J. Alonso-Martinez

Table-Based Direct Power Control: A Critical Review for Microgrid Applications

Direct power control (DPC) has increasingly gained attention in the last years as a robust and simple control scheme. Yet, the implications of some of the assumptions made by the original table-based DPC are not well explained in the literature. Here, a new formulation of DPC is presented, that allows to analyze the shortcomings of this kind of algorithms, mainly regarding the power limits in which table-based algorithms are valid. This is a key aspect for microgrid applications, as they require a wide range of possible active and reactive power setpoints in order to be able to control the systems voltage and frequency. A new table is then presented that allows to overcome those limitations. The proposed table is valid at any point of the inverters power limits.

Problem-Based Learning in Wind Energy Using Virtual and Real Setups

The use of wind energy is now an established fact, and many educational institutions are introducing this topic into their engineering studies. Problem-based learning (PBL), as a student-centered instructional approach, has contributed to important developments in engineering education over the last few years. This paper presents the experience of a problem-based learning approach within the context of teaching wind energy conversion systems for electricity generation at an Electrical and Electronic Masters degree level. Students were given the problem of finding the response of a wind turbine to a grid fault. Groups of three students worked on a cooperative learning project for 15 weeks, with the instructor providing resource assistance and information at all stages of the work. Two tools were designed to help the students: a virtual wind turbine simulator and a real wind turbine setup. Both experimental tools are described, and the results obtained by the students are discussed. The results show that the students valued both tools and were able to address problems at a high cognitive level.

A New Variable-Frequency Optimal Direct Power Control Algorithm

This paper presents a new formulation for direct power control (DPC) with several improvements over previous DPC formulations such as an exact discrete-time expression for the predicted power variations yielded by a given inverter switching state and the inclusion of the computing delay time in the model. Based on this formulation, a variable-frequency optimal DPC algorithm is presented that selects the inverter switching state that minimizes the power error. This algorithm shows an excellent precision in estimating power variations, therefore reducing power ripple and unwanted current harmonics, while retaining the fast dynamics inherent to variable-frequency DPC. This makes this algorithm suitable for interfacing distributed generation to microgrids, where a fast and accurate power control is desirable in order to perform voltage and frequency regulation. Additionally, studies are carried out regarding the average switching frequency and the influence of the model parameters on the algorithm robustness.

Battery Energy Storage System in smoothing control application of photovoltaic power fluctuations caused by clouds passing

This paper describes the power smoothing control of a hybrid system. The hybrid system is composed of a Battery Energy Storage System (BESS) and a Photovoltaic (PV) generator connected to the grid. The control allows to limit the ramp of the power fluctuations defined by system limits or standard specifications. The different tests shown in the paper demonstrate the relationship between the battery usage and the magnitude of the smoothing. A description of the main parameters for the smoothing control configuration is included. The study case describes the response of a smoothing control on real photovoltaic power fluctuations caused by a one-day clouds passing.

Dynamic Minimum Voltage Tracking in Electrochemical Batteries for Power Capability Estimation in Microgrids

This paper proposes a new algorithm to dynamically calculate the minimum battery voltage at which the peak power of a battery is drawn. This voltage limit is applied to calculate the corresponding discharging current in order to obtain the maximum instantaneous power from the battery. When the discharging current magnitude is greater than the aforementioned current, the discharging process becomes inefficient and the battery enters into what is referred to in this paper as a wasted energy process. This working condition can only be reached if the required application voltage is smaller than the calculated dynamic voltage for peak power. This situation can occur in a stationary application like a microgrid. In this case, unlike traditional methods, maximum power can be obtained at a lower current rate in a more efficient discharge process. The proposed power capability estimation algorithm in discharge processes is based on the partnership for new generation vehicles (PNGVs) specifications, but making use of the calculated dynamic minimum voltage. This algorithm was validated on an 11 Ah 210 Ni-Cd battery stack and compared with the classical PNGV algorithm. Results show that the wasted energy process can be avoided with the proposed algorithm and that around 3% more energy can be extracted at a higher power rate.

State of charge estimation model for Ni-Cd batteries considering capacity and efficiency

This paper proposes a new model for state of charge estimation in Ni-Cd batteries, considering the various definitions of capacities and efficiencies that take place in the charging and discharging of the battery. First, the main variables used to determine the capacity of an electrochemical battery are described. Second, coulombic efficiency in charging and in discharging processes is studied. Then, the different capacities defined and efficiencies considered are mathematically related and included in the estimation model. With the new proposed model, the effect of temperature and current rate on the capacity of the battery is analysed. These two parameters are considered to be the most influencing parameters. Finally, a characterization test is carried out on a single 11 Ah Ni-Cd cell and the results are used to verify the performance of the proposed model.

A new representation model of standard and available active materials for electrochemical batteries

The main goal of this paper is the introduction of a predictive model to calculate the remaining run-time of a battery. It is not meant to increase precision or compete with other methods but to be a reliable model by using a new formulation, more representative of the active material used zones. This paper defines two different states of charge (SOC) based on the concepts of standard and available capacities, named SOCs and SOCav, respectively. Concepts such as uncharged and undischarged capacities and loss charge efficiency are included as well. With these new definitions, an estimation algorithm is proposed. It is implemented as a runtime SOC estimation model based on an ampere-hour counting electrical model. The characterization of SOC model parameters of an 11 Ah Ni-Cd battery is also presented. These tests combine non-standard charge or discharge processes with end-of-charge/discharge detection methods that avoid overcharge or over discharge. Four case studies are carried out: two on a single battery cell and the other two on a 210 battery stack. The results show a good performance, with an approximate improvement of the estimation of 5%. They also show the importance of differentiating standard and available SOC in order to calculate the available capacity.The main goal of this paper is the introduction of a predictive model to calculate the remaining run-time of a battery. It is not meant to increase precision or compete with other methods but to be a reliable model by using a new formulation, more representative of the active material used zones. This paper defines two different states of charge (SOC) based on the concepts of standard and available capacities, named SOCs and SOCav, respectively. Concepts such as uncharged and undischarged capacities and loss charge efficiency are included as well. With these new definitions, an estimation algorithm is proposed. It is implemented as a runtime SOC estimation model based on an ampere-hour counting electrical model. The characterization of SOC model parameters of an 11 Ah Ni-Cd battery is also presented. These tests combine non-standard charge or discharge processes with end-of-charge/discharge detection methods that avoid overcharge or over discharge. Four case studies are carried out: two on a single batter...

State of charge, hysteresis and room temperature effects on the Ni-Cd battery modeling

Electrochemical batteries are complex devices to model whose static and dynamic responses are highly dependent on the value of its internal and external variables. This paper aims to determine the different importance of considering three of them (state of charge, hysteresis and room temperature) on the battery modeling accuracy. The battery model used in the paper is based on an electrical model composed of three different circuits: the current-voltage circuit, the state of charge circuit and the hysteresis circuit. The first circuit models the polarization of the battery, based on a second order Thévenin model. The last two circuits model the cycling and the hysteresis battery processes. Circuit parameters are dependent of the internal and external variables considered. Finally, the proposed model and characterization tests have been carried out and validated for a single cell of Ni-Cd of 11 Ah of capacity.