— Welcome JUNE 2021 Technical Sharing Session Lionel Ng, Global Training Specialist © Copyright ABB. All rights reserved. Short Circuit Calculation IEC 60909 Short Circuit Calculation IEC 60909 IEC 60909 Part 0 to 4 • Part 0 Calculation of currents • Part 1 Factors for the calculation of short circuit currents according to Part 0. • Part 2 Data of electrical equipment for short-circuit current calculations • Part 3 Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth • Part 4 Examples for the calculation of short-circuit currents June 7, 2021 Slide 3 Short Circuit Calculation IEC 60909 IEC 60909-0 Short-circuit currents in three-phase a.c. systems • Terms and definitions • Short-circuit currents • Calculating Assumptions • Short-circuit impedances of electrical equipment June 7, 2021 Slide 4 Short Circuit Calculation IEC 60909 Terms & Definitions • Short-circuit “Accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal or close to zero” • Short-circuit current “Overcurrent resulting from a short-circuit in an electric system” • Prospective short-circuit current “Current that would flow if the short circuit were placed by an ideal connection of negligible impedance without any change of the supply” June 7, 2021 Slide 5 Short Circuit Calculation IEC 60909 Short-circuit currents June 7, 2021 Slide 6 Short Circuit Calculation IEC 60909 Short-circuit currents June 7, 2021 Slide 7 Short Circuit Calculation IEC 60909 Short-circuit currents June 7, 2021 Slide 8 Short Circuit Calculation IEC 60909 Calculation assumptions The calculation of maximum and minimum short-circuit currents is based on the following simplifications. • For the duration of the short circuit there is no change in the type of short circuit involved, that is, a three-phase short-circuit remains three-phase and a line-to-earth short circuit remains line-to-earth during the time of short circuit. • For the duration of the short circuit, there is no change in the network involved. • The impedance of the transformers is referred to the tap-changer in main position. • Arc resistances are not taken into account. • Shunt admittances of non-rotating loads shall be neglected in the positive-, the negative and the zero-sequence system. • Line capacitances shall be neglected in the positive- and negative-sequence system. Line capacitances in the zerosequence system shall be taken into account in low-impedance • Earthed networks having an earth-fault factor (see IEC 60027-1) higher than 1,4. • Magnetising admittances of transformers shall be neglected in the positive and negative sequence system. June 7, 2021 Slide 9 Short Circuit Calculation IEC 60909 Method of calculation Equivalent voltage source at the short-circuit location June 7, 2021 Slide 10 Short Circuit Calculation IEC 60909 Short-circuit impedances of electrical equipment June 7, 2021 Slide 11 Short Circuit Calculation IEC 60909 Short-circuit impedances of two-winding transformer The positive-sequence short-circuit impedances of two-winding transformers ZT = RT + jXT with and without on-load tap-changer can be calculated from the rated transformer data as follows: June 7, 2021 Slide 12 Short Circuit Calculation IEC 60909 Short-circuit impedances of Overhead lines and cables The positive-sequence short-circuit impedance ZL = RL + jXL may be calculated from the conductor data, such as the cross-sections and the centre-distances of the conductors. The effective resistance per unit length RL’ of overhead lines at the conductor temperature 20 °C may be calculated from the nominal cross-section qn and the resistivity ρ : June 7, 2021 Slide 13 Short Circuit Calculation IEC 60909 Calculation of Short circuit currents The equivalent network is solved according to standard electro-technical rules (circuits in series / in parallel) Zknet ZcMV ZTR ZcLV Ik Ik = June 7, 2021 Slide 14 Un 3 (Z knet + Z cMV + Z TR + Z cLV ) Un Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Distribution network – It is necessary to know the network short-circuit power • From 250MVA to 500MVA Un 30kV • From 700MVA to 1500MVA Un > 30kV • According to IEC 60076-5 Z knet June 7, 2021 Un2 = = Sknet Slide 15 Un 3 Iknet Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Generator short circuit impedance – It is necessary to know the • Rated apparent power Sn • Rated voltage Un • Subtransient reactance X”d from 10% to 20% smooth rotor (isotropic machines) from 15% to 30% salient pole rotor (anisotropic machines) " 2 x U X "d = d n 100 Sn June 7, 2021 Slide 16 Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Transformers – It is necessary to know the • Rated apparent power Sn • Primary rated voltage V1n • Secondary rated voltage V2n • Short-circuit voltage uk%(Between 4% to 8%) • According to IEC60076-5 Z TR June 7, 2021 Slide 17 uk % U22n = 100 Sn Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Asynchronous motors • In case of short-circuit it functions as a generator with a x”d from 20% to 25% • a current equal to 4-6 times the In can be assumed as contribution to the short-circuit • the minimum criteria for taking into consideration the phenomenon (Ik short-circuit without motor contribution) June 7, 2021 Slide 18 Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Example Sk = 500 MVA UMV= 20 kV ZcMV = 0.01 