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From: jgd@rsiatl.UUCP (John G. De Armond)
Newsgroups: sci.electronics
Subject: Re: Watt Hour Meters
Message-ID: <1115@rsiatl.UUCP>
Date: 11 Jan 90 10:51:20 GMT
Reply-To: jgd@rsiatl.UUCP (John G. De Armond)
Distribution: usa
Organization: Radiation Systems, Inc. (a thinktank, motorcycle, car and gun works facility)
Lines: 82

In article <1452@umigw.MIAMI.EDU> wb8foz@mthvax.cs.miami.edu (David Lesher) writes:
>
>One question I never asked a utility when I should have...  
>
>Virtually everything causes inductive power factor problems. You
>correct for it (typically) with capacitors (some old-timers may call'em
>condensers, but I think those belong to steam engines and HVAC units :-}).
>
>What does the utility think if you offer to correct other people's
>leading PF by having too many farads? This came up when I had to
>improve the pf at a pump station. It had one each: 500, 1000, and 1600
>hp 4160 volt motors. We went to great expense to switch the correct #
>of cans in for each motor. But if we had left the worst case setup
>connected at all times, we would have helped out with all the leading
>factor from the nearby farms. I never was able to get my boss to
>understand this, and so we never did get any feedback from the
>utility.
>
>
>So anyone with direct knowledge of a utility's position on this
>case? I suspect that the posture would be the same penalty, if
>only because their pf meter didn't read direction, only scaler
>amount.


I can't state utility policy but I can discuss the engineering issues
involved.  In short, you'd do as much harm with your capacative 
reactance as does the neighbors with inductive reactance.   Here's why.

Your excess capacity causes leading VARs to flow through your
substation transformer, through the distribution wiring, and through
the transformers feeding the inductive load.  Wattless amps 
caused by leading PF cause the exact same IR heating as lagging PF.
In other words, you've not done anything helpful but have 
hurt the overall situation by causing leading VAR heating your
power feed.  But it gets worse.

The power companys have pretty accurate models for calculating what
a PF will be given a typical distribution of loads.  Using this
model, they can correct the PF close to the source.  You will commonly
see this done via capacitor banks mounted on distribution poles.
Here in Marietta, Cobb EMC tends to correct in a residential setting
on a neighboorhood boundary and in light industrial areas, at the 
industrial park feed.  This isolates the wattless current and IR
losses to the pole pigs and the low voltage (7200 v or 14.4 kv) distribution
wiring.  This is a good compromize between the cumulative losses
in the pole pigs versus the cost of capacitor banks.

In order to size this correction capacity, some underlying assumptions
have to be made.  For instance, that a typical house will have a 
heat pump of between 3 and 5 horsepower as the primary reactive load.
Or that an office building will have primarily inductive loads in the
form of air conditioning and fluroscent lamps.  

They cannot anticipate that some well intentioned customer is feeding
leading VARs back into the line, especially of the magnitude you
mentioned.  In other words, they want you to be typical.

The best thing you can do is to correct the PF to unity.  This will
make the utility happy and will reduce your energy costs.  Realize that
while wattless current itself represents no energy consumption and
therefore is not metered, the heat loss in the conductors and 
transformers IS energy and you DO have to pay for it.  Conceptualize 
the equivelant circuit of an inductor and a resistor in series.  
The larger proportion of impedance that is represented by the
resistance, the larger proportion of the amperage is representative of
work energy.  In other words, the phase vector rotates toward 0 as
the reactive component becomes less significant.

So because you have to pay for the real watts dissipated as IR losses
in wiring and transformers, you should correct the PF as close to the
source as possible.  I like to see capacitor banks mounted right at
the site of the motor downstream of the starter.  This has a side
benefit of reducing arcing in the starter and prolonging contact life.

John


-- 
John De Armond, WD4OQC                     | The Fano Factor - 
Radiation Systems, Inc.     Atlanta, GA    | Where Theory meets Reality.
emory!rsiatl!jgd          **I am the NRA** |