Dynamic Rotor Balancing

Dynamic Rotor Balancing

Our vibration analysis deter-mines the mass and aero-dynamic imbalance and will lead to a significant reduction of your wind turbine vibrations and fatigue level.

Benefits of
Dynamic Rotor Balancing

Increase of efficiency
Increase of energy production
Increase of lifetime and return on investment
Reduction of vibration and fatigue
Reduction of damages and down-time
Reduction of cost of energy


Rotor Vibration Analysis

Determination of mass and aerodynamic imbalance


A well-balanced rotor, as well as accurately adjusted blade angles are fundamental requirements for the optimal operation of wind turbines, through their positive impact on energy and life cycle costs. Early and periodic imbalance checks and corresponding technical counter measures help to optimise availability and yield. This also effectively prevents a reduction of turbine lifetime caused by rotor imbalance.

Expenses for periodic rotor balancing usually amount to a mere fraction of the costs associated with the repair and yield loss caused by rotor imbalance. There are two different primary causes of rotor imbalance, namely mass imbalance and aerodynamic imbalance.

Mass Imbalance is caused by uneven mass distribution along the blades.

Aerodynamic Imbalance is a consequence of differing aerodynamic pro-perties between rotor blades, often caused by blade angle deviations.

Often, rotors exhibit both types of imbalance in combination. Statistics from long-standing imbalance measurements on more than 2000 turbines show that on average 4 out of 5 turbines are affected by rotor imbalance beyond acceptable limits

Mass imbalance as well as aerodynamic imbalance cause an unwanted increase in turbine vibrations and thus are one of the main causes of dynamic problems of today's wind turbines. With years of experience and extensive research BerlinWind can reliably detect both types of imbalance by analysing their vibrational signature and initiating appropriate technical counter measures.

For state-of-the-art imbalance assessment our engineers have years of experience measuring not only axial and lateral nacelle vibrations, but also tower top torsional vibration. This allows aerodynamic imbalance to be reliably identified and eliminated and hence, prevents the falsification of mass imbalance measurements as a consequence of the superposition of aerodynamic effects.

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Figure left: Mass imbalance causes nacelle vibrations in axial and lateral directions. Thanks to a sophisticated 3-point sensor system, aerodynamic imbalance can be reliably identified in the complex 'mixture' of vibrations | Figure right: Impact of successively taken measures for vibration reduction of a wind turbine on level of axial, lateral and torsion vibrations.

After correcting the aerodynmic imbalance by adjusting blade angles, which is usually done while our engineers are on-site to perform vibration-based validation for quality control, the mass imbalance is determined. For this purpose, several defined states of mass imbalance are generated by attaching test weights to the rotor. The vibrational signature for each state is recorded while the turbine is in operation.

Given the results of several measurement runs for different imbalance levels, the magnitude and position of the mass imbalance are determined and validated. For quality control, a final vibration measurement is recommended to validate the final counter weights mounted on the rotor. Based on the measurement results, BerlinWind gives specific and comprehensive recommendations for appropriate technical measures to ensure safe, profitable and low-wear operation of the turbine.

Our in-house developed measurement systems and procedures combined with our experience conducting measurements on more than 110 different wind turbine types, ranging from kW, up to and exceeding 10 MW, form the basis upon which suitable measurement procedures for reliable assessment of new turbine types with complex control systems can be developed.

Since 2013 BerlinWind has been developing and publishing quality criteria for reliable rotor balancing, e.g. based on the international standard on in-situ balancing of large rotors (DIN ISO 21940-13:2013). We stand for high-quality vibration measurements and high diagnostic safety in the challenging field of turbine rotor balancing and participate in related committees, e.g. for the guideline VDI 3834 on vibration measurement of wind turbines, in which part 1-2015 containes an annex on rotor balancng.

Brochure (PDF)



Blade Angle Measurement

Blade Angle Measurement

Our high-precision, optical measuring method (camera- or laser-based) allows us to determine blade angle misa-lignments (relative/absolute) with an accuracy of 0.1°.

Benefits of
Blade Angle Measurement

Increase of aerodynamic rotor performance
Increase of energy production
Increase of return on investment
Reduction of vibration and fatigue
Reduction of damages and down-time
Reduction of cost of energy
Reduction of noise emissions


Measurement of relative/absolute blade angle deviation

high-precision optical measuring method

with an accuracy upt to 0.1 degree


Having correctly adjusted blade angles is a fundamental requirement for the optimal operation of a wind turbine. By precisely measuring and adjusting blade angles, operating loads can be reduced which helps to avoid damage and extend service lifetime as the aerodynamic performance of the rotor improves. Not only availability but also yield and operating costs can be optimised. It is also possible to reduce noise emissions by fine tuning the aerodynamics. When assessing blade angles it is important to differentiate between relative and absolute blade angle deviations, though many rotors exhibit both types.

