Pass through current

[Isetech]

I am trying to understand pass through current.

Here is an example of a 3.6 kva (3900watt) inverter.

Would I be correct in saying that if the inverter is operating with the grid and solar power, then the current which can pass through the inverter continuous is 35 amps.

The 35 amps would be made up of the total power feeding the non essential and essential load and charging the battery?

The rated output power of 15.7 amps is the current which would be measured across the load terminals only.

Max output 18A, would be an AC-1 resistive load or AC-3 inductive load?



AC Output Data
Rated AC Output and UPS Power 3600W
Max. AC Power 3960W
Peak Power (off-grid) 2 times of rated power, 10 S
AC Output Rated Current 15.7A
Max AC Output Current 18A
Max Continuous AC Passthrough 35A
Power Factor 0.8 leading to 0.8 lagging
Output Frequency and Voltage 50/60Hz; 220/230/240Vac (single phase)
Grid Type Single Phase
Current Harmonic Distortion THD<3% (Linear load<1.5%)

[Isetech]

It is going to be interesting to see the wave form of this unit when I connect a power quality analyser and how it responds to the load tests.

[GCE]

I am trying to understand pass through current.

Here is an example of a 3.6 kva (3900watt) inverter.

Would I be correct in saying that if the inverter is operating with the grid and solar power, then the current which can pass through the inverter continuous is 35 amps.

The 35 amps would be made up of the total power feeding the non essential and essential load and charging the battery?

The rated output power of 15.7 amps is the current which would be measured across the load terminals only.

Max output 18A, would be an AC-1 resistive load or AC-3 inductive load?



AC Output Data
Rated AC Output and UPS Power 3600W
Max. AC Power 3960W
Peak Power (off-grid) 2 times of rated power, 10 S
AC Output Rated Current 15.7A
Max AC Output Current 18A
Max Continuous AC Passthrough 35A
Power Factor 0.8 leading to 0.8 lagging
Output Frequency and Voltage 50/60Hz; 220/230/240Vac (single phase)
Grid Type Single Phase
Current Harmonic Distortion THD<3% (Linear load<1.5%)

You would size the supply to the inverter as 35 Amp because the inverter, without solar , could supply the output rated at 15,7Amp and charge the batteries.

With solar and depending on settings the solar will supply essential loads first then charge batteries and only then feed the grid ( non essential) .

I do not load the essential side to full capabilities of 15,7 amp so that my battery reserve for grid outage is kept at a minimal and rather use the battery to store excess solar and feed back to the grid over the evening peak when electricity per unit is at it's highest due to time of use tariffs .

The clients need to maximize return of investment from the solar and batteries and just use the backup facility to take the pain out of load shedding.

The reason you would use a 5KVA inverter when supplying a small non essential load is to be able to get the grid tie function up with solar feedback and supplemented with battery storage feedback once the sun goes down.
The back up non essential loads are just a bonus feature that should be used for essential only. Not fridges, pumps washing machines, printers etc. I would put security systems , lights, data connectivity, PC's as essential and due to load shedding in this country TV and decoder.

As a rough estimate solar produces 5hrs x KW installed per day - So you install 5 KW you will get around 5hrs x5Kw x 30days = 750KWH per month.

If the client is only using 400Kwh ( units) of electricity a month he will struggle to get a reasonable payback time on his investment.

If he uses 100Kwh per month then he will be maximizing his return if he uses the solar smartly.
If he goes 10Kw of panel the payback period increases - In my mind I want to reduce my electricity bill drastically but not give my solar energy to the utility to make money on. If I reduce my bill from R2,5K a month down to R0.6 K then I am achieving what I want.
If I only have sufficient funds to buy 3 KW of solar then I still achieve decent savings and full return on investment.

Lets take the 1000KWH a month user. If he moves bigger loads to day time , like dishwasher , washing machine , ironing, cooking, geysers etc and maximizes the solar produced during the day he can then reduce battery storage.
If he insists on doing the bigger loads at night then he needs to store more from the day , bigger batteries, to get maximum use and payback. The more batteries the longer the payback

The new wash machines , stoves etc all have timer functions that you can set to go at different times of the day to use solar - Don't wash on a cloudy day , run dishwasher on a sunny day etc.

With solar it is a mindset change which often happens naturally because the client is watching his unit through an app.

