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Sheath Bonding Design Guide for High Voltage Cables

Solidly, single-point, and cross-bonded cable systems designs are explained

Purpose of cable sheath bonding

High-voltage power cables are provided with an outer concentric conductor in the form of a metal screen and/or a metal sheath which surrounds the main conductor and insulation layer. Metal screens and metal sheaths are collectively referred to as the sheath. The sheath also includes any metallic armour layer(s) in the cable. The purposes of a cable metal sheath include to provide fault current return path, return path for the induced capacitive charge currents, earth (zero touch voltage or thereabouts) potential for human safety, and moisture barrier for cable insulation.

For high-voltage cable circuits carrying more than about 500 A, sheath bonding arrangements are implemented to reduce (often significantly) sheath current losses. Whilst the bonding arrangements come with an additional cost for equipment and maintenance, they will often result in considerably smaller cable sizes to be used for supplying the same load currents. The metal sheath bonding arrangement selected for high-voltage cable systems has the second highest impact on cable current ratings, second only to the external thermal resistance (the installed environment) to the cables. High voltage cable current ratings are calculated using the IEC 60287 Standard series.

A sheath bonding system is used for protecting the insulation of various cable system components during normal operation as well as from transient overvoltage from lightning, switching and fault surges. Moreover, the sheath bonding arrangement is also important from a safety point of view.

In this report we explain the following types: solid bonding, single-point bonding, cross-bonding, and impedance bonding.

Sectionized cross-bonding is the most common special bonding arrangement for HV and EHV transmission lines except if the line length is short then single-point bonding is mostly used.

Refer to separate articles for earth continuity conductor (ECC) design, sheath voltage limiter (SVL) selection, and bonding lead selection.

Losses in the cable sheath

The magnetic field that is generated from the current flow in the main conductors of cables creates a “transformer” effect that induces a current in the sheath. Due to these induced currents in the sheath two types of losses can occur: circulating current losses and eddy current losses. The presence of current losses in the sheath causes additional heating inside the cable resulting in reduced current-carrying capacity of the cables conductor.

The eddy currents circulate radially and longitudinally of the cable sheaths; and are generated on similar principles of skin and proximity effects. The eddy currents are generally of smaller magnitude compared with circulating current and they are generated in the cable sheath irrespective of the bonding system of single core cables.

If circulating currents are present in the sheath (depending on the sheath bonding arrangement) their current magnitude is generally much greater than that of the eddy currents.

Circulating currents occur in the sheath for solid bonding of single core cables and multicore cables where an individual sheath surrounds each conductor core. For sheaths that are single-point or cross-bonded there are no circulating currents, however eddy current losses still occur.

Sheath bonding designs

This technical report explains the different sheath bonding configurations for high-voltage cables used in transmission systems. This report should assist engineers and stakeholders to select the most suitable (meaning functional, safe, and cost-effective) sheath bonding arrangement for their high-voltage cable systems projects.

Solid bonding

Method

With this bonding arrangement, the sheaths, screens, and armour (if present) are solidly earthed at both ends of the cable system (see Figure 1). There may also be bonds at intermediate points along the cable route (refer to Figure 2), which ensure safety in case one of the end bonding points get disconnected, potentially resulting in excessive induced voltages.

Figure 1. Solid bonded cable system

Figure 2. Solid bonding system with intermediate earthing points

Application

Solid bonding is typically used for short lengths with low current rating requirements, low and medium-voltage cable systems (typically up to 66 kV), and submarine cables. Generally, this method is not used for transmission systems due to the relative advantages of other bonding arrangements, such as lower losses.

Advantages

Simple and low-cost solution.
The magnetic field external to the cable is low due to the opposing sheath current direction concerning the conductor currents.
Minimal maintenance is required.
The sheath's voltage is considered at the earth’s potential at every point.

Disadvantages

Design Considerations

Use two independent parallel earth bonding leads at each bonding point (end or mid) to reduce the likelihood of disconnection.
Bonding leads at the earth points must be capable of withstanding the fault currents.
Depending on the length and other parameters, intermediate earthing points may be included.
The magnitude of the circulating current is independent of the circuit length.

Single-point bonding

Method

In a single-point bonded cable system the cable sheaths of all three phases are solidly bonded together and earthed at one point only whilst the other end is isolated from ground (see Figure 3). Note that this method is applicable to short-line lengths. However, an alternative scheme that can be applied to long line lengths is multiple single-point bonding in which the three cable sheaths are solidly bonded and grounded to an earth continuity conductor (ECC) at one end of a section and connected to ground through sheath voltage limiters at the other end, done at multiple locations along a route (see Figure 4).

