Ct Sizing Calculation(sample)-areva Manual

ALASSAR CT SIZING CALCULATION / SAMPLE CALCULATION 1.0

2.0 3.0

4.0

5.0

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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 MAXIMUM INTERNAL FAULT CURRENT :35000A DETAILS OF CURRENT TRANSFORMER (DATAS GIVEN BY CT MANUFACTURER) TYPE OF CT: CLASS X TRANSFORMATION RATIO :3000/1A, 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

5.1

CONNECTED RELAY BURDEN : 0.08VA at Is=1A RESISTANCE : ρι /ɑ SPECIFIC RESISTANCE OF COPPER ;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) :5.88 Ω/Km RESISTANCE AT 90⁰C :2RlXL =2X5.88X0.185 =2.1756 Ω =ρι/ɑ LEAD RESISTIVE BURDEN = ρι/ɑx Isn x Isn =2.1756 VA TOTAL CONNECTED BURDEN ( 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≥ 2xVs Vs>K.If.(Icts /Ictp)(Rct+2Rl)= Vs> 1x35000x0.000333x(6+2.2556) =96.30V (STABILITY FACTOR K IS TAKEN AS 1 FOR Vk/Vs LESS OR EQUAL TO 16) MINIMUM STABILITY VOLTAGE(Vsmin.)= 96.30V HENCE ACTUAL STABILITY VOLTAGE (Vs) =100V MAX.SYMM .SHORT CIRCUIT CURRENT=35000A SECONDARY WINDING RESISTANCE =6Ω TOTAL CONNECTED BURDEN TO CT (2Rl+Rr)=2.2556Ω CT PRIMARY RATED CURRENT (Ictp) = 3000A CT SECONDARY RATED CURRENT(Icts) =1A THE ACTUAL STABILITY VOLTAGE SETTING =100V

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 THAN STABILITY VOLTAG Vs.A RATIO OF 4 OR 5 WOULD BE APPROPRIATE. (ACCEPTED KNEE POINT VOLTAGE:500V for high speed operation) 7.0 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 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) = 309.6A 1 RELAY OPERATING CURRENT IN TERMS OF MINIMUM CURRENT FOR AN EARTH FAULT 10.32% ….(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. 8.0 CALCULATION OF STABILISING RESISTOR FOR (64R) STABILISING RESISTOR (Rst) = Vs RELAY BURDEN IS =100 0.08 (RELAY BURDEN IS NEGLIGIBLE AND NOT CONSIDERED) 0.1 =1000Ω POWER RATING OF( Rst)= 4xVs² /Rstab =40W HENCE (Rst)=1000Ω 9.0 CALCULATION OF MAGNETISING CURRENT(Ie) ASSUMING THAT THE EFFECTIVE FAULT SETTINGFOR TRANSFORMER REF SHALL BE BETWEEN 10 60% OF RATED CURRENT OF PROTECTIVE WINDINGFOR SOLIDLY EARTHED 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 PRIMARY SEC. CLASS FREQ SHORT SECONDARY CURRENT CURRENT AS TIME WINDING (Ipn) (Isn) PER THERMAL RESISTANCE BS CURRENT (Rct) AT (Ith) 75⁰C 3000A 1A X 50HZ 35KA/1 6Ω SEC.

12.0 RELAY VOLTAGE SETTING

NOMINAL CHARACTORSTICS

(PAGE:-3) MINIMUM HIGHEST KNEE SYSTEM POINT VOLTAGE VOLTAGE

500V

0.66KV

RECOMMENDED METROSIL TYPE

C β Single pole relay Up to 125V 450 0.25 600A/S1/S256 RMS REFERENCES:- SP-1107 DOCUMENTS/ MICOM P122 APPLICATION GUIDE P12XEN AP/A96/IEC 600441/BS 3938 K.T. MENON, ALASSAR ELECTRIC LLC, MUSCAT