Scenario

Transmission utility in the Middle East with a fleet of strategic transformers (380 kV, up to 500 MVA), operating under high load and extreme climatic conditions.
A diagnostic campaign highlighted the widespread presence of DBDS and corrosive sulfur compounds in the insulating oils, with potential risk of metallic sulfide formation and consequent failures.

Technical characteristics

• 22 units (transformers, autotransformers, reactors)
• Power rating: 60–500 MVA
• Voltage: 380 kV
• Total oil: approx. 1,710,000 kg; up to 130,000 kg per unit

Problem

Analyses revealed initial concentrations of DBDS between 135 and 171 mg/kg and TCS up to 189 mg/kg.
According to CIGRE guidelines, such values combined with high temperatures and heavy load result in a high risk of corrosion and dielectric degradation.
The utility required an effective long-term solution compatible with service continuity.

Solution

In cooperation with GCC Lab, the utility selected the Sea Marconi selective depolarization, considered the most effective technique for removing DBDS and sulfur compounds, avoiding the limitations of traditional clay-based treatments.

The intervention was performed on-load, using Decontamination Modular Units (DMU) operating 24/7, featuring:

• closed-loop circulation of the entire oil volume;
• chemical–physical treatment using solid reagent and absorbent materials;
• integration of filtration, degassing, and dehydration;
• dedicated connections to the various compartments (main tank, OLTC, conservator).

Outage windows were limited to a few hours for connection/disconnection.

Results

In the first 12 months, 8 transformers were treated, totalling 809 tonnes of oil.

Results after treatment:

• DBDS < 5 mg/kg (below detection limit and well below the required < 10 mg/kg)
• TCS < 2.5 mg/kg
• Chemical–physical properties (water, TAN, DF, IFT, breakdown voltage) fully compliant with IEC 60422 ed. 4; many values comparable to new oils
• No significant alarms during treatment
• Effectiveness confirmed after 1 year: oils non-corrosive on copper and silver, DBDS stable < 5 mg/kg, parameters within IEC limits

The treatment allowed the utility to keep critical units in service, reducing operational risks and improving long-term oil quality.

3 single-phase transformers installed in a thermoelectric power plant in southern Spain

Technical characteristics of the transformer:

Manufacturer .: xxx
Year: 1977
Power [MVA]: 216
Voltage [KV]: 220/22
Mass of oil [Kg]: 15,650 each.

Periodic analyzes of the oil showed a condition of hydration of the cellulosic insulation. The estimated water content in solid insulation is between 3 and 4% by weight, a typical condition for equipment with more than 30 years of service.

Treatment details

The process was carried out with the transformer on-load.
The temperature of the oil in the tank (top oil) was kept constantly above 70 ° C through the use of an auxiliary heating equipment.
Simultaneously the oil was recirculated through a vacuum dehydration equipment. Overall, about 15 liters of water were extracted from each device.

Duration of the intervention: 6 weeks (2 for each transformer)

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Technical specifications of transformer:

Manufacturer: xxx
Year: 2013 (new)
Power [MVA]: 185
Voltage [kV]: 380/150
Oil mass [kg]: 50.940

Transformer empty following transport to destination site.

Twin transformers installed in a quarry:

Manufacturer: xxx
Year: 1980
Power [MVA]: 250
Voltage [kV]: 400
Oil mass [kg]: 41,000

Following a problem associated with excessive gas formation diagnosed by another supplier, the latter asked Sea Marconi to drain the oil, keep the transformers under vacuum for a specified period, then refill them with oil and carry out oil treatment.

The procedure presented severe logistical constraints as the transformers were located in a quarry. Sea Marconi was able to exploit the modest size and modularity of its decontamination units for the oil treatment.

Sea Marconi implemented the following measures:Draining the oil from the transformer, refilling the transformer under vacuum and oil treatment

 Duration of treatment: 3 days (T1) and 4 days (T2)

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Technical specifications of transformer:

Manufacturer: xxx
Year: 2012 (new)
Power [MVA]: 80
Voltage [kV]: 227/21/21
Oil mass [kg]: 26.700

Following an initial request to fill the new transformer, when the Sea Marconi technicians arrived they noticed there was free water inside the transformer.

Sea Marconi carried out a rather complex operation in three separate stages:

1. Dehydration of the papers by the direct “hot oil + vacuum” drying technique
   This phase involves applying a vacuum to the transformer tank, filling it with hot oil to cover the windings, draining the transformer rapidly and repeating the process until the established values are achieved (water < 10mg/kg; dielectric strength > 70kV; amount of water extracted < 0.25 litre/24 hours).
2. Refilling and treatment
   This phase involves applying a vacuum to the transformer tank, filling it with oil and subsequent physical treatment of the vents

The process took just over two weeks, during which 2.85 litres of water was extracted from the oil/transformer system.

Hydroelectricity company. The transformer is located in the Adamello peaks (Lombardy).

