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turbogenerator operation
most text books say a turbogenerator is absorbing MVARS from the system when operating in leading zone of the generator reactive capability region...

In most of the text books it is described that a turbogenerator is absorbing MVARS from the system when operating in leading zone of the generator reactive capability region. Generator is designed to provide power to the system and not to take power from the system. If someone can kindly explain this phenomenon to me(absorbing VARS?)!

Again in some of the threads a lot is mentioned about under-excited operation of a turbogenerator. Most of the textbooks have a mention as many authors have explained that in the lagging region, the stator flux directly opposes the field flux whereas in the leading region the stator flux assists the rotor field flux. Hence the terminal voltage is increased. The vectorial diagrams prove what it is said in the text-books and what is said by authors. But what happens physically? How does the armature reaction which was in direct opposition in the lagging region, starts assisting rotor flux in the leading region? Vectorially it is proved and I can satisfy myself. If one of the knwoledgable authors on this forum can throw some light on what practically happens, it would be great.

As one of the authors has explained like this :-When one reduces the excitation on the rotating magnetic field, the flux field of the rotor "shrinks". Which allows the flux field of the armature to "expand." The flux distributions get "disturbed" when the excitation is reduced too much, and this is what causes the concentrated heating which leads to the unwanted growth (things grow with heat!) which causes unwanted things to happen.

Agreed that the rotor flux "shrinks", but the magnitude of the flux on the stator depends on the field strength, so how does the armature flux "expand". If the author can kindly explain this I shall be grateful. The latter part will be clear then, that if there is expansion of flux, there will be core saturation, flux fringing and hence end winding heating.

By Nathan Boeger on 19 November, 2006 - 4:45 pm

Check out reactive power in Wikipedia. You'll find that "true" AC power is a vector with real (Watts) and reactive (Volt-Amp-Reactive, VARs) components. The idea is that your usable power for most applications is real (Watts) and that the reactive power doesn't get used and causes "waste". When the current "leads" or "lags" the voltage (remember we're dealing with AC) you get a reactive component that has capacitive or inductive properties. For a given load (real, Watts) you will have a greater energy loss due to higher current (AC i^2R analog, I think...) with a greater reactive power. You can read more about this under "power factor". There are cases when you want to create a slight lead or lag such as paralleling generators. I think most systems go for a (slighly capacitive) power factor around .9ish. This would be a good post to our power engineers and godly EEs.

The simple explaination when you start hearing: armature, stator, or shunt windings, field fluxes, and excitation energies/voltage, and vector fields growing/shrinking are that they refer to ways of controlling the power factor (lead/lag) of a motor/generator by applying some current or voltage. The details are based on the motor/generator type and engineering configuration.

Nathan Boeger
"Design Simplicity Cures Engineered Complexity"

First of all, what is being described here is not exclusive to turbogenerators--it's applicable to any AC generator (more correctly called an alternator, but commonly referred to as a "generator") driven by any type of prime mover (torque-producing method). Generators convert torque into electrical energy (power), which can easily be transmitted long distance over wires, which can then be converted back into torque by electric motors. (Come to think of it--that's why electrical energy is produced: so torque can easily be transmitted over long distances from a place where it is "abundant" to a place or places where it is not available or can be used productively--notwithstanding for light and heat, and for many: air conditioning (which requires torque!). Some of the earliest large prime movers were hydro turbines, and the electrical energy they produced was transmitted to factories and cities which were loacted some distance away from the large rivers and falls.) Where the torque that is input to the generator comes from is unimportant--turbine (steam- or gas- or hydro- or wind turbine), reciprocating engine, steam engine, they're all torque-producing methods.

Generators can be a watt (MW) load on the system--and they can also be a VAR load on the system. If the torque being applied to the generator rotor which is connected to an electrical grid is insufficient to maintain synchronous (rated) speed then the generator acutally becomes a motor. (Any torque produced by the prime mover over and above that necessary to maintain synchronous speed is converted by the generator, when connected to a grid, into watts (MW, depending on the size of the prime mover and generator.) This is sometimes called "motorizing" the generator--or "reverse power" because energy in the forom of watts is flowing INTO the generator and back into the prime mover instead of OUT of the generator. For some prime movers, this is VERY bad (like steam turbines, in particular) and there are reverse power detecting relays which will trip (open) the generator breaker when the reverse power exceeds a pre-set level to protect the prime mover.

In the same way, if the excitation being applied to the generator rotor is insufficient to keep the generator terminal voltage equal to the grid voltage, then VARs will begin to flow into the generator, and the generator will become a VAR load on the grid. (That's why this author doesn't understand why people want to operate their generators in a leading power factor configuration...unless they are trying to reduce the grid voltage by reducing would sure be nice if someone would explain the reasoning behind the "necessity").

