Analysis Of Current Carrying Capacity Of Ehv Single Core Cables

09 Dec 2024

ABSTRACT:

India, with its rapidly growing population, expanding industrial base, and accelerating urbanization, faces an increasing demand for electricity. To meet these demands, the country’s power transmission infrastructure has to be both robust and efficient. Among the key components in ensuring reliable power delivery across the country are Extra High Voltage (EHV) power cables, which play a critical role in long-distance power transmission, especially in situations where overhead lines may not be viable or desirable.

EHV power cables are designed to transmit electricity at voltage levels typically 220 kV or 400 kV in India. These cables are typically used for bulk power transmission in regions where traditional overhead power lines face challenges due to environmental, safety, or logistical factors. In India, the importance of EHV power cables cannot be overstated, particularly in the context of ensuring stable, reliable, and efficient power delivery across diverse geographies, environments, and industries. In the present situation the underground cables has become the only solution for electric transmission in urban areas or crowded cities when the ROW for overhead transmission lines are not available. During designing and selection of EHV cable, many electrical parameters aspects must be taken into consideration like: Cable voltage, short circuit calculation, cable ampacity, laying configuration, cable length, grounding and bonding of cables etc. Cable earthing /grounding is a very important aspect as it decides the induced voltage generation in the cable sheath which causes overheating and leading to decrease the cable ampacity. This paper provides in-depth analysis of various parameters affecting the current carrying capacity / ampacity of EHV single core cable.

KEY WORDS: Underground Cable, Conductor, Insulation, Grounding, Bonding, Current Carrying Capacity

 

 1.0 INTRODUCTION

Electric power can be transmitted or distributed either by overhead system or by underground cables. The underground cables have several advantages such as less liable to damage through storms or lightning, low maintenance cost, less chances of faults, smaller voltage drop and better general appearance. However, their major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with the equivalent overhead system. For this reason, underground cables are employed where it is impracticable to use overhead lines. Such locations may be thickly populated areas where municipal authorities prohibit over headlines for reasons of safety, or around plants and substations or where maintenance conditions do not permit the use of overhead construction .The chief use of underground cables for many years has been for distribution of electric power in congested urban areas at comparatively lower moderate voltages. However, recent improvements in the design and manufacturing have led to the development of cables suitable for use at high voltages. This has made it possible to employ underground cables for transmission of electric power for short or moderate distances. (Refer figure 1)

An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover. Although several types of cables are available,the type of cable to be used will depend upon the working voltage and service requirements. In general, a cable must fulfil the following necessary requirements:

1. The conductor used in cables should be tinned stranded copper or aluminium of high conductivity. Stranding is done so that conductor may become flexible and carry more current.

2. The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.

3. The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed.

4. The cable must be provided with suitable mechanical protection so that it may with stand the rough use in laying it.

5. The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.

 

 1.1 CONSTRUCTION OF EHV CABLES

1. The main conductor which is responsible for current transferring could be stranded or segmented depending on the conductor cross section. (Refer figure 2)

2. The conductor screen is a semiconducting screen to achieve homogeneous field within insulation.

3. The insulation is XLPE (cross-linked poly ethylene) with dielectric properties surrounding the cable conductor, the surface of the insulation should be clean from any dust or voids which can cause partial discharge leading to complete breakdown.

4. The insulation screen is a semiconductor screen to achieve homogenous electric field within insulation.

5. The metallic screen is the moisture barrier and provides the main grounding for the cable against short circuit current in case of faults or the capacitive charging current.

6. Outer protective jacket is the mechanical protection for the metallic sheath and anticorrosion layer, usually it consist of polymeric material PE (Polyethylene) or PVC (poly vinyl chloride).

Several factors influence the current carrying capacity of power cables are Conductor Material,Cable Size, Insulation Type,Ambient Temperature, Installation Conditions ,Length of the Cable, Frequency of Current, Cable Arrangement (The way cables are arranged, such as in groups or bundles and number of circuits & Depth of laying of cables). Last but not the least Earthing / Bonding Methods have also impact on the current rating capacity of the cable.

 

2.0 Power Cable Laying Methods

Depending on factors like the type of cable, environmental conditions, load requirements, and the desired longevity of the installation, various methods can be used for laying power cables. Among the most common techniques are direct burial, ducting, trefoil, and flat cable laying methods. Each of these methods has distinct advantages, limitations, and suitable applications.

