MEC&F Expert Engineers : CAUSE AND CONTRIBUTING FACTORS OF FAILURE OF GEARED WIND TURBINES, Part 1

Thursday, July 27, 2017

CAUSE AND CONTRIBUTING FACTORS OF FAILURE OF GEARED WIND TURBINES, Part 1

Wind turbine on fire

The geared wind turbines continue to be plagued by numerous gearbox (more accurately the bearings within the gearbox), blade, mechanical, weather-related (e.g. lightning), design and maintenance issues.

Just yesterday Siemens announced that the wind projects are not viable without heavy government subsidies. The failure of the bearings located within the gearbox is the most significant problem associated with the turbines This year, Siemens, a main manufacturer of wind turbines, reported a charge of 48 million Euros for inspecting and replacing defective main bearings in some onshore wind turbines. Structural and mechanical failures (which can result in a tower collapse) are primarily due to control system errors and lack of effective maintenance.

If it was not for the government subsidizing of these systems, they would have never been built that way. This blog addresses the major causes of failure of wind turbines.

Wind turbines generate electricity through the wind-induced rotation of two to three aerodynamic blades located around a rotor. The rotor is connected to the main shaft that is connected to a generator that in turn spins to create electricity. We have seen wind turbines smaller than 100 kW and as large as 6,000 kW. The wind turbines can generate electricity to run a single piece of equipment (e.g. a water irrigation pump) at a particular facility or can produce electricity for sale to a power grid.

The annual wind turbine capacity of the United States continues to grow exponentially. This growth is fueled by investment tax credits, federal goals of mandatory generation of electricity from renewable sources, rising energy demand, and other factors.

The biggest wind turbine manufacturers include General Electric, Vestas, Siemens, Clipper, Mitsubishi, Suzlon, Alstom, and Gamesa.

Crews lift a blade assembly onto the nacelle of Gamesa's G9X-2.0-megawatt turbine at the NWTC.

The main components of a wind turbine system are:

· Tower, made from tubular steel, concrete, or steel lattice;

· Foundation for the turbine’s tower, nacelle and rotor blades;

· Nacelle; it houses the mechanical, electrical, electronic and other components of the turbine;

· Rotor Blades that rotate and cause the rotor to spin;

· Transformer;

· Generator that produces AC electricity from mechanical (rotational) energy; usually an induction generator;

· Rotor; it is formed by the blades and the hub;

· Brake that stops the rotor;

· Tail, but not all turbine types have tails;

· Gearbox that changes between the low and high gear shaft to increase the rotational speed to 1,000-1,800 rpm;

· Electronic control panel ;

· Shafts (low and high speed) connecting the rotor to the generator

· Blade power control system (controls the blade pitch and the yaw);

· Anemometer (wind speed control) and wind vane (wind direction control)

Anatomy of a wind turbine

A commercial, utility-scale wind energy generation facility typically consists of tens to hundreds of wind turbines capable of generating hundreds of megawatts of renewable energy. In addition to the wind turbines, other facilities associated with a wind farm project include access roads, temporary crane paths, underground power collection lines, aboveground generation tie lines (gen-tie), collector substations, interconnection switch yard, several permanent and temporary meteorological towers, and operations and maintenance building.

A 6,000-kW offshore wind turbine

The useful life of these wind turbines is supposed to be 20 years. However, the turbines have been plagued by numerous problems and this 20-year life is rather a utopia than anything else. Gearboxes and bearings in wind turbines, more than those in any other application, tend to fail prematurely. In fact, at some wind projects, up to half of all bearings inside the gearboxes fail within a few years.

There are several reasons for this, including the poor understanding of gear functioning during storms and gusty winds, relative immaturity of the technology and industry, the rapid evolution of turbines to extra-large sizes, poor understanding of turbine loads, and an emerging (and largely unexplained) failure mode in turbine bearings called axial cracking.

