Reactive power 2

[mab2008]

What is the advantage of reactive power (VAR)? How it is controlled from the power plants? Is it possible to transmit real power (KW) wihtout KVAR?

 

[rbulsara]

david:

I am still rereading what I wrote, but since you replied, it is true for a just a condensor.

Can a generator be overexcited to a leading power factor while still supplying a load?

 

[davidbeach]

Yes, a generator can be under excited to to produce a leading power factor while connected to the grid, that is it can consume VArs. But it can get into stability trouble rather quickly running leading.

Supplying additional VArs to the system requires the generator to be over excited causing a more lagging power factor.

Generators connected to the grid are generally run very close to unity pf as there is someone buying Watts but nobody buying VArs. If the prime mover W rating is close to the generator VA rating, producing many VArs reduces the number of Watts that can be produced without exceeding the Amp limit of the generator.

Generator (Watts out) VArs out = lagging; VArs in = leading

Not a Generator (Watts in) VArs out = leading; VArs in = lagging.

 

[pwrtran]

Divid, from the power flow point of view you are correct! It seems too hard to find a example that makes your statement false ;-) Now I may have one - how about a impluse generator in the charging stage?

 

[NAZ55]

I like davidbeache's definition as well. To further refine his comments on watts and vars. Load consumed by an inductive equipment is often shown in +ve with both watt and var in first quardrant i.e. both flowing in positive direction where as load supplied will be shown with -ve and hence against the flow.

 

[Bronzeado]

Hi folks,

I would like to remind you that the real and reactive power are components (so, maths) of the resultant instantaneous power wave. These components do not exist as separate entities but they can be conveniently considered for purpose of engineering analysis.

As Hoxton said, leading/lagging depends on the reference.

I think the best way to refer to reactive power should be to label as inductive or capacitive vars. In combining the two in analytical work it is important to consider one positive and the other negative regardless the convention employed.

Regards,

Herivelto

 

[QBplanner]

Generator (Watts out) VArs out = lagging; VArs in = leading

Not a Generator (Watts in) VArs out = leading; VArs in = lagging.

Davidbeach:
is it from IEEE /IEE?
From system point of view, it does not matter for MW generator or synchronous condenser as long as it generates Var to system we call it running at lagging pf otherwise it absorbs Vars then we call it running at leading pf. we've been using it for a long time for system planning point of view. it seems you have different definition of leading /lagging in terms of machine.

 

[eyec]

i just want to know who pays for supplying reactive power to keep a line from collapsing.

 

[GTstartup]

If a generator is motorized it can still export vars, so strictly speaking vars and watts do not always have to be in the same flow direction

 

[waross]

Large users pay for VARs in the form of penalties for poor power factor. These users may avoid the penalties by supplying their own VARs at the point of use by the installation of capacitors and/or synchronous condensers or over excited synchronous motors.
The cost of supplying VARs to residential users is averaged over the customer base and factored into the KWHr charges.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

Usually transmission companys pay for vars by adding capacitor banks, or wresseling there power plant managers to produce vars.
But also under loaded transmission lines in the under ground, or EHV catogory also produce vars (When highly loaded the same transmission lines consume vars).

 

[ScottyUK]

If a generator is motorised then surely it is no longer a generator but a synchronous motor, in which case exporting vars and consuming power makes it a leading machine, per David's rule posted earlier.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[peetey]

I like to think of vars as the "magnetizing or non-working portion of power" Generally vars are needed to establish the magnetic field necessary for inductive devices to perform work. Sychronous generators are pretty cool because of their abillity to vary the PF over a given range. As a utility we try to maintain a good PF on our transmission and distribution system. From a customer load stand point we like 4th quadrant phase angles because watts delivered are + and vars received are negative and they offset the inductance of our system, like capacitors do also.

--peetey

 

[GTstartup]

If a generator is motorised then surely it is no longer a generator but a synchronous motor

.

That's a whole 'nother thread

 

[kamenges]

I like the way that Peetey and bronzeado talk about vars. Completely abstracting reactive power from real power can get very confusing very quickly.

The way that makes most sense to me is that vars are stored power. It is power that is available to return to the source. Leading and lagging simply define phase. It's more like vars are traded than produced or consumed.

Keith

 

[bdn2004]

http://www.circuit-magic.com/acpower.htm

It's referenced on Wikipedias definition of reactive power - which is worth reading too.

 

[waross]

I think that this is a little misleading;

Sychronous generators are pretty cool because of their abillity to vary the PF over a given range.

It is the nature of AC systems that the load controls the power factor. If the load demands VARs then VARs will flow. (Actually the current phase angle changes, nothing is "created" but VARs are a mathematical description of current phase angle related to watts that is relatively easy to work with.)
The shifted phase angle may be corrected at the load or at some place on the distribution system with capacitors.
A single, islanded, synchronous generator can not change the phase angle of the connected load. When two or more generators are connected in series, They may be adjusted so that the phase angle of the current through one of them is zero, (unity power factor, no VARs, etc.) The phase angle of all the other generators on the system will then shift to compensate for the machine that is not producing its share of the VARs.
VARs are imaginary, a mathematical fiction.
BUT, express a load in ohms and Henrys, and try to calculate the farads needed to correct the phase angle to 25.8 degrees (PF = 0.9) without ever reducing the numbers to kW, KVA or KVARs. The calculations are so much easier when you use the imaginary VARs to express the phase shift between the current and the voltage, as it relates to the power flowing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rmw]

Let me throw a hand grenade out into the room and see if anyone will jump on it.

I essentially understand all the above from the vector diagrams to the PF meters on the panels and the excitation settings on the AVR's.

I understand that a synchronous compensator operating near a load that needs vars can keep vars from having to be transmitted over long distances hence saving the I2R losses on the transmission lines.

But what I don't understand is how a synchronous compensator located on the end of a transmission line somewhere (not necessarily near a highly inductive load) will raise the voltage on the line by operating as a synchronous condenser and (is it eating or creating?-excuse the ME terminology) vars.

I could really use some help with that.

rmw

 

[davidbeach]

Condenser is another term for capacitor - VArs out.

 

[rmw]

David,

Can you elaborate more on what that means? I understand that capacitors shift the phase, not 'how' but 'that' and that condensers serve that function but don't understand how that relates to the line voltage being raised at that time.

The question comes from encountering a synchronous condenser way out in the boondocks way away from any industrial loads that might have had an inductive influence but the condenser was put there to raise the voltage on the end of the line.

To my warped ME way of thinking, it would have made more sense to produce real power and put 'volts' on the line loosely said.

So I didn't understand the function of the sync condenser was in that occasion.

rmw

 

[roydm]

When I was an apprenitice I was told "the customer is penalized for bad power factor because reactive Amps still load up the system" i.e. transformers and switches need to be oversized and I2 losses still occur with reactive power.
I remember some parts of the plant had pf of 0.3, has anyone seen lower?
Interesting Topic
Roy