 

A. Direct Burial Method:

Direct burial is the process of laying electrical cables directly into the ground without the need for conduits or ducts. The cables are typically buried at a specified depth, ensuring safety and protection from physical damage and environmental factors. This method is often employed in outdoor installations where space constraints or cost considerations make it less feasible to use underground ducts. (Refer figure 3)

A. In Duct Method:

The duct method involves laying power cables inside protective conduits (ducts) that are buried underground. These ducts provide a protective barrier around the cable, shielding it from physical damage, moisture, and environmental factors. The ducts can be made from a variety of materials such as PVC, HDPE (High-Density Polyethylene), concrete, or steel, depending on the application requirements. (Refer figure 4)

 

 

B. Trefoil Cable Laying Method:

Trefoil laying refers to the arrangement of three cables in a triangular formation, often used for highvoltage (HV) or medium-voltage power cables. The trefoil configuration optimizes the magnetic fields generated by the current flowing through the cables, reducing losses and minimizing the risk of overheating. This method is highly effective in both underground and aerial installations, especially when cables need to be grouped tightly together.

B. Flat Cable Laying Method:

In the flat laying method, cables are laid in a horizontal or vertical flat formation. Typically, the cables are arranged in parallel, with each cable maintaining a specific distance from the others. This configuration is suitable for both mediumand low-voltage cables, and it is commonly used in scenarios where the space is wide enough to accommodate multiple cables in a single row. (Refer figure 5) The choice of cable laying method depends on several factors, including environmental conditions, cost considerations, space availability, and the long-term maintenance needs of the electrical system. Direct burial offers a low-cost and efficient solution for areas where physical damage risks are minimal, but it can pose challenges for long-term maintenance. The duct method provides a more flexible and protective solution but comes at a higher installation cost. Trefoil and flat cable laying methods each offer distinct advantages for managing heat dissipation, reducing electromagnetic interference, and optimizing the configuration for multi-phase systems.

  

3.0 EATHING & BONDING OF POWER CABLES

One of the key considerations when laying power cables, especially in low- and medium-voltage systems, is ensuring the safety of the installation through earthing and bonding systems. These systems help protect the cable, electrical equipment, and people from electrical faults and potential hazards like electric shock, fire, or damage due to fault currents. Any sheath bonding or grounding method must perform the following functions:

1. Reduce or eliminate the sheath losses

2. Limit sheath voltages as required by the sheath sectionalizing joints.

3. Maintain a continuous sheath circuit to permit fault-current return, and adequate lightning and switching surge protection

 

3.1 SINGLE POINT BONDING

The simplest form of special bonding consists in arranging for the sheaths of the three cables to be connected and grounded at one point only along their length.Typically the supply or source end, is connected to the earth or ground. The other end of the cable is left floating or not grounded. At all other points, a voltage will appear from sheath to ground that will be a maximum at the farthest point from the ground bond. The sheaths must therefore be adequately insulated from ground. Since there is no closed sheath circuit, except through the sheath voltage limiter (if any), current does not normally flow longitudinally along the sheaths and no sheath circulating current loss occurs (sheath eddy loss will still be present) shown in figure 6.

 

3.1.1 Types of single point bonding

Types are divided according to cable length , if the cable length is less than 500 m or one drum length then one side bonding is applied, but if the cable length is from 300m to 1000m long cable section then split bonded system (mid point bonding system) is applied. The induced voltages in different bonding systems are shown in figure 7.

Advantage:

The ampacity is higher compared with both sides bonding, as there are practically no circulating current in cable sheath.

Disadvantage:

1. As the screen is open, there are no circulating currents, but there is an induced voltage appearing at the open cable screen, where sheath voltage limiters must be installed in order to protect the cable outer jacket against transient overvoltage.

2. The length of one section is limited as the magnitude of the induced voltage is increasing with length.

3. During a ground fault on the power system, the zero-sequence current carried by the cable conductors could return by whatever external paths are available. A ground fault in the immediate vicinity of the cable can cause a large difference in ground potential rise between the two ends of the cable system, posing hazards to personnel and equipment.

 

For this reason, single-point bonded cable installations need a parallel ground conductor, grounded at both ends of the cable route and installed very close to the cable conductors, to carry the fault current during ground faults and to limit the voltage rise of the sheath during ground faults to an acceptable level. The parallel ground continuity conductor (PGCC) is usually insulated to avoid corrosion and it is transposed, if the cables are not transposed, to avoid circulating currents and losses during normal operating conditions. The sheath voltage reduction rate of transposed PGCC is better than with no transposed PGCC.

