Review of dc-dc converters for multi-terminal HVDC transmission networks

This study presents a comprehensive review of high-power dc-dc converters for high-voltage direct current (HVDC) transmission systems, with emphasis on the most promising topologies from established and emerging dc-dc converters. In addition, it highlights the key challenges of dc-dc converter scalability to HVDC applications, and narrows down the desired features for high-voltage dc-dc converters, considering both device and system perspectives. Attributes and limitations of each dc-dc converter considered in this study are explained in detail and supported by time-domain simulations. It is found that the front-to-front quasi-two-level operated modular multilevel converter, transition arm modular converter and controlled transition bridge converter offer the best solutions for high-voltage dc-dc converters that do not compromise galvanic isolation and prevention of dc fault propagation within the dc network. Apart from dc fault response, the MMC dc auto transformer and the transformerless hybrid cascaded two-level converter offer the most efficient solutions for tapping and dc voltage matching of multi-terminal HVDC networks

Power Balancing in Cascaded H-Bridge and Modular Multilevel Converters Under Unbalanced Operation: A Review

Multilevel Voltage-Source Converters (VSC) based on modular structures are envisioned as a prominent alternative for grid and industry applications. Foremost among these are the Cascaded H-Bridge (CHB) and the Modular Multilevel Converter (MMC). In this context, depending on the application and the power conversion structure, unbalanced operating conditions can be asked to the converter. Previous investigations regarding the operation and the solutions for modular structures under unbalanced conditions have already addressed this topic, but information is dispersed over a wide number of sources. This paper identifies, classifies, and analyzes the intercluster active power balancing strategies for the adequate operation of the most commonly used modular structures in some typical unbalanced operating scenarios: the Static Synchronous Compensator (STATCOM) under unbalanced voltage and/or current conditions, the unequal power generation in large-scale photovoltaic (PV) power plants, and the uneven power distribution in a battery energy storage system (BESS). Each of the applications has been independently studied so as to provide a comprehensive analysis of the alternative techniques found in the specialized literature, clearly explaining their respective strengths and drawbacks. Several future challenges have been identified during the study, which will involve greater research effort in this key research topic

DC fault ride-through capability and STATCOM operation of a HVDC hybrid voltage source converter

HVDC transmission systems are becoming increasingly popular when compared to conventional AC transmission methods. HVDC voltage source converters (VSC) can offer advantages over traditional HVDC current source converter topologies; as such, it is expected that HVDC-VSCs will be further exploited with the growth of HVDC transmission. This paper presents the DC fault ride through capability and new STATCOM modes of operation for the recently published Alternate Arm Converter (AAC), intended for the HVDC market. Operation and fault ride through of the converter during a local terminal to terminal short circuit of the DC-Link is demonstrated; during the fault STATCOM operation is also demonstrated

Solar Photovoltaic and Thermal Energy Systems: Current Technology and Future Trends

Solar systems have become very competitive solutions for residential, commercial, and industrial applications for both standalone and grid connected operations. This paper presents an overview of the current status and future perspectives of solar energy (mainly photovoltaic) technology and the required conversion systems. The focus in the paper is put on the current technology, installations challenges, and future expectations. Various aspects related to the global solar market, the photovoltaic (PV) modules cost and technology, and the power electronics converter systems are addressed. Research trends and recommendations for each of the PV system sectors are also discussed.Junta de Andalucía P11-TIC-7070Ministerio de Ciencia e Innovación TEC2016-78430-

Mitigation of power quality issues due to high penetration of renewable energy sources in electric grid systems using three-phase APF/STATCOM technologies: a review.

This study summarizes an analytical review on the comparison of three-phase static compensator (STATCOM) and active power filter (APF) inverter topologies and their control schemes using industrial standards and advanced high-power configurations. Transformerless and reduced switch count topologies are the leading technologies in power electronics that aim to reduce system cost and offer the additional benefits of small volumetric size, lightweight and compact structure, and high reliability. A detailed comparison of the topologies, control strategies and implementation structures of grid-connected high-power converters is presented. However, reducing the number of power semiconductor devices, sensors, and control circuits requires complex control strategies. This study focuses on different topological devices, namely, passive filters, shunt and hybrid filters, and STATCOMs, which are typically used for power quality improvement. Additionally, appropriate control schemes, such as sinusoidal pulse width modulation (SPWM) and space vector PWM techniques, are selected. According to recent developments in shunt APF/STATCOM inverters, simulation and experimental results prove the effectiveness of APF/STATCOM systems for harmonic mitigation based on the defined limit in IEEE-519

