Oil Regeneration

INSULATING OIL

Transformers play an essential role in power transmission and distribution. Electricity generated by power plants cannot be supplied to residential and industrial consumers directly. Voltage is increased for long-range transmission and must be reduced before this power can be used for running industrial machinery or powering air conditioners and computers in households. Such transmission of electricity would be impossible without transformers. This means that transformer reliability becomes crucial for uninterrupted supply of electricity to factories, offices and living-rooms around the world.

The all-important factor in transformer reliability is the condition of its internal insulation system. Transformer insulation consists of both solid (cellulose) and liquid (oil) insulation. Transformer oil serves as a coolant and dissipates heat generated by transformer windings and protects cellulose insulation. The stresses of transformer operation influence the insulation system, and, if left unattended, this insulation fails, causing catastrophic and very expensive failures.

The dielectric oil is influenced by elevated temperature and various chemicals, most of all by oxygen, which it may come in contact with, whether from atmospheric air or from water impurities in the oil. Oxygen naturally causes oxidation of oil, which results in formation of all kinds of undesirable substances, such as acids or oxides, as well as volatile hydrocarbons. This process is referred to as aging of oil. Instead of protecting the paper and the metal parts of the transformer, transformer oil with high acid content attacks them. Cellulose decomposes, forming water and oxygen which in turn accelerate oxidation of oil, causing a runaway process of transformer deterioration.

These effects are well known: there are remedies that help slow or even entirely stop such processes. Transformers require regular inspections and preventive maintenance. An essential part of such inspections is sampling and analyzing the oil to ensure that it can still perform its functions well. Transformer maintenance also includes processing the oil to remove water, solid particles and gases, which contribute to aging of the oil and damage the transformer.

While removing water, gas and particulars from the oil, the heat and vacuum purification process alone cannot remove the acidic compounds and sludge present in aged oil. It would seem that the only solution is to change the entire load of oil in the transformer.

Changing the oil may seem like a good option, but it is not without its limitations. When oil is saturated with solved impurities, they cannot solve in the oil anymore and settle in the oil-impregnated paper and in the windings of the transformer. If the old oil is removed, and the transformer tank is filled with new oil, these impurities will solve in this fresh new oil, contaminating it from day one. Besides, changing oil in a transformer involves all kinds of costs, from the direct cost of purchasing new oil to transportation and disposal of old oil. Note also that the transformer must be taken off line for the duration of this operation, further increasing the costs with the downtime. There is also the environmental effect to consider: used transformer oil contains environmentally hazardous substances and requires special disposal. Besides, fossil oil resources are not limitless, and it makes sense to conserve them when possible.

In order to reliably remove acids and other undesirable compounds from the transformer, oil regeneration is required. Transformer oil regeneration involves passing the oil through a layer of sorbent, which traps the contaminants in its pores. After percolating through the sorbent, the aging products are removed from the oil; its condition is improved to as good as new.

The only problem with regeneration of transformer oil by adsorbent is that the adsorbent quickly becomes saturated with the contaminants, ceasing to function. The sorbent in the regeneration unit must be unloaded, disposed of and replaced with new sorbent, which places obvious limitations on oil regeneration capacity and imposes additional costs associated with such replacement.

GlobeCore engineers set out to develop a system of oil regeneration which would comprehensively purify the oil and solve the problem of sorbent saturation at the same time. The research resulted in creation of the CMM-R system: a complete solution for regeneration of transformer oil which also does not require frequent and costly changes of adsorbent material.

The CMM-R oil regeneration unit operates as follows. The unit consists of a number of columns with adsorbent. It also includes a degassing unit to remove gases, water and particulate matter from the oil. Contaminated oil passes through the adsorbent and comes out visibly clearer and lighter; from there, it goes into the degasser and then back to the transformer. In the end, all performance parameters of the oil are vastly improved, from acid number and dissipation loss factor to dielectric strength and gas content.

