non-zero vars indicates reactive current flow is taking place, and represents current flow that is performing no useful work, but which increase the voltage drops in you feeders
Watts (real power) := Volt-Amps * Cosine (phase angle)
Vars (reactive power):= Volt-Amps * Sine (phase angle)
while the Power Factor is defined as:
PF:=Cosine (phase angle between the voltage and the current)
positive megavars require copper in the generator to carry extra current (more heating)
negative megavars too.
Positive megavars can result in very high system voltage and over-excitation.
negative megavars can result in low system voltage, and going below the under-excitation limit which is very dangerous because the generator can fall out of synchronism and get mechanically damaged... badly. not a pretty sight.
cheers.
a.
As mentined by Hacksaw and abeltio, VARS do not do useful work and the electric utility usually does not charge the customer for vars unless their power factor drops to a level of 80% or lower. Induction loads (electric motors and similar) use vars. Mathematically it is called imaginary power, but the effect is very real. All equipment used to deliver power (wires, transformers, etc.) must be sized by KVA (which takes into account watts and vars). Like a pipe and water flow, if the pipe has foam in the water, the quantity of water transferred is reduced. The same is true with electricity, only so much kva (within the rating of the equipment) can be transmitted without overheating the equipment. As customers use more vars the utility may charge them a penalty. To improve their power factor (inside and outside their facility), customers and/or utilities will install capacitor banks. If they raise their power factor high enough (usually 95% or greater) the utility will provide discounts on the utility bill. (The billing discount/penalty is only possible if the load is large enough and the right type of metering is installed.)
I had an old professor explain it this way. He said that all VARs did was heat up the transmission wire, and that when there was danger of ice hanging on the wires, then it was time to change the power factor, and "ship" some vars to heat the wires, and melt the ice. Maybe he was over simplifying it so that us mechanicals could try to get a grasp on something that even a lot of electricals do not understand.
One other real aspect of reactive power is that it is a necessary component of power transmission, and there is a real tug of war right now with independent power producers in the USA because no one gets paid for "shipping" reactive power, and none of the independents want to do it, where as the regulated utilities recognized it as a fact of life, and accomodated it by the way they operated certain units to generate the reactive power a system needed. That is about all I can comprehend about the matter as an "iron piler".
We get paid for producing Mvars by the grid company and can be instructed to produce leading or lagging Mvars inside the capability curve of the generators and transformer.
I'm Working, in a Electric Generation Plant, the last
week We had a bad experience with Generator, quicly the
automatic excitation was zero, mvars too and after this
we saw that mvars changed to negative value.
At the next day, we worked out(out service) the generator,
for inspection and this one, done equal failure.
What's your aswers to these experiences ?
buzz - As rms says it is a necessary component of power transmission for voltage control. Somehow the Independent System Operator or equivalent must provide incentives and penalties so that proper vars are provided where needed. Similar thing for spinning reserve. That's my guess.
javi1972 - I understood you to say you lost excitation and now your generator is damaged. My guess is that possibly there was a pole slip since real power wasn't reduced and effective power transmission capability was diminished by reduced generator voltage (P ~ |V1||V2|/|X|). Obviously protection should function to protect generator from damage under these circumstances.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
Assuming that the questioner understands that AC induction motors require a magnetic field to run, I'll answer this power plant triva question by saying that "basically" the magavars, or reactive power is provided to that motor to create and maintain the magnetic field so that it can use the real power, KW, to turn the motor shaft.
Because the system is AC (cyclic), the rate of energy exchange between the generator and induction motor (to provide the buildup and decay of the field) is the reactive power.
Electricpete,
Been out for a few weeks so forgive the delay. I understand all about VARs but was wondering if your generating plant bills for VAR's? None of ours do (we are federal though so our superiors are not real business savvy). I always wondered why they did not since the utilities do bill the end-users for this.
some power grids will pay for vars, but you have to have you head examined to sell them by adjusting the in plant generators.
the wear and tear on the equipment far out weighs any percieved benefit. at a minimum you experience a lot of undervoltage trips (increased voltage drop due to the vars being shoved around), etc.
the best bet is to run as close to unity pF as possible and perform the pF correction in other ways.
Re: "the best bet is to run as close to unity pF as possible and perform the pF correction in other ways" -- that's an interesting statement -- My uneducated guess would have been that generators would be happiest with perhaps a 90% lagging load. . . .
Can't leading loads cause generator problems, problems with voltage regulators and oscillations and voltage instability? Wouldn't we like a bigger cushion than running at unity?
We have a plant that is a peaking plant (4 generators) so we are constantly providing and absorbing VAR's. We don't charge for the VAR's we provide. There are cases when basically the only thing the generator is providing is VARs with next to no Watts. Heck, some of the generators fight each other with one supplying and one absorbing, when all the operator has to do (remotely operated) is balance these out. However, they don't because they don't have too. Now if we were billing for this, they would pay much closer attention to keep the customers happy. It sounds like only the federal companies are ones providing free VARs based on the above comments.
Federal companies? Meaning federally owned? There's at least one private utility I'm aware of that doesn't bill for apparent power: ComEd in Chicago (part of Exelon) has never charged any of their customers a single cent for a var. Free vars for all. kW/kWh billing only. Not a cap in sight.
Yeah federally owned (by Department of Interior). I am new to this government thing so not sure how it is all structured (generation, marketing, etc, etc, etc). None of our facilities get reimbursed for VARs so no monies added to the treasury for VAR usage even if the company that sells to the industry does charge for VARs.