Hybrid cascaded modular multilevel converter with DC fault ride-through capability for HVDC transmission system

A new hybrid cascaded modular multilevel converter for high-voltage dc (HVDC) transmission system is presented. The half-bridge (HB) cells are used on the main power stage and the cascade full-bridge (FB) cells are connected to its ac terminals. The main power stage generates the fundamental voltages with quite low switching frequency, resulting relatively low losses. The cascaded FB cells only attenuate the harmonics generated by the main power stage, without contribution to the power transfer. Thus, the energy storage requirement of the cascaded FB cells is low and the capacitance of FB cells is reduced significantly. Due to the dc fault reverse blocking capability of the cascaded FB cells, the proposed topology can ride-through the pole-to-pole dc fault. In addition the soft restart is achieved after the fault eliminates, without exposing the system to significant inrush current. Besides, the average-value model of the proposed topology is derived, based on which the control strategy is presented. The results show the feasibility of the proposed converter

A monopolar symmetrical hybrid cascaded DC/DC converter for HVDC interconnections

With the rapid development of voltage source converter (VSC) based high voltage direct current (HVDC) transmission, it is an irresistible trend that HVDC grid will come into being. High-voltage and high-power DC/DC converters will serve as DC transformers in HVDC grid to interconnect DC lines with different voltage ratings. This paper proposes a monopolar symmetrical DC/DC converter which is composed of cascaded half-bridge sub-modules (SMs) and series-connected IGBTs. This hybrid topology features low capital costs, high efficiency, small footprint, and bidirectional power transfer capability. Operation principle, parameter design, and the control strategies of this topology are introduced. A 480MW, ±500kV/±160kV monopolar symmetrical DC/DC converter is simulated to verify its performance and evaluate the efficiency. In addition, a downscaled prototype rated at 2.4kW, ±300V/±100V has been built and tested. Experimental results further validate the effectiveness of the proposed DC/DC converter

Sizing and Short-Circuit Capability of a Transformerless HVDC DC-DC Converter

This work aims at optimizing the converter design of the double-T MMC DC-DC converter in terms of transmitted power per submodule and also in terms of transmitted power per silicon area, while, at the same time, providing the capability to block dc faults. Firstly, the converter operation is described and the optimal values of the inner ac and dc voltages that minimize device power rating are derived. Next, the submodule topology is analyzed and a thorough study on the converter capability for blocking fault currents is carried out, showing that the converter is able to isolate dc faults both at the input and at the output of the converter. Finally, the previous analytical study is verified by means of detailed PSCAD simulations

Hybrid Smart Transformer for Enhanced Power System Protection Against DC With Advanced Grid Support

The traditional grid is rapidly transforming into smart substations and grid assets incorporating advanced control equipment with enhanced functionalities and rapid self-healing features. The most important and strategic equipment in the substation is the transformer and is expected to perform a variety of functions beyond mere voltage conversion and isolation. While the concept of smart solid-state transformers (SSTs) is being widely recognized, their respective lifetime and reliability raise concerns, thus hampering the complete replacement of traditional transformers with SSTs. Under this scenario, introducing smart features in conventional transformers utilizing simple, cost-effective, and easy to install modules is a highly desired and logical solution. This dissertation is focused on the design and evaluation of a power electronics-based module integrated between the neutral of power transformers and substation ground. The proposed module transforms conventional transformers into hybrid smart transformers (HST). The HST enhances power system protection against DC flow in grid that could result from solar storms, high-elevation nuclear explosions, monopolar or ground return mode (GRM) operation of high-voltage direct current (HVDC) transmission and non-ideal switching in inverter-based resources (IBRs). The module also introduces a variety of advanced grid-support features in conventional transformers. These include voltage regulation, voltage and impedance balancing, harmonics isolation, power flow control and voltage ride through (VRT) capability for distributed energy resources (DERs) or grid connected IBRs. The dissertation also proposes and evaluates a hybrid bypass switch for HST module and associated transformer protection during high-voltage events at the module output, such as, ground faults, inrush currents, lightning and switching transients. The proposed strategy is evaluated on a scaled hardware prototype utilizing controller hardware-in-the-loop (C-HIL) and power hardware-in-the-loop (P-HIL) techniques. The dissertation also provides guidelines for field implementation and deployment of the proposed HST scheme. The device is proposed as an all-inclusive solution to multiple grid problems as it performs a variety of functions that are currently being performed through separate devices increasing efficiency and justifying its installation