Inverter "ugly AC"?

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lbwireman

lbwireman

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Long Beach, CA
With the recent passage of California's Solar Initiative, we have been getting a lot of calls from customers who want to install PV systems. For this reason, we've been "crackin' the books" and "burnin' the midnight oil" to come current with the latest tech in this area. So, I couldn't help but notice Mike's (iwire's) comment on another thread re: the quality of inverter created power.

"I think that once you start looking at inverters that actually put out a true AC sine wave the motor generator sets start looking much better cost wise.

Many inverters put out ugly AC."

The "big 3" in the PV world are SMA America's "Sunny Boy" line, Fronius and Xantrex. If you see this post, Mike, I'd appreciate hearing your thoughts on these products and the quality of the power they produce. Especially with regard to their effects on sensitive electronics, etc. Actually, I'd welcome anyone's thoughts, knowledge, opinions, etc on this topic. Thanks,
Sean
 
The older-style inverters, which oscillated at 60 Hz, put out a square wave. The newer ones put out what is called a modified square wave. Square waves are notoriously noisy. See more here.


z_waveform.gif
 
got_nailed said:
I own 6 outback inverters and I’m living off the grid. I have had no problems with dirty noise in the AC.
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This is where the expression, "DUH!", comes into play. You have true sine wave inverters. That's high-dollar equipment.
 
Thank you mdshunk. So you have solved the problem of the.

And outback dose have grid tie inverters that will work with the California's Solar Initiative.
 
My understanding of these grid-interactive inverters is they produce much cleaner sine waves than your sundry off line generator, because they must bee frequency agile to align with the grid. In fact, using a local mechanical generator to drive the grid is a recipe for disaster. The grid-interactive inverters I'm aware of operate like electronic power factor correctors, but bidirectionally, supplying current in phase with the line frequency voltage, or slightly leading. I expect you will be "boning up" on this subject for a long time to come because this stuff is in the infancy stage. There is discussion about local interactivity with the grid using signals on the power line to prevent power islanding and to request the inverters to provide leading or lagging current to the grid.
 
Twodollar,
In fact, using a local mechanical generator to drive the grid is a recipe for disaster.
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Why? That is exactly the case for all small generators that are connected to the grid.
Don
 
Twodollar said:
There is discussion about local interactivity with the grid using signals on the power line to prevent power islanding and to request the inverters to provide leading or lagging current to the grid.
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Any inverter thats listed for grid inter tie already has anti-islanding provisions. Basically this class of inverter internally phase locks to the grid, and then at each half-cycle after the zero cross it waits for the utility voltage to rise a few volts, which proves to the inverter's satisfaction that there is something else out there providing power, and at this point the inverter ramps up for the half-cycle.
 
don_resqcapt19 said:
Why? That is exactly the case for all small generators that are connected to the grid.
Don
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Maybe you are referring to ATS generators?

A system of multiple prime movers (of the 5kW variety) driving the power grid without central control of frequency, I see problems. These don't have enough inertia to deal with load steps on the grid to keep the frequency stable and their feedback system could not react fast enough to a large load step either. The power grid is normally driven by a few large turbines with remote load-frequency control feedback to their governors.

And practically, maintenance on the moving parts is a cost.
 
the question now is will there be too much harmonics going around the grid once there are enough inverters (and converters) connected to it.

recently, an elevator vendor insists that his elevator can only operate on a perfect sine wave. with 'dirty power' becoming the norm in the future, I believe it was time to change some standards regarding how 'normal' equipment (like small pumps and fans) can stand so much harmonics in the grid
 
IEEE 1547 currently applies and needs to be followed. Any one working with PV systems should have a copy and read it. I think this will answer all your questions.

Keep in mind IEEE 929 was withdrawn in 2006, and no longer applies.
 
All grid tied inverters are pure sine wave inverters. They would not be able to connect to the grid if they were not. Look at IEEE 1547.
 
dbuckley said:
Any inverter thats listed for grid inter tie already has anti-islanding provisions. Basically this class of inverter internally phase locks to the grid, and then at each half-cycle after the zero cross it waits for the utility voltage to rise a few volts, which proves to the inverter's satisfaction that there is something else out there providing power, and at this point the inverter ramps up for the half-cycle.
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But these methods rely on an assumption that there is a main source of power to the grid and the local generation is a small fraction. That could change in the future and more sophistocated methods of anti-islanding will be necessary.
 
