Project-Report-Busduct-Design-Cost-estimation.pdf

Evaluation, Estimation & Costing of Bus ducts

Project report/Technical paper

2009

Evaluation, Design Estimation & Costing of Bus ducts

Prepared by

Pankaj Kumar Rajput

©Pankaj Kumar

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Table of Contents Table of Contents ................................................................................................................................................ 1 1.

Title of the Project ................................................ ....................................................... ................................ 3

2.

Objectives of the study .................................................... ................................................ ............................ 3

3.

Methodology used for carrying out the study ......................................... ....................................... ............ 3

4.

Statement of the Problem ...................................................... ............................................. ........................ 3

5.

Input data/Structure/Questionnaire ................................................. ........................................ .................. 3

6.

Analysis/Solution/Description ........................................................ ...................................... ....................... 4 A.

Type of busbars: ................................................... ........................................................ ........................... 4

B.

Busduct design & costing: ...................................................... ............................................ ..................... 7 a)

Introduction to busduct design ................................................. ............................................ .............. 7

b)

Basic constructional details of air insulated nonsegregated phase enclosed Busducts .................... 7

c)

Basic design parameters:.................................................... ............................................... .................. 8

d)

Components of a busduct: ..................................................... .............................................. ............... 8

e)

Design calculation of busduct: busduct:............................................................................................................. ............................................................................................................. 9 i.

Aluminium bus bars ............................................................ ............................................. ................ 9

ii.

MS sheet for enclosure............................................... ..................................................... .............. 13

iii.

FRP sheet for support insulators ...................................................... ................................... .......... 16

f)

Other design validations: ................................................ .................................................. ................. 16

g)

Calculation of weights of raw materials: ................................................. .................................. ........ 17 i.

Aluminium bus bars ............................................................ ............................................. .............. 17

ii.

MS sheet for enclosure............................................... ..................................................... .............. 17

iii.

FRP sheet for support insulators ...................................................... ................................... .......... 18

h)

Calculation of cost of busduct: ...................................................... ...................................... .............. 18

i)

Cost model – MS Excel: .......................................................... ........................................... ................ 20

7.

Final Results .......................................................... .................................................... ................................. 22

8.

Conclusion ................................................................ ................................................. ................................ 22

9.

Scope of Future Study ......................................................... ............................................. ......................... 22

Appendix 1: Derating factors for busbar ........................................................................................................... 23 Appendix 2: Standard Aluminium busbar sizes and their Ampacity table ........................................................ 24 Appendix 3: Indoor busbars – Minimum clearances ......................................................................................... 25 Appendix 4: Modulus of Inertia between busbars ............................................................................................ 26 References: ........................................................................................................................................................ 27















Project Report 1. Title of the Project Evaluation, design, estimation & costing of Bus ducts

2. Objectives of the study 

Study of types & design parameters of B usbars



Study of cost components of Busducts



Preparation of a cost module for Busducts for estimation of approximate cost

3. Methodology used for carrying out the study 

Study of various types of busbars from techni cal books & internet



Selection of the type of Busduct for which design & cost approximation tool is to be developed



Collection of preliminary & basic design data of standard Busduct from boo ks & internet sites.



Preparation of components breakdown structure of selected Busduct



Perform design & cost calculation of selected busduct



Development of cost module in MS Excel software



Testing of the module by taking output o utput for a selected rating of Busduct



Validation of cost output by taking a feedback from any one manufacturer of bus duct

4. Statement of the Problem 

To minimize the errors in cost approximation by the project estimation engineers in estimating the cost of the equipment i.e. bus duct in our case, during planning phase of the project.



To improve the process of price negotiation by working out a target cost based on the cost of components used in manufacturing of Busducts. Conventional way of negotiation is collecting offers from the suppliers and negotiating on the basis of cost provided by the suppliers.

