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.


Friday, January 24, 2014

Performing a Solar Site Analysis

The site analysis is a critical junction in the process of selling and installing a solar system.  Proper information gathering leaves your installation team with everything they need to know to prepare for the install.  Leave out the wrong piece of info and there could be last minute design changes, emergency trips for more equipment, and an overall lack of professionalism in the eyes of your customer.  A good site analysis will give you all of your design information in an efficient and structured manner and leave a lasting impression on your customer.

Roof

The roof analysis begins by noting the roof type and condition.  If it’s a flat roof, make sure you get the roofing material correct.  This is important for ballasted systems (common on flat roofs) where roof pads may be needed to protect the roof from sharp metal edges in the solar racking.  If it’s pitched, the same question on roofing material needs to be answered.  Shingle roofs are most common, but if it’s a metal roof you need to note whether it’s corrugated, pro-panel, or a standing seam roof.  Tile roofs can be flat tiles or curved Spanish tiles, each require their own specialized attachment.  Getting the roof right will ensure you get the proper roofing protection, whether that is flashings, roof pads, or tile hooks. 

Obstructions can come from a variety of sources.  Parapet height needs to be measured for southern roof edges.  Vents, pipes, skylights, and other roof obstructions need to have their location noted if they will affect the solar window in your planned mounting area.  Rule of thumb is 2.5 times the height of the obstruction is the length of the shadow it will case on the winter solstice, read my blog post on calculating shade spacing for a more in-depth analysis.  One way to make measuring obstruction locations easier is to use two measuring tapes or one tape and a long string.  Lay the string/tape out in a perpendicular or parallel line from your obstruction then use your measuring tape to take measurements off of this straight line.  This way, complicated areas and weird geometry can be accounted for.

One thing you can do to save time on the roof is do “roof triage” where you focus your measurements and analysis to the areas of the roof you know will fit solar, and ignore/do a cursory examination of the rest of the roof.  This can be especially relevant if there are a large obstructions off-property shading the roof like trees or buildings, or if there are so many roof obstructions an array just won’t fit there.

Sometimes for roof mounted jobs the AHJ/permitting office will require framing diagrams of the underlying roof structure, even for flat roofs.  To ensure you have this information, take pictures of the attic or refer to the building plans to find the framing direction and spacing on the roof.  This also helps you plan out your roof penetrations and framing layout for your solar system.

Something many installers forget when they’re looking at a roof is how they plan to run wire and conduit within the solar array and back to the point of connection.  Sometimes you are forced to break the array up over multiple roof sections, and running strings or branch circuits across the roof can get expensive.  It’s very important to consider the conduit run and location, it can be a very visible and expensive element on the roof unless properly designed. 

Ground

An analysis of a site for a ground mount is pretty straightforward.  Begin by looking for sufficient roof for the solar array.  The big thing to look out for is a septic system or leech field which is usually placed in an open area directly adjacent to the house (sound familiar?).   Underground lines and pipes all need to be accounted for in the array location as well, and avoided at all costs.  Make sure the point of connection on the building is known, because the trenched wire run back to that location needs to avoid these as well.  Something that may not be on your radar when doing a ground mount is the water table and drainage.  If there’s a drainage path or high water table, a solar array cannot be installed on the ground there.  Lastly, the utility drop and any easements on the property are off limits for solar, so stay away!

The ground and substrate material needs to be considered if you are making ground penetrations/footings, solid rock or dense clay may require specialized groundwork equipment to install your system.  You should be noting what grading or groundwork may be required in any case, so when the install happens the site is properly prepared for the solar array.

BOS (Balance of System)

BOS is the rest of the solar equipment needed for an installed system to function properly.  For the purpose of this post, BOS is AC inverters, disconnects, and meters.  The first thing to look for on the outside of the building is the location of the Main Distribution Panel and Billing Meter.  After that, start looking for working space for all of your electrical equipment.  Usually, there will be a solar inverter or subpanel, a single throw safety switch, and a meter socket that all need to fit on a single wall.  Things that need to be avoided are gas lines, water canales and spigots, AC units, existing electrical equipment, and doors and windows.  Additionally, the southern and western exposures should be avoided if possible.  These building faces soak up the sun during the hottest part of the day and can adversely affect system performance by reducing voltage and efficiency of the wiring.

Most AHJ’s/utilities will have specific requirements for BOS location and placement, follow their requirements above all else. Many of these requirements have to do with equipment line of sight to the billing meter/point of connection and accessibility of equipment for utility workers.  Couple these requirements with avoiding gas and water lines, and you can find yourself with little to no space to install your BOS on some jobs.  In those cases, you may have to make an external “equipment rack” out of metal strut to mount your solar equipment, an expense that has to be accounted for.

