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Stone Marmot Goes Solar: Part 5, Installing The Grid Tie Portion
By Sid of Stone Marmot
So, the last week in October, 2009, I gave Harrimans the go-ahead and a down payment for installing a solar electric system on our house. Three weeks later they notified me that they had all the plans, permits, and parts with one exception: They still hadn't received the battery backup inverter, an SMA SI5048, which wasn't expected for another couple of weeks. So they planned to install all the grid tie stuff first and add the battery backup stuff after the SI5048 was received.
They installed the photovoltaic (PV) panels, the grid tie inverter, and all the appropriate wiring and disconnects in less than a day and a half. Figure 1 shows the ten 200 Watt Sanyo HIP-200BA19 PV panels mounted on my south facing roof section. These panels are electrically identical to the older HIP-200BA3 panels, with the only change I could find being a slight change in width due to a slight change in the frame. These panels give the minimum of 2000 Watts total I desired.
Most all of the larger modern inverters have maximum power point tracking (MPPT), which automatically adjusts the load characteristics of the inverter on the panels so that the PV panels operate at the point where they are outputting the maximum possible power for the present light and temperature conditions. This maximum output power point can vary significantly with the type, orientation, and temperature conditions of the PV panels. Consequently, you want all the panels tied to a particular MPPT inverter to be matched as close as possible, that is, the same manufacturer and model and mounted in the same plane with approximately the same light and temperature conditions.
Figure 1 shows that these panels take up almost the entire south roof. We might be able to squeeze two more panels in with a bit of effort, but these additional panels would probably overhang the roof ridge caps. We wouldn't want to add panels on the other roof sections to the existing inverter because these panels would experience much different light and temperature conditions, giving the inverter conflicting information on what the maximum power point should be. This would probably cause the inverter to operate in a condition that is not optimum for any of the panels, assuming the inverter is able to find a stable operating point and doesn't start oscillating. Even mounting the additional panels on angled standoffs on the east or west roof sections so that they would face south and be at the same angle as the panels on the south roof section would still result in different temperature conditions for the added panels than those of the panels on the south roof. So ten panels is the practical maximum possible on our south roof, with twelve panels being the maximum possible with a bit of effort.
Figure 1 - Solar panels mounted on south roof.
Figure 2 shows the two additional power disconnects added on the outside of our house by the power meter. One is for disconnecting the direct current (DC) output of the solar panels from the inverter. This is required by many communities so emergency personnel, such as firemen, can shut off all electrical power in the house before they perform their emergency tasks. The other is an alternating current (AC) disconnect between the inverter output and the rest of the AC circuits in the house. This makes it possible for the power company to disconnect the solar electric system from the utility power grid in case they have to do maintenance on the local grid. This disconnect is also often required in many communities, though the power company will probably just remove the power meter to absolutely guarantee that the solar installation can't back feed the power grid during maintenance conditions.
Figure 2 - The two additional power disconnects added on the outside of our house.
Figure 3 shows the SMA Sunny Boy inverter mounted inside the garage by the main breaker panel. The inverter's 240 VAC output goes to the outside disconnect first, and then to an added breaker in the panel before tying to our AC power within the house. The smaller box at the bottom of the inverter provides a DC output disconnect for the sloar panels that is inside the garage.
Figure 3 - SMA Sunny Boy inverter mounted inside the garage by the main breaker panel.
The inspections, an electrical and a structural inspection, both went without any problems. This particular electrical inspector was exceptionally thorough, removing the covers off of everything and tracing all the wiring against the permit drawings, which was commendable and unusual. But one thing he missed and I didn't notice until the inspection was that Harrimans installed the wrong inverter; they installed an SB4000 and the contract and permit drawings called out an SB3000. The SB3000 and SB4000 are the same size and shape and look identical, with the only external clues to distinguish them being a very small print label on the side of the unit and the model number being flashed across the screen for a few seconds during power up.
Now in most cases this would not be a problem and would actually be a free upgrade, as the SB4000 costs about 25 % more and handles 33 % more power than the originally specified SB3000. The SB4000 could allow significant future expansion of the number of solar panels. But, as discussed previously, we have little room for expansion on this south roof. Also, I ran our particular Sanyo PV panels through the inverter manufacturer's compatibility software (http://www.sma-america.com/en_US/service/downloads/dl/U0Rlc2lnbl8xLTUyX0ludGVybmV0/download.html) and found out that the total output voltage of the PV array would vary from 240 VDC to about 374 VDC over worst case conditions. The SB3000 was specified for correct operation between 200 to 400 VDC, whereas the SB4000 was specified for 250 to 480 VDC operation. So our particular PV array would operate well within the SB3000 specifications but outside the SB4000 specs.
I contacted Harrimans about this. They said they checked the compatibility of the panels and the SB4000 with SMA software and found the SB4000 was compatible with between 4 to 8 Sanyo HIP-200BA19 PV panels per series string, with my system having 5 per string. It turns out they used SMA's online software (http://www.america.sma.de/newstringsizing.aspx), which you run online from the SMA website, whereas I used SMA's download software, which runs on your own computer. I tried the online software and got the same result as Harrimans. But then I noticed a “Predicted Outputs” button in the lower right hand corner of the results. I clicked on this button and the resulting data showed that the minimum voltage at 104 degrees F outdoor temperature from these Sanyo panels would be 236 VDC, lower than the 250 VDC minimum shown for the SB4000. In fact, this data showed that only 6 to 8 of these Sanyo panels per string was compatible with the SB4000, which conflicts with the 4 to 8 panels per string results shown on the previous web page. This same software showed that the SB3000 was compatible with 5 or 6 of these Sanyo panels per series string, as the downloaded software did.
I called SMA's toll free help line and the technical person who responded was very emphatic about using the SB3000 and not the SB4000 in our installation, though she wouldn't say what the consequences of using the SB4000 would be. So I called Harrimans and pointed out that the data for both compatibility software programs indicated that the lowest possible voltages out of the Sanyo PV array was outside the specified operating range for the SB4000. They agreed to replace the SB4000 with the originally specified SB3000.
Note that the system was operating correctly at that time with the SB4000. But it was also the end of November, when temperatures tend to be cooler in Florida. The low voltage condition would tend to occur during hotter temperatures, such as those typical during the Florida summer. In fact, most people would probably never notice the problem, but may only notice they weren't getting as much power out of the system as expected during the summer. But without any previous experience with PV systems, most might think this is normal.
Note also that I don't blame Harrimans for this problem as they did check for compatibility and the SMA software did initially indicate that five of these Sanyo panels per string was compatible with the SB4000. The problem is really with SMA in that they have inconsistent results in their software, which SMA needs to fix. Once the problem was pointed out, Harrimans did swap the inverters with no fuss.
So now we had a working grid tied solar electric system. But we didn't let it operate all the time. Why not? The answer to that is the subject of the next article.
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