Strategies to Keep Them Turning
The SCADA alarm flashes red as the blades slowly come to a stop. The site manager gets a call – what now? A generator is failing. Again. Sadly, this scenario plays out somewhere in North America every day of the year. Why is this happening and what can be done? Generators stop working for several reasons and, of course, it might be an unforeseen design flaw, but most failures are both predictable and preventable. With proper cause analysis combined with good planning and execution of maintenance procedures, the failure rates could be cut by almost 50%. In addition, serial defects due to non-optimised designs or poor installation can be managed and solved efficiently once discovered.
By Kevin Alewine, Director of Renewable Energy Services, Shermco Industries, USA
Larger turbines, ranging up to 3MW, have experienced different types of failures, including collector ring flashover, rotor lead failures and loose or damaged stator wedges. Although these different failure modes are encountered, the incident rate is about the same or higher than with smaller turbines. The larger generators, although appearing more mechanically robust, continue to be plagued by inadequate design and, often, poor installation. There are still failures related to rotor winding damage, but these are mostly based on severe over-speed conditions and not directly related to generator design or manufacturing.
Insulation systems must be adequate for the electrical stresses encountered during operation, but must also be durable enough for the mechanical and environmental challenges of the application. Wind turbines, by their very nature, are subjected to the vagaries of both vibration and the weather. Often, cost reduction initiatives have limited the amount and type of insulation materials utilised. As these are generally low voltage machines, there is often less emphasis put on the long-term durability of the coil design, coil insulation and choice of impregnating resins when compared to high voltage machines of similar size and output. Very few engineers would consider using these types of insulation systems for industrial machines, even for low voltage applications. Understanding this and bringing to bear a long history of repairing and rebuilding motors and generators for industrial applications, quality remanufacturing facilities can often produce generators with improved reliability compared to the original OEM designed system.
Now to the real culprit affecting generator reliability and availability: premature bearing failures. In the early years, there were many failures related to transient electrical currents induced in the rotor shaft. These currents would discharge though the ball bearings, creating destructive pitting at the interface with the bearing race. These transient voltages were mostly the result of having such a high current source available, sometimes aggravated by the high voltage spike effects of PWM (pulse width modulated) converters often used in conjunction with the generator, especially if they were attached over long cable runs to the base of the turbine. With the improvements in shaft grounding methods and the advent of ceramic hybrid and otherwise insulated bearings, this type of failure has become much less common.
The industry is now plagued by more traditional bearing failures caused by vibration, alignment and rotor balance issues and, most importantly, improper maintenance. While good rotor balance is normally the responsibility of the OEM or the remanufacturer, proper alignment during installation and verification at regular intervals are key maintenance points. Confirming and maintaining proper alignment is possibly more critical in wind turbines than in industrial applications due to the inherent instability of turbines and the difficulty of periodic inspections. A simple laser alignment check can save thousands of dollars in repairs and lost production.
A basic rule of rotating machinery maintenance is that either under or over-greasing ball bearings results in dramatically reduced life. The results of under-greasing are intuitive and obvious – increased friction, heating and rapid failure. Over-greasing is just as destructive, but in more subtle ways and is much more common. An overfilled bearing does not roll freely, causing a loss of efficiency. Additionally, the trapped excess grease can harden to a shellac-like substance making the problem very complicated to remedy. This also dramatically increases wear and can make it difficult for the old grease to exit when fresh is inserted. All of this often results in the excess grease migrating into the stator assembly where it can contaminate the windings. Even if the grease itself is not conductive, it can attract and hold dirt and other particulates that are both abrasive and hygroscopic. Often, auto-lubrication equipment is installed on generators. While generally that is seen as an improvement, over-lubrication is very common with automated equipment as the units must also be maintained and adjusted properly. With the growing popularity of permanent magnet generators and their promise of lower operating costs, maintenance issues will not magically disappear. These new designs will experience the same failure rate if not properly maintained, and the repairs are probably even more complicated.
Any repair that cannot be done uptower is costly. Cranes, technicians, transportation, and surprise developments that delay the procedures such as high winds and other inclement weather conditions all contribute to the expense. Proper maintenance is the key to controlling and predicting long-term repair costs in any industrial or utility environment, and wind energy is no exception.
