Optimization of constant power control of wind turbines to provide power reserves

In several countries, the wind power penetration increased tremendously in the last years. As the current wind turbines do not participate in frequency control nor reserve provision, this may compromise the proper functioning of the primary control and the provision of power reserves. If no actions are taken, increasing levels of wind penetration may result in serious problems concerning the stable operation of the power system. This paper focuses on the provision of power reserves by wind turbines. For this service, the constant power control strategy is chosen as control strategy, as it gives a constant power output and has the ability to provide power reserves. In this way, the wind turbine behaves more like a conventional power plant. The choice of the power reference value is crucial as it determines whether or not a stable operation of the wind turbine is possible and power reserves can be provided. In this paper, an algorithm is proposed to obtain the range of possible reference values. By means of simulations, the optimal reference value to provide power reserves with a single wind turbine is obtained. Also, reserve provision in a wind farm is investigated. It is shown that the provision of power reserves with wind turbines using the constant power strategy is possible, especially in wind farms

Contribution of a smart transformer in the local primary control of a microgrid

In order to enable an easy participation of microgrids in the electricity markets, the smart transformer (ST) concept has been developed. The ST controls the power exchange between a microgrid and the utility network by only controlling its microgrid side voltage, instead of the conventional arrangement where new set points are communicated to all microgrid elements. When the voltage-based droop (VBD) control is implemented in the DG units, loads and storage elements, all microgrid units automatically respond to this change of microgrid voltage by altering their power output or consumption. However, this reference value of power exchange is dependent on (day-ahead) predictions of both consumption and (renewable) power generation. Hence, when these predictions prove to be inaccurate, the ST will still control the power exchange, but with consequently large variations of the microgrid voltage from its nominal value. It is suggested to take the real-time microgrid voltage into account when determining the reference power of the ST. This is presented in this paper by extending the ST's control strategy with a VBD control, such that the ST can contribute in the primary control. Simulations are included to analyze this primary control of the ST combined with VBD control of the other microgrid elements

Provision of Ancillary Services with Variable Speed Wind Turbines

In recent years, the amount of wind turbines in the power system has increased tremendously. As the current wind turbines do not participate in the provision of ancillary services such as frequency control and voltage control, this may compromise the proper functioning of the electric power system. However, since the modern wind turbines are equipped with a power-electronic converter, they can assist in the provision of ancillary services. To achieve this, additional control loops have to be added to the wind turbine controller. In this paper, an overview of the different ancillary services is given. The ability to provide them with wind turbines is discussed. Since frequency and voltage control are the most important, these two services are further elaborated. It can be concluded that wind turbines are suited to provide frequency control, especially when they are operated slightly below their maximum power point. They can also assist in voltage control, while operation in the maximum power point is usually possible, so few energy is lost. These are important outcomes, since wind turbines which provide ancillary services can contribute in allowing a higher penetration of renewable energy in the power system without compromising its proper functioning

OptiWind: grootschalige integratie van windturbines: hoe kunnen windturbines deelnemen aan de frequentieregeling in het net?

Alle veranderingen in het elektriciteitsnet van de laatste jaren zorgen voor heel wat nieuwe uitdagingen. Kunnen ook windturbines hier een rol in spelen? Verschillende onderzoeken en studies proberen hier een antwoord op te bieden. Zo is er OptiWind, een SBO-project gefinancierd door het IWT, waar de VUB, Sirris, KU Leuven, UGent, 3E en LMS International op zoek gaan naar een manier om de energie die wordt opgewekt door windturbines in het elektriciteitsnet te injecteren. Meer over de technische mogelijkheden van het net en van windturbines wordt in dit artikel uit de doeken gedaan

Three-phase primary control for unbalance sharing between distributed generation units in a microgrid

For islanded microgrids, droop-based control concepts have been developed both in single and three-phase variants. The three-phase controllers often assume a balanced network, hence, unbalance sharing and/or mitigation remains a challenging issue. Therefore, in this paper, unbalance is considered in a three-phase islanded microgrid where the distributed generation (DG) units are operated by the voltage-based droop (VBD) control. For this purpose, the VBD control, which has been developed for single-phase systems, is extended for three phase application and an additional control loop is added for unbalance mitigation and sharing. The method is based on an unbalance mitigation scheme by DG units in grid-connected systems, which is altered for usage in grid-forming DG units with droop control. The reaction of the DG units to unbalance is determined by the main parameter of the additional control loop, viz, the distortion damping resistance Rd. The effect of Rd on the unbalance mitigation is studied in this paper, i.e., dependent on Rd, the DG units can be resistive for unbalance (RU) or they can contribute in the weakest phase (CW). The paper shows that the RU method decreases the line losses in the system and achieves better power equalization between the DG unit's phases. However, it leads to a larger voltage unbalance near the loads. The CW method leads to a more uneven power between the DG unit's phases and larger line losses, but a better voltage quality near the load. However, it can negatively affect the stability of the system. In microgrids with multiple DG units, the distortion damping resistance is set such that the unbalanced load can be shared between multiple DG units in an actively controlled manner rather than being determined by the microgrid configuration solely. The unit with the lowest distortion resistance provides relatively more of the unbalanced currents

Effective capture of wind gusts in small wind turbines by using a full active rectifier

Small wind turbines have difficulties to start rotating at low wind speeds due to their relatively large rotor inertia and small starting torque. In case a permanent magnet generator is used, the rotor magnets will cause an additional cogging torque which makes starting even more difficult. By using the generator as motor from the moment a wind gust is detected, the turbine is able to accelerate much faster to reach the maximum power point. A maximum power point tracking algorithm is used to locate the optimal working point. At sufficiently large rotor speeds, the controller switches to the generator mode where the energy used for acceleration is recuperated, together with the additional energy captured from the wind gust. To control the generator power in both directions, an active rectifier is used in a back-to-back converter topology. In this paper, this wind capture strategy is simulated. The results show that the power output during a wind gust can be largely increased compared to common MPP-tracking

Grid voltage control with wind turbine inverters by using grid impedance estimation

Currently, distributed generation (DG) units usually do not participate in the primary grid voltage control and inject power regardless. Only when the grid voltage level surpasses a certain threshold, these units shut down to avoid any overvoltage problems. In this paper, a method is developed to control the grid voltage by wind turbines. This way, the grid voltage level improves, and the wind turbine can still generate power under suboptimal grid voltage conditions. The effect on the grid voltage depends on the grid impedance ratio R/X. In this paper, the effectiveness of the grid voltage control is improved by estimating the grid impedance ratio in real-time to inject active and reactive power accordingly. The method is applied to wind turbines, but can be used for other DG units as well

Impact of speed ripple on the back-Emf waveform of permanent magnet synchronous machines

Permanent Magnet Synchronous Machines (PMSMs) are frequently used in industry due to their high efficiency and favorable dynamic properties. Mechanical limitations and design considerations cause several harmonics in the flux and back-emf of these machines. The back-emf harmonics can be measured on the machine terminals if no stator current is present and the neutral point is accessible. The measured harmonics can then be included in a mathematical model of the machine. This measurement is often done for a constant speed. However, when a speed ripple is present, several new harmonics are introduced in the flux and back-emf. Although the existence of this phenomenon is intuitively clear, it has not yet been investigated in detail and no method exists to calculate these additional harmonics. Nevertheless, the impact of a speed ripple on the back-emf can become significant in some applications. Therefore, in this paper, a mathematical model is presented which allows to accurately calculate the additional back-emf harmonics in the presence of speed ripples. Also, it provides more insight in the interaction between speed ripples and harmonics. The mathematical model is extensively validated by means of simulations and experiments