, then any units may be shut down for efficient operation.
Here the frequency allowance lies from 48.5 to 51.5 Hz. The variation of frequency with load is controlled by controlling the turbine rpm as well as steam flow. This control process is known as turbine governing system.
Generator cooling system:
Normally generators are cooled by hydrogen gas. A cooling system consist some hydrogen cylinders, two pressure regulating valves, two isolation valves and a manifold.
Figure: Generator cooling system.
Ø Hydrogen gas is supplied from gas cylinders.
Ø Gas pressure is maintained at 100-125 psig by the 1st pressure regulating valve.
Ø Gas pressure is maintained at lower value of 30 psig by 2nd pressure regulating valve.
Ø Two insulation valves are used for insulation the supply.
Ø Finally hydrogen enters into generator through a manifold and cools the entire temperature.