Busbar Systems – An Overview

Busbar Systems comprise current-carrying conductors insulated for the desired system voltage, exposed or enclosed, along with required accessories, for transferring power from one location to another.  The conductors are either supported on insulators at discrete locations or restrained along their entire length.  The conductors may be bare or insulated (rated partially or fully). 

The choice of conductor material is between Copper and Aluminium, and these have been well-identified for different types of Busbar Systems.  The overlap in the selection, in a few applications, is often a matter of customer preference. 

The choice of enclosure material is between Aluminium and Steel.  Stainless steel and glass-reinforced plastics have been used for special applications.  Aluminum is, by far, the most preferred material for an enclosure. 

Support structures are mostly made of rolled steel sections for outdoor installation and, formed steel sections for indoor application.        

Insulating materials, made of porcelain, glass, and organic compounds, have established their niche applications.  Since the majority of the failures of a Busbar System can be traced back to the failure of the insulation, the choice of the Busbar System depends upon the perceived reliability and the comfort, the insulating material offers.

Processing of conductors and enclosures during manufacture vary significantly, based on the material and environmental conditions. 

Busbar Systems are designed to withstand specified dielectric, thermal, electrodynamic, and environmental stresses.  The design may also be constrained by performance criteria such as voltage drop and power loss that may be capitalized on.  The customer needs to be satisfied that the envisaged specified criteria for the impedance, ampacity, and short-circuit forces withstand capabilities, are met with calculations and substantiated by tests.    

  

Classification

Busbar Systems may be broadly classified as under:

A          Isolated Phase Bus (IPB)

A1         Naturally Air-Cooled IPB

A2         Forced Air Cooled IPB

Isolated phase bus form the interconnections between large generators and generator transformers up to a rated voltage of 36 kV.  They incorporate tap-off connections to auxiliary transformers, excitation systems, surge protection voltage transformer cubicles, and neutral grounding cubicles.  Generator circuit breakers may be installed between the generator and generator transformers for isolation and synchronization.  The inherent feature of the IPB is that they can withstand very large electrodynamic forces arising from high magnitude fault currents in the connection and have been tested for fault currents of over. 300 kA (r.m.s)

Naturally-Air-Cooled IPBs are recommended for a rated continuous current up to 25 kA.  Forced air-cooled IPBs have been manufactured for a rated continuous current up to 50 kA. 

B          Metal Enclosed Air Insulated Bus (Busduct)

B1         Non-Segregated Phase Bus

B2         Segregated Phase Bus

B3         Busbar Trunking

Busducts have been designed for voltage up to 36 kV, current up to 6.0 kA, and a fault level of 100 kA r.m.s.  They can also be designed for higher current rating and fault levels, for specific connections and are suitable for both indoor and outdoor installations

Non-segregated phase bus, are air-insulated, with bare or insulated conductors.  They are installed in medium and low voltage networks for bulk power transfer.  They form the connection between the transformer and the switchgear panel and between switchgear panels. 

Segregated phase bus, are also air-insulated, with bare or insulated conductors.  They are mostly installed in, medium voltage networks to reduce the probability of an inter-phase fault.  Connections between the generator and generator transformer for unit ratings up to 60 MW can also be made with a segregated phase bus. 

Busbar trunking (BBT) are mostly installed indoors for low voltage power distribution in shop-floor, machine shop, and similar locations.  They are also used as rising mains in high rise buildings and commercial complexes.  They are characterized by relatively low, rated continuous and fault currents, with facility for tap-off connections. 

C          Sandwich Bus (Busway)

C1          LV Busway for Power Connections

C2          LV Busway with Tap-Off for Rising Mains.

Busways are extensively used in low voltage networks, for power distribution in indoor and outdoor installations.  They are compact and can be procured from the retail market.  Inherent in the design, are the very high fault current withstand capabilities and low voltage drops, far exceeding the capabilities of Busducts of similar rating. 

