The protection for an electrical system should not only be safe under all service conditions but, to insure continuity of service, it should be selectively coordinated as well.
A C coordinated system is one where only the faulted circuit is U isolated without disturbing any other part of the system. R Overcurrent protection devices should also provide short- R TIME circuit as well as overload protection for system E components, such as bus, wire, motor controllers, etc. N T To obtain reliable, coordinated operation and assure that system components are protected from damage, it is necessary to first calculate the available fault current at Figure 1 various critical points in the electrical system.
Once the short-circuit levels are determined, the In Figure 2, note that the total short circuit current Ia is engineer can specify proper interrupting rating require- the summation of two components - the symmetrical RMS ments, selectively coordinate the system and provide current IS, and the DC component, IDC.
The DC component component protection. It decays to zero after a few General Comments on Short-Circuit Calculations cycles due to I2R losses in the system, at which point the Short Circuit Calculations should be done at all critical short circuit current is symmetrical about the zero axis. The points in the system. Maximum thermal and mechanical stress on - Transfer Switches the equipment occurs during these first few cycles.
It is - Load Centers important to concentrate on what happens during the first half cycle after the initiation of the fault. Normally, short circuit studies involve calculating a bolted 3-phase fault condition. From this calculation, other types of fault conditions can be obtained. Equipment intended to N break current at fault levels shall have an interrupting rating T sufficient for the system voltage and the current which is Ia - Asymmetrical RMS Current available at the line terminals of the equipment.
Available Short-Circuit Current. Service IP - Instantaneous Peak Current Equipment shall be suitable for the short circuit current available at its supply terminals.
Figure 2 Low voltage fuses have their interrupting rating Figure 2 illustrates a worst case waveform that 1 phase expressed in terms of the symmetrical component of short- of the 3 phase system will assume during the first few circuit current, I S. They are given an RMS symmetrical cycles after the fault initiation. Thus only the symmetrical power factor. For U. The reader From Table 8, note the following relationships.
See System system, first draw a one-line diagram showing all of the A, below Also, System B, for a double transformation, will sources of short-circuit current feeding into the fault, as well be studied. The impedance tables given in the Data Section System B can deliver a short-circuit of , KVA at the include three phase and single phase transformers, current primary of the first transformer.
These tables can be used if information from the be considered. With this available short-circuit information, begin to It must be understood that short circuit calculations are make the necessary calculations to determine the fault performed without current limiting devices in the system. Calculations are done as though these devices are Four basic methods will be presented in this text to replaced with copper bars, to determine the maximum instruct the reader on short circuit calculations.
This is necessary to These include : project how the system and the current limiting devices will - the ohmic method perform. The - the point to point method downstream, or load side, fuse will operate alone under a short circuit condition if properly coordinated. MVA , S. The symmetrical motor contribution can be Ohmic Method approximated by using an average multiplying factor Most circuit component impedances are given in ohms associated with the motors in the system.
To solve for the symmetrical motor the secondary voltage. KVA u tility Step Step 3. Step This multiplier will provide the worst case asymmetry occurring Step 5. Total all X and all R in system to point of fault.
When the average 3-phase multiplier is desired use column Ma. Step 6. Determine impedance in ohms of the system by: Step Step 7. The short-circuit current that the motor load can Esecondary line-line IS. Determine the motor load. Add up the full load motor currents. The generally Step See Step 7. Short circuit amperes can be affected by this tolerance.
For more finite details obtain R of utility source. More finite values involve vectorial addition of the currents. Note: The ohms of the circuit components must be referred to the same voltage. If there is more than one voltage transformation in the system, the ohmic method becomes more complicated. It is recommended that the per-unit method be used for ease in calculation when more than one voltage transformation exists in the system.
MVA , Actual motor contribution will be somewhat smaller than calculated due to the impedance of the feeder cable. Reflect X and R values of all components to the following steps apply: secondary side of transformer Step 1a. Summarize X and R values of all components on Vs2 V2 primary side of transformer.
The symmetrical motor contribution can be The per-unit method is generally used for calculating approximated by using an average multiplying factor short-circuit currents when the electrical system is more associated with the motors in the system. This factor varies complex. KVA utility Step This multiplier will provide the worst case asymmetry Step 4.
Step 5. Next, total all per-unit X and all per-unit R in system to point of fault. The asymmetrical RMS short-circuit current can be calculated as: Step 6. Determine the per-unit impedance of the system by: IS. The short-circuit current that the motor load can Step 7. Calculate the symmetrical RMS short-circuit current contribute is an asymmetrical current usually approximated at the point of fault.
Add up the full load Step The total asymmetrical short-circuit RMS current motor currents. For more finite details obtain per-unit R of utility source. Starting in , he led a research team in the field of industrial and distribution networks.
Since , as a project manager, he has been in charge of the technical development of electrical distribution services. He joined Schneider Electric in as a research engineer and has worked since in the Electrical Networks competency group of the Projects and Engineering Center.
MLVS Main low voltage switchboard. RL Line resistance per unit length. Sn Transformer kVA rating. Symbols A Cross-sectional area of conductors. U Network phase-to-phase voltage with no load. E Electromotive force rms value. Un Network nominal voltage with load.
Xa Equivalent reactance of the upstream network. Xsubt Subtransient reactance of a generator. Z 1 Posititve-sequence impedance of a network ip Maximum current first peak of Z 2 Negative-sequence or an the fault current. ZL Line impedance. Zup Equivalent impedance of the upstream Ik" Initial symmetrical short-circuit current network.
