calculation of short circuit current as per iec60909

7/30/2019 Calculation of Short Circuit Current as Per IEC60909

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Calculation of Short CircuitCurrent as per IEC 60909 std.

Muhammad Haekal

920016246


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Nature of Short Circuit Current

When a short-circuit occurs on a power

system :

1. Arcing & burning

2. Short circuit current occurs

3. System voltage drops

Short circuit current sources : Transformers,

Generators, Synchronous & Induction Motors


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Characteristic of short-circuit current

Far-from-genera

Far-from-generator short-

circuit current

Near-to-generator short-

circuit current


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Types of short-circuit current


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Equivalent voltage source at fault location F


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Short circuit impedances of a 3phase

system on a short-circuit location F


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Calculation of short-circuit currents

The short-circuit current can be considered as the sum of thefollowing two components:

1. the a.c. component with constant amplitude during the whole shortcircuit,

2. the aperiodic d.c. component beginning with an initial value A anddecaying to zero.

In the case of a far-from-generator short circuit, thesymmetrical a.c. currents Ik", Ib, and Ik are r.m.s. values andare nearly equal

In the case of near-to-generator, circuit breaking current Ib issmaller than the initial symmetrical short-circuit current Ikand the steady-state short-circuit current Ik is smaller thanthe symmetrical short-circuit breaking current 1,.


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Calculation of equipment impedances

If no accurate value is known for the resistance RQ of network

feeders, one may substitute RQ = 0,l XQ where XQ = 0,995 ZQt

(Eq.1) (Eq.2)(Eq.3)

(Eq.4)

(Eq.5)

(Eq.6)(Eq.7)

(Eq.8)


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Calculation of initial short-circuit

current (ik)

(Eq.9)

(Eq.10)

(Eq.11)

(Eq.12)


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Calculation of peak short-circuit

current (ip)

(for meshed networks)(if Z

(1

) = Z(2)

)

(Eq.13)

(Eq.19)

(Eq.14)

(Eq.15)

(Eq.16)

(Eq.17) (Eq.18)


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Calculation of DC component of short-

circuit current (id.c)

For meshed networks, the ratio R/X or X/R is to be determined by the method of equivalent

frequency. An equivalent impedance Zc = Rc + jXc, of the system as seen from the short-circuit

location is calculated assuming a frequency = 20 Hz (for a nominal frequency off = 50 Hz) or

fc = 24 Hz (for a nominal frequency off = 60 Hz). The R/X or X/R ratio is then determined

according to equation

Depending on the product f . t; where f is the frequency and t is the time, the equivalent frequency

f, should be used as follows:

(Eq.20)

(Eq.21)


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Example of calculation A low-voltage system with Un= 400 V and f= 50 Hz is given in figure 6. The short-

circuit currents Ik" and ip shall be determined at the short-circuit locations F1 to

F3. It may be assumed that the short circuits at the locations F1 to F3 are far-from-

generator short circuits


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Example of calculation Equipment impedance calculations (positive sequence impedance)

(Eq.2) (Eq.3)

(Eq.4)

(Eq.5)

(Eq.6)

(Eq.7)

(Eq.8)


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Example of calculation Equipment impedance calculations (zero sequence impedance)

(Eq.9)

(Eq.19)

Because the R/X-ratio of ZT2K+ ZL1+ ZL2 is higher than 0,3, it

is necessary to introduce the factor 1,15

(Eq.14)


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Example of calculation Using equivalent frequency method :

(Eq.21)

(Eq.13)

(Eq.9)

(Eq.21)

(Eq.13)


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calculation of short circuit current as per iec60909

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