Hi Everybody

Synch Gen can produce positive Mvar or use negative Mvar. What's AVR rule in this process? I mean after Gen connected to grid Gen terminal voltage is as same as grid voltage. Is this correct? In some references it mentioned that Gen terminal voltage as Pu for example is more than grid voltage so Gen produce Positive Mvar and if Gen terminal voltage be lower than grid it uses Negative Mvar. I have some questions here, please see:

http://hemmati65shabakeh.persianblog.ir/page/5)

1-Is Generator terminal voltage differ from Grid voltage after Gen is paralleled by Grid? why is it more than voltage grid?

2- What is AVR Drop and what happen in AVR that it causes Positive and Negetive Mvar? I mean MVAR is negative and using AVR we change it to 0 or Positive Mvar. I want to know how Avr do this exactly. You may say by changing PF form lag to lead or reverse. I want to know the mathematical relation of this changing or control loop of this process (Controllers, Electronic circuits,...)

3-We say Synch Gen Produce or use Mvar. How do physically the Gen do this? What happens that Gen produce or use MVAR as Capacitors or reactors? What do AVR over Excitation or Under Excitation mean and what are their rules in Mvar production and using?

I would be pleased if help me using mathematical relations of these process or introduce strong references in this field.

Best Regards

[email protected]

 

Saied,

Lots of questions--good questions! Unfortunately, I will only be able to give real-world operating examples in this area; I find detailed maths to be proofs of reality, and it can be very confusing if trying to explain reality with maths. (Also, we can't draw pictures or vectors or post graphic images/pictures on control.com.) What I can offer is to explain things in real-world, operating terms with very little maths.

It's presumed you have a good understanding of real power generation (watts) on an AC power system. That is, that a synchronous generator turns at a constant speed to produce a constant frequency. That relationship (speed versus frequency) is defined by the formula<pre>F=(P*N)/120</pre>where F = Frequency (in Hertz)
P = Number of poles (always an even number)
N = Speed of generator rotor (in RPM)

Synchronous speed is the machine speed that is proportional to frequency, so for a 50 Hz generator with two poles the synchronous speed would be 3000 RPM. When synchronized to a large or infinite grid with other generators and their prime movers, the speed of the generator--and it's prime mover--is locked into (synchronized with) the grid and can't go faster or slower than synchronous speed (which is a function of grid frequency). Since grid frequency is, or should be, stable and relatively constant, the speed of all generators synchronized to a grid--and their prime movers--is dictated by grid frequency. No single generator can go faster or slower than the synchronous speed proportional to the frequency of the grid to which it is connected. Magnetic forces inside the synchronous generator keep a generator rotor's magnetic field locked into synchronism with the magnetic fields of the armature (stator) which appears to be rotating (in exactly the same way electric motor rotors are magnetically coupled to the magnetic field of the motor stators). (This goes for "grids" consisting of as little as two generators and their prime movers or as many as hundreds or thousands of synchronous generators and their prime movers. Synchronism is synchronism; magnetism is magnetism.)

There is a certain amount of power (torque) required just to keep a generator and its prime mover (turbine; reciprocating engine; etc.) spinning at synchronous speed--both when the generator is not synchronized to a grid, and when it is synchronized to a grid. If a generator's breaker is open (that is, it is not synchronized to a grid) and the energy flow-rate into the prime mover driving the generator is increased above that required to maintain synchronous speed (in a sense, "over-torquing") the speed of the generator will increase.

And, if the energy flow-rate into the prime mover driving a generator is decreased below that required to maintain synchronous speed while the generator breaker is open (in a sense, "under-torquing") then the speed of the generator will decrease below synchronous speed.

If a generator's output breaker is closed (meaning the generator and its prime mover is synchronized to a stable grid (stable frequency and voltage)) and the amount of torque being produced by the prime mover is equal to that required just to maintain synchronous speed the electrical output of the generator will be zero (0) watts. If the amount of torque being produced by the prime mover and transmitted to the generator rotor increases (above that required to maintain synchronous speed--"over-torquing" in a sense) when the generator is synchronized to the grid the <i><b>tendency</i></b> would be for the unit speed to increase--but the speed can't increase because the magnetic forces at work inside the generator rotor keep the rotor locked into synchronism with the magnetic field if the generator armature (stator). The additional torque is converted by the generator into amperes flowing in the generator armature (stator) which causes the amount of electrical power being produced (the real power--watts) to increase. This is considered positive power (watts).

