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1: Introduction: The main current carrying parts in the substation include:

o Busbars o connectors and clamps o Power cables o Live parts of Equipments

These parts carry the normal load current continuously and are subjected high short-circuit currents occasionally. The conductor system is at higher voltage with respect to earth and is installed on insulators having adequate clearances and creepage distances. The main requirements of conductors, busbars, connectors and current carrying parts of the equipment include the following;

Normal current rating, the conductor shall be able to carry the specified normal current continuously with temp rise within specified limits. Therefore the cross-section of conductor should be adequate and resistivity of material should be low.

Short-timer current rating, the conductor system shall be capable of withstanding specified short-circuit current for a specified short-time. (1 second). The short-timer current is generally about20 to 25 times normal current rating. The spacing and strength of insulator supports should be adequate for the short-circuit with-stand.

Low electrical resistance of busbars, clamps, connector, Joints etc.

Mechanical strength, The following mechanical stresses are experienced by the conductors : a. Dead weight of conductor and associated components b. Short-circuit forces during peak of first major current loop of short-circuit c. Wind-loading, ice loading. d. Vibrations.

Conductors, connectors and associated hardware shall be corona free and free from

Radio interference (RI) and Television Interference (TI)

The conductor system shall have minimum number of joints. The resistance local of each heating joint should be below 15 micro-ohms to avoid local heating.

The conductor system should have minimum but adequate number of insulators

Conductor system should be economical.

Conductor system should be fully reliable.

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Figure 1: Installation of Busbar Conductors

2: Forms of Busbars 2.1: Busbars of Outdoor Switchyard: These are in the following forms:

ACSR conductors supported at each on strain insulators, such flexible busbars are used for very long spans and need, tall type or goal post type support structures at each end. In some cases, the strain insulators may be supported on the walls of a building or some tower:

Tubular Aluminium Conductors supported on post insulators made of porcelain. These are either welded or bolted to get extended lengths.

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Figure 2: Flexible (Strain) ACSR Busbars (FB) supported on Strain Insulator

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2.2: Busbars for Indoor Switchgear: These are in the form of aluminium of copper flats or tubular aluminium or copper pipes. These are supported on epoxy cast insulators.

2.3: Busbars for Generator Transformer Connections. These are in the form of Isolated Phase Bus-duct System in which rectangular/tubular/octagonal aluminium conductor of each phase is enclosed in hollow tubular. Aluminium enclosure. The conductor is supported on epoxy insulators. The enclosures of three phases are star connected and earthed at each end. Induced currents flow in enclosures. The magnetic field outside the enclosures cancels out. Therefore the forces between conductors during short-circuit current flow are negligibly small. The enclosures give magnetic shielding. The forces on insulators during short-circuits are reduced to about 10 per cent due to interaction between magnetic fields of the induced enclosure current and magnetic field of main conductor current. IPB is safe and reliable and is used universally for generator transformer connections.

3: Configuration of Busbars in Outdoor Substation: The conductors of busbars systems in an outdoor substation are of the following two types:

Rigid aluminium tubular bus conductors supported on post insulators. The conductors of three phases of each bus are placed in horizontal configuration.

The conductors of the three phase of each bus are placed in horizontal configuration. Table 1: Comparison between Rigid Bus System & Flexible Bus System Feature Rigid Bus System Flexible Bus System Cost Higher because of higher

conductor cost, post-insulators Lower

Land Area Requirement Larger Lower. Most of the equipment installed below the flexible bus

No. of Support structures -More No. -Simple -Amount of steel lesser

-less no. -complex -amount of steel higher

Structural deflection & strain More Less Cleaning of Conductors Insulators

Easy Difficult

Repair of Insulators Easy Difficult A suitable combination of rigid busbars and flexible busbars is selected for a particular substation. The choice between rigid bus and flexible bus is made after careful considerations of various aspects since the substation buses are the most important part in the substation. Table 7.1 gives comparison between the rigid buses and strain buses. The early substations were generally with flexible bus design. A flexible bus consists of flexible

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ACSR (Aluminium conductor steel reinforced) or All-aluminium alloy stranded conductors supported by strain insulators from each end. The flexible bus is held at higher level above the various substations equipment. The connections between the flexible bus and the terminals of substation equipment are by flexible conductors held in vertical or inclined plane. EHV substation is usually with rigid busbars. Rigid busbars are easy to maintain. They are at lower height. Connections to substation equipment are easy. A substation usually has a combination of Rigid Busbars and Flexible Busbars.

