Capacitances to ground in a substation 1

[PeterS84]

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

 

[davidbeach]

Within the tolerances of a protection system you can probably safely assume zero. Unless, of course, you have a fault between the CT and the breaker.

 

[PeterS84]

Thanks davidbeach. Does this apply to CB fail protection as well? I've done some simulations and it seems that under some conditions the charging current can be in the range of 0.05In and this can last 50-100ms (in those simulations I used capacitance of 5nF per phase which might be a bit excessive...).
If this is the case in real world then you may encounter spurious CB Failure operations if you set your I< threshold too low.

 

[davidbeach]

How far are you talking about anyway? At what voltage?

Really can't be that far from the CT on the bushing to the interrupting mechanism of the breaker. ;-)

 

[PeterS84]

Nope, not that far if CT is on the bushing. However for HV CTs are not always mounted on the bushing (at least that's what I heared).
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).

 

[davidbeach]

Well, I do spend a fair amount of time is substations, and practically every protection CT I know of is bushing mounted, either on a breaker or on a transformer. That applies at voltages from 15kV class to 500kV. Metering CTs are generally not bushing mounted. Generator neutral CTs are not bushing mounted as there is no bushing. But in substations/switchyards, I can't think of any exceptions in our system or in what I've seen of other companies' yards.

 

[DTR2011]

This sounds like a dead tank (ANSI) / live tank (IEC) comparison.

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.

 

[PeterS84]

Thanks for the information. Sorry if my previous response sounded a bit cheeky - I meant I don't go out to substations so have very little idea about substation design.
Perhaps I was looking at a metering CT...

 

[PeterS84]

Thanks a lot smallgreek. Since I don't have access to any layout drawings my question was more of an academic nature I was just curious of how much of an impact (if any) it may have on CBFail.
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.

 

[DTR2011]

You may want to read the paper from SEL on breaker failure relaying.

http://www.selinc.com/search/SearchPage.aspx?searchtext=352

, titled Advanced in Breaker Failure Protection, starting on pg. 8, this is discussed.

 

[PeterS84]

Thanks for the interesting paper!
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.