How do you force feed current to the grid from a Solar system
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Only Electrical Engineers that know the answer need to reply, and please no smoke just the facts.
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Re: How do you force feed current to the grid from a Solar system
welcome, try this explanation, http://en.wikipedia.org/wiki/Grid-tied_electrical_system
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Re: How do you force feed current to the grid from a Solar system
I do it every day, It is called a Grid Tie inverter that push the solar output to my main panel, if my consumption is less than my production it goes out to the utility transformer and is counted as production (like rolling the meter backwards) on the smart meter. I do believe the Grid Tie inverter has slightly more voltage then the grid. -
Re: How do you force feed current to the grid from a Solar system
Tell me how it pushes the current to the grid?solar_dave wrote: »I do it every day, It is called a Grid Tie inverter that push the solar output to my main panel, if my consumption is less than my production it goes out to the utility transformer and is counted as production (like rolling the meter backwards) on the smart meter. I do believe the Grid Tie inverter has slightly more voltage then the grid. -
Re: How do you force feed current to the grid from a Solar systemTell me how it pushes the current to the grid?
They explain it better than I could.The Inverter
The grid-tied inverter differs from the stand-alone variety in that the control circuit has to be able to operate in the presence of the existing grid voltage and force the grid to accept power instead of providing it. Because the grid is essentially a very low impedance voltage source, the inverter must be able to act as a current source, only allowing the desired amount of current to be sent into the grid. This process requires close control of the inverter output voltage. Generally, the inverter will control its bulk DC input voltage and use this to determine the output power level. The power level signal is then used to determine the output power, and the inverter output will be adjusted upward until this amount of power is delivered to the grid.
http://www.angelfire.com/biz/themill/hydrofiles/grid-tied.html -
Re: How do you force feed current to the grid from a Solar system
If you turned everything in the house off how much current would be sent to the grid? and at what voltage level?solar_dave wrote: » -
Re: How do you force feed current to the grid from a Solar system
Current flows from a higher voltage point to a lower voltage point as long as there's a conductor to carry it. Make the source a higher voltage than where you want the power to go and it goes.
No need to "force feed" it, although my Uncle, before he retired, farmed chickens and when asked how he force fed his chickens, he explained how he rammed a funnel down the chickens throat and poured in the chemicals. But then he wasn't an electrical engineer either.
Oh, and don't be too quick to think that only Electrical Engineers know anything about electricity. There are a lot of non-engineers on this forum who help a lot of folks on here every day, and will continue to help those who don't insult them.
We are a family.
Peace. -
Re: How do you force feed current to the grid from a Solar system
In this case the Voltage differential isn't necessary as the PV's are a current source: only Voltage matching is required (at the utility Voltage which is 240 VAC 60 Hz +/- tolerances in North America).
The amount of current that is sent depends on how much PV there is, how well-illuminated it is (insolation), the size and efficiency of the grid-tie inverter, and how much power loss there is int the system over-all.
If you check a single PV you will see that under full illumination it is able to produce its maximum current into a dead short. This is called "Isc"; Intensity (old term for current) at short circuit. The GTI converter the panel current from DC to AC and matches the power supplied to the grid specifications. Otherwise the panels "see" the grid as a resistance load, adjusted by the GTI to be 'ideal' for maximum power, and push current to it.
The systems do not care if there are load on or not: the current goes "where needed" according to the varying resistance of local loads and grid. The grid, having an extremely large power capacity, can take any power not utilized locally. -
Re: How do you force feed current to the grid from a Solar systemTell me how it pushes the current to the grid?
You could model the grid as a voltage source (the generator) and a resistance (the transmission line system.) At your end you have your own local voltage source. If your voltage source is larger than the voltage source at the generator side, current will flow back to the generator at a rate of I=(Vl-Vg)/Rt.
Or you could model it as a current source; same result, although now your voltage is set by Vdiff=Il*Rt.
In general you need a voltage difference to force a current to flow. If your grid's voltage swings from 160 to -160 during its AC cycle, and your source is synchronized and swings from 161 to -161, current will flow back to the grid. -
Re: How do you force feed current to the grid from a Solar systemThe systems do not care if there are load on or not: the current goes "where needed" according to the varying resistance of local loads and grid. The grid, having an extremely large power capacity, can take any power not utilized locally.
So, you are saying the grid will draw as much current as the inverter will allow. what determines if the loads in the house gets the current or if the grid gets the current. is he inverter a current source for both the house and the grid? or Voltage source for the house and a current source for the grid.
