Mutual Coupling between single core cables on same ladder

[Vegemite]

Folks,

thread238-447179 posted 10 Dec 2018 asked about relay behaviour when protection CTs were double earthed. Some testing at site has confirmed that there is a single earth reference for the CT secondary circuit. The CT earht5ing was not, therefore, responsible for the nuisance earth fault trip.

Since that post I have noticed that there is a 130 metre run of cable ladder carrying the pair of cable circuits, one of which, clr1-AA3,tripped when it erroneously measured an earth fault current of 60 A.

Please see Fig.1 which is the single line of the system in question marked with some flux linkage which is the subject of this query. The cables are 19/33 kV, 400 mm² 1c., Cu, XLPE, AWA and are arranged in trefoil.

Fig.2 shows the cable layout on the 600 mm wide cable ladder.

Fig.3 is the zero phase sequence network, as I see it. My questions are:

1. "How can I determine the flux from one circuit cutting each core of the opposite circuit?
2. Has anyone experienced similar earth fault operations in a tie circuit between two sections of a power station or substation busbar where there are several solidly earthed star windings on either side of the tie circuit?"

 

[rcw retired EE]

How is the zero sequence current measured - by summing the phase CT's or with a zero sequence CT that surrounds all phases?
Incorrect routing of the cable shield (screen) grounding leads through a zero sequence CT can cause false actuation, especially if the cable shields are grounded at both ends of the circuit. The CT picks up the ground current flowing in the shield from an external fault.

 

[waross]

As well as how is earth fault current measured, where is it measured.
Am I missing a detail on the drawings?
I suspect some type of unbalance, possibly on a circuit outside the power house is causing a neutral current to flow between grounds on the grounded wye transformers.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Vegemite]

Folks,

Thank you for your questions which are answered below.

The earth fault current is measured from the residual connection of the three phase CTs which is fed into the "Ground" input of a type SR750 relay.

The three phase CTs are installed on the bushings of a Schneider type YSF6 circuit breaker which precludes any possibility of passing screen wires through the CT window.

Figure 1 attached is revised to show the location of the relay which tripped.

 

[7anoter4]

This order is good only if all the cable phase are the same as really parallel cables.
The 2 central feeders are not the same since the currents run in opposite directions.
Then an unbalance between parallel conductors of the same phase could be present.
I don't know if that situation could influence the shield and armor currents but it could be.
My program is for parallel cables only.