Danish Power System Research Articles

The potential for arbitrage of wind and solar surplus power in Denmark

We have recently developed a simple yet powerful method to identify key properties of electricity systems with a high share of renewables. Here, our weather-driven methodology is described and applied to model the Danish power system with combined wind and solar energy gross shares of up to 100% of the total demand. We show that in a wind only scenario, surplus energy grows rapidly beyond gross shares of about 50%, while the potential for arbitrage of surplus renewable energy, i.e. demand-side management or high-efficiency storage, is very limited in this case. A scenario with a wind-solar energy mix of 80/20, on the other hand, both decreases the total amount of surplus and has a significantly higher potential for arbitrage of the remaining surplus. However, beyond gross shares of about 75%, only large-scale seasonal storage of, e.g. hydrogen, enables the use of Danish surplus wind and solar energy to cover the residual Danish electricity demand in both scenarios.

A Systematic Approach for Dynamic Security Assessment and the Corresponding Preventive Control Scheme Based on Decision Trees

This paper proposes a decision tree (DT)-based systematic approach for cooperative online power system dynamic security assessment (DSA) and preventive control. This approach adopts a new methodology that trains two contingency-oriented DTs on a daily basis by the databases generated from power system simulations. Fed with real-time wide-area measurements, one DT of measurable variables is employed for online DSA to identify potential security issues, and the other DT of controllable variables provides online decision support on preventive control strategies against those issues. A cost-effective algorithm is adopted in this proposed approach to optimize the trajectory of preventive control. The paper also proposes an importance sampling algorithm on database preparation for efficient DT training for power systems with high penetration of wind power and distributed generation. The performance of the approach is demonstrated on a 400-bus, 200-line operational model of western Danish power system.

Energy Comes Together in Denmark: The Key to a Future Fossil-Free Danish Power System

The transition of the Danish energy system to a system based only on renewable energy in 2050 carries many challenges. For Denmark to become independent of fossil energy sources, wind power and biomass are expected to become the main sources of energy. Onshore and offshore wind farms are expected to provide the majority of electricity, and biomass and electricity are expected to become the major sources of heating. On the way toward the 100% renewable goal in 2050, the Danish government has proposed a 2035 midterm goal to cover the energy consumption for power and heat with renewables.

Optimal Operation of Plug-In Electric Vehicles in Power Systems With High Wind Power Penetrations

The Danish power system has a large penetration of wind power. The wind fluctuation causes a high variation in the power generation, which must be balanced by other sources. The battery storage-based Plug-In Electric Vehicles (PEVs) may be a possible solution to balance the wind power variations in the power systems with high wind power penetrations. In this paper, the integration of plug-in electric vehicles in the power systems with high wind power penetrations is proposed and discussed. Optimal operation strategies of PEV in the spot market are proposed in order to decrease the energy cost for PEV owners. Furthermore, the application of battery storage based aggregated PEV is analyzed as a regulation services provider in the power system with high wind power penetrations. The western Danish power system where the total share of annual wind power production is more than 27% of the electrical energy demand is chosen as the studied case. The results show that an optimal operation of PEV in both spot market and regulation market can not only decrease the energy costs for PEV owners, but also significantly decrease the power deviations between West Denmark and Union for the Coordination of Electricity Transmission (UCTE) control areas.

Demand as Frequency Controlled Reserve

Relying on generation side alone is deemed insufficient to fulfill the system balancing needs for future Danish power system, where a 50% wind penetration is outlined by the government for year 2025. This paper investigates use of the electricity demand as frequency controlled reserve (DFR), which has a high potential and can provide many advantages. Firstly, the background of the research is reviewed, including conventional power system reserves and the demand side potentials. Subsequently, the control logics and corresponding design considerations for the DFR technology have been developed and analyzed, based on which simulation models have been built using the DIgSILENT Power Factory. The simulation studies of different scenarios confirm that the DFR can provide reliable performance of frequency control. Furthermore, relevant issues regarding implementing DFR in reality have been discussed.

On the synergy between large electric vehicle fleet and high wind penetration – An analysis of the Danish case

Increasing the level of wind power penetration beyond the present level in the Danish power system implies large challenges when it comes to energy management and system stability. Plug-in electric vehicles promise to contribute to the flexibility of the energy system by creating a link between the power system and the transportation sector and provide the possibility to make use of the inherent energy storage of a large electric vehicle (EV) fleet. The present work investigates the effects of different EV charging strategies on the balance between wind power production and consumption in a future Danish power system. The results show that an electrification of the transport sector will indeed reduce the excess of wind power, but additional mechanisms are needed if the full wind power potential in Denmark is utilized. Further it is foreseen that the vehicle-to-grid option (where the vehicle batteries are used as backup at times with little wind power production) will have very limited effects on the overall energy management and is more likely to be used only for regulation and reserve services, also in the longer perspective.

