Have you ever noticed the footnote on the standard recloser Time Current Curve?
'Tests conducted at 25 C (77 F)'
Have you considered the effect of temperature on your recloser co-ordination?
Although you have no control of the ambient temperature in which the reclosers operate, you cannot ignore its effects on recloser co-ordination.
Hydraulic reclosers are very temperature sensitive. For example, the TCC of a Type 'L' shifts approximately 2.5% per degree F; faster times for ambient temperatures above 77; slower times for colder temperatures. The actual variation is a natural log function of ambient temperature.
How have new hydraulic reclosers co-ordinated so successfully?
New hydraulic reclosers are tested and calibrated at the same factory using the same testing standard that controls current and temperature while measuring time. When you installed new reclosers, the TCC's shifted up and down with ambient temperature, but your co-ordination scheme functioned correctly because each TCC maintained its relative position in the scheme. In reality, you have always had two recloser co-ordination schemes; summer and winter.
Now what happens when you introduce repaired reclosers?
Consider the following example:
Assume you install a 50 amp Type 'L' recloser tested and calibrated at 87 F during the summer. The standard 50 amp TCC shown in black shifts approximately 25% slower to the red TCC when TCC's are compared at standard temperatures. (See 2nd graph from the top)
And assume you install a 70 amp Type 'L' recloser tested and calibrated at 57 F during the winter. The standard 70 amp TCC shown in black shifts approximately 50% faster to the blue TCC when TCC's are compared at standard temperatures. (See 3rd graph from the top)
Now look at the co-ordination carefully. (See 4th graph from the top) The reclosers have effectively traded positions on the ditribution line. The 70 amp recloser will lockout for all faults beyond the 50 amp recloser. Failure to consider temperature during testing and calibration results in loss of co-ordination!
Reclosers repaired in our shop are tested and calibrated in a temperature controlled oil tank; not in the oil in which the reclosers are finally tanked.
Our maintenance program works; simply because we consider all the variables!
Now, can you appreciate the difference in our repair process?