Not clear on power factor

[jriegle]

I understand that the lagging current does not do useful work and reduces
the supply line capacity to supply power. Adding PF correction capacitors of
the right amount balances out the inductive reactance by capacitive
reactance.

Does the effect of poor power factor show at the generator? I would say yes
because of the extra heat generated in the wires (conservation of energy).
If so, is that the only extra torque the generator needs is to over come the
extra heat loss in the lines from low power factor?

Although the transformer's capacity is also reduced by the effects of
lagging current, does the lagging current pass though the transformer to the
supply side?

I see some devices, such a equipment with a diode bridge and capacitor to
rectify and filter the AC supply to DC. These are rated to have low power
factor because the asymmetrical current draw due to very high crest factor
from the charching of the capacitor near the peak voltage. The current isn't
lagging, just distorted. Are they generalizing terms by calling devices with
high crest factor to be of low power factor? (i.e. It draws more VA than
real power consumed so it has a lower than 1.0 power factor)

In an experiment, I added 2uf of capacitance to a 9 watt magnetically
ballasted fluorescent lamp. This brought the measured VA of 18 down to 11
which seems like the power drawn should be (9 watts plus 2 watts lost in the
ballast). When I scoped the current wave form it had a distinct double hump
(not unlike the top part of the heart symbol). Wouldn't this asymmetry still
cause issues with utility equipment (transformers and such)?

Thanks!

 

[Don Kelly]

I understand that the lagging current does not do useful work and reduces
the supply line capacity to supply power. Adding PF correction capacitors of
the right amount balances out the inductive reactance by capacitive
reactance.

Does the effect of poor power factor show at the generator? I would say yes
because of the extra heat generated in the wires (conservation of energy).
If so, is that the only extra torque the generator needs is to over come the
extra heat loss in the lines from low power factor?

 

[daestrom]

I understand that the lagging current does not do useful work and reduces
the supply line capacity to supply power. Adding PF correction capacitors of
the right amount balances out the inductive reactance by capacitive
reactance.

Does the effect of poor power factor show at the generator? I would say yes
because of the extra heat generated in the wires (conservation of energy).
If so, is that the only extra torque the generator needs is to over come the
extra heat loss in the lines from low power factor?

Although the transformer's capacity is also reduced by the effects of
lagging current, does the lagging current pass though the transformer to the
supply side?

Yes, it shows at the generator. Although the I^2R in the
wiring/transformers/etc must be supplied by the generator (as extra torque
on the constant speed shaft as you say), there is another concern.

Generator armature windings are limited in the total kVA /MVA they can
carry. Capability curves for generators have three distinct regions, and
one of them is strict kVA/MVA loading. If a poor power factor is allowed,
then the generator can not be operated to full *real* power capabilities
because you reach the apparent power limit of the windings.

A second region of generator capability curves is maximum *field* current.
Running with a severe *lagging* power factor will require more field current
to maintain terminal voltage than would be required with unity power factor
and the same kVA/MVA load. To avoid burning up the field windings,
allowable kVA/MVA loading with a lagging power factor is lower than the
armature winding limit.

So you want as close to unity on the generator as you can get to avoid
having to reduce the *real* power component to stay inside the capabilities
curve. Thus you get the most kW/MW from your unit and create the most
revenue (assuming your prime-mover is well matched).

I see some devices, such a equipment with a diode bridge and capacitor to
rectify and filter the AC supply to DC. These are rated to have low power
factor because the asymmetrical current draw due to very high crest factor
from the charching of the capacitor near the peak voltage. The current isn't
lagging, just distorted. Are they generalizing terms by calling devices with
high crest factor to be of low power factor? (i.e. It draws more VA than
real power consumed so it has a lower than 1.0 power factor)

The strict definition of 'power factor' is W/VA. In the 'olden' days, this
was affected most by phase shift between voltage and current by inductive
loads (motors transformers and the like). Nowadays, with more non-linear
loads, the harmonics can cause poor power factor as much as inductive loads.

In an experiment, I added 2uf of capacitance to a 9 watt magnetically
ballasted fluorescent lamp. This brought the measured VA of 18 down to 11
which seems like the power drawn should be (9 watts plus 2 watts lost in the
ballast). When I scoped the current wave form it had a distinct double hump
(not unlike the top part of the heart symbol). Wouldn't this asymmetry still
cause issues with utility equipment (transformers and such)?

Yes, if allowed in large magnitude. Large industrial customers with
non-linear loads must purchase harmonic-filtering equipment and/or pay an
additional fee to the utility for such problems. IIRC, DC arc furnaces are
one such culprit.

HVDC transmission lines use SCRs for the AC-DC converter/inverter section.
These can create a lot of harmonics, so the installations also have some
*serious* harmonic filtering.

daestrom

 

[jriegle]

capacitors
Yes, it shows at the generator. Although the I^2R in the
wiring/transformers/etc must be supplied by the generator (as extra torque
on the constant speed shaft as you say), there is another concern.

