Electrical Engineering - parallel variac balancing chokes

Author

parallel variac balancing chokes

Daniel Indyk

2006-03-25, 4:21 am

Dear group,

after verifying to myself (using pspice) that a balancing transformer with
high mutual inductance between both halves of the winding (without being a
particualarly high inductance, even) gives an incredible reduction on
circulating current when used with 2 varaics in parallel ( 2 uncoupled
chokes also reduces Icirc...but the coupling reduces it yet further), can
anyone give me a nice explanation of what's going on. I've tried it myself
but seem to be stuffing up somewhere.

Many thx

Danny

Salmon Egg

2006-03-25, 6:21 pm

On 3/24/06 11:58 PM, in article
tK6Vf.15811$dy4.14904@news-server.bigpond.net.au, "Daniel Indyk"
<danbenfred@bigpond.com> wrote:

> Dear group,
>
> after verifying to myself (using pspice) that a balancing transformer with
> high mutual inductance between both halves of the winding (without being a
> particualarly high inductance, even) gives an incredible reduction on
> circulating current when used with 2 varaics in parallel ( 2 uncoupled
> chokes also reduces Icirc...but the coupling reduces it yet further), can
> anyone give me a nice explanation of what's going on. I've tried it myself
> but seem to be stuffing up somewhere.
>
> Many thx
>
> Danny
>
>
At this point, I just am not capable of understanding your post.

Bill
-- Ferme le Bush

Daniel Indyk

2006-03-26, 5:21 am

Dear Salmon Egg (Roe;)?),

thx 4 ur response. please see the following thread which may help u
understand my problem. many thx.
http://www.pupman.com/listarchives/...h/msg00473.html

Danny

"Salmon Egg" <salmonegg@sbcglobal.net> wrote in message
news:C04AF9A0.1F34E%salmonegg@sbcglobal.net...
> On 3/24/06 11:58 PM, in article
> tK6Vf.15811$dy4.14904@news-server.bigpond.net.au, "Daniel Indyk"
> <danbenfred@bigpond.com> wrote:
>
with[color=darkred]
a[color=darkred]
can[color=darkred]
myself[color=darkred]
> At this point, I just am not capable of understanding your post.
>
> Bill
> -- Ferme le Bush
>
>

daestrom

2006-03-26, 1:21 pm

"Daniel Indyk" <danbenfred@bigpond.com> wrote in message
news:tK6Vf.15811$dy4.14904@news-server.bigpond.net.au...
> Dear group,
>
> after verifying to myself (using pspice) that a balancing transformer with
> high mutual inductance between both halves of the winding (without being a
> particualarly high inductance, even) gives an incredible reduction on
> circulating current when used with 2 varaics in parallel ( 2 uncoupled
> chokes also reduces Icirc...but the coupling reduces it yet further), can
> anyone give me a nice explanation of what's going on. I've tried it myself
> but seem to be stuffing up somewhere.
>
> Many thx
>

Your problem is similar to that found in power production when two or more
generators are paralleled. While the flow of real power from/between
generators is controlled by frequency/prime-move controls, the sharing of
reactive load is done by voltage regulation.

The 'circulating currents' you speak of are a form of reactive load.
Ideally, with identical output voltages and identical internal impedances,
any reactive load supplied by the system is split equally between the
sources. And if there is no reactive load at all, then their would be no
reactive current (i.e. 'circulating current') flowing between the units.

In power generation, one generator can actually become a reactive load for
another generator if its output voltage is reduced below the line voltage.
Its power factor will decrease but the current will be leading the voltage
reference instead of the usual lagging. The result is large 'circulating
currents' between the two machines.

If you model the variacs as ideal adjustable AC sources with a series
inductive reactance, you can see the voltage drop in the internal reactance
for a given load. Because this reactance is inductive, the variac's final
output voltage is not always exactly in phase with the output of other
variacs, even though the supply voltage is common to both and the ideal
transformer's secondaries are in phase.

So if the two transformer secondaries are slightly different voltages, and
the final output voltage of both circuits must be the same (they are
directly connected), then the voltage drop in the internal reactances must
be different. For that to be true, the two variacs' internal reactances
must have different current flows, hence 'circulating currents'.

By putting a balancing transformer in series with both variacs, any phase
difference in variac output voltages cause circulating currents as before.
But the currents themselves cause the final output voltages to match through
both the internal reactance drop as before, but now also the balancing
transformer. The transformer forms a sort of 'proportional feedback' such
that a much smaller current is needed to bring the two final outputs equal.

Of course, if you hook up the transformer backward, well look out ;-)

Chokes have the disadvantage that the final output voltage has poorer
regulation. That is, as load on the bank increases, the voltage drop on the
chokes causes the output voltage to drop more than if they weren't there. A
balancing transformer can have low inductance (as you noted) so the voltage
won't drop as much with applied load, yet still limit circulating currents.

Hope this helps...

daestrom

danbenfred@bigpond.com

2006-03-26, 6:22 pm

Many thx Daestrom for ur reply. I'm slowly starting to understand, I
think. A major stumbling block is the significance of the mutual
inductance. Why does this mutual inductance make such a difference to
the effectiveness of the circuit?

kind regards

Danny

daestrom

2006-03-27, 10:21 pm

<danbenfred@bigpond.com> wrote in message
news:1143407907.746647.156420@j33g2000cwa.googlegroups.com...
> Many thx Daestrom for ur reply. I'm slowly starting to understand, I
> think. A major stumbling block is the significance of the mutual
> inductance. Why does this mutual inductance make such a difference to
> the effectiveness of the circuit?
>

The variacs have slightly different output voltages. The final outputs are
tied together (i.e. shorted together) so the variac voltage difference has
to be 'made up' in the choke/balance transformer.

With a choke, the only way to get a voltage drop is by having circulating
currents. But with a transformer, the 'voltage drop' on one side is
reflected as a 'voltage rise' on the other (transformer action if you want
to call it that).

Careful connections electrically means a small circulating current in one
direction can not only provide a voltage drop for that variac's output, but
also induce a voltage of opposite phase in the other variac's circuit to
'raise' it. So the amount of circulating current needed to 'balance' the
various voltages from variac to common output to other variac is much
smaller.

daestrom

danbenfred@bigpond.com

2006-03-28, 2:21 am

Daestrom

Ur absolutely correct. Thanks for a very nicely worded and thought out
reply. I ploughed in and did the maths for 3 cases. A balanced Tx,
Chokes, and the case of no balancing coils at all.

After making some assumptions in the 1st case as follows:

a) variac internal r << resistive load R

b) r and Xv (variac reactance)<<Xb which is half the balancing
transformer inductance

c) mutual coupling k=1 (so that Xb=M)

say voltage differential=dV

results in: Icirc= dV/2*Xb (almost completely reactive)

I think I should have seen this by inspection but I wouldn't have
trusted myself. I simulated this circuit and found that this formulae
came to within 6% accuracy..which is good enough. I also did the case
for uncoupled chokes and it verified exactly ur qualitative reply. If I
put in all the values I get closer, yet.

The case of no balancing coils (coupled or otherwise) resulted in a
burning hole in my red notebook. ;)

Ur nice answer (with ur name still attached) is going alongside my
quantitative solution in my red notebook.

Many thx

Danny

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