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Westinghouse UTT Upgrade Series: Part 2 of 3

Posted on March 14, 2024 by -

Upgrading your UTT, UTT-A or UTT-A70 to the UTT-B Style Selector Switch

In this installment, we will discuss upgrades for the selector switch assembly of your Westinghouse UTT Series load tap changer (LTC). These upgrades address areas of the selector switch that are prone to failure due to overheating and carbon buildup and are applicable to the UTT, UTT-A and UTT-A70 models.

Each phase of a 3-phase LTC contains a selector switch. Below are 3-D models of individual phase panels for each of the different UTT Series LTC models. Red arrows indicate areas where overheating and coking typically occur. These arrows also help visually identify areas where there are differences in the designs of the individual models.

Figures 1 and 2 below depict the flip side of the moving contact arm assembly in which the contacts are clearly visible. Arrows indicate areas of the moving contact arm assembly where overheating and carbon buildup are most likely to occur. Figure 1 depicts the UTT model and Figure 2 depicts the UTT-A/UTT-A70 models.

By upgrading the moving contact assembly of the UTT, UTT-A, UTT-A70 to the UTT-B design, the risk of overheating and carbon buildup in the contact areas is fundamentally reduced. Figure 3 below depicts the contact arm assembly of a UTT-B with the contacts exposed. The upgrade to the UTT-B design reduces the number of contact fingers from four (4) to two (2) and the number of non-wiping fingers from four (4) to zero (0).

In addition to the upgraded contact configuration, the collector rings on the UTT-B design are offered in an improved “split” design for phases A and B. Figure 4 below indicates the position of the collector rings. The split design allows for worn collector rings on phases A and B to be replaced in the field without untanking the LTC.

Form, fit and function are exactly the same between the different designs, but the selector switch upgrades presented above offer the following benefits:

  • 50% reduction in spring-loaded contact fingers, reducing risk of overheating and coking
  • Elimination of all non-wiping spring-loaded contacts, reducing risk of overheating and coking
  • Significant reduction in time required for collector ring replacement without untanking the unit

Each upgrade kit from Waukesha® Components comes with detailed instructions for installing the upgraded designs. We also offer standard and customized component kit cases. These cases offer the following unique set of benefits:

  • Parts are easier to pull from inventory and issue to the maintenance jobs
  • All key parts are included for easy and safe transport to the work location
  • No need for field personnel to keep lists of components consumed during maintenance
  • Cases provide a better means of protection and storage for the components
  • Quick and easy to replenish after completion of field maintenance

Our 30-year history of providing replacement parts for the majority of OEM LTCs has allowed us to develop the capability to confidently engineer, manufacture and support a myriad of design-enhanced replacement parts. You can be confident that the Waukesha® Components version of the UTT-B style selector switch components will match the performance of the OEM part at a fraction of the cost.

Similar upgrades are available for the UTT, UTT-A and UTT-A70 transfer switches. These other upgrades will be covered in detail in the third installment of this three-part Tech Tip series.

To learn more about all upgrades available for the UTT Series LTC 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 and oil circuit breakers, while also including one for the Waukesha® Components’ line of Transformer Health Products®.

​Westinghouse UTT Upgrade Series: Part 1 of 3

Posted on March 14, 2024 by -

Upgrading to the RMT-1 Style Reversing Switch

Did you know that many affordable upgrades are available for your Westinghouse UTT Series load tap changer (LTC) that can be installed during your next maintenance rotation? These upgrades are designed to extend maintenance intervals while improving reliability of the unit. This is the first in a three-part series intended to introduce you to upgrades currently available for the Westinghouse UTT series. This installment will focus on upgrading to the RMT-1 style reversing switch.

Upgrading to the RMT-1 style reversing switch is highly recommended since the RMT-1 style is a self-aligning design that offers a higher current (amps) capacity than any other UTT design. Table 1 below contains a summary of the designations and ratings of Westinghouse UTT LTCs:

To quickly identify what type of tap changer model you have, use the following guidelines:

  • UTT: Two windows into cam switch compartment front and side; position indicator from “ON POS” indication side
  • UTT-A: One window through swinging door; position indicator front “ON-POS” indication, open door
  • UTT-A70: One window through swinging door; side position indicator tilted down 30 degrees, “ON-POS” indication can only be seen when the cam switch compartment door is open
  • UTT-B: No window through swinging door; side position indicator tilted down 30 degrees, “ON-POS” indication can only be seen when the cam switch compartment door is open
  • RMT-1: No window through swinging door; side position indicator boss round

Below are 3-D models of phase panels for each of the different UTT Series LTC models. Red arrows indicate areas where overheating and coking typically occur. These arrows also help visually identify differences between the UTT models.

