WO2007123388A1

WO2007123388A1 - Multiple-run bus duct system

Info

Publication number: WO2007123388A1
Application number: PCT/MY2006/000001
Authority: WO
Inventors: Chih Bok Lew
Original assignee: Chih Bok Lew
Priority date: 2006-04-24
Filing date: 2006-08-16
Publication date: 2007-11-01
Also published as: TW200741757A; MY177110A; CN101473504A

Abstract

The present invention relates to a bus duct system for high ampere power transmission, each phase comprising one multi-run conductor, said multi-run conductor comprising a plurality of elongated bars running parallel to each other, characterised in that each elongated bar has a surface-to-volume ratio in the range of 0.45 mm2/mm3. to 1.15. The increased surface area reduces the skin effect ratio j which is dependent on the ratio of the width to the thickness of the bar, and as the thickness of the bar increases, the skin effect increases. Therefore, a multiple of thin copper strips are more efficient than a single thick one a conductor for alternating current.

Description

MULTIPLE-RUN BUS DUCT SYSTEM

FIELD OF THE INVENTION

The present invention relates to a multiple-run bus duct system. More particularly, the present invention relates to a multi-phase bus duct system for transmission of high ampere current.

BACKGROUND ART

Conventionally, bus. duct manufacturers concentrate ^"on making single bar current carrying conductors, instead of multiple bars, due to smaller size trunking. Copper conductor is also widely used compared to other types of conductors.

However, single bar conductors have a high copper ■ content. These conductors have problems of magnetic flux (back emf) cutting across the bar, reducing the intensity of current being carried in individual bars. The center of the bar experiences the most pronounced magnetic flux effect, causing the current to be concentrated at the skin of the conductor. This phenomenon is called 'skin effect'. Furthermore, magnetic flux from the conductors, affects adjacent bars, where the magnetic flux cutting across the adjacent bars, also causes back-emf.

As such, there is a need for a new type of current transmitting bus ducts which will use less copper for a given ampere rating and thus effect a cost saving in the manufacture of bus- ducts. SUMMARY OF THE INVENTION

Accordingly the present invention provides a bus duct system for high ampere power transmission, each phase comprising one multi-run conductor, said multi-run conductor comprising a plurality of elongated bars running parallel to each other, characterised in that each elongated bar has a surface-to-volume ratio in the range of 0.45 mm2/mm3. to 1.15.

The increased surface area reduces the skin effect ratio, which is dependent on the ratio of - the width to the thickness of the bar, and as the thickness of the bar increases, the skin effect increases. Therefore, a multiple of thin copper strips are more efficient than a single thick one a conductor for alternating current.

The present invention consists of certain novel features and a combination of parts hereinafter fully described and illustrated in the accompanying drawings and particularly pointed out in the appended claims; it being understood that various changes in the details may be without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of. facilitating an understanding of the invention, there is illustrated in the accompanying drawings the preferred embodiments from an^' inspection of which when considered in connection with the following description, the invention, its construction and operation and many of its advantages would be readily understood and appreciated.

FIG 1 shows the relationship between thickness of conductor and A.C current flow.

FIG 2 shows the relationship between thickness of conductor and Skin Effect Ratio.

FIG 3 is a cross sectional view of a bus duct using single bars.

FIG 4 is a cross sectional view of a multi-run conductors using 3 bars.

FIG 5 is a cross sectional view of a multi-run conductors using 11 bars.

FIG 6 shows the interconnection and tap-off points of a multi-run conductor.

FIG 7 shows the manufacturing procedure in housing the multi run conductors.

FIG 8 illustrates the tapping of current from a multi-run bus. duct.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a multiple-run bus duct system. More particularly, the present invention relates to a multi-phase bus duct system for transmission of high ampere current. Hereinafter, the Multiple-Run Bus Duct System shall be described according to the preferred embodiments of the present invention and by referring to the accompanying description and drawings. However, it is to be understood that limiting the description to the preferred embodiments of the invention and to the drawings is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications without departing from the scope of the appended claim.

In a bus duct system, two main factors affect performance. They are: l.Skin effect due to the characteristics of the copper conductor. 2.Heat dissipation due to larger surface area, hence, the surface area to volume ratio.

The given table below shows how the skin effect tends to be smaller as the width gets larger relative to thickness under given values of impedance and resistance..

Table 1: Skin effect ratio

Table 2 below clearly verifies that in a conventional 1 meter length busduct system, as the width gets larger, the surface area to volume ratio per meter increases. This is beneficial to reduce the skin effect ratio.

