ALASSAR (PAGE:-1)

CT SIZING CALCULATION / SAMPLE CALCULATION

1.0 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

2.0 MAXIMUM INTERNAL FAULT CURRENT :35000A

3.0 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

4.0 DETAILS OF RELAY MAKE : AREVA TYPE :P122 NOMINAL CURRENT:1A RELAY BURDEN(Rr) : 0.08 VA ( MFG. DATA)

5.0 5.1

CALCULATION OF BURDEN CONNECTED TO THE CT CORE 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 Ie<Is—Ir where Is =30%x2666.70x1/3000=0.2666 n Ir=relay pickup setting=(10% of 1A)=0.1 & n= No. of CTs in parallel=4 Ie< 0.2666 –0.1 =0.04165 4 HENCE CT MAGNETISING CURRENT≤4ma at Vk/2 ACCEPTABLE

10.0 CALCULATION OF METROSIL FOR (64R) AS PER SP-1107, CL.5.4.5, MAXIMUM PEAK VOLTAGE ACROSS CURRENT TRANSFORMER SECONDARY WIRING SHALL NOT EXCEED 2.5 KV.OR( NON LINEAR RESISTANCE) SHALL BE PROVIDEDTO LIMIT THE VOLTAGE DEVELOPED TO 2.5KV OR LESS. Vkmax.int.=Is (Rct+Rleadwire+Rstab)=35000x 0.000333x1011.88= 11793V

VOLTAGE ACROSS RELAY: = 6721V 6.72KV >2.5KV HENCE METROSIL REQUIRED

11.0 RECOMMENDED CT PARAMETERS FOR 4 NOS. CTs(3P&N) FOR (64R) (PAGE:-3)

RATED PRIMARY CURRENT (Ipn)

RATED SEC. CURRENT (Isn)

CT CLASS AS PER BS

RATED FREQ

RATED SHORT TIME THERMAL CURRENT (Ith)

MAXIMUM SECONDARY WINDING RESISTANCE (Rct) AT 75⁰C

MINIMUM KNEE POINT VOLTAGE

HIGHEST SYSTEM VOLTAGE

3000A 1A X 50HZ 35KA/1 SEC.

6Ω 500V 0.66KV

12.0

RELAY VOLTAGE SETTING

NOMINAL CHARACTORSTICS

RECOMMENDED METROSIL TYPE

C β Single pole relay

Up to 125V RMS

450 0.25 600A/S1/S256

REFERENCES:- SP-1107 DOCUMENTS/ MICOM P122 APPLICATION GUIDE P12XEN AP/A96/IEC 60044-

1/BS 3938

K.T. MENON,

ALASSAR ELECTRIC LLC,

MUSCAT