using a grid tie inverter without sending power to grid.

surveyor1

Hi folks
how can i setup a grid tie inverter to activate only when the load increases (ie load trip points) but not to export to the grid?

thanks in advance

Cariboocoot

Re: using a grid tie inverter without sending power to grid.

Sort of.
It depends on what you mean by grid tie inverter.
For example a hybrid (battery-based) GT inverter can have 'SELL' turned off, which will allow it to supply anything connected to its AC OUT without ever back-feeding to the grid.
A standard GT inverter can be connected with some switching to cause it to engage only when the known load is on, thus only supplying power to offset that load. The problems there are that it will only work in daylight and that it is an awful waste of investment since it would not be utilized all the time it could be.

If you are thinking of doing AC coupling of a standard GTI to your Radian to compensate for your occasional heavy daytime load, that too should work; the Radian can limit AC fed to it to prevent over-charging of its batteries (due to the bidirectional nature of the inverter).

surveyor1

Re: using a grid tie inverter without sending power to grid.

the ac coupling sounds feasible, but what happens on weekends when we are not using the heavy loads? How do you deal with this? Would this excess power cause a problem since the batteries will be charging from two sources?

Cariboocoot

Re: using a grid tie inverter without sending power to grid.

surveyor1 wrote: »

the ac coupling sounds feasible, but what happens on weekends when we are not using the heavy loads? How do you deal with this? Would this excess power cause a problem since the batteries will be charging from two sources?

No, because both charge sources are regulated. The Radian should shift frequency, causing the GTI to drop out, once the batteries are fully charged.

The absolute best system for AC coupling is SMA's Sunny Island and Sunny Boy inverters combination.

BB.

Re: using a grid tie inverter without sending power to grid.

When the battery bank is fully charged, the Radian should alter the frequency from 60 Hz to 61/59 Hz which will "knock" the GT inverter off line (5 minute minimum timeout). When the battery needs charging current, the Radian should go back to 60 Hz.

Note, I have not read the Radian manuals--But that is, typically, how AC coupled GT inverters are controlled with recent Off Grid inverters designed to support GT coupling (and the OG inverter has to be larger than the connected GT inverters).

SMA makes the Sunny Island family which does actually modulate the system AC frequency to control how much current the GT inverters will output (not just On/Off).

-Bill

surveyor1

Re: using a grid tie inverter without sending power to grid.

Guys,
outback DO NOT use the frequency shifting approach in their ac coupling. they use toddle relays. their manual also says that charging from gt coupling is not regulated. I would prefer something more certain especially if the OB relay happens to stick.
I am aware that sma is the best but "made in america" is my first choice!
what else can we do?

BB.

Re: using a grid tie inverter without sending power to grid.

Back to basics.

What are the loads (Watts, Watt*Hours per day, day/night/24 hour per day, etc.).

Do you have grid power or not?

Do you want backup power?

Use Radian for short term backup and a genset for days/weeks of backup power?

Why GT solar AC coupled to the Radian? If longer distance wire runs from solar array--Schneider/Xantrex makes a nice 600 VDC max MPPT solar charge controller. Midnite makes a 250 VDC max MPPT charge controller too (better than ~140-150 VDC MPPT controller limit, not as good as 600 VDC limit).

Other than SMA, GT charging is a "bang bang" controller... Maximum charging current, then nothing (cut AC power, turn on DC dump heater, etc.). Not the best for battery life.

The standard MPPT/PWM series type solar charge controllers are generally better at maintaining batteries at their proper charge.

I like to define the load, then the battery bank+Inverter, then finally the charging source(s). Lastly, what to do for backup power (generator, shed loads in bad weather, etc.).

-Bill

pechan

Re: using a grid tie inverter without sending power to grid.

surveyor1 wrote: »

Guys,
outback DO NOT use the frequency shifting approach in their ac coupling. they use toddle relays. their manual also says that charging from gt coupling is not regulated. I would prefer something more certain especially if the OB relay happens to stick.

what else can we do?

1. use the aux out on the radian to throw a contactor and disconnect the grid tie inverter.
2.Use a dump load on the ac side of the inverter to heat water etc.
3. Get a really big battery bank

Cariboocoot

Re: using a grid tie inverter without sending power to grid.

So Radian's specs have changed since the initial. Fair enough.

Frankly I wouldn't be too concerned about doing this, as per the info in your other thread about getting the Radian itself to handle the load (haven't checked that yet to see your response). Investing in additional panels & inverter for occasional loading isn't such a good way to spend money.

