ABSTRACT

 

      EN: 27 pages, 8 figures, 5 tables, 5 sources, 3 drawings.

      Subject of investigation – high-voltage bushing.

      Aim of the work – to learn to use engineering methods of calculation

of electrical devises with the constructor development and solving problem of producing all details and devise itself. The work on this term project is for better understanding for students of such course as "High voltage apparatuses" "SAD", and solving the problem of diploma designing.

      Bushes apply to a conclusion of wires of high potential from tanks of transformers, oil switches, and also for a lining of wires through walls of buildings.

This work was held with the usage of program package MathCAD. Usage of this program gives us possibility to accept more accurate results. The drawings were made by program package AutoCAD.

      For the realization of the purpose of this project was designed the high-voltage Bushing with RIP insulation.

      Given data:

Nominal voltage: U_n=66 kV;

Nominal current: I_n = 400 A; as soon as bushings are produced only for the nominal current 630 A we take this value as the input data.

Isolation:internal-external;

Environment: Air-oil;

Type of hermetization: Hermetic;

Type of apparatus: high-voltage bushing;

Load;P=5kN;

Device is installed on 600m 

BUSHING INSULATOR, HIGH-VOLTAGE BUSHING, CHINA COVER. INSULATION, RIP INSULATION, OIL-BARRIER INSULATION, CAPACITY PLATE, TRANSFORMER OIL 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

CONTENTS

TASK…………………………………………………………………………….…2

ABSTRACT 3

INTRODUCTION 6

1 CONSTRUCTIONS OF HIGH VOLTAGE BUSHINGS 7

2  DESCRIPTION OF CHOSEN CONSTRUCTION 13

3 CALCULATION OF INSULATOR 14

3.1 Calculation of isolation. 14

3.2 Calculating of china covers 19

3.3 Mechanical calculation 21

3.4 Heat calculation 22

4 DESIGN DESCRIPTIONS 25

CONCLUSION 26

LIST OF REFERENCES 27

 

INTRODUCTION

 

      Bushes are applied for terminal of wires of high potential from tanks of transformers, oil switches, and also for a lining of wires through walls of buildings. Hey are installed the bush on a cover of the tank or on a wall of the building the metal flange. Bushes can well work at height of no more then 1000м above sea and in an interval of temperatures from-40 up to +45° with relative humidity up to 85%. Corresponding designs of isolators can be used for other, heavier environmental conditions.

      Bushings are BI, intended for carrying out of current carrying parts of the device through his (its) earthed parts.

      Bushes (BI) have two executions: internal installation (either ends BI are indoors or the device) in accordance with GOST 20454-79 and external-internal installation (one end BI is outside of a premise or the device) in accordance with GOST 20479-83. From both cleaning cloth PI there can be a same environment (air -air) or different environments (air - oil or air - SFe gas).

      Aim of the work – to learn to use engineering methods of calculation

of electrical devises with the constructor development and solving problem of producing all details and devise itself. The work on this term project is for better understanding for students of such course as "High voltage apparatuses" "SAD", and solving the problem of diploma designing. 
 
 
 
 
 

      1 CONSTRUCTIONS OF HIGH VOLTAGE BUSHINGS

 

      Bushes (BI) have two executions: internal installation (either ends BI are indoors or the device) in accordance with GOST 20454-79 and external-internal installation (one end BI is outside of a premise (room) or the device) in accordance wilh GOST 20479-83. From both cleaning cloth BI there can be a same environment (air - air) or different environments (air - oil or air - SF$).

      Bushes (high-voltage bushings are BI, intended for carrying out of current carrying parts of the device through its grounded parts) in the elementary kind represent a cylindrical body from one dielectric or several layers of various dielectrics along which axis passes a current carrying core; outside in an average part the body is covered by the metal grounded flange serving for fastening of isolator to the case of the device or to walls of buildings.

      Bushes for outside installations have a china detail with far acting edges (wings) which are located in the top hamper of isolator intended for work in an outside atmosphere

      High-voltage bushings, as well as bushes, have external and an internal insulation. External insulation will consist of the top china cover taking place in an external atmosphere and tightly connected to the bottom china cover by the metal connecting cartridge and ring linings from rubber. High-voltage bushings can be established under a corner of an inclination to a vertical from 0-45 and 0 - 90° depending on their design.

      The top china cover has far acting edges (wings) intended for protection against a rain of sites of a surface, located under acting edges. Its both parts work for linear High-voltage bushing in the air environment, but one part settles down inside the building, and another outside.

