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!)
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
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.
remove the urine to answer
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.
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).
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.
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.