Generators in hybrid systems

schmek

Hi,

Some basic questions about the use of generators in systems with batteries and solar power. This applies to an off grid DC coupled system.
Can the generator power loads at the same time as solar power?

Also, I’ve noticed the generator is always connected to the inverter. Why is that? Why can’t the generator be connected directly to the load?

Estragon

It depends on the inverter/charger, but my understanding loads can supported by both generator and solar.

IIRC, my Outbacks dial back battery charging to support AC loads as needed. The internal transfer switch does pass AC from the generator to AC loads, one of which is the internal battery charger, so in a sense the generator is directly connected to AC loads when operating.

The charge controller supplies what DC it can to assist in charging but not for AC loads.

In my case, the inverter in charge mode doesn't invert, so loads are limited to generator output. My understanding is some inverter-chargers can support the generator by switching to invert mode to use DC to supplement generator output if needed, a feature that I would occasionally find useful as I have a relatively small diesel (4kw) with larger inverter capacity (7kw continuous).

mike95490

In my Schneider/Xantrex system, the inverter has 2 sets of Transfer Relays, one for GRID and one for GENERATOR.

It will attempt to run loads from the generator if it is on and any remaining generator capacity is used for battery charging.   Solar charging could be happening at the same time.  If AC loads increase past the generator's rating, the charging stops, and things switch into INVERTER mode, assisting the generator. Solar can still be charging the batteries while this happens.   As loads drop off, the inverter will switch back to charging the batteries while the generator has spare capacity.

Estragon

@mike95490 that's exactly the sort of function I was trying to describe.

schmek

Okay, but will generator power always go via the inverter? Is this because voltage and frequency have to match when powering a load from two energy sources?

mcgivor

The AC input, or generator input, to the inverter will be rectified to DC, which will in turn  be used for charging the batteries and or invented to support loads at the nominal output voltage. The AC input typically has parameters for voltage/frequency to qualify, the inverter can increase/decrease input  voltage to the nominal output  parameters, this method ensures a stable output voltage/frequency. The generator or grid input is therefore not directly passing through the Inverter but is electronically and magnetically isolated by the output transformer, otherwise known as galvanic isolation. Inverter chargers, because of the aforementioned sequence are intrinsically better than separate devices because the rectification/inversion process allows for power factor correction. Naturally the question of rectification then inverting would have losses, the answer is, there are always losses in conversion, but the benifits outweigh the losses in the form of stability, much like a frequency drive which also rectifies the input to be able to provide a variable AC output frequency. Difficult to explain, perhaps someone else can add or refine this explanation, or differ, whatever the case may be.

mike95490

Yes, the inverter/charger (mine is XW series) syncs to the generator (so the generator has to be reasonably stable).  The Gen input has relaxed Volt & Freq specs, because generators are never grid quality. 

You program your generator specs into the inverter, and when the sensed loading approaches the limits of the genset, the inverter (Schneider calls it Generator Support) begins to assist the generator with the loads.   The rapidity with which this happens depends on how you have configured your gear, when my system switches over to assist, there is nothing apparent, the genset does not bog down, lights don't flicker, all I can notice is the transformer hum from the inverter increases.  Metering indicates the assist has indeed happened.  For me, this is when I'm running the generator in winter, and the wife puts the toaster or hair dryer on, and I'm charging batteries.  Charging is disrupted for 2  or 3 minutes, then resumes seamlessly.

BB.

As McGivor says--A "simple" setup either uses the generator AC switched directly to the output (and charges the battery bank on the side), or switches over to AC Inverter output if the generator is off or has frequency/voltage output issues.

The more complex (battery based) Hybrid AC inverters are more complex. They will output the generator frequency and voltage, and the AC inverter will match the AC voltage and frequency. And will either charge the battery bank, or even "support" the genset (if you have a 2,000 Watt genset and a 3,000 Watt load, the AC inverter will supply 1,000 Watts of power to "help support" the genset).

Some Hybrid inverters (battery based) will have 2 AC inputs. One is generator AC input (as discussed). and a second AC Grid input--So that the Hybrid inverter, when the battery bank is full and the sun is shining and AC utility power is running, can go into Grid Tied mode and feed power back to the utility (just like a Solar GT Inverter setup).

It is amazing out complex these Hybrid AC inverters have become.

