When a turbine of any sorts is used for power generation then we are using the rotations to create electricity using magnets in the generator. This means there is no actual contact to cause friction right? Then why does the speed of the turbine decrease when we start using more power?

 

When you draw power from anything, that power has to come from somewhere - you can't get something for nothing.

With a rotating machine, there is a considerable amount of energy stored in the rotating parts due to inertia. If there is no change of shaft drive power in, any increase in electrical power out has to come initially from the stored kinetic energy - this depends on the square of speed, and the shaft will slow down as energy is removed. The change in speed is detected by the governor or other control system and used to increase the fuel or steam to the prime mover so that the overall energy balance is restored.

Note that this explanation applies to an isolated system - when a machine is connected to an external supply, the behaviour is more complex.

 

> When you draw power from anything, that power has to come from somewhere - you
> can't get something for nothing.

---- snip ----

> Note that this explanation applies to an isolated system - when a machine is
> connected to an external supply, the behaviour is more complex.

It does apply to an infinite bus system too. Although as you say it is more complex behavior.

 

> When a turbine of any sorts is used for power generation then we are using the
> rotations to create electricity using magnets in the generator. This means
> there is no actual contact to cause friction right? Then why does the speed
> of the turbine decrease when we start using more power?

Mechanically, Power = Torque X Angular velocity (1). This is where the rate of energy flow (Power) you start up with. When you increase the load to a generator, you just increase the opposing torque in Equation 1. From the conservation of energy, if no additional input energy is supplied to the prime mover then angular velocity will reduce. For the same power input power, 1% increase in opposing torque then the angular velocity of the prime mover will reduce by 1% too assuming damping component of the shaft remains the same (It is a matter of fact they are not the same for both cases).

In an electrical system be it an isolated or an infinite bus system, the change in mechanical frequency will be arrested by the so call governor speed droop. This gadget must be there for almost every prime mover that is attached to the system. But, more often a layman may not able see it unless he works on to understand the how the system works. The main function of this gadget is to bring the system frequency rate of change (df/dt) to zero in several seconds.

Many discussions about the droop even in this forum. I don't want to discuss it here again.

Back to your concern. I have explained mechanically how a prime mover behaves to regulate its shaft angular velocity via the governor speed droop under variable load condition. An electrical system is merely a slave in power system operations. In steady state, it can't produce more power than the limit given by Equation(1) above. When you add more load to a generator you are actually varying the torque angle of the generator. Your are changing something.

Maybe some of electrical engineers in this forum can add more about the what so call "the synchronism torque angle".

 

because of droop

 

Sourabh,

Just assume you are riding a bicycle with 5 kg weight. if weight increased to 10 kg. What will you do? You have to give more acceleration to maintain the speed.

The same thing will happen in generator when load increased. Governor will make the speed equal.

Take care
G.Rajesh