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CERTIFIED GROUNDING SOLUTIONS
Medium voltage cables typically used in renewable energy farms have polymeric insulation such as XLPE (cross-linked polyethylene) or EPR (ethylene propylene rubber). These cables have copper metal shielding and polyethylene sheaths of different densities.
When defects have been generated in the cable sheath during the installation process, humidity may penetrate. This humidity, in the presence of the electrical field generated by its current and in combination with its high operating temperature, starts a physicochemical process whereby water trees grow inside the plastic insulation, degrading its dielectric strength and causing ruptures.
In turn, defects in the installation of splices and terminals or exceeding the radius of curvature when installing the cables can all initiate a partial discharge activity inside the insulation, which grows relatively fast. Once a fault occurs, typically a perforation of the insulation is caused, which can withstand voltages between 4 and 12 kV for cables with a nominal voltage of up to 35 kV. These types of faults are known as high-resistance faults.
The grounding system must address low earth impedance as well as low resistance. A spectral study of lightning’s typical impulse reveals both a high and a low frequency content. The high frequency is associated with an extremely fast rising “front” on the order of 10 microseconds to peak current. The lower frequency component resides in the long, high energy “tail” or follow-on current in the impulse. The grounding system appears to the lightning impulse as a transmission line where wave propagation theory applies.
A single point grounding system is achieved when all equipment within the structure(s) are connected to a master bus bar which in turn is bonded to the external grounding system at one point only. Earth loops and differential rise times must be avoided. The grounding system should be designed to reduce ac impedance and dc resistance. The shape and dimension of the earth termination system is more important a specific value of the earth electrode. The use of counterpoise or “crow’s foot” radial techniques can lower impedance as they allow lightning energy to diverge as each buried conductor shares voltage gradients. Ground rings around structures are useful. They should be connected to the facility ground. Exothermic (welded) connectors are recommended in all circumstances.
Cathodic reactance should be considered during the site analysis phase. Man-made earth additives and backfills are useful in difficult soils circumstances: they should be considered on a case-by-case basis where lowering grounding impedances are difficult an/or expensive by traditional means. Regular physical inspections and testing should be a part of an established preventive maintenance program.
Site Audit & Surveys Include The Following:
Ground System Performance, Internal Bonding/Grounding, NEC Compliance, Electrical Safety, Lightning Protection System, & AC Surge Suppression.
A full written report with observations, recommendations, pictures and CAD of the grounding design recommendation is provided upon completion.
Our grounding systems design services help assure the installation begins with the proper foundation that follows through all the wayto the complete system installation . Our site surveys and design services exceed the industry standards and include detailed explanations with photos illustrating any issues identified along with the appropriate plan of action to solve any correction. Our engineering team provides site specific solutions providing you with Total Site and Facility Protection.
