CROSS BONDING DESIGN OF EHV CABLE SYSTEM

ABSTRACT

Underground power transmission and Gas insulated substation are increasing day by day due to right of way (ROW) and smart city initiative. This increases the demand of subterranean cable networks in a medium and long-distance power transmission. Cable is completely shielded by metallic sheath, which not only provide a ground potential surface over the cable, but also provide the short circuit current flow path during the grounding fault condition. However, during normal operating condition of the cable, the outer metallic sheath will also carry a current due to EMF induced onto it by the main current carry conductor. This undesirable current attributes towards increasing the sheath loss. Power loss minimization is one of the key indices for the utilities and industries due to stringent government policy and global initiative in energy management. The energy loss over the metallic sheath increases the cable temperature which results in reduction of cable ampacity. Proper grounding system design for the cable network can minimize the power loss without compromising safety aspect. Cross bonding of 3-phase cable system is a widely used method for minimizing the power loss over the cable sheath. In the present work an attempt has been made to provide insight of cross bonding design for single and multiple run of the 3-phase cable system.

KEYWORDS

Underground cable, grounding system, induced EMF, earthing, ampacity, cross bonding, power loss, link box

1) INTRODUCTION 

The current carry conductor of the power cable creates a time varying magnetic field which will imposes an EMF generation over the cable metallic sheath. It is important to keep the sheath voltage within a pre-specified limit [1, 2] for safety aspect. Earthing of the metallic sheath has to be done to provide ‘zero’ potential at cables outer surface. However, EMF generation due to main current in the cable conductor will induce a voltage; hence cable metallic sheath needs to be grounded at more than one point to limit the sheath voltage [2]. Earthing (sometime time refer as bonding) at multiple points of the cable network provides a current flow path due to induced EMF. Depending on the resistivity of the metallic sheath, this undesirable current produces ohmic loss over the metallic sheath. Part of this heat energy will dissipate on the main insulation layer of the cable. As a result, this heat energy will contribute to temperature rise of insulation layer in addition to heat energy produced due to ohmic loss in the main cable conductor. Network designer will consider this effect to limit the maximum current carry capacity (ampacity) of the cable. Apart from ampacity reduction, the ohmic loss over the metallic sheath is reduced the overall energy efficiency of the network. As a green initiative Raychem RPG has identified loss reduction as one of the key deliverables in their new product development portfolio. Different types of grounding system along with cross bonding technique in a 3-phase system are cost effective solution.

Cross bonding method [2, 3, 4] takes advantage of vector summation of more than one emf of different phase angle. Thus, the resultant EMF will be minimized or close to ‘zero’ which significantly reduces the undesired current flow and losses over the cable sheath. This method is very much useful when the cable network length is > 2km. Needless to mention, while there are more than one runs in parallel in cable network, EMF will have a mutual effect. This method is proven to be effective in such scenario provided design has considered mutual EMF effect.

2) RAYCHEM POWER LOSS REDUCTION PRODUCTS

Raychem has considered power loss reduction as one of the major drivers while developing new product and service solution for power transmission and distribution. It is offering compact connection system for different kind of transformers which reduces losses significantly. Compact distribution boxes offer not only ohmic loss reduction but also prevent power theft. Raychem provides guidance for cable grounding system design and cross bonding length design to minimize the power losses in cable and use cable network to its optimum ampacity. Three phases link boxes with and without sheath voltage limiter (SVL) is very much useful for transposition of cable and provide protection during the transient over voltages (Figure 1) [5]. Shield break joints are designed for cross bonding purposes, whereas straight through joints can be used while transposition is not required.

Fig. 1. Raychem link box for shield break cable joint

3) DIFFERENT BONDING SYSTEM

High voltage power cable should not be used without providing the solid grounding onto it. 

Metallic sheath grounding provides earth potential and also allow the short circuit current to ground through nearest grounding location. Several grounding systems are practiced depending on the cable network design.

a) Single point bonding- In this method cable sheath will be connected to the ground in one end of the cable system. The other end will be connected to the ground via a SVL (Figure 2). Due to single point earthing, current path is interrupted. Hence, power loss problem is   mitigated. However, metallic sheath potential will be raised as moving away from the solidly grounding end (Figure 2). This type of bonding   system needs a ground continuity conductor to minimize zero sequence impedance. This type of bonding system is useful when cable network   length is small(<500m).

Fig. 2. Single point bonding cable

b) Midpoint boding- In this type of bonding, the mid of the cable network will be connected to ground solidly and both the end terminal
will be connected via SVL. The maximum sheath voltage will be in the end terminal of the cable (figure 3). This can be used when there is a joint in the middle of the cable. This will not have circuiting current and also the sheath voltage will be less than the previous type of bonding. It can be utilized while cable length is ~ 1000 m.

