Determining AC Fault Current
AC fault current available from an electrical source can follow unintended paths back to the source, affecting cathodically protected structures. A solid, metallic connection between the faulting source and the cathodically protected structure can result in heavy current flow, but even an isolated pipeline or structure can pick up moderate currents flowing in the soil. This latter case is an example of a pipeline in a common corridor with a power line.
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In all cases, some value of phase-to-ground AC fault current must be determined. Some applications may not have notable AC fault current exposure, and the lowest standard fault rating may be chosen. Applications are listed below, with suggested methods of determining the AC fault current for each.
AC Fault Current by Application
For a discussion of AC fault current data, choose your site description or application:
- Insulated Joint Protection
- AC Voltage Mitigation on Pipelines
- Decoupling Electrical Equipment (e.g. motor-operated valve)
- Ship-to-Shore Galvanic Isolation
- Decoupling Power Company Ground from User Site/Station Ground
- Stray Voltage Mitigation at Farms (Primary-to-Secondary Isolation)
- Lead-Sheath or Pipe-Type Power Cable Decoupling from Substation Ground
- Blocking GIC/Transformer Neutral Decoupling
Exposure level: Since insulated joints are not usually directly tied via metallic connection to an AC faulting source, the fault currents are usually low to moderate. Higher exposure exists for insulated joints on pipelines serving power plants.
Calculation method:
A) For insulated joints in a common corridor with power lines, specialized consultants can model the expected fault current. DEI can recommend such AC mitigation specialists.
B) For common corridor sites with no other calculations available, a suitable percentage of the full power company fault current can be used as a guideline. Contact the power company transmission engineering department and ask for the maximum phase-to-ground fault current magnitude and duration at the location of interest. Compare DEI product ratings to the value, selecting a suitable rating percentage. Contact DEI if assistance is needed.
C) For sites not in a common corridor with power lines, and not in metallic contact with AC fault sources, the lowest DEI product AC fault rating of 3.7kA (for 0.5 sec at 60Hz, or 3.5kA at 50Hz) can be chosen.
AC Voltage Mitigation on Pipelines
Exposure level: Unless the pipeline has a direct, metallic connection to a power line grounding system (which is not recommended), the fault current is kept to low to moderate levels under most conditions due to the resistance offered by the pipeline coating and soil. Direct metallic bonding of the pipeline to the transmission tower will greatly increase the exposure of the pipeline and coatings to fault current and lightning, and is not recommended.
Calculation method: Manual calculations, or more commonly, analysis using specialized software, are used to determine the fault current magnitude that can be picked up by a pipeline system. Specialized consultants can assist in performing this analysis. An alternate method for estimation can be to use a percentage of the power company's maximum phase-to-ground fault current magnitude and typical duration. Contact the power company transmission or distribution engineering department to acquire this maximum value at the location of interest.
Decoupling Electrical Equipment (e.g. motor-operated valves)
Also for: Ship-to-Shore Galvanic Isolation
Exposure level: The grounding conductor of an AC voltage system is designed to carry the full phase-to-ground fault current in the event of a fault (e.g. shorted conductors or motor winding failures). Placed in this grounding conductor, the DEI device would be exposed to the entire available fault current, and should be rated for such. Certain DEI devices are certified for use in grounding conductors for electrical equipment.
Calculation method: The full available fault current in the AC circuit in question should be used. The methods for sizing include (in order of preference):
A) using the fuse or breaker clearing curve for the circuit in question,
B) comparison of the grounding conductor to fault withstand graphs for cables, or
C) calculation of the fault current at the transformer (worst case).
Each method is described below.
A) Comparison to fuse/breaker clearing curves
The next clearing device, such as a fuse or breaker, upstream (toward the source) of the circuit of interest can be compared to the DEI device ratings. The fuse or breaker "total clearing time" curve should be used, and a DEI device rating should be chosen to exceed these curve values. On a clearing curve graph, the DEI device ratings should appear to the right of the fuse/breaker clearing curve. In the case of a ship where upstream breaker data is not known, the main shore power breaker on the ship could be used to obtain an estimate of the AC fault current.
