I am very familiar with the standard definition which applies
to loads, having designed and built a true power meter myself
some 25 years ago, and having explained power factor many times
here over the years.

You'll have to point out a definition of power factor of a
supply, as that's not covered by the standard definition.
Google draws a blank too. The only related thing I could think
of would be a measure of the worse power factor load a particular
supply can drive, but even that's meaningless as supplies
are rated in [k]VA and in many cases can handle worst case
loads with a PF of zero.

--
Andrew Gabriel

I'm clearly trying to discover whether the power factor I measure is
affected in any way by the electricity network. If you put a purely
resistive load across the supply coming into your house, are you
guaranteed to measure no phase difference between the voltage and the
current? If the phase difference is zero, then the power factor of your
supply is 1.

AFAICS, a supply doesn't have a power factor. It is determined entirely
by the load downstream.

In the simplest terms, the generator is responsible for the alternating
voltage, but your load is responsible for drawing the current. Given the
available voltage at the point of connection, your load is the sole
determinant of the magnitude of the current that it will draw, and its
phase angle relative to the applied voltage. Therefore the PF is solely
a property of your load.


There's a whole load of stuff on the web about the lengths the National
Grid go to power factor correct their supply. Perhaps you should ring
them to tell them they are wasting their time?

I see your point, but let's build this up one step at a time.

The simplest case is a single generator and a single load, connected by
a loss-free line. As the consumer, you measure the power factor of
whatever load you choose to apply. Meanwhile, the supplier measures the
power factor of the load that *it* sees, back at the generator output.
In this very simplest case, the consumer's and the supplier's
measurements of PF are obviously the same.

Now add another consumer and another load. Each individual consumer
measures a PF that is determined by the current flowing into his own
individual load. The other consumer's load may affect your supply
voltage, but it does *not* affect the PF of your load (unless the load
happens to be non-linear - but that is still entirely a property of your
local load, not the supply).

Meanwhile, back at the generator, the supplier can measures the PF of
*its* load, as seen at the generator output terminals. This will depend
on the individual PFs of the two separate consumers' loads, line lengths
etc; but it does not depend on the generator - the generator itself
doesn't *have* a PF.

The complications arise when we have a distributed network with several
generators, a very large number of loads, propagation delays and line
losses. As you know, this is a hugely complex situation which includes
the possibility of generators not being synchronised, and not
necessarily contributing power to the network. This blurs the boundaries
between "generator" and "load", as the network has some characteristics
of both.

But even then, the same basic principles apply. Each individual
generator has only one set of output terminals, and sees the rest of the
network as its load. So it is still the *load* that has a PF - not the
generator. I'm not sure how the "network PF" is defined... but it still
doesn't affect you.

When you connect a load somewhere on the network, all the complexities
drop away - we're right back to basics with a single connection point.
Your load is the sole determinant of the current that it draws, and the
phase angle relative to the applied voltage is still determined entirely
by the characteristics of your load.

Whatever may be happening in the network upstream of your connection
point, it does not concern you. When you connect a load, it will affect
the voltage/current relationships somewhere back upstream in the supply
network (at least to some small extent) but it still doesn't affect the
PF of your load, as measured by you at your single point of connection.


[*] Another important detail is that the other load(s) must not affect
the waveform of the alternating voltage available to you. That is why
there are restrictions on the harmonic and non-sinusoidal content that
loads may throw back into the supply. However, I don't believe that is
where the original question was coming from.


Hope this makes sense, because the coffee mug's empty now.


--
Ian White

On Sat, 06 Oct 2007 16:30:57 -0700, wrote:

On 6 Oct, 20:00, John Evans wrote:
On Fri, 05 Oct 2007 17:17:00 -0700, wrote:
On 5 Oct, 23:51, Rumble wrote:
wrote:


It wasn't supposed to be a trick question. The supply has a power
factor by definition. The question is whether it equals unity or not.
Are you implying that a domestic supply is guaranteed to have a power
factor of 1?

