Explanation for Importance and/or application of the "3*Zero Seq Current" for LG and LLG F

[Wfg42438]

Hello Everyone,

I am in the middle of trying to further understand symmetrical components.

The SEL paper below has been a great resource.

https://cms-cdn.selinc.com/assets/L...etrical-Pt2_AH_20201019.pdf?v=20201020-155433

I notice power system programs allow one to see the sequence, phase currents or the "3 * I Zero Seq Current" results for LG or LLG faults.

I cant seem to be able to figure out the significance of or the typical application for the "3 * I Zero Seq Current" result.

Based on the paper above the only thing I can conclude is that "3 * I Zero Seq Current" is equal to the summation of the phase currents but i still don't have a clear picture of the end goal for this result is.

Can anyone shed some light on what this result is typically used for or an article that clarifies the same?

Could it be that this is perhaps used to identify the pick up value of ground protection?

 

[System Protection]

Yes, the 3I0 value would be used to set your ground relays.

Could also be used in ground grid studies, ground potential rise calculations, etc.

 

[Wfg42438]

@System Protection

The question that comes to mind is why use "3 * I Zero Seq Current" over the other available results (seq or phase currents for example)?

Would one example of this results relevance be for a fault on the secondary of a XFMR with Y-solid grounding since the neutral current would be "3 * I Zero Seq Current" which would be used to determine the pickup settings of the ground protection?

IF this example is valid is there any other example you can provide ?

 

[davidbeach]

We use 3I0 because I0 is 1/3 of the total residual current.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[DiscoP]

The main reason to use this is that 3*I0 only appears for ground faults so it can be set lower (and often faster) than an element based on negative or positive phase sequence that need to be set above normal load conditions and co-ordinate with all down-stream protection.

As others have said 3*I0 is a mathetical equivalent of the traditional ground fault detection using the residual connection of the phase CTs.

There are many text books and references on the application ground fault (earth fault) protection. Most advice in these guides apply to either a 3*I0 calculated ground fault or a measured one.

 

[JezNZ]

3 phases, interconnected via various sources or load impedances etc.

In an earthed/grounded system, when one phase or multiple phases are faulted to ground, current will 'flow' in all phases in-phase through the system into the fault. This in-phase current is the zero sequence current. In the unfaulted phases it is 'off-set' by I1 & I2.

Since this in-phase zero sequence current flows equally through all three phases, and forms a circuit through the ground. It should become obvious.

Current through the ground (system) = 3I0
(Zero-sequence) current through the phases = I0

Edits to clarify

 

[System Protection]

It "flows" in phase and equal magnitude through all the sequence networks.

I1=I2=I0

The current does not flow equally through all the phases (A,B,C)

 

[jghrist]

Since this in-sequence current flows equally through all three phases, and forms a circuit through the ground. It should become obvious.

True except for the becoming obvious part. In a single phase to ground fault, there is no net current in the unfaulted phases. The I0 is offset by I1 and I2 in the unfaulted phases.

 

[cranky108]

The three sequences are mathematical constructs to explain what really happens. The reality is one current flows in the faulted wire to ground. When you do the actual math (not the theory) the other two phases equal zero, which is what happens in the real world.

In mathematical constructs there are the equations and there is the actual numbers, where the equations may look wrong, until one runs the numbers to see some values are actually zero (or near zero).