Busways for LV power connections have been designed for a current rating up to 6300 A and a fault current of 200 kA (r.m.s). 

Busways with Tap-off are installed for power distribution in high-rise buildings and data centres.  Tap-off boxes with isolating and protective devices, deliver power to the designated area.

D          Cast Resin Bus (CRB)

D1          LV Cast Resin Bus  

D2          MV Cast Resin Bus

Cast resin bus is an extension of the design concept of Busway for application beyond the low voltage application to medium voltage and provides a higher degree of protection.  It can be installed underground in trenches and offers better resistance in a corrosive atmosphere.  Inherent in the design is its ability to withstand high fault current.

CRBs for low voltage applications are similar to the Busway except for a comparatively larger phase spacing.  They have been manufactured up to a rated current of 6000 A.  They need not have an external enclosure and can interface with Busway of a similar rating. 

CRBs for medium voltage applications are manufactured up to a rated voltage of 36 kV and are provided with a well-ventilated enclosure for indoor and outdoor applications.  They are manufactured in monoblock and segregated-block designs, depending upon the rated voltage.

E          Solid Insulated Bus (SIB)

E1          Cast Resin Bus – Partially or Fully Insulated (PIB or FIB)

E2          Resin Impregnated Paper Bus (RIPB)  

Solid insulated bus are single-phase, tubular conductors that are either cast in resin or insulated with resin-impregnated paper.  They are suitable for indoor and outdoor installations.

Cast resin tubular bus, are fully insulated up to a rated voltage of 3.6 kV and mounted on earthed structures.   They are partially insulated for 7.2 kV and are installed mounted on insulators and separated from other phases with a recommended, minimum air and ground clearances up to a rated voltage of 36 kV.       

Resin impregnated paper insulated tubular bus, are fully insulated for the rated voltage and, are very compact for medium and high voltage applications.  The voltage rating is limited to 170 kV.  Individual phases are shielded and earthed and mounted on earthed support structures.

F          Gas Insulated Bus (GIB)

F1          Gas Insulated Bus for Switchgear and Switchyard Interconnections

F2          Gas Insulated Transmission Lines (GIL or GITL)

Gas insulated switchgear (GIS) is now in use in many stations, not only in urban areas where space is a premium but also in polluted and coastal areas.  GIB, forms the interconnections between switchgear and, switchgear to transformers, both indoors and outdoors.  These are fabricated at works and installed at the site.  They are an extension of the GIS and operate at the same gas pressure.

Gas insulated transmission lines are expensive when compared to conventional transmission lines.  These can be buried underground, run in trenches, or installed overground.  These are also installed where interconnections cannot be carried out by air-insulated conductors or cables.   

G          High Voltage Open Bus (HVOB)

G1          High Voltage Rigid Bus

G2          High Voltage Strain Bus 

High voltage rigid bus, are installed in switchyards.  Tubular Aluminium pipes have been identified as the choice of conductors.  Switchyards are also designed with a combination of strain and rigid conductors, under rare circumstances. 

High voltage strain bus, are installed in transmission lines.  Reinforced stranded Aluminium is the identified choice of conductor material.  The conductor sizes have been standardized for voltage and current.

 

H          High Current Open Bus (HCOB)

H1          High Current DC Bus

H2          High Current AC Bus

High current open bus, are project-specific, with an identified conductor material.  Some connections may comprise a hybrid combination of Copper and Aluminium conductors considering the nature of the corrosive atmosphere in the area of installation.  Limiting the voltage drop is the most significant criterion in the design of the connection.  The connection comprises rigid as well as flexible conductors.     

High current DC bus, are installed in the extraction of Aluminium, Magnesium, and other metals.  The rated continuous current can be as high as 600 kA.  They are also installed in metal refining and Chlor-Alkali plants.  These buses form the connection between the rectifier and end cells and between cells.   

