mivey
Senior Member
OK.
At 30 degrees I have:
I_load_real = 66.14
I_load_reactive = 18.49
I_Load = 84.63
I_Gen_real = -122.48
I_Svc = -37.85
So take it home for me as I'm too distracted with other stuff to follow where you are headed.
Negative power factor and PV systems
- Thread starter tallgirl
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OK.
At 30 degrees I have:
I_load_real = 66.14
I_load_reactive = 18.49
I_Load = 84.63
I_Gen_real = -122.48
I_Svc = -37.85
So take it home for me as I'm too distracted with other stuff to follow where you are headed.
mivey
Senior Member
K8MHZ
Senior Member
- Occupation
- Electrician
While we calculate real and reactive current components, the fact remains that current flow through conductors is indifferent. When a load passes current from two current sources, the three-way node must obey KCL.
When a PV system (first path) is outputting more current than is used by the load (second path), in those instances, the excess current has to go through the third path, the service. During these instances, the PV system supplies both the real and reactive component currents of the load. When the PV system is outputting less currrent than the load demands, the utility supplies the balance. In the example I posed, that occurs after the 60? cycle instant, and continues until around the 120? cycle instant.
What this amounts to is that the PV system does transmit reactive current to and from the load, it is just the anomoly of how service power is measured that all VARs seem to be carried on the service-only conductors. In essence, both sources transmit real and reactive current... but because the PV system has its current restricted to a specific output waveform, the service meter measures some of the service-supplied real-power current as VAR.
You are correct. The measurement anomoly varies with PV system output and load demand changes.
The [service] power factor using your example's values is zero (0).
utilities and power factor
utilities and power factor
My 2 sents:
I thought utilities charged for apparent power, so they have you regardless. And most loads are inductive reactance, so the utility has to supply capacitance to balance out power to customers. Also heard 2nd hand of customers adding their own capacitance to balance their loads, and then just paying for the real power (unity power factor) and getting reactive power for free.
Not sure how that relates to inverter /PV power .
utilities and power factor
My 2 sents:
I thought utilities charged for apparent power, so they have you regardless. And most loads are inductive reactance, so the utility has to supply capacitance to balance out power to customers. Also heard 2nd hand of customers adding their own capacitance to balance their loads, and then just paying for the real power (unity power factor) and getting reactive power for free.
Not sure how that relates to inverter /PV power .
K8MHZ
Senior Member
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- Electrician
Utilities charge for watts, not for apparent power.
The myth that utilities charge for apparent power is being perpetuated by scam artists selling devices that claim to use capacitance to lower utility bills.
Horse hockey, it is. Pure and simple.
A watt meter measure watts. Period.
robbietan
Senior Member
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- Antipolo City
utilities charge you for low power factor, not kVAR consumption. for commercial and industrial customers.
residential accounts are charged only in kWHrs
mivey
Senior Member
Then perhaps it is better to state it as a "before" and "after" case.
The illustration I was trying to make was that the power factor is a measure of how efficiently the space in the conductor or the capacity of the equipment is being used to deliver real power and has nothing to do with the direction of the current or power flow. In other words, the space utilized when you have only real power flow vs. the space utilized when you have real & reactive flow (before and after reactive compensation).
mivey
Senior Member
Some do have charges based on apparent power. They are trying to charge for a poor power factor. In my experience, not too many use a kVA charge but many do have a kvar charge.
That is what we call kvar delivered vs. kvar received.
What they would get for free is the initial charging of the electromagnetic field. After that, the customer-owned capacitors and customer-owned inductors will trade energy back and forth during the cycle. Poor power factor due to harmonics are a different story.
For the displacement (non-harmonic) case, the customer could install a capacitor bank to handle the vars, or let the new PV inverters that can handle vars take care of it.
Not always.
That is the case in residential and small commercial where the utility does not individually monitor or penalize for excess kvar load. This has traditionally been the case because the cost of the meter as compared to the size of the load was excessive. Usually, an average rate class load profile is assumed (based on load studies) and the costs are allocated accordingly.
