Prolec GE

Maintaining Healthy Transformers and LTCs

Posted on March 14, 2024 by -

APPROPRIATE APPLICATION OF DEHYDRATING BREATHERS TO REDUCE MOISTURE IN TRANSFORMERS AND LTCS

For transformers and LTCs to operate at their optimum level, transformer oil, a key part of the insulation system, must remain free of impurities that will lower its ability to function properly and efficiently. Three key impurities contribute significantly to the aging rate of a transformer: heat, oxygen and moisture. At least one, the moisture level, has a solution.

Every non-conservator equipped free breathing transformer/LTC has a headspace above the transformer oil level that is filled with a gaseous mixture. This 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, the head space contracts, causing the transformer/LTC to exhale the differential volume of gas to the atmosphere. During the cooling cycle, the headspace expands, causing the transformer/LTC to inhale the differential volume from the atmosphere.

This air transfer without the use of a silica gel breather would contain the ambient level of moisture. This level could swing from 10% (Mojave Desert on a sunny day) to 99% during a foggy or rainy day anywhere in the USA. Why should we care? 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 which could lead to a transformer failure.

What about transformers with conservators, are they not immune to moisture since the oil is protected from contact with ambient air by the rubber bladder? Yes and No. The bladder, when intact, does protect oil from ambient air. Degradation/Service aging of the bladder material over time can impair the life of the bladder. In addition, since no easy way exists to inspect the bladder without actually opening the conservator tank, a rupture can occur and go unnoticed. In this case, the oil has the same exposure to ambient air and moisture as a free-breathing transformer without a conservator tank.

Therefore, what type of silica gel breather should be used? What are the pros and cons of each system?

A static, “dumb” silica gel breather has a relatively low upfront cost. However, it has a finite capacity to absorb moisture, and, therefore, its life is difficult to predict due to the multiple variables involved with the weather and equipment loading. This type of breather requires frequent visual monitoring since the unit lacks any type of self-monitoring. Once the capacity of the unit has been reached, silica gel must be replaced with new or recycled dry silica gel to avoid moist air from entering the equipment.

A properly designed auto regenerating dehydrating breather has more upfront expense but eliminates the need for frequent monitoring and replacement. These breathers are self-monitoring with the capability of remote reporting and will function for many years with only minor annual inspections.

In conclusion, protecting the oil in a transformer or LTC from moisture contamination is readily accomplished with minimal attention by using an automatic regenerating dehydrating silica gel breather, such as the Waukesha® Dual Column Breather.

AUTHOR:
Art Martin
Senior Product Engineer – Service & Components Division

PRODUCT INFORMATION ON THE NEW WAUKESHA® DUAL COLUMN BREATHER

The latest generation Waukesha® Auto Regenerating Dehydrating Breather System (Dual Column Breather is designed to remove moisture from the air entering transformer, LTC and conservator tanks as well as other sealed tanks. Regeneration is accomplished using Positive Temperature Coefficient (PTC) solid state heaters located in the central column and controlled by an adjustable timer and humidity sensor to provide automatic recharging of the silica gel desiccant, eliminating the need for manual intervention. The humidity sensor constantly monitors air from the in-service column and will force a column shift if the air stream humidity reaches the trigger point, regardless of the timer setting. The use of dual columns ensures that the unit does not have to wait for a “quiet time,” i.e. when the transformer in not inhaling, in order to regenerate. That enables the breather to have a fresh silica gel column continuously available for service. By adjusting the silica gel column regeneration cycle time, the system may be configured for various tank (air volumes) of 100 to 40,000 gallons or more.

Column “A” in Service

Column “B” in Service

During normal operation, air enters the breather through slots in the upper housing and passes through the desiccant to the center of the assembly. The center tube contains several holes along the entire length, forcing airflow to disperse through the maximum surface area of desiccant. Airflow then travels through the center tube, along a path—depending on the column in service—to the isolation solenoid and humidity sensor and eventually through the top port to the conditioned airspace.

During regeneration, a temperature regulating PTC heater element within the center tube of the column being regenerated, is energized to heat the desiccant to a specified temperature. Any moisture present in the desiccant is driven outward to the cooler borosilicate glass globe where it turns into condensate. The condensate runs to the bottom of the breather assembly where it is discharged through the water drain filter. In the arctic version, heat is automatically transferred to the drain to prevent freezing down to –50°C. During regeneration, the solenoid valve, located at the top of the breather assembly, isolates the column being regenerated while allowing the conditioned airspace to breathe through the column that is in-service. Once regeneration of the out-of-service column is complete (3 hours plus 30 minute cool down), the column is placed in stand-by mode. At the end of the in-service column cycle or if a column switch is triggered by the humidity sensor, the exhausted column is placed in regeneration mode and the stand-by column is placed in-service, ensuring a continuous supply of dry air to the conditioned airspace.

