Short Circuit According to the IEC 61660 - Open Electrical

07/05/2016

Accor di ng to the IEC 61660 - Open Electr i cal

According to the IEC 61660 From Open Electrical

Contents 1 Introduction 2 Calculating the Total Short-Circuit Current 3 Partial Fault Currents 3.1 Fault Current from Batteries 3.2 Fault Current from C apacitors 3.3 Fault Current from Rectifiers 3.4 Fault Current from DC Machines 3.5 Correction Factors 4 References

Introduction The scope of IEC 61660 is to describe a method for calcula ting short-circuit currents in DC auxiliary DC auxiliary systems in power plants and substations. Such systems can be can be equipped with the following equipment, acting as short-circuit current sources: rectifiers in three-phase AC bridge connection for 50 Hz; stationary lead-acid batteries; smoothing capacitors; DC motors motors with independent excitation. NOTE – Rectifiers Rectifiers in three- phase phase AC bridge connection for 60 Hz are under consideration. The data of other equipment equipment may be given giv en by the manufacturer. This standard is only concerned with rectifiers in three-phase AC bridge connection. It is not concerned with other types of rectifiers. The purpose of the standard is to provide a generally applicable method of calculation which produces results of sufficient accuracy on the conservative side. Special methods, adjusted to particular circumstances, may be used if they give at least the same precision. Short-circuit currents, resistances and inductances may also be ascertained from system tests or measurements on model systems. In existing DC systems the necessary values can be ascertained from measurements taken at the assumed short-circuit location. The load current is not taken into consideration when calculating the short-circuit current. It is necessary to distinguish between two different values of short-circuit current: the maximum short-circuit current which determines the rating of the electrical equipment; the minimum short-circuit current which can be taken as the basis for fuse and protection ratings and settings. For more information please refer to the standard itself IEC 61660-1 (https://webstore.iec.ch/publication/5700) .

Calculating the Total Short-Circuit Current

Figure 1. Equivalent circuit diagram for calculating the partial shortcircuit currents

Each DC source during the fault shall contribute to the total short-circuit current. The superposition principle is being applied. When one source is observed then the other ones are being disconnected and ignored. The potential DC sources are battery, rectifier, capacitor and machine. The partial short-circuit currents are calculated for each of those sources as follows: for 0

t

t p:

Where t p is the time to peak of the partial current and τ 1 is the rise time constant for the partial current source. for t p

t

Tk :

Where Tk is the fault duration time and τ 2 the decay time constant for the partial current source.

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According to the IEC 61660 - Open Electrical

And the total short-circuit current is the sum as follows:

for 0

t

Tk . And n DC is the number of the DC sources contributing the fault current, j is the observed DC source.

Partial Fault Currents Fault Current from Batteries

The peak short-circuit current is calculated as:

The quasi steady-state short-circuit current is calculated as follows:

The decay component is calculated as:

The rise-time constant (τ1B) and time-to-peak of short-circuit currents of batteries is taken from the diagram (Figure 10. in IEC 61660:1997). The time constant of the battery TB is assumed to be 30 ms. The decay-time constant (τ 2B) is assumed to 100 ms. R BBr is

Figure 2. Time to peak and rise time constant (Figure 10. IEC 61660:1997)

the sum of the battery internal resistance and the line (path) resistance up to the fault location (R BBr =0,9R B+R Br ). L BBr is the sum of the battery internal inductance and the line (path) inductance up to the fault location. Rise-time current , for 0 ≤ t ≤ t pB:

Decay-time current, for t pB ≤ t ≤ Tk :

And the total current from the battery is:

Fault Current from Capacitors

The peak short-circuit current is calculated using:

Where EC is the voltage of the capacitor terminal before the fault, and R CBr is the sum of capacitor and branch resistance, up to the fault location. The factor κ C depends on the eigen-frequency ω 0 and the decay coefficient δ, as follows:

LCBr is the inductance of the capacitor and common branch up to the fault location. a) If δ > ω 0: Figure 3. Factor k 1C to determine rise-time constant (Figure 14. IEC 61660:1997)