      High-voltage bushings divide on their design on tight, leaky and oil-filled. The internal insulation of tight high-voltage bushings has no message with an environmental atmosphere. At leaky high-voltage bushings the oil filling in them, has the message with an environment through an oil bar and drier of air. The last detain humidifying and oxidation of oil. Oil-filled high-voltage bushings tight, but have the general (common) oil system with transformers and chokes on which they are established.

      To destination distinguish high-voltage bushings for transformers, chokes, oil switches and for pass through walls and overlapping (linear high-voltage bushings).

      High-voltage bushings for transformers and chokes establish under a corner from 0 up to 60°, for oil switches - from 0 up to 15°, and linear high-voltage bushings - from 0 up to 90°.

      High-voltage bushings make or without additional capacity C2 (the measuring condenser), or with the measuring condenser for connection of devices of measurement of a voltage and let out(release) on rated voltage 66, 110,150, 220, 330, 500 both 750 KB and rated currents 200, 320, 400, 630, 800, 1000, 1250, 1600, 2000 and 2500 And.

      Besides let out high-voltage bushings with normal and amplified (strengthened) external insulation. In the latter case the top cover has more advanced surface at acting edges. Thus the leakage path length of a current on a surface of a china cover considerably increases. High-vokage bushings with amplified (strengthened) external insulation are used in areas with a dusty atmosphere.

      The majority of high-voltage bushings is intended for work at temperature from-45 up to +40°C on a voltage from 66 up to 500 KB at height no more than 1000 m above sea level, and high-voltage bushings on a voltage 750 KB - no more than 500 m inclusive can work. Amplified (strengthened) designs of high-voltage bushings can work and in more heavy conditions of operation.

      All high-voltage bushings should have strictly certain mechanical, thermal and electric parameters.

      Now several constructional types of bushings can be regarded.

      Bushings for power transformers and reactors are manufactured with two types of internal insulation of the capacitor type: solid (RIP Resin Impregnated Paper) and paper-oil (OIP Oil Impregnated Paper).

      Solid insulation.

      Internal insulation is the main structural part of the bushing and presents an insulation core, which is formed by winding insulating paper on the central pipe with sliced conducting leveling plates. Facings are used for optimal distribution of electric field in the radial and axial directions, which ensures the highest values of electric strength of both internal and external insulation, including a transformer placed on the bottom of the bushing.

      The isolating core is formed by crepe winding. Then the feedstock is saturated with epoxide compound under vacuum with the following solidification under pressure. This way all gas including are removed. 

      Figure 1.1 – The construction of bushing with solid insulation

      After thermal processing and glazing of the outer surface the isolating core forms a solid heart on which the coupling is installed by press fit. For better sealing between the coupling and the core the interfaces are filled with epoxy.

      Advantages:

• high reliability;
• long operation life.

Disadvantages:

• impossibility of recovery after breakdown.

 

      Paper-oil insulation.

      Internal insulation is the main structural part of the bushing and presents an insulation core, which is formed by winding dielectric paper on the central pipe with sliced conducting leveling plates. Facings are used for optimal distribution of electric field in the radial and axial directions, which ensures the highest values of electric strength of both internal and external insulation, including a transformer placed on the bottom of the bushing.

      Before assembling the core is dried under vacuum. After assembling bushing is processed by vacuum and saturated with transformer oil.

      Advantages:

• low cost;
• ability of automation of the producing process

      Disadvantages:

• construction is complex;
• combustibility;
• process of drying and impregnation is long.

 

      Figure 1.2 – The construction of bushing with paper-oil insulation 

      Gas-filled bushings can include either capacitor plates or cylinder-disk system. The isolation is presented by air, nitrogen, SF6 and mixture of nitrogen and SF6.

      Advantages:

• high isolating properties;
• high blowout properties;
• quick recovery of electric strength;

Disadvantages:

• can be used only in sealing bushings;
• risk of liquefaction process.

 
 
 
 

      Figure 1.2 – The construction of bushing with gas insulation 

      As a pesult of processing I choose oil-filled bushing with RIP insulation as it is the newest technology. The description is in the next paragraph.

 

2  DESCRIPTION OF CHOSEN CONSTRUCTION

 

      The designed high-voltage bushing on voltage 66 kV and nominal current 630 A will have next construction. As soon as bushings are produced only for the nominal current 630 A we take this value as the input data.

      The current carrying circuit of high-voltage bushing insulator, will comprise of a copper pipe. Lead will have three isolation gaps: RIP condenser isolation, transformer oil and a china insulator.

      We have the composition of next insulating materials: -internal insulation is the RIP insulation composed from bakelite paper and epoxy compound; external insulation are the china covers.

