Bus Zone Protection On Open Conductor Site
Bus Zone Protection On Open Conductor Site
(OP)
High impeadance bus zone protection with 3-ct's connected in parallel for each feeder feeding onto buswires with a high impeadance relay across is what I am use to seeing for this type of protection in metal clad switchgear. The other day whilst on an open conductor site I noticed 2 out of the 3 ct's per feeder were in parallel the 3rd was connected in reverse, they again supplied buswires with a high impeadance relay in circuit. I asked around for an explanation for the reversed ct, the only explanation being it allows the scheme to protect against phase - phase as well as earth faults.
My question is how?
My question is how?






RE: Bus Zone Protection On Open Conductor Site
Regards
Ralph
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RE: Bus Zone Protection On Open Conductor Site
RE: Bus Zone Protection On Open Conductor Site
Regards
Ralph
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376: Eng-Tips.com Forum Policies
RE: Bus Zone Protection On Open Conductor Site
I remembare from old manuals ( if I'm not mistake, it's Siemens and maybe ABB)some special wiring of High Impedance BB protection. This type of connection used in case of special summation CT's are installed before input to relay. This CT translate or inverte 3-ph current of each feeder to one current input to relay. It's not case of Isquaredr.
I assume in case of Isquaredr it's some mistake in installation or marking. As Jghrist saied, "Each phase has an independent impedance element".
Isquaredr, you can continue work with your system w/o any problem. It's standard configuration of HI protection and of course it's protected against phase to phase and earth faults.
Maybe for yourself, provide some secondary or primary test of protection with simulation of phase to phase and earth fault.
Good Luck
Slava
RE: Bus Zone Protection On Open Conductor Site
Connecting the positive polarity end of one phase to the neutral which had the opposite polarity connected on the other phases would certainly be unusual.
RE: Bus Zone Protection On Open Conductor Site
How does the scheme protect against busbar phase faults ?
I have come across an other similar scheme since the original post, both of which would have been constructed by the CEGB as it was called then in the UK around 1960.
RE: Bus Zone Protection On Open Conductor Site
RE: Bus Zone Protection On Open Conductor Site
RE: Bus Zone Protection On Open Conductor Site
Apparently with 1 of the ct's connected in reverse on each feeder the scheme also covers phase faults. This is what was said to me, I just cant figure out how it works.
RE: Bus Zone Protection On Open Conductor Site
Standard BBP is phase segragate protection.
3-CT's of bay in parallel is Holmogren connection, used for earth/ground fault protection.
HIR ( high impedance relay)have 3-voltage inputs, each one is parallel connection of all feeders (of course per zone) per phase.
Each input is completly independent from other.
BTW, same logic used in low impedance BBP: vectorial sum of
all zone currents for each phase, but this is provide by SW.
Of course it's stabilazed BBP with additional criterions.
Could you please send us some drw of relay connection.
What is name of relay, may be today is CEE relay?
I have some crazy idea, why it's work with reverse CT, but..
What is a setting of protection and what is delay?
Are your HIR have only one input?
Regards.
Slava
RE: Bus Zone Protection On Open Conductor Site
See attached link.
http://www
Fig 1 and 2
Are you have Fig 2 connection with one reverse CT?
Regards.
Slava
RE: Bus Zone Protection On Open Conductor Site
I think, I'm understand, what them do.
See fig 2 (earth fault BBP)
I0=Ia+Ib+Ic="0" ( I01+I02+..+I0n="0", n=number of bays)
in case of phase to phase fault it's also eq. to "0".
Now, we reverse connection of one CT.
Is=Ia+Ib-Ic= "2I" (Ia=Ib=Ic=I)
We build now some bay reference current. In the normal situation: (Is1+Is2+..+Isn="0").
In case of any fault: Is1+Is2+..+Isn : not eq. "0").
Reason????, may be economy and simplicity. In this way, them
reduce 3ph-currents of each bay to simple one-phase meas.
Regards.
Slava
RE: Bus Zone Protection On Open Conductor Site
RE: Bus Zone Protection On Open Conductor Site
I'm hope, that my excplanation is right.
It's intresting, first time I see this type of connection.
Regards.
Slava
RE: Bus Zone Protection On Open Conductor Site
RE: Bus Zone Protection On Open Conductor Site
Thanks for comments:
I need think about Iab fault option.
BTW, we don't have this problem in case of Iac ( Ia-(-Ic)) or Ibc (Ib-(-Ic)) or Iabc faults.
Regards.
Slava
RE: Bus Zone Protection On Open Conductor Site
RE: Bus Zone Protection On Open Conductor Site
There is one part of the scheme which I had overlooked in that the two ct's in parallel, one is centre tapped at half the ratio of the other two, the third of coarse still being connected in reverse.
Looking at a drawing is sometimes easier to explain and if I could upload one I would do.
RE: Bus Zone Protection On Open Conductor Site
Why only now, you tell us about half the ratio of center CT.
The change now things.
Siemens mix sum CT it's 2:1:3 ratio. here, we have 2:1:reverse.
We finished with Iab fault problem and with Iac and Ibc to ground fault problems. OK, we covered now all of type of faults.
Very cost effective way???? May be.
In all cases, after 40+, you can change it to some new digital relay or DTRS ( Don't Touch Running System).
Folks, I'm not recommend used this solution in your new systems.
For your information only:
ABB and Siemens still used such to this system, but with some sum CT per bay with new relays.
Regards.
Slava
RE: Bus Zone Protection On Open Conductor Site
Hope I am correct with this.
So lets say A-phase ct i= -1 (reverse connected), B-phase ct i=1 (normal connected)& C-phase ct i=2 (normal connected, half ratio)
for the following bus bar faults the current in the relay (i) would be:-
A-E, i=-1
B-E, i=1
C-E, i=2
A-B, i=2
A-C, i=3
B-C, i=1
A-B-C, i=2.6
All other faults out of zone will have a cancelling out effect rendering the scheme stable.
RE: Bus Zone Protection On Open Conductor Site
You are correct.
A-E, i=1 ( "-" is not important).
Lets go check phase to phase to ground/earth faults.
AB-E, i=0, still problem.
BC-E, i=3, passed.
AC-E, i=1, passed.
ABC-E, i=2.6, passed.
Something missing???
Check again.
Regards.
Slava.
Crazy, how time we spent for understand old scheme, but it's intresting.