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// A Texbook on POWER SYSTEM ENGINEERING
// A.Chakrabarti, M.L.Soni, P.V.Gupta, U.S.Bhatnagar
// DHANPAT RAI & Co.
// SECOND EDITION
// PART III : SWITCHGEAR AND PROTECTION
// CHAPTER 7: PROTECTIVE RELAYS
// EXAMPLE : 7.3 :
// Page number 596
clear ; clc ; close ; // Clear the work space and console
// Given data
MVA = 20.0 // Transformer MVA
overload = 30.0 // Overload of transformer(%)
kV = 11.0 // Bus bar rating(kV)
CT_trans = 1000.0/5 // Transformer CT
CT_cb = 400.0/5 // Circuit breaker CT
ps = 125.0 // Plug setting(%)
ts = 0.3 // Time setting
I_f = 5000.0 // Fault current(A)
t_margin = 0.5 // Discriminative time margin(sec)
// Calculations
I_sec_fault = I_f/CT_cb // Secondary fault current(A)
CT_cb_sec = 5.0 // Secondary turn
PSM = I_sec_fault/(ps/100*CT_cb_sec) // Plug setting multiplier
t = 2.8 // Time against this PSM(sec). From graph E7.1 in textbook page no 595
time_oper = t*ts // Operating time of feeder relay(sec)
I_ol = (1+(overload/100))*MVA*1000/(3**0.5*kV) // Overload current(A)
I_sec_T = I_ol/CT_trans // Secondary current(A)
CT_T_sec = 5.0 // Secondary turn of transformer
PSM_T = I_sec_T/CT_T_sec // Minimum plug setting multiplier of transformer
I_sec_T1 = I_f/CT_trans // Secondary fault current(A)
ps_T1 = 1.5 // Plug setting as per standard value
PSM_T1 = I_sec_T1/(CT_T_sec*ps) // Plug setting multiplier of transformer
t_T1 = 7.0 // Time against this PSM(sec). From graph E7.1 in textbook page no 595
time_setting = (time_oper+t_margin)/t_T1 // Time setting of transformer
// Results
disp("PART III - EXAMPLE : 7.3 : SOLUTION :-")
printf("\nOperating time of feeder relay = %.2f sec", time_oper)
printf("\nMinimum plug setting of transformer relay, P.S > %.2f ", PSM_T)
printf("\nTime setting of transformer = %.3f ", time_setting)
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