      Lower china cover will not have ribs.

      As analogical construction the bushing was chosen ГКТIII-60-66/630 (See on figure 2.1).But designing bushing will have more dimension.

      The high-voltage bushing comprise of the following elements: the contact clip - 1, the compensator of pressure - 2, the upper china tire casing - 3, a connective sleeve –  4, the lower china tire casing - 5, the shield - 6. 

      Figure 2.1 – Bushing on 66 kV.

 

      3 CALCULATION OF INSULATOR

      3.1 Calculation of isolation.

 

      For calculation of isolation the insulation gaps must be calculated.

      The composition of next materials is presented:

• Internal insulation is the RIP insulation composed from resin impregnated paper;
• External insulation is the china covers.

      The insulation gap through the thickness of bakelite paper δ and also length of china covers L to preserve the surface discharge in the air. The next methodic is used ([3], p. 138-157). 

      Figure 3.1 – Calculated gaps 

Phase operating voltage : 
 
 

      Figure 3.2 – A location of facings in high voltage bushing isolator 

      The cross-section of a current carrying pipe is defined proceeding from meaning of an initial current and an economic current density.

      In table 3.2 the sections of a current carrying pipe given for a choice are resulted.

I_max – the greatest working current;

I_n – nominal current;

J – the accepted current density (for copper conductors J=0.9-1.2 A/mm²). 

      Table 3.1 – Currents for definition of section of a current carrying pipe.

 

      Let us choose copper pipe with external diameter d_out and internal d_in :

d_out=30∙10^-3 m;

d_in=20∙10^-3 m.

      When calculating the isolation of bushings the condition E=const is used and the next equality is used:

U = 1,1U_c,

where U_c=140 kV is test voltage in the dry state, when there should not be creepages at the edges of the equalizing facings. The coefficient 1.1 takes into account the deviation of the test voltage caused by test conditions and measurement accuracy.

U = 1,1∙140=154 kV

      The number of insulation layers is defined by the formula:

,

where d_min is the minimal dielectric layer thickness;

E_max.calc. is the maximal radial tension.

      The number of insulation layers by the conditions of unstable ionization is: 

where Е_{r.max.calc.и} = 10,4×d^-0,55×U/U_ph= 135.3 kV/cm – is maximal calculated field tension, defined by the conditions of unstable ionization.

      The number of insulation layers by the conditions of creeping discharge is:

,

where e×d_.– is maximal calculated field tension, defined by the conditions of creeping discharge;

e - is the dielectric constant o RIP insulation taken as 4,2.

      The dielectric thickness is taken δ=0.1cm.

      The number of isolation layers is taken by the minimal value of maximal calculated field tension, so  
 

      So we can take n=14.

Length of a ledge of the core inverted in apparatus: 

where k₁=1.4 – depreciation factor of durability. 

      The sum of lengths of ledges of one part of high-voltage bushier: 
 
 
 

      Let’s check up whether the received meaning Σλ_e satisfies the condition 
 

      The condition is satisfied, so we leave λ_l=2.064 cm

      The full sum of lengths of ledges is: 

 

      Length of a facing at a core: 

where for minimum conditions x=4.1.

      Length of null facing of the core: 

and parameter  

      The radius of null facing is: 

      The radius of n^th facing is: 

=6.56 cm

      Parameter A is: 

     and parameter: 

     Maximal tension in x layer:

      ,

     where layer voltage

     Layer length x is: 
 
 

     Table 3.2 – The results of layers calculation

      3.2 Calculating of china covers

 

      Length of the top tire cover is defined by the formula: 

      Length of the lower tire cover is defined by the formula: 

then

      Length of coupling 

      Inside diameter of the coupling is taken: 

      Outside diameter of the coupling is taken: 

      Inside diameter of tires 

      Outside diameter of tires 

      Diameter by sheds of insulator is 

      Number of sheds is: 
 

      The distance between sheds of insulator is: 
 
 

      Table 3.3 – Dimensions for rib calculating

 

      Figure 3.3 – The profile of a rib of a china insulator 

      Wet flashover test: 

      Layer voltage is: 

      Layer density is: 

      Medium radial density taken at maximum: 

      Skeleton volume is: 

      Maximal flange density: 
 

      3.3 Mechanical calculation

 

      Bushes treat to an operation of the bending loading affixed by the end of the air end. At calculations on a mechanical strength it is supposed that all loading is perceived by a chine tire cover and the internal part of lead does not carry mechanical loading.

