AC Mains meter circuit design for energy sources....

[terahertz]

What circuit design is used to ensure that the renewable energy source does not begin to absorb energy from the grid (for instance, when its terminal voltage drops below the distribution line voltage)?

 

[Baluncore]

The changing voltage and polarity of the grid is known. The direction of current flowing between the grid and the generator can be measured. The grid voltage multiplied by current will change sign when energy flows the wrong way. The inverter that connects the generator to the grid is designed to transmit energy from generator to grid, so it turns off when there is no power being generated.

 

[Windadct]

There are a couple situations - and each scenario has a specific function built into the inverter. But a typical grid tie inverter, has no "local load" so if it is not able export energy to the grid, there is no reason it would absorb energy from the grid, except a small amount typically to bring the DC bus voltage up.

Looking at the Boosting Solar Inverter topology - if the grid voltage is higher than the Inverters output - the Inverter will become a basic rectifier and charge C3 to essentially the peak line voltage. -- But now the Boot Circuit (D8 & 2, T5 &6) are reverse biased boosts and energy will not flow Back towards the Solar Array (in this case).

Not all inverters have this boost circuit, but will have reverse blocking diodes. But your question points out why a Boost Converter is sometimes used - it boosts the PV voltage to a point above the Grid Voltage to allow the inverter to export power.

It is more common in large systems to "over panel" the PV array to ensure that the DC voltage is always above the grid voltage if there is any PV power available, and not use a boost Common to see 1000V DC PV array for 480 VAC, and 1500V DC is the hot topic today for 480 to 600VAC - usually tied directly to a Transformer to the grid.

 

[NTL2009]

...

It is more common in large systems to "over panel" the PV array to ensure that the DC voltage is always above the grid voltage if there is any PV power available, and not use a boost Common to see 1000V DC PV array for 480 VAC, and 1500V DC is the hot topic today for 480 to 600VAC - usually tied directly to a Transformer to the grid.

Why exactly is this done? Is it just more efficient to always be feeding into a lower voltage?

 

[anorlunda]

Don't forget that voltage magnitude relative to the grid controls imaginary power (VAR) . Phase angle controls real power flow (watts)

 

[Windadct]

There are a number of reasons - it starts with the need the to be able to harvest power over a wide DC supply range, the PV array voltage varies dramatically depending on solar intensity, temperature and loading. -- Looking at a MPPT tracking plot or PV Curve and you can see the first challenge.

For the inverter to work at all it needs DC voltage to be higher than the Peak Voltage of the AC tie. ( 480 V RMS ~ 670VDC) -- so for a 480V Grid, you need Vdc at 670V or higher.

The then to ensure you can harvest energy throughout as much of the day as possible - you add panels, both in parallel ( current capacity) and series ( string voltage) to make sure you have power available for as much of the day as possible.

The 1000V and 1500V systems - are often called Open Circuit voltages - because you can see in the PV curve - you can draw little to no power at the max voltage, the voltage quickly drops to ~90% or lower of the Open Circuit ( no load) voltage. So for a 1000 V system - you will design your inverter for max power from 650 to 900V.

The relative cost of the panels - to the cost of the project overall is coming down - by overpaneling you reduce the time to payoff of the systems, more KWH per day / year, etc..

Lastly - running at higher voltages - reduces the cabeling cost ( copper) for the installation. ( This is a bigger motivator then you would expect). But in general - for all of the systems involved you pay more (capital cost)for current capacity than voltage.

An interesting point that Anorlunda touches on - is that these system have the ability to do PF correction with their excess inverter capacity. This is referred to as VAR support - An optional function in this Solectria 500KW Inverter. ( LINK ) This is only possible because no energy is fed back to the PV array, due to contactors or blocking diodes.

 

[NTL2009]

There are a number of reasons - it starts with the need the to be able to harvest power over a wide DC supply range, the PV array voltage varies dramatically depending on solar intensity, temperature and loading. -- Looking at a MPPT tracking plot or PV Curve and you can see the first challenge.

For the inverter to work at all it needs DC voltage to be higher than the Peak Voltage of the AC tie. ( 480 V RMS ~ 670VDC) -- so for a 480V Grid, you need Vdc at 670V or higher. ...

I understand the MPPT issue - but I don't understand why you say the panel voltage must be higher than the AC peak. There are inverter designs that will step the voltage up. Is a step-up design is inherently less efficient? That's what I'm asking. A step up inverter could perform the MPPT function as well.

Lastly - running at higher voltages - reduces the cabling cost ( copper) for the installation.

Agree.

An interesting point that Anorlunda touches on - is that these system have the ability to do PF correction with their excess inverter capacity. This is referred to as VAR support - An optional function in this Solectria 500KW Inverter. ( LINK ) This is only possible because no energy is fed back to the PV array, due to contactors or blocking diodes.

But a step up inverter would not allow energy to be fed back to the panel either.

 

[Windadct]

The Boost inverter adds cost and impacts efficiency - I was referring to the inverter only design ( in commercial / utility scale this is the most common type). Also note - the Boost inverter ALWAYS is paying an efficiency price, where with an Inverter only design - you can design around the Nominal operating point for the best efficiency.

You see the Boost - converter in many (possibly most??) residential < 10KW designs. ( the factors get complicated - a 240VAC system can use MOSFET, where 480 typically IGBT - they have different loss characteristics, like Rds on vs Vce and dramatically different switching losses... ). Due to the switching losses - it is a little easier to make a MOSFET boost ckt ( high Fsw = smaller inductor & lower losses in the the inductor - etc) . There is no one best solution.

 

[Baluncore]

The simplest and most efficient converters are high voltage buck converters. By running cooler they can also be more reliable. A 1% improvement in efficiency increases the return on investment by more than 1% after costs. If that return can be gained by simply wiring PV cells in series rather than parallel, it would be silly not to do it that way.

 

[NTL2009]

The simplest and most efficient converters are high voltage buck converters. By running cooler they can also be more reliable. A 1% improvement in efficiency increases the return on investment by more than 1% after costs. If that return can be gained by simply wiring PV cells in series rather than parallel, it would be silly not to do it that way.

Thanks - that's what I was questioning. If a buck converter (a step-down converter) is more efficient in practice (I didn't know if this was true or not), then yes, it makes good sense to put the solar panels in series so that their voltage is typically > peak of the AC signal they are driving.

 

[Windadct]

An inverter IS essentially a Buck Converter. ( 4 or 6 of them - modulated to go from 0 to peak voltage)