Methods to provide power reserves for grid support by wind energy farms
To combat the climate change and its global consequences, the European Union enforces an increased installation of renewable energy sources power plants in the interconnected system, thereby decreasing the number of conventional power plants connected to the utility grid. The latter one endangers the frequency stability in the grid, since conventional power plants are involved in the provision of inertial power and primary control power. Moreover, the voltage support is negatively influenced, since the power plants coupled to the grid via synchronous generators no longer provide reactive power. In order to ensure a secure and robust operating point of the interconnected grid in the future as well, generation units like wind farms, which are decoupled from the grid via power converters, must be taken part in the provision of grid services.
The objective of this research project is the developement of a management, that is superordinated to the individual turbine controls and enables frequency and voltage support by a wind farm. This one is designed to optimally combine the objectives of maximum energy yield and provision of grid services as well as to limit the stresses on the structural components, in order to increase the lifetime and thus the economic efficiency of the wind turbines. For the investigations, a holistic modeling of a wind farm with its electrical, mechanical and aerodynamic couplings by 3D wind models is intended.
Contact: M.Sc. Benedikt Spichartz
Low-voltage ride through (LVRT) of DFIG-based wind energy converters
This project studies the behavior of wind energy converters with doubly-fed induction generators during short-term low-voltage grid faults. This type of grid faults can induce very high voltages and currents in the stator and rotor windings of the generator and as a consequence, the costly power electronic hardware connected to the rotor can be damaged. To prevent this, different control strategies and hardware are considered in order to maintain control of the wind energy converter during the fault, while protecting the power electronic converters. Furthermore active and reactive power must be controlled both during and after the fault in order to support grid voltage recovery. The aim is to develop suitable control strategies and hardware which can protect the wind energy converter system and simultaneously meet the most stringent LVRT requirements specified by the latest grid codes.
Contact: M.Eng. Pavlos Tourou
Management of Decentralized Energy Systems
The shift of the shares in Germanys electric power generation from fossil to renewable but also stochastically fluctuating energy sources results in a reduction of temporal flexibility on the generating side. Simultaneously the spatial distribution of energy supply changes. To previously purely central power plants decentralized and new central but remote central power plants have been added. This makes an expansion or modification of the network necessary. To mitigate these effects, the proportion of temporal flexibility in the system should be increased. Progress in information and communication technology now enables new approaches for the development of previously undeveloped flexibity potentials. Load and generation management can now be realized for smaller power ratings.
Integration Of Storage Systems In The Energy Supply Of Production Lines
At the operation of production lines, process depending peak power occurs, which can be considerably higher than the average power demand. Good examples for this are, the welding process that joins material with non continuous operation and the shredding process.
These fluctuations stress the grid infrastructure which leads to voltage and phase angle variations at the point of common coupling. Another problem is unbalanced load causes an unsymmetrical dynamic in the three phase power supply system.
The objective of the project is to design a suitable dynamic Energy Storage Systems (ESSs), which smoothes the load variations by an intelligent control strategy with a minimum of storage depth. By this, the power quality in the grid can be improved severely and flicker effects can be reduced to the minimum.
To realize this, the whole energy supply as well as the consumer should be described by a mathematical model, which is validated with the help of simulating tools.
These models effectively simplify the analysis of the ESS and give a strong physical intuition to the complete system and integrated into the existing model of the production lines energy supply and consumers. With this overall model, different approaches for the control of the ESS should be developed and optimized.
Low-loss sensors in the power engineering
The range of the sensor, which is responsible for the measurement of electrical variables, has been expanding year by year more and more. No matter what area is looked at, whether in power electronics, drive technology or in the automotive industry, the concept of sensor is not unknown "variable". The specification for the sensor (here: current sensor) has been thoroughly redesigned. Accordingly, the following aspects must be considered, as on the one side the potential-free transmission of measured value (long distance) and on the other hand, regarding to the influence of the sampling frequency of the measured value of current sensors in terms of the switching frequency of the converter.
For this reason, a new chip to should be developed for the measurement of currents in power electronic devices. This is located directly on a shunt resistor. From there, the measured data is sent to a control unit which controls the power electronics. Objective should be a high resolution and accuracy. Another aspect that is addressed in this work is about the influence of the sampling frequency of the measured value of current sensors in terms of the switching frequency of the converter. It will be analyzed how or whether the system behavior changes by the sensor.
