grid-tied system design questions

Hello everybody,

for now I want to make a design for a workshop of 6 kW. This system will supply the workshop during daylight time only, and in the night the workshop will be supplied using the public grid!

I suggested a design of on-grid system to avoid using batteries to reduce cost, however the system will not be tied to the public grid as we don't have an agreement to sell electrical energy to the utility. I will use an automatic switch to change between the 2 sources (solar system, and the public grid).

I have the following questions:
1. is there any technical problem is I don't connect the system to the grid?? for sure the over-consumption energy will be lost as I will not use it and in the same time will not sell it to the utility so there's no benefit of it!
2. as the workshop has many motors, so they need a start-up surge power (current). All on-grid inverters don't have the option of surge power which means the rated power is the max power that one can get of the inverter ( for example, for an off-grid inverter of 6KW rated power, one can get a surge power of 12KW for 5 seconds to enable motors of starting-up). any suggestions about overcoming such a problem??

waiting for your prompt response.

many thanks for you all in advance

Mina
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grid-tied system design questions

    Welcome to the forum Mina.

    Here's the first problem, and it's a big one: battery-less grid-tie inverters do not work without the grid. You either have a grid connection or you have batteries. Using a GT system but avoiding selling surplus power to the utility is very difficult. There really are no off-the-shelf solutions for that. It comes down to measuring power use and inverter output and making certain the latter never exceeds the former (as that would be sending power to the utility).

    How are you basing this 6kW design standard? Is this the maximum running Watts? (The start surges not withstanding.)

    Inverters usually have a start surge capacity, but you have to be careful about the numbers: if everything turns on at once (unlikely but possible) what will that power draw be? And not all inverters can maintain their rated surge for long enough to start a motor. Better ones can, cheaper ones can't.

    Someone is bound to ask how expensive and reliable the grid is where you are so that the economic feasibility of this project can be evaluated as well.
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    Many thanks for your prompt reply :)

    Actually I can connect the system to the grid but I will make no profit of that, but in case that I must connect the inverter to the grid to make it works properly I will!

    When I talked about connecting the inverter to the grid, I was asking about technical problems not money affairs!

    may you explain any technical problems that I may face if I don't connect the inverter to the grid?

    For the 6kW size, I calculated it by calculating the power of the appliances in the workshop not more.

    When I read the specs of an on-grid inverter there was nothing about surge capability.

    Can you suggest some good inverter manufacturers that I can rely on pls?

    Thanks
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: grid-tied system design questions

    A grid tie (utility interactive) inverter has no surge capability. Its job is to transfer 100% of the available solar energy from the array to the AC mains. (technically it is a constant power source or, roughly, a current source--Which has no surge capability).

    An off grid inverter is a voltage source... And it can supply (typically) ~2x its rated output power as surge (cycles to a few minutes, depending on brand/model)--And it gets this "extra energy" from the battery bank--Which is also a voltage source and can supply surge current/power.

    With a GT inverter, the utility grid acts for all the world like a giant AC battery bank. The Grid sets the voltage, frequency, and supplies needed current (or in the case of GT inverters, sinks excess current back to the grid for others to you--Aka turning the power meter backwards).

    A GT inverter does not usually have a throttle on it--It runs at 100% of available solar energy and there is no inbuilt system to measure power (and direction) at the utility meter.

    Since the GT inverter connects to the house mains, it means the local permitting department and the utility have the right/obligation to set requirements/review the design/and setup billing to support the operation of a GT inverter.

    Off Grid inverters are not made to connect their outputs to the AC utility... They are not made to "sync" with the grid (it would be like connecting an AC generator directly to the grid--Not a good idea at all).

    There are Hybrid Inverters that are Off Grid Inverters that have the ability to also go into GT mode and feed power back to the utility (some actually come sort of close to what you want--Not feed power back to the utility but supply a mix of solar + utility power to the loads, and if the utility "goes away", feed the loads directly--or at least a subset of the loads). However--I have not seen a hybrid type inverter that is really capable of meeting the legal requirements of running in hybrid mode and not "leak" a bit of power backwards at time to the grid (or leak more grid power than needed to the loads).

    There are Off Grid inverters with "generator support" that can run one way power from the grid and supply "off grid" power to the loads. It is probably the closest you will come to what you are asking....

