Industrial PhD programme
by Iván Arana Aristi
Defence:
”Switching Overvoltages in Offshore Wind Power Grids”
Date & time:
Wednesday, 29 February 2012,at 13:00
Location:
DTU, Building 306, Auditorium 38
Examiners
Professor Torbjörn Thiringer,
Chalmers University of Technology, Sweden
Senior Grid Planner, PhD, Torsten Lund,
Energinet.dk, Denmark
Associate Professor Tonny W. Rasmussen,
DTU Electrical Engineering, DTU, Denmark
Chairman
Associate Professor Bogi Bech Jensen,
DTU Electrical Engineering, DTU, Denmark
PhD thesis 
Period
December 2008 to June 2012
Principal Supervisor
Joachim Holbøll,
DTU Electrical Engineering, DTU, Denmark
Co-supervisors
Arne Hejde Nielsen,
DTU Electrical Engineering, DTU, Denmark
Troels Sørensen,
Dong Energy
Kim Høj Jensen,
Siemens Wind Power
Companies in cooperation
- Centre for Electric Technology (CET), DTU Electrical Engineering
- Electric Components (ELC), DTU Electrical Engineering
- DONG Energy A/S
- Siemens Wind Power A/S
Sponsors
- Danish Agency for Science Technology and Innovation
- DONG Energy A/S
Description
This project is expected to produce guidelines regarding the modelling of electrical equipment and methods to assess correctly the transient and temporary overvoltages known to appear in the collection grid of large offshore wind farms.
So far, there have been successful applications of large offshore wind farms in Denmark and the UK. However, the failure rate of components in offshore wind power plants is unpredictably high and several research projects focus on different aspects relating to this.
The theoretical focus of the project will be on understanding each electrical model, as well as investigating the causes of overvoltages and how overvoltages propagate through the collection grid. Both operational and topological aspects will be taken into consideration in order to identify critical overvoltages in the grid under different operating conditions.
A core subject will be the analysis of available measurements in offshore wind farms, with the aim to create digital models of the main electrical components in the collection grid. Special emphasis will be given to the models since it is very important to fully understand the program and the components in order to create a reliable, robust and sufficiently accurate model representation.
Once all the relevant components have been modelled, and there is an agreement between simulation results and measurements (model validation), transient and temporary overvoltage studies will be done in order to correctly estimate the potential risks. Then, based on these studies, protection schemes and systems will be evaluated in order to create the best technical solution. Finally practical guidelines regarding protection methods and required simulation studies at different stages of the life time of the wind farm (planning, construction and operation) will be created to ensure a more reliable protection system.
Transient overvoltages
Regarding the modelling of medium voltage and high voltage equipment for transient overvoltage studies / insulation coordination, there is fair amount of information and guidelines available in the literature about circuit breakers, transformers, cables, surge arrestors et.; this however tend to be mainly theoretical information, and in practice the information about these components is typically not available in sufficient detail for accurate insulation coordination studies. Furthermore, commercial simulation tools tend either to have limited facilities for this type of studies or require very specialized training and experience. In short, the credibility of this type of studies relies on the quality of the models and information that the user can get hold of, and even then, it depends on the simulation tools and the user’s skills. This creates additional concern and further work is necessary to design the collection grid of wind farm with the high reliability, needed in offshore conditions.
Temporary overvoltages
This type of overvoltages are gaining attention from wind farm owners and wind turbine manufacturers due to the large amount of sensitive power electronics in frequency converter wind turbines. The transmission system operators are also interested in this phenomenon, since situations in the grid connection system of offshore wind farms have occurred where the insulation systems have been stressed in situations never experience before. However, at the planning stage of a wind farm, the system designer has at best only a limited digital model from the wind turbine manufacturer in order to estimate the worst possible temporary overvoltage.
Since the overvoltage protection cannot be solved by one unique solution, three focus areas have been defined: (1) prevention of occurrences, (2) prediction of occurrences and (3) control of occurrences. These focus areas are not independent of each other, and they will be addressed during the entire project for both types of overvoltages.
- Prevention of occurrences. The system designer follows international insulation coordination standards to estimate the level of insulation required for the electrical equipment of the collection grid of offshore wind farms, at an early stage of a project. It is important to select an appropriate insulation system, based on an insulation coordination study in order to construct a reliable protection system. At the planning stage the switchgear, cables, park transformers, wind turbines and protection systems are designed and selected using results from simulation studies and the designer’s experience. Included in this Industrial PhD Project is the creation of guidelines to improve the selection of the equipment and the necessary studies to be realized before the construction process is started.
- Prediction of occurrences. Assessment of transient and temporary overvoltages in power systems are based on simulations, where the results depend on the simulation tool, the available information about the components, and the models used. This creates uncertainty since the characteristics of the models of the equipment are based on information that might not be sufficiently accurate for certain detail studies. As mentioned before one of the objectives of this project is to focus on the models of the electrical component in the collection grid, to fully understand each component and create appropriate models validated with measurements. At the end of the project, guidelines regarding how to model each component for accurate representation of transient and temporary overvoltages will be created. In this model guideline, the information that the developer is recommended to obtain from the component manufacturers will be addressed as well
- Control of occurrences. Once it has been established what are the insulation coordination properties and capabilities of the protection system in the collection grid (1) and it has been predicted what overvoltages can occur (2), then the next and obvious step is to address those occurrences that represent a risk. This focus area is aimed at controlling the occurrences by changing the system conditions, creating new protection methods and strategies in order to create a more secure and reliable system; either by adding or modifying protection devices, communication and measurements systems or control philosophies. There is a large difference between transient and temporary overvoltages, regarding their cause and the equipment at risk. Hence, the control strategies have to be tailored for each equipment, type of occurrence and operating condition of the system. As stated above, such new control strategies will be based on validated models of different components and systems. However it is the goal that at the end of the project, some of these new control strategies are to be designed, evaluated and proposed to be implemented and tested on a real wind power plant.
A this stage of the planning, some activities have been defined with relative high priority since they are expected to be very important in the overall project development. These activities are:
- Literature studies regarding current research, latest work and results in related areas
- Analysis of measurements realized in offshore wind farms
- Simulations of measurements realized in offshore wind farms
- An industrial experience in Siemens Wind Power or Siemens Power Transmission and Distribution
Contact
Joachim Holbøll, CET, DTU Elektro
Links
• ORBIT database
2009-02-23 iaa