B) Comparison of the grounding conductor to fault withstand graphs for cables
Device ratings can also be indirectly estimated by comparison to the AC fault capability of the conductor used on the circuit of interest. This is based on the assumption that the conductor was originally sized correctly for the available fault current, and will generally result in a conservative selection. Locate the conductor size used on your circuit, then compare the current/time values in the withstand graph to the DEI device, choosing a product rating that approximates or exceeds the conductor rating. As this is an indirect method of fault sizing, it may be appropriate in some cases to have a DEI product rated below the conductor withstand value, if other factors, such as total conductor length, indicate that the fault current is substantially lower than the maximum conductor withstand.
C) Calculations based on the source fault current at the transformer
Unless the user has fault data for the exact circuit of interest, a conservative estimate can be achieved by starting with the fault current at the site transformer. If desired, further refinement can be done by calculating current reductions due to conductor impedance. This method is most useful for smaller electrical facilities; use at larger facilities may provide overly conservative values.
The maximum fault current available at the transformer terminals is determined by the following formula, using data from the transformer nameplate, which can be supplied by the power company or a site engineer.
| Secondary full load current = |
Single Phase Transformer kVA / (Secondary kV)
|
OR
| Secondary full load current = |
Three phase transformer kVA / (Secondary kV)(√3)
|
Then
| Secondary fault current = | Secondary full load current x 100 / Percent transformer impedance |
This is the maximum fault current at the transformer, which will be reduced by conductor impedance. This reduction can be estimated using wire characteristics for various cross-sections and lengths.
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Decoupling Power Company Ground from User Site/Station Ground
Exposure level: A DEI device connected as the sole bond between a power company grounded neutral system and the secondary grounded neutral system at a station or other site with cathodic protection will be exposed to, and should be rated for, the full primary fault current from the power company distribution system.
Calculation method: The primary phase-to-ground fault current (magnitude and duration) at the transformer should be requested from the power company's distribution engineering department. If this is not available, the fuse clearing curve can be requested from the utility. A DEI product rating should be chosen that exceeds the "total clearing time" of the fuse or breaker. A graph of current versus time values should result in the DEI device to the right of the clearing device curve.
Stray Voltage Mitigation at Farms (Primary-to-Secondary Isolation)
Exposure level: A DEI device connected as the sole bond between a power company grounded neutral system and the secondary grounded neutral system at a farm or residence will be exposed to, and should be rated for, the full primary fault current from the power company distribution system.
Calculation method: The primary phase-to-ground fault current (magnitude and duration) at the transformer should be requested from the power company's distribution engineering department. If this is not available, the fuse clearing curve can be requested from the utility. A DEI product rating should be chosen that exceeds the "total clearing time" of the fuse or breaker. A graph of current versus time values should result in the DEI device to the right of the clearing device curve.
Lead-Sheath or Pipe-Type Power Cable Decoupling from Substation Ground
Exposure level: A DEI decoupler is typically used at each end of a power cable circuit, between the sheath/pipe and substation grounding system. As the sole grounding path, the DEI device may see the full phase-to-ground fault current on the transmission system, and should be rated for this value.
Calculation method: As this would likely apply only to transmission cable systems owned by power companies, the power company can turn to their transmission engineering department for the phase-to-ground fault current magnitude and duration. These values have already been modeled for the transmission system, and are readily available. Choose a DEI device AC fault rating that exceeds the modeled system fault current.
Blocking GIC/Transformer Neutral Decoupling
Exposure level: The grounded transformer neutral that is affected by GIC or DC currents would have a DEI decoupler placed in series in the grounding conductor. This subjects the decoupler to the full fault current available in the neutral.
Calculation method: The full available fault current should be used, which can be obtained from the power company substation or transmission engineering department for the phase-to-ground fault current magnitude and duration. These values should be readily available. Choose a DEI device AC fault rating that exceeds the modeled system fault current.