The Dual Column Breather system is shipped as a single integrated assembly and includes installed silica gel and accessories needed for mounting. The breather is constructed with a machined, anodized aluminum top, bottom and cast controls housing. Other components include heating elements, heat conductive fins, screen, condenser media and a filter vent system. Customer electrical and signal wiring is via conduit connections on the bottom of the control housing. The outer tube is optically clear borosilicate high strength glass. Sealed super bright LED lamps on the control cover provide clear visual indication of breather status, even in sunlight.

The Dual Column Breather systems feature an integrated PCB microcontroller that constantly monitors the condition of airflow through the breather. User adjustable, time-based controls regenerate the desiccant regardless of condition. Humidity sensing capability automatically overrides and regenerates the desiccant, if needed, between the set timer frequencies. Due to the dual column design, column regeneration is independent of the breathing status of the conditioned airspace.

The Dual Column Breather system is designed to be vertically mounted by means of four mounting tabs. Due to the dual column design, the mounting pattern is significantly different from our previous single column designs. An adapter plate is available from the previous mount pattern to that of the Dual Column Breather. An adapter kit is also available to accommodate the transition from previous cabled design to the conduit connections of the Dual Column Breather. The recommended connection to the conditioned space should be accomplished using standard flex hose and hose barbs (included), copper tubing, hard non-ferrous pipe or DIN 42562-5 flanged fitting.

In order to ensure that the Dual Column Breather system performs with exceptional reliability, it has been rigorously tested to IEC, EN, MIL and Prolec GE Waukesha internal standards. Five beta units are currently in service at diverse U.S. locations.

This new system features specifications designed to allow for ease of installation and operation:

  • Wide range of input voltage tolerance, 100–240 VAC, 50–60 Hz
  • Both time based and humidity based silica gel regeneration control
  • Local system status SuperBrite LEDs for visibility is bright sun
  • Remote monitoring of major system functions

Intelligent controls continuously monitor the status of the Dual Column Breather systems’ major components. The component’s status is reported via a combination of local LED signals and, for major faults, via an alarm relay, which may be monitored remotely. These include:

  • Normal Operation (Green LED)
  • Fast Mode operation (Flashing Green LED)
  • Regeneration in Progress (Yellow LED)
  • Humidity Sensor out of range (Blinking Yellow)
  • Regeneration Heater Fault (Blinking Red LED & Remote alarm)
  • Solenoid Fault (Red LED & Remote alarm)
  • Power Failure (No LEDs Lit & Remote alarm)

The Dual Column Auto Regenerating Dehydrating Breather requires minimal annual maintenance:

  • Check bottom drains for restrictions such as dust or other contaminants
  • Visually check the silica gel for contamination, particularly transformer oil, which will show up as a dark or blackened color (transformer oil contaminated silica gel must be replaced)
  • Clean the glass tube(s), if required
    • Use glass cleaner or soap and water as solvents may degrade the rubber seals

Regulating Heat in the Control Cabinet

Posted on March 14, 2024 by -

Appropriate Use and Application of Control Cabinet Heaters

Proper use of heaters in cabinets can improve reliability, efficiency and operational life of electrical equipment by preventing condensation inside the cabinet, thereby eliminating component and metal surface corrosion. However, the effects of too much heat and/or heat concentrated in one area of the cabinet can cause more harm than good. For anti-condensation purposes, the appropriate amount of heat needed should be based on the calculated power required (watts) to warm the enclosure to a temperature difference of 5 – 7 degrees Celsius above outside ambient temperature.

Two types of heater technology are typically available for control cabinet applications: strip heaters and self-regulating Positive Temperature Coefficient (PTC) heaters. Strip heaters have been around longer, are based on a fixed resistance element and therefore operate at a fixed wattage output. This type of heater requires a thermostat to turn the heater on below a set temperature and off above the set temperature to control the enclosure temperature. Typically, strip heaters only provide radiant heat which result in extreme temperature gradients with the heat concentrated on equipment closest to the heater, while condensation can still form in areas furthest away from the heater. Excessive localized heating caused by strip heaters may result in the unintentional thermal overload operation of protective devices, such as circuit breakers, deterioration of cable insulation, and warping / melting of plastic components such as wire ducts. Terminal blocks and wiring closest to strip heaters are exposed to high temperatures, possibly causing thermal damage. When wire insulation is damaged, risk of faults increases as does the risk of equipment failure, both of which can negatively affect personnel safety.

