Short-circuit studies are as necessary for any power system as other fundamental system studies such as power flow studies, transient stability studies, harmonic analysis studies, etc. Short-circuit studies can be performed at the planning stage in order to help finalize the system layout, determine voltage levels, and size cables, transformers, and conductors.
For existing systems, fault studies are necessary in the cases of added generation, installation of extra rotating loads, system layout modifications, rearrangement of protection equipment, verification of the adequacy of existing breakers, relocation of already acquired switchgear in order to avoid unnecessary capital expenditures, etc. “Post-mortem” analysis may also involve short-circuit studies in order to duplicate the reasons and system conditions that led to the system’s failure.
The requirements and extent of a short-circuit study will depend on the engineering objectives sought. In fact, these objectives will dictate what type of short-circuit analysis is required. The amount of data required will also depend on the extent and the nature of the study.
The great majority of short-circuit studies in industrial and commercial power systems address one or more of the following four kinds of short circuits:
— Three-phase fault. May or may not involve ground. All three phases shorted together.
— Single line-to-ground fault. Any one, but only one, phase shorted to ground.
— Line-to-line fault. Any two phases shorted together.
— Double line-to-ground fault. Any two phases connected together and then to ground.
These types of short circuits are also referred to as “shunt faults,” since all four exhibit the common attribute of being associated with fault currents and MVA flows diverted to paths different from the prefault “series” ones.
Three-phase short circuits often turn out to be the most severe of all. It is thus customary to perform only three phase-fault simulations when seeking maximum possible magnitudes of fault currents.
However, important exceptions do exist. For instance, single line-to-ground short-circuit currents can exceed three-phase short-circuit current levels when they occur in the vicinity of
— A solidly grounded synchronous machine
— The solidly grounded wye side of a delta-wye transformer of the three-phase core (three-leg) design
— The grounded wye side of a delta-wye autotransformer
— The grounded wye, grounded wye, delta-tertiary, three-winding transformer
For systems where any one or more of the above conditions exist, it is advisable to perform a single line-to-ground fault simulation. The fact that medium- and high-voltage circuit breakers have 15% higher interrupting capabilities for single line-to-ground faults should be taken into account, if elevated single line-to-ground fault currents are found.
Line-to-line or double line-to-ground fault studies may also be required for protective device coordination requirements. It should be noted that, since only one phase of the line-to-ground fault can experience higher interrupting requirements, the three-phase fault will still contain more energy because all three phases will experience the same interrupting requirements.
Other types of fault conditions that may be of interest include the so-called “series faults” (Anderson [B1]) and pertain to one of the following types of system unbalances:
— One line open. Any one of the three phases may be open.
— Two lines open. Any two of the three phases may be open.
— Unequal impedances. Unbalanced line impedance discontinuity.
The term “series faults” is used because the above unbalances are associated with a redistribution of the prefault load current. Series faults are of interest when assessing the effects of snapped overhead phase wires, failures of cable joints, blown fuses, failure of breakers to open all poles, inadvertent breaker energization across one or two poles and other situations that result in the flow of unbalanced currents.