Hey Ken & Grumpy
I started a new thread from the 7.5 VAC xfmr, because the subject has
drifted. You guys mentioned a DC component in pole pigs. Sometime ago
I heard that if an AC circuit had too many switching power supplies on
it, that would cause the power company all kinds of problems. What were
or are the problems and what was the evidence and what did the power
companies do to solve the problem?

--
Thanks & 73
Hank WD5JFR

On Thu, 30 Oct 2008 19:50:45 -0500, "Henry Kolesnik"
wrote:

Hey Ken & Grumpy
I started a new thread from the 7.5 VAC xfmr, because the subject has
drifted. You guys mentioned a DC component in pole pigs. Sometime ago
I heard that if an AC circuit had too many switching power supplies on
it, that would cause the power company all kinds of problems. What were
or are the problems and what was the evidence and what did the power
companies do to solve the problem?

There could be transformer core saturation problems if large half-wave
rectifier systems are used. In Europe old televisions had only a half
wave rectifier to generate the anode voltage for the tubes.
Fortunately, since the users would plug in their televisions both
ways, usually both half cycles were loaded within an area.



In a balanced three phase system the neutral current cancel out and
the neutral wire is loaded only then the phases are not equally
loaded. For this reason, the neutral wire is often smaller in large
underground cables.

Any full wave rectifier system generate strong odd harmonic currents
each time the capacitor is charged.

However, when each phase is loaded with switching mode power supplies,
which generate string 3rd harmonics, these harmonics do not cancel,
but instead the 3rd harmonic neutral currents from all three phases
add up, possibly overloading the neutral wire.

For this reason, at least in Europe, large power supplies must have a
power factor correction circuit to extend the conduction angle during
each half-cycle.

Paul OH3LWR

Howdy,

You're correct. The problem is mainly the result
of many single phase capacitor input power supplies
distributed across a three phase system.

In a capacitor input power supply, the line current
takes the form of narrow spikes, because the rectifiers
only conduct when the line voltage is higher than the
capacitor voltage. Many SMPS have a capacitor input
bulk supply at the front end. So the power factor is
low and there is much harmonic distortion which
includes triplens 3rd 6th 9th... harmonics. also called
zero sequence harmonics (they do not rotate.) As they
add in the neutral of a three-phase four wire system
difference between their half cycles is a DC component.
You can see that half-wave input would be the worst
as Paul mentioned.

The solution reduces THD and increases power factor.
For single phase input power supplies is either a
large inductor which causes the current to look more
like a rectangle. Or active PFC which is usually a
boost converter with no input capacitor and a control
circuit that causes the envelope of the current
pulses follows the shape of the line voltage. Most
often the active PFC is used but in low power supplies
I've seen the big inductor approach taken. There are
also other approaches like "valley fill" which force
current to be drawn when then line is less than
that needed to charge the capacitor. I don't see
these used much.

Before PFC was common this was a huge problem in places
with many crappy PC and VDT terminal power supplies (like
big office buildings full of VT100's.) Sometimes the
neutral conductor which was sized smaller than the phases
because they expect little imbalance would burn up.
Sometimes the DC component would saturate big transformers
and they would fry.

Triplens can be blocked by ungrounded-wye or delta
transformer connections which keeps them out of the rest
of the facility but they still do their dirty work on
the secondary side. So it's better to not draw distorted
line current. With three phase input power supplies
a simple boost converter without the PFC control circuit
improve the PF to about 0.96 and reduce the THD to eh,
maybe 25% or less. The current in phase leg will be
rectangular.

This explanation could be too simple. And as always
some of it might be wrong! But does it answer your
question?


73
Grumpy



"Henry Kolesnik" wrote in
:

Hey Ken & Grumpy
I started a new thread from the 7.5 VAC xfmr, because the subject has
drifted. You guys mentioned a DC component in pole pigs. Sometime ago
I heard that if an AC circuit had too many switching power supplies on
it, that would cause the power company all kinds of problems. What were
or are the problems and what was the evidence and what did the power
companies do to solve the problem?

Grumpy The Mule wrote:
This explanation could be too simple. And as always
some of it might be wrong! But does it answer your
question?

Thanks for your answer and explanation. This leads me to ask,
"Is there anything WE (the consumer) can do about it and if so, what?"

Thanks,

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

Howdy,


It's something to ponder. It appears there's not
much the individual consumer can do.