Conversely, if the amount of torque being applied to the generator rotor is less than that required to keep the unit spinning at synchronous speed then current will flow from the grid into the generator stator to keep the generator and prime mover spinning at synchronous speed. This is called reverse power--and the generator does actually become a synchronous motor when this occurs, and spins the prime mover (instead of the prime mover spinning the generator).

This is what electric generators do: convert torque into amperes. Electric motors, conversely, convert amperes into torque. Electricity is just how torque is transmitted from the prime mover driving the generator to the motor driving a load to produce useful work by producing torque. Most people have a great understanding of electric motors--at least that electric motors convert torque into amperes; they don't realize that electricity is just a transmission medium--for transmitting torque. And, most people have have a very hard time understanding that generators convert torque into amperes. One can't have motors converting amperes into torque if one doesn't generator converting torque into amperes. The prime movers driving generators are really doing the work of motors in far-flung locations--through the medium of electricity, which allows torque to be transmitted and distributed via wires. (Pretty cool, huh?)

Remember:<pre>P=Vt*Ia*pf</pre>where P = Power (Watts)
Vt = Generator Terminal Voltage (Volts)
Ia = Generator Armature (Stator) Current (Amperes)
pf = Power Factor (a value less than 1.0)

Presuming the power factor is 1.0 (unity), and knowing that generator terminal voltage is relatively constant when synchronized (because grid voltage is also supposed to be relatively constant just as frequency is supposed to be constant on an AC power system), synchronous generator power (watts; real power) changes when generator armature (stator) current changes. And, armature current is a function of torque being delivered from the prime mover. And, the torque produced by a prime mover is a function of the energy flow-rate into the prime mover (fossil fuel for a combustion turbine/engine; water for a hydro turbine; steam for a steam turbine; wind, for a wind turbine; etc.).

When speaking about reactive power <b><i>it can be </i>considered<i> to be</b></i> very much like real power: that reactive power is, or can be, "produced" and "consumed." (Generators "produce" real power; motors and other types of load (computers and computer monitors; televisions; lights; etc.) "consume" real power.)

The majority of the loads on an AC power system are motors and other inductive loads, resulting in a lagging power factor on the system (from the generator's perspective!). Most motors in the world are actually induction motors (they are inexpensive to build and operate)--and they "require," or "consume," reactive power (VArs) to operate properly. (Actually, it can be said that operating induction motors creates VArs; it all depends on the perspective and context of the discussion.)

<b>The net result of an increase of inductive-related reactive power on an AC power system is to shift the voltage and current sine waves of the system out of phase with each other.</b>

That's a really important concept to understand. And, if that phase shift between the voltage- and current sine waves is allowed to continue unchecked, then bad things begin to happen on the system (beginning with brown-outs and ending with black-outs). So, something has to be done to counter the effect of all those induction motors to ensure efficient transmission and distribution of power and to prevent brown-outs and black-outs and system issues.

Synchronous devices (generators and motors) can "produce" reactive power (lagging VArs), and they do so by changing excitation which is controlled by the "AVR" (Automatic Voltage Regulator). When a synchronous generator is not synchronized to a grid, there is a certain amount of excitation that's required just to make the generator terminal voltage equal to the grid voltage. (Kind of like the amount of torque required to maintain synchronous speed!) And, similarly, when the generator is synchronized to the grid there's a certain amount of excitation required to keep the generator terminal voltage equal to the grid voltage.

When a generator's output breaker is open (not synchronized to a grid) and the unit is running at its synchronous speed, the AVR can be (and usually is) adjusted to make the generator terminal voltage equal to the grid voltage. If the excitation is increased above the level required to keep the generator terminal voltage equal to the grid voltage (over-excited), the generator terminal voltage will increase above the grid voltage value.

And under the same conditions (unit at synchronous speed and open generator breaker), if the excitation is decreased below the level required to keep the generator terminal voltage equal to grid voltage (under-excited) the generator terminal voltage will decrease below the grid voltage value.

When a generator is synchronized to a grid with other synchronous generators and their prime movers (and I'm referring to a large grid with many synchronous generators and their prime movers), increasing the excitation on the generator rotor <i>tends to try to increase the grid voltage</i> but one generator on a large grid doesn't really have the capability to change the grid voltage (really, the voltage of all the other generators on the grid). Just like one generator on a large grid doesn't really have the capability to change the grid frequency (the speed of all the other generators and prime movers on the grid). What happens is that lagging reactive current (VArs) begins to flow in the generator armature (stator). And, the maxim is, "Lagging VArs feed a lagging load" (from a generator perspective). In this case, the synchronous generator is said to be "producing" reactive power (VArs), and the reactive power is said to be positive.