Table 2: Surface to Volume Ratio

Table 3: Comparision of single and multi-run bars

Refering to Table 3, values of surface to volume ratio per meter are much higher in a multi run system. Therefore, a multi run system is clearly more advantageous since the reduction in skin effect ratio is more pronounced.

For a conductor, the apparent resistance is always higher for alternating current (a.c.) than direct current (d.c). The conductor has problems of alternating magnetic flux (back emf) created by an alternating current which interacts with the conductor, reducing the intensity of current carried in individual bars. The current generated by the magnetic flux is going against the direction of normal current.

The center of the bar experiences the most number of lines of force, whereby the number of line linkages decreases as the edges are approached. The electromotive force produced in this way by self-inductance varies both in magnitude and phase through the cross-section of the conductor, being larger in the center and smaller towards the outside.

This causes the current to be concentrated at the skin of the conductor in which the back- emf is a minimum. This is either into the skin of a circular conductor or the edges of a flat strip, producing what is known as the 'skin effect'. The resulting non-uniform current density has the effect of increasing the apparent resistance of the conductor, thereby gives rise to power loss.

The 'skin effect' ratio can be defined as the ratio of the apparent d.c. and a.c. resistances i.e.:

S = Rf / R₀ where R_f = a.c. resistance of conductor R₀ = d.c. resistance of the conductor

S = skin effect ratio

The magnitude and importance of the skin effect increases with the frequency, size, shape and thickness of conductor, but is independent of the magnitude of the current flowing.

It should be noted that as the conductor temperature increases, the skin effect decreases which gives rise to lower than expected a.c. resistance at elevated temperatures. The effect is more noticeable for a copper conductor than an aluminum conductor of equal cross sectional area due to lower resistivity of copper conductor, especially in large bus bar sections.

The skin effect ratio of solid copper rods, the most used material for power transmission, can be calculated from the formulae derived by Maxwell, Rayleigh and others (Bulletin of the Bureau of Standards, 1912): S = [Square root Cl + χ⁴ / 48Yl + 1 when x <= 3

2

Where S = Skin effect ratio x = πd [Square root [2fUjLiQj!]]

P d = diameter of rod, mm f = frequency, Hz p = resistivity, μΩcm μ = permeability of copper (=1)

For HC copper at 20° C, p = 1.724 μΩcm, hence

x = 1.069 x lO-² dVf χ = 1.207 x lO^'2 VAf

Where A = cross sectional area of the conductor, mm²

Figs. 1 and 2 are drawn from formulae by Dwight and Arnold, which shows the value of skin effect for various conductor sections. Skin effect in tubular copper conductors is a function of the thickness of the wall of the tube and the ratio of that thickness to the tube diameter. For a given cross-sectional area, skin effect can be reduced by increasing the tube diameter and reducing the wall thickness.

For example, in the cases of copper tubes (Fig. 1), it is evident that it is desirable to ensure, where possible, low values of t/d and V(f/r). For a given cross-sectional area, the skin effect ratio for a thin copper tube is appreciably lower than that for any other form of conductors. Conclusively, copper tubes have maximum efficiency as conductors of alternating currents, particularly those of high magnitude of high frequency. Fig. 2 shows the values of skin effect for flat bars. The skin effect in flat copper bars is a function of its thickness and width. For a given cross-sectional area, the skin effect ratio for a thin bar or strip is usually less than in a circular copper rod but greater than in a thin tube. It is dependent on the ratio of the width to the thickness of the bar. As the thickness of the bar increases, the skin effect increases. Therefore, a thin copper strip (within the range of 2mm to 5mm) is more efficient than a thick one as an alternating current conductor.

Therefore, a bus duct system comprising multiple runs of copper bars is more cost effective due to large percentage of copper reduction. Table 4 shows the percentage of copper reduction in a multiple busduct system compared to conventional for different ampere systems.

Table 4: Copper savings for multi-run conductors

FIG 3 to FIG 5 illustrate cross sectional views of different multi-run systems using different number of bars for different widths and ampere systems.

In the manufacturing procedure, it is important to protect the multi-run system inside a fully hardened steel casing. This is to strengthen the mechanical strength in order to withstand short circuit current and increase heat dissipation. As shown in Fig. 7, the copper bars (1) are placed in an enclosed area made up of steel sheets. The steel sheets (2) are bended into C channels covering the multiple bars from all four sides. Inside the enclosed area, the multiple bars will be arranged together in a uniformly spaced-apart area, using pre-molded supports (3), made of bulk molding compound. These supports are placed on both ends of the bus duct and not_. at the center of the busduct.

With the copper bars clamped by the busbar supports (3) and placed in the enclosed steel sheets (2), the exposed ends (4) of the multiple bars will be mounted with pre-made steel jigs bolted onto the steel sheets, forming the final enclosed area Fig. 7.

