In order to understand problems with cable systems, knowledge of cable architecture, properties, and ratings are essential. However, more information is needed to effectively choose a cable system & ensure its good functioning. This information may include service conditions, the kind of load handled the mode of operation, and maintenance procedures.
The selection of the power cables for a particular application depends on a number of parameters. Selecting it is thus not an easy process. Given the wide range of cables on the market, selection additionally becomes difficult.
Here we will examine a few of the most important aspects that influence the selection of power cables in this post.
1). Rated Voltage
An electrical cable that can withstand a certain system voltage must be selected.
The recommended voltage of the power cable in an AC system is to remain equal to or higher than the system voltage.
Consider using the following formula to get rated voltage:
If each conductor is connected to ground at the specified cable voltage (V0), then
The cable rated voltage (V) between phase conductors which is written as:
V = √3 V0
The power system engineers pertain for earth fault withstand limitations and requirements determine the precise rated voltage of power cable that should be used.
According to IEC standards, there are three classifications
- Category A: It requires that the earth fault be repaired within a second.
- Category B: Earth faults are resolved within an hour for cables of the IEC-183 type and eight hours for cables of the IEC-502 type.
- Category C: It includes all systems that not applied out of A and B.
Cables with the same rated voltage as the system voltage may be used for categories A and B. The recommended cable voltage for Category C should, however, be greater than the system voltage.
2). Current Carrying Capacity
Every power cable has been developed to function at a certain temperature.
The kind of insulation and conductor material (copper or aluminium) affects the power cable’s ability to transmit current.
Therefore, copper conductor cable can carry more current than aluminium.
Since XLPE (Cross-linked polyethylene) insulation is superior to PVC (Polyvinyl Chloride) insulation, XLPE (Cross-linked polyethylene) cable has a higher current carrying capability than PVC (Polyvinyl Chloride) insulated cable.
Continuously using a cable over its rated current carrying capability reduces its lifetime because the insulation is more likely to break.
Operating temperature also affects the current carrying capacity. The current carrying capability of the cable decreases with temperature and vice versa.
3). Voltage Drop
This is part of the data sheet provided by a power cable manufacturer. The voltage drop throughout the power cable’s whole length is important. The unit of measurement is mV/A-m.
In order to acquire voltage at the terminal point that is about equal to supply side, the voltage drop/unit length of cable should be as low as feasible.
5). Derating Factor
Standard conditions for operation may not be achieved by a power wire in practical applications.
Therefore, this might have an effect on the present carrying capacity.
Ex: Deep-buried wires will have a lower current carrying capability than cables deployed in the air. Multiple elements, such as soil temperature and thermal resistance, have an influence on transmission.
In order to accomplish this, cables are given a derating factor in order to determine their true value for current carrying capacity.
Derating factor times the cable’s current carrying capacity under standard conditions equals actual current carrying capacity.
Therefore, the actual current carrying capacity for a 100 A cable with a 0.8 derating factor would be: 0.8 x 100 = 80 A.
6). Cable Availability
This has to be verified with the cable’s manufacturer or distributor. Cables are produced in segments of a specific minimum length. A 30m length of 300 mm2 cable will thus be more difficult to find than a 300m length of the same wire.
Additionally, the costs for the two amounts may differ significantly.
7). Withstand A Short Circuit
A power cable should be able to tolerate high current values in the event of a short circuit without causing any harm to the cable or insulation.
The specification of the linked protection device directly affects the choice of a power cable’s short circuit current withstand capability.
Ex: If a power cable is connected to a breaker that is programmed to trip at 1000 A in 1 second, it is necessary to choose a cable that can resist high current of 1000 A for 1 second.
8). Bending Radius
This may present a practical issue while installing. Multi-core cables of larger sizes may bend more easily than those of smaller sizes. As a result, a multi-core XLPE (Cross-linked polyethylene) cable of the same size has a larger bending radius than a PVC (Polyvinyl Chloride) cable.
A professional electrician could use separate single core wires to overcome this radius issue.
9) Conductor Type
Copper or aluminium is the two conductors that are most often used in cables. As is well known, the continuous current rating, the short time current rating, and the cost per unit length of a Copper cable are much greater than those of an Aluminium cable for the insulation, type, same voltage rating, cross sectional area, and method of installation.
10). Cables Types
- Unarmoured cables or
- Armoured cables
must be utilised for any underground cable installation; unarmoured cables are used for interior installations and above ground installations such as cable trays, pre-built concrete cable trenches, etc.
The armour may be a wire or a strip composed of aluminium or galvanised iron. Frequently, this armour is only linked to the plant’s earthing system at one end, usually the transmitting end.
11). Rating Factors
The general criteria for rating to be taken into consideration are as follows:
- Rating factors for changes in the ambient temperature, ground temperature, or duct temperature
- Group Rating Factor – Vertical Spacing
- Rating factor for the variation in soil thermal resistivity
- Horizontal Spacing Group Rating Factor
The cable manufacturers’ catalogues also provide all of these rating variables for a wide range of conditions.
12). Cable Construction
The type of power cable will depend on the type of the cable structure necessary for that installation. Meanwhile,
- Conductors,
- Cable arrangement,
- Insulation and finish covering
are all components of cable manufacturing.
Conductors: Depending on their manufacturing, the weather conditions it will be exposed to, and maintenance requirements, conductive materials like copper and aluminium are employed as conductors.
Cable arrangement: Conductors may be arranged in a single-conductor (or) three-conductor cable configuration. Single conductors are more readily installed and joined together. Additionally, they may be utilised to create several cable circuits. On the other end, ground wires in three-conductor cables are the way with the lowest impedance.
Insulation and finish covering: Installation type, service factors, ambient operating temperature, and the kind of load connected are often taken into consideration. Unusual conditions, such as a corrosive environment, the presence of oil & solvents, insect & rodent risks, very high temperatures, and the presence of ozone, may exist in certain installations.
13). Cable Operation
The insulation of the cable must be able to resist the voltage strains present under different operation conditions.
The correct phase-to-phase voltage determines the best cable insulation to use, & the general system category is divided into insulation levels of 100%, 133%, or 173%.
14). Shielding
Power cables with shields provide more protection than those with unshielded standards. The electric field is partially located in the insulation system & partially in the air in power systems without a metallic or shield covering.
Surface discharges will happen and ionise the air particles if the electric field is strong, like in the case of medium & high voltage.
Power cable shielding reduces the possibility of major safety risks and improves the dependability of cable circuits.
15). Other Factors
Aluminium conductor cables must be handled carefully since the metal quickly oxidises when exposed to air & forms a thin dielectric layer. In the installation of generating stations and substations, aluminium conductor cables are not utilised.
Aluminium has a high conductivity-to-weight ratio, making it a popular material for various application fields.
Large cables are highly inflexible and challenging to install, bend, and terminate.
