Wind Power

Comprehensive monitoring for challenging environments.

Taking a sample from a wind turbine is not the most straightforward task. The location is often remote and resources are limited.  Not only will you need results you can rely on, but also maximise information from every sample by including the patch test – a window into the wear particle and contamination levels. With a simple overview of the entire wind farm you can stay informed at a glance or drill down to histories of individual units using graphical trends and patch images to identify specific issues.

 

Service Categories

  • Package B
  • Wear Debris Analysis
  • Failure Investigation
  • Transformer Oils

This suite of tests provides a lot of detail about the condition of the oil and types of wear particles and contaminants present. Together with PQ Index and Elemental Analysis the Patch Test delivers a comprehensive overview of the wear situation.

Viscosity

Viscosity is a measure of fluids resistance to flow. Often defined as an ISO or SAE viscosity grade (such as VG 220 or SAE 15W/40). Typically measured at 40 or 100ºC. Each application will have a viscosity range suited to the task. Too high a viscosity can lead to lubricant starvation, wear in circulating pumps, increased temperatures and reduced efficiency. If viscosity is too low, the components will not be sufficiently separated resulting in excess friction and causing wear on the machinery. A change in viscosity could be due to:
  • Contamination with water/fuel/solvents/very small particles
  • Oxidation or ageing of the oil
  • Incorrect oil
  • Mechanical shearing of the oil

Acidity

Oil oxidises over time and becomes more acidic indicating the age of the oil. If the oil is too acidic it can damage metal components and further accelerate the ageing process. A rapid increase in the Acid Number may be due to:
  • Severe oxidation (often caused by overheating)
  • Depletion of additive package
  • Top up with a large volume of incorrect oil with a much higher base acidity e.g. hydraulic oil
  • Contamination with process fluids; or in case of engine oils with combustion products
 

Particle Quantifier Index (PQ)

  • A measure of total magnetic ferrous debris in the sample irrespective of particle size.
  • Does not detect non-magnetic ferrous debris e.g. rust.
  • Combine with Elemental Analysis and ISO Code for comprehensive assessment of the wear situation.

Elemental Analysis

Induction Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is used to measure the concentration of over 20 different elements in the oil. These include wear metals, additives and contaminants.

By monitoring wear metal concentrations the wear rate and its origin can be established. Trending additive levels ensures that the right oil is used and that it remains suitable to the task, while measuring levels of contaminants helps prevent severe wear and loss of function.

For grease and debris samples a Rotating Disk Electrode Optical Emission Spectrometer (RDE) is used. This eliminates a lot of cross-contamination issues and, as no dilution with solvents is required, allows for more accurate measurement of heavily contaminated samples which would settle at the bottom of the test tube if an ICP-OES was used.

 

Water Content

Excess water in the oil reduces the lubricating effectiveness by disrupting the oil film, accelerates corrosion (i.e. rusting of iron and steel surfaces), depletes and/or degrades additives and accelerates the aging (oxidation) of oil.  Where large quantities of water are present oil may become emulsified. The emulsions can combine with insoluble oxidation products to form sludge which impairs the operation and reliability of equipment. In addition excessive water if present as free water can promote bacteria growth or form hard deposits on bearing surfaces.

An increase in water content may be due to:

  • Leaking covers on equipment
  • Leaking oil coolers
  • Excessive leaking turbine gland steam seals
  • Condensation
  • Using water contaminated fluid for topping up
 

Particle Counting - Patch Test

Fluid cleanliness is particularly critical for hydraulic and turbine oils. High levels of particulates, especially if the particles are abrasive (e.g. silica), can increase wear of components and lead to reduced life and premature failure. It has been demonstrated that improving cleanliness by even a couple ISO Codes can lead to doubling of the expected life of a component. Conversely, contaminated lubricant will greatly reduce component lifespan and increase costs.

Fluid cleanliness is quantified by counting particles in prescribed ranges of particle sizes. It is typically expressed using an ISO 4406, a NAS 1638 or an SAE AS4059 cleanliness code.