Relative Blade Angle Deviation means the 3 blades exhibit differing angles relative to each other, such that they deliver different torque and thrust to the hub, which increase operational loads. This is a common cause of dynamic problems and damages. Considering ever increasing rotor diameters, tower heights and light-weight design, the damaging impact of even small relative blade angle deviation continues to grow.

Absolute Blade Angle Deviation means that one or more of the blades deviate from the manufacturer' design value. This adversely affects aerodynamic performance and impacts on production which may cause significant yield losses. Moreover - for larger absolute deviation - aerodynamic noise, blade vibration and asymmetric rotor loading may increase significantly, especially if the stall effect occurs.

In most cases, both types of deviation occur together. Statistics from longstanding imbalance measurements on more than 2000 turbines show that 3 out of 4 turbines exhibit blade angle deviations beyond acceptable limits. During optical blade angle measurement each blade's angle is measured individually at one or more radii to detect and quantify relative blade angle deviations (between the blades). By comparing the vaalues with the manufacturer's design value, the absolute blade angle deviation as well as twist error (deviation of angle distribution along the blade) is quantified.

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Figure: Relative blade angle errors of more than 0,3° cause a significant aerodynamic imbalance. Already 0,1° yield increasing vibrations, especially at large rotors.

Using high-precision optical measuring methods (ground-based camera or laser scanner on nacelle or heli-deck), BerlinWind is able to determine blade angle errors with an accuracy of 0.1° (deg). This precision allows for the best possible reduction of vibrations. Our measurements show that relative blade angle errors of more than 0.3° (deg) already lead to a distinct aerodynamic imbalance. For modern Multi-MW turbines with large hub heights, significant foundation cracks may be caused by blade angle deviation of more than 2° (deg) within less than five years, sometimes even structural issues.

Based on the measurement results, BerlinWind gives tangible and specific recommendations for action in order to ensure a through lowered vibration a safe, profitable and long operation of the turbine.

Brochure (PDF)



Camera-Based Measurement

Camera-Based Measurement

Our camera-based methods allow us to precisely measure movements of turbine struc-tures (tower/nacelle/blades) with high-resolution, sub-pixel accurate motion tracking.

Benefits of
Camera-Based Measurement

Simultaneous motion tracking at several locations
Simple setup without sensor installation in nacelle
Validation of prototype simulations
Root Cause Analysis


High-Precision / Sub-Pixel Accuracy

Ground-Based / Blade Root-Mounted Cameras

Natural Frequency / Modal Measurement

Tower-Blade Clearance


Since 2012 BerlinWind has been developing and successfully deploying camera-based systems to precisely measure the movements of the turbine structure.

In order to increase yield and reduce costs, blades and towers of modern wind turbines have become taller, lighter and slenderer than ever before. These developments, in using new materials and simulation techniques is set to leave established practices behind and break new technological ground. During the turbine certification process, simulation and validation of the turbine type's design is required to prove that critical dimensions (e.g. tower-blade clearance), parameters and further design assumptions are realistic and thus the safe operation of turbine throughout its planned service life can be expected.

For the safe and reliable operation of every serial wind turbine, these dimensions and parameters must of course lie within the defined limits. This ensures that lifetime consumption as well as operation and maintenance costs are kept to a minimum, even under continuous operation in remote areas. High precision, efficient and safe measurement systems are required to detect and avoid increased operational loads, e.g. due to resonance problems with respect to the natural frequency of the tower, or through elevated levels of rotor imbalance and blade angle deviation.