 

3.1 BOTH ENDS BONDING:

The metallic sheaths are grounded at least at the two extremes of the cable. This system doesn’t allow high values of the induced voltages in the metallic sheaths. When dealing with both ends bonding cables, sheath circulating current loss occurs because there is a closed circuit current. This type is applied for MV and LV cable system with short distance. (Refer figure 8 and 9)

 

Advantage:

1. The two cable ends are directly connected to earth with no sheath voltage limiter

2. No need for parallel continuity earthing cable

3. No stand voltage at the end of the cable

Disadvantage:

Establishing a closed circuit for the induced current causing over heat and consequent reduction in the cable ampacity.

 

3.3 CROSS BONDING METHOD

For underground circuits, longer than 1 km, the losses on the metallic sheaths can be minimized making in each joint a cross bonding of the cable shields. The most basic form of cross bonding consists of sectionalizing the cable into three minor sections of equal length and cross connecting the sheaths between each two-minor section. Three minor cable sections form a major section (Refer figure 10).

The natural points to establish the crossings are the joints, where appropriate cross bonding link boxes connected to the sheaths, these link boxes include SVL. This method provides balanced induced voltages which are separated at an angle of 120° so that there is no resultant circulating current. The methods of single end earthing, both end earthing, and cross bonding are integral to ensuring the safety, reliability, and efficiency of power cable installations. Each method has its own set of advantages and challenges, and the selection of the appropriate method depends on factors such as voltage levels, installation environment, fault tolerance, and cost considerations.

 

4.0 ANALYSIS OF CURRENT CARRYING CAPACITY

The current carrying capacity of power cables, often referred to as ampacity, is a critical aspect of electrical engineering and power transmission & distribution. It determines the maximum amount of electric current a cable can carry without exceeding its temperature limits, which could lead to insulation failure, energy loss, or even fire hazards. Understanding ampacity is essential for ensuring safety, reliability, and efficiency in electrical systems.

 

4.1 Factors Influencing Current Carrying Capacity

Several factors influence the current carrying capacity of power cables:

1. Conductor Material: The material used in the conductor (typically copper or aluminum) significantly impacts its resistance and thermal conductivity. Copper, with its higher conductivity, generally allows for greater current carrying capacity compared to aluminum.

2. Cable Size: The cross-sectional area of the conductor is directly proportional to its ampacity. Larger conductors can carry more current due to lower resistance.

3. Insulation Type: The type of insulation material used can affect the temperature rating of the cable. Higher-rated insulations allow for greater temperature tolerance and, consequently, higher ampacity.

4. Ambient Temperature: The surrounding temperature affects the cable’s ability to dissipate heat. Higher ambient temperatures reduce the current carrying capacity, as the cable can reach its maximum operating temperature more quickly.

5. Installation Conditions: The environment in which the cable is installed—such as whether it is buried underground, installed in conduit, or exposed to air—affects its heat dissipation capabilities. For example, cables installed in ducts or bundles may have reduced ampacity due to restricted airflow.

6. Length of the Cable: Longer cables experience higher resistive losses, which can affect their effective ampacity. Voltage drop over long distances must also be considered in the design of power systems.

7. Frequency of Current: The frequency of the electrical supply can also impact current carrying capacity. Higher frequencies may lead to increased skin effect, which alters the effective resistance of the conductor.

8. Cable Arrangement: The way cables are arranged, such as in groups or bundles and number of circuits (cables) & Depth of laying of cables, can lead to mutual heating effects, which may further reduce the ampacity of individual cables.

9. Earthing / Bonding Methods: The methods of single end earthing, both end earthing, and cross bonding have impact on the current rating capacity of the cable.

 

4.2 Standards and Calculation Methods

C. IEC 60287 outlines methods for calculating the current carrying capacity of insulated cables. This method considers the heat generated by the current flowing through the conductor and the heat dissipation to the surroundings. The balance between heat generation and dissipation defines the maximum permissible current. (Refer table 1)

 

 

4.3 Thermal Resistance of Power Cables

Thermal resistance, often denoted as Rth, measures a material’s ability to resist heat flow. It is defined as the temperature difference across a material divided by the heat transfer through it. In the context of power cables, thermal resistance can be understood through several key parameters:

1. Conduction: The primary mode of heat transfer in cables, conduction occurs through the cable materials, including the conductor, insulation, and sheath.

2. Convection: In cables installed in air or buried underground, heat transfer can also occur through convection. This process involves the movement of heat by the fluid (air or soil) surrounding the cable.

3. Radiation: While less significant compared to conduction and convection, radiation can play a role in heat dissipation, especially in hightemperature environments. The overall thermal resistance of a power cable can be expressed as the sum of the resistances from each of these mechanisms.

Factors Affecting Thermal Resistance

1. Material Properties: The thermal conductivity of the conductor (usually copper or aluminum) and the insulation material (often PVC, XLPE, or EPR) greatly affects thermal resistance. Materials with high thermal conductivity provide lower thermal resistance, allowing heat to dissipate more effectively.