There are several ways of obtaining the particle count with the most common being instrumental particle counting and the patch test method.

Instrumental Particle Count

Most instrumental particle counters relate a change in the amount of light (either visible or laser) transmitted through the fluid into a particle count using a stored calibration. When a particle flows between the light source and the sensor, the measured output drops and this is interpreted as a particle of a certain size. Some instruments scan over a particle and are able to capture its shape and outline. Other systems measure a pressure drop as the oil is passed through a series of sieves.

Patch Test

Another approach is to pass the fluid through a filter membrane and with the aid of microscopy to either count the deposited particles or perform a comparison with reference slides.

The advantage of the latter approach is that, as well as obtaining the ISO Code, the types of wear and contamination particles can be examined and captured, giving further insight into the types of wear or contamination. This method is also insensitive to air bubbles and water droplets, which can interfere with the readings of the instrumental particle counters. An example of such patches can be seen in the slideshow.

At STS we have invested in a new state of the art Leica motorised microscope and camera system with a range of illumination options. We are particularly excited about Darkfield Illumination - a way to light up the patch from all sides and get better definition of difficult slides and translucent contaminants in particular.

We are also enjoying the Z-stack feature - it combines in-focus areas from multiple images to achieve a single fully focused composite as shown below.

The overview/tile stitching facility lets us construct overviews of an entire membrane or ferrography slide. In the following overview we were able to use Dark Field illumination to successfully separate translucent particles from the woven 11µm membrane patch, which is also translucent.

The following overviews show differences in large (inner ring) and fine (outer ring) particle densities in two ferrography samples.

Analytical Ferrography is a technique for depositing and analysing wear particles contained in an oil or grease sample.

Analytical Ferrography

Analytical Ferrography is a technique for depositing and analysing wear particles contained in an oil or grease sample.

The sample is deposited onto a glass slide, with the particles trapped by strong magnets and the oil washed away with a suitable solvent. Both linear and rotary particle deposition systems exist. At STS preference is given to a rotary system, which has been developed at the company. It ensures good separation of particles over the three rings, with particles also being sorted by size with the larger particles settling out on the inner ring.

Once deposited the particles are analysed by a metallurgist, who is able to report on the relative quantities, types and sizes of particles present. A Particle Quantifier Index of the slide is also recorded. All of this is taken into account to produce a comprehensive report on the wear rate and situation.

You can find an example Ferrography Report here.

Filter Debris Analysis

Oil filters are essential to maintaining oil cleanliness and removing wear metals and contaminants. As they perform their function, however, they also remove some of the information about wear and contamination levels from the oil stream. This is then stored in the filter itself. Fortunately Filter Debris Analysis grants us access to this store of information. By extracting and analysing the entrained particles we can learn a lot about the wear situation or the source of contamination. Oil filters come in many shapes and sizes and how we treat an individual filter will depend on its size and construction. Typically a section of the outer cage is cut out and removed to enable access to the filter media. A section of the filter media is then removed and placed into a beaker. The beaker is then filled with solvent and an ultrasonic bath is used to agitate and extract the particles and any residual oil present. The solvent is then evaporated leaving the particulates and any residual oil. A portion of the debris is then deposited onto a filter membrane for microscopic analysis. The remaining oil/debris mixture is then analysed using a Rotating Disc Electrode Optical Emission Spectrometer to determine the elemental composition of the debris mixture. This gives a breakdown of the different wear metals and contaminants present. Where more detail is needed, the membranes containing the particles are analysed using a Scanning Electron Microscope with Energy Dispersive X-Ray facility. This provides information on the exact elemental composition of individual particles, which can then be matched to specific component metallurgies.

Analysis of the type, size, quantity and elemental composition of the wear debris can give vital clues to the failure mode. Examination and testing of the failed component can also help identify the root cause of failure.