Fig. 3 Midpoint bonding cable

c) Double point bonding- Here both end of the cable will be solidly earthed. The maximum potential due to sheath current will be in the middle of the cable (Figure 4). This type of system will have a circulating sheath current; however, the sheath potential will be limited. This can be used in case the cable length is not very high (&lt;1000m). This is generally used in a substation or industry.

Fig. 4 Double point bonding cable

d) Cross bonding- Here the cable will be bonded to the ground point in multiple locations. Within a minor section, the R–Y-B phase sheath will be transposed and at the end of the minor section (major section terminal) it will be connected to the ground (Figure 5). Minimum
3 minor sections and one major section are required. This can be used for a long cable network. This method will take advantage
that vector summation of 3 phase voltage will be ‘zero’ or close to zero. In other words, the resultant loss current will be minimum. This method needs link box to create transposition and provide safety ground via SVL. This required proper design of cable joint location
in a long cable network. This is widely used in a power transmission network.

Fig. 5 Cross bonding cable

e) Cross bonding with cable transposition- The sheath transposition alone always unable to bring down the vector sum of sheath voltage to ‘zero’. This considers effect of one phase current over other cable sheath voltage.

Fig. 6 Cross bonding cable with transposition

Here, cable core will be also transposed (Figure 6). This is ultimate bonding system. Here cable core position of different phase will be transposed. This needs a very careful design of transposition and joint location.

4. CROSS BONDING DESIGN
Minimum three minor sections is required to complete a transposition cycle between three phases, which will results a zero sheath voltage due to vector addition of voltage phasor. Induced voltage over cable sheath for a single circuit can be calculated using below formula (trefoil formation) [1],

Table 1. Input data and induced voltage 

Where I= Conductor current ω= 2πf
s= Axial spacing between 2 conductor d=
Geometric mean dia of sheath

Xac = k
Where k is a constant = 2ω10-7
S14 is the distance from cable 1 to
cable 4 S12 is the distance from cable 1 to cable 2 S15 is the distance from cable 1 to cable 5 S13 is the distance from cable 1 to cable 3 S16 is the distance from cable 1 to cable 6 A sample output chart (table 3), shows the joint location for a three-phase double
cable network,

Table 2. Output data sheet - Joint location

A Sample chart made for a single cable circuit
shown in table 1. The calculation sheet also
provide the joint location in the cable network
(table 2)
Similarly, cross bonding design can be done for
double cable network. The below formula can be
used,
E = Ia x jXaa + Ib x j Xab + Ic x J Xac V
Where,
E= Induced voltage
Ia, Ib, Ic = Current magnitude in a, b &amp; c phase.
Xaa = k

Table 3. Double circuit cable network Output – Joint location.

It needs to be noted that the screen voltage to be calculated for the rated fault current condition of the network. SVL for the link box has to withstand  the sheath voltage during the rated fault current condition. Table 4 shows an example of induced voltage during of phase-to-phase fault condition.

Table 4. SVL voltage calculation during a phase-to- phase fault

The cross-bonding link box is equipped with sheath voltage limiter (SVL) which protects the sheath from overvoltage during transient overvoltage situation. During switching or lightening overvoltage, the sheath voltage also becomes high. If there is a sufficient voltage difference between the cable sheath and earth, cable jacket will get punctured. This may cause moisture ingress in the cable. A SVL is limiting overvoltage on the cable sheath due to its nonlinear current- voltage characteristics. However, current carry capacity SVL is limited, so it cannot provide a path of fault current to the ground. With a limited energy capability of the SVL, a continuous path of fault current would lead to overheating and destruction of the SVL. Therefore, during fault current flowing through the sheath, the SVL shall be a highly resistive component and only during transient overvoltage its conductivity shall increase significantly to limit overvoltage. For the above example SVL to be triggered at 3 kV for the case discussed in table 4. In practical cases, a combined bonding system is often used by different utilities for long distance power transmission purpose. In such cases both or single end sections will be having single end bonding or midpoint bonding and remaining section will be having cross bonding design. Even though, cross bonding with transposition is the ultimate solution which is more like creates a three-core cable sheath effect, but due to practical difficulties this solution is not widely used. This also needs to be noted that the bonding location to be finally decided based upon the availability of joint bay in a cable lying way. The sheath voltage to be recalculated once more after getting the final location. The sheath voltage to be limited depending on country specific guidelines. For example, in India cable sheath voltage is limited to 65 V [1]. In European countries, the value is slightly higher side.

5. CONCLUSION

A detailed calculation method for cross bonding design for an underground long distance cable system has been developed based on the
sheath voltage calculation. Process has been proven while providing solution to transmission and distribution utilities. A guideline has been
given for selection of SVL considering the critical parameters.