In the NYC area, where the trains run on electricity, every couple of years someone manages to impale the third rail with an automobile at a grade crossing. If the auto also comes to rest on the running rails, it becomes a short to ground.
The trains don't stop, or slow down. The car melts.
Other way of looking at stiff system is that the reduction on source termial voltage is much less than it would be on a 'soft' system during a short circuit condition. Or Upon an short circuit a soft system voltage collapses easier than that of a stiffer system.
This is because of the ability of delivering short circuit currents which again is related to source impedance.
For example, utility source would generally be considered a stiff source, a stand alone generartor or a UPS source will be soft. A larger generator plant with multiple generator would be somewhere in between. Transformer sources can be thought of similar to generators.
Example: Your facility has a large motor. When it starts the lights dim, some equipment malfunctions, some HID lights go out. This would be termed as, "your facility does not have a stiff supply."
Its all relative and depends on the system or application. Just like comparing strengths of any two elements. An element strong enough for one application may not be suitable for other.
Consider a 50 KVA distribution transformer.
120/240 volt secondary.
Full load secondary current = 208 amps.
A transformer with a percent impedance voltage of 1.8% will deliver a symetrical short circuit current of 11,574 amps.
This would be considered a "stiff" system.
A transformer with a percent impedance voltage of 4% will deliver a symetrical short circuit current of 5208 amps.
This would be considered a fairly 'soft" system.
A transformer with a percent impedance voltage of 6% will deliver a symetrical short circuit current of 3472 amps.
This would be considered an even softer system.
There are other criterea;
When considering motors of 5hp or 10hp. the 1.8% transformer performance would be considered fairly stiff.
If a hypothetical 50hp. motor was started, the 1.8% transformer would be overloaded and the performance would be somewhat soft.
The normal impedances of large power transformers are somewhat higher than the impedances of distribution transformers.
Are we concerned with the systems stiffness under overloads or short circuits.
The stiffness of the system under normal loads and acceptable overloads is more dependant on the regulation of the transformer(s) than on the impedance voltage of the transformers.
The stiffness of the system during short circuits is more dependant on the percent impedance voltage than on the regulation of the transformers.
The impedance of the transformer is the vector sum of the reactance and the resistance of the windings. The regulation is mostly dependant on the resistance of the windings.
Therefore the X/R ratio has some small influence on the stiffnes of a transformer or system, depending on your point of view.
The plant engineer will judge the stiffness of his system by the performance during motor starting and overloads. If his transformer bank is 300% oversized, at 33% load on the transformer bank, the plant engineer will consider his system to be "stiff" and by it's performance at that plant it will be stiff even if the transformers are somewhat soft.
The transmission engineer and protective relay engineer will probably judge system stiffness under short circuit conditions.
By way of comparison, I may ask,
"What is the difference between a fast conveyance and a slow conveyance? Are we talking bicycles, motor cycles, planes, trains or automobiles?
How about a slow driver and a fast driver?
A slow boat to China, or a fast boat to China?
The definitive answer,
"It Depends".
respectfully
The car melting on the 3rd rails in NYC is pretty funny considering that most of the 3rd rail systems use underrunning 3rd rail where the contact shoe runs underneath the rail and there is a wooden or plastic insulation cap over the top of the rail. The automobile would need to impact hard enough to tear off the insulating cover which would also disrupt the supports.
I have never seen at at grade crossing for a 3rd rail system but I know that Boston's Orange line has 1 crossing on the north side of Massachusetts Bay. Does a section of 3rd rail go up and down with the crossing gates or is train momentum used to cross the gap in the 3rd rail.
Actually, if an at grade crossing is critical and changing to a bridge would be too expensive, you could set up a parallel set of narrow gauge tracks that support a mobile concrete barrier. That would rain on the parade of people who like to play chicken with trains.
The last time I considered standing on one and thought better of it, the third rail 'insulation' was wooden '1x' planking, not in good repair.
Gaps in the third rail shorter than the motive power's inter- truck spacing are not a problem, because there are four pickup shoes per railcar, one on each side of each truck. I forget if the M.U. connectors also carry drive power from car to car; they're certainly big enough to do so.
The shoes project laterally from the trucks at knee/bumper height above the running rail. The third rail is ideally positioned to impale an autombile that encounters it axially, and the insulation and third rail support stanchions are not sturdy enoough to seriously impede the process.
Yeah, I know, the automobiles are not supposed to be driving parallel to the tracks. Except of course for the commuters who got drunk on the train are in a hurry to get out of an icy parking lot. ;-)
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At least the third rail of the system was stiff!