Concept of bonding/earthing of power cables

E s ELEA ss R RE P

CONCEPT OF BONDING/ EARTHING OF POWER CABLES WITH SPECIAL EMPHASIS ON XLPE CABLES – ADVANTAGES

OF CO-AXIAL CABLES OVER

CONVENTIONAL SINGLE CORE

CABLES IN CROSS BONDING

SCHEME

ABSTRACT: In the present era power-system growth highly depends on the network laid for power cables as it has become difficult for utilities to find ROW for overhead transmission lines especially in metro cities. The most popular cable being used is

XLPE cable. The XLPE is the acronym of “Cross linked Polyethylene” insulation material.

To safeguard the cables, earthing/ bonding of metallic sheaths is essential. Unlike multicore cables, wherein two-point earthing is exercised, in the single core cable single point earthing is provided at the sending end and with SVL (Sheath Voltage Limiter) at the receiving end up to a short length say 3-4 km, however, if the route length is several km long and having unequal cable sections, cross bonding scheme is adopted. The selection of SVL and accurate earthing become very crucial so far power cable network design is concerned. The authors have written this paper highlighting the pros and cons of cross bonding scheme of sheaths with Co-axial cables against the conventional single core cables for transmission of Power to long distances having unequal cable section. Key words and Acronyms: XLPE Cable (Cross linked Polyethylenecable), SVL (Surge Voltage Limiter), Cross bonding, Co-axial cable. 1.0 Introduction: Prior to year 2018, XLPE Underground cables have assumed greater importance in the present-day scenario though, they are much costlier than the OH Transmission lines, on the following counts;

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a. Aesthetic sense; As UG cables are not visible from outside, these provide a clean and aesthetic view of the city / town where UG cables are laid. Clean India. b. They do not require ROW (Right of Way), unlike OH transmission lines. c. UG cables can be laid in the densely populated residential areas. d. Unlike Transmission lines, Underground cables are not exposed to air/atmosphere, thus negligible susceptibility for outages due to various atmospheric conditions like lightening, hurricane/cyclones/typhoons etc. e. Hazard to wildlife: Underground cables generally pose no hazard to wildlife as compared to overhead network. [1] f. Dielectric-Loss factor of XLPE cables is also less as compared to paper insulated and PVC insulated cables. g. Current carrying capability: It has capability of carrying large currents under normal(90oC), emergency (130o C) or short circuit (250o C) conditions. h. Last but not the least, it is most important component in the indoor GIS (Gas insulated Sub-stations) which are installed where space is the constraints. For the EHV application XLPE (Cross linked Poly Ethylene) cables are being extensively used because of their superiority over other underground cables. They are generally single core cables.

1.1 Brief description of the XLPE cables; XLPE is an abbreviation of “Cross linked Polyethylene”. Following is the chemical formula;

Fig.1: Chemical Formula of molecules forming XLPE bonding. Figure.2: XLPE Cable with corrugated sheath.

Note I. Conductor can be Milliken conductor or circular conductor of Aluminum or copper. II. Corrugated Aluminum sheath (shown in item no.6) facilitates sharp bending of the cable as well as can carry heavy Earth fault currents.

Fig.3: XLPE Cable with copper wire screen and Lead alloy sheath. Note: I. Unlike Aluminum sheath Lead sheath is not capable of carrying high earth fault currents due to high fault levels therefore Copper-wire screen is provided. 2.0 Salient Features of XLPE cables: a) Excellent Electrical and Physical Properties: Capability of carrying large currents. It has excellent resistance to thermal deformation and excellent aging process permit it to carry large current under normal(90OC), emergency (130o C) or short circuit (250o C). b) Ease of Installation:

XLPE cable withstands smaller radius bending and is lighter in weight, allowing for easy and reliable installation.

Further, more, the splicing and terminating methods for

XLPE cable are simpler in comparison with other kinds of cables. c) Free from Height Limitation and Maintenance

2.1 Short - circuit current capacity of conductors. The table 1, contains the maximum admissible short-circuit currents Ik,1s for conductors acc. to IEC 60949 with a duration of 1 second for the Copper and Aluminum conductors and insulation types-XLPE Vs Oil.[3]

Table1: Admissible short circuit currents. [2] 2.2 Life expectancy and general practice: Life expectancy of XLPE cable is > 40 years. Looking to expected load growth/demand and to avoid need for augmenting the cables from time to time, it is a general practice by the utilities to use higher conductor size. 3.0 Induced voltages on cable screens; 3.1 Electro magnetic voltage Induction; The curreent flow in the inner conductor of the cable creates a magnetioc field which induces voltage on to the own cable screen as well as on to the screen of neighbouring/ surrounding cables. If cable screens are connected forming a loop, by applying Ohm’s law a current flows in that loop as a result of induced voltage and the resistivity of the cable-screen material, thus produces additional power losses.

