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Archive for the ‘Inverter’ Category

 

Series vs parallel inverters

Tuesday, May 12th, 2009

Hi there.

I have a question on series and parallel inverters.

It’s my understanding that most solar panels are strung together in a series. The DC power coming off is usually high voltage and low current. The power is fed to inverter(s) which, of course, convert DC to AC.

With a series arrangement, there are dependencies between the panels which may reduce the overall performance of the system. For example, if one of the panels is slightly mismatched to the others, or one is shaded, then the overall system will operate at a corresponding lower performance. This is somewhat similar to Christmas tree lights – if one light is out, the entire string is out.

I understand that there are some parallel inverters which take DC from each panel. In this arrangement, one panel in the shade does not affect the overall performance of the system as much. Compared to series inverters, parallel inverters provide lower voltage but higher current.

Did I get this correct? Do parallel inverters work? Do solar installers give you choice of series or parallel inverters?

Thank you in advance.

Best regards,

Tom Stepien

Tom,

I’ll make it simple for you – there is no such thing as wiring inverter outputs in series. You also do not wire different groups of modules to more than one inverter input. You are mixing up how this really works. When we were finally allowed to legally do a grid-connect solar system, the inverters used were designed for higher voltage solar arrays than the battery based inverters using solar arrasy in the 12 to 48 violt DC range.

With a high voltage array, the wire sizes connecting the solar modules are much smaller and have a much lower line loss due to the high voltage. Its not unusual for these systems to have an open circuit voltage in the 400 to 450 volt DC range. Lets say you have a 4 kw array. You could wire this to a 4 kW inverter, or two 2 kW inverters each wired to the house panel which would be a parallel output arrangement.

Now if this 4 kW array was going to be used with the two inverters, you would wire half of the array, or 2 kW of array to each 2 kW inverter. It is important to understand that in this arrangement, each half of the array only has wire runs to one inverter, they cannot be wired together.

When deciding how to wire an array together, you can only wire in series the number of modules that will not exceed the high voltage input limit of the inverter. This means you may have ten modules at 100 watts in one string, in parallel with another string of 10 modules, making a total of 2 kW feeding a single inverter.

When we have more than one string, there are several things we do to avoid the balance problem you mentioned. For example, on a long east-west roof, we could wire all 10 modules of each string side by side in single rows east to west. However, if there is some side shading, we may have the last few modules in each string in the shade while the rest are in full sun, which means none of the strings will be providing very much power due to the shading of the end modules. If we had wired the 10 modules in series in a 2 X 5 arrangement on half the roof and feed one inverter, then do the same for the other half of the array and feed the second inverter, then at least one inverter will be near full power output if the other half were being shaded, since side shading in the early morning or late afternoon usually only affects half of the roof. Of course if there is no shading, then it would make little difference how we arranged the modules.

Finally, you are also correct that no two modules have the same exact voltage-current output curves, so during installation we usually try to place groups of modules on the ground facing the sun and quickly take output readings before the sun or clouds change. This cheap and dirty method will still tell you which modules have the higher and which have the lower outputs. I usually mark the backs with my readings, which makes it much easier to “balance” the separate strings when we start installing them. By balancing the strings that are in parallel, we are minimizing the balance problem you mentioned, as we want each parallel string to produce the same voltage or the inverter will not properly “track” the maximum power point of the array and we will have a lower overall solar output.

Hope this helped,

Jeff Yago

 

Inverter not charging battery bank

Sunday, May 10th, 2009

Jeff,

I have a Xantrex 2000 Charger/Inverter and 6 -12 volt, 250 amp hour batteries. I charge my Batteries from my generator and I am not connected to the Grid.

While the Inverter is charging, the volts read between 14 to 15 volts but go back down to 12 when the charging is complete. The Inverter only charges the Batteries to 12 volts and the Inverter shuts down at 10 volts, this leaves me with only 2 working volts.

How do I figure out if my Inverter/Charger is too small for my Battery Bank? Could I be missing an adjustment on the configure mode of my Inverter?

Thanks.

Mark

Mark,

As I see it, you have 2 possible reasons for this problem. You indicate the charging voltage is around 14 to 15 volts during charging which would seem to be good, but you did not say what the charging current was. For example, your inverter could be producing the right voltage output, but little or no current flow which would mean the batteries are getting little or no real charging amps going into them. My first check would be the generator and checking the amp loading of the charger.

