Alysson A. P. Machado

Central inverter topology issues in large-scale photovoltaic power plants: Shading and system losses

Most of the large-scale PV power plants are based on central inverters, in which the PV panels are concentrated in single or few MPPTs and connected to the grid through three-phase inverters. Although its high conversion efficiency, extracting the total available power from an association of a large number of PV panels represents an issue in these systems. The irregular shading effects added to the intrinsic limitations in series/parallel panel associations may result in severe limits on the power injected into the grid. This paper is focused on simulations and experimental results from a power plant subjected to controlled shading conditions. In addition of this work, an alternative multi-MPPT converter topology capable of dealing with such conditions is enunciated and proper references from it are also pointed out.

A new fault-tolerant realization of the active three-level NPC converter

The three-level (3L) neutral-point-clamped (NPC) voltage-source (VSC) converter has been the standard solution in medium-voltage range for industrial applications. Recently, the NPC converter structure was improved by adding two extra active switches per phase leg, thus overcoming one of its main drawbacks: the unequal power loss among the power semiconductors. The resulting structure became known as active NPC (ANPC) converter. The high availability of these power converters is mandatory, since their maintenance or replacement imply equipment downtimes incompatible with the requirements of the production processes. The fault-tolerant operation of such converters has been addressed in the technical literature, but the main proposed solutions result in more complexity and higher parts count. This work proposes a new fault-tolerant structure for the ANPC IGBT-based converter, using standard devices, with very low additional cost and small complexity. The main goal is to show that the ANPC converter can overcome different and successive failures in its power devices without loss of functionality, before its definitive downtime.

Forward dual-active-bridge solid state transformer for a SiC-based cascated multilevel converter cell in solar applications

Central inverters based on conventional topologies are the current preferred solution in solar farms because of their low cost and simplicity. However, such topologies have some disadvantages as poor maximum power tracking, use of bulky filters and low frequency transformers. A good alternative in this case is the SiC-based Cascaded Multilevel Converter (CMC), which provides a distributed MPPT control with reduced footprint and high flexibility. Each cell of a CMC usually has as an intermediate stage a solid-state transformer based on a Dual-Active-Bridge (DAB) DC-DC Converter. Due to the unidirectional power flow characteristic of the photovoltaic application and aiming further reduction in the converter footprint, this work proposes a Forward Dual-Active-Bridge (F-DAB) topology, which reduces the number of active switches. This paper shows through analytical, simulation and experimental results that the cell using an F-DAB is superior to other unidirectional topologies in two aspects: greater power density with the available power modules and simplicity of control.

Control of a SiC-based Cascaded Multilevel Converter cell for solar applications

Central inverters based on conventional topologies are the current preferred solution in solar farms because of their low cost and simplicity. However, such topologies have some disadvantages: poor Maximum Power Point Tracking (MPPT), and the use of bulky filters and Low Frequency (LF) transformers. An attractive alternative in this case is the Cascaded Multilevel Converter (CMC), which can provide a distributed MPPT control, allied with overall reduced footprint and high flexibility. A CMC cell using silicon carbide devices has been proposed and designed in previous works to incorporate three main functions: MPPT control of a Photovoltaic (PV) array, galvanic isolation through a Solid-State Transformer (SST) and control of grid power flow. This work proposes a closed-loop control strategy for each stage of the CMC cell and shows its validation thorough simulations. Experimental results are performed and presented in a single-phase 6.2 kW prototype cell. These results lead to the conclusion that the applied control techniques are suitable to the PV application.

