The fault that sizes the grid is not always the one with the largest total current: it is the one that leaves the largest current between the grid and the soil, combined with the clearing time that gives the lowest tolerable voltages. IEEE 80 recommends studying only the single line-to-ground and the double line-to-ground faults, inside and outside the substation.
Short answer
To size a substation grid, IEEE Std 80-2013 asks you to find the fault that produces the largest grid current (IG) and also to check the clearing time of each fault, because slow clearing reduces the voltages a person can tolerate. In practice two fault types are studied, the single line-to-ground and the double line-to-ground, at the location that sends the most current through the grid into the soil. There is no universal rule for the location: the system has to be analyzed. The duration of the fault is covered in fault clearing time and grid safety. With that current the substation grounding design with IEEE 80 continues.
Which faults are studied
Many types of fault can occur in a system, and the standard recognizes that there is no simple rule for knowing which one produces the largest current flow between the grid and the soil. It therefore recommends considering the probability of occurrence: simultaneous multiple faults can give larger currents, but they need not be considered if their probability is negligible. For practical reasons, the study is limited to:
- Single line-to-ground fault.
- Double line-to-ground fault.
In both, what matters for the grid is the zero-sequence current: it is the one that returns through the earth. The current sent into the soil is three times that zero-sequence current; the guide zero-sequence current explains what it is and how it originates.
How the fault type is chosen
The worst fault type for a grid is the one that gives the largest ground current, because the maximum grid current is proportional to it and current division is almost independent of the fault type.
The standard gives a criterion using the positive-, negative- and zero-sequence impedances at the fault point:
| Condition | Most severe fault |
|---|---|
| Zero-sequence impedance greater than positive (with negative equal to positive) | Single line-to-ground |
| Zero-sequence impedance lower than positive (with negative equal to positive) | Double line-to-ground |
Assuming that the negative-sequence impedance equals the positive is acceptable in most calculations, especially when a good part of the positive-sequence reactance up to the fault corresponds to static equipment and transmission lines.
Zero-sequence impedance depends on the transformer connection, as explained in zero-sequence impedance by transformer connection and in symmetrical components and the zero-sequence diagram.
Where the most severe fault occurs
The worst location can be on the high-voltage or the low-voltage side of the transformer, and inside the substation or outside, on a line at some distance. A fault is considered inside the substation when it involves a metallic structure bonded to the grid with negligible impedance.
The standard gives no universal rules and presents some possibilities:
- Distribution substation with the transformer grounded only on the distribution side. The maximum grid current usually occurs with a ground fault at the high-voltage terminals of the transformer. If the high-side fault source is weak, or if several transformers in parallel form a strong source on the low side, the maximum can occur with a fault on the distribution circuit.
- Fault at the low-voltage terminals of that transformer. The transformer contribution circulates through the grid conductors with negligible leakage into the soil, and does not affect the substation potential rise.
- Fault outside the substation, on a remote feeder. If the fault is far enough to be considered at remote earth with respect to the grid, a good part of the current returns to its source, the transformer neutral, through the grid, and contributes to the potential rise.
The standard illustrates the cases with four figures: a fault inside the substation with the local neutral grounded, with the neutral grounded at a remote point, with the system grounded at the substation and at other points, and a fault on the high side of a distribution substation with its typical current division.
Clearing time: the second half of the question
The duration of the fault depends on the protection scheme, the location of the fault and whether the primary or the backup clearing time is taken. That duration affects the decrement factor and also the tolerable touch and step voltages.
That is why the most severe fault for the grid is not always the one with the largest current. An example from the standard: a grounded delta-wye transformer fed from a weak source, with a fault several kilometers away on a rural feeder. The high-side current can be low, and if clearing depends on the backup of the high-voltage device on failure of the feeder breaker, it can take several seconds. The tolerable voltage for that case can be much lower than for a fault on the high side, and the feeder becomes the governing case in the design.
- Does the fault occur inside the substation or outside, at remote earth?
- Inside: Fault resistance equal to zero; it is the only safe assumption
- Yes: The single line-to-ground fault governs
- No: The double line-to-ground fault governs
- Outside: The current returns through the grid to the neutral; backup clearing is also evaluated
- Yes: This fault can govern even without the maximum current
- No: The fault with the largest grid current governs
- Inside: Fault resistance equal to zero; it is the only safe assumption
What the engineer decides
With the candidate faults defined, for each one the fault current, the split factor, the decrement factor according to its clearing time and the tolerable voltages are calculated. The combination that is most demanding for the grid is chosen. The starting data come from the short-circuit study, and the times from protection coordination.
How we do it at Tiza Group
Choosing the design fault requires modeling the network with its transformer connections, its lines with shield wires and the real clearing times. At Tiza Group we do that analysis within the substation grounding study, together with the short-circuit study and protection coordination, remotely and worldwide. See our engineering design and studies service or write to us.
Related guides
- IEEE 80: Substation Grounding Safety
- Maximum Grid Current (IG) and Split Factor
- Decrement Factor Df and Fault Asymmetry
- Short-Circuit Study: Sources, IEC 60909 and ANSI/IEEE
- Substation Grounding Design: IEEE 80 Method
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 15.3 and 15.8.
Frequently asked questions
How do you know whether the single line-to-ground or the double line-to-ground fault is the worst?
The zero-sequence impedance is compared with the positive-sequence impedance at the fault point. With negative-sequence impedance equal to positive, which is the usual case, the single line-to-ground fault is the worst if the zero-sequence impedance is greater than the positive, and the double line-to-ground fault is the worst if it is lower.
Is a fault inside the substation always worse than one outside it?
No. In a distribution substation with the transformer grounded only on the distribution side, the maximum grid current usually occurs with a fault at the high-voltage terminals of the transformer. But if the high-side source is weak or there are several transformers in parallel, it can occur with a fault on the distribution circuit.
Can the worst fault be one that does not have the largest grid current?
Yes. If clearing of a fault is slow, for example a remote fault on a rural feeder that depends on high-voltage backup, the tolerable voltages drop so much that this fault can be the most severe even though its grid current is not the maximum.