T

?????

Not so!

The numerical power factor eg 1, 0.9 etc - is a measure of the phase
angle between the LOAD current and the applied voltage. It is also the
measurement of the ratio of real power and apparent power or the ratio
of watts (W) to voltamperes (VA).

....


The power companies worry about the overall power factor of the supply
for obvious reasons.

You have just contradicted yourself.

T


No I haven't! I accidently missed out "generation" from "power ...
companies" when I mad a correction.



There are two seperate issues.

Consumers Power Factor.

This is the users responsibility and depends on the applied votage and
the phase angle of the consumers load current. (I know the supply
isn't truly sinusiodal and this clouds the issue a bit). If this is
too far from unity the energy supplier will want corrections made.


Power Factor as seen by Generation Plant.

This is the total of all consumers powers factors plus any losses in
the transmission system. The generation companies want this to be as
near unity as possible.


I still haven't seen a post the explains thet the supply has a power
factor by definition. How does it and what is this definition?

On 7 Oct, 02:05, John Rumm wrote:
wrote:
I'm clearly trying to discover whether the power factor I measure is
affected in any way by the electricity network. If you put a purely

IIUC, it is not effected by the network as such, although it may be
affected by other users of it.

resistive load across the supply coming into your house, are you
guaranteed to measure no phase difference between the voltage and the
current? If the phase difference is zero, then the power factor of
your supply is 1.

With purely resistive loads then it is not an issue. For loads with a
reactive component then the reactive component will typically be the
major influence on the power factor. However the quality of the waveform
that you are supplied with can further influence it.

There's a whole load of stuff on the web about the lengths the
National Grid go to power factor correct their supply. Perhaps you
should ring them to tell them they are wasting their time?

They go to some effort to mitigate the effects their users have on the
supply. A big industrial user (or the cumulative effect of many smaller
ones) pulling large currents from the supply that are not phase aligned
with the voltage, can end up distorting the supply waveform. This leaves
the waveform non sinusoidal and hence introduces other frequency
components into it. These will interact with the reactive elements of
any load differently than would a plain 50Hz supply.

--

Thanks for the informative answer. It seems that the power factor of
the supply into my house is guaranteed to be 1. However I believe the
mutual capacitance and inductance of the power lines and other
equipment has to be compensated for as well.

T

On 7 Oct, 09:15, wrote:
On 7 Oct,
wrote:

There's a whole load of stuff on the web about the lengths the
National Grid go to power factor correct their supply. Perhaps you
should ring them to tell them they are wasting their time?

Power factor is a function of the load. They can protect their supplies from
a poor load power factor by adding a load with a poor power factor out of
phase in the opposite direction.


Interesting. So the supply cable from the local transformer to my
house doesn't constitute a load. It must be superconducting?

T

On 7 Oct, 09:09, Ian White wrote:
I am very familiar with the standard definition which applies
to loads, having designed and built a true power meter myself
some 25 years ago, and having explained power factor many times
here over the years.

You'll have to point out a definition of power factor of a
supply, as that's not covered by the standard definition.
Google draws a blank too. The only related thing I could think
of would be a measure of the worse power factor load a particular
supply can drive, but even that's meaningless as supplies
are rated in [k]VA and in many cases can handle worst case
loads with a PF of zero.

--
Andrew Gabriel

I'm clearly trying to discover whether the power factor I measure is
affected in any way by the electricity network. If you put a purely
resistive load across the supply coming into your house, are you
guaranteed to measure no phase difference between the voltage and the
current? If the phase difference is zero, then the power factor of your
supply is 1.

AFAICS, a supply doesn't have a power factor. It is determined entirely
by the load downstream.

In the simplest terms, the generator is responsible for the alternating
voltage, but your load is responsible for drawing the current. Given the
available voltage at the point of connection, your load is the sole
determinant of the magnitude of the current that it will draw, and its
phase angle relative to the applied voltage. Therefore the PF is solely
a property of your load.

