The ever increasing progress of high-voltage high-power fully controlled semiconductor technology continues to have a significant impact on the development of advanced power electronic apparatus used to support optimized operations and efficient management of electrical grids, which, in many cases, are fully or partially deregulated networks. Developments advance both the HVDC power transmission and the flexible ac transmission system technologies. In this paper, an overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided. Selected key multilevel converter topologies are presented. Control and modeling methods are discussed. A list of VSC-based HVdc installations worldwide is included. It is confirmed that the continuous development of power electronics presents cost-effective opportunities for the utilities to exploit, and HVdc remains a key technology. In particular, VSC-HVdc can address not only conventional network issues such as bulk power transmission, asynchronous network interconnections, back-to-back ac system linking, and voltage/stability support to mention a few, but also niche markets such as the integration of large-scale renewable energy sources with the grid and most recently large onshore/offshore wind farms.

VSC-Based HVDC Power Transmission Systems: An Overview

This paper reviews the power electronic applications for wind energy systems. Various wind turbine systems with different generators and power electronic converters are described, and different technical features are compared. The electrical topologies of wind farms with different wind turbines are summarized and the possible uses of power electronic converters with wind farms are shown. Finally, the possible methods of using the power electronic technology for improving wind turbine performance in power systems to meet the main grid connection requirements are discussed.

A Review of the State of the Art of Power Electronics for Wind Turbines

http://distributedwind.org/assets/2013/05/Integrating-distributed-generation-into-electric-power-systems.pdf

Integrating distributed generation into electric power systems: A review of drivers, challenges and opportunities

The paper reviews different approaches, technologies, and strategies to manage large-scale schemes of variable renewable electricity such as solar and wind power. We consider both supply and demand side measures. In addition to presenting energy system flexibility measures, their importance to renewable electricity is discussed. The flexibility measures available range from traditional ones such as grid extension or pumped hydro storage to more advanced strategies such as demand side management and demand side linked approaches, e.g. the use of electric vehicles for storing excess electricity, but also providing grid support services. Advanced batteries may offer new solutions in the future, though the high costs associated with batteries may restrict their use to smaller scale applications. Different “P2Y”-type of strategies, where P stands for surplus renewable power and Y for the energy form or energy service to which this excess in converted to, e.g. thermal energy, hydrogen, gas or mobility are receiving much attention as potential flexibility solutions, making use of the energy system as a whole. To “functionalize” or to assess the value of the various energy system flexibility measures, these need often be put into an electricity/energy market or utility service context. Summarizing, the outlook for managing large amounts of RE power in terms of options available seems to be promising.

https://research.aalto.fi/files/6557604/lund_et_al_review.pdf

Review of energy system flexibility measures to enable high levels of variable renewable electricity

(1990). ENERGY FUNCTION ANALYSIS FOR POWER SYSTEM STABILITY. Electric Machines & Power Systems: Vol. 18, No. 2, pp. 209-210.

Energy function analysis for power system stability

The increasing penetration of variable-speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will require changes in the way the grid frequency is controlled. In this letter, a method is proposed to let variable-speed wind turbines emulate inertia and support primary frequency control. The required power is obtained from the kinetic energy stored in the rotating mass of the turbine blades.

/pdf/wind-turbines-emulating-inertia-and-supporting-primary-13qg1bzk50.pdf

Wind turbines emulating inertia and supporting primary frequency control

This paper presents a technical and economic analysis to evaluate the benefits and drawbacks of grid connecting offshore wind farms through a dc link. A first case, concerning a 100-MW wind farm, is thoroughly investigated and cases of larger wind farms (200 and 500 MW) are presented. Three different transmission solutions are compared: 150-kV ac, 400-kV ac, and high-voltage dc based on voltage sourced converters (VSC-HVDC). After a brief overview of the features of these connection solutions, the related operational aspects are evaluated. An economic assessment compares the dc connection option to the ac alternatives, taking into account the investment, operation, and maintenance costs, and the negative valorization of losses and energy not supplied. Economic assessment includes sensitivity analyses of parameters, which could impact the 100-MW wind farm: distance, component costs, dc converter reliability, and dc converter losses

HVDC Connection of Offshore Wind Farms to the Transmission System

This paper proposes a new simulation method that can fully assess the impacts of large-scale wind power on system operations from cost, reliability, and environmental perspectives. The method uses a time series of observed and predicted 15-min average wind speeds at foreseen onshore- and offshore-wind farm locations. A Unit Commitment and Economic Dispatch (UC-ED) tool is adapted to allow for frequent revisions of conventional generation unit schedules, using information on current wind energy output and forecasts for the next 36 h. This is deemed the most faithful way of simulating actual operations and short-term planning activities for a system with large wind power penetration. The problem formulation includes ramp-rate constraints for generation schedules and for reserve activation, and minimum up-time and down-time of conventional units. Results are shown for a realistic future scenario of the Dutch power system. It is shown that problems such as insufficient regulating and reserve power-which are typically associated with the variability and limited predictability of wind power-can only be assessed in conjunction with the specifics of the conventional generation system that wind power is integrated into. For the thermal system with a large share of combined heat and power (CHP) investigated here, wind power forecasting does not provide significant benefits for optimal unit commitment and dispatch. Minimum load problems do occur, which result in wasted wind in amounts increasing with the wind power installed

/pdf/impacts-of-wind-power-on-thermal-generation-unit-commitment-5cdpcdyu3w.pdf

Impacts of Wind Power on Thermal Generation Unit Commitment and Dispatch

The impact of large scale wind power generation on power system oscillations

The use of energy storage for increased operational flexibility is commonly regarded as a logical complement for systems with large amounts of wind power. The authors explore, the opportunities for energy storage for the integration of large-scale wind power into a future lay-out of the Dutch generation system, for which minimum-load problems are foreseen with high wind power penetrations. A central unit commitment and economic despatch model is extended with models for three large-scale energy storage technologies: pumped hydro accumulation storage (PAC), underground PAC and compressed air energy storage. Furthermore, an alternative solution is investigated, comprising the installation of heat boilers at selected combined heat and power locations (CHP) in order to increase the operational flexibility of these units. Results are shown for different wind power penetrations and scenarios. A cost-benefit analysis shows that the operation cost savings from energy storage increase with the amount of wind power installed. Taking into account the large investment costs, energy storage units are however unlikely to have a profitable exploitation. The installation of heat boilers at CHP locations is found to be more efficient and a promising solution for the integration of large-scale wind power in the Netherlands. A notable result is that for the Dutch system, the use of energy storage increases the system-s overall CO 2 emission levels because energy storage allows storing power from cheap coal plants for substitution of expensive gas during peak. Even though often proposed as a solution for wind power integration, energy storage in fact partly annuls CO 2 emission savings by wind power.

Wil L. Kling

Papers

Wind turbines emulating inertia and supporting primary frequency control

HVDC Connection of Offshore Wind Farms to the Transmission System

Impacts of Wind Power on Thermal Generation Unit Commitment and Dispatch

The impact of large scale wind power generation on power system oscillations

Integration of large-scale wind power and use of energy storage in the netherlands' electricity supply