[RE-wrenches] Power Factor (was Efficient step up, down transformers)

[Joel Davidson]

Hi Bob,
The inverter PF was 1.0. The grid was always less than 1.0 and often down to 
0.7. LADWP would not say what they did to fix the problem.
Joel Davidson

----- Original Message ----- 
From: "boB Gudgel" <boB at midnitesolar.com>
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Cc: "Joel Davidson" <joel.davidson at sbcglobal.net>
Sent: Tuesday, July 26, 2011 7:30 PM
Subject: Re: [RE-wrenches] Power Factor (was Efficient step up, down 
transformers)


> On 7/26/2011 4:42 PM, Joel Davidson wrote:
>> Power factor less than 1.0 can shorten the life of and damage equipment 
>> and reduce PV production. We installed a 200 kW PV system on a facility 
>> with a lot of 3-phase motors and monitored the PV and the grid. We 
>> measured around 0.7 PF being delivered by the Los Angeles Dept of Water & 
>> Power. The data was used as evidence by the PV client to negotiate 
>> compensation for lost power and get better quality power at that site and 
>> other facilities. Now I always monitor utility power on commercial 
>> systems for trouble-shooting.
>>
>> Joel Davidson
>>
>
>
> That's very interesting...   I would expect   a current-source type grid 
> tie inverter would
> be very close to PF of 1.0 (if that one WAS a CS type which I would think 
> it would be)
>
> Might be because the grid voltage at that facility is really distorted 
> because of the loads
> you mentioned.  Did you measure the PF of the line from the GT inverter 
> itself ?
>
> What kind of inverter system was this ??   A big one of course.
>
> Some GT inverters that are more of a voltage source will supply lower 
> power factor
> and tend to clean up the dirty voltage waveform.
>
> boB
>
>
>
>
>
>
>> ----- Original Message ----- From: "Exeltech" <exeltech at yahoo.com>
>> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
>> Sent: Tuesday, July 26, 2011 12:13 PM
>> Subject: Re: [RE-wrenches] Efficient step up, down transformers
>>
>>
>> Hello Ray,
>>
>> Wow.  I could do an another thesis on the topic of
>> power factor alone...
>>
>> There's no good way to fully explain this in depth
>> here .. so all we can do is scratch the surface.
>>
>>
>> First, a quick review.
>>
>> Power factor less than 1.0 causes current flow in a load
>> that's out of phase with the applied voltage.  The actual
>> power delivered to the load (and thus consumed by the
>> source, disregarding efficiency for now), would be:
>> Vac * Iac * COS(theta) .. where theta is the phase angle
>> differential between the voltage and current (with the
>> values for V and I in rms).
>>
>> Though the real power consumed in a reactive circuit
>> is going to be less than the real power consumed in
>> an equivalent resistive circuit (by virtue of the
>> phase angle), the amperage in the reactive circuit
>> is still whatever is flowing.
>>
>> Now, to answer your question...
>>
>> All inverters will operate at a lower efficiency
>> when connected to a load with lousy power factor
>> than if connected to a purely resistive load
>> under similar operating conditions (VA vs. watts).
>>
>> The actual efficiency reduction will depend also
>> on whether the low power factor is due to reactive
>> components (e.g. inductors or capacitors), or
>> due to a non-linear load, such as a switching power
>> supply in a computer, microwave oven, etc.
>>
>> Nonlinear loads present their own issues that I'll
>> skip for now.  Let's presume reactive...
>>
>>
>> That said...
>>
>> The majority of the losses in inverters under
>> these conditions would be resistive.
>>
>>
>> Here's why:
>> Inverters made with power transformers have large
>> amounts of copper wire, and consequently, wire
>> resistance.  When connected to a load with a poor
>> power factor, transformer-based inverters will not
>> perform as efficiently as they do when connected
>> to a purely resistive load.
>>
>> With a very low power-factor load, you could
>> theoretically be providing the max current from
>> an inverter at much lower real power output than
>> it's rated.  This additional loss, when taken
>> into account at an AC power with low real wattage
>> output, will make the inverter's efficiency
>> worse than with a resistive load.  Expect the
>> efficiency hit to be on the order of 2-5% for
>> transformer-based inverters (again, depending on
>> inverter's max wattage rating as well, as this
>> will impact the wire size used in its construction,
>> and thus the total wire resistance).
>>
>> Transformerless inverters don't have this large
>> mass of wire (thus much lower internal resistance).
>> Subsequently, they aren't nearly as affected by
>> low-power-factor loads - but even here, I'd expect
>> around 0.5-1% efficiency reduction, all other
>> conditions being equal.
>>
>>
>> Power factor is deceptively simple .. but is one of
>> the more complex aspects of power systems and
>> power equipment design.
>>
>>
>> Hope this helps.
>>
>>
>> Dan Lepinski
>> Sr. Engineer
>> Exeltech
>>
>>
>> P.S. Thanks for being a long-time Exeltech customer.
>> Much appreciated.
>>
>>
>>
>> --- On Mon, 7/25/11, R Ray Walters <ray at solarray.com> wrote:
