Wind Power Integration into Power Systems: Stability and Control Aspects

Power network operators are rapidly incorporating wind power generation into their power grids to meet the widely accepted carbon neutrality targets and facilitate the transition from conventional fossil-fuel energy sources to clean and low-carbon renewable energy sources. Complex stability issues, such as frequency, voltage, and oscillatory instability, are frequently reported in the power grids of many countries and regions (e.g., Germany, Denmark, Ireland, and South Australia) due to the substantially increased wind power generation. Control techniques, such as virtual/emulated inertia and damping controls, could be developed to address these stability issues, and additional devices, such as energy storage systems, can also be deployed to mitigate the adverse impact of high wind power generation on various system stability problems. Moreover, other wind power integration aspects, such as capacity planning and the short- and long-term forecasting of wind power generation, also require careful attention to ensure grid security and reliability. This book includes fourteen novel research articles published in this Energies Special Issue on Wind Power Integration into Power Systems: Stability and Control Aspects, with topics ranging from stability and control to system capacity planning and forecasting

Dynamic Phasor Modeling of Type 3 Wind Farm including Multi-mass and LVRT Effects

The proportion of power attributable to wind generation has grown significantly in the last two decades. System impact studies such as load flow studies and short circuit studies, are important for planning before integration of any new wind generation into the existing power grid. Short circuit modelling is central in these planning studies to determine protective relay settings, protection coordination, and equipment ratings. Numerous factors, such as low voltage situations, power electronic switching, control actions, sub-synchronous oscillations, etc., influence the response of wind farms to short circuit conditions, and that makes short circuit modelling of wind farms an interesting, complex, and challenging task. Power electronics-based converters are very common in wind power plants, enabling the plant to operate at a wide range of wind speeds and provide reactive power support without disconnection from the grid during low voltage scenarios. This has led to the growth of Type 3 (with rotor side converter) and Type 4 (with stator side full converter) wind generators, in which power electronics-based converters and controls are an integral part. The power electronics in these generators are proprietary in nature, which makes it difficult to obtain the necessary information from the manufacturer to model them accurately in planning studies for conditions such as those found during faults or low voltage ride through (LVRT) periods. The use of power electronic controllers also has led to phenomena such as sub-synchronous control interactions in series compensated Type 3 wind farms, which are characterized by non-fundamental frequency oscillations. The above factors have led to the need to develop generic models for wind farms that can be used in studies by planners and protection engineers. The current practice for short circuit modelling of wind farms in the power industry is to utilize transient stability programs based on either simplified electromechanical fundamental frequency models or detailed electromagnetic time domain models. The fundamental frequency models are incapable of representing the majority of critical wind generator fault characteristics, such as during power electronic switching conditions and sub-synchronous interactions. The detailed time domain models, though accurate, demand high levels of computation and modelling expertise. A simple yet accurate modelling methodology for wind generators that does not require resorting to fundamental frequency based simplifications or time domain type simulations is the basis for this research work. This research work develops an average value model and a dynamic phasor model of a Type 3 DFIG wind farm. The average value model replaces the switches and associated phenomena by equivalent current and voltage sources. The dynamic phasor model is based on generalized averaging theory, where the system variables are represented as time varying Fourier coefficients known as dynamic phasors. The two types models provide a generic type model and achieve a middle ground between conventional electromechanical models and the cumbersome electromagnetic time domain models. The dynamic phasor model enables the user to consider each harmonic component individually; this selective view of the components of the system response is not achievable in conventional electromagnetic transient simulations. Only the appropriate dynamic phasors are selected for the required fault behaviour to be represented, providing greater computational efficiency than detailed time domain simulations. A detailed electromagnetic transient (EMT) simulation model is also developed in this thesis using a real-time digital simulator (RTDS). The results obtained with the average value model and the dynamic phasor model are validated with an accurate electromagnetic simulation model and some state-of-the-art industrial schemes: a voltage behind transient reactance model, an analytical expression model, and a voltage dependent current source model. The proposed RTDS models include the effect of change of flux during faulted conditions in the wind generator during abnormal system conditions instead of incorrectly assuming it is a constant. This was not investigated in previous studies carried out in the real-time simulations laboratory at the University of Saskatchewan or in various publications reported in the literature. The most commonly used LVRT topologies, such as rotor side crowbar circuit, DC-link protection scheme, and series dynamic braking resistance (SDBR) in rotor and stator circuits, are investigated in the short circuit studies. The RTDS model developed uses a multi-mass (three-mass) model of the mechanical drive train instead of a simple single-mass model to represent torsional dynamics. The single mass model considers the blade inertia, the turbine hub, and the generator as a single lumped mass and so cannot reproduce the torsional behaviour. The root cause of sub-synchronous frequencies in Type 3 wind generators is not well understood by system planners and protection engineers. Some literature reports it is self excitation while others report it is due to sub-synchronous control interactions. One publication in the stability literature reports on a small signal analysis study aimed at finding the root cause of the problem, and a similar type of analysis was performed in this thesis. A linearized model was developed, which includes the generator model, a three mass drive train, rotor side converter, and the grid side converter represented as a constant voltage source. The linear model analysis showed that the sub-synchronous oscillations are due to control interactions between the rotor side controller of the Type 3 wind power plant and the series capacitor in the transmission line. The rotor side controls were tuned to obtain a stable response at higher levels of compensation. A real-time simulation model of a 450 MW Type 3 wind farm consisting of 150 units transmitting power via 345 kV transmission line was developed on the RTDS. The dynamic phasor method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for different sub- synchronous oscillation frequencies

Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

A New Approach on the Design and Optimization of Brushless Doubly-Fed Reluctance Machines

International audienceThe Brushless Doubly-Fed Reluctance Machine (BDFRM) is being considered as a viable generator alternative to be used in wind turbines. A literature review shows that there is still a lack of researches to define a design procedure to make this machine widely used in such application. This paper aims to address this issue by considering a new BDFRM design method using a reluctance network approach and the concepts of sizing and optimization models. It also presents a case study using the proposed methodology where the torque has increased significantly whereas the iron mass has been kept to a minimu

Wide-area monitoring and control of future smart grids

Application of wide-area monitoring and control for future smart grids with substantial wind penetration and advanced network control options through FACTS and HVDC (both point-to-point and multi-terminal) is the subject matter of this thesis. For wide-area monitoring, a novel technique is proposed to characterize the system dynamic response in near real-time in terms of not only damping and frequency but also mode-shape, the latter being critical for corrective control action. Real-time simulation in Opal-RT is carried out to illustrate the effectiveness and practical feasibility of the proposed approach. Potential problem with wide-area closed-loop continuous control using FACTS devices due to continuously time-varying latency is addressed through the proposed modification of the traditional phasor POD concept introduced by ABB. Adverse impact of limited bandwidth availability due to networked communication is established and a solution using an observer at the PMU location has been demonstrated. Impact of wind penetration on the system dynamic performance has been analyzed along with effectiveness of damping control through proper coordination of wind farms and HVDC links. For multi-terminal HVDC (MTDC) grids the critical issue of autonomous power sharing among the converter stations following a contingency (e.g. converter outage) is addressed. Use of a power-voltage droop in the DC link voltage control loops using remote voltage feedback is shown to yield proper distribution of power mismatch according to the converter ratings while use of local voltages turns out to be unsatisfactory. A novel scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is also studied. The effectiveness of the proposed approaches is illustrated through detailed frequency domain analysis and extensive time-domain simulation results on different test systems