DETERMINING CIRCUIT BREAKER SELECTIVITY USING LET-THROUGH CURRENT TO ADJUST TRIP TIME CURVE OVERLAYS
Publication Date: 2013-Jun-06
The IP.com Prior Art Database
The present invention comprises a method for determining the available current level to which a pair of series-connected breakers will be selective. A transfer function that models the current-limiting behavior of the branch breaker is developed. The transfer function is then used to adjust the trip-time curve of the main breaker on its current axis. The overlaid branch and main trip time curves intersect at the selectivity limit for the pair.
The present invention relates generally to electric circuit breakers, and more particularly to circuit breaker selectivity.
Normally, an electrical distribution network which serves critical loads must permit only a protective device nearest a fault to open during an over current event. In this way, the interruption of electrical service on other branches of the network is prevented. Pairs of circuit breaker samples typically need to be tested in order to determine their selective performance. Determining whether or not such a pair of series circuit breakers will exhibit this isolative behavior or selectivity, requires detailed trip-time and current-limiting characteristics of the breakers involved.
Typically, testing is performed to determine selectivity. A main breaker and branch breaker are connected in series, the former typically having a higher rated current than the latter. A short circuit test is conducted, which causes fault current to flow through both breakers. If only the branch breaker trips, the pair of breakers is considered to be selective at the test current level.
Conventional industry methods, utilize overlay trip time curves to determine the selectivity of the breakers they represent and the intersection of the curves is the selectivity limit of the two.
In another conventional method called "up-over-and-down", is used at discrete current levels to shift the trip time curve of a main breaker when used with a current-limiting branch breaker.
Further, in such conventional methods, the tripping energy (I^2t) and I^2t let-through of the main and branch breakers, respectively, can be compared to determine selectivity. The selectivity limit is the location of overlap of the two functions.
Without the use of analytical methods, short circuit testing is required at many discrete current levels to ensure selectivity throughout the range of operation of the breakers. However, the analytical methods demonstrate the selective performance (or lack thereof) continuously throughout the entire operating range.
It would be desirable to have an analytical method for determining circuit breaker selectivity, which significantly reduces the time, money, and effort required for testing.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an exemplary embodiment of the selective breaker system in accordance with the present invention.
Figure 2 is an exemplary embodiment of a graphical representation of a let through curve for a breaker in accordance with the present invention.