During cold weather, when a significant amount of heat is required to reach the thermostat setting, or in applications of continuous year-round use, a traditional strip heater is likely to drive the temperature of the area around the heater well above desirable levels. This can result in uneven heat distribution in the cabinet with areas measuring much colder temperatures further away from the strip heater.

PTC heaters, on the other hand, utilize a variable resistance element that changes resistance in response to changes in air temperature. As the air temperature decreases, the resistance of the heater element decreases allowing more current to flow through the heater, which leads the effective watts in heat to increase. Likewise, as the air temperature increases, the resistance of the heater element increases, allowing less current to flow through the heater, which leads the effective watts in heat to decrease. PTC heaters are available with fans for circulating the air in the enclosure to produce a more even distribution of heat throughout the enclosure. Due to the variable resistance element, PTC heaters do not require a thermostat to control operation, but thermostats can be used in special applications where ambient temperatures can exceed 40C and supplemental heat in an enclosure is not required or to increase power output when temperatures are lower than desired.

Below in the Max Temperature chart is a comparison between a strip (non-PTC) heater and a PTC heater, both without thermostats that can limit their power output. Depending on the ambient temperature, PTC heaters stabilize slightly above that ambient temperature, while strip heaters continue outputting as much heat as possible.

To Select a Heater

  1. Calculate the power (watts) needed for your particular enclosure size. For estimation of enclosure heat needed (based upon natural convection air moving less than 5 m/s), use this equation:Joules/Second = Watts = h x A x TWhere h = overall heat transfer coefficient W/(m^2K) – The value of h is difficult to calculate and is different for virtually every application; however, for rectangular outdoor enclosures with small amounts of venting and mounted to a vertical support, the typical value is between 5 and 10. Using 10 will represent a “worst case” scenario in a windier environment.A = Exposed surface area of enclosure (m^2)T = Temperature difference desired (K) – For anti-condensation purposes, typical value is equal to 5.A higher value may be used for particularly humid applications. EXAMPLE: A 3.5 foot wide, 4 foot tall and 1 foot deep cabinet mounted to a flat wall would have exposed surfaces equaling 29 ft^2 or 2.7 m^2. Watts = h x A x T = 10 x 2.7 x 5 = 135
  2. Draw a corresponding horizontal line on the selection chart (see Water vs. Heater Inlet Temperature chart below) based upon wattage calculated in Step 1 above.
  3. Determine the highest ambient temperature condition for the enclosure application and draw a corresponding vertical line at the bottom of the chart.EXAMPLE: The same cabinet in the example above is in a location where the higher air temperatures often reach 45°C. The vertical line should be drawn at 45°C and intersect with the 135 watt horizontal line in Step 2.
  4. Select the closest heater that intersects above and to the right of the drawn intersecting lines.EXAMPLE: The 200 watt heater would be selected for this application. NOTE – If high temperature operation would have been 65°C or higher, the 300 watt heater would have been appropriate for the application.

Measures can be taken to prevent issues like high cabinet temperatures, so remember your options: Properly calculate and select the number of heaters and the type of technology you need in the cabinet, use heaters with fans to evenly distribute heat and consider adding thermostats to your heaters instead of selecting continuous use heaters to prevent overheating your cabinets.

Transformer Life Extension

Posted on February 22, 2024 by -

Performing transformer life extensions (TLEs) is the practice of improving and upgrading an asset, returning it to near new status. TLEs provide a strategic fleet management option to overcome issues with budget, availability, site constraints and/or grid priorities. Current supply chain constraints and longer lead times magnify these issues, making TLEs a viable option for more asset managers.

As defined here, TLEs are done to upgrade and improve critical systems to add useful life to the asset. In contrast, a transformer rewind would be considered a full transformer rebuild. A rebuild is normally considered when a catastrophic event happens in the main tank and the unique nature of the asset requires a like-for-like replacement of the unit. Rebuilt units also serve as a viable alternative on the secondary market.

A TLE is an option when the main tank elements (core and coil) are in excellent shape, and the peripheral elements are contributing to the asset’s limitations. For example, an ideal transformer for a TLE would have DGA and electrical tests on the main transformer tank which indicate a healthy unit, but its arcing-in-oil load tap changer (LTC) is a potential failure point. In this instance, the main tank is healthy and could provide several years of additional service to the utility; however, the LTC’s condition may force a premature retirement of the asset.