Fig. 4 : Development of Induced voltage in the neighbouring cable screen/ sheath. 3.2 Induced voltage; Ui =Xm.I.L Where, Xm=Ω.M(Ω/km), Mutual inductive reactance between core and the sheath. I= in Amp ,current in the cable conductor, L= length of cable section in kms.

Fig. 5: Magnetic fields of neighboring 3 cables encompassing each other.

3.3 cable layout

Fig.6. flat cable layout

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Figure 7: Trefoil cable layout

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3.4 Dependence of Induced voltages; • Cable dimensions and cable lay-out

¾ Axial spacing between conductors(S). ¾ Cable sheath diameter(D). • Length of the cable section(L). • Network frequency (ω=2πf). • Short circuit current (worst case) • -Failure mode (Single phase, three phase). 3 .5 Graphical representation of dependance of induced voltages:Induced Voltages depend on ; • Cable length, • Frequency, • Conductor current, • Sheath diameter & • Axial spacing.

Fig8: Graphical representation of dependance of Sheath voltage on cable length,System frequency,Conductor current,Sheath diameter and axial spacing 3.6 Maximum Value of induced voltage ; • Limited by Electrical properties of outer sheath. • Withstanding voltage 10kV DC for 1 min as per IEC, • Over voltage due to short circuits must be below this value. • Higher values to be limited by SVL. • Typical SLVs are of 3kV and 6kV nominal values Note: ¾ Overload factor 1.36. ¾ Tolerance factor approx 1.1 ¾ Residual voltage = approx 3 x nominal voltage of SVL.

4.0 Earthing/Bonding methods and Induced voltages: 4.1 Both end Bonding: Both ends of the cable sheath are connected to the system Earth. Advantage: i. • With this method no induced voltages occur at the cable ends, which makes it most secure regarding safety aspects. Disadvantages; • Circulating currents would flow in the sheath as the loop between the two earthing points if closed through the ground. • These circulating currents are proportional to the conductor currents resulting into reduction of the ampacity of the cable.

Fig 10: Single Ended Bonding.[2]

Fig 9A: Both end bonding. [2]

Fig 9B: Both ends Bonding. [3] 4.2 Single-end bonding: Sending end of the cable sheath is connected to the system earth. The other end is open thus standing voltage shall appear which is induced linearly along the cable length. In order to ensure safety requirements, the open end of the cable sheath has to be protected with surge arrester (Sheath Voltage Limiter). The SVLs are basically a metal oxide varistors (MOVs) surge arrester which deflect switching and atmospheric surges but must not trigger in the short circuits.

4.3 ECC (Earth Continuity Cable); • In order to avoid potential raise in case of a failure, both earth points have to be connected additionally with an earth continuity cable. • To avoid/ minimize different earth potentials at both ends of the cable. • To ensure correct operation of SVL s. Requirements; • Ability to carry the single-phase short circuit current; • Must be placed in an induction-free manner to avoid difference in potential/circulating currents.

Fig 11A :3 HV cables in Trefoil formation directly earthed at sending end and SVL at receiving end with ECC.[3]

Fig 11B: Single End bonding with ECC Transposed at central cable at the middle of length to cancel out induced voltage.[3]

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• Placement: between the power cables ,70% of cable axial distance apart from Center cable.

Transposition: ECC is transposed to the other side in the middle of the cable system to cancel out induced voltage. • The ECC has to be insulated. 3.4 Single end bonding at a glance;

Aspects/ subjects

Safety

Circulating current Advantages Disadvantages

No circulating currents possible as there will not be an earth-loop and thus Ampacity of the cable is not impaired. Potentially dangerous as induced voltage occurs proportional to length of the cable section, frequency, conductor current with the length of the cable. Protection against accidental contact essential

Losses Since no circulating currents, thus additional losses will be minimal (eddy currents only).

Length limit Length of the cable section is limited as induced voltages have to be limited to certain values

Cost economics

Application Laying of an earth continuity cable alongside the Power cables for ensuring correct functioning of SVLs.

Typically for short HV connections but also for LV and MV In case of large phase distances.

