An OIP bushing uses concentric cylindrical capacitors in series between the central conductor and outer insulator housing of the bushing. An OIP bushing serves the same purpose as the other types of bushings do. That means it provides proper insulation between the live conductor and the grounded metallic tank of electrical equipment, such as power transformers, circuit breakers, and so on.
The advantage of an oil-impregnated paper bushing over an RBP bushing, or resin-bonded paper bushing, is that, because of capacitor grading, it facilitates a uniform voltage gradient distribution over the cross-section of the bushing. Obviously, the uniform electrical field intensity or voltage gradient distribution is essential in high-voltage applications. More specifically, we should use OIP bushings for the voltage class above 72.5 KV.
Construction of OIP Bushing
The main parts of an oil-impregnated paper bushing include condenser core, central conductor, insulating material, housing, oil expansion chamber, oil level gauge, mounting flange, sealing and clamping arrangement, etc.
Central Conductor
A copper, aluminum, or sometimes bronze rod or hollow pipe acts as the central conducting part of the bushing. It therefore carries the main current through the bushing. So the dimension of this central conducting rod or pipe must be such that it can carry the required current without overheating. Therefore, for designing a bushing, it is always essential to decide the proper diameter of the conducting rod or pipe.
Condenser Core
We wrap kraft paper with aluminum foil around the central conductor to make the grading capacitor. Then we wrap a number of such layers of kraft paper with aluminum foil one by one, depending upon the design parameters and voltage level of the bushing. We use oil-impregnated kraft paper for this purpose. That is why we call this type of bushing an oil-impregnated paper bushing or OIP bushing.
Construction of Condenser Core
The impregnation of oil in the kraft paper enhances the dielectric strength of the paper and its overall insulation strength. The outer layer of oil-impregnated kraft paper with aluminum foil is shorter than the adjacent inner layer. This is to maintain the equal capacitance of all the capacitors formed by the layers of oil-impregnated kraft paper and aluminum foil.
Housing of Oil Impregnated Paper Bushing or OIP Bushing
A hollow cylindrical porcelain housing covers the entire arrangement. The housing provides mechanical support and environmental protection to the capacitor grading system of the bushing.
There are two portions of the bushing: one is the outer portion, and the other is the inner portion. The length of the outer and inner portions both depend upon the minimum required distance from the live conductor to the grounded metallic part of the tank.
The outer portion of the porcelain housing has rain sheds to maintain the minimum creepage distance. On the other hand, the inner portion does not require such rain sheds, but that portion will also have sufficient length to maintain the minimum required clearance between the conducting part and the grounded tank.
Both the top and bottom of the housing are hermetically sealed. The entire arrangement inside the housing is filled with transformer oil.
Expansion Chamber
A gas-filled, mainly nitrogen-filled, spring-loaded chamber on the top of the bushing accommodates the thermal expansion and contraction of transformer oil. This portion of the bushing serves the purpose of the conservator tank of a transformer. Some manufacturers provide a bellow in the chamber to compensate for the thermal expansion and contraction of oil.
Oil Level Gauge
The expansion chamber contains a magnetic or prismatic-type oil level gauge to visually monitor the level of oil. When the manufacturer provides a bellow, the only option is a magnetic-type oil level gauge.
Mounting Flange
At the bottom of the air-side part of the bushing, there is a mounting flange made of aluminum alloy. This mounting flange serves the purpose of fixing and sealing the OIP bushing on the turret of the transformer tank. The flange contains a test tap or voltage tap used for capacitance or tan-delta testing. The mounting flange is also provided with a hanging arrangement for the bushing.
Manufacturing Process of OIP Bushing
Manufacturing process of the condenser core
First, a wrapping machine wraps the kraft paper around the central tube or rod. The number of turns in the wrapping depends on the thickness of the graded capacitor, which is calculated for proper grading. Manufacturers normally use a computerized program to calculate the thickness and length of each concentric cylindrical capacitor.
After wrapping the kraft paper for the required turns, the machine winds aluminum foil on the paper. Over that aluminum foil, the machine again wraps kraft paper of the calculated thickness and then applies another layer of aluminum foil. The same process continues according to the calculated thickness and length requirements.
Dehydrating Process
In this way, after completing the total wrapping procedure, manufacturers place the condenser core along with the central conducting pipe or rod in a vacuum oven or autoclave under higher controlled temperature. A vacuum is applied to remove moisture and trapped gases from the assembly.
Next, previously treated and filtered oil is injected into the vacuum chamber. Under vacuum conditions, the oil easily impregnates the paper insulation. The oil impregnation process eliminates voids or gaseous inclusions from the paper.
Final Assembly
After impregnation, we assemble the impregnated core into a porcelain or insulator housing. Then, we fit the top expansion chamber and bottom flange onto the bushing. The bushing is hermetically sealed from the top and bottom using gaskets.
Advantages of OIP Bushing
- It has much higher dielectric strength than a non-graded bushing of the same size.
- An oil-impregnated paper bushing is superior to resin-bonded paper bushings of the same size.
- It is a proven and reliable design.
- The capacitor grading distributes voltage uniformly, maintaining a uniform voltage gradient.
- It is highly suitable for high-voltage applications, mainly above the 72.5 kV level.
- The main advantage of an OIP bushing is that it is almost free from partial discharge.
Disadvantages
- Because of oil inside the bushing housing, it may develop oil leakage after long-term operation.
- Another limitation is storage. The entire graded capacitor system must always remain immersed in oil, so the bushing should be kept in a vertical position.
- Although an inclination of up to 30° is normally allowed, it should not exceed this limit.