Grid-Tie Inverter Theory and Operation Confusion

[J.P.M.]

Well Sunking may not have a great bedside manor but he does have a lot of electrical experience and I believe a Masters or Doctorate in the field, So I would say he has a lot more experience then most people on this forum.

FWIW, and speaking only from reading his stuff for about 10 years or so, I believe he also carries a P.E. license in electrical engineering which usually carries as much or more gravitas than graduate degrees to real nuts & bolts engineers.
Having both a license and advanced degrees raises credibility all around.
A license, among other things, qualifies one to render testimony as an expert witness in courts of law when discussing matters pertaining to their claimed areas of competence.
By the time your qualifications are picked over and your character insulted by the opposing side however, court testimony is a lot like qualifying for the license all over again but more of a PITA.

Anyway, this forum isn't a popularity contest. Some good answers are available if you're selective about who's posts you read and believe. Just don't look for a lot of hand holding.
You want a friend that always has nothing but warm and fuzzies and never challenges you, buy a dog. You want safe, straight and answers, stick around.

[bcroe]

There is a lot to be learned from SUNKING, best to think about that
and not personalitys. As another engineer used to technical reviews,
I find him rather entertaining. Bruce Roe

[SunEagle]

Tell me you're the troll of these forums by not telling me you're the troll of these forums.....

Well Sunking may not have a great bedside manor but he does have a lot of electrical experience and I believe a Masters or Doctorate in the field, So I would say he has a lot more experience then most people on this forum.

[cinoaz]

Wrong again. The current takes all paths. Next!.

Tell me you're the troll of these forums by not telling me you're the troll of these forums.....

[Sunking]

We do agree current will flow in the direction of least resistance (impedance).

Wrong again. The current takes all paths. Next!.

[cinoaz]

What really happens, voltage
available, is the current flows through ALL paths, each set by I=E/R.
Bruce Roe

Thanks bruce, yes indeed I understand. I did not uniquely define a single path vs multiple paths and that was a mistake.

[bcroe]

The saying is, current flows thru the path of least resistance. That is
WRONG, the truth is more complex. What really happens, voltage
available, is the current flows through ALL paths, each set by I=E/R.
Yes the largest part of the current flows through the smallest resistance.

Electronic devices will work over a range of conditions, defined by
the designer. Current solid state technology allows inverters to
perform safely and efficiently over a huge range, like 2:1 input
voltage and 100:1 available power. Compare that to any appliance
a century ago. But there are always practical limits. The ability to
throttle down power from a huge energy source like a large DC:AC
ratio is definitely limited, because that is completely counter to the
normal operating mode, of converting all available energy. Pushed
way outside the design range, the feedback might be unstable, or
even energy samples might be damaging.

But the GTI cannot see what actual panels are out there, only how
much energy is available. That is why my effective ratio can be
around 1.2:1, when the panels facing varied directions total 2.2:1.

Bruce Roe

[jflorey2]

We do agree current will flow in the direction of least resistance (impedance).

Not quite. Current always flows from higher potential to lower potential. Impedance matters because that limits how much current flows. As an example, if you want to drive 10 amps into your load center, and the impedance that your inverter "sees" looking into the loac denter is .01 ohms, then you have to get the voltage .1 volts higher than the load center's voltage to make that happen (V=IR.) As a practical note, most inverters are current source, and they simply try to feed that 10 amps - the voltage difference happens automatically.

If the Inverter is producing more power than the house consumes, from the Grid perspective, the resistance of current flowing into the breaker panel from the grid is greater than the resistance of current flowing from the breaker box into the Grid.

Any time you are discussing impedance it is very important to define where you are looking at that impedance, and what kind of impedance you are looking for. For the purposes of this discussion we will assume all impedances are "simple" resistances, because complex impedance (phase) is a whole 'nother thing.

Thus if you are looking into the panel from the grid, you can define the resistance as how much more current flows when you increase the voltage. (This goes back to V=IR again.) So from the perspective of the grid, adding solar generation increases the resistance seen, since now when you increase the voltage slightly, current goes up by less - since the grid is not supplying all the power. In that case, R=V/I. To be more accurate, ΔR=ΔV/ΔI (since we are talking about small changes here.)

