Capacitances to ground in a substation
Capacitances to ground in a substation
(OP)
Dear All,
In a substation we have protection CTs in one place and the circuit breakers in another. What I'm interested in is what can be the capacitance of the wire connecting both, or at least the distance between CTs and CB.
What I'm trying to understand is how much current (and for how long) will the CT be seeing after the CB has opened. Since there is some capacitance between CTs and CB, just after the CB breaks the current this capacitance will start charging to a higher voltage and effectively some charging current will be seen by CTs. I just wonder how much of current can there be and for how long.
Thanks in advance
In a substation we have protection CTs in one place and the circuit breakers in another. What I'm interested in is what can be the capacitance of the wire connecting both, or at least the distance between CTs and CB.
What I'm trying to understand is how much current (and for how long) will the CT be seeing after the CB has opened. Since there is some capacitance between CTs and CB, just after the CB breaks the current this capacitance will start charging to a higher voltage and effectively some charging current will be seen by CTs. I just wonder how much of current can there be and for how long.
Thanks in advance






RE: Capacitances to ground in a substation
RE: Capacitances to ground in a substation
If this is the case in real world then you may encounter spurious CB Failure operations if you set your I< threshold too low.
RE: Capacitances to ground in a substation
Really can't be that far from the CT on the bushing to the interrupting mechanism of the breaker.
RE: Capacitances to ground in a substation
I don't know about the distance - I was hopeing to get some input from people who actually visit a substation every now and then (unlike myself).
RE: Capacitances to ground in a substation
RE: Capacitances to ground in a substation
As davidbeach correctly stated in dead tank circuit breakers, the CT's are mounted on the bushing. More specifically, they are mounted at the bottom of the bushing (tank), which is grounded.
In live tank (HV CT) applications, the CT's can be located several meters from the breaker. The distance to ground is a function of the system voltage.
To PeterS84 - you can find the data you are looking for by examining the physical layout drawings for the substation. The drawings should specifically state the height and distance from CT to breaker.
My guess to the original question would be that the charging current would be largely influenced by what side of the breaker we are talking about. If, for example, it was the line side, and the line is a long cable, the value could be 10's to maybe a few hundred amps (very long line). If on the busbar side, consider 3 meter height and 2 meter distance to breaker and report back with findings.
I have seen live tank breakers in the USA (GE ATB) and am glad these are largely a thing of the past.
RE: Capacitances to ground in a substation
Perhaps I was looking at a metering CT...
RE: Capacitances to ground in a substation
A quick calculation for 3m high, 2m to the breaker gives around 0.2nF so not much. For the case of line side CT it sounds like CBFail could be impacted.
RE: Capacitances to ground in a substation
RE: Capacitances to ground in a substation
Although the phenomena they're describing there is caused by CT ringdown (energy trapped in the CT after CB has opened) so it's different to the transients caused by charging of the capacitance.