John Olav Tande

Large-scale wind power integration and voltage stability limits in regional networks

When planning and developing large-scale wind power plants in areas distant from the main power transmission system, voltage control assessments and reactive power compensation are increasingly important. Voltage stability of the regional network may be a main limitation with respect to maximum rating and operation of the wind power plant Technical constraints in relation to wind power integration in weak grids may in general be associated with limited thermal capacity in parts of the grid and/or the adverse effect wind power can have on voltage quality and stability. In certain situations, however, local constraints regarding development of new transmission lines or upgrading of existing lines can make it interesting to utilise the existing lines to a level which in worst case may imply operation beyond the normal technical constraints of the system. In this work, challenges and opportunities arising from situations as described above are analysed, and viable measures to enable secure and acceptable operation of large wind farms in remote areas close to the thermal capacity and stability limits of the power system, are pointed out. The paper presents results from computer analyses of a simplified, yet realistic, electrical power system with wind power integration, illustrating possible solutions to achieve this.

Multivariate Analysis of Transformer Resonant Overvoltages in Power Stations

In this paper, cable-transformer resonant overvoltages are studied on the low-voltage (LV) side of a 410-MVA generator step-up (main) transformer, due to ground fault and energization. The transformer is placed in a network configuration often found in Norwegian hydropower stations. The overvoltages are calculated when systematically varying the length of the high-voltage (HV) side cable and that of the cable between the main transformer LV side and the station transformer. This multivariate analysis is performed efficiently, using frequency-domain calculations and the inverse Numerical Laplace Transform. Time-domain simulations are used for analyzing the critical cases in detail. Both types of calculations make use of a blackbox, wideband model of the transformer, and novel procedures are introduced for initializing computations from correct 50-Hz initial conditions. Among the findings are that extreme overvoltages can occur on the station transformer if current-limiting reactors are placed in series with the cable between the main transformer LV side and the station transformer.

Integration of Wind and Hydropower Systems: Results of IEA Wind Task 24

In May of 2004, the IEA Wind Implementing Agreement (IA) established R&D Task 24, “Integration of Wind and Hydropower Systems.” Australia, Canada, Finland, Norway, Sweden, Switzerland, and the United States joined Task 24 with the goal of collaborating in the study of wind integration in a variety of electrical system configurations (load, generation, and transmission); hydro system configurations and characteristics; and market and operational configurations. Representing these countries were utilities and research organizations with the intent to understand the potential for and limiting factors in integrating wind into systems with hydropower. Case studies that analyze the feasibility, benefits, detriments, and costs of specific wind-hydro integration projects were the mechanism through which the goals of the task were addressed. The purpose of this article is to summarize the framework within which these studies were performed, and to present the key results and the general conclusions of the Task.

Cost Analysis Case Study of Grid Integration of Larger Wind Farms

This paper assesses the connection of a large wind farm to a regional power system with a weak link to the main transmission grid. The study demonstrates that utilization of modern wind turbine technology and automatic generation control schemes allow operation of large wind farms in weak grids. In the case study considered of a regional power system, the viable wind farm capacity may be increased from 50 to 200 MW.

Options for large scale integration of wind power

This article demonstrates options for large scale integration of wind power. Two cases are considered. One is considering a connection of a large wind farm to fairly week regional grid, and the other is considering the power system balancing of large magnitudes of wind power. It is demonstrated that local control actions enables quite large wind farms to be operated at fairly week grids, and that marked based balancing copes well with large magnitudes of wind power.

Balancing of Wind Power Variations Using Norwegian Hydro Power

This paper addresses the role of Norwegian hydro power to provide balancing power to a future wind dominated European power system. Two power market models, one simplified and one detailed are used to model possible responses of Norwegian hydro power to a wind driven exchange pattern for various amounts of exchange capacity. The case analysed assume a 2030 scenario for wind generation in Europe and an increase in exchange capacity between Norway and Europe from 2300 MW to 5800 MW. We find that the generation constraints and the exchange capacity, and not the aggregated reservoir size, are the most important limiting factors for the amount of balancing the Norwegian hydro power system can provide.

