Turbine Speed? 2

[tool32]

At the Power Station I work at,we have 4 Boilers,and 4 turbines.All Turbines/generators run at 3600 rpm.My question is,to increase load more steam is admitted to the turbine.What keeps the turbine/generator speed from increasing also?

 

[poetix99]

Rotating power is equal to (rotating speed) x (torque). More power might correspond to either an increase in speed OR an increase in torque. The controls of the turbine are set to regulate the steam admission valves in order to hold a (more or less) constant speed.

What actually happens in the case of the turbine-genrators is that the load imposed on the turbine by the generator first increases (by making a change to the generator exciter voltage) and then the valves open to admit more steam in order to try to hold the speed set point. (This might seem like a distinction without a difference.)

 

[00098]

The mechanical energy does not match the electrical energy due to inefficiencies in the steam turbine, the electrical generator and gearbox(if utilized).
Mechanical energy is developed through the turbine by virtue of high pressure steam passing through turbine blading to a lower pressure. The Mollier chart best illustrates the process. If the turbine was 100% efficient, the process would be an isentropic expansion (ie vertical line on the Mollier chart). The energy developed would be represented by the difference in enthalpy from the high to low pressure. Since we live in a real world, we could expect an efficiency (depending an the blading; Rateau, Curtis, etc.) of say 84%. This would mean that the enthalpy drop would be 84% of theoretical and would be shown by a left to right sloping line on the chart, terminating at the same low pressure.
Gearbox and generator losses generally range from 2 to 5%.
I will let "electrical fellows" elaborate on their respective theories.

 

[bluemax]

This is a great question. What happens is that the speed changer brings the unit to 3600 rpm, once the unit is synchronized the generator must run at system frequency which happens to be 3600 cpm. The electrical inertia of the system is so large compared to your machines, so your machine must stay in step, similar to meshing a big bull gear to a pinion, at the mesh point the surface speeds are identical. Now the speed changer has become a load changer once synchronized.

 

[sancat]

I really don’t know much about power plants, but I have some experience with variable torque systems:

Since generators are synchronous motors, they are like "geared" electrically to the frequency of the net. Lets suppose that the total loses of the turbine and motor when turning at 3600rpm are supplied with a torque of 1 N x m, then, if you let in a very small amount of steam, lets say 1 kg/min, that generate that torque, 1 N x m, then the turbine /generator system “electrical gear”will not "push" the "grid electrical gear", nor will it be pushed by it, and will not bring any power to the grid.

If you want to give power to the grid, then you open your steam valves, and this particular "electrical gear" pushes the big “grid gear”, bringing some power to the net. The amount given will be proportional to the steam admitted, and thus to torque as Poetix writes. It is similar to 4 folks pushing a car. While each folk could push with more or less force, all four will walk at the same speed, and the power of each one will be its speed multiplied by its force.

Here you are working against the "electrical inertia" of the whole grid, referred by Bluemax. If the case were of a single generator connected to the system, you will have a governor that follows with a PID control the electrical demands (voltage/frequency drops), and that is most times more unstable than a larger system.

It is also possible to use an asynchronous motor to generate electricity, but here the speed of the generator will be slightly higher than 3600 rpm due to slipping, the higher the slip the higher the power generated.

Sandro

 

[Romefu12]

Turbine-Generator operation relative to shaft speed is controlled by a block of steam valves called throttling valves or control valves. An electrical signal is fed to the Turbine Controller via a Speed Signal Pickup or Speed Signal Generator that monitors shaft speed at the turbine shaft. This signal is processed by the Turbine Controller and the control valves of the steam turbine begin to open. The valves will stop opening at synchronous speed (3600 rpm in your case) when the Speed Signal Generator sees 3600 RPM. This condition is called the Speed / No-Load condition. At the Speed / No Load condition the RPM will be stabilized by the Turbine Controller once certain conditions are met (i.e. turbine outlet temperature, vacuum, etc) and the Auto / Manual Synchronizer is activated. A Load Setpoint is chosen by the turbine operator and the Auto Synchronizer begins looking to match Frequency (60Hz) and Line voltage. Throttling adjustments are made to stabilize RPM, Voltage and Frequency are matched and the Unit Breaker closes instantaneously to synchronize the Turbine-Generator to the grid system. At this idle speed condition excitation current is applied to the rotor causing a lag in Turbine speed. This lag in RPM is then seen by the Speed Signal Generator. The signal is processed to open the control valves even further. This lagging and matching process will continue to ramp the Turbine up to the desired output or setpoint requested by the turbine operator. There will be a few changes in acceleration and matching along the way because of system response and/or optimization of the Turbine operating parameters.

The theory is that the more resistant torque applied by the generator excitation on the turbine shaft, the more shaft horsepower required by the turbine (more steam by volume for expansion to overcome generator torque).

That’s pretty much it in a nutshell. We can get really complicated with turbine controllers, condition monitoring, and Generator / Grid system monitoring but there are books that can cover that.

Hope this helps

Romefu12

 

[ilan7]

Tool32
You got a lot of suffisticated answeres.
I have a simple one.
When the Generator connected to the Grid, The grid is controlling the frequency and the generator speed. while pushing power into the system (Turbine and Generator) the power convert into current instead of speed. If the grid is strong - the frequency of the grid will keep the generator speed (3600 rpm at 60 Hz)steady. if the grid is weak (compare to the generator) the grid frequency will start to raise with the generator speed. If this happened the Generator Droop Control suppose to take over and reduce the power to keep the grid frequency at 60 hz.

 

[abeltio]

in layman's terms...
1. the grid frequency holds the generator at a speed given by the frequency and the number of pairs of poles of the gen... for a 2 pole gen @ 60Hz the speed is 3600rpm.

2. If the generator is big compared to the grid (> 10%) the grid frequency will be affected by the generator... whatever power is not used by the load (grid) will be used to increase the grid frequency...

3. The droop control of OTHER generators will UNLOAD the other generators to maintain the speed setpoint (frequency).
In a grid there usually is a SYNCHRONOUS DRIVER, the synchronous driver (usually a BIG UNIT compared to the grid)maintains a very tight control on the grid frequency and the rest of the generators are connected in droop to that synch driver.

4. The other fundamental player in maintaining the frequency is the excitation of the generator... while the excitation helps maintaining the grid voltage to the rated value... if a generator is "absorbing vars" that is... the vars have a negative value... there is a lower limit called the UEL (under excitation limit) that may result in the generator falling out of step (out of synch) and may cause mechanical damage to the generator.

Just adding to the general confusion...
HTH

saludos.
a.

 

[Cookie]

Having a turbine syncronized to the grid keeps the turbine/generator speed from increasing.
When you are not syncronized to the grid the speed of a turbine varies with steam flow.
When your turbine generator is syncronized to the grid by closing the tie breaker, the grid LOCKS the speed at 60HZ or 3600 rpm. Once the SPEED is "LOCKED IN" by the grid, steam flow to the turbine will change generator electrical output.