What is the resistance of the grid? -
Re: How do you force feed current to the grid from a Solar systemThe systems do not care if there are load on or not: the current goes "where needed" according to the varying resistance of local loads and grid. The grid, having an extremely large power capacity, can take any power not utilized locally.
So, you are saying the grid will draw as much current as the inverter will allow. what determines if the loads in the house gets the current or if the grid gets the current. is he inverter a current source for both the house and the grid? or Voltage source for the house and a current source for the grid.
What is the resistance of the grid? -
Re: How do you force feed current to the grid from a Solar systemSo, you are saying the grid will draw as much current as the inverter will allow.
However much current the GTI can produce, it all goes somewhere.what determines if the loads in the house gets the current or if the grid gets the current.
It's a matter of current balance: if the GTI is pushing 20 Amps, for example, to the wiring and the house has need of 10 Amps then 10 Amps goes to the house and the remaining 10 Amps goes to the grid, which still "appears" as a load to the GTI. If the house demands 30 Amps, all of the GTI's 20 Amp output goes to that and the grid makes up the 10 Amp difference as it is now a parallel power source to the GTI rather than a power sink.is he inverter a current source for both the house and the grid?
Yes, as per the balance described above.or Voltage source for the house and a current source for the grid.
The grid is a Voltage-based power source, the GTI is a current-based power source. So the grid will try to maintain a fixed Voltage against a varying current demand, including the "negative current" of being back-fed by the GTI. That will try to maintain its maximum current output so long as the grid is connected and the Voltage and frequency are within tolerance range. Outside of range it shuts down to prevent energizing disconnected lines (anti-islanding).What is the resistance of the grid?
Variable. You can't think of it as a fixed resistance as it is made up of both a power source which can be altered (generators coming on or going off) and a load demand which changes depending on what is turned on/off. Much of the GTI's proper functioning depends on the electrical inertia of the grid; it's resistance to change rather than resistance in Ohms, so to speak. -
Re: How do you force feed current to the grid from a Solar systemIf you turned everything in the house off how much current would be sent to the grid? and at what voltage level?
The voltage level is determined by the voltage of the utility, and by the resistance in the wiring between the inverter and the loads it serves. Roughly speaking, the voltage at the inverter will rise by the same percentage that it would drop if the inverter were a load instead of a source. The voltage will drop between the inverter and the farthest out parallel load connection that the inverter is delivering any power to. From there, it will rise again as you continuing moving out towards the utility. -
Re: How do you force feed current to the grid from a Solar systemis he inverter a current source for both the house and the grid? or Voltage source for the house and a current source for the grid.
Neither the utility's generators or the inverter know or need to know where the 'house' ends and where the 'grid' begins in order to operate. The inverter is simply connected in parallel with the utility and will feed whatever loads are likewise connected in parallel, starting with the closest along the wire path. The place where the power from the inverter leaves off and the power from the utility takes over may be inside your house or out somewhere on the grid. Which of those it is depends on the power being produced by the inverter (which in turn depends on the sun), and on the size of the loads that are connected on your side of your utility meter. From the generators' points of view (both inverter and grid), the utility meter is just an arbitrary point along the wires where the humans decided to measure something. -
Re: How do you force feed current to the grid from a Solar systemOnly Electrical Engineers that know the answer need to reply, and please no smoke just the facts.
You wanna see my BSEE diploma? -
Re: How do you force feed current to the grid from a Solar systemSo, you are saying the grid will draw as much current as the inverter will allow. what determines if the loads in the house gets the current or if the grid gets the current. is he inverter a current source for both the house and the grid? or Voltage source for the house and a current source for the grid.
What is the resistance of the grid?
The "resistance" of the grid is irrelevant; the output of the inverter is what it is irrespective of the grid impedance; such is the nature of a current source.
It's often a hard concept to grasp because virtually all the supplies of power that we encounter are voltage sources. Batteries and AC power from wall outlets are voltage sources; the voltage stays the same and the current drawn is dependent on the impedance of the load.
Grid tied inverters are not voltage sources, they are current sources. They export the same amount of current irrespective of the load, which is why they have to be connected to the grid to keep running; the grid clamps the voltage at line voltage and provides a virtually infinite current sink that can absorb all that the inverter can export irrespective of your local loads. -
Re: How do you force feed current to the grid from a Solar system
Another way to look at the grid is as a giant AC Battery Bank.
And your GT Inverter is just like a car alternator (plus rectifier).
