Design considerations for inertia emulating controllers used in variable speed wind turbines

A tendency to erect ever more wind turbines can be observed in order to reduce the environmental consequences of electric power generation. As a result of this, in the near future, wind turbines may start to influence the behavior of electric power systems by interacting with conventional generation and loads. Therefore, wind turbine models that can be integrated into power system simulation software are needed. In this contribution, a model that can be used to represent all types of variable speed wind turbines in power system dynamics simulations is presented. First, the modeling approach is commented upon and models of the subsystems of which a variable speed wind turbine consists are discussed. Then, some results obtained after incorporation of the model in PSS/E, a widely used power system dynamics simulation software package, are presented and compared with measurements.

This paper investigates the possibilities for frequency support by synthetic inertia from variable speed wind turbines. A model for representing synthetic inertia in PSS/E (Power System Simulator for Engineering) has been developed. It allows the user to relatively simple implement different control strategies to find the most suitable strategy for a particular power system. Furthermore, it can be used regardless of the existence of wind turbine models in the power system model and is connected separately to any arbitrary bus in the power system. The results from the simulations show that the frequency nadir is reduced when implementing frequency support by synthetic inertia from wind turbines. As the wind power integration increases the frequency nadir is further improved when wind turbines can provide synthetic inertia. Regarding recovery, the results indicates that in order to minimise the risk of an under-frequency event the energy should be taken during a frequency overshoot and/or distributed in time, i.e. not all wind power plants should recover at the same time. However, these studies of synthetic inertia provided by wind turbines are still in the early stage and further work will be performed.

Synthetic inertial frequency response has attracted increasing attention due to the paradigm shift in power generation from synchronous generators toward wind turbines. This paper presents a novel nonlinear controller to enable inertial response of a variable speed wind turbine (VSWT). The VSWT has a nonlinear model which cannot efficiently be dealt with by linear control techniques as it may experience large deviations from its operating point during the frequency support period. To design the controller, a nonlinear model for the wind turbine together with power system is utilized. The output power of the wind turbine is expressed in terms of the state variables, and its exact Taylor series expansion is used in control design procedure. A regular controller is first designed based on the input–output feedback linearization approach. Then, the regular controller is replaced by an adaptive one to accommodate parameter variations of VSWT. The proposed controller is then embedded in the detailed model of the VSWT in MATLAB/Simulink environment. Extensive investigations, including the effects of different controller parameters, are carried out. Impacts of the most important practical operational constraints in the VSWT inertia response are also analyzed. Results verify the superiority and appropriateness of the proposed controller.

Penghu Island is a small isolated power system with 70MW peak load and 30MW off-peak load. According to the operation records of the installed wind farms, Penghu has bettter wind conditions than any other site in Taiwan. The total capacity of wind farms in Penghu reaches up to 10.2MW, which means 35% penetration on wind power is possible during minimum load. The high level of wind power generation in the Penghu power system makes the decision of diesel unit commitment and control of the power system more difficult due to the high variability of wind power output in a time frame ranging from seconds to hours. This work investigates the significant factors that affect the system transient stability as high penetration of wind power is integrated. Additionally, the possible impact and coping strategies for the high wind power penetration are studied, and several specific suggestions are proposed to Taiwan Power Company. These strategies include revising the load shedding methods, unit commitment scheduling, wind turbine operation, the suitable energy storage system planning, and wind turbine protection.

Combined hydro-wind frequency control scheme: Modal analysis and isolated power system case example

A power generation scheme using a synchronous generator with an excitation control system driven by a variable-speed wind turbine is described. The proposed scheme has been analyzed over a wide speed range and under wind gusting conditions. Performance characteristics show that it is feasible to use variable-speed fixed-pitch wind turbines to generate electric power.

As wind power penetration into power systems increases, it poses a growing challenge to power system operation and stability. This is partly because wind power is highly time variable and cannot inherently contribute to power system frequency regulation. The provision of frequency support from wind turbines under changing wind conditions is examined in this paper. A simplified variable speed wind turbine model is developed. A modified turbine controller to deliver inertial and droop response is proposed. The interaction between inertial and droop response from wind turbines is examined under varying wind.

In the last two decades, the wind power generation has been rapidly and widely developed in many regions and countries for tackling the problems of environmental pollution and sustainability of energy supply. However, the high share of intermittent and fluctuating wind power production has also increased the burden of system operator for securing power system reliability during the operational phase. Moreover, the power system restructuring and deregulation have not only introduced the competition for reducing cost but also changed the strategy of reliability evaluation and management of power systems. The conventional long-term reliability evaluation techniques have been well developed, which have been more focused on planning and expansion rather than operation of power systems. This paper proposes a new technique for evaluating operational reliabilities of restructured power systems with high wind power penetration. The proposed technique is based on the combination of the reliability network equivalent and time-sequential simulation approaches. The operational reliability network equivalents are developed to represent reliability models of wind farms, conventional generation and reserve provides, fast reserve providers and transmission network in restructured power systems. A contingency management schema for real time operation considering its coupling with the day-ahead market is proposed. The time-sequential Monte Carlo simulation is used to model the chronological characteristics of corresponding reliability network equivalents. A simplified method is also developed in the simulation procedures for improving the computational efficiency. The proposed technique can be used to evaluate customers’ reliabilities considering high penetration of wind power during the power system operation in the deregulated environment.