5. Input data/Structure/Quest data/Structure/Questionnaire ionnaire 

Find out the details of various types of Busbars & Busducts



Prepare structured design evaluation of Busducts



Study & list down the components of Busducts



Get information & basic formula for weight & cost calculation of raw materials



Prepare structured bill of material & cost of various components of Busducts















6. Analysis/Soluti Analysis/Solution/Descriptio on/Description n Conductors are required for distribution of power from Generators to switchgear and loads. The main current carrying parts in an electrical system incl ude:



Busbars



Connectors & clamps



Power cables



Live parts of the equipments

These parts carry normal load current continuously and are also subjected to high currents during faults. The conductors are either provided with insulation or installed on insulators with adequate clearance & creep age distances. An aluminum or copper conductor supported by insulators that interconnects the loads and the sources of electric power in an electric power system is known as Busbar. Busbar . For higher current ratings, generally more than 800A, the higher temperature rise & losses in cables makes the design & execution difficult as it requires oversizing of cables. The busducts (enclosed busbars) provide an economical & technically superior solution.

A. Type of busbars: 1) Based on installation a) Outdoor Outdoor i.e. i.e. Open or enclosed busbars subjected to installation in open sky or outside of sub stations in open atmosphere b) Indoor Indoor i.e. i.e. Open or enclosed busbars subjected to installation inside the substation of closed atmosphere i.e. protected from rain, dust, vermin etc. 2) Based on type of conductor’s construction a) Flexible Flexible i.e. i.e. ACSR (Aluminium Conductor Steel Reinforced) or AAC (All Aluminium Conductor) conductors installed on insulators. These are generally used for outdoor installations. Flexible copper links are also used to connect enclosed rigid busbars for inside or outside installations. b) Rigid. Rigid. These These are in form of flats, channels or tubular pipes of Aluminium or Copper 3) Based on cooling media a) Air insulated i.e. Open or metal enclosed busbars b) Gas insulated i.e. Busbars enclosed in gas filled metal enclosure e.g. SF6 gas c) Oil immersed i.e. immersed i.e. Busbars enclosed in oil filled metal enclosure 4) Based on protective covering method a) Open busbars i.e. Busbars which doesn’t have any protective cover b) Enclosed busbars busbars i.e. Busbars of rigid Aluminium or copper conductors, supported on insulators, enclosed by sheet steel or aluminium sheets ducts.















5) Based on insulation between the phases, Busducts are of following types: a) Nonsegregated phase Busducts. Busducts. The conductors of three phases are in a common metal enclosure without any barrier between them. Busducts used for low voltage applications are usually nonsegregated phase busducts.

Pictorial cross sectional view of nonsegregated busduct b) Segregated phase Busducts. Busducts. The conductors of three phases are in a common metal enclosure with metal/insulated barriers (FRP) between them. This segregation minimizes the possibility of a short circuit between the phases. Busducts used for medium voltages i.e. above 1.1 kV, are usually nonsegregated phase busducts.















Pictorial view of segregated phase busduct c) Isolated phase Busducts. Busducts. For very large currents in generating stations or substations, where it is difficult to provide circuit protection, an Isolated Phase Busduct is used. Each phase of the circuit is run in a separate grounded metal enclosure, hence the phase are Isolated. The only fault possible is a phasetoground fault, since the enclosures are separated. This type of bus can be rated up to 50,000 amperes and up to hundreds of kilovolts.

















