Please explain the battery / Generator problem!

[Andy D]

First I am an EE, senior IEEE member with over 30 years in the TV broadcast industry including circuit design. I have done hands on work with 750kva UPS systems but I'm new to solar power.

I have searched the forum and basically determined what I want to do is not possible so be it. I just don't understand the technical reasons.

I am installing a grid tied 10kw system in Los Angeles. (via a dealer, not DIY). I own a 8kw 120/240 portable generator with an approved panel main breaker interlock. My solar will be line tapped via one of those new Siemens solar ready panels. So why can't the generator be used as a power source to fool the inverters into operation during a power outage?

Note I realize the complexity in a safe change over system and I can construct one with the appropriate relays (power contactors) and control logic to prevent any backfeeding. That's not the question here.

As I understand it the inverters sense the grid impedance and a local generator will be too high. I can perhaps fool that too electronically. But what I don't get is why can't the inverters simply throttle back in the case of low demand? An typical modern inverter is basically a PWM switch mode power supply. It is possible to design an inverter that works this way.

Why do solar inverters have to output full capacity all the time? Can't they be made with a throttle back mode that is generator friendly? I guess what I am missing is why the need for a battery in the mix? At night the inverters simply shut down and you run off the generator?

[Sunking]

I am installing a grid tied 10kw system in Los Angeles. (via a dealer, not DIY). I own a 8kw 120/240 portable generator with an approved panel main breaker interlock. My solar will be line tapped via one of those new Siemens solar ready panels. So why can't the generator be used as a power source to fool the inverters into operation during a power outage?

Real simple and if you are who you say you are will understand instantly. Solar panels and Grid Tied Inverters (GTI) are Current Sources and your generator is a Voltage Source. All that current has to have someplace to go, and the generator does react well when you try to pump current into it. They let out the magic smoke. Plus a generator voltage and frequency stability are way outside the operational range of a GTI. GTI's need the ultra impedance of the grid to look into so the current has someplace to go. Otherwise the voltage goes to infinity.

Told you it was simple.

[Andy D]

Real simple and if you are who you say you are will understand instantly. Solar panels and Grid Tied Inverters (GTI) are Current Sources and your generator is a Voltage Source. All that current has to have someplace to go, and the generator does react well when you try to pump current into it. They let out the magic smoke. Plus a generator voltage and frequency stability are way outside the operational range of a GTI. GTI's need the ultra impedance of the grid to look into so the current has someplace to go. Otherwise the voltage goes to infinity.

Told you it was simple.

Ok that explains it. Cheap! That's how they can sell a 5kw inverter for $1000 at wholesale prices. It's a current source and therefore a much simpler design!

That's how they must measure the load impedance, too little current draw or high output voltage as in open circuit, shutdown. Too much current draw as in a blackout, shutdown.

[Andy D]

OK seperate question?

How do grid tie converters safely deal with back feeding?

We know the infamous layman backfeed generator argument:
"My little generator cannot possibly power the city of Los Angeles so in a blackout with the main accidentally left on, it will simply trip it's local breaker"

The important utility and NEC response is:
"Suppose the failure is local only affecting a few houses. Your 10kw generator can certainly power a few refrigerators so you now have live load side energized 15kv lines laying in the street"

So if the GTI only senses load impedance, what if the line impedance is still low enough as in the above scenario to allow inverter turn on? How do they prevent this? Measure frequency drift, IOW, design with a very tight PLL? That would explain why GTI's are so frequency sensitive.

[Sunking]

This short paper gives a decent overview. In a nutshell to comply with UL1741 the GTI is monitoring both frequency and voltage. If the detector circuit senses any changes in frequency or voltage within a specified range has to trip off-line within .5 seconds. The range is quite narrow.

Having sad that there are GTI that can operate in a grid outage, they are called Hybrid Inverters. They have Transfer Switch and switch input source to battery power. They are very expensive and still require a generator for extended outages.

[Mike90250]

.....

Why do solar inverters have to output full capacity all the time? Can't they be made with a throttle back mode that is generator friendly? I guess what I am missing is why the need for a battery in the mix? At night the inverters simply shut down and you run off the generator?

Grid Tie inverters are designed to push all possible power to an "infinite grid" with thousands of loads that absorb the power. Your 7Kw genset cannot absorb any power, and your microgrid voltage will rise till overvoltage disconnect sensor shuts it down.
Only an inverter generator will be stable enough in frequency to be qualified as a "grid" for the inverter to connect to it, and the inverter genset will not like being overvoltaged, and will likely quickly die, along with appliances as the inverter peaks it's voltage.

You can accomplish this with a "Hybrid" inverter, but you will need batteries in the mix. Off Grid & Hybrid inverters can limit their output voltage & current, GT inverters do not.

I know the XW-6048 like I have, can do this.