High current AC bus, are installed in electric induction furnace and electric arc furnace.  Water-cooled, Copper conductors are installed at strategic locations.  

I          Cable Bus (CB)

I1          Low Voltage Cable Bus

I2          Medium Voltage Cable Bus

Cable bus, as an alternative to Busduct, has been recognized in many countries.  These have been installed for power connections in systems up to a rated voltage of 72 kV, rated current of 6000A, and a fault level of 100 kA r.m.s.  They are economical for outdoor installation with significant route length.  Cable bus, are maintenance-free and corrosion-resistant.   

J          Accessories 

J1          Busduct Accessories

J2          Generator Connection Accessories             

Only a few important Busduct accessories have been described in this book.  Generator connection accessories have been added since isolated phase bus, are essentially installed in power stations and interface with equipment for supplying power to auxiliaries and excitation systems, and provide inputs to control, protect, monitor and if required, isolate the generator. 

 

Statements made cover practical issues arising during design, manufacture, testing, installation, and commissioning.  Many practical solutions are based on techno-commercial considerations such as standardization, procurement optimization, and ease of manufacturing without compromising on the performance.

 

 [Several applications are very specific to certain industries, and products such as small power plug-in bus, underfloor bus for power distribution and lighting, multi-conductor trolley bus, flexible lighting bus, EOT crane bus, railways and tramways bus, and similar other applications.  These have been excluded from the scope of discussions.  Water-cooled bus for high current AC and DC connections are very specialized and need to be custom designed.  Since the requirements are few, these have also not been discussed.  Aluminium strain conductors on transmission lines for power transmission in MV, HV, EHV & UHV are also excluded from the scope of discussions.]

Design

Based on the mathematical models which consider ideal conditions, thermal, dielectric, mechanical, and environmental stress withstand capabilities of a Busbar System for given parameters can be analysed to predict its performance under service conditions.  Many formulae are empirical in nature and correction factors for site conditions are approximate at best.  Hence it is the norm to verify the offered design by conducting a series of type tests in a laboratory under controlled conditions.  It is almost impossible for a manufacturer to possess a type-tested design for a wide range of input parameters for projects.  It is, therefore, an accepted practice in the industry to validate an offered design by interpolating the data from the type tested design, backed up by calculations.  Calculations are also helpful in establishing the margin of safety.  

ANSI/IEEE and IEC are the most widely used international standards specified for Busbar Systems.  Most countries have aligned their National Standards with one of them.  Since most of the Busbar Systems are custom- built, they may not have a dedicated Standard.  In such cases, the product may be required to conform to a standard, for a similar product of a different voltage rating or, switchgear of similar rating.  The parameters specified in ANSI/IEEE and IEC may appear to differ from each other for the same product.  However, a thorough understanding of the product and its applications can clarify some of the perceived anomalies. 

Since Busbar Systems are project-specific, and there are significant engineering costs involved in the preparation of layout drawings for customer approval and manufacturing drawings for use on the shop floor.  Supporting arrangements of Busbar Systems are also very project-specific and require detailed drawings to be furnished to the project authorities.  Installation drawings are required for detailing the sequence of installation.  Project-specific quality assurance plans (QAP) for manufacturing and installation need to be prepared.  Project-specific manuals must be prepared for installation & commissioning (I&C) and operation & maintenance (O&M).  While many of the documents & component drawings can be pulled out from standard libraries, they still need to be compiled and customized. 

Manufacture

The industry is mature and most of the reputed manufacturers have optimized and adopted similar manufacturing processes, with marginal variations to cater to the salient features of their designs.   Extrusions are extensively used not only for better aesthetics of the products but also for ease of conformance to accurate dimensions.

Conductor and enclosure raw materials for the manufacture of Busbar Systems are readily available from the mill.  The sizes of sheets and extrusions can be standardized, to minimize the inventory, and products can be designed and manufactured for a wide range of requirements.