    However, any off grid (or hybrid) AC inverter + battery bank + charge controller + solar panels etc... Is expensive to run. If you live in a place with high power costs (like summer afternoons in California where the costs can exceed $0.50 per kWH in the afternoons), you may come close to breaking even. But, usually that is about it.

    Typically, a full off grid power system will cost you (20 year life for system installation, new batteries every ~7 years, new inverter+charge controller+electronics every ~10+ years, etc.) somewhere around $1-$2+ per kWH (some people do get a bit less). Or around 10x the cost of utility power for most people in the US.

    And a system that can supply 6kW power (12 kW peak) will not be cheap. Minimum of 600 AH @ 48 volt battery bank, around 4-5kWatt of solar panels, A MPPT charge controller, wiring, etc... And still need to figure out your loads... 6kW for 1 hour a day (6kWH) vs 3 kW for 10 hours per day (30 kWH) is a huge difference in battery bank and solar array sizing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grid-tied system design questions

    Might be best to use grid-tie inverter just to reduce the electric bill by the average load.

    Otherwise the requirements point to an off-grid system and as Bill said not a cheap one.
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    Many thanks for these valuable info.

    For the GT system, what's the real operation of it?
    I can understand 2 scenarios for its operation as follows and you tell which is the right one:
    1. the inverter generates 100% of the available solar energy, then it feeds the generated energy to the grid and this energy is counted and billed by the utility. the load consumes the energy it needs from the grid and this energy is counted and billed too. at the end of the month, the utility calculates the generated and consumed energy and see if you have deficit/ surplus to pay/get. that means the system is just a financial affair!

    2. the invereter generates 100% of the available solar energy, then it feeds the generated energy to the load (not the grid). then in case the inverter generates more energy than the load consumes, this grid will absorb this energy surplus (calculate and bill it too). in the other hand, when the inverter generates less energy than the load consumes, the grid will supply this energy deficit (calculate and bill it too). at the end of the month, the meter will show either the client has a surplus or deficit and charge him regarding to that!

    can you tell me either scenario 1 or 2 is correct?

    Also, can you tell me if there are inverters can output 380 V ??

    Mnay Thanks

    Mina
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    I'm planing just to reduce energy bill not more!

    In my country energy is supported and prices look to be very low
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grid-tied system design questions

    A standard grid-tie system pushes current to the wiring. It does not know nor can it control whether the power is going to local loads or out to the grid. If the loads consumer more than the inverter can produce the deficit is made up by drawing utility power. If the inverter produces more than the load demands the surplus is fed to the grid. How the utility handles this back-feeding for billing purposes varies with the particular utility. You'd have to contact yours to find out what the consequences would be. Usually a bi-directional meter is used, running up as the loads draw from the grid and going down if the inverter is back-feeding. In some cases separate metering is required: you get paid for the inverter output running through one meter and billed for the load use running through the other.

    Reducing the bill is easy, but normally solar comes with a fairly hefty price so you may not actually be saving any money by doing so. For example where I am we pay ten cents per kW hour, whereas a GT solar system would generate power at a cost about six times that. As such there's no point in installing solar. (I run an off-grid system at my cabin where I am now because there is no utility power here. In fact my Internet connection is via satellite.)

    380 Volts is an unusual number. What sort of installation is this? I'm thinking it's a 3-phase system, and that is another issue.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grid-tied system design questions

    I'm thinking you may need an inverter like this SMA Sunny TriPower: http://www.sma-australia.com.au/en_AU/products/solar-inverters/sunny-tripower/sunny-tripower-5000tl-6000tl-7000tl-8000tl-9000tl.html
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    Thanks for these valuable info.

    I can totally understand the 1st paragraph. I can tell you that in my country there's no agreement that the utility will pay me for any feed-back energy!
    Also, we don't have bi-directional meters. I'm thinking of connecting the inverter output after the meter (meter output) not before the meter (meter input). I believe that will reduced the consumed energy billed by the meter (let me know if that's right or wrong). The main concern for me is the feedback energy. As we use uni-directional meters, if the energy passed through the meter in the reversed direction, this will cause lead to the meter to be burnt or damaged anyhow?

    I know that my system will not reduce the cost noticeably, but it's a prototype system for a company that will be applied in this workshop as an Ad.

    May I know where the kWh is billed at $0.1??

