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1SDC007101G0202Technical Application PapersFebruary 20082MV/LV transformer substations: theory and examples of short-circuit calculation

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Index1 General information on MV/LV transformer substations1. Classic typologies ...................................... 21.2 General considerations about MV/LV transformers ................................................ 51.3 MV protection devices: observations about the limits imposed by the ...

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2 MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substations1. Classic types An electrical transformer substation consists of a whole set of devices (conductors, measuring and control ap-paratus and electric machines) dedi...

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3MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substationssecondary remains close to the phase value, thus guaranteeing higher safety for people and maintaining the insulation.Method 1 Substation with a single transformer...

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4 MV/LV transformer substations: theory and examples of short-circuit calculationMethod 2 1 General information on MV/LV transformer substationsSubstation with two transformers with one as a spare for the otherWhen the plant foresees installation of a transformer considered as a spare, the circui...

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5MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substations1.2 General information about MV/LV transformersThe transformer is the most important part of the trans-former substation. Its selection affects the configuration ...

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6 MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substationsOther important characteristics to be considered are those referring to the electrical parameters and, in addition to the usual quantities such as rated power, n...

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7MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substationsG1 Fault on one of the LV usersRegardless of the presence or absence of the bus-tie:with appropriate selection of the protection devices and according to normal L...

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8 MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substationsFigure 41.4 LV protection devices LV protection devices are located on the load side of the transfomer.The protection functions usually available on a LV device ...

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9MV/LV transformer substations: theory and examples of short-circuit calculation1 General information on MV/LV transformer substationsterize the inverse time-delay curve with characteristic I2t constant as those available for functions L - S – G. With reference to the protection function “L...

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0 MV/LV transformer substations: theory and examples of short-circuit calculationTable 11 General information on MV/LV transformer substationsANSI/IEEECodeFunction definitionSimbology correspondingto the Standard IEC 60617-7515051N50N6767NTime-delayed overcurrentInstantaneous overcurrentTime-dela...

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MV/LV transformer substations: theory and examples of short-circuit calculation2 Calculation of short-circuit currents2. Data necessary for the calculation Some general indications regarding the typical param-eters characterizing the main components of an instal-lation are given hereunder. Knowl...

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2 MV/LV transformer substations: theory and examples of short-circuit calculation2 Calculation of short-circuit currentsTransformer A MV/LV transformer with delta primary winding (Δ) and secondary winding with grounded star point ().The electrical parameters which are usually known and which cha...

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3MV/LV transformer substations: theory and examples of short-circuit calculation2 Calculation of short-circuit currentsSupply network (net)In the most cases, the installation results to be supplied by a medium voltage distribution network, whose supply voltage value Vnet and initial short-circui...

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4 MV/LV transformer substations: theory and examples of short-circuit calculation2 Calculation of short-circuit currentsAn example of calculation of the three-phase short-circuit current using the above described relationship is given hereunder.Short-circuit power and current of the supply networ...

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5MV/LV transformer substations: theory and examples of short-circuit calculation2 Calculation of short-circuit currents2.3 Calculation of motor contribution In case of short-circuit, the motor begins to function as a generator and feeds the fault for a limited time corre-sponding to the time nec...

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6 MV/LV transformer substations: theory and examples of short-circuit calculationSince each element with an impedance modifies the short-circuit current on the load side, and since a circuit-breaker is an element with an impedance of its own, the prospective current is defined as the current flow...

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7MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devices 3. Generalities about the main electrical parameters of protection and control devices Generally speaking, when it is necessary to analyse and select a protection and control...

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8 MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesMoulded-case circuit-breakersAir circuit-breakersH80000025060020002500320000000085()8585858522022087877565V80025060020002500320030300000000085852862862202208565L2000250030085...

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9MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesH80000025060020002500320000000085()8585858522022087877565V80025060020002500320030300000000085852862862202208565L200025003008585300656528624287875S40007575757575757575656565657...

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20 MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesMaximum protected lengthFor the secondary circuit of type TN-S on the LV side, the Standard IEC 60364 gives some indications for an approximate calculation to evaluate the m...

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2MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesan extreme thermal overload capacity of the machine in case of short-circuit. The Standard IEC 60076-5 indicates as a limit to the thermal stresses due to a short-circuit (ove...

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22 MV/LV transformer substations: theory and examples of short-circuit calculationMoulded-case circuit-breaker and switch-disconnector OT and OETL Table 3With reference to the highlighted combination between a circuit-breaker type T5..400 on the load side and a switch-disconnector type OETL200,...

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23MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devices3.5 Example of study of a MV/LV network Hereunder there is an example of how the analysis of a MV/LV installation can be carried out to evaluate the main electrical parameter...

