CT Clamp Innacuracy

[Lazy_pete]

Hi. I’ve recently had a Solis Inverter installed which seems to be working ok with its 3.8 kW PV array and 5 kWh battery.

What doesn’t seem to work is the metering. There’s an Eastron SDM120CT meter with the CT on the incoming mains cable. For some reason however the Active power reading on the meter is always 3-3.3 x lower than that reported by the SMETS2 smart meter gadget. Thus I think the house is drawing power from the grid which the inverter is not compensating for.

Solis support suggested I turn the clamp around so the active power was negative if the inverter was off grid - I tried that but when I put the inverter on it began both charging the battery and told me it was exporting to the grid simultaneously, which can’t be true ! Swapped it back around, so the arrow is pointing the ‘wrong’ way and things worked as normal, except for the CT meter under-reading.

Anyone come across this before ? Have any ideas on how to resolve ? I gather the inverter needs an accurate grid reading to function properly.

Cheers,

Pete

 

[740GLE]

Is there any scaling you can change of the CT?

 

[goldserve]

Sometimes referred to as turns ratio in a setting somewhere.

 

[LydMekk]

Look for something like this in the settings "2000:1"
Deye/Sol Ark/Sunsynk has this.

 

[SeaGal]

I don't believe the Solis has any such configuration. Unlike the Deye/Sol Ark/Sunsynk, the Solis uses an active (separate) power meter, not a CT clamp connected directly to itself.

The Solis supports either the Acrel or Eastron meters, each of which come with their own CT clamp and should work correctly out-of-the-box.

@Lazy_pete Welcome! You are correct, the inverter does need an accurate grid reading to function properly.

Two more questions...

1) You mention the reading being wrong - is that on the Eastron itself or on the Solis screen? If the latter, please check the Easton to see if that is correct. If the Eastron is correct, but the Solis is wrong, has the Solis been correctly configured to use the Eastron, rather than the Acrel meter? Also, is it configured for meter placement = grid and is the RS485 link showing as working on the Solis screen.

2) You say the CT clamp is on the incoming mains. Can you share an image of that? I am wondering if the CT clamp is around both Live and Neutral, rather than just the Live incoming meter tail? I am wondering if the CT clamp has been wrongly installed around both Live and Neutral, rather than just the Live incoming meter tail, as it should be?

 

[LydMekk]

The CTs are only clamped around the LIVE wire, not both live and neutral.
Depending on country/firmware it is often pointing to the inverter (can also be pointing to the grid meter but more seldom).

 

[SeaGal]

The CTs are only clamped around the LIVE wire, not both live and neutral.

That's what I was trying to get the OP to check - I have amended my wording to make it clearer, thanks.

 

[goldserve]

Looks like these digital meters also have a CT and has a rating of 1A or 5A but if the ct came with the product, it should be correct right? Just something else to check.

 

[Lazy_pete]

Hi folks - thanks so much for taking the time to reply - my notification emails must be setup wrong as I missed them, so apologies for delay in reply.

The CT clamp is around the single Live mains cable, between grid and meter. The only oddity I see with that is that the clamp has a 120A input rating but the meter is configured for a 100A rating. Pic below.

The inverter seems to work, and has an active RS485 link from the remote meter supplied by the Modbus radio supplied by the Eastron meter.

The most obvious inaccuracy is that the Eastron meter reading is always 3x lower than the reading taken from the Live app on the gadget that came with the Smart meter. I’ve made a table of the daily readings from Octopus usage vs Solis reported Grid import and on most days there is almost exactly 3.0 times more reported on Octopus than Solis.

My installer assured me everything is working fine, but seems now to accept that the import readings are wrong. I think they have only looked at the inverter when doing their load tests, not the Eastron.

The regularity of the inconsistency and the fact that it’s a ‘round’ factor of 3x makes me think there’s a config error somewhere !


The Eastron

 

[SeaGal]

1) You mention the reading being wrong - is that on the Eastron itself or on the Solis screen? If the latter, please check the Easton to see if that is correct.

Have you done ^^^^ ? As I also posted in your other thread?

 

[Lazy_pete]

Thanks Seagal - it seems to be the Eastron that’s incorrect to me. Reading 3x lower than the smart meter app. I’m not sure where to check the inverter meter settings - no where in the remote control app I can see - however that would only be relevant if the Eastron and the smart meter agreed in the first place, right ?

Apologies for adding topic to other thread as well - had forgotten about this post when I found the other thread.

 

[SeaGal]

Apologies for adding topic to other thread as well - had forgotten about this post when I found the other thread.

Let's keep the discussion over in this thread then...

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[RCinFLA]

Clamp-on CT sensors have a split core that must be tightly clamped together to avoid inductance leakage gap in the core. A small gap in the core results in a lower amperage reading.

You can manually squeeze the two core halves together to see if the amperage readings change. Sometimes a tie-wrap around the core will keep the sensor tightly closed.

Make sure the surfaces of the clamp-on core are clean before you close the clamp. Check the plastic latch is not worn or deformed so it does not fully pull the two core halves together.

Keep clamp-on CT sensor closed before installing them and keep them away from any magnets that may permanently magnetically bias the core.

Very high surge currents spikes may saturate CT sensor core creating a magnetic bias on core. This can screw up low current readings.

Passive CT sensors are not totally linear so accuracy depends on current where they are calibrated. Sense wires should be twisted together to cancel stray fields from other nearby wires. Twisted pair wires are about 100 ohms impedance which is why 100 ohm load burden resistor is often used to terminate the sensor wires in the inverter. This becomes more important the longer the sense wires are. Also consider the sense wire gauge and its series resistance in relation to the 100 ohm shunt load termination resistor.