Types of Bushings and Voltage ratings

Bushings are very important component of Transmission system. These are used of providing insulation to equipment.

There are various types of bushings available in the market, bushings are classified according to following:-

(i)                  Type of insulating material used

(ii)                Type of construction

(iii)               Insulation inside bushing

Let’s discuss about the same:-

(i)           Type of Insulating Material Used:-

This classification depends primarily on the application of the bushing, i.e. the purpose for which bushing is used. These can be further classified as:-

(a)    Air-to-oil bushing

In this type of bushing there is air is used as insulation at one end on bushing and oil acts as insulation at other end. In these bushings oil end is approximately half as long as air end as oil dielectric strength is more than two times than air at atmospheric pressure.

(b)   Air-to-air bushing

In This type of bushing  air acts as insulation at both ends and is used for applications where at end is open to atmospheric conditions and other is at indoor conditions.

(c)    Air-to-SF6 bushings

This type of bushing is used for SF6 insulated circuit breakers.

(d)   SF6-to-oil bushings

This type of bushing is used as shift from SF6 bus ducts and oil filled equipment.

(e)   Oil-to-oil bushings

This type of bushing as transition between Oil bus ducts and oil filled equipment.

(ii)                Type of Construction:-

There are two types of bushings depending upon type of construction, these are classified as below:-

(a)    Solid Type bushings also known as Bulk type bushings

(b)   Capacitance graded bushings also known as Condenser type bushings

Let’s discuss about these:-

(a)   Solid Type bushings:-

The kind of bushing, is normally made with a focal conductor and porcelain or epoxy covers at either end and is utilized essentially at the lower voltages through 25 kV.  These bushings are generally utilized as a part of utilizations extending from little distribution transformers and circuit switchgears to vast generator venture up transformers and hydrogen-cooled control generators.

The essential impediment of the strong bushing is its capacity to withstand 60-Hz voltages over 90 kV. Henceforth, its applications are restricted to 25-kV hardware appraisals, which have test voltages of 70 kV. Late applications require low halfway release confines on the 25-kV terminals amid transformer test and have brought about additional limitations on the utilization of this sort of bushing.

In these cases, either an extraordinarily outlined strong bushing, with one of a kind evaluating protecting that empowers low innate halfway release levels, or a more costly capacitance-reviewed bushing must be utilized.

(b)   Capacitance Graded Bushings:-

These bushings are utilized for basically all voltage evaluations over 25-kV voltage and has been utilized for bushings upto 1500-kV voltage.

These bushings utilizes directing layers at foreordained spiral interims inside oil-impregnated paper or some other protection material that is situated in the space between the focal conductor and the protector.

Diverse makers have utilized an assortment of materials and techniques for making capacitance-evaluated bushings. Early techniques were to embed concentric porcelain barrels with metallized surfaces or overlaid pressboard tubes with installed conductive layers. Later outlines utilized conductive foils, commonly aluminum or copper, in oil-impregnated kraft paper.

An elective strategy is to print semiconductive ink (distinctive producers have utilized diverse conductivities) on all or a portion of the oil-impregnated kraft-paper wraps.

The primary components are the focal round conductor, onto which the capacitance-evaluated center is wound; the best and lower protectors; the mounting rib; the oil and an oil-extension top; and the best and base terminals.

Capacitance-reviewed bushings include numerous more specialized and assembling subtle elements than strong bushings and are hence more costly. These points of interest incorporate the protection/directing layer framework, gear to wind the capacitor center, and the oil to impregnate the paper protection.

In any case, it ought to be noticed that the outspread measurement required for the capacitance-evaluated bushing is considerably less than the strong development, and this saves money on material inside the bushing also in the mechanical assembly in which the bushing is utilized. Likewise, from a down to earth stance, higher-voltage bushings couldn't in any way, shape or form be fabricated with a strong development.

(iii)             Type of Insulation inside Bushing

Still another order identifies with the protecting material utilized inside the bushing. As a rule, these materials can be utilized as a part of either the strong or capacitance-evaluated development, and in a few sorts, more than one of these protecting materials can be utilized as a part of conjunction. The accompanying content gives a short portrayal of these sorts:

1. Air-Insulated Bushings

Air-protected bushings for the most part are utilized just with air-protected mechanical assembly and are of the strong development that utilizes air at air weight between the conductor and the protectors.

2. Oil-Insulated or Oil-Filled Bushings

Oil-protected or oil-filled bushings have electrical-review mineral oil between the transmitter and the encasings in strong sort bushings. This oil can be contained inside the bushing, or it can be imparted to the contraption in which the bushing is utilized. Capacitance-reviewed bushings likewise utilize mineral oil, typically contained inside the bushing, between the protecting material and the protectors for the motivations behind impregnating the kraft paper and exchanging heat from the directing lead.

3. Oil-Impregnated Paper-Insulated Bushings

Oil-impregnated paper-protected bushings utilize the dielectric cooperative energy of mineral oil and electric evaluations of kraft paper to deliver a composite material with predominant dielectric-withstand attributes. This material has been utilized broadly as the protecting material in capacitance-evaluated centers for approximately the most recent 50 years.