The grid supplies current to your AC loads (and sets the local voltage 120/240 VAC). And as the GT inverter starts to "inject current" into the local home wiring, the current is either used by the local loads. As the GT output exceeds the local current needs, then, for all the world, it looks like the GT inverter starts to "recharge" the AC Utility power.
There is no "switching" or fancy power control needed... The system works pretty much like your car's electrical system. From a systems point of view--It is the battery that sets the voltage of the car's electrical system. The alternator/generator output varies on engine speed (or when engine is off, the battery supplies all power).... And as the alternator/engine spins faster, there is more available current to support all of the loads, plus recharge the battery bank (of course, the grid is "infinite" is size, so there is no charging voltage to control).
The typical GT Inverter is a "current source" that simple injects a sine wave current in phase with the AC voltage with the current equal to the Array Wattage (available sun) divided by the AC line voltage (with ~5% losses).
If you are familiar with larger emergency/backup AC generator installations (hospitals, etc.), the Current Injection is quite a different control loop in the GT inverter vs Synchronous AC power support as performed by AC Generators (really alternators).
AC Generators need to "sync" with the AC Line Frequency, then transfer over to the AC mains (if not in sync--You can cause a huge amount of damage to the gensets and AC mains). The AC generator now turns in sync with the AC frequency (1,800 or 3,600 RPM, or other integer frequencies) and to increase power output, more torque is applied to the genset (increase throttle). The AC genset then outputs more current to the AC mains and carries local loads/feeds back energy to the AC grid (and/or synchronizes with other paralleled AC generators).
The AC GT Inverter cannot support AC loads without an AC power source present.
Of course, there are many options out there, and many ways to accomplish ways of transferring power--It is a complex question/answer. And there are "different" products/methods out there that can be used.
Do you have specific question/application you are looking to address?
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How do you force feed current to the grid from a Solar systemDo you have specific question/application you are looking to address?
-Bill
I think his question was just what a lot of people wonder - how do the electrons from my PV system know where to go in order to preferentially service my loads? -
Re: How do you force feed current to the grid from a Solar system
The AC GT Inverter cannot support AC loads without an AC power source present.
So, if I have a 3000 watt Grid tie inverter and the grid is off I can't use the 3000 watts to supply current to any loads in the house.
Thanks ggunn and BB I think the picture is starting to come into focus. Where could one get a Schematic of a GT Inverter?
Do you have specific question/application you are looking to address?
We have a small 2 wheel enclosed trailer with the following items installed and used for a portable power source. 4 130 watt solar panels, outback FX 60 charge controller, xantrex pro sine 3 inverter, 8-6 volt wet cell batteries and a 4 KW Genset. We also just purchased 10, 235 watt solar panels and not sure if we want to do the grid tie or off grid system using the 10 panels which will determine what additional Equipment I will need to buy. We have an all Electric home 3ooo sq ft electric bill average about $300 per month. I am leaning toward the off grid side. -
Re: How do you force feed current to the grid from a Solar systemThe AC GT Inverter cannot support AC loads without an AC power source present.
So, if I have a 3000 watt Grid tie inverter and the grid is off I can't use the 3000 watts to supply current to any loads in the house.
That is pretty much correct... The typical or standard Grid Tied AC inverter (or also called Grid Interactive Inverter) just measures the AC line voltage and frequency. If they are within specifications, the GT inverter "injects" current into the AC mains and the energy flows to where ever it is needed. Just like your car's alternator will inject current into the cars wiring... The loads will consume power, and the battery will either absorb or supply current to keep the voltage at ~12.7 volts or so.
But, to confuse the issue a bit more, SMA now has a new GT inverter that has a smallish (something like 10 amps at 120 VAC?) outlet that can run small AC appliances from sunlight. Intended to supply AC power in an emergency. It does have its limitations (cannot supply power unless the sun is up and shining ~9am to 3pm) and if a bird flies overhead or clouds come by, the AC power will "go away" until the sun returns (GT inverter outlet may need to be reset too--You usually don't want your AC appliances to have AC power cycling to them or "browning out" from voltage sags).Thanks ggunn and BB I think the picture is starting to come into focus. Where could one get a Schematic of a GT Inverter?
You will not usually find a schematic for the Inverters--The companies want to keep that stuff relatively controlled... They make money from servicing their products and don't want a cheap mfg to make knockoffs of their products.
A GT Inverter is going to look sort of like a typical AC off grid inverter. Much of the way a GT inverter behaves is based on the control/feedback circuitry. And that is not going to be "obvious" from just having a schematic (control theory in engineering is a huge/complex subject with lots of math--it is also very interesting).