B. Busduct design & costing: Our focus of cost estimation study is Busduct i.e. enclosed bus bars.

We choose air insulated nonsegregated phase Busduct for our case study . a) Introduction to busduct design The dimensions of busbars are determined considering normal operating conditions i.e. rated current. The system voltage determines the phase to phase & phase to earth distance and also determines the height and shape of the supports to ensure adequate creepage clearance (the shortest distance between earthed end to the conductor, along the contour along external surface of insulator). Design validations are done to ensure that the busbar & supports are adequate to withstand the mechanical & thermal effects due to short circuit currents. Design validations are also done to ensure the temperature rise of conductors & enclosure is within safe limits. We also have to check that the period of vibration to the busbars themselves is not resonant with the system current frequency. During normal power frequency current flow, the conducting parts and associated insulating & other mechanical parts experience mechanical oscillations. The nature of such oscillations depends on operating frequency & the characteristics frequency of the equipment. In case of resonance, the structural parts are likely to fail. b) Basic constructional details of air insulated nonsegregated phase e nclosed Busducts NSPB consists of enclosure in rectangular form with conductors in the form of flats/channels of Aluminium/Copper material. These conductors are usually supported on FRP supports. Following are the basic constructional details of an air insulated nonsegregated phase busduct:















jointed by double doub le splice plates. p lates. A typical joint may use tape or heat shrink tubing to insulate when insulation is required. A 50x6 mm or other suitable size of Aluminium or bare copper conductor ground bus shall be installed and bolted to metal enclosure to provide continuous electrical ground. ii.

Enclosure The metal enclosures shall be made from suitable thickness (usually between 1114 gauge) Sheet steel, Aluminum or Stainless steel. Aluminium & Stainless steel being nonmagnetic materials results in low losses due to electromagnetic induction. Hence the enclosure size is less for Aluminium & Stainless steel in comparison to MS sheets. MS sheet enclosures are used up to current rating of 2500A and Aluminium or stainless steel sheet enclosures are in practice for ratings more than 2500A. Galvanized Iron (GI) sheet enclosures can also be used in place of MS enclosure particularly for outdoor application. Outdoor enclosures shall be additionally provided with rain canopy for water ingress protection. Enclosures shall be finished with bakedon polyester powder coat paint that results in a uniform thickness and glossable to withstand harsh environments. Standard color is ANSI61 light gray, special colors shall also be used if required. All enclosures should have removable covers secured with bolts for easy access to the joints for periodic inspection. Flexible joints shall be supplied in all straight bus runs at intervals of approximately 50 feet to allow for expansion when conductors are energized and carrying rated current. Minimum two numbers of space heaters with thermostats shall be provided to prevent moisture condensation and maintain cubicle temperature 5 C above the ambient. The busduct shall be provided with silica gel breathers, in all sections.

c) Basic design parameters: Following are the basic design parameters of an air insulated nonsegregated phase busduct:



Conductor material e.g. Aluminium or Copper



Conductor type e.g. Rectangular bar, channel or pipe



Normal current rating e.g. 1200, 1600, 2000, 2500, 3200, 4000 Amps. Etc.



Rated short circuit current e.g. 40kA fo r 1 Sec



Rated voltage e.g. 415, 460, 3300, 6600, 11000, 22000, 33000 etc.



Rated frequency e.g. 50 or 60 Hz



Rated Basic Insulation Level (BIL) e.g. BIL for 11kV system is 12/28/75 kV, kV, where 12kV

:

Highest system voltage

28kV

:

Power frequency withstand voltage

















Support Insulator e.g. FRP sheets, Post insulator – Epoxy or Porcelain



Enclosure



Silica gel breather & drain plugs



Anticondensing heater



Marshalling box for heater & CT (if used)



Misc hardware i.e. Nuts & bolts for jointing of bus bars & enclosure



Flexible links for connecting busduct with equipment e.g. Tin coated copper flexible links



Bends, disconnecting links (if required)



Rain canopy for outdoor installation



Wall frame assembly & support structure

e) Design calculation of busduct: Design & calculate the cost of Busduct of following specifications:

3000A, 3Ø 415V, 50Hz, 40kA for 1 sec Length – 15 – 15 Meters Assumptions: We Assumptions: We assume



Conductor of Busbar is Aluminium



PVC sleeves are used for busbar insulation



Support insulators are FRP sheets



Enclosure is of MS sheet of 2 mm thickness



Single earth bus of 50x6 mm Al shall be provided



1 silica gel breather is used at every 6 meters length of busduct



1 anti condensation heater is used at every 6 meters length of busduct



1 marshalling box for heater connection is used at every 6 meters length of busduct



There is no bend in the busduct



Busduct is used inside, hence rain canopy is not required

Calculation of size of components: i.

