Loads are variable parameters and they never remain constant. Based on the demands their intake varies. As a result, the power generated by the alternator needs to meet these demands.
Load Frequency control is a method that helps in maintaining the frequency of the network at its rated value i.e., 50Hz in India. This article deals with the process that goes about when the load varies in various cases.
Let us take an example,
Consider the initial state to be as follows,
Power generated = 200MW
Now it is observed that the,
Now the frequency will also be at its rated value. Moreover, the machine (Synchronous Generator) will run at its synchronous speed.
But, as we mentioned earlier the load of the system is never a constant. Thus there are two possibilities,
CASE1: THERE IS AN INCREASE IN LOAD
CASE2: THERE IS A DECREASE IN LOAD
Now let us see the detailed effects of each case.
CASE 1 : THERE IS AN INCREASE IN LOAD
Consider the demand increases and the load connected to the bus changes to 300MW. Now the values are,
Load connected to the bus = 300MW
Power generated = 200MW
Now it is observed that the
- When it is observed that 100MW is required from the load side, the energy that is needed is supplied by the stored kinetic energy of the machine. The K.E. is produced by the rotating parts of the machine:
Where,
K.E-Kinetic Energy (J/joules)
I-Moment of Inertia (is a constant)
ω - Angular velocity (rad/sec)
Thus, 100MW is supplied by the stored K.E and its value decreases. - As observed in the equation 1, the,
K.E.∝ ω2
Thus, when K.E. decreases, the speed of the machine also decreases. - The speed of the machine and its frequency are related by the relation,
Thus, when the speed decreases, it, in turn, reduces the frequency and if the new frequency is fnew, then,
fnew < 50 Hz
Which not acceptable for a stable operation.
CASE 2: THERE IS A DECREASE IN LOAD
Consider the demand decreases and the load connected to the bus changes to 150MW. Now the values are,
Load connected to the bus = 150MW
Power generated = 200MW
Now it is observed that the,
As a result the following occurs:
- When it is observed that 50MW is produced in surplus the generator side, the energy that is needed is absorbed is added to the stored kinetic energy of the machine.
The reason is, there needs to be a balance in energy (due to the Law of Conservation of energy).
Thus, 50MW is added to the stored K.E and its value increases. - As observed from the equation 1, when K.E. increases, the speed of the machine also increases.
- From equation 2, when the speed increases, it in turn increases the frequency and if the new frequency is fnew, then,
fnew > 50 Hz
Which is also not acceptable for stable operation.
The change in frequency can be sensed using a device called a Governor. The Governor detects the change in frequency and then insists to change the Steam valve position.
This is done because by adjusting the valve position based on the change in frequency, we can change the steam input to the turbine. As the turbine is mechanically coupled to the Synchronous Generator, by adjusting the speed of the turbine it in turn adjust the mechanical input to the rotor.
SUMMARY:
| Change in load | Change in frequency | Steam valve position | Steam input | Mechanical input to generator | Power generated by the generator |
|---|---|---|---|---|---|
| Increases | Decreased | Open more | Increased | Increased | Increased |
| Decreases | Increased | Close more | Decreased | Decreased | Decreased |
Now adjusting the mechanical input to the rotor, it also adjusts the Power Generated and this is how the change in load is met and the frequency is adjusted back to its rated value.
[Also Read: Synchronization of Alternator]
Author: Vaishnav Chathayil is pursuing his B.Tech. in Electrical and Electronics engineering at National Institute of Technology, Calicut.
