I am designing a solar system that I plan to implement Spring 2006. I have been reading the Xantrex manuals and am interested in the SW5548 grid tie unit. I looked at the PLUS series but the lack of grid tie capability while not a deal breaker would be missed. I’m interested in assembling a 120V/240V system with two stacked inverters. The features I’m interested in are listed below. From the manuals it appears that the unit will perform them but I’d like to get a second opinion. It is a lot of money to spend just to see if they work.

Desired Inverter Features:

1) AC Load Support.(Peak Load Shaving)

The maximum current allowed is set for the AC source. The inverter will use the batteries to provide any extra energy needed above this threshold.

2) AC Source Power Reduction.

The inverter uses any extra energy delivered to the batteries to reduce the energy requirements from the source. It essentially “SELLSâ€￾ the energy to the output side of the inverter. This should be done with out having to use the 5 minute rule that grid-tie requires.

3) Online Battery Charger.

Ability to charge batteries and perform AC load support when needed. The charger would initially back off as the load increased. Once the load increased to the threshold the inverter would switch to AC Load Support.

4) Uninterruptible Power Supply.

When the grid is unavailable the inverter will provide energy to its output using the batteries. Short outages of 5-30 seconds would not trigger a major battery recharge cycle.

5) Grid-Tie/Sell Back.

The inverter will sell any excess energy delivered to the batteries and not being used back to the grid or upstream loads.

One important feature is Item #1 above. I have a 5HP air compressor and a 3-TON AC unit that I wish to power at different times. My installation is about 300 ft from the main panel and seams to have limited surge capability. I would like to use the inverters to improve this. How would the inverter handle this type of load? Will it help start these devices?

I was told on some forums that the SW5548 is an old design and Xantrex may not be producing it much longer. The SW PLUS series would be replacing it but with out grid tie capabilities. Does anybody know what is the current and future status of this device?

I was also told on some forums that when Xantrex had to add the GridTie module to the system that this probably reduced its ability to provide Peak Load Shaving/AC support? How does the GridTie module effect these features? How much will the inverters improve my surge capabilities?

The last question is about the grid tie feature. When the AC unit or Air Compressor is started it puts a large load on the grid. Will this fluctuation cause the inverter to stop selling energy back to the grid? How sensitive is the GridTie feature? Will it have to wait 5 minutes before reconnecting and will the inverter be able to sell energy to the down stream loads during this period?

These are just a few issues I still have questions on as I narrow down my selection.

Thanks,

JAM

I have owned SW4048 since 1999 and last year added two SW Plus 5548 units series stacked.

Pretty happy with Trace units but don't try to call or contact Xantrex on any questions or issues. They make it a point of not talking to their customers. They want the dealers to take this on.

As to your numerous questions.

Most of your desires are covered with the Trace/Xantrex SW units.

1) First a little understanding on how the SW series operator. You can look up U.S. Patent 5,373,433 if you want to read more details.

There are three transformers that have their secondaries connected in series. The three primary side of each transformer is chopped by a MOSFET bridge providing either +, -, or 0 vdc to each transformer primary. The secondaries on the three transformer produce about 15 v, 45 v, and 135 v. With the input FET chopping, the small transformer produces -15 v, 0 v, or +15 v. The mid-sized transformer produces -45v, 0 v, or +45 vdc. The big monster transformer produces -95 v, 0v, or +95 v. By mixing the combinations this gives 27 possible voltage steps which are put together in proper timing to come close to a sine wave. It is not a pure sinewave but pretty close. There are 52 power FET's in the larger power SW series units that make up the three input H-bridge drivers.

The voltages I listed are approximate and depend on the battery voltage. As the voltage of the batteries go down, slightly different transformer stacking combinations are used to match the input AC waveform.

The advantage of this design is the FET's are chopped at a very low frequency (60 to 240 Hz) compared to PWM (high frequency pulse width modulation) designs therefore has a relatively low no-load idle current drain from batteries. Compare the SW series 16 watts to a 3 to 4 KW PWM sinewave inverter of about 50+ watts of no-load power consumption.

Other advantage is the transformers provide isolation allowing easy connection to main breakbox with its common ground configuration.

The disadvantage is the weight of the transformers. The SW5548's weighs in at nearly 150 lbs.