So, the net effect of increasing the excitation on a synchronous generator rotor is to cause reactive power (VArs) to flow in the generator stator windings. Continuing to increase the excitation increases the VArs (in the lagging direction). And, this lagging reactive power flow "feeds" the lagging load of the AC power system. This serves to help keep the voltage- and current sine waves of the AC power system from shifting too far out of phase with each other. The generator terminal voltage can change, but usually not by a perceptible amount (except under some unique conditions, when generator terminal voltage can change--but it's not common for generator terminal voltage to change appreciably when changing excitation while the generator is synchronized to a grid with other generators, and I'm referring to a large grid in this example with lots of generators and a large "load").

The generator is also said to be trying to "boost" the grid voltage, but since it can't really have much of an effect on grid voltage, the energy (real power--watts) being put into the generator rotor by the AVR is converted to reactive power (lagging VArs) by the synchronous generator.

Just like when the torque being applied to a generator synchronized to a grid with other generators and their prime movers tries to increase the generator rotor speed--but the speed is fixed by the grid frequency (and we're presuming a stable, constant grid frequency)--and the generator converts the additional torque to amperes and this is what makes the real power, watts, output of the generator increase, increasing the excitation applied to a synchronous generator rotor tends to try to increase the grid voltage, but since the grid voltage can't be changed (except in some unique situations, and even then, not by a "lot") the excitation energy is converted to lagging, positive reactive (VAr) power flow in the generator armature (stator) windings. Increasing excitation above that required to maintain equilibrium with grid voltage is called "over-excitation."

Conversely, if the excitation is dropped below that required to keep the generator terminal voltage equal to the grid voltage ("under-excitation), the generator is said to be "bucking" the grid voltage--or trying to reduce the grid voltage. But, again, in most cases it can't (change the grid voltage) and the result of this is to increase the leading reactive power (VArs) flowing in the generator armature (stator). This is said to be negative VArs, and in this case the generator is said to be "consuming" VArs. Decreasing excitation below that required to maintain equilibrium with grid voltage is called "under-excitation."

This is what synchronous generator operators see--and how they react to operating conditions. Grid voltage does change throughout the day, every day, generally, and so even if the real power output of a generator remains constant the operators are often required to change excitation in order to maintain a particular reactive power (VAr) setpoint or a particular power factor as, and because, grid voltage changes. Or, if the operators are asked to change the current reactive power (VAr) value or power factor, they change excitation which results in a tendency to try to change grid voltage but actually results in a change in VArs or power factor.

Again, if reactive power is thought of in similar terms as real power--which is reasonable to do--then production and consumption and positive and negative are reasonable terms to use when describing reactive power.

Reactive power doesn't do any useful work--but it does have an effect on the ability to do useful work. It also results in heat, in synchronous generators (armatures (stators)) and loads, as well, which must be considered and dealt with in the design and operation of electrical equipment (motors and generators). When excitation is greatly increased, then the generator rotor winding temperature will also increase--sometimes by too much. Also, if generator excitation is decreased by too much the strength of the generator rotor's magnetic field(s) may get too low to support synchronization--resulting in a condition known as "slipping a pole" which is VERY bad for the generator, the coupling between the generator and the prime mover, and also likely for the prime mover. (Many AVRs have over- and under-excitation limiters to try to prevent damage to the generator.)

Now, you can find all kinds of maths and formulae and vectors in textbooks and references and on the World Wide Web to describe what goes on inside the generator when torque and excitation are being varied. There's no need to try to go into all of that here (and it's difficult here because we can't "draw" or post pdfs or sketches on control.com). It's usually most helpful to understand what's really happening and how things are affected, and then to use the maths and vectors to prove or predict what will happen, particularly in unusual situations.

Hope this helps; it's very helpful to think of power--both real and reactive--as being produce and consumed. Induction motors and other inductive loads "consume" reactive "power", and synchronous electric machines (generators and motors) can "produce" reactive "power" by over-exciting the generator rotors. No; the generator terminal voltage doesn't appreciably change (and, under most, normal conditions, neither does the grid voltage).

