Importance of Dehydrating Breathers Applied to Power Transformers and On-Load Tap Changers

Posted: October 23, 2018

Proper maintenance of a power transformer’s insulation system plays a key role in helping ensure the transformer operates at an optimum level throughout its life, and transformer oil is a critical component of that insulation system. To the greatest extent possible, transformer oil must be protected from impurities that lower its ability to function as designed. Three key impurities contribute significantly to the aging rate of a transformer: heat, oxygen and moisture. This article provides a solution for dealing with excess moisture in transformer oil.

Every free-breathing transformer without a conservator/on-load tap changer (OLTC) has a headspace above the transformer oil level that is filled with a gaseous mixture. This configuration is depicted below in Figure 1. The headspace exists to allow for expansion and/or contraction of the oil volume due to load or environmental heating and cooling.

During the heating cycle, oil expands which causes the headspace to contract. This causes the transformer/OLTC to exhale the differential volume of gas to the atmosphere. During the cooling cycle, the transformer oil contracts, causing the headspace to expand, which causes the transformer/OLTC to inhale the differential volume from the atmosphere. This air transfer process is known as “breathing.”

dual column breather-free breathing

Figure 1: Free Breathing Configuration

Without treatment, air transferred during the breathing process will contain the ambient level of moisture found in the localized environment. This level can swing from 10% (think Mojave Desert on a sunny day) to 99% during a foggy or rainy day anywhere in the USA. Why is this important? Because moisture in transformer oil affects the dielectric breakdown strength of oil, the temperature at which water vapor bubbles are formed and the aging rate of the insulation materials (oil and paper). All of the aforementioned factors can lead to premature failure of a transformer.

By design, transformers with conservators are considered immune to moisture in air since the oil is protected from contact with ambient air by the rubber bladder inside the conservator. In practice, as long as the bladder remains intact, the transformer oil is protected from ambient air. However, service aging over time can lead to degradation of the bladder material, thus impairing bladder life. Additionally, there is no easy way to inspect the bladder without actually opening the conservator tank, which means a rupture can occur and go undetected. In this case, oil has the same exposure to ambient air and moisture as a free-breathing transformer without a conservator tank.

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Figure 2: Conservator Configuration

For each of the applications discussed above, the addition of a silica gel breather to the air transfer path will effectively protect the transformer/OLTC oil from ingress of moisture through air. As the transformer breathes, the silica gel serves to remove moisture from the air, thus delivering dry air to the transformer. Several types of silica gel breathers are available, each of which has specific pros and cons. Careful consideration of the application requirements is necessary in order to choose the right silica gel breather.

A static silica gel breather is the least complicated solution and has a relatively low upfront cost. However, the silica gel has a finite capacity to absorb moisture. As a result, this type of breather requires frequent visual monitoring, as once the capacity of the unit has been reached, the silica gel must be replaced with new or recycled dry silica gel to avoid moist air from entering the equipment. Additionally, timing for replacement of the silica gel is difficult to predict due to multiple variables involved with weather and equipment loading.

An auto regenerating single column dehydrating breather is a more advanced solution that has more upfront expense but eliminates the need for frequent monitoring and replacement of the silica gel. These breathers are self-monitoring and automatically “regenerate” the silica gel through a heating process that removes moisture from the gel during the regeneration period. In order to avoid exposing the oil to the moisture removed from the silica gel through regeneration, a resting period (non-breathing period) in which the breather is isolated from the protected oil tank is generally required. Normally these breathers also have the capability of remote reporting and will function for many years with only minor annual inspections.

Finally, one of the most comprehensive solutions is the Waukesha® Dual Column Breather (DCB). The DCB operates under the same principle as the auto regenerating single column breather, but the DCB incorporates a second column of silica gel into the design. Figure 3a below is a picture of the DCB. The addition of the second column offers additional flexibility to the operation of the breather. Figure 3b below offers a visual representation of the operational concept of the DCB. The addition of the second column ensures one column of dry silica is always available to dehydrate the air as the equipment breathes. The two column design is effective in key applications and across a wide range of oil volumes. The DCB is designed for conduit wiring and is suitable for wall or pipe mounting. An optional DIN 42562-5 mounting flange is available.

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Figure 3a: Waukesha® Dual Column Breather (DCB)


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Figure 3b: DCB Operational Concept

The DCB built-in controls allow it to actively manage the operational environment. The frequency with which the DCB regenerates is configurable to occur using discrete time-based intervals. Depending on application, the DCB can be set to regenerate every 2, 5, 10 or 20 days. The DCB also incorporates a function by which a regeneration cycle is forced if relative humidity in the air becomes excessive. This unique operational concept, coupled with the advanced configuration options, uniquely position the DCB to consistently dehydrate the air path without interruption. Table 1 below provides details on the built-in controls of the DCB:

dual column breather-built in controls

Table 1: DCB Built-in Controls

In addition to the advanced operational concept and built-in controls, the rugged design of the DCB incorporates the following key features:

  • All aluminum construction
  • Borosilicate glass globes
  • Super Bright LED status lights
  • Direct conduit wiring (no adapter box)
  • Sealed vertical latching solenoid
  • Silica gel ships installed
  • Optional heated drains available (for -50°C operation)

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Figure 4: DCB Rugged Design Features

Figure 4 above illustrates the rugged design features of the DCB. Each of these design features contribute to the DCB’s ability to reliably operate in the most challenging applications. By design, the DCB is the right choice to consistently maintain a dehydrated air supply to your equipment without interruption or regular maintenance.

dual-column-breatherWe welcome calls from customers seeking technical support on the DCB. If you need assistance, we will gladly work with you to support your needs. To learn more about the DCB and all components available from SPX Transformer Solutions Components Group, visit our website or contact a member of our sales team at 1-800-338-5526. Also, don’t forget about our library of easy-to-navigate, 3D catalogs designed to help you quickly identify and locate hard-to-find components for LTCs, oil circuit breakers and several of the SPX Transformer Solutions’ line of Transformer Health Products®.