An Example of Choosing 138 kV Line Circuit Breaker (photo credit: wilsonconst.com)
Let’s choose 138 kV line #3 circuit breaker
To better understand a process of high voltage equipment selection, let’s discuss an example of choosing 138 kV Line #3 Circuit Breaker, shown in Figure 1, assuming that the following information about the system is available:
Figure 1 – Power System for Example of 138 kV Breaker Selection
Continuous current for all the lines:
- Line #1 – 1,000 A,
- Line #2 – 1,500 A,
- Line #3 – 800A
3-phase fault current on the bus is 46.5 kA, contributions from the lines:
- Line #1 – 24.5 kA,
- Line#2 – 12kA,
- Line#3 – 10kA
Projected Substation load growth is 25%.
Available breaker ratings:
- Continuous current: 1,200 A, 2,000 A, 3,000 A
- Short circuit interrupting capability: 40 kA, 50 kA
Let’s see the solution…
Future load considering 25% growth: (1000 A + 1500 A – 800 A) x 1.25 = 2,125 A.
This load may be fed from Line #3 through its circuit breaker if lines #1 and #2 are switched off. So, the breaker should be sized to carry at least 2,125 A. Closest available continuous rating meeting this requirement is 3,000 A.
Maximum 3-phase fault current that Line #3 breaker needs to interrupt may be calculated by deducting from a total bus fault current a contribution from line #3, i.e. 46.5 – 10 = 36.5 kA
Closest short circuit current interrupting rating and meeting requirement to be at least 36.5 kA is 40 kA.
This is a summary of the selected circuit breaker ratings:
- Rated maximum voltage – 138 kV
- Rated continuous current – 3,000 A
- BIL – 650 kV
- Rated short circuit current – 40 kA
Reference: Fundamentals of Modern Electrical Substations (Part 3: Electrical Substation Engineering Aspects) – Boris Shvartsberg, Ph.D., P.E., P.M.P.