Tools

The tools you should bring to a site visit can change based on the type of roof and nature of the solar project.  For most any job, you’ll want a measuring tape or two to quickly dimension areas and locations of equipment.  A ladder and a way to quickly secure it to your vehicle are needed for any roof mounted project (sometimes two ladders are needed for large/tall roofs, make sure you have enough!).  It may not seem like much, but having a screwdriver or two is a big help for opening stubborn electrical panels.  For electronics, a camera is your safest bet for gathering effective site information.  A calculator is good to have, and if you can get one an Inclinometer is perfect for measuring roof pitches and drainage slopes.  Most of these can be found in modern smart phones and internet enabled devices, they are one of your best tools on the roof.  Finally, no discussion on site analysis would be complete without talking about the Solmetric Suneye.  If you can afford one, nothing is better at providing a comprehensive snapshot of nearby shading issues for the solar array.

Customer Interaction


This is the big one, the one that can differentiate you from the competition and make your customer’s solar installation experience one they’ll remember fondly.  The customer has been talking to their salesperson about a solar system, and now there’s a stranger at their house on top of their roof and poking through their electrical boxes.  Many times what was sold to the customer does not match up with what can be installed on their building, and it’s your job to help guide them to the final design and show them how it will work.  They’re paying thousands of dollars for an elaborate electrical system that they may not fully understand, so you have to be prepared for their requirements and suggestions.  Start by reading the customer, seeing how picky they are and how they respond to your installation plan.  Any reasonable suggestions or requirements they make should be followed as long as they are happy with the finished product and it follows local requirements.  If you can’t meet their requests, try to manage their expectations and explain why you have to do it a certain way.  As long as you are courteous and respectful, your point should be well-received.

Friday, January 17, 2014

Wire Sizing Tools and Resources

Hello ladies and gentlemen!  To wrap up my extended series on wire sizing and voltage drop in solar systems, I want to share some wire sizing tools/resources for calculating voltage drop that can help make your life easier.

Online Wire Sizing Configurators


This is an extremely simple voltage drop calculator that assumes no conditions of use similar to my examples for voltage drop in the previous post.  It allows you to input any voltage into it, which makes it unique among the free online voltage drop calculators I checked out.  It’s simple but easy to use and understand.


This calculator lets you select from a long list of preset voltages, and gives you the choice of DC, AC, or AC three phase.  The issue with this calculator is the list of voltages is very arbitrary, there are “holes” in its selection that prevent it from being a comprehensive tool.  Other than that, it lets you adjust conductor temperature to include derating due to conduit fill and ambient temperatures.  It also lets you select your conduit type.  All in all, it’s a very good calculator for AC voltage drop and an OK calculator for DC. 

Online Wire Sizing & Voltage Drop Information Resources

These are some of the resources I have used in the past for learning about voltage drop and wire sizing.  The information in these articles is extremely useful and in-depth.  I owe a great deal of my knowledge on this subject to them.



http://en.wikipedia.org/wiki/Voltage_drop#How_to_calculate_voltage_drop

Thursday, January 16, 2014

Episode 5 pt 3: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material).  This post will cover selecting the correct wire size based on your maximum solar current and calculating the voltage drop based on that.  I will be heavily referencing the NEC 2011 codebook throughout this post.

Conditions of Use

Before getting into selecting wire sizes, I want to cover conditions of use.  These are factors that need to be applied to the maximum current we calculated earlier based on environmental conditions.  The most obvious condition of use is ambient temperature, found in Table 310.15(B)(2)(a).  Based on most sources, the average high temperature for the location should be used.  This can be obtained from the ASHRAE handbook (the 2% value) or determined from weather data available online.  The other conditions of use are more complicated.  Conduit fill is an important consideration for large systems and long conduit runs, it is found in Table 310.15(B)(3)(a).  This is because wiring will generate heat during operation and the hotter it gets inside the conduit the worse the voltage drop will get.  Conduit distance above roof for roof mounted systems is very important as well, and can make the conductors even hotter, reference Table 310.15(B)(3)(c) for more information.  For the purpose of this post, conduit fill and conduit distance factors will both be assumed as 1 for power production.  Ambient design temperature will be fixed at 85° F, which gives a modifier of 1 on our current calculation for simplicity.