Many low cost generators have a voltage regulator that drops the peak to peak voltage of the output waveform when more loads are added to the generator as their way of controlling overloading. Although this method of load control does not affect lights and motor driven tools, most battery chargers and inverters in charging mode will stop charging completely. These chargers are designed to use the full 169 peak to peak volts (120 VAC is the average of this voltage) of the generator. In other words, battery chargers only use the “peaks” of the voltage waveform and if these peaks are cut off or reduced, there is a major drop in charging amps output.

My second guess is the batteries. If these batteries are either old in age, or are newer, but have experienced many extreme total discharge cycles, or go long periods with only a partial re-charge, then what you describe is a classic indication of a failing battery bank. Assuming you do have a good charging process, when a battery is in the condition I just described, it will get what we call a “skim charge” and shuts down the battery charging because the battery voltage goes up real fast which tells the charger to stop charging. However, the aging battery plates usually have very little lead remaining in contact with the acid as most of the plate area is covered with calcium buildup and no longer in contact with the acid. This in effect is like making your batteries much smaller, which causes the battery voltage to rise very fast under the heavy charging, yet little real charging is taking place. As soon as the charging process stops, the battery voltage will drop almost down to a totally discharged level of around 12 volts since it was never really re-charged, and will quickly drop even lower when a load is connected, which also is what you have described.

I would not only suggest replacing these batteries, but I would switch to a deep discharge type of battery like a 6 volt golf car battery or larger L-16 battery. This will give you more amp-hours of capacity with fewer batteries wired in series. Its always better to have only 1 or 2 battery strings in parallel, as one string will always end up getting most of the charge and most of the discharge when you have 3 or more strings in series.

Good luck,

Jeff Yago

 

Inverter Sizing

Sunday, May 3rd, 2009

Hello Jeff,

I appreciate you taking the time to answer questions.

I’ve been speaking to a number of people and reading on-line regarding Solar Photovoltaic systems and find myself confused with respect to the inverter sizing that I am hoping you can help me with.

Many sites seem to size the inverter to the maximum output of the solar panels. For example, for a string of 10 200 W panels, they recommend a 2 kW inverter. But based on the isolation of our area (Eastern NY), the panels will rarely operate at more then 20% or 25% of their rated capacity (i.e. 400 – 500 W).

Should the inverter be sized to the panels rated output or to what they will actually produce?

Assuming the inverter is sized to actual power production, if there is a really bright day and the power produced exceeds the rated capacity (e.g. 600W), how will, in general, inverters react?

Appreciate you help.

Regards,

Steven

Steven,

This might surprise you but the inverter size has nothing to do with the size of the solar array or the amount of sun. First decide what electrical loads the inverter must power during a power outage. If off-grid, determine what electrical loads will be on at the same time.

For example, you have a well pump, microwave oven, refrigerator, TV, and some lights you want to power from an inverter. Normally not all of these loads will be operating at the same time, so first decide what is the most likely worst case and that will determine to inverter size. Odds are the larger loads of the well pump and microwave oven will not both be on at the same time, but the refrigerator could cycle on while the well pump is pumping. You also would not normally have every light turned on at the same time so you might only count half of the lighting load.

Since most high quality inverters can handle a very large overload for a few seconds, there will be times when the well pump or other load might kick on but only last a few seconds. This is not as critical as it seems and should not harm a quality inverter, but could occasionally trip a circuit breaker.

After you know your peak electrical load and inverter size, check the inverter specifications and this will give you the highest battery amp draw, which will help size the wiring and battery bank. If you now add how many hours per day these loads will operate, this will determine how many amp-hours will be removed from the battery bank, which in turn will tell you how many amp-hours the solar array must put back per day. Most designers use a simple spreadsheet for this calculation.

For week-end cabins, you take the total amp-hours that will be used for the weekend visit, then consider that you will have several days of solar to make up the battery drain so you could actually get by with a solar array that is smaller than the actual daily usage.

I usually figure 4 hours of sun during the winter and 6 hours during the summer which when multiplied by the array size will give you solar amp-hours per day, but I suggest multiplying this answer by 75% to account for system losses, array losses, weather losses, etc. It is very rare that any solar array will produce 100% of its nameplate rating.

Good luck,

Jeff Yago

 

Wind Power Question

Monday, April 20th, 2009

Jeff,

I saw a catalog at NorthernTool.com, and there was a ad for a wind generator 120 volts , not DC and the ad stated “Run your meter backward”. The item number in this catalog is Model #44470. It states it also comes with an inverter built in.