Fault-tolerant Utility Interface power converter for low-voltage microgrids

The utility interface (UI) converter is a three-phase power conversion unit equipped with energy storage which governs the interaction between the utility grid and the microgrid. The non-interruptive operation of UI is desired due to its several functions: 1) in grid-connected operation it performs as a voltage-supporting unit regulating the power flow through the microgrid point of common coupling, and compensating reactive power, unbalance and distortions caused by loads. 2) Whereas in islanded operation it performs as a voltage-forming unit and sets voltage amplitude and frequency for the entire microgrid. 3) Moreover, the UI ensures seamless transitions from grid-connected to islanded operation, actively decoupling the microgrid and the mains. Therefore, the UI plays a crucial role in centralized microgrid structures, and being a single-point of failure its safe and reliable operation entails directly the overall microgrid reliability. Thus, fault tolerant operation of such converter is a very important issue. The three-phase active neutral point clamped based power converter topology is capable to survive to successive open-circuit faults. Besides, it takes into account the advantages of the multilevel topologies: improved waveform quality, reduced filter size and equal loss distribution in the semiconductors. Simulation results are shown to analysis the operation of the fault-tolerant UI under faults.

Mitigation of electric arc furnace transformer inrush current using soft-starter-based controlled energization

The energization of power transformers is a frequent operation in Electric Arc Furnaces (EAF). Magnetization inrush currents with 10 to 50 times the rated transformer current can flow during several line cycles. The magnitude of the inrush currents depends on parameters like the point on wave of voltage energization, residual flux in the core, transformer size and utility systems characteristics. This momentary high current results in system disturbances and reduction of the transformer life cycle. In this paper a scheme to mitigate the inrush current using controlled voltage ramp soft-starter-based circuit is presented. Simulation and experimental results prove the effectiveness of the soft-starter-based controlled energization.

Probabilistic Assessment and Evaluation of Transients in a Medium-Voltage Three-Phase Capacitor Bank Energized by Unsynchronized Vacuum Switchgears

This paper presents a case study of a medium-voltage three-phase capacitor bank (CB) energization, based on an unsynchronized switching scheme using three independent pole vacuum switches simultaneously commanded. The motivations are cost reduction and simplification of the switching scheme. The maximum current and voltage peaks during transients caused by connection of CB with three independent pole switches or a three-phase mechanically coupled switch are compared. A probabilistic assessment for the overcurrents and overvoltages based on Monte Carlo simulation is also performed. The obtained results show that three single-phase unsynchronized switches could be used in a CB, with good transitory response.

Probabilistic assessment and evaluation of transients in a medium voltage three-phase capacitor bank energized by unsynchronized vacuum switchgears

This paper presents a case study of a medium-voltage three-phase capacitor bank (CB) energization, based on an unsynchronized switching scheme using three independent pole vacuum switches simultaneously commanded. The motivations are cost reduction and simplification of the switching scheme. The maximum current and voltage peaks during transients caused by connection of CB with three independent pole switches or a three-phase mechanically coupled switch are compared. A probabilistic assessment for the overcurrents and overvoltages based on Monte Carlo simulation is also performed. The obtained results show that three single-phase unsynchronized switches could be used in a CB, with good transitory response.

Experimental results of a Thyristor Switched Series Reactor for Electric Arc Furnaces

Supplementary series reactors are widely used in high power AC electric furnace to allow furnace operation with longer arcs, lower currents and lower electrode consumption. Such reactors can be switched off in the final stages of the melting process when the electric arc is stable and arc ignition is not of great concern. Bypassing the reactors with the use of conventional high voltage circuit breakers is not practical due to the maintenance problems related to the great number of operations. This paper describes an oil-immersed thyristor switch with a proper design for installation at a high voltage industrial substation to switch on and off the series reactor of the electric arc furnace circuit in order to improve furnace productivity. The concept of thyristor switched series reactors, TSSRs, can also be used to minimize or even replace on load tap changing of the furnace transformer

Photovoltaic power plant submitted to voltage sags: Model and performance evaluation

This paper brings the PV model of a real PV farm installation with a total power of 917 kW. All the elements of the PV farm (panels and inverter) are described. Simulation on PSCAD and experimental results at the installation are reported. The PV model is submitted to voltage sags originated in the grid as well as their impact on the photovoltaic power plant.