PV System & Load Current

sun_day

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!

mcgivor

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.

jonr

If you are asking about where energy goes, I suggest doing all of your calculations in watts (not amps and not VA).

BB.

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

Estragon

@ 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)?

sun_day

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!

mcgivor

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

jonr

In other words, amp readings alone are usually worthless when is comes to understanding AC power flow.

sun_day

Hi guys,

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