IEC 60909 Short-circuit: Meshed vs Non-meshed

In our previous discussion, we introduced the terms used in IEC 60909 short-circuit calculations and compared them to more familiar ANSI/IEEE terms. This is summarized in the table below.

ANSI/IEEE C37 Series	IEC 60909
First Cycle/Momentary (1/2 Cycle Symmetrical RMS)	Initial (I”k)
Closing and Latching (1/2 Cycle Asymmetrical Peak)	Peak (Ip)
Interrupting (1.5 – 4 cycles)	Breaking (Ib)
Time-delayed (> 30 cycles)	Steady-state (Ik)

IEC 60909 ‘Equivalent Source’

IEC 60909 uses an ‘equivalent source’ technique where only one source is exciting the network at the fault location. All other sources are represented by their internal impedances. Let us look for example at the figure below. From this figure, we can see a faulted bus and how its represented in IEC 60909. Only one source exciting the network at the fault point while other sources in this case, the utility and generator, are replaced with their internal impedance.

The magnitude of this equivalent voltage source is the product of a voltage factor and the nominal system voltage, cV_n. This is done to account for the system prefault conditions. To determine the applicable voltage factor, click this link.

There might be a little confusion with regards to the concept of ‘equivalent source’. One might assume that only the value of the short-circuit current at the fault location needs to be calculated using the equivalent source and impedance. This may not be the case since one will need to calculate the individual contribution of the fault sources depending whether current path to the fault location are ‘meshed’ or ‘non-meshed’.

‘Non-meshed’ Current Paths

Unlike ANSI/IEEE, IEC 60909 uses multiplying factors specific for each individual source contribution such that the total short-circuit current at the fault location, may it be the initial, peak, breaking, or steady-state, is the sum of the respective individual contribution from the fault sources. In figure 2, the total short-circuit current is the sum of the individual currents, I₁, I₂, and I₃.

‘Meshed’ Current Paths

This is more like the ANSI/IEEE methodology, the impedance to the fault forms a meshed network such that the calculation of the short-circuit current uses the equivalent system impedance at the fault point instead. When calculating short-circuits, it is important to identify whether a source contributes to the fault through a ‘meshed’ or a ‘non-meshed’ current path. This is very important especially in the calculation of multiplying factors since, AGAIN, they are specific for each individual source contribution to the total short-circuit current at the fault location, may it be the initial, peak, breaking, or steady-state.

Figure 3 shows fault current sources from a ‘meshed’ and ‘non-meshed’ current paths. The contribution from the ‘meshed’ network which is I₁ should be determined from the equivalent impedance representing the ‘meshed’ network. The total short-circuit current is then calculated as the sum of the individual currents, I₁, I₂, and I₃.

Before we end…

…one bit of advise, when conducting short-circuit analysis using IEC 60909, always identify the ‘meshed’ current paths and then resolve them into an equivalent system impedance at the fault point. You would want to work with a simple network diagram such as the one shown in figure 2, right? Then you should always find a way to transform everything into ‘non-meshed’ current paths. This way, you can calculate the individual short-circuit contribution from different sources. AGAIN, multiplying factors are specific for each individual source contribution to the total short-circuit current at the fault location, may it be the initial, peak, breaking, or steady-state.

References

IEEE Std 551-2006 [The Violet Book]: Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems. (2006). S.I.: IEEE.

ETAP Enterprise Solution for Electrical Power Systems Online Help

Rodolakis, A. J. (1993). A Comparison of North American (ANSI) and European (IEC) Fault Calculation Guidelines. IEEE Transactions on Industry Application, 29(3). Retrieved from https://ieeexplore.ieee.org.