Symmetrical faults.

[johndaniel]

Hi folks
I am working on a short project to determine the detection of symmetrical fault occurence in loop system. Does anybody have a idea about this thing?

Thanks

 

[davidbeach]

An overcurrent element picks up in response to the fault. What more does your scheme need?

 

[johndaniel]

I want to find the direction of fault current in scheme.

 

[davidbeach]

Then use directional overcurrent.

 

[dpc]

In general, to determine the direction some type of 3-phase voltage sensing will be required at each overcurrent relay. The voltages are used to polarize the current detection.

 

[magoo2]

To my way of thinking, most faults are unsymmetrical. Maybe a three phase fault would be the only symmetrical fault.

Are you looking at ways to detect symmetrical faults or any fault?

 

[davidbeach]

Ah, come on, there's lots of types of symmetry.

While a 3-phase fault is indeed symmetrical about any phase, an SLG fault is symmetrical about the faulted phase and a 2LG or LL fault is symmetrical about the unfaulted phase.

 

[johndaniel]

I am looking for the ways to detect the three phase faults on my system. As for any unsymmetrical faults(LG, LL, LL-G) the dominance of negative sequence components are high which could be used to determine the direction. But what about the three phase fault?

 

[dpc]

In power systems, "symmetrical" generally refers to symmetry of the sine wave. Faults other than three-phase are generally referred to as "unbalanced".

With appropriate voltage sensing and polarization, directionality of three-phase faults can be determined even with electro-mechanical relays. There's even a standard ANSI function code: 67

Here, let me Google that for you:

https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=7273

 

[davidbeach]

Directionality for 3-phase faults (yes, I know that's the common usage of symmetrical, but the question seemed rather odd) is actually easier than for the rest of the bunch; all that's needed is just the phase angle between voltage and current. When there's a fault and no negative sequence current any modern relay will use a positive sequence directional element. Now that negative sequence is just a numeric function it is readily available, but in the bad old days, phase directionality required comparison of phase-phase voltage angles against phase current angles.