The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.

Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

This paper presents a new control scheme for an active front-end rectifier using model-based predictive control. The control strategy minimizes a cost function, which represents the desired behavior of the converter. Future values of currents and power are predicted using a discrete-time model. The active and reactive powers are directly controlled by selecting the optimal switching state. The main advantages of this method are that there is no need of linear current controllers, coordinates transformations or modulators. The rectifier operates with sinusoidal input currents and unity power factor. Simulation and experimental results are presented to verify the performance of the proposed power control scheme.

Direct Power Control of an AFE Using Predictive Control

This paper is related to faults that can appear in multilevel (ML) inverters, which have a high number of components. This is a subject of increasing importance in high-power inverters. First, methods to identify a fault are classified and briefly described for each topology. In addition, a number of strategies and hardware modifications that allow for operation in faulty conditions are also presented. As a result of the analyzed works, it can be concluded that ML inverters can significantly increase their availability and are able to operate even with some faulty components.

/pdf/survey-on-fault-operation-on-multilevel-inverters-1lv2auia1i.pdf

Survey on Fault Operation on Multilevel Inverters

The modern electric power systems are going through a revolutionary change because of increasing demand of electric power worldwide, developing political pressure and public awareness of reducing carbon emission, incorporating large scale renewable power penetration, and blending information and communication technologies with power system operation. These issues initiated in establishing microgrid concept which has gone through major development and changes in last decade, and recently got a boost in its growth after being blessed by smart grid technologies. The objective of this paper is to presents a detailed technical overview of microgrid and smart grid in light of present development and future trend. First, it discusses microgrid architecture and functions. Then, smart features are added to the microgrid to demonstrate the recent architecture of smart grid. Finally, existing technical challenges, communication features, policies and regulation, etc. are discussed from where the future smart grid architecture can be visualized.

Enhancing smart grid with microgrids: Challenges and opportunities

A sensorless three-level neutral-point-clamped inverter-fed induction motor drive is proposed in this paper. The conventional direct torque control (DTC) switching table fails to consider the circuit limitations, such as neutral-point-balance and smooth vector switching, caused by the topology of a three-level inverter. Two kinds of modified schemes for three-level DTC are proposed to solve these problems. They also provide performance enhancement while maintaining robustness and simplicity. Fuzzy logic control and the speed-adaptive flux observer (with novel gain and load toque observation) are introduced to enhance the performance of the system. The issue of large starting current is investigated and solved by introducing the technique of preexcitation. A 32-bit fixed-point DSP-based motor drive is developed to achieve high-performance sensorless control over a wide speed range. The effectiveness of the proposed schemes is confirmed by simulation implementation and experimental validation.

An Improved Direct Torque Control for Three-Level Inverter-Fed Induction Motor Sensorless Drive

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.

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

This study deals with direct torque control (DTC) of induction motor drives when a multilevel inverter is involved. The proposed control technique is finalised to reduce the amplitude of torque oscillations and to improve the motor dynamic response regardless of operating speed conditions while assuring a satisfactory input current distortion. This is achieved by using a special look-up-table built with a novel approach. The control itself is based on the application in each sampling interval of one or two voltage vectors suitably selected, in order to ensure low electromagnetic torque ripple in steady-state operations and fast dynamic behaviour in transient operations. In the algorithm, the converter voltage vectors selection is carried out by using a novel look-up table whose entry points are computed by means of a specialised relation, starting from the outputs of multilevel hysteresis controllers. The suggested method, suitable for a generic multilevel inverter, is detailed for a neutral point clamped three-level inverter. The proposed algorithm is validated by several numerical results, and compared with a DTC algorithm based on a traditional three-speed range look-up tables. Finally, some experimental results, obtained with a scale laboratory setup, are shown, discussed and compared with the numerical ones.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-epa.2014.0488

Generalised look-up table concept for direct torque control in induction drives with multilevel inverters

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.

Power Electronic Transformer application to grid connected photovoltaic systems

European energy policy guidelines recognise renewable energy sources the main mean to contrast the rapid fossil fuels depletion and the related global warming. Marine energy source represents an attractive and inexhaustible reservoir from which to draw. One of the major difficulties in integrating sea wave generation systems or equivalently wave energy converters (WECs) with existing electrical systems is the management of their generation intermittency. This is essentially due to the inherent nature of the sea wave source. Energy storage represents an effective enabling technology for mitigating such an effect. To this aim, this study proposes an efficient control strategy for embedded floating buoy generation systems with energy storage technologies in order to regularise the injected grid power while minimising the contractual power established by the distribution system operator. The control strategy has been tested numerically on a grid connected DC microgrid formed by a DC bus at which a floating buoy generation system is interfaced with an energy storage system, supercapacitors-based, having the purpose of smoothing the natural power fluctuations of the WEC.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rpg.2015.0093

Grid connection of wave energy converter in heaving mode operation by supercapacitor storage technology

This article proposes a modeling approach and an optimization strategy to exploit a third-harmonic current injection for the torque enhancement in multiphase isotropic permanent magnet synchronous machines with nonsinusoidal back electromotive forces. The modeling approach is based on a proper vector space decomposition and on the associated rotational transformation, aimed to properly select a set of stator current space vectors to be controlled. It is presented for a generic (i.e., asymmetrical, with an arbitrary angular shift) winding configuration. The injection strategy is related to the choice of a constant synchronous current set aimed at minimizing the average stator winding losses for a given reference torque by using the first and the third spatial harmonics of the air-gap flux density. The optimal solution has been found analytically and has been developed in detail for a selected set of asymmetrical winding configurations. Both the numerical and experimental results are in good agreement with the theoretical analysis.

/pdf/optimal-third-harmonic-current-injection-for-asymmetrical-34pasidko6.pdf

Gianluca Brando

Papers

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

Generalised look-up table concept for direct torque control in induction drives with multilevel inverters

Power Electronic Transformer application to grid connected photovoltaic systems

Grid connection of wave energy converter in heaving mode operation by supercapacitor storage technology

Optimal Third-Harmonic Current Injection for Asymmetrical Multiphase Permanent Magnet Synchronous Machines