Short Circuit Currents
The calculation of short circuit currents is a central task for Power System engineers, they are essential parameters for the design of electrical equipment and installations, the operation of power systems and the analysis of outages and faults.
Inspec keywords: earthing; shortcircuit currents; fault currents
Other keywords: shortcircuit current calculation; AC threephase HVsystem; neutral earthing influence; impedance calculation; lowvoltage system; double earthfault current; electrical equipment; DC auxiliary installation
Subjects: Power system protection
 Book DOI: 10.1049/PBPO051E
 Chapter DOI: 10.1049/PBPO051E
 ISBN: 9780863415142
 eISBN: 9781849190480
 Page count: 336
 Format: PDF

Front Matter
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1 Introduction
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This book deals with the calculation of shortcircuit currents in two and threephase a.c. systems as well as in d.c. systems, installed as auxiliary installations in power plants and substations. It is not the objective of this book to repeat definitions and rules of norms and standards, but to explain the procedure for calculating shortcircuit currents and their effects on installations and equipment. In some cases repetition of equations, tables and diagrams from norms and standards however are deemed necessary for easy understanding. It should be emphasised in this respect that the presentation within this book is mainly concentrated on installations and equipment in high voltage systems, i.e., voltage levels up to 500 kV. Special considerations have to be taken in the case of long transmission lines and in power systems with nominal voltages above 500 kV. The calculation of shortcircuit currents and of their effects are based on the procedures and rules defined in the IEC documents 61660, 60909, 60865 and 60781.
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2 Theoretical background
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A detailed deduction of the mathematical procedure is not given within the context of this book, but only the final equations are quoted. In general, equipment in power systems is represented by equivalent circuits, which are designed for the individual tasks of power system analysis. For the calculation of noload current and the noload reactive power of a transformer, the noload equivalent circuit is sufficient. Regarding the calculation of shortcircuits, voltage drops and load characteristic a different equivalent circuit is required. The individual components of the equivalent circuits are resistance, inductive and capaci tive reactance (reactor and capacitor), voltage source and ideal transformer. Voltage and currents of the individual components and of the equivalent circuit are linked by Ohm's law.
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3 Calculation of impedance of electrical equipment
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In general, equipment in power systems are represented by equivalent circuits, which are designed for the individual tasks of power system analysis, e.g., for the calculation of noload current and the noload reactive power of a transformer, the noload equivalent circuit is sufficient. Regarding the calculation of shortcircuits, voltage drops and load characteristic a different equivalent circuit is required. The individual components of the equivalent circuits are resistance, inductive and capacitive reactance (reactor and capacitor), voltage source and ideal transformer. Voltage and currents of the individual components and of the equivalent circuit are interlaced by Ohm's law, which is valid for the threephase system (RYBsystem) as well as for the system of symmetrical components (012system). A detailed deduction of the mathematical methods and equations is not given within the context of this section of the book, but only the final equations are quoted.
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4 Calculation of shortcircuit current in a.c. threephase HVsystems
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In this chapter, an outline of different types of shortcircuits in threephase a.c. systems is provided such as threephase shortcircuit,doublephase shortcircuit without earth/ground connection, doublephase shortcircuit with earth/ground connection and linetoearth (linetoground) shortcircuit. The method for calculating shortcircuit current in a.c. threephase highvoltage system is also provided.
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5 Influence of neutral earthing on singlephase shortcircuit currents
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The theoretical approach to calculate shortcircuit (s.c.) currents with symmetrical components in general and especially in the case of singlephase shortcircuit was explained in detail in Chapter 2. Current and voltages in case of shortcircuits with earth connection (e.g., singlephase shortcircuits) depend on the positive and zero sequence impedances Z_{1} and Z_{0}.
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6 Calculation of shortcircuit currents in lowvoltage systems
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IEC 60781 presents an application guide for the calculation of shortcircuit currents in lowvoltage radial systems. The shortcircuits are treated as farfromgenerator shortcircuits. This assumption is valid in the future as well, even with an increasing number of distributed generation units in lowvoltage systems.