For the potential-free transmission of measurement data which are received by an ASIC, a transmission path is used consisting of micro-controller, optocouplers and transmitting and receiving unit including power supply. The power supply is required because of the isolation and ripple high demands on the design. By designing a PCB layout the additionally high electromagnetic radiation must be observed, which occurs in circuits with modern power electronics. For the implementation of data transfer for the current sensor, a new modulation method (frequency pulse-width modulation (PWM-F)) was developed. Based on a doubly modeled PWM signal, the measured value will be coded with the modulation algorithm into the transmitted signal. For the investigation into the influence of the inclusion of current sensors, a simulated model of the developed sensor, will be installed in a system model consisting of a power converter (in this case the inverter), and the network. It should be analyzed if an increase of the sampling rate of the current sensor necessary when in the future the power converter will have a higher clock frequency (eg 64kHz).
Contact: Dipl.-Ing. Abdoulkarim Bouabana
Control Structures for Dynamic Damping of Torsional Oscillations in Energy Conversion Systems
Oscillations in wind turbines are an unwanted appearance that can be causes for expensive damages. In particular, oscillations in the drive system can cause damages to bearings, gear boxes and shafts. Causes of these oscillations are principally the changing in wind speed and the direction of flow, but also interactions of the various control systems of the wind turbine can be referred as a cause. State of the art for controlling these vibrations is usually the usage of damping systems that consist of tunable damping elements and absorb the energy of the vibrations. The objective of this work is the active damping of vibrations in drive trains for wind turbines, using new control strategies and methods, where the generator system with its limited tuning reserve is the main actor. This allows a reduction of the system complexity of the wind turbine. In addition, the efficiency of the wind turbine increases by using the generator system for oscillation damping.
The solving process first deals with the topology of the drive system and its modelling. The modelling is a key component, because the necessary order reduction is always accompanied with the loss of natural frequencies. Based on the models, a statement regarding to the penetration of the control variable on the individual components of the drive system can be made. In addition, conditions are described that particularly encourage or hamper active damping of vibrations.
The integration of a control concept for active damping of vibrations is another part of the solution procedure. For this purpose multivariable regulation concepts are examined for their effectiveness to the active oscillation damping.
Multi-functional power electronic tailored for electrical drives
The issue „Multi-functional power electronic tailored for electrical drives“ is aiming the energy efficiency of the power electronic of electric equipment in a line or any other conceivable drive train. With respect to that the energy efficiency in not only metonymic with the onsite power of the power electronics, but also for the impact to connected periphery equipment. According to this the power electronic should reduce the effect to adjacent systems to increase their energy efficiency. This should be realized with regard to the harmonic substance of the power electronic as well as to the reduction of the reactive power needs and reactive power feeding respectively. With an existing grid connection energy reclamation for example from switching off processes (exempli gratia: deceleration of a remnant system) should be made possible.
Influence of power supply quality in production systems
The mostly ﬂuctuating appearance of renewable energies and the lack of large-scale energy storages in the grid pose a challenge to be met concerning the maintenance of a consistent power supply quality. Furthermore, power electronics are increasingly used for diverse tasks in electrical supply systems and drive systems. They give the opportunity to provide demand speciﬁc electrical and mechanical energy supply in order to gain a highly dynamic system behaviour. However, due to the high-frequency switching of power electronics, which is inherited to the system, also unwanted pertubations are caused. These mentioned grid pertubations or - generally speaking - a lower power supply quality of the grid may have a severe impact on equipment up to malfunction. The research aim is to investigate these inﬂuences in production systems.
Electric drive concepts for the increase of the efficiency of energy recovery
Currently, the small range of battery electric vehicles avoids a larger acceptance in the community. A big part of the kinetic energy of a moving car could be revorered in the battery and, as a consequence, a significant extension of the range could be achieved, if braking processes were almost completely electrically executed. However, in most of the presently available electric vehicles the bigger part of the energy, which is available during braking, is still converted into thermal energy by the mechanical brakes. Often, only a fractional amount of the energy is fed back into the battery.
One reason for that is, that currents, which are a multiple of the rated current of the machine and the inverter, can occur for a few seconds. For cost reasons and limited space, oversizing of the inverter is not wanted.
The objective of this research project is to find power train topologies and control strategies, which offer maximum regenerative braking and therefore a rise of the range.
Contact: Dipl.-Ing. Philipp Spichartz