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Monday, 19 August 2019

WHAT IS GOVERNING OF TURBINE?

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine.  


Today we will understand here the basics of governing of a turbine with the help of this post. Once we will come to know the meaning of governing of a turbine, we will be able to easily understand the concept of governing of Pelton wheel turbine and governing of Francis turbine. 

Governing of turbines 

We have already understood that hydraulic turbines are basically defined as the hydraulic machines which convert hydraulic energy in to mechanical energy and this mechanical energy will be given to a generator to produce the electric energy. Generator will be directly coupled with the hydraulic turbine. 

In order to maintain the constant frequency of electric power output, the rotor of the turbine has to rotate with a constant speed and therefore it is needed to maintain the constant rotational speed of the turbine rotor. 

Now we must understand here that how to maintain the constant rotational speed of the turbine rotor. 

Rotational speed of rotor of a turbine will be dependent over the driving torque and resisting torque.
Driving torque will be provided by fluid flowing through the blade passages by its change of angular momentum and resisting torque will come from the electrical load. 

Balance between these two types of torque i.e. driving torque and resisting torque will enable the rotator of turbine to rotate at constant angular speed. 

When electrical load will be changed, electrical load might be increased or decreased depending on demand, speed of rotor of turbine will be changed if there is no provision to change the driving torque. Hence, frequency of electric power output will be changed due to change in rotational speed of rotor of turbine and it is not a desirable result. 

Let us take one case where electrical load is increased. Let us think what will happen due to increase in electrical load. Resisting torque will be increased and therefore for a given driving torque, speed of rotation of rotor of turbine will be decreased. In this case it will be required to increase the driving torque to boost up the rotational speed of rotor up to its original speed. 

Suppose if electrical load is decreased then will happen. Resisting torque will be decreased and therefore for a given driving torque, speed of rotation of rotor of turbine will be increased. In this case it will be required to decrease the driving torque to reduce the rotational speed of rotator up to its original speed. 

Therefore in order to restore the initial speed of rotation of rotor of turbine, we need to change the driving torque up to the desired value of resisting torque which is changed due to change in electrical load. 

As we know that energy given to the rotor of the turbine will be directionally proportional to the fluid flow rate, therefore change in driving torque will be done by change in fluid flow rate. 

If electrical load is increased, it will be required to increase the driving torque to boost up the rotational speed of rotor up to its original speed and driving torque will be increased by increasing the fluid flow rate. 

If electrical load is decreased, it will be required to decrease the driving torque to reduce the rotational speed of rotator up to its original speed and driving torque will be decreased by decreasing the fluid flow rate. 

Therefore, by controlling the fluid flow rate, driving torque could be changed to meet with the resisting torque to maintain the constant speed of rotation of rotor of turbine in order to produce the constant frequency of electrical power output. 

This is the basic principle behind the governing of all type of turbine. This process by which the speed of rotation of turbine rotor is kept constant will be termed as governing of a turbine. 

Governing of a turbine is very necessary as a turbine is directly coupled with electrical generator which is required to run at constant rotational speed in order to produce the constant frequency of electrical power output. 

Do you have any suggestions? Please write in comment box.  

Further we will find out, in our next post, governing of Pelton turbine

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google  

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