But watts is *not* volts times amps, in an AC circuit. There is a power factor in there to worry about. In the capacitor example, watts dissipated is zero (or close to it) but VA might be rather high.
Correct. Ohms Law.
On 4/12/2011 9:06 PM snipped-for-privacy@att.bizzzzzzzzzzzz spake thus:
That is *not* Ohm's Law. Where do you get that? Sheesh--you're trying to lecture *me* on this stuff???
*-- bud--
----------------- |Perhaps re-read ( or just read ) the last few posts. Your objection is |mostly agreement with items already covered.
Perhaps you should take reading lessons. Maybe you and harry could get group rates.
- You said "considering the street transformer as an infinite current supply" which no one does.
- As a result your calculation is meaningless.
- You said Canadian house panels were protected with fuses. I disagree. Perhaps a cite?
- You said "any approved O/C device in a panel these days is rated at
100kA". I asked for a cite - still missing.- Daestrom said adding PV systems to residences could result in an available fault current larger than the rating of existing service panels. It is certainly an interesting point, but not likely for reasons stated.
I did agree with daestrom that most US house panels are likely to have a
10kA IR.|Can you cite the percent impedance of the transformers
5% impedance would be common| or the code rules you discuss?
I didn't discuss code rules.
Your 'newsreader' is incompetent at treating sigs.
If you are talking about normal load ratings - what a useful revelation. I am sure no one had any idea...
There is a limit on the normal current for a transformer? I had no idea....
But heating is not a limit on fault current (which my post was almost entirely about).
Earth is not calculated because it is such a poor conductor. It may be necessary in some of the screwier UK electrical systems with an RCD main.
With minimal reading ability it is obvious that daestrom, mII (or whoever) and I talked about the fault current ratings of circuit breakers or fuses. Or did you think that houses have 10,000A services?
I worked them out 40 years ago then worked with them the last 40 years.
You really should learn to read and think. Maybe when cows fly....
Then you should look back and see that is precisely the issue being discussed here. You wrote, "But still - you can't push more electricity onto a network than the load is asking for (given that your invertors are functioning correctly I guess)."
David wrote, "That second statement is correct: you can't "push" electrons into the grid. But it doesn't matter *how* your inverters are working; it's a basic law of physics."
I have explained in simple terms (without getting into power factors, phase shifting, pulse width modulation, etc) the physics behind how you can push your power onto the grid whether it is asking for it or not.
You do not raise it up as high as you can, you raise it just enough to do the job. The ignition coil in your car raises the 12 volts of its battery to many thousands of volts to force a spark across the gap of its sparkplugs. You don't need thousands of volts to feed power into the grid.
Go back to the example I provided using two batteries. As I said, when they are connected in parallel the 12 volt battery charges the 11 volt battery and the voltage across them will measure somewhere between 11 and 12 volts. Connect a light to the batteries. You will measure a slight drop in the voltage but it will still be over 11 volts. That means the 11 volt battery is still being charged and all the power to light the light and charge the 11 volt battery is coming from the 12 volt battery. Connect more lights (load) to the batteries and you can drag the voltage down so that it is just over 11 volts. So long as the voltage across the two batteries is higher than the stand alone voltage of the 11 volt battery all the current going through the lights will be coming from the 12 volt battery. And it doesn't matter that the 12 volt battery has been dragged down to within a small fraction of a volt over the 11 volt battery, the lights see 11+ volts. Can you see now how the inverter can pump its power into the system? By having its voltage just a bit higher than the transformer, but well within the normal range of the line voltage, it can take over feeding the local water heaters, cooking stoves, air conditioners, lights, etc. No additional controlls are needed to reduce the current coming from the transformer. The voltage difference takes care of it.
As you saw above the current flow through the transformer will be reduced. The substation sees that as a reduced load and will behave the same way it would any other time the load goes away. No additional controls are needed.
Yes, you have said enough to make it clear how little you know.
When an electric motor is running it produces a back EMF that counters the flow of the current.
With resistance heaters the higher voltage flows more current, so you get more heat, which is what you wanted anyway.
I doubt that the regional sub-station is going to do that.
I didn't say that it couldn't be handled.