To throw even more into the mix, there are newer GT Inverters that do not use isolation transformers. This cuts down on costs and weight (less iron and copper in the GT inverters). The variations and solutions out there and how engineers/companies choose to address those markets is quite amazing (at least to me).Do you have specific question/application you are looking to address?
We have a small 2 wheel enclosed trailer with the following items installed and used for a portable power source. 4 130 watt solar panels, outback FX 60 charge controller, xantrex pro sine 3 inverter, 8-6 volt wet cell batteries and a 4 KW Genset. We also just purchased 10, 235 watt solar panels and not sure if we want to do the grid tie or off grid system using the 10 panels which will determine what additional Equipment I will need to buy. We have an all Electric home 3ooo sq ft electric bill average about $300 per month. I am leaning toward the off grid side.
Flat out, for costs and maintenance issues, Grid Tied is the way to go. Your true cost of power ($$$/kWH) with a GT System can meet or even beat the cost of electricity in states with high electrical costs (in summer afternoons, people that use >~1,000 kWH per month can pay upwards of $0.50 per kWH, a GT system can "make electricity" for ~$0.15 to $0.30 per kWH).
HOWEVER--There can be issues with the electric utilities... Many smaller utilities will not allow GT solar (illegal to install). And some (becoming more and more) are changing their rate structures... For me, I pay ~$4.50 per month and $0.10 to $0.30 per kWH, depending on time of day and season. And I get "full credit" if I generate kWH to the grid (i.e., I usually generate power at $0.30 per kWH and buy power at night at $0.10 per kWH--Makes my array look ~3x larger on summer weekday afternoons at peak time of use rates).
Now, utilities are starting to see losses from distributed solar power--And some have now changed the rates to something like $40 to $100 per month fixed charges and $0.06 per kWH for energy usage. In those cases, solar GT and even conservation may not make much sense (i.e., I conserve, I save $0.10-$0.30 or more per kWH--The new rate plan only saves $0.6 per kWH and leaves the fixed charged dominating the billing costs).
Which then makes you look at off grid power.... Normally, if you include installation+capital costs+battery/electric replacements (every ~10+ years or so), the cost of off grid power is in the range of ~$1-$2+ per kWH--Still 10x what utility power costs. Some folks have gotten their off grid power costs down to ~$0.50 or so per kWH--but that takes work+shopping around to get those prices).
Normally, I would suggest conservation first... It is almost always cheaper to conserve than to generate power. Insulation, double pane windows, modern A/C and/or Heat pump systems (even heat pump based water heating) can be very cost effective and even make the home a bit quieter/less drafty to live in.
And look at your utility's rate plans... Understand how they bill for power (low per month fixed charges, high per kWH charges; or the opposite) and if you can even connect grid tied solar.
In my region (California), it is actually illegal to install Off Grid Solar power for my home and leave the grid unless I pay the utility for the right to leave the grid (stranding charges--Utility took out 40 year loads "in my name" for distribution/transformer/power plants/ etc. and I need to "reimburse" the utility for their long term costs--at least that is how I understand the justification).
In general, I don't see GT or off grid solar power systems as an "investment"... The are just one of those things that you have to pay for if you want the power (and/or emergency backup power)--Just like buying a car is not an investment--Just a cost of living the lifestyle we choose.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: How do you force feed current to the grid from a Solar system
When it comes to getting a schematic for a GTI you would have to mean a specific unit as they are not all the same. And honestly if you have any need of a schematic for one you'd better have one of those BSEE's like ggunn has first. -
Re: How do you force feed current to the grid from a Solar systemThanks ggunn and BB I think the picture is starting to come into focus. Where could one get a Schematic of a GT Inverter?
If you are just talking about a one line block diagram that shows the guzintas and guzoutas and what sections of the inverter feed each other, many inverter manuals will have something like that. A full schematic is really hard to get and would be pretty much unintelligible to most people. -
Re: How do you force feed current to the grid from a Solar systemCariboocoot wrote: »When it comes to getting a schematic for a GTI you would have to mean a specific unit as they are not all the same. And honestly if you have any need of a schematic for one you'd better have one of those BSEE's like ggunn has first.
