ABB’s LEAP predictive service keeps the Middle East’s oil and gas platforms running smoothly
Large MV (medium-voltage) motors and generators are typically at the heart of industrial processes in the oil and gas industry, and plant owners want to invest in monitoring and maintenance programs that will keep them running with maximum efficiency and reliability.
Over the years, there has been significant progress from a reactive maintenance philosophy, through preventive maintenance to one based on condition monitoring. There are many motor and generator health monitoring programmes available that carry out periodic measurements to assess the general condition of the machine and its components.
Their main limitation is that they can give a reliable warning only when failure is imminent, without providing the crucial information regarding the expected remaining life. ABB’s Life Expectancy Analysis Program (LEAP) adds the vital dimension that enables an operator to know, with a high level of confidence, how long their motor or generator can remain in service before a failure. This systematic approach to maintenance management, when used in conjunction with the known time to repair or replace a component, then allows service to be performed during planned downtime, rather than during a costly emergency.
Stator winding health – the crucial factor
Surveys have shown that for larger motors, above 2000 kW, the most common cause of failure was in the stator (static) windings at nearly 61 percent. Since these larger machines are in the most widespread use in the oil, gas and petrochemical industries, ABB’s LEAP analysis is focussed on assessing the condition of stator windings.
Furthermore, the need for a new approach to monitoring the condition of stator winding insulation was clearly indicated – since these surveys identified that very few winding failures (just over 8 percent) were being detected under maintenance using existing, conventional techniques.
Predicting insulation ageing
Stator windings comprise a relatively complex multi-layer structure that, during operation, is exposed to thermal, electrical, mechanical and environmental stresses. On startup a large motor will draw an increased current – typically 5 x its normal operating current – this current peak causes differential expansion of the copper wire and the various layers of insulation. Over time this creates air gaps that result in weak points.
In service, the insulation material ages as shown by the light blue curve in Fig 1. The stresses due to both normal operation and abnormal events (such as transient currents) is represented by the dark blue curve. When the two curves cross an insulation failure will occur, resulting in expensive unplanned downtime. LEAP provides the operator with detailed information on the condition of the stator insulation so they can plan effective action to prevent the failure and increase the overall life of the machine as shown by the dotted line.
How is a LEAP analysis carried out?
To carry out a LEAP analysis, ABB takes a range of sophisticated instrumentation to site that enables a battery of tests to be performed on the stator winding insulation. This instrumentation is available throughout the world via ABB’s network of 17 global service hubs.
The basic tests are non-invasive, and in many cases they do not require stripping of the motor or generator, all that is required is access to the terminal box. All that is needed is for the equipment to be taken offline for a short period – typically one day. In addition to carrying out the tests, a range of basic information on the equipment duty will also be collected for use in the subsequent analysis.
The advantages of combining DC and AC techniques
The ABB tests provide a comprehensive fingerprint of the condition of the stator insulation. The capability to develop this test regime is a direct result of ABB’s significant investment in R&D for maintenance and service techniques. This is rare in the electrical equipment industry where manufacturers usually focus their R&D efforts almost exclusively on developing new products.
LEAP combines both DC and AC techniques since DC tests are sensitive to the surface condition of the insulation, while AC tests provide further information on its whole structure, including the internal condition.
The DC measurements provide a PDCA (Polarization/Depolarization Current Analysis). In addition to the leakage and absorption current, it provides an indication of the quantity and location of charge storage within the machine. It can also identify contamination even when conventional IR (resistance) and PI (Polarization Index) measurements have apparently satisfactory values. Overall, the PDCA test determines the condition of the winding insulation, ageing, looseness etc.
The AC measurements comprise: Non Linear Behaviour Analysis; Tan Delta Analysis and Capacitance Analysis; Partial Discharge Analysis. These:
- confirm the results of the DC tests
- assess the condition of the corona protection shield
- determine the extent of delamination or void content in terms of a percentage of discharging air volume to insulation volume
- assess the condition of the stress grading system at slot ends
- trend ageing effects
The LEAP site report provides a fingerprint for the machine comprising the following information:
- contamination of the stator winding (which increases stresses and reduces lifetime)
- ageing of insulation, ageing of resin, delamination.
- status of stress grading system
- status of corona protection system in the stator slot area
- PD (partial discharge) activity in other parts of the stator such as the winding overhang
LEAP test analysis
Once the fingerprinting tests have been carried out, the data collected is transferred to ABB’s Machines Service centre of excellence in India that has 12 years of experience in LEAP analysis.
During this time, it has compiled a vast database of information covering many thousands of different motors and generators across all manufacturers, not just ABB.
Here, the data is analysed to identify the process of insulation degradation, and stresses calculated to perform the Life Expectancy Analysis –indicating the key factors that will affect the machine’s lifetime.
A variety of LEAP packages are available, with the more advanced levels including either partial or complete dismantling of the machine to carry out a more detailed electrical and visual inspection, while the most sophisticated package also involves FEA (Finite Element Analysis) techniques.
The remaining lifetime is estimated with different confidence levels according to the package. Clear plans are provided for further inspections, maintenance, replacement or upgrades.
In some cases a simple cleaning out of contamination from the end winding areas or slots might be indicated. Other cases could show that the insulation had reached critical levels, calling for rewinding or replacement with spare coils. Since a major rewind project can take up to 12 months, this will have a significant impact on scheduling the future availability of the asset.
Predicted life expectancy
“It must be emphasised that LEAP is much more than a battery of tests”, says Ehab El-Naggar, ABB’s Regional Service Manager for Motors and Generators in India, Middle East and Africa. “Instead, it represents a coherent philosophy for the optimization of maintenance planning. Unlike condition monitoring techniques that rely on measurements over a period of time to indicate a trend, LEAP is a one-off measurement that can be performed at any stage during the life of a motor or generator. Even where no previous history is available LEAP can still provide an accurate indication of remaining life.”
Describing the typical results delivered by LEAP, Mostafa State, ABB’s sales manager for motors and generators in the UAE, Oman, Kuwait and Bahrain said: “A good example of LEAP in action is a 150 MW generator in Saudi Arabia. Our analysis showed that the stator insulation could be expected to last for at least 10 000 hours (a year is 8760 hours), so a planned outage for maintenance could be scheduled in 14 months.”
“In another example, again in Saudi Arabia, we were called in to investigate a large motor that had been in service for 20 years and was scheduled for a rewind operation in six months. Our LEAP analysis showed that the motor could continue in safe operation for at least 8000 hours. This enabled the outage to be put back significantly, increasing availability and reducing maintenance costs.”
Ehab El-Naggar concludes: “To date, ABB has established a reference list of over 300 machines throughout the Middle East where LEAP has been applied to great effect. LEAP is not just about ageing assets either. It can be usefully employed early in the life of a new asset to confirm correct installation and commissioning. In one case in Qatar, a test on six non-ABB motors installed in April 2008 showed that three had already developed insulation problems only a year later, enabling them to be dealt with under warranty.”
