The power grid is a massive interconnected network used to deliver electricity from suppliers to consumers and has been a vital energy supply. To minimize the impact of climate change while at the same time maintaining social prosperity, smart energy must be embraced to ensure a balanced economical growth and environmental sustainability. There fore, in the last few years, the new concept of a smart grid (SG) became a critical enabler in the contempo rary world and has attracted increas ing attention of policy makers and engineers. This article introduces the main concepts and technological challenges of SGs and presents the authors' views on some required challenges and opportunities pre sented to the IEEE Industrial Electronics Society (IES) in this new and exciting frontier.

The New Frontier of Smart Grids

DC fault protection is one challenge impeding the development of multi-terminal DC grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and DC fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode DC component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a DC to DC transformer where two converters operating in the proposed mode are coupled by an AC transformer for voltage matching and galvanic isolation. The proposed DC transformer is shown to be suitable for high-voltage high-power applications due to the low switching frequency, high efficiency, modularity, and reliability. The DC transformer facilitates DC voltage regulation and near instant isolation of DC faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

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Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

Although HVDC transmission systems have been available since mid-1950s, almost all installations worldwide are point-to-point systems. In the past, the lower reliability and higher costs of power electronic converters, together with complex controls and need for fast telecommunication links, may have prevented the construction of multiterminal DC (MTDC) networks. The introduction of voltage-source converters for transmission purposes has renewed the interest in the development of supergrids for integration of remote renewable sources, such as offshore wind. The main focus of the present work is on the control and operation of MTDC networks for integration of offshore wind energy systems. After a brief introduction, this paper proposes a classification of MTDC networks. The most utilized control structures for VSC-HVDC are presented, since it is currently recognized as the best candidate for the development of supergrids, followed by a discussion of the merits and shortcomings of available DC voltage control methods. Subsequently, a novel control strategy-with distributed slack nodes-is proposed by means of a DC optimal power flow. The distributed voltage control (DVC) strategy is numerically illustrated by loss minimization in an MTDC network. Finally, dynamic simulations are performed to demonstrate the benefits of the DVC strategy.

A Novel Distributed Direct-Voltage Control Strategy for Grid Integration of Offshore Wind Energy Systems Through MTDC Network

This paper presents an analytical design method for the dc collector systems of offshore wind farms and proposes an improved topology to overcome the limitations of the conventional series-parallel (SP) topology. During a severe failure in the SP topology, bypassing the faulty units imposes overvoltage on fault-free units and leads to the disconnection of the series branch from the collector system. By providing auxiliary connection paths, the suggested topology reduces the overvoltage of the units and hence maintains fault-free units operational following a severe failure. Hence, the efficiency of a wind farm increases considerably upon any failure occurrence. To show the merits of the suggested topology, the captured power during a failure has been calculated and fairly compared with that of the conventional SP topology, considering the excess cost of the suggested topology due to additional connection paths. To investigate the performances of both topologies and their feasibility, a collector system including 12 wind turbine generators has been simulated in time domain software environment. Then, various failure scenarios have been investigated to evaluate the transient behavior of the dc collector system following slight and severe failures. The study results reveal the feasibility of the proposed topology as a promising structure for future dc collector systems.

Topology Design for Collector Systems of Offshore Wind Farms With Pure DC Power Systems

For achieving the European renewable electricity targets, a significant contribution is foreseen to come from offshore wind energy. Considering the large scale of the future planned offshore wind farms and the increasing distances to shore, grid integration through a transnational DC network is desirable for several reasons. This article investigates a nine-node DC grid connecting three northern European countries—namely UK, The Netherlands and Germany. The power-flow control inside the multi-terminal DC grid based on voltage-source converters is achieved through a novel method, called distributed voltage control (DVC). In this method, an optimal power flow (OPF) is solved in order to minimize the transmission losses in the network. The main contribution of the paper is the utilization of a genetic algorithm (GA) to solve the OPF problem while maintaining an N-1 security constraint. After describing main DC network component models, several case studies illustrate the dynamic behavior of the proposed control method.

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Operation and Power Flow Control of Multi-Terminal DC Networks for Grid Integration of Offshore Wind Farms Using Genetic Algorithms

The transmission of bulk power over long distances is a very current topic in the worldwide energy sector. An excellent example is the transmission system of Madeira River Hydropower Generation Project in Brazil, which includes two 2,400-km transmission lines, designed to transmit 3,150 MW through each HVDC link. In fact, long transmission trunks enable the use of natural resources that are far from major load centers as well as provide the interconnection of large power systems.

