AVR and Governing Systems of Synchronous Power Generators
The large-scale synchronous generators have a tendency of variating its voltage and frequency due to load. When the load changes with the time, it feels the generator and it may affect the power system’s stability. The study of synchronous generators can be easily divided into two main categories, voltage regulation and speed governing. both of this controlling requirement is must to maintain the stability of the power system. Let’s look into one by one.
As already mentioned, when the load varies, that load change feels to the generator directly. As soon as load increases, generator speed droops and when the load decreases, it acts vice versa. We all know the generated frequency is directly proportional to the rotating speed of the generator. Since electricity provider has agreed to provide an almost constant frequency for their customers within allowable regulation, the guarantor speed should be restored. To restore the speed, it should be increased or decreased the acting force on the prime mover. To do this function with a minimum time along with optimum transient, we should use Governing Systems. This system controls the reactive power output of the generator.
In governing systems, they have two main operating modes.
- Free running operation
- Pre-setting operation
In the free running operation mode, when suddenly load increases, the rotating speed of the generator is suddenly dropped. At the same time, the rotating speed of the speed governor is dropped since it is directly attached to the generator axial. In the speed governor mechanism, there are heavyweights attached to the spring is rotating. Since rotating, there is a centrifugal force acting outside from the axis. That centrifugal force is balanced by the spring attached to the speed governor (see figure 1). Since the rotating speed drops, the distance between weights is got lower and ABC rod moves up from the C end. The linkage arm is raised, and pistons attached to the point D goes up. Due to high-pressure oil, the Main piston moves downward, and the flow valve is further opened. When the flow valve is opened, the force acting on the prime mover increased and the speed of the generator raises. with the restoring of the generator speed, the distance between two weights is increased. Then the linkage arm tries to get lower. This loop occurs continuously occur and as a result of this, the frequency is stabilized at a constant value which already decided. When the load suddenly decreased, the system acts vice versa.
The characteristic of turbine speed and mechanical power is called “Droop Characteristic”. This is defined as,
It is also can be represented as a graph of frequency against electrical power output.
Assume generator is delivering power at PC at 50 Hz in steady state. If the demand is increased up to PD, the frequency will be dropped according to the droop of the system. By increasing the reference point form Pref (0) to Pref (1), we can restore the frequency back to 50 Hz.
This free running mode is not the only operating mode. This mode is established in every power plant to respond for load changes as quickly as possible. The Pre-setting operation mode is done by manually. Here the reference is adjusted manually with respect to the load by adjusting the set screw. This operating mode is essentially used in large scale power plants as well as even the power plant is big or not the pre-setting operation is done through the free-running operation. This Governing process is involving with active power. Nowadays this kind of mechanical governing systems are replaced with electronic systems.
As we already discussed When the load changes, the frequency changes. When the frequency changes, the generated voltage across the output terminal changes. But the thing is the electricity provider has agreed to provide a constant voltage output to the customer and there is a maximum allowable tolerance. So, the electricity provider has to control the voltage output somehow. The easiest way of controlling output voltage changes the strength of magnetic flux crossing through the stator. To change the magnetic flux, it’s obvious that we should change the excitation current of the stator (field winging’s). This voltage controlling mechanism is done by the Automatic Voltage Regulator (AVR). Behind this mechanism, the AVR controls the reactive powers demanded by loads. To control the reactive power output, it is needed to control the excitation current. This excitation current changes the induced emf and changes the reactive power output. This process is totally done by power electronic circuits and this has 2 main operations as same as governing systems.
1. Free running mode
2. Pre-setting mode
In free running mode, output voltage varying according to the reactive power for a given reference output voltage. In the pre-setting mode, we directly change the output voltage reference to change the reactive power.
AVR also has a droop characteristic. It is defined as,
AVR and Governing system is to control has the same process, but they involve reactive power and active power respectively.
