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setting 67 relay

setting 67 relay

setting 67 relay

(OP)
Want to install a 67 relay in the bushing transformer, looking in direction system->transformer, as a remote backup protecion of  equipment.  Should be submitted to the following currents:

  1 - Current direction transformer->system:
  Load = 8000 A
  remote fault = 12000 A
  2 - Current direction system->transformer:
 Load = 100 A
 remote fault = 800 A
 Question:
1- Assume that relay must operate in direction system->transformer , which would be the pickup current  of the relay:
 Ipick > 100 A or Ipick> 8000 A?
2- The results of a program of short circuit will be  reliable when the fault current is much greater than the load current,
 which is not our case (load = 8000A, 800A = fault)?
 

RE: setting 67 relay

If the relay is directional, and set to only operate in the system->transformer direction, it needs to be set above 100A and less than 800A.

RE: setting 67 relay

Not quite clear what your "system" mean though...

But, let's say relay must see from system -> transformer, assume 100A is the full load and 800A is the end-of-line fault, you may chose the pickup setting > 200% of the full load, and < 50% of the end-of-line fault.

If your system has a fault-to-load ratio less than 4:1, you may use 21 element.  Or use load encroachment feature on your 50/51 element.

RE: setting 67 relay

(OP)
davidbeach ,
That was the setting selected(> 100A). But there were objections:
1- Relay is a multfunction numeric type, and we are using 67 function.The logic diagram of the function used (Figure 2-15 of the manual) shows that if it is adjusted under load (8000A) it will be in the starting state (Pickup) and become permanent depending only on the direction to take command of the trip.
The ideal is that the relay will start when overcurrent and right direction at the same time, not only due to overcurrent. Already referenced by the logic diagram that would only be possible through the internal logic(CFC).
2- The results of a program of short circuit are reliable only when the fault current is much greater than the load current, which is not our case (load = 8000A, short = 800A). In fact the actual current is obtained through the theorem of superposition where the final current is the sum of the load current plus the current produced by the short. In our case the current would continue in the same direction as the load would be the sum of 8000-800 = 7200A in the direction of the load. The relay would not operate.

pwrtran
I am eliminating the details of the project. The relay is already bought and function 67 was the only one available. I do not know about "load encroachment feature on your 50/51 element."

 

RE: setting 67 relay

If there is no generation beyond the transformer, you'll never trip.  If there is generation, you'll get outward current because the load will reduce significantly due to the voltage sag.  There will be a limit as to how far away the fault can be.

RE: setting 67 relay

What model of relay is this?

If I understand this correctly, your transformer has a sufficiently large enough source on it's secondary terminals that it can supply 12,000A of remote fault current to a system type fault...but the HV system is weak enough to only supply 800A?  Is that 800A a fault on the primary of the transformer or the secondary?  My assumption is the secondary.

I'm confused why bullet point #1 is an objection, that's how every 67 element I've set operates and I've never had a problem with them mis-operating.

RE: setting 67 relay

(OP)
1- There is no generation beyond the transformer.
If we assume bullet point #1, so relay operate only by direcional unit.
2- My concern is whether the calculations (800A) are feasible because of  theorem of superposition.Are they ?

We have to find another solution.
 

RE: setting 67 relay

(OP)
pwrtran ,
Could you say write down a reference about this application.

RE: setting 67 relay

Ok, I'm confused now.

Transformer has a high side and a low side.

What is the winding on the high side?
What is the winding on the low side?

Where is the relay, high side or low side?

Where is the positive sequence source, high side or low side?

RE: setting 67 relay

Looks like a misapplication.  Normally that fault will be fed from the generators and a relay on the ST transformer would see only a slight reduction in current.  Put a nondirectional overcurrent relay at the breaker above the small transformer.

RE: setting 67 relay

(OP)
Fault will be fed from the generators and System (very high contribution).
We do not permit relay of Auxiliary System trip Transformer units (4gen + ST). There are different maintenance crews to service and assist the transformer unit.
Transformet Unit relay should provide back up protection for auxiliary system.

 

RE: setting 67 relay

I agree with David.  If you have no generation on the secondary of the transformer you have no need for a directional relay.

RE: setting 67 relay

Sorry for the two posts in a row.

Why don't you put a relay on the breaker on the HV side of the small transformer with the 440V secondary?

RE: setting 67 relay

(OP)
There is this relay. But in a breaker failure on the HV side of the small transformer , there are backup from generator protection but not from transformer protection.

RE: setting 67 relay

Better to just have a breaker failure relay on the breaker for the small transformer and have it trip the big transformer?

RE: setting 67 relay

(OP)
We do not permit relay of Auxiliary System trip Transformer units (4gen + ST). There are different maintenance crews to service and assist the transformer unit.
 

RE: setting 67 relay

Sorry, too bad that procedures supersede good protection.

RE: setting 67 relay

(OP)
you guys tend to install breaker failure protection for circuit breaker of the auxiliary service  MV system belong to generator system like that?
 

RE: setting 67 relay

I really don't know if any of our plants would have breaker failure in that instance, but I do know that any thing that can trip the generator breakers could also trip the transformer high side breaker.  Changes in ownership between plant and high-side switchyard doesn't matter.

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