Design of a Wind Turbine Controller for Power System Stabilization
In many countries of the world wind power expands and covers a steadily increasing part of these countries’ power demand. Growing wind power has impacts on the power systems into which the wind turbines feed their power. As wind power penetration increases, the respective power system operators are concerned about the stability and reliability of their power systems. Therefore, more and more power system operators revise their gird connection requirements and issue grid connection requirements specifically made for wind turbines and wind farms.
Increasing wind power penetration in a power system means that wind turbines substitute the conventional power plants that traditionally control and stabilize the power system. If wind power penetration exceeds a certain level, wind turbines have to be involved in performing these control tasks. This is even more important since it has been recognized that wind turbines themselves have an influence on the dynamic behavior of the power system to which they are connected. A stable power system is also of great importance for wind turbines, as variations in grid voltage and frequency have considerable impacts on the operation of wind turbines.
A transient short circuit fault is a very common disturbance in a power system. Such a short circuit fault leads to sub-synchronous system oscillations that have to be damped before the system becomes unstable.
Traditionally such oscillations are damped by conventional power plants with synchronous generators, which are equipped with power system stabilizers. Power system stabilization with synchronous generators is an established technology, which is applied all over the world . If wind turbines are to take over such damping tasks they have to have a very effective means of controlling their electrical output power. A common wind turbine type is the fixed speed active-stall wind turbine, which has a pitch system that allows the turbine to vary the pitch angle of the blades. If an active-stall turbine is to limit its power, it pitches its blades to an angle where the airflow around the blades gets detached from the surface of the blades and becomes turbulent, i.e. the blade stalls. Active-stall turbines have directly grid connected squirrel cage induction generators and the reactive power demand of this type of generator is usually compensated by capacitor banks. For a description of the active-stall concept see. The pitch system of active-stall turbines has previously been tested and used in practice for various dynamic operating situations. In this article, a controller is presented that enables an active-stall turbine to use its pitch system to perform power system stabilization similar to conventional power plants.
For simulations of grid frequency oscillations to be realistic, a realistic grid model is required. One approach is to consider a model of a power system, which could possibly exist in reality; another approach is to use a model of a real system. In this project, the latter approach has been chosen to ensure the validity of the findings. The power system model used here is an aggregated model of the interconnected power system of the countries Norway, Sweden, Finland and eastern Denmark.
To introduce the problem to be dealt with a brief introduction to the phenomenon of power system oscillations and the general concept of power system stabilization is given in the beginning of this article. A simplified transfer function of the wind turbine is found from the turbine’s step response. With this transfer function, a PID controller for grid frequency stabilization is designed, using the root locus method. The performance of the wind turbine with the designed controller is assessed in simulations and the results are discussed.
The frequency in an AC power system is stable when the electrical demand plus the electrical losses equal the electrical generation in the system. An imbalance between generation and demand leads to rising grid frequency if the generation exceeds demand, and to dropping grid frequency if the demand exceeds the generation. The grid frequency finds a new equilibrium if there is either sufficient frequency sensitive load in the system, or if the generators are equipped with governors that adjust the prime mover power so the generators pull the frequency back to its rated value. Governor controllers, which control the mechanical power of the prime mover are used to control the steady state frequency of the system in all modern power systems.
If a change in load or in generation happens gradually, the frequency will deviate gradually. If a step change happens, the frequency will experience transient oscillations before it settles to its new equilibrium.
A transient short circuit fault can be considered a step change, as the short circuit current constitutes a step in load. If the short circuit happens close to a generator, the voltage at the generator terminals will be suppressed so the generator cannot export active power, hence a step change in generation occurs. In any case, a short circuit upsets the balance between load and generation in a step change. If, as described above, a synchronous generator (SG) cannot export electrical power during a short circuit fault it has to accumulate the mechanical energy, with which the prime mover drives the generator. A rotating machine can only accumulate energy by accelerating. Hence the generator accelerates during the fault, and, after the fault is cleared, it tries to export as much electrical power as possible to decelerate again. As a result, the rotor speed of the generator oscillates.
In an interconnected AC power system a fault in one area and the subsequent rotor speed oscillations of the SGs in this area lead to power swings (inter-area oscillations) between different areas in the whole system. Considering the Nordic power system, which is the system at hand in this project, a fault in eastern Denmark causes inter-area oscillations as far away in the system as in the inter-area link between northern Sweden and Finland.
Since the rotor in a SG rotates synchronously with the stator field, the rotor speed is the same as the electrical frequency. Hence, rotor speed oscillations are grid frequency oscillations, which have to be dampened before the whole system becomes unstable. In a conventional power plant, SG equipped with power system stabilizers dampen these oscillations. If future wind farms substitute a considerable amount of conventional power plants, these wind farms have to be involved in the damping of grid frequency and inter-area oscillations.
As described above, frequency oscillations are caused by an imbalance between generated power and consumed power. Hence, grid frequency oscillations (as well as inter- area oscillations) can be counteracted with a controlled active power injection into the grid. Since the wind turbine type considered here is an active-stall turbine, which is equipped with a squirrel-cage induction generator, the only means of controlling its output power is the pitch system that controls the aerodynamic power, which drives the generator. (Photo: vaxomatic).