Failure Investigation - Wear Debris

Depending on the failure mode (e.g. wear related or catastrophic failure due to overloading), there may be wear metal evidence present in the lubricant or in the oil filters. We are able to extract and analyse this wear debris through a collection of different analytical techniques. Extracted particles can be deposited on a filter membrane during the Patch Test or deposited on a glass slide to enable Analytical Ferrography. Elemental data can be obtained from bulk fluid analysis as well as from Scanning Electron Microscopy. The latter allows for individual particles to be interrogated and for data to be compared against machine metallurgy.  

Failure Investigation - Component Testing

In addition to wear debris analysis we are able to review failed components and engage a select group of partners to deliver a comprehensive analysis. Techniques available include the initial visual inspection, sectioning and metallographic analysis, hardness testing, elemental analysis via SEM-EDX, etc.

Just like lube oil analysis mentioned elsewhere a lot of value can be derived from transformer oil testing. The tests for dissolved gases and furfuraldehyde usually indicate the condition of the transformer whilst most of the other tests e.g. acidity, electric strength, fibres reflect the quality of the oil. Since it is not desirable to have any water present, that test can also indicate whether the dryers are functioning correctly.

Water Content (Oil Condition)

High water content will reduce the insulating properties of the oil, which may result in dielectric breakdown. It can cause breakdown of cellulose based paper insulations in the windings, together with accelerating corrosion.

Acidity (Oil Condition)

High acidity can result in corrosion and varnish deposits, together with degradation of oil and paper in the windings. Consequently, sludging can occur within the oil, thus reducing heat dissipation qualities of oil and causing overheating. Reduced insulating properties and an increase in water content normally go hand in hand with high acidity.

Electric Strength (Oil Condition)

Low electric strength indicates oil is no longer capable of performing the vital function of insulating under high electrical fields.  Poor electric strength is often linked with high water and fibre content.

Colour (Oil Condition)

An indication of level of oxidation and degradation of oil.

Fibres (Oil Condition)

Under high electric fields the behaviour of fibres can cause a dramatic reduction of insulating qualities of oil. Although invisible to the naked eye fibres are often introduced via poor maintenance techniques.

Polychlorinated Biphenyls (PCBs)

PCBs are insulating liquids used for their non-flammable properties, mainly in transformers, whether a fire would be unacceptable and as the dielectric in capacitates. Unfortunately the mineral oil used in transformers has become cross contaminated over the years. As PCBs are fairly non-biodegradable and tend to collect in food chains legislation was brought in to prevent wide-spread contamination. Liquids containing over 50 mg/kg must be classed as injurious substance and disposed of via costly high temperature incineration. It is therefore essential those responsible for plant know their level of contamination and have strict controls on oil movements to site.

Dissolved Gas Analysis (DGA) and Buchholz Gas Analysis

This is the single most important test performed on transformer oil and is used to determine the concentration of certain gases such as nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene and acetylene. The concentration and relative ratios of these gases can be used to diagnose certain operational problems and incipient faults in transformers, which may or may not be associated with a change in physical or chemical property of the insulating oil. For example, high level of carbon monoxide relative to other gases may indicate thermal breakdown of cellulose paper while hydrogen in conjunction with methane may indicate a corona discharge. Acetylene is considered a significant gas generated as it is formed in breakdown of oil at temperatures in excess of 700oC and can indicate a serious high temperature overheating fault.  

FFA (Furfuraldehyde)

This is a measure of the degradation of cellulose paper in the windings i.e. as the paper ages its degree of polymerisation (DP) reduces and thus so does its strength as it becomes more brittle. The DP of paper can directly related to the concentration of furan derivatives in the oil which are formed as a direct result of the breakdown of the polymeric structure of cellulose paper. New paper has a DP rating of 1250 while at 250 the paper is sufficiently brittle to fall away from the windings. It is therefore possible to use FFA to estimate the used life of the transformer as well as which will be dependent on factors such stresses on plant, overheating etc. FFA formation can also indicate low temperature overheating.

Metals in Oil

Elemental data helps to pinpoint the origins of faults for example iron and copper can be indicative of arcing between a copper contact and iron core.