There's a whole load of stuff on the web about the lengths the National
Grid go to power factor correct their supply. Perhaps you should ring
them to tell them they are wasting their time?

I see your point, but let's build this up one step at a time.

The simplest case is a single generator and a single load, connected by
a loss-free line. As the consumer, you measure the power factor of
whatever load you choose to apply. Meanwhile, the supplier measures the
power factor of the load that *it* sees, back at the generator output.
In this very simplest case, the consumer's and the supplier's
measurements of PF are obviously the same.

Now add another consumer and another load. Each individual consumer
measures a PF that is determined by the current flowing into his own
individual load. The other consumer's load may affect your supply
voltage, but it does *not* affect the PF of your load (unless the load
happens to be non-linear - but that is still entirely a property of your
local load, not the supply).

Meanwhile, back at the generator, the supplier can measures the PF of
*its* load, as seen at the generator output terminals. This will depend
on the individual PFs of the two separate consumers' loads, line lengths
etc; but it does not depend on the generator - the generator itself
doesn't *have* a PF.

The complications arise when we have a distributed network with several
generators, a very large number of loads, propagation delays and line
losses. As you know, this is a hugely complex situation which includes
the possibility of generators not being synchronised, and not
necessarily contributing power to the network. This blurs the boundaries
between "generator" and "load", as the network has some characteristics
of both.

But even then, the same basic principles apply. Each individual
generator has only one set of output terminals, and sees the rest of the
network as its load. So it is still the *load* that has a PF - not the
generator. I'm not sure how the "network PF" is defined... but it still
doesn't affect you.

When you connect a load somewhere on the network, all the complexities
drop away - we're right back to basics with a single connection point.
Your load is the sole determinant of the current that it draws, and the
phase angle relative to the applied voltage is still determined entirely
by the characteristics of your load.

Whatever may be happening in the network upstream of your connection
point, it does not concern you. When you connect a load, it will affect
the voltage/current relationships somewhere back upstream in the supply
network (at least to some small extent) but it still doesn't affect the
PF of your load, as measured by you at your single point of connection.

[*] Another important detail is that the other load(s) must not affect
the waveform of the alternating voltage available to you. That is why
there are restrictions on the harmonic and non-sinusoidal content that
loads may throw back into the supply. However, I don't believe that is
where the original question was coming from.

Hope this makes sense, because the coffee mug's empty now.

--
Ian White

Ian

Thanks for taking the time to provide such a comprehensive answer. I'd
realised that if all loads were in parallel then there would be no
mutual effect, but I was just wondering if the complexities of the
distribution network might introduce a phase shift.

I guess I'll just have to accept that the power factor of my kettle is
0.99!

T

On Sun, 07 Oct 2007 15:50:56 -0700, wrote:

Thanks for the informative answer. It seems that the power factor of
the supply into my house is guaranteed to be 1. However I believe the
mutual capacitance and inductance of the power lines and other
equipment has to be compensated for as well.

The supply to a premises can only be voltage. How closely the phase
relationship of the appliances' load currents follow the supplied
voltage determines the power factor.
The supply company doesn't supply a current - this is determined
purely by the load imposed by the consumer. IOW if you place no load
on the supply, there is no current, therefore no power - where does
power factor stand?
The effects of power lines/transformers etc are the worry of the
supply company.

--
Frank Erskine

wrote:

Thanks for the informative answer. It seems that the power factor of
the supply into my house is guaranteed to be 1.

Yes, the supply is going to be close to a true sine wave with stable
voltage and frequency. This is really all the suppliers can do. The
actual power factor is then dictated by the reactive components of any
load you apply.

However I believe the
mutual capacitance and inductance of the power lines and other
equipment has to be compensated for as well.

What the distribution network will seek to prevent is the waveform they
supply you being affected by the loads that are imposed on it by the end
users and by reactive elements within the distribution network itself.

