I appreciate Shoal’s new arc fault technology as shown in this advertisement, however I was shocked to see the photo of a man standing on PV modules.
This is a reminder that you can not stand on solar PV modules like this if you expect them to last 25 years. Standing on the modules creates micro fractures on the cells that will lead to premature failure. If you were ever in a difficult situation where you needed to step on a module, at least be sure all your weight it applied to the frame and not on the module surface (but even that is risky).
When you select the wrong transformer enclosure rating, moisture will get into the transformer enclosure and cause faults. NEMA 3R transformers are effective at keeping out rain, but sometimes no snow drifts.
This was a NEMA 3R transformer (some of the rain shields are removed in the photo to gain access) and supposed to be rated for outdoor locations and rain, but snow drifts were able to push up and under the shields and into the enclosure where it cause a significant fault. The system outage lasted for months while the insurance companies fought it out.
While conducting O&M on a Satcon 250kW central inverter, we noticed this fan with an object lodged between the blades.
After inquiring with the owner, we learned another O&M company was called in to repair a squealing fan bearing. Apparently, rather than replacing the fan they opted to stop the noise by stuffing cardboard inside the fan which preventing it from spinning. While that stopped the noise, it put the inverter at risk of overheating. We have seen a lot of corners cut over the years, but this may be the most egregious offense yet.
Learn from this owner’s mistake and only hire qualified technicians to repair your solar PV system!
In this installation, an AC panel board was located at ground level which combined the AC circuits from string inverters on the roof. We wanted to avoid entering the top of the enclosure with conduits, so a trough was used so that the feeders could enter through the bottom of the enclosure.
During O&M we often find combiner boxes and disconnect switches with water inside them, despite the box being NEMA 4 and fully gasketed, no top or side penetrations, and weather tight connectors properly installed.
Where is the water coming from?
The water enters the enclosure as moisture in the air, which later condenses into water. During the day, the bus bars heat up and warm the air in the enclosure. When air warms, the amount of moisture it can hold increases. At night, the air in the conduit or enclosure cools, and the water vapor in the air condenses. Each daily cycle only generates a tiny drop of water, but after several months this begins to add up.
Water may only travel downwards due to gravity, but moist warm air can enter and travel through a conduit and enter the enclosure from any direction. This is how water enters an enclosure even when there is no apparent way for water to run down and into the cabinet.
The solution is to seal the conduits where they enter the enclosure to prevent fresh air from getting inside the box. Traditional duct seal is effective and readily available. Expanding foam sealants area also available and very effective, however they must be approved for direct contact with the electrical cables.
Here are two examples of enclosures with only bottom entry conduits, but ground water entered the underground conduit, evaporated up the conduit into the enclosure, then condensed.
At this site, the intermodule wiring is zip-tied to the rails but the string homeruns are laying loose on the roof, which is poor workmanship. Over time the rough shingles will wear away on the insulation compromising its integrity and putting the system at risk for faults. In the winter, snow and ice will surround this wiring, and as the frozen ice tries to slide down the roof it will pull on the conductors. Wire must be kept up off the roof and secured to the rails.
Many roofs are designed to support thousands of pounds of HVAC units and large air handlers, so the additional weight of an inverter is feasible in many cases.
If the structural engineer determines the building can handle the load, the next thing to consider is how you will mount the inverter on the roof. One option is to cut out a section of roof membrane and insulation and install a flashed curb that is bolted directly to the roof deck below. Effective, but involves cutting a hole in the roof, which we always try to avoid.
Pure Power has a better solution that is more cost effective, easier to install, and that doesn’t involve cutting a hole in the roof.
You can place the inverter directly on the roof surface, using an outrigger pad to evenly distribute the weight across the roof membrane and insulation. Polyisocyanurate insulation is has a compressive strength of least 20 PSI, which converts to 2,880 PSF. A 100kW inverter weighs around 3,000 across a footprint of 15 square feet, which translates to an average load of 200 PSF, less than one tenth the compression strength of the roof insulation.
In the past, we have used 2” thick white outrigger pads (used to distribute the weight of crane outriggers) with a slip sheet which look great! One tip, locate near a wall so you can provide a lateral support to address the concern of tipping over during a hurricane.
The cost savings of Aluminum conductors is too good to ignore. Both the engineer and the electrician must respect the differences between copper and the less forgiving aluminum. However, if designed and installed properly, Aluminum can reduce the installed cost and perform just as well as copper.
The four steps and the pitfalls of aluminum will be discussed below
Strip insulation off conductors
Use a wire brush on the bare conductor strands
Apply No-Ox compound
Install a lug appropriate for the thermal cycling
Stripping the insulation – Extreme care must be taken when stripping the insulation from an aluminum conductor. Using a utility knife may be acceptable for copper, but it is not for aluminum. If the soft aluminum in nicked, over time those strands will break free and the lose strands will cause small shorts and sparks over the air gap of the nick, eventually leading to a failure.
Wire Brushing – You should wire brush the exposed conductor to remove oxidation before applying the oxide inhibitor and terminating the conductor. This step will remove any excessive oxide from copper or aluminum wire and remove any pieces of insulation or other contaminants that might interfere with your connection.
Oxidation – Oxidation is to aluminum what rust is to iron. Oxidation will increase the resistance of the connection. Anti-oxidation compound (AKA “no-ox”) will mitigate oxidation and promote a low impedance connection to the lug.
Thermal Cycling – Due to the thermal cycling from day to night, the conductors expand and contract. Aluminum expands and contracts more than copper and is much softer, so when mechanical lugs are used the cable can work itself loose. Mechanical lugs should not be retorqued as part of any routine maintenance procedure because repeated tightening of any set crew mechanical lug could result in the eventual biting through of the conductor which will ultimately cause a failure of the connection. This is why Pure Power Engineering strongly recommends the use of compression lugs, which will ensure a long life and secure connection.
Note: A hydraulic crimper for compression lugs runs about $2,000, but once you have the crimper the material and labor cost is equal for mechanical and compression lugs.
When deciding what size and quantity of combiner boxes to use, always remember to use the full capacity of what you are buying.
Example with external fused DC disconnect switches:
Suppose you have a 100kW system with 30 strings, and string has a short circuit current of 8.5 at STC. How many combiner boxes should you use?
Option 1 – (3) 10 string combiners:
Each 10 string combiner box will have a design short circuit current of 132A. You must round up to the next size DC disconnect switch, which is 200A which costs $700 materials plus $700 labor. Therefore you are buying three disconnects but only using 66% of the capacity on each. The total cost for the three disconnects is $4,200, with $1,400 of that ($466 per combiner box) being wasted unused capacity.
Option 2 – (2) 15 string combiners:
Each 15 string combiner box will have a design short circuit current of 199A. This would also go on a 200A disconnect, but uses the full capacity of the switch. The total for the two 200A disconnects is $2,800, and no money is wasted on unused capacity.
Small but wise decisions during the engineering stage make big differences on the cost of construction. In this example a properly value engineered design will save the client $1,400 (1.4 cents per watt), all while making no sacrifice on performance or quality. When you start the design of a project, before you start breaking the system up into sub arrays, keep in mind the sizes on the equipment and always try to use the full capacity of the equipment you have before adding the expense of another component.