There is a potential issue that can impact installations where a 44-kV 3-wire feeder ends up sharing poles, shortly after exiting the station, with an underbuilt 4-wire feeder (hence carrying a multi-grounded neutral) supplied from elsewhere. Thus this underbuilt neutral has no metallic connection to the 44-kV station grounding system in general. For this configuration, consider the scenario where somehow one of the 44-kV phase conductors establishes electrical contact with the underbuilt neutral. This could happen in a number of different ways, some being less probable than others. However once this occurs, fault current from the 44-kV feeder is injected into the distributed neutral-to-ground connections and forced to return through earth to the 44-kV supply station through its local grounding system (ground-grid). The result is that the predicted GPR for the 44-kV station can end up being higher for this scenario than for other fault scenarios normally considered in assessing adequacy of the 44-kV station’s grounding system. A higher GPR inevitably translates into higher step & touch potential hazards, which may not coordinate with permissible limits. The same may also be true outside the station, along the 4-wire feeder, due to the current injected into the multi-grounded neutral.

So the question is what to do about this? There are various thoughts on this.

1. Accept that the associated safety hazard is sufficiently improbable, comparable to other risks that are tolerated, and therefore do nothing to address the matter. This may correspond to a decision made (perhaps informally) in the past, and explain how we ended up having such pole-sharing arrangements with no specific counter-measures and without any incidents. Going forward though, a possible concern is that these exposures may be more frequent, either due to system expansion or for instance due to recently instituted practice (over the past 10 or so years?) of installing LDC metering cabinets for 44-kV feeders just outside the station (which may be bonded to the 4-wire neutral, for grounding safety). In view of this growing exposure, perhaps we need to reconsider whether the risk remains sufficiently improbable. Unfortunately there’s no established industry practice in North America for conducting a formal probabilistic risk assessment for grounding safety. However we’re aware of this approach being taken elsewhere (e.g. in Australia) which may warrant consideration here, if nothing else than to assess the relative risk.

2. If the status-quo is not acceptable, evidently some form of mitigation is required to reduce GPR at all relevant points of galvanic contact with earth (e.g. 44-kV station electrode as well as selected points along multi-grounded neutral on ROW). In turn, this means either improving grounding resistance or reducing the magnitude of fault current injected into the earth, as follows:

a) Grounding resistance improvement entails either increasing the size of the buried electrode, requiring availability of land, or gaining access to deeper soil layers exhibiting lower resistivity (e.g. ground well electrode).

b) Reducing injected fault current can be done in two ways:

(i) reduce available station fault current level, e.g using higher-impedance neutral grounding reactors (NGR). This would desensitize protections, reducing safety somewhat in case of high-impedance or remote-end feeder faults that may need to be countered through changes to protections. It may also mean departing from our past practice of utilizing a standard NGR value at all stations (hence impacting spares inventory).

reduce the fraction forced to return through earth/soil by providing an alternate galvanic path. Establishing a bond between the underbuilt neutral and the 3-wire station ground would serve the latter function effectively. Possible impediments for this approach involve costs, of course. But also, the 4-wire neutral outside the station may belong to a 3rd party (LDC) requiring their consent. There can be opposition in doing so by LDC on the basis that doing so would subject their customers to either higher or more frequent GPR incidents from faults at the 3-wire station. This concern however may be misplaced for two reasons. Firstly this exposure is no different than what is presently accepted on all feeders served from 4-wire stations. Secondly, not establishing this bond makes leaves their 4-wire customers worse off in case of such faults on the ROWs. Furthermore there’s a possibility that their neutral may not be sized adequately to carry the 44-kV fault current (hence needing to be upgraded).