The process takes time and requires many passes of the oil through the machine. However, the CMM-R comes equipped with a Transformer Safety System, a device which allows operation on an energized transformer. So while the transformer must be switched off to connect and disconnect the unit, for safety reasons, it can be energized for the duration of the treatment. The Transformer Safety System keeps track of the level of oil in the transformer during processing and stops the machine should the level of oil drop below the safety operating limits of the transformer.

This capability to regenerate oil in powered transformers is also important in that it addresses the problem of contaminants lodged in the paper insulation. In fact, if safety allows, it is recommended to operate the CMM-R with the transformer under power. When some of the contaminants are removed from the oil by the adsorbent and it returns into the transformer, it is no longer saturated and can solve some of the contaminants deposited in the windings. The vibrations and heat of the operating transformer promote and accelerate this process. The oil literally washes the contaminants out of the windings, resulting in a much better result that simply changing the oil only to have it contaminated by the impurities left over in the oil-impregnated paper insulation.

However, the capacity of the adsorbent to accumulate contaminants is not unlimited, and after a number of passes of the oil through the layer, it becomes saturated. The rate at which the adsorbent becomes saturated depends primarily on the degree of oil contamination. Initially, the oil coming out of the columns is light and clear, but becomes darker as the sorbent can no longer trap the impurities from the oil. At a certain point, the adsorbent becomes completely saturated, and the oil regeneration process must be stopped.

This is where the most important feature of the CMM-R oil regeneration unit comes into play. When the adsorbent is saturated and cannot regenerate oil any more, the unit automatically reverts to adsorbent reactivation mode. The adsorbent does not need to be extracted from the columns. Remaining oil is pumped out of the adsorbent columns, and the reactivation cycle starts. While the adsorbent columns are reactivated, oil processing can continue either by running the oil continuously through the degassing section of the machine or by switching to the second set of adsorption columns, if the machine is equipped with them. For instance, the CMM-12R is equipped with 12 columns, which means that while six of the columns are in reactivation mode, the other six can continue running and regenerating the oil without the need to stop the machine. The oil pumped out from the columns immediately before reactivation starts, is kept in a vessel separately from cleaner regenerated oil and is returned for processing once the reactivation cycle is complete.

Of course, after many cycles of oil regeneration and adsorbent reactivation, the material becomes exhausted entirely and must be replaced. The adsorbent used in the CMM-R oil regeneration unit can last for as many as 300 cycles of regeneration and reactivation before it must finally be replaced. This is the equivalent of roughly two years of operating the machine at full capacity. In the end of its service life, the adsorbent undergoes one last reactivation cycle which removes all oil and harmful substances and becomes completely environmentally safe. The exhausted sorbent requires no special disposal procedure.

The CMM-R oil regeneration unit is a relatively large plant, and is usually installed on a road-worthy trailer. The trailer is equipped with an air-conditioned operator compartment, from which the operator can oversee and manage plant operation. The unit is fully automated through a SCADA system and a PLC, and can be controlled from the screen of a PC or a laptop computer. Smaller oil regeneration plants can optionally be equipped with a touch screen control panel on the unit’s electrical control cabinet.

The insulation system of the transformer greatly benefits from oil regeneration. Unlike oil, solid insulation of the transformer cannot easily be repaired or replaced. The costs of replacing the oil-impregnated paper insulation are significant to say the least. However, oil regeneration removes the aggressive substances that cause deterioration of solid insulation, and greatly slows or even stops this process. Regular and timely transformer oil regeneration is a method that can extend the lifetime of a transformer by as much as twenty or thirty years by keeping its solid insulation in good condition.

The CMM-R oil regeneration unit greatly improves the reliability of the key element of power transmission and distribution, the transformer. Beside ensuring uninterrupted power supply to the customers, regular oil regeneration eliminates or minimizes many costs associated with transformer operation, such as the costs of oil change, repairs and, ultimately, the cost of premature replacement of the entire transformer.

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