Analysis

Design Analysis

ABB has access to original designs for more than 30 legacy brands and design knowledge of nearly 75 percent of the installed base of large power transformers in North America – including those from Westinghouse, GE, ASEA and BBC – and other predecessor technologies. All new ABB transformers are built using the same design concept, which incorporates standardized, service-proven components and modules, ensuring flexible, dependable and adaptable transformer designs.

Historical Review

ABB’s installed data system monitors a wide range of the company’s products. A plethora of data on transformers is available and is continuously updated, eg, current owner details and history. The system provides an important basis for the proactive detection of problems. For example, an analysis revealed about 700 potential cooler problems in the installed base of transformers. The search focused on 10 to 600 MVA transformers that were over 20 years old and had oil- and water-type coolers. Many failed completely due to leakages in these cooling systems, and one such failure resulted in a three-month production shutdown and lost revenue for the operator. Using the information in the installed base system, operators were contacted proactively and the systems could then be checked regularly.

Transformer Monitoring

Transformer monitoring is becoming an essential component of transformer management. It serves as an early warning system for any fault developing in the main tank and in the accessories, allowing an operator to evaluate the severity of the situation. Multiple transformers are connected to the operator’s network and can be monitored from a local control room or from remote working stations 7. Sensors measuring dissolved gases, moisture in oil, oil temperature, load current for each unit, and ambient temperature send data to the system via analog signals. The interface provides exact status information by generating a model of the transformer and its working condition and then comparing the measured parameters with the simulated values 8. Discrepancies are detected and potential malfunctions and normal wear in the transformer and its ancillaries are indicated. The monitoring system also tracks transformer alarms, recording an actual event as well as the sequence leading up to the alarm to assist operators in determining the root cause. The benefits of monitoring are substantial. A CIGRE study has shown that transformer monitoring can reduce the risk of catastrophic failures by 50 percent 2 [2]. Furthermore, it has been shown that early detection of problems can reduce repair costs by 75 percent and loss of revenue by 60 percent, and that annual cost savings equal to 2 percent of the price of a new transformer – ie, approximately $40,000 to $80,000 – can be achieved [3].

The strength of ABB’s Transformer Electronic Control, or TEC, monitoring system is that it receives all the relevant information from just a few multipurpose sensors. Other necessary parameters are calculated, adding only minimal complexity to the transformer. The end user is no longer forced to spend a lot of time sorting and interpreting data. In addition, the maintenance manager receives important information indicating the necessary actions for first-level maintenance.3

Condition assessment

ABB is the pioneer in highly customized condition assessment offerings. Its MTMP (Mature Transformer Management Program) is a state-of-the-art minimally invasive condition assessment process used to evaluate the power transformers in a customer’s fleet and to identify which units need to be replaced or refurbished and when. This process is implemented in three steps 9. It starts with a high-level fleet assessment based on easily accessible data, such as unit nameplate data, oil and dissolved-gas-in-oil data, load profile and history of the unit (transformer fleet screening) 9a. Next, a subset of the transformers identified in step one is examined in more detail (transformer design and condition assessment) 9b. Modern design rules and tools are used to evaluate the original design, and advanced diagnostic tests are performed to assess each of the principal properties of the transformer in a structured way. These include mechanical status, thermal status (aging of the insulation), electrical status of the active part and the condition of the accessories, such as tap changers, bushings, overpressure valves, air-dryer system, pumps and relays. The number of units identified for further analysis is typically limited to two or three out of a population of 100. At this stage (life assessment/profiling) 9c, highly specialized experts analyze the units using simulation tools. Detailed data is then sent to the end users’ operational managers, providing concrete information about whether a transformer can be overloaded, its nominal power or voltage rating increased or its lifetime extended [4].

Risk assessment

The risk assessment 6 is based on two variables. The first, risk of failure, is estimated using the input from the analysis phase, ie, age or time in service, transformer’s nameplate data (kV, MVA, etc.), application and loading practices, operational problems or issues, latest field-test data (eg, dissolved gas and oil analyses), availability of a spare transformer and spare parts. The second variable is the importance of a transformer in a network, indicating how much of the operator’s system will be out of service if a particular transformer fails. By comparing these two variables, different levels of urgency for maintenance actions can be defined 9a. The asset manager can then ensure that maintenance of high-risk transformers is prioritized.

Asset management scenarios

The risks for a transformer operator include not only the inherent technical risks but also the economic consequences of a possible fault, eg, the cost of non-delivered energy. With this in mind, ABB and a large operator co-developed an economical model that evaluates the lifecycle costs of a transformer fleet over a given period 6. The model takes into account four categories of costs related to the cost of ownership over the lifetime: investment, maintenance, operational and consequential costs. Comparative investment scenarios and sensitivity studies can be run by varying the replacement year or maintenance of the unit. For each scenario, the process shows the associated net present value. An optimization routine can also be used to automatically minimize the life-cycle costs of the population. The process outputs a list presenting the optimum time to maintain or replace the individual transformers or transformer groups. The net present value of the whole population of transformers is determined by looking at the condition of each unit and the maintenance actions selected to improve their condition. The operational manager can then evaluate different maintenance scenarios and obtain a summary of the payback of planned maintenance actions. The novel aspect of the method is that not only are maintenance costs considered but economical benefits related to the impact of maintenance on reliability are considered as well [5].

Maintenance packages

ABB provides personalized recommendations and support using available data and state-of-the-art tools and maintenance packages, as shown in 6. These include regular asset services, early-life inspection, midlife refurbishment and remanufacturing. For many operators midlife refurbishment has become very important as their transformers are aging. Midlife refurbishment is an extensive overhaul of a transformer to extend the remaining lifetime and increase reliability, and is typically performed after half of the expected lifetime. It involves several maintenance steps, including advanced diagnostics to check mechanical, thermal and electrical conditions. New or refurbished accessories such as on-load tap changers, bushings, pumps, temperature sensors, valves, gaskets and water coolers might be used. Refurbishment of the active part through, for example, cleaning, winding reclamping, connection retightening and installation of new parts, is often an aspect of a midlife refurbishment.