radial distribution system question

[immsk]

http://static.schneider-electric.us/assets/consultingengineer/appguidedocs/section5_0307.pdf

In the PDF linked above on page 3. they compare two different types of radial systems, figures 5-4 and 5-5, and say that:

"A more reliable and maintainable arrangement utilizing multiple primary feeders is shown in figure 5-5. In the
system of figure 5-5, each unit substation is supplied by a dedicated feeder from the service entrance switchgear"

I don't quite understand why Figure 5-5 is more reliable. Even though the only difference between the two systems is the addition of a switch before the transformer. Would some one care to enlighten me?

Thanks in advance.

Saad

 

[wroggent]

I imagine the first switch and fuse would be in the service entrance switchgear and and second switch would be near the transformer.That would facilitate isolation and easier access to the feeder cable and the transformer; this would allow for preventative maintenance to be done without disassembling anything (e.g., disconnecting feeder cable from transformer), but instead just flipping the switches. It's more reliable because it's more maintainable.

 

[MCF59s]

I agree with wroggent about the switch fuse locations though I think Schneider could have done a better job with the diagram. Here's my take:

Figure 5.4 - Each Load Interrupter switch/fuse combination is physically located next to the substation transformer. Since the only overcurrent protection device upstream of the primary conductors for each transformer is the medium voltage circuit breaker, a fault on either of these two feeder sections will trip the MV CB and cause a system wide outage. This is specifically why 5.4 is less reliable than 5.5

Figure 5.5 - Each Load Interrupter switch/fuse combination is physically located at the medium voltage switchgear (though not apparent in the diagram). The substation primary isolation switch is located next to the transformer. The conductors between the upstream fuse and the isolation switch make up a dedicated feeder to each transformer (whereas in the previous arrangement, they were essentially two sections of the same feeder). A fault on any single feeder will cause a power outage only to the loads within the upstream fuse's zone of protection (1/2 the system). This is more reliable than 5.4 because multiple simultaneous feeder faults are required to take out the entire system.

As far as maintainability goes, having switches at both ends of a feeder allows for complete isolation.

 

[Marmite]

I agree with MCF59s. It's very badly drawn as from a cursory glance both look very similar. For it to make sense as per the description for 5.5 the top level of switchgear all has to be part of the same switchboard, ie incomer and two fuse switches all on one board.

Regards
Marmite

 

[ScottyUK]

I think the entire section titled 'Expanded radial system' is being proposed as more reliable than the earlier examples of 'Radial' and 'Radial system with primary selectivity'. The example system shown in 5.5 isn't proposed as being more reliable than that in 5.4, it is just more maintainable. In fact it is almost certainly less reliable than 5.4 due to the introduction of the additional disconnectors, but I think the author's intention was to say that the schemes shown in diagrams 5.4 and 5.5 are more reliable than those shown in diagrams 5.2 and 5.3.

This probably belongs in the grammar forum - it's the text which is poorly written, not the drawings!