Current Transformer for MulitPlus II - can the cable be extended and what is the maximum cable run?

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raymiller asked Feb 9, '22

Current Transformer for MulitPlus II - can the cable be extended and what is the maximum cable run?

If I implement the Current Transformer (CT) recommended for the MulitPlus 2 and install it at the Grid interface point. What is the maximum cable distance?

I looked up the specification for the Current Transformer YHDC.com SCT013 100A:50ma shows this CT model is terminated at the measuring interface in a 10-ohm resistor to achieve a 1% accuracy. Another poster measured the MulitPlus 2 input resistance at 40ohms, I'm yet to confirm this.

With such a low impedance required for the CT would indicate the wiring Extention could be very limiting or the cable size (mm2) would need to be increased.

What is the recommended loop resistance of the CT wiring to keep the calibration of the CT in the Victron implementation?

Can the CT be calibrated in software?

Or has anyone else successfully increased the CT wiring over any distance?

Thanks in advance for any info.

ct

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raymiller commented · Feb 10, 2022 at 01:32 AM

I've just measured the "Burden" resistance (or the termination impedance within the MuitiPlus) of the Current Transformer in MulitPlus 2 at 60 ohms.

I am assuming then that Victron Engineering has programmed the MulitPlus to calibrate the recommended CT SCT013 100A:50ma with the changed "burden" 60-ohm resistor (from the 10-ohm shown in manufactures specification of the product).

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Answer 1 · 2022-02-09T12:35:13Z

Alexandra answered · Feb 09, 2022 at 12:35 PM

@RayMiller

Because the signal is not huge from the CTs you may find extensions can cause issues.

There will be noise, inaccuracies and other issues like unintended feedback. I have seen it discussed in various threads on the forums from trying to extend. The consensus on one thread was to use an energy meter for accuracy especially over a distance.

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Alexandra ♦ commented · Feb 09, 2022 at 12:38 PM

https://community.victronenergy.com/questions/32828/can-current-transformer-replace-energy-meter.html

Here is one thread where a person said they had some success using an article linked in his reply.

https://community.victronenergy.com/questions/32828/can-current-transformer-replace-energy-meter.html

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raymiller Alexandra ♦ commented · Feb 10, 2022 at 02:35 AM

Thanks for the thread references, so the CT cable can be extended with a couple of caveats.

- From what I found out about the MulitPlus II 60-ohm burden resistor, the total extra cable resistance can only be a small percentage of the 60ohms otherwise the accuracy of the CT will be severely diminished. This limits both the cable type and length, unfortunately.

A number of other CTs use a much higher burden resistor making it much easier to extend the CT to 50 metres without issues.

- the extension cable needs to be at least a twisted pair all the way up to the CT (even replacing the supplied CT cable with twisted pair) otherwise stray magnetic fields will cause errors with the wanted signal.

- One possible solution is to use a CT with a 4-20ma output and convert the 4-20ma current loop back down to the current equivalent current through the 60-ohm burden resistor. This solution is better-suited to long-distance and interference reduction in noisy environments.

- using a Victron recommended energy meter is one solution but the response time is many times that of a CT solution, which delays significantly response to load variation.

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