Before discussing the types of sheath bonding used in high voltage cable let us recall our knowledge regarding sheath and cause of induced voltage and current in the sheath of a HV cable. Actually, all types of bonding are used to limit the unwanted, induced voltage and current in a cable sheath.
What is a Cable Sheath?
Before discussing the importance of a cable sheath, let us first understand what it is. A cable sheath is the outer covering of a cable that basically protects the conductor and insulation from damage. It can be made of metallic materials like lead, aluminum, or copper, or non-metallic materials like PVC.
Now, why is a cable sheath used?
It is easy to understand why a non – metallic cable sheath is used. It is used for protecting the cable from physical damage like cuts, bends, and impacts. It also prevents moisture from entering the cable, which could harm the insulation. The non metallic materials like PVC is much economical to use as a cable sheath compared to a metallic sheath but till we need to use metallic cable sheath in high voltage system. It will be well understood, if we go through the advantages of metallic cable sheath one by one.
- A metallic cable sheath also protects the cable from physical damage like cuts, bends, and impacts, similar to a PVC sheath.
- It also prevents moisture from entering the cable, just like a PVC sheath.
- In addition to the above advantages, in high-voltage cables, a metallic sheath provides a few vital benefits. High-voltage underground or overhead cables often cross or pass near other high-voltage cables. A cable may also pass through the close vicinity of other high-voltage installations. Due to these situations, there may be influences from external electromagnetic fields. These external magnetic fields can affect the performance of the cable. Electromagnetic interference may also cause disturbances to nearby telecommunication lines. In high-voltage cables, a metallic sheath acts as a shield to control the electric field and reduce electromagnetic interference.
- If there is any fault due to insulation failure or puncture, the fault current must flow to the earth as easily as possible to ensure the smooth operation of protective relays in the power system. A metallic sheath is always connected to the earth, providing a path for fault currents to safely reach the ground.
- Since the heat conductivity of metal is very high, it easily absorbs the heat generated in the conductor through the insulation layer. It also facilitates the transfer of heat to the surrounding soil. Thus, the metallic sheath helps release the heat generated in the cable due to the current in the conductor.
Why is sheath bonding required for a high-voltage cable?
Before discussing why sheath bonding is important, let us first understand what sheath bonding means. Sheath bonding means connecting the metallic sheath of a cable to the ground or earth at one or more points. It is easy to understand why sheath bonding is required for a high-voltage cable. When current flows through a high-voltage cable, it creates a magnetic field around it. This magnetic field can induce voltage and current in the metallic sheath of the cable. If not controlled, this induced voltage and current can cause power losses, overheating, and damage to the cable. Sheath bonding helps to limit this induced voltage and current in the sheath. In high-voltage cables, sheath bonding also protects the cable from external electromagnetic fields. Without proper bonding, the cable can even affect nearby communication lines.
As we have already discussed, another important reason for sheath bonding is safety. Sheath bonding provides a smooth path to the fault current to ground, ensuring that the protective devices in the power system work correctly to isolate the fault.
What are the types of Sheath Bonding in High Voltage Cable?
Both Ends Bonding: As I have already told, if the metallic sheath is connected to the ground, the problem of induction might be solved. Normally, one first thinks of connecting both ends of the cable sheath to the ground. Yes, this is the simplest way of sheath bonding used for the purpose. But it has some inherent disadvantages. If the cable is long enough, due to the non-negligible impedance of the long cable sheath, there may be a significant voltage arising in the sheath towards the mid-portion of the length compared to the grounded ends. Also, the induced current in the sheath due to the induced voltage gradient will get a closed path through the ground. It creates unnecessary circulation of induced current and causes heating and power loss. This extra heating causes de-rating of the cable. But for very short length cables, preferably below 500 m, the sheath impedance is insignificant, hence induced current and heating due to induced current are negligible. So, for short length cable laying, the both-end bonding method is acceptable. Here, both ends of the sheath are solidly connected to the earth.
