|Courtesy of Tritech|
Short-circuit current sources and the impedances of these sources must be considered in the design of a coordinated power system protection. Below are sources of short circuit currents.
When a short-circuit occurs downstream of a synchronous generator, the generator may continue to produce output voltage and current if the field excitation is maintained and the prime mover continues turning the generator at synchronous speed.
The flow of short circuit current from the generator into the fault is limited only by the generator impedance and downstream circuit impedances. The magnitude of generator fault current depends on the armature and field characteristics, the time duration of the fault, and the load on the generator. The ability of a generator to supply current during a fault is a function of the excitation system.
(1) Some generator excitation systems do not have the ability to sustain short-circuit current. The magnitude of fault current is determined by the generator reactance, and, for such systems, can be essentially zero in 1.0 to 1.5 seconds.
(2) Static exciters derive excitation voltage from the generator terminals. Since static exciters do not sustain short-circuit current, protective devices on the system will not operate properly, or at all. Static exciters, therefore, are not recommended. Static exciters with current boost should be specified for applications requiring static excitation.
(3) Round-rotor generators with brushless exciters, typically above 10 MVA, can sustain short circuit current for several seconds. Salient-pole generators less than 10 MVA, also with brushless exciters, will typically sustain short-circuit current at 300 percent of generator full load amperes.
When a short-circuit occurs upstream of a synchronous motor, the system voltage goes to zero, and the motor begins losing speed. As the motor slows down, the inertia of the load is actually turning the motor and causing it to act like a generator.
The synchronous motor has a dc field winding, like a generator, and actually delivers short-circuit current into the fault until the motor completely stops. As with a generator, the short-circuit current is limited only by the synchronous motor impedance and the circuit impedance between the motor and the fault.
With one slight difference, a short-circuit upstream of an induction motor produces the same effect as with a synchronous motor. Since the induction motor has no dc field winding, there is no sustained field current in the rotor to provide flux as is the case with a synchronous machine. Consequently, the short-circuit current decays very quickly.
Supply transformers are not sources of short-circuit current. Transformers merely deliver short circuit current from the utility generators to the fault point. In the process, transformers change the voltage and current magnitudes. Transformer impedances will also limit the amount of short-circuit current from the utility generators.
Standard tolerance on impedance is plus or minus 7.5 percent for two-winding transformers and plus or minus 10 percent for three winding transformers. The minus tolerance should be used for short circuit studies and the plus tolerance for load flow and voltage regulation studies.