Peter Tavner

Comparison of direct-drive and geared generator concepts for wind turbines

The objective of this paper is to compare five different generator systems for wind turbines, namely the doubly-fed induction generator with three-stage gearbox (DFIG3G), the direct-drive synchronous generator with electrical excitation (DDSG), the direct-drive permanent-megnet generator (DDPMG), the permanent-magnet generator with single stage gearbox (PMG1G), and the doubly-fed induction generator with single-stage gearbox (DFIG1G). The comparison is based on cost and annual energy yield for a given wind climate. The DFIG3G is a cheap solution using standard components. The DFIG1G seems the most attractive in terms of energy yield divided by cost. The DDPMG has the highest energy yield, but although it is cheaper than the DDSG, it is more expensive than the generator systems with gearbox

An Industry-Based Survey of Reliability in Power Electronic Converters

A questionnaire survey was carried out to determine the industrial requirements and expectations of reliability in power electronic converters. The survey was subjective and conducted with a number of high-profile semiconductor manufacturers, integrators, and users in the aerospace, automation, motor drive, utility power, and other industry sectors. According to the survey, power semiconductor devices ranked the most fragile components. It was concluded that main stresses were from the environment, transients, and heavy loads, which should be considered during power electronic system design and normal operation. This paper has also highlighted that there is a significant need identified by the responders for better reliability-monitoring methods and indicators.

Condition Monitoring for Device Reliability in Power Electronic Converters: A Review

Condition monitoring (CM) has already been proven to be a cost effective means of enhancing reliability and improving customer service in power equipment, such as transformers and rotating electrical machinery. CM for power semiconductor devices in power electronic converters is at a more embryonic stage; however, as progress is made in understanding semiconductor device failure modes, appropriate sensor technologies, and signal processing techniques, this situation will rapidly improve. This technical review is carried out with the aim of describing the current state of the art in CM research for power electronics. Reliability models for power electronics, including dominant failure mechanisms of devices are described first. This is followed by a description of recently proposed CM techniques. The benefits and limitations of these techniques are then discussed. It is intended that this review will provide the basis for future developments in power electronics CM.

Cost-Effective Condition Monitoring for Wind Turbines

Cost-effective wind turbine (WT) condition monitoring assumes more importance as turbine sizes increase and they are placed in more remote locations, for example, offshore. Conventional condition monitoring techniques, such as vibration, lubrication oil, and generator current signal analysis, require the deployment of a variety of sensors and computationally intensive analysis techniques. This paper describes a WT condition monitoring technique that uses the generator output power and rotational speed to derive a fault detection signal. The detection algorithm uses a continuous-wavelet-transform-based adaptive filter to track the energy in the prescribed time-varying fault-related frequency bands in the power signal. The central frequency of the filter is controlled by the generator speed, and the filter bandwidth is adapted to the speed fluctuation. Using this technique, fault features can be extracted, with low calculation times, from direct- or indirect-drive fixed- or variable-speed WTs. The proposed technique has been validated experimentally on a WT drive train test rig. A synchronous or induction generator was successively installed on the test rig, and both mechanical and electrical fault like perturbations were successfully detected when applied to the test rig.

A Multilevel Modular Converter for a Large, Light Weight Wind Turbine Generator

In an onshore horizontal axis wind turbine, generator and converter are usually in the nacelle on the top of the tower, while the grid step-up transformer is placed at the bottom. Electric power is transmitted down through flexible cables of high current rating which are expensive and can suffer from large I2 R loss. An offshore wind turbine usually has to include the step-up transformer in the nacelle. This adds significantly to the mechanical loading of the tower even new designs aim to reduce the transformer size and weight. In either case, a transformer-less, high voltage, high reliability generating unit for nacelle installation would be an attractive technology for large wind turbines. This study presents a power electronic solution based on a permanent magnet generator design. A multilevel cascaded voltage source converter is developed to synthesize a high sinusoidal output voltage. The dc link voltages of inverter modules are balanced by rectifiers fed from isolated generator coils while the inverter switching strategy equalizes the power sharing between the modules. The switching strategy also reduces the low order harmonics to constrain the sizing of the dc link capacitors. The modulating effect between the ac and dc sides of the inverter is taken into account. This paper describes the generator-converter arrangement, analyzes the inverter switching effects and derives the switching strategy which is verified by simulation and laboratory experiment.