The larger customers have meters that measure both watt-hours and var-hours along with peak values. With the age of smart metering, this may even trickle down to residential and small commercial.
The excess kvar penalty may be a function of the power factor, peak kvar, or average kvar but they are all usually based on measuring the var and watt flow.
mivey
Senior Member
For residential and small commercial this is generally true.
Usually the case, but not always. Some residential accounts are actually on commercial rate structures that have var charges.
mivey
Senior Member
I mis-read that. Utilities normally do not charge residential and small power for a low power factor. For large commercial, they bill for low power factor and usually use the kvar as the billing determinant. Some do use a % power factor calculation that is the basis for increasing the kW demand charge or as a percent multiplier on the bill total, etc. but even these are usually based on a kvar reading as I explained here:
The power factor is usually a secondary calculation based on peak kW & peak kvar or kWh & kvarh. Even for the case where the minimum power factor is stored in the meter (I don't know of any that use this for billing), the power factor is based on the stored var & W values (for meters with higher harmonic accuracy, the var blocks are usually determined using the VA & W block measurements but that is not really important for this discussion).
For many, the power factor penalty is charged based on the amount of kvar that exceed a certain percentage of kW (1/3 or 1/2 or 2/3 or approximate pf of 95% , 90%, 85%, etc) using the kvarh & kWh divided by the number of hours in the billing cycle. This is the average method.
For the peak method, some utilities use the peak kvar and peak kW values that are stored (not so great for loads that manage their kW demand).
For either of these, the charge is usually something like a $/kvar fee.
For the utilities that use the % power factor, they usually use the kvar data in order to secondarily derive the average or peak power factor. Then they use the variance from some acceptable pf % as a multiplier on the real billing determinants or the bill $.
At any rate, there is no power factor without the kvar measurement because the power factor is a secondary calculation.
There are many other billing schemes to recover the reactive costs, including kVA demand charges, contract capacity charges, etc.
tallgirl
Senior Member
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- Glendale, WI
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- Controls Systems firmware engineer
I agree and disagree -- power factor does represent how efficiently the conductors are being used. But only by considering "direction" can you determine who is responsible for fixing any problems and how severe the problems are going to be.
As I understand it, the problem is being looked at by inverter manufacturers, which is a good thing.
mivey
Senior Member
I believe I have said that we must consider the direction of power flow so maybe we almost agree+agree. Power factor does not have a direction. Perhaps that is where you & I disagree. Also, I can figure who is responsible for what and the severity of the problem without having to put a sign on the power factor. I can do this with a meter that does not even record the power factor, much less puts a sign on the power factor.
Consider that in our industry there are at least three different ways signs are being added to the power factor number. You started out by asking what the utilities wanted so consider that utilities use four-quadrant metering that is consistent with IEEE measurement standards. We do this to get a complete picture of what is going on with power flow. Q1 & Q4 means we supply watts. Q2 & Q3 means we absorb watts.
As for reactive power, the average value is zero so it really doesn't have a sign either. Nevertheless, we use the concept of complex power representation to put a sign on the var component.
Reactive load in Q1 & Q2 means the load is inductive in nature, like most of the system and adds to our reactive load burden. By convention we call this delivered vars and assign a positive value.
Reactive load in Q3 & Q4 means the load is capacitive in nature unlike most of our system and decreases our reactive load burden (but can cause other issues). By convention we call this received vars and assign a negative value.
When speaking to utility people they will be thinking about Q1/Q2 vs Q3/Q4 if you mention a positive or negative power factor and you will have a very confusing conversation until they figure out what you mean.
I would respectfully recommend you use power conventions used by IEEE and the utility industry, at least while you are working with the utility. It might simplify things to do this on a regular basis since these DG systems will usually involve interaction with utility personnel. It is just my preference and I realize some PQ meters may use a sign notation on the pf (most I have seen do not).
It might make your life and the life of many other contractors a lot easier if they use the predominant standard in the generation and power delivery industry. Not all utility personnel will be well versed in the different usages out there.
