Nicole C. Foureaux

Application of solid state transformers in utility scale solar power plants

With the increasing participation of PV sources in the international energy market, more efficient inverter topologies and modules connections are desired. Large-scale PV plants (solar farms) use generally the central inverter topology, in which the PV modules are distributed into few maximum power point trackers. Although this topology has typically high efficiency, it has low MPP tracking performance. This paper proposes a modular converter based on solid state transformers (SST), in which high frequency transformers are used replacing the bulky medium-voltage line-frequency ones. In this technology, the converter output is provided by the series connection of the DC-DC-AC converter stages, supplied by independent PV module arrays. Among its many advantages, one can point out their reduced size and weight, improved reliability and easier maintenance of the system. This paper presents the proposed SST based converter topology for PV power plants and addresses some of its control issues.

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.

Low voltage PV power integration into medium voltage grid using high voltage SiC devices

High voltage high power semiconductor devices are being used for grid integration of renewable energy sources. 1200V, 100A SiC Mosfets, 10 kV SiC Mosfets and 10kV SiC JBS Diodes have proven to be beneficial for high voltage application. High Voltage SiC devices enable high switching frequency operation thus reducing size of passive elements. Scope of this paper focuses on an alternative approach for 0.9 MW PV power plant, which is currently being constructed in Brazil. Use of high power SiC devices for PV power plant for integration into 13.8 kV grid provides higher efficiency, reduction in size and volume.

Cascaded multilevel SST medium voltage converter for solar applications

Most of the large-scale PV power plants (solar farms) are based on central inverters, in which the PV modules are concentrated in few maximum power point trackers and connected to the grid through three phase inverters. Although this topology has typically high efficiency, it has limited performance in terms of capturing all the power available from the PV modules. This paper proposes a cascaded multilevel converter based on solid state transformers (SST) technology, in which multiple high frequency transformers replace the bulky medium-voltage line-frequency function. In the proposed alternative, the converter grid front-end consists of a series connection of H-bridge inverters supplied by independent PV module arrays through solid state transformers. Among the many advantages of the proposed solution, it is important to point out: multiple MPP trackers, reduced size and weight, improved reliability and conversion efficiency and easier maintenance. The proposed SST based converter topology for PV power plants and its main design and control issues is addressed.

SiC based cascaded multilevel converter for solar applications: Downscaled prototype development

Photovoltaic power generation is growing worldwide at an accelerated pace. A cascaded multilevel converter topology based on Silicon Carbide switches and high frequency components in association with multiple maximum power point tracking capabilities addresses common limitations of central grid inverters, targeting a raise in the overall efficiency of large-scale photovoltaic power plants. The topology of the converter cell is overviewed, and the design issues of a downscaled prototype utilizing 1,2kV Silicon Carbide semiconductor devices and high frequency transformer are discussed.

Utility-accessible external disconnect switch suppression in PV microgeneration: Brazilian scenario

Utility-Accessible External Disconnect Switch (EDS) is an AC switch used in electric power systems to disconnect Distributed Energy Resources (DER) from the main grid during maintenance. The EDS utilization is intended for line worker safety, since it is currently the only mechanism to ensure galvanic disconnection between DER and distribution network. Frequency inverters used in DER has a built-in switch, composed by a relay, called Integrated Disconnect Switch (IDS), and protective functions (anti-islanding, under/over-voltage and -frequency algorithms). In some countries, the EDS were recently considered unnecessary and redundant for PV systems, since the IDS provide DER galvanic disconnection when it detects utility outage or abnormal conditions. Moreover, the EDS suppression reduces the DER overall cost. The elimination of the EDS from PV low power installations was recently in discussion in Brazil. This paper aims to present a study on the reasons why the EDS should not be eliminated from PV grid-tied low-power installations. The existing standards, network maintenance procedures and the EDS cost/effectiveness are discussed.

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.

Command Generation for Wide-Range Operation of Hysteresis-Controlled Vienna Rectifiers

Hysteresis current controllers inherently present wide bandwidth and have been proposed for application in Vienna rectifiers switching at high frequencies. In such applications, near zero ac input currents are required at no load. Since the hysteresis band is limited by switching losses and delays, controlling low current amplitudes becomes an issue. In this paper, it is described as a suitable alternative to overcome such limitation, satisfying both the control of the power flow at no load and minimum current amplitude requirements. Test results on a 40-A/380-V Vienna rectifier are presented to support the theoretical analysis.

Command generation for wide range operation of hysteresis controlled Vienna rectifiers

Hysteresis current controllers inherently present wide bandwidth and have been proposed for application in Vienna rectifiers switching at high frequencies. In these applications, near zero ac input currents are required at no load. Since the hysteresis band is limited by switching losses and delays, controlling low current amplitudes becomes an issue in Vienna rectifiers. In this paper it is described a suitable alternative to overcome such limitation, satisfying both the control of the power flow at no load and the minimum current amplitude requirements. Test results on a 400A/380V Vienna rectifier are present to support the theoretical analysis.