Aluminium bus bars















Design validation We need to check if the busbars chosen



Safely dissipates the generated heat (copper loss) and hence the conductor temperature rise is within limit i.e. < conductor material softening temperature.



Voltage drop, at receiving end, is within limits



Mechanical strength Is more than the strain developed during short circuit

Temperature rise of conductor Short circuit current

= 40kA

Time duration

= 1 sec

Cross section of busbar

= 4000 mm2

Material of busbar

= Aluminium

Formula:#

T

=

2

2

C * (Isc/a) * (1+) * 10 * t

Where, T = Final conductor temperature after fault, in C C = Material constant i.e. 0.54 for Copper, 1.17 for Aluminium Isc = Short circuit Current, in Amps a = Cross section area of conductor, in mm2  = Temperature coefficient of resistivity at 20C 0.00393 for Copper 0.04003 for Aluminium (EIEM) 0.00364 for Aluminium alloy (E9IE – WP)  = Initial conductor temperature before fault, in C i.e. Ambient + permissible temperature rise e.g. in our case it’s 50+35 = 85 C t = Duration of fault, in sec # Ref: “Electrical substation – Engineering & Practice” Handbook by S S Rao.

Calculation: T

=

1.17 * (40000/4000)2 * (1 + 0.00364 * 85) * 10 2 * 1















Formula: The busbar reactance is not normally sufficiently large to affect the total reactance of a power system and hence is not included in the calculations when establishing the short circuit currents and reactive volt drops within a power system. The voltage drop is usually calculated from the circuit current & resistance of conductor.

Vd = = =

IRac IRdc* k I * ( * L/a) * k

Where, Vd = Voltage drop / phase I = Rated current Rdc = DC Resistance of conductor / phase Rac = AC Resistance of conductor / phase k = Correction factor for Skin & Proximity effect e.g. usually 1.2  = Resistivity, in mm2/m For Aluminium : 0.0287  at 0C, 0.034 at 75C For Copper : 0.01724  at 0C, 0.021 at 75C L = Conductor length, in m, i.e. 15 Meters in our case a = Cross section area of conductor/phase, in mm2 Calculation: Vd = = =

I * ( * L/a) * 1.2 3000 * (0.034 * 15 / 4000) * 1.2 0.459 V

Voltage drop in % Vd (%)

= = =

Vd * 100 / (VL / 3) 0.459 * 100 / (415 / 3) 0.19 %

The voltage drop is less than maximum permissible voltage drop i.e. 2%. Hence the size of busbar conductor is safe. Mechanical strength of busbar
















D

=


Span between insulators support, in cm We consider 1 meter span i.e. 100 cm

r

=

Distance between neighboring conductors (Phases), in cm

# Ref: “Electrical substation – Engineering & Practice” Handbook by S S Rao.

Calculation: Force between conductors, F

= = =

2.04 x (40 x 2.5/2)2 x (100 / 17) x 102 300 Kg 294.2 daN *

* 1 daN (decaNewton) = 1.01972 Kg The strain developed in busbar, due to bending moment by force occurred during short circuit, can be found as below:














I/v =


2 * [{(b * a3) / 12} + S * d 2] / (1.5 * a)

=

2 x [{(200 x 103) / 12} + (200 x 10) x 102] / (1.5 x 10)

=

433333 / 15 mm3

=

28.89 cm3

Calculation: Strain 

=

(F * L / 12) * [1 / (I / V)]

=

(294.2 x 100 / 12) x (1/28.89) daN/cm 2

=

84.86 daN/cm2

The maximum fibre strain for Aluminium is 1056 daN/cm. As the strain developed during short circuit is less than the maximum permissible strain, the considered busbar size is safe. ii.