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Twodollar said:
But these methods rely on an assumption that there is a main source of power to the grid and the local generation is a small fraction. That could change in the future and more sophistocated methods of anti-islanding will be necessary.
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I am completely lost as to where this change is coming from.

How far in the future - more than 100 Years?

The area I am in, "The Grid" has maybe 10 generators capable of a total of maybe 200MW - yes, we are an island - relatively small capacity compared to other grid interties in America. Are you suggesting this will be replaced, "in the future", by 40,000 5kw generators/inverters?

If so, that is certainly some interesting thinking. I would not feel competent to comment on that subject, but I would be listening.

carl

edited to clarify first statement about me being lost
 
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coulter said:
I am completely lost as to where this change is coming from.

How far in the future - more than 100 Years?
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One goal of distributed generation is to supply peak power demands. The california solar initiative may result in a huge demand for these systems. If you are familiar with the fubar'd power distribution in Calif., cries of NIMBY regarding new peak power generation, green power policy and the brownouts, distributed generation could very well become a large fraction of the base generation level in Calif. during peak demand hours. An interactive grid that can schedule load demands and generation, and supply a 60 Hz "heartbeat" to the inverters, which does not rely on syncing to the line voltage, will be vital. I do switch mode power supply and electronic pfc in 1kW range and know that International Rectifier and NXP are ramping production of IGBT's just to supply the expected demand for distributed generation. Mass production because of UL certification based on IEEE 1547 is the main driver. Distributed generation is the real deal and it's coming.
 
Twodollar said:
... If you are familiar with the fubar'd power distribution in Calif., ...
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A few years ago, shortly after the infamous rolling brownouts occured, I read a few papers on the causes. As I recall, the consenus was the power shortages came from the pricing controls set by the Peoples' Government of CA. There were no financial incentives to build power plants, and the articifically low rate structure didn't pay to purchase imported (wheeled) power. Inexplicably the Laws of God and Physics were not trumped by the laws of man and the lights got dim.
Twodollar said:
...distributed generation could very well become a large fraction of the base generation level in Calif. during peak demand hours. ...
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This is the part that interests me the most. My background doesn't give me any insights. In particular I am interested in the studies showing that this proposed distributed network is more efficient than normal sized utility production, optimized for efficiency.

Here is an example of where my thinking gets stuck:
In Feb 07 CA produced 13 X 10^9kwh (utility and private). http://www.eia.doe.gov/cneaf/electricity/epm/table1_6_a.html
Say that 10% is peak loading (That number seems really low to me, but right now I just guessing to get a handle on the reasoning).

This implies 1300 x10^6kwh/month = 45 x 10^6kwh/day for peaking. If the peaking can be spread out over 5 hours (makes the math easy), that requires (xxx200 millionxxx math error) 9 million of your 1kw power supplies (yes, I know they are not the same thing). Asuuming 4 people per household, every household in California will have one each 1kw units to meet the peaking.

This is why I am interested in seeing the papers showing the scale of economy and efficiency.

Then again, my math and reasoning could be really goofed up.:confused:

carl

edited to fix the math (wonderous thought - Feb has 28 days, not 1 day) and it could still be wrong
 
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Carl... I don't know about your math, but your logic is reasonable. It would certainly take 225,000 homes to take offset the place of one 225MW generator. Yes, there are economies to scale, but only up to a certain point.

But since each home is not going to use power consistently, or share it equally, I think due to diversity you may have to install 280,000 1KW units to offset a 225MW generator. 1KW units would definitely be peaking since that would not be enough to run everything in a house. Also, a PV system will cost roughly $10/kw (installed). This is considerably more expensive then a gas fired combustion turbine install.

I don't know who is figuring out the economics on this but personally, I would like to see it happen. It will make more money for the electrical industry as a whole.
 
Twodollar,
A system of multiple prime movers (of the 5kW variety) driving the power grid without central control of frequency, I see problems. These don't have enough inertia to deal with load steps on the grid to keep the frequency stable and their feedback system could not react fast enough to a large load step either.
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If they are connected to the power grid (utility) that provides the frequency lock. That frequency is very solid and there is no way small generation is going to make any real change. If the prime mover has enough power the generator produces power to the grid. It will share load based on the power available from the prime mover. As long as the prime mover is trying to turn the generator above slip frequency, it will produce power to the grid.
Don
 
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