Going the Distance

For very high power applications, it is imperative to limit switching losses in the power converter. Multipulse converters are made up from several converter modules, switching at the fundamental frequency, but properly phase shifted to produce a nearly sinusoidal output voltage. Robust controls for HVDC and FACTS systems based on multipulse converters were developed. The developed HVDC system makes the rectifier station to control the instantaneous active (real) power through the HVDC voltage link, whereas the inverter station controls the instantaneous reactive (imaginary) power generation. The control of real and imaginary powers is independent one from the other and there is no control signal that is shared by both converter stations. A complete model of a 100 MW, 33 kVac, 50/60 Hz, 24-pulse HVDC system was implemented in the electromagnetic transient program (EMTP/ATP). The static synchronous compensator (STATCOM) is a shunt type of FACTS controller. It regulates the voltage magnitude of a controlled ac bus by drawing variable reactive current from the power system. The fast dynamic of the STATCOM controller avoids high fluctuations of the dc voltage caused by transients. A complete model of a 100 MW, 33 kV, 60 Hz, 24-pulse STATCOM was implemented and the results have confirmed the robustness of the proposed voltage control.

Multipulse converters and controls for HVDC and FACTS systems

The transmission of bulk power over long distances is a very current topic. In fact, it allows the connection of remote power plants to the electrical system, a common situation in countries of continental dimensions - such as China, Russia, Brazil and Canada and also some regions in Europe and Asia -where hydroelectricity and other renewable energy resources are often far away from the large consumer areas. Another benefit by means of long transmission power corridors is the possibility of adding electrical and energetic benefits for the two interconnected power systems. Regarding solutions to transmitting bulk power over long distances, the High Voltage DC Transmission System (HVDC) and the Half-Wave Length Transmission System have been shown as adequate solutions, depending on the length of the transmission trunk. However, both HVDC and half-wave length transmission systems have some restrictions to derive fractional amount of power along the main power corridor, since they are intrinsically point-to-point transmission systems. In this sense, the paper presents some solutions to overcome this problem in HVDC systems, as well as in half-wavelength transmission systems by means of HVDC and HVAC tapping. Moreover, a FACTS device is presented as an alternative to provide fast response, active power flow control in the half-wavelength transmission system.

Power electronics applications in bulk power transmission over long distances

This work presents the experimental development of a four-wire single-phase to three-phase static converter for rural distribution system. The converter topology is based on a front-end full-controlled PWM rectifier that handles the DC bus voltage regulation as well as improves the input power factor correction. In order to provide the three-phase four-wire system, a four-legged PWM voltage source inverter (VSI) is used at the output. The rectifier-inverter control strategy has been implemented in a digital processor (DSP), where a proposed PLL algorithm performs the rectifier synchronism. Component specification guidelines and control strategies are described. Complete models are implemented in an electromagnetic transient simulation package (PSCAD/EMTDC/spl trade/), where the converter is connected to a rural distribution network and is supplying three-phase and single-phase loads. Simulation results show that this converter can ride through a typical voltage sag condition that is very often in rural areas. The converter topology is composed of 12 IGBTs and can be implemented by two off-the-shelf three-phase PWM inverters. This availability can offer significant cost reduction. Experimental results are provided.

Four wire single-phase to three-phase system for rural distribution network

This work presents the development of four-wire single-phase to three-phase static converters for rural distribution system. Component specification guidelines and control strategies of two proposed topologies are described. Both converters are based on a front-end half-controlled PWM rectifier that handles the DC bus voltage regulation as well as reduces the input current distortion. In order to provide three-phase four-wire system, two basic inverter topologies are considered: three-leg PWM-VSI with split DC link capacitors and four-leg PWM-VSI. Complete models are implemented in an electromagnetic transient simulation package (PSCAD/EMTDC ), where each converter is connected to a typical rural distribution network and supplies three-phase unbalanced loads. Performance analysis based on simulation results is carried out considering the occurrence of voltage sags.

A.F.C. Aquino

Papers

Going the Distance

Multipulse converters and controls for HVDC and FACTS systems

Power electronics applications in bulk power transmission over long distances

Four wire single-phase to three-phase system for rural distribution network

Single-phase static converters for rural distribution system