Under Ground Cables By Dr. P. Bala Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Known to Unknown Can you identify the things shown below? F1G.1 Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Introduction to Underground Cables Electric power can be distributed by underground cables. Advantages of underground cables : Less liable to damage through storms and lightening. Low maintenance cost. Less chances of faults. Better voltage regulation. Good general appearance. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Definition of a Cable An underground cable essentially consists of one or more conductors covered with suitable insulation surrounded by a protecting cover. F1G.2. A simple cable Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Definition of a Cable In a simple way, a cable can be seen as a combination of conductor, insulator and protective cover. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Requirements of Underground Cables Conductor used in cables should be copper or aluminium. Conductor must be stranded and tinned. Conductor size should be such that the cable carries the desired load current. Cable should cause voltage drop within permissible limits. (Contd…) Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Requirements of Underground Cables Cable must have proper thickness of insulation to give high degree of safety. Cable must be provided with suitable mechanical protection. Materials of a cable must be free from chemical and physical throughout. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Properties of Insulation materials used for Underground Cables High insulation resistance to avoid leakage current. High dielectric strength to avoid electrical breakdown. Good mechanical strength to have flexibility. Low coefficient of thermal expansion. Non-inflammable. (Contd…) Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Properties of Insulation materials used for Underground Cables Low permittivity. Immune from acid and alkalies. Non-hygroscopic i.e. should not absorb moisture. Low power factor. Cost should be economical. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Most commonly used Insulating materials in Underground Cables Impregnated paper Poly Vinyl Chloride Butyl Rubber Polythene Cross linked Polyethylene Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Comparison between Overhead Lines and Underground Cables S.no Aspect Overhead Lines Cables 1 Place of use Distribution and Transmission Only in Distribution 2 Cost of installation Cheap Costly 3 Requirement of towers…etc Compulsory Not required Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Comparison between Overhead Lines and Underground Cables S.no Aspect Overhead Lines Cables 4 Spacing between conductors In metres (more) In centimetres (less) 5 Charging current Less More 6 Cross section Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Comparison between Overhead Lines and Underground Cables S.no Aspect Overhead Lines Cables 7 Insulation requirement Less More 8 Safety 9 Appearance Not good Better Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Comparison between Overhead Lines and Underground Cables S.no Aspect Overhead Lines Cables 10 Voltage Regulation Good Poor 11 Radio interference More Less 12 Chances of Fault occurrence Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Comparison between Overhead Lines and Underground Cables S.no Aspect Overhead Lines Cables 13 Voltage stress on Insulation Less More 14 Heating 15 Efficiency Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Classification of Cables (a) According to Number of conductors (i) Single-core cables (ii) Two-core cables (iii) Three-core cables (iv) Four-core cables Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Classification of Cables (b) According to Voltage (i) Low-tension (LT) cables (up to 1 KV) (ii) High-tension (HT) cables (up to 11 KV) (iii) Super-tension (ST) cables (up to 33 KV) (iv) Extra High-tension (EHT) cables (up to 66 KV) (v) Extra super voltage cables (132 KV and above) Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Classification of Cables (c) According to Insulation and lead sheathing (i) Belted cables (ii) Horchstadter (H)- type cables (iii) Separate Lead (SL)- type cables (iv) HSL-type cables Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Classification of Cables (d) According to the methods of improving dielectric stress (i) Solid Type cables (ii) Oil Filled Type cables (iii) External Oil Pressure Type cables (iv) Gas Pressure Type cables Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables Low Voltage Cables are of two types (i) Single-core cables (ii) Multi-core cables Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables (i) Construction of a Single-Core Cable: Fig.1. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables Construction of a Single-Core Cable: These are used for voltages up to 1 KV. It consists of one circular core. Its core is made of tinned stranded copper or aluminium. Impregnated paper layers are used as insulation for this cable. A lead sheath is provided around the insulation to protect it from moisture. Jute is provided around the lead sheath as serving to avoid corrosion. