A city without a network

[Mbrooke]

What would you guys think of a large city (like Chicago) designed entirely around a main-tie-main approach? No secondary networks either.

A few down sides I see in networks:

1) The loss of more than one or two feeder cables under peak load conditions can result in the entire network needing to be shut down in that the remaining cables and transformers overload. This overloading can weaken cables making future failure most likely.

2) A ground fault with a stuck network protector (which is rather common) subjects cables to full line to line potential resulting in unnecessary dielectric stress which could lead to future failures.

3) Short circuit current is obscene

4) Relying on "burn clear" techniques greatly increase the incident energy- possibly damaging other cables/equipment in the vault- and releasing vast amounts of smoke and fire sometimes entering the basements of building producing high CO2.

5) The loss of any one transformer without causing a primary feeder breaker to open results in unnecessary stress on reaming equipment during other equipment outages increasing the odds of failure now and down the road.

Yes there is the advantage of maximizing assets while providing superior service continuity in the short term- but the long term has me pondering what I consider to be a self fulfilling prophecy.

Just thinking...

 

[protoslash]

1) the whole point of network system is to mitigate outage of one feeder. Some should handle even two feeder outage.
2) protector has fuse as back up. If fault is SLG and network tx primary is ungrounded, then the fuse won't work. You can install grounding transformer on the feeder to make sure SLG always have ground reference after feeder trips
3) yup
4) im not sure what you mean here, you taking about secondary cable?
5) What do you mean loss of one TX. If a TX faults, the primary feeder will open. If the TX is just out of service, yes rest of the network will pick up its load, but this is the entire point of a network system, all TX are oversized in case a few goes out.

 

[Mbrooke]

1) Correct, most large city networks can handle two feeders out not problem. Even 3 with a 5% voltage reduction.

2) Or high speed grounding switches... but I'm not sure that all POCOs use them. Some POCOs use delta-wye trafos, so a primary ground fault with a live on back feed condition occurs it will subject the cable to L-L voltage.

4) Yes, the secondary cables. The manhole will smoke, sputter and shot flames until POCO crews arrive. Cable limiters help, but only to a degree.

https://youtu.be/tJ1yEndX6ng?t=333

https://youtu.be/pFPphf4qSOM?t=320

5) TX that fails without a opening the primary feeder, or more common a network protector that fails to close back in. None functioning equipment is not immediately known.

 

[protoslash]

2) there will be more than one network transformers on a faulted feeder. These TX are loaded differently, so when feeder breaker opens, the most lightly loaded TX back feeds the higher loaded ones. The protectors trips in sequence from lightest loaded to heaviest loaded. The heaviest loaded protector in the end see TX excitation current of all other tx on the feeder which is sufficient trip itself. The surge in cable insulation voltage is not sustained for long.

4) i thought all secondary cables are equipped with current limiters

5) why won't tx fault trip the feeder?

Imo the biggest issue with network system is that it can't allow DG connection as the protectors are programmed to trip on very small reserve power flow.

 

[Mbrooke]

2) Not if a network protector sticks (fails to open). The ones that work will open as said- but not the stuck one(s).

4) Only modern networks or those which have been upgraded.

5) Not all TXs fail in short circuit- though granted such a failure is less common. More common is a network protector failing to close back in leaving an unloaded traof. This is typically not an issue, until equipment goes out of service elsewhere. The Long Island City network outage uncovered a lot of these problems.

 

[protoslash]

2) ok, i guess if it fails to open and no fault current, then either buy higher rating cable or grounding TX and force fuse to clear fault.

4) didn't know this

5) Ok, that makes sense. But this is easy to catch, these things are inspected for maintenance regularly.

 

[Mbrooke]

2) The cost is often prohibitive.


5) Well- try inspecting several thousand network protectors mostly in vaults or locked away on the customer's premises.

 

[Mbrooke]

Anyone know how new cities are done today? Or how they would be if the US was to start over today? I hear Con Ed wants to subdivide networks into smaller segments.

 

[JASGripen]

IoT Technology Microgrids in smaller networks with a lot more interaction between customer and utility.
Con Ed is trying to do it before the customers do it - good move from them.

 

[Mbrooke]

Can you give more detail on this? I'm curious on how it works.