I'm saying that a small-scale PV system is going to raise the local grid voltage for the homes connected to the same step-down distribution transformer. All the linear loads on the local grid will consume the extra power (probably about 250 to 500 watts per home, including the house with the PV system on the roof). The extra 250 to 500 watts will be divided up between the various AC motors (AC and fridge compressors, vent fans) and lights. They don't need the extra volt or two rise on their power line supply - the motors won't turn any faster and the lights will just convert those extra watts into heat more than light output.
The home owner with the PV system will get paid 80 cents / kwh for the
40-odd amps he's pushing out into the grid, but that energy will be wasted as it's converted disproportionately into heat - not useful work- by the linear loads on the local grid.
I don't see a rabbit around here.
I'm not claiming that pushing current into the local grid by way of raising the local grid voltage doesn't work.
I'm claiming that there won't be a corresponding voltage down-regulation at the level of the neighborhood distribution transformer to make the effort worth while for all stake holders.
Do you think they need one more thing they don't understand? ;-)
E=IR, certainly *IS* Ohm's law. I and E are proportional. You can't increase I without increasing E. Get it? I suppose not.
Wrong. You CAN increase I without increasing E. You have 3 variables in that formula, not just 2.
Apparently not.
Vaughn
>
From Wikipedia: "In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line."
Obviously when a local area is supplying power to the grid, power generation elsewhere will be reduced. And any voltage changes that results from that will be adjusted with line voltage regulators, if necessary.
How do you get the value 250-500W?
Motors will only increase their energy drain by raising the frequency, Plus a small loss due to internal resistance in the windings.
As for a resistive load, increasing the voltage from 120 to 125 volt will result in a power drain increase of about 8.5% or 8.5 W for a 100W light bulb, assuming 120V is the nominal voltage.
Remember though that the voltage increase on the step-down side of the transformer due to homeowners PV arrays will be less than 5 volt pretty much guaranteed. Local codes state a maximum voltage drop (7V in BC) over the lines to a house, at 80% load of service panel capacity.
Most households have a 200A service panel. A 10kW PV array is well below the service panel capacity.
And you cannot just look at the PV array output. You must take into account the local energy consumers as well. That will reduce the current going into the grid, and thus the voltage increase.
You claims are pretty vague, please explain what you mean by wasted.
By the way, there is some "waste" by just using the grid only as well. Losses everywhere in the grid.
What is your definition of worth while? And what do you know about the utility's voltage regulation policies?
The utilities _have_ to use voltage regulation due to demand changes.
Oh but there is. Look around and you will see an occasional unit that looks like a pole pig (transformer) with no low voltage wires attached, just the high voltage ones. This is either a line reactor of some kind (inductor or capacitor) or an on-load tap changer, which is an autotransformer that automatically changes winding taps as needed to maintain the output voltage within the 5 or 10% of spec.
If the load on it is say 50KVA and you switch on a GTI with a perfect 1.0 power factor and outputting 25KW the load on that transformer will drop to
25KVA, since all wiring has some resistance this will cause the voltage to rise the same as if half of the load were switched off, the tap changer responds after a delay period by changing taps which lowers the output voltage back down.
Dumbass, it's a fixed circuit.
You've only proved that you're just as stupid as David.
One? Complex numbers would be a start, but the list is apparently endless.
Plug a Kill-A-Watt (etc) voltmeter into an outlet and then switch on a load, like a heater or iron. See what the voltage drop is for that current. You can measure the drop at the breaker box by metering an outlet on a different breaker on the same side of the line. This will show you approximately how much your grid voltage changes with current, from or to the grid.
jsw
Names? Didn't your mother tell you how babyish that is?
You never defined the circuit, except perhaps in your own mind. I was responding to your statement about E=IR.
And you have proven yourself as a troll.
Bye Vaughn
I think he gets it directly from Ohms Law. V=3DIR.
Or, I =3D V/R
If V, the voltage is zero, then I, the current must be zero. Or, in other words, current will only flow if there is a difference in voltage.
