In a three phase power system, the type of faults that can occur are classified by the combination of conductors or buses that are faulted together. In addition, faults may be classified as either bolted faults or faults that occur through some impedance such as an arc. Each of the basic types of faults will be described and shown in Figure 1.
It must be noted that in a majority of cases, the fault current calculation required for the selection of interrupting and withstand current capabilities of equipment is the three phase bolted fault with zero impedance.
Let’s go through each of the four three phase faults //
- Three phase bolted faults
- Bolted line-to-line faults
- Line-to-line-to-ground faults
- Line-to-ground faults
A three phase bolted fault describes the condition where the three conductors are physically held together with zero impedance between them, just as if they were bolted together. For a balanced symmetrical system, the fault current magnitude is balanced equally within the three phases.
While this type of fault does not occur frequently, its results are used for protective device selection, because this fault type generally yields the maximum short-circuit current values.
Figure 1(a) provides a graphical representation of a bolted three phase fault.
Bolted line-to-line faults, Figure 1(b), are more common than three phase faults and have fault currents that are approximately 87% of the three phase bolted fault current.
This type of fault is not balanced within the three phases and its fault current is seldom calculated for equipment ratings because it does not provide the maximum fault current magnitude. The line-to-line current can be calculated by multiplying the three phase value by 0.866, when the impedance Z1 = Z2.
Special symmetrical component calculating techniques are not required for this condition.
Line-to-line-to-ground faults, Figure 1(c), are typically line-to-ground faults that have escalated to include a second phase conductor. This is an unbalanced fault. The magnitudes of double line-to-ground fault currents are usually greater than those of line-to-line faults, but are less than those of three phase faults.
Calculation of double line-to-ground fault currents requires the use of symmetrical components analysis. The impedance of the ground return path will affect the result, and should be obtained if possible.
Line-to-ground faults, Figure 1(d), are the most common type of faults and are usually the least disturbing to the system. The current in the faulted phase can range from near zero to a value slightly greater than the bolted three phase fault current.
Calculation of the exact line-to-ground fault current magnitudes requires the special calculating techniques of symmetrical components.
However, close approximations can be made knowing the method of system grounding used. On ungrounded distribution systems, the line-to-ground fault currents are near zero.
Line-to-ground fault current magnitudes in distribution systems with a solidly grounded system will be approximately equal to the three phase fault current magnitudes. Determining line-to-ground fault currents on long cable runs or transmission lines will require detailed ground return path impedance data and detailed calculation techniques.
Reference // IEEE Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems