This paper is intended to explain the Regeneration procedure recommended to be carried out prior to the addition of proposed Passivator agent.
The process of Reclaiming Transformer Oil involves the removal of Partials, Moisture, Gases, Acidity and other Oxidation Decay Products, polishing the oil and restores oil to like-new quality.
This process of Regeneration is simply the passing of Badly Deteriorated Transformer Oil through an Absorbent Bed. Several types of absorbents are available but the most common are Fuller’s Earth, Activated Alumina, Trisodium Phosphate and Activated Carbon.
The FILTERALL REGENERATION process utilises a blend of such Absorbents which is formulated and structured in order to provide the most efficient Absorbent Bed, whilst retaining an ability to be Reactivated once they become no longer active, due to the volume of Decaying Products which have been retained during the oil Reclaiming process.
The Temperature achieved during the REACTIVATION of the Clay Beds determines the degree of Internal Porosity and the High Surface Area of the Absorbent Beds determines its ability to absorb polar compounds & lighten the colour of service-aged oil. The unique crystalline structure or the bed material is thus very important and during extraction, the Absorbents are crushed, mixed and heat activated before being loaded into the Regeneration Columns.
Activated Regeneration Beds can absorb large quantities of acid. The amount of acid removed depends on many factors since absorption is a dynamic equilibrium process. Temperature, Flow Rates, Oil Viscosity, Residence Time and Initial Acid Level all affect the rate and capacity for Absorption.
Working on Line. While Transformer Oil can be Regenerated in just one pass of the oil through the rig, it takes more passes to sufficiently clean the Transformer Internals. In most cases, this work can be done safely and more efficiently while the Transformer being processed is Energised. The cleaning is more efficient because the 100/120 cycles coursed through an energised transformer, causes the laminations to vibrate, thus helping to loosen sludge from the transformer internals. The heat produced in an energised transformer will also aid in getting the oil temperature to the aniline point in a more, timely manner.
Specific factors must however be satisfied, prior to On Line Working being considered.
The most requirement prior to energised work, is to establish the present “health” of the transformer, and to this end, a DGA should be carried out on the oil. If this indicates the presence of combustible gasses, then work on line must not be done until the oil has been degassed & the cause of the gassing has been established.
Depending on the gasses identified, other problems with the transformer may be identified. Significant quantities of Acetylene indicate that there is arcing taking place in the transformer and this would definitely preclude on line work for example.
Similarly, if moisture analysis indicates greater than 6% moisture by dry weight, (a rule of thumb is higher than 50 ppm), on line working should not be done until first carrying out purification to reduce the moisture.
High levels of furan also suggest an inherent problem with the transformer and should be taken as a reason to prevent On Line Working.
Obverse, apart from such situations, there has to be a high level of awareness with regards to operator safety.
With regards to processing capacitates, the attached graph, which plots Initial Neutralization value against Clay Required, offers a useful reference in order to determine the Regeneration Ability of the Filterall Beds for Initial Acidity Levels when processing Transformer Oil. This assumes a Tank to Tank processing arrangement and that the Processed Oil is required to be “as new” after a single pass.
When processing (circulating) on a Transformer or Tank, although multi passes are required, the volume of oil which can be treated between Bed Reactivation will increase since the “feed” acidity level will be reduced with each pass.
**e.g. Oil with Initial Acidity of say 1.0 mg KOH/gram requires some 2lb of Clay to treat 1 gallon down to 0.05 mg KOH/gram in a single pass. When returned to the transformer, this “clean” oil will be mixed with “dirty” oil existing in the transformer causing the oil feed acidity level to reduce / be diluted.
In addition, the warm cleaner oil being returned via the top valve of the transformer, acts to flush the transformer internals and draw contaminates from out of the windings.
This process continues until oil content of the transformer satisfies the required parameters.
To satisfy the requirements prior to the addition of the passivator, it is suggested that up to some 8 passes may be required
In order that this detail can be related to existing National Grid Operated Regeneration Plant, we advise the Regeneration Bed Absorbent Capacities of each below:
MRP2 offers 2 Regeneration Columns Sets and each set of 9 Columns (1 set) holds some 1,000 kg (2,200 lbs) of Absorbent.
MDD3 and MDD4 both have 1 set of Regeneration Columns containing some 1,200 kg (2,700lbs) of Absorbent.
** So using basis as above ** with MDD3 or MDD4 you can treat 2,700/2 = 1,350 gallons (5,000 litres) between Bed reactivations in order to achieve required product in a single pass.
In practice however, when circulating on a Transformer, you can assume some 4 to 8 passes would be required & some 5,000 L processed per cycle initially, building up to 20,000 L for the final polishing pass. Required Contact Time between the Oil & the Clay Bed material will depend on total contamination of the initial oil (colour a good indication), so the flow rate at which MDD3 or MDD4 should be run during Regeneration will vary; 2,000 L/H is suggested as a start point & then it can be adjusted according to performance. In practice, it is best to adjust flow so that you can process the required litres & reactivate the columns on a 24-hour cycle. (Providing some 6 hrs processing availability per day when allowing <18 hrs for the Clay Reactivation process).
In order to increase efficiency and to satisfy proposed Passivator requirements, it is suggested that a dual clay treatment facility should be considered. This would allow processing to be carried out utilising 1 set of columns, whilst the 2nd set of columns are Reactivated.
Since the passivating procedure would not require a Vacuum to be pulled on the transformer and the processing rate would be reduced, the size of such allied equipment (oil heater & Vacuum pumps etc) could be reduced. This would allow a dual column arrangement to be contained within a trailer configuration very similar to MDD3 & MDD4 and accommodate Working on Line.
In addition, operator accommodation could be built on a separate chassis, so to allow for remote (off plant) operation. This operator accommodation could be configured so to allow it to be towed by a Land Rover type vehicle, which usually accompanies similar processing equipment, in order to supply operator transport whilst on site.
Factors to Consider when Regenerating in Service Transformer Oil in order to achieve best performance:
- Advantageous to Dehydrate the oil before it contacts the Absorbent in order to prevent water from wetting the Bed Material. It is considered an advantage if Initial Processing is carried out without going through the Clay Beds in order to ensure the oil is as dry as possible prior to carrying out Regeneration via the Clay Beds.
Also, Purification (filtration, dehydration & degassing) & Heating should be continued even when Regeneration via the clay columns is not possible.
- Ensure that the Oil coming out of the columns is dehydrated and degassed via Vacuum Chamber before being returned to the transformer or clean oil tank.
The Clay beds can hold up to 10% moisture, which can be passed on to the treated oil.
This is particularly applicable when re-circulating oil in a transformer.
- Regeneration of oil can remove any added Oxidation Inhibitor remaining in the oil to be processed, as well as contaminates existing in oil prior to treatment. Oxidised oil will however, have had its inhibitor consumed during the Deteriorating Process.
New Inhibitor or Passivator can be added once oil regeneration is completed, but should be processed via the filters and degasser only; the clay treatment facility must be bypassed.