    For the 380 V appliances, they are 3-phase motors. What's different in that system?

    many thanks

    Mina
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grid-tied system design questions
    Thanks for these valuable info.

    I can totally understand the 1st paragraph. I can tell you that in my country there's no agreement that the utility will pay me for any feed-back energy!
    Also, we don't have bi-directional meters. I'm thinking of connecting the inverter output after the meter (meter output) not before the meter (meter input). I believe that will reduced the consumed energy billed by the meter (let me know if that's right or wrong). The main concern for me is the feedback energy. As we use uni-directional meters, if the energy passed through the meter in the reversed direction, this will cause lead to the meter to be burnt or damaged anyhow?

    Problem with non bi-directional meter is that any surplus power you send back to the grid will run your bill up as the meter will go in one direction only regardless of which way the power is flowing! There is no easy way to stop the inverter output from flowing back to the grid if the loads are not taking all the output.
    I know that my system will not reduce the cost noticeably, but it's a prototype system for a company that will be applied in this workshop as an Ad.

    It may not be such a good promotion if the electricity bill goes up instead of down. :p
    May I know where the kWh is billed at $0.1??

    British Columbia, Canada. Land of cheap hydroelectric. :D
    For the 380 V appliances, they are 3-phase motors. What's different in that system?

    many thanks

    Mina

    For supplying 3 phase systems you have limited inverter options. In some applications you can feed one phase only, in others you must supply all three. Some inverters can be 'stacked' to supply 3 phase, others simply won't work. In this case the SMA I linked to looks like the logical choice as it is 3 phase, 50 Hz, and seems to have the 380 Volt range available. You should contact the company for exact details.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: grid-tied system design questions

    Which country? We have international posters that may be able to give you some advise for your country.

    380 VAC 3 phase at 50 Hz--We had a poster from Finland (as I recall) where that was (more or less?) standard home power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    For the issue of feedback energy which will run the meter up, this can be solved easily in my opinion. I can install power diode on the output of the meter which will act as a buffer permitting the energy (current) to flow in only one direction and prevents it from flowing in the other direction!

    Can that solution be considered??
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    I'm from Egypt, welcome to the land of Pharaohs :)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: grid-tied system design questions

    The math for AC power is actually quite complex.

    War
    of Currents - Wikipedia, the free encyclopedia


    Edison was an experimental researcher... Tesla was a genius at math. Edison was pushing DC power distribution and it lost out to the superior Westinghouse AC system (at least for basic power distribution--There are a lot of DC long transmission line/AC to DC to AC regional inter-tie systems).

    It is "difficult" to, out of off the shelf components, to make unidirectional AC power couplings (as far as I know, somebody will come with something I never have seen before).

    How much Power/Electrical experience do you have? Three Phase Power can be another set of complication.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    I have a bachelor in electrical engineering
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: grid-tied system design questions

    Ok, good.

    Remember that Voltage and Current are both vectors, and you have the "real" and the imaginary parts of the power flow. You have to not only know the magnitude of the voltage/current, you need to know the phase angle between them.

    So, whether power is fed back out the meter to the utility or fed to your main panel--It depends on the phase/amplitude of the current relative to the voltage sine wave.

    A "simple" Grid Tied inverter is a current source. It "follows" the AC wave form and outputs a sine wave current in proportion to the amplitude of the voltage waveform. In phase (power factor = 1.0). The GT inverter has no "knowledge" of any of the other current flow in your system.

    With smaller GT inverters, you simply put 3x single phase GT--One on each phase--To feed back the power to the panel/grid. In the US, usually there is a requirement to turn off all inverters if there is a single phase lost. And, something like a 10 kWatt limit on phase imbalance with GT inverters).

    Out of curiosity, do you know if you have Wye or Delta three phase power? Anything "unusual" like open Delta power (only two transformers to supply thee phase power)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: grid-tied system design questions
    For the issue of feedback energy which will run the meter up, this can be solved easily in my opinion. I can install power diode on the output of the meter which will act as a buffer permitting the energy (current) to flow in only one direction and prevents it from flowing in the other direction!

    Can that solution be considered??

    No, you can not control the power direction of Alternating Current with a diode.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: grid-tied system design questions
    ..

    When I read the specs of an on-grid inverter there was nothing about surge capability.

    ..