The camera-based measurement systems developed by BerlinWind are exceptional tools suited to validating simulations, as a supplement to load measurements and for detecting vibrational issues. This is due to their high displacement resolution, sub-pixel movement tracking accuracy and simple assembly without needing to install sensors in the nacelle. These systems deliver positions, movements/displacements and angles directly, avoiding the need to integrate and post-process acceleration signals. BerlinWind offers the following camera-based measurement procedures:

Ground-Based Cameras
- Tower clearance measurements
- Tower and blade natural frequency measurements
- Tower and blade modal measurements
- Nacelle vibration measurements for rotor imbalance check
- Displacement measurements for root cause analysis
- Calibration of strain gauges for load measurements

Blade Root-Mounted Cameras
- Measurements of blade twist change and blade deflection
- in operation, at stand-still and during blade tests


Ground-Based Camera Measurements

Horizontal displacement (movement) of the tower, nacelle and blades can be precisely measured with a camera mounted on the ground and directed upwards. One or more cameras can be quickly positioned near the base of the tower and the turbine vibrations recorded as video or sequence of images. Depending on the application, we evaluate the turbine movements at one or more defined locations using our in-house developed movement tracking and postprocessing software. To do this, existing visible parts of the structure are selected for tracking, or markers are placed in agreed locations.

Typical applications are :
- Nacelle vibration measurement for rotor imbalance check
- Vibration measurement for root cause analysis
- Tower and blade natural frequency measurement
- Tower blade modal measurement
- Cost-effective calibration of strain gauges for load measurement

Benefits :
- Resolution in millimetres, even for marking position at 100m of height
- Simultaneous measurements at multiple positions
- Simple measurement setup at ground level
- No ascent or installation of equipment required
- Robust algorithm to correct for yaw effects
- Direct measurement of position and displacement, instead of acceleration

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Figure left: Measurement of nacelle vibrations through displacement tracking of visible elements on the underisde of the nacelle (labelled TP1 and TP2) | Figure right: Measured nacelle displacement during emergency stop.

Using high-precision optical measuring methods (ground-based camera or laser scanner on nacelle or heli-deck), BerlinWind is able to determine blade angle errors with an accuracy of 0.1° (deg). This precision allows for the best possible reduction of vibrations. Our measurements show that relative blade angle errors of more than 0.3° (deg) already lead to a distinct aerodynamic imbalance. For modern Multi-MW turbines with large hub heights, significant foundation cracks may be caused by blade angle deviation of more than 2° (deg) within less than five years, sometimes even structural issues.

Based on the measurement results, BerlinWind gives tangible and specific recommendations for action in order to ensure a through lowered vibration a safe, profitable and long operation of the turbine.


Tower-Blade Clearance Measurement

Tower-blade clearance – the distance between the blade tip and the face of the tower – must conform to the design value at stand-still and must not exceed the limit value provided during certification, in order to avoid a collision between blade and tower. This measurement serves initially to validate the design simulation, but further periodic measurements in operation can also provide an indication of blade structural fatigue.

For tower-blade clearance measurements a high-quality camera is positioned at ground level, side-on to the rotor plane at a safe distance. This measurement, depending on the assignment, can be performed during operation or at stand-still. The sub-pixel accuracy is achieved using specially developed software.

Benefits :
- Resolution in centimetres for tower clearance of approximately 20 m
- Simple configuration at ground level, at a safe distance from the tower
- No ascent or installation of equipment required
- Robust algorithm to correct for yaw effects
- Safe measurement position

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Figure left: In-situ measurement of distance between tower and blade during passage | Figure right: Time-resolved evaluation in [mm] range of measurement


Measurement of blade twist and deflection in operation

Blade twist as well as edge-wise and flap-wise deflection are measured by cameras mounted at the blade root, which therefore rotate and pitch with the blade during operation. Simultaneous results for marked blade sections are provided by specially developed Tracking and Post-Processing Software.

This measurement procedure provides valuable results to validate simulations and allows the visualisation and investigation of various influences, such as gravity (self-weight), wind shear, thrust variation, gusts, blade passage or manoeuvres. Furthermore, this procedure can also be used in blade testing.

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Figure: Rotor blade with 8 markings and cameras mounted at the root | Figure small: Flap-wise and edge-wise blade deflection as a function of time [from 0s to 160s].

Benefits :
- Simultaneous measurement of blade twist and deflection in flap- and edge-wise directions
- Simultaneous results at multiple blade radii
- Camera secured with sufficient clearance to the blade root (rotating observer)
- Avoid interference from pitching and yawing
- no problems with very high blade tip speed (up to 300 m/s) relative to the ground
- Large number of recorded images per rotation
- Continuous measurement over a number of days, remote access monitoring
- Resolution depending on radius in the range of millimetres to centimetres (at blade tip)
- Tip deflection beyond 20 m at a distance of more than 100 m can be measured with a resolution of approx. 10 mm using tracking with sub-pixel accuracy – even finer for closer radii

Brochure (PDF)



Drive-Train Balancing

Drive-Train Balancing

Our in-house measuring sys-tem allows us to assess the level and location of mass imbalance (single- or multi-plane) at the fast shaft side. Generator rotor balancing.