      Width of a wall of a china tire cover is equal 35 mm;

      Moment of inertia: 
 
 

      where D=0.239 m, d=0.169 m – external and internal diameters of a china insulator accordingly.

      The moment of resistance to curving of bushes at round section: 
 
 

      Mechanical effort in section: 
 
 

      where M – flexing operating moment of an insulator: 
 
 

      The cross-section area of china cover: 
 
 

      Figure 3.4 – Dependence of limit strength of china at curving from cross-sectional area. 

      By the graph

      The condition is held 
 
 
 
 

      So the chosen parameters satisfy the request.

      3.4 Heat calculation

 

      At passage of a current on a current carrying core there is an allocation of heat in a core and in isolation owing to dielectric losses in it. At some temperature there is a thermal breakdown of isolation. Therefore it is important to define voltage at which there will be a breakdown of isolation, so-called thermal breakdown voltage which is defined: 
 
 

      where p₀ – losses in dielectric at temperature v₀, Wt/cm³;

      λ₁ – calorific conduction, Wt/(cm∙K);

      v_B – ambient temperature, °C.

      It is necessary to determine the following sizes: 
 
 
 
 
 

      where λ₂ – equivalent calorific conduction of a china cover and a layer of oil between RIP isolation and china, Wt/(cm∙K);

      r_i – outside radius RIP, cm;

      r₀ – outside radius of a current carrying pipe, cm;

      r_c – outside radius of a china cover, cm;

      k_T – factor of a heat dissipation from a surface of a china cover in air. 

Table 3.4 – Data for calculation of U_T.

 

Table 3.5 – Dependence of f_o(m).

 
 
 
 
 

      We can choose from the table by approximation f₀=0.6: 
 
 

      The received value should be in 1.5…2 times more then 1.1∙U_H.P. 

      So the demand is satisfied. 
 

 

4 DESIGN DESCRIPTIONS

 

      The job of the given academic year project consisted in constructing and calculation of high-voltage bushing insulator on the voltage 66 kV. The designed high-voltage bushing intended for installation on SF₆ factory-assembled switch-gear or switches - on voltage 330 kV. Bushing is installed on the height no more then 600 m on the sea level in normal pollution conditions. For a basis of projection it is chosen oil - filled hermetic high-voltage bushing insulator for transformers on voltage 66 kV.

      The current carrying circuit of high-voltage bushing insulator, comprise of a copper pipe. Lead has three isolation gaps: RIP condenser isolation, transformer oil and a china insulator. The upper part of high-voltage bushing insulator on voltage 66 kV will comprise of the following elements: the contact clip, the compensator of pressure, the upper china cover, a connective sleeve. The lower part: the lower china cover, the shield. Design of bushing is sealing. For indemnification of temperature changes of volume of oil the special equalizers built -in bushing serve. The internal insulation is executed as RIP isolation and placed in china covers filled by insulating oil. Bushing constantly is under superfluous pressure of oil.

On the connecting cartridge intended for fastening input are: clevis for rise of bushing, terminal for measurement of a tangent of a dielectric loss angle and capacities of an internal insulation which in a running time earthed the gate for adjustment of pressure and the measuring device with the manometer for the control of pressure over input. In the bottom part of bushing there is a shield. General view of construction is represented on the drawing 1. View of the lower china cover on sheet 2. View of current leading pipe on sheet 3.

 

CONCLUSION

 

      Result of execution of an academic year project became designed and calculated the high-voltage bushing insulator on 66 kV.

      Principal advantages of the given construction are that it is hermetic, hence there is no moistening and an intensive strain ageing of internal isolation of high-voltage bushing, it is reliable and safe. The following advantage of designed high-voltage bushing insulator is be relative the random distribution of voltage along a current carrying circuit. Disadvantage of the given construction is impossibility of recovery after breakdown. 

 

LIST OF REFERENCES

 
 

1. Methodical guide for course project on the "High-voltage apparatus" course for 8.092206 major - Electrical apparatus./Author A.Afanasiev.-Zaporozhue:ZNTU,2003. - 15 p.
2. Справочник по электрическим аппаратам высокого напряжения./ Под ред. Афанасьева В. В. - Л.: Энергоатомиздат, 1987.
3. Синявский В. Н. Расчет, конструирование и испытание изоляторов высокого напряжения. М.: Энергия, 1965.-112 с.
4. Справочник энергетика промышленных предприятий. Гольстрем В.А. ,Иваненко А.С.- К.: Техника»", 1997 г. - 562 с.
5. Дмитриевский В. С. Расчет и конструирование электрической изоляции. М.: Энергоиздат, 1981. 246 с.