The charts in Figure 2 show the common failure modes found in wind turbine generators, how often they occur and the size of the sampled groups. When compared to the results of a survey of rotating machines in the petrochemical industry developed by O.V. Thorsen and M. Dalva for the IEEE in 1995, the frequency of bearing failure is very similar. In that industry, the motors and generators are also difficult to maintain because of their locations, and this study helped illustrate the importance of good maintenance for that group.
So, what to do to assure maximum availability and reduce overall, long-term maintenance costs? The steps are the same as in any other application. Carefully review maintenance training and practices to assure proper planning as well as execution of the procedures. Invest in a maintenance team that can analyse the SCADA reliability information that is collected and give them the tools to do so. Consider expanding the data collection by utilising vibration monitoring or, at least periodic vibration surveys of the installed drive trains. Schedule maintenance as would any other utility, recognising that failures will happen; plan accordingly to minimise the surprises. Lastly, choose maintenance partner companies carefully. Whether for repairs in the field or remanufacturing facility, experience, quality programmes, tech support and financial stability are critical. Uptower repairs should comply as nearly as possible with the OEM specifications, and a properly remanufactured generator should meet or exceed the performance and reliability of a new machine. Keeping generators running for 20 years requires planning and investment for the long run, not just a short-term strategy to maximise the current financial period results. The goal is to keep them in the air, keep them turning, and if they must come down, make sure it is only once.
Biography of the Author
For over 30 years, Kevin Alewine assisted motor and generator companies with the evaluation and specifications of electrical insulation materials and systems to optimise the cost/performance values of their designs. He is now directing business development in renewable energy services for Shermco Industries, a leader in maintenance and repair of wind energy electrical components and power systems.{/access}
The SCADA alarm flashes red as the blades slowly come to a stop. The site manager gets a call – what now? A generator is failing. Again. Sadly, this scenario plays out somewhere in North America every day of the year. Why is this happening and what can be done? Generators stop working for several reasons and, of course, it might be an unforeseen design flaw, but most failures are both predictable and preventable. With proper cause analysis combined with good planning and execution of maintenance procedures, the failure rates could be cut by almost 50%. In addition, serial defects due to non-optimised designs or poor installation can be managed and solved efficiently once discovered.By Kevin Alewine, Director of Renewable Energy Services, Shermco Industries, USA
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}Over the past five years, Shermco Industries has repaired over 1,200 generators ranging from 660kW to 3MW and, although the root failure causes vary, the rate of failures has not substantially declined with the larger turbine designs. A review of the collected data from these repairs has revealed that the occurrence of wind turbine generator failure types actually closely mirrors failure modes in more traditional rotating machines, just at a greater incident rate.
As shown in the charts (Figure 1), bearing failures are by far the most common failure point in wind turbine generators, but there are other important failures to consider as well, as they are less predictable. In the older, smaller generators more failures occurred due to design issues, most likely caused by the change from 50 to 60 Hertz applications, as well as the wide range of ambient temperatures experienced in the western deserts and the cold winters in the northern prairies. Higher rotor speeds and other mechanical stresses were certainly the root causes of most of the failures, and catastrophic rotor failures invariably destroy the stator windings and possibly other components. This type of failure is less likely to be predicted by condition monitoring equipment, especially if only temperature sensors are utilised.Larger turbines, ranging up to 3MW, have experienced different types of failures, including collector ring flashover, rotor lead failures and loose or damaged stator wedges. Although these different failure modes are encountered, the incident rate is about the same or higher than with smaller turbines. The larger generators, although appearing more mechanically robust, continue to be plagued by inadequate design and, often, poor installation. There are still failures related to rotor winding damage, but these are mostly based on severe over-speed conditions and not directly related to generator design or manufacturing.
Insulation systems must be adequate for the electrical stresses encountered during operation, but must also be durable enough for the mechanical and environmental challenges of the application. Wind turbines, by their very nature, are subjected to the vagaries of both vibration and the weather. Often, cost reduction initiatives have limited the amount and type of insulation materials utilised. As these are generally low voltage machines, there is often less emphasis put on the long-term durability of the coil design, coil insulation and choice of impregnating resins when compared to high voltage machines of similar size and output. Very few engineers would consider using these types of insulation systems for industrial machines, even for low voltage applications. Understanding this and bringing to bear a long history of repairing and rebuilding motors and generators for industrial applications, quality remanufacturing facilities can often produce generators with improved reliability compared to the original OEM designed system.
