IEC 60909: ‘Near’ Generator Short Circuit Calculation
We talked about the ‘Far from’ generator short circuit calculation in our previous article but we never mentioned the conditions to consider a short circuit to be ‘far from’ or ‘near’ generator.
So really, when do we consider a short circuit to be ‘far from’ or ‘near’ generator?
According to IEC 60909, a short circuit is considered to ‘near’ generator when at least one synchronous machine contributes a current exceeding twice its rated current, I”k/IrG > 2, or synchronous and asynchronous motors contribute more than 5% of the initial short circuit current calculated without considering any motors.
Computational Significance for Generator Short Circuit Calculation
‘Near’ generator short circuit considers AC decrement in the subsequent calculation of short circuit currents. This means that the ‘near’ generator steady-state short circuit currents have smaller magnitude than the symmetrical short circuit breaking currents. These breaking currents are, in turn, smaller in magnitude than the initial short circuit currents.
The calculation of the initial and peak short circuit currents for ‘near’ generator short circuits is the same as the ‘far from’ generator short circuits. In determining the breaking and steady-state short circuit currents for ‘near’ generator short circuits, additional steps are required unlike ‘far from’ generator short circuits were initial, breaking, and steady-state short circuit currents are equal (I”k = Ib = Ik). It is important to determine which generators are ‘far from’ or ‘near’ the fault so that the additional steps in the calculation of the breaking and steady-state currents are only left to ‘near’ generator short circuit.
Symmetrical Short Circuit Breaking Current (Ib)
In the calculation of symmetrical short circuit breaking current for ‘near’ generator short circuit, AC decrement is accounted for by introducing a factor µ as shown in the equation below.
As with ANSI-approved standards, the breaking current depends on the contact parting time of the protective device or the minimum time delay tmin in IEC 60909 terms. This can be seen from the factor µ. The factor µ also depends on the ratio of generator initial short circuit current and rated current, I”kG/IrG. The following equations define the factor µ for a particular minimum time delay. For other values of minimum time delay, linear interpolation between curves is acceptable.
These equations apply to turbo generators, salient-pole generators and synchronous compensators excited by either rotating or static converters (provided, for static exciters, the minimum time delay is less than 0.25s and the maximum excitation voltage is less than 1.6 times rated load excitation-voltage). For all other cases, µ = 1.
It is also worth noting that the prerequisite for the identification of ‘far from’ or ‘near’ generator short circuits is preserved in the factor µ in that if the ratio I”kG/IrG is not greater than 2, µ is set to 1. This will set the breaking current equal to the initial short circuit current, a characteristic of ‘far from’ generator short circuit.
For short circuits involving ‘meshed’ current paths, determining µ from a single equivalent ratio I”kG/IrG is not applicable. In this case, setting the symmetrical short circuit breaking current equal to the initial short circuit current is permitted. This will affect the accuracy though will be more conservative.
The symmetrical short circuit breaking current contribution from asynchronous motors are quantified by introducing an additional factor q and replacing the ratio I”kG/IrG with I”kM/IrM. The factor q takes into account the rapid decay of the motor short circuit due to the absence of an excitation field.
where
PrM is the rated active power in MW
p is the number of pairs of poles of the motor
Take note that the factor q is limited to 1.
The total symmetrical short circuit breaking current is the sum of the contribution from individual sources,
Steady-state Short Circuit Current (Ik)
Steady-state short circuit current for ‘near’ generator short circuits is normally lower in magnitude than the symmetrical short circuit breaking current. It depends on the excitation system, the voltage regulator action, and saturation influences. Synchronous machines with static exciters fed directly from its terminals has zero steady-state contribution for short circuits on its terminals. This is because the field voltage collapses with the terminal voltage during fault. They only contribute to the steady-state short circuit if the there is an impedance between its terminals and the fault location, e.g., faults on the high-voltage side of the unit transformer in the case of power station units.
The calculation of the steady-state short circuit current is rather straightforward in that it depends only on the generator rated current and the excitation voltage. However, the procedures presented are only accurate for the case of one generator or power station unit supplying the fault. Maximum and minimum values are calculated in order to provide the range of the steady-state short circuit contribution. The minimum steady-state short circuit current is calculated based on a constant and unregulated excitation voltage using the following equation.
The maximum steady-state short circuit current is calculated based on maximum excitation voltage using the following equation.
The multiplying factor λmax depends on the whether the generator is a turbo or salient-pole generator; and the ratio of the maximum excitation voltage to the excitation voltage under normal load conditions (series 1 or 2).
Series 1 multiplying factor λmax is based on the highest possible excitation voltage which is 1.3 times the rated excitation voltage at rated apparent power and power factor for turbo generators or 1.6 times the rated excitation voltage at rated apparent power and power factor for salient-pole generators.
Series 2 multiplying factor λmax is based on the highest possible excitation voltage which is 1.6 times the rated excitation voltage at rated apparent power and power factor for turbo generators or 2.0 times the rated excitation voltage at rated apparent power and power factor for salient-pole generators.
References
BS EN 60909-0:2001: Short-circuit currents in three-phase a.c. systems Part 0: Calculation of currents (2002).
Kaskci, I. (2002). Short Circuit in Power Systems: A Practical Guide to IEC 60909. Weinheim, Germany: Wiley-VCH Verlag-GmbH.
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