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| Cable Oil Degassing Plants |
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| Oil Purification Plant for Insulating Oil |
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| General Specification: |
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This specification describes the technical layout, the performance and the workmanship of a vacuum insulation oil purification plant. |
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| Introduction: |
| The growing consumption of electrical energy continuously demands on higher degrees of durability and safety of operation of the generating, transforming, switching and conduction electrical devices. |
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The increasing transfer-power requires higher nominal voltages, which requests in a more sophisticated performance of the insulating materials. For electrical devices filled and operating with insulation oil, the quality and purity of the oil defines the lifetime and durability of the electrical apparatus, where especially the dielectric strength has a major importance.
Before the first filling and also during operation, - after defined cycles - , the insulation oil will be checked and tested regarding the electrical properties - and if necessary - reconditioned. This cleaning covers a degassing and dehydrating (drying) process under vacuum as well as a filtration. According to the specific requirements optionally additional adsorption or inhibitions treatment equipment may be applied.
The major parameters for defining the quality of the insulation oil are: |
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dielectric strength (breakdown voltage) kV/cm |
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Ohmic resistance O |
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water content (solved or dissolved) ppm |
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gas content Vol.% |
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impurities (particle size and numbers per volume) |
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turbidity |
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During operation, possible external sources for contamination are leakages of the housings, through which water, water vapour or gases impurify the insulation oil. Internal sources are the erosion of micro particles of the components and housing walls, diffusion of the residual humidity of the installed components or internal arcing which burns out the oil locally and generates floating ash particles. Even the oil ageing itself turns the probity of insulation negatively.
The oil properties for insulation oil, as well as the measuring methods are specified in the standards VDE 0370 or according the ASTM-Code D1533, D2945 and D877.
MARSHAL has designed and manufactured vacuum oil treatment plants and also entire insulation oil transfer and conditioning stations.
MARSHALbuilds the vacuum oil treatment systems in a stationary version, in a moveable version (on castor wheels) or in a fully mobile version on one or two axle trailers licensed for the public traffic.
The throughput capacity of the plants reaches from 200 l/h to 12,000 l/h. The regular systems are made from mild steel, special systems e.g. for silicon oil-treatment, are fully made of stainless steel. The plants are designed for treatment of oils on a mineral or silicon or ester base. |
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| The plants can be operated in the manual mode, semi-automatic or automatic mode and are equipped with the latest measuring and control instruments. MARSHAL is one of the major suppliers of the leading transformer companies and electric power generating or distributing |
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| Process Description |
The vacuum drying and degassing process is suitable for the treatment of insulation oil with a water contents of 50 ppm at 20°C and a gas contents of 10 Vol.% or respectively with a water contents of 100 ppm at 50°C. Oils with a water contents 100 ppm have to be predried with e.g. a centrifugal separator. The drying and degassing takes place in an evacuated container while the preheated oil is atomised and flows in a thin film over a huge Raschig-ring evaporator column.
The application of the vacuum process results in an increase of the water partial pressure and related reduction of the boiling point. E.g. the boiling point of water at an environment pressure of 5 mbar is at 5°C. Vacuum drying and degassing takes place in a temperature range where the thermal load of the oil is not critical.
The oil is heated up to approx. 50°C in order to improve the viscosity properties for smooth passing through the system, for thin film generating and even for preventing freezing of the internal water droplets while evaporating by the endothermic vaporisation energy. The degassing rate of the humidity or the permanent gas in the oil depends on oil temperatures, viscosity, absolute vacuum pressure, dwell time and oil film thickness.
At an absolute chamber pressure of 1 till 10 mbar and an oil temperature of 50 - 70 °C the water content is reduced from 50 ppm to < 7 ppm and the gas content is reduced from 10 Vol.% to 0,7 Vol.% in one passing. In order to increase the throughput or the drying properties several options as multi passing, serial connecting of degassing chambers as well as double stage pumping systems are useable.
For filtering the oil the system is equipped with mechanical filters on the inlet and outlet line. In the inlet line a metal sieve is installed, for fine filtering ceramic, cotton wool or plastic filters in candle or plate design are used, with 1 µm, 3 µm or 5 µm mesh size. The peroxides or micro molecular components, which are generated by the ageing process of the oil are exposed as sludge and are replaced by an absorption treatment with fuller's earth. |
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| Operation Mode |
| The plant serves for dehydrating, degassing and filtering of electric insulation oil with the following process sequences: |
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Oil filtering with preheating |
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Oil filtering without preheating |
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Oil purification with filtering and preheating |
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Oil purification without filtering, optional |
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Oil filtering without degassing, optional |
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External evacuation |
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| The plant is designed for operation in a semi-automatic or automatic mode with interlocks. |
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| Plant Layout |
| The plant can either be installed on a trailer, on a base plate with supports and steering rollers or base plate, or stationary mounted. |
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