Now to the real culprit affecting generator reliability and availability: premature bearing failures. In the early years, there were many failures related to transient electrical currents induced in the rotor shaft. These currents would discharge though the ball bearings, creating destructive pitting at the interface with the bearing race. These transient voltages were mostly the result of having such a high current source available, sometimes aggravated by the high voltage spike effects of PWM (pulse width modulated) converters often used in conjunction with the generator, especially if they were attached over long cable runs to the base of the turbine. With the improvements in shaft grounding methods and the advent of ceramic hybrid and otherwise insulated bearings, this type of failure has become much less common.
The industry is now plagued by more traditional bearing failures caused by vibration, alignment and rotor balance issues and, most importantly, improper maintenance. While good rotor balance is normally the responsibility of the OEM or the remanufacturer, proper alignment during installation and verification at regular intervals are key maintenance points. Confirming and maintaining proper alignment is possibly more critical in wind turbines than in industrial applications due to the inherent instability of turbines and the difficulty of periodic inspections. A simple laser alignment check can save thousands of dollars in repairs and lost production.
A basic rule of rotating machinery maintenance is that either under or over-greasing ball bearings results in dramatically reduced life. The results of under-greasing are intuitive and obvious – increased friction, heating and rapid failure. Over-greasing is just as destructive, but in more subtle ways and is much more common. An overfilled bearing does not roll freely, causing a loss of efficiency. Additionally, the trapped excess grease can harden to a shellac-like substance making the problem very complicated to remedy. This also dramatically increases wear and can make it difficult for the old grease to exit when fresh is inserted. All of this often results in the excess grease migrating into the stator assembly where it can contaminate the windings. Even if the grease itself is not conductive, it can attract and hold dirt and other particulates that are both abrasive and hygroscopic. Often, auto-lubrication equipment is installed on generators. While generally that is seen as an improvement, over-lubrication is very common with automated equipment as the units must also be maintained and adjusted properly. With the growing popularity of permanent magnet generators and their promise of lower operating costs, maintenance issues will not magically disappear. These new designs will experience the same failure rate if not properly maintained, and the repairs are probably even more complicated.
Any repair that cannot be done uptower is costly. Cranes, technicians, transportation, and surprise developments that delay the procedures such as high winds and other inclement weather conditions all contribute to the expense. Proper maintenance is the key to controlling and predicting long-term repair costs in any industrial or utility environment, and wind energy is no exception.
The charts in Figure 2 show the common failure modes found in wind turbine generators, how often they occur and the size of the sampled groups. When compared to the results of a survey of rotating machines in the petrochemical industry developed by O.V. Thorsen and M. Dalva for the IEEE in 1995, the frequency of bearing failure is very similar. In that industry, the motors and generators are also difficult to maintain because of their locations, and this study helped illustrate the importance of good maintenance for that group.
So, what to do to assure maximum availability and reduce overall, long-term maintenance costs? The steps are the same as in any other application. Carefully review maintenance training and practices to assure proper planning as well as execution of the procedures. Invest in a maintenance team that can analyse the SCADA reliability information that is collected and give them the tools to do so. Consider expanding the data collection by utilising vibration monitoring or, at least periodic vibration surveys of the installed drive trains. Schedule maintenance as would any other utility, recognising that failures will happen; plan accordingly to minimise the surprises. Lastly, choose maintenance partner companies carefully. Whether for repairs in the field or remanufacturing facility, experience, quality programmes, tech support and financial stability are critical. Uptower repairs should comply as nearly as possible with the OEM specifications, and a properly remanufactured generator should meet or exceed the performance and reliability of a new machine. Keeping generators running for 20 years requires planning and investment for the long run, not just a short-term strategy to maximise the current financial period results. The goal is to keep them in the air, keep them turning, and if they must come down, make sure it is only once.
Biography of the Author
For over 30 years, Kevin Alewine assisted motor and generator companies with the evaluation and specifications of electrical insulation materials and systems to optimise the cost/performance values of their designs. He is now directing business development in renewable energy services for Shermco Industries, a leader in maintenance and repair of wind energy electrical components and power systems.{/access}