4: Definitions

I. Busbars: Conductors to which a number of circuits are connected. II. Busbar Connections: The conductors that form the electrical connection between the

busbars and individual apparatus. III. Open Busbars: The busbar which does not have protective cover. IV. Enclosed Busbars: The busbar that is contained in a duct in a cover of any material. The

busbar enclosed in metal enclosures are called metal enclosed busbars. The enclosures are either of aluminium or sheet-steel.

V. Outdoor Busbars: An open or metal enclosed busbars designed for installation under open sky.

VI. Indoor Busbars: The busbars designed for indoor use. VII. Gas Insulated Busbars. Busbar enclosed in gas filled metal enclosure. Usually SF6 gas is

used. VIII. Oil immersed Busbars.

IX. Compressed gas insulated busbars. Enclosed in enclosures filled with gas at a pressure above atmospheric pressure (Gas insulated cables).

X. Rigid Busbars. Busbar supported on rigid post insulators. XI. Flexible or Strain Busbars. Busbars supported on strain insulators. These are generally of

ACSR conductors. XII. Enclosed Busbars: The busbars are rigid conductors of aluminium or copper, supported to

support insulators. The assembly is supported on fabricated rolled steel sections and is enclosed by sheet steel or aluminium sheets.

XIII. Non-segregated Bus-ducts: The conductors of three phases are in common metal enclosures without any barrier between them.

XIV. Segregated, Bus-ducts: The conductors of the three phases are in a common metal enclosures with metal/insulator bathers between them.

XV. Isolated Phase Bus Systems: The conductor of each phase is enclosed in a separate metal enclosure.

XVI. Isolated Phase bus System of discontinuous Type: The Isolated Phase Bus System in which the enclosures are made up of units of standard length. The neighbouring enclosures are insulated from each other and are electrically discontinuous.

XVII. Isolated Phase bus System of Continuous Type: The enclosures are electrically continuous throughout their length. The three enclosures are connected in star and earthed at each end. The enclosures are insulated from the terminal apparatus.

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Figure 3: Types of Enclosed Busbars

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5: Ratings of Busbars: 5.1: Rated Current (Normal Current Rating): The RMS value of current which the busbars can carry continuously with temperature rise within specified limits. The standard values are following (BS: 159-1957 Busbars). 200, 400, 600, 800, 1200, 1600, 2000, 2400, 3000. Amperes.

Figure 4: SF6 Gas Insulated Bus-duct (Tc= Thickness of Encloser)

The specified limits of temperature rise are:

Ambient Temperature peak (1 hour) ... 40°C Temperature rise permitted ... 40°C Short time current duration: 1 second or 3 seconds.

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5.2: Rated voltages. The rms values of voltage, between lines for which the busbars are intended. Standard values are the following (in kV): 0.415, 0.6, 3.3, 6.6, 11, 15, 22, 33, 44, 66, 88, 110, 132, 165, 220, 272, 400, 500, 765, kV r.m.s.

Figure 5: Connector between tubular bus & flexible bus

5.3: Rated Frequency Usually 50 Hz (60 in USA). 5.4: Rated short-time current. This corresponds to short time current rating of CBs/switches/isolators. It is defined as the rms value of the current which the busbar can carry, with temp rise within specified limits for a specified duration. 5.5: Rated Insulation Level. It is the combination of the following capabilities, which characterize the insulation of busbars:

Normal rated voltage Power-frequency withstand voltage Lightning impulse withstand, voltage Switching impulse withstand voltage

6: Design Aspects

The design of substation buses is based on : 1. Current carrying capacity 2. Short-circuit stresses 3. Electrical clearances.

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Figure 6: T-connector between ACSR flexible connectors

Figure 7: Flexible Connector between flat busbars

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The current carrying capacity of bus conductors is detected by the limits of temperature rise. The flow of current causes heating due to I2Rt effect. The temperature rise of conductors, terminals, connections joints short is within specified limits. Permissible Temperature Rise above 40°C ambient temperature.

For plain electrolytic copper 30°Over Ambient Temperature 40°For Plain Aluminium 30°Over Ambient Temperature 40°Hot spot Temperature Rise 35°

The temperature rise is dependent on several factors such as resistivity of material cross-section of conductor size and shape of conductor skin effect, proximity effect type of enclosure or open heating due to solar radiation Ambient temperature; wind.