Table-2 4.0 Cross bonding of sheaths of cables: 4.1.1 Cross bonding of sheaths with SVL at Bonding Location;

Figure 12A: Cross bonding methodology; development of induced voltages shown section-wise.[3] Please refer to figure 12A; • Generally used for long distance transfer of Power. • Minimum section of cables has to be 3 equal lengths. • Generally equal length of cables is cross bonded. • As could be seen from the figure 12A that there are 3 equal sections of the cables. The sheath of the cables of all the 3 phases are connected with each other in the cross-bonding system. • The sheaths of the cable of L1, L2 & L3 are earthed at the starting point of section 1and at the end of the 3rd section they are again earthed. 4.1.2 Transposing of cables and Cross bonding of sheaths thereof;

Figure 12 B; Cables transposed at each bonding point.[3] Please refer to figure 12B; (i) For very long cables lengths or parallel circuits, in addition to cross bonding of the sheaths, the conductor of each phase may also be transposed as shown in the figure 12 B.

4.2.1Cross bonding of the sheaths at the cable junction boxes; Cross bonding of sheaths done at the location of cable Jointing box through coaxial cable from each of the phase of the cables through coaxial cables through links with SVLs in the CB box as shown in the Figure 13A.

Fig 13A: Cross bonding of the sheaths of the individual phase cable. 4.2.2.1Cross bonding box.; It can be used for installation in the floor or can be mounted on the walls as the case may be. (fig;13B)

Fig 13B: Cross bonding box. 5.0 Cross bonding of sheaths of long cables lengths- Coaxial cable Vs Single core Cables; One of the FAQs by the customers to the executors of the Underground cables laying projects/ proposers is; “Why do you prefer using coaxial cables over single core cables for cross-bonding purposes of HV/EHV power cables.” The relevant explanation to this effect is being mentioned here-under; There is no restriction for extending power supply through Underground cables of lead length beyond 1500 meters as cross bonding of sheaths are in series. 5.1 Cross bonding of Sheaths; Advantages of Coaxial cable over single core cable. (i) It has been already mentioned that when all the 3-sections of cables have to be equal in length, there will be neither any induced voltages nor circulating current however, if all 3 sections are not equal then there will be circulating current due to development of induced voltages. (ii) Graphical representation effect due to unequal cable section lengths;

Fig 14: Graphical representation of Ampacity & screen losses due to unequal cable section. As already explained the importance and advantages of equal cable sections of all the three phases, however, the graph and the table shown in the figure 14 is explained below; (i) As could be seen from the table of Cross Bonding Length Ratio, against 2 in figure 14 Cable section “c” is longer than the section a & b, (ii) The ratio of all the cable sections should be 1:1. (iii) If the ratio of the cable section is not 1:1 there will be unequal induced voltages which will give rise to some circulating current. (iv) Due to the circulating current as explained in Sr. No.(iii), the Ampacity of the cable shall reduce. Such cases are generally found in the metro-cities. (v) In such cases due engineering is done, losses due to less Ampacity are taken into account and benefit thereof is passed on to the proposed consumer. 5.2 Comparison between cross bonding with Coaxial cable and Single core cable;

Co-axial cable Cross bonding Single core Cross bonding

Limiting length of the cable For long distance For short distance

Limiting of cable voltage above 132kV Cable section

Un-equality of cables section Cable sections 3 nos, in series of equal lengths to long distance Can be used as in equal 3 sections till the last span may be longer than the previous series of sections. Up to 100kV Cable sections 3 nos. of equal lengths in series for short distance.

Not possible.

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Ratio of Cable sections If more than 1:1, due to difference in induced voltages circulating current shall be produced resulting into lowering down of ampacity. Reactance/ Impedance Reactance low Effect of short circuit heavy current fault Transient voltage developed is lower therefore damage to cable insulation is overruled. Has to be 1:1 only

Reactance high Transient voltages shall be higher and may damage the cable insulation.

