Jacob Østergaard

Advanced Control Strategy of DFIG Wind Turbines for Power System Fault Ride Through

This paper presents an advanced control strategy for the rotor and grid side converters of the doubly fed induction generator (DFIG) based wind turbine (WT) to enhance the low-voltage ride-through (LVRT) capability according to the grid connection requirement. Within the new control strategy, the rotor side controller can convert the imbalanced power into the kinetic energy of the WT by increasing its rotor speed, when a low voltage due to a grid fault occurs at, e.g., the point of common coupling (PCC). The proposed grid side control scheme introduces a compensation term reflecting the instantaneous DC-link current of the rotor side converter in order to smooth the DC-link voltage fluctuations during the grid fault. A major difference from other methods is that the proposed control strategy can absorb the additional kinetic energy during the fault conditions, and significantly reduce the oscillations in the stator and rotor currents and the DC bus voltage. The effectiveness of the proposed control strategy has been demonstrated through various simulation cases. Compared with conventional crowbar protection, the proposed control method can not only improve the LVRT capability of the DFIG WT, but also help maintaining continuous active and reactive power control of the DFIG during the grid faults.

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.

Oscillatory Stability and Eigenvalue Sensitivity Analysis of A DFIG Wind Turbine System

This paper focuses on modeling and oscillatory stability analysis of a wind turbine with doubly fed induction generator (DFIG). A detailed mathematical model of DFIG wind turbine with vector-control loops is developed, based on which the loci of the system Jacobians eigenvalues have been analyzed, showing that, without appropriate controller tuning a Hopf bifurcation can occur in such a system due to various factors, such as wind speed. Subsequently, eigenvalue sensitivity with respect to machine and control parameters is performed to assess their impacts on system stability. Moreover, the Hopf bifurcation boundaries of the key parameters are also given. They can be used to guide the tuning of those DFIG parameters to ensure stable operation in practice. The computer simulations are conducted to validate the developed model and to verify the theoretical analysis.

A Comprehensive LVRT Control Strategy for DFIG Wind Turbines With Enhanced Reactive Power Support

The paper presents a new control strategy to enhance the ability of reactive power support of a doubly fed induction generator (DFIG) based wind turbine during serious voltage dips. The proposed strategy is an advanced low voltage ride through (LVRT) control scheme, with which a part of the captured wind energy during grid faults is stored temporarily in the rotors inertia energy and the remaining energy is available to the grid while the DC-link voltage and rotor current are kept below the dangerous levels. After grid fault clearance, the control strategy ensures smooth release of the rotors excessive inertia energy into the grid. Based on these designs, the DFIGs reactive power capacity on the stator and the grid side converter is handled carefully to satisfy the new grid code requirements strictly. Simulation studies are presented and discussed.

A Decentralized Storage Strategy for Residential Feeders With Photovoltaics

This paper proposes a decentralized storage strategy to support voltage control in low-voltage (LV) residential feeders with high photovoltaic (PV) capacity installed. The proposed strategy is capable of preventing overvoltage situations during high PV generation periods, by the use of locally controlled battery energy storage systems (ESS) at the PV system grid interface. The traditional way of operating a domestic ESS is based on charging the battery as soon as the PV generation exceeds the consumption, without taking into account overvoltage events during high PV generation hours; the proposed storage concept improves the traditional approach, thanks to the provision of voltage support. A novel method, based on voltage sensitivity analysis, identifies a common power threshold that triggers the ESSs activation in the feeder. A Belgian residential LV feeder is used as a case study. Time-series simulations based on 1-year load and generation profiles verify the method findings and quantify the ESS size in terms of storage power and energy level.

Variable speed wind turbines capability for temporary over-production

New control systems for Variable Speed Wind Turbines (VSWT) need to be developed in order to provide inertia response and frequency control to support the grid. This work studies the behavior and capability of VSWT for providing temporary active power overproduction. The study is conducted on a multi-megawatt VSWT with Doubly-Fed Induction Generator. It was found that it is possible to have an active power overproduction of 0.2 pu for at least 10 seconds, which could be useful for the grid operator for restoring a critical situation of grid frequency dip. Nevertheless it is important to be aware about the underproduction power and the associated recovery period that follow a VSWT overproduction operation. The results can be used in designing suitable control systems for VSWT inertia response and frequency control.

Driving Pattern Analysis for Electric Vehicle (EV) Grid Integration Study

In order to facilitate the integration of electric vehicles (EVs) into the Danish power system, the driving data in Denmark were analyzed to extract the information of driving distances and driving time periods which were used to represent the driving requirements and the EV unavailability. The Danish National Transport Survey data (TU data) were used to implement the driving data analysis. The average, minimum and maximum driving distances were obtained for weekdays, weekends and holidays to illustrate the EV users¿ driving requirements in different days. The EV availability data were obtained from the driving time periods to show how many cars are available for charging and discharging in each time period. The obtained EV availability data are in one hour time periods and one quarter time periods for different study purposes. The EV availability data of one hour time period are to be used for optimal EV charging study in energy power market. The EV availability data of quarter time periods are to be used to investigate the possibility of utilizing EVs for providing regulation power. The statistical analysis software, SAS, was used to carry out the driving data analysis.

Smart Demand for Frequency Regulation: Experimental Results

As renewable energy sources increase their penetration, the traditional providers of frequency regulation service, i.e., fossil fueled thermal power plants, will be displaced, motivating the search for novel providers such as demand-side resources. This paper presents the results of field experiments using demand as a frequency controlled reserve (DFCR) on appliances with programmable thermostats. The experiments conducted showed the response of a population of thermostatically controlled loads acting as normal reserves (up and down regulation) and disturbance reserves (up regulation only) as defined by the Nordic Grid Codes . In addition, industrial pump loads and relay-controlled loads were tested as DFCR. The tests show that a population of refrigerators was able to deliver frequency reserves approximately equal to their average power consumption. Electric space heaters in the autumn season were able to provide frequency reserves of a magnitude 2.7 times their average power consumption.

Information and Communications Systems for Control-by-Price of Distributed Energy Resources and Flexible Demand

The control-by-price concept fits well with controlling small-scale generation, storage and demand. In this paper, we investigate the required information and communications systems that are needed to realize the control-by-price concept for such units. We first present a proposal for overall infrastructure and subsystem design and secondly focus on the design and implementation of the end-user price-responsive controller, interfaces, and communications. The design and its applicability on existing devices is verified through laboratory tests with two cases: electric space heating thermostat control and a small combined heat and power unit. The results show that the price-responsive controller reduces the end users electricity cost, or increases his income respectively, by about 7%. At the same time, the price-responsive controller provides an interface for the transmission system operator to utilize distributed energy resources and flexible demand as a regulating resource. Furthermore, the results illustrate and verify the applicability of the concept and the proposed infrastructure for controlling distributed energy resources and flexible demand.

Reactive power and voltage control based on general quantum genetic algorithms

This paper presents an improved evolutionary algorithm based on quantum computing for optimal steady-state performance of power systems. However, the proposed general quantum genetic algorithm (GQ-GA) can be applied in various combinatorial optimization problems. In this study the GQ-GA determines the optimal settings of control variables, such as generator voltages, transformer taps and shunt VAR compensation devices for optimal reactive power and voltage control of IEEE 30-bus and 118-bus systems. The results of GQ-GA are compared with those given by the state-of-the-art evolutionary computational techniques such as enhanced GA, multi-objective evolutionary algorithm and particle swarm optimization algorithms, as well as the classical primal-dual interior-point optimal power flow algorithm. The comparison demonstrates the ability of the GQ-GA in reaching more optimal solutions.