Low frequency Inverters and in-rush current concern.

[再生可能 エネルギー]

I am reviewing for an acquaintance an installation proposal from an installer, not a DIY, for a small grocery store.
The daily power grid consumption is 60 kWh, under 220V, mostly to run 8 professional fridges and freezers, and indoor and outdoor lightings.

It seems to make sense to use Solar power to lower the electricity bill, but the Solar input should be limited to 10 kW
from requirement from the power company for the type of contract at that location.
The use of Power walls, also limited to 10kWh, is not yet decided because of the additional cost.

The installer choice is to use two 5KWA Huawei Sun2000 inverters.
Looking at the specifications of those inverters, the Max Output Current is
25A (on Grid) or 22.7A (off Grid) for each inverter,
based on the AS 4777.2: 21.7A Grid security guidance.

Doing some quick napkin calculations, the hourly consumption is about 2.5 kWh so under 220V there is an average of 12 Amp.
If the Fridges are running about 30% of the time (5 minutes On, 10 minutes Off) there are about 3 fridges running all the time,
so each fridge would use 800W or 4 Amp under 220V. When the fridge's motor is starting I estimate the In-Rush current to be around 5x or 20Amp.

My concern is that the type of inverters proposed seems to be a little bit on the border line regarding the In-rush current,
especially in case of short power failures of the Grid, which occurs quite frequently at that location.

I would be interested getting some opinion on this installation, in particular:

• I think that installing Relay Timers for each fridge should be recommended to avoid having too many motors re-starting at the same time.
• When running motors I read that it is recommended to use low frequency inverters, mostly because of the in-rush current.

 

[Steve_S]

Low Frequency inverters can handle 3x surge, while High Frequency can only do 2x on a "good day" and they don't like motors as such.
The best option is to modify the fridge Motors to use a SOFT-START Module to reduce that huge in-rush pull. It will be far easier for the inverter(s) your battery systems and even the health of the fridge compressors.

There are many solutions for SoftStart modules but be cautious and review the reviews etc... Honestly, get a good quality "Known Brand Name" with a good reputation.

A note on the battery system. Having more than 1 battery in parallel for such an application is pretty important, in fact, any installation that will pull heavy loads from a battery should be split out. Batteries in Parallel divide & share load & charge, so rather than hitting 1 battery with a 200A pull with 2 packs that would be 100A each, or if 4 packs that would be 50A each. This also adds fault tolerance & fall back which for a "food establishment" is terribly important. Should 1 pack cutoff for any reason the others carry on for example.