To obtain optimum and best performance, it requires high quality and purity of insulating oil during the time of use. To avoid increasing voltage and greater electrical stress, oil with good di-electric qualities and minimum residual moisture (water) content in oil should always be maintained.
Contaminated oil tends to lose its characteristics of di-electric strength, resistivity and loss factor.
The oil conditioning plant will be mounted on a compact under carriage with suspension springs and two pneumatic tyres with a tow bar to hitch the plant to a towing vehicle for towing the plant over motarable roads.
All the components are neatly and compactly arranged with adequate space. The complete plant is provided with a sheet metal casing with necessary doors providing access to all controls and is suitable for outdoor operations under all weather conditions.
All the components will conform to relevant IS standards or equivalent International Standards and in the absence of any such standards the components will be of good engineering design and of good workmanship and quality.
The filtration plant will consist of the following:
The plant will be provided with a suitable magnetic strainer at the inlet of the gear pump with necessary wire mesh perforated core to arrest all coarse particles. Provision is made to clean the strainer element without dismantling the complete strainer from the pipeline.
The inlet feed pump provided will be positive displacement type gear pump and has a built-in pressure relief valve to take care of any accidental pressure raises at the delivery side of the pump. A separate bypass valve is provided across the gear pump, so that the flow through the filter can be adjusted as required.
The heater tank will be of MS welded construction. The vessel is completely leak proof and is suitable for high vacuum and pressure application.
The electric heaters provided have an adequate capacity to heat the oil from the ambient temperature to the required oil conditioning temperature upto 60 deg. C.
The heater vessel is designed in such a way that a generously dimensioned heat transfer surface area is provided. This generous design affords a very low specific heat load of the order of < 2.0 Watts/sq. cm.
The heater vessel elements are designed such that they have a long life. These elements can be replaced without draining the heater vessel.
The heater vessel is efficiently designed with baffles to make the oil pass over the entire length of the heater tubes and avoid any chance of localized overheating of oil.
The total heating capacity will be divided into suitable number of thermostatically controlled heating stages and a digital temperature controller with an accuracy of at least +/- 2.0% deg. C is provided. The digital temperature controller is of adjustable type by using feather touch keys. The controller is housed in the control panel for easy observation and readjustment if required.
While dividing into number of heater banks, it is ensured that the total load is evenly balanced in all the three phases. A preset safety thermostat is set at higher temperature than the required oil conditioning temperature is provided on the heater vessel and this safety thermostat will take care of any accidental temperature rise of oil by putting off all the heaters when the oil temperature rises even above the higher set temperature. This thermostat is factory set and tamper proof. Suitable pressure relief valve is provided to take care of any accidental pressure rise over the specific safe level.
Dial type temperature gauges are provided before inlet pump and after the filter vessel to indicate the oil temperature before and after oil heating.
The filtering system essentially consists of a high efficiency non-hygroscopic throwaway type filter cartridge with large dust holding capacity. The filter medium is progressively denser in the direction of flow, which will ensure large dust holding capacity.
The filter vessel is so mounted as to afford facile replacement of cartridges without the use of any special tools or any special facilities. The vessel is leak proof and suitable for high vacuum and pressure application.
Necessary compound gauge is provided on the filter vessel to indicate the inlet pressure so that the condition (clean or choked) of the filter cartridges can be ascertained from the gauge reading.
The hot insulating oil even at temperature of the order of 100 deg. C will not affect the filter cartridges. There will not be any medium migration. After initial fitting of the filter cartridges these cartridges do not require any periodic adjustment till it is replaced.
Necessary aeration valve is provided on the filter vessel to aerate the vessel during draining.
Degassing chamber (single stage)
The degassing system is efficiently designed in such a way that the required parameters as mentioned are achieved.
The degassing chamber is of fully welded construction to preclude any possibility of leakage due to seal failure or improper assembly. The vessel is suitable for operation under high vacuum conditions.
The degassing chamber consists of efficiently designed column of reaching rings. The spread of oil in the degassing chamber is uniform and ensures maximum exposure of oil to vacuum.
All illuminated sight glass (inspection window) is provided on the degassing chamber through which oil flow in the chamber can be observed clearly.
The operating vacuum in the degassing chamber will be suitable to achieve oil parameters as mentioned.
