By Muhammad Faheem Meer
General Manager Business Development
Newage Cables (Pvt) Limited,
Formerly, Head of EHV Cable Projects – Siemens Limited, Saudi Arabia/UAE
Manager High Voltage Field Support – Saudi Cable Company, Saudi Arabia
Cables are the arteries of an electrical power system and carry the energy required for the successful operation of a plant or equipment. It is therefore, of prime importance to design cables suitable to transfer energy in the most efficient and reliable manner.
There are several types of cable designs available for carrying electrical energy in a given distribution system. The selection of a particular design may however, be influenced by the load requirements, site conditions, past experience and the policies governing the systems.
Below are some of the parameters which need to be carefully looked into before designing cables for a particular system.
1. CABLE (CONDUCTOR) SIZE SELECTION;
The selection of cable (conductor) size is based on the following factors;
1. Current Carrying (Load) Requirements
2. Voltage Drop Regulations
3. Short Circuit Rating
1.1 Current Carrying Requirements;
Current requirements are based on the required power of the system or equipment for which cable is being used. Current ratings of various conductors for different types of cables are mentioned in cable catalogues. However, the ratings are based on certain installation conditions (ambient temperature, laying formation, depth and soil resistivity for underground installation, grouping etc.) and it is necessary to adjust current ratings according to the applicable laying conditions by applying de-rating factors.
1.2 Voltage Drop Regulation;
Apart from carrying the current, it is also important for cable to maintain the system voltage at receiving end. Longer runs result in voltage drop at receiving end of cable and therefore, higher conductor size is required to reduce conductor resistance which results in reducing voltage drop.
In Pakistan, allowable voltage drop is 2.5% of system voltage, whereas, in some countries it allowed upto 5%.
1.3 Short Circuit Ratings;
Every electrical system is designed at a particular short circuit current level and it is imperative for the cable to match that level so that faults could be properly transferred to protection equipments. In case, cable short circuit level is lower than system’s, the fault will result in cable failure before it reaches to protection equipment.
In Pakistan, this important factor is usually ignored and results in expensive cable failures. Cable catalogues tabulate short circuit ratings of various conductor sizes and proper conductor size can be selected using tables.
2. CABLE CONSTRUCTION
2.1 Conductor Material
Copper and aluminium conductor cables are used in Pakistan. Consideration with respect to cost, installation, environmental conditions, and maintenance should be given importance while selecting conductor material. The requirements for aluminum conductors with regard to these factors are more critical than for copper. Conductivity of copper is much higher than of copper and therefore, thicker cables result in case aluminium is opted for conductors.
2.2 Conductor Arrangements;
Cable conductors can be arranged to form single or three/four conductors (cores) cables, and both formations have pros and cons associated with them.
Single core cables are easier to install and join and allow formation of multiple cable circuits. Single core cables provide easy maneuvering of phase adjustments according to site requirements. However, single core cables have higher reactance and may lead to high shield currents and heating of cable, in case it is shielded. Single core cables are also subject to considerable mechanical stress and resultant movement due to short circuit currents.
Three core cables have lower reactance and voltage stress is balanced because of equivalent spacing between conductors. However, three/four cores together restrict the maneuvering of cable phases.
The selection of using single or three core cables should be made based on the cable arrangements and available space for laying at site.
2.3 Insulation & Outer Sheath;
The selection of cable insulation and sheath material is normally based on the system voltage, type of installation, cable operating temperature, service conditions, type of load served and other applicable criteria. In many installations, unusual conditions such as corrosive atmosphere, high operating temperature, termite and rodent hazards, presence of oil, toxic solvents and presence of ozone, or combination of these may be prevalent.
Normal PVC insulated cables are rated at 70oC conductor operating temperature, where as XLPE insulated cables have 90oC operating temperature. Special insulation materials with 105oC operating temperature are also available. Similarly, special cable sheathing materials with oil/acid resistant properties are available. Additives to make anti termite and anti rodent sheaths can also be arranged.
Recently in Pakistan, special fire retardant and fire resistant cables are also in use, using fire-retardant PVC and low smoke zero halogen (LSZH) insulation and sheathing materials.
2.4 Additional Protections
Some cables may have additional protections against the electric, site and environmental requirements. Medium and high voltage cables have both metallic and non-metallic shields (screens) to reduce the ionization of air over insulation which may lead to flow leakage currents. These shields also strengthen short circuit ratings of cables.
There are many possible variations in cable design according to the operational requirements. Steel wire armouring is used for mechanical protection in case cables are directly buried in ground. In case of control cables, a concentric layer of copper wire and/or tapes is applied over bunched cores for electrical noise protection. Water barrier tapes are applied for ingress protection to cable cores. Mica tapes may be applied over conductors to block the fire propagation to cable conductors and to ensure circuit integrity in case of cables is put to fire.
4. CABLE OPERATIONS
The insulation of cable must be designed to withstand voltage stresses during normal operating and fault conditions. Therefore, selection of cable insulation should be made on the basis of appropriate phase-to-phase voltages and the insulation system categories which are classified to 100% & 133% and to 173% in some cases. Wapda for example uses 8.7/15 kV cables in their 6.35/11 kV distribution system (133%).
100% insulation level category is provided with relay system protection which normally clears ground faults within one minute.
133% insulation level category is provided with relay system protection which normally clears ground faults within one hour.
173% insulation level category is provided where time needed to de-energize the ground fault is indefinite.
5. CABLE INSTALLATION
Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served. A properly designed cable may fail because of poor installation arrangements at site.
A good understanding of local conditions, installation teams, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily.
Sites should be cleared from stones, pebbles, nails and sharp edge objects before laying cables. Cable should be pulled via proper cable winch using rollers, bends and cable stocking/pulling eye.
In case of harsh site conditions, steel wire armored cables with polyethylene (PE) sheaths are recommended which not only offer resistance to many toxic solvents, but also provide mechanical strength against rough site conditions at cable laying stage.
