The high penetration of wind energy in Spain has led us to consider, in this type of facility, certain aspects related to the quality of the power generated, their influence on the mains and behaviour in the continuity of supply to certain contingencies of network (voltage dips and micro-cuts).
SOTAVENTO GALICIA S.A., aware of this, has taken advantage of the benefits from our facilities to have different wind technologies implemented in the wind farm to conduct a comparative study of them from actual measurements in reference to the quality of waves and continuity of generation against disturbances.
The study was carried out by the Group of Electrical Engineering and Electrical Systems at the University of Vigo.
The objectives of this study include the following aspects in the present wind turbines at Sotavento Wind Farm
- Comparative study of wave quality depending on the type of turbine and wind farm level (power fluctuations, interharmonics)
- Simulation of the influence on the electrical grid (flicker, harmonics, voltage fluctuations …) of wind farms with different technologies. Comparison with the values obtained in the measurement step
- Study of energy efficiency of the wind farm and the various wind turbine technologies
- Analysis and simulation of wind turbines to electrical network disturbances (voltage dips, frequency variations, etc…)
- Investigation of the effects on the electricity mains grid that wind farms would have based on some of the existing technologies in Sotavento
- Estimation of the capacity of different technologies to suit the technical requirements of the regulations applicable at the time of project development
We have studied in detail eight wind turbine models that can be framed in five different technologies. The proposed studies go beyond the individual behavior of each machine, as it was evaluated using simulation, the behavior of composite wind farms for each one of these technologies, taking as reference the results obtained in the Sotavento machines.
The wind turbines that have been studied are:
- ECOTECNIA 44/640 kW
- 900 kW NEG MICON
- 600 kW IZAR
- MADE 660 kW
- 1,300 kW MADE: Asynchronous two speeds and fixed blade pitch
- 1,300 kW IZAR: Asynchronous two speeds and variable blade pitch (active stall)
- Gamesa G47/660 kW: Doubly fed induction generator and variable blade pitch
- MADE 800 kW: Synchronous generator and variable blade pitch
The proposed study has been divided into the following phases
- PHASE 0: Previous documentation
Stage preceding the study, knowledge of the Sotavento Wind Farm setting analysed. The most relevant data obtained at this stage were:
- Electrical and mechanical characteristics of wind turbines
- Characteristics of transformers and underground lines to the substation
- Substation characteristics
- Features of the lines of electrical energy evacuation
- In terms of its behaviour, it was necessary to know basic electrical parameters of the operation of the facility, either at the level of the machine and wind farm indifferent wind conditions:
- Historical wind conditions at the wind farm and machine level
- Active and reactive power levels at the wind farm and machine level
- Behaviour of the voltage at the various points of the installations
- PHASE A: Campaign and wave quality measures
The first consisted of carrying out a campaign of measures aimed at detailed aspects of wave quality and supply, both at wind farm and machine level. The purpose of this measurement campaign was divided according to the special characteristics of the measures to be under taken. On one side was the classic study of the parameters referring to the Spanish regulations (UNE-EN 50160:2001 “Voltage characteristics of electricity supplied by public distribution systems”). These measures covered aspects such as:
- Voltage variations (slow and fast)
- Rate of harmonic distortion
- Imbalance in the voltage supply
- Surges in voltage
The influence of the wind turbines on the electricity mains grid goes beyond the classical aspects of wave quality as those listed in the previous section, requiring the measurement of more variables. Below are listed some of these points:
- The random nature makes wind power fed into the grid follow a similar pattern. For this purpose it was necessary to superimpose effects like the tower shadow that causes low frequency oscillations superimposed on the random behaviour mentioned.
- The influence of power electronic equipment that endow the wind turbines are not only harmonic power but also inter harmonics, even at frequencies above those contained in the regulations at the time of project development. Therefore a device is needed capable of detecting high-frequency harmonics, which typically fall outside the bulk of the measurement range of the equipment.
- Doubly-fed wind turbines connected to the network or via an AC/AC link make a good demonstration of control of power delivered to the grid. Thus, fluctuations of the power injected into the grid are greatly diminished, as are the efforts to support mechanical components of wind variations. However, it was necessary to record the power output of the wind turbine and, in this case, the ability to achieve these purposes depended largely on the choice of appropriate control algorithms employed. All these aspects were of necessary consideration when it was time to study the influence on the electrical grid of continually growing wind energy.
The measurements to do this were based on a recorder that specifically allowed a sampling of high frequency and long duration of the wave forms at the output of the turbine. To this end, this project also envisaged the use of equipment developed at the Department of Electrical Engineering that covered these aspects. This campaign has included only those measures that were considered representative of turbines of a given technology.
- PHASE B: Study of energy efficiency
One of the fundamental aspects in any system of electric energy production is the evaluation of energy efficiency, defined as the degree of utilisation of energy resources that are available. The first aspect evaluated was the relationship between energy losses at the various points of its discharge, i.e., transformers, wires, lines, etc. At this point, the effect of the power converters on those technologies included was evaluated. This last aspect is of great importance because of the degree of market penetration of wind turbines with doubly fed asynchronous link or AC/AC. Another aspect is the comparative efficiency of production among different technologies in similar wind conditions. This is critical to all historical data information available at Sotavento, especially the detail of its synchronisation, whether it be the power fed into the grid or as the various wind measurements.
- PHASE C: Simulation of behaviour against electric disturbances
The continuing increase in the rate of saturation of wind power into the grid at the beginning of the century made it necessary to know its behaviour against normal disturbances. To this was added an increasing demand that wind farms guarantee some protection and contribute to maintaining the stability of the network. The behaviour against disturbances, and most importantly those related to voltage variations, is closely related to the technology used. Thus, to protect against a decrease of voltage, an asynchronous wind turbine reacts with intensities similar to those of direct starting of an engine, while for a doubly fed system, the more probable situation is for its disconnection to ultimately protect the electronic power. In this project an important simulation study was performed, not only establishing the behaviour of the Sotavento machines, but allowing it to predict, and therefore extrapolate the behaviour for a composite wind farm with technologically identical machines a soften occurs in wind farms different to Sotavento.In this phase the potential of the analyzed technologies were evaluated to suit the technical requirements from regulatory agencies in place at the time in which the project was developed, namely the RD 436/2004. In addition, we studied possible control strategies that would enable improved performance of wind turbines.
- PHASE D: Simulation of the impact on the grid
A complementary aspect to that mentioned in the previous section is the impact on the electricity mains grid having different wind turbine technologies. Tracking the overview of the conclusions, wind farms with various exclusive technologies with different wind and power conditions were both simulated, using the data obtained in the study of Sotavento Wind Farm. As for the electrical conditions, we have studied the influence of the characteristics of the connection point under current regulations.The above two phases (Phase C and Phase D) required a major effort of simulating the operation of the various wind turbine technologies. This involved the use of modeling, so it took a collection of both electrical and mechanical data of individual wind turbines, data available in the Sotavento Experimental Wind Farm.
The project was completed in January 2007.The results and conclusions regarding this study have been published in a report with the title of the project.
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