3-phase short-circuit current (Isc) at any point within a LV installation
In a 3-phase installation Isc at any point is given by:
U20 = phase-to-phase voltage of the open circuited secondary windings of the power supply transformer(s).
ZT = total impedance per phase of the installation upstream of the fault location (in Ω)
Method of calculating ZT
Each component of an installation (MV network, transformer, cable, busbar, and so on...) is characterized by its impedance Z, comprising an element of resistance (R) and an inductive reactance (X). It may be noted that capacitive reactances are not important in short-circuit current calculations.
The parameters R, X and Z are expressed in ohms, and are related by the sides of a right angled triangle, as shown in the impedance diagram of Figure G35.
The method consists in dividing the network into convenient sections, and to calculate the R and X values for each.
Where sections are connected in series in the network, all the resistive elements in the section are added arithmetically; likewise for the reactances, to give RT and XT.
The impedance (ZT) for the combined sections concerned is then calculated from
Any two sections of the network which are connected in parallel, can, if predominantly both resistive (or both inductive) be combined to give a single equivalent resistance (or reactance) as follows:
Let R1 and R2 be the two resistances connected in parallel, then the equivalent resistance R3 will be given by:
or for reactances
It should be noted that the calculation of X3 concerns only separated circuit without mutual inductance. If the circuits in parallel are close togother the value of X3 will be notably higher.
Determination of the impedance of each component
Network upstream of the MV/LV transformer
(see Fig. G36)
The 3-phase short-circuit fault level PSC, in kA or in MVA is given by the power supply authority concerned, from which an equivalent impedance can be deduced.
|Psc||U20 (V)||Ra (mΩ)||Xa (mΩ)|
A formula which makes this deduction and at the same time converts the impedance to an equivalent value at LV is given, as follows:
Za = impedance of the MV voltage network, expressed in milli-ohms
U20 = phase-to-phase no-load LV voltage, expressed in volts
Psc = MV 3-phase short-circuit fault level, expressed in kVA
The upstream (MV) resistance Ra is generally found to be negligible compared with the corresponding Xa, the latter then being taken as the ohmic value for Za. If more accurate calculations are necessary, Xa may be taken to be equal to 0.995 Za and Ra equal to 0.1 Xa.
Figure G36 gives values for Ra and Xa corresponding to the most common MV short-circuit levels in utility power-supply networks, namely, 250 MVA and 500 MVA.
(see Fig. G37)
The impedance Ztr of a transformer, viewed from the LV terminals, is given by the formula:
U20 = open-circuit secondary phase-to-phase voltage expressed in volts
Sn = rating of the transformer (in kVA)
Usc = the short-circuit impedance voltage of the transformer expressed in %
The transformer windings resistance Rtr can be derived from the total load-losses as follows:
so that in milli-ohms
Pcu = total load-losses in watts
In = nominal full-load current in amps
Rtr = resistance of one phase of the transformer in milli-ohms (the LV and corresponding MV winding for one LV phase are included in this resistance value).
Note: for an approximate calculation, in the absence of more precise information on transformer characteristics, Cenelec 50480 suggests to use the following guidelines:
- if U20 is not known, it may be assumed to be 1.05 Un
- in the absence of more precise information, the following values may be used: Rtr = 0.31 Ztr and Xtr = 0.95 Ztr
Example: for a transformer of 630kVA with Usc=4% / Un = 400V, approximate calculation gives:
- U20 = 400 x 1.05 = 420V
- Ztr = 4202 / 630 x 4% = 11 mΩ
- Rtr = 0.31 x Ztr = 3.5 mΩ and Xtr = 0.95 x Ztr = 10.6 mΩ
|Rated Power kVA)||Oil-immersed||Cast-resin|
|Usc (%)||Rtr (mΩ)||Xtr (mΩ)||Ztr (mΩ)||Usc (%)||Rtr (mΩ)||Xtr (mΩ)||Ztr (mΩ)|
The resistance of busbars is generally negligible, so that the impedance is practically all reactive, and amounts to approximately 0.15 mΩ/metre length for LV busbars (doubling the spacing between the bars increases the reactance by about 10% only).
In practice, it's almost never possible to estimate the busbar length concerned by a short-circuit downstream a switchboard.
The resistance of a conductor is given by the formula:
ρ = the resistivity of the conductor material at the normal operating temperature
ρ has to be considered:
- at cold state (20°C) to determine maximum short-circuit current,
- at steady state (normal operating temperature) to determine minimum short-circuit current.
L = length of the conductor in m
S = c.s.a. of conductor in mm2
|20 °C||PR/XLPE 90 °C||PVC 70 °C|
Cable reactance values can be obtained from the manufacturers. For c.s.a. of less than 50 mm2 reactance may be ignored. In the absence of other information, a value of 0.08 mΩ/metre may be used (for 50 Hz systems) or 0.096 mΩ/metre (for 60 Hz systems). For busways (busbar trunking systems) and similar pre-wired ducting systems, the manufacturer should be consulted.
At the instant of short-circuit, a running motor will act (for a brief period) as a generator, and feed current into the fault.
In general, this fault-current contribution may be ignored. However, if the total power of motors running simultaneously is higher than 25% of the total power of transformers, the influence of motors must be taken into account. Their total contribution can be estimated from the formula:
Iscm = 3.5 In from each motor i.e. 3.5m In for m similar motors operating concurrently.
The motors concerned will be the 3-phase motors only; single-phase-motor contribution being insignificant.
Short-circuit faults generally form an arc which has the properties of a resistance. The resistance is not stable and its average value is low, but at low voltage this resistance is sufficient to reduce the fault-current to some extent. Experience has shown that a reduction of the order of 20% may be expected. This phenomenon will effectively ease the current-breaking duty of a CB, but affords no relief for its fault-current making duty.
(see Fig. G39)
|Parts of power-supply system||R (mΩ)||X (mΩ)|
|Xa = 0.995 Za
|Circuit-breaker||Not considered in practice|
|Busbars||Negligible for S > 200 mm2, below use the formula:
|Xb = 0.15 mΩ/m|
|Circuit conductors[b]||[a]||Cables: Xc = 0.08 mΩ/m|
|Motors||See Motors (often negligible at LV)|
circuit current in kA
- U20: Phase-to-phase no-load secondary voltage of MV/LV transformer (in volts).
- Psc: 3-phase short-circuit power at MV terminals of the MV/LV transformers (in kVA).
- Pcu: 3-phase total losses of the MV/LV transformer (in watts).
- Sn: Rating of the MV/LV transformer (in kVA).
Usc: Short-circuit impedance voltage of the MV/LV transfomer (in %).
- RT : Total resistance. XT: Total reactance
Example of short-circuit calculations
(see Fig. G40)
- RT : Total resistance. XT: Total reactance. Isc : 3-phase maximum short-circuit current
Calculations made as described in figure G36