Wire Sizing

From my previous submission, maximum DC solar current is based on the solar panel’s Isc rating multiplied by 1.56.  The maximum AC solar current is the inverter’s maximum continuous current multiplied by 1.25.  Taking these maximum calculated currents, proper wire sizes for them can be selected using NEC 2011 wire sizing methods and tables.  These methods will assume conductors and terminals are rated for 75° C.  The first table to reference is 310.15(B)(16), where the 75° C Copper column will be used to select wire size based on the maximum calculated current.  For example, for a solar panel Isc rating of 8.5A, our maximum DC current is 13.26A.  Using table 310.15(B)(16), the minimum wire size #14 AWG.  For a solar inverter with a rated current of 25A, maximum AC current will be 31.25A.  This results in #8 AWG wire.  Conditions of use, if we had them, would be applied here to the overall ampacity of the wire to derate it and check against our maximum calculated current.  Conveniently, all of our conditions of use for this example are fixed at “1”.

Now that the wire size has been found for the maximum calculated current, it needs to be checked against worst case voltage drop in the conductor for the designed length of the circuit.  If the voltage drop would be too high the wire has to be upsized to the next size to correct for it.  The maximum voltage drop is 1.5% for DC and 1.5% for AC circuits based on my previous post. 

Calculating Voltage Drop

The heart of voltage drop calculations is Ohm’s law, which reads V (Voltage) = I (Current) x R (Resistance). The units are Volts, Amps, and Ohms, respectively.  Voltage drop calculations are based on values for Resistance (Ohms) per 1000 feet found in NEC tables in Chapter 9, Table 8 or 9 (DC circuits Table 8,  AC circuits Table 9). The value we need is for stranded, uncoated copper conductors.  This changes the formula from V = I x R to Voltage Change = I x (R / 1000’) x Total Circuit Length
Written another way, it can be expressed in terms of 1-way circuit length and converted into a percentage value:

Equation for Voltage Drop based on NEC 2011 Chaper 9 Tables 8 & 9


Going back to my example in Wire Sizing, let’s consider a DC and an AC wire run for a 100 foot distance and calculate the voltage drop for each.  For DC, we have:

#14 AWG Wire  300V String         I = 13.26A            L = 100 feet         R = 3.26 (NEC Chapter 9 Table 8)

VD = 2 x 100 x 13.26 x 3.14 / 1000 = 8.33V               8.33V / 300V = 2.78% Voltage Drop

Based on our maximum desired DC voltage drop of 1.5%, this obviously will not do.  We need to upsize the wire to get a better voltage drop.  Rerunning the numbers for a #12 wire gives:

VD = 2 x 100 x 13.26 x 1.98 / 1000 = 5.44V               5.44V/ 300V = 1.75% Voltage Drop

Close, but not quite what we wanted.  Obviously a #10 AWG wire is the way to go, but just to be sure:

VD = 2 x 100 x 13.26 x 1.24 / 1000 = 3.42V               3.42V / 300V = 1.10% Voltage Drop

That’s got it right there!  A #10 AWG wire will give us ~1.1% voltage drop for a 300V circuit over 100 feet (In general, I’ve found that a #10 wire is best for solar string wiring for most roofs).  Now that we’ve figured out DC voltage drop, AC voltage drop is pretty simple.  Using Table 9 of Chapter 9, we need to use the values in the column labeled “Alternating-Current Resistance for Uncoated Copper Wires”.  Going back to our earlier example:

#8 AWG Wire     240V AC               I = 31.25A            L = 100 feet         R = 0.78 (NEC Chapter 9 Table 9)

VD = 2 x 100 x 31.25 x .78 / 1000 = 4.875V               4.875V / 240V = 2.03% Voltage Drop

Again, we’re close but not quite where we want to be for this AC wire run.  Upsizing the wire to #6 results in:

VD = 2 x 100 x 31.25 x .49 / 1000 = 3.06V 3.06V / 240V = 1.28% Voltage Drop

Based on our examples, a #10 wire will give us a 1.1% voltage drop for a 300V, 8.5A string of solar panels over 100 feet.  A #6 wire will give us a 1.28% voltage drop for a 240V, 25A solar inverter over 100 feet.

Wire sizing and voltage drop calculations are an extremely important consideration for any solar designer or installer, and it’s essential to understand the fundamentals behind them.  The calculations and examples in this post have been kept simple on purpose, the subject can get complicated in a hurry when multiple conditions of use need to be applied.  In my next post, I will share some of my favorite tools and rules of thumb for calculating wire size and voltage drop.

References

NEC 2011
Conditions of Use: 310.15(B)(2), 310.15(B)(3)
Wire Ampacity: Table 310.15(B)(16)
Wire Resistance: Chapter 9 Tables 8 & 9

Monday, January 13, 2014

Episode 5 pt 2: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material).  Today I’m going to go over how to calculate your maximum current for a designed voltage drop in a solar system.

Based on last week’s article, the amount of voltage drop to design for in a solar PV system is usually around 3% unless otherwise specified.  This should be split between the DC and the AC conductors, so 1.5% maximum DC and AC voltage drops are the end goals.  If microinverters are being used, there are no DC conductors that need to be sized.  In that case, the AC voltage drop should still be kept below 1.5% to help with grid synchronization.  Now, it’s time to break out the NEC 2011 codebook and start figuring out how to size our wires.