4. Gum Bonded or - Impregnated Paper-Insulated Bushings

Gum fortified paper-protected bushings utilize a sap covered kraft paper to manufacture the capacitance-reviewed center, while gum impregnated paper-protected bushings utilize papers impregnated with tar, which are then used to create the capacitance-evaluated center. The last kind of bushing has prevalent dielectric attributes, equivalent with oil-impregnated paper-protected bushings.

5. Cast-Insulation Bushings

Cast-protection bushings are developed of a strong thrown material with or without an inorganic filler. These bushings can be both of the strong or capacitance-evaluated types, despite the fact that the previous sort is more illustrative of present innovation.

6. Gas-Insulated Bushings

Gas-protected bushings utilize pressurized gas, for example, SF6 gas, to protect between the focal conductor and the rib. It utilizes the same pressurized gas as the electrical switch, has no capacitance reviewing, and utilizes the measurements and situation of the ground shield to control the electric fields.

Bushing rating and mounting are depended upon type of interface.  HT Bushing above 1000 V are designed as per IS 2099

There are 5 types of interfaces:-

1.       Interface A1

2.       Interface B1 & B2

3.       Interface C1 & C2

4.       Interface D1 & D2

5.       Interface F1, F2 & F3

Short circuit ratings of all above mentioned bushings is as below:-

Interface Type	Contact Type	Current Rating(A)	RMS(kA)			Peak asymmetrical current rating kA
			1 Sec	2 Sec	3 Sec
A1	Pin and Socket	250	12.5	9.0	7.5	31
B1	Pin and Socket	250	12.5	9.0	7.5	31
B2	Pin and Socket	400	16.0	11.3	9.2	40
C1	Bolted	630	28.0	19.7	16.1	70
C2	Bolted	1250	75.0	53.0	43.3	>150
D1	Bolted	800	50.0	35.3	28.8	125
D2	Bolted	1250	75.0	53.0	43.3	>150
F1	Bolted	2500	-	-	-	-
F2	Bolted	630	28.0	19.7	16.1	70
F3	Bolted	1250	75.0	53.0	43.3	>150

Applications of different types of bushings:-

A1 Interface type bushings :-

These types of bushings are useful for equipment insulated with oil fluids such as Transformers, Switchgear, Capacitors.  There are following types of A1 interface type bushings:-
(i) 180AR-1

Voltage Rating- 12 KV, Current Rating- 250A, Dimensions are- 222 X 106 (height X Breadth)

(ii) K180AR-1

Voltage Rating- 24 KV, Current Rating- 250A, Dimensions are- 222 X 106 (height X Breadth)

(iii) 180AR-2

Voltage Rating- 12 KV, Current Rating- 250A, Dimensions are- 284 X 168 (height X Breadth)

(iv) K180AR-2

Voltage Rating- 24 KV, Current Rating- 250A, Dimensions are- 284 X 168 (height X Breadth)

(v) 180AR-3

Voltage Rating- 12 KV, Current Rating- 250A, Dimensions are- 171 X 55 (height X Breadth)

(vi) K180AR-3

Voltage Rating- 24 KV, Current Rating- 250A, Dimensions are- 171 X 55 (height X Breadth)

There is another interface A1 bushing known as In-Air bushing for dry type transformers, Motors , Switchgear and capacitors.

Types of Bushings are

(i)                  180A-24P-O

Voltage Rating- 12 KV, Current Rating- 250A, Creepage Distance (mm)- 630 mm

(ii)                180A-24P-O

Voltage Rating- 24 KV, Current Rating- 250A, Creepage Distance (mm)- 630 mm

Interface B Type Bushings:-

This is useful for Oil fluids such as Transformer, Switchgear & Capacitors.

There are following types of bushings:-

(i)                  400T1

Voltage Rating- 12 KV, Current Rating- 400A

(ii)                K400T1

Voltage Rating- 24 KV, Current Rating- 400A

(iii)               M400T1

Voltage Rating- 36 KV, Current Rating- 400A

(iv)              400AR-1

Voltage Rating- 12 KV, Current Rating- 400A

(v)                K400AR-1

Voltage Rating- 24 KV, Current Rating- 400A

(vi)              M400AR-1

Voltage Rating- 36 KV, Current Rating- 400A

(vii)             400AR-2

Voltage Rating- 12 KV, Current Rating- 400A

(viii)           K400AR-2

Voltage Rating- 24 KV, Current Rating- 400A

(ix)              M400AR-2

Voltage Rating- 36 KV, Current Rating- 400A

(x)                400AR-8

Voltage Rating- 12 KV, Current Rating- 250A

(xi)              K400AR-8

Voltage Rating- 24 KV, Current Rating- 250A

(xii)             M400AR-8

Voltage Rating- 24 KV, Current Rating- 250A

There is another type of Interface B type switch is specifically used for SF6 circuit breakers.

Similarly there are other Interface type bushings and have same applications only difference is in their mounting designs , voltage rating and current rating.