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TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE LINES Doc-No:G1408-AA-2030-J-0-101 Page 2 of 39 Rev - A Document Title : CT / VT calculations for 11kV switchgear for ADWEA contract no G1408 TABLE OF CONTENTS SECTION TITLE PAGE NO 1.0 OBJECTIVE...................................................................................................... 3 2.0 REFERENCES.................................................................................................. 3 3.0 SCOPE.............................................................................................................. 3 4.0 CT SIZING CALCULATIONS............................................................................ 3 4.1 33/11.55kV, 15MVA Transformer Incomer Feeder........................................................3 4.1.1 CT's for Metering and Transducers ..................................................................................3 4.1.2 CT's for Differential Protection & REF protection ..............................................................4 4.1.3 CT for Standby Earth Fault(SBEF) protection ...................................................................7 4.1.4 CT's for OverCurrent and Earth Fault protection ............................................................10 4.2 33/11.55kV, 20MVA Transformer Incomer Feeder......................................................13 4.2.1 CT's for metering and Transducers .................................................................................13 4.2.2 CT's for Differential Protection & REF protection ............................................................14 4.2.3 CT for Standby Earth Fault(SBEF) protection .................................................................17 4.2.4 CT's for Overcurrent and Earth fault protection ..............................................................19 4.3 33/11.55kV, 15MVA Transformer Bus section Feeder...............................................22 4.3.1 CT's for Metering and Protection .....................................................................................22 4.3.2 CT's for Transducers .......................................................................................................24 4.4 33/11.55kV, 20MVA Transformer Bus section Feeder...............................................25 4.4.1 CT's for Metering and Protection .....................................................................................25 4.4.2 CT's for Transducers .......................................................................................................26 4.5 Outgoing 11/0.433kV, 500kVA Transformer Feeder...................................................27 4.5.1 CT's for Metering and Protection .....................................................................................27 4.5.2 CT's for Transducers .......................................................................................................29 4.6 Outgoing 11kV, 5MVAr Capacitor Feeder...................................................................30 4.6.1 CT's for Metering and Protection .....................................................................................30 4.6.2 CT's for Transducers .......................................................................................................33 4.7 11kV Outgoing Feeder..................................................................................................34 4.7.1 CT's for Metering and Protection .....................................................................................34 4.7.2 CT's for Transducers .......................................................................................................37 5.0 VT SIZING CALCULATIONS.......................................................................... 38 6.0 CONCLUSION................................................................................................ 39 7.0 SUMMARY OF CURRENT TRANSFORMERS (Annexure-1) .......................... 39 8.0 SUMMARY OF VOLTAGE TRANSFORMERS (Annexure-2) .......................... 39 9.0 Relavent pages of Catalogues for Burden datas (Annexure-3) ................... 39 NOTE Refer DTS no 0012 dated 15.11.2003 for Relay Catalogues. TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE LINES Doc-No:G1408-AA-2030-J-0-101 Page 3 of 39 Rev - A Document Title : CT / VT calculations for 11kV switchgear for ADWEA contract no G1408 1.0 OBJECTIVE To establish Current transformer(CT) and Voltage transformer parameters at 11 kV level for all the 5 new substations. 