Integration of Vehicle-to-Grid in the Western Danish Power System

The Danish power system is characterized by a large penetration of wind power. As the nature of the wind power is unpredictable, more balancing power is desired for a stable and reliable operation of the power system. The present balancing power in Denmark is provided mostly by the large central power plants followed by a number of decentralized combined heat and power units and connections from abroad. The future energy plans in Denmark aim for 50% wind power capacity integration which will replace many conventional large power plant units. The limited control and regulation power capabilities of large power plants in the future demands for new balancing solutions like vehicle-to-grid (V2G) systems. In this paper, aggregated electric vehicle (EV)-based battery storage representing a V2G system is modeled for the use in long-term dynamic power system simulations. Further, it is analyzed for power system regulation services for typical days with high and low wind production in the Western Danish power system. The results show that the regulation needs from conventional generators and the power deviations between West Denmark and Union for the Coordination of Electricity Transmission (UCTE) control areas are significantly minimized by the faster up and down regulation characteristics of the EV battery storage.

Experience with voltage control from large offshore windfarms: the Danish case

AbstractThis paper gives an overview of the state of the art and lists future challenges to reactive power and voltage control in the Danish transmission system in relation to large offshore windfarms. Today, the reliable and stable operation of the Danish transmission system is based on the voltage and frequency control carried out at central, conventional power plants. Moreover, the control of some larger decentralized combined heat and power units is activated for voltage control and system balancing, which is specific for the Danish system. In the years to come, according to the government's goal of increasing the share of renewable energy sources in the Danish power system, the share of large offshore windfarms in the Danish power generation mix will increase greatly, replacing central power plants, including their control characteristics during periods of strong winds. Large offshore windfarms must therefore provide the transmission system with the necessary voltage and frequency control, e.g. ancillary services, and ensure secure operation of the power system through their contribution to system service. Danish experience, based on the operation of a system with two large offshore windfarms and several smaller ones, has shown that the efficient use of windfarms' reactive power and voltage control for the on‐land transmission system might be limited by several factors. Among such limiting factors are the reactive power and current capability limits of the electronic power converters and switchable capacitor banks of the offshore wind turbines, which are smaller than those of central power plants measured per unit of the active power rating. Combine this with the use of AC cables, tens of kilometres long, to connect the large offshore windfarms to the on‐land transmission system, the reactive power range available to the transmission system gets poor. The Transmission System Operator should already take such limiting factors and alternative solutions for efficient reactive power and voltage control, such as incorporation of a reactive power compensation unit at the on‐land point of connection or evaluation of a Voltage Sourced Converter‐High Voltage Direct Current instead of an AC connection, into consideration during the planning phase for a windfarm connection. Copyright © 2009 John Wiley & Sons, Ltd.

Simple Models for Model-based Portfolio Load Balancing Controller Synthesis

Abstract This paper presents a collection of models of so-called ‘effectuators', i.e., subsystems in a power plant portfolio that represent control actions with associated dynamics and actuation costs. These models are derived in order to facilitate higher-level model-based control synthesis of a portfolio of generation units existing in an electrical power supply network, for instance in model-based predictive control or declarative control schemes. We focus on the effectuators found in the Danish power system. In particular, the paper presents models for boiler load, district heating, condensate throttling and wind turbine effectuators. Each model is validated against actual measurement data. Considering their simplicity, the models fit the observed data very well and are thus suitable for control purposes.

Wind Power and Future Trading with Regulating Power

Regulating power is needed to compensate for unplanned deviations from production and consumption plans. The transmission system operator (TSO) in each country is responsible for keeping the electric system balanced, and activates regulation if needed to keep the frequency in the network stable. The need for regulations will likely grow when more wind power is built in the power system, as it is difficult to forecast wind power production and make accurate production plans. By using data on wind power prognosis errors from the West Danish power system, a need for regulating power in the Swedish system is predicted, with 4000 MW of new wind power installed. An attempt is made to forecast pricing on regulating power in Sweden based on the need for regulating power along with other aspects. The results show that the regulating power market energy turnover increases together with the monetary turnover following the large‐scale installation of wind power.

