Leading Power Factor - Voltage drop
Leading Power Factor - Voltage drop
(OP)
Hi, I am hoping you can help me understand this concept.
I've always known that generators operating a leading power factor lowers voltage drop and a lagging power factor increases voltage rise (as seen here: [url]http://bo oks.google .ca/books? id=nIcgB1h f90EC& pg=PA6& ;dq=%22ope rating+the +generator +at+a+lead ing+power+ factor%22& amp;hl=en& amp;ei=eI9 MTJaJHMP88 Aa1ldEz&am p;sa=X& ;oi=book_r esult& ct=result& amp;resnum =1&ved =0CDEQ6AEw AA#v=onepa ge&q=% 22operatin g%20the%20 generator% 20at%20a%2 0leading%2 0power%20f actor%22&a mp;f=false[/url] ), but never understood why.
How does the source/sink of VARs affect voltage rise/drop?
Thanks
I've always known that generators operating a leading power factor lowers voltage drop and a lagging power factor increases voltage rise (as seen here: [url]http://bo
How does the source/sink of VARs affect voltage rise/drop?
Thanks





RE: Leading Power Factor - Voltage drop
Think about how a generator would behave it it were islanded, and compare that with how it behaves when tied to a large system. In an islanded system raising the field increases terminal voltage. On a large system raising the field increases the reactive export of the machine (the generator becomes more lagging). Unless the system is very stiff compared to the generator then a lagging generator usually causes a local rise in system voltage.
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If we learn from our mistakes I'm getting a great education!
RE: Leading Power Factor - Voltage drop
RE: Leading Power Factor - Voltage drop
As a rough rule, the voltage profile is determined by reactive power.... reactive power flowing through series inductance of transmission lines and transformers creates voltage drop. Wherever you add vars usually tends to increase voltage at that location.
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RE: Leading Power Factor - Voltage drop
RE: Leading Power Factor - Voltage drop
The load flow equations are the first equation here:
http://en.wikipedia.org/wiki/Power_flow_study
As it turns out, the off-diagonal elements of J and J^-1 are very close to zero for typical series transmission elements which are primarily inductive.
That is the basis for "decoupling" the real and reactive power equations during "fast decoupled load flow" method and also very useful for quick back-of-the-envelope qualitative analysis as we are discussing.
When real and reactive power are decoupled, real power flow through a series inductive element is dependent upon (or controls, depending on your viewpoint) the voltage PHASE angle difference accross that element (and is independent of the voltage magnitude difference accross that element). More importantly, reactive power flow through a series inductive element is dependent upon (or controls, depending on your viewpoint) the voltage MAGNITUDE difference accross that element (and is independent of voltage phase angle difference).
So again the last sentence is the important one. Vars control voltage magnitude. If there is a net vars flowing out of a node where caps are added, then that node will be higher voltage than whatever node those vars flow to. By mapping out the var flow in the system you can get a pretty good idea of the voltage distribution.
Sorry, if it's not what you're looking for.
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RE: Leading Power Factor - Voltage drop
The off-diagonal elements which are close to zero are:
dP/d|V| and dQ/d(theta)
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RE: Leading Power Factor - Voltage drop
Due to electromechanical devices and miles of conductor, a power system is of course inductive. Adding capacitance reactance negates some of the inductive reactice, reducing the total impedance. A reduction of impedance will of course cause the voltage to rise.
RE: Leading Power Factor - Voltage drop
RE: Leading Power Factor - Voltage drop
1> Power factor correction reduces line current and line voltage drop.
2> An open circuited capacitive line or an unloaded line with capacitors connected at the open end will flow current towards the source. As a result the voltage drop will be seen as a voltage rise.
This is a gross simplification to illustrate the effects.
In real life the various voltage drops are at different phase angles and the calculations are a little more complex than simple addition or subtraction of voltage drops.
Bill
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"Why not the best?"
Jimmy Carter
RE: Leading Power Factor - Voltage drop
You are mistaken - perhaps this will help: http
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If we learn from our mistakes I'm getting a great education!
RE: Leading Power Factor - Voltage drop
I'm not certain I fully understand it, but will run through those calculations to see if it makes more sense.
I apologize for my error.
RE: Leading Power Factor - Voltage drop
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If we learn from our mistakes I'm getting a great education!
RE: Leading Power Factor - Voltage drop
My thought is that the current of the resonant harmonic (primarily) charges the capacitance, and all voltage losses are through the conductor resistance. As the current discharges, the voltage at the end of the line is highest, as it is now the souce and hasn't yet dropped across the line resistance back to the generator.
Am I on the right track or do I have a bit more to think about?
RE: Leading Power Factor - Voltage drop
The tricky part to understand is why is a long transmission line modeled as series inductor with shunt capacitance on each end (pi circuit). I don't have any handy explanation for that model.... but once you accept that model it's easy to see why the voltage at the open end is higher (the vars injected by the cap have only one direction to flow....toward the system end and this has to create higher voltage at the open end than the system end.
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