DETAILS TRANSFORMER RATING :2 MVA NOMINAL VOLTAGE :0.415KV NOMINAL FREQUENCY ; 50 HZ RATED SHORT CIRCUIT CURRENT : 80KA/1 SEC ( 42.3KA/3 sec) data by VOLTAMP TRANSFORMER IMPEDANCE :6.4% EARTH FAULT CURRENT : 4000A ( 1.5X2666.70A ) RATED CURRENT FROM NAME PLATE DETAILS OF VOLTAMP 2 MVA TRANSFORMER=2666.70A TRANSFORMER=2666.70A MAXIMUM INTERNAL FAULT CURRENT :35000A DETAILS OF CURRENT TRANSFORMER (DATAS GIVEN BY CT MANUFACTURER) TYPE OF CT: CLASS X TRANSFORMATION RATIO :3000/1A, Ie≤ Ie≤ 4ma CT RESISTANCE (Rct) : 6 Ω at 75⁰ C NUMBER OF CTs : 4 FOR REF PROTECTION DETAILS OF RELAY MAKE : AREVA TYPE :P122 NOMINAL CURRENT:1A RELAY BURDEN(Rr) : 0.08 VA ( MFG. DATA) CALCULATION OF BURDEN CONNECTED TO THE CT CORE
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CONNECTED RELAY BURDEN : 0.08VA at Is=1A RESISTANCE : ρι /ɑ SPECIFIC RESISTANCE OF COPPER COP PER ;ρ =0.0175 Ω/sqmm/m at 75⁰C LENGTH OF CABLE : 0.185 km AREA OF CROSS SECTION : (A) 4sqmm LEAD RESISTANCE AT 90⁰C(Rl) 90⁰C(Rl) :5.88 :5.88 Ω/Km RESISTANCE AT 90⁰C :2Rl :2RlXL =2X5.88X0.185 =2.1756 Ω =ρι/ɑ LEAD RESISTIVE BURDEN = ρι/ɑx ρι/ɑx Isn x Isn =2.1756 VA TOTAL CONNECTED BURDEN ( 2Rl+Rr)) 2Rl+Rr)) =2.1756+0.08 =2.2556Ω CT INTERNAL RESISTANCE (Rct) =6 Ω
6.0
KNEE POINT VOLTAGE(Vk minimum) REQUIRED Vs>Vsmin. KNEE POINT VOLTAGE Vk≥ Vk≥ 2xVs Vs>K.If.(Icts /Ictp)(Rct+2Rl)= /Ictp)(Rct+2Rl)= Vs> 1x35000x0.000333x(6+2.2556) =96.30V =96.30V (STABILITY FACTOR K IS TAKEN AS 1 FOR Vk/Vs LESS OR EQUAL TO 16) MINIMUM STABILITY VOLTAGE(Vsmin.)= VOLTAGE(Vsmin.)= 96.30V HENCE ACTUAL STABILITY VOLTAGE (Vs) =100V MAX.SYMM .SHORT CIRCUIT CURRENT=35000A SECONDARY WINDING RESISTANCE =6Ω =6 Ω TOTAL CONNECTED BURDEN TO CT (2Rl+Rr)=2.2556Ω (2Rl+Rr)=2.2556Ω CT PRIMARY RATED CURRENT (Ictp) = 3000A CT SECONDARY RATED CURRENT(Icts) =1A THE ACTUAL STABILITY VOLTAGE SETTING =100V
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Vk≥ Vk≥4xIsxRst =4x0.1x1000 = 400V (PAGE:-2) AS PER AREVA APPLICATION NOTES, TO OBTAIN HIGH SPEED OPERATION, THE KNEE POINT VOLTAGE OF THE CT Vk MUST BE SIGNIFICANTLY HIGHER HIGHER THAN STABILITY VOLTAG Vs.A RATIO OF 4 OR 5 WOULD BE APPROPRIATE. (ACCEPTED KNEE POINT VOLTAGE:500V for high speed operation) CT TYPE RECOMMENDED :CLASS X CT TRANSFORMATION RATIO : 3000/1A NUMBER OF CTs :4 RELAY SETTING Ismin : 10% AS PER SP-1107 CL.5.4.5 SELECTED MAGNETISING MAGNETISING CURRENT OF CT (Im) =0.004A CALCULATION OF MAXIMUM SENSITIVITY PRIMARY OPERATING CURRENT ( Iop) =Ipx(Is min+NxImag.Vs/Vk) Is =3000 (0.1+4x0.004x100/500) (0.1+4x0.004x100/500) = 309.6A 1 RELAY OPERATING CURRENT IN TERMS OF MINIMUM CURRENT FOR AN EARTH FAULT 10.32% ….(309.6/3000) ….(309.6/3000) THE RELAY OPERATING CURRENT IS WITH IN 10 TO 25% OF THE MINIMUM CURRENT OF AN E/F (AS PER SP-1107,CL.5.4.5 HENCE ,ACCEPTABLE . CALCULATION OF STABILISING STABILISING RESISTOR FOR (64R) STABILISING RESISTOR (Rst) = Vs RELAY BURDEN IS =100 0.08 0.08 (RELAY BURDEN IS NEGLIGIBLE AND NOT CONSIDERED) 0.1 =1000Ω POWER RATING OF( Rst)= 4xVs² /Rstab =40W HENCE (Rst)=1000Ω CALCULATION OF MAGNETISING MAGNETISING CURRENT(Ie) ASSUMING THAT THE EFFECTIVE FAULT SETTINGFOR SETTINGFOR TRANSFORMER REF SHALL BE BETWEEN 10 60% OF RATED CURRENT OF PROTECTIVE WINDINGFOR SOLIDLY EARTHED SYSTEM(REF.SP1107/CL5.4.5), SYSTEM(REF.SP1107/CL5.4.5), THE VALUE SELECTED FOR CALCULATION IS 30% Ie DRAWN BY CT AT Vs Ie2.5KV HENCE METROSIL REQUIRED
11.0
RECOMMENDED CT PARAMETERS FOR 4 NOS. CTs(3P&N) FOR (64R) RATED RATED CT RATED RATED MAXIMUM MINIMUM PRIMARY SEC. CLASS FREQ SHORT SECONDARY KNEE CURRENT CURRENT AS TIME WINDING POINT (Ipn) (Isn) PER THERMAL RESISTANCE VOLTAGE BS CURRENT (Rct) AT (Ith) 75⁰ 75⁰C 3000A 1A X 50HZ 35KA/1 6Ω 500V SEC.
12.0 RELAY VOLTAGE SETTING
NOMINAL CHARACTORSTICS C 450
β 0.25
(PAGE:-3) HIGHEST SYSTEM VOLTAGE
0.66KV
RECOMMENDED METROSIL TYPE Single pole relay 600A/S1/S256
Up to 125V RMS REFERENCES:- SP-1107 DOCUMENTS/ DOCUMENTS/ MICOM P122 APPLICATION GUIDE P12XEN AP/A96/IEC 600441/BS 3938 K.T. MENON, ALASSAR ELECTRIC LLC, MUSCAT