Coordinated control of an HVDC link and doubly fed induction generators in a large offshore wind farm

Doubly fed induction generators (DFIGs) are an economic variable-speed solution for large wind turbines while high-voltage dc (HVdc) transmission is being considered for the grid connection of some offshore wind farms. This paper analyzes the need for coordinating the control of the DFIGs and the HVdc link so that the two topologies can work together, giving system designers and operators a choice that may be useful in some applications. It is desired that individual generators be controlled for power tracking in a way similar to that used when they are connected directly to an ac grid, although a grid voltage reference for the DFIG control is no longer available as an independent source in this case. The study shows that machine control should explicitly maintain the flux level, which then allows the HVdc link to regulate the local system frequency and, indirectly, voltage amplitude. Interactions between DFIGs and the HVdc link are investigated and simulations performed to verify the proposed control strategy.

Condition Monitoring of the Power Output of Wind Turbine Generators Using Wavelets

With an increasing number of wind turbines being erected offshore, there is a need for cost-effective, predictive, and proactive maintenance. A large fraction of wind turbine downtime is due to bearing failures, particularly in the generator and gearbox. One way of assessing impending problems is to install vibration sensors in key positions on these subassemblies. Such equipment can be costly and requires sophisticated software for analysis of the data. An alternative approach, which does not require extra sensors, is investigated in this paper. This involves monitoring the power output of a variable-speed wind turbine generator and processing the data using a wavelet in order to extract the strength of particular frequency components, characteristic of faults. This has been done for doubly fed induction generators (DFIGs), commonly used in modern variable-speed wind turbines. The technique is first validated on a test rig under controlled fault conditions and then is applied to two operational wind turbine DFIGs where generator shaft misalignment was detected. For one of these turbines, the technique detected a problem 3 months before a bearing failure was recorded.

Reliability analysis for wind turbines with incomplete failure data collected from after the date of initial installation

Reliability has an impact on wind energy project costs and benefits. Both life test data and field failure data can be used for reliability analysis. In wind energy industry, wind farm operators have greater interest in recording wind turbine operating data. However, field failure data may be tainted or incomplete, and therefore it needs a more general mathematical model and algorithms to solve the model. The aim of this paper is to provide a solution to this problem. A three-parameter Weibull failure rate function is discussed for wind turbines and the parameters are estimated by maximum likelihood and least squares. Two populations of German and Danish wind turbines are analyzed. The traditional Weibull failure rate function is also employed for comparison. Analysis shows that the three-parameter Weibull function can obtain more accuracy on reliability growth of wind turbines. This work will be helpful in the understanding of the reliability growth of wind energy systems as wind energy technologies evolving. The proposed three-parameter Weibull function is also applicable to the life test of the components that have been used for a period of time, not only in wind energy but also in other industries.

Monitoring wind turbine gearboxes.

Concerns amongst wind turbine (WT) operators about gearbox reliability arise from complex repair procedures, high replacement costs and long downtimes leading to revenue losses. Therefore, reliable monitoring for the detection, diagnosis and prediction of such faults are of great concerns to the wind industry. Monitoring of WT gearboxes has gained importance as WTs become larger and move to more inaccessible locations. This paper summarizes typical WT gearbox failure modes and reviews supervisory control and data acquisition (SCADA) and condition monitoring system (CMS) approaches for monitoring them. It then presents two up-to-date monitoring case studies, from different manufacturers and types of WT, using SCADA and CMS signals. The first case study, applied to SCADA data, starts from basic laws of physics applied to the gearbox to derive robust relationships between temperature, efficiency, rotational speed and power output. The case study then applies an analysis, based on these simple principles, to working WTs using SCADA oil temperature rises to predict gearbox failure. The second case study focuses on CMS data and derives diagnostic information from gearbox vibration amplitudes and oil debris particle counts against energy production from working WTs. The results from the two case studies show how detection, diagnosis and prediction of incipient gearbox failures can be carried out using SCADA and CMS signals for monitoring although each technique has its particular strengths. It is proposed that in the future, the wind industry should consider integrating WT SCADA and CMS data to detect, diagnose and predict gearbox failures.Copyright

An industry-based survey of reliability in power electronic converters

A questionnaire survey was carried out to determine the industrial requirements and expectations of reliability in power electronic converters. According to the survey, power semiconductor devices ranked the most fragile components. It was concluded that main stresses were from the environment, transients and heavy loads, which should be considered during power electronic system design and normal operation. Further analyses suggest that power device reliability is a key issue and power electronic converter design is correlated with failure costs.