# Program for comparing current values

Hello,

I am designing a differential relay for 3-phase busbar protection. I have placed two CTs in parallel in each phase. I need a program to compare the two current values in case of a fault and send a trip signal if the values are not same. Can someone please help me with the programming?
I have no idea about it and tried to learn it, but didn't understand

I am going to show my ignorance by asking what a CT is. Presumably it is a device for measuring current. If so, what sort of output does it give and can you provide a link to the device ?

You mention "the two current values" but in a 3 phase supply won't there be 3 sets of two values ?

What have you tried so far ?

I must say, however, if this is some kind of safety system, then with your level of knowledge I am afraid that I would not trust it from what you have said and asked so far.

If CT is a current transformer then putting it in parallel with a phases will just short out the phase. It needs to be in seriese.

Once you have the readings then subtract the two values and test that the absoloute value of the result is greater than some threshold value to detect a miss match.

`````` if( abs(reading1 - reading2) > threshold ) { //do the trip stuff
``````

@UKHeliBob- A CT is basically a Current Transformer that measures the current value flowing through the phase. In my case, it is a 230/15 V transformer. I have connected 2 transformers for the same phase and creating a phase-to-ground. So I’m basically concerned about those 2 transformer current values.
I am still developing it as of now, not implemented it yet

@Grumpy_Mike I have attached the circuit diagram for your referance. Please have a look into it. I am concerned about only the 2 CTs connected across one phase as I’ll be creating a phase-to-ground fault

thunderarpit:
@UKHeliBob- A CT is basically a Current Transformer that measures the current value flowing through the phase. In my case, it is a 230/15 V transformer.

That is strange. The CT i know off have a specified current ratio with the secondary current often 5A. Using voltage transformers as current transformers does not work and is very dangerous. So are CT's themselves if handled improperly.
Unless you know what you are doing and have sufficient proficiency in electrical power and safety do not attempt this.

@nilton61 The current ratio of the CT is 5/1 A. Sorry I confused CT with the main supply transformer

First off that does not look like you are putting the current transformers in parallel with the load like you said.

A current transformer has a primary and a secondary, that is not shown on your circuit. I can not make sense of your circuit but it looks like you have the secondary windings for the current transformers in parallel. If this is so this is wrong.

Why is it wrong?

Why is it wrong?

Because you want to measure the outputs of each independent CT.

The following two statements, and the question above, suggest that this is not a project you should be undertaking.

I am designing a differential relay for 3-phase busbar protection.
...
I have no idea about it and tried to learn it, but didn't understand

thunderarpit:
Why is it wrong?

If you don't know then you are not ready to play with such high voltages.

I will be keeping the fault constant. As in, let's say tha fault is only beween phase R and ground. So, I'll need to calculate the 2 current values by the two CTs on phase R only, send them to the computer, calculate the difference and if it is not zero, then send a trip signal. So I won't be requiring the output of the other CTs, just the 2 connected on phase R

@jremington By the "I have no idea about it" part, I was referring to the programming involved. I haven't done arduino programming in the past, hence don't have an idea about it.

Lets see:

• You give incorrect designations of Cts
• You draw your scehematics in a unclear way
• You describe the problem in a less than clear way

Altogether this gives my the impression that your proficiency is less than required for this.

Hi,
Is this a school/university project, if so then please tell us.
Then send us the description of what the project is supposed to do that was given to you.

There is another thread on this forum with someone else doing a similar project.

Can you tell us what experience you have with electronics, programming and hardware.
We need this information so we can answer your questions on a level that you will be able to understand.

Why two CTs in parallel on each phase, just one on each phase is enough to measure current the current, with each outputting to the arduino via its own "level shifter".

Tom.....

@TomGeaorge Yes, this is the major project I am supposed to submit in my final year of engineering.
The project is supposed to be a busbar protection scheme. So whenever a fault occurs on any of the phases, the gate driver sends a trip signal to the anti-parallel thyristor circuit and the whole system shuts down.
I haven't had much experience with programming and hardware as such. But I did much of the soldering work involved in the project.
The two CTs are required so as to compare the values of current. Only one side will have the fault and the other side will be healthy for the same phase. And I have to compare those two current values.

Hello,

I am designing a differential relay for 3-phase busbar protection. I have placed three CTs (current transformers) in parallel in both sides. I need a program to compare the two current values in case of different faults(line to line and line to ground) and send a trip signal if this current values are not same. Can someone please help me with the programming?
I have no idea about it .

Hi,

mahipatsinh can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Why are you putting the CT outputs in parallel?
You need to monitor each phase separately.

Tom.....