Axial cracking in the gearbox of the wind turbine

The fact that the manufacturers of the turbines provide only a two-year warranty, is pretty good evidence of the reliability of this technology at this time. For example, the catastrophic gear box failures appear to be caused primarily by induced mechanical voltage straying through the gearbox, pitting the bearings. This has happened, in some cases, within 18 months after the turbine was placed into service.

Insurance for Wind Farms and Turbines

Like any piece of complex machinery operating under stress, turbines can fail. They break. They develop faults. They are improperly constructed. They are improperly maintained, and so on. And without the right care and protection policy in place, the resultant claim can quickly spiral out of control. For the owner and investor, this can lead to lost revenue and operational downtime; at worse, it means absorbing an increasingly daunting repair bill. We outline the top causes for turbine failure and explain what to do when things go wrong.

Wind farm insurance packages can include: construction insurance, physical damage, and third-party liability insurance coverage for delays in building of a wind farm, loss of earnings, and business interruption once the operation is running. Specifically, wind turbine coverage can compensate the policyholder for production losses if the wind farm’s annual wind levels fall below forecast.

Based on our investigations, we list below the most commonly encountered causes of wind turbine failures:

· Bearings and Gearbox issues – this is the Achilles Heel of the Gear-Driven Turbine

· Lightning strikes

· Blade design, manufacturing and installation issues

· Mechanical Breakdown (generators and transformers, burning the windings due to overspeed, etc.)

· Hydraulic failures

· Wind turbine and wind farm electric systems

· Grid failures

· Nacelle fire

· Improper handling during transportation, construction and improper assembly

· Human error(s) in O&M, construction and design

· Turbine collapse

· Natural Catastrophic events

· Yaw motor events

· Poor O&M arrangements

· Axial Stress

· Foundation damage

· Icing

· Accumulation of bugs, dirt and other debris

· For offshore turbines, the power converter suffers from high failure rates

We will address these failure modes below.

Fires

In a typical year we expect to see total losses – typically caused by a fire – whereby the unit can no longer be repaired and is declared a total loss. In these instances, the most common causes are internal component failure or a buildup of material in lubricants. This can start an escalating spiral of sequential events and a rather spectacular – if not expensive – mechanical fire.

Fires are the second highest cause of catastrophic turbine failure

Extreme Weather

In occasional circumstances, extreme weather is also responsible for failure – whereby the wind speed and the elements simply become too much for the engineering dynamics of the machine. Brakes fail, blades seize up and the chain of events continues to make things worse.

Root Causes of Generator Failure

The main root causes of failure of generators at wind turbine sites include but are not limited to:

· Failure to follow recommend maintenance practices regarding the lubrication procedures, collector systems, etc.; mechanical or electrical failure of bearing, rotor lead failures, cooling system failures leading to excessive heat and fire.

· Lightning strikes, wind loading, weather extremes, lubricant contamination, thermal cycling, etc.

· Misalignment and other improper installation, excessive vibration, voltage irregularities, convertor failure, improper grounding, overspeed that results in burning of the windings, etc.

· Manufacturing and/or design failures, such as, loose components (wedges, banding), inadequate electrical insulation, transient shaft voltages, poorly designed/crimped lead connectors, rotor lead failures, the presence of other components inside the nacelle that complicate service, etc.

· For generators that are less than 1,000-kW, the most common failure mode is damage to rotor, following by stator, bearings, collector rings and miscellaneous generator failures. For generators that are between 1,000- and 2,000-kW, the most common failure mode was associated with the bearings, followed by collector rings, rotor, stator, cooling system, rotor leads and miscellaneous failure modes. For generators that were greater than 2,000-KW, the most common failure mode was associated with bearings, followed by stator, stator wedge, rotor, rotor leads, collector rings and miscellaneous failures.

Maintenance is the critical factor affecting machinery life. Proper repairs are also critical to the reliability and longevity of the turbine generator.

Damage to the generator windings due to over-speed and subsequent overheating of the windings

Then there are the gearbox and blade lightning strikes.

Again, these create a spectacular display but also a spectacularly large loss – with the resultant damage often requiring either extensive turbine down time and a complex replacement or repair.

