Jan Van de Vyver

Droop Control as an Alternative Inertial Response Strategy for the Synthetic Inertia on Wind Turbines

In several countries, the wind power penetration increased tremendously in recent years. To ensure the proper functioning of the power system, some grid operators already require the capability to provide inertial response or primary control with wind turbines. This paper discusses the emulated inertial response with wind turbines by means of the synthetic inertia and the droop control strategy. The behavior of the synthetic inertia strategy is determined by the inertial and the droop constant, whereas droop control only has a droop constant. When these strategies are used, it is important to tune the control parameters depending on the power system to which the wind turbines are connected. Simulations show that it is possible to enhance but also to deteriorate the frequency response of the system, dependent on these parameters. For different power system compositions, the optimal inertial constant is always close to zero. This way, the synthetic inertia strategy reduces to a fast droop control strategy. This is an important outcome, as it means that no differentiation of the frequency is needed to obtain an optimal inertial response from the wind turbines, which is beneficial in terms of robustness. Consequently, droop control is a viable alternative for the synthetic inertia.

Joule losses and torque ripple caused by current waveforms in small and medium wind turbines

Grid-connected small and medium wind turbines are a promising technology to further increase the share of renewable energy sources. Although their impact on the environment is proven to be advantageous, the cost effectiveness still needs to increase. In this paper, the impact of the passive diode rectifier on the Joule losses in the generator is investigated. Passive diode rectifiers cause unwanted harmonics in the current waveform and a low power factor, increasing the losses. The capacitance value at the output of this rectifier has a large influence on these harmonics. The amount of harmonics could be reduced by including an inductance in series with the generator terminals. However, this results in a higher reactive power demand, which also affects the losses. In this paper, the impact of both the capacitor and inductance values on the losses are investigated.

Online estimation of the power coefficient versus tip-speed ratio curve of wind turbines

Wind turbines are one of the main sources of renewable energy worldwide. Although large wind turbines in the MW range are popular, small wind turbines of several kW can also have a valuable contribution since they can be installed in a more decentralized manner. However, the energy yield of these small wind turbines still needs improvement. The performance of the Maximum Power Point Tracker (MPPT) has a crucial impact on the energy yield. In order to design and set-up the MPPT, knowledge of the Cp(λ) curve of the turbine blades has an important added value. However, this curve is often not known as it requires a sufficiently large wind tunnel or complex computational fluid dynamics calculations. In this paper, a new alternative method is presented to estimate the Cp(λ) curve by measuring the wind speed, generator voltage and current. This method can be applied on any turbine, small or large, during normal operation and does not require an intervention in the turbine itself, e.g., for placement of additional sensors. The Cp(λ) estimation method is implemented on a lab-scale wind turbine emulator to obtain experimental validation.

Congestion-induced wind curtailment mitigation using energy storage

Allowing the connection of additional renewable energy sources (RES) in areas with limited transmission capacity is becoming of a serious concern. Building new transmission lines only provides a long-term solution to cope with this issue due to the fact that it takes much longer time (up to 5-10 years) compared to time needed to build new wind farms (about 1 year). Storage is proven to be an effective solution to make maximal use of existing grid infrastructures in the short-term. This paper proposes a cost-benefit optimization formulation for optimally sizing the storage in a wind-storage system which is connected to an external spot market via limited transmission lines. A small test system is studied in order to find the optimal size of storage to avoid congestion by allowing revenue to be generated only via reducing the congestion-induced wind curtailment. Additional revenue streams can be also included to maximize the monetary value of the wind-storage system.