The original question, as repeated, was if a PV Array was sized to have a much larger AMP value than what is rated on a specific individual Inverter input (with Voltage being within range) but still within the range of the total Inverter output, will the Inverter self limit how much amps it uses on that individual input

Yes. Internally the inverter has a current sensor to monitor that. It has to have a current sensor, since the MPPT algorithm needs to know both voltage and current. It also uses that current sensor to limit maximum current through the system to a safe level.

From a purely theoretical perspective you can put a megawatt of PV on a 7600 watt GTI (provided maximum voltages are not exceeded) and have it work. The inverter will limit the current to a safe level. However, from a practical perspective, the inverter designer has to design for worst case faults, like an internal short that places a short across the incoming PV lines. In that case, a megawatt of power would cause a very rapid unscheduled disassembly. To avoid this, they add safety circuits (fuses and breakers) that will open and prevent fire/explosion. They need to have a maximum fault current to design these safety circuits to, so they come up with a maximum safe PV power that can be attached to the inverter. Generally it's between 120% and 200% of the inverter's rating.

But again, that's not because the inverter won't work above that. That limit is there so that if there is an internal fault, the inverter fails safely.

To cut costs, the inverter divided the total input into 3 individual input lines and limited the inputs to specific values (360v @ 15amps). When combined, the 3 inputs can total the max output of the inverter. However, if you decided to put the full wattage into ONE input line, will the inverter self limit to 15amps on that line

Most of the time - yes.

Most inverters of that sort have two sections. The first is the MPPT section, where one or more MPPT converters boost the voltage to a voltage that works with the inverter section. That voltage is called the link voltage. For 240VAC it has to be at least 360 volts, but is often higher. This DC voltage is then converted into AC by the inverter. In this design each MPPT section limits to a specific current.

Some inverters take a different approach. The Solaredge, for example, uses optimizers on the panels themselves to generate ~400 volts DC and do the MPPT function. All of the 400V strings are then fed to an inverter-only device that converts it to AC. In this case, the inverter itself must limit current, since there can be many different string/panel configurations used with a Solaredge system.

or just try to maximize output, thus increasing AMPs on the input beyond its rated capability and go poof.

In all cases, putting too much current on a string inverter will not cause it to go poof. If you exceed the maximum power rating AND have an internal fault, though, you may see unexpected problems.

The consensus is the Inverter just does Volts x Amps until Max output is reached, regardless of how many inputs it has. Its up to the installer ro ensure the PV arrays don't exceed the input limits, the inverter doesn't clip itself on the inputs.

The inverter does clip (limit) its maximum power.

[cinoaz]

Sorry, but you do not know how a GTI works or anything about Impedance. That is why you're confused.

First, if I knew how everything works then why would I post in this forum to begin with? Thanks for the constructive correction.

In addition, I may have been incorrect but not completely wrong. We do agree current will flow in the direction of least resistance (impedance). If the Inverter is producing more power than the house consumes, from the Grid perspective, the resistance of current flowing into the breaker panel from the grid is greater than the resistance of current flowing from the breaker box into the Grid. This delta is created by the Inverter. There is a portion of correctness in that the Inverter changes the impedance of the panel when viewed from the Grid's perspective.

The original question, as repeated, was if a PV Array was sized to have a much larger AMP value than what is rated on a specific individual Inverter input (with Voltage being within range) but still within the range of the total Inverter output, will the Inverter self limit how much amps it uses on that individual input or does it just try to reach max output by doing Volts x Amps regardless of what those values are.

To cut costs, the inverter divided the total input into 3 individual input lines and limited the inputs to specific values (360v @ 15amps). When combined, the 3 inputs can total the max output of the inverter. However, if you decided to put the full wattage into ONE input line, will the inverter self limit to 15amps on that line or just try to maximize output, thus increasing AMPs on the input beyond its rated capability and go poof.

The consensus is the Inverter just does Volts x Amps until Max output is reached, regardless of how many inputs it has. Its up to the installer ro ensure the PV arrays don't exceed the input limits, the inverter doesn't clip itself on the inputs.

