When dealing with medium voltage (MV) cables it is important to consider whether to include cross bonding joints in an installation. Cross bonding joints cause a reduction in the induced currents circulating on the metallic screen of a MV cable. Cross bonded grounding schemes are generally used in high voltage (HV) cable systems above 72.5kV voltage class while MV cable systems are usually solidly grounded without interchanging the cables screens of the different phases. A solidly grounded system requires the metallic screen to be directly connected to ground potential at both extremities of the circuit, ie each end of the cable. The solidly grounded circuits, especially on longer runs and under high energy outputs become subjected to these circulating currents which diminishes the performance of the installation.
High circulating currents on the metallic screen of the cable can have negative consequences on cables and their accessories. Cables become subject to high energy losses, overheating and reduced current carrying capacity, and overloading / damaging of the grounding connections of terminations and screen connections in joints. Cross bonding schemes can be used to mitigate the effects of induced circulating currents, by establishing an insulated interruption between the joint screen wires as detailed below.
If the screen is solidly bonded to earth on both ends of the cable, a closed electrical circuit is built through the ground that allows induced AC current to flow continuously. Other methods to reduce this circulating current include construction of the cable (3 core or single core), laying a trefoil arrangement, and running the cable at reduced load capacity. While these methods will not balance the circulating currents to zero, they can minimize them to an acceptable level.
Cross bonding joints can be particularly useful in renewable energy farms, where the MV cables are under high loading capacity and can run over 10km in one circuit. This would allow for cables to be run to near full load capacity without diminishing the lifespan of the cable and its accessories. This provides a much more cost effective and safer solution compared to designing these installations with a solid grounded system.
We provide a full assessment of your installation by use of thermography and electrical testing methods to determine the condition of your MV cables. We can retrofit cross bonded joints into your electrical installation where excess circulating currents are present increasing the lifespan of your cabling and accessories.
If you would like to know the condition of your MV cables, give us a ring today to discuss what we can do to help you manage your assets.