Each utility will have its own process for evaluating assets and determining which assets are TLE candidates, but the four main considerations are as follows:

  • Unique designs and location constraints
  • Health of the candidate units
  • Asset classification
  • Fleet status and availability of new units

When the design of the existing unit or substation does not allow for a change in footprint, a TLE may be the only option. For these assets, TLEs can help overcome physical constraints. In urban areas where transformers can be located inside structures, replacing a transformer can pose significant challenges.
New designs can change the height, width, or length dimensions, changes that may not conform to existing codes. If the main tank is viable, a TLE will allow the asset to be upgraded in place. Managing the TLE within the existing footprint can eliminate the need for a new substation project.

The health of the main tank is the primary driver when considering a TLE. You should evaluate trend data as well as point-in-time reports to determine the viability of the unit. Both DGA and routine electrical tests should be evaluated. Additional data can also help evaluate the health of the unit, including fault history, PD monitoring data, surge counts, historical load profile, etc. Once the data analysis is complete and the unit is determined to be a candidate, a visual inspection should be performed to determine if any internal issues exist.  The decision to perform a TLE is subjective, and each utility will have a threshold for the minimum acceptable condition to move forward.

Two Primary Considerations:  Accounting Treatment and Economic Viability
Each utility will have a lower and upper bound for TLEs. The lower bound may be driven by accounting and asset management rules that prescribe the minimum requirements for capitalization of the project. These requirements can be expressed in terms of dollars vs. asset value or in terms of the number of systems addressed. Accounting principles must be considered as well as regulatory requirements and PUC rate case requirements when establishing parameters to evaluate capital reinvestment in an existing unit.

The upper bound is the crossover point where investment in the existing asset no longer makes financial sense. With any capital project, there will be a cost/benefit calculation. Generally, the viability of the TLE diminishes as the value of the upgrades approaches the value of a new asset. Certain assets will not warrant a TLE. The cost of updating an older asset compared to the cost of a new asset and the ability to establish a longer depreciation schedule on the new asset will naturally drive a decision for the new asset.

Situations exist where the status of the utility fleet, spares, and upcoming expansion projects will influence the TLE decision. When the available budget or number of assets becomes finite and the current assets outstrip available resources, extending the life of existing assets can overcome the shortfall. This can be a temporary strategy to minimize the impact of point-in-time conditions, or it can augment a new unit strategy. TLEs allow utilities to manage expansion, non-viable unit replacement, and service levels to customers.

Once assets are evaluated and identified for TLE, scope of work elements are identified. Each utility will have its own process for evaluating the elements to include in a TLE, but here are six considerations:

  • Load tap changer: upgrade or retrofit
  • Electrical controls: wiring, terminal blocks, gauges, fuses, relays, breakers, and heaters
  • Electrical load handling: bushings, arresters, and leads
  • Fluid cooling and breathing: radiators, fans, pumps, coolers, conservator, and breathers
  • Monitoring and controls systems: alarms, protection, and asset monitoring
  • External tank upgrades: painting, crack repair, and leak mitigation

LTC Updates
One of the weakest systems on a transformer is the LTC. The LTC is a common—and often catastrophic—failure point. Arcing-in-oil units require periodic inspection and maintenance to ensure proper operation, and older vacuum systems do not benefit from the design improvements realized over the last 50 years. Moving from either of these systems to newer technology can extend the life of the transformer and minimize ongoing maintenance costs. Retrofitting the LTC with a new vacuum unit is a significant upgrade to the transformer. However, the ability to upgrade to new vacuum technology depends on the design of the transformer. Each unit must be evaluated and inspected to verify the unit qualifies for an LTC retrofit. Once validated, a complete set of engineering drawings is produced to include the new vacuum LTC. The LTC retrofit is a significant project that can take 30+ weeks from identification to commissioning; however, the total transformer outage time to perform the upgrade is typically only two to three weeks, including LTC throat wall retrofits.

Transformers that utilize outdated, maintenance intensive vacuum tap changers also benefit from upgrades that take place during transformer life extensions. First generation vacuum systems may require more frequent maintenance or adjustment to perform properly. Current generation LTCs improved the reliability of the systems, but older units may require upgrade or replacement to overcome limitations of the earlier designs.

Controls Upgrades
Electrical controls within the transformer are frequently upgraded during the life extension process. Gauges that monitor performance of the unit’s temperature, pressure, tap position, and fluid level are replaced with modern equivalents that have outputs for external monitoring. Control cabinets on field-aged transformers normally have multiple modifications performed by field crews that sometimes render the original control drawings inaccurate. The life extension process involves a reengineering effort that can include replacement of control cabinet wiring as well as components within the control cabinet of an outdated design that are no longer produced, such as fuses and terminals.