Solar by itself pays back at around 4 years , add 10Kw of batteries and suddenly it is 12 years.

We did a tender for 380KW of solar on carport structures , trenching tar repairs etc which came to 10 Million - We calculated the payback period taking into account tariff increases, reducing max demand etc and a conservative payback was 6,5Years and the end the return of investment allowing for inverter replacements every 7 years was +/-70 Million. No battery storage
We did another on 60Kw roof mount system without taking into account tariff increases and it was sitting at 4 years .They then asked for 120Kwh of battery storage and return went to 8 years

[Isetech]

That explains why one of the installations where the installer used 2.5 mm wire supply to the inverter thinking it would only use pull 20 amps, ran into problems wit the cabling getting hot.

So the supply to the inverter input should be at least a 6 mm and the output can be 2.5 mm or 4 mm depending on the distance from the inverter to the essential DB.

[Isetech]

Do all hybrid inverters feed back into the non essential or grid?

Do you know of any other inverters which use a CT to prevent feedback into the grid ?

If a CT is not installed will all hybrid inverters automatic feed back into the non essential side and the grid?

[GCE]

Do all hybrid inverters feed back into the non essential or grid?

Do you know of any other inverters which use a CT to prevent feedback into the grid ?

If a CT is not installed will all hybrid inverters automatic feed back into the non essential side and the grid?

Some suppliers and installers are calling hybrid inverter and inverter that can take , battery , solar, mains power and feed to essential load only with a combination depending on settings.

A true Hybrid will grid tie as well.

Some inverters do not require a CT on the mains when producing solar, but then you cannot limit the feedback depending on the utility requirements. According to NERSA you cannot feed more than 4,6Kw back per phase and you cannot imbalance your phases more than 4,6Kw

When using battery mode at night and using the batteries to feedback to supplement utility usage you will need a CT to monitor mains otherwise it will not know how much to push back to zero your utility input.

The majority of true Hybrids use a CT as far as I am aware - It becomes an option when it is str grid tie inverter only.

[Isetech]

This topic has had me thinking, I have never taken the charge current into account when selecting cabling for inverters. Most of the inverters we install never operate at higher than 50 % capacity and in most cases not more than 30 % due to the backup period required. This is why I have also been looking into using smaller inverters with bigger batteries, so saved on the inverter cost and use the saving to invest in bigger battery capacity.

If I understand this correctly, a little more consideration must be taken into account when selecting cable sizes for both the battery and the supply.

IF you have a 5 KVA unit which is generally rated at around 20 amp, and this doesn't include the charging current, then a 2.5 mm cable would certainly not be considered, a min of a 4mm cable would be required.

If a 0.5C battery (100amp/hr) is installed then the charge current would only be around 50 amps, this could explain why some batteries are supplied with 16 mm cables and not 25 mm cables.

The charge current on the AC side for a 0.5C battery would only be around 10 amps, however the charge current for a 1 C battery would be around 20 amps.

I am yet to see a system running at full capacity continuously, in most cases they run at around 10% capacity, loading for a short period of time. It makes no sense to run a system at 80 % capacity unless you have a wall full of batteries. I am sure there are systems running at capacity.

This will also require consideration when selecting a generator, throw in a by Hybrid bi directional inverter and the non essential also have to be considered.

[Isetech]

it simple terms: Pass through current is the max current that can flow from the grid to the load, while the power is switched on.

The changeover relays inside the inverter are designed to carry, using an 8 kw Sunsynk inverter as an example 50 amps continuous through the relay in both directions.

We need to understand that when the inverter operating under "normal" conditions the power source is from the grid, not the inverter. YOu will notice a voltage fluctuation when in grid mode and a clean, stable voltage when in islanding mode.

So the power bypasses all the electronics and filters built into the inverter while in grid mode, basically in through the relay and out. the relay, only the power required to "keep the lights" in side the inverter will be taken from the grid.

When i islanding mode, that relay is open, no pass through current (trying to simplify this information, not easy as I thought)

We need to understand that these units are not "online" devices, hence the time delay.

Most inverters are not designed or suitable for UPS type applications, where they need to operate on line and dont switch from grid to battery, or require a really fast switching time (less than 20 mS).

I have a complaint we are dealing with at the moment where a consulting company supplied a customer with an inverter and battery (AGM) and got their sib contractor to install.

Some might ask the question, does the pass through current include the solar power.