The degassing chamber is provided with a suitable optical level monitor which will control the oil/foam level in the degassing chamber. The level monitor provided senses both oil and foam level.
The optical level monitor will switch off the inlet gear pump in the event of oil/foam level raising above the desired level. This foam level control is necessary to prevent the contamination of the vacuum pump oil with the insulating oil.
The oil level in the degassing chamber is maintained to the minimum so that maximum free fall height is available for the oil to fall through irrespective of height of the degassing chamber. By this, the oil is exposed to the high vacuum to the maximum extent so that degassing and dehydration is complete.
There is an air relief valve in the degassing chamber for reducing the vacuum in the system in the event of excessive foaming of the oil.
The oil conditioning plant is provided with required capacity of vacuum to ensure that the required parameters as mentioned are achieved.
Vacuum performance can be enhanced through use of a roots booster pump.
This pump is provided with gas ballast valve to reduce the vacuum during operation and to prevent contamination of lubrication oil of pump with moisture, which is removed from the transformer oil in the initial stage.
The plant is provided with suitable facility to ensure that, in case of power failure the vacuum pump is not sucked into the degassing chamber.
The oil after purification, dehydration and degasification in the degassing chamber held under vacuum, will be drawn out by a discharge pump, which is of centrifugal pump glandless type.
The above centrifugal pump is a glandless pump without any rotary seal. This glandless feature is to ensure total isolation of the conditioned oil from atmosphere, which in turn is necessary or maintaining the quality of the conditioned oil.
This discharge pump has adequate capacity to deliver the required quantity of oil from the degassing chamber under vacuum and is capable of producing a head of 10 meters.
All valves in oil lines are ball valves with Teflon seating and with stainless steel balls. These valves are leak proof and are suitable for vacuum and pressure application.Ball valves are provided for inlet and outlet. A bypass valve is also provided to afford internal circulation of oil within the plant itself.
Two numbers mechanical non-return valves are provided, one before the degassing chamber and the other after the outlet pump so that in case of power failure or shutdown, flooding of degassing chamber or the plant with the oil from the storage tank is prevented, thereby preventing mixing of conditioned and unconditioned oil.
Aeration valves are provided on the filter vessel and the degassing chamber.
A ball valve is provided at the outlet of the outlet pump for drawing samples of conditioned oil.
All pipes are of good quality and all oil and vacuum connections are in the form of flanged joints with ‘O’ rings, ensuring full dealing efficiency at all times with minimum maintenance.
A compound (pressure-vacuum) gauge is provided at inlet of the filter vessel to check the condition (clean or choked) of the filter cartridges.
A dial vacuum gauge (Glycerin filled) of range (0-760 torr) is provided on the vacuum line to measure vacuum. Dial type temperature gauges are provided before inlet pump and after the filter vessel.
This filter is provided with a suitable single point draining system.
A sturdily built easily accessible control cabinet housing all the electrical control gears like contactors, circuit breakers, selector switches, isolators, digital temperature controller, interlocking features, etc are provided. All the wiring is neatly routed and all wire terminals are suitably identified with number ferrules.
The control panel is provided with a mimic diagram indicating the state of operation of vacuum pump, heaters, gear pump, glandless centrifugal pump, etc. the mimic diagram is provided with required indicating bulbs with switches.
The wiring provided is of adequate capacity to handle appropriate load currents continuously.
All electric motors driving the various pumps are TEFC squirrel cage induction motors and will operate on 415 Volts +/- 15%, 3 phase, 50 Hz, AC supply and are suitable for direct on line starting through a starter. For glandless centrifugal pump, the motor is integral with pump. All other motors generally conform to IS 325 with class of insulation F.
The gear pump is interlocked with the heater elements in such a way that unless the gear pump is switched on, the heater elements cannot be energized.
The gear pump and level monitor in the degassing chamber are interlocked in such a way that when the oil/foam level in the degassing chamber increases beyond the desired level, the gear pump is put OFF, so that there is no more oil flow into the degassing chamber.
The digital temperature controller provided puts OFF the connected heater bank when the set temperature is reached.
A preset safety thermostat is provided on the heater vessel which will switch OFF al the heaters in case of any accidental temperature rise of oil above the safe level, so that any likely-hood of fire hazard is fully eliminated.