R. Rizzo

A high performance control technique of power electronic transformers in medium voltage grid-connected PV plants

With reference to medium-voltage grid-connection of large photovoltaic (PV) plants, the paper proposes and analyses a high performance solution based on a power electronic transformer (PET). It Includes dc-links and multilevel converters either in low-voltage or in high-voltage side, and a medium-frequency (MF) transformer. Together with the very reduced sizes of the whole conversion apparatus, an important feature of the proposed topology is the permitted presence of unbalanced voltages on the different dc-links corresponding to the partitions of the PV plant. This is enabled by the multilevel cascaded H-bridge configuration of the low-voltage side converter, in order to take into account the nonuniform distribution either of temperature or (mainly) of solar radiation on the different cells arrays, together with eventual different kind or age of used cells. another advantageous feature is the good level of power quality indexes on grid quantities, ensured by a multilevel NPC front-end: mainly low values of either harmonic distortion, or voltage and current dissymmetry. this occurs even if the commutation frequency of the switching devices of this converter is not high (1÷2 kHz). However, the paper is mainly focused on the control technique of the multilevel converters connected to the primary and secondary windings of the MF transformer, imposing multistep- or square-wave voltages. Some simple control algorithms are presented in order to maximize the power generated by every partition of the plant (also in unbalanced operations) together with the minimization of the joule losses of the central conversion unit in quasi-stationary operating conditions.

An efficient architecture of a PV plant for ancillary service supplying

The use of Distributed Generation (DG) units for grid ancillary services, in particular for smart grid applications, has been deeply considered in the last few years. It has been demonstrated that DG units can produce not only active power, which is obviously the principal role, but also they can provide ancillary services such as: voltage control, spinning reserve, backup supply, harmonic compensation, peak shaving and others. Recently, the use of Photovoltaic (PV) DG units has been proposed for reactive power ancillary service production; the concept has been fully demonstrated, highlighting the potentiality of PV DG units as ancillary service provider. In this paper, some improvements are proposed for a PV DG unit as provider of ancillary services in terms of single components modeling and global optimization of the system. The capability of a PV plant of generating both active and reactive power is demonstrated.

Power Flow Control Strategy for Electric Vehicles with Renewable Energy Sources

The paper analyses the benefits given by a compound storage system with batteries and supercapacitors on board of road electrical vehicles supplied by fuel cells. A control strategy for the management of the energy is developed for the maximization of the vehicle autonomy and for the increase of the electrochemical battery life. The controller is based on two finite state machines and it is implemented on the DC/DC converters that interface fuel cells and supercapacitors to the batteries. Experimental tests made on a sample electrical drive fully validate the effectiveness of the suggested control technique.

An overview of Power Electronic Transformer: Control strategies and topologies

In the last years the interest towards Power Electronic Transformers (PET) is increasing. These new conversion apparatuses perform either voltage transformation or power quality functions, using power electronics both on primary and secondary sides of a transformer operating at medium frequency. In the technical literature are proposed several circuital configurations, with different control strategies. However these are not exhaustive yet, especially if is considered that PET can be utilized in a lot of different fields of application. Simplicity, effectiveness and high-resolution of the control are important aspects which can strongly improve performance of PETs. The paper presents an overview of the main topologies of converters for PET with a Medium Frequency (MF) transformer, and the control strategies are analyzed.

Power Electronic Transformer application to grid connected photovoltaic systems

Grid-connected photovoltaic systems utilize static converters which influence the efficiency of the system. The energy conversion depends on the architecture of the converter, different solutions are available and H-bridge multilevel converters seem to be an optimal solution also for the power quality. In the paper is proposed an architecture that includes a Power Electronic Transformer which is practically an isolated high-frequency link AC/AC converter that substitute a conventional transformer. An MPPT control technique is presented and validated by simulation implemented on a photovoltaic system with H-bridge n-levels converter and Power Electronic Transformer. The simulation results confirm that the control is able to effectively track the Maximum Power Point and to stabilize immediately in the new steady-state condition.

