http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

This paper provides an overview of grid code technical requirements regarding the connection of large wind farms to the electric power systems. The grid codes examined are generally compiled by transmission system operators (TSOs) of countries or regions with high wind penetration and therefore incorporate the accumulated experience after several years of system operation at significant wind penetration levels. The paper focuses on the most important technical requirements for wind farms, included in most grid codes, such as active and reactive power regulation, voltage and frequency operating limits and wind farm behaviour during grid disturbances. The paper also includes a review of modern wind turbine technologies, regarding their capability of satisfying the requirements set by the codes, demonstrating that recent developments in wind turbine technology provide wind farms with stability and regulation capabilities directly comparable to those of conventional generating plants.

A review of grid code technical requirements for wind farms

The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

/pdf/dc-microgrids-part-ii-a-review-of-power-architectures-69j8nn2rpd.pdf

DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

https://web.stanford.edu/group/efmh/jacobson/Articles/I/DJEnPolicyPt2.pdf

Providing all global energy with wind, water, and solar power, Part II: Reliability, system and transmission costs, and policies

Today, a characteristic of the operating regime of small wind turbines is that they do not obtain the maximum power efficiency. Taking into account that that the operability margin can, in general, be enhanced, this paper sets out to develop algorithms designed to extract the maximum power. First, an analysis is made of existing algorithms and as a result, a new algorithm is proposed to track the maximum power point (MPPT). This algorithm is based on an improvement in another previously defined in the literature. The proposed algorithm resolves the problem of Hill-climb searching algorithms in tracking the maximum power point, when the wind speed drops sharply. Lastly, the proposed algorithm is valid under any wind conditions and is, moreover, an economical alternative as it does not require any additional sensor for it to operate correctly.

Maximum power extraction algorithm for a small wind turbine

Wide-bandgap (WBG) transistors provide better switching performance and higher operating temperatures compared to state-of-the-art Si devices and are suited for high-frequency applications due to very short switching times. The main obstacle for implementation of WBG transistors at full potential is the high-frequency oscillation in voltage and current during switching transients. Oscillations arise from resonance due to parasitic and device inductances and capacitances. Introduction of WBG transistors depends on the elimination of these oscillations and their negative effect on the performance of power converters. Good layout practice is mandatory, but there is a limit to the reduction of these parasitics, and often, slowing of the semiconductor switching time must be applied. This article presents a simple methodology for the attenuation of the negative effects of WBG transistor high-frequency oscillations without increasing rise and fall times. The proposed methodology is based on determination of the source of feedback resonant frequency between gate and power loops using network analyzer measurement on printed circuit board and utilization of a tuned $RLC$ filter. Experimental application of the methodology shows a direct relationship between loop resonant frequency and voltage and current oscillations. The proposed method reduces power losses, high-frequency oscillations, and electromagnetic interference.

Wide-Bandgap Semiconductor HF-Oscillation Attenuation Method With Tuned Gate $RLC$ Filter

Joint cost of energy under an optimal economic policy of hybrid power systems subject to uncertainty

Droop control method is usually used when distributed energy sources are connected in parallel. The absence of communications between the distributed energy resources and its flexibility are some of its advantages. Moreover, a fictitious impedance can be added which improves the performance of the original technique. This fictitious impedance has to be properly designed in order to obtain a good stability and performance of the system. On the other hand, a restoration control that brings frequency and voltage amplitude of the microgrid to their nominal values is used in some cases. In this article, a linear model of a droop controlled microgrid with fictitious impedance and restoration control is implemented. Thanks to this linear model, an optimal fictitious impedance can be designed. Moreover, the influence of a proposed restoration control on the whole system can be studied. This way, the dynamic of the restoration control can be properly chosen. The stability as well as nominal values of frequency and voltage amplitude are guaranteed. Simulation results of the system confirm the validity of the proposed design.

Stability analysis and design of droop control method in dq frame for connection in parallel of distributed energy resources

Many modulation techniques have been developed along the years to provide in medium and high-volta ge converters a high power quality and minimum switching frequency. This paper introduces some popular modulation techniques for multilevel converters, whose major challenges are o btain the previous two features. The modulation strategies th at are presented are specially focused in the three-phase three-level Neutral-Point-Clamped convert (NPC), which is one of the most used topology in industry and in renewable applicat ions.

/pdf/review-of-modulation-algorithms-for-neutral-point-clamped-2hw8gw8lbc.pdf

Jon Andreu

Papers

Maximum power extraction algorithm for a small wind turbine

Wide-Bandgap Semiconductor HF-Oscillation Attenuation Method With Tuned Gate $RLC$ Filter

Joint cost of energy under an optimal economic policy of hybrid power systems subject to uncertainty

Stability analysis and design of droop control method in dq frame for connection in parallel of distributed energy resources

Review of Modulation Algorithms for Neutral-Point-Clamped Multilevel Converter