Kjetil Uhlen

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.

Application of linear analysis for stability improvements in the Nordic power transmission system

This paper presents research work on stability assessment and stabilizer design that has been carried out for the Finnish transmission system operator, Fingrid. The aim of the work has been to assess observability and controllability of low frequency inter-area modes in the Finnish transmission grid, and to design stabilizing controls to improve damping of critical modes. Challenges in system modeling are described, and main results from linear analyses are presented and compared with nonlinear dynamic simulations. The results are seen to correspond well with real system observations. Modal analysis and nonlinear simulations are applied to verify design of power system stabilizers of some important generators in Finland. Similar methods are also used for retuning and verification of stabilizers on the Fennoskan HVDC link between Sweden and Finland. The stabilizer performance is demonstrated in terms of stability improvements. A main conclusion is that linear techniques can be of great benefit when designing damping controllers for large power systems. It is also illustrated, with the use of validated models, how linear tools can be used in operation planning for determination of power transfer limits.

Monitoring amplitude, frequency and damping of power system oscillations with PMU measurements

Since 2005, Statnett (the Norwegian transmission system operator), ABB and Sintef Energy Research, have worked together on designing, implementing and deploying a Wide Area Monitoring System (WAMS) completely integrated with Statnettpsilas SCADA system. This paper focuses on the application for on-line monitoring of amplitude, frequency and damping of power oscillations as implemented in the WAMS.The power transfer capability in the Nordic transmission system is at times limited by the risk of instability due to inter- area power oscillations. With the WAMS in operation detailed operational data is available to alert operators and support identification of potentially unstable system conditions. In the past, such power swings would often go by unnoticed, and therefore this application can be of critical importance. WAMS recordings from a recent system disturbance leading to temporary sustained power oscillations are presented and used to discuss the ability to provide early detection of power oscillations in terms of amplitude, frequency and damping of the dominant mode. The main conclusion is that the system is able to identify accurately the amplitude and frequency of the main oscillatory modes. Identification of damping is difficult in an on-line application, particularly when the magnitude of the oscillations is similar to that of random load variations. The present measurements, however, show that as the magnitude of these oscillations increase, the indicator on damping also becomes more reliable.

Control concepts to enable increased wind power penetration

This paper addresses problems encountered in planning and operation of wind farms in areas with favorable wind conditions, but long distances to load centers and relatively weak transmission systems. The paper presents an overview of several control problems and challenges to he considered in order enabling increased wind power penetration. This includes control objectives related to voltage/reactive power control and active power control, including management of network constraints. Realistic examples are provided to illustrate proposed solutions with respect to coordinated control of wind farms and their interactions with the existing transmission and generation system. Results from computer analyses of a simplified, yet realistic, electrical power system model are used to illustrate two possible control concepts to achieve increased wind power integration. These are reactive power control and coordinated generation control.

Coordinating power oscillation damping control using wide area measurements

In this paper, the potential benefits of a Wide Area Control System for coordinated power oscillation damping control is investigated for the Nordic power system, with an overall motivation to facilitate increased power transfer limits.

The Potential of Integrating Wind Power with Offshore Oil and Gas Platforms

Offshore wind technology has developed rapidly and an offshore wind farm has the potential to power nearby offshore platforms in the future. This paper presents a case study of integrating a 20 MW wind farm which addressed the theoretical challenges of integrating large wind turbines into a stand-alone oil and gas platform grid. Firstly, the operational benefits of the 20 MW wind power integration were quantitatively assessed with regard to the fuel gas consumption and CO2/NOx emissions reduction. Secondly, the electrical grid stability after integration of the 20 MW wind power was tested by nine dynamic simulations that included: motor starts, loss of one gas turbine, loss of all wind turbines and wind speed fluctuations. Thirdly, the maximum amount of the wind power available for integration was identified by simulating critical operational conditions and comparing these to the governing standards. Integration of an offshore wind farm to an oil and gas platform is theoretically possible, but has not been proven by this study and many other operational and economic factors should be included in future feasibility studies.

Wind power integration in isolated grids enabled by variable speed pumped storage hydropower plant

This paper presents aspects of control and operation of a variable speed pumped storage power plant for integration of wind power in an isolated grid. A topology based on a synchronous machine and a full scale back-to-back voltage source converter is suggested for variable speed operation of the pump-turbine. With this topology, variable speed operation can be obtained in both pumping mode and generating mode, and reactive current can be controlled independently of the active power flow and the operation of the pump turbine. By utilizing the controllability of the variable speed system, power fluctuations from wind turbines can be compensated to limit the influence on the rest of the grid. At the same time the pumped storage can be controlled to take part in the frequency control of the system and also to control grid voltage or flow of reactive power. The operation of the proposed system is illustrated by simulations based on the situation on the Faroe Islands, where controllable energy storage could allow for higher penetration of renewable energy in the power system and by that reduced dependency on power generation based on fossil fuels.

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.

Estimation of Eastern Denmark's electromechanical modes from ambient phasor measurement data

In this paper we report on the preliminary results of a collaborative investigation effort between researchers in North America and Europe aiming to baseline the electromechanical modes and mode shapes of the Nordic system from synchronized phasor measurement data. We provide an overview on the Danish power grid and its interconnections, and describe the features of an experimental phasor measurement unit designed by DTU which is used to provide the measurements used in this investigation. Parametric and non-parametric block processing techniques are applied to 48 hours of ambient data recorded at the Radsted and Hovegård substations in Eastern Denmark. The estimated modes are in agreement with those shown in eigenanalysis studies, and other measurement-based investigations. More importantly, the emergence of a new 0.8 Hz mode in Sjælland is reported.