Generator MVARS

M

mohammad saeed

as u said we can control the mvar by excitation and u said if mvar is zero that's is advantageous for the supplier. then y don't they always try to make it zero? that is always greater than zero and is positive.
 
mohammad saeed,

That is the ideal operating condition for the "supplier" (generator).

VAr flow and power factor will change if the system (grid) voltage changes, and it usually does change throughout the day. Operators will respond to system voltage changes by adjusting excitation to maintain the desired VAr/power factor. Because prolonged leading VArs/power factor operation is not good for most generators most plants will have their operators try to maintain a slight lagging VAr/power factor setpoint to provide some "margin" to prevent a leading VAr/power factor condition should the system voltage increase and the operators are not aware of the change.

A small (percentage-wise) amount of lagging (positive) VArs/power factor does not reduce power output by very much. Only when the amount of VArs increases such that the power factor starts to decease below approximately 0.80 will the ability of the unit to produce watts be adversely affected.

Remember: Power factor is really a measure of the efficiency of the generator at producing watts for the amount of torque being provided (really the total amount of power being provided to the generator, including the excitation). At a power factor of 1.0, the generator is not producing any VArs, so it's 100% efficient at converting the power being applied it into watts. As the power factor decreases (as VArs increase) the efficiency decreases.

Hope this helps!
 
i am confused about your point that positive and negative vars will reduce the watts capability.

ok capability is some what total of watts and vars.... i am not saying sum....
if positive vars are decreasing watts than negative wars should increase it.. isn't so

i hope i had conveyed what i want to say
 
Have a look at any explanation of the "power triangle."

VA, the hypotenuse of the triangle, is the total power, which is the algebraic sum of the squares of both watts and vars. If the VA is held constant, and VArs are increased (positive or negative) the watts will decrease.

The same thing can be seen in the reactive capability curve for any generator--which is effectively the power triangle at multiple loads.

Most generators--but not all of them--are not built to "absorb" VArs from the grid. And, consequently, most are not operated with negative VArs.
 
VA is the square root of the sum of the squares of the real (Watts) and reactive (VArs) sides of the power triangle.

Sorry for any confusion.
 
And "negative" VArs are a recent phenomenon--brought on by digital meters and HMI displays. A lagging VAr (positive, from the generator perspective) is the same as a leading VAr (negative, from the generator perspective) in the power triangle; sign doesn't mean anything. (Even if it did, the square of a negative value is a positive value.)

Remember, the TOTAL power (real plus reactive) is VA. If the if the VA is held constant but the VArs are increased (lagging or leading), the watts will decrease.

It's like the difference between pedaling a bicycle with properly inflated tires versus pedaling one with improperly inflated tires. To go the same speed on the bicycle with under-inflated tires takes more torque than the bicycle with properly inflated tires, or if the same amount of torque is applied to the crank on both bicycles the speed of the one with the under-inflated tires will not be the same as (it will be slower than) the one with properly inflated tires.

And if the tires are over-inflated the bicycle doesn't have the same traction, so while it may be possible to go slightly faster on the over-inflated tires prudence dictates a decrease in speed for safety, not to mention the increased risk of blowout.

Excessive leading (negative) VArs means an increased risk of stator end-turn overheating--as well as a risk of slipping a pole ("loss of traction"), for the majority of synchronous machines.

TANSTAAFL (translation) There Ain't No Such Thing As A Free Lunch

Even with negative VArs.
 
I understand how a motor would consume reactive power and that a generator would produce reactive power, but how does a generator consume reactive power?

Is this generally poor practice to operate generators this way?

Is this because the excitation is too low?

I am new to a powerplant here and two wheels are operating in parallel yet I noticed one wheel operates with a negative reactive power and another operates with positive reactive power, with both putting out the same MW. This struck me as odd and would like to know the consequences of operating this way.

Thanks
 
LCH,

Every generator is supplied with something commonly called a "Reactive Capability Curve", sometimes called a "D-curve" (because it has a shape like a capital D). The Reactive Capability Curve defines the limits of operation for the generator--for real power (watts), lagging VArs ("positive" reactive power), and leading VArs ("negative" reactive power). This document defines whether or not a generator might be damaged if it is being operated with a certain reactive power flow ("positive" or "negative") at a certain real power output.

Yes; a leading power factor (leading VArs at the generator terminals) is generally caused by "under-excitation." And, a lagging power factor (lagging VArs at the generator terminals) is generally caused by "over-excitation." When the generator excitation is at the value that makes the generator terminal voltage exactly equal to the grid voltage, the power factor will be unity (1.0), and there will be zero VArs (lagging or leading).

Many times at a plant, two generators are connected to a common bus ahead of (upstream) of a step-up transformer. When this happens (when there is no reactance between generators) sometimes excitation systems require special tuning and configuration in order to be able to "balance" the reactive power between them.

