As use 6 wires for 3 CTs? We typically connect the CT star point in the circuit breaker cabinet and then use a 4 wire cable to run to the control house.
Assuming this is not a question of best location to implement single-point-grounding for CT circuits.
With 6 wires, balanced phase current has to flow back to the star point before it adds to zero. If the star point is at the CT, the distance is short and there is little resistance (burden).
For some applications, there is an advantage to running 2 wires per phase / 6 wires total in twisted pair from the CTs to the control house.
An example of that would be for HV metering applications where the secondary current level is very low, as in the case of Extended-Range CTs. These CTs can have secondary current ranges from 2.5mA to 10A. Using twisted pairs inside a grounded sheath guards against EMF impact on the current signal.
Scottf-What size wires would you run as twisted pairs? I have only seen twisted pairs in wires sizes 18 gauge and smaller, which would be too small to carry 10 A.
Not sure of the source, but I know it exists. We have utilities that use it with #10 or #12 conductor. The normal requirement is a twist every approx 1 foot.
Upon a very quick search, I found this, but I'm not sure if it is twisted or not.
A twist every 1 foot sounds similar to our typical 4 conductor #10 control cable. I am not sure why three separate pairs would perform better than a 4 conductor cable if both cables have the twist per foot. I had been thinking of 18 gauge instrumentation than has 10+ twists per foot and ethernet cable that has 60+ twists per foot.
Thank you all for great suggestion and comment. In the mean time i have come across a case wherein very high fault on A phase( 250A secondary current on 5 A relay) caused sufficient voltage across healthy CT secondary of remaining two health phase to drive them into saturation and caused maloperation. needless to say it relatively rare occurrence
please see attached sketch
One of the possible solution was not to run common neutral wire
dfdt,
The voltage developed across the CT secondary in faulty phase comes to ~400V. But, the voltage across the healthy phase CT / relay circuits is 125V only.
If CT rated knee point is >125V, the CTs in healthy phases will be offering very high impedance and there will not be current in those circuits (except the magnetising currents corresponding to 125V).
This is true with 6-wire scheme as well (with minor changes due to change in lead burden).
This is my view.
Without studying on it too much, I believe the twisting would be much more effective with each phase pair being twisted vs only have 4 wires and thus 2 twisted pairs. In the later scenario, 1 phase would be tested with another phase and 1 phase would be twisted with the neutral/common. I don't think that would be very effective at negating effect from adjacent electrical fields.
After tracing a two substation outage to a bad crimp making up a CT star point neutral, I have wondered about the choice to use four conductors rather than six. When I counted the crimps, I came up with a higher number using four conductor.
Hi Raghu
I agree with you view point. In this case other two healthy phase CTs had remanent flux from previous high DC offset fault and hence were quickly driven into deep saturation without corresponding high primary current. The relay in question is low impedance bus protection scheme which was meant to restrain for this through fault but did not.
remnant flux from previous fault - can be relevant only if the faults are successive without much gap between them (that allows remnant flux to die down).
