Power Output from Generators

Unreadable...

but if you end with the sentence "I am completely against the concept that turbine produces active power and excitation current produces VAR."...

no comments...

I found it completely readable and I am a lifelong english class flunky.

Hello SMCTR,

Would you have a explanation of how that is actually possible? True power by turbine and VAR by excitation? So that means if you increase the excitation current, it will have no effect on the true power but the VAR only?

The AVR (I am assuming you mean Automatic Voltage Regulator?) Just reads the voltage and tries to keep it stable regardless of the variations in the amp loads regardless of whether its leading lagging or perfectly in phase.

The generator itself mechanically doesn't really care how well or poorly the amps and volts phasing lines up with each other.

Think of it as like your vehicle axle transferring torque to a tire. all it sees is the twisting force. It doesn't care where exactly on the radius of the tire the torque force is being transferred to something else.

Surely... simply, active power is proportional to the mechanical torque on the shaft and reactive power proportional to the excitation...

Think on this and you will find the answer....
What have you to do to increase the active power of a generator ¿!? And how to change the power factor (or reactive load) ¿!?

Does that mean increasing the excitation current has no effect on true power? Because increasing the excitation current ultimately increases the magnetic field strength.

http://en.wikipedia.org/wiki/Synchronous_condenser

I think I put my question in the wrong way. Simply, I am very much confused with the statement that turbine produces active power and excitation current produces VAR.

I don't know what you mean by "active power".....

http://en.wikipedia.org/wiki/Volt-ampere_reactive

By active power, I meant true power or real power.(Active power = PF * Apparent power)

Simply, imagine a generator coupled to ideal inductance only. One gets plenty of amps and no watts. Those amps have to travel down a cable to the load but they don't do any work there; the power factor is zero.

It is the load characteristics that determine what the generator produces and when.

Electrical and thermal watts[edit]

In the electric power industry, megawatt electrical (abbreviation: MW_e^[14] or MWe^[15]) is a term that refers to electric power, while megawatt thermal or thermal megawatt^[16] (abbreviations: MW_t, MW_th, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GW_e).^{[notes 3]}

For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MW_t of heat, which creates steam to drive a turbine, which generates 648 MW_e of electricity (a numerical energy conversion efficiency of 648/2109 = 0.307, or 30.7%). The difference is due to the inefficiency of steam-turbine generators and the limitations of the theoretical Rankine Cycle.

Above from Wikipedia search Watt. may answer your question ?

Regards JD

Excitation simply produces the field used to generate the output of the generator. The current used to create the excitation field is taken from the output. This means the generator produces the total output excitation and all.

Are you asking how excitation affects VAR? Are you asking how it affects the output phase?

Yes. And is VAR only the function of excitation? and Watt only the function of torque ? It seems so conflicting with my knowledge of EE?

Excitation is a controlling mechanism for var.....var is a function of reactive(inductive or capacitive) components in the electrical system....Watt is the measurement of current produced by the generator.... The power factor of a load tells us what fraction of the apparent power is in the form of Real Power and performs actual work. ...A high power factor is desirable since it minimizes the amount of reactive power needed by the load, reducing heat losses and maximizing efficiency.....

noticed _{the mathematics won't make sence if you take it under a fine revision - it's because a lot of stuff is inconsistently (for universal) defined ... it'll give a result on bases determined by it's coverage/consistency that'll be however not enough for some . . . ? logical systems .... blah, blah ...}

i gave also a GA -- the CR4 4X-ample is inconsistent for such rating ((_{g.d. idiots}))

The prime mover operates at a fixed RPM. For example, 1800 RPM for a 60hz genset. When the AVR sees an increased load (current demand), sensed by drop in RPM and hence in voltage, it signals the prime mover to increase torque to maintain RPMs. I dont believe the genset changes in excitation in response to the load vector. Pls correct me if im wrong!

Thank you very much powersolutionsFBD. This has a very satisfactory answer to my question. Would you have a link to the article or the book that I should refer to? I want to go into more details regarding this topic and relate your statements with mathematics.

