Bus Zone Protection On Open Conductor Site

[isquaredr]

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?

 

[RalphChristie]

That would be the first time I've even heard about a bus zone scheme connected like that. You sure the reversed CTs were not incorrectly marked? Can't see how such a scheme will stay stable, especially when loaded.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[jghrist]

Each phase has an independent impedance element. If all CTs from one phase were reversed, it should make no difference. The relay operates by current going through the high impedance creating a voltage. It doesn't care which direction the current flows.

 

[RalphChristie]

Yes, yes, you are quite right - if there is a element on each phase. Although I can't see any real advantages in doing it on such a way.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[slavag]

Hi.
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

 

[jghrist]

Even though the relays have independent phase elements, connecting one phase CT reversed would be strange and I can't think of a reason to do so. One result would be to have continuous current in the CT neutral between terminals if four wires are used for the CT connections. The CT secondary currents would not add to zero with balanced load.

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.

 

[isquaredr]

I can confirm it is the same phase ct reversed on each incoming / outgoing circuit and I agree as long as no fault exists on the busbars there will be zero voltage on the bus wires.
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.

 

[davidbeach]

isquaredr, look at the instruction manual for a Basler BE1-87B or an SEL-587Z for a good description of how a high impedance bus differential system works. Both manuals are free, but both sites require you to register before you can download.

 

[jghrist]

Essentially, the relay works by passing the sum of the CT currents through a high impedance. If the fault is external to the bus, the sum of CT currents is zero because any current flowing into the bus through one terminal flows out of it through the other terminals. For a bus fault, the current in one or more terminals flows toward the bus without being balanced by current flowing away from the bus. The sum of the CT currents causes a high voltage which operates the relay.

 

[isquaredr]

I understand a standard bus bar protection scheme ie 3-ct's in parallel per feeder or core balance ct but this system only protects for ground faults.
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.

 

[slavag]

No,absolutely,No.
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

 

[slavag]

Hi.
See attached link.

http://www.ceerelays.co.uk/brochures/CEE/IAG7014 & IAG7034.pdf

Fig 1 and 2
Are you have Fig 2 connection with one reverse CT?
Regards.
Slava

 

[slavag]

O.K.
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

 

[isquaredr]

Yes Slava fig 2 is correct with 1-ct connected in reverse per bay, the 2-bus wires connect to a high impeadance voltage operated relay (atracted armature type)

 

[slavag]

O.K. Isquaredr.
I'm hope, that my excplanation is right.
It's intresting, first time I see this type of connection.
Regards.
Slava

 

[jghrist]

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").

I'm not sure that this is true in the case of a phase-to-phase fault. In this case, Ia = -Ib and Ic = 0, so Is = Ia+Ib-Ic = -Ib+Ib-0 = 0

 

[slavag]

Jghrist.
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

 

[davidbeach]

If such a system truly exists - it would be a prime example of penny wise and pound foolish - it would have to be the middle bus with the reversed CT connection. The scheme would work for phase-phase faults for any two adjacent phases and blind to faults that involve only the two outer phases. The likelihood of no ground involvement and no middle phase involvement is quite low.

 

[isquaredr]

Davidbeach, I can confirm that such a system, indeed many systems exist of this type. Im not quite sure "penny wise and pound foolish" is the correct phrase to use as the scheme is 40+ years old, I would rather consider it a unique and very cost effective way of protecting busbars at minimum expense.
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.

 

[slavag]

Nu NU Isguarder.
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