I suppose a person could direct their purchases toward
devices that don't add to the problem. Like buying
PC power supplies with PFC. Early CF bulbs were pretty
bad but they're better now, though incandescent bulbs
will likely always be better in this one regard. Some
thyristor controls are still horrible, like dimmers and
thermostats. But I didn't see how the consumer could know
this.

Being aware of the energy savings when considering the
price of a product is something the consumer can do.
If it doesn't make economic sense then Ok, don't buy it.
But consider the cost of ownership as well when making
the decision.

Working to have power factor, total harmonic distortion
and efficiency ratings required on the equipment name
plate along with volts, amperes and watts. Is something
individual consumers could do. Sometimes the industry
needs a little nudge.

Keeping equipment in good repair can help. We nearly
burned two taps off a 500KVA transformer at work. It
was installed improperly which took some time to discover
by way of insanely high THD under load. All that time
the transformer was burning up power heating the taps
and reducing efficiency of the equipment it supplied.
This applies to farms and homes as well. Might prevent
a fire as well.

From a ham perspective. Hmmm. perhaps build a PFC power
supply for that big linear amplifier! Or use a PFC battery
charger and run the shack off battery power

One last note, high PFC does not insure low THD but they
usually go hand-in-hand because of the methods employed.
A person could build a device to provide excellent PF and
crappy THD. I've never seen any product that does this
but it is possible.

I'm sorry I can't offer more or better suggestions.


73
Grumpy



(Geoffrey S. Mendelson) wrote in
:

Grumpy The Mule wrote:
This explanation could be too simple. And as always
some of it might be wrong! But does it answer your
question?

Thanks for your answer and explanation. This leads me to ask,
"Is there anything WE (the consumer) can do about it and if so, what?"

Thanks,

Geoff.

On Oct 31, 12:17*pm, Grumpy The Mule wrote:
Howdy,

It's something to ponder. *It appears there's not
much the individual consumer can do.

I suppose a person could direct their purchases toward
devices that don't add to the problem. *Like buying
PC power supplies with PFC. *Early CF bulbs were pretty
bad but they're better now, though incandescent bulbs
will likely always be better in this one regard. *Some
thyristor controls are still horrible, like dimmers and
thermostats. *But I didn't see how the consumer could know
this.

Being aware of the energy savings when considering the
price of a product is something the consumer can do.
If it doesn't make economic sense then Ok, don't buy it.
But consider the cost of ownership as well when making
the decision.

Working to have power factor, total harmonic distortion
and efficiency ratings required on the equipment name
plate along with volts, amperes and watts. *Is something
individual consumers could do. *Sometimes the industry
needs a little nudge.

Keeping equipment in good repair can help. *We nearly
burned two taps off a 500KVA transformer at work. *It
was installed improperly which took some time to discover
by way of insanely high THD under load. *All that time
the transformer was burning up power heating the taps
and reducing efficiency of the equipment it supplied.
This applies to farms and homes as well. *Might prevent
a fire as well.

From a ham perspective. *Hmmm. *perhaps build a PFC power
supply for that big linear amplifier! *Or use a PFC battery
charger and run the shack off battery power

One last note, high PFC does not insure low THD but they
usually go hand-in-hand because of the methods employed.
A person could build a device to provide excellent PF and
crappy THD. *I've never seen any product that does this
but it is possible.

I'm sorry I can't offer more or better suggestions.

73
Grumpy

(Geoffrey S. Mendelson) wrote :



Grumpy The Mule wrote:
This explanation could be too simple. *And as always
some of it might be wrong! *But does it answer your
question?

Thanks for your answer and explanation. This leads me to ask,
"Is there anything WE (the consumer) can do about it and if so, what?"

Thanks,

Geoff.- Hide quoted text -

- Show quoted text -

PF and THD can be a real problem when you generate your own
electricity.
About three years ago we installed a new UPS at work that went crazy
whenever we switch from line to generator. The installers said we
would have to install a generator 4X as large as the one we have to
take care of the problem. Considering our present generator was a
120KW unit going to 500KW was out of the question. We measured the PF
and THD for the input to the UPS and it was ridiculously high but the
installers insisted this was normal for the size generator we had.
Long story short we kicked them and their UPS out and got a different
one that worked flawlessly with our equipment.

Jimmie