# va

• posted
on transformers they can be rated in va what dose this really mean in simple
terms
• posted
It tells you the handling capacity which you get by multiplying Volts times Amps, irrespective of the phase angle between them, and indicates the maximum voltage and current combination that that the transformer can handle before there is a risk of damage.
You have to be very careful here. If you have an inductive load, then the apparent current is out of phase (the Power Factor, as it is known, is the cosine of the angle of phase difference) because the inductive load is acting like a generator and is sending energy back to you. The net effect is that the power handling of the transformer in Watts is a lot less than that indicated by the VA rating.
You probably would prefer to know the power handling capacity in Watts. (As we all do when we first encounter this matter). The power handling capacity in Watts is the same as the va product if you only have resistive loads (eg, incandescent lights).
To find your power handling capacity for other loads, take the va product and multiply it by the Power Factor.
Unfortunately, you won't know what the Power Factor is until you measure it, a sort of chicken and egg situation, and this is where some experience comes in.
Sorry, but there isn't a simple black-and-white explanation for what you want to know.
(I myself remain amused by the assertion of power engineers that they can elect whether the load on a power station is reactive and can select the MVars. How they do this is beyond me (Any takers?) because my understanding is that the vars are an attribute of the load an not of the generator. Must come from having a light current background!)
• posted
Way out of my field (embedded systems) here but...
?q=%22shunt+capacitor+bank%22
• posted
---------------- Vars are an attribute of the load as you say -but - in a system with more than one generator, the distribution of both real and reactive load between generators can be controlled. In that way an attempt to increase speed of a prime mover increases the power output and attempting to raise voltage will increase the reactive output of that unit (in both cases, reducing the (complex) load on other generators). This is what the power engineers are referring to.There are practical limits such as running one machine as a var source and another as a sink is not generally a good idea and can lead to problems (Mum, where are the candles?).
There is an optimal sharing of load, real and reactive between the generators which will minimise real and reactive "losses". A typical "load flow" analysis for a system might involve 100 to 10000 non-linear simultaneous equations and a great deal of effort has gone into efficient algorithms for this analysis. -- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer
• posted
AND, vars can be 'generated' by some devices other than generators at a power station. Capacitor banks and synchronous condensers are commonly used in large switch-yards/substations. If some of the vars needed to supply the load come from these sources, the power generators themselves can operate closer to unity pf.
daestrom
• posted
Guys, this is neat stuff! Comming from an electronics background (now processor logic verification) I find such discussions interesting and mind enhancing. I certainly understand capacitors and their role here. However...
Q: How much energy is lost in a mechanical monster like a synchronous condenser (obviously they're cheaper than the equivalent capacitor). The ones I've seen seem to be *huge*, so the mechanical loss must be significant.
Really! I'm interested in expanding my view (I *hated* anything to do with power in my college days).
• posted
----------snip---------->
-------- I would eyeball it as about 5% or less of its rated KVA (i.e. 10MVA ->500KW) depending on size (mechanical and electrical losses). Mechanical losses would be in the order of 1-2%. The synchronous machine is more flexible than a capacitor bank in that it is continually adjustable rather than in steps and can also go lagging to some extent. However a prime competitor is the combination of a saturable reactor in parallel with a capacitor bank. Quebec Hydro has (had?) some of these at Rimouski For compensation on a 735KV line) which could go from 84MVAR lead to 0MVAR lead within 0.1 seconds and could provide 200MVAR lag for 5 seconds . Since these date back to the mid to late 60's, I'm sure that improved (and larger) versions now exist. They are not small.
• posted
I'd agree with Don's assessment. About 5% of the KVA rating in KW. Interestingly, near me is an *ancient*, four-unit coal plant. In the late 60's, early 70's, they built a couple of oil-fired plants next to them. Shutdown most of the coal plants and sold for parts. But kept the main generators and some equipment to run two of the former generators as synch-condensers. Helped a lot on the stability of the line for the newer, larger, oil-fired units. Then in early 90's, they converted the oil boilers to use NG. But those *ancient* (circa 1923) generators are *still* being used as synch-condensers. They really got their money's worth out of them.
daestrom

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