EFFECTS OF PHOTOVOLTAICS ON DISTRIBUTION SYSTEM VOLTAGE REGULATION
Peter McNute, Josh Hambrick, and Mike Keesee
National Renewable Energy Laboratory, Golden, Colorado Sacramento Municipal Utility District, Sacramento, California
ABSTRACT
As grid-integrated photovoltaic (PV) systems become more prevalent, utilities want to determine if, and at what point, PV systems might begin to negatively impact the voltage regulation of their distribution systems. This paper will briefly describe voltage regulation methods in a utility distribution system. It will also take a preliminary look at the distribution impacts of high penetrations of grid-integrated PV systems being installed and operated in a Sacramento Municipal Utility District community. In particular, the issue of excessive service voltage and excessive substation voltage due to the reverse power flow from exporting PV systems will be examined. We will also compare measured data against modeled data.
Index Terms - Photovoltaic (PV), Distribution System, Voltage Regulation
INTRODUCTION
A three-year, joint project between the National Renewable Energy Laboratory (NREL) and Sacramento Municipal Utility District (SMUD) began in March 2008 to analyze the distribution impacts of high penetrations of grid-integrated renewable energy systems, specifically PV-equipped SolarSmartSM Homes found in the Anatolia III Residential Community (hereafter referred to as Anatolia) in Rancho Cordova, California. SolarSmart Homes combine high-efficiency features along with rooftop-integrated 2.0-kWac PV systems with no energy storage. When completely built out, Anatolia will have 795 homes, 600 of which will be SolarSmart, eventually amounting to 1.2 MWac potential generation. (So far, only 115 homes have been built for 238 kWac potential generation.) In particular we are investigating if there will be excessive service voltage or substation voltage due to reverse power flow from exporting PV systems.
VOLTAGE REGULATION METHODS
The primary distribution voltage needs to remain within ANSI C84.1 limits: 114 V to 126 V or 11.4 kV to 12.6 kV, over the length of the feeder. The service voltage is provided to the customer meter and includes any voltage drop from the primary service through the distribution transformer and service conductors. The electric supply system is designed so that the voltage is within the 110 V to 126 V range (Range A) most of the time and infrequently within the 106 V to
127 V range (Range B). The utilization voltage is the operating voltage system and loads are designed to operate within. During heavy loading, SMUD may adjust the substation voltage as high as 12.75 kV to account for the voltage drop at the end of the feeder.Load Tap Changer
A load tap changer (LTC) effectively varies the transformer turns ratio to maintain the transformer secondary voltage at the substation as primary-voltage changes occur due to changes in loading of the transmission system, or as the load on the transformer itself varies. The Anatolia substation voltage is automatically controlled by a LTC on the secondary of the substation transformer, managed by a voltage regulating relay. The band center is 123 V (12.3 kV lineto- line at the substation). The bandwidth is +/- 1.5 V (or 3.0 V) total. The time delay for adjustments is 60 seconds. The LTC compensates for added load by increasing the band center linearly from 123 volts at no load to 126 volts at full load (about 20 MVA or 1,200 A).