2.0 REFERENCES a) Bay control unit REF542 Plus catalogue b) 11kV switchgear vendor (ABB FJH) Single line diagram no. N611844/1 to N611844/8 c) Ducab cable catalogue d) ESI standard 48-3 and Clients/Consultant Recommendations e) Relay catalogues 3.0 SCOPE To establish the requirements of CT/VT parameters such as CT/VT ratio, VA burden, knee point voltage, accuracy class and magnetising current for CTs on the 11 kV switchgear at all the sub-stations. 4.0 CT SIZING CALCULATIONS 4.1 33/11.55kV, 15MVA Transformer Incomer Feeder 4.1.1 CT’s for Metering and Transducers CT Ratio: 800-900 / 1A Type of Metering: It is proposed to use Bay Control Unit(BCU) type REF542 Plus. Transformer full load current on secondary side ( considering tap setting at – 15% ) = 15000/( √ 3*11.55*0.85) = 882.12 Amps Hence it is proposed to provide 800-900 / 1A CT for metering. Total burden of bay control unit = 0.1 VA (Refer REF542 Plus catalogue) Maximum burden of Current Transducer = 2 VA (Actual is 0.2VA) Maximum burden of Power Transducer = 2 VA Maximum burden of Power factor Transducer = 2 VA The transducers are located in LDC cubicle. Minimum length of cable = 40 meters(As per equipment layout) between CT and transducers TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE LINES Doc-No:G1408-AA-2030-J-0-101 Page 4 of 39 Rev - A Size of cable = 4 sqmm Resistance of cable per = 4.61 Ohms (as per Ducab cable catalogue) km at 20 deg C Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.42 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.42 x 1 2 = 0.42 VA Total burden = Burden of BCU+Burden of transducers+Burden due to lead resistance = 0.1 + 6 + 0.42 = 6.52 VA Considering 25% future margin Total burden required =1.25 x 6.52 = 8.15 VA Hence a standard burden rating of 15 VA is chosen for a Tap of 800A and 30VA chosen for a Tap of 900A. The CT Accuracy Class selected is CL. 0.5S as per the specification requirements. The instrument security factor (ISF) selected is less than or equal to Five(5) as per the specification requirements. Abstract Therefore a CT of CL. 0.5,15VA is proposed for 800-900/1A and CL. 0.5,30VA is proposed for 800-900/1A.The CTs are with a factor of safety less than or equal to 5. 4.1.2 CT’s for Differential Protection and REF Protection: CT Ratio: 900/1 Type of relay: RET 316*4 (ABB make) Knee Point Voltage for Calculation for Differential Protection: Fault current rating of 11 kV switchgear = 31.5 kA As per ESI Standard 48-3 enclosed the formulae for calculating Knee Point Voltage is V K = I f * N * (R ct +R l ) Where I f = fault current N = Turns Ratio R ct = CT secondary resistance R l = Lead resistance Maximum fault current carrying capacity of 11 kV switchgear is 31.5 kA ,which can be considered for knee point calculation. TENDER NO. G1408 – FIVE (5) NEW 33/11KV PRIMARY SUBSTATIONS WITH ASSOCIATED 33KV OVERHEAD LINES & 33KV CABLE LINES Doc-No:G1408-AA-2030-J-0-101 Page 5 of 39 Rev - A I f = 31500 Amps N = 1/900 Rct = 7 Ω (As per Manufacturers Recommendations) The differential relay is mounted on 33kV relay panel which is located in control room. Minimum length of cable = 40 meters(As per equipment layout) between CT and relay Size of cable = 4 sqmm Resistance of cable per = 4.61 Ohms (as per Ducab cable catalogue) km at 20 deg C Resistance of cable at = 4.61(1+0.00393 (55-20) ) 55 deg C, considering temperature correction factor = 5.244 ohm/km Total resistance = 2 x 5.244 x 40 / 1000 (Lead & Return conductors) = 0.42 Ω Burden due to lead resistance = Total resistance x (CT secondary current) 2 = 0.42 x 1 2 = 0.42 VA Therefore, substituting the above values in the formula for V k , V K = 31500* (1/900) * (7 + 0.42 ) V K = 259.7V V K > 400V Therefore a CT of 900/1A, CL. X, V K > 400V is proposed. Imag shall be 30mA @ Vk/2 Recommendations from Consultant/Client for Differential Core requirement : Further, to avoid mal-operation on energization of power transformer and in connection with fault current that passes through power transformer, the rated secondary voltage has to satisfy the following conditions: Condition-1 The core may not saturate for current lower than 30 times the Power Transformer rated current at connected burden. This ensures stability also with heavy DC saturation. Vk ≥ 30 * I nt * (R ct + R l + R r / I r 2 ) Where, I nt = Main CT secondary current corresponding to rated primary current of power transformer R ct = CT secondary resistance R l = Lead resistance R r = Relay burden
