Flexitest Switches Induced Currents

[mgtrp]

Hi,

The utility that I work for often uses ABB Flexitest switches for isolating CT and VT circuits from protection relays.

I have observed in some relays that even with the CT shorting switches opened (but the circuit still live - i.e., current passing through the shorted CT's), there is still observable current on the relays, in the region of 0.5 to 1 A secondary. I presume that this is due to induction. Talking to our technicians, this is not altogether uncommon.

This makes it particularly awkward when trying to test some relay elements, especially if the current is sufficient to fool the relay into thinking that the circuit breaker is not open (relays that use both 52a contact and current to determine circuit breaker status).

Does anyone else have experience with this? Are there any convenient methods for reducing this induced current?

Thanks.

 

[DTR2011]

In my experience, this has to do with poor design of the FT Switch.

#1 Is this a single relay circuit, or are there multiple relays and FT's in series.

#2 If this is a single Ft - relay circuit, where are the neutral jumpers?

I have seen so many poorly designed FT circuits in the last 10 years. What is worse is when it it shown to powers that be, the problem is ignored because "thats the way we have always done it"

I think the FT is a great "flexible test switch", buit much has to to be learned about its proper application and design.


I dont think you have an issue of induction. I think you have an issue of too many neutral jumpers in the CT circuit, but need more details to confirm.

 

[waross]

Can you measure the resistance of the switch? Given the low burden of modern relays, is it possible that the shorting switch and the relay element are dividing the current as will two resistors in parallel?
As a test, wire a test switch and relay element in series on the test bench and apply a current. Then measure the voltage drop across each device with a sensitive voltmeter. See if the result correlates with the load current and the current that the relay element sees when the shorting switch is closed.

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

 

[JG2828]

With the correct application of the FT switch it is physically impossible to see current or voltage on the relays through the FT switch. There is a pretty large air gap.
I believe smallgreek meant to say "poor application" of the FT switch and not poor design.

See page 37 of the attached FT switch catalog for proper application.

Can you upload part of a schematic showing how you have it wired?

 

[mgtrp]

Typical schematic is attached.

I presume that the system earth should be on the CT side of the switch, negating the need for a second earth on the relay side?

 

[JG2828]

Well there's your problem...

You need to remove the following jumpers:
7 - 11
11 - 15
15 - 3
and also remove the extra ground on 5.
Keep the jumer on 3 - 5.
Remember that there should only be one ground on a CT circuit.

Please be sure to never open circuit a CT, it is very dangerous. Make sure you short at the terminal block and check for current before any re-wiring.

 

[JG2828]

I have drawn in the two paths that the A phase current will take using your schematic. As you can see, the current will divide into the red and blue paths based on the difference in electrical resistance in the relay CT coil and in the FT switch.

See page 37 in the catalog I attached in my above post for the porper application and wiring of the FT switch.

 

[mgtrp]

Thanks. Seconds too late though, I just finished making the exact same sketch for our technician, the only difference being that I used blue on the relay side and red on the CT side.

 

[davidbeach]

Wiring should be:

IA - TB-29C-A-1 - 29C-A-4
IA1 - 29C-A-3 - Z01
IA2 - Z02 - 29C-A-5

IB - TB-29C-A-2 - 29C-A-8
IB1 - 29C-A-7 - Z03
IB2 - Z04 - 29C-A-9

IC - TB-29C-A-3 - 29C-A-12
IC1 - 29C-A-11 - Z05
IC2 - Z06 - 29C-A-13

I0 - IN - TB-29C-A-4 - 29C-A-14 - 29C-A-10 - 29C-A-6

Ground TB-29C-A-4

That's it. No need, purpose, or benefit to test switches on the residual. And always EXACTLY one ground on a CT circuit.

 

[JG2828]

davidbeach,
I agree that there is no need to have the risidual wired to the test switch in this schematic. I was trying to provide the easiest fix so that the schematics would not change much and there would not be much re-wiring.

I do not agree with your wiring though. While it will work, it is not the industry standard or per the FT switch instruction book. The FT switch should be wired so that the CT polarity flows through the test jack of the FT switch. In this case 6,10,14 or 18.
Your wiring would have it on 4 8 and 12.
It is necessary to have the proper polarity at the test jack when using test tools like the "in service test plug" which is often pre-wired to a wattmeter.

You can reference page 37 of the manual for an example.

 

[davidbeach]

You may be right about the in-out order; it will be a few days before I can check our standards, but it probably doesn't matter. When testing the test set would have A-phase current leads connected to 3 and 5, B to 7 and 9 and C to 11 and 13. The test jack doesn't really play into it since both poles are used.

But in any case:

No jumpers on the relay side of the test switch.

No residual through the test switch.

Exactly one ground. Typically this should be at the first point the CT circuit lands on a terminal in the control house.

 

[JG2828]

I usually draw the FT switch as shown in this attached pdf. It makes the drawings cleaner and easier to read.
Especially if you have multiple relays in the same current circuit.

Maybe this will help someone down the road.

 

[JG2828]

Corrected a mixup on the relay inputs.

 

[mgtrp]

Thanks.

For closure, it turns out that the as-designed wiring was based on the BC Hydro standard, but poorly implemented. The BC Hydro design does incorporate two grounds, but only one is in the circuit at any one time - see attached.