A Sensorless Control of H-bridge Multilevel Converter for Maximum Power Point Tracking in Grid Connected Photovoltaic Systems

There is a high development of grid-connected photovoltaic systems that utilize static converter which influence the efficiency of the system. The energy conversion depends on the architecture of the converter, different solutions are available and H-bridge multilevel converters seem to be an optimal solution also for the power quality. It is important to utilize a proper architecture of converter but also to set up a control optimized to have the energy conversion at maximum efficiency. The persistence of the maximum efficiency condition is related, depending on the atmospheric conditions, to the tracking of the Maximum Power Point (MPP) by modifying the operating conditions of the system (Maximum Power Point Tracking -MPPT). In the paper is proposed a sensorless control set up to deliver the maximum power to the grid in presence of variations of incident irradiation on the photovoltaic arrays. The control technique is presented and validated by simulation implemented on a photovoltaic system with H-bridge 5-levels converter. The simulation results confirm that the control is able to effectively track the MPP and to stabilize immediately in the new steady-state condition.

A sample application of supercapacitor storage systems for suburban transit

The advance in energy storage technologies allows to improve significantly the performances of electrified light transportation systems. This is aligned with the massive requirement of competitiveness compared to other transportation technologies in terms of sustainable energy and high reduction of environmental impact. The installation of stationary storage devices, as widely recognized, allows the recovery of the braking energy for increasing the energy efficiency as well as lower line voltage drops. In the paper, the storage system under investigation is based upon a bidirectional dc-dc converter with supercapacitors, characterized by high power density. The main parameters of the storage system are determined by employing an optimization technique, which in a quite general way is capable of taking into account contemporaneously several aspects in an integrated manner. Some simulations of lines of a suburban railways in Southern Italy managed by Sepsa Company are presented, pointing out how supercapacitors can be practically designed with the proposed technique.

High Efficiency Wind Generators with Variable Speed Dual-Excited Synchronous Machines

Wind power seem to be one of the most interesting technology for electrical energy generation by renewable sources. It is necessary to set up carefully the wind turbine taking into account that the wind speed is variable with the time and there is a fluctuation of power output of wind energy conversion systems. There are many technical solutions adopted to increase the system efficiency by utilizing proper generators. In the paper the case of variable speed dual-excited synchronous machine is investigated. For its control possibilities, this machine is particularly suitable for variable-speed constant frequency applications like wind power generation systems. In the paper, after presenting the mathematical model of the wind turbine and the generator, a control technique for output power maximization is proposed. Results obtained by numerical simulations are discussed in the paper to demonstrate that using the proposed control algorithm it is possible to achieve a significant increment on the power generated in comparison with that obtained using an unregulated induction machine.

An optimized algorithm for torque oscillation reduction in DTC-induction motor drives using 3-level NPC inverter

The paper presents a direct torque control DTC for induction motor drive using a 3-level NPC inverter. The control is optimized for torque oscillation reduction. The control is described evidencing the good performance validated by means of simulation. The results are reported and discussed in the paper.

A PV plant simulator for testing MPPT techniques

This paper introduces a versatile and reliable photovoltaic systems simulator. It includes the main components of a photovoltaic plant: a PV string and a PWM controlled boost chooper. The key feature of this simulator is the ability to consider different and non-uniform irradiation and temperature conditions (partial shading and partial heating of the strings). All the different I-V and P-V or P-I characteristics can then be determined in such non-uniform irradiation and temperature conditions, with the aim to try different MPPT algorithms. The simulator also allows to verify an enhanced version of the Incremental Conductance algorithm (IncCond) where, in order to reach the real absolute maximum power condition, the reference control current of the boost chopper are periodically reset to a given percentage of the short circuit current at standard conditions. After this perturbation, the steady state power values are registered and compared to establish the final value of reference current that allows the IncCond algorithm to reach a steady state at the real abolute maximum power transfer.