why VPD is so important for power transformer production and who are the major suppliers in China
Vapor Phase Drying (VPD) is one of the most critical processes in power transformer manufacturing because it ensures the complete removal of moisture from the insulation system (mainly cellulose paper and pressboard). Moisture is one of the biggest enemies of transformer reliability.
Below are the key reasons why VPD is so important:
- Moisture Severely Reduces Insulation Strength
Transformer insulation systems are mainly composed of cellulose materials (paper, pressboard) combined with transformer oil.
- Even 1–2% moisture in cellulose can dramatically reduce dielectric strength.
- Water can cause partial discharge, insulation failure, and internal faults.
VPD reduces the moisture content in insulation typically to below 0.5%, which is required for high-voltage and extra-high-voltage transformers.
- Extends Transformer Lifetime
Moisture accelerates cellulose aging through hydrolysis.
Effects of moisture:
- Faster polymer chain degradation
- Reduced degree of polymerization (DP)
- Shorter transformer lifetime
Proper drying through VPD can extend transformer life from 20 years to 40+ years in many cases.
- Much Faster Than Conventional Vacuum Drying
Traditional hot air or vacuum drying is slow because heat transfer inside the winding is inefficient.
VPD uses vaporized solvent (usually kerosene) which:
- Condenses on the cold insulation surface
- Releases large latent heat
- Penetrates deep into the winding
Result:
- Uniform heating
- Much faster drying
Typical time comparison:
Drying Method Typical Time
Hot air drying 10--20 days
Vacuum drying 5--10 days
VPD 2--5 days
- Deep Drying of Thick Insulation Structures
Large power transformers have very thick insulation structures:
- Thick pressboard cylinders
- Multi-layer windings
- Large core-coil assemblies
VPD provides deep penetration heating, allowing moisture to migrate out from the inner insulation layers, which is difficult with other drying methods.
- Essential for High Voltage Transformers
For ≥220 kV transformers, and especially 400 kV / 500 kV / 765 kV, VPD is considered industry standard equipment.
Most major manufacturers use it, such as:
- Siemens Energy
- Hitachi Energy
- GE Vernova
- TBEA
Without proper drying, the transformer cannot pass impulse and AC withstand tests.
- Improves Production Efficiency
VPD systems also improve manufacturing efficiency:
- Shorter production cycle
- More consistent insulation quality
- Lower risk of factory test failure
For factories producing large power transformers, VPD often becomes the production bottleneck, so its reliability and capacity are critical.
In simple terms:
If winding is the “heart” of a transformer, insulation dryness is its “life”.
VPD is the most effective way to guarantee that life.
Since you mentioned earlier that you are working on transformer equipment tenders for Siemens projects, VPD equipment is often one of the highest-value and most technically evaluated machines in transformer factory investment.
If you’d like, I can also explain:
- Why many transformer factories prefer VPD over Hot Oil Spray (HOS)
- The key technical parameters customers evaluate when bidding VPD systems
- Which companies dominate the VPD equipment market globally
These points are often very important in transformer equipment tenders.
In the power transformer industry, the Vapor Phase Drying (VPD) equipment market is relatively small and specialized, with only a limited number of experienced suppliers globally. Transformer manufacturers usually choose suppliers with long references and proven drying technology, because VPD is critical for insulation quality.
Below is a practical industry view of the major suppliers.
- Major Global Suppliers of VPD Equipment
These companies are widely recognized suppliers to large transformer factories worldwide.
European Technology Leaders
- Micavac (Germany)
- One of the earliest developers of VPD technology
- Known for patented JET evaporator technology
- Many installations in Europe and Asia
- Often considered top-tier technology
- Piller Blowers & Compressors (Germany)
- Supplies vacuum systems and drying solutions for large transformer plants
- Often integrated into large transformer manufacturing lines.
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- Major Chinese VPD Equipment Suppliers
China now has several strong suppliers, mainly serving Chinese transformer factories and export projects.
Leading Chinese Suppliers
A Shanghai Trihope
- Turnkey supplier for transformer factories
- Provides VPD systems, core cutting lines, foil winding machines, etc.
- Equipment installed in many factories globally.
B Zhongshan Kaixuan Vacuum Technology
- Supplies VPD plants used by large transformer companies and utilities.
These Chinese suppliers are very competitive on price and increasingly used in:
- Southeast Asia
- Middle East
- Latin America
- Africa
For example, Shanghai Trihope has supplied one set of VPD to PEL Pakistan, the leading power transformer manufacturer in South Asia. The equipment has been running well and customer issued appreciation letter.