Essential Differences Between IEC 60909 and ANSI/IEEE C37 Series
Before we start our discussion on IEC 60909, let us first review the concepts in ANSI/IEEE C37 series on short-circuit study.
Our previous discussions on short-circuit analysis and circuit breaker selection were grounded on the ANSI/IEEE Standards. We started with a brief overview of power systems faults and introduced the widely accepted equation for a short-circuit current. Then we moved to the derivation of ANSI/IEEE multiplying factors for the circuit breaker closing and latching duty.
Next, we discussed several concepts affecting the calculation of circuit breaker interrupting duty for medium- and high- voltage circuit breakers such as circuit breaker rated cycle, AC and DC decay, separate X and R networks, remote and local fault contributions, and circuit breaker rating structure. After that, we moved on to the calculation of kAIC rating for low voltage circuit breakers. We wrapped the whole series with a step-by-step manual calculation and computer simulation using ETAP software.
In this series, we will try to explore how short-circuit calculations are addressed by other International Standards. We will focus our discussion on IEC short-circuit calculation methods particularly the IEC 60909. Before diving into the calculations, it is recommended to review the following terms used in IEC 60909 and how they relate to the short-circuit currents defined in ANSI/IEEE standards. This way, we’ll be able to understand IEC 60909 better.
Maximum Short-circuit Current (Imax)
The short-circuit current used to evaluate circuit breaker interrupting and peak rating derived using the voltage correction factor, Cmax.
Minimum Short-circuit Current (Imin)
The short-circuit current used as basis for protective relay settings derived using the voltage correction factor, Cmin.
IEC 60909 recommends applying ‘Voltage Factors’, Cmax and Cmin, to the prefault nominal system voltage in order to account for the system prefault conditions. The following table shows the voltage factor for different nominal system voltages.
In ANSI/IEEE standards, prefault load currents are neglected since they are assumed to be of much smaller magnitude than the short-circuit currents. Therefore, prefault voltages are assumed to be rated system voltages. In other words, ANSI/IEEE standard assumes a voltage factor of 1.0. However, IEEE Std 551 allows the use of the operating voltage under actual conditions as the prefault voltage which could exceed the customarily assumed 1.0 pu.
Initial Short-circuit Current (I”k)
The RMS value of the symmetrical short-circuit current at the instant of the fault given that the impedance does not change. In ANSI/IEEE, this is equivalent to the ½-cycle RMS symmetrical short-circuit current or the value of the short-circuit current based on the subtransient network impedances.
Peak Short-circuit Current( Ip)
The maximum instantaneous value of the short-circuit current. This is equivalent to the closing and latch peak duty in ANSI/IEEE standard.
Symmetrical Short-Circuit Breaking Current (Ib)
The RMS value of an integral cycle of the symmetrical AC component of the short-circuit current at the instant of circuit breaker contact separation. In ANSI/IEEE standard, this value is equivalent to the interrupting duty.
Steady-state Short-circuit Current (Ik)
The RMS value of the short-circuit current that remains after the transient component has completely decayed. In ANSI/IEEE, this pertains to the RMS value of the symmetrical short circuit current at >30 cycles derived from the steady-state network.
References
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.
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