Sn = 400 kVA U= 20/0.4 kV uk% = 4 % ZcLV = 0.06 June 7, 2021 Slide 19 U Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Example U Z u20 kV Z u 400 V ( ( ) Sk = 500 MVA UMV= 20 kV Zu = 0.32 m 2 U2 20 10 3 = = = 0.8 Sk 500 10 6 ) 2 ( 400 ) = Z u20 kV (20000 )2 = 3.2 10 −4 V1 V2 N1 N2 I2 I1 P V R June 7, 2021 Slide 20 I V I Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Example U Z cMV 400 V June 7, 2021 2 ( 400 ) = Z cMV 20 kV (20000 )2 Slide 21 Zu = 0.32 m = 4 10 −6 ZcMV = 0.01 ZcMV = 410-3 m Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Example U Zu = 0.32 m ZcMV = 410-3 m 2 (400 ) = 0.016 uk % UnLV 4 ZT = = 100 Sn 100 400 10 3 2 June 7, 2021 Slide 22 Sn = 400 kVA U= 20/0.4 kV uk% = 4 % ZT = 0.016 Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Example U Zu = 0.32 m ZcMV = 410-3 m ZT = 0.016 ZcLV = 0.061 m June 7, 2021 Slide 23 ZcLV = 0.061 m Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Example Sk = 500 MVA UMV= 20 kV U Zu = 0.32 m ZcMV = 410-3 m Ik = Ik = June 7, 2021 Un 3 (Z u + Z cMV + Z T ) Un ZT = 16 m 14.1kA 3 (Z u + ZcMV + ZT + ZcLV ) Slide 24 ZcLV = 0.061 m 13.6kA Un = 400 V Short Circuit Calculation Basic Principles Common Management Methods Substation with single transformer Typical single line diagrams June 7, 2021 Slide 25 Short Circuit Calculation Basic Principles Common Management Methods Substation with two transformers (One as spare for the other) – The use of mechanical interlock between two ACBs June 7, 2021 Slide 26 Short Circuit Calculation Basic Principles Common Management Methods Substation with two transformers which operate in parallel June 7, 2021 Slide 27 Short Circuit Calculation Basic Principles Common Management Methods Substation with two transformers which operate simultaneously on two separate half-busbars. (Two incoming with bus-coupler) Mechanical Interlock between three ACBs June 7, 2021 Slide 28 Short Circuit Calculation Basic Principles Management philosophy for the protections To ensure continuity of service to sound parts of the installation – Circuit breaker L1 must trip and clear the fault June 7, 2021 Slide 29 Short Circuit Calculation Basic Principles Management philosophy for the protections To ensure continuity of service to sound parts of the installation – Circuit breaker I LV1 must trip and clear the fault – If CLV is closed I LV2 must trip as well. Depending if CLV is an ACB or just a bus coupler. June 7, 2021 Slide 30 Short Circuit Calculation Basic Principles Management philosophy for the protections To ensure continuity of service to sound parts of the installation – Circuit breaker I LV1 must trip and clear the fault – If CLV is closed I LV2 must trip as well. Depending if CLV is an ACB or just a bus coupler. June 7, 2021 Slide 31 Short Circuit Calculation Basic Principles Short Circuit Calculations Single Transformer U MV Level 22kV 750MVA Z22kV = (22k)2 750M = 0.6453 Ω 4002 Z400V = 0.6453 x 220002 ZTX = 6 100 x 4002 1.6M = 0.2133 mΩ 1600kVA 22/0.4kV 6% = 6 mΩ ZTotal = 6.2133 mΩ 400 Isc = = 37.168kA 3 x 6.2133m June 7, 2021 22kV 750MVA Slide 32 Isc Short Circuit Calculation Basic Principles Short Circuit Calculations two transformers in parallel MV Level 22kV 750MVA Z22kV = (22k)2 750M = 0.6453 Ω 4002 Z400V = 0.6453 x 220002 6 ZTX = 100 x 4002 1.6M = 0.2133 mΩ = 6 mΩ ZTotal = 6.2133 mΩ Isc = 400 3 x 6.2133m 1600kVA 22/0.4kV 6% = 37.168kA ISC Total = 37.168 x 2 = 74.336kA June 7, 2021 U 22kV 750MVA Slide 33 Isc Total U Short Circuit Calculation Basic Principles Short Circuit Calculations two transformers in parallel MV Level 22kV 750MVA Z22kV = (22k)2 750M = 0.6453 Ω 4002 Z400V = 0.6453 x 220002 5 ZTX = 100 x 4002 8ooK = 0.2133 mΩ = 0.01 Ω ZTotal = 10.2133 mΩ Isc = 400 3 x 10.2133m 800kVA 22/0.4kV 5% = 22.611 kA ISC Total = 22.611 x 2 = 45.22 kA June 7, 2021 U 22kV 750MVA Slide 34 Isc Total U Short Circuit Calculation Basic Principles Short Circuit Calculations Single Transformer ZTX = Isc = 6 100 x 400 3 x 6m 4002 1.6M U = 6 mΩ = 38.49kA 1600kVA 22/0.4kV 6% Isc June 7, 2021 Slide 35 22kV MVA = ??? Short Circuit Calculation Basic Principles Time Current curves of Circuit breakers – L function protection against overload L – S function protection against delayed short-circuit – I function protection against instantaneous shortcircuit – G function protection against earth-fault June 7, 2021 Slide 36 S G I Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Technical Application Paper June 7, 2021 Slide 37 http://new.abb.com/low-voltage http://new.abb.com/low-voltage/business/epc Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Electrical Installation Handbook Determination of short-circuit current downstream of a cable as a function of the upstream short-circuit current (Table 6.4). June 7, 2021 Slide 38 Determination of shortcircuit current downstream of a cable as a function of the upstream short-circuit current (Table 6.4). June 7, 2021 Slide 39 Short Circuit Calculation Basic Principles EasyPower Short Circuit Calculation Example June 7, 2021 Slide 40 Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Manual Verification • Example 1 June 7, 2021 Slide 41 June 7, 2021 Slide 42 Short Circuit Calculation Basic Principles Calculation of Short Circuit Currents Manual Verification • Example 1 June 7, 2021 Slide 43