-Bill

schmek

Okay. Consider the situation where i don't want to charge batteries with the generators. Is it possible to connect generators directly to the load, in the sense that they can power loads directly withouth going through an inverter? In this way, is it possible that solar power and generator power one load at the same time?

BB.

Yes...

A simply AC relay or switch can do the job. Here is a nice packaged relay with time delay (start up the genset and 8 seconds later, the relay switches from AC Main/AC Inverter power to Genset power) and put your loads on the output that you want to support with the genset or AC inverter as needed:

https://www.solar-electric.com/pomaxpmautrs.html

Of course, there are very complex "transfer switches/genset controllers" that do all sorts of things (start/stop genset, switch over power, run the genset every couple of week for self test, shutdown cranking if genset does not start, check oil level and pressure, monitor temperatures, keep local genset battery charged, etc.).

To power one AC load with both genset and solar power at the same time... Gets a bit more complex. And there are multiple solutions.

• Genset "charges" => battery bank, Solar panels "charge" => Battery bank. Battery bank => AC load
• Hybrid AC inverter (Schneider/Xantrex or Outback, and others). Genset => AC loads through Hybrid inverter (assume genset has stable 60 Hz frequency and 120/240 VAC output). Solar panels => charge battery bank. AC inverter "supports" Genset output with energy from battery bank (and from solar panels).
• Can be done but not recommended (AC mains, you can feed back AC energy safely. Gensets, if you try to feed back AC energy will destabilize genset and possibly cause damage). Get a frequency stable inverter genset (Honda eu2000i, etc.) and use Grid Tied Inverter(s) (solar panels => GT inverter => AC Genset output). As long as AC loads are greater than GT Inverter output (no back-feeding genset), all is OK. May not work will for non-inverter gensets (mechanically govern)--These do not have 60 Hz +/- 0.5 Hz stability and GT inverters will not sync. Also some "cheap" AC gensets have poor wave form (not good sine-wave) and GT inverters will not sync.
• If you have a large industrial genset--Some of these will sync with AC mains and support AC mains loads (frequently done during weekly/monthly testing/exercising of gensets--Instead of wasting power to test, use power to run part of hospital load during test).
  

It is almost always wrong to say something "can't be done" in engineering. Somebody somewhere will usually figure out a way. Now, should it be done and/or is it cost effective--That is a different set of questions.

Instead of talking "in theory"... Do you have a specific application you wish to solve? Perhaps it will be less frustrating for you. Size of loads, complexity (manual vs automatic), variable needs (sunny summer weather vs gloomy winter weather), attended vs unattended system, diesel vs gasoline vs propane vs very hot or very cold weather, etc.... An answer for a 200 Watt load will be different for a 2,000 or 200,000 Watt load.

-Bill

schmek

Okay, but how is it when a diesel generator and solar power both goes to one electrical switchboard with several loads attached to it. Can then solar and diesel generator power loads induvidually as long as their voltage is right?

BB.

It depends on your local electrical standards how it is done.

In some countries we use a common neutral bus in our panels (120\240 vac split phase systems). Electrically it is sort of possible to share the neutral conductor between a generator and an inverter. However the genset and the inverter are not usually phased locked and a shared neutral will overheat from too much current.

More or less the switch box needs to switch all power leads at the same time from source A to source B.

Generally these are specialized switch gear for larger applications. Not just something that is picked up from the local hardware store.

Here is an example of a home manual transfer switch that let's us choose utility or generator power for up to 10 circuits.

Notice some are two switches tired together. This is intended for two hot circuits (240 vac for single phase North American power).

https://www.homedepot.com/p/Reliance-Controls-10-Circuit-30-Amp-Manual-Transfer-Switch-Kit-310CRK/205793178

Bill

mike95490

In the Schneider XW series, battery charging can be enabled or disabled. 

mcgivor

schmek said:

Okay, but how is it when a diesel generator and solar power both goes to one electrical switchboard with several loads attached to it. Can then solar and diesel generator power loads induvidually as long as their voltage is right?

Generally it's bad practice to have more than one source in an enclosure not intended for such an application, a transfer switch for example where unavoidable , additionally it may violate electrical codes depending on location. The two sources cannot be connected together if not synchronized as @BB has explained in detail, this is why an inverter charger is useful as it consolidates the two sources seamlessly.