7: Thermal Expansion The length of conductor tends to increase with the increase in temperature due to thermal expansion of materials. This is an important factor in the designing of busbars of long lengths and heavy currents. The aluminium tubular bus conductors expanded by 0.09% in length for a temperature rise of 38°C. The longitudinal expansion of conductors produces bending stresses on support insulators, isolators and equipment terminals. Insulators are weak in tension and are liable to crack. To prevent such damage, expansion joints, are provided with rigid buses and connections. The clamps for supporting rigid buses should have a provision of sliding the buses. Long tubular buses have a tendency to vibrate. This is reduced by placing a piece of flexible ACSR conductor inside the tubular conductor. For flexible bus of long spans, vibration dampers are installed at each end at a distance of about 2 m from insulators. The connection between rigid tubular bus and. Equipment terminals is via semi-rigid, expansion joint, so that busbars do not strain the equipment.

8: Method of Jointing The total length of long tubular busbar conductors is obtained by jointing a number of pieces by means of

-clamps -welding.

The length of individual piece of conductor depends upon the limit of manufacturing facility and transportation by truck/trailer up to site.

The rigid busbars are supported on post insulator. The clamp for joining two busbar pieces is mounted on the post insulator by means of bolts. These bolts are screwed into the uppermost metal cap on the post-insulator. The clamp is tightened by means of nut-bolts and spring washer to provide a grip on conductor surface. The provision for expansion can be made in the design of the clamp. Welding of two busbar sections requires a high degree of skill and special site facility.

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Aluminium welding is a specialized job. However, welding of aluminium busbars is an accepted practice in todays' substations. Suitable number of expansion joints should be provided in the length of the conductor. Adjacent bus-sections are prepared for welding. Sleeve of exact dimensions and adequate length is used for welded joint.

9: Clamp and Connectors These are also used for connecting the busbars with the equipment terminals. Bolted joints are preferred. Bimetal Jointing: When aluminium conducting surface is to be jointed with copper surface, the joint gets damaged due to electrolytic action, particularly in presence of moisture. Following methods are used for preventing such corrosion:

Figure 8: Connector between equipment Terminal (T) & Flexible Conductor (F)

1. Use of special jointing compound in the joint. 2. Using insulating washer between visible gaps between two adjacent surfaces.

Use of welded bimetal strip of aluminium and copper between the joint 10: Oxidation of Layer While preparing the joint, the mating surfaces should be thoroughly cleaned by means of emery-paper and wire brush to remove the thin oxidation layer. Oxidation inhibiting grease is immediately applied to each surface. Thereafter the joint is made. The nut-bolts should be tightened by means of torque spanner to adequate pressure. Spring washers should be used to provide positive locking of nut-bolts. The joint should have sufficient grip so that resistance is less than a few micro ohms.

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11: Design Considerations for Clamps and Connectors Applications

to connect conductors with equipment terminals to connect two conductors to provide conducting support to provide non-conducting support to provide support for suspended/tensioned conductors.

Clamp and connectors are either current carrying type or non-current carrying types.

12: Requirements Temperature rise of clamps and connectors should be lesser than that of the associated

busbar/ conductors. The clamps and connectors should have enough mechanical strength to withstand the

dynamic loads occurring during short-circuits, erection. Clamps and connectors should be resistant to corrosion due to moisture, oxygen, etc. Clamps and connectors for EHV substations should be free from corona and

radio/television interference. Resistance per connection shall not exceed 20 micro-ohms. If resistance is higher, prepare

the surfaces again and make the joint.

13: Configurations of clamps and connectors: Typical configuration of clamps and connectors used in substations include the following:

Tee-connector for connecting ACSR flexible conductor to ACSR tap conductor. Tee-connector for connecting twin ACSR conductor to aluminium tubular bus. Parallel-Groove connectors for connecting two ACSR flexible conductors parallel. Fixed type bus post clamps for supporting tubular bus on post insulators. Sliding type bus post clamp for supporting tubular conductors on post insulators. Expansion type flexible bus post damp for supporting and joining two busbar lengths on

to a post insulator.

Connector between ACSR conductor and equipment terminal. Connector between tubular bus section and equipment terminal. Spacers for double-ACSR conductors and quadruple ACSR conductors. Hardware for string insulator assembly.

Table 2: Reference Data for Clamps and Connectors and Hardware Fittings

Conductor Tension 1000 kg/conductor Wind Load 560 kg

Forces due to short-circuit 1600 kg