    That's because PV panels cannot produce a surge on demand. At the same time a cloud goes by, you try to start your motor. No power for it.
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  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    that looks amazing
    you own this company? what's your main business?
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    how i may start a new thread ??
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    Attachment not found.
    this is page 1 of the datasheet of the energy meter, pls chk
    01.png 104.8K
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    Attachment not found.
    this is page 2 of the datasheet of the energy meter, pls chk!
    (last page)
    02.png 103.2K
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    can you read the datasheet pls and tell me?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: grid-tied system design questions

    I am not quite sure what you are asking--If you are asking if the meter can run "forwards" and "backwards"... It is unclear to me.

    It says that it is bi-directional--Which may mean that is can run forward and backward--However, it also has "revenue guard"--And one of the common methods is to program the meter to only run "forwards". The utility can order/program the meters to have different behaviors (they are just "computers" and software now).

    In days past, people would plug the meter in right side up for 15 days, and then upside down for 15 days--And the meter would register zero energy use--So one of the methods to prevent this was to make the meters to only run forwards... No matter the true direction of energy flow (as Inetdog said in your other thread).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    many thanks for your valuable info.

    my question is "will this bidirectional meter work properly with my grid-tied system?"

    I need another info, can you send me a connection diagram shows how i tie the inverter to the grid?? ( i need a diagram for a real project if it's possible)

    Many thanks
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: grid-tied system design questions
    many thanks for your valuable info.

    my question is "will this bidirectional meter work properly with my grid-tied system?"

    I need another info, can you send me a connection diagram shows how i tie the inverter to the grid?? ( i need a diagram for a real project if it's possible)

    Many thanks

    Now your in the realm of the local utility rules and requirements along with the permitting requirements. Here the NEC (National Electrical Code) is used as a "guide" for attachment.

    Here in general the grid tie is run back into the main panel by attaching it via a proper sized beaker to the breaker box. Some required an external disconnect and solar production meter between the inverter and the breaker.
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    Re: grid-tied system design questions

    On a very short term basis (like per cycle), meters give you credit for power sent to the grid as long as the net use is positive. That's why you aren't charged for reactive power use.

    I am available for custom hardware/firmware development

  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    many thanks dear,

    in the issue of connecting the system to the grid, how i will make it in the reality?? i mean is it something like parallel operation of generators or transformers or what?
    can you help pls??

    Thanks
  • minabenyamin
    minabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    I'm from egypt and we don't have a net meter agreement to sell power to the government.
    actually I don't want to sell power to the government, I only want to reduce my consumption.
    However, reduction in consumption may result from feeding power to the grid (even if I will not be paid for).
    For example, if I consumed 500 kWh in a month and I produced 100 kWh in the same month so I will be billed for only 400 kWh.
    Let's make it more clear, assume that I'm not at home in the morning and the system is generating energy with no consumption. assume that morning the reading of the meter was 2,500 kWh and the system generated 20 kWh, this means the reading will go down to 2,480 kWh or not??

    I hope I made things clear!

    Thanks
  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    Re: grid-tied system design questions
    I'm from egypt and we don't have a net meter agreement to sell power to the government.
    actually I don't want to sell power to the government, I only want to reduce my consumption.
    However, reduction in consumption may result from feeding power to the grid (even if I will not be paid for).
    For example, if I consumed 500 kWh in a month and I produced 100 kWh in the same month so I will be billed for only 400 kWh.
    Let's make it more clear, assume that I'm not at home in the morning and the system is generating energy with no consumption. assume that morning the reading of the meter was 2,500 kWh and the system generated 20 kWh, this means the reading will go down to 2,480 kWh or not??

    I hope I made things clear!

    Thanks

    Yes but what you are failing to grasp is that if your meter cannot determine the direction of the power, you could end up being BILLED for power that you put BACK into the grid. It has happened to people here who installed a GT system and didn't realize it.

    Now if you will NEVER generate more power than you will use, then technically you would not have this issue, but the chance of that is unlikely. For example, it is mid-day, you are generating 2500watts but you are only using a total of 1000watts. So the other 1500watts is going back into the grid and the meter will just see that as 1500watts and you will be billed as if you CONSUMED 1500watts even thought you actually generated it and put it back into the grid.

    So you MUST be sure that the meter will "run backwards"...
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