Below-Grade Intelligence: A Necessity for Efficient Underground Asset Management and Healthy Power Grids

By Amir Kalantari

In February 1998, Auckland, New Zealand, experienced one of the most severe blackouts in the entire 20th century. A series of four power failures left the city’s most important business district in the dark for nearly five weeks. The investigation that followed singled out the insufficient appreciation of soil conditions as one of the primary causes of this crisis. Specifically, it has been pointed out that “the actual backfill soil resistivities (along the cable route) exceeded by up to 500% the values specified by the cable manufacturer.” The soil thermal resistivity is a critical factor in rating power cables. Hence, one can easily imagine the sheer scale of threat when cables are operated on the assumption that the design values hold true.

An oftentimes underrated and worrisome reality worth revisiting is that soil is a “living” thing. Its properties, including thermal, electrical and mechanical, not only do vary spatially (even over short distances and within limited areas), but also vary over time, due to seasonal transition and weather variations. Soil electrical resistivity, for example, can reportedly vary by a factor of 5-1000 times. Another example are changes in soil thermal resistivity, which can rise beyond 300%. Now, there are several factors determining the precise level of variations, but the two pivotal ones, i.e. soil moisture content and soil temperature, are weather related and driven by changes in air temperature and precipitation levels. The larger the swing in the latter two factors, the wider the variation in soil properties.

Such continuous variations in soil properties, when gone unnoticed, can present serious challenges for the operation of power grid assets and, particularly, underground infrastructures including the following;

• a substation grounding grid, which is safe in summer, but unsafe in winter (putting personnel at risk),
• an underground cable collector system in a wind farm or solar plant that can deliver up to full capacity during wintertime, but is hampered during summertime leading to cable failures or lost revenue,
• a cathodic protection system in a power plant which leaves underground structures under or over-protected considering seasonal soil resistivity variations leading to premature structural failures.

Also, many “above-grade” HV equipment and installations depend on sound and reliable underground infrastructures for their proper operation, or their functionality would be compromised and take a hit. Think about ground fault protection malfunction, equipment insulation failures, service interruptions, etc. Once they occur, these problems may lead to the shut down of an entire installation (or even an entire interconnection).

Despite the grave consequences of inadequate treatment of soil in power engineering practices, the telling insights from industry suggest otherwise. The subject is not considered a criticality, and it also seats at the bottom of the priority lists in many power projects, reminding the old adage “out of sight, out of mind”. On the flip side of the coin, for above-grade installations, it is becoming (or has become) a normal practice to adopt technologies such as dynamic line rating, inlet air cooling fog systems, quasi real-time load and wind forecasts, market clearing, etc. All (or part of) these practices seek to optimize resource utilization in a smart attempt to best respond to various manifestations of weather variations, e.g. air temperature, wind speed, air pressure, etc. in day-to-day power system operation. But not so much in response to soil properties variations! It is safe to say that incorrect soil (electrical) resistivity is an underrated critical pillar in power engineering practices, during both design and operation phases. Nevertheless, in an industry survey conducted by CEATI among utilities, 50% of the surveyed participants believe that soil is a high-risk area for grounding grids (and about 40% think it is of medium-risk). Yet, other industry insights reveal that the resistivity figures, which are still the basis of power cable ratings (in many cases regardless of the spatial and seasonal variations), follow the principle of “one-size-fits-all ‘standard’ soil (thermal)” cases.

In order to mitigate the risks associated with seasonal soil properties variations, there should be some type of tracking mechanisms in place. Such mechanisms may either monitor and/or forecast variations in thermal, electrical and mechanical properties of soil and, thus, adjust the system operation accordingly or alarm some operator for the need of system upgrades. One approach could be that of monitoring thermal resistivities along cable routes (at least, at hot spots such as buildings, roads and pavements, trees, or in proximity to other cables, etc.), or variations of soil electrical resistivity at reference locations close to HV installations. Alternatively, a “system-wide cathodic protection” could be adopted; such as the Dawson Power initiative (wireless communication between the field where corrosion data are collected and analysed and a web data center generating alert messages and recording historical trends). As far as technology is concerned, the current metering and communication platforms can very well support such a transition. The best scenario though is to first capture the range of variations in soil properties as closely to the actual phenomenon as possible and as early as at the design stage (preferably via a mix of, for instance, year-long, pre-installation investigation and inspecting historical trends within the lifetime of power assets). Then, at the operation phase, monitoring systems may be installed and forecasting systems can be integrated for timely adjustments in/on time.

What increases the risk of variation in soil properties for power infrastructure is the unprecedented rate of extreme weather events driven by climate change and global warming. We are experiencing more and more droughts, floods, heat waves, water level variations in lakes and underground water tables, ice-storms, elongated summer or winter periods, etc. These phenomena will only serve to stress and underline the significance of getting more intelligent with continuous variations, not only in the above-grade weather manifestations, but also in the below-grade variations in soil properties. As we venture our way into the future, we drive more and more power lines underground, tolerate less and less service interruptions and develop more and more complicated underground facilities. Hence, the significance of soil for healthy power infrastructures is just on the rise.

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