"Boost" and "buck" (terms certainly put into the lexicon by some Texan...) are sometimes used to describe under- and over-excitation, respectively. When the excitation being applied to the generator exceeds that required to maintain the generator terminal voltage equal to the grid voltage, the generator is said to be "boosting" the grid voltage, or, attempting to raise the grid voltage. The generator is then producing (supplying) VARs to the grid.

When the excitation being applied to the rotor (field) is insufficient to maintain the generator terminal voltage equal to the grid voltage, then the generator is said to be "bucking" the grid voltage, or, in effect, trying to reduce it. The net effect is to cause VARs to flow "into" the generator instead of out of the generator.

Any time current flows through a conductor, a magnetic field is generated. When the generator is connected to the grid and current is flowing through the stator (armature) conductors, magnetic fields associated with the armature windings are generated by virtue of the fact that AC current is flowing.

The VOLTAGE on the generator stator is a function of the excitation being applied, and voltage does NOT--by itself--produce flux. You can have 11,283 volts on a conductor, and if there is no current flowing there will be "no" flux field around the conductor. So, when the generator is NOT connected to the grid, the generator terminal voltage is a function of rotor field strength (and speed! but since alternators--in most parts of the world, anyway--operate at a fairly constant speed it's purely a function of rotor field strength, which is a function of excitation).

As a generator is "loaded", current flowing through the stator increases. As the current flow increases, so does the magnetic flux associated with the stator (armature) windings.

The stator (armature) flux "reacts" with the field flux--and generators are designed so that as long as the flux reactions remain with a particular range, the generator will operate with minimal destructive heating issues.

However, when the rotor field flux is reduced "excessively" (i.e., the generator is running in an "excessive" underexcited condition), the flux field of the stator "expands" and "overpowers" the flux field of the rotor, and BAD heating happens.

That's what is meant by "armature reaction"--the interaction ("fighting" almost) of the stator flux and the field flux. A perfect example of armature reaction is what happens as a generator is "loaded." Usually, as a generator is loaded if the operator does NOT watch the VAR or power factor meter as the generator load is increased, VARs will be reduced and may even begin flowing into the generator, and the power factor will swing towards unity (1) and may even begin to decrease in the leading direction. This is because the strength of the magnetic field of the armature is increasing as current flow through the armature windings increases--and it tends to "overcome" or reduce the effective strength of the rotor field. Unless the Automatic (AC) voltage regulator is very well adjusted to the grid tendencies, the generator terminal voltage will decrease slightyly because of the reaction between the armature field and the rotor field as the armature field strength increases. So, that's why operators usually have to increase excitation as a generator is loaded--to increase rotor field strength which is being effectively reduced as the stator (armature) field strength increases as current flow through the stator (armature) increases.

As has been mentioned previously, there is a fine Internet-based resource with some very "down-to-earth" descriptions of electrical power-production fundamentals at Use the search function to look up "armature reaction" and you will find some very excellent information presented in a very easy-to-understand format.


I thank markvguy very much for his patience to explain the under-excited operation of turbogenerator.

I really understand that the protective relays wont take the generator into that zone, moreover units dont usually enter into that region.
However, this was purely my curiosity to know how fluxes look in the underexcited region. Nearly all books elaborate the lagging, zpf and upf operation along with vectors and fluxes of rotor and stator. However for the leading zone a mention "armature reaction supports the rotor main field" is always mentioned which does not show any diagram of the fluxes. To make it more complex for me, I was told by someone that generator never absorbs reactive power, rather generator only delivers reactive power.
Anyways I will try to absorb most of what you have written and probably will ask for your help again before giving "Under-excitation" a final call.

Thanks Sir!

Don't believe everything you're told--even in this forum. Question things that don't seem correct, even in this forum.

Could you please cite a reference which refers to underexcited operation as "supporting rotor main field"? Again, this author has never read that anywhere--that he can remember.


explanation to question raised by markvguy, Nov 28, 2006 7:27 pm

As per the canteach reference stated by one of the authors in one of the threads on this forum, it is mentioned in article 4.8.4, Page 77, "the stator current creates resulting flux that directly assists the field flux". I used "support" instead of 'assist". Maybe I was wrong and should have used the correct word here.

Thanks to markvguy for that detailed explanation on leading side of the capability curve and stator end-iron heating.

One more thing that is mentioned in most of the technical journals/ books is that the generator absorbs reactive power ( in light loading conditions such as night times specially for hydro plants) or again in the leading zone operation. It is also said that reactive power flows from the node where magnitude of voltage is higher to a node where the voltage magnitude is lesser. But still in the generator which is supplying some MW load , the current is flowing outside the generator terminals. How does this reactive power flow? Does the generator really absorb it in the leading zone operation? I shall be grateful if you can share some ideas.