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Why Use Shielded Cable in Substations?

March 14, 2017
The right shielding for substation wire and cable continues to be a critical factor in enabling substations to become smarter, more sophisticated and more reliable.

The right shielding for substation wire and cable continues to be a critical factor in enabling substations to become smarter, more sophisticated and more reliable.  New sensors, distributed computing, microprocessor-based relays and other sensitive electronics increasingly appear behind substation switching panels and even in switchyards. But all that new functionality has created big challenges for engineers and technicians -  solid-state circuitry, whether home grown by the utility or imported from other industries has a legacy of false operation and even limited survival in the harsh substation environment unless properly “hardened” and shielded.

The need for shielded wire goes back to the late 1800’s. Interference between telegraph and telephone wires sharing the same poles led to the practice of greater spacing and using twisted pairs for service drops. That did the job until radio came along and turned the lines into unintended antennas. Reportedly, radio broadcasts sometimes emanated from overhead telephone wires! Engineers and physicists knew that, although twisted pairs could exclude stray magnetic fields, they didn’t exclude electric fields. Some sort of metal covering was required, sort of a miniature Faraday cage surrounding the conductors.

Simply wrapping a grounded third conductor around the length of the twisted pair seemed to work well enough. Then braided tinsel and braided wire shielding was developed to provide greater mechanical flexibility. But soon the fledgling electric power industry was putting a growing number of electric distribution lines on the same poles as telegraph and phone lines.  Inductive coupling, capacitive coupling and ground loops led to low frequency “hum” on phone lines, and sometimes even shock from telephone and telegraph handsets! 

We now classify all these interactions as electromagnetic interference (EMI). Radio frequency interference (RFI) refers to EMI that occurs within the radio frequency spectrum. Interference through induction and capacitive coupling are other classes of EMI. But whatever we call these impacts, electric power substations can have them all!  Sixty Hz electric and magnetic fields and stray ground currents; higher frequency electromagnetic noise caused by breaker and air-switch operations; lightning strikes and even occasional insulator flashovers – hands down, the substation environment remains the biggest shielding challenge in the electric utility.

For the most part, challenges have been met by careful engineering and wire and cable manufacturing innovation. As a result, modern shielding design on control and communication wires can prevent the signal carried on the protected conductor(s) from radiating and interfering with other conductors or causing safety issues. Shielding can stop external signals and electrical noise from interfering with the protected conductor(s). And, of course, properly chosen shielding can protect conductors and insulation from abrasion and rough handling during field applications.

One shield design may not meet the needs of all applications. For example, braided shields offer excellent flexibility. However, high frequency, short wavelength interference can pass through the braid openings. Foil shielding provides better protection from high frequency interference but can have too much electrical resistance for lower frequency protection. A combination of braid and foil would offer great broadband protection but then there’s the issues of cost, weight and flexibility. And on it goes. Besides simple foil or braid, there are many other shield designs offered by manufacturers. Two types, Longitudinally Corrugated Tape (LCT) and Helical Copper Tape are frequently used in substations:

Longitudinal Corrugated Tape Shielding - A flat metal tape is corrugated at right angles to its length and then formed into a cylinder around a cable core or inner jacket. This design provides highly effective shielding across all but lowest frequencies. LCT shield design allows rapid stripping and termination.

Helical Copper Tape - A flat metal tape is applied, with overlap, around bundled conductors. In addition to wide-spectrum protection, this construction can withstand rough handling and a tighter bend radius without damage.

Choosing the right configuration from all that manufacturers offer can be confusing. But experienced engineers know that a careful and knowledgeable selection process is required.

The benefits of shielding in higher voltage power cables are somewhat different than in their low voltage cousins. The largest benefit is the protection of the cable insulation. Without the shield, and depending on surroundings, the irregularities in capacitive current can produce large variations in the voltage distribution across the insulation. In turn this can lead to corona discharge, rapid insulation degradation and eventual failure. Reduction of capacitive current also has the benefit of reducing the chance of stray currents getting into nearby equipment and even causing electric shock. The shield may be constructed of metallic coated tape or some other conductive of semi-conductive material. If the cable is multiconductor then each of the conductors has its own shield, as does the group.

Shielding experts have developed methodologies to work through the variables and can recommend optimal shielding systems for specific electrical and physical substation environments. For the most part, industry-wide standards provide guidance with built in conservative margins. Still, it behooves the substation engineer, designer and technician to understand basic shielding principles. As is sometimes said, shielding design is an art, as it was a century ago. But, thank goodness, it’s also rational and understandable.

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