If I recall, your additional loads are not one large item such as an A/C unit but multiple small items adding up? They would not make as good a use of any type of AC coupled system as would a single large load. You are better off with increased PV on the existing bank, providing the inverter can handle the total.

ggunn

Re: using a grid tie inverter without sending power to grid.

surveyor1 wrote: »

Hi folks
how can i setup a grid tie inverter to activate only when the load increases (ie load trip points) but not to export to the grid?

thanks in advance

Strictly speaking, you can't. Grid tied inverters have no way of differentiating between local loads and the grid. As others have said, you can set up a grid interactive system with batteries to do this, but you are talking about a system which is twice as expensive (or more) and much more complex than a comparatively sized GT system.

bill von novak

Re: using a grid tie inverter without sending power to grid.

surveyor1 wrote: »

I am aware that sma is the best but "made in america" is my first choice!

The SMA TL series is made in America.

alexpi01

Re: using a grid tie inverter without sending power to grid.

Hi guys! Very usefull thread for me! It's very close (maybe even identical) to my situation.
As I am a mechanical engineer and I didn't underestood 100% the discution I am begging for help from you.
I own a house in a place without a grid and I already built a 0.65 kW PV system (4 pannels + 24V/200Ah batteries + PV controller + 2kW true syn inverter) which is working satifactory.
Now I am planning to extend. And I want to do it in a clever way.

I need to use more energy in the daylight and to spare the energy from the batteries to be used in the evening/night.
The first thought was to "expand" by modules consisting in 1(or maybe 2) pannels with an own grid tied inverter, using my original instalation as a "driver" grid.
I figured that those grid tied inverters would "pump" the energy only if it's needed. Accordingly with the comments of this thread, this seems not possible.

Of course I still have the "classical" hard/expensive way: more pannels -> bigger PV controller -> bigger battery bank.

I wonder if you, as experts, can you suggest me another solution?

stephendv

Re: using a grid tie inverter without sending power to grid.

alexpi01 wrote: »

Of course I still have the "classical" hard/expensive way: more pannels -> bigger PV controller -> bigger battery bank.

That is actually the cheaper way. AC coupling, as described in this thread would need another grid tie inverter to work, and they're much more expensive than simply buying a bigger charge controller. If you're going to expand beyond 1kW of panels then it starts to make economic sense to buy an MPPT charge controller, which will mean that you can harvest more power in winter- which is exactly when most people need the extra power.

NorthGuy

Re: using a grid tie inverter without sending power to grid.

alexpi01 wrote: »

Of course I still have the "classical" hard/expensive way: more pannels -> bigger PV controller -> bigger battery bank?

This is a better way because you may not need more batteries. The batteries are necessary to support your loads, and if the current bank supports your loads well, the benefits of adding more batteris will never justify the cost. Moreover, since more of day-time loads will be supported directly by solar, you may need less batteries.

GT inverter may be more efficient than the standard MPPT+Inverter pathway, but only by a small margin. The important thing is that the "MPPT controller->Inverter" pathway doesn't use batteries, which gives you an opportunity to use energy directly.

Cariboocoot

Re: using a grid tie inverter without sending power to grid.

Since most batteries can take a charge rate up to 20% of their capacity it is entirely feasible to add panels without increasing the battery bank size.

You have 200 Amp hours @ 24 Volts now. If the 650 Watts of panel is on an MPPT controller it should produce about 20 Amps. Therefor you could as much as double the size of the array. Depending on what the controller is you may not need a new one. If you do it would be better to replace with a single larger controller, providing the new panels can match the old ones. You would need to rewire the array and change the down-lead as each parallel panel connection will need its own over-current protection and the wiring from the array will need to be able to handle the increased current.

Trying to get AC coupling to work without using equipment specifically designed to do it is a nightmare.

bill von novak

Re: using a grid tie inverter without sending power to grid.

Cariboocoot wrote: »

The absolute best system for AC coupling is SMA's Sunny Island and Sunny Boy inverters combination.

Just a note -

The above won't work for his specific application because the Sunnyboy remains on-grid and thus cannot alter the (grid) frequency. The throttling only works when the system is islanded (i.e. not connected to the grid.) Normally not an issue since most people _want_ to export to the grid when the grid is connected.

ggunn

Re: using a grid tie inverter without sending power to grid.

In order to have a grid tied system that will never export power to the grid, you must have some piece of equipment series connected between your meter and your main distribution panel (service panel) through which all the power into and out of your local loads must pass and which can detect and regulate the direction of current flow. A normally installed GT inverter has no way of distinguishing your household loads from those outside your meter, i.e., the grid.