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right way to terminate CT signals

gary36

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I have to design an energy meter/protection relay for a customer. I am receiving 4 current inputs. In my board I am planning to use current transformer followed by the bleed resistor in secondary to convert current to voltage. My customer wants a barrier terminal strips in the rear side of the unit. I had been thinking of having double sided barrier screw terminals, with rear terminals used as inputs and the other side of screw terminals can be used for connecting the loop wire from the current transformers. Not sure about the scheme. My input range is 0-75A. Besides my customer wants PCB board that shall insert into the backplane and easily detachable on demand. I felt this as conflicting requirements. Passing 75A on a PCB for current inputs is not a good choice. Hence loop wire around CT will terminate directly on a screw terminal. In that case the board (housing CTs) cannot be removed like a backplane board. Hence require advice on the right way to terminate CT inputs to meet my customer requirements.
 

wwfeldman

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maybe you can mount torroidal current transformers around the current carrying wire
and connect the outputs thereof to the board with a simple connector - maybe a 25 pin D=Shell?
a twisted shielded pair from each torroid.

these Pearson current transformers may have a built in resistor and hence have a voltage output
there are mounting holes:

look at number 5101

look at number 1025

no barrier strips, no exposed currents, no weird routing of current carrying wires
easily detached, even with power on, as the voltages entering the board are small, and no current
cable from current transformer to board some sort of coax, so some noise immunity
 

crutschow

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You, of course, know that a current transformer output should never be left open circuited, otherwise damaging voltages can be generated.
If not a resistor that back-to-back Zener diodes in series can be used to limit the voltage without affecting the measurement when the measurement resistor is connected.
 

SunnySkyguy

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maybe you can mount torroidal current transformers around the current carrying wire
and connect the outputs thereof to the board with a simple connector - maybe a 25 pin D=Shell?
a twisted shielded pair from each torroid.

these Pearson current transformers may have a built in resistor and hence have a voltage output
there are mounting holes:

look at number 5101

look at number 1025

no barrier strips, no exposed currents, no weird routing of current carrying wires
easily detached, even with power on, as the voltages entering the board are small, and no current
cable from current transformer to board some sort of coax, so some noise immunity
Do we know if the fundamental frequency of this energy meter is above 160 Hz?

Also if there are any failure modes with Partial Discharge then the high bandwidth is useful for detecting such events as an early warning to dielectric breakdown failure from aging. e.g. 1ns risetime 300 MHz even better.
 
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gary36

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these Pearson current transformers may have a built in resistor and hence have a voltage output
there are mounting holes:

Since I need 8 channels, size seems to be bigger and not sure how would they respond to 50 Hz signals. Besides require screw terminals for field connections. I do not think end user would be comfortable with BNC. Not so often used in electrical substations
 

FvM

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The Pearson current probes are expensive high precision instruments. Definitely not suited for industrial instrumentation or energy meters.

I fear, the application specification isn't yet very clear. Energy meters with 100 A current range would normally use external industry standard CT with 5A secondary current. 75A is at the edge. If I consider to integrate 75 A current transducers into the instrument, I would probably use dI/dt (Rogowski) sensors.
 

gary36

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Energy meters with 100 A current range would normally use external industry standard CT with 5A secondary current.
FvM
You are right. The nominal current is 5A and fault currents can go upto 75A. In field we have another CT(100:1) that steps down kA of fault currents to 75A.
So 7500A is seen as 75A by the energy/protection device in question. Nominal is 500A (5A at the instrument side). I am struggling with choosing proper terminations for CT and most of the commercial units have customized connectors not generally readily available in market
 

FvM

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Fault currents will be interrupted by fuses. Your PCB should be designed for the respective transformed I²t, but not for 75A continuous current. Standard 5A PCB traces will be usually sufficient. The same with terminals and measurement shunt or secondary CT.
 

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