Assessment of Power Quality Characteristics of Wind Farms

In this paper the main parameters to assess the power quality of grid embedded wind farms are presented. International standards to assess and quantify the power quality of grid connected wind turbines exist for some years now, and are here extrapolated to wind farms aggregates when possible being the correspondent methodologies identified in the document. Recently, the grid code requirements posed a novel challenge to this technologic area, particularly since they were issued with national or local objectives and without particular normalized global concerns. The form how the international standards are evolving in order to cope both with the power systems industry local requirements, but also with the global wind turbine manufacturers principles is addressed in the paper.

Connection scheme for north sea offshore wind integration to UK and Norway: Power balancing and transient stability analysis

In recent years there have been plans to develop large scale offshore wind farms in the North Sea. Dealing with variability of power from such wind farms is an important issue and should be addressed properly in the project developments. An HVDC connection between the offshore wind farm and Norway grid enables the offshore wind farms to benefit from the large hydro generation capacity of Norway for power balancing services. In this paper a connection scheme for an offshore wind farm in the Dogger Bank area, which makes use of the Norwegian hydro capacity for power balancing, is studied. It is demonstrated with PSCAD simulation how such interconnection can help in dealing with power fluctuations from the wind farm.

Fault Ride-Through Testing of Wind Turbines

Grid codes concerning low-voltage fault ride-through capabilities of wind turbines are not (yet) harmonized, varying from country to country. Full-scale field testing of wind turbines with respect to all such codes may therefore not be practical. IEC 61400-21 is currently under revision, and the last committee draft (CD 2006) presents a standardized physical test for characterizing a wind turbines response to a voltage-dip. The results of such physical tests may be used to validate numerical models of the turbine, which in turn can be used for modelling grid code compliance assessment of particular country codes. Success with such modelled validation will provide confidence in numerical simulations. The hypothesis of this paper is that the use of numerical models, that have been validated against the standardized tests, can reduce the number of physical full-scale tests needed. This paper assesses to what degree a validated simulation model is capable of predicting the fault ride-through capabilities of a fixed speed induction generator. This is done by comparing measurements on a wind-turbine emulator in a laboratory with predictions from a validated simulation model of the laboratory emulator. Simulations and laboratory measurements show excellent agreement. The validated simulation model accurately predicts the fault ride-through capability of the direct grid connected induction generator. It is concluded that the proposed methodology is promising and thus has the potential to increase the efficiency of grid code assessments. Full-scale field tests will still be required to validate the modelling completely, but using numerical models reduces the number of field tests needed.

Planning and Operation of Large Offshore Wind Farms in Areas with Limited Power Transfer Capacity

At many locations with excellent wind conditions the wind farm development is hindered by grid issues. Conservative assumptions are often applied that unnecessarily limits the wind power installation. This paper shows that significantly more wind power can be allowed by taking proper account of the wind power characteristics and facilitating coordinated power system operation. A systematic approach is developed for assessing grid integration of wind farms subject to grid congestions. The method is applied to a case of connecting offshore wind farms to regional grid with hydro generation (380 MW) and loads (75–350 MW). The tie to the main grid is via a corridor with limited capacity (420 MW). With conservative assumptions (i.e. no changes in scheduled hydro generation or control of wind power output) the wind power installation is limited to 115 MW. The system operation is simulated on an hourly basis for multiple years taking into account the stochastic variations of wind speed and hydro inflow as well as the geographical distribution of wind farms. The simulation uses a control strategy for coordinated power system operation that maximises wind penetration. By using the developed methodology the wind power capacity can be increased from 115 MW to at least 600 MW with relatively little income reduction from energy sales compared to a case with unlimited grid capacity. It is concluded that coordinated operation allows for the integration of surprisingly large amounts of wind power. In order to realize the increase in transfer capability, it is essential to take account of the power system flexibility and the stochastic and dispersed nature of wind power. The presented methodology facilitates this and represents a rational approach for power system planning of wind farms.