Finally (and I know you're ready for this to be over!), there is a direct relationship between speed and voltage and excitation, but since we're primarily talking about synchronous generators when synchronized to a stable grid (frequency, and voltage) the relationships with speed can be effectively disregarded. Actually, everything affects everything else--and maths can really make this very hard to understand; but if we disregard some effects while we concentrate on others while we're trying to grasp the big picture before drilling down into the finest points, things can be much easier to understand. Yes; EMF and back- or counter-EMF are very important--for designers and AC power system regulators/engineers, but for simple operators and for those just beginning in the industry/study we have to start somewhere. And understanding real-world effects and then getting down to the proofs of the causes sometimes helps understand things.

Again, hope this helps!

 

Saied...

If you want additional info I suggest you read:

http://www.control.com/thread/1026245130

Regards,
Phil Corso

 

Saied...

Unfortunately, you're being drawn into a long-running dispute between Mr. Phil Corso and myself.

If you need maths and formulae and vectors and EMFs and counter-EMFs and back-EMFs and load angles and air gaps to understand, you can write to him at [email protected], provide him with some information about yourself (school/corporate affiliation), provide him with your email address, and he wll be happy to take this thread off-line so that he can enlighten you using maths and formulae and vectors and such. In particular, you should ask him for a copy of his widely-read paper 'The Physics .... of Armature Reaction'

I wish you the best of luck.

I would like to clarify a point I made poorly in my previous post:

The net result of an increase of inductive-related reactive power on an AC power system is to shift the voltage and current sine waves of the system out of phase with each other.

What I tried (poorly, I'm afraid) to say was:

The net result of an increase in the reactive loads on an AC power system (motors; flourescent lights; etc.) is to shift the voltage- and current sine waves of the system out of phase with each other.

That's why VArs are important--because if someone doesn't pay attention to the reactive load on an AC power system bad things will start to happen, and if left unchecked, blackouts can occur.

To "supply" or produce VArs, a synchronous generator's excitation must be increased above the amount required to maintain the generator terminal voltage equal to the grid voltage. This is called "over-exciting" the generator, because more excitation than required is being applied to the generator rotor, and this results in lagging VArs, which feed a lagging load on the AC power system.

To "consume" VArs, a synchronous generator's excitation must be decreased below the amount required to maintain the generator terminal voltage equal to the grid voltage. This is called "under-exciting" the generator, because less excitation than requires is being applied to the generator rotor, this results in leading VArs, which are supplied by the AC power system.

Does the generator terminal voltage actually change when increasing or decreasing the excitation above or below that required to maintain the generator terminal voltage equal to the grid voltage? Minimally, usually less than 1- or 2 percent. Considering that most industrial- or utility synchronous generators operate in the 11-13.8 KV range (or, 11,000 Volts to 13,800 Volts), a couple of hundred volts is a couple of percent, and unless one is using a digital voltmeter with a high degree of accuracy it's virtually impossible for an operator to tell. (By the way, the limit of most excitation systems, "AVRs", is usually +/-5% of generator nameplate (rated) terminal voltage. So, the ability of the excitation system to change the generator terminal voltage isn't much more than 5% above or below rated voltage.)

Hope this helps you in your endeavour, Saied.

 

Saied...

I suggest you avail yourself of Control.com Archives. You will find a wealth of meaningful data.

Just use the Control.Com "Search the Site" Tab, by typing in 'MVAr' or 'kVAr'.

Phil Corso

 

CSA... hy did you suggest to Saied that he obtain a copy of my "Physics of... Armature Reaction" paper?

By your own volition you refuse to address it! Too bad! If you had read it you'd know it doesn't address the AVR impact on MVAr!

 

Saied

Try this thread--particularly Bruce Durdle's and nic's comments; especially nic's comment with the formula: S=P+jQ.

 

Dear All,

Really a nice contributions made by all. the more times i'm reading the more no of queries i'm getting and again searching for them, its just like a continuous learning thing.

ok coming to the point I want to contribute a small (just a small) thing like to my understanding about how to understand Torque to amperes relation.

like basic law energy can neither be created nor destroyed. so the ampere requirement if you consider as energy requirement which is load which is already existing (motors, lighting etc). So what we are doing is simply providing or exciting the stator (Copper which when excited allows its valence electrons to release charge) through the rotor (field excitation) to meet the ampere requirement.

Hope i'm not wrong in my understanding. If wrong please correct.

 

Saied

> Try this thread--particularly Bruce Durdle's and nic's
> comments; especially nic's comment with the formula: S=P+jQ.

Seems I forgot to include the link....

http://control.com/thread/1360687812

nic's excellent response was made on 12 February 2013. Actually, Bruce Durdle's responses and peterjh101 response were all very helpful.