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables Advantages of a Single-Core Cable: It is very simple in construction. Electrostatic Stresses developed are very small. Large conductor cross-section is available. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables (ii) Construction of a Multi-Core Cable: Fig.2. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables Construction of a Multi-Core Cable: These are used for voltages up to 6.6 KV. It consists of either circular cores or shaped cores. Its core is made of tinned stranded copper or aluminium. Impregnated paper layers are used as insulation between the cores. Paper belts are provided around the insulation. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables Construction of a Multi-Core Cable: (contd…) A lead sheath is provided around the insulation to protect it from moisture. Around the lead sheath jute is provided to protect it from corrosion and mechanical injuries. Galvanized steel wire is provided as armouring. Fibrous material like jute is provided around the armouring as serving to avoid corrosion. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of Low Voltage Cables Advantages of a Multi-Core Cable: A single multi-core cable can be used as replacement for the group of single-core cables. Cost of installation is low when compared with single-core cables’ installation. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of H.T. Cables High Tension (H.T.) Cables have a belt of insulation around the cores’ group. So, H.T. Cables generally called as Belted Cables. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of H.T. Cables Construction of a High Tension Belted Cable: FIG.1 Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of H.T. Cables Construction of a High Tension Belted Cable These are used for voltages up to 11 KV. It consists of group of circular cores. Its core is made of tinned stranded copper or aluminium. Cores are separated by Impregnated paper insulation layers. Another layer of impregnated paper tape called Belt Paper is wound around the insulated cores. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of H.T. Cables Construction of a High Tension Belted Cable:(contd…) The gap between the insulated cores is filled with jute so as to give circular cross-section to the cables. The belt is covered with lead sheath to protect the cables from the entry of the moisture. The lead sheath is covered with a layer of galvanized steel wire as armouring. Finally Steel armouring is surrounded by a layer of serving. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables Super Tension (S.T.) Cables can be classified into three types. H – Type Cables SL – Type Cables HSL – Type Cables Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (i) Construction of a Hochstadter (H) Type Cable: Fig.1 Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (i) Construction of a Hochstadter (H) Type Cable: These are used for voltages up to 33 KV. The cores are insulated by layers of impregnated paper. This insulation on each core is covered with metallic screens like Aluminium foils. The cores are laid in such a way that metallic screens make contact with each other. An additional conducting belt is wrapped around the cores . Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (i) Construction of a Hochstadter (H) Type Cable: The cable has no insulating belt. This cable is further provided with lead sheath, bedding, armoring and serving. All the core sections and conducting belt are kept at earth potential . The electric stresses are radial there fore the dielectric losses are reduced . Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables Advantages of a Hochstadter (H) Type Cable: The perforations in the metallic screen will gives complete impregnation to the cable to avoid entry of air. Metallic screens increase the heat dissipating capacity of cables. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (ii) Construction of Separate Lead (SL) Type Cable: Fig.2 Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (ii) Construction of Separate Lead (SL) Type Cable: These cables can be used for voltages up to 66 KV. This type of cable is basically a H-type cable but the screen around each core is replaced by its own lead sheath. Each core can be treated as a separate single core cables. In this type of cables there is no overall lead sheath. Only armouring and serving are provided. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (ii) Advantages of Separate Lead (SL) Type Cable: Bending of cables becomes easy due to the elimination of over all lead sheath. It reduces the core to core break down due to the use of separate sheaths. Oil leakages are less on hilly routes due to elimination of filler spaces containing compound. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (iii) Construction of a HSL Type Cable: Fig.3 Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Construction of S.T. Cables (iii) Construction of a HSL Type Cable: This cable is a combination of both H and SL type cables. In this type of cables, each core is rounded with impregnated paper, metallic paper screen and then separate lead sheath. Then the cores are provided with filler, bedding, armouring and finally serving. Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet

Prepared by Dr. P. B. Chennaiah, Associate Professor, EEE Department, AITS, Rajampet