Evaluation of the MPPT performance in small wind turbines by estimating the tip-speed ratio

In this paper, a method for online estimation of the tip speed ratio (TSR) of small wind turbines is presented, in order to evaluate the MPPT performance. Unlike obvious methods, for which an encoder or resolver is needed, the presented method only requires measurements of the wind speed and the generator current. Firstly, the accuracy of the presented method is evaluated by using a wind turbine emulator. The obtained results show that the method is suitable for accurately determining both the shaft speed and the TSR of a small wind turbine. This method of estimating the TSR is promising given its simplicity and applicability in the full range of synchronous generators. In addition, by comparing the estimated TSR with the optimum TSR, the MPPT performance can be evaluated. Secondly, the TSR of a commercially available small wind turbine is measured by using the presented method and the MPPT performance is evaluated. In conclusion, the MPPT of the tested turbine can be improved, allowing a considerable increase in the yearly energy output.

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.

Comparison of wind turbine power control strategies to provide power reserves

Due to an increasing amount of wind turbines in the power system, which have a volatile power output, the need for flexibility increases. This flexibility can be obtained by ancillary services. Therefore, the provision of ancillary services with wind turbines is gaining interest. As they currently do not participate in the frequency control, this might compromise the proper functioning of the power system. The modern turbines are equipped with a power-electronic converter, so they can be used to provide a variety of ancillary services when the appropriate control strategies are used. In this paper, the provision of active power reserves for frequency control is considered. Three state-of-the-art power control strategies, i.e., constant power control, linear slope control and percentage control are compared to determine their ability to maintain reserves. It is concluded that all the strategies are able to provide reserves, but linear slope control and percentage control perform the best, especially when the wind turbines are aggregated in wind farms.

OLTC selection and switching reduction in multiple-feeder LV distribution networks

On-Load Tap Changers (OLTCs) are increasingly being considered to control the voltage in Low-Voltage (LV) distribution grids. Today, most OLTC control strategies are based on secondary transformer measurements, while a Field Measurement Based (FMB) control could increase the quality further. In this paper, field measurements, performed by a Distribution System Operator (DSO), are used to simulate the voltage quality improvement derived from implementing an OLTC with FMB control. Unfortunately, FMB control results in an unacceptable number of tap changes, so, in order to improve FMB control, two strategies to reduce the number of tap changes are presented. Next, an OLTC selection procedure is clarified. Both the voltage range of the OLTC and the tap position occurrence show that several OLTCs are not suited to increase the voltage quality. A voltage quality analysis shows that a well-selected OLTC with FMB control result in a significant increase in voltage quality, which allows more Distributed Energy Resources (DERs) to be connected in the existing LV distribution grid.

Energy yield losses due to emulated inertial response with wind turbines

In several countries, the wind power penetration increased tremendously in recent years. As the current wind turbines do not participate in inertial response nor primary control, this may compromise the proper functioning of the power system. This paper discusses the emulated inertial response with wind turbines. In literature, several strategies to provide inertial response with wind turbines are available. However, if they are operated in their maximum power point, emulated inertial response results in energy yield losses. These are caused by the suboptimal operation of the wind turbines during the inertial response. This paper focuses on the calculation of the energy yield losses for different cases. In this way, it is possible to estimate their importance and the loss of revenue for the operator of the wind turbine. It is shown that the energy yield losses are very low for both strategies, for all simulated cases. This is an important result, as it shows that the different inertial response strategies only have a minor effect on the total energy yield of the turbine.

Maximum efficiency current waveforms for a PMSM including iron losses and armature reaction

Permanent Magnet Synchronous Machines (PMSMs) are popular due to their high efficiency and favorable dynamic properties. In practice, these machines often have a back-emf waveform which deviates from an theoretically ideal sinusoid or trapezoid. In the past, current waveform shaping techniques were developed for these machines to calculate non-conventional current waveforms to reduce the torque ripple. However, in the light of global warming, energy efficiency has become more important than torque ripple in many applications. Therefore, current waveform shaping techniques can be used to focus on maximum efficiency. In this paper, a technique is presented which maximises the efficiency of both the PMSM and the drive simultaneously. A time domain approach is used instead of the classical frequency domain approach. It allows to include many machine aspects which had to be neglected previously, i.e., armature reaction, reluctance, zero-sequence components and iron losses.