6.0 Short circuit considerations.

Fig 15: Co-axial cable (i) Since, it is a coaxial cable the reactance will be low and the Impedance too will be low, However, if single core cable is used the impedance of the cable is higher than Coaxial cable. (ii) In case of short circuits heavy current shall flow and thus the Transient voltage developed shall be low in the Co-axial cable as compared to the single core cable. (iii) Due to low transient voltages the possibility of damage to the cable /cable -sheath is over ruled. (iv) As per standards it should not be more than 10 meters. (v) If more than 10 meters then complete engineering is to be done for reducing the Impedance of the Co-axial cable. 7.0 Selection of proper SVL; The SVLs are very important equipment used in single end bonding of screen/sheath as well as in the cross-bonding schemes. Once the over voltages are known on each cable section in power frequency mode (normal operation and short circuit) then appropriate SLV is selected as follows; a) The limiter should be suitable for continuous operation with an applied voltage equal to the shield/sheath standing voltage under either normal or emergency loads.[4] b) The surge voltage limiter must be designed to dissipate the energy associated with the transient over voltages impressed upon.[4] c) The maximum Continuous operating voltage Uc of the SVL must always be higher than the maximum induced voltage at the cable section during short circuit (this is extremely important failing which SVL will be destroyed). Note: It is recommended to use 3kV SVL for 66kV cables and 6kV SVL for 132 and above. For example, if the Uc is say 4kV then SVL to be used should be of 6kV.[2] d) The residual voltage Ures of SVL must be lower than the impulse withstand- voltage of the cable outer jacket and joint outer protections, typically considering the recommended15-20% safety margin. Note: The function of SVL is to protect the cable outer jacket if residual voltage of SVL is more than the cable outer jacket, the outer jacket will fail in case of the fault. 8.0 Conclusion; It should be strictly borne in mind that Power transfer through cables is many folds costlier than the overhead transmission lines, therefore all the following necessary precautions should be followed scrupulously; (i) Detailed DPR should be prepared visiting the site. (ii) Cable size should be proposed taking care for any expected augmentation of load in that area. (iii) Type of sheath bonding to be required. (iv) In case of long cable lengths, whether only cross bonding shall serve the purpose or transposing of cables is also required along with the cross bonding. (v) Whether cross bonding shall be done with single core cables or with the co-axial cables. (vi) Cross bonding at a glance;

(vii) Appropriate kV rating of SVLs to be used. (viii) Scrupulous supervision of cable laying is very essential. (ix) Last but not the least it is essential to grout cable markers to identify the route of the cable. References: 1. CEA’s Guidelines for use of under-ground cable system and Overhead conductor system along with cost benefit analysis. 2. Brugg Cables; High Voltage XLPE Cable Systems Technical

User Guide. 3. Hitachi- ABB; Cable sheaths over voltage protection-

Application note 3.1. 4. IEEE Std.575-2014; IEEE guide for bonding shields and sheaths of single conductor Power cables through 5kV through 500kV. EM

Dr. RAJESH KUMAR ARORA obtained the B. Tech. & Master of Engineering (ME) degrees in Electrical Engineering from Delhi College of Engineering, University of Delhi, India in 1999 and 2003 respectively. He completed his PhD in grounding system design from UPES, Dehradun. He is also certified Energy Manager and Auditor and has worked in 400kV and 220kV Substation for more than 14 years in Delhi Transco Limited (DTL). He has also worked as Deputy Director (Transmission and Distribution) in Delhi Electricity Regulatory Commission (DERC) for 03 years and 06 months. He has also given his contribution in the OS department of DTL for more than 2 years and rendered his services in the SLDC of Delhi Transco Limited (DTL) also. Presently he is working in D&E (Design and Engineering) department of DTL. His research interests include high voltage technology, grounding system, protection system, computer application and power distribution automation.

–Author–

Er. K.K. Murty

K.K. Murthy holds a degree of B.E. (Hons) in Electrical Engineering obtained from the University of Jabalpur in the year 1968. He was a former Chief Engineer and Head of Department (Testing & Commun.) in M.P. Power Transmission Co. Ltd. Jabalpur (India)., he was a member of the panel of Expert Professionals at the Central Power Research Institute (CPRI), Bangalore, from 2008 to 2012. Prior to this, he worked as an Advisor (Testing) at SOUTHCO, a DISCOM in the State of Odisha, a metering consultant to M.P. Electricity Regulatory Commission and a Course Director for the Graduate Electrical Engineering Trainees at the Training Institute of M.P. Power Transmission Co. Ltd., Jabalpur. He has published many technical articles in the national and international journals and presented technical papers at various national and international conferences pertaining to the Power Transformers and other equipment of power sector. He is a member of India’s Society of Power Engineers (MSPE), a Fellow of Institution of Engineers, India (FIE), a Chartered Engineer (CE) and a Member CIGRE’ India. He had been awarded a plaque by the Institution of Engineers (India), Kolkata, in Oct. 2015, in recognition of his eminence and contribution to the profession of Electrical Engineering at the National level.

–Author–

Dr. Rajesh Kumar Arora