MS sheet for enclosure














Minimum required height of enclosure, H


= 2 x (t + j + h) + (a 2c) = 2 x (2 + 25 + 100) + 200 – 2 x 20 = 414 mm

Considering dissipation of heat generated, supporting systems, rectangular shape etc., we take the dimensions as below: Width of enclosure, W

= 600 mm

Height of enclosure, H

= 450 mm

Design validation We need to check if the temperature rise of enclosure chosen is within the limit i.e. its surface area is sufficient to dissipate the generated heat during normal situations. Temperature rise of enclosure Maximum rated current

= 3000 A

Cross section of busbar

= 4000 mm2

Material of busbar

= Aluminium

We will first calculate the heat/copper losses occurred in conductor and then the temperature rise of enclosure due to this heat. The walls of the enclosure dissipate the heat by both radiation & convection. Formula – Copper/heat loss generated:















Formula – Heat dissipation / Temperature rise:#

T = St =

P / ( * St) P / ( * T)

Where, T = Temperature rise of enclosure from ambient in C e.g. 20C P = Losses in bus conductor & enclosure, in Watt  = Specific heat dissipation in Watt/m2 12.5 W/m2 for naturally cooled MS surface. 6.0 W/m2 by Radiation & 6.5 W/m2 by convection. 20 W/m2 for forced cooled surface St = Heat dissipating surface area # Ref: “Electrical machine design” Handbook by AK Sawney & Chakrabarti.

Calculation: Copper loss per unit length/phase, Pc

= 2 *  * L * a * 1.2 = (0.78)2 * 0.034 * 10 6 * 103 * 1000 * 4000 * 1.2 = 99.3 Watt

Total loss per unit length of conductors, P

= 3.45 * Pc = 342.5 Watt

Surface required for dissipation of heat loss, St = P / ( * T) = 342.5 / (12.5 * 20) = 1.37 m2














iii.


FRP sheet for support insulators Number of Busbar/Phase

= 2 numbers 200x10 mm

Busbar support size (L x B x H), in mm = (Width of enclosure  2 x 5 mm) x Available standard width x standard thickness of FRP sheet = (600  10) x 200 x 8 = 590 x 200 x 8 Design validation We need to check if the FRP sheet insulator chosen withstand the electrodynamics forces of busbar during flow of short circuit currents. We have already found out the force between main conductors due to short circuit: Force between conductors, F

= 300 Kg/m

Force acting on FRP insulator, F1

= F x Span = 300 Kg/m x 1 m = 300 Kgs

Area of contact between busbar & FRP plate is Groove depth x Thickness x No of supports Now, Groove depth Thickness

= 20 mm = 8 mm

















Where, f n = Natural frequency of busbar, Hz  = Maximum deflection, mm 

=

5w * L4 / (384 * E * I)

Where, w = Weight per unit length of busbar, N/mm (Aluminium  2.7 x 103 N/mm) L = Busbar length between supports, supports, mm = 1000 mm mm E = Modulus of elasticity (Aluminium  71 x 103 N/mm2) I = Moment of inertia of busbar section, mm4 = 2 * [{(b * a 3) / 12} + S * d2] = 433333 mm4 

 f n

=

5 x (2 x 2.7 x 103) x (1000)4 / (384 x 71 x 10 3 x 433333)

=

2.286 x 103 mm

=

18.04 /  (2.286 x 103)

=

377 Hz















= 4216 cm3 Weight of enclosure, per unit length

= Volume * Density = 4216 cm3 * 7.81 gms/cm3 = 32.9 Kgs

Considering wastage of 5%, weight of enclosure

iii.

 35 Kgs/m

FRP sheet for support insulators Size of FRP support insulator, Width of support insulator, w

= 600 – 10 = 590 mm

Height of support insulator, j

= 100 mm

Thickness of sheet, t

= 8 mm

Number of insulators per unit length

=2

Cross sectional area of FRP sheet

=wxj = 590 x 100

Volume of one FRP insulator sheet

= 59000 mm2 x 8 mm














i.


