Tying secondary systems together
Tying secondary systems together
(OP)
Why are the lower voltages of transmission systems often tied together? For example, where you have 345kv/115kv, 400kv/132kv, or 500kv/161kv, the lower voltage system is often tied together with interconnecting transmission lines even though the majority of the generation is at the high voltage level ie 345kv and above.
Second why isn't 115kv, 138kv, 161kv, ect refereed to as sub-transmission even though today these voltages today are stepped directly down to distribution, ie 13.8kv?
Second why isn't 115kv, 138kv, 161kv, ect refereed to as sub-transmission even though today these voltages today are stepped directly down to distribution, ie 13.8kv?






RE: Tying secondary systems together
In most of North America everything 100kV and above is subject to the same set of NERC requirements. It may function as sub-transmission, but it is regulated as transmission. Not much to be gained by treating it differently. In our case, if it's transmission enough to get line relays (including our 57kV) the settings are done as though all the PRC requirements apply. That way there's a common expectation of how it works regardless of voltage level.
RE: Tying secondary systems together
RE: Tying secondary systems together
RE: Tying secondary systems together
RE: Tying secondary systems together
In a nut shell, the critical Clearing Time (CCT) of the system becomes small, as the impedance between the two stations is very less.
When you tie the system though the sub-transmission lines, the transformers and the lower voltage line offer higher impedance and hence the system becomes more elastic. The CCT improves. Thus even if the primary relay fails to operate, the back up relay can clear the fault, without the cascade tripping, grid collapse.
With this arrangement, there could be some system oscillations and power swing, but better than the system break up.
RE: Tying secondary systems together
Sometimes I have heard 115 kV referred as sub-transmission.
RE: Tying secondary systems together
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Tying secondary systems together
I get what you are saying about raidal vs looped, but don't forget independent yet electrically paralleled lines from the same bulk substation. One line will fault without a total interruption of load- just the voltage sag before the fault clears.
@Krisys: Good point- I remember that being touched in another thread. I guess this would be an example of increasing critical clearing time.