Burden and current limiting resistor calculations 5000A/5A CT transformer

Hi guys, I planned to design a current measuring device for 1MVA and also 500KVA generator, then concluded through my calculations that 5000A primary current rated CT should do the job, with popular 5A Secondary current output.

I want to capture and output the measured current values with Raspberry Pi through popular ADS1115. I am confuse of how to go about the Analog frontend circuitry to convert the Signal to unipolar and which is also obvious i must regulate the current somehow.

I need your help concerning this, am new to designing device that can measure AC signals related parameters.

Welcome, @HgO , to the OEM forum.

Not knowing your output voltage, I can’t comment on that.

What do you want to know about your current? Is it just the rms value you need, and what sort of waveform does it have - i.e. is it a nice clean sine wave, or does your load draw currents at higher harmonics of your (presumably) 60 Hz frequency?

Because one thing that stands out to me on the ADS1115 data sheet is 860 samples per second. This equates to 14.33 samples per 60 Hz mains cycle, so in theory it’s only going to measure up to the 7th harmonic, and if you have any harmonics above that, you’ll need very good filtering to get both the 7th harmonic and to avoid aliases of higher harmonics.

If you want to use the ADS1115 in single-ended (unipolar) mode, then you’ll need to follow more or less what we do with the Arduino / ATMega 328P - bias the input so that with no current, the input voltage to the ADC sits at the midpoint of its range, then remove that standing bias in the software. There’s a lot of basic theory and practical (if not fully engineered) examples that you can look at in the ‘Learn’ section here.

You will most likely need to use the most sensitive input range available, otherwise the power dissipated by your burden resistor will be considerable.

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Thanks for the reply Robert,

My aim is to get the rms value of the current, and I think the current waveform should be sinusoidal in nature, but all that would have to be considered in my code, Am yet to buy to the CT, I just made some assumptions base on what will work for my design. I saw CT online rated as 0.72KV

The Arduino like connection is what I thought of, but i just have to be sure how to handle the 5A max input current.

I agree, it should be. But it would be good to know how far it deviates from that. You will only know when you start monitoring, unfortunately, therefore I suggest you allow for a peak current somewhat greater (say 10 - 15%) that the rms value would indicate. You also need to consider the peak inrush current in your loads - though the capability of your generator to support a large inrush will be limited, that might not be true of the 1 MVA source if it’s a transformer on a high voltage grid supply.

Even if you don’t need to measure the true peak current, you must still design the analogue curcuitry so as to protect the ADC input from an excessively high voltage and current into the ADC input itself.

That will be the insulation rating. The operating voltage of the cable through the c.t. must be lower than that.

If you look at the diagram of the Arduino input circuit in ‘Learn’, the 5 A c.t. secondary current flows around the loop comprising the c.t., its secondary connection cables and the burden resistor or instrument(s). It is the voltage developed across the burden resistor that is sampled by your ADC.

I suggested using the lowest available voltage range for the ADC, hence the lowest value of burden, to reduce its power rating, hence its heating and its subsequent resistance change with temperature, which gives you a calibration problem.
Were you to be using our emonTx, that needs 1.1 V rms across the burden to use the full 3.3.V input range of the ADC. If you had 5 A flowing to create that voltage, the burden would be dissipating 5.5 W, so you’d need a resistor rated to at least 25 W to keep its temperature rise (and resistance change) to a sensible value.

One further point I must warn you about: the sort of c.t. you will buy must NEVER be left on a conductor that might be energised, without its burden or without the secondary winding being short-circuited. If the secondary is open-circuit and a primary current flows, there is a very good chance that a dangerously high voltage will be generated by the secondary winding, the insulation will break down and the c.t. will destroy itself.
So never use a plug connector on that sort of c.t. I always specified hook crimps on the cables and spring-loaded terminals, so even if the terminal screw worked loose, the wire would not fall out and leave the c.t. open-circuit.

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Thanks for the reply once again Robert.

I guess the emontx is a device right?

Can you tell me more about it.

Price and features.

See the Shop page. There’s a link at the top of this thread.

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Ok, I will do that