[Sunking]

I understand the principles of a Grid-Tie Inverter (GTI),

A GTI is (first question) a dumb device. It doesn't know anything about "load." Its job is to take whatever is connected to its input and drive as much power out as possible. If it can provide more power than the house needs, that raises the Impedance of the home panel above that of the grid, and power flows out to the grid. When the Inverter produces less power, it lowers the Impedance of the house panel, and power flows from the grid to the home. I think this is correct.

Sorry, but you do not know how a GTI works or anything about Impedance. That is why you're confused. The only thing you got right is the GTI is a dumb power converter. The Inverter has zero effect or control on the Impedance of your home load panel. You control the Impedance of your home panel with the crap you turn on and off. Turn everything off, and your home's Impedance is infinite or open circuit, and no current flows. Start turning crap on, impedance lowers and current flows. Where it comes from, no one knows except Dr. Ohm and your utility electric meter.

The GTI, Uttility, and your Service Entrance form a current node. Think of the GTI as a 3 GPM water pump without control over where the water goes. The utility is a two-way supply or load you control with back pressure. The house is a sink that can take water from either supply or both. When you turn everything off, the GTI water has to go somewhere, so it builds up pressure forcing the water to flow out on the utility. As you turn things on, the pump can keep the pressure up and supply your needs. Now you turn all your crap on, the pressure drops, the GTI cannot keep up, and now the house is getting water from both the utility and GTI.

[Ampster]

And that was my thought. I know we like to think these inverters are more than they are. I tend to believe they are way simpler than we are led to believe. Just Volts x Amps until it reaches max output. It's a matched set, the inverter and panels. Both have to be matched with each other and limitations adhered to.

More than likely the only risk of poof is by over voltage on the input side. It is also important when matching current specs with charge controllers to understand whether the limit is on the input side or the output side of the charge controller section. In your case with multiple MPPT controllers going to an inverter whose input voltage may not be known the input current could be significantly different than the output current. That can lead to confusion about how much over paneling could be tolerated or which configuration would be optimal.

[cinoaz]

poof

And that was my thought. I know we like to think these inverters are more than they are. I tend to believe they are way simpler than we are led to believe. Just Volts x Amps until it reaches max output. It's a matched set, the inverter and panels. Both have to be matched with each other and limitations adhered to.

[davidcheok]

davidcheok
gti input #1 = pv array 360v@30amps = 10kw (contacts on input are rated for 360v@15amps)
g

poof

[Ampster]

Yes, and that is precisely the question. If the inverter is trying to hit 10Kw output, and 30amps is available on the PV line, will the Inverter just keep sucking up the AMPs until something gives?

VOLTS x AMPS = WATTS and regardless of Volts or Amps, the inverter just does the math. So if WATTS = 10Kw, and VOLTS are set to 360v, will the inverter ramp up the AMPs it consumes until it reaches 10Kw, even tho the input is only rated to 15amps, or, will the Inverter self limit to 5Kw output and not PULL more than 15amps on that input?

As I mentioned earlier I have no idea how the three MPPT sections share their output. You will have to do that research to answer the question. There are two distinct sections of that device, the MPPT section and an inverter section. The MPPT section converts the solar energy to a voltage and Amperage compatible to the inverter section.

[bcroe]

The system DC operating voltage is set by the panel array. It
must be set up so the open circuit voltage, and the MPPT voltage,
over the temperature range, are withing the inverter specs. Then
the inverter will draw as much MPPT current as the panels can
deliver. But if more panel energy is available than the inverter
rating, it will throttle back the current drawn to limit output power
to its rating. This of course will cause panel voltage to rise
closer to open circuit voltage.

The issue with a very large DC to AC situation, is the ability
of the inverter ability to throttle back. Instability could result
and possible damage, if this is taken to an extreme.

Something elso not discussed, is the EFFECTIVE DC:AC ratio.
While adding up all the panels here gives a ratio higher than 2,
it is impossible for all the varied orientation panels to all be
facing the sun at the same time. So the primary panels keep
changing over the day, and the effective DC:AC is around 1.2:1.
This arrangement can double the effective hours per day under
good sun, and double the output with light dispersed by varied
levels of clouds. Inverters here have been happy with this
arrangement for a decade. Bruce Roe