Welding processes are well defined and product-specific.  Manual welding is carried out with the help of jigs and fixtures.  Automated processes are adopted wherever feasible to provide a good and consistent quality of the weld.  Welder certification is carried out as per the norms and procedures laid out in standards.  Weld inspection is a significant activity in the Bus industry.

Resin casting, coating, and impregnation of the conductor is a very critical process.  The infrastructure and the process, are product specific.  

Since Busbar Systems are custom designed, the manufacturing process is labour intensive for most types.  (Busbar trunking, sandwich bus, and cast resin bus along with their accessories such as tap-off boxes and supporting arrangements are exceptions to these as they have been standardized and their manufacture, automated.  These products are sold in the retail market.) 

Testing

Procedures for type and routine testing of Busbar Systems are carried out as defined in applicable standards, or, in accordance with applicable switchgear standards if a dedicated Standard does not exist.  In many instances, configurations of the test sample are mutually agreed upon between the manufacturer and the project authorities. 

The testing is carried out, on a relatively small representative section of the Busbar System of specific configuration, under controlled conditions in a laboratory.  Further, only a section of the Busbar System can be tested and not the entire connection comprising series elements such as bends, flexible laminates, and links on the conductor and bends and bellows on the enclosure.  Adapter Boxes at the equipment interface are not type tested.  It should further be emphasized, that it is impossible to test all ratings of Busbar Systems of different configurations, conforming to different international specifications.  Test results, nevertheless, provide useful information when designing the total system and for validating the offered design.

ANSI/IEEE standards and IEC recommendations are internationally recognized for product certification.  They specify different preferred ratings for, system rated voltage, rated current, rated fault current, rated basic insulation level, and temperature rise for a Busbar System among various other parameters.  Nomenclature for different type-tests may vary with the standards.  The minimum specified size of the test sample and configuration are not the same in many standards.  Their definitions of parameters are different.  Their design philosophy and acceptance norms are different.  It is, therefore, suggested that the product be defined to only one of the standards since all the other companion specifications will then be well co-ordinated.  

A product, type tested as per one standard, can in most cases, be justified to conform to the other standard, although to a different assigned rating, by comparing and interpolating with industry-accepted empirical formulae. 

 Installation & Commissioning

Busbar Systems are custom-built products that are manufactured at works and installed at project sites.  Installation is a major activity that ensures the delivery of the intended performance of the product.  Products such as Busduct, Busway, Solid Insulated Bus, High Voltage Open Bus, Cable Bus require, only the assembly of sections and supporting them at the site as per the installation manuals.  Cast Resin Bus installed outdoors, requires epoxy casting at joints which is a specialized activity.  There are, however, Busbar Systems such as Isolated Phase Bus and Gas Insulated Bus that require significant and extremely critical welding operations to be carried out at the site.  In High Current Open Bus, welding of the cast and extruded bars at the site is one of the most significant operations of the entire project.     

In many of the sites, it may not be practical to assemble and install the Busbar System as envisaged in the layout drawings.  The mismatch can be attributed to the variation in the location of the foundation of the interfacing equipment or tolerances in dimensions or both.  Despite the inherent built-in flexibility in the design of the product that caters to the above contingencies normally encountered in such projects, there may be instances when the manufactured sections and components need replacement, site modification, or additional components.  Such modifications are carried out after due deliberations and consultations with the manufacturer.       

Busbar Systems are static devices and many of them may not be maintained for the life of the equipment.  This puts an onerous responsibility on the installation activity.  Depending upon the environmental conditions, some of the Busbar Systems may be subjected to routine maintenance with a periodicity determined by past experiences.  In general, maintenance may be limited to cleaning the surface of the insulators and bushings and replacement of rubber components and anti-condensation heaters, if required.  Joints may be subjected to inspection when installed in a severely corrosive atmosphere for enclosed, as well as open Buses.  These may have to be cleaned and if warranted, a protective coat applied.