Short-circuit current
1 Terms and Definitions
The following terms and definitions correspond largely to those defined in IEC 60 909. Refer to this standard for all terms not used in this book. The terms short circuit and ground fault describe faults in the isolation of operational equipment which occur when live parts are shunted out as a result.
? Causes:
1. Overtemperatures due to excessively high overcurrents. 2. Disruptive discharges due to overvoltages.
3. Arcing due to moisture together with impure air, especially on insulators.
? Effects:
1. Interruption of power supply. 2. Destruction of system components.
3. Development of unacceptable mechanical and thermal stresses in electrical operational equipment. ? Short circuit:
According to IEC 60 909, a short circuit is the accidental or intentional conductive connection through a relatively low resistance or impedance between two or more points of a circuit which are normally at different potentials.
? Short circuit current:
According to IEC 60 909, a short circuit current results from a short circuit in an electrical network.
It is necessary to differentiate here between the short circuit current at the position of the short circuit and the transferred short circuit currents in the network branches.
? Initial symmetrical short circuit current:
This is the effective value of the symmetrical short circuit current at the moment at which the short circuit arises, when the short circuit impedance has its value from the time zero. ? Initial symmetrical short circuit apparent power:
The short circuit power represents a fictitious parameter. During the planning of networks, the short circuit power is a suitable characteristic
number.
? Peak short circuit current:
The largest possible momentary value of the short circuit occurring. ? Steady state short circuit current:
Effective value of the initial symmetrical short circuit current remaining after the decay of all transient phenomena. ? DC aperiodic component:
Average value of the upper and lower envelope curve of the short circuit current, which slowly decays to zero. ? Symmetrical breaking current:
Effective value of the short circuit current which flows through the contact switch at the time of the first contact separation. ? Equivalent voltage source:
The voltage at the position of the short circuit, which is transferred to the positive-sequence system as the only effective voltage and is used for the calculation of the short circuit currents. ? Superposition method:
The superposition method considers the previous load of the network before the occurrence of the short circuit. It is necessary to know the load flow and the setting of the transformer step switch. ? Voltage factor:
Ratio between the equivalent voltage source and the network voltage Un,divided by 3.
? Equivalent electrical circuit:
Model for the description of the network by an equivalent circuit. ? Far-from-generator short circuit:
The value of the symmetrical AC periodic component remains essentially constant.
? Near-to-generator short circuit:
The value of the symmetrical AC periodic component does not remain constant. The synchronous machine first delivers an initial symmetrical short circuit current which is larger than twice the rated current of the synchronous machine.
? Positive-sequence short circuit impedance:
The impedance of the positive-sequence system as seen from the position of the short circuit.
? Negative-sequence short circuit impedance:
The impedance of the negative-sequence system as seen from the position of the short circuit.
? Zero-sequence short circuit impedance
The impedance of the zero-sequence system as seen from the position of the short circuit. Three times the value of the neutral point to ground impedance occurs here. ? Short circuit impedance:
Impedance required for calculation of the short circuit currents at the position of the short circuit.
For the same external conductor voltages, a three-pole short circuit allows three currents of the same magnitude to develop between the three conductors. It is therefor only necessary to consider one conductor in further calculations. Depending on the distance from the position of the short circuit from the generator, here it is necessary to consider near-to-generator andfar-from-generator short circuits separately.
For far-from-generator and near-to-generator short circuits, the short circuit path can be represented by a mesh diagram with AC voltage source, reactances X and resistances R (Figure 1.2). Here, X and R replace all components such as cables,conductors, transformers, generators and motors.
Fig. 1.2: Equivalent circuit of the short circuit current path in
the positive-sequence system
The following differential equation can be used to describe the short circuit
p??? 1.2 Short circuit path in the positive-sequence system
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