There is also an unwanted feedback element at work here. Say you place a
capacitive load across your supply, then the amount of current lead will
dictate the power factor. However if the supplier lets through an amount
of harmonic noise on the supply at entry to your property, then the
reactance presented by your load would be different (i.e. lower) for the
higher frequency noise component of the the supply waveform. The lower
power factor now being more likely to hinder the ability of the network
to supply the next customer with a clean supply.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

wrote:
On 7 Oct, 09:15, wrote:
On 7 Oct,
wrote:

There's a whole load of stuff on the web about the lengths the
National Grid go to power factor correct their supply. Perhaps you
should ring them to tell them they are wasting their time?

Power factor is a function of the load. They can protect their supplies from
a poor load power factor by adding a load with a poor power factor out of
phase in the opposite direction.


Interesting. So the supply cable from the local transformer to my
house doesn't constitute a load. It must be superconducting?

Sorry, Tom, I'm afraid you still haven't quite got it.

Power factor is ONLY defined looking in the DOWNstream direction from
the connection point. That is an absolute - "by definition", no
exceptions, not negotiable.

The only power factor that you can measure is for the load that YOU have
connected, DOWNstream of YOUR connection point.

The supply cable is part of the power company's load, but is not part of
YOUR load because it is upstream from you. Therefore it is definitively
not involved in your power factor measurements.

Other details have been mentioned, but you do need to get hold of that
basic principle before you can ever understand how the other details fit
in.

(Those details are mainly about "pathological" types of loads that can
modify the waveform of a weak incoming power supply. They can only do
that because in some sense those loads are also acting as generators,
so the definitions of "upstream" and "downstream" become blurred. But
for normal passive loads, those definitions remain clear and absolute.)


--
Ian White

On Sun, 07 Oct 2007 15:54:47 -0700, wrote:

On 7 Oct, 09:15, wrote:
On 7 Oct,
wrote:

There's a whole load of stuff on the web about the lengths the
National Grid go to power factor correct their supply. Perhaps you
should ring them to tell them they are wasting their time?

Power factor is a function of the load. They can protect their supplies from
a poor load power factor by adding a load with a poor power factor out of
phase in the opposite direction.


Interesting. So the supply cable from the local transformer to my
house doesn't constitute a load. It must be superconducting?

T



You still haven't got it.


The power factor(PF) that a consumer are concerned about is the one
your total load produces. This is measured with reference to the
supply terminals to your premises. The additional PF produced by the
cable , Grid System etc, are only of concern to the power companies -
in fact you can't "see it". You are charged for the power you use.

The supply to your premises consists of a nominal sinusoidal EMF of
240v (assuming single phase and UK), a nominal frequency of 50Hz
(assuming UK) and the ability to supply an amount of current. It can't
have a PF - by definition - as there is no current flowing into your
premises until a load is connected. Therefore no power is dissapated.

Current is required to produce power, and that, with its phase angle
relative to supplied EMF, is what the PF is all about.

It is meaningless to say that the supply has a PF of 1 (I and V in
phase) as it implies that there must always be a resitive load drawing
current somewhere beyond your load. If there wasn't a load, there
couldn't be a current drawn, power dissaptated or a PF to measure!

The PF is related to the co-sine of the phase angle between supplied
EMF and your load current. A PF of 1 is zero degrees phase shift and
the PF of a purely reactive load is zero and the phase angle is 90
deg. Zero PF doesn't mean zero current. A practical circuit has an
impedance giving an angle and PF somewhere between zeo and unity.
Hopefully nearer the latter.

The following link gives a good description and also explains why the
power companies are concerned about the PF.

http://en.wikipedia.org/wiki/Power_factor


The power companies are concerned with the overall PF - consumers,
transmission lines etc - as a poor PF means that more power is
required for a given useage. These losses are further compounded by
distorted waveforms, cbale resistance etc.