It is the plant operator's responsibility to know the capability of the equipment being operated and to operate it in accordance with the requirements of management, ownership and any contractual commitments. <i>In general,</i> in the absence of any contractual commitments most of the plants I have been to operate their generators at a very slight positive (lagging) power factor (very few lagging VARs). This to allow the operator(s) time to react to changes in VAr flow and correct back to a slight positive power factor (grid voltage does change throughout the day, usually, which will cause the reactive power/power factor to change even if the real power (watts) is steady throughout the day.

Some power plants have the ability to operate the generator(s) in VAR- or Power Factor control mode, which automatically adjusts excitation as required to maintain a VAr- or Power Factor setpoint. )Some grid operators are now forbidding power plants to use either of these modes.)

It is generally considered a poor practice to operate generators such that they are consuming VArs. The effect of VArs on an AC system is to shift the voltage and current sine waves out of phase with each other. If the shift gets too great then power transmission is negatively impacted. So, it's necessary to produce some VArs to keep the AC system transmitting power efficiently. If generators consume VArs instead of producing them they can add to the negative effects of reactive power on the grid.

In some parts of the world where the load is predominantly capacitive (as opposed to most of the world where the load is predominantly inductive) some generators are built to be operated to consume VArs, and some generators are operated as "synchronous condensers" to consume VArs (and in some parts of the world synchronous condensers also produce VArs; just depends on the design of the machine and the needs of the operator/grid in that area).

Lastly, most power plants I've worked at don't have VAr-hour meters which would measure the VAr production for revenue purposes. So, the power plant doesn't generally get paid for producing VArs (which would require a VAr-hour meter) and since producing lagging VArs consumes power (watts) most power plants want to operate at close to unity power factor (zero VAr flow) to make as much money as possible (to keep as much power flowing through the watt-hour meter as possible!).

Hope this helps!
 
Dear all

what a nice discussion of VAr and Watt. In a traditional love story of Adamkhan & Durkhani when a farmer starts telling in evening to another and when the sun rises next day the story ended. Then the listener farmer says, " please tell me which one in Adamkhan & Durkhani was male. (Actually he was sleeping).

We are supplying 12MW @11KV rated, power to 14Km away grid. My gen terminal voltage remains 12.5KV, PF leading 0.9, and VArs negative. To get positive VArs, Lagging PF I need to increase more terminal volts by excitation, thats are already 12.5 instead of 11KV.

Is some body can tell me.

Is negative VArs, Leading power factor harmful for generator?
May I increase terminal voltage more then 12.5KV to get positive VArs this will lead to damage the PTs, cables etc.?
May I stop power generation and bear financial loss?

Regards
Shaheed
 
Shaheed,

Seems you were sleeping while reading the above posts.

The reactive capability curve for your generator defines the limits of operation.

The specification sheets for your PTs define the maximum voltages and the saturation voltages they can withstand.

There's just too much you haven't told about the configuration and operation of your site. How long has the situation existed? If for just a short while, what has changed to cause this new operating condition?

Are there any transformers between the generator and the grid connection point? Do they have adjustable taps which could be used to help change the voltage seen at the generator terminals?

Actually, the best thing you can do is to have a proper power system study performed to take into account all of the equipment, the equipment capabilities, the transmission lines and equipment, and make recommendations for improving the condition of operation of the equipment.
 
S

Shaheed ul Islam

Yes you are right but the story was too long, why I have given the example.

I don't know the capability curve of reactive power of generator. PTs maximum voltage are 12KV.

1- The situation exist for past 3 months.

2- The grid distribution is in around 120km area and facing power shortfall. To overcome the problem grid commence load shedding. when a heavy loaded feeder goes to load shedding the grid volts get rises.

3- There is no transformer in between grid and generator.

4- Yes tape at grid is adjustable but if they reduce voltage, the receiving end of consumer drop up to 150V instead of 220V, in case heavy loaded feeder gets on. So they do not alter the tape.
We have recommended a voltage regulator transformer ( auto transformer) but you know the cost of regulator transformer of 12MW would be much high! although the working is in progress.

Regards
 
Shaheed,

Although I dislike the use of maths and vector diagrams here's a link to a pretty good explanation of "generator capability curves" (also known as reactive capability curves):

http://sari-energy.org/PageFiles/Wh...Colombo,_Sri_Lanka/Course_ppts/lecture_19.pdf

(Remove any spaces inserted by the forum software when pasting into your preferred search engine.)

The curves should be in the instruction manual provided with the generator. If not, try contacting the manufacturer, or post your generator make and model here to see if anyone has a similar generator who could share their curve with you.

My own personal estimation is that when you are running at full load and high voltage it's probably not very good for the machine or the auxiliaries (there's that whole volts-per-hertz thing, too)--but until someone can resolve the overloading issue in a better manner you are probably stuck with the current situation.

If you want assistance from the grid regulators in protecting your equipment--and in maintaining the reliability of your generator-set for the grid--get a copy of the capability curve and use it to help you make your point. Data don't lie.

And it's much more convincing than whinging and pointing to free advice from an Internet forum when dealing with regulators.

Best of luck!
 
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