There were extreme logistical difficulties associated with reaching the operating site: the access route was a steep slope at high altitude and there was more than a metre of snow. Helicopter was the only possible means of transporting the oil treatment machinery, so there were stringent weight constraints.

Sea Marconi overcame the stringent logistical constraints to access the operating site by using a new design of mobile processing unit (DMU).These new units are designed and manufactured to be lightweight (estimated empty weight is 590kg), and smaller in size (L 1,560 x W 1,000 x H 1,440 (mm), so they’re easy to handle. It also means they can be transported by road with no need to unload them from the trailer, so they can be taken to areas with very tight access points (off-shore platforms, quarries, bunkers, underground power stations, etc.)

Because the Sea Marconi decontamination unit is lighter in weight and smaller in size, it was suspended from the helicopter with a chain hooked through its anchoring rings for transportation to the power station.

Specifically, there are two devices that perform the following functions: oil is sucked out and pumped back in (draining and refilling), oil is heated to a preset temperature, solid suspended particles are filtered out, and the transformer is degassed and dried under vacuum.These functions can also be carried out whilst the oil is circulating in a continuous, closed circuit between the unit and the transformer.
The control and safety systems installed mean the system can even be used without intervention by the operator.The unit is designed to operate at the following settings:

• Nominal flow: 160 ÷ 600 l/h
• Maximum heating temperature: 80°C
• Oil temperature in the degassing chamber: 60 ÷ 70°C
• Heating capacity ?t = 40°C at 600l/h
• Max pressure: 6 bar
• Absolute pressure in the degassing chamber: < 1 mBAR at full performance
• Filtration: 2 microns

The unit is mounted on a stainless steel AISI 304 frame and is fitted with wheels, fork guides for lifting with a forklift truck, anchoring rings for lifting by crane, and corner blocks for anchoring to road trailers.
All elements of the hydraulic circuit, including connection points for the flexible suction and delivery pipes, are contained in a sealed tank to contain any of accidental loss of oil.
The hydraulic system, degassing chamber, filtration bodies and heater are made of AISI 304 stainless steel.

Primary alluminiun factory: 8 Rectifier Transformers (Sweden)

Nameplate data of transformers:

Manufact: xxx
Year: 1930
Power [MVA]: 19
Voltage [KV]: 13
Oil Mass [Kg]: 15.000
Oil type: mineral blended

Selective depolarization treatment (Chedcos) in order to remove the DBDS + dehalogenation treatment (CDP Process) in orderr to remove the PCBs both from oil and transformers. The treatments were performed on-site, on-line and on-load (transformer in servise and under charge during the treatment).

The treatments were carried out with the following guarantees: DBDS < 10 mg/kg, PCBs < 2 mg/Kg restoring also the other oil parameters as suggested by IEC 60422 ed. 4, 2013

For this case, Sea Marconi has done a detailed study using multidetection diagnostic tools from which emerged the following critical factors:

1. PCBs above the threshold due to cross contamination caused by previous treatment performed by unskilled operator in a sensitive environment (water for cultivation)
2. Corrosion C2 (NON DBDS & Corrosive sulfur) with DBDS equivalent = 70 mg/Kg and CCD test result as corrosive without DBDS
3. corrosion C4 (NON sulfur corrosion & metal dissolution), due to presence of copper in organ metallic form
4. dielectric properties of oil non standard

there are no electrical faults, there are thermal defects but only a normal aging of the insulation solid

Sea Marconi proposed and performed and integrated treatment based on selective depolarization (Chedcos) and dehalogenation of PCB in oil (CDP Process). The treatment was performed on-site and on-load (transformer in service and under load) with a total duration of 22 days, of which 16 for the treatment part.

The solution Sea Marconi won and convinced primarily for the modularity of the treatment unit (DMU), with reduced weight and size and therefore extremely easy to handle in an area with extreme logistical constraints.
Not only, the service Sea Marconi was able to heal, with just one treatment, the problems of corrosivity and the problems of PCB contamination, both the poor dielectric oil.

The brilliant results achieved are the result of a mix of excellence:

• Innovation and technical treatment plants and transformer oil,
• the use of specific reagents patented,
• the competence of technicians Sea Marconi,
• certification of the analytical test results

This case brings together a number of peculiarities that make it unique worldwide. It has required a major effort to research, development and innovation. This has generated a series of advanced methodologies focused to multidetection; moreover this case has allowed us to study new treatment methodologies and instrumental configurations for monitoring entirely new.

Since this case is expected to publish some results in trade magazines from IEEE and Analitcal chemistry.

Nameplate data of transformer:

Trasformatore: grid transformer
Manufact: xxx
Year: 1970
Power [MVA]: 70
Voltage [KV]: 225
Oil mass [Kg]: 38.000

Dealogenazione (CDP Process®) treatment in order to remove the PCBs, performed on-load (this means that the transformer is in service and under load during the treatment)