Which Oil Purification is Best for 110kV Transformers: Single-Stage vs Double-Stage?

The use of a Double-Stage vacuum Oil Purification for a 110kV transformer is no longer considered a luxury, but rather a necessary requirement. Such a unit possesses efficiency, dehydrating/degassing properties, and the capability to operate under a vacuum in the transformer’s casing. The latter cannot be obtained with the use of a single-stage oil purification unit. When viewed from the cost-benefit point of view, a Double-Stage vacuum Oil Purification is a better investment than a single-stage unit.

Firstly, Single-Stage vs Double-Stage: what is the difference?

Feature / Metric	Single-Stage Vacuum Oil Purification	Double-Stage Vacuum Oil Purificationr
Number of Vacuum Pumps	1 unit (Backing Pump)	2 or more units (Backing Pump + Roots Booster Pump)
Ultimate Vacuum Degree	Generally ≤ 133 Pa	Can reach ≤ 5 Pa or lower
Dehydration Capability	Removes trace water; suitable for routine maintenance	Deep dehydration; rapidly removes micro-moisture
Degassing Capability	Meets requirements for standard transformers	Exceptionally strong; rapidly reduces dissolved gas content
Oil Treatment Limits	Breakdown voltage ≥ 50 kV  Trace water ≤ 20 ppm	Breakdown voltage ≥ 70 kV  Trace water ≤ 5 ppm
Functional Applications	Routine oil maintenance, small-to-medium transformers	Overhaul of HV/EHV transformers, on-site vacuum pumping & oil filling

Similarities

The two systems take advantage of the physical law whereby the boiling points of water and gases drop significantly in the vacuum state. Through the process of heating the insulating oil, both systems get rid of moisture, gases, and mechanical impurities from the transformer oil. Thus, the systems restore or improve the breakdown voltage or the dielectric strength of the oil.

• Heating System: Reduces oil viscosity and moisture evaporation.
• Filtration System: Consists of coarse and high precision filters for the removal of metal dust, carbon black, and other impurities.
• Oil Transfer System: Helps transfer the cleaned oil to the transformer.
• Control System: Equipped with pressure protection, infrared liquid level control, and temperature control.

Differences and Commonalities

A “Double-Stage” method isn’t merely a combination of two devices; rather, it signifies a major advancement in terms of vacuum depth and separation effectiveness.

• Single-Stage Vacuum Oil Purification Method: Uses one vacuum pump (in most cases, the rotary vane pump). Such pumps serve to create negative pressure in the vacuum chamber and evacuate the vaporized water along with gases to the atmosphere. The working vacuum is -0.08 MPa to -0.095 MPa (the ultimate vacuum level – 133 Pa). The oil goes through only one vacuum evaporation separation chamber, which limits the size of the evaporating zone and the time spent in it. In this case, after purification, the breakdown voltage of the oil may reach approximately 50 kV.
• Double-Stage Vacuum Oil Purification Method: Employs two-staged evacuation technology with the utilization of a backing vacuum pump + Roots booster pump. Although the Roots pump itself isn’t capable of discharging gases into the atmosphere independently, it has excellent air suction ability during low-pressure conditions. Roots performs an initial “intensive evacuation,” while the backing vacuum pump follows up with “exhausting.” The vacuum pressure level achieved is over -0.099 MPa with an ultimate vacuum being ≤ 5 Pa, or even ≤ 1 Pa. The interior is usually designed as a Double-Stage three-dimensional flash evaporation system (or features an independent secondary enhanced vacuum separation chamber). The oil is filtered through two zones with different vacuum levels, which can control the trace water in the oil to below 3–5 ppm, easily achieving a breakdown voltage of above 70 kV. Its evacuation rate is several to dozens of times faster than a single-stage unit. Beyond oil filtration, it can also function as an independent vacuum pumping station for the transformer body, used for vacuum drying and on-site vacuum oil filling.

Secondly, What are the Hard Requirements of 110kV Transformers for Oil Quality？

This voltage level comes under the HV category. As per the national and industry standards like GB/T 7595 or DL/T 596, the standards for 110kV transformer oils are very stringent, both for newly filled oil or for the existing oil in operation, which are:

• Dielectric Breakdown Voltage: Should generally be ≥ 40 kV for normal operational oil, while ≥ 50 kV is mandatory for fresh oil or after overhaul.
• Moisture Content (Trace Water): Strictly should not exceed 20 ppm (with even more stringent limits up to 15 ppm in high-quality operations).
• Gas Content: For transformers having voltages above 220kV (Extra-high Voltage or EHV), stricter standards apply (≤ 1%). However, in the case of 110kV transformer oils, even then, harmful gases such as acetylene and hydrogen need to be removed with utmost caution.

Finally, Which One is Suitable for 110kV Transformers?-Strongly Recommend Double-stage Vacuum Oil Purifier?

There are three primary reasons why a Double-Stage system—rather than a single-stage one—is far better suited for 110kV transformers:

1. High Vacuum is the Guarantee for Dielectric Strength

110kV transformers demand high insulation strength. Because a single-stage purifier has a limited vacuum degree, it struggles to thoroughly extract deep, dissolved trace water. Conversely, the high vacuum (≤ 5 Pa) of a Double-Stage purifier breaks the binding bonds between water and oil molecules. This achieves deep dehydration, easily boosting the oil’s breakdown voltage to 60–70 kV.

2. Adaptation for “On-Site Vacuuming” and “Vacuum Oil Filling.”

During the installation or major overhaul of a 110kV transformer, the transformer body itself must undergo vacuum evacuation and vacuum oil filling.

A single-stage purifier’s evacuation rate is too slow to handle the massive task of vacuuming a transformer body.

A Double-Stage purifier, equipped with a Roots pump, delivers an immense evacuation rate. It can pull a vacuum on the transformer body while simultaneously filtering and filling oil, multiplying operational efficiency.

3. Time Costs and Grid Operational Safety

Using a single-stage purifier on 110kV transformer oil often requires numerous filtration cycles just to barely meet minimum standards, which takes an immense amount of time. During emergency repairs or scheduled outages, time is money. The high-efficiency dehydration and degassing of a Double-Stage purifier ensure that the oil reaches premium quality in just one or a very few cycles, maximizing the limited maintenance window.

When can a single-stage purifier be used as a compromise?

While a Double-Stage unit is highly preferred, a single-stage vacuum Oil Purification can serve as a backup or transitional choice under the following specific conditions:

Routine daily maintenance in non-core areas: Where the transformer’s oil metrics are already relatively good, and the operation is just for periodic inspection and filtration.

Extreme budget constraints with ample cycle time: Where you can accept letting the Oil Purification run continuously for days and nights to slowly nudge the oil quality upward.

Small-capacity 110kV transformers: Where the oil volume is small, and there is no rigid requirement for a high evacuation rate.