Second important point on the SW series is the syncronizing and matching of the AC grid or generator input waveform. First thing SW series do is match the inverter voltage and phase to the incoming grid or generator.

This allows the SW inverter to run in parallel with the incoming AC power. This is normal mode unless the input power is lost at which time it actually is subjected to an overload (or open) and immediately (within half cycle) releases the input grid or generator relay from the parallel inverter. The inverter takes over the output load.

With this configuration the only difference between pulling power to charge the battery, or pushing power to the input AC source or output load is whether the transformer voltage stacking is a little higher or lower then input AC power.

This means you have a battery charger or ability to sell power to grid.

The original SW series did this without a Grid-Tie-Interface before regulations where established to address some issues with selling into grid. The primary issue is called 'islanding' which is a condition where you try to push power into an open power grid. This may be the case where a lightning strike to a power pole has caused the high voltage side breaker for a pole-mount transformer to pop open. The 'island' will be the group of houses that are connected to the secondary side of that transformer. It typically is several of your neighbors but could be more. 99.99% of the time your SW inverter will be overloaded by your neighbors' load and immediately release the grid connection relay. If there is light enough load the SW series would continue to push power into the open grid 'island' There are several other not-so-good effects of 'islanding but I think this gives you the main points.

Another important point, because of the quantizing step voltage limitations (27 steps to select from) the charging voltage never exactly match the needs. For example, assume your batteries are fully charged at float voltage level and take 0.3 amps D.C. of constant input current to maintain the float voltage level. If you have an ampmeter on your battery line you will see the SW series bouncing from + currents of 0 to 8 amps and -currents of 0 to -8 amps. The actual numbers vary depending on the input AC voltage and it transformer ratio'd down voltage. To avoid over or under charging, the SW series duty cycles the 'push' and 'pull' currents to the battery to average out the net current. For example, if it pushes 4.3 amps into battery half the time and pulls 4.0 amps half the time, the net result is the 0.3 amps you needed to maintain the float voltage. I believe this 'breathing' is actually good for lead-acid batteries.

So it is easy to imagine where the + battery charge current power comes from (from grid) but where do you think the - battery charge backwash is going, into the grid. So during normal charging you are effectively 'selling' momentarily to the grid, or pushing into your output AC loads.

I do not think they have changed the design to prevent this. They would have to dissipate a lot of power inside the unit to avoid pushing these over-charge compensations back to grid.

If I take this a step farther and imagine I have a PV charger directly on my batteries and the SW inverter set to a charge float voltage level lower then the PV is elevating the battery voltage to then the SW series would try to push the excess to the grid. I have turned down the float setting on the SW momentarily and watched it suck up current from the battery. I have not left it this way for an extended time so maybe the unit senses the net push current and eventually temporarily modifies the internally used float setting to keep the push to pull ratio near 50%. This would defeat the ability to 'sell' without intentionally setting up 'sell' on the inverter.

As to powering your air conditioner, the main problem is startup current on the AirCond compressor. I can pretty much say the SW5548 will not take the startup current unless you stay below 50 amps AC startup surge which is not likely for a 3 ton unit. The Plus series software allows more overload then the original SW series. This however comes with a momentary voltage slump on the Plus units. Refer to the SW Plus manuals available on-line in appendex A for the surge capability of the SW Plus series. A SW Plus 5548 will likely startup and run the AirCond. There will be a quarter to half second slump in AC voltage perhaps down to 80 to 90 volts as compressor starts. You might think if you want your refridgerator subjected to this momentary slump every time the AirCond kicks on.

The way the SW series handles all setting for charge current rates is done by measuring AC current input. There are no DC current measurement capability on SW series.

If you set max input AC current to 30 amps and draw an output load of 40 amps the inverter will pull from the batteries to make up the 10 amp AC input shortfall (load shaving).

If you also set max AC charge rate to 10 amps (about 28 amps DC into a 48 v battery system) it will first steal from the charging to supply output load. In above example, the charge rate would begin to drop as load exceeded 20 amps AC, go to zero at 30 amp load, and suck from battery above 30 amp load.

I think I covered most of your questions. As to the new grid tie adaptor, I think it is a 'band-aid' adaption to meet new regulations and salvage existing SW series design. Since I do not have interest in selling to grid I am probably prejudgist but I have not heard good things about the GTI adaptor box.