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Issue #211 February 2008
Intelligent Energy Solutions
Solar-Powering the Circuit Cellar
Part 3: Wiring & Electronics
by Steve Ciarcia
Start | Back to the Solar Panels | Wiring and Inverters | It's All About Architecture | Maximum Power Point Tracking | So, Does It Work? | Conservation | Soaking Up Some Photons | Sources & PDF
WIRING AND INVERTERS
With all that information about the panels, what values are important when we select wire and inverters? Basically, think big.
Seriously, solar system configuration is an exercise in accommodating worst-case conditions. While we might discuss array output power at nominal conditions, the wiring from the arrays and the inverter input range must be sized for the highest short-circuit currents and open-circuit voltages.
The roof array is configured as two series strings of 10 panels connected in parallel. The nominal current and voltage of this configuration is 400 VDC at 10.5 A. The open-circuit voltage and short-circuit current rating of the panels increases these values to 477 V and 11.5 A. Similarly, the pole-mounted SPR-205 arrays are configured with two series strings of eight panels connected in parallel and would be 320 VDC at 10.26 A, 383 VDC, and 11.06 A, respectively. That basically says that the inverter inputs and the wiring insulation have to handle 500 VDC. However, the national electric code requires a 125% safety margin applied to the worst-case voltage and current values when configuring the wiring and circuit breakers. For the pole-mounted arrays, the requirement is 478 VDC and 13.83 A. For the roof array, it is 596 VDC and 14.4 A. Because Sunlight Solar Energy fully understood that “bulletproof” was the primary decision factor on this installation, they decided that the neatest and cleanest way to deal with three separate arrays of this size was to use three separate conduits connected to three separate inverters. The wire connecting the arrays to the inverters was about 100¢. Like I said in the first article, using the higher voltage in a grid-tied system eliminates many of the IR losses associated with low-voltage battery backed off-grid systems. In my case, we used #6 copper wire (rated for 37 A) from each array to the inverters and #4 copper wire (rated for 60 A) from the combined inverter AC outputs to the service entry panel. The wiring losses for the total system were less than 1%.
The inverters were chosen with the same accommodation to safety margins. One SPR-3300x was used for each pole-mounted array and one SPR-5000x was connected to the roof array. The inverters are manufactured by Xantrex Technology and are functionally equivalent to their GT3.3 and GT5.0 inverters. Photo 4 shows how Sunlight Solar Energy mounted them to an inside garage wall (with room for accessories). ;-)
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| Photo 4—The three SunPower/Xantrex inverters are mounted on a garage wall where it is cool and offers easy access to the inverter data ports. |
When selecting inverters, it’s important to keep in mind that when we are talking about solar panels, we are discussing DC watts of input power to an inverter, and when we are talking about the inverters, we are discussing AC power output after all the wiring and conversion losses. Applying 3,200 W from a pole array to a GT3.3 rated for 3,300 AC watts output is not running the inverter at maximum level. In fact, Xantrex suggests that arrays up to 3,600 W STC can be safely connected to the GT3.3. Similarly, the 4,200 W from the roof array applied to the GT5.0 is well inside its range. It is more expensive to use an overrated inverter, but as long as the array output voltages stay within the inverter’s maximum power point tracking (MPPT) range, you get the added benefit of less output power derating caused by high heatsink temperatures when the solar system is really cranking. As Figure 2 shows, the advantage of running a GT5.0 inverter at 80% of rated output is that it can take more heat before derating its output. I will probably add a thermostatically controlled fan to each inverter but it may not be necessary. While I haven’t used the system during the summer yet, the heatsinks seem barely warm running at 9 to 10 kW with 80°F ambient. Because I have the inverters located in a cool garage, the fans could be just more overkill.
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| Figure 2 —GT5.0 power output derating as a function of temperature. |
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