Benefits of
Drive-Train Balancing

Reduction of vibration and fatigue
Reduction of drive-train and generator damages
Increase of generator service life after up-tower repair


Single or multi-plane balancing of the high speed shaft side.

Generator rotor balancing e.g. after repair.


Mass imbalance of the drive-train's high speed side with brake, coupling and generator can cause significantly damaging vibrations. Even small imbalance levels need to be balanced due to the much higher rotational speed, e.g. uneven brake abrasion is a relevant issue. For generators on solid (concrete) foundations, e.g. the international rotor balancing standard DIN ISO 21940-11 provides the relevant balancing quality grades. However, the wind is a fluctuating driving force most nacelle main frames are a more flexible base, so this step-by-step in-situ balancing requires a high quality level (see also DIN ISO 21940-13).

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Figure left: Correction masses at the back side of rotor | Figure right: Polar plot of rotor speed ramp with passage of numerous natural frequencies of drive train .

BerlinWind has developed a system to assess the level and location of mass imbalance along the high speed shaft side. The system consists of the BerlinWind BalancingBox with the related proprietary specialist measurement and analysis software. Three acceleration sensors are installed, in the planes of the two generator bearings and at the gearbox housing close to its shaft outlet. The additional optical rotor speed sensor is imperative according to the above standards, in order to determine exactly by order analysis the rotor imbalances as well as the correction masses and their locations to reduce imbalance below to the required balancing quality grade.

Multi-plane balancing has been successfully deployed in the field, e.g. after a significant generator rotor repair up tower of a turbine type for which replacement generators are no longer available. Furthermore this system can extend the service-lifetime of a generator by reduced vibration.



Load Measurement

Load
Measurement

Our engineers measure and analyse the actual structural loads on the tower and com-ponents. Additionally we can compile and assess all rele-vant load spectra.

Benefits of
Load Measurement

Root Cause Analysis
Comparison between design and field load spectra
Wide range of measurement devices available
Load validation for lifetime extension


Measure and Analyse structural loads on tower and components

Compile and asses load spectra


Measuring and analysing the loads exerted on the tower structure can provide an important comparison between design loading and actual loading experienced by the turbine at the specific site. This can be accomplished by installing an array of strain gauges at critical loading sections on the tower, which collect data over a certain scope-related period to include a range of wind conditions.

In parallel further data can be measured, e.g. acceleration data collected during vibration measurements can be twice integrated to obtain equivalent displacement data. In conjunction with data from additional sensors or operational data, equivalent load spectra of the tower-nacelle system under the operation conditions present during measurement can be compiled, classified and analysed. This demonstrates, e.g. the significant load impact of blade angle deviation.

Tailored short- and long-term measurement campaigns with more than 100 sensors of various types, duration of several years and with up to 1 GB data per day were accomplished, for root cause analysis load validation (e.g. after turbine component modification, retro-fitting or structural changes). It is also feasible to assess the real turbine's load relevant for lifetime and impact of measures for load reduction taken, e.g. in the course of lifetime extension.

Brochure (PDF)



Technical Consulting

Technical Consulting

BerlinWind offers independent expertise on technical con-sulting and root cause anal-ysis. We have excellent know-ledge of all types of turbines and its structure mechanics.


The impressive growth of the wind energy sector over the last decades has not only led to an increase in wind turbine size but also to new turbine concepts and a more complex design in general. Today, more and more technical issues occur during wind turbine operation. These are among others due to shorter design cycles (due to market pressure and competition) and smaller safety factors chosen in course of the design (saving of material).

BerlinWind offers independent expertise and precise root cause analysis for arising technical issues. We serve our clients with well directed troubleshooting on problematic turbines and clear advices for optimisation or further development of wind turbines. BerlinWind’s staff has excellent knowledge of all types of wind turbines especially in the areas of rotor dynamics and structural mechanics. Our experts have been involved in many international projects, helping our clients not only to identify the root causes of technical issues but to develop individual and sustainable solutions for technical issues of wind turbines.

Brochure (PDF)



Contact us

Your questions are important to us. For further information, please do not hesitate to call us or send us an email. Whether you have a question about our services, pricing or your project's status, our team is ready to answer your questions and inquiries. We will get in touch with you as soon as possible.

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