Earthing power loss 2

[Power0020]

In any power system, the earth fault current goes to several kiloamperes and earthing resistance of 0.5 ohm is quiet common at a substation.

if that is the case, a circulating current between source and fault of say 10 kA and a resistance of 0.5 + 0.5 = 1 ohm leads to a power loss of about 100 MW! (I^2R)

Faults can last for up to 2 seconds and a reasonable power is dissipated, the question is :

where does this huge amount of power go? and if this is the case, the fault power is so high to cause generator rotor angle to stall (What happens is usually the opposite!)

I believe something is wrong with my understanding, any clue?

 

[waross]

The fault energy is wasted as heat in the ground, the conductors in the ground fault path, in transformer windings, in any arcs and in the generator windings. If the fault is large in relation to the generator a self excited generator may suffer voltage collapse. Some installations use some form of field boosting to mitigate voltage collapse so that the fault current will remain high enough to ensure reliable operation of protection circuits.
A high magnitude fault may result in an instantaneous trip.

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

 

[dpc]

In your calculation, you are neglecting power factor. Faults have low power factors, so much of the fault MVA is reactive power, not real kW. You can't assume that all of the current gets converted to hear.

 

[waross]

I guess I should have said;
The fault energy is wasted as I²R losses in the path of the fault current.
Less ambiguous and such calculations will give true power wasted in the fault, not VA, KVA or MVA.

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

 

[Skogsgurra]

It may well be "huge amount of power", but that doesn't mean a huge amount of energy. A dead short more often trips the breaker in one second than in two and that means that energy in the (assumed) 1 ohm total resistance would not be more than, say, 30 kWh. That is not a lot of energy and the heating effect will only be noticable in the "narrow sections" - if there are any.

Also, most power systems of any magnitude do not cause very large currents during a line-ground fault. The realy heavy currents flow during a line-line fault. And then, the path resistance is not even close to 1 ohm. Current is limited by generator reactance, not by the resistance.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[crshears]

Thanks, Bill and Skogs, for being as fussy with the wording as you were; hence the LPS's to both of you! Imprecise terminology has screwed up many a project, and probably will continue to...

Our utility has begun responding to the increasing amount of distributed generation on our low-tension systems by employing load encroachment relays, which use feeder power factor to discriminate between faults and high loadings based on the notably more inductive nature of fault current.

As for self-excited generators suffering voltage collapse during faults, this is part of a continuum where slow-acting automatic voltage regulators [AVRs] are poorer than quick-acting AVRs, which are poorer than those with field-forcing and/or "transient-stability excitation control."

In this vein, similarly to how aircraft have a 'minimum equipment list' detailing just how much stuff in a given plane can be broken before it's no longer permitted to fly, IESOs have rules stating that no more than a certain number of generators within a given area [as based on system studies] can remain in service with their AVRs on manual, or with resistor field control, before the system fault response characteristic is adversely affected. Since loss of generation is loss of revenue, overtime has routinely been approved to have AVRs repaired overnight so the unit will be available again during the following day's higher primary demand period.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

One case of energy loss through earth is HV DC Transmission where nearly 4000 A will be flowing through earth path continuously (nearly 1600 KMs) during earth return path operation.I know one case where due to high resistance found in the path(rocky regions) the Dc current entered in to AC transmission lines. It entered through the grounded neutral of AC transformers, traveled through 220 kV AC lines and then returned back to earth path through the neutral of a far away transformer.Mean while it saturated the transformer core creating abnormal sound etc.