standards / manuals / guidelines for small hydro development - AHEC

4.8.3 Simplified Methods for calculation short circuit currentSimplified methods calculating fault current to fix short circuit rating of circuitbreakers have been recommended by standardizing agencies over years as given below.Simplified conservative method of calculation were recommended by protective devicescommittee of AIEEE for general use of the industry. It was recommended that rigorousmethods be used when specifically required. The method is based upon determination ofan initial value of rms symmetrical current (ac component) to which followingmultiflying factor are applied for application purposes (table 3.5).Table 3.5General Generator current breakers/shortcircuit more than 500,000 kVAA. 8 cycle breaker 1.0 1.15 cycle breaker 1.1 1.23 cycle breakers 1.2 1.32 cycle breakers 1.4 1.5B. Mechanical stressesand mandatory duty 1.6Accordingly the steps involved for determining short circuit rating of circuit breakersare as follows as per the AIEE simplified method:1. Determine highest value of rms symmetrical current for any type of fault equalE/X 1 phase fault or 3E/2X 1 +X 0 for ground fault whichever is greater.2. Multiply this current by appropriate factors from table 3.5.3. The resulting interrupting and momentary current should be used to select theavailable normal rated circuit breaker.8.4 E/X Simplified method as per IEEE std. C37010-1999The AIEEE simplified method was further referred in IEEE std. 37010-1999. Thissimplified method is now recommended. For short circuit duty of circuit breakers unlesscomplex more accurate studies are warranted.In these studies generating station and transmission lines interconnected with the systemare represented in detail and the system is represented by equivalent system.For small hydro say 5 MW unit size connected to regional grids, the grid size can beassumed to be infinite size and calculation carried out accordingly.Calculation based on simplified method of calculating short circuit current for theapplication of recommended by IEEE std. 37-01-1999 are given as an example andcompared with calculation made according AIEE committee report.4-19