Microinverter Application for Off-Grid PV Systems

Here is an unusual twist to the micro-inverter story: What happens when you try to build a micro-inverter based off-grid system?

Approximately 5%-30% of collected energy is lost or wasted while an inverter converts DC power from solar PV to AC power. Inverter efficiency is vital to the solar PV system efficiency because central or string inverters determine both the amount of DC energy collected from a solar array and the demand placed on transformers and capacitors to complete the process.

Micro-Inverters offer an effective efficiency solution because they are attached to one (or two with dual Micro-Inverter) solar PV panel(s) instead of an entire solar array. The overall effect reduces the need for large transformers, large electrolytic capacitors, and eliminates the need for fans (as cooling loads are much lower).

How Micro-Inverters work: Micro-inverters produce grid-matching power at the back of each solar PV panel. Arrays of panels are connected in parallel to each other and then connected to the grid feed so that a single panel outage will not lead to an entire system outage (engaging grid dependency at the meter).

Can I use a micro-inverter for an off grid PV application?

To answer this question it is necessary to analyze the difference between grid-tie and off-gird solar PV systems with regard to how inverters work.

Micro-inverters obtain optimum power by performing MPPT (Maximum Power Point Tracking) for each connected panel while the MPPT for string inverters is based on overall collected performance.

The difference between tracking maximum power through a string inverter and Micro-Inverter lies in the difference between the collection of DC energy (300-350 VDC) produced from a solar array and the individual energy produced from each solar PV panel (25-30 VDC) per panel.

As mentioned, the central/string inverter MPPT is influenced by the lowest power performing solar panels, which will dominate the algorithm and lower MPPT but works interchangeably with grid-free systems.

Grid-tie vs. Off-Grid

In a grid-tie system, the DC output of solar PV panels is converted to AC electricity via micro-Inverters and synchronized to the utility grid.

Off-grid systems also pull DC energy from solar PV panels into the inverter for AC conversion; however, with off-grid systems DC energy first flows from the solar PV array to a charge controller (optional) to a battery source before going to the inverter.

Presently there are no micro-inverters available for off grid solar PV systems. The main reason as illustrated in the comparison above, is that with off grid systems there is nowhere for excess energy (energy that would be sent back to the grid) to go. 

This means in an off grid scenario excess energy produced by micro-inverters would cause either a system breakdown or become a potential fire hazard because the power they are designed to harness would be more than that which is required through appliance demand.

New Solutions

For several years, Xantrex, now Schnieder Electric, has been offering a "package" comprised of their XW series charge controller and their GT series inverter. The XW and the GT work in tandem to ceate an AC mini-grid, which can charge batteries and provide AC power off-grid or during a grid failure (White Paper attached).

Problem: The GT has never really recovered from reliability problems that started in 2005 and does not seem to be available for purchase as of this writing (10/6/2011). 

Proposal: Can the XW's frequency shifting capabilities be leveraged to work with Enphase inverters?

Douglas Caldwell, an engineer in California, has implemented a micro-grid design which combines the Xantrex XW with Enphase micro-inverters. Mr. Caldwell used the AC coupling features of the XW to build his system with the help of Xantrex (Schneider) Tech Support.

Important: To prevent battery damage in a mini-grid AC system, use only firmware which has the AC coupling feature implemented. Use the firmware version (or higher) listed in Table 1 for your model. For firmware upgrade instructions, see the Xantrex XW Config User’s Guide (Document Part Number 975-0365-01-01) available on www.schneider-electric.com.
AC Coupling Module with Line Frequency Variation Pattern - from linked Xantrex white paper.

The AC Coupling Module is a control strategy that varies the line frequency according to a predetermined pattern to cause a grid-tie inverter to cease producing power to avoid overcharging the system battery. The Xantrex XW executes a pattern generator algorithm that varies the line frequency linearly to avoid overload.

Figure 2 on page 6 shows the effect of the frequency generation function where the Xantrex XW changes the mini-grid frequency with a linear rate of change of 0.4Hz/s. When the charge bulk voltage is exceeded, the frequency linearly decreases until the GT Inverters cease converting power to the AC mini-grid.

While the Xantrex XW and GT Inverter are in AC connected/coupling mode, the Xantrex XW changes the frequency only when the Charge Bulk Voltage setting is exceeded. This setting is user-adjustable in Custom battery mode.

The internal maximum frequency adjustment range when the Charge Bulk Voltage setting is exceeded is:

North American models: fLINE=[60-55]Hz

European models: fLINE=[50-45]Hz

Micro-grid development connects battery back up at the grid-tie so that excess energy could return to the grid but energy loss would draw from the battery backup instead of the grid.

Micro-Grid Research Data and Development- With a growing focus on both climate change and global energy demand, research and development for Micro-Grid and Smart Grid deployment will be an influence in the future of renewable energy.

Summary

This is promising information for off-grid and micro-grid development. Please confirm compatibility with all manufacturers before trying any of these ideas.

Resources:

Comments

What is anti-islanding and why is it built into grid-tied inverters?


Think of anti-islanding as a performance standard for grid-tie inverters which requires the inverter to cease exporting energy to the grid in the case of grid failure. Without it, each system would become its own "energy island", attempting to export energy to all of the grid-connected devicesout there. If such operation only carried the risk of damaging the inverter, utilities probably wouldn't care. However, in the case of a black out, a utility employee could be working on the power lines, in which case, the lines should be dead, for their safety. This feature isn't necessary in off-grid situations, since there is no connection to devices or cables other than those in the building.

Thanks.  That makes sense.  MG


For several years, Xantrex, now Schnieder Electric, has been offering a "package" comprised of their XW series charge controller and their GT series inverter.

I'm a bit confused by this statement, having not read the white paper as yet. I'm assuming this should read 'comprised of their XW series inverter/charger and their GT series inverter." Correct?

Yes, the white paper makes it clear: It's the XW series inverter/charger and the GT series inverter depicted in various applications.