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24 MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesIn Figure 3 there is a representation of the time/current diagram with the following curves:Curve 1: tripping curve of the MV overcurrent protection; Curve 2: characterist...

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25MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesPassive load L- rated current of the load: InL =PnL3x= 54Ax V2n=300 x 000000x cosj3 x 400 x 0.8Active load M- rated current of the motor:InM =PnM3x= 60Ax V2n=90 x 000000xη ...

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26 MV/LV transformer substations: theory and examples of short-circuit calculationof Figure 5 is obtained, where curve 4 represents the short-circuit current value, on the LV side, affecting the MV devices. 0.kAkA0kA00kAs0.sE-2sE-3s0s00sE3sCurve 2Curve Curve 3Curve 4Ik x 2sFigure 5From the course...

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27MV/LV transformer substations: theory and examples of short-circuit calculationUpstreamVersionT6T7N,S,H,LS,H,LReleaseTM ELELDownstreamT5N,S,H,L,VTMELIu [A]In [A]400630630400800 800 000 250 600600250800320400500630320400630303030303030303030TTTTTTTTTTTTTTTTTTTTT3 Choice of protection and control...

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28 MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesProtections against earth faultProtections against earth faults shall be studied now.In case no earth fault protection is present in the trans-former star point, the overcur...

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29MV/LV transformer substations: theory and examples of short-circuit calculation3 Choice of protection and control devicesIf the zero-sequence protection is present, its tripping threshold shall be lower than the threshold 5N defined by the utility company and declared in the electrical con-nect...

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30 MV/LV transformer substations: theory and examples of short-circuit calculationAnnex ATable 1: Oil transformerSnTR [kVA]ki =ipinrushInTRτinrush [s]5000602504006300006002000542220980.00.50.200.220.250.300.350.400.45Table 2: Cast resin transformerSnTR [kVA]ki =ipinrushInTRτinrush [s]2002503540...

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3MV/LV transformer substations: theory and examples of short-circuit calculationAnnex AExample: Considering as example an oil transformer with rated power SnTR=630kVA and primary rated voltage Vn=0kV, the calculation of the primary rated current gives a value of InTR = 36.4A.With reference to the...

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32 MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BExample of calculation of the short-circuit currentThe study of the short-circuit currents is one of the clas-sic problems plant engineers have to face; knowledge of the values of such currents is fundamental...

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33MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BB Method of symmetrical componentsThis method is based on the principle that any set of three vectors may by resolved into three sets of vectors: - a balanced positive sequence set formed by three vec-tors of...

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34 MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BThe installation typology represented by the single line diagram of Figure may be significant of a generic in-dustrial plant, where a unique overall outgoing feeder has been considered for simplification. On...

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35MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BOnce identified the three sequence networks, the calcu-lation of the short-circuit currents for the different fault typologies can be carried out:Three-phase faultSince the three-phase fault is a symmetrical f...

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36 MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BSingle-phase faultAs regards the single-phase fault a distinction must be made between: - single-phase fault to earth, then return through the protection conductor, being a distribution system of TN-S type- ...

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37MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BTwo-phase fault The equivalent positive sequence impedance is:ZdEq.B = ((ZdTR ZdTR2) + Zdnet) (ZdG + ZdC2) + ZdC= 0.003 + i . 0.0046 ΩThe equivalent negative sequence impedance is:ZiEq.B = ((ZiTR ZiTR2) +...

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38 MV/LV transformer substations: theory and examples of short-circuit calculationThree-phase faultThe equivalent positive sequence impedance is:ZdEq.B = ((ZdTR ZoTR2) + Zdnet + ZdC2) (ZdG ) = 5.653 . 0-4 + i . 0.0035 ΩThen the three-phase fault current value is:Ik3D == 65.9 . 03 ∠ - 80.82°...

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39MV/LV transformer substations: theory and examples of short-circuit calculationAnnex BTaking into consideration the fault in A, the network schematising the contribution of the short-circuit powers is the following:By the reduction of the elements in series – in parallel, the following expres...

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40 MV/LV transformer substations: theory and examples of short-circuit calculation vk% short-circuit voltage in percent pk% short-circuit power in percent Vn rated voltage Sn rated power In rated current V1n primary rated voltage V2n secondary rated voltage X”d subtransient reac...

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Technical Application PapersFebruary 200821SDC007101G0202 February ’08Printed in Italy6.000 - CALDue to possible developments of standards as well as of materials, the characteristics and dimensions speciﬁed in this document may only be considered binding after conﬁrmation by ABB SACE.http:...