Blade Failure Modes

The main causes of failure of turbine blades include: lightning strikes, foreign object damage, poor design, material failure, power regulator failure. A combined thermal and stress analysis of a lightning strike model of typical wind turbine blade material (including E-glass composite layups) shows that the fiberglass material immediately surrounding the lightning attachment location becomes damaged due to plastic deformation. Depending on the magnitude and number of lightning strikes, the blade has the potential to fail under an extreme static gust load, under fatigue, or a combination of the two.

Turbine blade damaged by lightning strike.

Accumulations of bugs, oil, and ice on the blades will also reduce power as much as 40%. Regular cleaning of the blades has become a maintenance requirement. Included in the hours of down time for cleaning the blades is ice built-up when the ice causes the airfoil shape to be changed and the turbine cannot produce power.

Broken wind blade

Bearing Failures - The Achilles Heel of Geared Wind Turbines

No matter what type of turbine model, mainshaft bearings are common failure points. The main reason: spherical roller bearings are not the optimal bearing configuration. Large amount of radial internal clearance (RIC) are needed to facilitate original bearing assembly at the manufacturer. Although this simplifies the assembly at the turbine OEM, this clearance is not well suited for handling axial loads. The proper bearing configuration would be a pre-loaded tapered roller bearing, but this would have increased both turbine assembly time, as well as purchased cost.

As a result of this configuration, the thrust from the wind causes the mainshaft to move axially towards the gearbox until the clearance has been absorbed, hence unseating the upwind row while the downwind row now sees the majority of the load.

The primary failure mode of the mainshaft bearing is macropitting (spalling) of the downwind race. The unloaded upwind row will then skid and skew as result of having no roller tractions, creating a second failure mode of micropitting. The micropitting is also called ‘grey staining’ or ‘frosting’.

This consists of microscopic cracks only a few microns deep (about .0001 inches). Individually these cracks are too small to be visible. As they accumulate they appear as grey stains on the roller surface. Eventually the bearing roller starts to shed its cracked and weakened surface losing a small bit of its precision tolerance. Furthermore, this contaminates the oil with microscopic super hard steel particles most of which are too small to be filtered out. Why does grey staining begin? Typically it is a breakdown of the oil film that separates the rollers from the races.

There are many opinions in the public domain summarizing common indications of specific operating conditions in conjunction with premature failures in wind turbine applications:

· periods of heavy and dynamic loads/torques – leading to vibrations and rapid load changes (e.g. transient raceway stress exceeding 3.1 GPa, heavy loads of 15,000 per year, impact loads). Such transient events can include: grid loss, high wind shutdowns, wind gusts, curtailments, control malfunctions, generator short circuits, resonant vibration, misc emergency stops. Although these reversals are infrequent, they can be severe.

· depending on turbine type, additional radial and axial forces by the rotor, axial motion of the main shaft – leading to dynamical loading, higher stresses of gearbox components especially in the first stage

· occasional connecting and disconnecting of the generator to/from the power grid – leading to torque reversals and bouncing effects (which e.g. can lead up to 2.5-4 times higher nominal torque and impact loads)

· rapid accelerations/decelerations and motions of the gearbox shafts

· misalignment, structural deformations (nacelle hub, housings)

· lubricant compromise between needs of gears and bearings as well as between low and high speed stages, insufficient oil drains and refill intervals

· harsh environmental conditions – possible large temperature changes and consequently larger temperature differences between the bearing inner ring and housing than expected when starting up, dust, cold climate, moisture and salt water (especially for off-and near-shore turbines)

· idling conditions – leading to low load conditions and risk of skidding damage (adhesive wear) and wear in the low running stage

· conflicting design needs, e.g. increasing rolling element size will increase the load capacity but simultaneously increase the risk for cage-and roller slip and sliding damage

As stated above, bearings may fail due to other reasons not covered by best practice standards and from other industrial experiences.

Statistical evaluations of onshore and offshore wind turbines indicate clearly a correlation between failure rate, wind speed and heavy and fluctuating loads.