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7 Double earthfault and shortcircuit currents through earth
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Chapter 7 discusses the double earth fault and shot circuit current through earth. Branch shortcircuit currents can flow through earth in the case of unbalanced short circuits with earth connection, i.e., linetoearth (singlephase) shortcircuit and doublephase shortcircuit with earth connection. Singlephase shortcircuits are the dominating fault type in power systems with earthed neutrals and are leading to the maximal branch shortcircuit currents flowing through earth.
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8 Factors for the calculation of shortcircuit currents
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Several factors for the calculation of shortcircuit (s.c.) currents have been introduced in previous sections, the origin of which will be explained within this section. · Voltage factor c_{max} and c_{min} for different voltage levels as per Table 4.1. · Correction factor using the %/MVA or the p.u.system as mentioned in Chapter 2. · Impedance correction factors for synchronous machines, power station units and transformers as per Tables 3.2, 3.3, 3.5 and 3.6. · Factors for the calculation of different parameters of the shortcircuit current based on the initial shortcircuit current as per Chapter 4. The factors are necessary as the method of the equivalent voltage source at the shortcircuit location is used for the calculation of shortcircuit currents which is based on some simplifications such as neglecting the load current prior to fault, assuming the tapchanger of transformers in middleposition, calculating the impedance of equipment based on the nameplate data or on data for rated operating conditions and neglecting voltage control gear for generators and transformers. The main task of shortcircuit analysis is to determine the maximal shortcircuit current which is one of the main criteria for the rating of equipment in electrical power systems. It is obvious that the parameters of the shortcircuit current as calculated with the equivalent voltage source at the shortcircuit location will differ from those currents, which may be measured during shortcircuit tests or may be calculated with transient network analysing programmes. In order to obtain results on the safe side without uneconomic safety margin the correction factors will be applied. Detailed deductions of the various correction factors are given in IEC 609091:199110.
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9 Calculation of shortcircuit currents in d.c auxiliary installations
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The calculation of shortcircuit currents in d.c. auxiliary installations, e.g., in power plants and substations is dealt with in IEC 616601. Contrary to the approach for the calculation of shortcircuit currents in a.c. threephase systems, the determination of the exact time course of the shortcircuit current is needed besides the calculation of defined parameters.
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10 Effects of shortcircuit currents
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Calculation methods for the thermal and electromagnetic effects of shortcircuit currents are dealt with IEC 616602, which is applicable to shortcircuit currents in d.c. auxiliary installations in power plants and substations and IEC 608651, related to threephase a.c. systems.
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11 Limitation of shortcircuit currents
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The expansion of electrical power systems by new power stations and new lines (overhead transmission lines and cable circuits) results in an increase of shortcircuit currents due to an increase in sources feeding the shortcircuit and due to a reduction of system impedance. The improvement of existing installations and the replacement of equipment are necessary, in case the permissible shortcircuit current will be exceeded. Measures to limit the shortcircuit currents can also be realised which might be more economic than the replacement of equipment and installations. Different measures have to be taken into account such as measures affecting the whole system (higher voltage level), measures concerning installations and substations (separate operation of busbars) and measures related to equipment (Iplimiter).
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12 Special problems related to shortcircuit currents
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Interference between overhead lines, communication circuits and pipelines is caused by asymmetrical currents, which may be due to shortcircuits, asymmetrical operation or asymmetrical design of equipment, especially asymmetrical outline of overhead line towers with respect to pipelines and communication circuits. This interference is based on inductive, ohmic and capacitive coupling between the shortcircuit path (e.g., overhead line) and the circuit affected by interference (e.g., pipeline). Normal operating currents, respectively voltages, cause magnetic as well as electric fields which are asymmetrical in the vicinity of overhead lines which may cause interference problems in the longtime range.
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13 Data of equipment
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A summary of relevant data of equipment is given in IEC 609092:1992. The data is based on a survey carried out by IEC TC 73.
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Back Matter
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