Generator armature windings are limited in the total kVA /MVA they can
carry. Capability curves for generators have three distinct regions, and
one of them is strict kVA/MVA loading. If a poor power factor is allowed,
then the generator can not be operated to full *real* power capabilities
because you reach the apparent power limit of the windings.

A second region of generator capability curves is maximum *field* current.
Running with a severe *lagging* power factor will require more field current
to maintain terminal voltage than would be required with unity power factor
and the same kVA/MVA load. To avoid burning up the field windings,
allowable kVA/MVA loading with a lagging power factor is lower than the
armature winding limit.

So you want as close to unity on the generator as you can get to avoid
having to reduce the *real* power component to stay inside the capabilities
curve. Thus you get the most kW/MW from your unit and create the most
revenue (assuming your prime-mover is well matched).


The strict definition of 'power factor' is W/VA. In the 'olden' days, this
was affected most by phase shift between voltage and current by inductive
loads (motors transformers and the like). Nowadays, with more non-linear
loads, the harmonics can cause poor power factor as much as inductive loads.

Yes, if allowed in large magnitude. Large industrial customers with
non-linear loads must purchase harmonic-filtering equipment and/or pay an
additional fee to the utility for such problems. IIRC, DC arc furnaces are
one such culprit.

HVDC transmission lines use SCRs for the AC-DC converter/inverter section.
These can create a lot of harmonics, so the installations also have some
*serious* harmonic filtering.

daestrom

Thank you for the detailed response.

I did not realize poor power was such a problem at the generator!
John

 

[jriegle]

Back in thoe old days, when ac/dc tube radios were the norm, I would
ocassionly let my mind wander to worry about dc flowing in a transformer. If
all the radios pere plugged in correctly, the half-wave rectifier used would
draw a dc current component. This would tend to saturate the transformer.

IIRC a typical radio of this kind consumed no more than 30 W. 18 of these
would be to run the heaters. Must used simple capacitor input folowed by an
RC stage. This meant that rectifier current was a series of fairly narrow
spikes at 60 (not 120) Hz.

While I doubt that these small radiow posed a problem for the power
companies, larger power draws using half-wave rectifiers, especially with
capacitor input filters could present a problem.

Can anyone enlighten me on how much power companies worried about such
things?

Bill

The plug on these were not polarized or grounded in nearly every case. I
would think that in all the homes with the radio on the chances that the
power for rectifier would be drawn off the positive half of the cycle or the
negative half would average out to 50/50. I scoped the current wave form
from a transformerless tube radio and, yes the wave for is asymmetrical, but
the spikes are rather smooth looking tops to a regular sine wave. The
current limiting characteristics of the rectifier tube kept the crest factor
low relative to the solid state rectifier that lets "anything go".

Several years back, halogen flood lights had a diode in the base because it
was easier to design a 82 volt halogen bulb than a 120 volt one of the
needed characteristics. Apparently these weren't an issue either.
John

 

[Don Kelly]

Back in thoe old days, when ac/dc tube radios were the norm, I would
ocassionly let my mind wander to worry about dc flowing in a transformer. If
all the radios pere plugged in correctly, the half-wave rectifier used would
draw a dc current component. This would tend to saturate the transformer.

IIRC a typical radio of this kind consumed no more than 30 W. 18 of these
would be to run the heaters. Must used simple capacitor input folowed by an
RC stage. This meant that rectifier current was a series of fairly narrow
spikes at 60 (not 120) Hz.

While I doubt that these small radiow posed a problem for the power
companies, larger power draws using half-wave rectifiers, especially with
capacitor input filters could present a problem.

Can anyone enlighten me on how much power companies worried about such
things?

Bill

In the old days- not at all. Non-linear loads were such a small part of the
total load that the effects were lost in the background "fuzz". For larger
loads where it would make a difference, half wave rectifiers weren't used.
The number of such large non-linear loads has increased rapidly in the last
25 years, to the point where harmonic content and interaction between
harmonic sources is a real problem.

 

[daestrom]

There usually were instructions to revrse the plug if you got hum. That is,
one position would give better performance than the other.

Some of the sets were death traps. One side of the line may have been
connected directly to the chassis. Correct polarization was a safety factor.
In some cases, even if plugged in optimally, the chassis could be hot via
the string of heaters if the set were not turned on.

My brother (an old radio fan) tells me that one side of the plug was tied to
the chassis inside the wooden case. So if it was the side that went to
'neutral' you effectively grounded the chassis through the service panel.
If the other side, then you had the chassis 'floating' at 120V above ground
and picking up all the noise in the neighborhood.

As far as all the radios in the neighborhood giving a DC component, I think
they would all have to be plugged in 'right' and all on the same 'hot' phase
in every home. This would be quite a fluke to happen ;-)

daestrom