Tying secondary systems together

[Mbrooke]

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?

 

[davidbeach]

Tied together for flexibility and redundancy. Single outages are much easier to deal with.

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.

 

[Mbrooke]

Understandable- however would not multiple 115kv lines from each 345kv substation to each distribution substation be satisfactory in fulfilling redundancy?

 

[davidbeach]

Could be. But I can string a bunch of stations together between bulk power transformers with a bit of network meshing and possibly have more line redundancy with fewer lines compared to an all radial 115kV system, and gain some bulk transformer redundancy as well. There's probably also a lot of regional/corporate preference involved too. Different companies come up with different answers to the same problems.

 

[Mbrooke]

And more capacity with less load on the 345kv lines? I've seen power flow studies where some of the power will actually run "parallel" to the 345kv system via the 115kv loops. RAS to trip the loop on a loss of 345kv.

 

[krisys]

From the power system stability point of view, the system gets extremely rigid, if you tie all of them at higher voltages. i.e. the fault level will be extremely higher. In case in any section, if the primary protection (specifically Zone-1) fails to clear the fault, the generation can go out of step very easily.

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.

 

[bacon4life]

Even with automatic reconfiguration of radial lines, there are still momentary interruptions after a fault clears but prior to the reconfiguration taking place. With looped systems, customers should not experience an outage for a transmission fault. Note that customers on the looped system may experience more frequent voltage sags from transmission faults.

Sometimes I have heard 115 kV referred as sub-transmission.

 

[crshears]

My utility calls everything > 100 kV [phase-to-phase] transmission and everything < 50 kV sub-transmission; we don't have anything in between.

CR

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

 

[Mbrooke]

@Bacon- in the US refered to as sub-transmission or outside? Any idea under what conditions?

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.