Capacitors
Capacitors are reactive power sources that affect the voltage by supplying leading reactive current that compensates for the lagging reactive current of the load. The Anatolia substation has six three-phase, 1,800kVAR capacitor banks (hereafter referred to as capacitors). The capacitors are computer controlled using an algorithm developed by SMUD called Capcon. During normal operations, one capacitor is turned on each time the VAR flow exceeds 900 kVAR (from the substation bank), and a capacitor is switched off each time the VAR flow exceeds -1,200 kVAR. These set points are modified on days hotter than
100oF, but this setting can be manually overridden depending upon weather forecasts. These settings automatically adjust for abnormally high or low bulk system voltages.SMUD ANATOLIA III OVERVIEW
Distribution System Description
Anatolia is a residential community served by individual single-phase lateral circuits from a three-phase primary feeder which connects to a
20 MVA, 69 kV/12.47 kV delta-wye transformer at the Anatolia-Chrysanthy Substation (hereafter referred to as Anatolia-Chrysanthy or substation). The feeder has several points along the line that allow for switching. The length of the feeder to the switching cubicle furthest from the substation is about 18,895 feet and the furthest distribution transformer, 5K7, is about 22,360 feet from the substation. In Anatolia, there are a total of 85 singlephase pad-mounted distribution transformers: 23 are 75 kVA and 62 are 50 kVA. Also connected to the same substation feeder is a rendering plant fed by two 1,500kVA transformers, consuming between 20 and 200 kW, a water storage plant, and a residential community with an estimated 1,000 homes between the substation and Anatolia. There are two additional feeders that also connect to the 20-MVA transformer that have an additional estimated 1,000 customer loads each.SolarSmart Homes Description
SolarSmart Homes are advertised as able to reduce annual residential electric bills by 60%. They combine cost-effective, energy-efficient features and a rooftop PV system. Typical SolarSmart Home features include: radiant barriers to reflect summer heat that would otherwise enter the attic and cause greater need for air conditioning;
90%-efficient furnaces; 14 SEER / 12 EER HVAC systems; compact-fluorescent lighting; ENERGY STAR-qualified windows; and independent third-party verification, required to confirm all energy-efficiency measures are installed and operate correctly.A 2.0-kWac PV system can generate a major portion of the electrical energy consumed in the home. The majority of the Anatolia PV systems comprise 36 SunPower 63-watt SunTile roof-integrated modules feeding into a SunPower SPR-2800x positive-ground, grid-connected inverter. The inverter can be remotely accessed by SunPower if the homeowner elects to connect it via the internet. The orientation of the PV systems ranges from southeast to southwest.
MONITORING
The distribution system is monitored at the substation feeder using a DAQ Electronics ART-073-79-0 Supervisory Control and Data Acquisition (SCADA). The SCADA is ANSI-class high-accuracy, better than 0.5%. The sampling rate is every two seconds with an average recorded every five minutes. Four distribution transformers are monitored using PMI Eagle
440 monitors having an accuracy better than 1.0%. The sampling rate is 256 samples per cycle with an average recorded every five minutes. Four homes are monitored at the service panel using PMI iVS-2SX+ power monitors having an accuracy better than 1.0%. The sampling rate is 128 samples per cycle with an average recorded every five minutes.Solar and metrological data are also collected from a solar/meteorological station installed at the substation. One-minute-interval data from the solar/meteorological station are collected automatically daily.
ANATOLIA RESULTS
SMUD wanted to investigate what effect reverse power flow from exporting PV systems would have on service and substation voltage regulation. Figure 1 shows the Home-to-Substation voltage difference (top) and solar irradiance (bottom) on a clear, cool day, Saturday, March 7, 2009, representative of a day with relatively low load and high local PV penetration. Penetration is defined as the amount of PV output divided by the load at a particular point in time.
At night the substation voltage ranged between 0.4 V to 0.7 V higher than the home voltage. This is representative of a typical circuit with voltage drops through the line and transformer impedances. During daylight hours, this reversed and the home voltage rose as high as 0.7 V, or 0.6%, greater than the substation voltage. The voltage amplitude was 124.5 V at its peak, so it remained well within ANSI C84.1 limits. Due to heavy home loads in the morning, the PV system did not begin to export to the distribution system until almost noon. In March 2009, there was a 2.0-kWac PV system on the home, 30.1 kWac of PV on the transformer (twelve SolarSmart Homes), and 238 kWac of PV on the distribution system (115 SolarSmart Homes). Preliminary estimates of PV penetration levels on the feeder were 11% to 13% under lightly-loaded conditions (2.0 MW) and around 4.0% of the total substation transformer load. Since the PV penetration levels are still relatively low, there were no adverse effects on voltage regulation.
Modelling Anatolia
To measure the effects of PV on voltage regulation, a Distributed Engineering Workstation (DEW) model was created that included the distribution transformers, secondary, and service connections. Once validated using measured field data, the model will be used to determine acceptable levels of PV penetration.
The feeder contains significant loads which are not part of the system under evaluation including a rendering plant, water storage facility and approximately 1,000 residential customers. Additionally, the substation transformer and LTC are shared by two unmonitored feeders.
For initial testing and model verification, the unknown residential loads were represented as a lumped spot load. Eventually, the residential loads will either be calculated from load research statistics or distributed based on distribution transformer size. The rendering plant load was represented as a spot load based on recorded data.