That's what I understand as well. In the past, PV arrays have not really been on the radar scope for the most part. As they start becoming a significant portion of the system, they start getting more attention.
tallgirl
Senior Member
- Location
- Glendale, WI
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- Controls Systems firmware engineer
These aren't power quality meters, they are watt-hour meters -- they report positive power factor for "import" and negative power factor for "export".
It's been a few weeks since I returned the Shark 100 I had on loan from a client, but I'm fairly confident it worked the same way as the WattNode (Continental Control Systems) I have presently.
I'd love to stick with industry conventions, but I'm sort of stuck with the devices the industry produces ...
It's been a few weeks since I returned the Shark 100 I had on loan from a client, but I'm fairly confident it worked the same way as the WattNode (Continental Control Systems) I have presently.
I'd love to stick with industry conventions, but I'm sort of stuck with the devices the industry produces ...
K8MHZ
Senior Member
- Occupation
- Electrician
Wouldn't better terminology be 'power sum' instead of factor. Negative power sum, to me, would be a better description.
I ran across another term I am not comfortable with, but will be forced to adopt it. 'Hysteresis' as used in the difference between two settings in a hybrid inverter.
This is what I was taught before I took the solar class:
"hys?ter?e?sis/ˌhistəˈrēsis/
Noun: The phenomenon in which the value of a physical property lags behind changes in the effect causing it."
This is from my textbook:
"Voltage Regulation Hysteresis (VRH) is the voltage difference between the voltage regulation (VR) setpoint and the array reconnect voltage (ARV) setpoint."
In control systems class, the difference between two setpoints was simply referred to as a span.
I don't see a difference in setpoints as being anything like the phenomena of hysteresis, and feel that the solar industry standard is forcing us to use the term improperly.
They also create new words to be used in place of words that mean the same thing. For instance, the solar buzz word for solar irradiation is insolation. Not only does it sound too much like 'insulation' and 'isolation', what other form of irradiation would we be talking about while on the subject of PV systems than solar?
gar
Senior Member
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- EE
110630-1337 EDT
Following is a useful discussion on hysteresis:
http://www.lassp.cornell.edu/sethna/hysteresis/WhatIsHysteresis.html
If you have some device or process where the output does not proportionally follow the input, then you have some form of hysteresis.
For example: a comparator device may trigger at +x on a positive slope input change, and at -x on a negative slope input. See
http://www.analog.com/en/special-linear-functions/comparators/products/index.html
and the heading "Adjustable Hysteresis".
Then look at http://www.analog.com/static/imported-files/data_sheets/ADCMP580_581_582.pdf
Figure 9 shows amount of hysteresis vs setting resistance.
These two AD references may not define "hysteresis", but in the industry it has a well known meaning.
.
Following is a useful discussion on hysteresis:
http://www.lassp.cornell.edu/sethna/hysteresis/WhatIsHysteresis.html
If you have some device or process where the output does not proportionally follow the input, then you have some form of hysteresis.
For example: a comparator device may trigger at +x on a positive slope input change, and at -x on a negative slope input. See
http://www.analog.com/en/special-linear-functions/comparators/products/index.html
and the heading "Adjustable Hysteresis".
Then look at http://www.analog.com/static/imported-files/data_sheets/ADCMP580_581_582.pdf
Figure 9 shows amount of hysteresis vs setting resistance.
These two AD references may not define "hysteresis", but in the industry it has a well known meaning.
.
Electric-Light
Senior Member
So these are more like 0 centered ammeters inside trains and such that shows + for power draw, and - for regenerative braking intended to show the direction of power flow.
tallgirl
Senior Member
- Location
- Glendale, WI
- Occupation
- Controls Systems firmware engineer
No, they really are watt-hour meters. The Shark 100 is a revenue-grade, serial number traceable, kilowatt-hour meter complete with all the doo-dads and hoo-hahs that such things have. The WattNode is intended to be used as a kilowatt-hour meter, but without the stringencies required for true revenue grade metering.
Both devices include some method of measuring the power factor. The Shark 100 gives the power factor for all active phases, while the WattNode provides the power factor for each individual phase. And in both cases, as with various on-line conversations on the subject, "positive power factor" refers to energy being received, while "negative power factor" refers to energy being delivered.
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