The Use of Link Boxes in HV Cable Systems
Link boxes are an important part of modern cross linked polyethylene (XLPE) and self contained fluid filled (SCFF) transmission cable circuits. Link boxes are also known as grounding boxes or sheath bonding boxes. They are used to provide a waterproof environment for the sheath bonding connections that make it possible to maximize cable load ratings by minimizing sheath current flow. These connection schemes are particularly important for single conductor cable arrangements as they counteract the effects of unbalanced magnetic induction. The two most common connection arrangements are cross bonding, and single point grounding schemes.
Regardless of the connection arrangement, there must be an electrical path between the sheath of each cable and ground. Generally, this is achieved by providing a ground connection inside the link box for each section of the sheath circuit. All of the links inside the box are removable, allowing the sheath sections to be easily isolated for electrical testing. Link boxes are usually installed at the locations where sheath continuity is interrupted. This includes cable joints that contain sheath sectionalizing insulators, and at terminations. In the case of terminations stand off insulators, also known as mounting insulators, interrupt the sheath continuity.
Modern link boxes are designed to precise specifications to ensure they can withstand the high voltages and currents that are present on the cable sheaths during system faults. The boxes must also be completely watertight to prevent water ingress. Depending on the requirements of the system, they can be mounted vertically or horizontally, in a pit, vault, handheld, manhole or on a support structure. In outdoor installations weatherproof boxes are used, while for underground installations it is necessary to use submersible boxes. Custom designed link boxes can be built for special cable arrangements, including those where a spare cable has been installed for circuit security.
The link box covers can incorporate a window to facilitate visual internal inspections without the need to open the box. This is helpful for maintenance as it eliminates the need to replace the cover sealing gasket when performing routine visual inspections.
Link Boxes Are Not Always Required
Sheath bonding schemes are not normally required for three conductor cable installations as the magnetic fields cancel when the phase currents are balanced. Therefore, the sheaths of three conductor cables are usually grounded at each end of the circuit. However, the use of link boxes is recommended at the grounding points in order to facilitate routine electrical testing and fault locating techniques.
Using Sheath Voltage Limiters (SVL)
In single conductor cable systems installed before the 1970s, sheath connections for bonding schemes were usually made using simple bolted connections wrapped with layers of tape. In many older cable circuits, it was common to find that flashovers had occurred at the sheath interruptions at joints and terminations. These flashovers would have occurred when steep fronted waves were transmitted during switching surges, lightning strikes or electrical faults. To address this situation, CIGRE published Electra 28 (later known as Electra 128) describing the use of sheath voltage limiters (SVLs) installed inside link boxes. The document specified stringent electrical requirements for the link boxes and the voltage limiters.
For effective performance of the SVLs it is important that they be as close as possible to the insulator that interrupts sheath continuity. This requires the use of short bond leads. As a result, the link boxes are normally mounted very close to the joints and terminations. Link boxes used at joints often contain SVLs and links for the sheaths of all the phases. Depending on the bonding scheme, some link boxes at joint locations may be sheath grounding points and will not have SVLs. Since the spacing between terminations can be large, it is customary to use a box with a single link for each termination. The boxes at terminations may or may not contain SVLs depending on the details of the sheath bonding scheme.
The selection of SVLs for sheath bonding systems requires precise calculations factoring in the cable characteristics, the physical arrangement, maximum loading, fault current levels and fault duration. Properly selected SVLs will protect the cable jacket and sectionalizing insulators from over-voltage by connecting the sheath to ground during any significant steep fronted waves that occur at the start of system disturbances. The SVLs are designed to prevent conduction during the subsequent power frequency stage of a system disturbance.
For more detailed information there are several documents issued by technical organizations, among the most relevant:
- IEEE Guide for Bonding Sheaths - IEEE P575 D10. This is still a draft document under preparation.
- CIGRE - Electra 128 - "Guide to the Protection of Specially Bonded Cable Systems against Sheath Overvoltages".
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