Figure 3. Single-point bonded cable system

Figure 4. Sectionalised single-point bonded cable system

Application

Single-point bonded cable systems are in common use for high-voltage transmission and as an alternative to multi-point bonding. A single-point bonded cable system is generally used in substations when the cable route is relatively short and where it is uneconomical to use the cross-bonded method.

Advantages

Single point bonding provides improved cable current ratings since the induced circulating current losses in the sheath are eliminated.
Only one length of the cable is required which makes single-point bonding an economical solution.
The current does not flow longitudinally along the sheaths. This is because there is no continuous closed-loop electrical path for the sheath. Consequently, the sheath circulating current losses are eliminated. (Note that the sheath eddy losses will still be present).

Disadvantages

This method causes voltage to rise along the length of the cable and these voltages are proportional to the conductor current and cable length. For single-core cables this voltage increases with the individual phase spacing.
To reduce this voltage an ECC is installed that runs parallel to the cables and this significantly adds to the cost of the cable system
Care must be taken to insulate the sheath at the free end to avoid danger from the induced voltages.

Design Considerations

In a single point bonded cable system a voltage is induced on the sheath of the cable during operation, progressively increasing with the distance away from the earthed point. This induced voltage reaches a maximum at the farthest point away from the earthed end. Therefore, the maximum cable section length is governed by the permissible sheath standing voltage allowed at the isolated end.

The induced voltage on the sheath must be calculated to not exceed an acceptable touch voltage limit for safety reasons. Most countries have a national standard that governs the maximum standing voltages allowed at power frequencies, as this is an important issue, particularly for maintenance workers, and these voltage limits are typically in the range of 35-400 V depending on the country and the regulations applied.

At points farthest away from the earthed end, the sheath must be effectively insulated from earth for the life of the cable system. If the anticipated surge voltage level is greater than 75% of the sheath insulator’s basic insulation level (BIL), a sheath voltage limiter (SVL) should always be utilised. The SVL is commonly connected between the open-ended sheath and the earth point to protect the cable sheath from overvoltage during transients caused by switching, lighting or faults.

An ECC is installed in parallel with the cables of a single-point bonded system except where both ends of the cable share a common earthing system. The relative location of the ECC with respect to the phase conductors should be carefully considered and the ECC should be transposed in the middle of the cable system.

Cross-bonding

Method

The cross-bonding cable system is a widely used method for minimising the power loss over the cable sheath. Cross-bonding method takes advantage of vector summation of three phase induced voltage 120 degrees out of phase with each other. Thus, the resultant voltage will be minimized close to zero which significantly reduces the undesired current flow and losses over the cable sheath. In this method sheaths are sectionalized into minor sections and then cross-connect to neutralize the voltage in three successive minor sections as shown in Figure 5. (The three minor sections together are referred to as a major section).

Figure 5. Cross-bonded cables without transposition

It is recommended to transpose the cable conductors as well to properly neutralize the induced sheath voltages (see Figure 6). However, there may be some practical constraints in transposing large and heavy high-voltage cables.

Figure 6. Cross-bonded cables with transposition

Types

There are numerous variations of cross-bonding, some of which are mentioned below:

Continuous Type of Cross-bonding

In continuous type of cross-bonded systems, the sheaths are cross bonded at the end of each minor section throughout the whole cable route while the three sheaths are solidly bonded and grounded only at the two ends of the route.

Sectionalized Cross-bonding

In this type of bonding, three consecutive minor sections form a single major section. This is repeated throughout the route length to make multiple major sections in series. At the junction of two major sections and at the ends of the circuit, the sheaths are solidly bonded together and earthed, while between two minor sections sheaths are simply cross bonded using SVLs.

Direct Cross-bonding

In this type of cross-bonding the metallic sheaths are transposed without SVL and consequently any link boxes. The main advantage of this scheme is that it is simple and easy to maintain as it needs fewer accessories. The drawback of this scheme is that the disconnection of the sheath becomes difficult due to the unavailability of link boxes and in case of testing or fault location, the bonding cable may need to be cut instead of simple disconnection through a link box.