The load on the other feeders was modeled using a spot load placed immediately after the LTC. Since the additional feeders share the LTC, the load on those feeders will affect the position of the tap as well as the regulation set-point. While the load on the other feeders will not greatly affect the voltage profile of the system under study, these loads may affect the voltage regulation.
To verify the topology of the model and to ensure the switching devices are represented in their correct states, the electrical distances from the substation of the measurement points were compared against the values estimated from a GIS map. Table 1 describes this comparison. All modeled electrical distances are within 3.5% of the estimated distances as determined from GIS drawings.
Next, the behavior of the secondary-side of the distribution transformer was validated against real data. The voltage rise measured between the secondary of the distribution transformers and the service entry to the homes will be affected by the loads and generation of the other, unmonitored homes that share the secondary connection.
Figure 2 shows the simulated and measured voltage rise from Home 3 to the secondary of Transformer 3 (8K6). The data reflects roughly a month of data where Home 3 is net exporting real power with an inductive load between 0.18 kVAR and 0.22 kVAR. The secondary system was simulated assuming uniform generation from all customers connected to the secondary of the transformer. The generation from the homes was increased while maintaining a constant inductive load of 0.2 kVAR. The homes were modeled as constant current loads.
The variation in measured voltage rise data is largely due to uncertainties with the other customers sharing the secondary connection as well as the resolution of the voltage measurement device at the home. Figure 2 indicates that the model reasonably reflects the behavior of the actual system. With improved monitoring and meter accuracy, the model could be better validated and, if necessary, adjusted to more accurately reflect the system.
Currently, there are too many uncertainties on the circuit to perform any meaningful whole system validation. Efforts are under way to increase the monitoring on the system so that the overall model may be better validated. This includes adding meters at critical points to eliminate many of the unknown loads described above.
CONCLUSIONS
After one year of monitoring the Anatolia SolarSmart Homes Community, there was no excessive service or substation voltage due to reverse power flow from exporting PV systems. Preliminary estimates of PV penetration levels on the feeder were 11% to 13% under lightly-loaded conditions (2.0 MW) and around 4.0% of the total substation transformer load. Since the PV penetration levels were relatively low, there were no adverse effects on voltage regulation. It was possible to see the effects of the PV systems on the voltage at the individual homes and the distribution transformers. This slight voltage rise was approximately
0.6% on clear days in comparison to the normal drop of -0.6% without the PV exporting. The DEW model that has been developed reasonably reflects the behavior of the actual system. The model will be used to determine acceptable levels of PV penetration.================
Photovoltaic Specialists Conference (PVSC)
2009 34th IEEE Issue Date: 7-12 June 2009 On page(s): 001914 - 001917 Date of Current Version: 17 February 2010
They have stated that the servo-loop stability of grid voltage regulation -may- become questionable at over 20% uncontrolled solar input.
For once I agree with Harry. We can forget about generators and distributions systems. Just take two 12V batteries and connect them in parallel to a 12ohm resistor. Under Homeguy's theories, I don;t know what he thinks would happen. But clearly he thinks if we put a second AC power source on a distributions system, it has to be at a higher voltage to "push" current out.
So, what happens with the two batteries? Under the laws of physics the rest of us use the voltage would remain at
12 volts and BOTH batteries would be supplying part of the 1 AMP flowing through the resistors.
You are claiming that any electricity produced by PV arrays that goes onto the local grid just gets wasted because putting it on the grid raises the voltage a tiny amount. I think that's what he meant by saying "it doesn't work". That is you're saying that PV arrays that have net current flowing into the grid don't work, because the energy somehow just gets dissapears.
There is SO much wrong in your analysis, that I don't know where to begin. But here's a start. You claim that with a slightly higher voltage, an AC motor in an HVAC compressor won't turn any faster and hence the additional power is wasted. What you've completely overlooked is that power is P=3DVI, or power is voltage times current. Give that motor an extra half a volt and I'll bet it's current decreases by a corresponding amount.
As Bud said a while back, you're new analysis must be devastating to all the power companies in the world.
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