Why would you need a inverter for 120 volts?

Second will this piece of equipment really work as it is in this ad?

If it does work as it is in the ad what if anything else will you need to set it up and does it come under the Federal law that the power company is required to let you use it in the system as long as they are advised of the equipment being on line?

Thanks

Bill Wilson

Bill,

I saw an ad for a car that can go 120 miles per hour, but that does not mean I can drive it down the Interstate at that speed if I buy it. In other words, the ad you reference may say you can turn your meter backwards, but you cannot do it legally unless you install all of the required safety dis-connects as required by your local utility, you complete their application form, and then they replace your existing electric meter with a model that records each in and out electric flow separately. Some states have other requirements like providing the utility with a copy of your homeowners liability insurance policy and having a lockable handle exterior disconnect switch for their use.

Although this company is known for their quality wind products, I don’t like having the inverter mounted 75 feet up a pole and subject to all the weather extremes. The inverter is the weak link in most of these systems and I want a system where this is at ground level and have as few electronic components as possible up on a tall tower and in the weather.

Most small wind systems generate DC power for battery charging. What they have done is mounted a DC to AC inverter inside the wind turbine to convert the DC electricity from the turbine into 120/240 VAC which can feed back into the utility grid. However, you cannot legally just plug this into the wall outlet and “turn your meter backwards”.

On another note, for those readers wanting to do something like this, please understand that the power output from any wind turbine is “wild”, in that it jumps constantly all over the place as the wind changes direction and flow rate. You cannot use a standard DC to AC inverter with a wind turbine since the voltage and current goes to such extremes every few seconds. Most inverter manufacturers make a “modified” version of their solar inverters to work with wind turbines, and most are designed for ground mounting in a protected area.

Also note that there are very few areas of the US with enough wind to make these worth the cost and the easiest way to tell is if there are already other wind turbines nearby. Those areas of the US with lots of wind year-round are usually already covered with wind turbines. Although you might be the first in your area, you may want to check a wind resource map for your state first before paying out $6000 plus.

Good Luck,

Jeff Yago

 

DC/AC Converters

Monday, March 30th, 2009

Jeff,

Thanks for offering this service.

I am building my own wind turbine for the home. This will be a grid-tied system that will plug into an outlet to supplement the grid. Can you recommend an affordable converter? I saw the Smart Sine Wave Inverter, but that costs $300. Is there a less expensive alternative?

Thanks,

Mark

Mark,

I think its great that you want to tackle a project like building a wind turbine. If you are going to use it to charge a set of batteries for a backup power system then that would be the way to go, but you will not be able to connect it to the grid.

Just a small point, a “converter” is used to convert AC to DC, and “inverter” is used to invert DC to AC.

First, the voltage and current output from a wind turbine is all over the chart as the wind keeps changing by the minute, which requires a very special DC to AC inverter designed for this type power input, plus be certified to feed power back into the grid. The lowest cost inverter I have seen that is designed for this type application and certified for grid connection costs around $2,500.00, plus circuit breakers and wiring. Your local utility will not allow you to feed power back into the grid unless the inverter has these UL and IEEE certifications, and you will not find this level of quality in any $300 inverter..

Sounds like you are on a tight budget, so I would stick with charging batteries and powering some emergency DC lights.

I will point out that many who ask me these type questions totally ignore my advice and go on and waste a lot of their time and money to find out the hard way, so either way, good luck with your project.

Jeff Yago

 

AC Grid Tie-in

Monday, March 23rd, 2009

Jeff,

Is there a UL product out there similar to a DC grid tie in inverter that would work for an AC generator?

I am aware of how the DC is converted with an inverter to AC and the phase angle is adjusted as well as the phase voltage to maintain proper alignment of electric main power. I have seen wind generators plugged directly into a breaker in the main panel as a grid tie in and initially was confused on how this was causing a dead short. However, I understand how it is possible now (there is an inverter in the wind generator head which adjusts Phase angle and Phase voltage to allow simultaneous feeding), but, haven’t been able to find anything for AC generators.

What I am looking for is something that will go between my AC generator and the main panel that will allow me to directly connect to the panel without a dead short, and allowing back feeding to the main line. Also an automatic shut off or transfer switch that would turn off the generator’s power supply from pumping electric back into the power grid when the main power goes out.

