Voltage drop calculations for CCVT's and Current TF's?

1. Oct 21, 2015

this1ssteve

I have not found a straightforward explanation on how to perform voltage drop calculations for cables used for instrumentations purposes. Suppose you have Current Transformer (CT) and Potential Transformer (PT) connections coming into a relay (IE SEL421). If we have a cable that runs from the yard equipment to the panels, let's say 300 Feet, using a #9 cable rated at 0.000729 ohms/ft, and we want the CT current to be 5A and the Potential coming into the relay to be at 67V, how do we determine this?

2. Oct 21, 2015

Staff: Mentor

Welcome to the PF.

What's a CCVT? And what's a TF?

3. Oct 21, 2015

dlgoff

I looked at the specs of the relay. Will the terminals even except a 9 AWG wire? Anyway, why are you needing to determine a voltage drop? Here's a snip from the relay's data sheet for the CT & PT inputs.

If you don't get 67 volts from the PT when at the nominal line voltage, surely the relay software can provide a way to compensate I would think.

Regards

4. Oct 22, 2015

this1ssteve

Berkeman:
-CCVT = Capacitive Coupling Voltage Transformer (used in Electrical Substations to step down huge potentials for metering purposes for protective relays)
-TF= Short for Transformer

Dlgoff:
So here is the scenario. We have a huge substation with very long runs between outdoor equipment and indoor relays. Let's say a protective relay needs to measure voltage and current on a line. It utilizes CCVT's (essentially to step down the huge voltages from the kilovolt level to 115V/67V) and CT's (Current transformers that are located on the bushings of a circuit breaker to step down the current from say 2000A to 5A) to be inputted into a protective relay in the control house. Well lets say the control house is located 1000 feet away and those cables are being ran with #9 AWG conductors from the outdoor equipment to the control house. With such a large run, there would be some sort of voltage drop to take into consideration. If there is too much loss (over 5% or so) you would need to size your cable to a bigger gauge. On a side note, the #9 conductor feeds into a panel, which has interconnecting wires that fit the size needed for the relay input.

I suppose my question is:
1) Would voltage drop be needed to be taken into consideration? It's not like the current or potential is being applied to a load, it is being used for measurement purposes, much like an ammeter or voltmeter on multimeter.
2) If so, how do we go about doing this? For CT's, I am not sure if im doing the secondary voltage current correctly (Vsec=Vpri *(Nsec/Npri) ). For the CCVT's, I am clueless.