Cost of raw materials Cost of raw material

= Length of busduct * Cost of raw material/meter

Cost of raw material per meter

= Weight * rate

Busbar cost – Rs/meter

= 35 Kg x 145 Rs/Kg = 5,075 /

Enclosure cost – Rs/meter

= 35 Kg x 75 Rs/Kg = 2,625 /

FRP insulator sheet cost – Rs/meter

= 1.8 Kg x 500 Rs/Kg = 900 /

Cost of raw material, Rs/meter

= 5,075 + 2,625 + 900 = 8,600 /














i)


Cost model – MS Excel:

Bus Ducts - Cost Model Inputs Busbar material Enclosure material Rate Ra te of Al Alum umin iniu ium m Rate Ra te of Co Copp pper er Rate of stee steell HR Shee Sheets ts for Encl Enclosur osure e Rate of FRP shee sheets ts for supp support ort insu insulato lators rs Curr Cu rren entt ra rati ting ng Am Amp p TPN or TP Encl En clos osur ure e wi widt dth h Encl En clos osur ure e he heig ight ht Enclos Enc losure ure she sheet et thi thickn ckness ess FRP she sheet et in insul sulato atorr wid width th FRP she sheet et in insul sulat ator or hei height ght FRP she sheet et in insul sulato atorr thi thickn ckness ess

Aluminium Sheet steel 110 11 0 200 20 0 30 500 3000 30 00 3 600 60 0 450 45 0 2 590 100 8

Rs/Kg Rs/K g Rs/K Rs /Kg g Rs/Kg Rs/K g Rs/Kg Rs/K g Amp Am p mm mm mm mm mm mm















Outputs Actual current current rating of busbar

3,080

Amps

Current density of busbar material Density of busbar material Density of enclosure material Rate of busbar material including fabricatio fabrication n Rate of enclosure material including fabrication, painting

0.77 2.70 7.81 145.00 75.00

Gms/Cm3 Gms/Cm3 Gms/Cm3 Rs/Kg Rs/Kg

Busbar X-section mm^2 Weight of busbar per m Cost of busbar per m Enclosure weight per m Cost of enclosure (inclussive painting) per m Insulator Plate Wt Cost of Insulator plates per m Misc hardware cost i.e. Nuts, bolts, gaskets etc Total material cost

12,300 34.87 5,056 34.6 2,593 0.89 892 854 9,395

mm2 Kg/m Rs/m Kg/m Rs/m Kg/m Rs/m Rs/m Rs/m

















7. Final Results 

Various types of bus ducts were studied



3 phase, 415V, 50Hz, 3000A indoor nonsegregated phase busduct was designed and the design parameters were validated by various calculation methodologies



Detailed costing was done for above busduct















Appendix 1: Derating factors for busbar Derating factor for bus bar, k

=

k1 * k2 * k3 * k4 * k5 * k6

Where,



Coefficient k1 is a function of the number of bar st rips per phase for: For 1 bar, 2 3 bars,

k1 = 1 table below:















Appendix 2: Standard Aluminium busbar sizes and their their Ampacity table

Aluminum Bus Bar Amperes for 6101T61 Alloy 57% IACS Conductivity Chart Below Bar Sizes (Inches)

1 Bar DC

60 Hz AC

2 Bars DC

60 Hz AC

3 Bars DC

60 Hz AC

4 Bars DC

60 Hz AC















Appendix 3: Indoor busbars – Minimum clearances

Table 3.1  Indoor busbars : Open or enclosed  Clearances for voltages up to 33kV Rated voltage rms

Minimum clearance to Earth Open

Enclosed

Minimum clearance between Phases Open

Enclosed















Appendix 4: Modulus of Inertia between busbars 

Modulus of inertia – One bar per phase

Moment of Inertia, I

= (b * a3) / 12 3

Modulus of Inertia, I/v = [(b * a ) / 12] / (a / 2)

















Project-Report-Busduct-Design-Cost-estimation.pdf

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