Are you talking about the situation where you have more than one generator paralleled? In this case, increasing the throttle setting on one machine will cause that generator to assume more of the KW load (real power). If you increase the voltage setting on one machine, the result will be cross current between the two machines without affecting how the load is shared.

My question deals with a synchronous generator. When the load increases on a power plant ( either active or reactive), how does the turbine and genset react to the increase ? Does it see the real and reactive power differently and react to these situations individually? If so what are the variables that are changed and how does it affect the output power ( both active and reactive) ? Are these powers dealt separately? What are the theory and mathematics involved with it ?

OK, so we are talking about a generator paralleled, in this case with the entire grid. I noticed the effect a long time ago paralleling two engine alternators where increasing the throttle increased the KW, but increasing the voltage generated reactive current (cross current) between the two generators. I spent a lot of time trying to figure out what was going on.

Let me describe my mental picture. The magnetic field of the rotor can be represented by 3 vectors pointed outwards (phasors), 120 degrees apart. The stator field, likewise, can be represented by 3 vectors spaced at 120 degrees. Each vector represents a phase. These vectors are rotating at the AC frequency, e.g. 60 revolutions per second.

Now if the generator is supplying power to the grid, its rotor field will be slightly leading the stator field. The angle that it leads depends on the torque supplied by the prime mover (engine). So the real power supplied by the generator can be represented by the vector difference between the rotor phasors and stator phasors. For small angles, the difference vectors are at right angles to the rotor and stator phasors as can be seen from the diagram below.

Now if one generator has a higher voltage setting (higher excitation, larger rotor field), current is induced from this generator to the other generator through the stator windings. The difference in excitation is parallel to the rotor phasors (because one set is longer than the other) and perpendicular to the rotor-stator difference vectors representing the real power produced by the generator. This is the reason that increasing the voltage generates reactive power (cross currents) between the two generators.

The turbine speed controls generation frequency not voltage magnitude.

The generator excitation controls the voltage magnitude and power output.

Whether the grid is inductive or capacitive (VARS) is dependent on several factors including line conductor length, line conductor spacing, type of load attached to/on the distribution lines, relative humidity, temperature, wind presence and speed.

The generator control operates by power factor (leading or lagging) as selected by the operator via excitation.

Capacitive = Leading PF

Inductive = Lagging PF

Many times generation plants are required to operate their facility in a leading or lagging operation mode to offset what is happening on/to the grid or at another generation node in/on the grid.

The prime mover (the turbine) is mechanical and produces rotational movement based on: (Power In) minus (Mechanical Losses) = Power Out

This mechanical power cannot be electrically reactive however variations in the turbine speed can definitely affect the generator frequency which in turn can cause huge reactive disturbances in/on the grid. The type or characteristic of the disturbance is dependent on the grid condition/status at the time of the disturbance.

The larger the generation facility, the larger the disturbance experienced on the grid.

Hope this helps.

Thank you for the answer SHOCKHISCAN. So if the active load increases, it has effect on the speed of the turbine and the operator reacts to it by increasing the fuel and keeping the excitation current constant. And if the reactive load increases, it has effect on the magnetic flux and the operator reacts to it by increasing the excitation current and keeping the speed constant.

Would the reverse mechanism work?

Do you have links to articles or books where I can mathematically and electrically relate these statements?

Yes, the reverse mechanism will work but is limited by operator error and reaction time.

There is a myriad of free tutorial information available on the internet if you will do a search on turbine generator control philosophy or generator control or EHC.

Here is a good place/document to start with.

http://site.ge-energy.com/prod_serv/products/tech_docs/en/downloads/ger3658d.pdf

This document contains simple block diagrams that illustrate the interconnection between gas turbine and generator control as well as vital easy to understand information.

In many cases you can download the operator manual for TG sets form various manufacturer sites that get deep into the details.

As far as the math and formulae for control is concerned; it depends on the type of prime mover. (Gas Turbine, Steam Turbine, Diesel Driven, ETC.)

Thank you. May be I can make my thanksgiving useful from this article.