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Excitation system
What is an excitation system is and what role it does play in the start up of a prime mover like a gas turbine


Was wondering what an excitation system is and what role it plays in the start up of a prime mover like a gas turbine.

I'm not an electrical engineer so I would appreciate it if you could use tech jargon along with maybe a dumbed down explanation.

Don't be afraid of google and the almighty wikipedia!

Here's the skinny. Excitation establishes the field, so that when the turbine rotates the rotor through the field you have generator action. Without an established magnetic field, you have no generator action.

Any synchronous generator requires some means of "exciting" (powering) the field of the generator, and the field is generally the part that is rotated by the prime mover. The field, when excited and rotated past the stationary conductors of the stator of the synchronous generator, causes voltage to be generated at the synchronous generator terminals.

The amount of voltage generated at the generator terminals is a function of speed and the amount of excitation supplied to the rotor (usually through slip rings, sometimes through a "brushless" or rotating exciter). Since most synchronous generators run at a fairly constant speed (in most parts of the world) the amount of generator terminal voltage is a function of the amount of excitation supplied to the rotor. More excitation equals more terminal voltage.

As for the part the excitation system plays in the start-up of a gas turbine, well, that kind of depends on the type of gas turbine and the starting means used to start and accelerate the gas turbine.

For the majority of gas turbines which use an electric motor (480 VAC or 6 KV, etc.) or diesel engine through a torque converter to start and accelerate the turbine, the excitation system does nothing to assist with or enable the starting of the gas turbine. The excitation system must be enabled at some point during the start-up to produce generator terminal voltage so that the unit can be synchronized to the grid to supply power to the grid.

Some gas turbines have the ability to be started independently of the electric grid and use power from the generator terminals during acceleration to drive cooling fans and pumps. These machines enable the excitation system early in the acceleration (at approximately 50%, sometimes less, sometimes more) and special transformers and connections to drive the fans and pumps. Most units do not have this capability.

Newer, very large gas turbines use the synchronous generator as a large motor for starting and accelerating the unit. In this application, the excitation system is used to produce the power for the rotor to allow the "motorizing" of the generator.

So, the role played by the excitation system in the start-up of a gas turbine can vary depending on the type of gas turbine and how it might be started.

That's a general description for GE-design heavy duty gas turbine-generators. I'm sure other manufacturers have different but similar applications.

If you wonder about how spinning a magnetic field can produce voltage, by all means, use Google and to research electricity and electrical power generation. Another good source is (no www. prefix) which has some excellent information on basic power generation.

Let us know if you need more information.

By sumonhlaing on 24 December, 2013 - 8:58 am

i want to know that what type of controller use in excitation system of synchronous generator in hydropower plant. how to work this controller? example. PLC, PI, PID, Fuzzy

Dear sir,

what may be the cause of power factor fluctuation of generator?

In our plant, 2 generators (2MW + 3MW) are paralleled in Island mode with power factor regulator in manual mode. problem is, normally power factor remains 0.8lag but suddenly PF of both generators goes down to 0.7 lag and after few moments it comes back at 0.8 lag again. This repeats intermittently. why does it happen?

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What else happens at the same time?

If by "island mode" you mean that your plant is powered by only the two generators described, with no other connections to any external supplies, then there must be a balance between the VArs produced by the generators and those consumed by the connected load. So if the power factor goes from 0.8 to 0.7, you are producing ( on a 5 MW base) 0.57 MVAr more. Something in the load will be taking the extra reactive power while the pf is low - this could be some switching operation such as a motor starting. Is there an increase in power shortly after the drop in pf for instance?

Dear Bruce Durdle,
No, there is no increment of power. Active power remains same at the time of pf fluctuation as before.

1 out of 1 members thought this post was helpful...


Bruce, May I answer?

If there is absolutely no load change during power factor fluctuation, the PF control is malfunctioning. Try keeping PF control in auto for better results.

The Power Factor is a function of the MVar. If your unit or turbine is connected to the grid, importation of Mvar leads to a negative MVar and thus power factor tending to negative and by so doing, your machine tends to act like an inductive motor.

At unitary Power factor, i.e if the power factor is equal to 1, it means your machine is very efficient even as it can't be 100% efficient due to losses (iron core and hysteresis loses). So by this, it implies that what you are generating is exactly what you giving out. To make the Mvar, PF to tend to 1, excitation is reduced and thus reducing load.

Excitation in power generation means to produce magnetic field. This is required for the production of voltage (emf) as per Faraday law. DC power is supplied to the windings of the generator by either rotating or stationary equipment which are called exciters.