Using a mixing machine, the epoxy (in liquid form), silica/quartz (in powder form) and hardener (in liquid form) will be mixed and poured into the enclosed area from the topside of the steel sheet. After this procedure is completed, the top steel cover (5) will be placed on and securely bolted, hi approximately 24 hours, the mixture inside the steel enclosure will be fully hardened.

Another method of installation without the epoxy, silica and hardener is by just using the epoxy material in powder form. The copper bars will be pre-heated and than placed into a fluidized tank filled with epoxy powder. Upon dipping the copper bar into the tank, the powder will melt and adhered onto the copper bar forming a coat of epoxy insulation material. This bar will than be placed back into the oven at approximately 200 degree C for 1 hour curing and eventually form a coated surface that has high mechanical strength, heat resistance and dielectric strength.

When the copper bars are done, this semi finished product will than be manually placed into the steel enclosure making it a complete product.

Obviously the advantage of having a multibar is to reduce the copper content and yet obtain the same current capacity as the conventional type of busduct. However there are relevant standards that govern the capability of the designs and final product. There are 2 basic categories which are applicable:

1. Low voltage system - applicable standards IEC 60439-1 and IEC 60439-2

2. Medium voltage system - applicable standards IEC 60694

The criteria of 'pass' is where a maximum temperature limit is applicable to the busducts at a rated current.

If a particular busduct is rated at 2000 ampere, hence this rated current will be injected into the busduct system. During this current injection, the operating temperature of the busduct will rise and the rise of this temperature will judge the 'pass' criteria. For a low voltage system, a maximum rise of temperature on the steel housing is 55 degree C off the ambient temperature. For more details please refer to the relevant standards.

This method of testing the conventional and multi-run system will verify the effectiveness of the invention.

As can be observed, multi-run conductors has increased the amount of surface area by splitting the bar. In high rise buildings each floor tap power off these busducts and along the busducts there^' are opening holes for power distribution. In the busduct construction, there is a forming which will make a tap off point. Refer to Fig 8. This tap off will only be on the top copper bar which will distribute the current. If the distribution of power will only by on the top copper bar along the entire busduct length, the top copper bar will be overloaded. Hence a solution is needed to balance all the copper bars.

This solution is by stamping a profile which allows a welding process to be done at the particular tap off point. Refer to Fig 6. Having the balancing points, the can be shared among all the multibars. provided that the balancing point is exactly below the tap off point. If the balancing point is before the tap off point, this will cause a short bottle neck on the top bar. The advantage of the multiple bar busduct system is that power loss will be reduced and hence transmission of current will be increased. This is most advantageous for high ampere systems, i.e. 1750A, 2000A, 2500A, 3000A, 4000A, 5000A and 6000A. Furthermore, cost of installing multiple bars will be reduced due to large percentage of copper reduction savings.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.

Claims

1. A multi-phase bus duct system for high ampere power transmission, each phase comprising a plurality of elongated bars running parallel to each other, characterised in that each elongated bar has a surface-to-volume ratio in the range of 0.45 to 1.15 mmVinm³'

2. A bus duct system as in claim 1, wherein the elongated bars of each conductor are joined with each other at substantially regular interval by an electrically conductive material, said intervals corresponding to a tap-off point, characterised in that the joints are aligned with said tap-off point and^' perpendicular to the length of said elongated bars.

3. A bus duct system as in claim 1, wherein the elongated bars are substantially rectangular.

4. A bus duct system as in claim 3, wherein the thickness of the elongated bars is within the range of 2mm to 8mm and the width of the elongated bars is within the range of 25mm to 200mm.

5. A bus duct system as in claim 1, wherein the elongated bars are shaped as rectangular bars, hollow squares, solid rods, hollow rods, U channels, C channels, I channels, I beams, solid triangular or hollow triangular bars.

6. A method of stacking multi-bar phases, comprising the steps of; a. producing the multi-bar phases by arranging a plurality of elongated bars in parallel and joining them at regular intervals corresponding to tap-off point intervals, characterised in that the joints are aligned perpendicular to the length of said elongated bars. b. Bending steel sheets (2) into C channels covering the multi-bar phases (1) from all four sides, forming an enclosed area; c. Arranging the multi-bar phases (1) together in a uniformly spaced-apart area using pre-molded supports (3); d. Affixing steel jigs onto the steel sheets, covering the exposed ends (4) of the multi-bar phases; e. Pouring a mixture of epoxy, silica and hardener into the enclosed area; f. Placing and bolting the top steel cover (5).

7. A method of stacking the multi-bar phases according to claim 6, wherein all the multi-bar phases have uniform lengths, thickness and widths.