Below is a different view of the moving selector switches for the UTT-A/A70 and UTT. Red arrows again point out common areas most affected by overheating and coking.

Now that we have shown the different model types with their amperage ratings and how to identify them, we will describe the benefits of upgrading your standard reversing switch to the RMT-1 style.

The purpose of the reversing switch is to select raise or lower connection of a tapped winding section. Each phase will contain a reversing switch on a 3-phase LTC. The figures below show a side-by-side comparison of the standard reversing switch designs and the RMT-1.

Form, fit and function are exactly the same between the different designs, but the RMT-1 offers the following benefits:

  • Offers increased current rating of 1320 amps vs 1000 amps
  • Operates at lower temperatures for a given load current
  • Self-aligns for better mechanical operation and smoother transition when
    passing through neutral
  • Moving contact utilizes six pair of contact fingers with high spring pressure vs two pair for the UTT-B model and four pair for the UTT/UTT-A/UTT-A70 models, allowing more current paths with oil between them for better thermal performance
  • Eliminates static, spring-loaded mating contact surfaces and replaces with moving contact surfaces that help ensure contact filming and heating are less likely to occur

Each upgrade kit from Waukesha® Components comes with detailed instructions for installing the upgraded designs. Our 30-year history of providing replacement parts for the majority of OEM LTCs allowed us to develop the capability to confidently engineer, manufacture and support a myriad of design-enhanced replacement parts. You can be confident that the Waukesha® Components version of the RMT-1 style reversing switch will match the performance of the OEM part at a fraction of the cost.

Similar upgrades are available for the UTT, UTT-A and UTT-A70 selector and transfer switches. These other upgrades will be covered in detail in the second and third installments of this three-part Tech Tip series.

To learn more about all upgrades available for the UTT Series LTC, 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 and oil circuit breakers, while also including one for the Waukesha® Components’ line of Transformer Health Products®.

Why Oil Analysis is Important to the Health of Your Load Tap Changer

Posted on March 14, 2024 by -

Maintaining your load tap changer fleet is crucial to the continuous, safe operation of your transformers. Over time, contacts and other moving parts can start to show wear from repeated use or become damaged from intense heating inside the LTC. Many countermeasures exist for preventing LTC malfunction and critical failure, evolving over the years as the industry continues to study the effects of tap changer operations in oil.

Time-based maintenance, while oftentimes effective for routinely inspecting and maintaining all parts in good condition, can become expensive quickly, especially with large LTC fleets. Draining oil, opening the unit(s) and inspecting for wear or damage adds time and money that can be avoided with other, more sophisticated diagnostic tools.

Dissolved Gas Analysis, or DGA, analyzes the gasses captured within the oil, such as hydrogen, methane, ethane, ethylene and acetylene, to form a good diagnostic tool for the LTC. Prior to 1995, LTC DGA was considered of no value. Even today, dissolved gas and oil quality analysis are not widely used on a regular basis to assess and troubleshoot LTCs. Annual oil sampling for DGA coupled with online oil filtration systems can extend major maintenance intervals to 10+ years. As temperature inside the LTC increases, the amount of combustible gasses created via arcing and heating are dissolved into the oil, increasing over time. By studying the rise and fall of these gasses, detection of low, moderate and severe heating conditions are found without the cost of opening the unit for visual inspection. Severe heating conditions can lead to a critical fault and malfunction of the tap changer.

The type of fault can be indicated by the dominant gas in the LTC, based on an increasing fault temperature that differs for each combustible gas. The ratios of each gas indicate the presence of heating, coking and arcing. While no industry accepted standards exist, we can analyze these gasses over time to form an idea of what is happening inside the tank.

