Also if you would like I can clarify what I meant by delaying new substations or using smaller substations:
In general failure of power transformers today is much less frequent then decades past. Multiple failures at once are rare. Thus as a result any particular system with multiple transformers theoretically only needs one spare.
Picture a typical 180 MVA load pocket fed by 3 different 138kv-25kv substations with 25kv overhead distribution feeders. Each substation supplies 60MVA of peak load. In a none looped system each substation will have two 60MVA transformers. Load is normally evenly split between the two transformers, so for the failure of either transformer results in the full 60MVA being placed across the remaining unit. Hence the selected MVA rating. This is the typical design and mode of operation for most US utilities. Any further increase in load will require more transformers or replacing the existing units with larger ones.
Now, picture a system with recloser loop automation on each feeder circuit. The system can be programmed so that the loss of any 60MVA transformer results in automatic "load rolling". When a transformer fails at substation #1 (for example), the source reclosers on various feeders attached to the substation with the failed transformer open, and the tie reclosers from feeders emanating from other substations close. 12 MVA of load is transferred to substation #2 (6 MVA across each of two transfomer) and another 12 MVA is transferred to substation #3. In total 24 MVA of load is removed from the 60MVA load chunk normally served by substation #1. Thus, when the feeders that were normally being supplied by the failed transformer are picked up by the remaining working unit, it will only see 36MVA of load.
Therefore we have two options:
1. Assuming no load growth will take place in our 180MVA load pocket we can select 6 25/30/36 MVA transformers instead of 6 40/50/60MVA transformers. This greatly saves cost, space and reduces fault current. In systems where larger transformers force short circuit currents to exceed 10,000amps being able to get at or below 10ka is a huge blessing and cost savings on many levels. Not having to consider reactors alone is a blessing.
2. Lets say your 180MVA load pocket was designed as a radial system, 60MVA units. In the next 5-10 years peak load is projected to increase to 250MVA but not beyond that for 25 years. In a radial system you will need larger transformers at each substation (85MVA) or an extra 60MVA at each substation. You could in theory only add a transformer on one or two of the subs instead of all 3 by redistributing the load (feeder size) so the sub that can accommodate the extra transformer sees more loading... but that means some feeders will loose more customers for any fault and still anything that involves substation expansion or renovation costs millions. However, by adding recloser loops and a simple SCADA system over those 5 years you can delay substation expansion for 30 years by investing peanuts. Peanuts that may even pay for themselves in keeping customers on when a car hits a pole or bad whether strikes. For some utilites being able to re-use existing substations alone has justified recloser loops.
No different then what line reclosers and substation breakers have been doing for 90 years. Most faults are temporary like wind blown conductors and squirrels shorting out insulators. In such cases there is often no visible indication of the outage (except maybe a toasted squirrel a half mile away) so auto re-closing is routine practice for overhead power lines.
