Feed back on this system, Please

Sun Fun
Sun Fun Registered Users Posts: 16
OK,

I am slowly getting to grips with this system. Can somebody take a look at the drawing attached and make any comments on any obvious or not so obvious mistakes or futur problems.

https://www.yousendit.com/download/dVlybUpUQ0NsUitGa1E9PQ

Also, should the solar panels be earthed? If so, Why?

And finaly is there a best angle for solar panels? We have good coverage facing south currently at about a 45 degree angle.

Oh and one last small detail, when batteries are overcharged does the voltage increase and if so potentially by how much.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Feed back on this system, Please

    There are two places that Solar PV systems are earthed... One is the frame of the solar panels and the second in the common negative (or positive if positive ground--such as telecom systems) ground bus connection.

    There are several issues with grounding... One is multiple ground points. When this happens (say you ground the "-" at the array, the "-" at the battery bank, and the "-" at the load--say a Ham radio rig). What can happen is a nearby by lightning strike will create a voltage field in the earth... And your three ground points (xxx feet a part) will now have voltage/current induced into your ground system.

    Another problem with multiple grounds is you can have shared current (say you have a ground "bus" and a "-" bus. If you ground both the battery and the load--you have shared current flowing through both sets of wires--generally avoided as ground is supposed to be a safety current path--not a sharing of current flow. A grounded lightning strike can now flow both through your safety ground and your "-" bus--possibly causing more damage).

    Some here live in lightning prone areas (Florida)--such as Solar Guppy. He prefers not to ground the PV Panel frames as this just makes them more attractive to lightning strikes. If I recall correctly, he just disconnects his array's from the charge controllers/inverters until the storm has gone by.

    Grounding is a very complex issue, may involve electrical codes and safety issues. And mixing different grounding philosophies can be worse than no grounding at all. If you search here for solar PV grounding--you will find lots of threads and discussions.

    Regarding battery voltage and overcharging... Typically, the batteries will increase out-gassing (of Hydrogen and Oxygen) as they overcharge (a danger in itself--if there is improper venting and/or if done to sealed AGMs which are very sensitive to overcharging damage).

    And, if there is enough current--the battery temperature increases, and their voltage falls (because of the increased temperature). It is possible to cause "thermal run-a-way" if the controller sees the voltage drop and increases current even more.

    Another reason for having a charge controller with a remote battery temperature sensor to reduce the possibility of thermal run-a-way.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Sun Fun
    Sun Fun Registered Users Posts: 16
    Re: Feed back on this system, Please

    Just when I thought it was getting easy!!

    I understand the issue of grounding, mixing the different philosophies and lightning strikes but you didnt explain why the panels should be earthed. Assuming we are in a low risk lightning zone.

    And did you say the negative on the batteries should be earthed too?!?!?

    (Perhaps your friend in Florida could conect a remote relay switch to his barometer and save him all that trouble when there is a storm;))

    Even though the batteries at present without a regulator and vulnerable to overcharge how can I tell if they actually are? And could that have been the reason the last inverter blew up!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Feed back on this system, Please

    When it said to "ground the panels" there are really two grounds to be discussed...

    One it the exterior metal frames of the panels--and the other is the DC wiring (power leads) from the panels.

    In general, the NEC looks at grounding for several reasons:

    1. To prevent a metal surface from accidentally being "energized" by an internal power to frame ground (don't knock somebody on their butt because they touched and exposed piece of metal).
    2. To provide a solid connection to ground/power return--so that if there is an internal short to metal case of some kind--there is enough excess current that will blow a fuse / pop a breaker (a reason that there is never a fuse/breaker in the ground/return lead--an open fuse/breaker could leave the "return" path energized).
    3. To help reduce the chances of lightning from entering the building--keep it on the outside of the building and keep the occupants safe(r).
    4. For DC systems--to help prevent electrolysis of metal buried in the ground (if there is a "positive" leak of voltage/current, anything "negative" of that will have accelerated corrosion--such as water pipes, other utility services).
    5. For elevated metal structures (such as masts, antenna, etc.)--there is actually a (roughly) uniform electric field around the earth of ~100-300 volts/meter (much more when under electrical storms). And if the mast/antenna was isolated/ungrounded--a nasty (even dangerous) high voltage capacitive charge can build up over minutes/hours. A solid ground will reduce that risk.
    The problem with lightning--there is a whole bunch of energy and the chances are that much of the wiring will vaporize and some of the energy will still get into the structure.

