Guru
Joined: 02/02/2017
Location: AustraliaPosts: 1464
| Posted: 11:57am 08 Mar 2019 |
Part 27: Why we make them big and beefy. (Inverters, that is)
In earlier post here, I showed that my house needs a relatively low power supply
for most of the day. I chose to supply this low level with the solar power system and let the high power loads be supplied by the street supply.
This has enabled me to supply about 50% of the house power from the solar array over the entire year. This is Melbourne. Cloudy, overcast Melbourne.
I see we only need about 5 to 10A at 50V for most of the time, then we only need an inverter that can handle 500W. Right?
I have 2 fridges on the solar supplied circuit. When a fridge switches on due to it's thermostat, there are large current peaks.
How large?
This is the recorded data from my system.
Green line is the smoothed or average current drawn by the inverter.
Brown is the peak current seen at any time.
We can see my house needs less than 10A most of the time, but very often, the inverter has to handle 50 A peaks when a fridge turns on.
That is why we build inverter boards that can deal with 2 or 3kVA or more when running modest loads.
First, today where quite modest power levels were provided by the inverter.
And last weekend, when I ran the pool pump. On top of this load, we can
see the fridges peaks when they switched on.
Over building the inverter is a good idea. The PWM based systems can run at quite high efficiencies under low loads. In my case the 250W to about 500W is at about 93% efficiency. This means at worst I waste 7% of 500W or 35W in heat. No fans need to come on to cool things.
The reason the brown and green curves don't align at low power levels is that
the current sensor has a lot of noise. The green curve is time averaged current, suppressing the noise effectively, the brown curve is recording the peak value seen at about a 1000Hz sample rate.
wronger than a phone book full of wrong phone numbers