As a protection engineer for Metropolis Light and Power (MLP) your job is to ensure that the transmission line and transformer circuit breaker ratings are sufficient to interrupt the fault current associated with any type of fault (balanced three phase, single line-to-ground, line-to-line, and double line-to-ground). The MLP power system is modeled in the PowerWorld simulator case ‘DesignCase.pwb'. This case models the positive, negative, and zero sequence values for each system device. Note that the 69/138kV transformers are grounded Y on the low side and delta on the high side; the 138/345kV transformers are grounded Y on both sides. In this design problem, your job is to evaluate the circuit breaker ratings for the three 345kV transmission lines and the six 345/138kV transformers. You need not consider the 138kV or 69kV transmission lines, or the 138/69kV transformers. Design Procedure: 1. Load DesignCase.pwb' into Power World Simulator. Perform an initial power flow solution to get the base case system operating point. 2. Apply each of the four fault types (balanced three phase, single line-to-ground, line-to-line, and double line-to-ground) to each of the 345kV buses and to the 138kV buses attached to 345/138kV transformers to determine the maximum fault current that each of the 345kV lines and 345/138kV transformers will experience. 3. For each device select a suitable circuit breaker from Table 7.10 (given in the project package). Each breaker that you select should (a) have a rated vpltage larger than the maximum system operating voltage, (b) have a rated continuous current at least 30% larger than the normal rated current for the line, and (c) have a rated short circuit current larger than the maximum fault current for any type of fault at the bus where the breaker is located. This conservative practice of selecting a breaker to interrupt the entire fault current, not just the contribution to the fault current through the breaker allows for future increases in fault currents. Since higher rated circuit breakers cost more, you should select the circuit breaker with the lowest rating that satisfies the design constraints. For circuit breaker selection procedure, please refer to Lecture Slides for Week 7 (on circuit breaker selection) and Section 7.5 of the textbook.
Simplifying Assumptions: To simplify the analysis, several assumptions are made: 1. You need only consider the base case conditions given in the ‘DesignCase.pwb' case. 2. You may assume that X/R ratios at each bus is sufficiently small (less than 15) so that the de offset has decayed to a sufficiently low value (see Lecture Slides for Week 7 on circuit breaker selection or Section 7.5 of the textbook for details). 3. As is common with commercial software, including PowerWorld Simulator, the delta-Y transformer phase shifts are neglected.