Heaters in the control cabinet along with relays and breakers used for auxiliary power handling are frequent sources of unplanned maintenance for field personnel during inspections on aged assets. The TLE investment offers replacement of these 20 to 50-year-old electrical items. Replacing the auxiliary control devices also provides the opportunity to decrease auxiliary voltage from 480V to 240V to accommodate modern safety practices that require arc flash protective equipment for 480V environments.

Electrical Load Handling
Bushings can also be upgraded as part of the TLE process. Due to their flammability, oil-impregnated paper (OIP) bushings can cause significant damage to other components, the main transformer, and even other systems in the substation. Upgrades to resin-impregnated paper (RIP) bushings can provide lower partial discharge, zero headspace, better physical characteristics, and are non-flammable while also offering lower external current leakage and reduced risk of flashover in contaminated coastal environments. Utility design parameters and preferences will influence adoption of this technology; however, a retrofit with RIP bushings can provide an upgrade to the transformer.

External arresters used for lightning and over voltage protection are normally replaced during the TLE upgrades. Aged porcelain-insulated arresters are frequently upgraded to new polymer sheathed designs that offer better performance in industrial and coastal environments. System conditions normally evolve during a transformer’s life cycle, which could provide an opportunity to optimize the arrester’s maximum continuous operating voltage, fault magnitude, and energy ratings for current requirements. Lower temperature operation along with replacement of the bolted electrical connections will reduce arrester maintenance and call-out visits in the future.

Improved Cooling Performance
Upgrades to the cooling system offer multiple benefits to the life of the transformer. Both fans and pumps are rotating assemblies and performance can degrade or fail over time. Upgrading to new fluid handling equipment will help restore the cooling capabilities originally designed into the transformer. In addition to ensuring all components are working, upgrading to modern, higher CFM, lower amp draw, sealed bearing fans—or even adding fans to the system—will help maintain operating temperatures in the main tank. Pump performance is also critical to maintaining the appropriate cooling.

Radiators used in transformer construction evolved from tube-based designs to more efficient plate-based coolers. Radiator performance increases from header pipe design, cooler efficiency, and cleanliness might add years to the life of a field-aged transformer. Modern radiators are more durable with longer lifespans, and most are zinc-coated to further extend life. Radiators will be replaced as part of a scheduled TLE.

Replacement of the conservator bladder, the Buchholz relay, and sudden pressure relay will restore the transformer’s oil handling and fluid-based protection systems. Leaks in the rubber conservator bladder increase moisture contamination within the transformer’s main tank and cause premature degradation of
in-tank insulation. Breathers are often upgraded to self-drying models to reduce O&M costs associated with replacing desiccant. If the transformer is nitrogen blanketed rather than a conservator design, replacement of the cylinder regulator and cylinder itself will be evaluated along with the potential addition of a nitrogen generator system.

Monitoring
The addition of real-time monitoring equipment as part of a TLE upgrade provides the transformer owner the opportunity to reduce maintenance visits. Gas-in-oil monitoring, partial discharge, remote temperature outputs, and bushing monitoring are useful additions to consider when performing a proactive life extension. Trending data that can be accessed remotely helps identify issues early before a critical failure occurs.

External Considerations
Aged transformers frequently suffer from fluid leaks and external tank degradation, such as rusting and welding cracks. The re-gasketing of fluid handling equipment on the transformer during a TLE might also include an external inspection that results in a recommendation to paint the outside of the transformer’s main tank. In salt air contaminated environments, paint deterioration is a frequent maintenance item that can be greatly improved with a deep cleaning and full “top to bottom” recoating. Failed welds and joints can be repaired on-site to decrease future maintenance visits and third-party contractor expenditures.

With a proactive and calculated approach to TLEs, a utility can improve the overall quality of its fleet. TLEs can provide reductions in forced outages, unit failures and maintenance. They can also improve quality of service, maximize spend, and allow for greater flexibility when dealing with spares and expansion. Transformer life extensions are another tool to ensure the reliability of the grid and high service levels while adding years to the life of the assets.

After the life extension, you could have:

  • Solid main tank (tested and inspected prior to project)
  • Modern load tap changer (a weak link in transformers with arcing-in-oil or early vacuum LTCs)
  • New controls and information devices
  • Improved electrical load handling (potentially safer and more reliable than existing equipment)
  • Better cooling performance
  • Upgrades to real time monitoring, and
  • Refreshed externals