Interesting lesson on a diesel-fed 12.5 kv distribution line

[Eleanor White]

Heard this from an engineer who worked on a
rural diesel generator system feeding an
18-mile, 12.5 kv distribution line.

The line had two sets of main line fused
disconnects out on the main line, some miles
from the generating station.

(Pretty much all of the load was domestic, in
a couple of small villages, with a few small
motels and shops. No industry.)

The diesels were three 800 kw units, with
two units paralleled much of the day, and some
of the night. These were 600 volt units.

An experiment was scheduled to see what sort
of fault current was available at the diesel
station, so as to review the fuse coordination
scheme.

At the scheduled time (middle of the night,)
the disconnects at the power plant were opened,
and a disconnect with a test fuse, one side
grounded, was closed in on the unloaded
generators. (Using the station's 600 volt
main breaker.)

The first test used a 7 amp fuse, rather light.

The 7 amp fuse did NOT blow, instead, the
600 volt side breakers on the generators tripped.

The experiment was cancelled as it was clear that
the short circuit current situation was far
different from distribution lines fed by large
transmission lines.

What is very interesting is that the disconnects
out on the line had 80 amp fuses closer to the
station, followed by 50 amp fuses further out.

Line crews reported that indeed, the 80 and 50
amp fuses had indeed blown during various storm
activity, yet, the diesels could not blow a
7 amp fuse right outside the station.

My estimate is that the impedance of the line
between the station and the mid-line fuses allowed
the generators to not drop their voltage, which
made it possible to more slowly melt a heavy
fuse.

So effective short circuit current was very low
at the station, and actually ROSE with distance
from the station.

Live and learn?

Eleanor White

 

[waross]

Thanks for sharing Eleanor.

My estimate is that the impedance of the line
between the station and the mid-line fuses allowed
the generators to not drop their voltage, which
made it possible to more slowly melt a heavy
fuse.

So effective short circuit current was very low
at the station, and actually ROSE with distance
from the station.

That sounds reasonable.
I am guessing that these are self excited sets.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

Actually, if you have the generator specifications, the available fault current can be calculated.
Unless these are very very small.

800kW units should have specifications available, or lost.

Likely, the 7 amp fuse was slower than that of the generator breakers. Has happened.

One thing is that most faults are very reactive, and maybe the generators can't provide reactive current very well. Larger units have a thing called field forcing that will attempt to support the voltage during a fault and help with post fault recovery.

 

[Mbrooke]

While I can't add much more than what has been said, I will say thank you for posting this!

 

[FreddyNurk]

I'm certainly skeptical that there is more fault current available away from the source than there is close in.
There are definitely effects related to impedances and the amount of reactance in the fault, and often enough the further away from the station the fault is, the more resistance there is in the fault, which is generally quite effective at stalling generators.

For the story to be accurate, one would need to ensure that none of the other protection (undervoltage, underfrequency and so on) is the reason for the circuit breaker tripping, rather than overcurrent from the breaker. I have also come across plenty of situations where it is expected that the generator will behave like a much larger source, and the decrement behaviour is ignored, which leads to overly ambitious settings.

I do know that for some overhead lines, when fed by a step up in this sort of arrangement, there isn't enough current to significantly damage a grounded line, which would certainly have some bearing on protection coordination. I have also seen overhead lines with a maximum prospective fault current not much greater than the load current, which makes it very difficult to be able to detect a faulted line without using additional protection functions.

EDMS Australia