>>> From: R Ray Walters <ray at solarray.com>
>>> Subject: Re: [RE-wrenches] Efficient step up, down transformers
>>> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
>>> Date: Monday, July 25, 2011, 4:10 PM
>>> Hi Dan;
>>>
>>> I agree that a 2000 VA inverter isn't equal to a 2000 watt
>>> unit.
>>>
>>> The focus on VA here though was not about looking at the
>>> rating of the inverter, but instead noting that a load with
>>> a high reactive component might draw more energy than a
>>> purely resistive load in a battery based inverter system. AC
>>> watts might be equal, but my understanding (perhaps false?)
>>> is that the inverter efficiency decreases with PF.
>>>
>>> SInce you're in the inverter manufacturing biz, perhaps you
>>> could better explain this to the rest of us.
>>>
>>> My fundamental question is: how does power factor effect
>>> the efficiency of an inverter?
>>>
>>> A better understanding of this issue would not only help us
>>> with the current issue of transformers vs. big wire,
>>> but also help us better size battery based systems to all
>>> types of AC loads.
>>>
>>> Thanks in advance for your time,
>>>
>>> R. Walters
>>> ray at solarray.com
>>> Solar Engineer
>>> (and long time Exeltech user)
>>>
>>>
>>>
>>> On Jul 25, 2011, at 1:41 PM, Exeltech wrote:
>>>
>>> > Focusing on VA ratings would not be advisable.
>>> >
>>> > Here's why.
>>> >
>>> > Watts are real power. This would be actual DC
>>> > volts times actual DC amps. Measuring DC volts
>>> > and DC amps as you're presently doing is a good
>>> > approach. It yields a valid result.
>>> >
>>> > On the other hand, volt-amps ("VA") are nothing
>>> > more than real AC watts at some unspecified power
>>> > factor. Without knowing the associated power
>>> > factor, a VA value is meaningless.
>>> >
>>> >
>>> > For example:
>>> > Let's say I'm a customer, and I tell you I need
>>> > a 1250 VA inverter.
>>> >
>>> > You offer to sell me a 625 watt inverter. Will
>>> > it work?
>>> >
>>> > Answer: Only if it's a true sine-wave inverter
>>> > and my load power factor is 0.5 or worse (<0.5).
>>> > I'm ignoring the possibility the inverter may not
>>> > be designed to work with a load power factor that
>>> > low .. but that's another topic.
>>> >
>>> >
>>> > Example #2: a 1 kW-rated sine-wave inverter
>>> > operating with a resistive load that has a PF of
>>> > 1.0 is truly generating 1,000 watts.
>>> >
>>> > At a load PF of 0.8, it becomes a 1250 VA
>>> > inverter. At a PF of 0.5, it's magically a 2000 VA
>>> > inverter.
>>> >
>>> > Unfortunately, "VA" is used by marketing
>>> > departments to> inflate the actual output wattage
>>> > and make an inverter (or other device) seem more
>>> > capable than it really is.
>>> >
>>> > In the end, the inverter won't generate more than
>>> > 1,000 watts, but incorporating a power factor of
>>> > less than 1.0 into the spec makes it seem to the
>>> > untrained eye like they're getting more bang for
>>> > the buck.
>>> >
>>> > Real watts = real watts. Period.
>>> >
>>> > ---
>>> >
>>> > Lars mentioned at the onset of this thread he
>>> > measured a no-load current of 15.6 amps flowing
>>> > into the transformer.
>>> >
>>> > It's very possible there could be that many amps
>>> > flowing through the transformer primary to build,
>>> > then reverse the magnetic field in the core.
>>> > This would mean, however, the current is 90
>>> > degrees out of phase with the voltage, hence
>>> > no real power being consumed.
>>> >
>>> > The majority loss at that point would be the
>>> > resistive> loss in the copper, which would be
>>> > on the order of a few watts - not kilowatts.
>>> >
>>> > If he's in fact measuring real power somehow, and
>>> > there's no load on the transformer secondary, he's
>>> > got a defective transformer.
>>> >
>>> >
>>> > Dan
>>> >
>>> >
>>> >
>>> > --- On Mon, 7/25/11, R Ray Walters <ray at solarray.com>
>>> > wrote:
>>> >
>>> > From: R Ray Walters <ray at solarray.com>
>>> > Subject: Re: [RE-wrenches] Efficient step up, down
>>> > transformers
>>> > To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
>>> > Date: Monday, July 25, 2011, 12:20 PM
>>> >
>>> > I thought (based a previous thread) that we needed to
>>> > be concerned with VA output of inverters, not wattage,
>>> > at least on the transformer based machines like the SW.
>>> > I realize this wouldn't be an actual load if on grid.
>>> > In these situations, I just measure DC amps into the
>>> > inverter to determine the actual consumption,
>>> > irregardless of power factor, AC efficiency, etc.
>>> >
>>> > R. Waltersray at solarray.com
>>> > Solar Engineer
>>> >
>>> >
>>> >
>>> >
>>> >
>>> > On Jul 25, 2011, at 9:29 AM, Exeltech wrote:
>>> > Measuring current without the phase relationship
>>> > between the current and voltage isn't indicative
>>> > of the actual power consumed. Likely what you
>>> > were> seeing is the eddy current in the transformer
>>> > core.
>>> >
>>> > If so, it's 90 degrees out of phase with the voltage,
>>> > and the real wattage consumed with no load connected
>>> > is minimal.
>>> >
>>> > Dan
>>> > Sr. Engineer
>>> > Exeltech
>>
>>
>