Current Calculations

Before getting into voltage drop and circuit distances, the first thing we need to figure out is how much current to design for.  Looking at solar panel and inverter data sheets, there’s a few different values for current we can use.  On the DC side, our starting point will always be the solar panel’s Isc current rating at STC.  This is based on NEC article 690.8, Circuit Sizing and Current.  We are concerned with 690.8(A)(1) and 690.8(B)(1)(a) for DC current calculations, which state:

690.8(A)(1): The maximum current shall be the sum of parallel module rated short-circuit currents multiplied by 125 percent.

690.8(B)(1)(a): Overcurrent devices, where required, shall be rated to carry not less than 125 percent of the maximum current calculated in 690.8(A).

While these talk about overcurrent devices, our circuits will have to be sized based on these same requirements.  Now, looking at the solar panel’s Isc rating, it has to be multiplied by 125% per 690.8(A)(1).  Then to size wire for it, an additional 125% multiplier needs to be added per 690.8(B)(1)(a).  Taken together, the DC current multiplier for wire sizing is 1.56 times Isc.

On the AC side, the inverter’s rated continuous current (make sure it’s for the correct AC voltage!) needs to be used for wire sizing calculations.  Like on the DC side, there’s a factor that needs to be applied to find our AC wire sizing current.  Looking at the same NEC article, the inverter output circuit current can be found using 690.8(B)(1)(a) and again referencing 690.8(B)(1)(a), which together state:

690.8(A)(3): The maximum current shall be the inverter continuous output current rating.

690.8(B)(1)(a): Overcurrent devices, where required, shall be rated to carry not less than 125 percent of the maximum current calculated in 690.8(A).


Unlike on the DC side, only one 125% multiplier needs to be applied on the AC current to find our maximum current for wire sizing.  The final AC current multiplier for wire sizing is 1.25 times rated continuous current.  Next, we’ll select wire sizes and calculate voltage drop based on our solar current calculations.  To keep post length down, I’m breaking it up into multiple submissions.

Saturday, January 4, 2014

Episode 5 pt 1: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material). Today I’m going to look at designing for voltage drop and how to design for it so your solar array functions properly.

The maximum voltage drop you should design for in a system is not set in stone.  For commercial systems, it can be specified in the bid documentation, but for smaller systems you are usually left trying to figure out the best value that will deliver the most energy without breaking the bank on wire.  While the National Electric Code (NEC 2011) does not explicitly provide a maximum permissible voltage drop for solar PV systems, you can find a reference point in Articles 210 and 215 in fine print notes.  They state that overall voltage drop should not exceed 3% for branch or feeder circuits and 5% overall.  Based on this, most solar systems are designed to never exceed 3% overall voltage drop as a feeder circuit, with no more than 1.5% voltage drop on the AC side.

The reasons you want to keep voltage drop low in your solar system are multiple. 

First, the obvious reason is wasted energy and wasted dollars. A solar array is a major investment, and if an upgrade from #12 to #10 wire will provide a significant decrease in voltage drop it will pay for itself throughout the life of the system.  A difference between 1% and 2% voltage drop between your inverter and the interconnection is a difference of 50W for a 5000W system, and it quickly adds up the larger it gets.

On the DC side of the inverter, you need to hit a defined voltage window.  If you size your solar strings to the lower end of that range, a high voltage drop could put you outside of the maximum power point tracking window of the inverter. This is at its worst in hot weather, when voltages are already low and the inverter is hot.  This is also some of the best production time for solar PV systems all year, optimum performance is critical and your DC wiring needs to be sized with that in mind.

AC voltage drop is possibly more important than DC for solar PV systems.  The issue is one with the electric grid and your inverter.  Your service voltage can vary depending on several conditions including weather and time, and the inverter is constantly working to match up with that voltage.  This is why on inverter data sheets you see an AC voltage range the inverter is capable of producing, typically -12% / +10%.  For a 240V inverter, this gives an upper voltage limit of 264V.  Grid voltages as high as 250V or higher have been seen for 240V services. iIf your AC wire was sized with a 5% voltage drop your inverter would see 262.5V, almost its limit and at risk of going into fault.  This is why low AC voltage drop is extremely important, and most commercial bids and lease agreements require 1% or less.


Now that you have a guideline for how much voltage drop you want to design for in your solar system, how do you do it?  Next week I’ll be digging into the NEC 2011 codebook for wire sizing methods and solar current calculations.  I'll also share some of my favorite online/mobile tools and rules of thumb to help make this as smooth and painless as possible.  Happy New Year!