Example Solution

Solution: For HV side and IV side backup relay settings; it is necessary to know the fault currents contributed by ICTs for IV side and HV side bus faults respectively. For calculations of these currents; ICT and source percentage reactance data shown in Table-1 and Table-2 of problem statement has practically no use. Still reactance data is included in problem statement only for readers who wants to validate calculations for the ICTs fault current contribution (for details see Appendix-A). For other readers ICT fault currents are readily made available and as shown in Table-2.

Other readers may directly refer to Table-1 and Table-2 for bus fault levels and fault current contributions by ICTs respectively. Also readers are requested to go through the Appendix-A to visualize the fault current contribution shown in Table-2 for different fault situations.

Table-1: Bus Fault Levels
SrNo.	Bus.	Three Phase	Single Phase
1.	HV Bus	2878 MVA	2056 MVA
2.	IV Bus	2659 MVA	2024 MVA

Table-2: Fault Current Contribution by ICTs
SrNo.	Fault Type	Bus	ICT Name	HV Side Current	IV Side Current
1.	Three Phase	HV	ICT-I 200 MVA	1119	1865
2.			ICT-2 100 MVA	606	1010
3.		IV	ICT-I 200 MVA	1953	3255
4.			ICT-2 100 MVA	1058	1763
5.	Single Phase	HV	ICT-I 200 MVA	1114	1192
6.			ICT-2 100 MVA	835	1044
7.		IV	ICT-I 200 MVA	1376	2927
8.			ICT-2 100 MVA	1116	2433

An Excel template sheet is developed for calculations of ICT HV side and IV side backup relay settings in line with information given here. By using –

1)    Excel sheet

2)    ICT capacities and CT ratios as shown in example

3)    and fault current data for Table-1 and Table-2

backup settings calculations carried out and displayed here. Readers are requested to try them by hand calculations for better understanding of the setting methodology. Also it is recommended to try setting calculations for ICT-2   using Excel sheet available in toolbox.