What do they mean by crossover system no good?

Hi all,

I am trying to design a 4500W on-grid solar system in Singapore. I submitted my proposal design to my local utility and they came back with below comments. I am not a trained electrician and therefore appreciate if someone can help plzzzzzz.:confused:

I attached the drawing here. Appreciate if someone can shed some lights. Thanks in advance. Vinnie

Below is utility guys' comments;

we note your PV system is a change-over design rather than parallel on grid connection. Please clarify whether the 32A 2P changeover is auto or manual changeover switch. Also, E&M interlock is required.

From your schematic, please note the following:

1. The standalone supply from the PV source is not reliable as the solar power is intermittent in nature.

2. For change-over design, the loads would be subjected to frequent supply interruption during changeover which may cause unwanted nuisance to the end user.

3. Due to low short circuit current of the PV system, the 32A 2P MCB at the load side may not be able to electrically protect the load from earth fault scenario during PV supply mode.

To resolve the above, you may consider using parallel on grid connection instead of change-over design.

Comments

  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: What do they mean by crossover system no good?

    Pure grid-tie inverters cannot run without AC grid or other AC source to lock onto. Your transfer switch idea will not work. Another problem, your total PV array Vmp = 616 V, Voc = 740 V. Typical GT inverters are 450V nominal, 600V limit. You may fry the inverter. What brand and model is it? Can you find an electrician or PV installer to help you?
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: What do they mean by crossover system no good?

    My question is "what is it that you want to do?".

    Are you trying to save money on power/go green? Does your utility support Grid Tied / Net Metered power? It sounds like they may--Then a Grid Tied Inverter which simply connects in parallel to your home wiring is the best price/performance for the average user. This system does not provide backup power in the event of a power failure (you still would need a generator and/or batteries+off grid inverter for emergency backup power). Thing of Grid Tied system using the Utility like a giant AC battery and the GT inverter both carries local loads and can "recharge" the utility (turn the utility meter backwards) during the day and during the night the utility supplies energy to your loads... There are no transfer switches required and the sharing of solar/GT power and utility power is all transparent (automatic). A very simple, reliable, and maintenance free system.

    If you are trying to go "Off-Grid" with the grid (or AC Generator) as a backup--Off-Grid systems are used where there is no grid connection, or sometimes where the utility does not support Grid Tied/Net Metered systems. This is the most expensive (price per kWH) way of generating electricity. Usually only done when there is no other type of power solutions available. Uses a large battery bank to store power during the day and provide power to your AC loads through an off-grid inverter.

    If you have utility power and they support net metering, and you want backup emergency power, a Hybrid Inverter system may be a good solution for you... Basically, it is an inverter system which can do both Grid Tied and Off-Grid operation. If you are familiar with computer backup power supplies (UPS--Uninterruptable Power Supply)--This is a large UPS system where the solar panels recharge the battery bank. There is a transfer switch that can use either Utility Power, or an Off Grid Inverter, or even a backup AC generator. And it also has a Grid Tied function where excess power from the solar panels charging the battery bank can be sent back out to the power grid (Grid Tied or Utility Interactive operation).

    I am sorry I don't have any good diagrams that describe the three different setups--The Xantrex/Schneider XW Hybrid Inverter system is a good example of the last type of system. About the best diagrams I have found are in the manual (page 129 of this PDF Download)--but they are a bit confusing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: What do they mean by crossover system no good?

    Tks alot, guys! What I m trying to do is a pure grid tie with net metering. During night,we will switch back to the grid or during days when the sun is not there, the house will turn to grid. We will not have battery at all or other energy source beside grid. You mentioned something about parallel, do that mean I shud have 2 strings of 10 panels in series connect to a combined box?
    I m using Eversolar 5.4kw utility inverter
    Cheers.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: What do they mean by crossover system no good?

    you want a straight grid tie system. there's no switching needed with a gt system as your excess solar power will turn the meter backwards and during low and no solar times you just consume from the grid as normal.
  • BB.BB. Super Moderators, Administrators Posts: 29,497 admin
    Re: What do they mean by crossover system no good?

    Think of a car electrical system... The "battery" is your "Grid Power" and the engine driven alternator/generator is your GT Solar System. (yes, the utility really is the equivalent of a giant AC battery bank).

    Your loads "don't care" were the power comes from. The "Battery" sets the system voltage (utility does same with voltage/frequency" and the Alternator (GT Inverter) just supplies power when the motor is running.