One ratio to consider is ethylene over acetylene for arcing-in-oil type LTCs (both resistive and reactive) to predict when inspection of the load tap changer should occur. Because certain gasses appear more frequently as the fault temperature increases, we can detect low, moderate and severe heating conditions over time based on how much more ethylene is present compared to acetylene. These ratios are independent to the number of operations in the tap changer and also resistant to changes due to loss of gasses to the atmosphere. For vacuum-type LTCs, tracking the total combustible gas count over time can give some indication as to when the unit should be inspected.

Another ratio that can be considered is the Stenestam Ratio, which looks at the sum of methane, ethane and ethylene compared to acetylene.

Important to note that for each ratio calculation, a minimum count of gasses needs to exist before the ratio is effective. With small counts of gas, the ratio can vary wildly up and down the spectrum and have misleading results.

The Duval triangle is another great tool for trending heating and arcing gasses for LTCs. By plotting the gasses on the triangle, units moving from the “N” region (normal operation condition) to a fault condition can be detected. Coupled with infrared scans while in service, units headed towards failure can be reasonably confirmed.

Five zones of abnormal operation or faults are identified in the Triangle for LTCs of the oil type:
T3 = severe thermal fault with heavy coking of contacts (T > 700°C);
T2 = severe thermal fault with coking of contacts (300°C < T < 700°C);
X3 = fault T3 or T2 in progress, with light coking or increased electrical resistance of contacts;
D1 = abnormal discharges of low energy D1;
X1 = abnormal discharges of low energy D1, or thermal fault in progress;
N = Normal Operation

Note: Minimum gas levels should be >10ppm to apply this analysis method

While industry guides exist for analyzing DGA results, model-specific industry accepted limits are NOT available today. Generating such guidelines requires a significant number of LTCs and samples to form sound statistical results. Gas generation rates can vary due to model, design vintage, breathing type, frequency of operation and even maintenance practices across a fleet.

With all the discussion above, DGA is only half of the LTC condition assessment equation. Oil quality analysis is equally important for determining the health of your tap changer. Measuring key parameters such as inter-facial tension (IFT), acidity and water content, these tests provide an oil quality index and relative saturation that can be utilized as an additional indicator for chemical changes in the oil. Fluid quality index – (Acidity x 1000) / IFT – coupled with comparing the N2/O2 ratio in oil on free breathing units can detect the onset of sludge formation and contact filming before coking and contact damage occurs.

As moisture and oxygen in the oil increase, the heat from loading acts as an accelerant to oil oxidation and sludge formation. This contact sludge, or film, adds resistance to the contact which, in turn, increases heating even further. As this resistance rises, the rate of heating and sludge increases as well, leading to “thermal runaway” and formation of coke.

Oil chemistry affects can be mitigated. Monitor oil quality and DGA annually and trend results. Determine oil quality index – (Acidity x 1000) / IFT – and replace oil when quality index is greater than 18 during maintenance. Use online oil filtration on arcing-in-oil reactive and resistive-type LTCs. On units flagged by oil analysis, use infrared in conjunction with oil testing data to verify temperature differences between the LTC and main tank.

CASE STUDY: WAUKESHA® UZD® – RESISTIVE-TYPE LTC WITH ARCING SELECTOR SWITCH

For an example of how both DGA results and oil quality data can be utilized as tools for LTC assessment, let’s take a look at the data from this UZD:

The ethylene over acetylene ratio of 0.11 is “normal” according to the accepted guidelines, but the fluid quality index of 22 triggered an inspection of the tap changer (should be below 18 under normal operation). If DGA results alone were used, this unit likely would not have been flagged for inspection, as it appears to be in the normal operation range when plotted on the Duval Triangle.

Normal!!

Upon opening the unit, one can clearly see that this was a good catch, as film was beginning to form on many components of the LTC. Thermal runaway/coking may have unexpectedly occurred before the next maintenance cycle.

Using traditional DGA alongside oil quality data, Waukesha® Components has formulated a patented algorithm to analyze your fleet data and create a prioritized listing of units most in need of maintenance before significant damage occurs. With a large population of load tap changers and enough statistical data points, Waukesha® Components can provide a FREE fleet consultation, giving a detailed, targeted approach to maintenance greatly exceeding industry standard DGA analysis guidelines.