Cross-bonding of Short Lines

Some circuits are long enough to raise the sheath-induced voltage to an unacceptable level but not so long to require proper cross-bonding with three minor sections. If these circuits are solidly bonded at both ends, then they will have a high circulating current therefore the sheaths are cross bonded with SVL at the middle location making two minor sections. This method reduces the circulating current in the sheath to some level compared to solid bonding.

Cross-bonding in Tunnel Installations

In tunnel installations, cable joints and bonding systems are easily accessible. Taking advantage of this proximity of the cable joints, the SVLs are connected across sheath sections with leads as short as possible. Direct cross bonded is generally used to connect the bonding leads to the sheaths of different phases.

Although in this arrangement the cross-bonding leads do not carry surge current, these leads must be sized to carry system short circuit currents. Compared to regular cross-bonding where the SVLs are connected in a star arrangement, the SVL in tunnel installation is connected in a delta arrangement. Due to higher induced voltages during normal and transient conditions, a higher-rated voltage for the SVL is required in this type of installation.

Applications

For longer line lengths, or when sheath voltages become excessive due to very high fault currents, cross-bonding is generally preferred and is the most widely used form of bonding.

Advantages

Compared to single-point bonding, a cross-bonded configuration has the advantage of not being length limited.
Circulating currents are significantly reduced, resulting in lower sheath losses and higher cable current ratings.
Besides inhibiting sheath currents during the normal balanced load operation, the sheaths do form a continuous path from end to end of the cable circuit and are grounded at both ends. Therefore, during ground faults, sheath currents can flow across the length, thus eliminating the need for a separate ECC.
In addition to the economy via eliminating the ECC, the cable sheaths function more effectively as screening conductors during earth faults than a parallel ECC. Hence, the voltages induced in parallel cables, communication lines, pipelines, fences, etc., are reduced during earth faults in a cross-bonded system versus a similar single-point bonded system.

Disadvantages

Design Considerations

For a cross-bonded configuration a minimum of three minor sections are needed (except for short lines as previously mentioned). Thus, cross-bonding is not possible with cable circuits comprised of only one or two lengths of cable. For such circuits, single-point bonding would have to be employed unless it were feasible to reconfigure the circuit into three sections. Moreover, in practice, there may be some circulating currents present because the lengths and cable spacings in each section may not be exactly equal.

The length of each section and cable spacings are limited by the induced voltage levels and the voltage differences between the sheaths and the earthing points.

For cross-bonded cable circuits the highest sheath-to-earth voltages are caused by two-and three-phase faults. If the cross-bonded cable circuit is inserted in the middle of an overhead line, single-phase faults produce the highest sheath-to-earth voltages that increase with earth impedances.

Impedance bonding

Method

In this technique of bonding scheme an impedance, such as a reactor or a resistance, is inserted into the sheath current path for the purpose of limiting fault currents or load losses. This impedance can consist of simple reactors or devices such as saturable reactors and bonding transformers.

In the transformer sheath bonding, both ends of each cable sheath are electrically connected to a three-phase sheath bonding transformer as shown in Figure 7.

Figure 7. Impedance bonding via sheath bonding transformers

Application

Impedance bonding methods are generally considered less satisfactory compared to other methods. Therefore, they are not recommended for general use. However, bonding transformers can be economical in some isolated cases such as when suitable balancing for cross-bonding is impossible and single-point bonding is unacceptable (for instance, no empty duct is available for an ECC). Similarly, this method can be used when a spare cable (a fourth cable for a single circuit or a seventh cable for a double circuit) is installed. In this case, reconnecting the cross-bonding whenever the spare cable is needed is a lengthy and complex operation, whereas reconnecting of bonding transformers is simple and straightforward.

Advantages

Disadvantages

Additional vault space is required.
The impedance devices are relatively expensive since they must be designed to withstand fault currents
During normal operation, third harmonics can be induced into the sheath, and these can cause interference on nearby telecommunication lines.
Stray direct currents entering through the grounding can cause saturation of the iron cores and upset the operation of the reactors or transformers.

Design Considerations

To provide ground connections, the impedance devices are typically designed with centre taps or grounding points. The sheath bonding transformers must be designed so that they will not saturate because of induced sheath voltages produced by normal and short-term emergency operating currents. The cable sheaths are also connected to local ground using sheath voltage limiters.

Conclusions

Sheath bonding is one of the most important design aspects, requiring careful consideration, for high voltage power cable transmission. Cable sheath bonding serves a multitude of purposes and has a significant impact on the overall cost of the system and on the current rating of the cables.