Navy Electrician’s school Commercial Radio Telephone License with a Radar endorsement diploma’s from 5 electronics schools after leaving the Navy been to several classes for various pieces of equipment machines using electronic controls worked for 30.5 years IBEW Industrial Electrician repairing electronic equipment. I have a good background in Electrical and Electronics But I still have a lot to learn that’s why I came here. I don’t want opinions or sales pitch information from anyone that has not had some formal training, and I think the last few post have been from poster with the knowledge that I am looking for. Sorry if I offended anyone -
Re: How do you force feed current to the grid from a Solar system
Navy Electrician’s school Commercial Radio Telephone License with a Radar endorsement diploma’s from 5 electronics schools after leaving the Navy been to several classes for various pieces of equipment machines using electronic controls worked for 30.5 years IBEW Industrial Electrician repairing electronic equipment. I have a good background in Electrical and Electronics But I still have a lot to learn that’s why I came here. I don’t want opinions or sales pitch information from anyone that has not had some formal training, and I think the last few post have been from poster with the knowledge that I am looking for. Sorry if I offended anyone
Not offended, but it would maybe clear up how they do what they do, I was not thinking of modifing the InverterNo offense intended, but what would you do with it? Even if you were an EE (I'm assuming you aren't), you wouldn't have much if any success in modifying an inverter to make it do something it's not designed to do. I certainly would not attempt something like that.
If you are just talking about a one line block diagram that shows the guzintas and guzoutas and what sections of the inverter feed each other, many inverter manuals will have something like that. A full schematic is really hard to get and would be pretty much unintelligible to most people. -
Re: How do you force feed current to the grid from a Solar system
We're just making sure. Sometimes it is wise to err on the side of caution and warn people about getting in to these things without the proper background knowledge.
As for me, I've already shredded all my qualifications on account of all the "Internet experts" out there who know more. <--- joke. -
Re: How do you force feed current to the grid from a Solar systemNavy Electrician’s school Commercial Radio Telephone License with a Radar endorsement diploma’s from 5 electronics schools after leaving the Navy been to several classes for various pieces of equipment machines using electronic controls worked for 30.5 years IBEW Industrial Electrician repairing electronic equipment. I have a good background in Electrical and Electronics But I still have a lot to learn that’s why I came here. I don’t want opinions or sales pitch information from anyone that has not had some formal training, and I think the last few post have been from poster with the knowledge that I am looking for. Sorry if I offended anyone
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Re: How do you force feed current to the grid from a Solar system
I will add that there are in fact several electrical engineers on the forum, including some involved in the design of inverters. However it is entirely their choice as to how much they wish to participate on any given thread: we are not going to point out anyone's qualifications. -
Re: How do you force feed current to the grid from a Solar systemCariboocoot wrote: »I will add that there are in fact several electrical engineers on the forum, including some involved in the design of inverters. However it is entirely their choice as to how much they wish to participate on any given thread: we are not going to point out anyone's qualifications.
I understand I visit several Forums and without knowing the poster's qualifications You don't know if you are getting good or bad information, but after a few post you can usually tell, if you haven't electrocuted yourself or destroyed something based on the bad information that was posted. It would be nice if a poster would state his or her qualification when making a post to give it some creditability.
I do appreciate good information and I thank those that have given it here on the forum, and I am sure I will have more questions later for those qualified to give the answers. -
Re: How do you force feed current to the grid from a Solar systemYou don't know if you are getting good or bad information, but after a few post you can usually tell, if you haven't electrocuted yourself or destroyed something based on the bad information that was posted. It would be nice if a poster would state his or her qualification when making a post to give it some creditability.
I cannot agree with that. You always can hire someone with "credibility" to do a job or consultation for you. Internet is not a place where cheapskates can get professional advise for free. On the Internet, you will find multitude of opinions which will help you learn something and bring you to the point where you can make decisions on your own. -
Re: How do you force feed current to the grid from a Solar system
If you hang around for a while and read lots of the posts you will be able to tell who the 'fielded' posters are and also realize when you are given good advice for making a reasoned decision. Others are not 'book learned' but have a wealth of hands-on give advice freely of their experiences, good and bad.
If any advice is not good, one or more members will say what their opinion is; sometimes (usually) we even agree to disagree but civilly.
KID #51B 4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada -
Re: How do you force feed current to the grid from a Solar systemI cannot agree with that. You always can hire someone with "credibility" to do a job or consultation for you. Internet is not a place where cheapskates can get professional advise for free. On the Internet, you will find multitude of opinions which will help you learn something and bring you to the point where you can make decisions on your own.
With bad information you only learn to make another mistake, but I guess we do learn from mistakes. I was not asking for advice or how to put a system together. I just wanted to know how the grid tie inverter sent the current to the grid. I would not perform any task based on information obtained from a forum. Am I a cheapskate maybe? I understand you not wanting to give out any free professional advice.
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