Hypothetical question in regards to system design and protection 6

[Mbrooke]

Would it be possible to design an entire power system where all 500kv, 345kv, 230kv and 115kv lines are only protected via step distance? No POTT, DUTT, DCB, or any communication between relays- of course very short lines would have differential. Perhaps the real question is- what can theoretically be done to increase the critical clearing time of a large power system? Ie GSU impedance, conductor size, generator inertia... I know this question is off the wall (for North America), but rather in regards to a developing country. Can the laws of physics even allow for this?

 

[bacon4life]

Transformer excitation current and capacitor current increase with the square of the voltage, which limits the amount of high-voltage these components can handle. Moving generators close to loads reduces stability issues.

 

[Mbrooke]

And automatic tap changers can certainly accelerate that curve as they draw more current to keep distribution bus voltages at 1.05pu?


Also, for those who have seen more stability reports then I have, what is the highest critical clearing time you have encountered for 345, 400 and 500kv? On average I have seen plenty of 115kv lines that can handle 34+ cycles CCT, while a lot of 230 and 345 seem to hang around 8-10 cycles. This has always mystified me.


In terms of maximum voltage, if I have a 500kv system, as long as I do not exceed an actual voltage of 550kv continuous I am technically fine since it does not exceed the ANSI listed rating?

 

[bacon4life]

In the system QBplanner mentioned, there are at least a couple of factors contributing to the shorter critical clearing time for higher voltages. 1) When a line is opened to clear a fault, it reduces the transfer capability between the over speed generator and the load. The 33% reduction in the 500 kV system is much larger than the 14% in the lower voltage system. 2) On a 115 kV line, a fault depresses voltage in a localized area, so only a limited number of generators over speed. On a 500 kV line, the voltage in a whole region is depressed, leading many generators accelerating.

 

[Mbrooke]

Would this be the lower system impedance mentioned here? Ie having 8 500kv lines over 3 for a 2,500MW flow gate increases CCT?

Also, the less time the fault persists, the less acceleration in degrees (guessing), so the more likely the generator can survive a sudden change impedance on the flow gate? This last paragraph in particular is a big question for me...

http://files.engineering.com/getfil...e07-0812c2dbd20b&file=equal_area_analysis.jpg

 

[bacon4life]

Yes, the pre and post fault curves as shown in the diagram are closer together for lower voltage lines where there are more lines in parallel.

Finally found some more data on the Chief Joseph brake. It operates for 30 cycles at 1400 MW, so it operates AFTER the fault rather than during the fault.

 

[Mbrooke]

Does anyone have any curve or data at what point a generator will go from slowing down to accelerating as the impedance of the "load" drops? Will a high R and low X fault still cause acceleration of the generator? This is one area that has peaked my interest.