A Large Wind Power System in Almost Island Operation—A Danish Case Study

In Denmark, incorporation of wind power into the power system is increasing whereas power generation from the large conventional power plants is reducing. The common concern has become maintenance of reliable and stable operation of the Danish power system with a large amount of wind power - the large wind power system. At present, maintaining the voltage stability of such a large wind power system relies on strong ac connections to neighboring power systems with sufficient reactive power and voltage control and on control of central power plants in Denmark. This paper presents the results of a stability investigation carried out by the Danish transmission system operator (TSO), Energinet.dk, for the case of almost island operation of the Western Danish wind power system. The main ac lines connecting Western Denmark to the synchronous area of the Union for the Co-ordination of Transmission of Electricity (UCTE) were out of service and the wind power generation was high. The investigation has shown that the reactive power and voltage control of the domestic power plants is essential for maintaining short-term voltage stability of the Western Danish wind power system and becomes even more relevant when ac connections to the strong transmission system of the UCTE are out of service. The results presented in this paper may also be relevant for the operators of the power systems that are in island ac operation and with plans for significant commissioning of wind power - Great Britain, Ireland, Tasmania, etc.

Large penetration of wind and dispersed generation into Danish power grid

The present Danish power grid is characterised by large penetration of wind power and local combined heat and power (CHP) units. Today, there are two large offshore wind farms commissioned in the Danish power grid. Construction of two more offshore wind farms (430 MW) with grid connection to the Danish transmission power grid will be by the year 2009. The Danish power grid is undergoing a change from a classical power network with few centralised large power plants into a power network with a significant amount of wind power and dispersed generation. Presently, there must be at least three large power plant units in operation in Western Denmark to control voltage and frequency and maintain security of the power system operation. The technology change to wind power and dispersed generation implies that power supply and control from the large power plants – and, consequently, also ancillary services such as, e.g. voltage and frequency control capability – have to be reduced in favour of dispersed generation and wind. Such a change constitutes a challenge with respect to maintaining security of the power system operation in different operational situations, and creates a need for new solutions with voltage and frequency control and compliance with planned power balance between the Danish power system and the neighbouring areas. This article focuses mainly on issues with regard to short-term voltage stability of the power grid.

Synchronized Phasor Measurements of a Power System Event in Eastern Denmark

Two sets of synchronized phasor measurements 200km apart have been performed during a planned outage of a double 400-kV tie-line between Eastern Denmark and Southern Sweden. The interconnection between Eastern Denmark and Southern Sweden is comprised of a double 400-kV line and a double 132-kV line. The outage of the 400-kV tie-line weakened the Eastern Danish power system and excited power oscillations in the interconnected power systems. During this event prototype, phasor measurements units gave the opportunity of real-time monitoring of positive sequence voltage and current phasors using the satellite-based global positioning system. Comparisons between real-time recordings and results from dynamic simulations with Power System Simulator for Engineering are presented. The main features from the simulation analysis are successfully verified by means of the corresponding synchronized phasor measurements.

Fixed-speed active-stall wind turbines in offshore applications

A large offshore wind farm in the East Danish power system was commissioned in 2003 at Rodsand. The power capacity of the wind farm is 165 MW divided between 72 wind turbines. For this large offshore application, robust and well-known wind technology has been chosen in the form of fixed-speed, active-stall wind turbines equipped with induction generators. In this paper, maintaining and improving the short-term voltage stability is discussed and systematized in terms of this wind technology. Copyright © 2004 John Wiley & Sons, Ltd.

Nuclear power and coal-fired CHP

The competitiveness of nuclear power in a power system with a substantial amount of coal-fired combined heat and power (CHP) is discussed using the expectedstructure of the Danish power system at the beginning of the next century as an example. The system efficiencies for CHP production are analysed, the structure of two simulation models is described, and the net savings of each marginal nuclear unit are calculated. The results are presented as probability distributions of the net savings taking into account the uncertainties of selected parameters. The method may be used for other very capital intensive power producing technologies.

Potential for a Danish power system using wind energy generators, solar cells and storage

SYNOPSIS Some overall properties of a power system in a northern climate based mainly upon renewable energy are studied. The power system consists of wind energy generators and solar cells in combination with a storage system and a back-up system. The type of power plants constituting the back-up system are not specified. The back-up system could be used to generate power only when the rest of the system is unable to cover the load. The predicted performance of the solar/wind power system is calculated on the basis of hourly meteorological data measured in Denmark during the years 1959 to 1972. It has been established from this data that to maximise the fraction of annual load covered by solar and wind energy the ratio of solar to wind production capacity is approximately 40% solar to 60% wind. With only small changes in the energy provision, however, the ratio could be varied between 20:80 and 60:40. The saving in energy resulting from provision of various storage capacities has been studied. This saving...

A combined wind and hydro power system

A high degree of coverage of electricity demand by wind energy conversion is possible only with access to energy storage. For regions with both hydro and wind resources, the hydro power system and the wind power system may complement each other and alleviate the need for any other energy storage. Scandinavia is such a region, and this article uses a simulation model to assess the conditions under which all the electricity needs of Norway and Denmark could be satisfied by a combination of the existing Norwegian hydro power system with a hypothetical all wind-based Danish power system.