I am looking for an AC grid tie in device that will not cause a dead short.

Your thoughts would be appreciated,

Rick Conaway

Rick,

I can’t imagine why anyone in the world would want to do what your are suggesting, although it is technically possible. Yes, there are special “in phase” monitors that will sync a generator with the grid, but this is only dome with very larger generators like you would find in hospitals or military bases. The main reason they do this is they are on a time of day rate and if they are approaching a peak demand period, by running their generators in sync with the grid they can save hundreds of thousands of dollars per year because these rates usually carry over the high penalty for the next 11 months. In addition, this switchgear to sync the generator with the grid costs more than your house and requires all kinds of coordination between the utility engineers and your installer. You will also be required to carry a huge liability insurance policy listing the utility as the insured in case something goes wrong and your power feeds back into a down line and kills a lineman.

Inverters for solar and wind systems have special circuits that make sure power is not fed back into the grid during a power outage, and the inverters must supply this power within a very narrow window of voltage and cycles. Most smaller generators are not that easy to maintain a constant output under varying loads.

Now that you know the legal issues involved with doing this, I will give you the simple reason why nobody in their right mind would ever do this:

If you count the cost of fuel, generator maintenance, repair parts, added oil changes, and annual service, you will be paid about 20% of what your actual costs will be, because you can never ever generate electricity cheaper than a utility. The only reason many of these solar and wind systems are doing this is either they are receiving some type of grant or tax credit, or they have been given a special feed-in tariff rate.

Many people still install solar systems as some systems also offer emergency backup, or at least do not need constant fueling or make noise like a generator. The national average cost today to generate electricity from a solar system is 35 cents/kWh. The national average for grid electricity is 9 cents per kWh. You figure it out.

Good luck,

Jeff Yago

Jeff,

Thanks for the insight and your knowledge on these subjects is amazing. Everyone at times has an idea and can’t figure out why the situation is the way it is; until they ask an experienced individual.

Thank you again. So if I wanted to have some sort of “economical” unit (and I use that term loosely), we would be best off using a UL device DC based unit (solar, wind, hydro) with an inverter monitoring the phase angle and voltage back to the main panel which would shut off when the main power goes out? If we wanted any sort of power when the main power goes out we would be using a battery system or a separate backup AC generator/transfer switch which would not back feed to the main power until main power comes back on. We would then turn off the generator and manually switch the transfer switch back to main power?

I guess I am in the same boat as most people trying to figure out what will work best for my individual circumstance. Oh, yeah I’m not a millionaire so the in phase monitoring for an AC generator would be out of the question.

I will probably be investing in a solar or wind powered DC unit with an inverter for my needs. Would you recommend any companies that you have had a positive experience working with for these devices. Probably, a 2KW-5KW max output would be what my financial situation will allow.

Jeff I can’t thank you enough,

Rick

Rick,

Any inverter designed for grid tie in the US should have the automatic transfer function built in. If you select a battery based inverter, it will also include a built-in transfer switch to disconnect from the grid. Some battery based inverters include a “second” transfer switch to allow also connecting a generator and the grid, and will switch to the generator when the grid is down and the battery charge is low. If you select an inverter that does not have the second transfer switch, then you will need a generator that includes its own transfer switch panel.

Good Luck,

Jeff Yago

 

Voltage and frequency limits for utility-interactive inverters

Thursday, March 12th, 2009

Hi Jeff

A problem has come up in a couple of states involving inverter voltage and frequency limits. Common inverter voltage and frequency limits are +/- 10%, and utility voltage and frequency limits are +/- 5%. This has had more than one utility thinking about limiting the number of acceptable inverters to like one or none.

I have told people that this problem is asking the tail to wag the dog, that the inverter follows the utility, not the other way around. One utility I worked with removed the voltage and frequency limits from its net metering agreement, just leaving the UL 1741 compliance requirement.

What do you think about this problem? Thanks!

David Ryan PE

Dave:

Normally, inverters control voltage and frequency much tighter than the grid, and I am not aware of any UL1741 certified inverter that can’t hold extremely tight to the grid it “sees” as this is how they control their shoving of amps back onto the grid. Normally this is more a problem with inverters when the local utility cannot hold their voltage in range in a local area. In most cases the utility is not even aware of this issue until somebody installs an inverter, since the inverter is looking for much tighter control by the grid and won’t connect. Most home appliances and lights do not require the grid to be this exact and so nobody reports a problem if it does not affect their appliances and lights. Yes, this could cause long term appliance damage but not short term.