    Regarding your batteries and how to tell if they are overcharged... Normally, with deep cycle battery chemistry--you end up adding lots of distilled water every month or two if overcharging. With your batteries, because they are Lead/Calcium variants (related to "maintenance free" car batteries)--they out-gas much less, so "use" less water. So--that won't work.

    You can get a cumulative Amp*Hour meter (such as from here)--Batteries are pretty Amp*Hour efficient (as opposed to power efficient). Basically if you take 1 Amp*Hour out, you need to put 1 Amp*Hour back. (power is you take about 1 Amp*Hour*12.5 volts = 12.5 Watt*hours; and put back 1 Amp*Hour*14.2 volts = 14.2 Watt*hours -- so they are typically much less than 100% energy efficient).

    And, there is the 100% correct device which is that Battery Monitor--however those are not cheap and you still should but your money into a charge controller (ideally, with remote battery temperature sensor) first.

    The simple answer--put a charge controller on the battery bank. Even a "cheap one" will be much better than no charge controller at all.

    Regarding blowing the Inverter up? Over-voltage on the input (probably over 15.5 volts for a decent inverter) may cause damage. But, under voltage (which causes higher input current) is not to be taken lightly either (low voltage of 10.5 volts or less can cause an inverter to overheat).

    Also, nearby lightning activity could cause damage too. Although--our host/Admin here says that, interestingly, most inverter failures around lightning is on the AC output stage--not the low voltage DC input...

    But it could also be almost anything else too (bad component, improper design, bad assembly, static damage to components, etc.).

    For me to take any more detailed guesses--would be meaningless. If it is an expensive inverter (i.e., True Sine Wave vs MSW), in might be worth sending into for failure analysis/repair.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Sun Fun
    Sun Fun Registered Users Posts: 16
    Re: Feed back on this system, Please

    thanks again bb. i can see clearly now.

    we do intend to buy a new charge controller. the inverter problem was reported as a factory fault but i suspect bad installation, no regulator no earthing, no fuses and mixture of cable sizes...

    and what about this earth to neutral. did i understand you correctly. should the battery neutral be bolted to earth?

    and if you dont mind me asking, why?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Feed back on this system, Please

    I am not sure what you mean by "battery neutral"...

    The battery usually has its "-" terminal (or negative bus bar/common point in larger systems) tied to earth or safety ground. This is done, by code, to prevent an accidental short (from + to ground) making the "-" wiring "hot" with respect to ground. This is so people working on the system are not "surprised". Isolated "small" systems (neither + or - grounded) are usually fine and safe (regardless of what the code says). It is also possible, in an isolated system for the "common" voltage to rise (or fall) with respect to ground if there is other voltages present (say an inverter) or static charge build up (so both + and - get charged by hundreds of volts away from ground)--again, another reason circuits are ground referenced. Also, grounding is done to help prevent lightning induced damaged (see lightning / grounding threads--not everyone agrees that grounding is helpful).

    "Neutral" grounding is typically an AC term in the USA... The Neutral lead is the "white" wire in an 120/240 VAC split phase system (or a single 120 VAC leg). NEC code requires the Neutral (typically the center tap of a 120/240 VAC transformer from the utility pole) to be grounded in the main fuse panel to earth. Again, for many of the same reasons we talk about in the DC grounding discussion.

    However, in an Off-Grid system--the "Neutral" wire cannot be grounded in many systems, depending on the inverter type and battery grounding (or not).

    Basically, the lower cost inverters are Modified Square (or Sine) Wave inverters which are not electrically isolated from the battery feed (no internal isolation transformer--cheaper to build). If you ground the battery "-" lead, and the Inverter "Neutral Lead", you will blow the inverter as it creates a dead short between the battery and ground through the inverter's electronic switching.

    In systems with DC loads (such as radios and lighting), plus a MSW inverter--I would ground the battery "-" bus (or to metal frame such as in an RV) and let the AC lines "float". This will allow you to use 12 volt DC accessories, such as a car radio, which use the metal case as the ground (in cars and RV's, the metal body/frame is the negative lead). Leaving the battery floating and grounding the AC "Neutral" will create lots of issues for the DC side of the system (including safety issues).

    If you have a more expensive TSW (True Sine Wave) inverter--most can safely ground the Neutral lead to follow NEC / Building Code requirements. TSW inverters have the internal transformers (needed to make the sine wave output) and therefore have the internal isolation from battery to AC output.

    Grounding of the AC output neutral lead is not required for the inverter to operate correctly--and is usually left floating in small systems. If you need to ground the AC Neutral--don't ground it with a MSW inverter, and check the manual (or with the Mfg.) of a TSW inverter to make sure it is OK.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset