IEC 60909: ‘Far from’ Generator Short-Circuit

We started our discussion on IEC 60909 by introducing the different terms used in the standard and compared them to ANSI-approved standards. Then we presented the concept of ‘meshed’ and ‘non-meshed’ currents paths and its significance in the calculation of initial, peak, breaking, and steady-state short-circuit currents. In this article, we will go deeper into IEC 60909 by introducing the concept of ‘far from’ and ‘near’ generator short-circuit.

Let’s start by reviewing our discussion on short-circuit analysis according to ANSI-approved standards. The concept of ‘local’ and ‘remote’ multiplying factors was introduced in order to account for the AC and DC decrement in the calculation of the interrupting duty at the fault point. While the local multiplying factors account for both AC and DC decrement, remote multiplying factors account only for the DC decrement. The calculation of the interrupting duty takes in the application of a weighted multiplying factor accounting for the ‘local’ and ‘remote’ generators through the concept of ‘No AC Decay’ ratio or NACD.

So how does this relate to ‘far from’ and ‘near’ generator short-circuit in IEC 60909?

The calculation of initial, peak, breaking, and steady-state short currents depend on the application of the correct multiplying factor. As was discussed in our previous article on IEC 60909, 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. The application of the correct multiplying factor depends on whether the fault is ‘far from’ or ‘near’ generator.

‘Far from’ Generator Short-circuit

As with the definition of the remote source in ANSI-approved standards, ‘far from’ generator short-circuit presents no AC decrement. In other words, the magnitude of the symmetrical AC component of the fault remains constant. Let’s look at the fault currents with ‘far from’ generator short-circuits.

Initial Short-circuit Current (I”_k)

For each individual source contribution with impedance, Z_n, for ‘non-meshed’ current paths or the complex equivalent impedance for a ‘meshed’ current path, the initial short-circuit current is calculated using

The total initial short-circuit current is the sum of the contribution from individual sources,

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Peak Short-circuit Current (I_P)

IEC 60909 defines the peak short-circuit current as the maximum instantaneous value of the short-circuit current. This is equivalent to the closing and latch peak duty in ANSI-approved standards. Peak short-circuit current is calculated by introducing a crest factor, Κ, to the initial short-circuit current, I”_k.

The determination of the crest factor rests on whether the fault is coming from ‘non-meshed’ or ‘meshed’ current paths. Click here to review the concept of current paths.

‘Non-meshed’ Current Paths

For each individual source contribution, the crest factor is calculated using

The crest factor is dependent on the X/R ratio of the impedance of the between the source and the fault.

‘Meshed’ Current Paths

As with the ‘non-meshed’ current paths, the calculation of the crest factor for involving ‘meshed’ current paths is somewhat the same except for the choice of the X/R ratio. IEC 60909 suggests three methods to determine the crest factor involving ‘meshed’ current paths. These are the following:

• Dominant X/R ratio
  • The X/R ratio is taken from the branch with the highest X/R ratio but given that this branch carries at least 80% of the fault current contribution.
• Equivalent X/R ratio
  • The X/R ratio is taken from the complex equivalent impedance with the crest factor calculated using

• Equivalent Frequency
  • The X/R ratio is calculated according to the following equation,

It is important to note that the crest factor involving ‘meshed’ current paths is limited to 1.8 and 2.0 for low- and high-voltage networks, respectively.

The total peak short-circuit current is the sum of the contribution from individual sources,

Symmetrical Short-circuit Breaking Current (I_b)

The calculation of the symmetrical short-circuit breaking current for ‘far from’ generator faults is quite straightforward since no AC decrement is present. With this, the breaking current is equal to the initial short-circuit current.

The total symmetrical short-circuit breaking current is the sum of the contribution from individual sources,

Steady-state Short-circuit Current (I_k)

For ‘far from’ generator faults, the symmetrical AC component of the short-circuit currents remains the same throughout the duration of the fault. Thus, the steady-state short-circuit current is equal to the initial short-circuit current.

The total steady-state short-circuit current is the sum of the contribution from individual sources,

Short-circuit analysis in IEC 60909 involving only ‘far from’ generators faults is very simple and straightforward because of the absence of the AC decrement. However, for ‘near’ generator faults, AC decrement becomes significant. In our next discussion, we will focus on the calculation of short-circuit currents for ‘near’ generators faults.

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.

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