The only time we have ever seen a problem with an inverter not connecting when it was not a problem with the grid is when the line losses between the inverter and point of connection back at the meter was so long that the voltage drop required increasing the voltage output from the inverter. However, even then, the higher inverter output voltage is usually dropped back in line with the grid voltage after passing back through this higher line loss. I have also seen problems when the homeowner or business has a very big load like an AC compressor or large well pump on a panel that was already borderline and that also was the point of connection for the inverter. Every time this large load kicks in, the inverter will see a high voltage sag and drop line. Then after waiting the required 5 minutes it will try to re-connect, and this can cycle so much the inverter will never send much solar power back on the grid.

As you know, it is against code, utility rules, and the inverter manufacturers UL listing for an installer to re-adjust these limits on an inverter, and to do this requires inputting special program key the inverter manufacturer will not provide the installer unless they submit a written form for authorization with the utility. Since this program key will only work for the specific inverter having the problem, I am not aware of any way any installer can go around and change these limits on all of their installations on a whim.

I do not think this is an issue for more utility regulation. I think if the grid can supply power within allowed tolerances to the homeowners site, and if installers install inverters mounted very close to the meter base, and select a wire sized for no more than a 1% loss, than there is no reason for this to be a problem for either party. Any unusual exception to the rule can be handled by existing permission forms that are issued only after review.

You are right that the inverter follows the grid, with the exception of a high resistance in the wire run between the inverter and the meter, and this is normally a reflection on the installers sizing of the installation. Don’t forget, the installer may be connecting the inverter to the grid at an existing sub-panel and think this couldn’t be a problem, but forgot to check that there may be a long wire run back to a main panel, that there may be another line loss back to the meter base, and all this existing wiring was installed years ago based on a 5% wire loss and have many connections that now have a high resistance causing the large voltage drop I am talking about.

Hope this helps,

Jeff Yago

 

Choosing a solar panel for maximum efficiency

Monday, March 2nd, 2009

Hello Jeff,

Putting together an off-grid system, I am inquiring as to how best choose/purchase the solar panels for optimal efficiency in relation to their voltage output. There are panels that put out similar wattage, however the vp/imp can vary greatly.

Let’s imagine for example the system objective is approximately 2kw using a 24v to 110v inverter with battery back-up.

Which would be the best solar panels to use for maximum efficiency?

* 12 – 160w panels @ 24vp/6.80imp
* 10 – 190w panels @ 26.7vp/7.12imp
* 10 – 190w panels @ 17.8vp/10.96imp
* 10 – 200w panels @ 53.8vp/3.16imp

Thanks!

Jon

Jon,

As we say on the web site, we cannot answer specific design questions because there are too many variables we would not know about your specific application, and we are trying to keep this a free service. However, I can provide some general answers that should help you narrow down your choices.

First, there are many things that go into the design process and the first is inverter input limits if grid tie, and solar charge controller input limits if battery based. In other words, if your solar charge controller has a limit of 75 volts DC input, this will require the number of solar modules wired in series to be well below this high limit. The NEC Article 690 provides more specific design guidelines related to safety multipliers that you must use.

The maximum input amp rating of the solar charge controller (or inverter if grid tie) will determine the maximum number of parallel strings and total amps from the array after the required NEC design multipliers are added.

Once you know the maximum voltage and maximum current your system can safely handle, this will define the solar array that your system can handle. Once you know this maximum total array wattage, you want to select the highest quality module you can buy that has the lowest cost per watt.

Take the total delivered price of a specific module and divide by nameplate wattage to get the $cost/watt.

If this process does not give you the solar array you want, then you will need to make changes to the inverter if grid tied, or the solar charge controller if a battery based system. Normally I first select the exact solar array wattage and physical size I want for a specific budget, then I select the inverter if grid tie, or the solar charge controller if battery based, but it sounds like you are working backwards and trying to base you design on comparing output voltages and amp ratings of the modules to fit a specific requirement.

In other words, its like taking a car and trying to replace the engine with a bigger engine to get more power. However, if you do not also upgrade the transmission, tires, drive shaft, fuel system, exhaust system, and brake system, then the vehicle cannot take advantage of the higher horsepower.

Hope this helps,

Jeff Yago

 
 


 
 

 
 
 
 
 
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