    There is no "switching" from the battery or alternator to run the loads based on when the "engine" is running or not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SkiDoo55SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    Re: What do they mean by crossover system no good?

    For your inverter and what you are trying to do you would need 2 strings of 10 panels so as not to exceed your inverter Max Voc of 680 VDC. Depending on inverter you could bring each individual string into the wiring box or combine them and run an properly sized wire for run and current to the inverter. The MPPT tracking range is 125 V to 530 VDC. You would also just eliminate the Change-over switch and back feed the inverter output to your MCB with an appropiate C/B estimate 30 amp 2 Pole, upon approval of your local authority. THe inverter will feed it's production to either your uses or feed the grid. It's operation is seamless and requires no actions upon your part after approval from utility and installation of a net-metering meter or a production meter
    Hope this helps.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
  • System2System2 Posts: 6,290 admin
    Re: What do they mean by crossover system no good?
    SkiDoo55 wrote: »
    For your inverter and what you are trying to do you would need 2 strings of 10 panels so as not to exceed your inverter Max Voc of 680 VDC. Depending on inverter you could bring each individual string into the wiring box or combine them and run an properly sized wire for run and current to the inverter. The MPPT tracking range is 125 V to 530 VDC. You would also just eliminate the Change-over switch and back feed the inverter output to your MCB with an appropiate C/B estimate 30 amp 2 Pole, upon approval of your local authority. THe inverter will feed it's production to either your uses or feed the grid. It's operation is seamless and requires no actions upon your part after approval from utility and installation of a net-metering meter or a production meter
    Hope this helps.

    Thanks alot, SkiDoo55. I believe I am one step closer to seeing some light.
    Referring back to what my local authority were saying (u can see my submitted drawing in first post) - I quote below:
    Below is utility guys' comments;

    Utility: We note your PV system is a change-over design rather than parallel on grid connection.

    Vinnie: So I guess if I do 2 strings of 10panel in series, this will sort the above comment?

    Utility: The standalone supply from the PV source is not reliable as the solar power is intermittent in nature. For change-over design, the loads would be subjected to frequent supply interruption during changeover which may cause unwanted nuisance to the end user.

    Vinnie: Initially, my proposal was to have 20 panels connected in series straight into the inverter. And if the sun is not shining, then the system will automatically switch back to grid. Then what makes the utility guys make the above comment?

    Utility: Due to low short circuit current of the PV system, the 32A 2P MCB at the load side may not be able to electrically protect the load from earth fault scenario during PV supply mode.

    Vinnie: What are they saying?

    Finally, they (the utlity guy) sent me a sample drawing and said I should take close reference to it. Can anyone tell me what is the key difference between theirs and mine.

    Many thanks in advance,
    Vinnie:blush:
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: What do they mean by crossover system no good?
    Utility: We note your PV system is a change-over design rather than parallel on grid connection.

    Vinnie: So I guess if I do 2 strings of 10panel in series, this will sort the above comment?

    No, the arrangement of the panels is a separate issue it has nothing to do with their comment about change-over. As BB and Neil have said, the problem is that "chnage over" switch you have in the diagram, it should not be there.
    Utility: The standalone supply from the PV source is not reliable as the solar power is intermittent in nature. For change-over design, the loads would be subjected to frequent supply interruption during changeover which may cause unwanted nuisance to the end user.

    Vinnie: Initially, my proposal was to have 20 panels connected in series straight into the inverter. And if the sun is not shining, then the system will automatically switch back to grid. Then what makes the utility guys make the above comment?

    Again, it's the changeover switch. Grid tied inverters need to see an AC frequency and voltage in order to produce any power. When that switch is disconnected from the grid, your system won't produce any power because the inverters won't see AC line voltage and will therefore shutdown.
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: What do they mean by crossover system no good?
    stephendv wrote: »
    Grid tied inverters need to see an AC frequency and voltage in order to produce any power. When that switch is disconnected from the grid, your system won't produce any power because the inverters won't see AC line voltage and will therefore shutdown.

    That is a safety issue really as if you energize the grid while they have it disconnected for some type of service it is a hazard for the people working on the grid.

    All this depends on your end game, what are you really trying to achieve? DO you want UPS type service so when the grid goes down you have battery backup power, or are you just tired of the power bill every month?

    UPS type Battery backup is super expensive to protect against an occasional outage.
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