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Dry vs. Liquid-Filled Power Transformers

constructionWith the variety of high-voltage power transformers available today, clients must consider a range of factors before selecting equipment. Although transformers are still categorized depending on whether they use wet or dry dielectric materials, numerous options are available for fast track power high-voltage applications. These are a few variables that will help to determine if dry or liquid filled power transformers are your best choice to use at new or existing sites.

  • Maintenance
  • Initial costs
  • Operating costs
  • Operating noise
  • Recyclability
  • Required capacity
  • Voltage losses
  • Site selection
  • Equipment size
  • Safety requirements
  • Environmental risks

Liquid-filled Transformers

The wet-type power transformer have been around for nearly a century, but recent technical advances have improved the overall practicality of these products. This proven technology is cost-efficient, versatile and easy to maintain. With an average lifespan of 25 years and a potential service life of 35 years, they last longer than dry transformers while offering numerous other benefits.

For decades, flammable and environmentally damaging dielectric fluids were a major drawback. Oil-filled transformers required containment troughs and other safety precautions. Indoor installations were regarded as risky due to potential fire hazards and exposure to the dielectric fluids. However, many alternatives to mineral oil are now available. Natural esters, silicone and hydrocarbons are recommended alternatives to flammable mineral oil. Some fluids are renewable, nontoxic and biodegradable.

Advantages of Liquid-filled Transformers

  • New dielectric fluids have high flash points and minimal impact on the environment.
  • High-voltage clients can recycle or remanufacture liquid-filled transformers easily.
  • A wet-type power transformer is smaller and more efficient than dry products.
  • Liquid-filled transformers are suitable for indoor use because operating noise is lower.
  • These transformers last longer than dry-type products.
  • Numerous recycling options are available.

Disadvantages of Liquid-filled Transformers

  • Not all wet-type power transformers have the same safety benefits.
  • Fluid testing and on-site maintenance is required for reliability.
  • Additional protocols are required to manage fire and environmental risks.
  • Oil-filled transformers may have lower cooling ratings than some cast-coil products.
  • Insulating fluids must be appropriate for the application.

Dry-type Transformers

The dry-type power transformer has been traditionally used for low- and medium-voltage applications in buildings. Improved safety made dry-type transformers preferred for indoor power distribution. However, these systems have some drawbacks and limitations.

Air-cooled transformers are larger than liquid-filled products with the same capacity and voltage ratings, so they are not the best choice for a mobile substation or a site where space is limited. Air natural transformers have ratings up to 3 MVA. With higher capacities, dry transformers must use air-blast systems to cool the core and windings. Due to these limitations, many experts recommend 2.5 MVA as the changeover point for dry and wet products.

Advantages of Dry-type Transformers

  • Air-cooled transformers are practical for low- and medium-voltage sites.
  • Dry-systems eliminate environmental risks related to leaks and spills.
  • Cast coils outperform conventional transformers in extreme conditions.
  • Some cast-coil products offer extraordinary resistance to surges and short circuits.
  • Air-cooled systems are often preferred at indoor sites where safety is a priority.

Disadvantages of Dry-type Transformers

  • Dry-type transformers are more expensive to purchase and to operate.
  • Repairs are difficult, and end-of-life recycling is limited.
  • Coils can pose fire risks when they are not properly cleaned.
  • Annual maintenance usually requires deactivation and downtime.
  • Electrical losses associated with dry transformers are significantly higher.
  • Heat generated by losses must be controlled with a cooling system.
  • Dry transformers have high operating noises, so they are not suitable for all indoor applications.

The advantages and disadvantages of each option depend on the application as well as the client’s budget, safety requirements and existing infrastructure. For more information about selecting transformers for mobile or conventional substations, call 713-733-3999. Houston-based Equisales Associates is a leader in fast track power, equipment sales and field services.





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