Abstract- This paper introduces a linear structure of wind turbine, operator and the pitch angle controller. Then, a new method of adaptive fuzzy on the variable speed wind turbines was provided and it was compared with PID and Fuzzy Logic Control ler and the simulation results were analyzed. So when the results of each simulation using PID and Fuzzy controllers and adaptive fuzzy PID controllers provided, the observed advantages and disadvantages of the rotor speed and wind turbine power can be introduced. It can be seen that in PID method the high overshoot is discussed as a disadvantage while it is overcome by using fuzzy controller and overshoot will decrease. The time of reaching to a sustained speed increases slightly and in adaptive fuzzy PID controller, the less overshoot has provided a good and effective performance for system response. The use of adaptive fuzzy PID controller causes the system does not have any steady-state error and at all moments of time, the response rate is better than PID and fuzzy controller. The importance of the amount of overshoot and rate fluctuations is that by reducing these parameters in addition to reducing the cost of preventive care, maintenance and depreciation, the fluctuations in electricity generated by induction generator also greatly reduced.

It is now recognized that many large wind farms will employ doubly fed induction generator (DFIG) variable speed wind turbines. A number of such wind farms are already in operation and more are planned or under construction. With the rising penetration of wind power into electricity networks, increasingly comprehensive studies are required to identify the interaction between the wind farm(s) and the power system. These require accurate models of doubly fed induction generator wind turbines and their associated control and protection circuits. A dynamic model has been derived, which can be used to simulate the DFIG wind turbine using a single-cage and double-cage representation of the generator rotor, as well as a representation of its control and protection circuits. The model is suitable for use in transient stability programs that can be used to investigate large power systems. The behavior of a wind farm and the network under various system disturbances was studied using this dynamic model. The influence of the DFIG control on the stability of the wind farm was also investigated by considering different control gains and by applying network voltage control through both stator side and rotor side converters.

Power flow calculation is a basic tool for power systems operation and control. Considering static frequency regulation characteristics of power systems, dynamic power flow (DPF) gives more precise results of system operation with frequency deviation than conventional power flow. The increasing penetration of wind energy into power grid makes it necessary to take wind power into consideration in DPF calculation. The operational traits and frequency regulation characteristics of major types of wind turbines are discussed, including fixed speed wind turbines and variable speed wind turbines with integrated control strategy combining over-speed control and droop control. With the primary frequency regulation characteristics of wind turbines, a simplified DPF algorithm is proposed in this study for power systems integrating wind power generation. The IEEE 30-bus system is modified to verify the proposed method considering different levels of wind power penetration.

A control method for variable speed wind turbines (VSWTs) supplying islanded parts of electrical networks is presented. Active power/frequency and reactive power/voltage droops are applied in order to determine the active, reactive power production, thus downscaling to the VSWTs the conventional control concepts of the power plants. Two types of VSWTs comprising doubly fed induction generators or synchronous generators are considered. Electrical, aerodynamic and structural detailed dynamic models were developed and combined with the proposed control strategies ensuring fast regulation of the frequency and the voltage in the islanded mode of operation. The obtained models are used for the simulation of a representative simplified distribution network supplied by VSWTs.

The applications of wind energy develops much more rapidly than the other renewable resources such as solar, geothermal etc. in the 21st century. It becomes the third core energy resource following non conventional fuels as oil and chemical. Electrical energy generated by wind power plants is the fastest developing and most promising renewable energy source. The wind is a clean, free and inexhaustible energy source. The origin of wind is simple. Wind turbines are classified into two types as fixed speed and variable speed wind turbine. A variable speed wind turbine provides more energy than the fixed speed wind turbines, reduce power fluctuations and improve reactive power supply. Basically direct drive PMSG (Permanent Magnet Synchronous Generator) and DFIG (Double Fed Induction Generator) are used in variable speed wind turbine generator. In this paper, the Matlab simulations of a Wind Energy Conversion System with PMSG and variable-speed wind turbine have been discussed.

With increasing of the capacity and flexibility of the wind turbine generator system (WTGS), it is more and more important to make the operation even better and to reduce the load while the wind speed is above the rated speed of WTGS. The individual pitch control can solve the above two problem efficiently. But the control of WTGS is very complicated because of the fluctuation and intermittence of wind speed and the complexity of power electronics. In classical control, the controller design needs accurate mathematical model, but this kind of model of the wind turbine generator system is very difficult to get because of the nonlinear characteristics. Using fuzzy controller, we can simplify and avoid the sophisticated mathematical model. Combining with the studied method and results of other scholars about pitch control, we put forward the individual pitch control based on fuzzy PI control and used to control the wind turbine so that it can be more stable and meanwhile its operation load can be reduced, and the fluctuation of the power is minimized. Firstly, we use oscillation acceleration of one bladed as input and PI controller to get one pitch angle; secondly, we use rotation speed as input and use fuzzy PI controller to get a pitch angle; and thirdly, we use a distributed controller to get a pitch angle; finally, combine the three pitch angles and used as the real control pitch angles of the three blades. The simulation results are given in this paper.

Frequency response is a key concern for power system security. Usually, it is estimated in a deterministic approach. But in most cases, power systems behave stochastically. Moreover, presence of wind power imposes further variability on generation profile of a power system. Hence, it appears more pragmatic to address these uncertainties during an evaluation of frequency response. This paper develops an index to assess frequency response of a power system with high wind power penetration. A probabilistic method is used for this purpose. K-means algorithm, a well-known data mining technique is employed. The developed index is called a ‘probabilistic frequency response index’. A realistic interconnected power system is investigated for a case study. Impact of increased wind power penetration on probabilistic frequency response index is analysed.