Grid tie Inverter current measurement

[Ian v N]

Hi to everyone. I am new on this forum, and I do not see any existing threads on this specific issue that I have.

I have recently inspected a 40kW 3Phase grid tie inverter installation that is purely used to supply a 3-phase 400VAC water pump. The grid is supposed to supply the deficit when the Solar power is less than what the pump requires. The pump draws 38Amps per phase. Since the Grid, Inverter and pump are all connected to a common bus on all 3 phases respectively, the wiring can be compared with a Capital - T for each phase.

When I measure the current simultaneously on all 3 'branches', I get the following readings per phase: The pump 'branch' =38A, the Inverter 'branch' = 36A, and the Grid 'branch' = 30A. The inverter LCD display at the time indicated about 24kW. I expected the Grid to only supply the shortfall of 2Amps to the pump, but now I see 30Amps.

What can the explanation be for this? I believe that the inverter LCD reading can be trusted, but yet the Amps does not make sense. Any advise, please?

[Mike90250]

What is the spec for the pump running at it's rated load ? Does that match the 38A you measured ?
Sometimes, clamp on meters are not very accurate, and if there are other cables nearby, they can interfere with the reading.
Are there any other loads besides the pump ? Try measuring with the pump off, or at night, eliminate a source of error.

[Ian v N]

Thanks Mike. The pump's current drawn are within expectation (22kw motor). The only load is this pump. I used good quality fluke clamp meters. I think this 30Amps from Grid opposed to the expected 2Amps might be too big a difference for mere instrument inaccuracies...I am still confused about this readings. I am sure there is a technical explanation but just not sure what.

[solarix]

I suspect your amp readings are not phased the same. btw, is this 3-phase a delta or a wye configuration?

[Ian v N]

Hi, I took all readings simultaneous on the same phases of the 3 branches. It is a 4 wire connected inverter. ( L1, L2, L3 and Earth wire. Same as the Grid. The motor is delta configured.

[bob-n]

Could it be that currents are out of phase (bad power factor)? That's the only way to explain currents not balancing.

If the motor is not heavily loaded, it will have high current, but out of phase with voltage. Alternately,, is it possible that your inverter is delivering power out of phase with the grid? They normally are set for nearly perfect sync (very high power factor), but...

[Ian v N]

Hi, My understanding and calcs on bad power factor (phase angle) is that the current will be excessive in relation to the motor kw rating. This specific motor probably at a power factor of 0,8. Unity power factor would see it taking closer to 28Amps. But it still is not solving my issue.
The inverter is surely in sync with grid since it would probably blow up if not.

[bob-n]

Ian,

Actually, the inverter doesn't have to be in sync with the grid and it wouldn't cause any problems if it wasn't in sync. Grid-tie inverters just send out current and the grid is a massive low-impedance sink of that current. It works most efficiently when in sync.

I'm not saying that this is a phase angle problem, but if your readings are accurate, I'm at a loss to come up with another explanation.

[solarix]

Yes, the voltage waveforms for an on-grid inverter have to be sync'd, but the current is just whatever is needed to push the power. The current waveforms are probably out of sync - you need a O-scope to really see what is going on....

[inetdog]

The current to the motor will include a substantial amperage 90 degrees out of phase with the voltage. This current causes the total current to be out of phase with the applied voltage. The difference between the product of RMS voltage and RMS current (apparent power) and the product of the in phase current with the voltage is represented by the Power Factor (PF) of the motor.This will not be a constant, but rather will be higher (closer to 1) when the motor is fully loaded. It is not unreasonable for a lightly loaded motor to have a power factor as low as .5.
The grid tie inverter, unless specifically designed for output PF control (as required of new installations in Hawaii), will deliver current only in phase with the voltage.
In a grid interactive situation with such an inverter, the inverter will supply only the in-phase current requirement of the motor, with the out-of-phase component coming from the grid.
The grid will deliver this current without registering any power consumed. So you really should look at that power meter reading over time to see what is going on if you cannot use a phase sensitive current or power meter or an oscilloscope.

A standalone inverter, on the other hand, will have to deliver the out-of-phase current all by itself, and to do this you may need to install a more powerful inverter than you would initially expect. (Plus of course the surge to start the motor.)
A typical generator will have two ratings. The real power in kW it can deliver and the total current including reactive (out-of-phase) current, and described the kVA rating. The most common spec will involve a kW rating which is 80% of the KVA rating, corresponding to a load PF of .8.

[Ian v N]

Thank you guys.
Much appreciated all your advice. The bottom line, it seems, is that I need better test instruments or have to study the powermeter readings over a longer period. Thanks again.