PV System & Load Current

sun_daysun_day Registered Users Posts: 23 ✭✭
edited November 2017 in Grid Tie and Grid Interactive Systems #1
Heya. Have a grid-tied PV system with no energy storage that is connected in parallel with a building load. The PV system is to supply the load first before the load draws power from the utility grid.

We were measuring the current of phase A at the load and PV system side. These were the readings,

L1 Current at Load Side with PV System Off: 800A
L1 Current at Load Side with PV System On: 430A
L1 Current at PV System Side: 600A

Based on the above values, if we sum up the PV system and load side (with PV system on) current, it will be 1030A. Now, when PV system is off, the current at load is only 800A.

My question - Since PV system is off, the building draws 800A from the utility grid to power the load. When PV system is on, this 800A should maintain at this value (assuming consistant and after combining grid and PV).

In this case, we have a summation of 1030A from both PV and grid. The building only require 800A to power its load. Where does the 230A goes to? Shouldn't PV system supply the full 600A to the load and the load should only draws current of 200A from the utility grid?

We have measured this several times and the results are the same.

Will appreciate if someone shed some light or point me some direction. Thank you!
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  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #2
    Can you provide further information, there is mention of L1, assumption this is line one, is this a 3 phase system, or single phase with neutral, there could be phase imbalance, or neutral carrying current due to imbalances between lines, just some thoughts.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
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  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    #3
    If you are asking about where energy goes, I suggest doing all of your calculations in watts (not amps and not VA).

    I am available for custom hardware/firmware development

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  • BB.BB. Super Moderators, Administrators Posts: 33,319 admin
    #4
    Note: If you are measuring the DC Battery bus current (12/24/48 vdc?), there is a direction with a DC Current Clamp Meter (or should be a +/- direction on Battery Monitor+shunt--if used).

    If you are measuring the DC output of the solar array--You have two major loads--The Battery Bank (charging) and the other loads (AC inverter, any DC loads, etc.). Those loads are variable, If the battery bank is full, then you may have little charging current. If the other loads are heavy (AC inverter, etc.), those may siphon current from battery charging.

    Using Watts can make things a bit clearer (i.e.: 100 Amps * 12 VDC = 1,200 Watts = 10 amps * 120 VAC).

    In any case, need a bit more information and clarification about your question(s).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #5
    @ BB - OP said "no energy storage", so battery loads/losses presumably not a factor.

    Inverters are typically rated around 85% peak efficiency, so 600a in would be 510a out at peak efficiency, which could be a factor.

    Is this a micro-inverter system (so measuring ac current from array)?
    Off-grid.  
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  • sun_daysun_day Registered Users Posts: 23 ✭✭
    edited November 2017 #6
    mcgivor said:
    Can you provide further information, there is mention of L1, assumption this is line one, is this a 3 phase system, or single phase with neutral, there could be phase imbalance, or neutral carrying current due to imbalances between lines, just some thoughts.
    This is a 3 phase, 3 wire system - no neutral is involve.
    Estragon said:
    @ BB - OP said "no energy storage", so battery loads/losses presumably not a factor.

    Inverters are typically rated around 85% peak efficiency, so 600a in would be 510a out at peak efficiency, which could be a factor.

    Is this a micro-inverter system (so measuring ac current from array)?
    This is not a micro-inverter. It is a normal string inverter. The measured point for current is measured at the PV system connection point inside the building main ACDB.

    Apologise for the insufficient information. Please let me know if any of you require more details, I will try to provide. Thanks!
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  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #7
    If the loads are induction motors there is the power factor to take into consideration, apparent current is not actual current, this link explains this and how to calculate the actual current, as well as explaining phase ballance and other relevant information  http://myelectrical.com/notes/entryid/8/three-phase-power-simple-calculations


    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    #8
    In other words, amp readings alone are usually worthless when is comes to understanding AC power flow.

    I am available for custom hardware/firmware development

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  • sun_daysun_day Registered Users Posts: 23 ✭✭
    #9
    Hi guys,

    Sorry for the late reply! Thanks for the inputs at least I know where I can start to read!
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