Friday, January 31, 2014

System Design with 1000V 3-phase string inverters

Much has changed in the solar PV industry in the past five years.  Solar panel efficiency has grown incredibly while prices have dropped so much that incentives are hardly needed to make solar projects financially viable.  New solar mounting technology allows for automatic grounding from the solar panel, with universal clamps and advanced ballasted and penetrating designs.  However, by far the biggest advances have been in inverter technology.  Inverter efficiency has steadily increased, voltage windows have gotten wider, and dual MPPT inputs are now a standard.  By far the biggest advance has been the approval of a true 1000V DC voltage window.  Coupled with dual MPPT inputs and a wide input voltage window, commercial system design is more flexible and adaptable than it has ever been.

1000V Strings

1000V strings are a direct improvement over 600V strings and the most visible improvement in new system design.  Nominal string voltage increases 66% and allows for smaller wire sizes in the array.   #10 wires can be used for string runs over three hundred feet long and still have a voltage drop under 1.5% for 9A strings.  This allows you to get away with one DC wire size for most projects, allowing you to keep things simple for your purchasers and installers.  In the past, inverter placement was more restrictive because long string runs needed to be eliminated to keep wire costs from getting out of control.  Now, inverter location is more flexible thanks in part to higher voltage and efficiency in the string wiring.

Size & Weight

The other factor that makes the new 1000V inverters so adaptable in placement is their size and weight.  Older central three phase inverters were extremely heavy and required a concrete pad for mounting/wiring them from the bottom.  Even a 30kW three phase central inverter could weigh over 1000 pounds and require a structurally engineering concrete pad be installed.  By comparison, a new 30kW 1000V 3 phase string inverter will weigh as little as 120 pounds and can be wall-mounted on a roof by two installers without any additional equipment.  You can either distribute 1000V inverters throughout the array or centralize them at one location, whatever is best for the project and its requirements.  This allows for much more flexibility when it comes to planning your install.

Reduced complexity

A new feature of the 1000V three phase string inverters is integrated string combiners in their wiring boxes.  This can be done because they have such a high input voltage, it reduces the number of strings required to reach rated power, so they’ve fit enough fused inputs into the inverter to handle almost any installation.  Because they are transformer-less, fuses are required on the positive as well as the negative conductors, so the manufacturers are making it easier for everyone by including integrated positive and negative fuse blocks.  Since the string combiners are already included in the inverters, external DC combiner boxes and inverter-integrated recombiners are eliminated from the design. 

Reduced BOS costs

Inverter-integrated string combiners save money two ways.  First, combined DC runs require larger wires that are exponentially more expensive than the wiring used for individual strings.  Using more of the same type and size wire usually allows you to save money by purchasing in bulk while also saving by only purchasing smaller wire.  Second, external DC combiners and inverter-integrated recombiners are both eliminated from the design.  For commercial projects, these are not cheap.  Specialized DC combiner boxes with integrated disconnects can be over $1000 each, and recombiners can cost several thousand dollars depending on the fuses or breakers required.  While specialized DC electrical equipment is eliminated from the design, it forces the inclusion of something to combine the three phase AC outputs of the inverters.  Three phase AC has significantly less line loss than DC, a cost savings on wire.  Additionally, three phase distribution equipment is less expensive, more available, and easier to work with for your electricians.  By moving the power distribution from the DC side to AC, your electricians and installers will thank you for giving them something more familiar and safer to work with.

Adaptability

1000V three phase inverters have unmatched adaptability for system design.  Dual MPPT inputs allows for two different string sizes in the solar array for each inverter.  In multi-inverter installations, which are the design standard with these inverters, you can have several different string sizes because each inverter’s DC input is isolated.  Pair the dual MPPTs with a wide voltage window, and these inverters can handle almost anything.  Some 1000V three phase inverters have DC voltage windows spanning over 800 volts, and can handle string sizes from 12 to 24 solar panels.  With a huge voltage window and two MPPT inputs to play with, the options available for the system designer and these inverters are amazing.

Restrictions

Designing with 1000V three phase inverters isn’t without its limits.  They are restricted to “inaccessible” solar projects, which means roof-mounted, solar carports, or utility projects that are behind the fence and not a part of the consumer grid.  Commercial ground mounts are not included, but you may be able to get an exception.  Some local municipalities or AHJs will not approve 1000V inverters in any solar project, so check what local requirements are before proceeding with a design with 1000V inverters.  The other big knock against these inverters is they are designed for a 277/480V grid only, no 208V models are on the market currently.  This can be overcome by purchasing an external step-down transformer from 480 to 208V.  Since so much money is saved in wire and DC equipment, a transformer can be added to a 1000V design and it will still be cheaper than a competing 600V central design without a transformer.

Conclusion

By eliminating larger wires and external combiner boxes, you are effectively simplifying the entire installation and reducing complexity and opportunity for error.  Replacing combined string runs with string wires makes wire pulls easier and saves money by allowing bulk purchases of smaller wire.  Installing various DC enclosures and managing/organizing wire sizes can add up over the course of a large project, and by using 1000V string inverters labor and equipment costs are directly and noticeably reduced.  Moving electrical distribution to AC instead of DC makes things easier, cheaper, and more effective.  Wide voltage windows and dual MPPT inputs make these inverters a no-brainer whenever you can use them.  It’s truly a solar revolution that we are living in, and 1000V three phase string inverters take full advantage of that.


2 comments:

  1. Thanks so much for this article, this is just what I needed today!
    Electric Energy

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  2. Wouldn't having two regulated DC/DC converters sharing outputs keep the same advantages with respect to independent inputs, yet gain simplicity because you don't need to worry about phase alignment in the inverters?
    Combining on a DC link bus allows you to add much more than two DC/DC converters in parallel, highlighting how much simpler they are.

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