Single End Bonding: In single-end bonding, the metallic sheath is grounded at only one end of the cable. This helps to avoid the problem of circulating currents in the sheath. Without a closed path, the induced current cannot flow continuously, so there is no unnecessary heating or power loss. But there is another issue. If the cable is long, a significant voltage can build up at the open end because there is no direct path to the ground. This high voltage can stress the sheath insulation.
To solve this, Sheath Voltage Limiters (SVLs) are used at the open end. SVL is a device that remains inactive during normal conditions, but if the induced voltage crosses a certain limit, it provides a temporary path to the ground. This protects the sheath from high voltage without allowing continuous current flow. Thus, single-end bonding with SVL is suitable for medium-length cables where induced voltage is high, but continuous sheath currents need to be avoided.
Mid Point Bonding: As I have already told, both-end bonding is a simple way to connect both ends of the cable sheath to the ground. But for long cables, it has some disadvantages due to sheath impedance, which causes significant voltage build-up and circulating currents, leading to power loss and heating. Single-end bonding solves this problem by grounding the sheath at only one end, avoiding continuous current flow. However, it may cause high induced voltage at the open end, especially for longer cables.
Now, to balance between both-end bonding and single-end bonding, mid-point bonding is used. In this method, the sheath is grounded at the mid-point of the cable length, while both ends remain insulated from the ground. This way, the cable is divided into two equal halves. Each half has its own induced voltage, but since both ends are open, there is no continuous current flow in the sheath.
The key point here is that the induced voltage at each half is much lower compared to the total length of the cable. As a result, the problem of high induced voltage at the ends (like in single-end bonding) is reduced. Also, since there is no direct connection at both ends, circulating current is avoided, which solves the problem seen in both-end bonding for long cables.
But still, for very long cable lengths, the induced voltage in each half may not be negligible. In such cases, Sheath Voltage Limiters (SVLs) can be used at the open ends to protect the sheath from transient overvoltages. So, mid-point bonding provides a balanced solution, especially for medium-length cables, where both-end bonding causes high losses, and single-end bonding causes high induced voltage at one end.
Cross Bonding: As I have already told, both-end bonding is the simplest method where both ends of the cable sheath are grounded, but it leads to circulating currents and power losses for long cables. Single-end bonding solves the problem of circulating currents by grounding the sheath at only one end, but it can cause high induced voltage at the open end. Mid-point bonding balances these two by grounding the sheath at the center, reducing voltage build-up and avoiding continuous currents. But for very long cables, even mid-point bonding may not be enough.
Here comes cross bonding, a more advanced method used for long high-voltage cables. In cross bonding, the cable length of all three phases is divided into three equal sections. The metallic sheaths of these sections are connected in such a way that the vector sum of induced voltage in all three sections is zero or very nearer to zero. This is done by connecting the sheath of one section to the sheath of another section of the next phase in a crossed manner.
For example, In three-phase systems, there are three cables, each carrying one phase of the current – Phase R, Phase Y, and Phase B. The total length of each cable is divided into three equal sections. The sheath of R Phase in the first section is connected to the sheath of Y Phase in the second section, and then to the sheath of B Phase in the third section. Similarly, the other two phases are also cross-connected in a similar pattern. Every cross bonding point is connected to the ground with SVLs.
Since the induced voltages are 120 degree apart from each other, the total induced voltage is ideally zero along the entire cable length. Also, there is no continuous current flow in the sheath, so no extra heating or power loss occurs. The cable can carry more current without being derated due to sheath losses.
However, cross bonding is more complex than other methods. It requires proper planning and installation. Special joint boxes are needed at the points where the sheaths are cross-connected. But for long cables, especially those above 1 km, cross bonding is the best choice. It avoids the disadvantages of both-end bonding, single-end bonding, and mid-point bonding, making it suitable for high-voltage power transmission over long distances.