2. Geometric Configuration: The cross-sectional area of the conductor, insulation thickness, and overall cable diameter influence how heat spreads throughout the cable. A larger conductor area typically results in lower resistance and better heat dissipation.

3. Installation Conditions: The environment where the cable is installed—whether in open air, conduit, or buried underground— affects thermal resistance. Cables in open air benefit from better convection compared to those buried in soil, where thermal resistivity is often higher due to lower air movement.

4. Temperature Ratings: Cables are designed to operate within specific temperature ranges. Exceeding these limits can increase thermal resistance due to changes in material properties, potentially leading to insulation breakdown or reduced conductivity.

The unit of thermal resistance in the context of power cables is typically expressed in degrees Celsius per watt (°C/W). This unit indicates how many degrees Celsius the temperature will rise per watt of power dissipated as heat. In some cases, especially in specific engineering contexts, thermal resistance may also be expressed in kelvins per watt (K/W), as the difference between Celsius and Kelvin is constant when discussing temperature differences. Overall, both °C/W and K/W are used interchangeably to describe thermal resistance in power cables.

 

 4.3 Ampacity Calculation – Case Studies

As explained above there are several factors which influence the current carrying capacity of power cables i.e. Conductor Material, Cable Size, Insulation Type, Ambient Temperature, Installation Conditions ,Length of the Cable, Frequency of Current, Cable Arrangement (The way cables are arranged, such as in groups or bundles and number of circuits & Depth of laying of cables). Last but not the least Earthing / Bonding Methods (The methods of single end earthing, both end earthing, and cross bonding have impact on the current rating capacity of the cable). The case studies showing impact of all these factors are given in table 2, table 3 , table 4 and table 5 respectively.

TABLE 2- AMPACITY VARIATION FOR SINGLE / DOUBLE CKT, TREFOIL / FLAT FORMATION/ EARTHING BONDING METHODS

 

 

Table 2- provide information about ampacity variation for single / double ckt, trefoil / flat formation/ earthing bonding methods. It can be seen that the maximum current capacity is achieved with single circuit having single/ cross end bonding with flat formation laid in ground.

 

 

TABLE 3- AMPACITY VARIATION FOR DIRECT/ DUCT BURIED, TREFOIL / FLAT FORMATION/ EARTHING BONDING METHODS

 

Table 3- provide insight about ampacity variation for single circuit with trefoil / flat formation/ earthing bonding methods. This provides good information about ampacity variation in duct buried cables.

 

TABLE 4- AMPACITY VARIATION FOR TREFOIL / FLAT FORMATION (WITH DIFFERENT PHASE SPACING)

 

Table 4- provide insight about ampacity variation with trefoil / flat formation of various phase to phase spacing.it can be seen that the increase in the phase spacing in flat formation has positive results.

TABLE 5- AMPACITY VARIATION FOR TREFOIL / FLAT FORMATION (WITH DIFFERENT DEPTH OF LAYING)

 

Table 5- provide insight about ampacity variation with trefoil / flat formation of various depth of laying of cables. It can be seen that with increase in the depth the ampacity of cable decreases.

 

 

5.0 CONCLUSION

During designing and selection of EHV cable, many electrical parameters aspects must be taken into consideration like: Cable voltage, short circuit calculation, cable ampacity, laying configuration, cable length, grounding and bonding of cables etc. Cable earthing /grounding is a very important aspect as it decides the induced voltage generation in the cable sheath which causes overheating and leading to decrease the cable ampacity. This paper has provided in-depth analysis of various parameters affecting the current carrying capacity / ampacity of EHV single core cable. Several factors influence the current carrying capacity of power cables are Conductor Material,Cable Size, Insulation Type,Ambient Temperature, Installation Conditions ,Length of the Cable, Frequency of Current, Cable Arrangement (The way cables are arranged, such as in groups or bundles and number of circuits & Depth of laying of cables). Last but not the least Earthing / Bonding Methods have also impact on the current rating capacity of the cable The case studies with different scenarios has provided useful information for field engineers.

REFERENCES

[1] IEC 60228 Conductors of Insulated Cables.

[2] IEC 60287 Current Capacity of Cables

[3] IEC 60067 Power Cable with Extruded Insulation – Test Method and Requirement

[4] IEC 60949 Calculation of Thermal Permissible Short Circuit Current

[5] IEEE 575 Guide for Bonding Shields and Sheaths of Single-Conductor Power Cables Rated 5 kV through 500 kV

[6] CIGRE 283 Special Bonding of High Voltage Power Cables

[7] BICC Cable Hand Book