IEC Subscripts Ir Rated current of a generator. G Generator. Is Design current. SO Generator set without on-load tap changer. T Transformer.
It is intended for radial and meshed low-voltage LV and high-voltage HV circuits. The aim is to provide a further understanding of the calculation methods, essential when determining short-circuit currents, even when computerised methods are employed.
Summary 1 Introduction p. Two values of the short-circuit protection against short-circuits wherever there current must be evaluated: is an electrical discontinuity. This most often c The maximum short-circuit current, used to corresponds to points where there is a change determine in conductor cross-section. The short-circuit v The breaking capacity of the circuit breakers current must be calculated at each level in the v The making capacity of the circuit breakers installation in view of determining the v The electrodynamic withstand capacity of the characteristics of the equipment required to wiring system and switchgear withstand or break the fault current.
The maximum short-circuit current corresponds The flow chart in Figure 1 indicates the to a short-circuit in the immediate vicinity of the procedure for determining the various short- downstream terminals of the protection device. The direction of current is chosen arbitrarily See IEC This distance is not voltages HV, LV and the series-connected necessarily physical, but means that the wiring systems with different cross-sectional generator impedances are less than the areas A and lengths.
Fault far from the generator This is the most frequent situation. The transient When a fault occurs between A and B, the conditions are those resulting from the negligible impedance between these points application of a voltage to a reactor-resistance results in a very high short-circuit current Isc that circuit. This voltage is: is limited only be impedance Zsc. Figure 8 illustrates the two extreme cases for The transient current-development conditions the development of a short-circuit current, are in this case modified by the variation in the presented, for the sake of simplicity, with a electromotive force resulting from the single-phase, alternating voltage.
The successive effect of the three reactances leads to a gradual reduction in the This short-circuit current i t is maximum for a short-circuit current which is the sum of four closing angle corresponding to the zero-crossing components see Fig.
Note that the decrease in the generator reactance is faster than that of the aperiodic component. This is a rare situation that can cause saturation of the magnetic circuits and interruption problems because several periods occur before the current passes through zero. Ib is the value of the T"d: Subtransient time constant short-circuit current at the moment interruption is T'd: Transient time constant effective, i.
Time tmin Practically speaking, information on the minimum time delay is the sum of the minimum development of the short-circuit current is not operating time of a protection relay and the shortest essential: opening time of the associated circuit breaker, i. In other words, it is assumed that the This method is used essentially for final circuits elementary impedances of two successive with origins sufficiently far from the source. It is sections in the installation are sufficiently similar not applicable in installations supplied by a in their characteristics to justify the replacement generator.
This approximation may be networks, radial or meshed, up to kV. All network when the impedances or the Isc in the feeders as well as the synchronous and installation upstream of the given circuit are not asynchronous machines are replaced in the known, to calculate the minimum short-circuit calculation by their impedances positive currents and the fault currents at the end of a sequence, negative-sequence and line.
It is based on the assumption that the zerosequence. Conductor reactance is neglected for sizes under mm2. It is taken into account for large 1. More technical in nature, it accuracy and its instructive value, given that implements the symmetrical-component principle 1. These fault remains phase-to-earth impose limits for which the calculations are valid c For the entire duration of the short-circuit, the but usually provide good approximations, voltages responsible for the flow of the current facilitating comprehension of the physical and the short-circuit impedance do not change phenomena and consequently the short-circuit significantly current calculations.
The short-circuit occurs away far from the generator, assumptions used in this document are as the actual position of the transformer regulator or follows: tap-changers does not need to be taken into c The given network is radial with nominal account voltages ranging from LV to HV, but not c Arc resistances are not taken into account exceeding kV, the limit set by standard IEC c All line capacitances are neglected c The short-circuit current, during a three-phase c Load currents are neglected short-circuit, is assumed to occur simultaneously c All zero-sequence impedances are taken into on all three phases account c During the short-circuit, the number of phases involved does not change, i.
Short-circuit and the lines see Fig. Calculation of Isc3 is therefore equal to all the impedances through which Isc essential for selection of equipment maximum flows from the generator to the location of the current and electrodynamic withstand capability. In this case, the source is less than Zsc for example, at the short-circuit current Isc2 is less than that of a terminals of a star-zigzag connected transformer three-phase fault: or of a generator under subtransient conditions.
In this case, the phase-to-neutral fault current U 3 may be greater than that of a three-phase fault. Except when rotating machines are involved Phase-to-neutral short-circuit clear of earth reduced zero-sequence impedance , the short- circuit current Isco is less than that of a three This is a fault between one phase and the phase fault.
Network impedances c Internal transformer impedance c Upstream network impedance The impedance may be calculated on the basis Generally speaking, points upstream of the of the short-circuit voltage usc expressed as a power source are not taken into account. Initially, Zup and ZT may be considered Practically speaking, for LV and conductors with comparable to their respective reactances. The cross-sectional areas less than mm2, only short-circuit impedance Zsc is therefore equal to the resistance is taken into account the algebraic sum of the two.
The error increases in proportion to the conductors in LV applications, depending on the transformer rating wiring system practical values drawn from French v The cable capacitance with respect to the earth standards, also used in other European countries. Generally speaking, the capacitance of a HV three-phase cable with a and, for HV applications, between 0.
See UTE C As above, If one of the values, RL or XL, is low with respect the impedance ZL curve may be considered to the other, it may be neglected because the identical to the asymptotes, but for cable cross- resulting error for impedance ZL is consequently sectional areas less than mm2 and greater very low.
Impedance of rotating machines.
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