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b) If δ < ω0:

c) If δ = ω 0: Figure 4. Factor k 2C to determine decay-time constant (Figure 15. IEC 61660:1997)

Where the time-to-peak is t pC. And the rise-time constant is:

And the decay-time constant is:

And coefficients k 1C and k 2C are taken from the diagrams/tables (defined in Figure 14. IEC 61660). The quasi steady-state current of the capacitor is considered to be 0. Rise-time current , for 0 ≤ t ≤ t pC:

Decay-time current, for t pC ≤ t ≤ Tk :

And the total current from the battery is:

Fault Current from Rectifiers

The quasi steady-state short-circuit current I kD of a rectifier in three-phase AC bridge connection is:

Where Un is the nominal system voltage on AC side of rectifier, Z N is the network impedance AC side, UnTLV and UnTHV are transformer rated voltages of low and high oltage side, respectively. The factor λ D is calculated using:

The peak short-circuit current is calculated using:

And the factor κ D and

is calculated using:


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The time-to-peak is calculated for all values κ D ≥1,05 as follows: for

it is

for

it is

(ms) (ms)

The rise-time constant for rectifiers is: For κ D >= 1.05 :

For κ D < 1.05 :

The suitable approximation is given as:

The decay-time constant is calculated using:

Fault Current from DC Machines

The quasi steady-state short-circuit current is calculated using:

Where LF is the field inductance and LOF is the unsaturated field inductance at no-load. This equation is valid only if the motor speed remains constant during the duration of the short-circuit fault. Otherwise I kM = 0.

Figure 5. Factor κ M for determining the peak short-circuit current i pM (Figure 17. IEC 61660:1997)

Figure 6. Factors for determining t pM, τ1M for nominal and decreasing speed (Figure 18. IEC 61660:1997)

The armature time constant is calculated as:

The time constant of the field circuit is calculated as:


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And the mechanical time constant is calculated as:

The eigen frequency is calculated as:

The decay coefficient is calculated from:

The peak short-circuit current:

The factors k 1M, k 2M, k 3M and k 4M are taken from the diagrams (Figure 18, 20, 21 in IEC 61660). The factor κ M is taken from the diagram (Figure 17 in IEC 61660).

Figure 8. Factor k 3M for determining the rise-time constant t1M for decreasing speed

Figure 9. Factor k 4M for determining the decay-time constant t 2M for decreasing speed

(Figure 20. IEC 61660:1997)

(Figure 21. IEC 61660:1997)

The time-to-peak in case when τ Mec≥10τF:

And the rise-time constant:

The decay-time constant: τ2M = τF when n=nn=const. τ2M = (k 4M)(τMec)(LOF/LF) when n→0 In case when τ Mec<10τF then the time-to-peak is taken from the diagram/table (Figure 19. IEC 61660).

The rise-time constant and the decay-time constant τ 1M and τ2M are calculated using:

Rise-time current , for 0 ≤ t ≤ t pM:


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Where t p is the time to peak of the partial current and τ 1 is the rise time constant for the observed voltage source. Decay-time current, for t pM ≤ t ≤ Tk :

And the total current from the DC machine is:

Correction Factors

Due to the fact that all non-observed sources at the time are neglected along with their branches it is suggested to use correction factors, which are supposed to improve total results. Each calculated correction factor is multiplied with the partial fault current of the each source, as follows:

Where I j is the initial partial fault current and σ j is the correction factor, both for the source "j".

Corrected resistance for the each source

Y refers to the branch (Br).

References IEC 61660: Short-circuit currents in d.c. auxiliary installations in power plants and substations - Part 1: Calculation of short-circuit currents. For more information please refer to the standard itself IEC 61660-1 (https://webstore.iec.ch/publication/5700) .

Related topics:

Short Circuit Calculation according to the ANSI/IEEE 946 Retrieved from "htt p://www.openelectrical.org/wiki/index.php?title=According_to_the_IEC_61660" Categories: Calculations | Protection This page was last modified on 9 May 2015, at 01:38.


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